Effect of austenitizing temperature on microstructure formation of quenched X65 micro-alloyed pipeline steel

The effect of austenitizing temperature on the microstructure formation in micro alloyed X65 pipeline steel was investigated using an optical microscope (OM), transmission electron microscope (TEM) and microhardness measurement. The results show that the austenite grains coarsening temperature of X65 steel reaches 1100 degrees C. When austenitization is carried out below 1000 degrees C, a large number of fine microalloying carbides precipitate during quenching within some a ferrite laths, with a uniform orientation. With austenitizing temperature increasing, the amount of precipitated carbides increases. Such fine carbide precipitates play significant strengthening for the steel. When the austenitizing temperature exceeds 1000 degrees C, no carbide precipitation occurs in a ferrite laths, because microalloying solute atoms have too low mobility to precipitate under such conditions. This results in that X65 steel exhibits the maximum microhardness when quenched from 1000 degrees C.