Effects of Carbon Content and Austenite Grain Size on Retained Austenite Fraction in Stir Zone of Friction Stir Welded 6%Ni Carbon Steels

Friction stir welding (FSW) was performed under the two welding conditions (rotation speed-traveling speed) of 150 rpm–100 mm/min and 200 rpm–400 mm/min using 6 mass%Ni steels with different carbon contents from 0.14 mass% to 0.63 mass%. The slightly lower peak welding temperature and the higher cooling rate were predicted under the condition of 200 rpm–400 mm/min. The effects of carbon content and prior austenite grain size on retained austenite fraction in the stir zones were evaluated. When carbon content was 0.30 mass% or more, a fine microstructure consisting of lath martensite and retained austenite was formed in the stir zone. Irrespective of welding conditions, the amount of retained austenite increased with the increase of carbon content. More retained austenite was obtained at the stir zone under the 200 rpm– 400 mm/min condition, which resulted from rapid cooling and finer prior austenite grains compared with the condition of 150 rpm–100 mm/min. The effect of the prior austenite grain size on the amount of retained austenite was successfully extracted, and the finer austenite grain size was concluded to play an important role to stabilize austenite by analyzing the difference in the martensitic start temperatures predicted based on the chemical composition and the retained austenite fraction based on Koistinen-Marburger equation.