Thermo-mechanical stability of austenite in post weld heat treated super martensitic stainless steel weld metal

Full text: Super martensitic stainless steels (SMSS) were developed based on the classical martensitic stainless steels (11-14% Cr), reducing the C content and increasing the Ni and Mo contents. SMSS have been developed as a technological alternative of a lower cost with good mechanical properties and corrosion resistance to be applied in the petrochemical industry and pipelines to transport oil and gas, both onshore platforms and 'off-shore'. Semiautomatic welding process is recognized as a very interesting technological alternative for the welding of these materials and their industrial implementation has grown in recent times. In these steels, post-weld heat treatments (PWHT) are usually necessary to adjust the final properties of weld deposits, based on microstructural evolution (control of martensite, martensite tempered, austenite, delta ferrite, carbides contents, etc.). The PWHTs used are single and double tempering to ensure a full martensite tempered and maximize the retained austenite. This is the most important phase in these steels because it can modify the hardness, tensile properties, toughness and corrosion resistance, reaching high values for certain heat treatments. Austenite contains a high density of dislocations and enrichment of Ni and C with respect to the matrix. One of the main characteristics of this phase is to be stable at room temperature and does not transform to martensite with subsequent cooling. The mechanical stability of the austenite is defined as the susceptibility of austenite to transform into martensite, as induced by plasticity. An austenite of higher mechanical stability needs more strain to transform into martensite than that of lower mechanical stability. Regarding the mechanical stability of the austenite in SMSS has not been widely studied in the literature. At low temperatures, retained austenite is less stable and transforms to martensite easily. Under mechanical load, cooling below ambient temperature or combinations thereof retained austenite can be transformed into martensite. This fact can modify the structural integrity of welded component. In this sense, the study of the mechanical stability of the austenite in SMSS weld metals is an aspect that presents scientific and technological relevance and there is scarce information published about it. The aim of this work is to analyze in -situ the stability of the austenite in SMSS weld metals under mechanical loads, for different welding conditions and PWHT's using a XTMS station. Evaluation of the effect of temperature and strain rate in the mentioned stability is also considered. In this sense, samples with different welding conditions and PWHT were mechanically tested in a Gleeble machine, at different temperatures (60 to -20 °C) and strain rates (0,1 mm/min to 10 mm/min), with in-situ monitoring of austenite evolution. It could be determined the austenite transformation during mechanical testing, obtaining a critical strain, for each temperature and strain rate. The mechanisms involved in the austenite transformation in this material was discussed. (author)