Influence of Upset Pressure on Microstructure and Mechanical Properties of Friction-Welded AISI 4140 Low Alloy Steel Pipes

Abstract

In the present study, low alloy steel AISI 4140 pipe in reheated, quenched and tempered (RQT) condition was friction-welded using 6.4 MPa upset pressure followed by post-weld heat treatment as per the manufacturing practice prevailing in the drill pipe industry and compared the same with friction welds made at different upset pressures in the RQT condition and in the as-received condition to evaluate bond quality. The weld region consisted of a mixture of tempered martensite and ferrite in case of RQT condition and mixture of lath martensite/upper bainite and ferrite in the as-received condition. As the upset pressure was increased, the martensite lath size decreased in both as-received and RQT conditions. In general, as the upset pressure was increased, the average microhardness increased. The average microhardness of the material adjacent to partially deformed zone showed lower hardness due to tempering effect because of weld heat. The upset pressure 6.4 MPa showed better notch tensile strength ratio confirming better ductility of the weld zone. The impact strength decreased as the upset pressure increased in both as-received and RQT conditions. But as-received condition showed significantly lower impact strength due to higher carbide content at the weld interface.