1. Nitrocarburizing of Austenitic Stainless Steels
Rhushikesh Mane#1, Prathamesh Thakar#2, N. R. Anand#
#Department of Metallurgy and Materials Science, College of Engineering, Pune-411005
1rhushikesh.mane12@gmail.com
2prathamesh.thakar@gmail.com
Abstract - The increasing demands placed upon internal combustion engines have led to the development of some new grades of austenitic stainless steels for valves of I.C. Engines. To improve their resistance to abrasive wear, these steels are nitrocarburized. AISI 304 is most widely used and basic grade of austenitic stainless steel. So in current project work this grade was used for experimentation. The steel was nitrocarburized at 5700 C and subsequently oxidized in salt bath furnace for period of time. After this treatment different mechanical tests and characterizations were done and their results were subsequently analyzed. Vickers’s micro hardness has shown fourfold increase in hardness of surface. Due to the contribution of this hardness, wear resistance of treated sample was increased by two orders of magnitude. Samples were further characterized by SEM, EDAX and XRD.
Keywords – Nitrocarburizing, AISI 304, Hardness, Wear, Characterization
I. INTRODUCTION
Austenitic stainless steels are a subclass of stainless steels containing 16-25% Cr, 8-20% Ni and other alloying elements. Cr increases strength and corrosion resistance making steel surface stainless. Being stainless steel, their surface contains a self-healing oxide film, which prevents further diffusion of oxygen through it, hence, resist further oxidation. Ni stabilizes austenite at room temperature, increasing the high temperature strength with good toughness. Due to high temperature strength and oxidation resistance at high temperature, stainless steels are suitable for high temperature applications. Valves of I.C. Engines is one such application where the working temperature is 7500C [1]. But for valves, following additional properties are required:
 High surface hardness
 Resistance to seizing and galling
 Good wear resistance
To increase these properties, steels are nitrocarburized.
Nitrocarburizing processes are those thermochemical treatments which involve the diffusional addition of both nitrogen and carbon to the surface of ferrous materials at temperatures completely within the ferrite phase field. It can be done in any gas or liquid medium which can provide nascent nitrogen and carbon at the surface of the steel. These interstitial elements dissolve in the steel [2]. After the solubility limit is exceeded, carbonitrides are formed. These carbonitrides are of two types- γ’ (Fe4(C,N)) and ε (Fe2-3(C,N)). If the amount of interstitials is more than 6%, γ’-phase converts to ε-phase, which has improved hardness and wear resistance. The combination of γ’ and ε phases is observed as white zone with an optical microscope; under white zone, depending on the steel constitution, elements and their percentage alloy nitrides are present. Especially, Ti, Al, V, Cr, Mo and W can combine with N to form metallic nitrides. This white zone or compound zone has higher hardness than the interior part of the material. The layer next to the compound layer consists of fine scale precipitates of carbonitrides in the matrix of ferrite or austenite, depending on the initial microstructure [3].
Austenitic stainless steels contain austenite (γ-phase) at room temperature, according to phase diagram of 18/8 stainless steels. In low alloy steels, microstructure consists of ferrite, pearlite and alloy carbides depending on alloying elements. When nitrogen absorption takes place in BCC ferrite, it changes to FCC austenite at 5700C. This FCC austenite has higher solubility for interstitial elements; hence, to form carbonitrides, higher amount of interstitial content is required and consequently the time for treatment increases. Hence, nitrocarburizing of low alloy steels is always carried out at temperature of 5700C or below. But in austenitic stainless steels, austenite is already present in the room temperature microstructure, this 5700C has no meaning. Even then, in many research papers, nitrocarburizing of austenitic stainless steels is carried out at this temperature. This is because at lower temperature, though solubility is lower, there will be less driving force of diffusion and kinetics will be sluggish. At higher temperatures, diffusion will be faster but solubility will be higher, hence time required will be more. There are some more problems in nitriding or any treatment of these steels involving diffusional addition of atoms from external atmosphere into the surface. Stainless steels possess an adherent film of Cr2O3 which cannot be reduced without a highly reducing environment, which acts as a barrier to diffusion. Also, stainless steels contain higher amount of alloying elements which tend to decrease kinetics of the process; hence, the process is slow [4]. This problem of oxide layer can be tackled by pickling the samples in HCl prior to treatment. When salt bath is used, the time of

2. treatment is reduced. This may be attributed to the continuous contact of the molten salts with the reduced steel surface which prevents the re-oxidation of the surface. Also, molten salts have high activities of interstitials [5].
When austenitic stainless steels are nitrocarburized, addition of interstitial elements takes place in FCC austenite. Due to this, expansion of lattice takes place which is commonly referred as expanded austenite or S-phase. Due to the distorted structure of this phase, hardness is more than the original austenite, which increases the wear resistance. Also, due to presence of compressive stresses in this layer, micro cracks formed during wear are closed. Further enrichment of this surface layer by interstitial elements tends to form compounds like carbonitrides of chromium, iron and other alloying elements depending on their composition.
In the current experiment, AISI 304 is selected as it is most widely used and basic grade of austenitic stainless steels. This AISI 304 steel is nitrided in salt bath furnace at 5700C for different periods of time. Different properties that were studied of these samples included hardness and wear rate. Characterization was done using XRD, SEM and EDAX.
II. EXPERIMENTAL WORK
For the purpose of nitrocarburizing, a long bar of AISI 304 steel having diameter 10 mm was cut in pieces of length 50 mm. These samples were polished on emery papers and pickled using a solution containing HCl to remove oxide layer. Nitrocarburizing treatment was carried out at 5700C for 60, 90, 120 minutes, which was followed by oxidizing at 4000C in salt bath furnace. The microhardness of the samples was measured in VPN transverse to the treated surface using a load of 50 gm with dwell time of 10 seconds. Micrographs were taken after etching the polished samples using a freshly prepared solution containing 10 gm FeCl3, 30 ml HCl and 120 ml distilled water. Further, samples were characterized by SEM and EDAX. X-ray diffraction analysis was carried out on the treated surface using Cu Kα wavelength (1.5418 nm). Scanning was done from the angle of 10o to 100o at the step size of 0.1o per second.
To measure the wear resistance, treated samples were tested using pin on disc type wear testing machine. The pin was the treated specimen of height 10 mm and diameter 10 mm and disc was made up of SAE 52100 steel. A normal load of 60 N and sliding velocity of 2.4 m/s was used. Slide distance was kept constant at 3000 m. To check the surface morphology of the samples, stereographic examination was carried out.
III. RESULTS AND DISCUSSION
Optical microscope and SEM examination of AISI 304 steel showed three layers- outer oxide layer, nitride layer or compound layer followed by diffusion zone containing nitride precipitates in the matrix of austenite. Fig. 1 shows the SEM image of the sample nitrocarburized for 120 minutes. Below the diffusion zone, the base matrix containing only austenite was observed.
Since nitrocarburizing involves diffusion of C/N to the surface, it is expected to observe the diffusion of these elements from the surface to the interior depending on the temperature, time and the chemical composition of the base material.
EDAX analysis done on the sample nitrocarburized for 120 min showed the compositional changes from layer to layer.
Fig. 2 shows EDAX peaks for O, C/N, Cr, Ni and Fe in layer 1, so it is oxide layer containing Fe, Ni and Cr oxides and some amount of carbonitride. Fig. 3 shows major peaks for Cr, Fe and minor peaks for C/N and O from layer 2, but absence of Ni peak; so it is Cr, Fe- carbonitride layer containing some amount of oxygen which decreases the amount of interstitial elements required to form compounds. Absence of Ni in above layer 2 may be attributed to the diffusional migration of strong
Fig. 1 SEM image of sample nitrocarburized for 120 min
Layer 1
Layer 2
Layer 3
Layer 4
Fig. 2 EDAX peaks from layer 1

3. nitride forming elements like Cr and Fe to this layer decreasing the amount of Ni.
Fig. 4 shows major peaks for Cr, Fe, Ni and very minor peaks for C/N, from layer 3; so this layer is made up of carbonitrides embedded in matrix of base material (diffusion zone).
Layer 4 has peaks for Fe, Cr, Ni but absence of C/N; so it must be base material containing austenite.
Fig. 6 reveals the X-ray diffraction of AISI 304 steel. It shows the presence of oxides (Fe3O4, Fe2O3, Cr2O3), nitrides (γ’, ε, CrN, (Fe, Cr)2N), austenite and some minor peaks for S- phase.
Microhardness testing showed that the maximum hardness obtained was 1186 VPN in the white layer. Fig. 7 shows that the hardness gradually decreases as the distance from the surface increases; the hardness of the base microstructure was found to be around 300 VPN. The surface hardness was found to be independent of the duration of treatment, but the case depth
Fig. 3 EDAX peaks from layer 2
Fig. 4 EDAX peaks from layer 3
Fig. 5 EDAX peaks from layer 4
Fig. 6 XRD peaks for from treated sample
Fig. 7 Hardness profile of treated samples S1(60 min), S2(90 min), S3(120 min)

4. increased as the duration of nitriding increased as shown in Fig.
8
Fig. 9 shows the variation of wear volume with the time of
nitrocarburizing.
Wear of untreated bare material is decreased by three
orders of magnitude, indicating the effectiveness of
nitrocarburizing treatment in improving the dry sliding wear
resistance of the austenitic stainless steel. Also, during wear
testing of untreated steel, significant amount of plate like wear
debris is formed which again scratches the wear surface and
increases the abrasive wear. Table 1 shows the data obtained
from the wear test.
The higher wear resistance can be explained from two
facts, namely, high surface hardness and high compressive
stresses produced in the case during the treatment. Wear volume
is proportional to surface hardness of mating surfaces as per
Arched’s equation. High compressive stresses in the surface
tries to close the cracks produced during wear.
Fig. 10 shows the stereograph of the sample nirocarburized
for 90 min. Presence of porosity can be observed in the form of
white patches. This could be associated with the oxide layer.
The porosity is beneficial when there is wear of surfaces in
lubricated condition, as it stores the lubricant through the
capillary action.
Fig. 11 and Fig. 12 are the stereographs of the weared
surfaces of untreated sample and the sample nitrocarburized for
120 min respectively. These stereographs indicate that the wear
occurred in severe mode in case of untreated sample,
characterized by surface plastic deformation, shearing, material
transfer and abrasion. The stereograph of treated sample shows
mild mode of wear characterized by abrasion and surface
polishing.
Fig. 8 Variation of case depth with time of treatment
Fig 9 Variation of wear height with time of treatment
Name of
sample
Wt loss in
gm
wear,
mm3/m
specific wear rate
0 min 0.4193 1.8E-02 2.24692E-07
60 min 0.0041 1.7E-04 7.95936E-09
90 min 0.0037 1.6E-04 7.85691E-09
120 min 0.0033 1.4E-04 7.07534E-09
Table 1 Variation of wear with time of treatment
Fig. 10 Stereograph of sample nitrocarburized for 120 min

5. IV. CONCLUSION
The following conclusions can be drawn from the present study:
1) The SEM investigation have shown that the salt bath nitrocarburizing followed by oxidizing of austenitic stainless steel AISI 304 leads to the formation of three layers, namely, oxide layer, nitride layer and diffusion zone on base material.
2) Oxide layer consists of oxides of Fe and Cr, while nitride layer contains different mixture of nitrides. At higher depths, nitrides only begin to form diffusion zone as a result of increasing concentration of iron in this region.
3) These results were verified by EDAX and XRD analysis.
4) As an effect of treatment, surface hardness shows fourfold increase. Maximum hardness of 1186 VPN was obtained during experiment.
5) Wear resistance of steel was increased by a magnitude of two orders.
ACKNOWLEDGEMENT
We immensely thank Dr. M.J. Rathod, Head, Department of Metallurgy and Materials Science, College of Engineering, Pune for allowing us to carry the project out and also providing us technical information and for given suggestions about the project. We especially thank Dr. Hosmane for helping us in the project and solving our difficulties. We also thank Mr. Iyer (M/s Metallurgia) for helping us.
Without the support of all the above mentioned people, we would never have completed our project.
REFERENCES
1. Marianne Kurz, Edith Ittlinger, Daimler Chrysler AG, (1999), “ Formation of nitride layers in the nitrocarburizing of Engine valves”, Praktische Metallographie 36 4
2. ASM handbook, volume 04, Heat treating.
3. MetinUsta et al. (2004), “Nitriding of AISI 316L surgical grade stainless steel in fluidized bed reactor”, Vacuum- Surface engineering, surface instrumentation & vacuum technology.
4. Vijendra Singh: Heat treatment of metals, Reprint edition, Standard Publishers, Delhi, (2007).
5. Prabhudev: Handbook of Heat Treatment of Steels, Tata McGraw Hill Publication, Delhi.
Fig. 11 Stereograph of wear surface of untreated sample
Fig. 12 Stereograph of wear surface of steel nitrocarburized for 120 min