Quenching process, quenching medium and cooling method

The quenching process is a heat treatment process in which the steel is heated to a temperature above AC3 or AC1 for a certain period of time, and then cooled at an appropriate speed to obtain martensite and (or) bainite structure.
The purpose of quenching is to improve the hardness, strength and wear resistance to meet the performance of the parts. Quenching process is the most widely used, such as tools, measuring tools, molds, bearings, springs and automobiles, tractors, diesel engines, cutting machine tools, pneumatic tools, drilling machinery, agricultural machinery, petroleum machinery, chemical machinery, textile machinery, aircraft and other parts. Use quenching process.

(1) Quenching heating temperature

The quenching heating temperature is determined according to the composition, structure and different performance requirements of the steel. Hypoeutectoid steel is AC3 (30-50°C); eutectoid steel and hypereutectoid steel are AC1 (30-50°C).

If the quenching heating temperature of hypoeutectoid steel is lower than AC3, the steel has not been completely austenitized at this time, and there is some untransformed ferrite. After quenching, the ferrite still remains in the quenched structure. The hardness of ferrite is low, so that the hardness after quenching is not up to the requirement, and it also affects other mechanical properties. If the hypoeutectoid steel is heated to a temperature much higher than AC3 for quenching, the austenite grains will be significantly coarsened, which will damage the properties after quenching. Therefore, the quenching heating temperature of hypoeutectoid steel is AC3 (30-50℃), which not only guarantees sufficient austenitization, but also keeps the austenite grains fine.

The quenching heating temperature of hypereutectoid steel is generally recommended as AC1 (30~50℃). In actual production, the temperature should be appropriately increased by about 20°C according to the situation. Heating in this temperature range, its structure is fine-grained austenite and some fine and uniformly distributed undissolved carbides. After quenching, except for a small amount of retained austenite, its structure is uniformly distributed fine carbonized material points on the flaky martensite matrix. Such a structure has high hardness, wear resistance, and relatively less brittleness.

The quenching heating temperature of hypereutectoid steel cannot be lower than AC1, because the steel has not yet been austenitized at this time. If heated to a temperature slightly higher than AC1, the pearlite completely transforms into austenite, and a small amount of cementite is dissolved into the austenite. At this time, the austenite grains are fine, and the carbon mass fraction is slightly higher than the eutectoid composition. If the temperature continues to increase, the secondary cementite will continue to dissolve into the austenite, causing the austenite grains to grow and the carbon concentration to increase, which will increase the tendency of quenching deformation and increase the microcracks of the quenched structure And brittleness increases. At the same time, due to the high carbon content of austenite, the amount of retained austenite after quenching increases, which reduces the hardness and wear resistance of the workpiece. Therefore, it is inappropriate for the quenching heating temperature of hypereutectoid steel to be too much higher than AC1, and it is even more inappropriate to heat to a temperature of ACm or above which is fully austenitized.

When selecting the quenching heating temperature of the workpiece in production practice, in addition to complying with the above general principles, the chemical composition, technical requirements, size and shape, original structure of the workpiece, heating equipment, cooling medium and many other factors should also be considered. Make appropriate adjustments. For alloy steel parts, the upper limit is usually taken, and the lower limit is taken for parts with complex shapes.

The quenching heating temperature selected for the new strengthening and toughening process is different from the commonly used quenching temperature. For example, sub-temperature quenching is the quenching of hypoeutectoid steel after austenitization at a temperature slightly lower than AC3, which can improve toughness, reduce brittleness transition temperature, and eliminate temper brittleness. For workpieces made of materials such as 45, 40Cr, 60Si2, the heating temperature for sub-temperature quenching is AC3-(5~10℃).

High temperature quenching can obtain more lath martensite or increase the strength and toughness of all lath martensite. For example, 16Mn steel is quenched at 940°C, 5CrMnMo steel is quenched at 890°C, and 20CrMnMo steel is quenched at 920°C. The effect is better.

High-carbon steel is quenched by low-temperature, rapid and short-time heating, and the quenching heating temperature of high-carbon steel is appropriately reduced, or the method of rapid heating and shortening the holding time can reduce the carbon content of austenite and improve the toughness of steel.

(2) Holding time

In order to make the internal and external parts of the workpiece complete the structure transformation, carbide dissolution and austenite composition, it is necessary to keep the quenching heating temperature for a certain time, that is, the holding time.

(3) Quenching medium

The medium used for quenching and cooling of the workpiece is called the quenching cooling medium (or quenching medium). The ideal quenching medium should have the condition that the workpiece can be quenched into martensite without causing too much quenching stress. This requires slow cooling at the temperature above the “nose” of the C curve to reduce the thermal stress caused by rapid cooling; the cooling rate at the “nose” should be greater than the critical cooling rate to ensure that non-Martensite supercooled austenite does not occur Body transformation; under the “nose”, especially when Ms is at the same temperature, the cooling rate should be as small as possible to reduce the stress of tissue transformation.