The metallurgical phase transformations in ROQ-tuf AD690 due to the MMA welding process

Abstract

Phase transformations in steel are of profound commercial importance due to the fact that they influence the physical and mechanical properties of the end product. During the casting process, which occurs at temperatures in excess of 1500°C, the microstructure of the steel is austenitic. During slow cooling the austenite will decompose into lower temperature microstructures such as ferrite, pearlite or hard, brittle cementite, the latter becoming increasingly present the higher the carbon content of the steel. Steel manufacturers, however, make use of additional forming processes such as rolling, quenching and reheat or tempering to impart specific mechanical or metallurgical properties to a particular steel. If the cooling rate is increased, as would be the case during a quenching process, the austenite will decompose forming a needle like microstructure called bainite in addition to the other microstructures. A further increase in the cooling rate will result in the diffusionless transformation of part of the austenite to form martensite, a hard brittle microstructure. If heat was now to be applied to the steel, as in a tempering process, some of the martensite formed during the quenching process will again transform into a softer microstructure such as pearlite or ferrite.