Elsevier

Materials Letters

Volume 214, 1 March 2018, Pages 240-242
Materials Letters

Exceptionally high strength and good ductility in an ultrafine-grained 316L steel processed by severe plastic deformation and subsequent annealing

https://doi.org/10.1016/j.matlet.2017.12.040Get rights and content

Highlights

  • 316L steel with a nanocrystalline microstructure was processed by HPT.

  • The effect of annealing on the tensile properties was studied.

  • Annealing to 740 K yielded a pronounced embrittlement and a strength reduction.

  • Annealing to 1000 K led to a good combination of strength and elongation to failure.

  • The excellent tensile behavior was caused by the very fine austenitic microstructure.

Abstract

An investigation was conducted to evaluate the effect of annealing at different temperatures on the tensile properties of ultrafine-grained 316L stainless steel processed by high-pressure torsion (HPT). A “moderate-temperature” annealing at 740 K resulted in reduced strength and elongation due to the annihilation of mobile dislocations. A “high-temperature” annealing at 1000 K yielded a remarkably good combination of yield strength (∼1330 MPa) and elongation to failure (∼43%) which can be attributed to the almost full reversion of α′-martensite formed during HPT into γ-austenite while the grain size remained very fine with a value of about 200 nm.

Introduction

The 316L stainless is a frequently used material in both medicine and industry due to its excellent properties, such as a good corrosion resistance and the low susceptibility to neutron absorption [1]. The main crystalline phases in 316L steel are the face-centered cubic (fcc) γ-austenite and the body-centred cubic (bcc) α′-martensite. The good ductility of γ-austenite is usually accompanied by a relatively low strength which can limit its applications under high loads. At the same time, the high strength α′-martensite with bcc structure exhibits low ductility. Therefore, the ability to process 316L steel with high strength and good ductility is a challenging task.

The strength of ductile austenitic steels can be increased by grain refinement using severe plastic deformation (SPD) techniques such as high-pressure torsion (HPT) and hydrostatic extrusion [2], [3]. However, during SPD there is a simultaneous phase transformation from γ-austenite to α′-martensite [2]. It was suggested that annealing after SPD may yield fine austenite grains [4], [5] so that a combination of high strength and good ductility is feasible. The present research was initiated to study the tensile properties of samples processed by HPT and annealed at different temperatures. Our study shows that a combination of HPT-processing and subsequent annealing to 1000 K yields an exceptional combination of high strength and good elongation to failure by comparison with the available literature data for 316L steel.

Section snippets

Material and methods

316L stainless steel samples with a coarse-grained single phase γ-austenite structure were HPT-processed for 20 turns at room temperature (RT) with an applied pressure of 6.0 GPa and rotation speed of 1 rpm. The thickness and the diameter of the HPT-processed disks were ∼0.75 mm and ∼10 mm, respectively. In former studies, the microstructure and the phase composition evolution during HPT processing and subsequent annealing were investigated in detail [2], [5]. Differential scanning calorimetry

Results and discussion

The yield strength, the ultimate tensile strength, the uniform elongation and the elongation to failure for the initial coarse-grained sample, for the HPT-processed specimens and for the samples annealed to 740 and 1000 K were determined from the tensile stress-strain curves (not shown) and the evolution of these data is plotted in Fig. 1. In order to explain the changes in the tensile properties of 316L steel during annealing, the evolution of the γ-austenite fraction, the grain size and the

Conclusions

  • 1.

    HPT processing dramatically increased the yield strength while the elongation to failure significantly decreased. The extremely high strength and limited ductility of the HPT-processed sample were attributed to the high fraction of α′-martensite, the small grain size and the high dislocation density.

  • 2.

    DSC annealing of the HPT-processed sample to a moderate temperature of 740 K led to a pronounced embrittlement and strength reduction. The reduction of ductility was most probably caused by the

Acknowledgements

The authors are indebted to the Hungarian Scientific Research Fund, OTKA, Grant No. K 109021 for financial support. Three of the authors were supported by the European Research Council under ERC Grant Agreement No. 267464-SPDMETALS (PHRP, YH and TGL). Park would like to acknowledge support from the NRF-2016-Fostering Core Leaders of the Future Basic Science Program/Global Ph.D. Fellowship Program (2016H1A2A1909161).

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