An 18-Ni 300 grade maraging steel was processed by selective laser melting and an investigation was carried out on microstructural and mechanical behaviour as a function of aging condition. Owing to the rapid cooling rate, the as-built alloy featured a full potential for precipitate strengthening, without the need of a solution treatment prior to aging. The amount of reversed austenite found in the microstructure increased after aging and revealed to depend on aging temperature and time. Similarly to the corresponding wrought counterpart, also in the selective laser-melted 18-Ni 300 alloy, aging promoted a dramatic increase in strength with respect to the as-built condition and a drop in tensile ductility. No systematic changes were found in tensile properties as a function of measured amount of austenite. It is proposed that the submicrometric structure and the phase distribution inherited by the rapid solidification condition brought by selective laser melting are such that changes in tensile strength and ductility are mainly governed by the effects brought by the strengthening precipitates, whereas the concurrent reversion of the γ-Fe phase in different amounts seems to play a minor role.

Aging Behaviour and Mechanical Performance of 18-Ni 300 Steel Processed by Selective Laser Melting

Microstructural evolution and mechanical properties of maraging steel produced by wire + arc additive manufacture process

The sequence of austenite reversion during overageing in 18 Ni (350) maraging steel was examined and its effects on the mechanical properties were evaluated. Austenite with different morphological features were identified at different stages of overageing. The reverted austenite caused decrease in the yield strength (YS) and ultimate tensile strength (UTS) and increase in the tensile ductility. Though the presence of austenite appeared to be beneficial to impact toughness in the initial stages of overageing, severe embrittlement was noticed in samples subjected to prolonged ageing. The observed deterioration in toughness with continued overageing was associated with the coarsening of intermetallic precipitates formed during the early stages of ageing. Electron probe micro analysis (EPMA) carried out on fracture surface identified the embrittling species as precipitates rich in titanium and nickel.

Effects of austenite reversion during overageing on the mechanical properties of 18 Ni (350) maraging steel

This work investigates the solution phenomenon, aging behavior and room-temperature mechanical properties of maraging steel manufactured by selective laser melting (SLM). Different heat treatment experiments, including solution treatment (ST), direct aging treatment (DAT) and solution + aging treatment (SAT) are designed. Microstructure analysis indicates that ST and SAT will eliminate the cellular and lath structures, but DAT has little effect on these. The content of austenite increases with the addition of DAT temperature and holding time. While austenite is almost undetectable in ST and SAT samples. Meanwhile, both the elongation and toughness of the samples with DAT gain a slight improvement with the temperature increasing. Importantly, DAT yields similar microhardness, tensile strength and impact toughness to SAT, although the resultant microstructures are completely different. The results demonstrate that DAT can achieve the similar mechanical properties to SAT samples. Samples with high mechanical properties (microhardness of 653.93 HV and ultimate strength of 2126.30 MPa) have been obtained by DAT at 520 °C for 6 h as well as solution treatment at 900 °C for 1 h and aging treatment at 520 °C for 6 h. This investigation reveals the evolution regularity of microstructure, microhardness, tensile performance and impact toughness of maraging steel manufactured by SLM after different heat treatments.

Effect of heat treatment on the microstructure and mechanical properties of maraging steel by selective laser melting

This experimental study investigates the combined effect of the three primary Additive Manufacturing (AM) build orientations (0°, 45°, and 90°) and an extensive array of heat treatment plans on the plastic anisotropy of maraging steel 300 (MS1) fabricated on the EOSINT M280 Direct Metal Laser Sintering (DMLS) system. The alloy's microstructure, hardness, tensile properties and plastic strain behaviour have been examined for various strengthening heat-treatment plans to assess the influence of the time and temperature combinations on plastic anisotropy and mechanical properties (e.g. strength, ductility). A comprehensive visual representation of the material's overall mechanical properties, for all three AM build orientations, against the various heat treatment plans is offered through time – temperature contour maps. Considerable plastic anisotropy has been confirmed in the as-built condition, which can be reduced by aging heat-treatment, as verified in this study. However, it has identified that a degree of transverse strain anisotropy is likely to remain due to the AM alloy's fabrication history, a finding that has not been previously reported in the literature. Moreover, the heat treatment plan (6h at 490 °C) recommended by the DMLS system manufacturer has been found not to be the optimal in terms of achieving high strength, hardness, ductility and low anisotropy for the MS1 material. With the use of the comprehensive experimental data collected and analysed in this study, and presented in the constructed contour maps, the alloy's heat treatment parameters (time, temperature) can be tailored to meet the desired strength/ductility/anisotropy design requirements, either for research or part production purposes.

Plastic anisotropy of additively manufactured maraging steel: Influence of the build orientation and heat treatments

Sheet material specimens from 18Ni (250-grade) maraging steel were gas tungsten-arc welded using two different filler wires: one matching the base material in composition and the other with higher cobalt but lower molybdenum and titanium contents. Welding was carried out both in constant-current and pulsed modes. Post-weld aging was performed at three different temperatures, viz., 425, 480 and 520°C. Metallographic characterization revealed pronounced segregation, presumably of Ti and Mo, along interdendritic and intercellular boundaries in the weld metal produced with filler of matching composition. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Segregation and austenite reversion were not noticed when Ti and Mo contents were reduced in the filler wire, except at the highest aging temperature used. The use of pulsed welding resulted in some grain refinement, but the effect was not considerable.

Microstructural changes during welding and subsequent heat treatment of 18Ni (250-grade) maraging steel

Gas tungsten arc welding was performed on 18Ni (250 grade) maraging steel sheet using two different filler wires: one of the same composition as the base material and the other containing more cobalt and aluminium and less molybdenum and titanium. Weld specimens were then aged under four different sets of conditions. After metallographic characterisation, mechanical properties including hardness, tensile strength and ductility, and fracture toughness were evaluated. Results showed that use of the matching filler material led to lower strength but higher ductility than in the other case; this was attributed to the presence of reverted austenite in the former (caused by segregation, especially of molybdenum) at the fusion zone substructure boundaries. In both types of weld metal, a re-solution treatment followed by aging at 480°C resulted in optimum tensile properties. Fracture toughness of the aged weldments was in general close to that of the parent material aged at 480°C; some deterioration occur...

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

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.

S. Sundararajan

Papers

Microstructural changes during welding and subsequent heat treatment of 18Ni (250-grade) maraging steel

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

Stress corrosion cracking of maraging steel weldments