Influence of post-weld heat treatments on microstructure and mechanical properties of gas tungsten arc maraging steel weldments

Abstract

The response to post-weld heat treatment of an 18%Ni (250 grade) gas tungsten arc weld metal has been investigated. The post-weld heat treatments are (a) direct aging at 480°C/3 h/air cooling, (b) solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling and (c) homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling. Metallographic characterisation of fusion zone revealed pronounced segregation of titanium and molybdenum along the interdendritic and intercellular boundaries. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Solutionised treatment at 815°C does not remove the segregation. Homogenisation treatment (1150°C/1 h/air cooling) succeeded in making the composition become homogenised. Mechanical properties including tensile, hardness and impact toughness were evaluated. Tensile test results showed that directly aged weldments exhibited lower strength but higher ductility than the other cases; this was attributed to the presence of reverted austenite. Homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling resulted in optimum tensile properties. A substantial increase in fusion zone toughness was observed after homogenisation+solutionising+aged condition due to a decrease in the content of austenite content compared to the directly aged condition. The reduction in microsegregation by diffusion of alloying elements from cell boundaries to the cell during homogenisation treatment is responsible for the decrease in austenite content.

Mechanical and Microstructural Characterization of Plasma Welded Joints of 18 Ni-300 Maraging Steels Subjected to Repairs

“…about 1 mm from the FZ. Venkateswara Rao et al (2010;Fanton et al, 2014) found a similar increase in the region close to the FZ interface in maraging steels. This same behavior was observed when making 1.…”

Current research and developments in welding of 18% nickel maraging steel

“…28 The HAZs also show lower hardness compared to BM, which is due to grain coarsening in the case of HAZ-1 and due to the presence of RA in the case of HAZ-2. 28 PWHTs increase hardness of the welded joint due to precipitation reaction. The DA condition shows a lower hardness in the FZ and HAZ, compared to the aged BM.…”

“…34,35 Welding is followed by post-weld heat treatment (PWHT) to elevate the strength of the FZ to that of the base metal (BM). 28 The commonly used PWHTs for maraging steel include direct ageing (DA), solutionizing + aging (SA), homogenizing + aging (HA), and homogenizing + SA (HSA). [36][37][38] Most of the conventional welding operations like GMAW, GTAW, laser beam welding (LBW), plasma welding, electron beam welding (EBW), and laser hybrid welding (LHW) achieve sound and crack free joints, with welding efficiencies in terms of strength exceeding 90%.…”

“…The microstructure of the welded joints exhibits cellular dendritic structure. 28 In fusion welding, the dendrites become coarser due to thermal fluctuations. TEM images of the weldments also consist completely of lath martensite structure with higher dislocation density.…”

An investigation on surface finish and flank wear in hard machining of solution treated and aged 18 % Ni maraging steel

“…However, in typical applications requiring high strength with almost double the hardness, they are strengthened using aging [2]. The maraging steels can be welded in both the solution annealed condition as well as the maraged condition; however, post-weld heat treatments become essential to relieve the residual stresses [3,4].…”