Common Defects in Heat Treatment
Do you know what are the common defects of heat treatment?
Do you know the causes of common defects in heat treatment?
Do you know how to deal with common defects in heat treatment?
If you don’t know, please don’t miss this article, DUHUI will tell you in detail.
Heat treatment is to subject the forged and machined bearing rings to high-temperature treatment. It directly affects the uniformity of carburization in the bearing ring and can improve the wear resistance and hardness of the bearing, and is an important factor affecting the reliability and life of the bearing.
Due to improper heat treatment processes, defects such as overheating, underheating, overburning, quenching soft spots, quenching cracks, oxidative decarburization, deformation and cracking often occur.
Overheating
Overheating means that during the heat treatment process of the bearing, the heating temperature is too high or the heat preservation time is too long, resulting in problems such as coarse grains, insufficient hardness, cracks, brittleness, etc., locally or entirely in the bearing, which has a great impact on the performance and service life of the bearing.
The amount of retained austenite in the overheated structure increases, and the dimensional stability decreases accordingly. Due to the overheating of the quenched structure, the crystals of the steel are relatively coarse, which will reduce the toughness of the parts and reduce the impact resistance. Eventually, the bearings will suffer from fatigue, deformation, cracking, and other problems during use, affecting the accuracy and reliability of the bearings. Serious overheating may even cause quenching cracks.
Overheated parts can be remedied by normalizing or complete annealing.
The main measure to prevent overheating is to strictly control heating temperature and heating time.
Underheating
During the heat treatment process, the quenching temperature is low or the cooling is poor, resulting in excessive troostite structure in the microstructure, which is called underheated structure.
Underheated tissue will cause the local or overall hardness and wear resistance of the bearing to drop sharply, ultimately affecting the life of the bearing.
Underheated parts can be remedied by spheroidizing annealing.
The main measure to prevent underheating is to strictly control the heating temperature.
Overburning
During heat treatment of bearings, due to excessive heating temperature, the austenite grain boundaries melt or the grains are oxidized.
Overburning will significantly reduce the comprehensive mechanical properties of the bearing steel and make it brittle, thus affecting the service life of the bearing.
Burnt parts can only be scrapped and cannot be repaired.
The main measure to prevent overburning is to strictly control the heating temperature, heating time, and atmosphere.
Quenching soft spot
During the quenching process of the bearing, due to local decarburization during forging, insufficient quenching heating temperature, insufficient insulation, poor cooling or improper preparation of the quenching medium soda water, oil on the surface of the soda water, etc., there are local hardness areas on the surface of the parts that are not soft enough. , called quenching soft spot.
Quenching soft spots seriously reduces the wear resistance and fatigue strength of parts, affecting the bearing life.
Parts with quenching soft spots can be corrected by rework and re-quenching.
Quenching soft spots can be prevented by increasing the quenching heating time and cooling capacity or using a hot soda water solution.
Quenching cracks
During the quenching process, when the stress generated by quenching is greater than the material’s fracture strength and exceeds the plastic deformation limit, cracks will occur. Quenching cracks are cracks caused by internal stress in bearing parts during the quenching, heating, or cooling process. Quenching cracks are also irreparable defects.
The causes of quenching cracks may be related to the quenching process and the material itself. The existence of internal stress is the main reason for the formation of quenching cracks.
Reasons for the formation of quenching cracks may include:
1. Before quenching, the workpiece was not preheated or the preheating heat treatment was improper.
2. During the quenching process, the heating method is improper, the heating is uneven, the quenching temperature is too high, the cooling method is improper, the cooling is uneven, or the cooling rate is too fast.
3. There are defects on the surface of the workpiece, such as cracks and scratches; or there are defects inside the steel, such as slag inclusions, severe non-metallic inclusions, white spots, shrinkage cavity residues, etc. These will form several stresses during quenching
4. The workpiece has severe surface decarburization and carbide segregation.
5. After the workpiece is quenched, the tempering is insufficient or not tempered in time.
Treating the above causes of quenching cracks can effectively reduce the probability of quenching cracks.
Oxidative decarburization
During the heat treatment process, if the bearing is heated in an oxidizing medium, oxidation will occur on the surface of the workpiece under high-temperature conditions, reducing the carbon content on the surface of the workpiece and causing surface decarburization.
Generally, the higher the heating temperature, the longer the holding time, the more intense the oxidation, and the deeper the decarburization layer.
Oxidation will damage the surface metal of the workpiece, affect the size and surface quality of the workpiece, and reduce the strength of the steel. Decarburization of the workpiece surface will lead to a decrease in the hardness, strength, wear resistance, and corrosion resistance of the workpiece.
Generally, there is a certain machining allowance left for workpiece processing. When the depth of the decarburization layer is within the machining allowance, it can be removed through the cutting process. However, if the depth of the decarburized layer exceeds the machining allowance range, it can only be scrapped.
Workpieces that have been pretreated by annealing or normalizing have a large machining allowance and the surface decarburization layer can be removed.
The quenched workpiece has a small machining allowance and a high hardness, which is difficult or impossible to remove by cutting, and can only be scrapped.
In order to avoid wasting materials and saving man-hours, we need to take preventive measures in advance before quenching.
Ways to reduce or prevent oxidative decarburization:
(1) For workpieces
Perform surface treatment on the workpiece to ensure there are no impurities or oxide layers on the surface.
Apply a layer of protective coating to the surface of the workpiece.
Sealed and heated in stainless steel foil packaging.
(2) For technology
Under the conditions of ensuring quenching requirements, reduce the heating temperature and shorten the holding time as much as possible.
Heating in salt bath furnaces, protective atmosphere furnaces, and vacuum furnaces.
Heating using a protective atmosphere.
Deformation and cracking
During heat treatment of bearing parts, there is thermal stress and organizational stress. This internal stress can superimpose or partially offset each other. It is complex and changeable because it can change with the heating temperature, heating speed, cooling method, cooling rate, and part shape. It changes with the change in size, so heat treatment deformation is inevitable. As long as the deformation can be controlled within a certain range, there is no problem. Mild deformation can be corrected through mechanical processing, but severe deformation can only be scrapped.
However, surface cracks on parts that lead to surface or overall cracking are not allowed and are generally directly scrapped (except for micro-cracks inside high-carbon steel quenched parts that can be eliminated after tempering).
The fundamental cause of heat treatment deformation and cracking is the internal stress formed during heat treatment, especially during quenching, which is generally 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.
Causes of bearing deformation and cracking:
1. Internal stress
If the heating temperature is too high, the austenite grains will become coarse and unevenly distributed, and the internal stress distribution will also be uneven, which can easily lead to stress concentrations.
If the heating speed is too fast, the thermal stress will increase, and the thermal stress and tissue stress will interact, making it easy to deform and crack.
2. Material properties
The internal structure of the steel is uneven and the material has internal defects, and there are material weaknesses.
Steel has a low elastic limit and strength limit.
Steel has poor thermal conductivity.
3. Other reasons
Workpiece surface quality: surface finish, surface residual stress, surface oxidation, and decarburization, etc.
According to the reasons for bearing deformation and cracking, the following preventive measures are proposed:
1. Carry out corresponding annealing or normalizing before quenching to refine the grains homogenize the structure and reduce the internal stress during quenching
2. Strictly control the quenching heating temperature to prevent overheating defects and reduce thermal stress during quenching.
3. Tempering should be performed immediately after quenching to eliminate stress and reduce the brittleness of the workpiece.
4. Reasonable selection of materials to improve the plasticity and strength of materials.
5. Surface treatment of workpiece.