Secondary Hardening Ultrahigh Strength Steel Ferrium S53, The Organization And Mechanical Properties Study

From the second half of the 20th century until now, ultrahigh strength steel has became a specialized branch of material science and engineering after a half century development, which was developed from original low alloy ultrahigh strength steel to high alloy secondary-hardening ultrahigh strength steels which are mainly strengthened by precipitation of M2C carbide. In this thesis, the mechanical properties, the microscopic structure of Ferrium S53, a new secondary hardening ultra-high strength steel has been studied.Ferrium S53 is a new secondary hardening ultra-high strength stainless steel with nominal composition as 0.21C-14Co-10Cr-5.5Ni-2Mo-1W-0.3V, developed by the QuesTak at the end of last century. It can be used as a structure steel because of the high strength (Rm=1930Mpa, Rp0.2=1580Mpa) and good toughness (KIC=55MPa·m1/2). The Ferrium S53's mechanical properties are equal to the 300M steel and it has better corrosion resistance than 300M steel.Based on the Ferrium S53 a heat of experimental steel was smelting and the mechanical properties, the microscopic structure and precipitate phase were studied by mechanical properties tests, metallographic microscope, scanning electron microscope, transmission electron microscopy and X-ray diffraction.After tempering at 200℃lots of s-carbides are precipitated in the martensite, the strength rises while toughness falls slightly. After tempering at 300℃, much coarsed cementite lamina are precipitated in martensite, the strength continue rises and toughness falls. With the aging temperature increasing, cementites are dissolved, the M2C carbide and the reverted austenite precipitate between the matensite lath. The tensile strength and hardness increase rapidly, the toughness roughly remain. When the aging temperature achieve to 560℃, the size of M2C carbide and the reverted austenite are grows up, and the M23C6 carbides precipitated, the mechanical properties of the experimental steel fall down. The Increase of steel's strength is mainly depend on the precipitation strengthening of M2C carbides during tempering process and martensite matrix with high density dislocations. The high-purity metallurgical process and the reverted austenite precipitated at the boundaries of lath matensite and grow up into thin-film shape along the lath boundaries, which improve the toughness.