Common Defects During Heat Treatment

 

Heat treatment is a very important link in the material processing process. It changes the internal structure of the material by controlling the heating, insulation and cooling processes, thereby improving its performance.

However, some defects may appear during heat treatment, affecting the final quality of the material. The following are some common heat treatment defects, their causes and corresponding treatment methods:

 

Overheat

Overheating usually occurs during the heat treatment of bearings. When the heating temperature is too high or the holding time is too long, the local or overall structure of the bearing will change. The grains become coarser, the hardness decreases, and cracks and brittleness may even occur. These changes can significantly affect bearing performance and service life.

The increase of retained austenite in overheated structures will lead to a decrease in dimensional stability. In addition, due to overheating of the quenched structure, the crystals of the steel become coarse, the toughness of the parts is reduced, and the impact resistance will also be reduced. These problems may eventually lead to fatigue, deformation, and cracking of the bearing during use, seriously affecting the accuracy and reliability of the bearing. In extreme cases, overheating may even lead to quenching cracks.

 

For bearing parts that have overheated, normalizing or complete annealing can be used to remedy them. These heat treatment methods can readjust the structure of the bearing and improve its performance.

 

To prevent overheating, the most critical measure is to strictly control the heating temperature and heating time. Specifically, the following measures can be taken:

• Optimize the heat treatment process: According to the material and performance requirements of the bearing, formulate reasonable heat treatment process parameters, including heating temperature, holding time, and cooling method.
• Use advanced heating equipment: Using equipment that accurately controls heating temperature and time, such as high-frequency induction heating furnaces, can more effectively avoid overheating.
• Strengthen process monitoring: During the heat treatment process, strengthen the monitoring and recording of parameters such as temperature, time, and cooling rate, and detect and handle abnormal situations on time.

 

Underheat

Underheating is a problem that cannot be ignored during heat treatment, especially in the field of bearing manufacturing. Due to low quenching temperature or poor cooling, excessive troostite structure, that is, underheated structure, may be formed inside the bearing. This organizational state poses a serious threat to the performance and life of the bearing.

Specifically, underheated tissue will cause a sharp decrease in the hardness and wear resistance of the bearing. Hardness is an important indicator of bearing resistance to external pressure and friction, while wear resistance is directly related to the service life of bearings. When a bearing has underheated tissue, its local or overall hardness is reduced, making the bearing prone to deformation and wear when it is subjected to pressure and friction, thus seriously affecting the normal operation of the bearing.

 

To remedy underheated parts, a common method is to perform spheroidizing annealing. Spheroidizing annealing is a heat treatment process that uses heating and cooling processes to form a spherical distribution of carbides in the bearing. This method can eliminate or reduce the impact of underheated tissue, improve the hardness and wear resistance of the bearing, and thereby restore its performance.

 

However, preventing underheating is even more critical.

• The main preventive measure is to strictly control the heating temperature. During the heat treatment process, an appropriate heating temperature range should be determined based on the material and performance requirements of the bearing.
• In addition, maintenance and inspection of heating equipment are also crucial to ensure that it can accurately control the heating temperature and avoid underheating caused by temperature fluctuations.

 

Overburn

Overburning is an extremely serious problem during bearing heat treatment. It usually occurs when the heating temperature is too high, causing the austenite grain boundaries to melt or the grains to be oxidized. This phenomenon causes great damage to the comprehensive mechanical properties of the bearing steel, making it fragile and thus seriously affecting the service life of the bearing.

 

Once a bearing is overburned, its performance has been irreversibly damaged. Therefore, overburned parts can often only be scrapped without any remedial measures to restore their original performance. This not only increases production costs, but may also affect the normal operation of the entire production line.

 

In order to prevent the occurrence of overburning, a series of strict measures must be taken.

• First, the heating temperature must be strictly controlled to ensure that the bearing is heat treated within a suitable temperature range. This requires precise temperature control equipment and experienced operators to ensure temperature accuracy and stability.
• Secondly, the control of heating time is also crucial. Excessive heating time may cause changes in the internal structure of the bearing, thereby increasing the risk of overburning. Therefore, a reasonable heating time range needs to be developed based on the material and size of the bearing.
• In addition, the choice of heat treatment atmosphere also has an important impact on preventing overburning. Different atmospheres may have different effects on bearing oxidation and melting. Therefore, when selecting a heat treatment atmosphere, it is necessary to consider the material and heat treatment requirements of the bearing and select an atmosphere that can reduce oxidation and melting.
• In addition to the above measures, regular maintenance and inspection of heat treatment equipment are also important measures to prevent overburning. The accuracy and stability of the equipment directly affect the control of heating temperature and time, therefore, it is necessary to ensure that the equipment is in good working condition.

 

Quenching soft spot

The presence of quenched soft spots has a serious impact on the performance and life of the bearing. These soft spots will significantly reduce the wear resistance and fatigue strength of the parts, making the bearings more prone to wear and breakage during use, thus shortening their service life.

 

There are many reasons for the occurrence of quenching soft spots.

• Local decarburization during forging is one of the main reasons, which will lead to a reduction in the carbon content of this part of the material, thereby affecting its hardness and strength.
• In addition, insufficient quenching heating temperature, insufficient heat preservation, or poor cooling will also cause the material to fail to achieve the desired hardening effect and form soft spots.
• The selection and preparation of quenching medium are also critical. Improper preparation of soda water or oil on the water will affect the quenching effect and produce soft spots.

 

To solve the problem of quenching soft spots, the following measures can be taken:

• Rework parts with quenching soft spots and re-quench them. By re-quenching, soft spots can be eliminated and the part’s hardness and strength restored. However, it should be noted that rework and re-quenching may increase the risk of stress and deformation of the parts, so process parameters need to be strictly controlled during the operation.
• Increase the quenching heating time and increase the cooling capacity. By extending the heating time, the material can be fully austenitized and the quenching effect can be improved. At the same time, increasing the cooling capacity can speed up the cooling rate of the material, which is beneficial to the formation of a more uniform hardness distribution.
• Use hot soda water solution as quenching medium. Hot soda water solution has better cooling performance and stability, which can improve the quenching effect and reduce the occurrence of soft spots. However, it should be noted that the preparation and use of hot soda water solution requires strict control of temperature and concentration to avoid adverse effects on parts.

 

In addition, To avoid the occurrence of quenching soft spots, the following points need to be noted:

• The occurrence of decarburization is strictly controlled during the forging process to ensure that the carbon content of the material is stable.
• Select the appropriate quenching heating temperature and holding time to ensure that the material is fully austenitized.
• Regularly check the status of quenching equipment and media to ensure they are in good working order.
• Conduct training and assessment for operators to ensure that they master correct quenching operation skills.

Quenching soft spots is a key issue that needs to be paid attention to during the bearing manufacturing process. By understanding the causes and taking corresponding preventive measures, the occurrence of soft spots can be effectively reduced and the performance and life of the bearing can be improved.

 

Quenching cracks

Quenching cracks are indeed irreparable defects formed during the quenching process due to the internal stress exceeding the fracture strength of the material. Such cracks have a serious impact on the performance and life of critical components such as bearing parts. The occurrence of quenching cracks will not only reduce the strength of the parts but may also cause the parts to break during service, causing serious safety accidents.

 

To reduce the probability of quenching cracks, we can start from the following aspects:

• First, ensure proper pre-treatment before quenching. This includes preheating, which helps reduce temperature gradients during quenching and reduces internal stresses. The preheating temperature and time need to be precisely controlled according to the type of material and the size of the workpiece.
• Secondly, optimize the heating and cooling methods during quenching. Heating should be uniform to avoid local overheating; quenching temperature should be controlled within an appropriate range to avoid excessive temperature leading to rapid tissue transformation and excessive internal stress. The cooling method also needs to be selected reasonably to ensure that the workpiece can be cooled evenly and to avoid stress concentration caused by too fast cooling rate.
• In addition, use high-quality steel to ensure that there are no defects such as slag inclusions and non-metallic inclusions inside the steel. These defects may become the source of stress concentration. At the same time, check the surface of the workpiece to ensure that there are no defects such as cracks and scratches. These defects may also cause cracks during the quenching process.
• Finally, the tempering treatment after quenching is also key. Tempering can eliminate quenching stress, stabilize the structure, and improve the toughness of the material. The tempering temperature and time need to be determined according to the type of material and the performance requirements of the workpiece to ensure the best tempering effect.

 

Oxidative decarburization

During the heat treatment process of bearings, when heated in an oxidizing medium, the surface of the workpiece is indeed prone to oxidation at high temperatures. This oxidation will lead to a reduction in the carbon content on the surface of the workpiece, thereby causing surface decarburization.

Heating temperature and holding time are key factors affecting oxidation. Generally, the higher the heating temperature and the longer the holding time, the more intense the oxidation will be and the deeper the decarburization layer will be. Therefore, during the heat treatment process, the heating temperature and holding time need to be strictly controlled to avoid excessive oxidation and decarburization.

The effect of oxidation on the workpiece is significant. It will cause damage to the surface metal of the workpiece, which not only affects the size and surface quality of the workpiece but also reduces the strength of the steel. Especially when decarburization occurs on the surface of the workpiece, its hardness, strength, wear resistance and corrosion resistance will be significantly reduced. This is undoubtedly very disadvantageous for parts such as bearings that need to withstand high loads and friction.

 

To avoid performance degradation caused by decarburization of the workpiece surface, a series of preventive measures need to be taken during the heat treatment process.

• First, ensure that the surface of the workpiece is clean and free of impurities and oxide layers to reduce the occurrence of oxidation.
• Secondly, a layer of protective coating can be applied to the surface of the workpiece, or stainless steel foil can be used for packaging, sealing, and heating to isolate the workpiece from contact with the oxidizing medium.
• In addition, optimizing the heating process is also key. The heating temperature and holding time should be reduced as much as possible while ensuring the quenching requirements.

 

Deformation and cracking

Heat treatment deformation and cracking are common process defects during heat treatment.

Mild heat treatment deformations can usually be corrected through subsequent mechanical processing to meet the use requirements of the parts.

However, when the deformation reaches severe levels, due to the difficulty and high cost of correction, scrapping is often the only option, which undoubtedly increases production costs and material waste.

What’s more serious is that if the parts have surface cracks or overall cracking during the heat treatment process, this will directly lead to the scrapping of the parts. This kind of cracking is usually not allowed, except in some special cases, such as micro-cracks that appear inside quenched parts of high-carbon steel and can be eliminated after tempering.

 

The root cause of these problems is mainly related to the internal stress formed during heat treatment, especially the thermal stress and structural stress generated during the quenching process.

Generally, it is related to two factors: stress size and material properties.

• If the internal stress does not reach the elastic limit of the material, the material will elastically deform.
• When the internal stress exceeds the elastic limit of the material and is lower than the strength limit, the material will undergo plastic deformation.
• When the internal stress reaches the strength limit of the material, the material will break.

 

To reduce the risk of heat treatment deformation and cracking, several measures can be taken.

• First, optimize the heat treatment process parameters, such as selecting appropriate heating speed and cooling method, to reduce the generation of stress.
• Secondly, pay attention to the structural design of the parts and avoid overly complex or unreasonable structures to reduce the possibility of stress concentration.
• In addition, the use of appropriate preparatory heat treatment and subsequent treatment processes, such as normalizing, annealing, and tempering, can also effectively reduce the risk of deformation and cracking.