Corrosion and Protection Research Lab, Northwestern Polytechnical University, Xi an , China

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1 Acta Metall. Sin.(Engl. Lett.) Vol.21 No.1 pp21-29 Feb IMPROVEMENT OF MECHANICAL PROPERTIES OF MARTENSITIC STAINLESS STEEL BY PLASMA NITRIDING AT LOW TEMPERATURE Y.T. Xi, D.X. Liu, D. Han Corrosion and Protection Research Lab, Northwestern Polytechnical University, Xi an , China Z.F. Han Xi an Shaangu Power Co. Ltd, Xi an , China Manuscript received 7 May Introduction A series of experiments were carried out to study the influence of low temperature plasma nitriding on the mechanical properties of AISI 420 martensitic stainless steel. Plasma nitriding experiments were carried out for 15 h at 350 CbymeansofDCpulsed plasma in 25%N 2+75%H 2 atmosphere. The microstructure, phase composition, and residual stresses profiles of the nitrided layers were determined by optical microscopy and X-ray diffraction. The microhardness profiles of the nitridied surfaces were also studied. The fatigue life, sliding wear, and erosion wear loss of the untreated specimens and plasma nitriding specimens were determined on the basis of a rotating bending fatigue tester, a ball-on-disc wear tester, and a solid particle erosion tester. The results show that the 350 C nitrided surface is dominated by ε-fe 3N and α N, which is supersaturated nitrogen solid solution. They have high hardness and chemical stabilities. So the low temperature plasma nitriding not only increases the surface hardness values but also improves the wear and erosion resistance. In addition, the fatigue limit of AISI 420 steel can also be improved by plasma nitriding at 350 C because plasma nitriding produces residual compressive stress inside the modified layer. KEY WORDS Martensitic stainless steel; Plasma nitriding; Low temperature; Mechanical properties AISI 420 is a martensitic grade stainless steel known for its high corrosion resistance and shock resistance based on a native surface oxide layer and high plasticity. It has been widely used in compressor blades and industrial blower blades. But low hardness and poor wear resistance usually lead to a short lifetime in industrial applications with intensive wear, especially when the air that flows, containing hard particles, erode the blades. In addition, blades bear many loadings at work such as cyclic loading generated by airstreams and rotating. This often leads to blade failure and sudden engine break. So the resistances to wear and fatigue are very important to stainless steel blades [1 3]. Nitriding, which is an effective technique for strengthening the surface of materials to improve their surface hardness and tribological properties, has been developed over many years. However, nitriding affects the corrosion property of a stainless steel due to the Corresponding author. Tel: ; Fax: address: sdxyt1978@mail.nwpu.edu.cn; sdxyt1978@163.com (Y.T. Xi)

2 22 diffusion of nitrogen into the steel surface. This has been well demonstrated in nitriding of austenitic stainless steel. For example, nitriding at a temperature above 500 C can produce a thick nitrided layer on the surface, which significantly improves the surface hardness and wear properties. However, corrosion resistance of the austenitic stainless steel surface is dramatically reduced after nitrided due to the formation of CrN and the depletion of free chromium in the steel matrix [4]. On the other hand, some studies show that the formation of chromium nitride can be avoided by nitriding at a low temperature of approximately 400 C because of the enough low mobility of the chromium. And an expanded austenite called γ N or S-phase can be obtained on the surface of the austenitic stainless steel. Due to the superb properties of the S-phase, the low temperature nitrided austenitic stainless steel has a very high surface hardness, a good wear resistance, and more importantly an excellent corrosion resistance [1,4 7]. In recent years, many studies have been carried out on austenitic stainless steel using the low temperature plasma nitriding technology, which is rapidly gaining industrial applications. In contrast, only limited studies have been focused so far on the detailed analysis of martensitic stainless steel, especially AISI 420 [8]. Though Kim [9] reported that the expanded martensite in comparison to S-phase also had been obtained at temperatures around C. However, systematic reports on the mechanical properties of the expanded martensite and the influences of low temperature nitriding have not been seen in publications. The wear and the erosion behavior as well as the fatigue properties are of great importance for industrial blades. The purpose of this article is to obtain a nitrided layer consisting of the expanded martensite by plasma nitriding at a low temperature about 350 C for 15 h and to study the structure, wear behavior, erosion resistance and fatigue properties of the modified layer. 2. Experimental Procedures AISI 420 martensitic stainless steel was selected as the substrate material with the following composition (wt pct): C, 0.19; Si, 0.28; Mn, 0.20; S, 0.007; P, 0.028; Cr, 12.65; Cu, 0.11; Ni, 0.12; and Fe, balance. The material was annealed at 860 C for 4h, oil quenched at 970 C for 3 h, and tempered at 670 C for 6 h. Disk-type specimens of 30 mm in diameter and 10 mm in thickness were cut from a bar. Before nitriding, the specimen surfaces were first fine-ground using wet SiC paper from 240 down to 1000-grit finish and polished mechanically. And finally, the surfaces were thoroughly degreased with acetone and ultrasonically cleaned. Fatigue testing specimens were machined to the configuration showninfig.1. The specimens were placed on the sample holder of DC-pulsed glow discharge plasma nitriding equipment, behaving as a cathode with respect to the chamber wall. Sputter cleaning was performed in an 80%Ar+20%H 2 atmosphere at about 250 Cfor1h,which would remove the oxide layers formed on the surface of the specimens. AISI 420 martensitic stainless steel was plasma nitrided for a period of 15 h at a substrate temperature of 350 C. The treatment gas was 25%N 2 +75%H 2 and the chamber pressure was controlled at 600 Pa. The substrate temperature was controlled to an accuracy of ±5 C, using a photoelectric thermometer. After the completion of the nitriding, specimens were slowly cooled in the chamber to room temperature in the presence of nitrogen flowing gas to minimize surface oxidation.

3 23 The composition and microstructure of thenitridedlayersformedontheaisi420 martensitic stainless steel at 350 C were characterized by an XJL-03 optical microscopy (OM), an X pert-pro X-ray diffraction (XRD), and a HITACHI S-570 scanning electron microscope (SEM). The microhardness of the nitrided layer was measured by an HV-1000 Knoop microhardness tester with a load of N. The residual stress profiles of the nitrided layer were determined by four-point method and chemical denudation method with D/max-2200 X-ray diffraction (XRD). In order to measure the residual stress profile of the nitrided layer, the sample Fig.1 Rotating bending fatigue testing specimens. Fig.2 Schematic diagram of solid particle erosion equipment. was denudated for 5 10 min in the 10% HNO 3 solution after each XRD measuring and the thickness of the sample was measured by a micrometer calipers. By this mean, the residual stress profile at the depth direction would be carried out. Sliding wear tests were carried out by means of a ball-on-disc wear tester without lubrication. During the tests, the stainless steel disc rotated against a stationary Si 3 N 4 ball with a diameter of 4.75 mm. A sliding speed of 6.91 m/min was used in the tests with a N constant load. The wear properties were evaluated by measuring the widths of the sliding tracks left on the surface by SEM and the wear loss by an analytical balance (accuracy of 0.1 mg). The wear mechanisms were analyzed by observing the wear surface by SEM. Erosion tests were carried out at room temperature with a modified tester according to the ASTM-G76-04 standard test method, as shown in Fig.2. The equipment was designed to feed abrasive particles into a high velocity air stream, which propelled the particles against the specimen surface. By controlling the air supply pressure the solid particle velocity could be controlled, which was measured by double rotating disk methods. In this study, the tests were carried out at impingement angles of 30 and a speed rate of 25 m/s for 6 min according to the reality industrial blades erosion phenomenon [2,3].The angular Al 2 O 3 particles in the size ranging from µm were used in the tests. Mass

4 24 losses were calculated by difference between the initial and final weight, and measured with a precision analytical balance (accuracy of 0.1 mg). SEM was used to examine the morphology of erosion surfaces. Fatigue tests were carried out with a PQ-6 rotating bending fatigue tester at room temperature. The rotating frequency was found to be 3000 rpm or 50 Hz, by applying Little s method, on estimating the median fatigue limit [10]. In order to determine the fatigue strength the used fatigue lifetime was considered to be at cycles because the AISI 420 martensitic stainless steel belongs to ferrous metals. That is to say, when the sample reaches cycles and does not fracture, the maximum fatigue strength is defined to the fatigue limit of the specimens. The minimum number of effective fatigue specimens is Results and Discussion 3.1 Microstructure characterizations Fig.3 shows the microstructure of a cross section of the AISI 420 stainless steel surface nitrided at 350 C for 15 h. A white layer of 90 µm in thickness is produced with the evident diffusion layer. The microhardness measurements are performed on the cross-sectioned sample, as shown in Fig.4. The microhardness of AISI 420 stainless steel is 289HK 0.025, while that of nitrided layer is 1229HK The latter is 4.3 times as much as the former. From the nitrided layer to the base material, the microhardness reduces homogeneously and shows a graded distribution because of the generation of the diffusion layer. Consequently, the specimens have high load carrying ability and the bond strength, between the nitrided layer and the base material due to the graded distribution of the microhardness and composition. The structure of the nitrided surfaces was analyzed by means of X-ray diffraction using CuK α radiation. Fig.5a shows that the untreated sample has produced two diffraction peaks of α(110) and α (200) in the range of Fig.5b indicates that the 350 C nitrided surface consists mainly of ε Fe 3 N iron nitride. A well defined diffraction peak, indicated as α N in Fig.5b, can also be apparently seen at The α N phase has high hardness and chemical stabilities, which is the supersaturated nitrogen solid solution. Solid solution of nitrogen expands the lattice structure of α Fe and shifted Fig.3 Microstructure of plasma nitrided specimens(with indentations of microhardness). Fig.4 Microhardness profile of the plasma nitrided layer.

5 25 Fig.5 X-ray diffraction patterns for the untreated (a) and plasma nitrided (b) AISI420 stainless steel. Fig.6 Residual stress profiles of the nitrided layer. the (110) peak (44.8 ) toward a lower diffraction angle [8]. The α N phase can be regarded as the expanded martensite suggested by Kim et al. [9], because this peak cannot be matched with any possible phases including iron, iron nitride, and chromium nitride in the nitrided surface. No CrN peaks are detected by XRD analysis, confirming that nitriding at 350 C prevents the formation of chromium nitride in the surface of the AISI 420 stainless steel. The residual stress profile of the nitrided layer was measured by four-point XRD method and chemical denudation method, as shown in Fig.6. Fig.6 shows the residual stress changing with depth z. On the specimens surface, the residual stress for the nitrided specimens is 617 MPa. That is to say, the residual stress of the surface is residual compressive stress. At depth z=15µm, the residual stress is 75 MPa and the residual compressive stress changes to residual tensile stress. The formation of these stresses inside the modified layer after plasma nitriding are mainly because of the diffusion of nitrogen and the formation of large lattice parameters phases in the nitrided layer [7]. 3.2 Wear behaviors and mechanisms Fig.7 and Fig.8 indicate the micrographs of the wear tracks of both the untreated and nitrided surfaces after a test at a load of N for 30 min. Comparison of the worn surfaces reveals quite different wear behavior toward untreated and nitrided specimens. The untreated steels have suffered severe wear. Soon after the start of the test, metallic worn debris and heavily scored surfaces were observed. A mixture of adhesion, abrasion,

6 26 Fig.7 Wear morphologies of AISI 420 stainless steel. Fig.8 Wear morphologies of the sample nitrided at 350 C. and plastic deformation was observed in and around the wear track. Fig.7 shows several parallel grooves and scratches, which are typical of a plough worn mechanism. On the contrary, the nitrided material showed polishing of the grinding marks, producing a smooth oxidized surface with a rapid transition towards a mild oxidative wear mechanism. The worn out surfaces of the nitrided AISI 420 stainless steel specimens have only shown mild abrasive wear and smooth appearance, as shown in Fig.8. Little plate-like metallic wear debris can be observed. The Fig.9 Friction coefficient of the untreated and nitrided specimens. widths of the wear scar of unnitrided specimens are 0.55 mm, twice more than that of the nitrided specimens. The weight loss of the nitrided specimens is 0.13 mg, only as much as 3% of that of the untreated specimens. As can be seen in Fig.9, the friction coefficient of nitrided specimens is 0.65, while that of untreated specimens is about 0.8. The friction coefficient of nitrided specimens is very stable; while that of untreated specimens is gradually increasing during wear tests. As a result, the reduction of the friction coefficient will improve the wear property of AISI 420 stainless steel surfaces.

7 27 All nitrided specimens exhibited a lower friction coefficient and a lower wear rate than unnitrided steel. The improvement of tribological properties for plasma nitrided AISI 420 steels is the result of a combination of microstructure and higher surface hardness of the nitrided case. The nitrided case imparts the resistance to plastic deformation to the specimens and allows oxide films to develop on the contact surface. Because oxide film is more steady than the nitrided layer, nitrogen atoms in the nitrided surface can be replaced by oxygen during the wear process based on free-energy theory [11]. This adherent oxide layer formed on the surface of nitrided specimens acts as a lubricating layer preventing metalto-ball contact, decreasing the friction coefficient, reducing adhesive wear, and therefore, reducing their wear. So a mild oxidative wear is the main mechanism dominating wear of the nitrided steel under low applied loads. In addition, the super-saturated nitrogen can introduce extremely high compressive residual stresses in the nitrided layer. These compressive residual stresses tend to impede microcrack formation during wear [12,13]. 3.3 Erosion behaviors and mechanisms The mass loss of unnitrided specimens is 8.86 mg, while that of nitrided specimens is 2.35 mg. From the results, it can be seen that the 350 C nitrided AISI420 stainless steel specimens are more resistant to erosion than unnitrided specimens. In order to gain some insight into the mechanisms of material removal, SEM is performed on the eroded surface as shown in Fig.10. Fig.10a illustrates the topology of the impact zone of the unnitrided sample. The large number of cutting lips and parallel grooves indicate the extrusion of platelets from the impact zone. The surfaces of unnitrided specimens generate severe erosion. On the contrary, in Fig.10b, relatively few cutting lips and narrow parallel grooves are observed in the impact zone of nitrided specimens, which is typical of a mild cutting wear mechanism. The 350 C nitrided surface is dominated by both ε Fe 3 Nandα N, which not only have high hardness but also introduce high compressive residual stresses. It can impede the ploughing and cutting wear generated by solid particle erosion [7]. Therefore, the erosion resistance can be improved obviously by nitriding at low temperature. 3.4 Fatigue properties The fatigue limit results of untreated and plasma nitrided specimens are given in Fig.11. The average maximum fatigue strength of the nitrided specimens is MPa, whereas the fatigue limit of untreated specimens is MPa, both at same cycles. This accounted for Fig.10 Solid particle erosion-wear morphologies of untreated (a) and 350 C nitrided (b) specimens.

8 28 Fig.11 Fatigue strength testing results of unnitrided (a) and nitrided (b) specimens. Fig.12 Fatigue fractographs of unnitrided (a) and nitrided (b) samples. more than 25% increase in fatigue limits by low temperature plasma nitriding. According to force diagram analysis of rotating bending fatigue tests, the maximum bending stress generates at the surface of fatigue specimens [14]. But the high residual compressive stress inside the modified layer after plasma nitriding can impede the generation of fatigue cracks at the nitrided layer because the internal compressive stress is superimposed to the external load and leads to a reduction of the effective stress in tension. So the fatigue crack source is transferred to the subsurface [14,15]. Fig.12 shows the fatigue fractographs of untreated and plasma nitrided specimens. From the ractographs, on the untreated specimen, it can be seen that the fatigue crack is initiated at the surface. On the contrary, on the low temperature plasma nitrided specimen, the fatigue crack source is located in the sub-surface. So the fatigue limit of AISI 420 stainless steel is increased obviously by low temperature plasma nitriding. 4. Conclusions Low temperature plasma nitriding of AISI 420 stainless steel at 350 C for 15 h increases surface hardness to 1229 HK The 350 C nitriding surface is dominated by ε Fe 3 N and α N,which is supersaturated nitrogen solid solution. They have high hardness and chemical stabilities. A reduction in wear and friction coefficient is obtained by means of 350 C plasma nitriding. The improvement of tribological properties is the result of a combination of microstructure, high surface hardness, and high compressive residual stresses of the nitrided case. The nitrided case improves resistance to plastic deformation

9 29 of the specimens and allows oxide films to develop on the contact surface. The severe metallic wear regime is replaced by a much milder wear behavior through plasma nitriding. The large number of cutting lips and parallel grooves can be seen in the topology of the impact zone of the unnitrided sample. On the contrary, there are relatively few cutting lips and narrow parallel grooves in the impact zone of 350 C nitrided specimens, which is typical of a mild cutting wear mechanism. So the erosion resistance of AISI 420 stainless steel can be improved by 350 C plasma nitriding because of the hard surface and high residual compressiveness in modified case. The fatigue strength in the high cycle fatigue range is increased due to 350 Cplasma nitriding. The plasma nitriding can push crack sources to the subsurface in high cycle bending fatigue tests due to the formation of high residual compressive stresses in the nitrided case. Acknowledgements This work was supported by National Natural Science Foundation of China (Nos and ) and National High Technical Research and Development Programme of China (No. 2007AA03Z521). REFERENCES [1] I. Alphonsa, A. Chainani, P.M. Raole, B. Ganguli and P.I. John, Surf. Coat. Technol. 150 (2002) 263. [2] M.R. Khajavi and M.H. Shariat, Eng. Fail. Anal. 11 (2004) 589. [3] T.J. Carter, Eng. Fail. Anal. 12 (2005) 237. [4] C.X. Li and T. Bell, Corros. Sci. 46 (2004) [5] Y. Sun and T. Bell, Wear 218 (1998) 34. [6] L. Wang, B. Xu Bin, Z.W. Yu and Y.Q. Shi, Surf. Coat. Technol. 130 (2000) 304. [7] E. Menthe, A. Bulak, J. Olfe, A. Zimmermann and K.T. Rie, Surf. Coat. Technol (2000) 259. [8] C.X. Li and T. Bell, Corros. Sci. 48 (2006) [9] S.K. Kim, J.S. Yoo, J.M. Preiest and M.P. Fewell, Surf. Coat. Technol (2003) 380. [10] R.E. Little, ASTM STP511 (American Soc. for Testing and Materials, 1972) p.29. [11] P. Corengia, G. Ybarra, C. Moina, A. Cabo and E. Broitman, Surf. Coat. Technol. 187 (2004) 63. [12] C. Pinedo and W. Monteiro, Surf. Coat. Technol. 179 (2004) 119. [13] P. Corengia, F. Walther, G. Ybarra, S. Sommadossi, R. Corbari and E. Broitman, Wear 260 (2006) 479. [14] D. Odhiambo and H. Soyama, Int. J. Fatigue 25 (2003) [15] C. Allen, C.X. Li, T. Bell and Y. Sun, Wear 254 (2003) 1106.