Shielded Active Gas Forge Welding (SAG-FW) is a solid state welding process for metal pipes, rods and other elongate products. As a part of an extensive qualification programme for SAG-FW for casing applications a series of welds have been completed for the pipe steel quality Vallourec & Mannesmann VM50. While the microstructure of the base material (BM) is tempered martensite, both natural cooling and quench-tempered microsturctures were evaluated in order to assess the need for post-weld heat treatment (PWHT). Forge welds were subjected to careful and independent metallurgical analysis and mechanical testing by Det Norske Veritas (DNV). For as-welded specimens the heat affected zone (HAZ) consists of ferrite and pearlite. Hardness values for the HAZ are 160 HV10 while for the base material inner and outer wall are 150 and 175 HV10 respectively. As-welded tensile tests show a drop in yield from 390 to 370 MPa. Charpy testing of the welds at −20°C reveals a fully ductile structure with toughness of 150 J. The HAZ is entirely in the region with increased wall thickness.
In an extended study of mechanical properties of the VM50 forge welds results from a notched tensile test were used to determine the damage parameters for the Complete Gurson Model (CGM) using finite element modeling for calibration. In order to evaluate the quality of the damage model Single Edge Notched Bending (SENB) tests were performed and studied using finite element models. The toughness of the as-welded weld is moderately reduced due to welding, but it remains ductile with high fracture resistance. The moderate reduction of fracture resistance can be explained by microstructure change.