Effect of surface carbonitriding by plasma immersion ion ...€¦ · condition and with surface carbonitriding by plasma immersion ion implantation were cyclically strained under

Effect of surface carbonitriding by plasma immersion ion implantation on the fatigue behaviour of 316L austenitic stainless steel

K. Obrtlík, Y. Jirásková, J. Man & J. Polák Institute of Physics of Materials, AS CR, Czech Republic

Abstract

Cylindrical specimens of 316L austenitic stainless steel in an as-received condition and with surface carbonitriding by plasma immersion ion implantation were cyclically strained under plastic strain control. The micro-hardness depth profile of the hardened surface layer was measured. Mössbauer spectroscopy and X-ray diffraction were used to study the phase composition of the treated layer. The stress-strain response and fatigue life data of both materials were obtained. The surface relief and fracture surfaces were examined to study fatigue damage mechanisms. Two domains separated by the transitional plastic strain amplitude could be distinguished in the fatigue behaviour of the treated steel. In the low amplitude domain, the main crack originates in the subsurface region resulting in a prolonged lifetime. In the high amplitude domain, the main crack initiates at the surface and the fatigue life of the treated steel is reduced compared to the untreated one. Keywords: austenitic stainless steel, surface carbonitriding, fatigue life curves, stress-strain response, plasma immersion ion implantation, crack initiation.

1 Introduction

Plasma immersion ion implantation is one of advanced methods of plasma engineering. This is a non-line-of-sight technique for the surface modification of industrial components, offering several inherent advantages over conventional beam-line ion implantation (Wei et al. [1]). One of the key advantages is the capability to treat efficiently irregular-shaped specimens without complex specimen or ion beam manipulation. The nitrogen and/or carbon ions to be

Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII 13

© 2005 WIT Press WIT Transactions on Engineering Sciences, Vol 49, www.witpress.com, ISSN 1743-3533 (on-line)

implanted into the near surface of the materials are extracted directly from the plasma in which the components to be processed are immersed. The beneficial effect of plasma nitriding or carburizing on the fatigue resistance of metallic materials in high cycle region has been shown many times. Both the fatigue limit and the fatigue life are increased [2-8]. A few low cycle fatigue data have shown that the beneficial effect of a nitrided layer decreases with increasing strain amplitude [3,8-12]. At high plastic strain amplitudes even the detrimental effect of surface nitriding has been reported. It can reduce the fatigue life in service loading with overload cycles [3,10,12]. In this paper, experimental results are reported on the fatigue life and the cyclic stress response of 316L austenitic stainless steel in an as-received condition and after surface carbonitriding by plasma immersion ion implantation. Surface damage observation and fracture surface examination contribute to elucidate differences in the fatigue behaviour of untreated and treated materials.

Table 1: Chemical composition of 316L steel (in wt. %).

C Si Mn P S Cr Ni Mo Fe

0.018 0.42 1.68 0.015 0.001 17.6 13.8 2.6 rest

2 Experimental

Austenitic 316L stainless steel (Uddeholm, Sweden) was obtained in the form of a 25 mm thick plate with a chemical composition as shown in Table 1. The material was in the as-received conditions, i.e., solution annealed at 1080°C and then water quenched. The heat treatment resulted in an average grain size of 39 µm (obtained using the linear intercept method without counting twin boundaries) and a hardness HV 10 = 145. Figure 1 shows the micrograph of the steel in a section parallel to the rolling axis. Cylindrical specimens with threaded ends had a gauge diameter and a length of 8 and 14 mm respectively (see Fig. 2). After machining, the specimens were annealed in a vacuum at 600 °C for 1 hour to relieve stress and the gauge section was mechanically ground and electrolytically polished. Then, plasma immersion ion implantation (PIII) was applied in the Mark I device at TU Clausthal [1]. The chamber was pumped down to a pressure below 5 x 10-6 mbar to minimize oxygen contamination and then filled with nitrogen/methane in a ratio of (25/75) %. The working pressure was approximately 3 x 10-3 mbar. The plasma was generated by 290 W of r.f. power at 13.6 MHz applied to an immersed antenna at the end of the vacuum chamber. High voltage pulses with an amplitude of -40 kV and a pulse length of 100 µs were applied to the specimens. The pulse frequency was controlled to maintain a treatment temperature of 400 °C and to achieve the treatment time 3 h. The phase composition was obtained by Conversion Electron Mössbauer Spectroscopy (CEMS) and re-emitted γ-rays Mössbauer Spectroscopy (CXMS) (Maddock [13]) performed at room temperature. The surface range, from which

14 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII


the 7.3 keV conversion electrons can escape after recoil-less resonant absorption of γ-rays in the 57Fe nuclei, is approximately 300 nm (CEMS). The re-emitted γ-rays can penetrate in the depth of about 10-30 µm thus the information on the phase composition using CXMS is in this range. The calibration was performed in both measuring modes against the standard pure α-iron foil. Computer processing of the measured spectra was carried out using the CONFIT program package (Žák [14]). The discrete single- and double-line components represent the paramagnetic (pm) phase. The six-line components and/or hyperfine induction distribution are used for the ferromagnetic (fm) phases. The individual phases were established on the basis of hyperfine parameters, hyperfine induction (fm), isomer shift, and quadrupole splitting, [mm/s] (fm, pm) by comparison with literature and in-house data. The phase contents are determined according to the corresponding sub-spectra intensities (fm, pm), i.e., from the iron atomic fractions supposing identical Lamb-Mössbauer factor for all phases present.

Figure 1: Microstructure of 316 steel in the section parallel to the loading axis.

Figure 2: Shape and dimensions of specimens (in mm).

The phase composition was confirmed by an analysis of the diffraction pattern collected using the diffractometer PANalytical X´Pert PRO equipped with a conventional X-ray tube (Co Kα radiation, 40 kV, 30 mA, point focus), an X-ray monocapillary, and a multichannel detector X'Celerator with an anti-scatter shield. This setup enabled phase analysis from 0.1 mm spots in several hours. Thus the measurement could be achieved directly on the modified surface of the sample used for the fatigue test. The penetration depth is approximately 10 µm. The measured data were analysed using HighScorePlus software and JCPDS PDF-4 database. The micro-hardness depth profile was measured using the Fischerscope H100 depth sensing indentation tester equipped with the Vickers indenter. The universal hardness HU was obtained from the loading/unloading curves.



Surface treated and untreated specimens were cycled in a computer-controlled electrohydraulic testing system in a symmetrical push-pull cycle in strain control. The strain was measured and controlled using a sensitive longitudinal extensometer with a 12 mm base. The strain rate of 3x10-3 s-1 and plastic strain amplitude, εap, derived from the width of the hysteresis loop, were kept constant in each test. When 5000 cycles were applied, the sinusoidal cycling with an average strain rate of 10-1 s-1 per cycle was imposed. The total strain amplitude in sinusoidal cycling was adjusted periodically with the “outer closed loop” using the computer in such a way that the plastic strain amplitude was kept constant and equal to that in slow rate cycling. The hysteresis loops were saved to disk memory. A few treated specimens were used to study surface damage evolution. Cycling was periodically interrupted and the specimen was removed from the testing machine for observation. Light and scanning electron microscopes were used to study surface relief evolution and fracture surfaces.

3 Results

3.1 Surface layer characterization

The simultaneous carbon/nitrogen implantation yields a complex phase composition. The analysis of the CEMS spectra revealed a presence of ε-FeN (15 %), Fe3C/Fe5C2 (24 %) in addition to dominant austenite (59 %). Moreover, a small amount of oxide (2.0 % Fe2O3) was detected as well. The ε-nitride (7.9 %) and carbide (3.5 %) phases were also obtained from the CXMS spectrum analysis. The comparison of hyperfine parameters of ε-FeN obtained from CEMS and CXMS reflects the higher nitrogen content in the layers close to the surface. The same phases were detected using the X-ray diffraction pattern analysis. Figure 3 shows the micro-hardness depth profile measured on the section perpendicular to the tensile axis using loads of 4 and 10 mN. The micro-hardness in the depth of 3 µm is about as twice as high than that of the core. The thickness of the surface treated layer was estimated to 10 µm. A decrease in micro-hardness confirms satisfactorily the change in the phase composition towards the bulk due to the decrease in content of implanted elements found by the Mössbauer spectroscopy phase analysis.

3.2 Stress response and fatigue life

Figure 4 shows the stress amplitude σa as a function of the number of cycles N for both treated and untreated specimens. The character of these hardening/softening curves is similar in both materials in the domain of low and medium plastic strain amplitudes. Cycling with the highest plastic strain amplitudes results in a continuous hardening in the untreated material. Plasma surface treated specimens show an initial rapid hardening followed by the maximum stress amplitude and by the cyclic softening stage.



Figure 3: Micro-hardness depth profile of a surface treated specimen measured in three different locations (load 10 mN).

Cyclic stress-strain curves measured on the specimens of treated and untreated steel are plotted in Fig. 5. The stress amplitude at half-life is plotted vs. the applied plastic strain amplitude. The stress response of both materials is almost identical with the exception of the highest plastic strain amplitude. The presence of the hard surface layer causes a reduction of σa at half-life in the specimen cycled with the highest εap. The reduction is consistent with the hardening/softening curves in Fig. 4 and can be explained by the different damage mechanism operating in the treated and untreated 316L steel.

Figure 4: Fatigue hardening/

softening curves of the treated (full symbols) and untreated (open symbols) 316L steel.

Figure 5: Cyclic stress-strain curve of the surface treated and untreated steel.

εap

σ a [M

Pa]



Table 2: Parameters of the Manson-Coffin and Basquin curves for the surface-treated and untreated 316L steel.

Fatigue life curves for the surface-treated and untreated 316L steel specimens are plotted in Figs. 6 and 7. The bilogarithmic representation in Fig. 6 shows that the Manson-Coffin law

log 2Nf = (1/c) log εap - (1/c) log ε ,f (1)

fits well the experimental data for both materials. The fatigue ductility coefficient, ,

fε , and the fatigue ductility exponent, c, were evaluated using regression analysis and are given in Table 2. Both Manson-Coffin curves intersect with each other at a transitional plastic strain amplitude, εapt, and this results in a transitional number of cycles to fracture, Nft = 1420. Thus, the plasma implantation gives rise to the fatigue life reduction for the high amplitude domain (εap > εapt) and to the fatigue life increase for the low amplitude domain (εap < εapt). For the lowest plastic strain amplitudes the fatigue life of the plasma treated steel augments 17 times in comparison with the untreated material.

Figure 6: Manson-Coffin diagrams for the treated and untreated steel.

Figure 7: Fatigue endurance curves for the treated and untreated steel.

The stress amplitude at half-life, σa, is plotted vs. the number of cycles to fracture, Nf, in Fig. 7 in a bilogarithmic representation. It can be seen that both

Material ε,f c σ,

f [MPa] b

316L 0.58 ± 0.11 - 0.534 ± 0.017 2110 ± 740 - 0.189 ± 0.007

316L + implantation 0.140 ± 0.027 - 0.355 ± 0.011 1250 ± 570 - 0.128 ± 0.006

316L + implantation

316L

316L + implantation

316L



Basquin dependences are curved in the low stress amplitude domain. Thus, the Basquin law

log 2Nf = (1/b) log ,fσ – (1/b) log σa (2)

fits experimental data well only in the high amplitude domain – see Fig. 7. The fatigue strength coefficient, ,

fσ , and the fatigue strength exponent, b, are given in Table 2. Both Basquin curves intersect at the number of cycles slightly higher than Nft. The reason is the decrease of σa at half-life in treated specimens in comparison with untreated ones in the highest amplitude domain (see Fig. 5).

3.3 Surface relief and fracture surface observation

Figure 8 shows the surface relief of a treated specimen cycled to 75% of the fatigue life with εap = 2x10-2. A net of surface cracks (see Fig. 8a) and pronounced slip markings (see Fig. 8b) are typical for the fatigue damage in the high amplitude domain. The cracks growing almost perpendicular to the loading axis interact and the main crack is formed. An examination of the fracture surface revealed that the main crack originated beneath the surface in specimens strained with low plastic strain amplitudes. Figure 9 shows the fracture surface of a specimen cycled with εap = 5x10-4. The white arrow in Fig. 9 denotes the site of crack initiation. Subsurface crack initiation is typical for the low amplitude domain.

Figure 8: Surface relief of a treated specimen cycled to 75% Nf with εap = 2x10-2. (a) Surface crack network, (b) details of slip bands and cracks. SEM, loading axis vertical.

4 Discussion

The detailed study of the fatigue life, cyclic stress-strain response and fatigue damage of surface plasma carbonitrided and untreated 316L stainless steel has shown an appropriate effect of surface treatment.



Figure 9: Fracture surface with subsurface fatigue crack initiation. εap = 5x104, SEM.

The effect of the plasma surface treatment on the fatigue behaviour depends on the plastic strain amplitude. Manson-Coffin curves and the corresponding Basquin curves of the treated and untreated specimens intersect with each other at a transitional amplitude (see Figs. 6 and 7). Therefore, the surface carbonitriding by PIII is detrimental for higher amplitudes and beneficial for lower amplitudes than the transitional ones. In the low amplitude domain, the lifetime of the treated steel is more than an order of magnitude longer than that of the untreated steel. Similar effect of surface treatment on the fatigue behaviour of metallic materials is also reported by Qian and Fatemi [3] in SAE 1045 hot-rolled steel, by Hussain et al. [10] in maraging steel and by Obrtlík and Polák [12] in 316L austenitic stainless steel. The transitional number of cycles to fracture (see Figs. 6 and 7) found in these works depends both on material and surface layer. It can be a suitable characteristic of the surface treatment quality. The present results show that the carbonitrided layer is strained mainly elastically in the low amplitude domain and the core material accommodates the applied plastic strain. Plastic strain localization in the core material results in crack initiation taking place in the subsurface region (see Fig. 9). Crack initiation in the subsurface region is more difficult in comparison with that in the surface grains due to constraint effects. Moreover, the initiated cracks propagate more slowly in the bulk than at the free surface, e.g. resulting from a lower stress intensity factor and the lack of oxidation. Thus, the plasma immersion ion implantation results in a prolonged fatigue life in the low strain amplitude domain. In the high amplitude domain, both the core material and the hardened surface layer are deformed plastically. The fatigue surface cracks having a depth equal to the thickness of the carbonitrided layer initiate within several percent of the lifetime. Stress and strain concentration ahead of the shallow but long surface cracks results in the fatigue crack growth into the core material. The main crack is formed by linking fatigue cracks at the surface within several percent of the lifetime - see Fig. 8. Therefore, plasma surface treatment is detrimental in the high amplitude domain manifesting itself in the fatigue life reduction in comparison with the untreated steel.



During plasma surface treatment, compressive residual stresses are introduced into the treated layer a result of an increase in nitrogen/carbon content [2,3,5,6]. For equilibrium, tensile stresses are generated in the core [2,6]. The residual stress is superimposed to the applied stress amplitude and leads to a reduction of the effective stress in tension in the case and to an enhancement of the tensile effective stress in the core. However, residual stresses were found to relax in the course of cyclic straining [3]. The effective relaxation observed implies that the influence of residual stresses on the fatigue behaviour of surface treated materials is not significant.

5 Conclusions

(i) The effect of surface carbonitriding by plasma immersion ion implantation on the fatigue behaviour of 316L austenitic stainless steel strongly depends on the applied plastic strain amplitude. Two domains separated by the transitional plastic strain amplitude can be distinguished in the fatigue life curves and in the fatigue damage mechanisms.

(ii) The hardened layer suppresses the surface crack initiation in the low amplitude domain and the dominant crack originates in the subsurface region. It results in the prolonged fatigue life in comparison with the untreated steel. The beneficial effect of the plasma surface treatment increases with a decreasing stress and strain amplitude.

(iii) In the high amplitude domain, the surface layer is deformed plastically and the main crack initiates at the surface. The fatigue life of the treated steel is reduced compared to the untreated one.

Acknowledgements

This research was financially supported by the grant No. 106/03/1265 of the Grant Agency of the Czech Republic, the grant No. 1QS200410502 of the Academy of Sciences of the Czech Republic and by the Czech Ministry of Education, Youth, and Sports under the Project No. ME 373. The authors express their thanks to Dr. V. Buršíková from MU Brno for the micro-hardness measurements and to Dipl.-Ing. K. Ritter from TU Clausthal (Germany) who has performed the PIII.

References

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Effect of surface carbonitriding by plasma immersion ion ...€¦ · condition and with surface carbonitriding by plasma immersion ion implantation were cyclically strained under