Annealing behavior of nano-crystalline austenitic SUS316L produced by HPT

Abstract

Nano-crystalline (NC) 316L austenitic stainless steel sample was prepared by means of High Pressure Torsion (HPT) and post-deformation annealing. Uniaxial tensile tests at room temperature showed the nano-crystalline sample exhibits extreme high yield strength up to 2230 MPa, which is the highest ever reported in the literature.

Introduction

316L austenitic stainless steel is one of the most widely used engineering material due to its excellent corrosion and oxidation resistance and good formability. However, the low mechanical strength and poor anti-friction properties of this material are the main obstacles hindering its application. Much attention has been drawn in the past decades to strengthen the stainless steel and various approaches have been developed, such as varying its chemical compositions to induce solid solution hardening [1], [2] and grain refinement [3], [4], [5].

To get higher strength, several techniques have been developed on refining grains of 316L stainless steel via plastic deformation and/or subsequent recrystallization, including mechanical milling (MM) [3], cold rolling (CR) [4], and severe plastic deformation (SPD) [5]. 316L powders were mechanically milled and sintered at 1173 K for 3.6 ks, forming a bulk sample with an ultrafine-grained structure (average grain size ∼250 nm) [3]. Its hardness is up to 6.2 GPa, which is about 4.4 times that of coarse-grained (CG) counterpart. An ultrafine-grained 316L sample was produced by means of heavy cold working at 77 K followed by annealing at 973 K, of which the yield strength (σ_y) is as high as 1280 MPa [4]. However, during these processes martensite transformation occurred as well as the grain refinement. Presence of a considerable amount of martensite in the processed samples deteriorates corrosion resistance of this material [6]. Thereby, it is of significance to develop a processing technique not only effectively refines grains but also keep the austenitic structure of 316L. Chen et al. produced a nano-crystalline layer of 20 nm thickness by means of the SMAT technique and reached the yield strength of 1450 MPa for the 316L without martensite phase. However, the ductility was drastically decreased to only about 3.4±0.4% [7].

High Pressure Torsion (HPT) is a recently developed technique that can effectively induce grain refinement into nanometer regime of bulk materials [8], [9], [10], [11] and is the most versatile processing technique for laboratory applications [12]. Since the geometry of the sample is constant during the deformation process, different processing parameters (applied strain, strain rate, deformation temperature, and pressure) can be easily performed on the samples. Thus, nano-crystalline (NC) samples produced by HPT are free of contamination [9].

In this work, NC 316L was produced by means of the HPT technique. Mechanical properties of the NC 316L samples were investigated with respect to its microstructural characteristics.