Effect of the material surface hardness on the erosion of AISI316

Abstract

Erosion due to impact of hard and abrasive particles is a common phenomenon observed in many industrial situations. A number of operating variables such as the impact angle and impact velocity affect the erosion process. The rate of erosion is influenced by the relative hardness of the target material and the particles impacting on the surface.In the present work, erosion of austenitic stainless steel has been examined under varying conditions. The hardness of the steel specimens was changed by cold rolling and case hardening of the surface by nitriding. Several specimens having different hardness were prepared using the above techniques. These samples were tested in an air jet impingement tester (as per ASTM G76 standard). Silica sand particles having mean particle size of 100, 145 and 200 μm were used for impingement on the steel surface of varying hardness. The specimens were tested at impact angles varying between 15° and 90° and the impact velocity of the particles was changed between 20 and 32 m/s. The tests were also conducted at specimen temperature varying between the room temperature to 150 °C. The eroded specimens were examined in an electron microscope to study the material removal process. This paper discusses the results of this extensive test program to study the erosive wear behavior of AISI 316 austenitic stainless steel specimens.

Introduction

Solid particle erosion caused due to the impact of high velocity particles entrained in a fluid stream is a common occurrence in many industrial situations. Erosion of fan and gas turbine blades, bends and valves in bulk material handling applications and surfaces of bins and hoppers are some of the numerous such situations. Improving the erosion resistance of engineering materials requires an understanding of the mechanism by which the material is removed. In case of ductile materials, the material removal is through cutting and ploughing while in case of brittle erosion it is through flake fragmentation and removal of flakes.

Fig. 1 illustrates the variation of erosion with impact angle for two different materials and is typical of the early work carried out to investigate the influence of these variables.

Both the materials showed very significant differences in both erosion rate and the effect of impact angle. A ductile material suffers maximum erosion at an impact angle of about 20° and offer good erosion resistance to normal impact, whereas brittle materials suffer severe erosion under normal impact (90°) but offer good erosion resistance at low angle, glancing impact. These relationships can be used to explain a number of observed phenomena in erosive wear, and are particularly useful in predicting the possible behavior in new and untried situations, Tilly [1].

Erosive wear usually occurs if the particle hardness is greater than the material hardness [2]. The improvement in the hardness with ductility of the material results in increased resistance to erosion [3]. Over the last few decades the solid particle erosive wear behavior of the materials by improving their erosion resistance by cold working and heat treatment has been discussed in detail. In a review [4], Sundararajan concluded that repeated experiments have clearly shown that neither heat treatment nor cold working of the target has any effect on erosion resistance. In another review [5] Naim and Bahadur reported that the prior cold working of the samples increases the incubation period for the onset of erosion. Also due to an increase in the initial level of cold working, a higher rate of erosion was observed for both the normal and the oblique impact conditions. Goretta investigated the erosion resistance of copper, nickel and 304 stainless steel with sharp alumina particles [3]. He concluded that work hardening improved the erosion resistance of the copper, which has high ductility. Higher hardness also improves the erosion resistance of the material if it retains sufficient ductility. Rao et al. evaluated the resistance of a laser-surface-melted 0.4% C low alloy steel to solid particle erosion [6]. Erosion tests were carried out at two impact velocities (46 and 96 m/s) and three impact angles (30°, 60° and 90°). The results indicated that laser surface hardening does not improve the erosion resistance of 0.4% C steel.

Molian et al. compared the erosion rate of untreated sample with laser heat-treated sample [7]. By comparison they revealed that the erosion rates of laser heat treated sample was substantially lower than that of untreated sample and they also found that the material removal is through micro machining and ploughing in untreated condition and through inter granular cracking in laser heat treated condition. Chen et al. [8] studied the solid–liquid erosion behavior of ion-nitrided S48C carbon steel and pure titanium and Ti6Al4V alloys. The loss of material due to erosion for ion-nitrided metals reduced considerably in S48C. Both titanium and Ti6Al4V showed no significant change due to their limited case depth and brittle compound surface layer. The compound layer did contribute erosion resistance at low-angle impingement. It has been observed from the literature survey, that, limited studies have been carried out on the influence of cold rolling and nitriding on the erosive wear behavior of the target materials like steels. However, for the same target material no work has been reported on the comparative erosive wear behavior investigation of the cold rolling and nitriding. Therefore, the present work was undertaken to examine‘ the potential of these methods for reduced erosive wear behavior of the target material. SS316 was selected for the purpose of this investigation. The erosive wear behavior of the cold rolled and nitrided specimens of varying hardness was investigated for a wide range of operating parameters such as the impact velocity, mean particle size, impact angle and temperature of the specimen. The material removal mechanism of the eroded samples was analyzed using scanning electron microscope.