Common Heat Treatment Process Of Steel Casting Parts

4. tempering: tempering is the quenching or ignition after casting heating to Ac, below a selected temperature, insulation after a certain time, at the appropriate rate of cooling, make the quenching or ignition after unstable tissue into stable organization, eliminate quenching (or ignition) stress and improve the plasticity and toughness of cast steel a heat treatment process. Usually the process of quenching and high temperature tempering treatment is called quality adjustment treatment. The quenching steel casting must be tempered in time, and the hot steel casting must be tempered when necessary. The performance of the steel casting after tempering depends on the tempering temperature, time and times. With the increase of the tempering temperature and the extension of the time, in addition to eliminating the quenching stress of the steel casting parts, the unstable quenching martensite is converted into the tempering martensite, support or sotensite, so that the strength and hardness of the cast steel are reduced, and the plasticity is significantly improved. For some alloy elements containing strong carbide formation (such as chromium, molybdenum, vanadium and tungsten, etc.), the phenomenon of increasing hardness and toughness decrease at 400~500℃ is called secondary hardening, that is, the hardness of cast steel in the tempering state reaches the maximum value. Generally, the medium alloy casting steel with secondary hardening characteristics needs to be tempered repeatedly (1~3 times).
The tempering of steel casting parts can be divided into low temperature tempering and high temperature tempering according to different temperature.
(1) Low-temperature tempering: generally carried out in the temperature range of 150~250℃. After tempering can be air cold, oil cold or water cold. The purpose is to eliminate the quenching stress while retaining the high strength and hardness of the casting. Mainly used for carburization, surface quenching and wear-resistant steel casting parts requiring high hardness.
(2) High temperature tempering: the high temperature tempering temperature is 500~650℃, and cooled after appropriate insulation time. It is mainly used for the organization of cast steel after quenching or regular ignition, so that it has high strength and good toughness of carbon steel and low and medium alloy steel casting. The aling brittleness is a problem that must be paid attention to when making the tempering process of alloy steel castings. Can occur in both of the following temperature ranges.
Brittleness at 250 to 400℃: Cast steel quinto martensitissue produces tempering brittleness in this temperature range. If slightly higher than this brittle temperature area tempering, this tempering fragility can be eliminated. And later in the above temperature range, there will be no tempering brittleness, so it is often called the first type of tempering brittleness.
Brittleness occurs at 400~500℃ (even 650℃): this occurs for most low alloy cast steel, that is, the high temperature tempering brittleness of cast steel occurs. A steel casting already brittle in this temperature range are reheated to 600. Above C (or 650℃) and then quickly cooled in water or oil to eliminate this brittleness. However, the casting that has eliminated the fragility, such as heating to the temperature that produces the tempering brittleness, the brittleness will appear again. This is often called the second type of tempering fragility.
5. Solid solution treatment: Solid solution treatment is a heat treatment process of heating the casting to the appropriate temperature and insulation, fully dissolving the excess phase, and then rapid cooling to obtain the oversaturated solid solution. The main purpose of the solid solution treatment is to dissolve the carbide or other precipitated phase in the solid solution to obtain an oversaturated monophasic organization. General austenitic stainless heat-resistant steel, austenitic manganese steel and precipitation hardened stainless heat-resistant steel castings