Quenching and tempering defects and prevention

In the heat treatment process, the scrap rate caused by the quenching process is often high, which is mainly due to the large thermal stress and organizational stress formed at the same time during the quenching process. In addition, due to the inherent metallurgical defects of materials, improper material selection, wrong materials, poor structural and technological properties in design, defects formed during cold and hot processing, and other factors, they are easy to be exposed in the quenching and tempering process, so the quenching The defects after tempering must be systematically classified and inspected.

Quenching defects and prevention

The most common defects during quenching of steel parts include quenching deformation, cracking, oxidation, decarburization, insufficient or uneven hardness, surface corrosion, over burning, overheating, and other metallographic structures unqualified according to the quality inspection standards.

1. Quenching deformation and quenching crack

In actual production, effective preventive measures should be taken according to the causes of quenching deformation and quenching cracks.

(1) Try to heat evenly and correctly

When the shape of the workpiece is complex or the size of the section differs greatly, it is often deformed due to uneven heating. For this reason, before the workpiece is loaded into the furnace, asbestos rope shall be used to block or bind the holes and abrupt changes in the cross section that do not need to be hardened to improve the heating conditions. For some thin-walled rings and other easily deformed parts, specific quenching fixtures can be designed. These measures are conducive to uniform heating and cooling.

When the workpiece is heated in the furnace, it should be placed evenly to prevent the single side from being heated. It should be placed flat to prevent the workpiece from deforming due to its own weight in the high temperature plastic state. For long and thin parts and sleeve parts, the vertical suspension heating of well furnace or salt furnace shall be adopted as far as possible.

Limiting or reducing the heating speed can reduce the temperature difference of the workpiece section and make the heating uniform. Therefore, for large forging dies, high speed steel and high alloy steel workpieces, as well as parts with complex shape, uneven thickness and small deformation requirements, preheating or heating speed limiting measures are generally adopted.

Reasonable selection of quenching heating temperature is also the key to reduce or prevent deformation and cracking. Selecting the lower quenching temperature and reducing the temperature difference between the workpiece and the quenching medium can reduce the cooling rate at the high temperature stage of quenching cooling, thus reducing the thermal stress during quenching cooling. In addition, coarse grains can be prevented. This will prevent deformation and cracking.

Sometimes, in order to adjust the volume deformation before and after quenching, the quenching heating temperature can also be appropriately increased. For example, CrWMn, Cr12Mo and other high carbon alloy steels are often used to adjust the heating temperature, change the martensite transformation point to change the residual austenite content, so as to adjust the volume deformation of parts.

(2) Basic principles for correct selection of cooling method and cooling medium

Pre cooling shall be adopted as far as possible, that is, before the workpiece is quenched into the quenching medium, it shall be cooled to Ar as slowly as possible to reduce the temperature difference in the workpiece. ‚ On the premise of meeting the requirements for the depth and hardness of the hardened layer, the quenching medium for cooling the slow steamed bread shall be used as far as possible.

Reduce the cooling rate below Ms point as much as possible. Grading or isothermal quenching process shall be reasonably selected and adopted.

(3) The basic principle of correctly selecting the way and running direction of quenching workpiece immersed in quenching medium

The most uniform cooling shall be ensured as far as possible during quenching. ‚ Quench in the direction of minimum resistance.

When free cooling, it is difficult to ensure the dimensional accuracy of the thin drawing ring parts, thin plate parts, cam discs and bevel gears with complex shapes produced in large quantities. To this end, press quenching can be adopted, that is, the parts are placed in a special press mold, and then cooled after a certain pressure (oil injection or water spraying). Because the shape and size of the parts are limited by the mold, the deformation of the parts may be limited within the specified range.

Carry out timely and correct tempering In production, a considerable number of workpieces cracked not during quenching, but because they were not tempered in time after quenching. This is because in the quenching stay process, the micro cracks existing in the workpiece, under the effect of great quenching stress, fuse and expand until their size reaches the critical crack size of fracture, so that delayed fracture occurs. Practice has proved that quenching without cooling to the end and tempering in time are effective measures to prevent cracking. For high carbon steel and high carbon alloy steel with complex shape, timely tempering after quenching is particularly important.

The distortion of the workpiece can be corrected by straightening, but it must be within the allowable range of the plasticity of the workpiece. Sometimes, special correction fixture can be used for correction during tempering heating. The volume deformation can sometimes be corrected by supplementary grinding, but this is only limited to the reduction of the hole and groove size and the increase of the outer garden. Quenching volume deformation is often unavoidable. However, as long as the deformation law is mastered through experiments, the required geometric dimensions can be obtained after quenching by properly modifying the deformation before quenching according to its expansion and contraction. Once the workpiece has quenching cracks, it shall be scrapped.

2. Oxidation and decarburization, overheating and overburn

3. Insufficient hardness

The reasons for insufficient hardness of 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 carbide can be seen in tool steel.

（2） ‚ Surface decarburization causes insufficient surface hardness. The hardness measured after the surface layer is removed is higher than the surface layer.

（3） ƒ The cooling rate is not enough, and the distribution of black troostite along the grain boundary can be seen in the metallographic structure.

(4) The hardenability of steel is not enough, and the large section cannot be hardened.

(5) When interrupted quenching is adopted, the hardness is reduced due to insufficient cooling or self tempering when the quenching time in water is too short, or after it is taken out of water, the quenching time in air is too long and then transferred to oil.

(6) The quenching temperature of tool steel is too high, and the amount of retained austenite is too much, which affects the hardness.

In case of insufficient hardness, the causes shall be analyzed and corresponding measures shall be taken. For insufficient hardness caused by too high or too low heating temperature, in addition to tempering the existing defects and reheating and quenching for remedy, the furnace temperature measurement and control instruments shall be strictly managed, and the calibration and maintenance shall be carried out regularly according to the metering transmission system.

4. Uneven hardness

Uneven hardness means that the workpiece has soft spots after quenching. The reasons for soft spots are as follows:

（1）  The workpiece surface has scale and dirt;

(2) Impurities in the quenching medium, such as oil in water, cause soft spots after quenching;

(3) When the workpiece is cooled in the quenching medium, the cooling medium is not stirred enough to drive away the grooves of the workpiece and the bubbles formed at the large cross-section in time, resulting in soft spots;

(4) The surface carbon concentration of carburized parts is uneven, and the hardness is uneven after quenching;

(5) Before quenching, the original structure 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 and reheating can be carried out once, and quenching can be carried out under the conditions of proper cooling medium and cooling method. For the latter two cases, if no processing is carried out after quenching, it will be difficult to remedy the gap once it appears. For workpieces that have not been formed and machined, in order to eliminate carbide segregation or coarseness, forging in different directions can be used to change their distribution and shape. The coarse structure can be annealed or normalized again to refine and homogenize the structure.

5. Organizational defects

Some parts, according to the service conditions, have certain requirements on the metallographic structure in addition to a certain hardness. For example, the size of martensite after quenching of medium carbon or medium carbon alloy steel can be rated according to the standard atlas. Excessive martensite size indicates that the quenching temperature is too high, which is called overheating structure. There are also provisions for the number of free iron strands, too much indicates insufficient heating or insufficient quenching cooling speed. Others, such as tool steel and high speed steel, also stipulate the austenite grain size, the amount of retained austenite, the amount and distribution of carbides. The causes of these defects should also 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 structural defects are also related to the original structure before quenching. For example, coarse martensite can be produced not only when the quenching heating temperature is too high, but also because of the heredity of the overheated structure left by the hot working before quenching. Therefore, annealing and other methods should be used to eliminate the overheated structure before quenching.

Tempering defects and prevention

Common tempering defects in production include: too high or too low hardness, uneven hardness, and deformation and brittleness caused by tempering.

The tempering hardness is too high, too low or uneven, which is mainly caused by too low, too high or uneven furnace temperature. The high hardness after tempering may also be due to the short tempering time. This kind of problem can be controlled by adjusting the tempering temperature. The uneven hardness may be caused by excessive charging at one time or improper selection of heating furnace. If the 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.

The deformation of the workpiece after tempering is often caused by the unbalanced internal stress of the workpiece before tempering, stress relaxation or stress redistribution during tempering. Deformation after tempering shall be avoided, or multiple straightening and multiple heating shall be adopted, or pressing tool tempering shall be adopted.

After decarburization on the surface of high speed steel, network cracks may form during tempering. Because after surface decarburization, the specific volume of martensite decreases, so that multi-directional tensile stress is generated and network cracks are formed. In addition, if the high carbon steel parts are heated too fast during tempering, the surface is tempered first, and the specific volume decreases, resulting in multi-directional tensile stress and network cracks. The brittleness after tempering is mainly caused by improper tempering temperature or insufficient cooling rate after tempering (the second type of tempering brittleness). Therefore, tempering temperature and cooling mode should be correctly selected to prevent brittleness. Once temper brittleness occurs, the first type of temper brittleness can only be tempered at another temperature by reheating and quenching; The second type of temper brittleness can be eliminated by reheating and tempering, and then accelerating the cooling rate after tempering.