Effect of Carbides on Crack Behavior within Rolls under Cold Rolling Condition

The effects of carbides on crack behaviors in high-vanadium high-speed steel and high-chromium cast iron rolls under cold rolling condition are studied by using scanning electron microscope and transmission electron microscope. Results show that the crack initiation and propagation depend on fine-structure in carbide, carbide/matrix interface and morphology of carbide. Dislocation in M7C3 tends to occur in the process of steel rolling owing to the stacking fault structure of M7C3 in high chromium cast iron, resulting in dense crack-initiation sites in M7C3. After propagating a short distance, the dense cracks joint each other to form main crack, causing in rapid failure of roll. There are a lot of MC-type nanometer carbides, enriched Mo, evenly distributed in VC of high speed steel, which can pin dislocation in VC, resulting in forming dislocation loop to absorb rolling energy. So it is difficult for crack to initiate inside VC. Crack mainly initiates at the VC/matrix interface, and propagates along spherical surface of VC. Crack blunting occurs when it propagates to the side of VC. The partial coherent relation between VC and austenite matrix can retard the initiation of cracks located at VC/matrix interfaces, and good morphology of VC is helpful to crack blunting, which contribute to enhancing fatigue performance and service life of roll.