Effects of Surface Treatments on Fatigue of Powder-Forged Steels

Abstract

Fatigue of plain-carbon steels and low alloy steels in the as-powder-forged condition and after different surface treatments, namely nitriding (950K), nitro-carburizing (1020K), and carbonitriding (1330K), was undertaken at room temperature using rotating cantilever specimens. Tensile properties in both the as-forged and surface treated condition were used to assess the fatigue stress/UTS ratios of the materials. Quantitative television microscopy (QTM) was used to examine the nature, size, and distribution of non-metallic inclusions and pores in both as-forged and surface-treated materials on polished (unetched) surfaces. Some run-out fatigue specimens were also studied to evaluate porosity changes after fatigue at 10 Hz; the results indicated an increase in porosity mainly due to opening of collapsed pores under fatigue conditions. Although carbonitriding produced the largest improvement in fatigue strength (200% increase compared to untreated material) the nitrocarburizing of low alloy steels also produced beneficial effects. The influence of grain size and porosity on the fatigue behaviour of surface treated materials has been evaluated. Fractographic examination showed that crack initiation sites were invariably either non-metallic inclusions or pores located at the case/core interfaces. X-ray structural analysis of the compound layers showed the presence of ε-nitride with traces of Fe4N and α-Fe in the nitrided materials and ε-carbonitride with peaks of Fe16N2 in the nitrocarburized alloys. Dimensional growth, between 0·10 and 0·52% occurred in all surface treated materials.