An investigation into the weldability of higher strength linepipe for the American Gas Association has been successfully completed in a collaborative project between TWI and the Edison Welding Institute in Columbus, Ohio
TWI and the Edison Welding Institute (EWI) have successfully investigated the weldability of a new, higher strength grade of pipeline steel for the American Gas Association (AGA).
The main objective of the project was to establish safe welding procedures for X80 grade steel. Although such high strength steel has only recently been included in pipeline materials and welding procedure national standards it has been produced for some time by steel producers but not used in practice. To provide confidence in the material, its welding characteristics and welding mechanical properties needed to be established. This formed the basis of TWI's approach to AGA.
The high yield strength of X80 means that thinner wall pipe can be used to meet the stress requirements imposed on the pipe wall by high pressure gas. Thinner wall pipe, in addition to requiring less steel, demands less welding time. Higher strength linepipe steel therefore offers great advantages.
The steels are manufactured using thermomechanical processing, undergoing a specialised heat treatment to enhance strength without increasing alloy content. However, the thermal cycle associated with welding can soften an area adjacent to the weld and lower the strength locally.
The most problematic aspect was that of achieving the same yield strength in the weld as in the plate. An elaborate heat treatment cannot be performed on a weld metal as it can with plate, so to match the plate strength the weld metal has to rely on alloying additions. This, however, increases hardenability with the associated increase in risk of weld metal hydrogen cracking.
TWI found that X80 materials can be welded successfully using conventional stovepipe welding techniques with a modest preheat.