Exothermic flux forge welding of steel tubulars

Abstract

Welding processes inevitably alter the local microstructure and in turn affect the properties. For many grades of steels that require high strength, ductility, and toughness, it is difficult to maintain this combination of properties after welding. While full part heat treatments can sometimes be used to recover the microstructure and properties, this approach is impractical for welding of tubular strings in service. Therefore, advanced welding and localized post weld heat treatment methods are needed that can economically produce high integrity welds in tubular strings while maintaining strength, ductility, and toughness property requirements. A novel exothermic flux forge welding method is introduced for solid-state welding of steel tubulars and aspects of the development are discussed including constituent and heating rate effects on self-propagating high-temperature synthesis of metal and oxide products. The exothermic flux forge welded process was investigated for solid-state welding of a high strength low alloy (HSLA) steel and American Petroleum Institute (API) Q125 grade high-strength casing with a 14-inch (355.6 mm) outer diameter and 0.866-inch (22 mm) wall thickness. Post weld heat treatment approaches, including a multi-step heat treatment that included an intercritical heating stage, were investigated on the welded steel for their effects on microstructure and properties. Welded steel performance was characterized by methods including tensile, bend, impact energy absorption, and strain life fatigue testing. Corrosion response was investigated by salt spray and by submersion in NACE Solution A. Heat affected zone microstructure and hardness were also examined along with bond plane fracture using the nick break method. The results include achievement of (1) material specified strength, ductility, and toughness requirements in welded Q125 steel and (2) demonstration of strain life fatigue mean reversals to failure in welded HSLA steel of 2.5 times higher than gas tungsten arc welded specimens.