Optimization of selective laser melting parameters and influence of post heat treatment on microstructure and mechanical properties of maraging steel

Maraging behavior of an Fe-19.5Ni-5Mn alloy II: Evolution of reverse-transformed austenite during overaging☆

This paper investigates the effect of austenitizing temperature on mechanical properties of X00CrNiCoMo9-9-5-3 corrosion resistant maraging steel. The results show that solution treatment recrystallization temperature of this experimental steel is 850 °C. After a solution treatment below 850 °C, the experimental steel will change to form austenite via the non-dispersive phase transition from α′ to γ, which inherits the forging structure high density defects. Then the experimental steel can cool to form high strength martensitic steel. Moreover, high hardness austenite increases the resistance to phase transition in the cooling process and increases the content of retained austenite, thus guaranteeing excellent low temperature toughness of maraging steel.

Effect of austenitizing temperature on the mechanical properties of high-strength maraging steel

Maragin behavior of an Fe-19.5Ni-5Mn alloy. III: Mechanical properties

The effect of austenitizing temperature on mechanical properties of 00Cr9Ni9Co5Mo3 corrosion resistant maraging steel has been investigated. The results show that the recrystallization temperature of experimental steel is 850 °C. The austenite formed by non-diffusion reverse α'→γ transformation inherit high density defects of the forging structure when solution treatment temperature is below recrystallization temperature. High-hardened martensite is formed during the cooling, the strength keeps a high level. Meanwhile, high-hardened austenite improves the resistance of γ→α' transformation in the cooling process, which leads to more retained austenite in the steel and guarantees good low temperature toughness. The increasing of the pretreatment can further improve the low temperature toughness. When the pretreatment is added, the low temperature toughness is improved further, and the mechanical properties of materials are stable with the fluctuation of pretreatment temperature.

Effect of heat treatment temperature on the mechanical properties of low-temperature high strength maraging steel

1.

At the normally used rates of heating the α»γ transformation occurs in the aged martensitic matrix, which together with phase work hardening from the α»γ transformation causes the formation of austenite with an increased hardness. With an increase in the austenitizing temperature the strength of the austenite drops, approaching the strength of the austenite in unaged steels.




2.

The formation of a large quantity of austenite, up to 30%, after short heating in the α»γ transformation does not reduce the strength of N18K9M5T steel. The loss of strength with long holds in the α »γ region may be explained by overaging of the α-matrix. It is also possible that there is a loss of strength in the austenite.




3.

If in the α»γ transformation the steel does not lose strength, the presence in the structure of a large quantity of austenite does not increase the impact strength.




4.

The form of the austenite particles and their distribution in the α-matrix is determined by the quantity of austenite and does not depend upon the temperature and holding time at which this quantity is obtained.




5.

The increase in the strength of the austenite after heating to temperatures close to the Ac3 (720–820°C) is inherited by the martensite, preserved after low temperature aging up to 500°C, but disappears after high temperature aging above 530°C.

Some rules for phase transformations in 000N18K9M5T steel

1.

The α→γ transformation in steel N18K9M5T occurs on holding at 500–730°C or on heating at rates below 30–50 deg/sec by way of martensite decomposition in the two-phase α+γ region with redistribution of alloying elements between ferrite and austenite.




2.

The main reason for the stable austenite formed on holding in the α→γ temperature range is the redistribution of alloying elements (enrichment of austenite in nickel).




3.

The rate at which martensite decomposes is very high, and therefore during heating to quenching temperature or during welding there is always formed some stable austenite enriched in nickel, which leads to residual austenite in the structure. The amount of residual austenite can be reduced by homogenizing annealing for 30–60 min at 820°C.

The α→γ transformation in maraging steel N18K9M5T

The main reasons for the low plasticity and ductility of maraging steel 00N18K9M5T in large sections are the coarse-grained structure resulting from heating for hot working and the precipitation of carbides or titanium carbonitrides in the form of a network in the grain boundaries.

S. S. Ryzhak

Papers

Some rules for phase transformations in 000N18K9M5T steel

The α→γ transformation in maraging steel N18K9M5T

Heat treatment of large forged and pressed bars of steel 00N18K9M5T

Effect of heat treatment conditions on the properties of steel 000Kh11N10M2T