Deep cryogenic treatment of AISI 302 stainless steel: Part I – Hardness and tensile properties

Abstract

The effects of deep cryogenic treatment (DCT) on the static mechanical properties of the AISI 302 austenitic stainless steel were investigated through experimental testing. The results of the tensile and hardness tests are discussed and compared to data and microstructural observations from the DCT literature concerning the same class of steel. In addition, the influence of two important treatment parameters, such as the soaking-time and the minimum temperature, is analysed through a full factorial design of experiments (DOE) and by means of a first approximation model in order to obtain confirmation and suggestions about the possible use of the DCT as a standard practice to improve the mechanical properties of stainless steels. A particular focus is given to the registered changes of the elastic modulus and of the hardness as representative measures of two different deformation mechanisms.

Introduction

The use of deep cryogenic treatment (DCT) on metallic alloys has grown during the last decades, starting from the USA and expanding now to Europe and India as a practice to enhance mechanical properties. Since the mechanisms behind this treatment is not still clear at all, it is difficult to predict its effects on a particular alloy. As a consequence, a specific experimental testing is needed for each material to be treated. Interesting positive effects were noticed on tool steels [1], on carburized steels [2], [3], [4], [5], on cast irons [6], [7], [8] and on other materials [9], [10], as resumed in a wide bibliographic review [11].

In particular, when focusing on austenitic stainless steels, the overall picture of DCT effects is not very clear. While the DCT effects on tool steels and on carburized steels are often ascribed to the retained austenite elimination, to the precipitation of fine dispersed carbides and/or to a change in residual stresses, a totally different mechanism is claimed to act on austenitic stainless steels by the available literature, which consists of two main research works. The first one was conducted at the Precision and Intelligence Laboratory of the Tokyo Institute of Technology [12], [13], [14]. After measuring the M_s (martensite-start temperature) of a stainless steel with the acoustic emission technique, the material specimens were cooled down until 3 K above such temperature and then returned to the room temperature. Before the treatment, all the specimens were pre-strained (from 2% to 10%) in order to increase the dislocation density. The experimental testing did not pointed out any effects on the tensile properties of the tested materials (AISI 304 and AISI 316). However, cryotreated samples showed longer life in the high-cycle fatigue regime. No details are reported about fatigue data dispersion and, unfortunately, the effects on hardness were not analysed in these works. The authors of these investigations claimed that, by controlling the dislocation density and the treatment temperature, it is possible to control the amount and the size of the martensite in the material, obtaining a dislocation pinning effect at the intersection of two partial dislocations. The hypothesis was supported by the transmission electron microscopy (TEM) observation of nano-martensite that was indicated as responsible for the prolongation of the nucleation phase. The second research work about DCT on austenitic stainless steels was published by Singh et al. [15], [16]. The DCT effects on hardness and on crack propagation rates of welded cruciform joints made by AISI 304L austenitic stainless steel were measured in this case. An extension of the crack initiation process in DCT specimens was noticed by the authors and it was ascribed to the presence of strain-induced martensite, which formed during the cooling process, in agreement with [12], [13], [14]. In addition, the Vickers hardness of the material was increased of about 19% with the DCT. The authors observed a different residual stress configuration near the welded metal after DCT, with a relieving of the tensile residual stresses and the generation of a beneficial compressive stress field.

The analysis of the above reported state of art points out two main aspects of DCT that need for a deeper investigation. The first one involves the impact of the DCT on the elastic modulus and on the hardness. It must be noticed that the tensile comparison reported in [12] is given through the whole σ–ε plot, on a scale where also a difference of several tens GPa on the Young’s modulus cannot be highlighted. In addition, the claimed dislocation-pinning mechanism should be noticed also on the hardness of the treated material, as in the case of carburized and tool steels. Secondly, it is not clear if the above results are strictly related to the particular cryotreatment performed in the first research work or if a generic standard DCT could produce the same effects. The present article resumes the methodology and the results of an experimental test campaign that was performed at the Politecnico di Torino in order to clarify the above aspects.