L80 pipe steel microstructure assessment using ultrasonic testing

The potential exists to use ultrasonic shear velocity for real-time microstructure assessment of the quenching step in the heat treatment of L80 steel pipe. L80 steel samples were austenitised and subsequently cooled in different quench mediums (water, oil, heated oil, air and furnace) to produce microstructures ranging from primarily martensite to coarse ferrite/pearlite mixed structures. Following heat treatment, the samples were ultrasonically tested, tensile and hardness tested and metallographically examined. The shear wave velocity was observed to increase as the underlying microstructure of each sample changed from primarily martensite, to primarily bainite and finally to coarse ferrite + pearlite. The measured shear wave velocity exhibited an inverse linear dependence on both yield strength and microhardness.