Effect of Pre-Strain on Mechanical Properties of Pressure Vessel Grades: Assessment of Stress Relieving/Post Weld Heat Treatments Efficiency at Regenerating Properties

Abstract

During fabrication of large pressure vessels, thick plates are submitted to numerous process phases that may affect the initial (i.e. as delivered) properties of the material. Regarding the advantages (both technical and economical) of cold forming process, this technique is largely preferred and widely spread. Modern forming presses and rollers are now sufficiently powerful to roll very thick plates (typically up to 250mm thick) devoted to ultra-heavy pressure equipments. As force does not really constitute a limitation anymore, current limitations are now focusing on maximum admissible strain in materials. This particular limit is linked to: - Intrinsic maximum deformation admissible by the material (given by tensile tests), - Regulation rules coming from construction codes. From a practical point of view, the actual limitation comes from the construction codes that are very severe. Main codes (ASME Boilers and Pressure Vessels Construction Code from American side and EN 13445 Unfired Pressure Vessels Construction Code from European side) both give a limit equal to 5% strain for using material in “as-strained” condition without any heat treatment. Above this limit, the philosophy differs from one code to another. While European Code requires a full quality treatment of the strained material (Normalisation or Austenitization / Tempering), American code only requires Tempering, allowing fabricators the possibility of using the mandatory Post Weld Heat Treatment (PWHT) (needed by welded zones) as a tempering treatment to improve welded zone toughness and to regenerate material properties. The purpose of this contribution is to review the effect of pre-strain on mechanical properties (Hardness, Tensile and Toughness transition curves) for different strain levels and to evaluate the ability of typical PWHT to regenerate material properties. Results presented in this paper are based on both recent studies on the most common up-to-date materials as well as on historical data collected in the last decades. This study clearly demonstrates that the required PWHT is efficient enough to regenerate all material properties and that there is no need to apply a full quality heat treatment, even for the highest level of strain. This benefits both the fabricator and the end user as it implies reducing costs and risks of components deformation while maintaining the necessary level of service properties.