Austenite reversion in 18 Ni Co‐free maraging steel

Abstract

The mechanism of austenite reversion in 18 Ni Co‐free maraging steel (250 grade) has been established by conducting extensive X‐ray diffraction (XRD) and transmission electron microscopy (TEM) under differently aged conditions. It has been proposed that contrary to the precipitate dissolution mechanism suggested for the initiation of austenite reversion in 18Ni‐8Co‐5Mo type maraging steels, the initiation of transformation of martensite to austenite in this type of maraging steel is due to the diffusion of Ni from matrix to the dislocations and other defect structures on prolonged/high temperature ageing. This results in local enrichment of Ni which lowers both AS and MS temperatures of the region. Lowering of these transformation temperatures is responsible for the early reversion of martensite to Ni‐enriched stable austenite which, on subsequent cooling to room temperature, does not transform back to martensite.

Microstructure and mechanical properties of a 2000 MPa grade co-free maraging steel

“…[20,21] To date, research on Co-free maraging steels has concentrated on T-250 and other grades with 1800 MPa strength level. [2,14,16,[22][23][24][25] Discrepancy among the results exists similar to that for 18Ni Co-containing maraging steels. For example, Vanderwalker believed that in T-250, austenite nucleated first in martensite, followed by nucleation of Ni 3 Ti from the austenite site, [22] contrary to Vasudevan et al's work showing Ni 3 Ti to be the only precipitation phase.…”

“…When aged at high temperature, nickel gradually segregates to the martensite lath boundaries as well as localized areas inside the laths, for example, dislocations and stacking faults. [16,18] Lattice reconfiguration in these regions leads to the formation of austenite. In the intermediate and late stages of aging (after 3 hours) at the high temperature, some precipitates coarsen significantly locally inside the martensite laths, forming nickel-rich regions, leading to the transformation to islandlike isolated reverted austenite.…”

“…Moving on to precipitation strengthening mechanisms, there are a dislocation looping mechanism [2,14] and a shearing mechanism. [3,15] Many authors observed reverted austenite of different morphology under overaging or special treatment, its orientation relationship with martensite varying in the range of Nishiyama-Wassermann (N-W) [3,16,17] and Kudjumov-Sachs (K-S) [3,6,17,18] types. In terms of mechanical properties, Rack and Kalish [19] and Zhu et al [20] found embrittlement in 18Ni (350 ksi grade) maraging steels after low-temperature aging at 673 to 723 K. Overaging decreases strength, ductility, and toughness, [14,19] but there were also reports of improved strength and toughness due to reverted austenite.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Dense, fine, and complex microstructures form in maraging steels during aging treatment, with precipitation having complicated diffraction patterns. These, coupled with compositional variations of different steels, have resulted in differing opinions among the research literature.…”

Age hardening and mechanical properties of a 2400 MPa grade cobalt-free maraging steel

“…Over the past half-century, two major types of maraging steels have been developed: [1][2][3][4][5][6][7][8][9] the 18Ni maraging steels and the cobalt-free maraging steels. Among the two types, the 18Ni maraging steels are in a more advanced and mature stage of development and applications, with maximum strength levels reaching 2400 MPa, accompanied by good toughness and ductility.…”