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

Bestimmung des Aufmischungsgrades beim Laser-Pulver-Auftragschweißen mittels laserinduzierter Plasmaspektroskopie (LIPS)

1.

For the Cr−Ni−Co−Mo steels investigated the character of the martensitic transformation changes with decreasing MS (induced by slight changes in chemical composition) — the isothermal transformation is superposed on the athermal transformation.




2.

The rate of the isothermal transformation increases with decreasing MS and evidently for this reason the susceptibility of austenite to thermal stabilization increases in the process of cooling and during repeated heating after quenching.




3.

For steels with>15% retained austenite after the original quenching the cooling conditions after austenitizing must be strictly controlled (cooling with compressed air, for example), since the amount of retained austenite may be affected by stabilization and destabilization processes and also stresses that occur in the process of cooling during quenching.




4.

Steels of the transition class investigated cannot be heated in the temperature range of 200–500° (welding, cutting, et al.) before the cold treatment.

Stabilization and destabilization of austenite in Cr−Ni−Co−Mo maraging steel

1.

The mechanical properties of the transition class stainless steel 03K12N5KM in the quenched condition are determined by its phase composition and do not depend to any significant extent on its Co and Mo contents.




2.

For the same steel in the aged condition, Co and Mo additions have an effect if the austenite content is less than 50%. If it is higher than 50%, then the mechanical properties of the steel are the same as in the quenched condition.




3.

The maximum aging effect was observed for the steel with an austenite content of 10–20%, with the Co content increased from 8–10% to 12%, and the Mo content increased from 4–5% to 6%. The ulitmate tensile strength of such a steel is 30% higher and the impact toughness is 3 times lower.




4.

The ductility and impact toughness of the steel studied are determined by its strength, which depends to an equal extent on the phase compostion and on the total concentration of alloying elements in the hardening phase.




5.

To obtain maximum impact toughness, the Co and Mo concentrations in the steel have to be restricted to the minimum value which provides the necessary strength level.

Effect of residual austenite and content of hardening elements on the mechanical properties of transition class stainless steels

1.

The martensite-aging stainless steel 05Kh12K14N5M5T-BD after hardening at 1050°, cold treatment at −70°C, and aging at 520°C is characterized by σ0.2 ≈ 1650 MPa, σu ≈ 1700 MPa,a1≈0.4 MJ/m2 and has in its structure about 10% of residual austenite.




2.

The results of statistical processing of the analysis of the mechanical properties of 05Kh12K14N5M5T-BD steel show that after heat treatment by a standard process it has stable and high strength properties (75% of σ0.2 values are higher than 1600 MPa), and unstable impact strength (a1=0.1–0.5 MJ/m2).




3.

The second hardening at 750°C preceding aging stabilizes about 10% of the residual austenite therby increasing the impact strength level.




4.

An increase of the second hardening temperature from 730 to 770°C results in a considerable change in the strength and resistance to crack propagation. Therefore, for the use of 05Kh12K14N5M5T-BD steel, it is necessary that the heat-treatment conditions are followed precisely.




5.

The stabilization of austenite by heating to 560–670°C results in resoftening of steel and an intense increase ofa1 andaT at +20 and −50°C. A partial or complete transformation of austenite into martensite during the impact tests at −196°C does not increase the impact strength. This can be achieved when the structure of steel prior to repeated heating contains ∼5% of austenite.

Mechanical properties of stainless martensite-aging steel 05Kh12K14N5M5T-VD

1.

Retained austenite from the direct \gg\arM transformation reduces the susceptibility to brittle fracture of cast maraging stainless steels of the Cr\t-Ni\t-Co\t-Mo type; austenite obtained in the reverse transformation has no such effect.




2.

Increasing the amount of retained austenite in steel 00Kh12N5K13M6 from 5 to 30% leads to an increase of the toughness (a n \t-70 increases from 0.5 to 3.5 kg-m/cm2), lower tendency to cracking (a p +20 increases from 0.5 to 1.5 kg-m/cm2, KIc increases from 170 to 300 kg/mm3/2), and lower angle of deflection of notched samples. The strength characteristics, especially the proof stress, decrease (σ0.2 declining from 170 to 130 kg/mm2).




3.

For the steels investigated there are certain limits at which the amount of austenite ensures the best strength and toughness.

For maraging Cr−Ni−Co−Mo steel with 18–28% austenite the toughness is high (a n −70 ≥3.0 kg-m/cm2,ap≥1.5 kg-m/cm2) at a strength level of σb ⪰ 170 kg/mm2, σ0.2 ⪰ 130 kg/mm2.




4.

To obtain the necessary quantity of austenite within such narrow limits proved to be possible under production conditions only by means of the magnetic method of checking the melting process, with use of a modified MKL instrument.

V. L. Nikol'skaya

Papers

Effect of retained austenite on the properties of cast stainless steels

Stabilization and destabilization of austenite in Cr−Ni−Co−Mo maraging steel