Wear of Laying Head Pipes in a Wire Rod Mill

The failure analysis on the premature wear of a “Laying Head Pipe” in a Wire Rod Mill has been presented. The hot-rolled wire rods subsequent to finish rolling pass through the “Laying Head Pipe” which rotates and lays the wire rods in the form of coils for air cooling to achieve the final properties. A worn-out pipe and a thermo-mechanically treated (TMT) re-bar have been analyzed. The material of the pipe is ASTM A335-P5 grade of seamless alloy steel pipe used for high temperature service. The microstructures of the wear groove of the alloy steel pipe show predominantly ferrite and globular cementite/carbide particles along with scales, while that away from the wear groove shows coarse tempered martensite matrix. EDS analysis confirms the presence of alloy carbides near the wear groove. Microhardness profile shows reduction in hardness toward the inner surface of pipe; hardness at the inner surface of the pipe becomes lower than the surface hardness of the TMT re-bar, exhibiting tempered martensite matrix. Softening at the inner surface of the pipe wall occurs due to a rise in temperature/ over-tempering in contact with the passing hot wire rods (900 °C) which causes transformation of martensite into ferrite and coarse globular cementite.