Module 37
Surface hardening
Lecture 37
Surface hardening
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NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
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 2 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.
Performance
Processing: Properties micrstructure
Material Fig 1: Tetrahedron showing the interrelation between material, its processing route and its property on the performance of a component.
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 3 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
NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
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 I beam A3 flange A1 + P web T Flange Slide 1 web Web: least stressed: thin Ms + M Flange: high stress: thick time I Beam: hot rolled sections Case: where we have % C ~ 0.2 to learn to live with it. Surface hardening: Which is stronger ? exploit such features.
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 4 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:
NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
• 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:
5 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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NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
Induction hardening Place the job inside an induction coil & pass high frequency ac. Surface gets heated due to skin effect. Job High Heating: local surface near Coil AC the coil gets heated = resistivity Slide 2 = frequency d = magnetic permeability Higher frequency: lower depth of hardening (d). On quenching only surface becomes martensite.
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.
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NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
Structure & properties Homogeneous T for Sph austenite Grain Ann growth
Plate (HC) T T martensite Needs %C time Much higher austenitization time Slide 3 tempering required as hold time is short Cm globules Spheroidising annealing is done in matrix before induction hardening Spheroidising Hard surface & tough core. annealing Compressive residual stress at surface & tensile at core
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 8 of surface crack. However martensite is brittle. It may be surrounded by retained austenite as well. Therefore induction hardening must be followed by tempering.
NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
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
(a) (b) C2 Sample T2 A3 C1 T1 T A1 + cm 85% char coal + 15% energizer C0 %C CS BaCO3 = BaO + CO2 Slide 4 (c) C S CO2 + C = 2 CO (d) C 0 2 CO = C (Fe) + CO % t 2 C CS : Function ( Temp) x 9 Depth: F (T,t)
NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
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 (a) CC x 0 erf CC CS C0 S 0 2 Dt x Q C DDC 0 exp S RT erf (0.5) 0.5 C0 Slide 5 xDt (b) d x
-4 Problem: T=927 C, t=10hrs, C0=0.2, D0= 0.7x10 2 m /s Q=157 kJ/mole, CS=1.2 Estimate x. erf(z) = z
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 10 effective diffusion distance d. It corresponds to the distance at which the concentration of carbon (C) is given by
NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
(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.
6.67
%C
Cs 2 Fig 2: Shows the concentration of carbon & microstructure as a C0 d x function of distance in a case carburized steel sample.
P Thin cm layer
P + cm P P + Initial network network structure
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. 11
NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
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 12 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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NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
Carburizing Fig 3: Shows the temperature at which core refining temperature (CR) is done. It is converted into 100% fine austenite. CR The case too is exposed to the same temperature. It HT may consist of either 100% fine austenite or a mixture + cm of austenite and cementite. This would depend on its + CH carbon content. In sthi case it should be a mixture of A 1 austenite and cementite. On quenching the austenite in the case would transform into martensite although a + cm part may remain untransformed. It would also have globules of carbides. The core may transform into 0 martensite if the hardenability of steel is high. Core %C Case Otherwise it may consist of ferrite, fine pearlite and martensite.
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: 14 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
NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | | 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.
Carb Core DQ Case Fine Gas / CR T grain liquid CH steel carb T Slide 6 carb T0 CR CH T Pack M Ms f carb % C Phase diagram helps in the time selection of proper temperatures for CR & CH
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 15 steel may make core refining process redundant. Therefore gas or liquid carburizing process may be used as a single step case hardening method.
NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
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.
T T
Ms
Ms
Mf Ms Mf
Mf Log (time) %C core %C %C case 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.
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: 16 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).
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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.
A3 Fe4N cm Slide 7 600 A1 500 cm VHN NH3 = 2N (Fe) + 3H2 x
Brittle white layer (Fe4N) is very hard. Can be removed by lapping. It is detrimental. Can be avoided by controlling process parameters.
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 17 the last heat treatment given to steel.
NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
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
910 C
680 C ’ ’ ‘ 590 C Slide 8 ’ 2 wt % N Fe4N
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NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
Kinetics of nitriding
Nitralloy 1Al 1.5Cr 0.2Mo Cr-Ni-Mo
VHN Slide 8 steel
time Presence of nitride forming elements increases kinetics of nitriding
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.
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NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
Effect of surface hardening on fatigue life
Nitriding
Shot peened Slide 10
No surf hard.
Endurance limit
No. cycles to failure
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 20 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. NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
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.
1147 T 910 850 0.8 0.26 910 910 723 850 2 1147 850 ; 1.16 723 2 0.8 1147 723
∆ 1.16 0.26 0.9
0.1 Ci 0.8 Cs 2.0 %C 21
NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |
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.
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NPTEL Phase II : IIT Kharagpur : Prof. R. N. Ghosh, Dept of Metallurgical and Materials Engineering || | |