Influence On The Mechanical Properties Of Low Alloy Ultra High Strength Steel With Nonhomogeneous And Unstable Structure

Low alloy ultra high strength steel is widely applied in the aviation, aerospace,weapon industry and civil facilities. Because of the exploitation conditions different, the operational performance also requests multiplicity. This article researched on the mechanical properties and fracture behaviors of 30CrNiSiMoV and 35SiMnCrMoVA steel employed in non-normal conditions.After normal heat treatment,30CrNiSiMoV steel carries on bending test(including notch and non)and Charpy notch impact in room temperature, 100°C, 200°C, 250°C, 300°C and 360°C respectively, by means of the metallography, SEM, TEM analyzes the microstructure and the fracture characteristic. The results indicated that: The non-notch bending strength (σbb) is 2316MPa at room temperature, between 100°C ~300°Cσbb is lower than it, 250°C falls to the minimum value 1749MPa, but 360°C elevates to 2469MPa, outdoes the room temperature value. Under various temperatures notchσbb all is lower than the corresponding non-notch, displays the notch sensitive tendency. The impact toughness first rises after the experiment temperature increment to fall, obtains the maximum (58J/cm2) at 200°C, falls to the room temperature value at 300°C, and at 360°C falls to the minimum value (36.8J/cm2). Between 100°C~250°C plastic deformation mainly is the slipping in the breaking process, but 250°C the slip bands congregate on the martensitic lath grain boundary cause to widen the breadth, the fracture characteristic demonstrates the plastic deformation slightly in the initial break stage, which appears the intensity and the plasticity simultaneously drop. In 300°C~360°C the plastic deformation is accompanied by the twin deformation in the martensitic lath transgranular, the microstructure presents unstable characteristic, carbides trend to be strain-induced precipitation. It is the primary factor to enhance the bending strength and reduces the impact toughness. 35SiMnCrMoVA steel is studied on the obdurability rule and crack behavior by the tensile experiment and the crack opening displacement (COD) test after different temperature (880°C and 900°C) quenching andmedium and high temperature tempering, and the metallography, SEM, TEM analyzes the microstructure and the fracture characteristic. The result indicated that: The microstructure is a non-homogeneous mix structure of the tempering troostite and the tempering sorbite between tempering in 460℃~ 500℃and carbides centralizing precipitation occurs on the original lath martensitic grain boundary that is developing. COD is the least, namely fracture toughness is lowest. The microstructure refining would enhance its fracture toughness but do not alter the rule with the temper temperature, and the influence on the plasticity and intensity including the hardness is not obvious. Researched on the notch effect, strength and toughness and fracture behavior of the nonnhomogeneous and unstable structure shows that, these steels employ in abnormal temperature or tempering in medium and high temperature, the notch effect and fracture toughness must be considered especially.Studied on the influence of the unsteady and non-homogeneous microstructure of low alloy ultra high strength steels on the mechanical properties and fracture behavior demonstrates that, this kind of steels employ in the unusual regulation conditions must be considered its notch effect and toughness specially.