introduction to surface hardening of steels* · 2020. 10. 3. · introduction to surface hardening...

Introduction to SurfaceHardening of Steels*Revised by Michael J. Schneider, The Timken Company, and Madhu S. Chatterjee, Bodycote

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 upon impact is useful inparts such as a cam or ring gear, bearings or shafts,turbine applications, and automotive componentsthat must have a very hard surface to resist wear,along with a tough interior to resist the impact thatoccurs during operation. Most surface treatmentsresult in compressive residual stresses at the sur-face that reduce the probability of crack initiationand help arrest crack propagation at the case-coreinterface. Further, the surface hardening of steel

can have an advantage over through hardeningbecause less expensive low-carbon and medium-carbon steels can be surface hardened with mini-mal problemsof distortion and cracking associatedwith the through hardening of thick sections.There are two distinctly different approaches

to the various methods for surface hardening(Table 1):

� Methods that involve an intentional buildupor addition of a new layer

� Methods that involve surface and subsurfacemodification without any intentional buildupor increase in part dimensions

The first group of surface-hardening methodsincludes the use of thin films, coatings, or weldoverlays (hardfacings). Films, coatings, andoverlays generally become less cost-effectiveas production quantities increase, especiallywhen the entire surface of workpieces must behardened. The fatigue performance of films,coatings, and overlays may also be a limitingfactor, depending on the bond strength betweenthe substrate and the added layer. Fusion-welded overlays have strong bonds, but the pri-mary surface-hardened steels used in wearapplications with fatigue loads include heavycase-hardened steels and flame- or induction-hardened steels. Nonetheless, coatings andoverlays can be effective in some applications.With tool steels, for example, TiN and Al2O3coatings are effective not only because of theirhardness but also because their chemical inert-ness reduces crater wear and the welding ofchips to the tool. Some overlays can impart cor-rosion-resistant properties. Overlays can beeffective when the selective hardening of largeareas is required.This introductory article on surface harden-

ing focuses exclusively on the second groupof methods, which is further divided into diffu-sion methods and selective-hardening methods(Table 1). Diffusion methods modify the

chemical composition of the surface with hard-ening species such as carbon, nitrogen, orboron. Diffusion methods may allow effectivehardening of the entire surface of a part andare generally used when a large number of partsare to be surface hardened. In contrast, selectivesurface-hardening methods allow localized hard-ening. Selective hardening generally involvestransformation hardening (from heating andquenching), but some selective-hardening meth-ods (selective nitriding, ion implantation, andion beam mixing) are based solely on composi-tional modification. Factors affecting the choiceof these surface-hardening methods are dis-cussed in the section “Process Selection” in thisarticle.

Diffusion Methods ofSurface 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 toenhance the diffusion of hardening elementsinto the surface and subsurface regions of apart. The depth of diffusion exhibits a time-temperature dependence such that:

Case depth ¼ Kffiffiffiffiffiffiffiffiffiffiffi

Timep

(Eq 1)

where the diffusivity constant, K, depends ontemperature, the chemical composition of thesteel, and the concentration gradient of a givenhardening element. 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 include

several variations of hardening elements (suchas carbon, nitrogen, or boron) and of the

ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and ProcessesJ. Dossett and G.E. Totten, editors

Copyright # 2013 ASM InternationalW

All rights reservedwww.asminternational.org

* Revised from S. Lampman, Introduction to Surface Hardening of Steels, Heat Treating, Vol 4, ASM Handbook, ASM International, 1991, p 259–267

Table 1 Engineering methods for surfacehardening of steels

Layer additions

� Hardfacing:¡ Fusion hardfacing (welded overlay)¡ Thermal spray (nonfusion-bonded overlay)

� Coatings:¡ Electrochemical plating¡ Chemical vapor deposition (electroless plating)¡ Thin films (physical vapor deposition, sputtering, ion

plating)¡ Ion mixing

Substrate treatment� Diffusion methods:

¡ Carburizing¡ Nitriding¡ Carbonitriding¡ Nitrocarburizing¡ Boriding¡ Titanium-carbon diffusion¡ Toyota diffusion process

� Selective-hardening methods:¡ Flame hardening¡ Induction hardening¡ Laser hardening¡ Electron beam hardening¡ Ion implantation¡ Selective carburizing and nitriding¡ Use of arc lamps

process method used to handle and transport thehardening elements to the surface of the part.Process methods for exposure involve thehandling of hardening species in forms suchas gas, liquid, or ions. These process variationsnaturally produce differences in typical casedepth and hardness (Table 2). Factors influen-cing the suitability of a particular diffusionmethod include the type of steel (Fig. 1), thedesired case hardness (Fig. 2), case depth(Fig. 3), the desired case profile, and cost.It is also important to distinguish between

total case depth and effective case depth. Theeffective case depth is typically about two-thirds to three-fourths the total case depth.(In some cases, the depth to the hardness valueof 50 HRC or five points lower than the surfacehardness is also specified.) The required

effective depth and the measurement techniquemust be specified so that the heat treater canprocess the parts for the correct time at theproper temperature.

Carburizing and Carbonitriding

Carburizing is the addition of carbon to thesurface of low-carbon steels at temperatures(generally between 850 and 980 �C, or 1560and 1800 �F) at which austenite, with its highsolubility for carbon, is the stable crystal struc-ture. With grades of steel engineered to resistgrain coarsening at high temperatures andproperly designed furnaces such as vacuum fur-naces, carburizing above 980 �C (1800 �F) ispractical to dramatically reduce carburizing time.

Hardening is accomplished when the high-carbonsurface layer is quenched to form martensitic casewith good wear and fatigue resistance superim-posed on a tough, low-carbon steel core. Of thevarious diffusion methods (Table 2), gas carburi-zation is the most widely used, followed by gasnitriding and carbonitriding.Case hardness of carburized steels is primar-

ily a function of carbon content. When the car-bon content of the steel exceeds approximately0.65%, additional carbon has no effect on hard-ness but does enhance hardenability. Carbon inexcess of 0.65% may not be dissolved, whichwould require high temperatures to ensurecarbon-austenite solid solution. Higher levels ofcarbon in the case will impact microstructuralproperties that can enhance performance charac-teristics such as wear, sliding contact fatigue,

Table 2 Typical characteristics of diffusion treatments

Process Nature of case

Process temperature

Typical case depth Case hardness, HRC Typical base metals Process characteristics�C �F

Carburizing

Pack Diffused carbon 815–1090 1500–2000 125 mm–1.5 mm(5–60 mils)

50–63(a) Low-carbon steels,low-carbon alloy steel

Low equipment costs; difficult to controlcase depth accurately

Gas Diffused carbon 815–980 1500–1800 75 mm–1.5 mm(3–60 mils)

50–63(a) Low-carbon steels,low-carbon alloy steels

Good control of case depth; suitable forcontinuous operation; good gas controlsrequired; can be dangerous

Liquid Diffused carbon andpossibly nitrogen

815–980 1500–1800 50 mm–1.5 mm(2–60 mils)


Faster than pack and gas processes; canpose salt disposal problem; salt bathsrequire frequent maintenance

Vacuum/LPC Diffused carbon 815–1090 1500–2000 75 mm–1.5 mm(3–60 mils)


Excellent process control; bright parts;faster than gas carburizing; highequipment costs

Nitriding

Gas/LPN Diffused nitrogen,nitrogencompounds

480–590 900–1100 125 mm–0.75 mm(5–30 mils)

50–70 Alloy steels, nitriding steels,stainless steels

Hardest cases from nitriding steels;quenching not required; low distortion;process is slow; is usually a batchprocess

Salt Diffused nitrogen,nitrogencompounds

510–565 950–1050 2.5 mm–0.75 mm(0.1–30 mils)

50–70 Most ferrous metalsincluding cast irons

Usually used for thin hard cases < 25 mm(1 mil); no continuous white layer;most are proprietary processes

Ion Diffused nitrogen,nitrogencompounds

340–565 650–1050 75 mm–0.75 mm(3–30 mils)

50–70 Alloy steels, nitriding steels,stainless steels

Faster than gas nitriding; no white layer;high equipment costs; close casecontrol

Carbonitriding

Gas Diffused carbon andnitrogen

760–870 1400–1600 75 mm–0.75 mm(3–30 mils)

50–65(a) Low-carbon steels,low-carbon alloy steels,stainless steels

Lower temperature than carburizing (lessdistortion); slightly harder case thancarburizing; gas control critical

Liquid (cyaniding) Diffused carbon andnitrogen

760–870 1400–1600 2.5–125 mm(0.1–5 mils)

50–65(a) Low-carbon steels Good for thin cases on noncritical parts;batch process; salt disposal problems

Ferriticnitrocarburizing

Diffused carbon andnitrogen

565–675 1050–1250 2.5–25 mm(0.1–1 mil)

40–60(a) Low-carbon steels Low-distortion process for thin case onlow-carbon steel; most processes areproprietary

Other

Aluminizing (pack) Diffused aluminum 870–980 1600–1800 25 mm–1 mm(1–40 mils)

< 20 Low-carbon steels Diffused coating used for oxidationresistance at elevated temperatures

Siliconizing bychemical vapordeposition

Diffused silicon 925–1040 1700–1900 25 mm–1 mm(1–40 mils)

30–50 Low-carbon steels For corrosion and wear resistance,atmosphere control is critical

Chromizing bychemical vapordeposition

Diffused chromium 980–1090 1800–2000 25–50 mm(1–2 mils)

Low-carbon steel,< 30; high-carbonsteel, 50–60

High- and low-carbon steels Chromized low-carbon steels yield a low-cost stainless steel; high-carbon steelsdevelop a hard corrosion-resistant case

Titanium carbide Diffused carbon andtitanium, TiCcompound

900–1010 1650–1850 2.5–12.5 mm(0.1–0.5 mil)

> 70(a) Alloy steels, tool steels Produces a thin carbide (TiC) case forresistance to wear; high temperaturemay cause distortion

Boriding Diffused boron,boron compound

400–1150 750–2100 12.5–50 mm(0.5–2 mils)

40–> 70 Alloy steels, tool steels,cobalt and nickel alloys

Produces a hard compound layer; mostlyapplied over hardened tool steels; highprocess temperature can causedistortion

(a) Requires quench from austenitizing temperature. Source: Ref 1

390 / Case Hardening of Steels

and rolling contact fatigue. Too high a carbonlevel can result in excessive carbide formationand carbide networking or massive carbides thatmay be detrimental to performance. Therefore, itis important to understand the carbon profileneeded and define it when necessary.Case depth of carburized steel is a function

of carburizing time, the steel chemistry, andavailable carbon (carbon potential) at the sur-face. When prolonged carburizing times areused for deep case depths, a high carbon poten-tial produces a high surface-carbon content,which may thus result in excessive retainedaustenite or free carbides. These two micro-structural elements can have adverse effectson the distribution of residual stress in thecase-hardened part. Consequently, a high car-bon potential may be suitable for short carbur-izing times but not for prolonged carburizing.Selection of carbon potential also depends onthe carburizing response of a particular steel.

Carburizing Steels

Carburizing steels for case hardening usuallyhave base-carbon contents of approximately0.2%, with the carbon content of the carburized

layer generally being controlled at between 0.7and 1% C (Ref 2). However, surface carbon isoften limited to 0.9% (Ref 3) because too higha carbon content can result in retained austeniteand brittle martensite (due to the formation ofproeutectoid carbides on the grain boundaries).Most steels that are carburized are killed

steels (deoxidized by the addition of alumi-num), which maintain fine grain sizes to tem-peratures of approximately 1040 �C (1900 �F).Steels made to coarse grain practices can becarburized if a double quench is introduced toprovide grain refinement. Double quenchingusually consists of a direct quench followedby a requench from a lower temperature.Many alloy steels for case hardening are now

specified on the basis of core hardenability.Although the same considerations generallyapply to the selection of uncarburized grades,there are some distinct characteristics in carbur-izing applications.First, in a case-hardened steel, the harden-

ability of both case and core must be consid-ered. Because of the difference in carboncontent, case and core have quite differenthardenabilities, and this difference is muchgreater for some grades of steels than for

others. Moreover, the two regions have differ-ent in-service functions to perform. Until theintroduction of lean alloy steels such as the51xx, or 86xx series, with and without boron,there was little need to be concerned about casehardenability because the alloy content com-bined with the high carbon content alwaysprovided adequate hardenability. This is stillsomewhat true when the steels are directquenched from carburizing, so that the carbonand alloying elements are in solution in the caseaustenite. In parts that are reheated for harden-ing and in heavy-sectioned parts, however,both case and core hardenability requirementsshould be carefully evaluated.The hardenability of the steels as purchased

is the core hardenability. Because these low-carbon steels, as a class, are shallow hardeningand because of the wide variation in the sectionsizes of case-hardened parts, the hardenabilityof the steel must be related to some critical sec-tion of the part, for example, the pitch line orthe root of a gear tooth or the largest inscribedcircle of a cross section such as a bearing. Thisis best accomplished by making a part of a steelof known hardenability, heat treating it, andthen, by means of equivalence of hardness,relating the hardenability in the critical sectionor sections to the proper positions on theend-quench hardenability specimens, for bothcarburized and noncarburized part. Finally,the relationship between the thermal gradientand the carbon (hardenability) gradient duringquenching of a carburized part can make a dif-ference in the case depth measured by hardness.That is, an increase in base hardenability canproduce a higher proportion of martensite fora given carbon level, yielding an increasedmeasured case depth. Therefore, a shallowercarbon profile and shorter carburizing timecould be used to attain the desired result in achosen steel.Core Hardness. A common mistake is to

specify too narrow a range of core hardness.

Fig. 1 Types of steels used for various diffusion processes

Fig. 2 Spectrum of hardness obtainable with selected diffusion processes of steel

Fig. 3 Categorization of diffusion processes by typicalcase depth

Introduction to Surface Hardening of Steels / 391

When the final quench is from a temperaturehigh enough to allow the development of fullcore hardness, the hardness variation at anylocation will be that of the hardenability bandof the steel at the corresponding position onthe end-quenched hardenability specimen. Oneway to alter this state of affairs is to usehigher-alloy steels. In the commonly used alloysteels having a maximum of 2% total alloy con-tent, the range for the core hardness of sectionssuch as gear teeth is 12 to 15 HRC points.Higher-alloy steels exhibit a narrower range;for example, in 4815 the range is 10 HRCpoints, while in 3310 it is 8 HRC points.Narrow-range steels are justified only for severeservice or special applications.In standard steels purchased to chemical

composition requirements rather than to hard-enability, the range can be 20 or more HRCpoints; for example, 8620 may vary from 20to 45 HRC at the 4=16 in. position. The 25-pointrange emphasizes the advantage of purchasing(cost) to hardenability specifications to avoidthe intolerable variation possible within theranges for standard-chemistry steels. Anotherway to control core hardness within narrowlimits without resorting to the use of high-alloysteels is to use a final quench from a lower tem-perature, so that full hardness in the case will bedeveloped without the disadvantage of exces-sive core hardness.Gears, Bearings, and Low-Distortion

Applications. Gears and bearings are almostalways oil quenched or high-pressure gasquenched, because distortion must be held tothe lowest possible level. Therefore, alloy steelsare usually preferred. The lower-alloy steelssuch as 4023, 5120, 4118, 8620, and 4620, witha carbon range between 0.15 and 0.25%, arewidely used to achieve satisfactory results. Inmost applications, 8620 or 5120 are preferred.The final choice, based on service experienceor dynamometer testing, should be the leastexpensive steel that will do the job. Anothersteel, 1524, could be considered; although notclassified commercially as an alloy steel, ithas sufficient manganese to make it oil hardenup to an end-quench correlation point of 3=16.For heavy-duty applications or heavy cross

sections requiring core strength, higher-alloygrades such as 4320, 4817, and 9310 are justifi-able if based on actual performance tests.Rather than the bench tests, actual life testingof gears in the same mountings used in serviceto prove both the design and the steel selectionis particularly important.The carbonitriding process extends the use of

carbon steels such as 1016, 1018, 1019, and1022 into the field of light-duty gearing bypermitting the use of oil quenching in teeth ofeight diametral pitch and finer. Steels selectedfor such applications should be specified silicon-killed or aluminum-killed fine grained to ensureuniform case hardness and dimensional control.The core properties of gears made from thesetypes of steel resemble that of low-carbon steel,oil quenched. In the thin sections of fine-pitch

teeth, this may be up to 25 HRC. The carboni-triding process is usually limited, for economicreasons, to maximum case depths of approxi-mately 0.6 mm (0.025 in.). In some bearingapplications, 52100 materials are also carboni-trided to enhance galling properties.Non-Gear/Bearing Applications. In other

applications, when distortion is not a majorfactor, the carbon steels described previously,water quenched, can be used up to a 50 mm(2 in.) diameter. In larger sizes, low-alloysteels, water quenched, such as 5120, 4023,and 6120, can be used, but possible distortionand quench cracking must be avoided.

Carburizing Methods

While the basic principle of carburizing hasremained unchanged since it was first intro-duced, the methodology has gone through con-tinuous evolution. In its earliest application,parts were simply placed in a suitable containerand covered with a thick layer of carbon pow-der (pack carburizing). Although effective inintroducing carbon, this method was exceed-ingly slow, and as the production demand grew,a new method using a gaseous atmosphere wasdeveloped. In gas carburizing, the parts are sur-rounded by a carbon-bearing atmosphere thatcan be continuously replenished so that a highcarbon potential can be maintained. While therate of carburizing is substantially increased inthe gaseous atmosphere, the method requiresthe use of a multicomponent atmosphere whosecomposition must be very closely controlled toavoid deleterious side effects, for example, sur-face and grain-boundary oxides. In addition, aseparate piece of equipment is required to gener-ate the atmosphere and control its compositionor liquids, such as methanol, which must bevaporized. Despite this increased complexity,gas carburizing has become the most effectiveand widely used method for carburizing steelparts in high volume.In efforts required to simplify the atmosphere,

carburizing in an oxygen-free environment atvery low pressure (vacuum carburizing) hasbeen explored and developed into a viableand important alternative. Although the furnaceenclosure in some respects becomes more com-plex, the atmosphere is greatly simplified. A sin-gle-component atmosphere consisting solely of asimple gaseous hydrocarbon, for example, meth-ane or acetylene, may be used. Furthermore,because the parts are heated in an oxygen-freeenvironment, the carburizing temperature maybe increased substantially without the risk ofsurface or grain-boundary oxidation. The highertemperature permitted increases not only thesolid solubility of carbon in the austenite butalso its rate of diffusion, so that the timerequired to achieve the case depth desired isreduced. When carburizing at temperatures over980 �C (1800 �F), properly engineered steelchemistries are recommended to mitigate thepotential for grain coarsening.

Although vacuum carburizing overcomessome of the complexities of gas carburizing, itintroduces a serious new problem that must beaddressed. Because vacuum carburizing is con-ducted at very low pressures, and the rate offlow of the carburizing gas into the furnace isvery low, the carbon potential of the gas in deeprecesses and blind holes is quickly depleted.Unless this gas is replenished, a distinct nonuni-formity in case depth over the surface of thepart is likely to occur. If, in an effort to over-come this problem, the gas pressure is increasedsignificantly, another problem arises, that of free-carbon formation, or sooting. Thus, to obtaincases of reasonably uniform depth over a part ofcomplex shape, the gas pressure must beincreased periodically to replenish the depletedatmosphere in recesses and then reduced againto the operating pressure. Clearly, a delicate bal-ance exists in vacuum carburizing: The processconditions must be adjusted to obtain the bestcompromise between case uniformity, risk ofsooting, and carburizing rate. Surface area andalloy content of the component are two importantconsiderations of vacuum carburizing.A method that overcomes the major limita-

tions of gas carburizing yet retains the desirablefeatures of a simple atmosphere and a higherpermissible operating temperature is plasma orion carburizing.To summarize, carburizing methods include:

� Gas carburizing� Vacuum carburizing or low-pressure

carburizing� Plasma carburizing� Salt bath carburizing� Pack carburizing

These methods introduce carbon by the use ofgas (atmospheric gas, plasma, and vacuum car-burizing), liquids (salt bath carburizing), or solidcompounds (pack carburizing). All of thesemethods have limitations and advantages, butgas carburizing is used most often for high-vol-ume production because it can be accurately con-trolled and requires minimal special handling.Vacuum carburizing and plasma carburizing

have found applications because the absenceof oxygen in the furnace atmosphere thus elim-inates grain-boundary oxidation. Salt bath andpack carburizing are still done occasionallybut have relatively little commercial impor-tance today (2013).Process characteristics of the aforemen-

tioned carburizing methods fall into two gen-eral groups:

� Conventional methods, which introduce car-bon by gas atmospheres, salt baths, or char-coal packs

� Plasma methods, which impinge positivecarbon ions on the surface of a steel part(the cathode)

The main difference between the conventionaland glow-discharge (or plasma) methods is the


reduced carburizing times in plasma-assistedmethods. The quickly attained surface satura-tion also results in faster diffusion kinetics.Furthermore, plasma carburizing can producevery uniform case depths, even in parts withirregular surfaces (Ref 4, 5). This uniformityis caused by the glow-discharge plasma, whichclosely envelops the specimen surface,provided that recesses or holes are not too small(Ref 5).With the conventional methods, carburiza-

tion always takes place by means of a gaseousphase of carbon monoxide; however, eachmethod also involves different reaction and sur-face kinetics, producing different case-hardeningresults. In general, with conventional methods,carbon monoxide dissociates at the steel surface:

2CO ! CO2 þ C (Eq 2)

The liberated carbon is readily dissolved by theaustenite phase and diffuses into the body of thesteel. For some process methods (gas and packcarburizing), the carbon dioxide produced mayreact with the carbon atmosphere or pack char-coal to produce new carbon monoxide by thereverse reaction of Eq 2. Because the reactioncan proceed in both directions, an equilibriumrelationship exists between the constituents(Fig. 4). If the temperature is increased at con-stant pressure, more carbon monoxide is pro-duced (Fig. 4). In turn, the equilibriumpercentages of carbon monoxide and carbondioxide influence the carbon concentrations insteel (Fig. 5).Quantitative algorithms for estimating case

depth from carburization often focus on makingthe proportional relation of Eq 1 explicit(Case depth ¼ K ffiffiffiffiffiffiffiffiffiffiffiTimep ) for gas carburizationonly (Ref 2, 3). However, even in gas carburi-zation, the kinetics of carbon diffusion givesan incomplete picture of carburizing. A

comprehensive model of gas carburization mustinclude algorithms that describe:

� Carbon diffusion� Kinetics of the surface reaction� Effects of steel chemistry� Kinetics of the reaction between endogas

and enriching gas� Purging (for batch processes)� Atmosphere control system

Reference 8 discusses possible modeling ofeach of these factors for gas carburization.The effects of process variables are also cov-ered in the article “Gas Carburizing” in thisVolume.Selective Carburizing. Sometimes it is nec-

essary to prevent carburization on certain areasof a part. For example, carburization preventionmay be necessary on areas to be machined fur-ther after heat treating, or to prevent a thin areafrom being carburized all the way through itssection, thereby becoming brittle (Ref 3). Pre-venting carburization in selective areas can bedone with mechanical masking, stop-off com-pounds, or copper plating (see the article “Stop-off Technologies for Heat Treatment” for moredetails). Close attention to cleanliness andhandling is needed to achieve the desired stop-off, and application instructions should beclosely followed to achieve effective and goodresults. In copper plate thicknesses of approxi-mately 0.03 mm (0.001 in.) are required.After case hardening, selective areas of the

workpieces also can be “softened” by inductiontempering. External threads of gears or shaftsare typical applications. This method is notsuitable for steels of high hardenability.Another method of selective hardening is toremove case prior to quenching. The areas ofthe part that were machined after the carburizecycle will only show the core hardness because

the case layer had been removed prior toquench.

Carbonitriding

Carbonitriding is a surface-hardening heattreatment that introduces carbon and nitrogeninto the austenite of steel. This treatment is sim-ilar to carburizing in that the austenite composi-tion is changed and high surface hardness isproduced by quenching to form martensite.However, because nitrogen enhances harden-ability, carbonitriding makes possible the useof low-carbon steel to achieve surface hardnessequivalent to that of high-alloy carburized steelwithout the need for drastic quenching, result-ing in less distortion and minimizing potentialfor cracks. In some cases, hardening may bedependent on nitride formation.Although the process of carbonitriding can

be performed with gas atmospheres or saltbaths, the term carbonitriding often referssolely to treatment in a gas atmosphere (seethe article “Carbonitriding of Steels” in thisVolume). Basically, carbonitriding in a saltbath is the same as cyanide bath hardening. Inboth processes, nitrogen enhances hardenabilityand case hardness but inhibits the diffusion ofcarbon. In many instances, carbonitriding ofcoarse-grained steels is more appropriate thancarburizing, because of the lower temperaturesand shorter cycle times.Like carbon, nitrogen is an austenite stabi-

lizer. Therefore, considerable austenite may beretained after quenching a carburized part. Ifthe retained austenite content is so high that itreduces hardness and wear resistance, it maybe controlled by reducing the ammonia contentof the carbonitriding gas either throughout thecycle or during the latter portion of the cycle.Another result of excessive nitrogen content inthe carbonitrided case is porosity (see the arti-cle “Carbonitriding of Steels” in this Volume).

Nitriding and Nitrocarburizing

Nitriding is a surface-hardening heat treat-ment that introduces nitrogen into the surfaceof steel at a temperature range (500 to 550 �C,or 930 to 1020 �F), while it is in the ferriticcondition. Because nitriding does not involveheating into the austenite phase with quenchingto form martensite, nitride components exhibitminimum distortion and excellent dimensionalcontrol. Nitriding has the additional advantageof improving corrosion resistance in salt spraytests.The mechanism of nitriding is generally

known, but the specific reactions that occur indifferent steels and with different nitridingmedia are not always known. Nitrogen has par-tial solubility in iron. It can form a solid solu-tion with ferrite at nitrogen contents up toapproximately 6%. At approximately 6% N, acompound called gamma prime (g0), with a

Fig. 4 Equilibrium diagram for reaction 2CO !C + CO2at pressure of one atmosphere. Source: Ref 6

Fig. 5 Equilibrium percentages of carbon monoxideand carbon dioxide required to maintain

various carbon concentrations at 975 �C (1790 �F) inplain carbon and certain low-alloy steels. K = 89.67.Source: Ref 7


composition of Fe4N, is formed. At nitrogencontents greater than 8%, the equilibrium reac-tion product is E compound, Fe3N. Nitridedcases are stratified. The outermost surface canbe all g0, and, if this is the case, it is referredto as the white layer (it etches white in metallo-graphic preparation). Such a surface layer isundesirable; it is very hard but is so brittle thatit may spall in use. Usually it is removed; spe-cial nitriding processes are used to reduce thislayer or make it less brittle. The E zone of thecase is hardened by the formation of the Fe3Ncompound, and below this layer there is somesolid-solution strengthening from the nitrogenin solid solution (Fig. 6). The Fe3N (E) formedon the outer layer is harder than Fe4N, whichis more ductile. Controlling the formation ofeach of these compound layers is vital to appli-cation and degree of distortion.The depth of case and its properties are

greatly dependent on the concentration and typeof nitride-forming elements in the steel. In gen-eral, the higher the alloy content, the higher thecase hardness. However, higher-alloying ele-ments retard the N2 diffusion rate, which slowsthe case depth development. Thus, nitridingrequires longer cycle times to achieve a givencase depth than that required for carburizing.Figure 7 shows some typical cycle times fornitriding versus case depth relationship forcommonly used materials, such as Nitralloy135M, Nitralloy N, AISI 4140, AISI 4330M,and AISI 4340.Nitrided steels (Ref 9) are generally

medium-carbon (quenched and tempered) steelsthat contain strong nitride-forming elements suchas aluminum, chromium, vanadium, and molyb-denum. The most significant hardening is

achieved with a class of alloy steels (nitralloytype) that contain approximately 1% Al (Fig. 6).When these steels are nitrided, the aluminumforms AlN particles, which strain the ferrite lat-tice and create strengthening dislocations. Tita-nium and chromium are also used to enhancecase hardness (Fig. 8a), although case depthdecreases as alloy content increases (Fig. 8b).The microstructure plays an important role innitriding, because nitrogen can readily diffusethrough ferrite, and a low carbide content favorsboth diffusion and case hardness. Usually, alloysteels in the heat treated (quenched and tem-pered) conditions are used for nitriding (Ref 6).Nitriding steels used in the United States fall

in one of two groups: aluminum-containingNitralloys and AISI low- or high-alloy steels.There is, however, a wide gap between thecharacteristics of these two groups of steels,which, in Europe, is filled by CrMo and CrMoV

steels with 2.5 to 3.5% Cr. Chromium providesgood hardenability and higher hardness innitrided case than AISI low-alloy steels.Molybdenum resists softening on temperingso that high strengths can be retained evenafter tempering at well over the nitriding tem-perature. It also minimizes susceptibility toembrittlement during nitriding and increaseshardenability and hot hardness. Vanadium per-mits easier control of heat treatment and giveshigher hot hardness.For surface hardness and toughness, the

nitrided CrMo and CrMoV steels occupy aposition in between Nitralloy 135M and AISIlow-alloy steels. Because of lower case hard-ness, these materials are less brittle. Further-more, they are less sensitive to grinding cracksand have higher hardenability. Also, they canbe heat treated to higher core hardness prior tonitriding. For example, 3.5Cr-AlMo, a British

Fig. 6 Nitride case profiles for various steels. Source:Ref 1

Fig. 8 Influence of alloying elements on (a) hardness after nitriding (base alloy, 0.35% C, 0.30% Si, 0.70% Mn) and(b) depth of nitriding measured at 400 HV (nitriding for 8 h at 520 �C, or 970 �F). Source: Ref 6

Fig. 7 Nominal time for different nitrided case depths. Source: Ref 9


Steel (EN 40C), can be heat treated to 375to 444 Brinell hardness in sections up to63.5 mm (2.5 in.), whereas Nitralloy 135Mcan be heat treated to only 248 to 302 Brinellhardness in that size. In addition, the steels with2.5 to 3.5% Cr come with low nonmetallicinclusions (higher cleanliness), even in the air-melted condition, whereas the aluminum-con-taining steels, such as Nitralloy 135M, requirevacuum melting or degassing to achieve similarcleanliness. In general, the cleaner the material,the lower the distortion during any hardeningprocess. Nitrided gears made from air-meltedCrMo steels produce negligible distortion.Process methods for nitriding include gas

(box furnace or fluidized bed), liquid (saltbath), and plasma (ion) nitriding. In a surveyof 800 commercial shops in the United Statesand Canada, 30% offered nitriding services, ofwhich (Ref 10):

� 21% offered gas nitriding� 7% offered salt bath nitriding� 6% offered fluidized-bed nitriding� 5% offered plasma nitriding

The advantages and disadvantages of thesetechniques are similar to those of carburizing.However, process times for gas nitriding canbe quite long, that is, from 10 to 130 h depend-ing on the application, and the case depths arerelatively shallow, usually less than 0.5 mm(0.020 in.). Plasma nitriding allows fasternitriding times, and the quickly attained surfacesaturation of the plasma process results in fasterdiffusion. Plasma nitriding may clean the sur-face by sputtering.Nitrocarburizing is a surface-hardening

process that uses both carbon and nitrogen,but with more nitrogen than carbon, whencompared to carbonitriding (see the article“Carbonitriding of Steels” in this Volume).Carbonitriding produces a martensitic case withnitrogen levels less than carbon levels. In con-trast, nitrocarburizing involves higher levels ofnitrogen with a compound layer. There aretwo types of nitrocarburizing: ferritic and aus-tenitic (Ref 11). Ferritic nitrocarburizing occursat lower temperatures in the ferritic temperaturerange and involves diffusion of nitrogen intothe case. Austenitic nitrocarburizing is a morerecently developed process with process tem-peratures in the range of 675 to 775 �C (1245to 1425 �F). It also uses much higher ammoniaadditions and thus higher nitrogen levels in thecase. This allows the formation of a surfacecompound zone, which is not typical of thecarbonitriding process. Austenitic nitrocarburiz-ing differs from ferritic nitrocarburizing in theability for deeper case depths with a betterload-carrying capability but may result ingreater part distortion because of the higherprocessing temperatures and the requiredquenching process. Although ferritic and aus-tenitic nitrocarburizing have higher processingtemperatures than does nitriding (Table 2), they

have the advantage of being suitable for plaincarbon steels.

Applied Energy Methods

Surface hardening of steel can be achievedby localized heating and quenching, withoutany chemical modification of the surface. Themore common methods currently used toharden the surface of steels include flame andinduction hardening. However, each of thesemethods has shortcomings that can prevent itsuse in some applications. For example, the dis-advantages of flame hardening include the pos-sibility of part distortion, while inductionhardening requires close coupling betweenthe part and the coil (especially when usinghigh frequencies), which must be preciselymaintained.Flame hardening consists of austenitizing

the surface of a steel by heating with an oxy-acetylene or oxyhydrogen torch and immedi-ately quenching with water or water-basedpolymer. The result is a hard surface layer ofmartensite over a softer interior core with aferrite-pearlite structure. There is no change incomposition, and therefore, the flame-hardenedsteel must have adequate carbon content forthe desired surface hardness. The rate of heat-ing and the conduction of heat into the interiorappear to be more important in establishingcase depth than the use of a steel of highhardenability.Flame-heating equipment may be a single

torch with a specially designed head or an elab-orate apparatus that automatically indexes,heats, and quenches parts. Large parts such asgears and machine toolways, with sizes orshapes that would make furnace heat treatmentimpractical, are easily flame hardened. Withimprovements in gas-mixing equipment, infra-red temperature measurement and control, andburner design, flame hardening has beenaccepted as a reliable heat treating process thatis adaptable to general or localized surfacehardening for small and medium-to-high pro-duction requirements.Induction heating is an extremely versatile

heating method that can perform uniform sur-face hardening, localized surface hardening,through hardening, and tempering of hardenedpieces. Heating is accomplished by placing asteel ferrous part in the magnetic field gener-ated by high-frequency alternating current pass-ing through an inductor, usually a water-cooledcopper coil. The depth of heating produced byinduction is related to the frequency of thealternating current, power input, time, part cou-pling and quench delay.The higher the frequency, the thinner or more

shallow the heating. Therefore, deeper casedepths and even through hardening are pro-duced by using lower frequencies. The electri-cal considerations involve the phenomenaof hysteresis and eddy currents. Because

secondary and radiant heat are eliminated, theprocess is suited for in-line production. Someof the benefits of induction hardening are fasterprocess, energy efficiency, less distortion, andsmall footprints. Care must be exercised whenholes, slots, or other special geometric featuresmust be induction hardened, which can concen-trate eddy currents and result in overheatingand cracking without special coil and partdesigns. For details, see the articles “InductionSurface Hardening of Steels” and “InductionHeat Treating Systems” in this Volume.Laser surface heat treatment is widely used

to harden localized areas of steel and cast ironmachine components. This process is some-times referred to as laser transformation harden-ing to differentiate it from laser surface meltingphenomena (Fig. 9). There is no chemistrychange produced by laser transformation hard-ening, and the process, like induction and flamehardening, provides an effective technique toharden ferrous materials selectively. Othermethods of laser surface treatments include sur-face melting and surface alloying. Laser surfacemelting results in a refinement of the structuredue to the rapid quenching from the melt. Insurface alloying, elements are added to the meltpool to change the composition of the surface.The novel structures produced by laser surfacemelting and alloying can exhibit improved elec-trochemical behavior.Laser transformation hardening produces thin

surface zones that are heated and cooled veryrapidly, resulting in very fine martensitic micro-structures, even in steels with relatively lowhardenability. High hardness and good wearresistance with less distortion result from thisprocess. The laser method differs from induc-tion and flame heating in that the laser can belocated at some distance from the workpieces.Also, the laser light is reflected by mirrors tothe focusing lens, which controls the width ofthe heated spot or track.Molian (Ref 12) has tabulated the character-

istics of 50 applications of laser transformationhardening. The materials hardened includeplain carbon steels (1040, 1050, 1070), alloysteels (4340, 52100), tool steels, and cast irons(gray, malleable, ductile). Because the absorp-tion of laser radiation in cold metals is low,laser surface hardening often requires energy-absorbing coatings on surfaces. Reference 12lists some energy-absorbing coatings.Typical case depths for steels are 250 to

750 mm (0.01 to 0.03 in.) and for cast irons areapproximately 1000 mm (0.04 in.). The flexibil-ity of laser delivery systems and the low distor-tion and high surface hardness obtained havemade lasers very effective in the selective hard-ening of wear- and fatigue-prone areas on irregu-larly shaped machine components, such ascamshafts and crankshafts.Electron beam (EB) hardening, like laser

treatment, is used to harden the surfaces ofsteels. The EB heat treating process uses a con-centrated beam of high-velocity electrons as an


energy source to heat selected surface areas offerrous parts. Electrons are accelerated and areformed into a directed beam by an EB gun.After exiting the gun, the beam passes througha focus coil, which precisely controls beamdensity levels (spot size) at the workpiece sur-face and then passes through a deflection coil.To produce an electron beam, a high vacuumof 10�5 torr (1.3 � 10�3 Pa) is needed in theregion where the electrons are emitted andaccelerated. This vacuum environment protectsthe emitter from oxidizing and avoids scatteringof the electrons while they are still traveling ata relatively low velocity.Like laser beam hardening, the EB process

eliminates the need for quenchants but requiresa sufficient workpiece mass to permit self-quenching. A mass of up to eight times that ofthe volume to be EB hardened is requiredaround and beneath the heated surfaces. Elec-tron beam hardening does not require energy-absorbing coatings, as does laser beam harden-ing. Processing considerations and propertychanges associated with EB hardening are cov-ered in Ref 13 and 14 and in the article“Electron Beam Surface Hardening of Steels”in this Volume.

Other Methods

Diffusion coatings are deposited either byheating the components to be treated in contactwith the powder coating material in an inertatmosphere (solid-state diffusion) or by heating

them in an atmosphere of a volatile compoundof the coating material (out-of-contact gas-phasedeposition, or chemical vapor deposition). As thecoating bond is developed by diffusion, the bondstrength is enhanced.Solid-state diffusion methods include pack

cementation, which is the most widely employeddiffusion coating method and includes coatingsbased on aluminum (aluminizing), chromium(chromizing), and silicon (siliconizing). Sub-strate materials include nickel- and cobalt-basesuperalloys, steels (including carbon, alloy,and stainless steels), and refractory metals andalloys. Diffusion coatings for wear resistanceare also based on boriding (boronizing) andthe thermoreactive deposition/diffusion process.Of these, boron and titanium treatments offerhigh levels of hardness (Fig. 2), while alumi-num, chromium, and silicon treatments are pri-marily used for corrosion resistance.Boriding involves the diffusion of boron into

metal surfaces for the enhancement of hardnessand wear resistance. Boriding is most oftenapplied to tool steels that may be hardened byheat treatment. Boriding techniques includemetallizing, chemical vapor deposition, andpack cementation. For additional information,see the article “Boriding (Boronizing) ofMetals” in this Volume.Titanium Carbide. With process tempera-

tures in the range of 900 to 1010 �C (1650 to1850 �F), titanium and carbon will diffuse toform a diffused case of titanium carbide duringchemical vapor deposition. This treatment ismost commonly applied to tool steels and

hardenable stainless steels. Because the treat-ment is performed above the austenitizing tem-peratures of these steels, the core must behardened by quenching.Ion implantation is a surface-modification

process in which ions with very high energyare driven into a substrate. Ions of almost anyatom species can be implanted, but nitrogen iswidely used to improve corrosion resistanceand the tribological properties of steels andother alloys. Although the nitrogen content ofalloy surfaces is increased by both nitrogenion implantation and plasma nitriding, majordifferences exist between the two processesand the surface modifications they create. Themajor difference is that ion implantation canbe performed at room temperature.Ion implantation machines accelerate ions,

generated by specially designed sources, at veryhigh energies (from 10 to 500 keV). In contrast,the energy of ions and atoms in plasma nitrid-ing is much lower (

variety of tool steel applications (Ref 16), andthe implantation of 52100 and 440C bearingswith titanium and/or nitrogen to improve roll-ing-contact fatigue resistance (Ref 17–19). Inthe latter applications, titanium was found toreduce the coefficient of friction, and nitrogenwas found to raise hardness by intermetalliccompound formation. Additional informationon ion implantation is given in Ref 20 and inCorrosion, Volume 13 of ASM Handbook, for-merly Metals Handbook, 9th edition.Surface hardening with arc lamps is used

in applications that involve surface remeltingor surface hardening by solid-phase recrystalli-zation. Examples include the surface remeltingof cast iron and the large-area remelting of tita-nium in the presence of nitrogen or methane toproduce titanium carbides in the surface layer.In the surface remelting of cast irons, lasersare also used. Another area in which arc lampsare finding application is in the selective hard-ening of the edges on agricultural sweeps andtilling equipment blades.Surface treating using white light from a

high-power arc lamp offers several advantagesover traditional methods and the beam techni-ques. For example, arc lamp treatment canachieve higher surface radiation intensities thancan flame heating, making the procedure fasterand less likely to cause distortion. Comparedwith induction hardening, arc lamp treatmentallows much larger distances between the partand heat source, providing more flexibility intreating irregularly shaped surfaces. Unlike theEB treatment, this method does not require theuse of a vacuum chamber, and an arc lampcan deliver greater power to the part surfacethan can a laser beam.However, with the arc lamp method, signifi-

cant power loss is encountered if arc radiationis concentrated onto surface areas smaller thanthe arc itself. Therefore, the illuminated spoton the sample surface should always be largerthan the arc. This necessitates extremely higharc power to achieve the surface intensitiesneeded for thermal treatment. Such high poweris achieved in a very small space with speciallydesigned arc lamps.

Process Selection

The benefits of the most common methods ofsurface hardening are compared in Table 3.Flame and induction hardening are generallylimited to certain families of steels, such asmedium-carbon steels, medium-carbon alloysteels, some cast irons, and the lower-alloy toolsteels. There is no size limit to parts that can beflame hardened, because only the portion of thepart to be hardened need be heated. Flame hard-ening is generally used for very heavy cases(in the range of approximately 1.2 to 6 mm,or 0.6 to 0.25 in.); thin case depths are difficultto control because of the nature of the heatingprocess. Diffusion methods are compared inTable 2.

Transformation hardening introduces surfacecompressive residual stresses, which are benefi-cial for fatigue strength. In selective hardening,however, some residual tensile stress will existin the region where the hardened zone meetsthe unhardened zone. Consequently, selectivehardening by methods such as flame or induc-tion heating should be applied away from geo-metric stress concentrations. Both nitridingand carburizing provide good resistance to sur-face fatigue and are widely used for gears andcams. In terms of bending fatigue resistance,the ideal case depth appears to be reachedwhere the failure initiation point is transferredfrom the core to the surface (Ref 21). However,specification of required case depth is a com-plex subject, which is briefly discussed inRef 21 for carburized steels.

REFERENCES

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

2. G. Krauss, Steels: Heat Treatment and Pro-cessing Principles, ASM International,1990, p 286

3. C. Wick and R.F. Vielleux, Ed., Materials,Finishing and Coating, Vol 3, Tool andManufacturing Engineers Handbook,Society of Manufacturing Engineers, 1985

4. B. Edenhofer, M.H. Jacobs, and J.N.George, Industrial Processes, Applicationsand Benefits of Plasma Heat Treatment,Plasma Heat Treatment, Science and Tech-nology, PYC Édition, 1987, p 399–415

5. W.L. Grube and J.G. Gay, High-RateCarburizing in a Glow-Discharge MethanePlasma, Metall. Trans. A, Vol 91, 1987,p 1421–1429

6. K.-E. Thelning, Steel and Its Heat Treat-ment, 2nd ed., Butterworths, 1984, p 450

7. ASM Committee on Gas Carburizing, Appli-cation of Equilibrium Data, Carburizing and

Carbonitriding, American Society forMetals, 1977, p 14–15

8. C.A. Stickels and C.M. Mack, Overviewof Carburizing Processes and Modeling,Carburizing Processing and Performance,G. Krauss, Ed., ASM International, 1989,p 1–9

9. A.K. Rakhit, Nitriding Gears, Chap. 6,Heat Treatment of Gears: A PracticalGuide for Engineers, ASM International,p 133–158

10. W.L. Kovacs, Commercial and EconomicTrends in Ion Nitriding/Carburizing, IonNitriding and Ion Carburizing, ASM Inter-national, 1990, p 5–12

11. D. Herring, Comparing Carbonitriding andNitrocarburizing, Heat Treat. Prog., April/May 2002

12. P.A. Molian, Engineering Applications andAnalysis of Hardening Data for Laser HeatTreated Ferrous Alloys, Surf. Eng., Vol 2,1986, p 19–28

13. R. Zenker and M. Mueller, Electron BeamHardening, Part 1: Principles, ProcessTechnology and Properties, Heat Treat.Met., Vol 15 (No. 4), 1988, p 79–88

14. R. Zenker, W. John, D. Rathjen, andG. Fritsche, Electron Beam Hardening, Part2: Influence on Microstructure and Proper-ties, Heat Treat. Met., Vol 16 (No. 2),1989, p 43–51

15. G. Dearnaley, Ion Implantation and IonAssisted Coatings for Wear Resistance inMetals, Surf. Eng., Vol 2, 1986, p 213–221

16. J.K. Hirvonen, The Industrial Applicationsof Ion Beam Processes, Surface Alloyingby Ion, Electron and Laser Beams, L.E.Rehn, S.T. Picraux, and H. Wiedersich,Ed., ASM International, 1987, p 373–388

17. D.L. Williamson, F.M. Kustas, and D.F.Fobare, Mossbauer Study of Ti-Implanted52100 Steel, J. Appl. Phys., Vol 60, 1986,p 1493–1500

18. F.M. Kustas, M.S. Misra, and D.L. Wil-liamson, Microstructural Characterization

Table 3 Relative benefits of five common surface-hardening processes

Process Benefits

Carburizing Hard, highly wear-resistant surface (medium case depths); excellent capacity for contact load; good bendingfatigue strength; good resistance to seizure; excellent freedom from quench cracking; low-to-medium-cost steels required; high capital investment required

Carbonitriding Hard, highly wear-resistant surface (shallow case depths); fair capacity for contact load; good bendingfatigue strength; good resistance to seizure; good dimensional control possible; excellent freedom fromquench cracking; low-cost steels usually satisfactory; medium capital investment required; improved saltcorrosion resistance

Nitriding Hard, highly wear-resistant surface (shallow case depths); fair capacity for contact load; good bendingfatigue strength; excellent resistance to seizure; excellent dimensional control possible; good freedomfrom quench cracking (during pretreatment); medium-to-high-cost steels required; medium capitalinvestment required; improved salt corrosion resistance

Inductionhardening

Hard, highly wear-resistant surface (deep case depths); good capacity for contact load; good bending fatiguestrength; fair resistance to seizure; fair dimensional control possible; fair freedom from quench cracking;low-cost steels usually satisfactory; medium capital investment required

Flame hardening Hard, highly wear-resistant surface (deep case depths); good capacity for contact load; good bending fatiguestrength; fair resistance to seizure; fair dimensional control possible; fair freedom from quench cracking;low-cost steels usually satisfactory; low capital investment required


of Nitrogen Implanted 400C Steel, NuclearInstruments and Methods in PhysicsResearch B31, North-Holland, 1988,p 393–401

19. F.M. Kustas, M.S. Misra, and P. Sioshansi,Effects of Ion Implantation on the Rolling

Contact Fatigue of 440C Stainless Steel,Ion Implantation and Ion Beam Processingof Materials, G.K. Hubler, O.W. Holland,and C.R. Clayton, Ed., MRS SymposiaProceedings, Materials Research Society,Vol 27, 1984, p 675–690

20. A.H. Deutchman et al., Ind. Heat., Vol 42(No. 1), Jan 1990, p 32–35

21. 21. G. Parrish, The Influence of Microstruc-ture on the Properties of Case-CarburizedComponents, American Society for Metals,1980, p 159–160, 164–165


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