Tensile properties and fracture toughness of 18Ni (250 grade) maraging steel weldments

Influence of aging temperature on strength and toughness of laser-welded T-250 maraging steel joint

Abstract: This paper presents an investigation on the strength and toughness of laser-welded T-250 maraging steel joint, aimed at elucidating the influence mechanism of aging temperature on microstructures and mechanical properties of the joints. The results showed that as the aging temperature increased, the ultimate tensile strength of welded joints increased, reaching a maximum of 1640.5 MPa at 520 °C, and then decreased. The static toughness of welded joints decreased at first and increased later with the aging temperature increasing. The minimum of 38.8 MJ m −3 for the static toughness was obtained at 560 °C. There were two types of reverted austenite respectively distributing in grain boundaries and in the matrix of martensite, due to the change of aging temperatures. This study underscores that the Ni 3 (Ti, Mo) precipitate and reverted austenite are the critical factors influencing the strength and toughness of welded joints. The Ni 3 (Ti, Mo) precipitate in the weld metal improves the strength of welded joints remarkably as its volume fraction increases. The reverted austenite in grain boundaries is harmful to the toughness of welded joints, while the reverted austenite in the matrix is beneficial to the toughness of welded joints because of its finely dispersive distribution and its ability to prevent crack propagation. Increasing the amount of reverted austenite in the matrix is an effective way to improve mechanical properties of laser-welded maraging steel joints.

Investigation into Microstructures of Maraging Steel 250 Weldments and Effect of Post-Weld Heat Treatments

Abstract: This study was undertaken to gain a better understanding of microstructures obtained by multipass gas tungsten arc welding in maraging steel grade 250. Metallography and microhardness measurements were carried out on sheet and welded joints in as-welded and post-weld aged conditions. It was found that there was a significant amount of reverted austenite formed on cell boundaries of weld metal after aging at 758-823 K for 3-5 h, and was stable at room temperature. Aging at higher temperatures led to an increase in the continuous network of patchy austenite along the cell boundaries. The reason for the above, in our opinion, is the concentrational heterogeneity which characterizes the microstructure of maraging steel welds. No reverted austenite was observed in as-welded specimens. Solution annealing at 1093 K for 1 h did not completely eliminate the chemical heterogeneity associated with weld structures. However, homogenizing at 1373 K produced homogenous structure that on subsequent aging produces austenite-free lath martensitic structure.

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 bu...

Stress corrosion cracking of maraging steel weldments

Abstract: The stress corrosion characteristics of 18 wt-%Ni (MDN-250) maraging steel and its weldments made under different welding conditions have been investigated. The threshold stress intensity factor K ISCC in stress corrosion conditions has been determined in 3.5 wt-%NaCl environment for the base metal and weldments. The fractured surfaces were analysed to study the types of fracture during stress corrosion cracking in base and weld metals. Fracture toughness tests were carried out and the results obtained from these tests have been compared with K ISCC values.

Strengthening of cobalt-free 19Ni3Mo1.5Ti maraging steel through high-density and low lattice misfit nanoscale precipitates

Abstract: The concept of low lattice misfit and high-density of nanoscale precipitates obtained through solution treatment was adopted to obtain ultrahigh strength maraging steel without compromising elongation. An “ultrahigh strength-high toughness” combination was successfully obtained in 19Ni3Mo1.5Ti maraging steel with ultimate strength of ~1858 MPa and static toughness of ~110 MJ m−3. Maraging steel had extremely high density (2.3 × 1024 m−3) of nanoscale precipitates with minimum lattice misfit of less than 1% at the solutionization temperature of 820 °C. Two kinds of nanoscale precipitates, namely, η-Ni3(Ti,Mo) and B2-Ni(Mo,Fe) contributed to ultrahigh strength. The size of nanoscale precipitates governed the movement of dislocations, cutting versus by-passing. Theoretical estimate of ordering and modulus contribution to strengthening suggested that ordering had a dominant influence on strength. The toughness was closely related to the characteristic evolution of nanoscale precipitates such that the high density of nanoscale precipitates contributed to increase of elastic deformation and low lattice misfit contributed to increase of uniform deformation. The nanoscale size and low lattice misfit of precipitates were the underlying reasons for the high-performance of maraging steel. Moreover, the combination of high-density of nanoscale precipitates and low lattice misfit is envisaged to facilitate the futuristic design and development of next generation of structural alloys.

The strength and fracture toughness of 18 Ni (350) maraging steel

Abstract: The influence of microstructure on the strength and fracture toughness of 18 Ni (350) maraging steel was examined. Changes in microstructure were followed by X-ray and neutron diffraction and by optical and electron microscopy. These observations have been correlated with the fracture morphology established by scanning electron microscopy. Air cooling this alloy from the austenitizing temperature results in a dislocated martensite. During the initial stage of age hardening, molybdenum atoms tend to cluster (forming preprecipitates) and the cobalt assumes short range ordered positions. Subsequent aging results in Ni3Mo and σ-FeTi with overaging being associated with the formation of equilibrium reverted austenite and Fe2Mo. The fracture behavior is examined in terms of elementary dislocation precipitate interactions. It is suggested that the development of coplanar slip in the underaged conditions leads to its increased stress corrosion susceptibility and decreased fracture toughness. The optimum aged condition is then associated with cross-slip deformation. The fracture behavior of the overaged condition is a dynamic balance between a brittle matrix and the ductile (crack blunting) reverted austenite.

The fracture toughness behaviour of a 18 Ni (300 grade) maraging steel in various solution treated and aged conditions

Abstract: Obtention de differentes microstructures par des traitements de vieillissement et des traitements de mise en solution a differentes temperatures. La presence de particules de carbure de titane est tres nefaste vis-a-vis de la tenacite et a plus d'importance que les autres caracteristiques microstructurales

Effects of solution treatment temperatures on microstructure, tensile and fracture properties of Co-free 18 Ni maraging steel

Abstract: The study presents the influences of solution treatment temperatures (S.T.T.) on microstructure, tensile and fracture properties of 18 Ni cobalt free maraging steel. The results are thoroughly discussed in light of the prevailing microstructural conditions as revealed by optical and transmission electron microscopy, and detailed scanning electron fractography. It is found that tensile strength in both unaged and aged conditions is independent of solutioning temperature and, consequently, grain size. However, the tensile ductility, CVN and K IC in aged condition get severely affected with the increase in S.T.T. because of a change occurring in the martensitic lath morphology from blocky to stringer type