Effect of shot peening on metastable austenitic stainless steels

Abstract

In this work, shot peening was performed in a metastable austenitic stainless steel EN 1.4318 (AISI 301LN) in order to evaluate its effect on austenite to martensite phase transformation and also the influence on the fatigue limit. Two different steel conditions were considered: annealed, i.e., with a fully austenitic microstructure, and cold rolled, consisting of a mixture of austenite and martensite. X-ray diffraction, electron back-scattered diffraction and focus ion beam, as well as nanoindentation techniques, were used to elucidate deformation mechanisms activated during shot peening and correlate with fatigue response. Results pointed out that extensive plastic deformation and phase transformation developed in annealed specimens as a consequence of shot peening. However, the increase of roughness and the generation of microcracks led to a limited fatigue limit improvement. In contrast, shot peened cold rolled specimens exhibited enhanced fatigue limit. In the latter case, the main factor that determined the influence on the fatigue response was the distance from the injector, followed successively by the exit speed of the shots and the coverage factor.

Introduction

Metastable austenitic stainless steels can be considered as TRIP (Transformation Induced Plasticity) steels because plastic deformation, either during forming or under service conditions, can lead to a strain-induced transformation from austenite to martensite [1]. Two types of martensite may form in austenitic stainless steels: ε and α'. ε-martensite has a hcp crystallographic structure, while α’ has a bcc one [2]. The typical transformation sequence can be summarized as γ→ε→α’, where the γ→ε transformation has been proposed for austenitic stainless steels deformed under tension, as well as by rolling [3], [4]. On the other hand, the direct transformation of austenite into α’-martensite, γ→α’, has been observed too, as found elsewhere [5]. These phase transformations may act as reinforcing mechanisms which make those steels to be candidate materials for the automotive industry, particularly for body-in-white construction, because they combine excellent formability and crash-absorbing capability, together with good corrosion resistance [6].

During service life of a component, fatigue failure may occur. Among a great variety of surface treatments, shot peening is one of the most widely used techniques to increase the resistance of metal parts to fatigue in a large range of industries, such as automotive, aerospace and petrochemical [7], [8], [9]. It consists of blasting high velocity small beads on the metal component to generate surface hardening and compressive residual stresses which oppose to the nucleation of cracks and also exert a closure effect on those cracks already nucleated, avoiding their propagation [10], [11]. Shot peening usually produces detrimental surface effects too, such as increasing roughness, nevertheless it has been shown that the average effect of both beneficial phenomena is more important than those induced by superficial detrimental effects [12], [13]. Recent studies [14], [15], [16], demonstrated that the generation of a nanograined layer over specimens’ surface results in a fatigue strength improvement. However, this is true as long as the density and size of surface defects induced by shot peening remain not significant. Otherwise the degradation of the fatigue resistance behavior of excessively shot peened components have been clearly observed [17], [18], [19].

Numerous investigations have shown the beneficial effects of shot peening on austenitic stainless steels [20], [21], [22], [23], [24], describing the role of residual stresses on fatigue life. However, scarce information exists related to metastable austenitic stainless grades. Kleber et al. [25] measured the content of martensite induced by shot peening, as a function of depth from the surface, using magnetic Barkhausen noise, while Peyre et al. [26] quantified the amount of martensite induced by both conventional shot peening and also laser peening in order to correlate it with the pitting corrosion resistance. In the present work, the relationship between microstructural changes induced by shot peening and fatigue behavior of the metastable steel was studied. Two different steel conditions were selected: annealed (with fully austenitic microstructure), and cold rolled (with a biphasic microstructure composed by austenite and martensite). Effect of shot peening was evaluated not only considering the induced martensite but also the influence of the pre-existing martensite produced by cold rolling.