It is well known that the heat treatment process of maraging stainless steel is aging treatment at around 500 ° C after solution treatment, wherein the solution treatment temperature has an important influence on the final toughness. Studies have shown that relatively low solution treatment temperature is beneficial to improve performance. Especially improve the stress corrosion resistance [1]. However, the low solid solution treatment temperature remains more χ phase and σ phase deteriorates toughness [2-4], Novikov et al. attempt to reduce the Co and Mo content of Cr-Ni-Co-Mo maraging stainless steel, resulting in lower solids The enthalpy phase and σ phase are no longer left in the solution treatment temperature, which significantly improves the toughness, especially the ultra-low temperature impact toughness [3]. The authors try to reduce the solution temperature of Cr-Mo-Ti maraging steel with relatively low Cr content. The advantages of mechanical properties of low temperature solution treatment materials were compared.

1 Test materials and methods

Test steel Cr-Mo-Ti maraging aging steel, smelted with 25Kg vacuum induction furnace, the main alloying element content: Cr: 8.92%wt, Ni: 12.64%wt, Mo: 1.46%wt and Ti: 1.37% Wt, the smelting blank is forged into a 40×40mm billet at 1150~850°C, and the phase transition temperature measured by the Formastor-D automatic phase change measuring instrument is: AC1=625°C, AC3=740°C, Ms=60°C. Stretching, V-notch impact blanks were cut on the billet. The blank samples were divided into two groups. The first group was treated with 800 ° C low temperature solution treatment, the second group was treated with conventional 1000 ° C higher temperature solution, and the two groups of solution treated samples were cold treated at -73 ° C × 2 h, followed by The two groups of solution treated samples were subjected to 450, 480, 510 and 540 °C for 4 hours, and the final processed tensile specimens were Φ5 mm in gauge length and 25 mm in gauge length. The impact specimens were 10×10×. The standard V-notch impact test of 55 was finally tested at room temperature tensile properties and impact toughness by WE300B tensile tester and JBN-300B impact tester respectively. The mechanical properties of the same heat treatment state were averaged with three test samples.

2 experimental results and analysis

2.1 Austenite formation and changes in its grain morphology

The solution treated by the solution treatment was cut into a metallographic sample, and the prior austenite grain boundary was boiled by H2SO4 and KMnO4 aqueous solution. The metallographic microscope observation showed that the 800 °C low temperature solution treatment had exceeded the AC3 temperature (740 ° C), but However, the grain morphology and size of the forged deformation are inherited (Fig. 1a), indicating that austenite is formed by the non-diffusion α'→γ reverse transformation mechanism. In addition, since the Cr content is lower than that of the conventional maraging stainless steel, the low-temperature solution treatment at 800 ° C is mainly composed of an austenite matrix, and no obvious residual second phase is observed. Due to the high density of austenite formed by non-diffused α'→γ, the austenite with higher relative solution temperature is recovered and recrystallized [5], so the solution treatment at 1000 °C is completely closed. Relatively uniform polygonal austenite grains (Fig. 2b).

2.2 Influence of solution treatment temperature on mechanical properties

The change of degree shows the same law, that is, the aging at 450 °C is still under-aged, the peak of yield and tensile strength occurs at 480 °C, and the aging temperature is further increased into over-aging, that is, the yield and tensile strength decrease; however, the plasticity and impact toughness With increasing aging temperature, it is always rising. It is worth noting that the peak tensile strength at 480 °C is 1773 and 1760 MPa, which belongs to the category of ultra-high strength steel. The aging strength at 510 °C is only 4.1% lower than that at 480 °C. And 3.0%, but the V-notch impact power increased by 29.0% and 45.8%, respectively, so the 510 °C aging is an optimized aging temperature.

Comparing the mechanical properties of different temperature aging, it can be seen that the yield and tensile strength of the solution treated at 800 °C and 1000 °C are similar, but the V-notch after aging at 450 °C, 480 °C, 510 °C and 540 °C The impact toughness is 37.2%, 46.3%, 30.7% and 18.4% higher than that of the solution treated at 1000 °C, so the strength and toughness of the 800 °C low temperature solution treatment material is very obvious. In order to clarify the better toughness of 800 °C low temperature solution treatment, X-ray diffraction analysis was carried out on the best 510 °C aging samples with strong toughness. It can be seen that the final residue/reverse of the sample at 800 °C low temperature solution treatment The relative intensity of the transformed austenite diffraction peak is significantly higher than that of the solution treated at 1000 °C (Fig. 3). The relative residual intensity of the diffraction peak is calculated. The final residual/reverse transformed austenite volume fraction of the 800 °C low temperature solution treated sample is calculated. 15.3%, and the solution treated at 1000 °C is 9.0%. Obviously more residual/reverse transformed austenite causes the material with low temperature solution treatment at 800 °C to have higher impact toughness, but does not reduce yielding. Tensile strength, which is related to the phase transformation involved in the low temperature solution treatment at 800 °C. First, the solid solution treatment at 800 °C inherited the forged structure, that is, transformed into a high defect density austenite in the form of α'→γ non-diffusion. The high-density defect austenite martensitic transformation resistance is large, and there are more retained austenite after cooling and cold treatment. On the other hand, since the formed martensite inherited this high-density defect again, it is advantageous to form the reverse transformation austenite by diffusion at the time of aging, and thus the final residual/reverse transformation austenite amount is necessarily higher. In addition, the high density of martensite defects after 800 °C low temperature solution treatment is beneficial to the aging treatment to enhance the dispersion distribution of precipitates, enhance the age strengthening effect of martensite, and make up for more residual/reverse transformation austenite. Softening effect.

3 Conclusion

3.1 800 ° C low temperature solution treatment to form austenite by non-diffusion α' → γ, the morphology and size of the austenitic grain of genetic forging, and the solution treatment at 1000 ° C due to recrystallization, forming a relatively uniform polygon recrystallization Austenite grains.

3.2 After aging between 450 °C and 540 °C, the samples treated at 800 °C and 1000 °C have similar strength, but the solution treatment at 800 °C forms austenite with non-diffusion α'→γ, and its high density defect eventually inherits to Markov. In the body, it not only enhances the effect of strengthening the aging, but also increases the residual/reverse transformation of austenite, so it has better toughness.

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