Microstructural characterization of plasma nitrided austenitic stainless steel

Microstructural characterization of plasma nitrided austenitic stainless steel

Surface and Coatings Technology 132 Ž2000. 270᎐274 Microstructural characterization of plasma nitrided austenitic stainless steel X.L. XuU , L. Wang,...

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Surface and Coatings Technology 132 Ž2000. 270᎐274

Microstructural characterization of plasma nitrided austenitic stainless steel X.L. XuU , L. Wang, Z.W. Yu, Z.K. Hei Institute of Metal and Technology, Dalian Maritime Uni¨ ersity, Dalian, 116024, PR China Received 8 June 1999; received in revised form 17 July 2000; accepted 17 July 2000

Abstract The microstructure of the layers produced by plasma nitriding austenitic stainless steel at different treatment temperatures Ž400 and 500⬚C. were studied by transmission electron microscopy ŽTEM. together with X-ray diffraction ŽXRD. and scanning electron microscopy ŽSEM.. The results show that the microstructures were composed of ‘expanded austenite’ Ž ␥ N . and ␣ Žferrite . q CrN following plasma nitriding at lower and higher treatment temperatures, respectively. The former contains stacking faults and deformed twin substructures, while the latter is made up of colonies displaying a lamellar structure. Kurdjumov᎐Sachs or Nishiyama᎐Wassermann orientation relationships between the ␣ and CrN layer were observed. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Plasma nitriding; Austenitic stainless steel; Microstructure

1. Introduction In order to raise the wear-resistance of austenitic stainless steel, many surface modification technologies have been used, such as plasma nitriding, plasma source ion implantation ŽPSII. and plasma immersion ion implantation ŽPIII. w1᎐5x. Plasma nitriding technology has been commercially used in industrial production w6᎐8x to improve the fatigue, anti-corrosion and tribological properties of steels. However, for austenitic stainless steel, the nitriding process may reduce the corrosion resistance owing to changes in the structure of the surface layers w9᎐11x. There are different microstructures corresponding to different treatment temperatures, on which the properties of the plasma nitriding layer are dependent. It is well known that the forma-

tion of an ‘S’ or ‘expanded austenite’ Ž ␥ N . w1,11,12x phase layer without CrN precipitation should maintain the good corrosion resistance of stainless steels. Although many studies on the structure of plasma nitrided layers on austenitic stainless steel have been done by X-ray diffraction ŽXRD. and scanning electron microscopy ŽSEM. w3,6,13x and by transmission electron microscopy ŽTEM. w14x, this is not enough to fully understand the nature of the plasma nitrided layer on austenitic stainless steel which is considerably different from those of the usual ferrite steels w15x. In this study the microstructures of plasma nitrided layers with different phase constituents on austenitic stainless steel were studied by TEM.

2. Experimental methods U

Corresponding author. Tel.: q86-411-472-7975; fax: q86-411467-1395. E-mail address: [email protected] ŽX.L. Xu..

An annealed AISI 304 austenitic stainless steel was used as the substrate material in this study. The composition of the material was Žin wt.%. C: 0.15, Mn: 2.0,

0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 9 0 5 - 1

X.L. Xu et al. r Surface and Coatings Technology 132 (2000) 270᎐274

Si: 1.0, Ni: 8, Cr: 18, Fe balance. Plasma nitriding was carried out in a commercial plasma-nitriding furnace made in China. The samples were plasma nitrided for 1 h at ; 400᎐420⬚C and 500᎐520⬚C in a 4N2rH 2 atmosphere at a total pressure of 0.3 Pa. The samples for TEM were prepared by grinding and mechanical polishing from the untreated side to 40 ␮m, followed by ion-thinning from the untreated side to make thin foils of the nitrided layer. The TEM observations were made with a Hitach 800 transmission electron microscope at an acceleration voltage of 175 kV. The phases present in the plasma-nitrided layer were determined by X-ray diffraction on a Rigaku Drmax-IIIA diffractometer, in which the Seemann᎐Bohlin diffraction geometry and a fixed incident angle of 10⬚ were used. The radiation used was CuK ␣ Ž ␭ s 0.1542 nm.. SEM observations were made by a JEOL-35 CF scanning microscope.

3. Results and discussion SEM micrographs of polished cross-sections through plasma nitrided layers produced at the two different temperatures are shown in Fig. 1 Ž400⬚C. and Fig. 2 Ž500⬚C.; the layers are, respectively, approximately 4 and 6 ␮m thick.

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Fig. 2. SEM morphology of plasma nitrided sample Ž500⬚C..

and associated stress caused by the nitrogen remaining in solid solution in the fcc lattice. From Fig. 3c, it can be concluded that the plasma nitrided layer produced at 500⬚C is composed mainly of the ␣-ferrite and CrN phases. It is suggested the CrN would precipitate when raising the treatment temperature Ž500⬚C.. It should be noted that although the X-ray penetration depth is estimated to be approximately 2.5 ␮m under our diffraction condition, XRD results can be representative of the whole layers due to the homogeneity of phases constituents of the layers Žseen from Fig. 1 and Fig. 2.. 3.2. TEM results

3.1. XRD results XRD patterns of the plasma nitrided and untreated samples are shown in Fig. 3. It is clear that the untreated sample ŽFig. 3a. is single phase ␥-Fe and its peaks are sharp which is typical of annealed materials. From Fig. 3b, it can be seen that for the plasma nitrided sample at 400⬚C, all peaks were shifted to lower angles and were broadened considerably. Peaks appear at 2␪ s 41.925⬚, 46.625⬚ and 71.775⬚ corresponding to inter-planar spacings of ds 0.215 nm, 0.194 nm and 0.131 nm, respectively. The shift and broadening of the peaks are associated with the ‘expanded austenite’ Ž ␥ N . phase w1,5x produced by nitrogen supersaturation

Fig. 1. SEM morphology of plasma nitrided sample Ž400⬚C..

The submicron structural features of the plasma nitrided layer produced at 400⬚C are shown in the TEM bright field and dark field images of Fig. 4. Heavy stacking fault and high dislocation density regions were observed. Electron diffraction patterns ŽEDP. indicated that an fcc phase with a lattice parameter of 0.372 nm, which is greater than that expected of the austenite Ž ␥ . parent phase, had been produced. This is in agreement with the XRD results. In other words, the expanded austenite Ž ␥ N . had formed due to supersaturation with nitrogen atoms when plasma nitriding at 400⬚C. Stacking faults are a prominent feature in ␥ N . The increase in stacking faults may result from the effects of nitrogen, which should be identical with the effects of hydrogen on stainless steel. When charging hydrogen into type 304 stainless steel, a solid solution supersaturated with hydrogen is formed. Hydrogen expands the austenite lattice, producing dislocations and stacking faults because hydrogen lowers the stacking fault energy of austenite w16x. As either hydrogen or nitrogen atoms supersaturated in an fcc lattice of austenite tend to position themselves at octahedral interstitial sites, it can be expected that both will have analogous effects on austenite. When alloys with a low stacking fault energy produce faults, they do so in clusters or bundles w17x. It should be noted in Fig. 4 that the field images exhibit two sets of bundles and there are two sets of

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Ž1 1 1 . C rN r r Ž0 1 1 . ␣ ; w0 1 1 x C rN r r w1 1 1 x ␣ a n d Ž111. CrNrrŽ110.␣ ; w110xCrN rrw001x ␣ . At this temperature, the mobility of the substitutional atoms is sufficiently large that equilibrium should be approached rapidly. The diffusivity of substitutional atoms is greatly increased as the treatment temperature is raised and the bonding between Cr and N is greater than that of other substitutional atoms with N, so the CrN can nucleate and grow. It is suggested that the nitrogen solid solution in austenite ␥ŽN. would decompose into ␣ and CrN if the temperature is increased. From its pearlite-type morphology, it can be concluded that the transformation of ␥ŽN. ª ␣ q CrN forms via a cellular mechanism. Collins et al. w20x suggest that the bcc phase could be martenitic and is produced by residual

Fig. 3. XRD patterns of austenite stainless steels. Ža. Untreated, Žb. after plasma nitriding at 400⬚C, Žc. after plasma nitriding at 500⬚C.

diffuse streaks in EDP which can also be attributed to a bundle substructure. By trace analysis of TEM images and EDP w18x it can be observed that the two streaks are, respectively, normal to the each of two sets of bundles. Based on many observations, it has been found that there are typically a lot of bundles with a twinned relationship in the ␥ N matrix ŽFig. 5.. Their morphology is different from that of growth twins, which do not show a bundled substructure, so it is suggested that these are generally deformation twins caused by stress. It is concluded that the strain associated with nitrogen supersaturation is responsible for deformation twins and the associated stacking fault substructure. The morphology of the plasma nitrided layer produced at 500⬚C is shown in Fig. 6. It is made up of colonies displaying a lamellar structure. Analysis by EDP indicates that each of the colonies consists of ␣ Žferrite or martenite. and CrN. Both phases in each colony have Kurdjumov᎐Sachs ŽK᎐S. or Nishiyama᎐Wassermann ŽN᎐W. relationships w19x, which are

Fig. 4. TEM results showing stacking fault substructure of ␥ N phase produced by plasma nitriding at 400⬚C: Ža. bright field, Žb. dark field, Žc. electron diffraction pattern of Ža. along w112x zone axis.

X.L. Xu et al. r Surface and Coatings Technology 132 (2000) 270᎐274

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ª ␥ q CrN should occur by a cellular mechanism. It is, however, difficult to explain the formation of the ␣ phase in the process of precipitating CrN, only taking account of the effect of the element Cr. The effect of other alloying elements should also be taken into account; however, further research is need. Due to the precipitation of CrN, the corrosion-resistance of the layer is greatly decreased Žsee Ichii et al. w12x.. This is shown qualitatively in the etched microstructures of Fig. 1 and Fig. 2, where the ␣ q CrN layer ŽFig. 2. suffers severe pitting corrosion when the Žuntreated. substrate does not; in contrast, the ␥ N layer still shows good corrosion-resistance ŽFig. 1. although the Žaustenite . substrate has suffered pitting corrosion under the stronger etching conditions employed in this case; Žetching solution: CuSO4 .5H 2 O Ž20

Fig. 5. Deformed twin substructure of the ␥ N phase: Ža. bright field, Žb. dark field, Žc. electron diffraction of Ža. along w345x zone axis Ž020 T twin diffraction..

internal stress. However, it is well known that transformations by the cellular mechanism are controlled by diffusion. From the morphology of the lamellar structure, it is suggested that this is most likely to be ferrite rather than martenite. There are no reasonable and detailed explanations on the mechanism of the transformation of ␥ ª ␣ in precipitating CrN in previous studies. According to the known effects of alloying elements on the stability of ␥ and ␣ , it can be shown that Cr is a ferrite stabilizing element. Hence, the CrN precipitation should make ␥ more stable. It therefore seems more reasonable that the transformation of ␥ŽN.

Fig. 6. Microstructure of CrNq ␣ produced by plasma nitriding at 500⬚C: Ža. morphology, Žb. EDP ŽK᎐S relationship., Žc. EDP ŽN᎐W relationship..

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g. q HCl Ž100 ml. q H 2 SO4 Ž5 ml. q H 2 O Ž80 ml., etching time in Fig. 1 was longer than that in Fig. 2..

4. Conclusions Plasma nitrided layers on austenitic stainless steel treated at 400⬚C consist of the ␥ N or S-phase. The layers show stacking faults and deformed twin substructures, which are attributed to the decreased stacking fault energy resulting from nitrogen supersaturation. Plasma nitrided layers on austenitic stainless steel treated at 500⬚C are made up of colonies of ␣ q CrN, which exhibit a lamellar substructure caused by a cellular transformation mechanism. The two phases in each colony exhibit K᎐S or N᎐W relationships with respect to each other.

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