The scrap rate caused by the quenching process in the heat treatment process is often high. This is mainly because large thermal stress and structural stress are formed during the quenching process. In addition, due to factors such as inherent metallurgical defects in the material, improper material selection, wrong materials, poor structural processability in design, and defects formed during cold and hot processing, they are all easily exposed during the quenching and tempering process. Therefore, the defects of parts after quenching and tempering must be systematically analyzed and investigated.
Quenching defects and prevention
The most common defects in steel quenching are quenching deformation, cracking, oxidation, decarburization, insufficient or uneven hardness, surface corrosion, overburning, overheating, and other metallographic structures that are unqualified according to quality inspection standards.
1. Quenching deformation and quenching cracks
In actual production, effective preventive measures should be taken according to the causes of quenching deformation and quenching cracks.
(1) Try to achieve uniform heating and correct heating
When the shape of the workpiece is complex or the cross-sectional dimensions vary greatly, it is often deformed due to uneven heating. For this reason, before loading the workpiece into the furnace, asbestos ropes should be used to block or tie holes and cross-section mutations that do not require quenching to improve their heating conditions. For some thin-walled rings and other easily deformed parts, specific quenching fixtures can be designed. These measures are conducive to both uniform heating and uniform cooling. When the workpiece is heated in the furnace, it should be placed evenly to prevent one-sided heating. It should be placed flat to avoid deformation of the workpiece due to its weight in the high-temperature plastic state. For slender parts and sleeve parts, vertical hanging heating in a well furnace or salt furnace should be used as much as possible. Limiting or reducing the heating speed can reduce the temperature difference in the cross-section of the workpiece and make the heating uniform. Therefore, for large forging dies, high-speed steel, and high-alloy steel workpieces, as well as parts with complex shapes, uneven thickness, and small deformation requirements, preheating or limiting the heating speed is generally used. Reasonable selection of quenching heating temperature is also the key to reducing or preventing deformation and cracking. Selecting the lower limit quenching temperature and reducing the temperature difference between the workpiece and the quenching medium can reduce the cooling rate in the high-temperature stage of quenching cooling, thereby reducing the thermal stress during quenching cooling. In addition, it can also prevent coarsening of grains. This can prevent deformation and cracking. Sometimes, to adjust the volume deformation before and after quenching, the quenching heating temperature can also be appropriately increased. For example, high-carbon alloy steels such as CrWMn and Cr12Mo often use the adjustment of the heating temperature to change their martensite transformation point to change the residual austenite content to adjust the volume deformation of the parts.
(2) Basic principles for correctly selecting cooling methods and cooling media
- Use pre-cooling as much as possible, that is, before the workpiece is quenched into the quenching medium, cool it as slowly as possible to near Ar to reduce the temperature difference in the workpiece.
- Under the premise of ensuring that the hardened layer depth and hardness requirements are met, use a quenching medium with slow cooling as much as possible.
- Slow down the cooling rate below the Ms point as much as possible.
- Reasonably select and use graded or isothermal quenching processes.
(3) Correctly select the basic principles for immersing the quenching workpiece in the quenching medium and the direction of operation
-When quenching, the most uniform cooling should be ensured as much as possible. Quench in the direction of minimum resistance. It is difficult to ensure the dimensional accuracy requirements of thin ring parts, thin plate parts, cam plates, and bevel gears with complex shapes produced in large quantities during free cooling. For this reason, press quenching can be adopted, that is, the parts are placed in a special press mold, and then cooled after a certain pressure (spraying oil or water). Since the shape and size of the parts are limited by the mold, the deformation of the parts may be limited to the specified range. Timely and correct tempering In production, a considerable number of workpieces do not crack during quenching but crack due to untimely tempering after quenching. This is because during the quenching dwell process, the fine cracks in the workpiece merge and expand under the action of large quenching stress, so that their size reaches the critical crack size of fracture, resulting in delayed fracture. The practice has proved that not cooling the quenching to the end and tempering in time are effective measures to prevent cracking. Tempering after quenching is particularly important for high-carbon steel and high-carbon alloy steel with complex shapes. The distortion of the workpiece can be corrected by straightening, but it must be within the range allowed by the plasticity of the workpiece. Sometimes, it can also be corrected by using a specific correction fixture during tempering heating. Volume deformation can sometimes be corrected by additional grinding, but this is limited to the reduction of hole and slot size and the increase of the outer field. Quenching volume deformation is often inevitable. However, as long as the deformation law is mastered through experiments, it can be appropriately corrected during the forming process before quenching according to the amount of expansion and contraction, so that the required geometric dimensions can be obtained after quenching. Once the workpiece has quenching cracks, it will be scrapped.
2. Oxidation and decarburization, overheating and overburning
3. Insufficient hardness
The reasons for the insufficient hardness of the quenched workpiece are as follows.
(1) The heating temperature is too low and the holding time is insufficient. When checking the metallographic structure, undissolved ferrite can be seen in hypoeutectoid steel and more undissolved carbides can be seen in tool steel.
(2) Surface decarburization causes insufficient surface hardness. The hardness measured after grinding off the surface is higher than the surface.
(3) The cooling rate is not enough. In the metallographic structure, black troostite can be seen distributed along the grain boundaries.
(4) The hardenability of the steel is not enough, and the large cross-section cannot be hardened.
(5) When interrupted quenching is used, the residence time in the water is too short, or after being taken out of the water, it stays in the air for too long and then transferred to oil, resulting in a decrease in hardness due to insufficient cooling or self-tempering.
(6) The quenching temperature of tool steel is too high, and the amount of residual austenite is too much, which affects the hardness.
When insufficient hardness occurs, the cause should be analyzed and corresponding measures should be taken. Among them, the insufficient hardness caused by the heating temperature being too high or too low, in addition to tempering the defects that have occurred and reheating and quenching to remedy, the furnace temperature measurement and control instruments should be strictly managed, and regular calibration and maintenance should be carried out according to the metering transmission system.
4. Uneven hardness
Uneven hardness means that the workpiece has soft spots after quenching. The reasons for the quenching soft spots are as follows:
(1) There are scales and dirt on the workpiece surface;
(2) There are impurities in the quenching medium, such as oil in water, which causes soft spots after quenching;
(3) When the workpiece is cooled in the quenching medium, the stirring of the cooling medium is insufficient, and the bubbles formed in the grooves and large cross-sections of the workpiece are not driven away in time, resulting in soft spots;
(4) The carbon concentration on the surface of the carburized part is uneven, and the hardness is uneven after quenching;
(5) The original structure before quenching is uneven, such as severe carbide segregation, or the original structure is coarse, and the ferrite is distributed in large blocks.
For the first three cases, tempering can be performed once, reheating can be performed, and quenching can be performed under the conditions of the appropriate cooling medium and cooling method to remedy. For the latter two cases, if no further processing is performed after quenching, once the pitting occurs, it is difficult to remedy. For workpieces that have not yet been formed, to eliminate carbide segregation or coarseness, forging in different directions can be used to change its distribution and morphology. For coarse structures, annealing or normalizing can be performed again to make the structure refined and uniform.
5. Organizational defects
Some parts, according to service conditions, in addition to requiring a certain hardness, also have certain requirements for metallographic structure, such as the size of martensite after quenching of medium carbon or medium carbon alloy steel, which can be rated according to the standard atlas. If the martensite size is too large, it means that the quenching temperature is too high, which is called an overheated structure. There are also regulations on the number of free ferrites, too much indicates insufficient heating or insufficient quenching cooling speed. Others, such as tool steel and high-speed steel, also have corresponding regulations on austenite grain size, residual austenite, carbide quantity, and distribution. For these organizational defects, the causes of their occurrence should be analyzed according to the specific quenching conditions, and corresponding measures should be taken to prevent and remedy them. However, it should be noted that some organizational defects are also related to the original structure before quenching. For example, coarse martensite can be produced not only by too high a quenching heating temperature but also by the inheritance of overheated tissues left by hot working before quenching. Therefore, annealing and other methods should be used to eliminate overheated tissues before quenching.
Tempering defects and prevention
Common tempering defects in production include too high or too low hardness, uneven hardness, deformation, and brittleness caused by tempering, etc.
Too high, too low, or uneven tempering hardness is mainly caused by too low, too high, or uneven furnace temperature. Too high hardness after tempering may also be caused by too short a tempering time. Such problems can be controlled by adjusting the tempering temperature, etc. The reason for uneven hardness may be due to too much furnace loading at one time or improper selection of heating furnace. If tempering is carried out in a gas medium furnace, there should be an air circulation fan in the furnace, otherwise the temperature in the furnace cannot be uniform.
Deformation of the workpiece after tempering is often caused by unbalanced stress in the workpiece before tempering, stress relaxation during tempering, or stress redistribution. To avoid deformation after tempering, adopt multiple straightening and multiple heating, or use press tempering and other measures.
After the surface of high-speed steel is decarburized, a network crack may be formed during the tempering process. After the surface is decarburized, the specific volume of martensite decreases, resulting in multi-directional tensile stress and forming a network crack. In addition, when high-carbon steel parts are tempered, if they are heated too quickly, the surface will be tempered first, the specific volume will decrease, and multi-directional tensile stress will be generated, resulting in a network crack. The emergence of brittleness after tempering is mainly due to improper tempering temperature or insufficient cooling speed after tempering (the second type of tempering brittleness). Therefore, to prevent the emergence of brittleness, the tempering temperature and cooling method should be correctly selected. Once tempering brittleness occurs, for the first type of tempering brittleness, it can only be eliminated by reheating and quenching, and then tempering at another temperature; for the second type of tempering brittleness, it can be eliminated by reheating and tempering, and then accelerating the cooling speed after tempering.
