Unit 03 - Materials

5/24/2018 Unit 03 - Materials

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M a n uf a c tu re O f I ro n

Iron ore is obtained from the earth and consists of iron oxide with impurities. The impurities, which vary in quantity

with different ores, are Silica, Alumina, C alcium, Manganese, Phosphorus and Sulphur.

The o re is c o nve r te d t o p ig ir on in a b la st fur na c e. The fur na c e is c ha r ge d w it h a lt er na te la ye rs o f c o ke , o r e a nd

limes to ne and air at ab o ut 1 0 0 0 F and 1 5 - 2 0 lb / sq in b lo wn t hr o ugh. W hen t he fur nace is w or k ing no r mally t her e is a

gr adual incr eas e in t emp erat ur e fr o m t o p t o b o tt o m. O x ygen is r emo ved f ro m t he o r e and at ab o ut 2 4 0 0 F t he met albecomes incandescent and starts to absorb carbon. This lowers the melting point of the metal which now melts rapidly

and r uns d o wnw ar d s o ver glo wing co k e, abs o rb ing mo r e car bo n, t o t he b o tt o m o f t he fur nace w her e it is t app ed o ff

and r un int o " pigs " o r t aken d ir ectly t o t h e s t eel mak ing p lant . T he limes to ne w hich is a flux fo r ms a s lag w hich ab s or b s

s o me o f t he imp ur it ies . The s lag lies o n t o p o f t he mo lt en met al and is t app ed o ff fr o m t ime t o t ime. P ho s p ho r us and

manganese contents are unchanged by slagging.

The pig iron produced contains from 3% to 4% of carbon and quantities of Silicon, Sulphur, Phosphorus and

M anganes e, d ep end ing o n t he q ualit y o f t he o r e.


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M a nufa c ture o f S te e l - Ac id a nd B a s ic P ro c e s s e s

S t eel is es s ent ially an allo y o f ir o n and car bo n and is p r od uced b y t he r emo val o xid atio n o f t he imp ur it ies fr o m mo lt en

pig iron or from a molten mixture of pig iron and scrap metal. The presence of these impurities makes the metal weak

and brittle.

T h e p ri nc i pa l p ro c e s s e s o f ma n uf a c tu re a r e :i. Bessemer Converters

ii. O p en H ear th F ur naces

iii. Electric Furnaces

B es s emer C o nver ter s and O p en H ear th F ur naces ar e gener ally emp lo yed b ut fo r high gr ad e allo y s t eels t he E lectr ic

F ur nace may b e us ed . The t erms "acid " and "b as ic" r efer t o the fur nace linings us ed and t he nat ur e o f t he s lag

produced. If the pig iron has a low phosphorus content an acid lining - silica can be used and in this case the

phosphorus content will not alter. To remove phosphorus lime is added to the charge and a basic slag is formed. This

slag would react with an acid furnace lining so a basic lining magnesiteor dolomite is used.

W hen t he char ge in t he s t eel mak ing fur nace is r eady t he s t eel is in an o xid is ed co nd it io n and deo xid atio n is neces sar ybefore pouring into ingot moulds. This is achieved by adding manganese which has a strong affinity for oxygen. The

manganese oxide formed is insoluble and mainly floats out as slag, any remaining within the ingot being in the form of

harmless inclusions. Prior to pouring into the ingot moulds, additions of carbon, silicon and manganese are made to

give the steel the required composition.


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B e s s e me r P ro c e s s

The molten iron is run into the converter, the blast air turned on and converter tipped to the upright position. The

imp ur it ies ar e o xid is ed i n t he o r der s ho wn in t he ab o ve gr ap h. P ho s p ho r us w hen o xid is ed w ill no t fo r m a s t ab le

phosphate unless there is an excess of lime. Under these conditions, the phosphorus is not removed. To remove the

phosphorus, lime must be added to the charge and a basic lining used as previously stated. The stable phosphate

formed is removed with the slag. When the manganese, silicon and carbon are reduced to a low percentage, as

ind icat ed by t he co lo ur o f t he flame, t he co nver ter is t app ed . B y t he ad d it io n o f an allo y o f manganes e ar e o b tainedand harmful oxides eliminated as far as possible.

B a s i c O xyg e n P ro c e s s

B as ic o xygen p lant has lo wer ins t allat io n co s ts and incr eas ed p r od uct io n co mp ar ed w it h o p en hear th fur naces and t he

d evelo p ment o f s uch p r oces s r epr es ent s a maj or t echno lo gical ad vance in t he ar t o f s t eel mak ing. S ever al d iffer ent

t yp es o f fur nace ar e b eing us ed in d iffer ent p art s o f t he w or ld ; ho wever , t he t wo i llus t rat ed in F igur e b elo w ar e t yp ical.

B ot h t he Linz D onaw it z* ( L- D ) and t he R ot o r co nver ter have cer tain s imilar it ies in t hat ( 1 ) t hey co ns is t o f s t eel s hells

lined with high-grade basic refractories, (2) they take charges consisting of steel scrap, molten pig iron and limestone,

and (3) they both employ oxygen blasts to remove the impurities by oxidation.


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H ow ever , w hile t he L- D co nver ter r emains up r ight d ur ing it s w or k ing cycle and us ed o xygen b lo wn o n t o t he s ur face

of the charge, the Rotor converter is mounted horizontally and slowly rotated while oxygen is blown both into and on

top of the molten charge.

The fur na c e r ea c tio ns a re s imila r t o t ho se o f t he o p en he a rt h fur na c e, b ut t he who le p ro c es s is muc h fa s te r a nd no

ext ernal heating is r eq uir ed b ecaus e o f t he s tr o ngly exo t her mic nat ur e o f s o me o f t he r eact io ns . V er y high- g rad e s teel

is produced quite inexpensively by this process.


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O pe n He a rth P ro c e s s

M o lt en met als o r p ig and s cr ap ir o n is char ged int o t he fur nace. The air and g as ar e t ur ned and f lux ad d ed. S ince t he

ga s a nd a ir a re c o nt ro lle d t he p ro c es s c a n b e s lo w t he e xt ent r e quir ed .

This allo ws t he t aking o f s amp les t o o b s erve p r ogr ess . T he flap d o o r s ar e s wit ched ab o ut 3 0 minut e int ervals t o

mak e us e o f t he r egener ative chamb er s. The r eq uir ed d eo xid ers and carb o n ar e ad d ed as in t he B es s emer p r oces ses .

T he s lo w r ate o f co o ling w hich t akes p lace in t he ingo t mo uld s , gives r is e t o a co ur s e gr ain s tr uct ur e and co ns eq uent ly

the steel is weak and brittle. Refinement of the grain structure is accomplished by hot working.


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E l e c t r i c F u r n a c e s

The main ad vant age o ffer ed b y t he electr ic fur nace is t he neut ral nat ur e o f t he heat s o ur ce. The mo s t co mmo n t yp e o f

electr ic fur nace is t he d ir ect- ar c t yp e w her e heat co mes fr o m ar cing b etw een t he lo ng gr aphit e elect ro d es and mo lt en

charge. It is the efficient method and modern steel plants use this method.


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P la in C a rbo n S te e ls

W hen ir o n is heat ed w it hin t he s o lid s t at es, it exhib it s s ever al d is t inct changes . The t emp er at ur e at w hich t hes e

changes t ake p lace, ar e t ermed ar res t p o int s. Tw o o f t hes e p o int s ar e d ue t o changes in t he at om p att ern and t he

temperatures at which the changes occur are termed upper and lower critical temperatures. Above upper critical

temperature carbon is completely soluble in solid iron and below lower critical temperature carbon is insoluble in solid

ir o n. Lo wer cr it ical t emp er at ur e is co ns t ant 7 2 3 C b ut up p er cr it ical t emp er at ur e d ep end s o n t he car bo n co nt ent o f

the steel.

The structures found in slowly cooled carbon steel are:

Austenite

A solid solution of carbon and iron, i.e. the metal is homogenize and

Ferrite

M ay b e r egar ded as p ur e ir o n. T his mat er ial is s o ft and d uct ile.

Cementite

Iron-carbide compound which is hard and brittle.

P e a r l i t e

A s t ruct ur e in w hich t he gr ains ar e co mp o s ed o f layer s o f fer rit e and cement it e and w hich is fo r med fr o m aus t enit e o f

0 . 8 3% car bo n co nt ent


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D e ta il O f B la s t F urna c e


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T h e B e s s e m e r C o n ve r te r


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B a s ic O pe n He a r th P ro c e s s

Consider specimens of steel, containing 0.4%C, 0.83%C and 1.2%C being slowly cooled from the austenitic state.

Refer to figure above.

I r on w it h 0 . 4 %C : A t X fer rit e o f co mp o s it io ns X 1 b egins t o p r ecip it ate mak ing r emaining aus t enit e r icher in car bo n

until carbon content becomes 0.83% at 723C and transformation from austenite to pearlite takes place. The final

s t ruct ur e is co mp o s ed o f gr aining o f fer rit e and gr ains o f p earlit e. I r on w it h 0 . 8 3% C : Tr ans fo r mat io n o f aus t enit e t o

pearlite begins and ends at 723C.

Final structure is all pearlite.

iron with 1.25C: At Y cementite is precipitated at austenite grain boundaries making the remaining austenite less rich

in car bo n unt il it s car bo n co nt ent co ns is t s o f p earlit e w it h cement it e at t he gr ain b o und ar ies .

The mechanical properties of slowly cooled plain carob steels depend upon the proportions of ferrite, pearlite and

cementite in the final structure. Figure 2 shows the relationship between carbon content, mechanical properties,

structure and uses of plain carbon steel in the normalised condition.

E ffect s o f o t her element s p r es ent in p lain car bo n s t eels

(These elements being residual from the refining process)

M a nga ne se - up to 1 %


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Silicon - up to 0.3%

S u lp hur - up t o 0 . 05 %

P ho s p ho r us - u p t o 0 . 0 5%

Manganese is an essential constituent since it deoxidises the steel, ensures freedom from blowholes and combines

with sulphur present. See Sulphur, Silicon has little effect on mechanical properties in low carbon steels but should notexceed 0 .2 % in high car bo n s t eels s ince it t end s t o b r eak d o wn cement it e int o gr aphit e o r fer rit e.

Sulphur may exists as ferrous sulphide or manganese sulphide. Ferrous sulphide appears at grain boundaries and is

hard and brittle at low temperatures and has a low melting point, giving rise to crumbling during hot working.

Manganese sulphide exits as soft inclusions, which produce no harmful effects. Phosphorus is soluble in steel and

r esult s in t he ap p ear ance o f a har d , b r it tle co ns tit uent . F o r t his r easo n, t he p ho s p ho r us o f s t eel s ho uld n o t b e allo wed

to exceed 0.05%.

T he mechanical p r o per ties o f p lain car bo n s t eel can b e var ied co ns id erab ly b y heat t reat ment . Ther e ar e t hr ee b as ic

methods:

1. Hardening

2. Tempering

3. Annea ling and N ormalising

Hardening involves heating the steel until it is 30-50C above upper critical temperatures followed by quenching. If

the rate of cooling is rapid enough insufficient time is available for the structural changes from austenite to ferrite,

pearlite and cementite to take place and an extremely hard and brittle constituent known as Martensite is formed.

Less rapid cooling results in an intermediate structure called Troostite. The extent to which hardening can be carried

o ut d ep end s o n s ectio n t hick nes s . T his is called mas s effect.

Temp er ing is emp lo yed t o r elieve q uenching s tr es ses and t o t o ughen t he s t eel. The s t eel is heated t o 2 0 0 C - 4 0 0 C

below the lower critical temperature followed by cooling. The higher the tempering temperature the more closely will

the structure revert to the slowly cooled state.


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Annealinga.

P r o cess o r s ub cr it ical annealing is car ried o ut o n co ld w o rk ed lo w car bo n s t eel t o r elieve int ernal s tr es s and

to soften the material. The steel is heated to about 50C below lower critical temperature and then allowed to

cool.

b.

Full annealing is employed on steel casting and hot worked steels to obtain grain refinement and ductility. The

steel is heated to about 50C above upper critical temperature and then allowed to cool slowly in the furnace.

c.

Normalising differs from full annealing in that the metal is allowed to cool in still air. Tensile strength and

impact values are higher than the figures obtained by annealing.

d.

Spheroidising annealing is used for softening high carbon steel to facilitate machining. The steel is heated to

just below lower critical temperature. Due to surface tension effects, the cementite assumes globular formwhile the remainder reverts to ferrite. After shaping, the pearlite-cementite structure can be restored by heat

t reatment . The heat t reatment r anges fo r p lain car bo n s t eels ar e s ho wn in figur e 3 .

e.

Case-Hardening - Where an article requires to have a hard wear-resisting surface, together with a tough

c o re , it is ne c es s ar y t o e mp lo y a lo w c a rb o n s te e l a nd c a se ha rd e n t he s ur fa c e t o o b t ain t his d ua l s tr uc tur e.

The car bo n co nt ent at t he s ur face is enr iched b y s ur ro und ing t he co mp o nent s in mat erials r ich in car bo n, such as a

mixt ur e o f char coal and b ar ium car bo nat e; t he p ieces t o b e car bo nis ed ar e p laced i nt o b o xes w it h t he car bo nis ing

materials and sealed. The boxes are then placed in a furnace and brought to a temperature around 900C and held at

t ha t t emp e ra tur e fo r a t ime d e pe nd a nt o n t he d e pt h o f c a se r eq uir e d. A ft er s ix t o e ight ho ur s, a lo w c a rb o n s te e l ma y

ha ve 0 .9 % c ar bo n to a d ep th o f a bo ut 0 .0 40 %.

A p a rt w hic h ha s b e en c a rb o nis e d a t s uc h a t emp e ra tur e will ha ve a c o ar se gr ain s tr uc tur e whic h mus t b e r efine d .

This is achieved by reheating the parts above the critical point and quenching in oil. This treatment results in refining

t he s tr uc tur e o f t he c o re . If it is d e sir ed , the c a se c a n b e t emp e re d at 1 5 0 C . The c a se is t he n ha rd e ne d , i. e . he a te d to

7 7 0 t hen q uenched in w at er. I t can t hen b e t emp er ed , i. e. h eat ed t o 2 0 0 C d ep end ing o n har d nes s r equir ed .

Cyanide Hardening

This met ho d o f har d ening is as fo llo ws :

The co mp o nent s ar e immer sed in a b ath o f S o d ium C yanid e at a t emp er at ur e j us t o ver 9 0 0 C . C yanid e co nt ains

car bo n and nit ro gen b o th o f w hich ar e active car bo nis ing agent s . I n a ver y s ho r t t ime, t he co mp o nent s can b e

r emo ved fr o m t he b ath and q uenched and a r easo nab ly t hick har d case w ill b e p r esent . I n t wo h our s , a cas e o f ab o ut

0 . 2 22 " can b e fo r med , at a t emp erat ur e o f 9 0 0 C . The t reatment is co mp let e aft er q uenching in o il o r w at er. T he

advantages of this method are:

1.

S p eed at w hich har dening can b e car ried o ut .

2.

D is t or t io n can b e k ep t t o a minimum ( wo r k s us p end ed o r in b as ket ).

3.


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The bright finish of the parts is maintained.

Disadvantages - Expensive, dangerous fumes, salt clings to parts causes splitting during quench, require gloves and

goggles, salt contaminated quench bath.

I nd uc t i o n H a r de n i n g

This p r oces s is car ried o ut b y heat ing t he s ur face t o b e har d ened b y a s ur r ound ing ind uct io n co il t o a t emp er at ur e o f

between 750 and 850C. ~When the temperature has been reached the heating current is switched off and work isq ue nc he d b y a s p ra y o f w at er .

Advantages

a.

H eat can b e ap p lied wher e r eq uir ed fo r co rr ect lengt h o f t ime.

b.

Heat may t ake o nly a few s econd s t her efo r e s mall o xid is atio n.

c.

Interior may be cool-minimising distortion.

d.

Good for mass production.


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Flame Hardening

F o r s te e ls wit h a c a rb o n c o nt ent b e twe e n 0 . 3 a nd 0 . 5% a nd me d ium c a rb o n a llo y s te e ls . I t c o ns is ts o f he a ting a nd

surface with one or more acetylene blowpipes and quickly following with special quenching jets.

1.

Job fixed - jets move

2.

Jets mixed - job moves

3.

Job rotates - jets move longitudinally

4.

Job heated quickly then blowpipe removed and quenched.

A d v a n t a g e sa.

No distortion

b.

High degree of surface hardening

c.

Core unchanged (may be heat treated for toughness and ductility first).

Parts should be tempered or stress relieved after quenching. Used for gears, brake drums, tyres, axles.


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Gas Carburizing

In this method carbon dioxide and other gases containing carbon are circulated through a furnace chamber into which

t he p ar ts have b een p laced . The fur nace may b e gas fir ed o r electr ically heated . This p r oces s is q uick er t han p ack

carburizing and compared to liquid carburizing may be controlled more in respect of case depths. The main drawback

is t he high init ial co s t o f t he eq uip ment .


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He a t T r e a tme n t Af te r C a r bu ri zi ng

C a s e h a r d e n i n g S t e e l s

Both plain carbon and alloy steels are used for case-hardening. The carbon content should not exceed 0.3% if

r easo nab le co r e t o ughnes s is t o b e maint ained . M anganes e is ad d ed t o cas e har d ening s t eels t o aid car bur is atio n and

incr eas e d ep th o f har d ening b ut s ho uld no t exceed 0 . 9 % s ince it t end s t o caus e q uench cr ack ing. S ilico n co nt ent is

kept below 0.3% since it causes graphitisation. Alloy Case-hardening steels usually contain nickel or nickel and

chromium. Nickel retards grain growth reducing the need for post-hardening heat treatment. Nickel also increases the

s t rengt h and t o ughnes s o f t he co r e. C h r omium may b e ad d ed t o incr eas e har d nes s o f t he cas e b ut s mall amo unt s o nly

a re us e d b e ca us e o f t he t end e nc y t o c a us e gr a in gr owt h.

C a s t Iro n

Ge ne ra lly a ny ma te ria l ma d e up p rima r ily o f ir on wit h a b out 2 % o r mo re o f c a rb o n is c o ns id e re d t o b e c a st ir on, w it h

mo s t co mmer cial allo ys co nt aining fr o m ab o ut 2 . 5 % t o 3 . 8 % car bo n.

The p ro p er tie s o f c a st ir on a r e:1.

Low melting point

2.

Good fluidity in the mould

3.

Rigidity

4.

Good water resistance

5.

High compressive strength

6.

Easily machined if suitable composition is chosen.

White a nd G re y C a s t Iro n

The t erms ' w hit e' and ' gr ey' refer t o t he fo r m in w hich t he car bo n co nt ent exis t s. I n w hit e cas t ir o n t he car bo n is

combined as cementite and the iron produced is white, hard, brittle and unmachinable. In grey cast iron the carbon is

fr ee in t he fo r m o f gr aphit e flak es and t he ir o n is gr ey, s o ft and eas ily machined . B o t h cement it e and fr ee gr aphit e ar e

present in 'mottled' cast iron. Unless graphitisation is sever, the matrix or background will in every case be pearlite.

The fa c to rs a ffe c ting t he fo rm t ak e n b y t he c a rb o n a re :

1. r ate o f co o ling

2. chemical compositions

Rapid cooling tends to produce cementite and a white iron while slow cooling allows the precipitation of graphite in a

grey iron.

S ilic o n a id s t he fo rma tio n o f gr ap hit e a nd is us e d fo r t he p ur po se o f p ro d uc ing a s o ft ir on in t he c a se o f t hin s e ct io ns


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where cooling is accelerated. Sulphur stabilised cementite making the iron hard and brittle but its effect may be

suppressed by the addition of manganese. When high fluidity is required phosphorus is used but the iron produced is

weak and brittle but is useful for ornamental purposes.


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M a l le a b le C a s t I ro n s

M alleab le cas t ir o n is mad e up by annealing w hit e ir o n cas tings at a t emp er at ur e o f ap p ro ximat ely 9 0 0 C fo r fo ur o r

five days. The graphite gathers into clusters in a ferrite-pearlite matrix. The disadvantages of this process are the time

taken and its limitation to sections of less than 2 inch thick.

S p he ro i da l G ra phi te C a s t I ro n ( S G Ca s t I ro n)

C as tings w it h t he gr ap hit e in glo b ular fo r m can b e o b tained b y ad d ing s mall amo unt s o f magnes ium t o t he lad le b efo r e

casting. The metal is then annealed giving a structure consisting of globules of carbon in a ferrite matrix. Such castings

can replace steel castings and forgings.

E l e me nt s R e s idua l F ro m T he R e f ining P r o c e s s I n P la i n C a rbo n S t e e ls A nd T he

E f fe c t s O f T h e s e s E l e me n ts

M a n g a ne s e ( u p t o 1 %)

Manganese is an essential constituent since it deoxidises the steel, ensures freedom from blowholes and combines

w it h any s ulp hur p r esent t o o ffs et t he ill effect o f t he s ulp hur ( s ee b elo w) .

N o te : S te e l is no t c la ss e d a s a n A llo y S te e l unle s s it c o nt ains mo re t ha n 1 % M anga ne s e.

S il ic o n ( up t o 0 . 3 %)

S ilico n has lit tle effect o n mechanical p r op er ties in ' lo w car bo n s t eels ' b ut s ho uld no t exceed 0 . 2 % in high car bo n

steels, since it tends to break down 'Cementite' into 'Graphite' or 'Ferrite'.

Sulphur

Sulphur may exist as 'ferrous sulphide' or 'manganese sulphide'. Ferrous Sulphide appears at grain boundaries and is

'hard and brittle' at low temperatures and has a low melting point, giving rise to crumbling during hot working.

Manganese Sulphide exists as soft inclusions, which produce no harmful effects.

P h o s p ho r u s (u p t o 0 . 5 % )

P ho s p ho r us is r egar d ed as a d eadly imp ur it y in s t eel.

T he G r a in B o u nd a r y

S ince t he ' o r ient atio n o f t he at oms ' in each gr ain is d iffer ent , t he at oms at t he ' G rain B ound ary' canno t b e ar ranged o n

a r egular s p ace lat tice.

Since the grain boundary structure is different from that of the grains, the properties will also be different.

At a relatively 'low temperature' the grain boundaries are 'stronger' than the grains.

At 'elevated temperatures' the grain boundaries are 'weaker' than the grains.

Therefore a 'Fine Grained Structure' will give 'higher' strength and hardness at 'room temperatures'.

At 'higher temperatures' a 'Course Grain' is preferred.


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These remarks apply to pure metals and solid solution alloys, but, may not apply to certain alloys with grain boundary

impurities.

This t yp e o f fr actur e at r o om t emp er at ur e is us ually ' A cro s s t he G rain' ( Tr ans crys t alline) . W her eas at t he elevat ed

temperatures the fracture may be 'Along the Grain Boundaries' (Intercrystalline).

I t c a n b e s e en t ha t t he p ro p er tie s o f a me ta l w ill b e go ve rne d b y t he a mo unt o f gr ain b o und a ry, i. e . The Gr ain S ize .

The co nt ro l o f t he ' G rain S ize' is t her efo r e imp o r tant in t he w or k ing and heat t reat ment o f met als and allo ys .


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R e c r y s t a l l i s a t i o n

W hen a cer tain t emp er at ur e is r eached t he d is t or ted gr ains ar e r ep laced b y new fine p o lygo nal gr ains and s o ft ening

occurs. The phenomenon is known as 'Recrystallisation'.

The r ecrys t allis atio n t emp er at ur e fo r a p ar ticular met al o r allo y w ill d ep end o n a numb er o f facto r s:

(a) The d egr ee o f p r io r co ld w or k ing

( b ) The ad d it io n o f o t her element s

(c) The annealing time.


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Ho t W o rk ing

This takes place 'above' the recrystalline temperature (Lower Critical Range for Steel). In Hot-Working, deformation

and recrystallisation occur simultaneously so that the rate of softening is greater than the rate of work hardening. The

important factor is the finishing temperature. Hot working should be finished at a temperature 'just above' the

temperature of recrystallisation so that a 'Fine Grain Size' is obtained.

If the finishing temperature is 'too high' then 'grain growth' will occur whilst the metal is cooling above therecrystallisation temperature.

If the finishing temperature is 'too low' then 'work hardening' will be the result.

D e fini tio n O f Ho t W o rk ing

'Working that does not produce residual stres se s' and also as

'Working at a te mperature above that of re crys tallisation'.

The chief processes of hot-working are hot-rolling, forging and extrusion. In general, simple shapes e.g. sheet, plate,

rod, etc., are usually hot rolled whilst more complicated shapes are forged. Non-ferrous sections and tubes are

usually made by extrusion.


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C o ld W o rk ing

Cold working destroys the lattice structure with its regular crystal planes along with deformation can readily occur.

The metal thus becomes 'harder' and the characteristic 'ductility is lost'. Hardening due to cold-working is referred to

as 'working hardening'.

D e fi nit io n O f C o ld W o rk ing

'Working that produces residual stresses' and also as

'Working at a temperature below that of recrystallisation'.

Cold working e.g. rolling, drawing or pressing is usually carried out on previously hot-worked metals and alloys. It is

frequently the finishing stage in production. The effect of cold working is to break down the crystal structure

'elongating the grains in the direction of working'.

E x a m pl e s o f W o r k H a rd e ni ng

Copper piping suffers from continual expansion and contraction which leads to hardness and brittleness (Removed by

Annealing).

Shackles, Chains, Lifting Gear, etc., develop surface hardness due to cold working and must be annealed at regular

intervals.


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G ra de s O f M ild S te e l

D ea d M ild S te e l

C ar b o n co nt ent 0 . 0 7 to 0 . 1 5% . I t is s o ft and d uct ile and is us ed fo r ho t and co ld - r o lled s tr ip fo r p r es s ings , r o d and

wire for nails, rivets and solid-drawn tubes.

I t d o es no t machine w ell b ecaus e o f it s s o ft nes s , b ut it w eld s eas ily.

I t has a Tens ile S t rengt h o f up t o 3 5 0 N / mm2 and an Elo ngat io n o f 3 2 %.

M ild S te e l

C ar bo n co nt ent 0 . 1 5 t o 0 . 3 %. I t is har d er and les s d uct ile t han d ead mild s teel and mo s t o f t he s t eel p r od uced falls

int o t his r ange. I t is us ed fo r cas e har d ening s t eels , b o iler and s hip ' s p lat e, s t eel s t ruct ur al s ectio ns s uch as j o is t s,

channels, angles, bars for machining and forging and for steel castings.

Mild steel welds easily and has good machining properties


M e dium C a rbo n S te e l

Carbon content of 0.3 to 0.6%. It is harder, stronger and less ductile than mild steel and is more difficult to weld and

machine.

It is used for components which need good load-bearing qualities and a resistance to abrasion.

Forging for general engineering purposes, connecting rods, axles, gears, crankshafts, fishplates.



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Al lo y S t e e l s

Plain carbon steels have the following limitations:

High tensile strength cannot be combined with good values for toughness and ductility.

Large sections cannot be effectively hardened due to the 'mass effect'

Poor resistance to oxidation, corrosion and creep at high temperatures.

To overcome these limitations alloy steels have been developed which are invariably stronger and/or have much

greater resistance to oxidation, corrosion, creep and fatigue. Alloy steels are more expensive than plain carbon steels

because of the cost of alloying elements and they are often more difficult to manipulate into shape and to machine.

Nickel and chromium are frequently used together since the grain growth tendency of chromium is checked by thegrain refinement due to nickel; also, the graphitising effect of nickel is offset by the stabilising effect of chromium.

Chromium imports a low rate of oxidation at all temperatures whilst nickel limits grain growth and the brittleness

arising there at high temperature. Nickel-chromium alloys are particularly suitable for high temperature work.

( s ee lat er no t e o n ' N imo nic S er ies ' . )


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A u s t e n i t i c S t e e l s

A us t enit ic S t eels ar e a gr o up o f allo y s t eels w hich co ns is t ent ir ely o f aus t ent ic F C C cr ys t al s t ruct ur e o f ir o n and

alloying elements, even after the steel has been slowly cooled to room temperature. Austenitic steels usually contain a

high chromium and nickel content and in such cases are frequently referred to as Austenitic Stainless Steel. Such steels

have a low Thermal Conductivity (i.e. therefore used for cryogenic and high temperature valve spindles), very high

resistance to corrosion, oxidation and creep - and they also have a very good resistance to Sulphide stress cracking

and Hydrogen embracement (i.e. valves used for chemical cargoes etc). Austenitic steels are generally used wherestrength, corrosion and oxidation resistance are required at high temperature (e.g. exhaust valves, high temperature

valve spindles, exhaust turbo- blowers, turbine nozzles and turbine rotor blading), and for low temperature (cryogenic)

and chemical cargo operations.

Austenite is relatively soft (e.g. Brinell Hardness approximately 180: Mild Steel 250, High Carbon Steel 800) and

d uct ile and no n- magnet ic. The s tainles s s t eels ar e uns uit able fo r w eld ing and s uffer fr o m a d efect k no wn as ' W EL D

D EC A Y' - w hich caus es co rr o sio n b r ought ab o ut b y t he p r ecip it atio n o f chr o mium car bid es in s o me p ar t o f t he

structure which then sets up a galvanic cell with the remaining 'stable' structure.


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T i t a n i um A ll o y s

I n t erms o f it s o ccur r ence in t he Ear th' s C r us t , t it anium is a r elat ively ab und ant met al and , o f t he Engineer ing M etals ,

only aluminium, iron and magnesium are more plentiful. The amount of titanium ore within the range of mining

o p erat io ns is ab o ut 1 0 0 t imes gr eater t han t hat o f co p per . H o w ever , t he ver y high affinit y o f t it anium fo r b o th o xygen

and nitrogen makes it difficult to extract whilst the molten metal itself reacts with all known refractories. Thus titanium

is a n e xp e ns ive me ta l b e ca us e o f it s c o st o f e xt ra c tio n a nd fo rming r at he r t ha n a ny s c ar cit y o f it s o re s .

H igh p ur it y t it anium has a r elat ively lo w t ens ile s t rengt h ( 2 16 N / mm2 ) and a high d uct ilit y ( 5 0% ) b ut t he s t rengt h can

be raised considerably by alloying with elements such as aluminium carbon, chromium, molybdenum, etc. Titanium is a

gr eyis h w hit e met al no t unlik e s t eel in ap p ear ance and has a s tr engt h t o w eight r atio n mid way b etw een aluminium and

s t eel. I t has a ver y go o d co r ro s io n r esis t ance.

The relative density of titanium is only 4.5 and suitable alloys based on it have a high specific strength. Moreover

creep properties up to 500C are very satisfactory whilst the fatigue limit is also high. Titanium alloys therefore find

application in the compressors of jet engines. In turbine engineering generally titanium alloys, being of low relative

density, impose much lower centrifugal stresses on rotors and discs for a given blade size. Titanium alloys are also

ver y r es is t ant t o ero s io n b y s et s t eam and o t her med ia s o t hat s t eam t ur b o gener ato r s, gas t ur b ines and heatexchanger p lat es all mak e us e o f t hes e allo ys .

Because of the high specific strength, high temperature resistance and good corrosion resistance generally, many

titanium alloys find use in supersonic aircraft - including 'Concorde' - as structural forging. Such an allow is IMI Ti690

( 11 S m; 2 . 25 A; 4 M o; 0 . 2S i) whic h d e ve lo p s a s tr e ngt h o f 1 3 00 N / mm2 .

'Nimonic' Services are not strictly speaking steels but contain basically 75% Nickel and 20% Chromium stiffened

with small amounts of carbon, titanium, aluminium, cobalt and molybdenum. Used where very high creep strength at

elevated temperature is required.

A llo y S t eel us ed up t o 1 00 0 K ( e. g. S t eam T ur b ines )

Nimonic Alloys used up to 1300K (e.g. Gas Turbines)

'Stellite' is the trade name of an alloy consisting of cobalt, chromium, molybdenum, tungsten and iron. Varying

proportions of some or all of the metals are used to produce alloys with various characteristics. Stellite is extremely

hard, corrosion resistant and has good resistance to loss of strength at high temperature. In diesel engines Stellite is

us ed t o s ur face t he face and s eats o f exhaus t p o p pet valves . S t ellit e is ap p lied b y w eld ing o r s p ray p r oces s o nt o t he

valve face and s eat, w hich mus t b e p r eheated .


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Alloying Elements

Thes e ar e ad d ed t o p lain car bo n s t eels t o imp r ove t heir exis t ing p r op er ties and als o t o int ro d uce new p r o per ties s uch

as corrosion resistance.

N ic k e l ( 1 to 8 %)

Increases strength and toughness with little loss of ductility, giving good erosion resistance. Nickel is a 'grain refiner',

forming a fine grained material, but fortunately it is also a 'powerful graphitiser'. It is used in amounts up to 5%; as a

grain refiner in case-hardened steels. Larger amounts are used in stainless steel and heat resistance steels.

M a n g a n e s e ( 1 t o 2 % )

Reduces the ill effect of Oxygen and Sulphur and 'increases strength'.

S il ic o n ( up t o 1 %)

Reduces the ill effect of Oxygen and Sulphur and 'increases strength'. Also gives good casting fluidity (up to 0.3%)

and u sed in s o me heat r esis t ing s t eels ( up t o 1 % ).

C h ro m iu m ( 0 . 2 5 t o 1 8 % )

Used in small amounts in constructional steels, tools steels, and ball races. Used in large amounts in stainless steels

and heat resisting steels. Induces 'hardness', improves resistance to erosion, corrosion and high temperatures.

Increases strength with unfortunately increases brittleness due to increasing of grain size.

M o l y bd e nu m ( 0 . 5 t o 1 % )

Increase strength, heat and creep resistance at high temperatures. Used for superheater tubes, turbine rotors, etc.

V a na d iu m ( 0 . 2 t o 4 % )

Used in steels required to retain hardness at high temperatures e.g. Hot Forging Dies. Increases strength and fatigue

resistance. Used in conjunction with Molybdenum for boiler tube material.

F a t i g u e

F atigue failur e is caus ed b y ' r ep eat ed s t res s cycles ' s uch as r ever se o r alt ernat ing s tr es ses , r ep eated s t res s es andfluctuating stresse s:

Rever sal o f d ir ectio n o f B end ing o r To r sio n

Alternating Compressive and Tensile Stresses

Application and Removal of Stresses

V ar iat io n o f t he I nt ens it y o f S t res s


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The ap p ear ance o f t he mat erial aft er a fat igue failur e is s ho wn b elo w:

D uct ile mat er ials d is p lay a s mo o th zo ne and a cr ys t alline zo ne, b ut t hes e zo nes ar e les s mar k ed , in t he cas e o f b r it tle

materials.

The s mo o th zo ne is p r od uced b y t he gr ad ual p r o gr es sio n o f t he fat igue cr ack and t he r ub b ing o f t he s ur faces o f t hecrack.

The crystalline zone is produced by the sudden fracture when the remaining sound material can no longer withstand

t he inc re a se d s tr es s c a us e d b y t he r e duc tio n in t he a re a c a rr ying t he lo a d.

A fatigue crack starts at some point of stress concentration. If the material is subjected to 'Intermittent Stressing' the

s ur face o f t he s mo o th zo ne w ill d is p lay a numb er o f cur ved lines .


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F a c t o r s T ha t I nf lu e nc e F a t ig u e S t re n g th O f A C o mpo n e nt

G r a in S ize W he n a ma te ria l ha s a ' c oa rs e gr a in' it ha s ' p oo re r' fa tigue r es is ta nc e t ha n whe n it ha s a fine gr ain.

C o mp o ne n t S h a pe

A fat igue cr ack o ft en s t ar t s s o me p o int o f s t res s co ncent rat io n and s o s tr es s r is ing feat ur es s uch as an ab rup t changeof section e.g. sharp corners, keyways, etc., should be avoided if possible.

S u r f a c e F i n i s h To o ling mar ks act as s t res s r ais er s and so the fat igue r esis tance o f a co mp o nent can b e imp r o ved by p o lis hing et c.

R e s idua l S t re s s e s Stressed produced by the machining operations will affect the fatigue resistance of a component. Tensile Stresses

'reduce' the resistance. Compressive Stresses 'improve' the resistance.

C o r r o s i o n C o r ro s io n p r o duces a p it ted s u rface and s o i nt ro d uces s t res s r ais ers . T he fat igue s tr engt h o f a co mp o nent is r ed ucedvery considerably by corrosion.

Temperature At high temperature materials tend to lose their strength and also suffer 'grain growth' and so fatigue strength falls

when the temperature is high.

C r e e p The term 'Creep' is used to describe the 'slow plastic deformation' that occurs under prolonged loading, usually at

high temperatures.


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F a c t o r s T h a t I nf lu e nc e C r e e p

Temperature The r ate o f cr eep i ncr eas es w it h t emp er at ur e

G ra in S iz e

A co ur s e gr ained mat er ial has a b ett er cr eep r esis t ance t han o ne w it h a fine gr ain.

C r eep is a fo r m o f s lip w hich o ccur s w hen met al is s ub j ect ed to a t ens ile lo ad at high t emp erat ur e. C r eep can b e

defined as the continuing deformation with the passage of time, in materials subjected to constant stress. This

d efo r mat io n is p las tic and o ccur s even t ho ugh t he acting s t res s is ' b elo w t he yield s t res s ' o f mat er ial. A t lo w

t emp er at ur es t he r ate o f cr eep is ver y s mall b ut at higher t emp er at ur es it b eco mes incr eas ingly imp o rt ant . F o r t his

reason creep is commonly regarded as a high temperature phenomenon associated with steam plant, gas-turbine

technology and turbo-c harger blading.

TIME

P la s tic S tr ain fr om 0 t o X w hic h O c c ur s in Thr e e S ta ge s:

P r imar y o r Tr ans ient C r eep ( 0 to P ) b eginning at a fair ly r ap id rat e w hich t hen d ecreas es w it h t ime as s t rain -

hardening sets in.

S eco nd ary o r S t ead y- S t at e C r eep ( P to S ) in w hich t he ' r ate' o f s t rain is fair ly unifo r m and at it s lo wes t value.

T ert iar y ( ter shar i) C r eep ( S t o X) in w hich t he ' r ate' o f C r eep incr eas es r ap id ly s o t hat fr actur e o ccur s at X . This

stage coincides with necking of the material.

During Primary and Secondary Creep, plastic deformation takes place due to slip associated with dislocations

movements within the grains. This leads to work hardening which, at high temperatures, is balanced by thermal

softening. The dislocations eventually move out of the grains and into the grain boundaries. Tertiary Creep coincides

with the initiation of micro-cracks at the 'grain boundaries', which leads to necking and the consequent rapid failure of


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the material. Hence, at the higher temperatures, fine grained material creep more than course grained material since

fine grained material contains more grain boundaries per unit volume.

C a s t Iro ns

Ge ne ra lly a ny ma te ria l ma d e up p rima r ily o f ir on wit h a b out 2 % o r mo re o f c a rb o n is c o ns id e re d t o b e ' c as t ir on' .

M o s t co mmer cial allo ys co nt ain fr o m ab o ut 2 . 5 % t o 3 . 8% car bo n.

The p ro p er tie s o f c a st ir on a r e:

1.

Low melting point

2.

Good fluidity in the mould

3.

Rigidity4.

Good wear resistance

5.

High compressive strength

6.

Easily machined if suitable composition is chosen.


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C o p pe r Al lo y s

The alloying elements used have a fair degree of solubility and within this range, the solid solutions formed are soft

and ductile and are suitable for cold working. Further additions of the alloying element result in the formation of hard

particles, accompanied by an increase in strength and a decrease in ductility. Such alloys are suitable for hot-working

and for castings.

C o ppe r - Zi nc (B ra s s e s )

Up to 39% zinc - cold working alloys, soft and ductile

3 9% - 4 5 % zinc - ho t w or k ing allo ys

ab o ve 4 5 % - mat er ial b eco mes b r it tle.

Tin improves corrosion resistance.

Manganese is added for the deoxidisation producing sounder castings and for improving tensile strength.

Aluminium increases tensile strength and corrosion resistance.

N ickel increases corrosion and erosion resistance.

C o pp e r- T i n ( B ro n z e s )

Ad d it io ns o f up t o 1 0 % t in ar e s o lub le in co p per and fo r m s o ft d uct ile allo ys w hich can b e co ld - w o rk ed . F ur t her additions of tine result in the appearance of a hard constituent, such alloys being suitable for hot-working and for

castings. The cold worked alloys are used mainly for coinage.

Admiralty Gunmetal (88: 10: 2, Cu: Sn: Zn)

Zinc is present as a deoxidiser and increases fluidity. This alloy is used mainly for castings requiring strength and

corrosion resistance. An additional of 1% lead may be used to improve pressure tightness. The alloy may be used for

bearings.

1 5 % Tin - B ro nze A llo y

C as t 1 5 % t in b r onze is s uit able fo r b earings .

P ho s pho r - B ro nze s

Phosphorus increases strength and corrosion resistance.

A llo ys co nt aining up t o 8 % and 0 . 3 % p ho s p ho r us can b e co ld w o r ked and ar e us ed fo r ins tr ument s p rings and s t eam

turbine blading. Cast alloys contain up to 13 tin and up to 18% phosphorus are used mainly for bearings.

Al umi ni um - B r o nz e s

Alloys containing up to 9.4% aluminium are soft and ductile and are suitable for hot and cold working. Further


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ad d it io ns r esult in t he ap p ear ance o f a har d co ns t it uent and ar e us ed fo r t he ho t w or k ed co nd it io n and fo r cas tings .

Aluminium bronzes have good mechanical properties, wear, fatigue and corrosion resistance. The 7% aluminium alloy

often contains small additions of nickel, iron and manganese and is used for marine condenser tubes.

The 10% alloy with additions of nickel, iron and manganese is used for propellers.

C o ppe r N ic k e l Al lo ys ( Mo ne l M e ta ls )

C o p p er and nick el fo r m s o lid s o lut io ns fo r all p r o po r tio ns . T hes e allo ys may b e ho t o r co ld w o r k ed an d h aveexcellent co r ro s io n r esis t ance. The co p p er - r ich allo ys ar e k no wn as cup r o- n ick els . (7 0 : 3 0 : C u: N i) us ed fo r

condenser tubes (Cupro-Nickel)

(29.5: 68.5: 1: 1, Cu: Ni: Fe: Mn) Monel Metal. This alloy combines good mechanical properties and high corrosion

resistance.

B e a r i n g M e t a l s

Alloys used for bearing should have a low co-efficient of friction, be sufficiently hard to resist wear, tough towithstand shock loading, have strength to support the working load, be sufficiently plastic to allow self alignment and

have high thermal conductivity to dissipate heat when running. The alloys employed usually consist of hard particles in

a s o ft mat rix. The har d p ar ticles p r ovid e t he w ear r es is t ance w hile t he mat rix p r ovid es t he mechanical p r op er ties .

C o p p e r B a s e B e a r in g Al lo y s

P h o sp ho r - b r onzes co nt aining 1 0 - 1 3 % t in and 0 . 3 - 1 % p ho s p ho r us and t in b r onzes co nt aining 1 0 - 1 5 % t in ar e us ed

fo r b earings w her e lo ad ing is heavy. L ead b r o nzes ar e us eful in t hat t hey have a high r es is t ance t o wear and have

good thermal conductivity enabling the bearings to remain cool. Sintered bearings are made by heating a compressedpowder mixture of 90% copper and 10% tin with an addition of graphite. These bearings being semi-porous retain

lubricant.

Y o rc a lbro - C o ppe r - Zi nc A ll o y

22% zinc 2% aluminium 0.04% arsenic.


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Alloys used for bearings should have the following characteristics:

1.

H ave a lo w co - efficient o f fr ict io n

2.

Be s ufficient ly har d t o r es is t w ear

3.

Be tough to withstand shock loading

4.

H ave s t rengt h t o s up p o rt t he w or k ing lo ad

5.

Be sufficient plastic to allow self alignment6.

Have a high thermal conductivity to dissipate heat when running.

The alloys employed usually consist of hard particles in a soft matrix. The hard particles provide the wear resistance

while the matrix provides the mechanical properties.


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C o p pe r B a s e B e a r i ng Al lo y s

Phosphorous Bronzes containing 10-13% Tin and 0.3-1% Phosphorous and Tim Bronzes containing 10-15% Tin

ar e us ed fo r b earings w her e t he lo ad ing is heavy.

Lead Bronzes are useful in that they have a high resistance to wear and have good thermal conductivity enabling the

bearings to remain cool.

S lunt ered b earings ar e mad e b y heat ing a co mp r ess ed p o wd er mixt ur e o f 9 0 % co p per and 10 % t in w it h an ad d it io n

of graphite.

These bearings being semi-porous retain lubricant.


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C o p p e r

C o m p o s i t i o n

Nearly Pure

C o p p er is malleab le and d uct ile eit her w hen co ld o r ho t and can b e r o lled int o s heet s o r d r awn o ut int o wir e.

W hen t he met al has w or k exp end ed o n it as s t at ed o r b y hammer ing, i t b ecomes har d and b r it tle, r end er ing it

uns uit ab le fo r p r act ical p ur p o ses . To s o ft en t he co p per it is heated unt il it acq uir es a r ed ap p earance w hen it is

quenched in cold water.

P r o p e r t i e s

I t is s o ft a nd d uc tile . I t is e a sy t o s ha p e. I t is r es is ta nt t o o r dina ry c o rr os io n whe n in c o nt ac t wit h wa te r o r s te a m, b ut

o wing t o it s s o ft nes s it is uns uit able fo r us e w it h s t eam o r high p r ess ur e and t emp er at ur e.

As it is a go o d co nd uct or o f heat it is r equir ed t o b e lagged t o p r event heat lo s s. Is go o d electr ical co nd uct or and isw id ely us ed in electr ical w or k . I ts gr eates t us e is in allo ying w it h no n- f er r ous met als in t he p r od uct io n o f b r onze,

gunmetal, muntz metal, nickel-c oppe r and other alloys.


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C o p pe r B a s e A ll o ys - Ad va n ta g e s

Good Mec hanical Properties

High Electrical and Thermal Conductivity

V ery R es is t ant t o C o rr o sio n and W ear

C an b e eas ily F o r med and M achined

Can be easily Joined by soldering, brazing, Welding

C a n b e e a sily p o lis he d a nd p la te d

P r ess ing and F o r ging Temp er at ur es ar e lo wer t han F er ro us M etals .


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D e zi nc i fi c a tio n O f B ra s s

This is o ne o f s e ve ra l fo rms o f s e le c tive c o rr os io n w hic h a ffe c ts s o me a llo ys . I n t he c a se o f b ra s s it is t he ' r emo va l o f

t he Zinc c o nt ent ' o f Br a ss ( a c o pp e r zinc - a llo y) b y t he s e a w at er o r ho t fr e sh w at er a nd le a ving b e hind a ' p or ous a nd

leak ing s p o nge o f co p per ' .

The char act eris tic ap p ear ance o f a ' d ezincified b r ass ' is t he co p per y co lo ur o f t he affected ar ea and s mall p it ho les .

S ingle p has e b r as s allo ys ( 7 0 % C u: 2 9 % Zn: 1 % S n) can b e inhib it ed agains t d ezincificatio n b y t he ad d it io n o f a s mall

amount of arsenic.

D up lex allo ys s uch as M unt z M etal i. e. 6 0 / 40 B r ass ( 6 0% C u: 4 0 % Z n) ' C anno t b e inhib it ed' , alt ho ugh t he ad d it io n

o f 1 % Tin will r et ar d t he c o rr os io n.

I n gener al, d ezincificat io n w ill no t o ccur t o b r ass es in w hich ar senic has b een ad d ed and w ho s e Z inc co nt ent is les s

t han 3 7 %.

Aluminium Brass (75% Cu: 22% An: 2% Al) will eliminate dezincification as a protective film is formed which even if broken will reform itself.


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C o p pe r Al lo y s

The alloying elements used have a fair degree of solubility and within this range, the solid solutions formed are soft

and ductile and are suitable for cold working. Further additions of the alloying element result in the formation of hard

particles, accompanied by an increase in strength and a decrease in ductility. Such alloys are suitable for hot-working

and for castings.

C o ppe r - Zi nc (B ra s s e s )

Up to 39% zinc - cold working alloys, soft and ductile

3 9% - 4 5 % zinc - ho t w or k ing allo ys

ab o ve 4 5 % - mat er ial b eco mes b r it tle.

Tin improves corrosion resistance.

Manganese is added for the deoxidisation producing sounder castings and for improving tensile strength.

Aluminium increases tensile strength and corrosion resistance.

N ickel increases corrosion and erosion resistance.

C o pp e r- T i n ( B ro n z e s )

Ad d it io ns o f up t o 1 0 % t in ar e s o lub le in co p per and fo r m s o ft d uct ile allo ys w hich can b e co ld - w o rk ed . F ur t her additions of tine result in the appearance of a hard constituent, such alloys being suitable for hot-working and for

castings. The cold worked alloys are used mainly for coinage.

Admiralty Gunmetal (88: 10: 2, Cu: Sn: Zn)

Zinc is present as a deoxidiser and increases fluidity. This alloy is used mainly for castings requiring strength and

corrosion resistance. An additional of 1% lead may be used to improve pressure tightness. The alloy may be used for

bearings.

1 5 % Tin - B ro nze A llo y

C as t 1 5 % t in b r onze is s uit able fo r b earings .

P ho s pho r - B ro nze s

Phosphorus increases strength and corrosion resistance.

A llo ys co nt aining up t o 8 % and 0 . 3 % p ho s p ho r us can b e co ld w o r ked and ar e us ed fo r ins tr ument s p rings and s t eam

turbine blading. Cast alloys contain up to 13 tin and up to 18% phosphorus are used mainly for bearings.

Al umi ni um - B r o nz e s

Alloys containing up to 9.4% aluminium are soft and ductile and are suitable for hot and cold working. Further


43/104

ad d it io ns r esult in t he ap p ear ance o f a har d co ns t it uent and ar e us ed fo r t he ho t w or k ed co nd it io n and fo r cas tings .

Aluminium bronzes have good mechanical properties, wear, fatigue and corrosion resistance. The 7% aluminium alloy

often contains small additions of nickel, iron and manganese and is used for marine condenser tubes.

The 10% alloy with additions of nickel, iron and manganese is used for propellers.

C o ppe r N ic k e l Al lo ys ( Mo ne l M e ta ls )

C o p p er and nick el fo r m s o lid s o lut io ns fo r all p r o po r tio ns . T hes e allo ys may b e ho t o r co ld w o r k ed an d h aveexcellent co r ro s io n r esis t ance. The co p p er - r ich allo ys ar e k no wn as cup r o- n ick els . (7 0 : 3 0 : C u: N i) us ed fo r

condenser tubes (Cupro-Nickel)

(29.5: 68.5: 1: 1, Cu: Ni: Fe: Mn) Monel Metal. This alloy combines good mechanical properties and high corrosion

resistance.


Alloys used for bearing should have a low co-efficient of friction, be sufficiently hard to resist wear, tough towithstand shock loading, have strength to support the working load, be sufficiently plastic to allow self alignment and

have high thermal conductivity to dissipate heat when running. The alloys employed usually consist of hard particles in

a s o ft mat rix. The har d p ar ticles p r ovid e t he w ear r es is t ance w hile t he mat rix p r ovid es t he mechanical p r op er ties .

C o p p e r B a s e B e a r in g Al lo y s

P h o sp ho r - b r onzes co nt aining 1 0 - 1 3 % t in and 0 . 3 - 1 % p ho s p ho r us and t in b r onzes co nt aining 1 0 - 1 5 % t in ar e us ed

fo r b earings w her e lo ad ing is heavy. L ead b r o nzes ar e us eful in t hat t hey have a high r es is t ance t o wear and have

good thermal conductivity enabling the bearings to remain cool. Sintered bearings are made by heating a compressedpowder mixture of 90% copper and 10% tin with an addition of graphite. These bearings being semi-porous retain

lubricant.

Y o rc a lbro - C o ppe r - Zi nc A ll o y

22% zinc 2% aluminium 0.04% arsenic.


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Alloys used for bearings should have the following characteristics:

1.

H ave a lo w co - efficient o f fr ict io n

2.

Be s ufficient ly har d t o r es is t w ear

3.

Be tough to withstand shock loading

4.

H ave s t rengt h t o s up p o rt t he w or k ing lo ad

5.

Be sufficient plastic to allow self alignment6.

Have a high thermal conductivity to dissipate heat when running.

The alloys employed usually consist of hard particles in a soft matrix. The hard particles provide the wear resistance

while the matrix provides the mechanical properties.


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E l e c t r o -C h e mi c a l C o r r o s i o n

This t yp e o f co r ro s io n co ver s all fo r ms o f ' W et C o r ro s io n' i. e. w her e t he met al is in co nt act w it h a liq uid o r even

mo is t at mo s p her e. I n t he electr o - chemical t heo r y it is as s umed t hat all met als have a t end ency t o d is s o lve o r co rr o d e,

when the metal discharges 'positively-charged' particles called 'ions' into solution. This leaves the metal with a

char act eris t ic ' negat ive char ge' o r p o tent ial. T he gr eater t he negat ive p o tent ial, t he gr eater is t he t end ency o f t he met al

t o d is s o lve o r co r ro d e.

The corrosion resistance of metals is governed by their position in the 'Electro-Chemical Series' in which metals are

arranged according to their electrode potentially. The results are expressed relative to 'Hydrogen' which is taken as

Zero.

The t end ency o f each ind ivid ual met al t o co r ro d e is r elat ively s mall, b ut is gr eatly incr eas ed w hen it is in co nt act w it h

a d is s imilar met al in t he p r esence o f a co nd uct ing liq uid , r efer red t o as t he ' E lectr o lyt e' . A cur rent w ill flo w b etw een

t he t wo met als s ince t hey ar e at d iffer ent p o tent ially. C o r ro s io n o f t he o ne ' higher in t he t able' ( A no d e) w ill b e

acceler ated , w hils t t he met al ' lo wer in t he t able' ( C atho d e) w ill b e p r o tected .

The r at e o f c o rr os io n is go ve r ne d b y t he r e la tive a re a s o f t he A no d e a nd the C a tho d e. In ge ne r al fo r a give n a re a o f A no d e, t he at tack incr eas es in s ever it y, t he gr eater t he ar eas o f t he ad jacent C at ho d e.

F o r elect ro - chemical co r ro s io n t o o ccur t her e mus t b e a C at ho d e, and A no d e and an E lectr o lyt e.

A no d ic E n d ( C o r ro d e d )

M e tal Ele ctrode Pote ntial Volts

Sodium - 2.71

Magnesium - 2.40

Aluminium - 1.70Zinc - 0.76

C hromium - 0.56

Iron - 0.44

C admium - 0.40

Nickel -0.23

Tin - 0.14

Lead - 0.12

Hydrogen 0.00

C opper + 0.35

Silver +0.80

Platinum +1.20

Gold +1.50

C a t h o di c E n d ( P r o te c t e d )


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Pitting

P it ting is an examp le o f t he d iffer ent ial aer at io n effect. The init ial d ep res s io n o r p it in t he s ur face may b e t he r es ult o f

s ever al fact or s e. g. a b r eak in a p r o tective film o r s cale, o r t he s o lut io n o f a no n- met allic inclus io n d ue t o electr o lyt ic

action.

O nc e a p it is fo rme d t he c o rr os io n p ro c ee d s r ap id ly s inc e t he s ur fa c e o f t he me ta l ( C at ho d e) ha s a gr ea te r a c ce s s t o

' o xygen' t han t he b as e o f t he p it ( Ano d e).

C o r ro sio n is a c ce le r at ed b y t he fa c t t ha t t he s ur fa c e a re a o f t he C a tho d e is c o ns id e ra b ly gr e at er t ha n t ha t o f t he

Anode.

The co r ro s io n p r od uct s accumulat e at t he mo ut h o f t he p it and as s is t co rr o sio n b y mak ing o xygen d iffus io n mo r e

difficult.


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C o rro s io n O f M e ta ls I n S e a W ate r

W it h d is s imilar met als in s ea w ater , galvanic actio n r es ult s and t he mo r e ano d ic met al co rr o d es .

Any ma te ria l in t he Ta b le b e lo w is a no d ic t o t ho se b e lo w it .

i. e . S t ee l is a no dic t o b r onze in s e a wa te r a nd t he re fo re it w ill c o rr od e . W e c a n s a y t ha t t he s te e l ha s give n ' c at ho d ic

protection' to the bronze.

An o di c E n d O f T a b le ( C o r ro d e d)

Magnesium

Aluminium

Zinc

Mild Steel

Manganese Steel (without oxide film)

Admiralty Brass

Copper

Aluminium Bro nze

Gunmetal

Cupro-N ickel 70/30

Nickel

Stainless Steel (with oxide film)

Monel Metal

Graphite

Titanium

C a t ho d ic E n d O f T a b le ( P ro t e c t e d)

To minimise galvanic effect:

1.

C ho o s e mat er ials clo s e t o each o t her in t he s eries .

2.

M ak e t he k e y o r ma in c o mp o ne nt o f a mo re no b le me ta l ( i. e . c a tho d ic ).

C o rr o s io n And D e po s i ti on

M o st o f t he met al eq uip ment in mar ine p o wer p lant s is mad e up o f s t eel o r co p per allo ys ( b ras s , co p per - nick el,

bronze and others). All of these metals will dissolve slowly in water unless the water is properly treated. This is called

corrosion.

S o me o f t he mo s t imp o rt ant k ind s o f co r ro s io n d amage w hich can o ccur in mar ine p o wer p lant eq uip ment ar e:a.

Thinning o f t he t ub e met al. This is t he r es ult o f co r ro s io n t hat is co nt inuo us and o ver a fair ly lar ge ar ea o f

met al. This k ind o f a d amage is als o called gener al co r ro s io n. Thinning can p r ogr ess t o t he p o int at w hich t he

met al can no lo nger co nt ain t he int ernal p r ess ur e w hich may caus e t he met al t o s well and event ually b ur s t.


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b.

P it ting - w hen o nly a s mall ar ea o f met al is co r ro d ed t he r esult is a d eep ho le called a p it . I f p it ting co rr o sio n

is no t co nt ro lled , s ome p it s may go all t he w ay t hr o ugh t he met al. This caus es leak s . W hen t hes e ar e many p it s

clo s e t o get her , t hey may b ecome co nnected . The effect o n t he met al is t he s ame as t hat o f gener al co r ro s io n.

c.

Corrosion cracking is another form of corrosion which can effect certain materials. In general, alloys, which

ar e mixt ur es o f met als ar e mo s t s us cept ib le t o cr ack ing. S t ainles s s t eel and b r ass s uch as A dmir alt y ar eparticularly susceptible to cracking under certain conditions. cracking is a form of corrosion which occurs

alo ng a ver y nar ro w b and t hr o ugh t he met al.

d.

S o me met al allo ys ar e s us cept ib le t o exfo liat io n o r d e- allo ying. B ot h o f t hes e t yp es o f co rr o sio n ar e

associated with the selective reaction of only one of the metals in a metal alloy. Exfoliation generally occurs in

feedwater heaters. Nickel is selectively oxidised from the copper-nickel alloy tubing leaving layers of copper

metal and nickel oxide. Brasses are mixtures of copper and zinc. When de-alloying occurs, zinc is removed

from the metal leaving a spongy mass of copper behind. This is commonly referred to as dezincification.

e.

Emb r it tlement is an effect o f co rr o sio n t hat changes t he p hys ical p r op er ties o f a met al. S o me co r ro s io n

reactions cause metals to lose their normal strength and ductility and become brittle and weak. Embrittlement

canno t b e s een b y ins p ect ing a b o iler t ub e t hat has no t failed . H o wever , an emb r it tled t ub e t hat has failed w ill

have a cr ys t allized ap p ear ance at t he ed ge o f t he p o int o f failur e and us ually t her e w ill b e no evid ence o f

bulging.

I n S ectio n I I s o me o f t he chemical r eact io ns o f co rr o sio n w ere co ver ed. T hes e w ill help y o u t o und ers t and s o me

o t her as p ect s o f co r ro s io n w hich fo llo w.

The s t ud y o f co rr o s io n co ns id ers r eact io ns b etw een a mat er ial and it s envir o nment . F r o m t he s tand p o int o f t he p o wer

plant water chemist, the study of corrosion also includes suppression of corrosion by altering or controlling the

environment to which steam power plant materials are exposed. In order to understand the suppression of corrosion,

one must first understand its causes.

T he ear lies t s t ud ies o f co r ro s io n o f ir o n ( s teel) s ho wed t hat p r act ically t he o nly facto r w hich limit s t he life o f ir o n is

o xid atio n. A ll o f t he chemical p r oces ses b y w hich ir o n is co r ro d ed, eaten aw ay, o r r us ted ar e co ver ed b y t his t erm.

Before proceeding the term oxidation and its counterparts reduction should be clearly understood. The word

oxidation implies the chemical reaction of a substance with oxygen. This is true, but this is a very specific application

o f t he t erm. I t als o has a much b r oad er and mo r e imp o rt ant meaning.

T he t erm r ed uct io n is o ft en t ho ugh o f as a r eact io n t hat invo lves t he r emo val o f o xygen fr o m a s ub s tance. A gain, t his

is t rue b ut it is als o o nly a s p ecific ap p licatio n o f t he t erm. R ed uct io n als o has a much b r o ad er and imp o r tant meaning.

The t erms ar e o ft en ab b reviat ed as "R ED O X" b ecaus e t hey ar e clo s ely r elat ed .

S imp ly s tat ed o xid atio n invo lves t he lo s s o f elect ro ns b y a s ub s tance and red uct io n o f t he gain o f electr o ns b y a

substance. By this definition the element oxygen does not have to be involved in an oxidation-reduction reaction at all.

The r easo n fo r t his clo s e r elat io ns hip o f t he t erms s ho uld als o b e ap p ar ent : I f o ne s ub s tance gives up electr o ns ano t her

substance must gain them.

In experiencing oxidation, uncharged iron atoms pass into solution and become iron ions. This change involves the

at oms giving up elect ro ns . The o xid atio n o f ir o n is t her efo r e electr ic in nat ur e b ecaus e o f t he flo w o f elect ro ns .

O x id atio n and co r r os io n ar e t her efo r e electr o chemical p r o ces s es . O n t his b as is co r ro s io n can b e lo o k ed at in a

similar fashion to other electrical processes.


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The b a sic na tur e o f c o rr os io n is a lmo s t a lw ays t he s a me . A F lo w o f e le c tr ic it y o c cur s b e tw ee n c e rt ain a re a s o f a

metal surface through a solution capable of conducting an electric current. This electro-chemical action causes the

e a ting a wa y o f me ta l a t a re a s whe re t he e le c tr ic c ur re nt le a ve s t he me ta l a nd t he me ta l a to ms e nt er t he s olut io n a s

ions.


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An o de s a n d C a t ho d e s

The p r esence o f a s o lut io n w hich can co nd uct an elect ric cur r ent , and elect ro lyt e, is o ne o f t he fir s t r eq uir ement s fo r

corrosion. An electrolytic solution is any liquid that contains ions. Remember that ions are electrically charged atoms in

solution and that even pure water contains both charged ions and negatively charged hydroxyl ions in equilibrium.

Because of this, solutions of salts, acids and alkalis are all good electrolytes.

I n ad d it io n t o an elect ro lyt e, tw o electr o d es - an ano d e and a catho d e - ar e r eq uir ed fo r co r ro s io n. T he elect ro d es

ma y c o ns is t o f t wo d iffe re nt t yp e s o f me ta l o r t he y ma y b e d iffe re nt a r ea s o n t he s a me p ie c e o f me ta l. I n e it he r c a se ,

fo r co r ro s io n t o o ccur t her e mus t b e a d iffer ence in elect rical p o tent ial b etw een t he t wo elect ro d es o r ar eas s o t hat

electricity will flow between them. In addition to the portion of the electrical circuit made up of electrolyte, the circuitmus t b e c o mp le te d b y a me ta llic p a th b e tw ee n t he t wo e le c tr od e s. I f t he y a r e o n t he s a me p ie c e o f me ta l t he re is a n

inher ent cir cuit . I f t hey ar e s ep ar ate p ieces o f met al t hey mus t b e co nnect ed in s o me manner .

W ha t t ak e s p la c e a t t he a no d e in a c o rr os io n c e ll w he n c o rr os io n o c cur s?

Positively charged atoms of metal detach themselves from the surface and enter into solution as ions, while the

corresponding negative charges, in the form of electrons, are left behind in the metal (oxidation).


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The d etached p o s it ive io ns b ear o ne o r mo r e p o s it ive char ges . I n t he co r ro s io n o f ir o n, each at om r eleas es t wo

electrons and becomes an iron ion carrying two positive charges. The released electrons travel through the metal to

t he c a tho d e a re a .

W ha t t ak es p la ce a t the c at ho de ?

The electr o ns r eaching t he s ur face o f t he catho d e b y p as sing t hr o ugh t he met al cir cuit meet and neut ralize s o me

positively charged hydrogen ions which were present in the electrolyte. In losing their electric charge by gaining

electrons the hydrogen ions become neutral atoms (reduction). They then combine to form hydrogen gas.

The co nver sio n o f hyd r ogen io ns t o hyd r o gen at oms and t hen t o hyd r o gen gas r es ult s in a d ecreas e in hyd r ogen io ns

in t he elect ro lyt e. This incr eas es t he alk alinit y o f t he electr o lyt e in t he ar ea o f t he catho d e.

The ionic and cationic (REDOX) reactions discussed so far can be written as follows:

Ano dic Re ac tio n - F e - > F e+ + + 2 e

Cathodic Reaction - 2e+2H+->2H->1H2

F r o m t he ab o ve it is eas y t o s ee t hat s ever al envir o nment al facto r s can b e var ied t o affect co r ro s io n r ate. I f fo r

ins t ance t he hyd r o gen io n co ncent rat io n is incr eas ed ( p H r ed uced ) t he r ate o f co r ro s io n is lik ely t o incr eas e s ince

t her e ar e mo r e hyd r ogen io ns t o r eceive elect ro ns at t he cat ho d e. C o nver s ely if t he s o lut io n is mad e mo r e alk aline ( b y

r ed uc ing t he H io n c o nc e nt ra tio n - p H inc re a se d ) t he o f c o rr os io n c a n b e r ed uc e d.


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Further by reducing the concentration of dissolved material in the electrolyte the conductivity of the electrolyte is

r ed uced and t he r esis tance is incr eas ed . An incr eas e in r esis t ance imp ed es t he flo w o f cur r ent and t he co rr o sio n r ate

o f an immer s ed mat erial can b e r educed .

I t is imp o rt ant t o no te t ha t a no d es a nd c a t ho d es c a n o c cur r a nd o mly o n a p ie c e o f me ta l. T his c a n b e illus tr at ed b y

placing a piece of steel in a hydrochloric acid solution. Upon immersion of the piece of steel in the acid solution, the

vigorous formation of numerous hydrogen bubbles is observed. Hydrogen is evolved seemingly from the entire surface

w it ho ut t he ind icat io n o f eit her catho d ic o r ano d ic ar eas . T his is , in fact, t he cas e s ince t he ano d es and cat ho d es s hiftfrom time to time during corrosion under these conditions.

The d evelo p ment o f an ano d e o n a met al s ur face may r es ult fr o m a var iet y o f micr o sco p ic s ur face co nd it io ns

including local impurities in the metal, surface imperfection, orientation of grains in the metal, localized stresses and

var iat io ns in envir o nment . I t is imp o r tant t o r ealize t hat it is no t alw ays p o s sib le t o d efine t he p r ob ab le ano d ic andc atho dic a re as in a s ys te m s uc h a s a s te am p owe r p la nt .

I t w as s t at ed t hat t he co nver sio n o f hyd r ogen io ns t o u nchar ged ( neut r al) hyd r o gen at oms w hich co mb ine t o f or m

hyd r ogen gas is t he maj or catho d ic r eact io n o f co r ro s io n. I n acid s o lut io ns s uch as hyd r ochlo r ic acid , co rr o sio n

proceeds rapidly because of the high concentration of free hydrogen ions available for the cathodic reaction. In neutral

o r alk aline s o lut io ns , t he numb er o f hyd r o gen io ns is gr eatly r ed uced and it is p o s sib le fo r a micr o sco p ic layer o f

hyd ro ge n ga s t o fo rm a film o n t he c a tho d e. The c o at ing o f hyd ro ge n o n a c a tho d e s lo ws d o wn t he c o rr os io n r at e b y

acting as an insulation and preventing hydrogen ions in the liquid from accepting electrons at the cathode surface. This

phenomenon is called cathodic polarization.


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I f o xygen is p r es ent in s o lut io n, it can r eact w it h hyd r ogen at t he cat ho d e s ur face fo r ming w at er b y t he fo llo wing

reaction:

4H++02+4e->2H20

W hen t his o ccur s , co r r os io n is acceler ated s ince t he cat ho d e canno t b eco me p o lar ized and co r ro s io n r ates ar e

increased.

The fo rma tio n o f a c o rr os io n p ro d uc t film o n t he s ur fa c e o f a n a no d e is a fo rm o f a no d ic p o la r iza tio n. I n t his c a se , t he

corrosion product acts as an insulation preventing further reaction and thereby inhibiting corrosion.

1.

Ther e ar e many t yp es o f co r ro s io n p r od uct films w hich can fo r m. I n s t eel s ys t ems w it ho ut o xygen p r esent ,

fer ro us io ns p r od uced at t he ano d e r eact w it h hyd r o xyl io ns in t he w at er fo r ming fer ro us hyd r o xid e. T his is t he

fir s t s tep in t he fo r mat io n o f magnet it e ( F e30 4 ) w hich is a d es ir ab le and ver y p r ot ect ive film. I r on is actually

converted into magnetite by the following complex sequence of chemical reactions.

i.

F e + 2H+ = Fe+ ++H2

ii.


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2F e + 6 H+ = 2F e+ ++ +3 H2

iii.

F e + 20 H- = F e(O H)2

iv.

2 Fe + 6 0H- = 2 Fe (O H)3

v.

F e(O H ) 2 + 2 F e(O H )3 = F e3 O 4 + 4H 2O

or

3 F e + 4 H2 O = F e3 O4 +4 H2

I t can b e s een fr o m t he ab o ve r eact io ns t hat t he fo r mat io n o f fer ro us hyd r o xid e and magnet it e is d ep end ent

on the presence of sufficient free hydroxal ions. The of conversion is also affected by temperature. The higherthe temperature the more rapid the conversion.

2.

I n t he p r es ence o f o xygen, fer ro us hyd r o xid e may no t b e co nver ted t o magnet it e b ut may fo r m fer ric

hyd ro xid e ins te a d. F e rr ic hyd ro xid e is a t yp ic a l fo rm o f r us t. I t is no t ve ry p ro te c tive b e ca us e it is p o ro us a nd

d o es no t ad her e t ight ly t o t he s ur face o f t he ano d e. A s a r esult , co r ro s io n can p r ogr ess b eneat h t he fer ric

hyd r o xid e s ur face. F er ric o xid e ( F e2O 3 ) is ano t her fr eq uent co rr o s io n p r od uct fo r med in t he p r esence o f

oxygen. Ferric oxide films are not very protective.

3.

I n c o pp e r s ys te ms , t he d e sir ab le p ro te c tive o xid e w hic h c a n o c cur a t t he a no d e if c up ro us o xid e ( C u2 O ). I n

t he p re s enc e o f o xyge n c up ric o xid e ( C uO ) w hic h is no t a s a d he r ent a nd p ro te c tive a s c up ro us o xid e c a n

form.

4.

The co r ro s io n p r od uct s o f ir o n and co p per can o ft en b e id ent ified by t heir co lo ur . The fo llo wing is s uch a lis t:

Product Formula Colour

Ferrous Hydroxide Fe(O H)2 White

Ferric Hydroxide FeO (O H) Yellow or O range

Ferric O xide Fe2O 3 Red or BrownMagnetite Fe3O 4 Black

Anodic polarization is frequently called passivation. Without the formation of protective oxide films on steel or copper

allo y s ur face, cor r os io n w ould b e exces sive b ecaus e t hes e met als ar e ver y r eact ive w it h w at er. I t is t her efo r e o ne o f

the water chemist's major goals to maintain water conditions that will achieve and maintain surface passivity of steel

and copper equipment.

a.

O ne o f t he fir s t as p ect s o f co r ro s io n p r o tect io n is t o p r event high hyd r ogen io n co ncent rat io ns ( lo w p H) . H igh

hydrogen ion concentrations accelerate corrosion because they accelerate the cathodic reaction, theacceptance of electrons by hydrogen ions. Conversely, maintaining suitably high hydroxyl ion concentrations in

s o lut io n in s t eel s ys t ems help s t o achieve ano d ic p o lar izat io n b y t he fo r mat io n o f a p r ot ect ive co rr o sio n

product film. Low pH also leads to the formation of soluble corrosion products whereas high pH encourages

the formation of insoluble and protective corrosion products.


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b.

O xygen s ho uld b e exclud ed fr o m p o wer p lant s ys tems b ecaus e it t end s t o d ep olar ize cat ho d ic ar eas b y

reaction with the hydrogen film as well as to impair anodic polarization by leading to the formation of

unprotective corrosion products. These are important concepts of corrosion and corrosion prevention.

They have a b r oad r ange o f ap p licat io n in p o wer p lant w ater management .


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W ha t a re t he c a u s es o f S pe c ific C o r ro s io n R e a c tio ns t ha t O c c ur in S te a m P o we r

P l a n t s ?

The fo llo wing w as a gener al d is cus s io n o n t he nat ur e and caus es o f co r ro s io n. The fo llo wing mat er ial co ver s mo s t o f

t he s p ecific caus es o f s p ecific co r ro s io n r eact io ns t hat o ccur in s t eam p o wer p lant s :

O xy g e n C o r ro s i o n

O xyge n c a n p la y a ma jo r r ole in t he c o rr os io n o f p o we r p la nt e q uip me nt . I t is o ne o f t he mo st und e sir ab le

contaminants which enter a condensate feedwater-boiler water system. It accelerates corrosion by cathodic

d ep olar izat io n and b y d es tr o ying t he p as sivit y o f s t eel o r co p per s ur faces .

De-aerating feedwater heaters are provided to remove oxygen from feedwater to prevent corrosion. Oxygen

s cavenger s s uch as hyd r azine o r s o d ium s ulp hit e ar e us ed to r emo ve t he t race amo unt s o f o xygen no t r emo ved b y t he

de-aerator. Hydrazine is highly reactive with oxygen. Feedwater treatment with hydrazine prevents oxygen from

corroding both the feedwater system and the boiler. Sodium sulphite may be added to boiler water to prevent oxygen

corrosion.


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C a r b o n D i o x id e C o r r o s i o n

C arb o n d io xid e can caus e co r ro s io n. Mo s t o f t he car ob dio xid e in p o wer p lant w ater is fo r med ins id e t he

evap o rat or s . H eat caus es car bo nat es and b icar bo nat es in t he b r ine t o "b r eak d o wn". C ar b on d io xid e t hat is fo r med in

t his w ay leaves t he evap o rat or w it h t he s t eam and caus es co r ro s io n o f co nd ens ate r etur n lines .

The fo llo wing r eact io ns s ho w t he manner in w hich C O 2 is fo r med b y t he d ecomp o s it io n o f b icar bo nat e and

carbonate.

NaHCO3 Heat NaOH + CO2

Na2CO3 + H2 Heat 2NaOH + CO2

C ar b on d io xid e gas caus es co nd ens ate p ip ing co r ro s io n b ecaus e it fo r ms an acid in w at er. The fo llo wing r eact io ns

s ho w ho w car bo n d io xid e fo r ms car bo nic acid in w ater .

C O 2 + H2 O > H2 CO 3

or

C O 2 + H2 O > H+ + HC O 3 -

Note the formation of free hydrogen irons which define an acid condition.

C ar b on d io xid e in co nd ens ate acceler ates co rr o sio n in s ever al w ays . F ir s t, it r educes t he p H o f t he co nd ens ate,

t her eby acceler ating t he catho d ic r eact io n. S eco nd , at t he ano d e car bo n d io xid e has o t her affect s. T he r ed uct io n in

pH by the formation of carbonic acid reduces the hydroxyl ion concentration which impairs the formation of protective

ferrous hydroxide and magnetic. It also leads to the formation of ferrous bicarbonate which is a highly soluble

compound and has not passivating effect. These effects are similar in copper systems.

Ac i d C o r ro s i o n

A cid co rr o sio n ( no t r esult ing fr o m t he p r es ence o f car bo n d io xid e) can als o o ccur in b o iler s . W hen co nd ens er leak age

o ccur s o r b o iler feed wat er b ecomes co nt aminat ed w it h car ryo ver fr o m t he mak eup evap o rat or , o ne o f t he maj or s alt s

t hat is int ro d uced t o t he b o iler w ater is magnes ium chlo r id e. The co mp o s it io n o f s eawat er w as s ho w n in S ectio n I .

Magnesium is the second most plentiful cation in seawater and chlorides are the most plentiful anion.

When magnesium chloride enters high temperature boiler water, the magnesium ions react with the phosphates andhydroxyl ions in the boiler water. Magnesium ions are so reactive with hydroxyl ions at high temperature in boiler

w at er t hat magnes ium hyd r o xid e p r ecip it atio n w ill o ccur unt il t he b o iler w at er p H d r o ps t o ap p ro ximat ely p H - 4 . 0 .

This can b e p r event ed b y p r op er b o iler w at er t reatment . W hen t he p H value is no t car efully co nt r olled , hyd r o xyl io n

concentrations are reduced to low levels. This results in hydrochloric acid attack to the metal surfaces.

M cC l2 + 2 H2 O H e at M g( O H) 2 + 2 HC I

and

F e ++ + 2 HC l H ea t F e (C l) 2+ H2

The p r esence o f d ep os it s o n b o iler t ub e s ur faces can lead t o co ncent rat io n o f hyd r o chlo r ic acid u nd er neath t he

d ep os it s . W hen t his o ccur s t he co rr o sio n r ates b eco me ext remely high and s erio us d amage can o ccur in a ver y s ho r t

time.


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C a u s t i c C o r r o s i o n

C aus t ic co rr o sio n o r co r ro s io n r esult ing fr o m t he p r esence o f s o d ium hyd r o xid e can o ccur in b o iler s . I t w as p r evio us ly

stated that maintaining elevated hydroxyl ion concentrations is desirable for both steel and copper surfaces. This is

t rue , b ut t he r e is a r ange o f c o nd it io ns ( pH va lue s ) a b ove w hic h t he p re s enc e o f fr e e hyd ro xyl io ns c a n b e d a ma ging

to these materials.

S o d ium hyd r oxid e ( N aO H) is o ne o f t he maj or chemical ad d it ives us ed in t he t reatment o f b o iler w at er. The p ur p o se

of sodium hydroxide is to maintain the hydroxyl ion concentration in the optimum range for the formation of good

protective magnetite on steel surfaces. Its other role is to help in the formation of non-adherent sludge instead of scale

w hen har d nes s ent ers t he b o iler w at er.

Excessive amounts of sodium hydroxide can, however, lead to corrosion. This is particularly true of ultra-high

pressure boilers. If there is too much sodium hydroxide present at a steel surface, this chemical can react with the steel

to form a soluble material which can then precipitate as a loose, porous magnetite deposit. The following reactions

illus t rat e t he manner in w hich it can o ccur :

F e+ 2 N aO H N a2 F eO 2 + H2

3Na2FeO2+4H2O 6NaOH+Fe3O4

The normal concentrations of sodium hydroxide maintained in boilers are not harmful. However, it is possible for

sodium hydroxide to concentrate in localized areas of boilers, thereby leading to localized corrosion. This occurs

when heavy layers of deposits form on the boiler tubing, causing sodium hydroxide to be concentrated under the

d ep os it s at t he met al s ur face.

C aus t ic co rr o sio n can als o o ccur w hen b o iler t ub e s ur faces b eco me s t eam b lank eted b ecaus e o f eit her exces sive

boiling or separation of steam and water in horizontal or inclined tubes.

Exces sive b o iling can r esult fr o m ver y high heat t rans fer r ates w hich can o ccur w hen b ur ner s ar e mis aligned in a

furnace and the flames impinge on the boiler tubing. In this case, boiler water containing sodium hydroxide can splash

o nt o t he s t eam b lank eted s ur face and as t he w at er b o ils o ff t he s o d ium hyd r o xid e co ncent rat io ns can b eco me

excessive.

A similar effect sometimes occurs in horizontal or inclined tubing if there is insufficient mass velocity in the tubing to

k e ep the s te a m a nd wa te r we ll mixe d . I f t he w at er s e pa ra te s a nd flo ws a lo ng t he b o tt om o f t he t ub e a nd t he s te a m

a lo ng t he t op o f t he t ub e , wa te r c a n s pla s h o nt o t he ho t d ry up p er s ur fa c e. A s t he liq uid b o ils fr om t he d ro p le t o n t he

hot upper surface, excessive sodium hydroxide concentrations can occur.

Because caustic corrosion is a very significant problem in ultra-high pressure boilers, the ultra marine program

eliminated free hydroxide from the boiler water by using the co-ordinated phosphate method of treatment.

Hy dr o g e n D a ma g e

In addition to the foregoing types of boiler corrosion which result in the localized thinning of boiler tubing, ultra-high

pressure boilers are susceptible to a form of attack which damages the internal structure of the metal causing it to

become brittle. This type of attack is called hydrogen damage.

Hydrogen damage occurs when there is a very rapid corrosion reaction in ultra-high pressure boilers. Hydrogenat oms fo r med at a cat ho d e ar e s mall eno ugh t o ent er t he b o iler met al. W hen t hey have ent ered t he met al t ub ing t he

hyd r ogen at oms r eact w it h t he car bo n w hich is no r mally p r esent . The r eact io n p r o duct o f car bo n and hyd r ogen is

met hane. M ethane is a ver y lar ge gas mo lecule and it caus es int ernal p r ess ur e t o fo r m w it hin t he met al o f s t eel t ub ing.

Thes e high met hane p r es s ur es caus e gr ains o f s t eel t o s ep ar ate and event ually caus e cr ack s in t he met al t ub ing.


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C + 4H C H4

Hydrogen damaged boiler tubing has been most frequently found after incidents of acid corrosion resulting from

s eawat er co nd ens er leak age an

Unit 03 - Materials

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