Different effects of carbon and nitrogen on precipitation behavior and mechanical properties in austenitic stainless steels

Abstract

Austenitic stainless steels, which are the most common type of stainless steels have good weldability and formability, and can be used in a wide range of temperatures. However, precipitation of carbides and nitrides may have detrimental effects on their mechanical properties. Fe-15Cr-15Mn-4Ni based steels were investigated in this research with different contents of 0.2C, 0.2N and 0.2C+0.2N. Precipitation behavior was observed by aging the specimens in temperature range from 600 OC to 1000 OC for up to 240 hours. Precipitates of M23C6 and Cr2N were formed. The specimens containing 0.2% of carbon formed M23C6 along grain boundaries and within grains depending on the aging condition. Cr2N, however, was formed only at grain boundaries in the specimens containing 0.2% of nitrogen. The specimens containing both carbon and nitrogen formed both carbide and nitride, the fractions of which are different. TTP (Time-Temperature-Precipitation) diagrams were constructed based on the observation. Moreover, tensile test was conducted to identify the effect of precipitation on mechanical properties. Specimens with carbon showed more decrease in elongation and increase in ultimate tensile strength (UTS), whereas ones with nitrides showed no distinct change in elongation and UTS. The different effects of nitrogen and carbon are discussed in detail.