Maraging steels based on iron–nickel martensite constitute a very important family of high-strength steels, which distinguishes itself by demonstrating an unparalleled combination of excellent fabricability and high strength and fracture toughness after heat treatment. Heat treatment of these steels has now been perfected to ensure consistently high levels of strength, ductility, and toughness in a variety of product shapes and sizes. Cobalt-free variants have been commercialized as part of efforts to save production costs. Further knowledge has been generated on 18% nickel maraging steels regarding phases precipitating during aging, thermal embrittlement, thermal cycling and austenite reversion/retention and their effect on mechanical properties. The paper reviews the progress made over the last one and half decades.

Progress in understanding the metallurgy of 18% nickel maraging steels

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.

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

We elucidate here the impact of aging temperature on microstructural evolution and strengthening behavior on low lattice misfit cobalt-free maraging steel. The best combination of high-strength (1850 MPa) and high-toughness (125.4 MJ m−3) was obtained at the optimal aging temperature of 520 °C, without sacrificing ductility. Electron back scattered diffraction studies suggested that preferred orientations of {101}, fraction of high-angle grain boundary (HAGB) and total length of grain boundary per unit area (μm/μm2) were increased with increase of aging temperature, which was beneficial to both strengthening and toughening of maraging steel. The strengthening contribution from the precipitates was transformed from shearing mechanism to bypass mechanism when the aging temperature is greater than 520 °C. The aging tempered steel of 520 °C provided maximum strengthening increment of 1463 MPa through shearing mechanism, while granular reverted austenite at this temperature contributed to high toughness.

Aging phenomenon in low lattice-misfit cobalt-free maraging steel: Microstructural evolution and strengthening behavior

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

This paper elucidates here the strength and toughness behavior of T-250 maraging steel welded by laser beam under different approaches of three post-weld heat treatments, i.e. aging (A), solutionizing+aging (SA) and homogenizing+solutionizing+aging (HSA). The microstructures of the weld metals with A and SA processes both comprised of finely dispersive Ni 3 (Ti, Mo) precipitates, small martensite lath and reverted austenite along the grain boundary. However, in the weld metal with HSA process, it exhibited the same Ni 3 (Ti, Mo) precipitate with the large martensite lath and the absence of reverted austenite. The ultimate tensile strength and static toughness of the welded joint with HSA process were 1350.6 MPa and 63.8 MJ m −3 , respectively. The static toughness has been remarkably improved from 71% to 91% of the applied parent metal compared with that of the welded joint with A process. The present study underscores that the Ni 3 (Ti, Mo) precipitate and martensite are significant to ensure the high strength of welded joints. Due to its inconsistent deformation with the matrix of martensite, the reverted austenite has a notable influence on the toughness of welded joints. It shows that the post-weld heat treatments of HSA process can influence the mechanical behavior of welded joints by eliminating the reverted austenite.

Effect of post-weld heat treatments on strength and toughness behavior of T-250 maraging steel welded by laser beam

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

Thomas Tharian

Papers

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