A comparison of cold forming, hot forging, and induction bending as methods of producing duplex stainless steel elbows for high-pressure pipework

Abstract

Two methods are commonly used to produce elbows for use in complex, high-pressure pipework: hot forging and cold forming. The former is expensive and produces an elbow that has an excessively thick wall section necessitating a large amount of preparation prior to fabrication welding. The thick wall sections also present a challenge for post-forming heat treatment and are a particular concern when the parent material is a duplex stainless steel as there is a risk that an intermetallic phase material will be formed. Cold forming is significantly less expensive and produces a finished product that requires much less preparation prior to welding; however, there are concerns regarding the acceptability of the metallurgy of the finished item. A third method of producing elbows, induction bending, is less frequently used and, while being disadvantaged by marked changes in wall section the finished elbow offers the prospect of a solution with similar cost and availability advantages to cold forming. This paper therefore briefly describes the three processes. It then compares the geometry, metallurgy, corrosion resistance, and mechanical properties of the products of each method, and commercial comparison is made. It is concluded that cold forming offers several distinct advantages over its rivals and it is recommended that this means of production be given favourable consideration by piping engineers when specifying elbows for complex subsea pipework.