The ultra-refinement treatment of austenite grains generally refers to the treatment method of refining the grain size of steel to more than 10 grades as 'grain ultra-refinement' treatment. After quenching after ultra-fine treatment, the steel can obtain high specified non-proportional elongation stress σp0.2, toughness and low ductile-brittle transition temperature. 1 The ultra-rapid heating method is mainly realized by using an energy source with ultra-rapid heating. Such as high-power electric pulse induction heating, electron beam heating and laser heating all fall into this category. 2 The rapid cycle heating and quenching method is shown in Figure 1. First, the parts are rapidly heated to above Ac3, and then rapidly cooled after a short time of heat preservation, and this cycle is repeated many times. Since the austenite crystals are refined every time they are heated, the grain size of the 45 steel is refined from grade 6 to grade 12 after 4 cycles. Figure 1. The process of 45 steel using rapid cycle heating and quenching method 3. The deformation heat treatment method is shown in Figure 2. The process is to first heat the steel to a temperature slightly higher than Ac3 to austenitize it, and then perform hot rolling to make the austenite strongly deform, and then keep it isothermally for an appropriate time, so that the deformed austenite occurs. Recrystallization begins, and quenching is carried out before the grains start to grow. Fig. 2 Deformation heat treatment method for obtaining ultra-fine grains Ultra-fine treatment of carbides At present, in addition to the ultra-fine treatment of austenite grains in production, the ultra-fine treatment of carbides in high carbon steel is also subject to General attention. This is because the size, morphology, distribution and quantity of carbides have a significant effect on the mechanical properties of steel such as toughness, fatigue strength, hardness and wear resistance. 1 High-temperature solidification quenching + high-temperature tempering (ie high-temperature quenching and tempering treatment), quenching after high-temperature solid solution can not only inhibit the precipitation of proeutectoid carbides, but also the martensite + retained austenite structure obtained by quenching is tempered at high temperature Afterwards, spherical carbides can be obtained with a uniformly dispersed distribution. 2 High-temperature solid solution isothermal treatment Some studies have proposed that the high temperature solid solution is first heated at 1040 °C for 30 min, followed by isothermal treatment at 625 °C or 425 °C, so that flaky pearlescent (625 °C isothermal) or bainite can be obtained. (425 ℃ isothermal) structure, and finally quenched and tempered according to the usual process. Quenching to control the microstructure and composition of martensite and bainite 1 Ultra-high temperature quenching of medium carbon alloy steel After quenching at normal temperature, a mixture of flaky martensite and lath martensite is generally obtained organize. The presence of flaky martensite is detrimental to the fracture toughness of the steel. Increasing the quenching temperature of medium carbon alloy steel is beneficial to obtain more lath martensite after quenching. 2. Low-temperature and short-time heating and quenching of high-carbon steel When using ordinary quenching process, high-carbon steel often obtains a flaky martensite structure, which has high brittleness. However, if the carbon content of austenite during quenching and heating is properly controlled, a lath-like martensite-based structure can also be obtained after quenching, so that the steel has good toughness while maintaining high hardness. 3 The cooling rate during continuous cooling to obtain the quenching of the composite structure Generally, the bainite transformation always starts preferentially in the carbon-depleted region. The increasing carbon content in austenite increases the chemical stability of austenite and makes it difficult to transform. Quenching to retain an appropriate amount of plastic second phase in the steel of free ferrite exists. Therefore, normal quenching or tempering (sometimes normalizing) is often required before sub-temperature quenching to obtain structures such as martensite, bainite, tempered sorbite, and sorbite. (1) The heating temperature of sub-temperature quenching is in the α+γ two-phase region. Due to the low temperature and the finely dispersed insoluble carbon and nitride points in the steel, the growth of austenite grains is greatly affected. The hindering effect makes the austenite grains very fine at this time. (2) The various elements contained in steel can be divided into two categories: elements that expand the γ region (such as carbon, manganese, nickel, nitrogen, etc.) and elements that reduce the γ region (such as phosphorus, antimony, tin, silicon, etc.). Figure 3 shows the phase diagram of two types of binary iron-based alloys. Fig. 3 Phase diagram of binary iron-based alloys (a) Enlarge the elements in the γ region (b) Reduce the elements in the γ region (3) For steels containing aluminum, niobium, vanadium, Qin and other elements, when heated in the sub-temperature region, the There are a small amount of fine dispersed carbides and nitrides. When tempering after quenching, they can be used as nuclei for carbides to precipitate in the crystal, thereby reducing the intergranular precipitation of carbides.This is very beneficial to improve the toughness of steel. 2 Heat treatment to control the morphology, quantity and stability of retained austenite By adjusting the quenching heating temperature, cooling specifications (including the temperature and time of isothermal treatment) and tempering process, the morphology, quantity and stability of retained austenite can be controlled to a great extent. Distribution, quantity and stability. Fig. 4 Effect of retained austenite content on contact fatigue life of GCr15 steel (Source: TAIYITONG)