conten! weigh! rt.1

O,O4QO,! '-i--+-

00100:~J~~J:~~ooosa

t~

~

in production at the Dillinger Hütte steel plant.2

max. S-content rt.J

005,

004i

003

~EN1

002

OO~~

t l I ~

_~k~~5~_~o;NI0025-::-:q-, + i

EN 10~JZ ,EN 0201(SPH ~IEN~01137::- '

D28, EN101 3L. EN 1+1 (SPHl2')

EN 10149 «3151;--1-

EN 101~'355\ ENf131"&" -;-ö22S;EN 0131 QL. Q 1 ---,_.~

10~5(Z): !good ~'nd.fdI-' ,

0001 -~ i I I ,0.00 001 0.02 0.03 0.04 0.05

mu. P-content (%)

Figure 2. Permissible phosphorus and sulfurcontents tor selected grades of steel; figures inbrackets indicate yield strength class.

S teel plant technology has undergone significant

improvements in recent decades.1 Between 1996

and 2002, Dillinger Hütte has spent more thanUS$ 150 million on further improving the facilities in its steelplant. The investments were aimed at cast reduction andenhanced productivity, but also for improved process controland better steel quality.

Figure 1 outlines how harmful trace elements in the steelcould be gradually reduced as a result of improved process-es and facilities, (for example by ladle metallurgy and vacu-um treatment). Other targets were homogeneous properties,achieved by replacing ingot with sophisticated continuouscasting, and the very specific achievement of an aimedchemical composition.

However, fabricators and users are less concerned withthe details of steelmaking, taking more interest in the advan-tages of modern steels, and in what risks may result fromusing cheep material of inferior quality.

Status of modern steelmakingToday, the LD process can achieve sulfur contents below0.0010;0 (10 ppm), and contents of nitrogen below 0.005%and phosphorous below 0.010;0 are also attainable. Evencontents of less than 0.005% can be assured in specialcases. However, in the case of structural and pressure ves-sei steels such as EN10025, EN10028, EN10113, ASMEand ASTM, modern steelmaking permits significantly lowerlimits than those specified in corresponding national andinternational material standards. Same examples ofEuronorms are given in Figure 2.

The permissible sulfur and phosphorus content limitsdecrease as demand for strength, toughness, formability andsafety in structural steels increases (Figure 2). The purchase ofsteels that merely fulfil the standard requirements may result inconsiderable difficulties when processing and in service.

The effects of sulfurDue to the high affinity between sulfur and the manganesealloyed to structural steels, little dissolved sulfur exists in ametal's solid state. Instead, sulfur is essentially present in theform of sulfide inclusions. As sulfur content increases, so taodoes the amount and size of these sulfides. Moreover, theyare becoming ever more harmful to the metal's material

properties.Taking normalised structural steel with a yield strength of

355 MPa as an example, Figure 3 shows how impact ener-gy has been improved in recent decades, thanks to theremoval of sulfur to the minimum level technically achiev-able. The upper shelf of impact energy has almost beenincreased by a factor of 10.

Lamellar tearingLamellar tearing is a defect that occurs in susceptible platesor beams when shrinkage stresses result in excessively highloads in the through thickness direction. Such cracks arelocated parallel to the plate surface, and are caused by nonmetallic inclusions (sulfides, silicates and oxides) alignedparallel to the surface by the rolling process. It is easy to per-ceive that high sulfur and oxide contents promote the risk oflamellar tearing.

Figure 4 indicates that low sulfur contents have afavourable effect on striction in the through thickness tensiletest, particularly when combined with sulfide shape control(which prevents elongation of the sulfides during hot rolling).

Sour gas resistant steelsNon alloyed and low alloy steels are susceptible to crackingif exposed to fluids containing hydrogen sulfide. Such

REPRINTED FROM HYDROCARBON ENGINEERING MAY 2003

0OO1~ ---0, .

1~ 1- 1870 18751~ 1- 1M 1995 2000 2005Year-Figure

1. Limits of same trace elements achieved

1960 1965 1970 1975 1980 1985 1990 1995

Vear, '

Figure 3. Notched impact energy achievable in theparent material as a result of improvements'in steelmaking and the corresponding sulfur contents., .

impact sl\,srgy (-165"C)200 ~ ~~ ~A+watercooling

150.,

~-~-\ --c- ~ C ~UI.t.d welding)

\ ~A+.lrcooling\100,

0..so, ~<>~

~~ HAZ + PWHT.~ .~ ~ ~cooling 216'Clh

O~ ~ A + tuma.. cooling~ ~ -OHAZ+PWHT.

..; rnm',csmQI;ng .0.000 0.005 0.010 0015 0.020

Ivlroge Phosphorus content %

Figure 5. Low temperature steel containing 9%nickel, the effects of phosphorus content and thecooling rate after tempering the impact energy ofthe parent material and HAZ microstructure.4

The removal of phosphorus in steelmaking is more difficultthan is the Gase with sulfur, as it can only be extracted by selec-tive oxidation during oxygen blowing in the converter process.Removal by secondary metallurgy is not feasible. As it is notfixed in stable inclusions, at least same phosphorus is dissolvedin the steel matrix in asolid state, and at a sufficiently high tem-perature. Solubility decreases at lower temperatures, and phos-phorus atoms tend to segregate and precipitate primarily atgrain boundaries. Grain cohesion is thereby weakened, underunfavorable conditions embrittlement is obtained, and evenintercrystalline fracture can occur. Phosphorus segregation ismost effective at 450 -500 "C, during heat treatment, slow cool-ing or when at a high service temperature.

Figure 5 shows how the impact energy was affected byintergranular embrittlement für a steel containing 9% nickel,intended to be used at cryogenic temperature.4 In this inves-tigation, phosphorus concentrations at the grain boundariesencompassed different bulk compositions of 0.001 -0.013%and different cooling rates. Due to its coarser microstructure,the HAZ specimen proved to be more susceptible than thefine grained parent material. A similar effect has also beencaused by, amongst others, tin and antimony.5

Embrittlement du ring high temperature serviceEmbrittlement at high service temperatures is often simulat-ed and studied by applying a special heat treatment ('stepcooling'). The temperature shift in impact testing, from brittleto ductile fracture, is examined before and after such treat-ment. Watanabe related the shift of the impact transitioncurve to the concentrations of silicon, manganese, phospho-rus and tin present in the steel.6 It was observed that steelswith a J factor of less than 100 were not prone to embrittle-ment. As a sufficient amount of silicon and manganese arenecessary to match the tensile properties, only low phos-phorus and tin concentrations are able to satisfy this value.The formula für the J factor is:

J = [(Mn+Si)*(P+Sn)*10000]

Dillinger Hütte's test did not fully confirm Watanabe's for-mula, but it remains in many specifications für CrMo alloyedsteels für use at high temperatures. However, it is importantto note that this formula is not applicable to weid metals.

Effects 01 trace elements on

weidabilityHydrogen induced cold cracking in the HAZCarbon equivalents (predominantly CE, Pcm and CET)7 arenormally used to characterise the weidability of a steel (Table 1).

They do not entail the use of sulfur and phosphorus, asthese elements are not relevant in this regard. Same yearsaga, it was argued that sulfides were trapping hydrogenintroduced by welding, with the result that steels containingvery little sulfur were suspected to be more susceptible tocold cracking. Hence, they should require high er preheat fürsafe welding. In order to clarify this matter, cold crackingtests8 were performed on various steel grades containing dif-ferent levels of sulfur. As can be seen in Figure 6, the criticalpreheat temperatures were indeed independent of the sulfurcontent, sinGe the experimental results were satisfactorily

predicted by the formula.In the context of preheat, another type of traGe element

should also be cons[dered, including typical alloying ele-ments such as Cr, Mo, cop per or nickel, which are uninten-tionally added in this Gase. Depending on the scrap that isused für steel production, considerable amounts of such ele-ments may be present in the finished steel. EN 10113 and10028 permit 0.3% of Cu, Ni and Cr and 0.10% of Mo (DIN10028,0.08%), totalling less than 0.70%. When using CET,

conditions may occur in certain areas of oil and gas produc-tion,

in pipelines, and in petrochemical (particularly desulfuri-

sation) plants. Practical experience was reported at the 3rd

Dillingen 'pressure vessel and boilermaking' colloquium.3 A

sulfur content below 0.001 % results in a high level of resis-

tanGe to sour media, provided the material shows homoge-

neous microstructure and little segregation.

The influence 01 phosphorus onsteel propertiesIt has lang been known that phosphorus effectively increas-

es the tensile properties in ferrite, but has an adverse effect

on the toughness of steel. This chemical element causes

severe segregation, first during solidification of the steel, and

secondly in the solid state to grain boundaries.Consequently,

even a moderate bulk concentration can lead

to a significant embrittlement under critical conditions.

REPRINTED FROM HYDROCARBON ENGINEERING MAY 2003

Solidification crackingA 'solidification crack' is a hot crack that occurs at the end ofthe solidification of (weid metal) melts that have been sub-jected to shrinkage induced tensile stresses. Therefore, inthe context of welding, solidification cracking is primarily aproblem of the weid metal and its composition.1o The chem-ical composition of the parent material only plays an impor-tant roje in welding processes that produce a high level of

mixing. TIG welding without filler wire, submerged arG weld-ing with a deep penetration, electron beam welding, andlaser welding should also be mentioned. It is apparent fromTable 2 that all formulae to quantify susceptibility to crackingascribe a particularly severe effect to sulfur and phosphorus.This is due to the fact that both elements accumulate in theresidual melt during solidification, and form phases with lowmelting points. In order to minimise the risk, steels intendedfür laser welding should have particularly low sulfur andphosphrus contents.11,12

HAZ hot crackingAn 'HAZ hot crack' is an intercristalline hot crack that occursas a result of the liquefaction of low melting point phases atgrain boundaries under the influence of tensile stresses. Thecauses are primarily sulfides of iran and nickel. Steels withan adequate manganese conte nt have no tendency to HAZhot cracking unless their sulfur content is excessively high.The problem is solved if the sulfur is fixed in thermally stableinclusions by means of a calcium treatment.

Figure 7. Impact energy for the HAZ of steelS355N; Offshore grouped according to theirphosphorus contents. Tests in the las weldedl con-dition at -40 .c.

the preheat temperature has to be raised by 25 °C if the mostharmful elements (Mo, Cr and Cu) reach the upper tolerancelimit. As the steelmaker is not obliged to mention in the cer-tificates these contents if they were not added intentiona/ly,the welding engineer will be unaware of any such 'traces',and may consequently prescribe unsafe welding conditions.In this respect, steels produced tram 100% scrap by theelectric arG process cause more cancern than steel producedvia the blastfurnace and converter process.

Hot crackingA 'hot crack' is a discontinuity that occurs during welding,casting or hot forming at temperatures at which individualareas of the material are in the two phase 'liquid/solid' range,and at which tensile stresses occur simultaneously.9

Toughness in the HAZ

The influence of phosphorusThe results of 121 submerged arG welded joints producedwith a heat input of 3.0 kJ/mm were available to assist in

assessing the effect of phosphorus on the toughness in theHAZ. The welded plates were made of a normalised 355MPa yield strength offshore steel, tor which weidability had tobe proven in the Gase of the thickest plate per heat of mate-rial delivered. The tests were performed in the as-weldedcondition. The target composition tor the steel was kept con-stant, but heat to heat variations allowed assessment of theeffect of individual chemical elements. The impact valueswith notch position at the weid fusion line of all heats with thesame phosphorus content were averaged, and these valuesare shown in Figure 7. A slight trend towards lower HAZimpact energy with increased phosphorous is observable,but the effect is not pronounced. As all material was used inthe as-welded condition, the effect of phosphorus afterPWHT was not tested.

Embrittlement and cracking du ring post weidheat treatmentNot only does PWHT relieve residual stresses, but HAZ and

deposited materials are also tempered, mostly by improvingtheir properties. However, undesirable precipitation and seg-regation of tramp elements may occur simultaneously. Thebasic metallurgical processes have recentiy been sum-marised in a literature survey.15

As with phosphorus, same metallic impurities haveproved to be harmful in grain boundary contamination; in par-ticular arseniG, tin and antimony. Steels produced by an elec-tric arG process generally contain significantly higheramounts of these elements than steels produced via a con-verter,16 and are therefore more prone to such embrittlement.If fabrication and service conditions become critical, thisshould be laken into account when ordering steel.

In the Gase of the HAZ, during PWHT an accumulationand reprecipitation of phases dissolved at high temperatures

REPRINTED FROM HYDROCARBON ENGINEERING MAY 2003

Glose to the fusion line can occur. These include sulfides

(particularly those at relatively low manganese contents),17

nitrides (AI and V) and carbides (Nb, Mo and Cr).

In the Gase of rapid cooling, renewed precipitation is sup-

pressed, with the result that the atoms remain in a supersatu-

rated solution in the matrix. During PWHT, or whilst al ready

heating, dissolved atoms can diffuse and precipitate. The former

austenite grain boundaries offer particularly favourable sites for

thin layers of such precipitates. This can weaken grain cohesion

and may, under unfavourable conditions, cause reheat cracking.

Surprisingly high local concentrations, (almost completely cov-

ering the former austenite grain boundary surface) were report-

ed, although the bulk concentrations of phosphorous were IOW.18

ConclusionThe contents of sulfur and phosphorus permitted by most

standards are far in excess of the values that are currently

easily achievable with modern steelmaking technology. Today,

improved material properties and safer processing can justify

the slightly higher price of properly desulfurised steels.

Reducing phosphorus to concentrations significantly

below 0.015% necessitates considerable extra Gasts to the

steel plant. A technical advantage is achieved only für partic-

ular applications. Steel merely fulfilling the requirements of

the standards can cause serious problems and extra Gasts in

fabrication and service.

References1. BANNENBERG, N.. 'Sekundärmetallurgische Behandlung für die

Erzeugung von Grobblechen', Stahlund Eisen 118 (1998) No. 7,pp. 33 -36.

2. BERGMANN, B.. BANNENBERG, N. and. STREIßELBERGER, A..'Entwicklung der Metallurgie und der HerstellungvonDruckbehälterstählen in den letzten 50 Jahren'. Dif/ingen colloquium onpressure vessel and boilennaking, 24th September 1996.

3 ZANKOWSKY, U., 'Werkstofftechnische Erfahrungen mit wasserstoffind-uzierter Korrosion in der PCK RaffinerieSchwedt', 3rd Dillingencolloquium 'pressure vessel and boilermaking', 9'" November 2000.

4. FURUKIMI, O. and SAlTO, Y., 'The effect of grainboundary segregationand heat treatment on toughness of 9% Ni steel and its welded joints',ISIJ International, Vol. 30 (1990), no. 5, pp. 390 -396.

5. MAST, R., et al, 'Grain boundary segregation of antimony in iron basealloys and its effect on toughness', Steel Research 70 (1999) no. 6,pp. 239 -246.

6. WATANABE, J. et al, 'Temper embrittlement of 2.25 Cr -1 Mo PressureVessle Steel', ASME 2,g11 Petroleum Mechanical EngineeringConference, Dallas, USA, September 1974.

7. EN1011 -2:2001 -5: Arc welding of ferritic steels.8. SCHUBERT,J., Kaltrißsicherheit tiefentschwefelter hochfester

schweißbarer Baustähle, Report AJF D21TU-Bergakademie Freiberg1993.

9. Stahl-Eisen-Prüfblätter SEP 1100: 'Begriffe im Zusammenhang mitRissen und Brüchen', September 1992.

10. HANUS, F., 'Erfahrungen zum Heißrissverhalten von unlegierten Stähiendes Apparatebaus', DVS -Sondertagung Schweißen im AnlagenundBehälterbau 1993 -DVS Volume 151, pp.60 -68.

11. DEVILLERS, L., and KAPLAN, D., 'The metallurgical aspects of laserwelding of structural steels', Welding in theWorld, Vol. 29 (1991)No. 11/12, pp. 334 -337.

12. WEGMANN, H. and HOLTHAUS, M., 'Laserstrahlschweißen hochfesterBaustähle mit Reh bis 960 N/mm2 und Blechdicken bis 10 mm', Stahlund Eisen 118 (1998) no. 11 pp. 121 -127.

13. BAILEY, N. and SAVAGE, W.F.. 'Solidification cracking of ferritic steelsduring submerged arG welding', The Welding Institute, Cambridge, 1974.

14. 'Laser Welding in Ship Construction', Final project summary report.L-ship/PSG(95),1995.

15. DHOOGE, A. and VINCKIER, A.. 'La fissuration au rechauffage -revuedes etudes reGentes (1984 -1990)', Welding in the World, Vol.30 no. 3/4,pp.45-7,1992.

16. BANNENBERG, N. and LACHMUND, H.. 'Comparative assessment ofLD converter and Electric Arc Fumace steelmaking in terms of removalof undesirable trampelements', Internat reporl LA\990E, 1999.

17. GRABKE, H.J.. 'Surface and grain boundary segregation on and in iron',steel research 57(1986) no. 4S, pp. 178 -185.

18. TAMAKI, K.. 'Temper embrittlement in HAZ of Cr -Mo Steels', Welding inthe World, Vol. 43, No. 2 (1999) S. pp. 36 -48

19. GRABKE, H.J.. 'Surface and grain boundary segregation on and in iron,steel research' 57 (1986) No. 4S, pp. 178 -185.

iDILLINGER HÜTTE GTS

H.avy Fabricatlon Division

Full Power at Dillingen !

Dillinger Hütte GTS not only offers plates inextreme dimensions, but also provides fabri-cation services fOT plate dimensions whichexceed its customers' own fabricationcapabilities. The recent installation of ODe of theworld's most powerful roll-bending machineshas significantly enhanced Dillinger's processingequipment It now allows cold forming on plate-sizes where in former times solely hot formingbad to be chosen. This is of particular interest fOTquenched and tempered steels which,increasingly, will playamore and moresignificant role in pressure vessel design in thefuture.

Your contacts foT enquiries:

Phone Facsimile E-Man addressJean-Paul BERNARD +496831472193 +496831473346 jean-paul.bemard@di11inger.bizJoel MICHEL +496831472813 +496831473346 joe1.michel@di11inger.biz

PostaladdressDillinger Hütte GrS, Heavy Fabrication Division

P.O. Box 1580,66748 Di11ingen, Gennany