Defects of quenching and Prevention
In the heat treatment process, the reject rate caused by the quenching process is usually higher. This is mainly in the quenching process, and at the same time, the formation of greater thermal stress and structural stress, in addition, it is easy to be exposed in the quenching and tempering process because of the inherent metallurgical defects, improper material selection, wrong materials, poor structural workability in design, and defects formed during cold and Hot Processing , therefore, the parts and components quenching, tempering defects must be systematic analysis and adjustment.
Defects of quenching and Prevention
The most common defects during quenching are quenching deformation, cracking, oxidation, decarburization, insufficient or uneven hardness, surface corrosion, over-burning, over-heating and other disqualification of the metallographic structure according to the quality inspection standard.
1.Quenching distortion and quenching crack.
In actual production, effective preventive measures should be taken according to the cause of quenching deformation and quenching crack.
(1) heating evenly and properly as far as possible
When the shape of the workpiece is complex or the section size is different, the deformation is often caused by uneven heating. In order to improve the heating condition of workpieces, we can design the special quenching fixture for some thin-wall ring and other parts. These measures are both conducive to uniform heating and uniform cooling.
When the workpiece is heated in the furnace, it should be placed evenly to prevent one side from being heated, it should be laid out flat to avoid deformation due to self-weight in the high-temperature plastic state. To the slender parts and sleeve parts as far as possible to use well-type furnace or salt furnace vertical suspension heating.
Limit or reduce the heating speed, can reduce the workpiece section temperature, so that heating uniform. Therefore, for large-scale forging die, high-speed steel and high alloy steel parts, as well as parts with a complex shape, uneven thickness, and small deformation, preheating or limiting, heating speed measures, are generally used.
Reasonable choice of quenching heating temperature is also the key to reduce or prevent deformation and cracking. By selecting the lower limit of quenching temperature and reducing the temperature difference between the workpiece and quenching medium, the cooling speed at the high-temperature stage of quenching can be reduced, thus the thermal stress during quenching cooling can be reduced. In addition, grain coarsening can be prevented. This prevents deformation from cracking.
Sometimes in order to adjust the volume deformation before and after quenching, the quenching temperature can also be appropriately increased. For example, Crwmn, CR12MO, and other high carbon alloy steels, the volume deformation of parts can be adjusted by adjusting the heating temperature and the martensite transformation point to change the content of residual austenite.
(2) proper selection of cooling method and the basic principle of the cooling medium
As far as possible, using pre-cooling, namely before quenching into the quenching medium, as far as possible slow cooling to AR, in order to reduce the temperature difference in the workpiece.
On the premise of meeting the requirements of depth and hardness of the hardened layer, the quenching medium of cooling slowly steamed bread was adopted as far as possible.
Slow the cooling rate below Ms as much as possible.
Reasonable selection and adoption of classification or ISOTHERMAL quenching process.
(3) the basic principle of how to choose the way and direction of immersion quenching medium
When quenching should try to ensure that the most uniform cooling. ‘quenched with minimum resistance.
Mass production, sheet shaped parts, complex shaped cam discs and bevel gears, etc. , are difficult to ensure dimensional accuracy during free cooling. To this end, you can take press quenching, that is, the parts in the special press mold, add a certain pressure after cooling (spray oil or water) due to the shape and size of the parts by the mold, so the deformation of the parts may be limited within the prescribed scope.
Timely and correct tempering in production, a considerable part of the workpiece, not in the quenching of cracking, but rather, because after quenching not timely tempering and cracking. This is because, during the quenching residence process, the micro-cracks existing in the workpiece are fused and expanded under great quenching stress, so that the size of the micro-cracks reaches to the critical size of the fracture, resulting in the delayed fracture. The practice has proved that the quenching is not cold to the end and timely tempering is an effective measure to prevent cracking. For High Carbon Steel and high carbon alloy steel with a complex shape, timely tempering is especially important after quenching.
The distortion of the workpiece can be corrected by straightening, but only within the limits of the plasticity of the workpiece. It may also be corrected by tempering using a special calibration fixture. Volumetric deformation can sometimes be corrected by additional grinding, but only if the hole or groove size is reduced and the field is enlarged. Quenching volume deformation is often inevitable. However, if the defamation law is known by experiment, the deformation can be corrected properly before quenching according to the amount of expansion and contraction, and the desired geometric size can be obtained after quenching. Once the quenching crack occurred in the workpiece, it is scrapped.
2. Oxidation and decarburization, overheating and burning
3. Lack of hardness
The causes for the lack of hardness in the quenched workpiece are as follows.
(1) the heating temperature is too low and the holding time is insufficient. In the metallographic examination, insoluble ferrite can be found in hypoeutectoid steel and more insoluble carbides can be found in tool steel.
(2) Surface decarburization causes insufficient surface hardness. The hardness measured after abrading the surface is higher than the surface.
(3) the distribution of black troostite along the grain boundary can be observed in the metallographic structure due to the insufficient cooling rate.
(4) steel hardenability is insufficient, section big place hardenability.
(5) when interrupted quenching is adopted, the residence time in the water is too short, or after being removed from the water, the residence time in the air is too long and then transferred to oil, the hardness is reduced due to insufficient cooling or self-tempering.
(6) tool steel quenching temperature is too high, the amount of residual austenite is too much, affect the hardness.
When the hardness is insufficient, the reasons should be analyzed and the corresponding measures should be taken. Besides tempering the defects and reheating and quenching the defects, the temperature measuring and controlling instruments should be strictly controlled, and the measurement and transmission system should be corrected and repaired regularly.
Uneven hardness is the workpiece after quenching soft point, quenching soft point caused by the following reasons:
(1) there are scale and dirt on the surface of workpieces;
(2) the presence of impurities in the quenching medium, such as oil in water, produces a soft spot after quenching;
(3) when the workpiece is cooled in the quenching medium, the agitation of the cooling medium is not enough, and the soft spot is produced because the groove of the workpiece and the bubbles formed at the large cross-section are not removed in time;
(4) uneven carbon concentration on the carburized surface and uneven hardness after quenching;
(5) the original microstructure is not uniform before quenching, for example, there is serious carbide segregation, or the original structure is coarse, and the ferrite is distributed as a large block.
In the first three cases, a tempering, re-heating, cooling medium and cooling methods under the conditions of the quenching remedy. On the latter two cases, such as quenching after no longer processing, once there is a notch, it is difficult to remedy. The distribution and shape of the unformed workpiece can be changed by forging in different directions in order to eliminate the carbide segregation or coarseness. The coarse tissue can be annealed or normalized again to refine and homogenize the tissue.
Some parts, according to the service conditions, in addition to a certain hardness, but also on the metallographic structure requirements, such as medium carbon or medium carbon alloy steel quenched martensite size requirements, according to the Standard Atlas Rating. The large size of martensite indicates that the quenching temperature is too high, which is called the overheated structure. There is also a rule for the amount of free iron cable body. Too much indicates insufficient heating or insufficient quenching cooling rate. Other steel, such as tool steel, high-speed steel, corresponding to the austenite grain size, the amount of retained austenite, the amount and distribution of carbides. The causes of these defects should be analyzed according to the quenching conditions, and corresponding measures should be taken to prevent and remedy them. However, it should be noted that some tissue defects are also related to the original structure before quenching. For example, coarse martensite can be produced not only when the quenching temperature is too high, but also because of the heredity of overheated structure left by the heat processing before quenching.
DEFECTS IN TEMPERING AND PREVENTION
The common tempering defects are too high or too low hardness, uneven hardness, as well as tempering deformation and Brittleness.
Tempering hardness too high, too low or uneven, mainly due to tempering temperature is too low, too high or furnace temperature caused by uneven. Too high hardness after tempering may also be due to too short tempering time. Such problems can be controlled by adjusting the tempering temperature, etc. The cause of uneven hardness may be due to overloading or improper selection of heating furnace. If the tempering in the gas furnace, the furnace should be airflow circulation fan or furnace temperature cannot be uniform.
The deformation of workpieces after tempering is often due to the stress imbalance before tempering, stress relaxation or stress redistribution during tempering. To avoid tempering deformation, or use multiple straightening multiple heating, or the use of tempering press and other measures.
After decarburization of the surface of high-speed steel, reticular cracks may be formed during tempering. Because after surface decarburization, the specific volume of martensite decreases, resulting in multi-directional tensile stress resulting in a network-like crack. In addition, when high-carbon steel is tempered, if the surface is heated too quickly, the specific volume decreases, resulting in multi-directional tensile stress To produce a network of cracks. The appearance of Brittleness after tempering is mainly due to the improper tempering temperature is chosen or the insufficient cooling rate after tempering (the second kind of tempering Brittleness). Therefore, to prevent the emergence of Brittleness, tempering temperature and cooling methods should be properly selected. Once the temper Brittleness, the first kind of temper Brittleness, only by re-heating quenching, selected temperature tempering; for the second kind of temper Brittleness, can be re-heating tempering, and then accelerate the cooling rate after tempering to eliminate.