Establishment of heat treatment cycles for forge welded API L80 tubular joints

Abstract

Microstructure design and heat treatment cycle optimization are two vital activities in any metal forming process which involves high working temperature. Much emphasis is given within these activities to achieve desired structural and mechanical properties of the end products. In this paper an attempt has been made to establish innovative and efficient heat treatment cycles for forge welded API L80 tubular joints. A requirement is that the heat treatment is completed within 5 to 6 minutes after welding. The L80 alloy studied here is a medium carbon steel that has abundant oilfield applications. In order to assess optimal heat treatment for the weld zone, displaying a highly transient and nonuniform temperature distribution, continuum modelling of the process has been performed. Forge welding is a process in which two mating surfaces of pipes are heated (within a small confined depth from the contacting surfaces) to a certain temperature and joined by applying a pressure. The whole process is carried out in the solid state producing a weld without weld metal and with narrow heat affected zone (HAZ), which distinguish it from some of the more conventional welding processes available to produce tubular joints. Specific mechanical properties of forge welded L80 tubular joints were obtained by a unique approach to heat treatment and microstructural design at joints as well as HAZ. Heat treatment cycles were estimated in SINTEF’s Smitweld Thermal Cycle Simulator^® to compare with the actual forge welding process. A detailed analysis of specimens subjected to Smitweld simulation and forge welding was carried out to study compatibility and to establish optimum heat treatment conditions for forge welding of L80 tubular joints.