The effects of varying soaking times at two hardening temperatures on material properties and morphology for the martensitic ferritic steel S165M

Abstract

The purpose of the thesis was to examine if the material properties and/or the morphology of a martensitic ferritic stainless steel were altered if exposed to varying soaking times at two distinct hardening temperatures. The background for the thesis was the need to analyze and verify the optimality of the hardening processes currently performed at NOMAC – Norwegian Material Center of Expertise AS. The analyzed steel was hot-forged S165M (W. nr. 1.4418, X4CrNiMo 16-5-1). The two hardening temperatures were 1000 °C and 1030 °C. The soaking times analyzed were 1, 2, 5, 10 and 20 hours, for both temperatures. Both as-quenched and tempered samples were examined.

Standardized mechanical testing was performed, and included tensile stress testing, hardness Vickers testing and Charpy-V impact testing. The mechanical properties remained statistically consistent. Neither the as-quenched nor the tempered samples showed any indication that the soaking times directly altered said properties. The two different temperatures produced statistically equivalent results. The majority of the observed differences in properties between as-quenched and tempered samples were as expected. The gathered impact energy values did not differentiate between as-quenched/tempered samples, leading to a debate regarding their validity. The general conclusion is that the mechanical properties are not correlated with longer/shorter soaking times.

The material morphology was analyzed using optical light microscopy, and SEM (including EDS and EBSD analysis). The use of different temperatures had little to no effect on the final microstructure. Morphologically, the material experienced substantial changes when exposed to longer soaking times. Longer soaking times introduced the growth of the average martensitic (i.e. prior austenitic) grain diameters, where the growth exceeded 80 % after a soaking time of 20 hours. The longer soaking times also caused a reduced ferrite fraction for samples hardened at 1000 °C (the fraction of ferrite was 0.19 in sample soaked for 1 hour, 0.11 in samples soaked for 20 hours). This caused a subsequent increase in the martensite fraction. The general ferritic grain shapes were also affected with longer soaking times, transforming from lacy/vermicular grain shapes into globular grains. With regards to material properties, several of these changes counteract each other. It is therefore believed that the combination of the martensitic grain growth and the reduction of ferrite content yields approximately constant mechanical properties. How these morphological changes alter the mechanical properties when occurring individually is not examined.

The results of the thesis indicate that S165M is a remarkably resistant alloy, producing consistent qualities regardless of the soaking time and/or the hardening temperature.