The Flash Butt Welding (FBW) technique is developed by the Russians. They have used FBW for onshore pipelaying in the USSR for the last 20-30 years. The technique has for different reasons not been used in the Western part of the world. The main problem has been low toughness in the welded zone as well as uncertainty regarding soft heat affected zones in modern processed steels.
McDermott (McD) who have the license for use of the FBW technique In the Western world, have been looking for a project where the FBW technique could be used. Use of FBW would give a faster pipe laying speed, and economical benefits. A programme for development of the FBW technique was therefore started by Statoil and McDermott. This programme has consisted of two phases, the first was a study of X65 pipe materials behavior under simulated FBW thermal cycles. In the second phase, real flash butt welds have been tested. These welds were made with pipes delivered to the Statoil linepipe specification. Through the test programme, post weld heat treatment parameters have been developed by means of induction heating to a maximum temperature between 850 and 1000 °C.
36"OD x l"WT pipes from four different steel mills have been used for testing of the FBW technique.
Traditional mechanical testing has been performed on FBW coupons made from all steels, while wide plate testing were only executed with welds from two steels.
The chemical composition of the steels used during these tests are given In Table 1, while Table 2 summarize the tensile properties.
(table 1 and 2 are available in full paper) The test programme was not identical for all coupons. However, all welds have been tested with transverse tensile and side bend specimens. The Charpy-V test programme has as a minimum consisted of six specimens in the fusion line and three specimens one millimeter into the HAZ. For a few welds additional Charpy-V testing have been performed further into the HAZ. After completion of the Charpy-V test programme a retest was performed on some samples, which through the initial test programme showed excessive scatter in the Charpy-V results. A CTOD testing programme in the fusion line, by means of Bx2B specimens were also performed on FBW coupons from all steels. The CTOD testing were performed on several welds with varying Charpy-V levels.
The hardness (HV5) indentations were positioned one millimeter below the outer and inner surface starting at the fusion line and measured till the base metal hardness was reached. In addition the extension and position of the soft zones were measured by hardness indentations for a few selected welds.