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

The austenite to martensite transformation in a semi-austenitic stainless steel containing 17 wt%Cr, 7 wt-%Ni and 1 wt-%Al was investigated with vibrating sample magnetometry and electron backscatter diffraction. Magnetometry demonstrated that, within experimental accuracy, martensite formation can be suppressed on fast cooling to 77 K as well as on subsequent fast heating to 373 K. Surprisingly, martensite formation was observed during moderate heating from 77 K, instead. Electron backscatter diffraction demonstrated that the morphology of martensite is lath type. The kinetics of the transformation is interpreted in terms of athermal nucleation of lath martensite followed by thermally activated growth. It is anticipated that substantial autocatalytic martensite formation occurs during thermally activated growth. The observation of a retardation of the transformation followed by a new acceleration during slow isochronal (i.e. at constant rate) cooling is interpreted in terms of the combined effect of the strain energy introduced in the system during martensite formation, which thermodynamically and/or mechanically stabilises austenite, and autocatalytic nucleation of martensite.

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Thermally activated growth of lath martensite in Fe–Cr–Ni–Al stainless steel

In salient effects, still crystals of solids that switch between phases acquire a momentum and are autonomously propelled because of rapid release of elastic energy accrued during a latent structural transition induced by heat, light, or mechanical stimulation. When mechanical reconfiguration is induced by change of temperature in thermosalient crystals, bursts of detectable acoustic waves are generated prior to self-actuation. These observations provide compelling evidence that the thermosalient transitions in organic and organic-containing crystals are molecular analogues of the martensitic transitions in some metals, and metal alloys such as steel and shape-memory alloys. Within a broader context, these results reveal that, akin to metallic bonding, the intermolecular interactions in molecular solids are capable of gradual accrual and sudden release of a substantial amount of strain during anisotropic thermal expansion, followed by a rapid transformation of the crystal packing in a diffusionless, non-displacive transition.

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Acoustic Emission from Organic Martensites

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

1.

Heating to 525–950°C after austenitizing at 1050°C reduces substantially the stability of austenite. Destabilization of austenite is explained by a depletion of the solid solution in chromium and carbon by chromium carbide precipitation.




2.

Prolonged holding at room temperature after austenitizing or heating at up to 500°C stabilizes the austenite. The martensitic transformation after thermal stabilization has the following peculiarities: Undercooling of the austenite down to −196°C is possible at relatively slow cooling rates; the transformation of austenite to martensite proceeds isothermally after a certain in induction period. The effect of isothermal temperature and holding time on the transformation rate is represented by a curve showing a maximum.




3.

These rules do not apply to an ordinary martensitic transformation of unstabilized austenite.




4.

Thus, in thermally stabilized steel Kh15N9Yu the martensitic transformation proceeds so as to indicate that its kinetics is determined by the thermal vibrations of the atoms.

The martensitic transformation in stainless steels of the austenitic-martensitic class

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

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.

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

1.

A thermomechanical treatment with a 90% reduction below the recrystallization temperature of constructional steels with a broad range of stable austenite can develop σb=280 to 300 kg/sq.mm at δ5=6% which by far exceeds the properties of these steels after ordinary or stepped hardening followed by low tempering.




2.

The strength of a steel thus treated increased with carbon up to 0.5% C; at still higher carbon, the tensile strength fell and the fracture was semi-brittle or brittle.




3.

The optimum ratio of strength and ductility after such a treatment was achieved in these steels after tempering at 100°C; at higher tempering, both strength and ductility decreased.




4.

An increase of the deformation from 50 to 90% during a themomechanical treatment in the zone of stable austenite below recrystallization increased the strength considerably.




5.

A 50% reduction in the same conditions improved greatly the ductility but rather little the strength. The advantage of such a deformation is that it can be carried out at relatively high temperatures, even above recrystallization.




6.

Thermomechanical processing increased the brittle strength of steel.




7.

Such processing is accompanied by refinement of the martensite platelets and by a substantial decrease of the areas of coherent scattering (size of blocks). The structural refinement was apparently one of the factors which increased the resistance to plastic deformation as well as the brittle strength.

L. M. Pevzner

Papers

The martensitic transformation in stainless steels of the austenitic-martensitic class

Some rules for phase transformations in 000N18K9M5T steel

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

Thermomechanical treatment of steel for high strength