Glow-discharge nitriding of sintered stainless steels

Ž .Surface and Coatings Technology 139 2001 251�256 Glow-discharge nitriding of sintered stainless steels T. Bacci�, F. Borgioli, E. Galvanetto, G. Pradelli Dip. di Meccanica e Tecnologie Industriali, Uni�ersita di Firenze, �ia S. Marta 3, Firenze 50139, Italy` Received 19 September 2000; accepted in revised form 19 January 2001 Abstract The glow-discharge nitriding process is particularly suitable to harden the surface of sintered stainless steels components, owing to the high porosity levels of these materials. On wrought austenitic stainless steels this treatment produces a metastable phase, known as supersaturated austenite or S phase, which has shown high hardness values and good corrosion resistance. In the present paper the influence of glow-discharge nitriding process on the microstructural and mechanical properties of AISI 316L austenitic sintered stainless steel has been evaluated and it is compared with the results obtained with ion-nitrided martensitic Ž . Ž .AISI 410 and ferritic AISI 430L sintered stainless steels. The ion-nitriding treatment, performed at 773 K for 4 and 8 h, produces modified surface layers. The microhardness profiles show high hardness values in the modified layers and a steep decrease to matrix values, thinner hardened layers and lower hardness values are observed on AISI 316L samples, in comparison with AISI 410 and AISI 430L samples. The S phase is detected on the modified layers of the ion-nitrided AISI 316L samples. The crystallographic characterisation has shown that a face centred tetragonal lattice as base for this phase fits well the diffraction spectra, in respect of the ‘traditional’ face centred cubic lattice usually adopted, since the lattice shows a strong distortion, in spite of this, the d-spacing values calculated with a f.c.t. base show a good agreement with literature data, when the used f.c.c. indexing is modified for the f.c.t. lattice. � 2001 Elsevier Science B.V. All rights reserved. Keywords: Sintered stainless steel; Ion nitriding; S phase; X-ray diffraction; Lattice parameters 1. Introduction Glow-discharge treatments provide a useful tool for modifying the surface layers of sintered steel compo- nents in order to improve their surface hardness and wear resistance. In fact, conventional processes, such as gas and salt bath ones, become complicated for sin- � �tered materials, owing to the presence of porosity 1�4 . As a matter of fact, after salt bath treatments, a careful cleaning of the sintered components has to be per- formed in order to avoid corrosion by salts trapped in the pores. In gas processes, the interconnected porosity promotes the diffusion of gaseous species deep in the �Corresponding author. Tel.: �39-055-4796503; fax: �39-055- 4796400. Ž .E-mail address: [email protected] T. Bacci . bulk of the material, causing an excessive hardening penetration, so that the sintered component becomes brittle and dimensional changes can happen. These problems can be overcome only by closing the pores � �with sealing pre-treatments 3 . On the other hand, in a glow-discharge process the peculiar mechanism of sur- face enrichment of active species reduces the influence of pores on the characteristics of the modified layer, so that a better control of hardness and case depth can be performed and pores-closing treatments are not usually � �needed 5�7 . Owing to its characteristics, this treat- ment appears to be suitable especially for sintered stainless steel components that show high porosity lev- els due to the poor compressibility of stainless steel � �powders 8,9 . The glow-discharge nitriding treatment can be used for improving the surface hardness and wear resistance 0257-8972�01�$ - see front matter � 2001 Elsevier Science B.V. All rights reserved. Ž .PII: S 0 2 5 7 - 8 9 7 2 0 1 0 1 0 1 0 - 6 ( )T. Bacci et al.�Surface and Coatings Technology 139 2001 251�256252 Table 1 Chemical composition of the sintered samples Ž .Material Composition wt.% C Cr Ni Mn Mo Si AISI 316L 0.02 17 13 0.2 2.2 0.8 AISI 410 0.2 12 � 0.2 � 0.8 AISI 430L 0.02 18 � 0.2 � 0.8 of stainless steels, especially of the relatively soft � �austenitic ones 10 . Studies regarding the nitriding of wrought austenitic stainless steels have shown that a metastable phase forms as consequence of the nitriding � �treatment 11�17 . This phase, produced by different nitriding techniques and designed as supersaturated or � � � �‘expanded’ austenite, � , 11�13 , S phase 14,15 and,N � �recently, as m phase 16,17 , has shown high hardness � �values and good corrosion resistance 11,15,16 . The aim of the present research was to evaluate the influence of glow-discharge nitriding treatment on mi- crostructural and mechanical characteristics of AISI 316L austenitic sintered stainless steel and to perform a crystallographic characterisation of the metastable S phase, formed in the modified layer. Glow-discharge Žtreatments were also performed on martensitic AISI . Ž .410 and ferritic AISI 430L sintered stainless steels. 2. Experimental procedure Ž . ŽParallelepipeds 10�10�50 mm of austenitic AISI . Ž . Ž .316L , martensitic AISI 410 and ferritic AISI 430L stainless steels, having the composition as reported in Table 1, were obtained as pre-alloyed powders, mixed with 1% Acrawax, for the martensitic steel, graphite powder was added to the blend, in order to achieve a final carbon concentration of 0.2 wt.%. The paral- lelepipeds were compacted at 600 MPa and sintered at 1473 K for 30 min in a hydrogen atmosphere. Table 2 Porosity and microhardness values of the as sintered samples Sample type Porosity HV HV0.025 1.0 Ž .vol.% AISI 316L 12�1 153�4 71�5 AISI 410 15�2 509�7 178�8 AISI 430L 9�1 171�4 73�5 Ž .Prismatic samples 4.5�10�24 mm were prepared from the parallelepipeds by cutting and then were ground and polished. Glow-discharge treatments were performed in a plasma equipment, similar to industrial ones. The sample holder was connected to the cathode of the glow-discharge power supply, the treatment tem- perature was controlled by a chromel-alumel thermo- couple inserted into the sample holder. The ion-nitrid- ing chamber was cleaned by means of alternate nitro- gen filling and evacuation, the samples were cleaned by cathode sputtering. Treatments were performed at 773 K for 4 and 8 h, at a pressure of 10 mbar. With the aim of trying to maximise the amount of S phase in the modified layer, ion-nitriding was carried out by using a gas composition of 80 vol.% N and 202 � �vol.% H 15 .2 The microstructure of the treated samples was ex- amined by optical metallographic techniques, scanning Ž .electron microscopy SEM and microprobe analysis, glyceregia reagent has been used to delineate the mi- Žcrostructure. X-ray diffraction analysis CuK� radia- .tion was performed to identify the phases present in the surface layers, diffraction patterns were collected using a diffractometer in Bragg-Brentano configuration and were analysed by means of a fitting program using � �Rietveld method 18 . Microhardness measurements ŽKnoop indenter, 25 gf, Vickers indenter, 25 and 1000 .gf were carried out on the modified layers and on the matrix. Ž . Ž . Ž .Fig. 1. Microstructure of as-sintered AISI 316L austenitic a , AISI 410 martensitic b and AISI 430L ferritic c stainless steels. ( )T. Bacci et al.�Surface and Coatings Technology 139 2001 251�256 253 3. Results and discussion 3.1. Characterisation of the as sintered materials The microstructure of the as sintered materials is shown in Fig. 1, AISI 316L has an austenitic structure, AISI 410 a lath martensite one, while AISI 430L shows a ferritic structure. Porosity has been evaluated using optical metallographic techniques and it is reported in Table 2. High porosity values are shown by the sam- ples, in accordance with the poor compressibility of stainless steels powders, due to the high content of � �alloy elements 8,9 , optical microscopy analysis shows that the porosity is interconnected and still open. ŽMicrohardness measurements Vickers indenter, 25 .and 1000 gf were performed on the samples before the glow discharge treatments, hardness values are re- ported on Table 2. While the 25 gf testing load allows to evaluate the ‘true’ hardness, i.e. the hardness of the steel particles, by using 1000 gf testing load the mea- sured value can be referred to an ‘apparent hardness’, since it depends also on the porosity, as a matter of fact, the ‘apparent’ hardness values are �2�3 times lower than the ‘true’ hardness. 3.2. Ion-nitrided materials The stainless steel samples were treated by ion- nitriding process for 4 and 8 h in order to evaluate the influence of treatment time. The glow-discharge nitriding process produced a modified surface layer, consisting of an outer com- pound layer and an inner diffusion layer, the mi- crostructure and mechanical characteristics of these layers depend on the steel employed. Fig. 2 shows the diffraction spectra of the surface Ž .layers of AISI 316L austenitic a , AISI 410 martensitic Ž . Ž .b and AISI 430L ferritic c stainless steels, ion- nitrided at 773 K for 4 h. As shown by the diffrac- tograms, the compound layer of the AISI 316L austenitic stainless steel consists essentially of ��-Fe N4 Ž . Ž .f.c.c. and CrN f.c.c. , small amounts of �-Fe N2 � 3 Ž . Ž .hex and traces of Fe O magnetite, cubic are also3 4 detected. It can be suggested that, owing to the pres- ence of austenitic phase, the formation of �-Fe based nitride, as ��-Fe N, is promoted. Microprobe analysis4 shows high chromium and molybdenum content where Žnitrides are present and a low nickel one 64 wt.% Cr, .8.5 wt.% Mo, 2.3 wt.% Ni , in accordance with the fact that chromium and molybdenum are strong nitride forming elements, while nickel is not. It can be hy- pothesised that the alloy elements substitute iron atoms in the nitrides, so that mixed nitrides form, as observed � �by other authors 19 . By increasing treatment time, no detectable change in compound layer constitution was found. The compound layer thickness, determined from SEM observations, increases as treatment time in- Ž . Ž .creases, it ranges from �4 t�4 h to �6 t�8 h �m. On AISI 316L samples, the diffraction analysis shows also the presence of peaks that can be ascribed to the phase designed as supersaturated or ‘expanded’ � � � �austenite, � 11�13 , S phase 14,15 and, recently, asN � �m phase 16,17 . In this paper, the less committal designation ‘S’ phase has been preferred, since both its Ž .Fig. 2. X-ray diffraction spectra of AISI 316L austenitic a , AISI 410 Ž . Ž .martensitic b and AISI 430L ferritic c sintered stainless steels samples ion-nitrided at 773 K for 4 h. ( )T. Bacci et al.�Surface and Coatings Technology 139 2001 251�256254 Ž . Ž . Ž .Fig. 3. Micrographs of the modified layer of AISI 316L austenitic a , AISI 410 martensitic b and AISI 430L ferritic c sintered stainless steels samples ion-nitrided at 773 K for 8 h. crystallographic structure and formation conditions are not yet completely clarified. The diffraction analysis of the AISI 410 martensitic and AISI 430L ferritic stainless steels shows that their compound layers are similar, they consist of iron and chromium nitrides, ��-Fe N, �-Fe N and CrN, traces4 2 � 3 of iron oxide, Fe O , are also detected. By increasing3 4 treatment time a slight increase in Fe N amount is4 observed. The diffraction peaks of �-Fe phase, present in the diffusion layer, are clearly detectable only on AISI 430L samples treated for 4 h and their intensity decreases as time increases, owing to the growth of the compound layer. On the other hand, the absence of �-Fe peaks on AISI 410 samples points out that the compound layer extends to all the thickness explored by X-rays. The X-ray diffraction results are in accor- dance with the microstructural analysis, thinner com- Žpound layer is observed on AISI 430L samples from Ž . Ž . .�4 t�4 h to �7 t�8 h �m in respect of AISI Ž Ž . Ž . .410 ones from �12 t�4 h to �15 t�8 h �m . Microprobe analysis, performed on the modified layers and on the matrix, shows that a chromium enrichment is present in the first microns of the modified layers of AISI 410 and 430L samples, pointing out that the chromium nitride, CrN, is essentially confined in the compound layer. The microstructure of the modified surface layers of Ž . Ž . Ž .AISI 316L a , AISI 410 b and AISI 430L c samples, ion-nitrided at 773 K for 8 h, is shown in Fig. 3. The diffusion layer of the treated samples consists in iron and alloy elements nitrides precipitates dispersed in matrix crystals rich in nitrogen. The chemical etching acts especially on the nitride precipitates present in the treated steels, on the AISI 316L samples, a strong etched line, roughly parallel to the surface, is present and separates the modified region from the matrix. The diffusion layer, as revealed by the chemical etch, ap- pears to be homogeneous in all the treated samples and no marked effect of the open and interconnected porosity on nitrogen diffusion paths has been observed, in accordance with the low influence of pores shown in glow-discharge treatments. On AISI 316L samples, a Žrelatively thin diffusion layer �40 �m for 8 h treat- .ment time is produced, on the contrary, the marten- sitic structure of AISI 410 samples is able to improve nitrogen diffusion, so that a high diffusion layer thick- Ž .ness is achieved �110 �m for 8 h . In AISI 430L samples, a thinner diffusion layer has been observed Ž .�85 �m for 8 h . Microhardness measurements were performed on the modified layers and on the matrix of treated samples. Ž .The microhardness profiles of AISI 316L a , AISI 410 Ž . Ž .b , AISI 430L c samples, nitrided for 4 and 8 h at 773 K, are shown in Fig. 4. All the samples show high hardness values in the modified layers and a steep decrease to matrix values. The microhardness data are in accordance with morphological observations, thin hardened layers are achieved on austenitic samples, while thick layers are produced on martensitic ones. As time increases, higher hardness values near the surface and thicker hardened layers are obtained, it must be pointed out that AISI 430L ferritic samples show a marked increase in hardness values near the surface with treatment time, as it is expected as a consequence of an increase in amount of nitrides and of the growth of the compound layer. Moreover, it should be noted that AISI 410 samples show a marked decrease of matrix hardness values, in comparison with the values of untreated samples, in accordance with the occurrence of a tempered marten- site structure as a consequence of the ion-nitriding treatment. 3.3. X-ray diffraction analysis of the S phase X-ray diffraction analysis shows that the S phase, detected on the treated AISI 316L austenitic samples, have peculiar features, if an austenite face centred ( )T. Bacci et al.�Surface and Coatings Technology 139 2001 251�256 255 Ž .Fig. 4. Microhardness profiles of AISI 316L austenitic a , AISI 410 Ž . Ž .martensitic b , AISI 430L ferritic c sintered stainless steel samples ion-nitrided for 4 and 8 h at 773 K. cubic lattice is chosen as the indexing base, the lattice Ž .parameter evaluated with the 2 0 0 diffraction line is Ž . Žalways greater than that calculated from the 1 1 1 , 2 . Ž .2 0 or 3 1 1 lines, moreover, the phase is textured in ² :the 111 direction. These features of the S phase have been observed on 3 xx series stainless steels nitrided with various techniques, from glow-discharge nitriding � � � �14,16 to plasma immersion ion implantation 11,12 , � �low pressure arc discharge 13 and reactive magnetron � �sputtering 20 . In consideration of the above men- tioned lattice distortion of the S phase, in the present Ž .paper a face centred tetragonal lattice f.c.t. has been chosen to reproduce the S phase pattern, as also sug- � �gested by other authors 15,17 . The f.c.t. lattice, tex- ² :tured in the 111 direction, was found to fit better the diffraction patterns of all the treated samples than a f.c.c. lattice, as is shown in Fig. 5. The interplanar d-spacing values of the four most intense peaks of the S phase, present in the spectra, the lattice parameters a, c and the c�a ratio have been evaluated for the treated samples and are reported in Table 3. Moreover, the d-spacing values and the a lattice parameter of the � phase of the untreated samples have been reported as a reference, it has to be recalled that, in a f.c.c. lattice, Ž . � 4the 2 0 2 plane belongs to the 2 2 0 plane family, that is usually indicated in indexing diffraction spectra. The d-spacing values of S phase in sintered samples show a good agreement with the data present in litera- ture, relative to wrought AISI 316L stainless steel � �nitrided with various techniques 11,13 in the range 690�723 K, these data refer to a f.c.c. lattice, but they can be compared with the data reported in this paper using the same indications as for � phase. The S phase shows an increase of cell volume of �27%, in respect of the parent � phase. By increasing the treatment time a slight decrease of the a, c parameters has been observed, it can be suggested that the nitrides present in the compound layer inhibit the nitrogen diffusion in the substratum, so that the nitro- gen amount solubilised in the S phase is reduced. 4. Conclusions The glow-discharge nitriding treatment is a useful Table 3 d-spacing values, a, c lattice parameters and c�a ratio of the S phase Ž . Ž . Ž . Ž .Sample type d 1 1 1 d 2 0 0 d 2 0 2 d 3 1 1 a c c�a Ž . Ž . Ž . Ž . Ž . Ž .nm nm nm nm nm nm �0.0001 �0.0001 �0.0001 �0.0001 �0.0002 �0.0005 �0.002 Untreated 0.2075 0.1797 0.1271 0.1084 0.3595 � � Nitrided 0.2251 0.1963 0.1373 0.1182 0.3927 0.3844 0.979 Ž .t�4 h Nitrided 0.2247 0.1962 0.1370 0.1181 0.3925 0.3829 0.975 Ž .t�8 h ( )T. Bacci et al.�Surface and Coatings Technology 139 2001 251�256256 Fig. 5. Fitting of the S phase present in the X-ray diffraction pattern of an AISI 316L sample, ion-nitrided for 4 h at 773 K, by using a f.c.t. Ž . Žlattice a�0.3927 nm, c�0.3844 nm and a f.c.c. lattice a�0.3927 .nm . tool for hardening the surface layers of stainless steel sintered components. By performing treatments at 773 K for 4 and 8 h on sintered AISI 316L austenitic, AISI 410 martensitic and AISI 430L ferritic stainless steels samples, modified surface layers are produced, consist- ing of an outer compound layer, in which iron and alloy Ž .elements nitrides ��-Fe N, �-Fe N, CrN are pre-4 2 � 3 sent and an inner diffusion layer, where nitride precipi- tates are dispersed in matrix crystals rich in nitrogen. In all the treated samples the microhardness profiles show high hardness values in the modified layers and a steep decrease to matrix values, thinner hardened lay- Ž .ers �40 �m, t�8 h and lower hardness values Ž .�1140 HK are observed on AISI 316L samples,0.025 Žin comparison with AISI 410 �110 �m, 1360 HK ,0.025 . Žt�8 h and AISI 430L �85 �m, 1370 HK , t�80.025 .h samples. 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