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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.

Small scale material testing offers many advantages compared to large or full scale testing. Small scale tests are usually simpler and quicker to perform than full scale tests, and as a consequence they are cost-effective -the concomitant speed of screening has implications for product commercialization (notably the time to market and the time in market). However, in order to make sure that small scale tests produce relevant information for the full scale process, it should always be demonstrated that the material is exposed to thermo-mechanical and environmental conditions similar to those of the full scale process. The objective of the work described in this article has been to demonstrate the feasibility of miniaturizing a hot forging process. In the original full scale process, a medium carbon steel pipe with Outer Diameter (OD) of 200 mm and wall thickness of 12 mm is heated locally by high-frequency induction heating coil before being forged to a plastic strain of approximately 1 and subsequently cooled in air. In the small scale test, specimens with OD 12 mm and wall thickness 3 mm are heated in a similar way by induction heating coil and forged. The heating and cooling rates (as well as gaseous environment) are controlled in order to carefully recreate the temperature history of the full scale test. Finite element simulations of heating, forging and cooling phases clearly demonstrate how to design the small scale test. If the specimen is heated by an induction coil during the cooling phase, it is possible to compensate the greater heat losses to the surroundings experienced by the small scale specimen.

Shielded Active Gas Forge Welding is a fully automatic high speed welding process for metals. It was invented in the early 1980s, but has since then been significantly improved and commercialized for mainly casing and pipeline applications for the oil and gas industry. The method consists of three main steps: (1) localised heating of the mating surfaces, (2) forging and joining of the mating surfaces and (3) heat treatment of the weld. An entire welding cycle can be completed in two minutes, independent of dimension. The method has been used for welding a great range of alloys, and it produces a weld with properties similar to those of the base material.