Observation of embrittlement in Cr-Mo Steels

Abstract

The technical importance of temper cmbrittlement anses from thc fact that many of the common low alloy steels exhibit an increase In thelr ductilc-to-brittle transllion tempcrahlre after being heated in the range of 350"-575'C or slowly cooled through this temperature range. Low-Alloy steels, in seyer,,1 fomls, arc used extensively in a ~ariety of applications th<lt involve exposure to elevated temperaturcs and, possibly, reactive cnvirorunenls. Cr-Mo steels arc widely used low-alloy steels whose typical applicatlOn~ include eqUIpment for gencration of electric power, chemical process equipment, etc. Thc properties of Cr-Mo steels change consider"bly due \0 long timc service at the tempcrature range of 370' to 650°C. In this present investigation the importance of gram Slte in influencing the bntt1e fractures of 2.25Cr-IMo steels having tempered martensitie microstructures is discussed by measuring the microscopic fracture stress (arl at -196"C. For doing this three different grain sizes have been produced by changing austenitizing temperature. The amount of grain boulld~ry impurity element segregation has <1150been changed by isothermal embritt1cment at 520"C for (hfferent periods as 100 and 200 hours, It has been fOClndthat grain size controls the brittle fracture mode of the embrittled steels. For lower grain size fracture mode is always dominated by transgrallular cleavage frachlre. however, it changes to mixed mode (transgranular cleavage and intergranular decohesion) in association with the deterioratlOn of toughness value ofthe steel when the grain size is increased.