Ultra high strength steel ” is defined relative to the requirements of the era of technical progress and changes in the. Generally speaking, the yield strength of 1 370MPa ( 140 kgf/mm2 ) and above, tensile strength is 1620 MPa ( 165 kgf/mm2 ) above the alloy steel called ultra high strength steel. According to their degree of alloying and microstructure consists of medium carbon low alloy martensitic hardening ultrahigh strength steel, carbon in alloy two precipitation hardening type ultra high strength steel, high carbon in alloy of Ni – Co type ultra high strength steel, ultra low carbon martensite aging hardening high strength steel, austenite precipitation hardening.
Medium carbon low alloy martensitic hardening type ultra high strength steel ( MART ) is in low alloy quenched and tempered steel is developed on the basis of the alloy elements, generally not more than 6% of the total. Main brands include traditional chromium molybdenum steel 4340 ( 40CrNiMo ), carbon content of 0.45% nickel chromium molybdenum vanadium steel D6AC ( 45 CrNiMoV ), carbon content of 0.30% chromium manganese silicon nickel steel ( 30CrMnSiNi2A ), on 4340 steel basis through the addition of silicon (1.6% ) and V ( 0.1%) and developed 300M steel ( 43CrNiSiMoV ) and containing no nickel manganese molybdenum vanadium or chromium molybdenum vanadium. Through vacuum smelting reduction in steel impurity element content, improve steel transverse ductility and toughness of the alloy elements in steel, low content, low cost, simple production technology, is widely used in aircraft landing gear, the engine shaft, frame, bolt of high strength, solid rocket engine shell body and chemical pressure vessel.
In the carbon in alloy two precipitation hardening type ultra high strength steel is from 5%Cr die steel moved to. Because of its high temperature tempering state has high strength and good plasticity and toughness, heat resistance, stable, for aircraft landing gear, rocket shell. A typical steel grades for H11 and H13. Its main components are: C 0.32%–0.45%; Cr 4.75%–5.5%; Mo 1.1%–1.75%; Si 0.8%–1.2%.
High carbon in alloy Ni – Co ( 9Ni–4Co– ××) type ultra high strength steel, is in has high toughness, low brittleness transition temperature of 9%Ni type low temperature steel is developed on the basis of. In 9%Ni steel is added in order to improve the drilling steel Ms ( martensitic transformation temperature ), reduce the residual austenite in the steel, at the same time, drilling in the nickel in solid solution strengthening effect, also by drilling to obtain steel self tempering characteristics, so this kind of steel with excellent welding performance. In this type of carbon steel reinforcement. Steel also contains a small amount of chromium and molybdenum, in order to produce dispersion strengthening effect of tempering. The main brands are HP9-4-25, HP9-4-30, HP9-4-45 and modified AF1410 ( 0.16%C-10%Ni-14%Co-1%Mo-2%Cr-0.05%V ) etc.. This kind of steel mechanical properties of high. Stress corrosion resistance, good technological property and welding performance, widely used in aviation, aerospace and submarine light body and other products.
Ultra low carbon martensite aging hardening high strength steel, usually called maraging steel. The steel matrix for ultra low carbon nickel iron or iron nickel cobalt martensite. Its characteristic is, martensite formation does not require the fast cooling, variable temperature and isothermal forming; have the body-centered cubic structure; hardness of about HRC20, plastic good; then heating did not appear in low carbon martensite tempering phenomenon occurs, and the great reversal of variable temperature hysteresis, so can be in a higher temperature of martensite matrix in vivo aging hardening. In such a high Ni in martensite containing can cause aging strengthening of alloy elements, with the help of aging, from supersaturated Martensite Precipitation dispersive intermetallics, to obtain high strength and high toughness steel. According to the content of nickel, maraging steel is divided into 25%Ni, 20%Ni, 18%Ni and 12%Ni type.18%Ni application is broad, containing molybdenum, titanium and other reinforcing elements of ultra low carbon iron nickel ( 18%) – drill ( 8.5% ) alloy, including 3 brands: 18%Ni ( 200), 18%Ni ( 250), and 18%Ni ( 300) ( 200, 250, 300 for the tensile strength grade, unit : Ksi). This kind of steel is made by intermetallic compounds because of the precipitation of steel reinforcement. Borrow without carbon martensite matrix to obtain the high plasticity, finally achieves very high strength plastic with. This kind of steel has good formability, weldability and dimensional stability, heat treatment process is also simpler, used in aviation, spacecraft components and cold extrusion, cold stamping die.
Half austenitic precipitation hardeningis a kind of high alloy ultra high strength steel, such as common 17-7PH ( OCr17Ni7Al ), PH15-7Mo ( OCr15Ni7Mo2Al ) and AFC-77 ( 15Cr15Mo5Co14V ) etc.. This kind of steel after solid solution treatment, cooling to room temperature for austenite, after processing, cold processing or heating to 750 ℃ by adjusting process, phase transformation from austenite to martensite. Finally at 400-550 ℃ aging, will be in the tempered martensite matrix dispersed second phase strengthening organization of ultra high strength steel. This kind of steel at 315℃when used for a long time, because of intermetallic compound precipitation and the brittle materials, so the use of temperature below 315 ℃to limit. This kind of steel is mainly used in the manufacture of aircraft components, pressure vessels and the high stress corrosion of chemical equipment parts.