Age hardening of an extremely ductile martensitic steel???

Heat Treatment of Maraging Steels

MARAGING STEELS derive their strength from the formation of a very low-carbon, tough, and ductile iron-nickel martensite, which can be further strengthened by subsequent precipitation of intermetallic compounds during age hardening. The term marage was coined based on the age hardening of the martensitic structure. Maraging steels are highly alloyed low-carbon iron-nickel martensites that possess an excellent combination of strength and toughness superior to that of most carbon-hardened steels (Fig. 1). As such, they constitute an alternative to hardened carbon steels in critical applications where high strength and good toughness and ductility are required. Hardened carbon steels derive their strength from transformation-hardening mechanisms (such as martensite and bainite formation) and the subsequent precipitation of carbides during tempering.

Fig. 1 Strength/toughness combination of 18 Ni maraging steels compared to conventional high-strength carbon steels.

Solution Annealing

The martensitic matrix of maraging steels is prepared for later age hardening through a heat-treating procedure commonly referred to as a solution anneal. Solution annealing entails heating the alloy significantly above the austenite finish (Af) temperature, holding a sufficient time to place the alloying elements in solid solution, and then cooling to room temperature. The most common solution-annealing cycle for the 18 Ni Marage 200, 250, and 300 alloys involves heating to 815 °C (1500 °F) for 1 h followed by air cooling.

Age Hardening

A typical age-hardening treatment after solution annealing usually consists of reheating the alloy into the temperature range of 455 to 510 °C (850 to 950 °F), holding at this temperature for 3 to 12 h, and air cooling to room temperature. In typical treatments at 480 °C (900 °F), the 18 Ni Marage 200, 250, and 300 grades are held 3 to 8 h, whereas the 18 Ni Marage 350 grade is usually held 6 to 12 h at 480 °C (900 °F). The 18 Ni Marage 350 grade can also be aged for 3 to 6 h at 495 to 510 °C (925 to 950 °F). The use of marage steels in applications such as die casting tooling requires the use of an aging temperature of approximately 530 °C (985 °F) to provide an overaged structure that is more thermally stable.

Historical Development

Age hardening of martensite, or maraging, depends on the occurrence of a thermal hysteresis of phase transformations, whereby the reversion of martensite to austenite during reheating occurs at a higher temperature range than the temperature range for martensite formation during cooling. Although knowledge of this thermal hysteresis in Fe-Ni alloys was observed as early as 1927, the first extensive research toward development of commercial iron-nickel maraging steels was conducted in the late 1950s by the International Nickel Company (currently Inco Ltd.). This research culminated in the development of the 20 and 25% Ni maraging steels. In addition to nickel, these two alloys contained 0.3% Al, 1.4% Ti, and 0.4% Nb, which resulted in precipitation hardening of the low-carbon martensitic structure when aged at 425 to 510 °C (800 to 950 °F). Both alloys were reported to exhibit good combinations of strength and ductility at hardness levels of 53 to 56 HRC; however, as reported by Hall, these alloys were abandoned because of their brittleness at extremely high strength levels.

Subsequent work on the iron-nickel system by Decker, Eash, and Goldman revealed that the martensite formed in this binary system could be hardened to appreciable levels through the addition of cobalt and molybdenum. A nickel level of 18% was chosen for this alloy system because nickel levels significantly greater than 18% resulted in the retention of austenite in the as-quenched condition. By the early 1960s, three new maraging steels based on the Fe-18Ni-Co-Mo quaternary alloy system were introduced. These were the 18 Ni Marage 200, 250, and 300 alloys, which are capable of achieving yield strengths of approximately 1380, 1725, and 2000 MPa (200, 250, and 290 ksi), respectively, in combination with excellent ductility and toughness. The nominal compositions of these grades are shown in Table 1. In general, the strength levels attained by these alloys are determined by the cobalt, molybdenum, and titanium contents of each alloy. These three alloys replaced the phased-out 20 and 25% Ni maraging steels.

Table 1 Nominal compositions of standard commercial maraging steels.