Research On Strength-Toughness Mechanism Of Low Nickel Ultra High Strength Maraging Steel

The strength-toughenss mechanism of the new Fe-Ni-Cr-Mo-Ti system00Ni13Cr9MoTi,00Ni14Cr7Mo3Ti maraging steel has been researched in this paper. It is using metallographic microscope, scanning electron microscopy and other modern materials analysis and testing methods to study the influence of different heat treatment on the austenite grain size, microstructure, residual austenite, the tensile properties and impact toughness, and to determine the most excellent heat treatment system, provide a theoretical basis for the actual production and process parameters.First of all, the effect of solution temperature on structure and mechanical properties of00Ni13Cr9MoTi maraging steel has been researched. The results show that:00Nil3Cr9MoTi marging steel at800℃solution treatment through non-diffusion reverse α’â†'γ transformation, but the reverse austenite does not recrystallize, still in the reply phase. From850℃the reverse austenite begins to recrysallize, forming smaller recrystallized grains, when the austenitizing temperature up to950℃, the recrystallization was completed, forming a complete closure of relatively uniform polygonal austenite grain. The grain morphology, grain size and high density defects due to forging are inherited after solid solution treatment at800℃, and the martensite inherits these defects which enhances the aging strengthening effect and increases the residual/reverse austenite, so the material has an higher combination of strength and toughness. Recovery and recrystallization of austenite lower the defect density when the steel was solutionized at high temperature, which decreases the residual/reverse austenite, thereby the toughness declines. But at the same time which makes up for the decline of the maraging strengthening effect, so the strength of the material does not reduce.Second, the effect of below the recrystallization temperature on structure and mechanical properties of OONi13Cr9MoTi maraging steel has been researched. The maraging steel was solution treated at un-recrystallized temperature (800℃) and after the recrystallized temperature (1000℃), then respectively aged at450℃,480℃,510℃and540℃. The results show that below the recrystallized solution treatment could be improve the strength and toughness.Third, the effect of solution temperature on structure and mechanical properties of00Ni14Cr7MoTi maraging steel has been researched.00Ni13Cr9MoTi maraging steel after the pretreatment of950℃, the austenite recrystallization, the distortion-free equiaxed grains replace the grain deformation in the forging process, eliminating the defects left in the organization in the process of forging. Followed by solution treatment at different temperatures. Low temperature (800℃) solution treatment, the material does not occur recrystallization and inherits the pretreatment organization, then which has the best mechanical properties.The solution treatment temperature of this kind of maraging steel like00Ni13Cr9MoTi,00Ni14Cr7MoTi is lower than the traditional18Ni maraging steel and Maraging stainless steel (Cr>12%). Maraging steel after low temperature solution treatment, the reverted austenite does not recrystallize, a large number of defects during non diffusion phase transformation are inherited by the martensite matrix, which is a important way to improve the combination of strength and toughness.