Module 37 Surface hardening Lecture 37 Surface...

NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |

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Module 37

Surface hardening

Lecture 37

Surface hardening


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Keywords : Non‐uniform properties in engineering components, methods of surface hardening, shot

peening, hard facing, induction hardening, surface modification by diffusion, carburizing, post

carburizing heat treatment, core refining, case hardening, origin of residual stress, nitriding, Fe‐N phase

diagram, effect of surface hardening on fatigue resistance of engineering components

Introduction The last six lectures were devoted to heat treatment of steel. We have seen that hardening

followed by tempering gives the best combination of strength and toughness. Formation of

martensite is primarily responsible for the development of very high strength in steel. However

you need to cool a component made of steel very fast to get martensite both at its surface and

at its centre. Although it may be rather easy to achieve a high cooling rate at the surface but

maintaining a high cooling rate at the centre may be extremely difficult particularly if the

section size of the component is large. Therefore the microstructure at the centre of a thick

section is likely to be different from that at its surface. There are several applications where we

do not need uniform microstructure or property across the section. For example components

like turbine shaft, gear, spindle and axle need to have a hard surface but a soft core. In general

high strength means low toughness. If the section size of a component is too high to be fully

hardened we may still have a soft core. It might be one of the methods of fulfilling such a

criterion. There are several other ways the strength or the hardness of the surface can be

increased without adversely affecting the toughness of the core. Some of the most common

techniques are as follows:

Induction hardening

Case carburizing + case hardening

Nitriding

Shot peening

Hard facing, coating or surface alloying

In this module we shall learn about some of these. The properties of steel or any other

engineering material depend a great deal on the processing route that is followed and its

composition. For example we know that cast metals have coarse inhomogeneous grains

with preferred orientation. Subsequent processing consisting of homogenization, forging,

rolling and annealing may result in a uniform fine grain structure having isotropic

properties. As a result there may be a substantial improvement in its strength and ductility.

In the case of steel the possible options are much more varied. In short all materials may

have an inherent base property or microstructure which can be altered or improved by

adopting an appropriate processing route. The performance of a component depends


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primarily on the properties of the material of which it is made. The interrelation between

the four is best described by the tetrahedron shown in fig 1.

Non‐uniform properties in engineering components: It is extremely difficult to have a uniform microstructure within a material unless it is extremely

thin. This is because the evolution of microstructure within a material depends on the local

processing parameters. Often it is difficult to maintain identical conditions at every point within

a material unless it is extremely thin. The effect is more pronounced in the case of steel that

goes through a solid state transformation during cooling. The cooling rate within a component

is a strong function of its section size. It is impossible to maintain identical cooling rate within a

component of finite dimension. Therefore we have to live with non‐uniform properties in

engineering components since it cannot be avoided in components of reasonable thickness.

This is illustrated with the help of an example in slide 1.

Steel I beam is one of the most common structural components. A cross section of I beam is

shown in slide 1. Its flange which is highly stressed is thick but its web where the stress is not so

high is relatively thin. I beams are made by hot rolling at a temperature while the structure of

steel is austenite. On completion of rolling I beam is allowed to cool in air. The average cooling

curves of the flange and the web of the beam have been super imposed on the CCT diagram of

0.2%C steel. The cooling rate within the web is a little faster than that in the flange. Therefore

the microstructure of the web is likely to be finer than that in the flange. It is likely to have

relatively less %ferrite but more %pearlite in comparison to those in the flange. The web is

therefore expected to be stronger than the flange although it is not necessary to make it

Fig 1: Tetrahedron showing the interrelation between material, its processing route and its

property on the performance of a component.

Processing:

micrstructure

Material

Properties

Performance


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stronger. Here is an application where the differential properties of an engineering component

are not being properly exploited. There are several other uses where intentionally the structure

of the material is altered either by local change in composition or by local deformation or by

adopting differential cooling rates at different places so as to improve its performance as an

engineering component. We would look at a few such examples.

Non uniform properties in engineering components

flange

I beam

web

Web: least stressed: thin

Flange: high stress: thick

I Beam: hot rolled sections

% C ~ 0.2Case: where we have to learn to live with it. Surface hardening:

exploit such features.Which is stronger ?

T

time

webFlange

Ms

+ M

A3

A1

+ P

Surface hardening: why & how? Components like gear, shaft or spindle need a hard / wear resistant surface but a soft / tough

core. Section size of such components is often too large to be uniformly hardened even on

severe quenching. More over the time lag between the transformations at the surface and the

core results in an unfavorable tensile residual stress at the surface. Recall the general thumb

rule that the region that transforms later is likely to have compressive residual stress. The

surface is likely to transform first in steel having the same composition all through its section.

Therefore surface would have residual tensile stress. Depending on its magnitude it may lead to

cracking or distortion. The presence of residual tensile stress is also harmful as it would reduce

fatigue life of critical components like turbine shaft or landing gear of an aircraft. The purpose

of surface hardening is to develop a hard surface with compressive residual stress, to improve

its wear resistance, to increase its fatigue life and to avoid susceptibility to distortion and

cracking. The most commonly used methods of surface hardening are as follows:

Slide 1


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• Shot peening: general applicable to all metals • Coating / hard facing • Surface (local) heating & cooling: steel • Surface diffusion & subsequent treatment

Shot peening: Shot peening technique is applicable to all metals and alloys that are amenable to plastic

deformation. The part to be hardened is placed in a chamber where extremely fine hard

particles moving at a high speed keep striking at its surface. The energy of the moving particles

is high enough to cause local plastic deformation at its surface. The stress on the material a

little beneath the surface is not high enough to cause plastic deformation. However it would be

under elastic stress as long as the shot peening process continues. When it stops residual stress

would develop at the surface because of the elastic recovery that occurs in the region a little

beneath the surface. Proper control on the process parameters such as the particle size, its

kinetic energy, the angle of incidence and the time may be necessary to develop favorable

residual stress pattern at the surface. It is compressive in nature. Therefore it would inhibit

crack initiation. Landing gears of aircrafts are subjected to shot peening to develop residual

compressive stress on its surface. Even automotive gears, following carburizing, are subjected

shot peening to raise the value of compressive residual stress (to as high as 1000 – 1200 MPa),

particularly at depths of 30 – 40 microns. This help resist crack propagation during service as

result of fatigue loading.

Hard facing: Engineering components that are required to resist solid particle erosion, abrasion, fretting or

cavitation are usually given a hard surface coating. This consists of a fine dispersion of hard

metal carbides in a compatible metal matrix. Thermal spray is the most commonly used

technique to apply such coatings on the component. There are specially designed setups with

spray guns that suck the coating material along with oxygen and fuel gas that ignites into a

flame to melt the matrix of the coating material while it deposits on the surface of the

component. The most commonly used coating materials are mixtures of chromium or tungsten

carbides in either cobalt or nickel‐chromium alloy matrix. Hard facing is also a commonly used

technique to salvage worn out parts so that they could be reused.

Induction hardening: a method based on local heating followed by cooling: This is applicable only for steel. An induction coil is used to heat the component to be

hardened. Only the surface gets heated. Its microstructure transforms into austenite from a

mixture of ferrite and cementite, but the structure of the core remains intact as it remains cold


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all through the process. Once the temperature of the surface attains a specified level the

power is switched off and the job is cooled rapidly by quenching it in water. Heat extraction

rate is much faster than that in a conventional hardening process. This is because only a thin

region near the surface of the component gets heated. The total amount of heat stored is so

less that it can be easily extracted by the quenching medium. The centre of the component

does not get heated at all. The cold core acts as a sink for the heat stored within the thin region

near the surface. Thus it also helps attain a very high cooling rate at the surface. Once the

process is complete the microstructure of the surface gets transformed into martensite while

that at its core remains unaltered. Induction heating is extremely fast and the time the

component spends above A3 temperature is very short. Therefore to ensure complete

transformation of ferrite to austenite the peak temperature should be a little higher than the

normal austenitizing temperature used for conventional hardening heat treatment. The total

time spent above A3 may still not be long enough to have homogeneous austenite within the

entire hot section. The composition of martensite nucleating in inhomogeneous austenite may

vary. Therefore hardness of induction hardened steel component may often be higher than

that in through hardened steel having identical composition. One of the main advantages of

induction hardening is good surface finish and little distortion. It can be applied to all grades of

steel. Alloy addition is not necessary. Induction hardening is very effective for surface hardening

of plain carbon steel having 0.35‐0.70%C. The salient features of induction hardening are as

follows:

• Heat the surface to a temperature above A3 (austenitic region) • Core does not get heated : the structure remains unaltered • Surface converts to martensite on quenching. • Fast heating & short hold time: needs higher austenization temperature • Martensite forms in fine inhomogeneous grains of austenite • Applicable to carbon steels (0.35 – 0.7C) • Little distortion & good surface finish


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Induction hardening

Place the job inside an induction coil & pass high frequency ac. Surface gets heated due to skin effect.

Heating: local surface near the coil gets heated

d

= resistivity

= frequency

= magnetic permeability

Higher frequency: lower depth of hardening (d).

On quenching only surface becomes martensite.

High AC

Job

Coil

Slide 2 shows with the help of a diagram the setup needed for induction hardening. It consists

of a high frequency AC power source and water cooled induction coil surrounding the job to be

hardened. Only the surface gets heated due to skin effect. The expression for the depth of

penetration of the field is given in slide 2. It depends on the frequency of power source, the

resistivity (increases with temperature) and the magnetic permeability of steel (It decreases

significantly as the temperature goes beyond Curie point). There will be a sudden increase in d

as the temperature goes beyond 750°C. The only parameter that can be controlled is the

frequency. Higher the frequency lower is the depth of penetration. You could select a lower

frequency to get a higher depth of hardening.

Slide 2


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Structure & properties

Spheroidising annealing

Cm globules in matrix

Plate (HC) martensite

Needs tempering

T

%C time

T

Much higher austenitization time required as hold time is short

T for Sph Ann

Spheroidising annealing is done before induction hardening

Hard surface & tough core. Compressive residual stress at

surface & tensile at core

Grain growth

Homogeneous austenite

Slide 3 explains with the help of a set of diagrams what should be the ideal microstructure at

the surface (or the case) and the core of a component and how these can be attained. The case

preferably should have high carbon martensite to ensure the maximum possible hardness. The

core on the other hand should have globular cementite in a matrix of ferrite. This is the

structure that has the highest toughness. Such a microstructure can be developed in steel by

prolonged soaking (thermal exposure) at a temperature a little below A1 (eutectoid

temperature). The process is known as spheroidising annealing. The appropriate temperature

range for spheroidising annealing has been marked on the phase with the help of a filled

rectangle. It denotes the range of temperature and composition of steel suitable for induction

hardening. If the temperature keeps oscillating around A1 the process of spheroidization is

faster. Spheroidizing annealing should always precede induction hardening.

The peak temperature to be used for induction hardening of eutectoid steel has been marked

with the help of a solid (filled) circle on the phase diagram given in slide 3. This is much higher

than 760°C. There is a time temperature transformation diagram for austenitization just beside

the phase diagram in slide 3. Note that the hold time at this temperature is not long enough to

form homogeneous austenite. It means %C in austenite may not be the same at all places. This

suggests that on quenching, martensite would nucleate within inhomogeneous austenite. This

is accompanied by volume expansion. However the core beneath it would not let it happen. As

a result there residual compressive stress would develop at the surface. This inhibits nucleation

of surface crack. However martensite is brittle. It may be surrounded by retained austenite as

well. Therefore induction hardening must be followed by tempering.

Slide 3


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Surface diffusion & subsequent treatment: The main objective of surface hardening is to have a hard surface but a soft core. Recall that the

hardness of steel depends only on the concentration of carbon in steel. Therefore it may be enough to

have high carbon only at the surface. This can be achieved by increasing the concentration of carbon in a

component made of low carbon steel by allowing carbon to diffuse into it. The rate at which carbon can

diffuse depends on its concentration gradient and its diffusivity in steel. A high concentration gradient

can be maintained only if there is a significant difference in the concentration of carbon at the surface

and the core. A high temperature would certainly ensure high diffusivity. One of the ways to achieve this

is to heat low carbon steel (< 0.2%C) kept in a packed bed to around 1000°C. The process is known as

pack carburizing. The basic principles of the process have been explained with the help of a few

diagrams in slide 4. The sketch (a) shows what would be best temperature for carburization. There are

two dotted horizontal lines on the same sketch. These are labeled as T1 and T2. Note that %C in steel is

C0. At room temperature its microstructure consists of ferrite and pearlite. When it is heated to T1

pearlite transforms into austenite. The sample now consists of ferrite and austenite. Out of the two the

ferrite is saturated with carbon. It is impossible to maintain any concentration gradient between its

surface and its centre. However the austenite in low carbon steel at this temperature is not saturated

with carbon. C1 denotes the likely difference in %C that can be maintained at T1. The microstructure of

low carbon steel at T2 is 100%austenite. The solubility limit of carbon in austenite is much higher than

C0. C2 denotes the likely difference in %C that can be maintained at T2. C2 is much greater than C1. Therefore selection of T2 as the carburization temperature is more appropriate.

Pack carburizing: LC steel

85% char coal + 15% energizer

BaCO3 = BaO + CO2

CO2 + C = 2 CO

2 CO = C (Fe) + CO2

CS : Function ( Temp)

Depth: F (T,t)

T

%C CS

T1

C0

%C

x

CS

C0

t

(a) (b)

(c)

(d)

A1

A3

+ cm

SampleT2

C2

C1

Slide 4


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The sketch (b) in slide 4 shows a schematic representation of how the sample is placed within a mixture

of BaCO3 (energizer) and char coal. At 1000°C, the carburization temperature, BaCO3 decomposes to

give CO2. In the presence of excess carbon CO2 is converted into CO which later decomposes in the

presence of Fe leaving a film of nascent carbon on the surface of the steel component. This

reacts with Fe in steel to form Fe3C. The concentration of carbon in austenite near the carbide

layer is given by the phase diagram. In this case it is CS. It is a function of temperature. The

sketch (c) gives a schematic map showing carbon concentration gradient. The sketch (d) gives

the expected concentration versus distance (x: from the surface to the centre) plots at T2. It is a

function of both temperature (T) and time (t).

Carburization depth

0

0

0

2

exp

(0.5) 0.5

S

C

C C xerf

C C Dt

QD D

RT

erf

x Dt

xCS C0

CS

C0

xd

Problem: T=927 C, t=10hrs, C0=0.2, D0= 0.7x10-4

m2/s Q=157 kJ/mole, CS=1.2 Estimate x. erf(z) = z

(a)

(b)

Slide 5 describes a method of estimating the concentration of carbon in a steel specimen as

function of distance from its surface. At the carburization temperature the concentration of

carbon at the surface is maintained at CS whereas far away from the surface it is C0. It can

therefore be visualized as a semi‐infinite diffusion couple as shown in sketch (a). The

concentration profile at any instant may be described by Fick’s second law of diffusion as

shown in equation 1.

0, 0 ∞, 0 (1)

The solution of Fick’s equation is given in slide 5. The sketch (b) in slide 5 describes the

concentration of carbon as function of distance at a given instant of time. It also defines an

effective diffusion distance d. It corresponds to the distance at which the concentration of

carbon (C) is given by

Slide 5


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(2)

On substitution of equation 2 in the expression for the concentration profile given in slide 5 it

can be shown that the effective diffusion distance d is given by:

√ (3)

Where Q = activation energy, R = universal gas constant. It suggests that different combination

of temperature and time can give identical effective diffusion distance. The temperature of

carburization is usually within 950°‐1000°C and the time is around 8‐9hours. Figure 2 shows the

concentration of carbon and microstructure of steel specimen on air cooling after pack

carburization.

A thin layer of carbide may form at the surface which is in contact with the carburizing mixture.

This helps maintain %C in austenite at a fixed level (CS). Its magnitude depends on the

temperature of carburization. It may be around 1.2% just beside the thin carbide layer. There

after it decreases as you move towards the centre. A typical plot describing how %C would vary

from the surface to the centre is also given in fig 2. Apart from CS & C0 it depends on the

temperature and the time.

%C

x C0

Cs

d

2

Thin cm layer P

6.67

P + cm network

P + network

Initial structure

P

Fig 2: Shows the concentration of carbon & microstructure as a function of distance in a case carburized steel sample.


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Gas / liquid bath carburizing: A major limitation of pack carburizing is poor control over temperature & carburization depth.

On completion of the process the steel parts are cooled slowly. Direct quenching is not possible

as the job is surrounded by carburizing mixture packed in a sealed box having high thermal

mass. This can be overcome by using gaseous or liquid carburizing medium.

Gas carburization is done by keeping the samples at the carburizing temperature for a specified

time in a furnace having a mixture of carburizing and neutral gas. CH4 and CO are the most

commonly used carburizing gas. It is usually mixed with de‐carburizing (H2 and CO2) and neutral

gases (N2). This helps maintain a close control over carbon potential. It should be enough to

maintain %C at in the range 1.0‐1.2% at the surface. High concentration of CH4 / natural gas and

high velocity should be avoided. In the presence of Fe the carburizing gases decompose to

produce nascent carbon that diffuses into steel.

CH4 = C (Fe) + 2H2

2CO = C (Fe) + CO2

It provides excellent control over the furnace temperature and atmosphere (carbon potential).

Samples after carburization can be directly quenched.

Liquid carburization is done by keeping the job in a salt bath consisting of ~8% NaCN + 82 BaCl2

+ 10 NaCl. It allows precise temperature control and rapid heat transfer. Carburization takes

place due to the formation of nascent carbon. The chemical reactions that occur in the

presence of Fe are as follows:

BaCl2 + NaCN = Ba(CN)2 +NaCl

Ba(CN)2 = C (Fe) + BaCN2

The sample can be quenched immediately after carburization.

Post carburizing heat treatment: Carburization is often done at a much higher temperature than those used during hardening.

This is primarily to hasten the process. However the exposure to such a high temperature may

give rise to very coarse austenite grains. This on subsequent transformation is likely to have

adverse effects on the ductility and the toughness of the core. To get the best combination of

hard case & tough core multiple stages of heat treatment may be necessary. These are known

as:

Core refining


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Case hardening Core refining: Core refining consists of heating a case carburized component to the normal hardening

temperature corresponding to the carbon content of its core. This is around 30°‐ 40°C above A3.

Usually %C in the core is around 0.2% whereas that at its case is around 1.0%. At this

temperature the core should consist of 100% fine homogeneous austenite. The structure of the

case during this stage would depend on its carbon content. Figure 3 may help you predict its

structure during the core refining stage of the process. In this case the microstructure would

consist of austenite and cementite. After carburization the case is likely to have a structure of

consisting of brittle cementite network surrounding pearlitic regions. This type of structure is

susceptible to brittle failure. Therefore it is undesirable. During core refining the pearlite in the

case transforms into austenite and most of the pro‐eutectoid cementite present may dissolve in

it. The continuous network of carbide breaks down into dispersed particles of irregular shapes

which subsequently transform into globules (spheroids). The main driving force for the

conversion of the shape of the carbide is the reduction of its surface energy (surface area per

unit volume).

After core refining the components are quenched. This is necessary to prevent the formation of

brittle network of carbide in the case. The austenite in the case has a much higher

concentration of carbon than that in its core. Its Mf temperature is likely to be lower than room

temperature. Only a part of it may convert into martensite. Therefore the microstructure of the

case after quenching should consist of high carbon martensite, retained austenite and globules

of un‐dissolved cementite. The microstructure of the core would depend on several factors like

section size of the component, quenching severerity (H), the composition or the hardenability

of steel. In the case of plain carbon steel it is likely to be a mixture of ferrite, fine pearlite and

martensite whereas in the case of alloy steel it might consist of low carbon martensite.


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The need for an additional heat treatment adds to the cost of the final product. Core refining is

necessary to restore the initial fine grain structure in this zone before carburizing so that it

regains its toughness. The current trend in industrial carburizing is the adoption of very high ( ~

1030 deg. C ) temperature in order to drastically reduce the time and have very high

productivity. In this case, the grain size of austenite increases excessively. To avoid the same, a

small amount of Nb ( ~ 0.04) is added these steel. The presence of NbC at prior austenite grain

boundaries prevents excessive grain growth during carburization. Therefore these steel do not

need this additional heat treatment.

Case hardening:

The main purpose of this stage is to harden the case. Therefore the component after case

refining is heated to 30° – 40°C above A1 (see fig 3). At this temperature the case consists of

austenite and globules of un‐dissolved carbide. The structure of the core during this stage of

heat treatment should have ferrite and austenite. The concentration of carbon in austenite

should be the same as that of the eutectoid. After proper soaking at this temperature the

component is quenched. The case on quenching should consist of (mostly) martensite, un‐

dissolved carbide and a little retained austenite. The core on the other hand may have mostly

ferrite with islands of high carbon martesite.

Tempering:

After case hardening the components must be tempered. This gives better micro‐structural

stability. The high carbon matersite both in the case and in the core transforms into more

stable low carbon martenste and carbide. The retained austenite too decomposes into a

Fig 3: Shows the temperature at which core refining

(CR) is done. It is converted into 100% fine austenite.

The case too is exposed to the same temperature. It

may consist of either 100% fine austenite or a mixture

of austenite and cementite. This would depend on its

carbon content. In this case it should be a mixture of

austenite and cementite. On quenching the austenite in

the case would transform into martensite although a

part may remain untransformed. It would also have

globules of carbides. The core may transform into

martensite if the hardenability of steel is high.

Otherwise it may consist of ferrite, fine pearlite and

martensite.

+ cm

+ + cm

A1

0 %C

HT

Carburizing

temperature

Core Case

CR

CH


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mixture of ferrite and carbide. The decomposition of austenite is accompanied by an expansion

in volume. This may have adverse effects on the performance of the component. Therefore it

must be removed by tempering before it is put into service.

T

% C

T0

MsMf

CoreCase

CR

CH

Phase diagram helps in the selection of proper

temperatures for CR & CH

DQFine grain steel

Pack carb

carb

T

CHCR

time

T

Carb

Gas / liquid carb

Case hardening by pack carburizing and subsequent stages of hardening is a long drawn

process. It does not allow fast cooling (quenching) after carburization. This results in the

formation of a brittle network of cementite in the case. Apart from this the core consists of very

coarse austenite grains because of the prolonged thermal exposure at the carburizing

temperature. This may adversely affect the ductility and the toughness of the core. This

problem can be avoided by resorting to liquid or gas carburizing process. Both of these provide

a much better control on the parameters (temperature and activity of carbon at the surface)

affecting the kinetics of carburization. The main advantage of this process is that it allows direct

quenching. It means the component can be hardened by a single stage process if liquid or gas

carburizing method is adopted. Slide 6 shows a set time temperature diagrams for the two

carburizing processes. The main reason for the core refining heat treatment was to restore fine

austenitic grain structure within the core. This can be avoided by selecting inherently fine grain

steel for case carburizing treatment. These are aluminum killed steel. It has fine globular oxides

of aluminum at its grain boundaries. This prevents austenite grain growth during the

carburizing stage of the heat treatment. The grain growth is also inhibited by the presence of

very small amounts of strong carbide formers like Nb, V, and Ti. Use of inherently fine grain

steel may make core refining process redundant. Therefore gas or liquid carburizing process

may be used as a single step case hardening method.

Slide 6


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Origin of compressive residual stress at the surface: One of the main reasons for case hardening is to develop compressive residual stress at the

surface of components that are subjected to fatigue loading. The volume expansion that

accompanies austenite to martensite transformation is primarily responsible for the

development of residual stress in steel on quenching. A general thumb rule is that the region

that transforms last has a compressive stress. In the case of a carburized steel there is a large

difference in the concentration of carbon at the surface and that at the centre. The Ms and Mf

temperatures of the two regions are widely different (see fig 4). The difference is so large that

all though the surface on quenching cools faster it transforms to martensite later than the core.

This is explained with the help of schematic diagrams in fig 6.

Figure 6 clearly shows that although the surface cools faster it transforms completely into

martensite much later than the core. Since it transforms last it should be under compression.

This is the reason why case hardened components have compressive residual stress.

Nitriding: If steel is heated in an environment of cracked ammonia it picks up nitrogen. Nitrogen like

carbon forms interstitial solid solution with iron. If it is present in excess it forms nitride (Fe4N).

Log (time)

T

Ms

Mf Ms

Mf

T

Ms

Mf

%C %C case %C core

Fig 6: The sketch on the left shows the cooling curves at the case and at the core of a component

on direct quenching from the carburization temperature. The sketch on the right shows the effect

of %C on Ms and Mf temperatures. %C at the case is much higher than that at the core. Therefore

its Ms and Mf temperatures are much lower than that of the core.


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It is extremely hard and brittle. However if it remains as a dispersed phase within a matrix of

ferrite or martensite it increases the hardness of steel significantly. Slide 7 explains the process

of nitriding with the help of a set of diagrams. The sketch on the left shows the temperature to

which hardened steel is heated. It is below the lower critical temperature. Nascent nitrogen

that forms at the surface of steel as ammonia comes in contact with Fe. This diffuses into iron

lattice and form nitride as and when the amount of nitrogen in steel exceeds its solubility limit.

The presence of alloying elements having high affinity for nitrogen increases nitrogen pick up.

The formation of nitride within the matrix results in a substantial increase in the hardness of

steel. The sketch on the right shows the location of a brittle Fe4N layer. This is extremely hard

and brittle. As you move away from it the amount of nitride goes on decreasing. The hardness

too decreases with distance as shown in the same diagram. The preferred thickness of the

hardened layer is around 20m. The hardness of the nitride layer is usually in the range of

1000‐2000Hv.

Nitriding

Nitriding treatment: done in ferritic region. No phase transformation. Hardness of thin surface layer ~ 20

m can be in the range 1000-2000 VHN.

600 500

Fe4N

NH3 = 2N (Fe) + 3H2

Brittle white layer (Fe4N) is very hard. Can be removed by lapping. It is detrimental. Can be avoided by controlling process parameters.

VH

N

cm

cm

A3

A1

x

The formation of the brittle layer should be avoided. It is also known as the white layer. It is

detrimental. It is prone to cracking. It can be removed by grinding or lapping. Its formation can

also be suppressed by proper control over the process parameter such as the partial pressures

of ammonia and H2 (or the activity of nitrogen at the surface of the sample) and the

temperature. Nitriding of steel is carried out only after it has been hardened and tempered. It is

the last heat treatment given to steel.

Slide 7


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Iron – Nitrogen phase diagram: Slide 8 gives the relevant part of a schematic binary phase diagram of Fe‐N. It has a striking

similarity with that of Fe‐C system. The solubility of nitrogen in iron like carbon is much higher

in austenite than that in ferrite. If nitriding is done at 500°C the nitrogen pick up or the diffusion

occurs in ferrite or the BCC form of iron. The diffusivity of N in ferrite is higher than that in

austenite. However the solubility of N in ferrite is low. Diffusion would occur under a low

concentration gradient. The process is very slow. It may need very long hours of thermal

exposure in an environment of active nitrogen. Depending on the depth of hardening the

exposure time may range from 10 to 50 hours. Like carburizing there are special salts bath for

nitriding in liquid environment. A typical liquid nitriding bath may consist of a mixture of Na / K

cyanides, cyanates and carbonates. At the nitriding temperature the cyanate decomposes to

release nascent nitrogen.

4 NaCNO = Na2CO3 + 2NaCN + 2N

The nascent nitrogen is very active. It diffuses into iron. When the solubility limit is exceeded it

forms nitride. The Fe‐N diagram helps understand the process of nitriding. The temperature of

nitriding is not very high. There is no transformation involving significant change in volume.

Therefore the problem associated with residual stress leading to cracking or distortion in not a

major concern.

Fe-N phase diagram

’

680 C

Fe4N

590 C

’

2

910 C

wt % N

’

‘

Slide 8


19

Kinetics of nitriding

Cr-Ni-Mo steel

Nitralloy

VH

N

Presence of nitride forming elements increases kinetics of nitriding

time

1Al 1.5Cr 0.2Mo

The presence of alloying elements having high affinity for nitrogen significantly improves the

kinetics of nitriding. This is illustrated with the help of a diagram in slide 9. Common alloying

that significantly improves the kinetics of nitrogen pick up are Cr, Al and Mo. A popular nitriding

grade of steel has 1%Al, 1.5%Cr and 0.2%Mo. It is known as nitralloy.

Effect of surface hardening on the fatigue life of steel: Engineering components like crack shaft, rotors, landing gears, governor valve spindle and

many other similar components that are subjected to cyclic loading are prone to fatigue failure.

Failure occurs after a certain numbers of cycles of loading depending on the magnitude of

stress amplitude. The fatigue resistance of an engineering material is best described by S (stress

amplitude) ‐ N (number of cycles to failure) plots. A typical shape of S‐N curve is shown in slide

10. The plots for steel are asymptotic. There is a stress range for every material below which a

component made of this is expected to have infinite life. This is known the endurance limit of

the material. Fatigue failure occurs only under tensile loading. Most often failure originates

from the external surface of a component. Therefore fatigue life of engineering components

can be improved either by introducing residual compressive stress on its surface or by

increasing the yield strength of the surface.

Slide 8


20

Effect of surface hardening on fatigue life

No. cycles to failure

Nitriding

Shot peened

No surf hard.

Endurance limit

Slide 10 shows with the help of a diagram the effect of surface hardening on the S‐N curves of

steel. The dotted lines represent the endurance limit. Shot peening introduces residual

compressive stress on the surface and increases the yield strength by stain hardening.

Therefore shot peening raises the endurance limit of steel. Nitriding treatment is usually given

after case hardening treatment. It further improves the hardness of the surface. Therefore it

gives the highest possible endurance limit.

Summary: In this module we learnt about various methods of surface hardening. The methods used can

be divided into two groups. One that is based on the modification of the surface either by

coating or by cold work and the other that is based on either heat treatment or a combination

of heat treatment and modification of the composition of the surface. Shot peening and hard

facing comes under the first category. These are more generic and can be applied to all metals

and alloys. The other category includes induction hardening, case hardening and nitriding.

These are applicable only for steel. The structural changes that take place during the various

stages of treatment have been explained with the help of schematic diagrams. The effect of

these on the evolution of surface residual stress and the fatigue resistance of steel has been

explained. Surface hardening offers an opportunity for more efficient use of materials. There

are several applications where the case is required to be strong but the core should be soft and

tough. It helps raise endurance limits of metals and alloys.

Slide 10


21

Exercise:

1. Induction heating followed by quenching is a common method of surface hardening of

steel. Can it be applied to an alloy steel having 18Cr8Ni0.15C?

2. Can steel having 0.1% carbon be case carburized at 850⁰C?

3. Cite three main reasons for surface hardening of steel.

4. Explain why core refining heat treatment may not be required for case carburized

aluminium killed steel.

5. What is the white layer on steel that forms during nitriding?

Answers:

1. No. 18Cr8Ni0.15C is austenitic steel. It cannot be hardened by heating & quenching.

2. It would carburize but the process would be too slow. At 850⁰C it will have ferrite

austenite structure. Solubility of carbon in ferrite is very small. Only the austenitic

region will pick up carbon. The concentration gradient for carbon to diffuse into

austenite is also less. Since both temperature & concentration gradients are low rate of

carbon pick up will be extremely slow. Therefore carburization at 850C is not

recommended. The following figure illustrates how %C at the interfaces can be

estimated.

910 850910 723

0.8 0.26

22 0.8

1147 8501147 723

; 1.16

∆ 1.16 0.26 0.9

723

910 850

1147

0.1 0.8 2.0 Cs Ci

%C

T


22

3. Three most important reasons for surface hardening are: (i) to have hard surface but

soft (tough) core in components like gears, shafts etc. (Hardening is often accompanied

by loss of toughness.) (ii) to overcome section size effect which makes it difficult to get

the required surface hardness in large sections by quenching and (iii) to get a favorable

residual stress on the surface which would inhibit crack initiation.

4. Purpose of core refining treatment is to get fine austenite grain in case carburized steel.

Aluminum killed steel are resistant to austenitic grain growth. Aluminum reacts with

dissolved oxygen to form oxide particles during solidification. These are located along

austenite grain boundaries and restrict their movements. In such steel grain growth

during carburization heat treatment may not be significant. This is why core refining

treatment may not be necessary. Steels having micro alloying elements like Nb, V, and Ti

too are resistant to grain growth. These too do not need core refining treatment.

5. Nitriding is done on steel after it has been hardened and tempered. Sample is heated to

around 500⁰C which is lower than the eutectoid transformation temperature in Fe‐N

phase diagram. The eutectoid consists of ferrite and Fe4N. While some N would diffuse

through ferrite to form fine carbo‐nitrides some may form a white nitride layer made of

Fe4N at the surface. This is hard and brittle. It is harmful and it must be removed by

lapping.

Module 37 Surface hardening Lecture 37 Surface...

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