It is necessary to understand the process of machine tool parts during quenching, heating, annealing, cooling, etc.

Parts heat treatment processability refers to the feasibility and economy of heat treatment production on the premise of meeting performance requirements. The heat treatment process of machine tool parts involves not only the materials and structures of machine tool parts, but also is closely related to the production process and heat treatment process of machine tool parts. Therefore, it is very necessary to correctly formulate the heat treatment process, ensure the quality of machine tool parts and products, improve production efficiency and reduce production costs.

The heat treatment processability of machine tool parts mainly includes hardenability, hardenability, temper brittleness, overheat sensitivity, tempering stability, oxidative decarburization tendency, etc.

(1) Oxidative decarburization tendency. During the heating process of steel, due to the effect of the surrounding oxidizing atmosphere, the iron on the surface is oxidized into ferrous oxide, causing the steel surface to lose its original luster, which is called oxidation. Decarburization means that the carbon on the steel surface is oxidized into CO, CH4 and other gases, reducing the carbon content on the steel surface.

Oxidation causes the surface of the workpiece to lose its metallic luster, increases the surface roughness value, and decreases precision, affecting the size of the workpiece and reducing the surface quality. It also reduces the strength of the steel and other mechanical properties, increasing the possibility of quenching cracking and quenching soft spots. . Decarburization decarbonizes the surface of the workpiece, resulting in a significant reduction in the quenching hardness, fatigue performance and wear resistance of the steel parts. Severe oxidation and decarburization will cause the workpiece to be scrapped. Among various steel types, Si-containing steel has a greater tendency to oxidative decarburization.

In industrial production, oxidative decarburization of workpieces should be avoided as much as possible, and important parts are not allowed to have oxidative decarburization layers on the final parts. Therefore, during the production process, the processing allowance should be reasonably reserved and the processing flow should be properly arranged. In the heat treatment process, various heat treatment processes with less oxidation and decarburization should be actively adopted to control oxidation and decarburization to ensure the heat treatment quality of the workpiece and obtain Stable and reliable performance.

(2) Hardenability. The hardenability (hardenability) of steel refers to the highest hardness that the steel can achieve after quenching, that is, the hardening ability of the steel during quenching. It mainly depends on the carbon content of the steel (martensite). The higher the carbon content of the steel, the higher the hardness after quenching, and other alloying elements have less influence. Therefore, when the surface hardness of the workpiece is required to be high, medium carbon or high carbon steel should be selected; when the surface hardness is not required, medium carbon or low carbon steel is generally selected. It should be noted that the quench-hardenability and hardenability of steel are two completely different concepts.

(3) Tempering stability. The so-called tempering stability refers to the ability of quenched steel to resist the decrease in hardness and strength during the tempering process. Since the alloying elements hinder the diffusion of atoms, the decomposition and transformation speed of the structure of quenched carbon steel is slowed down during the tempering process, pushing the four stages of tempering transformation to higher temperatures, causing the hardness after tempering to decrease more slowly. Thereby improving tempering stability. Compared with carbon steel, alloy steel has higher strength and hardness under the same tempering temperature; while maintaining the same strength and hardness, alloy steel has a higher tempering temperature, a longer tempering time, and the internal stress is eliminated. Thorough, so it has high plasticity and toughness. In industrial production, alloy steel with high tempering stability should be used for workpieces that require complete internal stress relief and good strength and toughness. For workpieces with higher operating temperatures, steel types with high tempering stability should be selected. Generally, the maximum operating temperature is 50°C below the tempering temperature.

(4) Tempering brittleness. During the subsequent tempering process, the impact toughness of quenched steel is significantly reduced in the two temperature ranges of 250~350℃ and 400~sso℃, and the embrittlement phenomenon is called temper brittleness.

1. Low temperature temper brittleness. The brittleness that occurs when tempered at 250~350℃ is low temperature temper brittleness (the first type of temper brittleness). Almost all industrial steels exhibit this type of brittleness. The occurrence of this type of brittleness has nothing to do with the cooling rate after tempering, that is, tempering in the brittleness temperature zone will produce this type of brittleness regardless of fast cooling or slow cooling.

At present, there is no way to completely eliminate the first type of temper brittleness. Usually, tempering is avoided in this temperature range, or isothermal quenching is used instead of quenching and tempering, or a small amount of silicon and other alloying elements are added to the steel to reduce the brittleness. The temperature zone increases.

2. High temperature temper brittleness. The brittleness produced by slow cooling after tempering at 400~550℃ is called high temperature temper brittleness (the second type of temper brittleness). This type of temper brittleness is reversible, that is, if the steel that has developed this type of temper brittleness is reheated to above 650°C and then cooled quickly, the brittleness will disappear; if the steel is tempered and kept warm and then cooled slowly, the brittleness will reappear, so it will appear again. Called reversible temper brittleness. This type of temper brittleness mainly occurs in structural steel containing alloy elements such as Cr, Ni, Si, Mn, etc.

The way to prevent high temperature temper brittleness is to use advanced smelting technology (such as electroslag remelting steel, vacuum remelting steel, etc.) to improve the purity of the steel and minimize the content of impurity elements (such as S, P, etc.) in the steel. ; Or add W (1%), Mo (0.5%) and other alloying elements that can inhibit grain boundary segregation; avoid tempering between this temperature (450~650℃); if tempered in this temperature range, temper After fire insulation, it should be cooled quickly (such as water cooling or oil cooling) to inhibit the precipitation of impurity elements.

(5) Hardenability. The so-called hardenability refers to the ability of steel to obtain the depth of the quenching layer during quenching. It is an inherent property of the steel itself, and its size is usually expressed by the depth of the quenching layer under specified conditions (referring to a steel sample of specified size and shape, quenched under specified quenching cooling conditions). The deeper the quenching layer of the steel after quenching under specified conditions, the better its hardenability. For structural steel, it is generally specified that the vertical distance from the surface of the steel piece to the semi-martensitic zone (i.e. 50% zone M structure) is regarded as the depth of the quenching layer.

The hardenability of steel directly affects its mechanical properties after heat treatment. In addition, the amount of martensite in the quenched structure will also affect the yield ratio and fatigue strength of the steel. For workpieces that do not allow plastic deformation, it is generally desirable to have a higher yield-strength ratio to maximize the utilization of material strength. The greater the amount of martensite, the higher the fatigue strength of the steel after tempering.

The hardenability of steel causes the steel to produce size effects. Different cross-sectional sizes of parts result in different depths of the hardened layer, which also affects the surface hardness of the quenched parts. Therefore, full attention must be paid to the hardenability of the material, and materials should be selected rationally. For large sections or Important parts with complex shapes should use alloy steel with good hardenability. Add alloy elements Mn, Si, Cr, Ni and trace amounts of B to improve hardenability, while V, Ti, Mo, W, etc. can refine the grains. Function, they can ensure a good fit with high strength and high toughness along the entire cross-section, while reducing heat treatment deformation and cracking. Therefore, the processing route should be reasonably arranged according to the hardenability of the steel. When the size of the part is large and the hardenability is limited, in order to ensure the depth of the hardened layer, rough machining can be used first, then heat treatment, and then finish machining after heat treatment. Parts with large cross-section differences, such as large-diameter stepped shafts, can be roughly turned into shape first and then tempered to increase the depth of the hardened layer from the perspective of hardenability.

In short, the hardenability of steel is an important basis for formulating the heat treatment process. When quenching steel with good hardenability, a slower quenching medium and a slower cooling quenching process can be used to reduce the tendency of deformation and cracking of parts.

(6) Overheating sensitivity. It includes austenite grain growth, overheating, overburning, etc. When the steel parts are heated beyond the critical temperature, an austenite structure is obtained. As the heating temperature increases and the holding time is extended, the austenite grains will gradually grow. Different steel types have different smelting methods, and the austenite grains will grow longer. The general tendency is also different. When steel contains elements such as Mn, C, and P, it will promote the growth of austenite grains; the reduction of elements such as W, Mo, Cr, and trace elements such as Al, Ti, V, Zr, and Nb will cause the austenite grains to grow. There is a tendency to grow up.。 

Machine tool parts are closely related to the overall operation of the machine tool. Heat treatment processes such as quenching and return of machine tool parts directly affect the quality of machine tool parts, and thus the machine tool itself. Therefore, it is very necessary to understand the heat treatment process of machine tool parts. Zhengzhou Gou's is a manufacturer specializing in the production, manufacturing and sales of small induction heating equipment such as high-frequency quenching equipment, super audio frequency induction heating equipment, medium frequency induction heating power supplies. It has been engaged in induction heating work for many years and is very knowledgeable in the quenching and annealing of mechanical parts. Learn more. If you have questions in this regard, you can call us for more information.