Chapter_1 -Surface hardening processes.pdf

CHAPTER 1

Process Selection Guide

SURFACE HARDENING, a process thatincludes a wide variety of techniques (Table 1),is used to improve the wear resistance of partswithout affecting the more soft, tough interior ofthe part. This combination of hard surface andresistance to breakage on impact is useful inparts such as a cam or ring gear that must havea very hard surface to resist wear, along with atough interior to resist the impact that occursduring operation. Further, the surface hardeningof steel has an advantage over through harden-ing, because less expensive low- and medium-carbon steels can be surface hardened withoutthe problems of distortion and cracking associ-ated with the through hardening of thick sec-tions.

There are three distinctly different ap-proaches to the various methods for surfacehardening (Table 1):

• Thermochemical diffusion methods, whichmodify the chemical composition of the sur-face with hardening species such as carbon,nitrogen, and boron. Diffusion methods alloweffective hardening of the entire surface of apart and are generally used when a large num-ber of parts are to be surface hardened.

• Applied energy or thermal methods, whichdo not modify the chemical composition ofthe surface but rather improve properties byaltering the surface metallurgy; that is, theyproduce a hard quenched surface withoutadditional alloying species.

• Surface coating or surface-modificationmethods, which involve the intentionalbuildup of a new layer on the steel substrateor, in the case of ion implantation, alter thesubsurface chemical composition

Each of these approaches for surface harden-ing is briefly reviewed in this chapter, withemphasis placed on process comparisons tofacilitate process selection. More detailed infor-mation on the various methods described can befound in subsequent chapters.

Diffusion Methods of Surface Hardening

As previously mentioned, surface hardeningby diffusion involves the chemical modificationof a surface. The basic process used is thermo-chemical, because some heat is needed to en-hance the diffusion of hardening species into thesurface and subsurface regions of a part. Thedepth of diffusion exhibits a time-temperaturedependence such that:

Case depth � K �T�im�e� (Eq 1)

where the diffusivity constant, K, depends ontemperature, the chemical composition of the

Diffusion methods

CarburizingNitridingCarbonitridingNitrocarburizingBoridingThermal diffusion process

Applied energy methods

Flame hardeningInduction hardeningLaser beam hardeningElectron beam hardening

Coating and surface modification

Hard chromium platingElectroless nickel platingThermal sprayingWeld hardfacingChemical vapor depositionPhysical vapor depositionIon implantationLaser surface processing

Table 1 Engineering methods for surfacehardening of steels

© 2002 ASM International. All Rights Reserved.Surface Hardening of Steels: Understanding the Basics (#06952G)

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2 / Surface Hardening of Steels

steel, and the concentration gradient of a givenhardening species. In terms of temperature, thediffusivity constant increases exponentially as afunction of absolute temperature. Concentrationgradients depend on the surface kinetics andreactions of a particular process.

Methods of hardening by diffusion includeseveral variations of hardening species (such ascarbon, nitrogen, or boron) and of the processmethod used to handle and transport the harden-ing species to the surface of the part. Processmethods for exposure involve the handling ofhardening species in forms such as gas, liquid,or ions. These process variations naturally pro-duce differences in typical case depth and hard-ness (Table 2). Factors influencing the suitabil-ity of a particular diffusion method include thetype of steel, the desired case hardness, and thecase depth.

It is also important to distinguish betweentotal case depth and effective case depth. Theeffective case depth is typically approximatelytwo-thirds to three-fourths the total case depth.The required effective depth must be specifiedso that the heat treater can process the parts forthe correct time at the proper temperature.

CarburizingCarburizing is the addition of carbon to the

surface of low-carbon steels at temperatures(generally between 850 and 950 °C, or 1560 and1740 °F) at which austenite, with its high solu-bility for carbon, is the stable crystal structure.Hardening of the component is accomplishedby removing the part and quenching or allowingthe part to slowly cool and then reheating to theaustenitizing temperature to maintain the veryhard surface property. On quenching, a goodwear- and fatigue-resistant high-carbon marten-sitic case is superimposed on a tough, low-carbon steel core. Carburized steels used in casehardening usually have base carbon contents of approximately 0.2 wt%, with the carbon con-tent of the carburized layer being fixed between0.8 and 1.0 wt%. Carburizing methods includegas carburizing, vacuum carburizing, plasma(ion) carburizing, salt bath carburizing, andpack carburizing. These methods introduce car-bon by use of an atmosphere (atmospheric gas,plasma, and vacuum), liquids (salt bath), orsolid compounds (pack). The vast majority ofcarburized parts are processed by gas carburiz-ing, using natural gas, propane, or butane. Vac-uum and plasma carburizing are useful because

of the absence of oxygen in the furnace at-mosphere. Salt bath and pack carburizing havelittle commercial importance but are still doneoccasionally.

Gas carburizing can be run as a batch or acontinuous process. Furnace atmospheres con-sist of a carrier gas and an enriching gas. Thecarrier gas is supplied at a high flow rate toensure a positive furnace pressure, minimizingair entry into the furnace. The type of carrier gasaffects the rate of carburization. Carburizationby methane is slower than by the decompositionof carbon monoxide (CO). The enriching gasprovides the source of carbon and is supplied ata rate necessary to satisfy the carbon demand ofthe work load.

Most gas carburizing is done under condi-tions of controlled carbon potential by measure-ment of the CO and carbon dioxide (CO2) con-tent. The objective of the control is to maintaina constant carbon potential by matching the lossin carbon to the workpiece with the supply ofenriching gas. The carburization process iscomplex, and a comprehensive model of car-burization requires algorithms that describe thevarious steps in the process, including carbondiffusion, kinetics of the surface reaction, kinet-ics of the reaction between the endogas andenriching gas, purging (for batch processes),and the atmospheric control system.

Vacuum carburizing is a nonequilibrium,boost-diffusion-type carburizing process inwhich austenitizing takes place in a rough vac-uum, followed by carburization in a partial pres-sure of hydrocarbon gas, diffusion in a roughvacuum, and then quenching in either oil or gas.Vacuum carburizing offers the advantages ofexcellent uniformity and reproducibility be-cause of the improved process control with vac-uum furnaces, improved mechanical propertiesdue to the lack of intergranular oxidation, andreduced cycle time. The disadvantages of vac-uum carburizing are predominantly related toequipment costs and throughput.

Plasma (ion) carburizing is basically avacuum process using glow-discharge technol-ogy to introduce carbon-bearing ions to the steelsurface for subsequent diffusion. This process iseffective in increasing carburization rates,because the process bypasses several dissocia-tion steps that produce active soluble carbon.For example, because of the ionizing effect ofthe plasmas, active carbon for adsorption can beformed directly from methane (CH4) gas. Hightemperatures can be used in plasma carburizing,



Process Selection Guide / 3

Pro

cess

Nat

ure

of c

ase

Pro

cess

te

mpe

ratu

re, °

C (

°F)

Typ

ical

cas

e de

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Cas

e ha

rdne

ss,

HR

CT

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ase

met

als

Pro

cess

cha

ract

eris

tics

Car

buri

zing

Pack

Dif

fuse

d ca

rbon

815–

1090

(150

0–20

00)

125

µm–1

.5 m

m(5

–60

mils

)50

–63(

a)L

ow-c

arbo

n st

eels

, low

-ca

rbon

allo

y st

eels

Low

equ

ipm

ent c

osts

, dif

ficul

t to

cont

rol

case

dep

th a

ccur

atel

yG

asD

iffu

sed

carb

on81

5–98

0(1

500–

1800

)75

µm

–1.5

mm

(3–6

0 m

ils)

50–6

3(a)

Low

-car

bon

stee

ls, l

ow-

carb

on a

lloy

stee

lsG

ood

cont

rol o

f ca

se d

epth

, sui

tabl

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ntin

uous

ope

ratio

n, g

ood

gas

cont

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requ

ired

, can

be

dang

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sL

iqui

dD

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sed

carb

on a

ndpo

ssib

ly n

itrog

en81

5–98

0(1

500–

1800

)50

µm

–1.5

mm

(2–6

0 m

ils)

50–6

5(a)

Low

-car

bon

stee

ls, l

ow-

carb

on a

lloy

stee

lsFa

ster

than

pac

k an

d ga

s pr

oces

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can

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esa

lt di

spos

al p

robl

em, s

alt b

aths

req

uire

freq

uent

mai

nten

ance

Vac

uum

Dif

fuse

d ca

rbon

815–

1090

(150

0–20

00)

75 µ

m–1

.5 m

m(3

–60

mils

)50

–63(

a)L

ow-c

arbo

n st

eels

, low

-ca

rbon

allo

y st

eels

Exc

elle

nt p

roce

ss c

ontr

ol, b

righ

t par

ts, f

aste

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an g

as c

arbu

rizi

ng, h

igh

equi

pmen

tco

sts

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ridi

ng

Gas

Dif

fuse

d ni

trog

en,

nitr

ogen

com

poun

ds48

0–59

0(9

00–1

100)

125

µm–0

.75

mm

(5–3

0 m

ils)

50–7

0A

lloy

stee

ls, n

itrid

ing

stee

ls, s

tain

less

ste

els

Har

dest

cas

es f

rom

nitr

idin

g st

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,qu

ench

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not r

equi

red,

low

dis

tort

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proc

ess

is s

low

, is

usua

lly a

bat

ch p

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ssSa

ltD

iffu

sed

nitr

ogen

, ni

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unds

510–

565

(950

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5 µm

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5 m

m(0

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0 m

ils)

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0M

ost f

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us m

etal

sin

clud

ing

cast

iron

sU

sual

ly u

sed

for

thin

har

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ses

<25

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(1

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, no

whi

te la

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proc

esse

sIo

nD

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sed

nitr

ogen

, ni

trog

en c

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unds

340–

565

(650

–105

0)75

µm

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5 m

m(3

–30

mils

)50

–70

Allo

y st

eels

, nitr

idin

g st

eels

, sta

inle

ss s

teel

sFa

ster

than

gas

nitr

idin

g, n

o w

hite

laye

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gh e

quip

men

t cos

ts, c

lose

cas

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ntro

l

Car

boni

trid

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Gas

Dif

fuse

d ca

rbon

an

d ni

trog

en76

0–87

0(1

400–

1600

)75

µm

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5 m

m(3

–30

mils

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a)L

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n st

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allo

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ainl

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l

Low

er te

mpe

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re th

an c

arbu

rizi

ng (

less

dist

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light

ly h

arde

r ca

se th

anca

rbur

izin

g, g

as c

ontr

ol c

ritic

alL

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d (c

yani

ding

)D

iffu

sed

carb

on

and

nitr

ogen

760–

870

(140

0–16

00)

2.5–

125

µm(0

.1–5

mils

)50

–65(

a)L

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arbo

n st

eels

Goo

d fo

r th

in c

ases

on

nonc

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al p

arts

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tch

proc

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sal

t dis

posa

l pro

blem

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c ni

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ngD

iffu

sed

carb

on

and

nitr

ogen

565–

675

(105

0–12

50)

2.5–

25 µ

m(0

.1–1

mil)

40–6

0(a)

Low

-car

bon

stee

lsL

ow-d

isto

rtio

n pr

oces

s fo

r th

in c

ase

on lo

w-

carb

on s

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, mos

t pro

cess

es a

repr

opri

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Oth

er

Bor

idin

gD

iffu

sed

boro

n,

boro

n co

mpo

unds

400–

1150

(750

–210

0)12

.5–5

0 µm

(0.5

–2 m

ils)

40–>

70A

lloy

stee

ls, t

ool s

teel

s,co

balt

and

nick

el a

lloys

Prod

uces

a h

ard

com

poun

d la

yer,

mos

tlyap

plie

d ov

er h

arde

ned

tool

ste

els,

hig

hpr

oces

s te

mpe

ratu

re c

an c

ause

dis

tort

ion

The

rmal

dif

fusi

on

proc

ess

Dif

fuse

d ca

rbid

e la

yers

via

salt

bath

proc

essi

ng

800–

1250

°C(1

475–

2285

°F)

2–20

µm

(0.0

8–0.

8 m

il)>

70T

ool s

teel

s, a

lloy

stee

ls,

med

ium

-car

bon

stee

lsPr

oduc

es a

har

d co

mpo

und

laye

r, m

ostly

appl

ied

over

har

dene

d to

ol s

teel

s, h

igh

proc

ess

tem

pera

ture

can

cau

se d

isto

rtio

n

(a)

Req

uire

s qu

ench

fro

m a

uste

nitiz

ing

tem

pera

ture

Tabl

e 2

Typi

cal c

hara

cter

isti

cs o

f diff

usio

n tr

eatm

ents




because the process takes place in an oxygen-free vacuum, thus producing a greater carbur-ized case depth than both atmospheric gas andvacuum carburizing.

Salt bath or liquid carburizing is amethod of case hardening steel in a molten saltbath that contains the chemicals required to pro-duce a case comparable with one resulting fromgas or pack carburizing. Carburizing in liquidsalt baths provides a convenient method of casehardening, with low distortion and considerableflexibility and uniformity of control of the case.However, the expense and environmental prob-lems associated with disposing of salt baths,particularly those containing cyanide, have lim-ited the use of this process, although non-cyanide-containing salts have been developed.

Pack carburizing is the oldest carburizingprocess. In this case-hardening method, partsare packed in a blend of coke and charcoal with“activators” and then heated in a closed con-tainer. Although a labor-intensive process, packcarburizing is still practiced in some tool rooms,because facility requirements are minimal.

NitridingNitriding is a process similar to carburizing,

in which nitrogen is diffused into the surface ofa ferrous product to produce a hard case. Unlikecarburizing, nitrogen is introduced between 500and 550 °C (930 and 1020 °F), which is belowthe austenite formation temperature (Ac1) forferritic steels, and quenching is not required. Asa result of not austenitizing and quenching toform martensite, nitriding results in minimumdistortion and excellent control. The variousnitriding processes (Table 2) include gas nitrid-ing, liquid nitriding, and plasma (ion) nitriding.

Gas nitriding is a case-hardening processthat takes place in the presence of ammonia gas.Either a single-stage or a double-stage processcan be used when nitriding with anhydrousammonia. The single-stage process, in which atemperature of 495 to 525 °C (925 to 975 °F) isused, produces the brittle nitrogen-rich com-pound zone known as the white nitride layer atthe surface of the nitrided case. The double-stage process, or Floe process, has the advan-tage of reducing the white nitrided layer thick-ness. After the first stage, a second stage isadded either by continuing at the first-stagetemperature or increasing the temperature to550 to 565 °C (1025 to 1050 °F). The use of thehigher-temperature second stage lowers thecase hardness and increases the case depth.

Liquid nitriding (nitriding in a molten saltbath) uses temperatures similar to those used ingas nitriding and a case-hardening medium ofmolten, nitrogen-bearing, fused salt bath con-taining either cyanides or cyanates. Similar tosalt bath carburizing, liquid nitriding has theadvantage of processing finished parts becausedimensional stability can be maintained due tothe subcritical temperatures used in the process.Furthermore, at the lower nitriding tempera-tures, liquid nitriding adds more nitrogen andless carbon to ferrous materials than that ob-tained with high-temperature treatments be-cause ferrite has a much greater solubility fornitrogen (0.4% max) than carbon (0.02% max).

Plasma (ion) nitriding is a method of sur-face hardening using glow-discharge technol-ogy to introduce nascent (elemental) nitrogen tothe surface of a metal part for subsequent diffu-sion into the material. The process is similar toplasma carburizing in that a plasma is formed ina vacuum using high-voltage electrical energy,and the nitrogen ions are accelerated toward theworkpiece. The ion bombardment heats thepart, cleans the surface, and provides activenitrogen. The process provides better control ofcase chemistry, case uniformity, and lower partdistortion than gas nitriding.

Carbonitriding and Ferritic Nitrocarburizing

Carbonitriding introduces both carbon andnitrogen into the austenite of the steel. Theprocess is similar to carburizing in that theaustenite composition is enhanced and the highsurface hardness is produced by quenching toform martensite. This process is a modifiedform of gas carburizing in which ammonia isintroduced into the gas-carburizing atmosphere.As in gas nitriding, elemental nitrogen forms atthe work-piece surface and diffuses along withcarbon into the steel. Typically, carbonitridingtakes place at a lower temperature and a shortertime than gas carburizing, producing a shal-lower case. Steels with carbon contents up to0.2% are commonly carbonitrided.

Ferritic nitrocarburizing is a subcriticalheat treatment process, carried out by liquid,gaseous, or plasma techniques, and involves thediffusion of carbon and nitrogen into the ferriticphase. The process results in the formation of athin white layer or compound layer with anunderlying diffusion zone of dissolved nitrogenin iron, or alloy nitrides. The white layerimproves surface resistance to wear, and the dif-




fusion zone increases the fatigue endurancelimit, especially in carbon and low-alloy steels.Alloy steels, cast irons, and some stainlesssteels can be treated. The process is used to pro-duce a thin, hard skin, usually less than 25 µm(1 mil) thick, on low-carbon steels in the form ofsheet metal parts, powder metallurgy parts,small shaft sprockets, and so forth.

BoridingBoriding, or boronizing, is a thermochemical

surface-hardening process that can be applied toa wide variety of ferrous, nonferrous, and cer-met materials. The boronizing pack process issimilar to pack carburizing, with the parts to becoated being packed with a boron-containingcompound such as boron powder or ferroboron.Activators such as chlorine and fluorine com-pounds are added to enhance the production ofthe boron-rich gas at the part surface. Process-ing of high-speed tool steels that were previ-ously quench hardened is accomplished at 540 °C (1000 °F). Boronizing at higher tem-peratures up to 1090 °C (2000 °F) causes diffu-sion rates to increase, thus reducing the process time. The boron case does not have to bequenched to obtain its high hardness, but toolsteels processed in the austenitizing temperaturerange need to be quenched from the coatingtemperature to harden the substrate.

Boronizing is most often applied to tool steelsor other substrates that are already hardened byheat treatment. The thin (12 to 15 µm, or 0.48 to0.6 mil) boride compound surfaces provide evengreater hardness, improving wear service life.Distortion from the high processing tempera-tures is a major problem for boronized coatings.Finished parts that are able to tolerate a few thou-sandths of an inch (75 µm) distortion are bettersuited for this process sequence, because the thincoating cannot be finish ground.

Thermal Diffusion ProcessThe thermal diffusion (TD) process is a

method of coating steels with a hard, wear-resistant layer of carbides, nitrides, or carboni-trides. In the TD process, the carbon and nitro-gen in the steel substrate diffuse into a depositedlayer with a carbide-forming or nitride-formingelement, such as vanadium, niobium, tantalum,chromium, molybdenum, or tungsten. The dif-fused carbon or nitrogen reacts with the carbide-and nitride-forming elements in the depositedcoating to form a dense and metallurgically

bonded carbide or nitride coating at the sub-strate surface.

The hard alloy carbide, nitride, and carboni-tride coatings in the TD method can be applied tosteels by means of salt bath processing or flu-idized beds. The salt bath method uses moltenborax with additions of carbide-forming ele-ments, such as vanadium, niobium, titanium, orchromium, which combine with carbon from thesubstrate steel to produce alloy carbide layers.Because the growth of the layers is dependent oncarbon diffusion, the process requires a relativelyhigh temperature, from 800 to 1250 °C (1470 to2280 °F), to maintain adequate coating rates.Carbide coating thicknesses of 4 to 7 µm are pro-duced in 10 min to 8 h, depending on bath temper-ature and type of steel. The coated steels may becooled and reheated for hardening, or the bathtemperature may be selected to correspond to thesteel austenitizing temperature, permitting thesteel to be quenched directly after coating.

Surface Hardening by Applied Energy

The surface methods described in this sectioninclude conventional thermal treatments, suchas flame and induction hardening, and technolo-gies that incorporate high-energy laser or elec-tron beams. All of these methods may be classi-fied as simply a thermal treatment withoutchemistry changes. They can be used to hardenthe entire surface or localized areas. Whenlocalized heating is carried out, the term selec-tive surface hardening is used to describe thesemethods.

Flame hardening consists of austenitizingthe surface of steel by heating with an oxy-acetylene or oxyhydrogen torch and immedi-ately quenching with water. After quenching,the microstructure of the surface layer consistsof hard martensite over a lower-strength interiorcore of other steel morphologies, such as ferriteand pearlite. A prerequisite for proper flamehardening is that the steel must have adequatecarbon and other alloy additions to produce thedesired hardness, because there is no change incomposition. Flame-hardening equipment usesdirect impingement of a high-temperature flameor high-velocity combustion product gases toaustenitize the component surface and quicklycool the surface faster than the critical coolingrate to produce martensite in the steel. This isnecessary because the hardenability of the com-ponent is fixed by the original composition of




Characteristics Flame Induction

Equipment Oxyfuel torch, special headquench system

Power supply, inductor, quench system

Applicable material

Ferrous alloys, carbon steels, alloy steels, cast irons

Same

Speed of heating Few seconds to fewminutes

1–10 s

Depth of hardening

1.2–6.2 mm(0.050–0.250 in.)

0.4–1.5 mm (0.015–0.060 in.); 0.1 mm(0.004 in.) forimpulse

Processing One part at a time SamePart size No limit Must fit in coilTempering Required SameCan be automated Yes YesOperator skills Significant skill

requiredLittle skill

required after setup

Control of process Attention required Very preciseOperator comfort Hot, eye protection

requiredCan be done in

suitCost

Equipment Low HighPer piece Best for large

workBest for small

work

Table 3 Comparison of flame- andinduction-hardening processes

the steel. Thus, equipment design is critical tothe success of the operation. Flame-heatingequipment may be a single torch with a spe-cially designed head or an elaborate apparatusthat automatically indexes, heats, and quenchesparts. With improvements in gas-mixing equip-ment, infrared temperature measurement andcontrol, and burner rig design, flame hardeninghas been accepted as a reliable heat treatingprocess that is adaptable to general or localizedsurface hardening for small or medium-to-highproduction requirements.

Induction heating is an extremely versatileheating method that can perform uniform surfacehardening, localized surface hardening, throughhardening, and tempering of hardened pieces.Heating is accomplished by placing a steel part inthe magnetic field generated by high-frequencyalternating current passing through an inductor,usually a water-cooled copper coil. The depth ofheating produced by induction is related to thefrequency of the alternating current: the higherthe frequency is, the thinner or more shallow the heating. Therefore, deeper case depths andeven through hardening are produced by usinglower frequencies. The electrical considerationsinvolve the phenomena of hysteresis and eddycurrents. Because secondary and radiant heat areeliminated, the process is suited for productionline areas. Table 3 compares the flame- andinduction-hardening processes.

Laser surface heat treatment is widelyused to harden localized areas of steel and castiron machine components. The heat generatedby the absorption of the laser light is controlledto prevent melting and is therefore used in theselective austenitization of local surface regions,which transform to martensite as a result of rapidcooling (self-quenching) by the conduction ofheat into the bulk of the workpiece. This processis sometimes referred to as laser transformationhardening to differentiate it from laser surfacemelting phenomena. There is no chemistrychange produced by laser transformation hard-ening, and the process, similar to induction andflame hardening, provides an effective techniqueto harden ferrous materials selectively.

The process produces typical case depths forsteel ranging from 0.75 to 1.3 mm (0.030 to0.050 in.), depending on the laser power range,and hardness values as high as 60 HRC. Laserprocessing has advantages over electron beamhardening in that laser hardening does notrequire a vacuum, wider hardening profiles are

possible, and there can be greater accessibilityto hard-to-get areas with the flexibility of opti-cal manipulation of light energy.

Electron Beam Hardening. In electronbeam hardening, the surface of the hardenablesteel is heated rapidly to the austenitizing tem-perature, usually with a defocused electron beamto prevent melting. The mass of the workpiececonducts the heat away from the treated surfaceat a rate that is rapid enough to produce harden-ing. Materials for application of electron beamhardening must contain sufficient carbon andalloy content to produce martensite. With therapid heating associated with this process, thecarbon and alloy content should be in a form thatquickly allows complete solid solution in the austenite at the temperatures produced by the electron beam. In addition, the mass of theworkpiece should be sufficient to allow properquenching; for example, the part thickness mustbe at least ten times the depth of hardening, andhardened areas must be properly spaced to pre-vent tempering of previously hardened areas.

To produce an electron beam, a high vacuumof 10–3 Pa (10–5 torr) is required in the regionwhere the electrons are emitted and accelerated.This vacuum environment protects the emitter




from oxidizing and avoids scattering of the elec-trons while they are still traveling at a relativelylow velocity. Electron beam hardening in hardvacuum units requires that the part be placed ina chamber that is sufficiently large to manipu-late the electron beam gun or the workpiece.Out-of-vacuum units usually involve shroudingthe workpiece; a partial vacuum (13 Pa, or 10–2

torr), is obtained in the work area by mechanicalpumps.

Surface Hardening by Coating or Surface Modification

Plating or coating treatments deposit hardsurface layers of completely different chem-istry, structure, and properties on steel sub-strates and are applied by well-established tech-nologies such as electrodeposition, electrolessdeposition, thermal spraying, and weld hardfac-ing. In more recent years, coating or surface-modification methods long used in the electron-ics industry to fabricate thin films and deviceshave been used to treat steels. These includevapor deposition techniques and ion implanta-tion. Laser surface processing (melting, alloy-ing, and cladding) has also been carried out on steels. These various surface-engineeringtreatments can deposit very thin films (e.g., 1 to 10 µm for physical vapor deposition) or thickcoatings (e.g., 3 to 10 mm for weld hardfacing).

Hard Chromium PlatingProcess Description. Hard chromium plat-

ing is produced by electrodeposition from asolution containing chromic acid (CrO3) and acatalytic anion in proper proportion. The metalso produced is extremely hard (850 to 1000 HV)and corrosion resistant. Plating thickness rangesfrom 2.5 to 500 µm (0.1 to 20 mils).

Applications. Hard chromium plating isused for products such as piston rings, shockabsorbers, struts, brake pistons, engine valvestems, cylinder liners, and hydraulic rods. Otherapplications are for aircraft landing gears, tex-tile and gravure rolls, plastic rolls, and dies andmolds. The rebuilding of mismachined or wornparts comprises large segments of the industry.

Advantages:

• Low-temperature treatment (60 °C, or 140 °F)• High hardness and wear resistance

• Low coefficient of friction• Thick layers possible

Disadvantages:

• Poor thickness uniformity on complex com-ponents

• Hydrogen embrittlement• Environmental problems associated with

plating bath disposal. Chromium replacementcoatings, such as electroless nickel and ther-mal spray coatings, are being used increas-ingly.

Electroless Nickel CoatingProcess Description. The coating is

deposited by an autocatalytic chemical reduc-tion of nickel ions by hydrophosphite, amino-borane, or borohydride compounds. Currently,hot acid hypophosphite-reduced baths are mostfrequently chosen to coat steel. Heat-treateddeposit hardness exceeds 1000 HV.

Applications. Electroless nickel coatingshave good resistance to corrosion and wear andare used to protect machinery found in thepetroleum, chemicals, plastics, optics, printing,mining, aerospace, nuclear, automotive, elec-tronics, computers, textiles, paper, and foodindustries.

Advantages:

• Low-temperature treatment (<100 °C, or 212 °F)

• More corrosion resistance than electroplatedchromium

• Ability to coat complex shapes uniformly• Incorporation of hard particles to increase

hardness• Good solderability and brazeability

Disadvantages:

• Higher costs than electroplating• Poor welding characteristics• Slower plating rate, as compared to rates for

electrolytic methods• Heat treatment needed to develop optimal

properties

Thermal SprayingProcess Description. Thermal spraying is a

generic term for a group of processes in which ametallic, ceramic, cermet, and some polymericmaterials in the form of powder, wire, or rod arefed to a torch or gun with which they are heated to




Property or characteristic Coating type

Wire flame spray

Powder flame spray Electric arc Plasma spray

High-velocity oxyfuel (HVOF)

Bond strength, MPa (103 psi)

Ferrous metals 14 (2) 28 (4) 41 (6) 34+ (5+) 62 (9)

Nonferrous metals 21 (3) 21 (3) 41+ (6+) 34+ (5+) 70 (10.2)Self-fluxing alloys . . . 69+ (10+)(a) . . . . . . 62 (9)(b)Ceramics . . . 14–34 (2–5) . . . 21+ (3+) . . .Carbides . . . 34–48 (5–7) . . . 55–69 (8–10) 83+ (12+)

Density, % that of equivalentwrought material

Ferrous metals 90 90 90 95 98+

Nonferrous metals 90 90 90 95 98+Self-fluxing alloys . . . 100(a) . . . . . . 100(a)Ceramics . . . 95 . . . 95+ . . .Carbides . . . 90 . . . 95+ 98+

Hardness Ferrous metals 84 HRB–35 HRC

80 HRB–35 HRC

95 HRB–40 HRC

80 HRB–40 HRC

90 HRB–50 HRC

Nonferrous metals 95 HRH–40 HRC

30 HRH–20 HRC

40 HRH–80 HRB

40 HRH–40 HRC

100 HRH–55 HRC

Self-fluxing alloys . . . 30–60 HRC . . . . . . 50–60 HRCCeramics . . . 50–65 HRC . . . 50–70 HRC . . .Carbides . . . 50–60 HRC . . . 50–60 HRC 55–65 HRC

Permeability Ferrous metals Medium Medium High Low NegligibleNonferrous metals Medium Medium High Low NegligibleSelf-fluxing alloys . . . None(a) . . . . . . None(a)Ceramics . . . Medium . . . Low . . .Carbides . . . Low . . . Low Negligible

Coating-thickness limitation,mm (in.)

Ferrous metals 1.25–2.5(0.05–0.1)

1.25–2.5(0.05–0.1)

1.25–2.5(0.05–0.1)

1.25–2.5|(0.05–0.1)

1.25–2.5(0.05–0.1)

Nonferrous metals 1.25–5(0.05–0.2)

1.25–5(0.05–0.2)

1.25–5(0.05–0.2)

1.25–5(0.05–0.2)

2.5–5(0.1–0.2)

Self-fluxing alloys . . . 0.4–2.5(0.015–0.1)

. . . . . .(0.05)

1.25

Ceramics . . . 0.4 . . . 0.4 . . .(0.015) (0.015) max

Carbides . . . . . . 0.4 0.6(0.015) (0.015) max

(a) Fused coating. (b) Unfused coating

0.4

Table 4 Comparison of major thermal spray coating processes

Process

(0.025)

or slightly above their melting point. The result-ing molten or nearly molten droplets of materialare accelerated in a gas stream and projectedagainst the substrate to form a coating. Com-monly employed methods of deposited thermalspray coatings can be classified as wire flamespray, powder flame spray, electric arc, plasmaspray, and high-velocity oxyfuel (HVOF) spray.Process characteristics are compared in Table 4.

Applications. Thermal spray coatings areused for prevention against wear, corrosion, oroxidation. Table 19 in Chapter 11 lists a widevariety of wear-resistant applications for ther-mal spraying. Thermal spray coatings, particu-larly HVOF coatings, are being used increas-ingly to replace chromium electrodeposits.

Advantages:

• Most metals, ceramics, and some polymerscan be sprayed.

• Significant substrate heating does not occurwith most thermal spray processes.

• Worn or damaged coatings can be strippedwithout changing the properties or dimen-sions of the part.

• Localized treatments are possible.

Disadvantages:

• Line-of-sight process employed.• Most sprayed coatings contain some porosity.• The adhesion of sprayed coatings is generally

poor, compared to other processes.




• High-quality coatings on reentrant surfacesproduced with difficulty.

Weld HardfacingProcess Description. Welding is a solidifi-

cation method for applying coatings with corro-sion, wear, and erosion resistance. Weld-overlaycoatings, sometimes referred to as hardfacing,offer unique advantages over other coating sys-tems in that the overlay/substrate weld providesa metallurgical bond that is not susceptible tospallation and can easily be applied free ofporosity or other defects. Welded deposits ofsurface alloys can be applied in thicknessesgreater than most other techniques, typically inthe range of 3 to 10 mm. Most welding processesare used for application of surface coatings, andon-site deposition can be more easily carried out,particularly for repair purposes.

Applications. Hardfacing applications forwear control vary widely, ranging from verysevere abrasive wear service, such as rockcrushing and pulverizing, to applications tominimize metal-to-metal wear, such as controlvalves where a few thousandths of an inch ofwear is intolerable. Hardfacing is used for con-trolling abrasive wear, such as encountered bymill hammers, digging tools, extrusion screws,cutting shears, parts of earthmoving equipment,ball mills, and crusher parts. It is also used tocontrol the wear of unlubricated or poorly lubri-cated metal-to-metal sliding contacts, such ascontrol valves, undercarriage parts of tractorsand shovels, and high-performance bearings.Hardfacing also is used to control combinationsof wear and corrosion.

Advantages:• Inexpensive• Applicable to large components• Localized coating possible• Excellent coating/substrate adhesion• High deposition rates possible

Disadvantages:

• Residual stresses and distortion can causeserious problems.

• Weld defects can lead to joint failure.• Minimum thickness limits (it is impractical to

produce layers less than 2 to 3 mm thick)• Limited number of coating materials avail-

able, compared to thermal spraying

Chemical Vapor Deposition (CVD)Process Description. Chemical vapor dep-

osition involves the formation of a coating on aheated surface by a chemical reaction from thevapor or gas phase. Deposition temperatures aregenerally in the range of 800 to 1000 °C (1470to 1830 °F). The most widely deposited wear-resistant coatings are titanium carbide (TiC),titanium nitride (TiN), chromium carbide, andalumina. Thicknesses are restricted to approxi-mately 10 µm due to thermal expansion mis-match stresses that develop on cooling.

Applications. The use of the CVD processfor steels has been largely limited to the coatingof tool steels for wear resistance.

Advantages:• High coating hardness; for example, TiN

coatings have a hardness of 2500 HV.• Good adhesion (provided the coating is not

too thick)• Good throwing power (i.e., uniformity of

coating)

Disadvantages:

• High-temperature process (distortion a prob-lem)

• Shard edge coating is difficult due to thermalexpansion mismatch stresses.

• Limited range of materials can be coated.• Environmental concerns about process gases

Physical Vapor Deposition (PVD)Process Description. Physical vapor depo-

sition processes involve the formation of a coat-ing on a substrate by physical deposition ofatoms, ions, or molecules of the coating species.There are three main techniques for applyingPVD coatings: thermal evaporation, sputtering,and ion plating. Thermal evaporation involvesheating of the material until it forms a vapor thatcondenses on a substrate to form a coating.Sputtering involves the electrical generation ofa plasma between the coating species and thesubstrate. Ion plating is essentially a combina-tion of these two processes. A comparison of theprocess characteristics of PVD, CVD, and ionimplantation is provided in Table 5.

Applications. Similar to CVD, the PVDprocess is used to increase the wear resistance oftool steels by the deposition of thin TiN or TiCcoatings at temperatures ranging from 200 to




ProcessProcessing

temperature, °CThrowing

power Coating materialsCoating applications and special features

Vacuum evaporation RT–700, usually <200 Line of sight Chiefly metal, especially Al (a few simple alloys/a fewsimple compounds)

Electronic, optical, decorative,simple masking

Ion implantation 200–400, best <250 for N

Line of sight Usually N (B, C) Wear resistance for tools, dies,etc. Effect much deeper thanoriginal implantation depth.Precise area treatment,excellent process control

Ion plating, ARE RT–0.7 Tm of coating.Best at elevatedtemperatures

Moderate to good

Ion plating: Al, other metals(few alloys). ARE: TiN and other compounds

Electronic, optical, decorative.Corrosion and wearresistance. Dry lubricants.Thicker engineering coatings

Sputtering RT–0.7 Tm of metalcoatings. Best >200 for nonmetals

Line of sight Metals, alloys, glasses, oxides. TiN and othercompounds(a)

Electronic, optical, wearresistance. Architectural(decorative). Generally thincoatings. Excellent processcontrol

CVD 300–2000, usually600–1200

Very good Metals, especially refractory TiN and other compounds(a),pyrolytic BN

Thin, wear-resistant films onmetal and carbide dies, tools,etc. Free-standing bodies ofrefractory metals andpyrolytic C or BN

PVD, physical vapor deposition; CVD, chemical vapor deposition; RT, room temperature; ARE, activated reactive evaporation; Tm, absolute melting temperature.(a) Compounds: oxides, nitrides, carbides, silicides, and borides of Al, B, Cr, Hf, Mo, Nb, Ni, Re, Si, Ta, Ti, V, W, and Zr

Table 5 Comparison of PVD, CVD, and ion implantation process characteristics

550 °C (400 to 1025 °F). This temperature rangeis much more suitable for the coating of toolsteels than the temperatures required for CVD.

Advantages:• Excellent process control• Low deposition temperature• Dense, adherent coatings• Elemental, alloy, and compound coatings

possible

Disadvantages:• Vacuum process with high capital cost• Limited component size treatable• Relatively low coating rates• Poor throwing power without manipulation

of components

Ion ImplantationProcess Description. Ion implantation

involves the bombardment of a solid materialwith medium- to high-energy ionized atoms andoffers the ability to alloy virtually any elemen-tal species into the near-surface region of anysubstrate. The advantage of such a process isthat it produces improved surface propertieswithout the limitations of dimensional changesor delamination found in conventional coatings.

Applications. For steels, the most commonapplication of ion implantation is nitrogen-

implanted tool steels used for forming and cut-ting tools. Titanium plus carbon implantationhas also proved beneficial for tool steels.

Advantages:

• Produces surface alloys independent of ther-modynamic criteria

• No delamination concerns• No significant dimensional changes• Ambient-temperature processing possible• Enhance surface properties while retaining

bulk properties• High degree of control and reproducibility

Disadvantages:

• Very thin treated layer (1 µm or less)• High-vacuum process• Line-of-sight process• Alloy concentrations dependent on sputtering• Relatively costly process; intensive training

required, compared to other surface-treatment processes

• Limited commercial treatment facilitiesavailable

Laser Surface ProcessingProcess Description. Laser surface pro-

cessing involves the melting of a surface with a




laser, with or without surface additions. Withlaser surface melting, melting and controlledcooling are combined to refine the microstruc-ture or to produce an amorphous (or nearlyamorphous) structure. No external material isadded during this process. The composition andproperties of the surface can also be modified byadding external material via powder injection orwire feed. External material can also be placedon the surface by powder deposition, electro-plating, vapor deposition, or thermal spray, thenincorporated by laser scanning. The nature ofincorporating material in the modified surfacevaries depending on laser processing parame-ters, such as energy density and traverse speed.Alloy, clad, and composite surface layers maybe formed in this way.

Applications. Although laser surface pro-cessing has not reached commercial signifi-cance for steels, various carbon and low-alloysteels, tool steels, and stainless steels have beenlaser processed to varying degrees of success.See Chapter 11 for application examples.

Advantages:

• Rapid rates of processing produce novelstructures in the surface region not possiblewith conventional processing.

• For laser cladding, low weld-metal dilutionrates and distortion, compared with compet-ing arc welding methods

Disadvantages:

• High capital equipment costs• Some substrate materials are not compatible

with the laser thermal conduction require-ments.

Important Considerations for Process Selection

Performance Requirements. The key toproper selection of surface-hardening tech-niques is in the identification of the performancerequirements for a given surface-modifiedmaterial system in a given application. Not onlymust the properties of the surface be consideredbut also the properties of the substrate and theinterface between the surface and substrate. Insome systems there is a gradual change in prop-erties between the surface and interior, as, for

example, in nitrided and carburized compo-nents, while in others there is an abrupt change,as, for example, for parts where a coating ofvapor-deposited titanium nitride has been de-posited on steel. Such interface characteristicsmay significantly influence the performance ofa surface-modified system.

The performance requirements of surface-engineered systems may vary widely. Forexample, heavily loaded systems, such as bear-ings and gears, require deep cases to resistrolling contact and bending stresses that resultin fatigue damage. Other applications mayrequire only very thin surface films to resistnear-surface abrasion or scuffing or to reducefriction between moving surfaces. Many ofthese requirements are based on complex inter-actions between applied static and cyclic stressstates and gradients in structures and propertiesof the surface-modified systems (see, for exam-ple, the discussion of bending fatigue strengthof carburized steels in Chapter 2).

Design Constraints. The component designconstraints include consideration of the size andshape of the component, because they mayaffect surface-treatment process capabilities.Will the component fit in the coating equip-ment? Can a line-of-sight process be used? Dosmall holes or channels require a process withhigh throwing power? What kind of maskingwill be required to prevent coating unwantedareas? Is the temperature required by the surfacetreatment compatible with the temperature lim-itations of the component material? What kindof postcoating treatment, including heat treat-ment and finishing, will be required?

Economic Analysis. The economic issue offundamental importance is a cost/benefit analy-sis. This analysis should be based on the fulllife-cycle costs of the surface treatment, includ-ing the process costs (preparation, application,finishing, quality control, waste disposal), uti-lization costs and benefits (productivity ofcoated components), and added value of anyproducts produced or treated. Other economicfactors that must be considered include theavailability of the process, number of compo-nents to be treated, quality-control require-ments, delivery schedules, and so on. Note thatthe analysis should not be based on just the ini-tial cost of the surface treatment. Life-cyclecosts are as important when comparing the costof various surface treatments as in comparing asurface treatment to an untreated surface.




Fig. 1 Range of hardness levels for various materials and surface treatments. EB, electron beam; HSLA, high-strength, low-alloy

Process Comparisons

Hardness versus Wear Resistance. Thewear processes that are usually mitigated by theuse of hard surfaces are low-stress abrasion,wear in systems involving relative sliding ofconforming solids, fretting wear, galling, and,to some extent, solid-particle erosion. Unfortu-nately, there are many caveats to this statement,and substrate/coating selection should be care-fully studied, with proper tests carried out ifnecessary. Coating suppliers should also beconsulted. Chapter 3 provides additional infor-mation on wear processes and the means to pre-vent specific types of wear.

Figure 1 shows typical ranges in hardness formany of the surface-engineering processes usedto control wear. All of the treatments shown inthis figure have hardness values greater thanordinary constructional steel or low-carbonsteel. The surface-hardening processes that relyon martensitic transformations all have compa-rable hardness, and the diffusion treatments thatproduce harder surfaces are nitriding, boroniz-

ing (boriding), and chromizing. The hardestmetal coating is chromium plate, although hard-ened electroless nickel plate can attain valuesjust under that of chromium. The surfaces thatexceed the hardness of chromium are the cer-mets or ceramics or surfaces that are modifiedso that they are cermets or ceramics. Theseinclude nitrides, carbides, borides, and similarcompounds. The popular solid ceramics usedfor wear applications—aluminum oxide, siliconcarbide, and silicon nitride—generally havehardnesses in the range of 2000 to 3000kg/mm2. As shown in Fig. 1, when materialssuch as aluminum oxide are applied by plasmaspraying or other thermal spray process, theyhave hardnesses that are less than the samematerial in solid pressed-and-sintered form.This is because the sprayed materials containporosity and oxides that are not contained in thesintered solid form.

Cost must be weighed against the perform-ance required for the surface-treatment system.A low-cost surface treatment that fails to per-form its function is a wasted expense. Unfortu-




Fig. 2 Approximate relative costs of various surface treatments

nately, it is nearly impossible to give absolutecomparative costs for different surface-engineering options. Often, a range of prices isoffered for a particular job from different,equally competent candidate suppliers. Proba-bly the most important factor that relates to costsof producing a wear-resistant surface on a part ispart quantity. Treating many parts usuallyallows economies in treatment and finishing.

Another consideration when assessing sur-face-treatment costs is part size. There are some critical sizes for each surface-treatmentprocess above which the cost of obtaining thetreatment may be high. A number of surfacetreatments require that the part fit into the workzone of a vacuum chamber. The cost of vacuumequipment goes up exponentially with chambervolume.

Other factors to be considered are:

• The time required for a given surface treat-ment

• Fixturing, masking, and inspection costs• Final finishing costs• Material costs• Energy costs

• Labor costs• Environmentally related costs, for example,

disposal of spent plating solutions• Expected service life of the coating

Because of these various factors, it is difficultto compare costs with a high degree of accu-racy. Figure 2 provides some general guidelinesfor cost comparisons.

Distortion or Size Change Tendencies.Figure 3 shows the surface temperatures that are encountered in various surface-engineeringprocesses. As indicated in the figure, theprocesses are categorized into two groups: onegroup produces negligible part distortion, andthe other group contains processes that havevarying potential for causing distortion. Obvi-ously, if a part could benefit from a surfacetreatment but distortion cannot be tolerated,processes that require minimal heating shouldbe considered.

Coating Thickness Attainable. Figure 4shows the typical thickness/penetration capabil-ities of various coating and surface treatments.As indicated in the figure, some surface-engi-neering treatments penetrate into the surface,



Fig. 4 Typical coating thickness/depth of penetration for various coating and surface-hardening processes. SAW, submerged arcwelding; FCAW, flux cored arc welding; GMAW, gas metal arc welding; PAW, plasma arc welding; GTAW, gas tungsten arc

welding; EB, electron beam; OAW, oxyacetylene welding; FLSP, flame spraying; PSP, plasma spraying

Fig. 3 Maximum surface temperatures that can be anticipated for various surface-engineering processes. The dashed vertical line at 540 °C (1000 °F) represents the temperature limit for distortion for ferrous metals. Obviously, a temperature of 540 °C

(1000 °F) would melt a number of nonferrous metals, and it would cause distortion on metals such as aluminum or magnesium. How-ever, this process temperature information can be used to compare the heating that will be required for a particular process. EB, elec-tron beam



and there is no intentional buildup on the sur-face. Other surface treatments coat or intention-ally build up the surface. This is a selection fac-tor. Can a part tolerate a buildup on the surface?If not, the selection process is narrowed to thetreatments that penetrate into the surface. Otherfactors affecting the thickness of a given surfacetreatment include dimensional requirements,the service conditions, the anticipated/allowablecorrosion or wear depth, and anticipated loadson the surface. Questions or concerns related tocoating thickness should be discussed with thecontractor. Available specifications should alsobe reviewed.

SELECTED REFERENCES

• K.G. Budinski, Surface Engineering forWear Resistance, Prentice-Hall, 1988

• J.R. Davis, Ed., Surface Engineering forCorrosion and Wear Resistance, ASMInternational and IOM Communications,2001

• G. Krauss, Advanced Surface Modifica-tion of Steels, J. Heat Treat., Vol 9 (No.2), 1992, p 81–89

• S. Lampman, Introduction to SurfaceHardening of Steels, Heat Treating, Vol4, ASM Handbook, 1991, p 259–267




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