Failure of super 13Cr stainless steel due to excessive hardness in the welded joint

Supermartensitic stainless steels (SMSS) with 11-13%Cr were developed to show higher corrosion resistance, toughness and weldability than conventional martensitic stainless steels (MSS). The main compositional characteristics of SMSS's, in a comparison to common MSS's, are the extra low C content, Ni and Mo addition. This new sub-family of stainless steels has been used in the oil and gas offshore production, as tubulars and casings in the wheel. Despite of the extra low C (<0.03%), the mechanical resistance of SMSS's is maintained high due to the addition of Mo (up to 2.0%wt.), and other elements, such as N, W, Nb and or Ti. As a consequence of the extra low C, the martensite of SMSS's is relatively soft and tough, which makes these steels, in theory, much more resistant to cold cracking in the heat affected zone (HAZ) of welds. Nevertheless, for applications in saline environments with CO2 and/or H2S a post weld heat treatment (PWHT) is necessary to reduce the peak of hardness in the HAZ. In the present work, the failure of a mandril for chemical products injection in the wheel made of SMSS is reported. The failure occurred in a welded joint between two heavy forged pieces. The hardness of the HAZ was higher than the minimum allowable to sour service conditions, indicating that the PWHT was not performed or it was done uncorrectly.