The technical requirements for heat treatment are generally indicators for quality inspection of heat treatment and are relatively simple to label on forging drawings. In addition to requirements for hardness and deformation, some forgings also require local heat treatment. For surface strengthened forgings, the depth of the hardened layer and the hardness of the core are also technical requirements. The technical requirements for heat treatment should be aimed at meeting the service performance of forgings.
Hardness is the most important quality inspection indicator for the heat treatment of forgings, and it is the only technical requirement for many forgings. This is not only because hardness testing is fast, simple, and does not damage the forging but also because other mechanical properties can be inferred from the hardness value. Some heat treatment process parameters are also determined based on the required hardness value of the forging. Therefore, a reasonable determination of the hardness value after heat treatment will endow the forging with the best performance, which plays an important role in improving quality and extending service life.
When determining the hardness, designers usually calculate the stress distribution on the forging based on the load it bears during operation, consider the safety factor, and propose strength requirements for the material. Determine the hardness value that forgings should have after heat treatment based on the relationship between strength and hardness. When determining the hardness, it is necessary to avoid copying the data in the manual and pay attention to the actual working conditions and failure forms of the forging. For example, when using the same cold working mold on high-precision punching machines, the mold hardness is required to be higher; however, if the accuracy of the punching machine is poor and the impact energy received by the mold during operation is large, in order to avoid blade breakage or breakage, appropriately reducing the hardness of the mold can actually prolong its service life. The hammer rod of a 10t large forging hammer made of 40CrNi or 35CrMo was mistakenly believed to have been subjected to high impact energy, resulting in a low hardness and a shortened lifespan. According to the failure analysis, the hammer rod belongs to fatigue fracture. Increasing the hardness value of the hammer rod from 241-270HBW to 38-43HRC significantly increases its service life.
In addition to requiring hardness values, other mechanical performance indicators must also be specified for certain important forgings.
- 1) Reasonable combination of strength and toughness. Usually, the strength and toughness of steel materials increase and decrease with each other. For structural forgings, primary impact toughness is commonly used as a safety criterion, pursuing high toughness indicators without sacrificing strength, resulting in mechanical products being bulky and bulky, with a short lifespan. On the contrary, for molds, in order to improve wear resistance, high hardness and strength (torsional strength) are pursued, while the effect of toughness on reducing mold blade breakage and fracture is ignored, and the service life is not long. Therefore, investigation and analysis should be conducted on the working conditions and failure forms of forgings, and the strength and toughness indicators that should be selected for forgings should be determined based on a reasonable combination of strength and toughness.
- 2) Properly handle the relationship between material strength, structural strength, and system strength. The strength indicators of various materials are measured using standard specimens, which depend on the microstructure state of the material (including surface state, residual stress, and stress state). Size factors and notch effects influence the structural strength of forgings, while the system strength is related to the interaction between other forgings. There are significant differences among these three, such as the high fatigue strength of the smooth test rod of the material, but the fatigue strength of the actual object may be very low. Therefore, it is appropriate to determine the mechanical performance indicators of certain important parts based on simulation test results.
- 3) The strength matching of the assembly should be reasonable. Numerous experiments and practical applications have shown that when the combination components (such as worm gears, chain sprockets, balls and collars, and transmission gears) achieve optimal strength matching, their service life can be extended. For example, the hardness of the ball should be 2HRC higher than that of the ring, and the surface hardness of the driving gear of the rear axle of a car should be 2-5HRC higher than that of the driven gear seat. The same type of steel is treated with the same method to form friction pairs with the same hardness, but the wear resistance is relatively poor.
- 4) Forgings with surface strengthening should have a reasonable matching of core and surface strength. When surface strengthening parts (such as carburizing quenching, carbonitriding quenching, nitriding, induction quenching, etc.), when the depth of the hardening layer is certain, the core should have appropriate strength to achieve the optimal matching state between the core and surface strength, to ensure that the forging has a high service life. If the core strength is too low, the transition zone is prone to generating fatigue sources, leading to a decrease in fatigue performance. If the core strength is too high, the residual compressive stress on the surface is small, and the fatigue life is short.
- 5) The impact of environmental media. Forgings made in special environmental media, such as corrosion and high temperature, should adopt corresponding mechanical performance indicators, such as stress corrosion threshold, creep limit, and endurance strength.
The determination of the depth of the hardened layer should consider the principles of the service performance, failure mode, and energy conservation of the forging.
For parts with wear failure as the main cause, the depth of the hardening layer should be determined based on the design life and wear rate of the forging. Generally, it should not be too thick, especially if the surface hardening layer of the mold is too deep, which can cause blade collapse or fracture.
For parts with fatigue failure as the main failure, the depth of the hardening layer is determined based on factors such as surface strengthening method, surface strength, load form, and the shape and size of the parts to achieve the optimal hardening rate. For carburized and carbon-nitrogen co-carburized gears, the optimal hardening rate is 0.1-0.15.
In order to save energy in heat treatment, the hardened layer should not be too deep. Some experts have studied the hardened layer and believe that the depth of the hardened layer for carburizing quenching and high-frequency induction quenching is generally specified to be too deep. If the depth of the hardened layer can be appropriately reduced, it can significantly save energy consumption.
The microstructure of various materials after different heat treatments can be evaluated according to national or industry standards, such as martensite rating of medium carbon steel and medium carbon alloy steel, carbide, residual austenite, and core ferrite rating of carburized or carbonitriding. The microstructure level of qualified products should be indicated in the technical requirements, and these standards should be strictly enforced. The standards should be updated through testing based on the working conditions and failure forms of the parts so as to improve product quality continuously. There are many research achievements on the relationship between microstructure and performance, such as the influence of ferrite morphology and relative amount on mechanical properties in quenched microstructure, the discussion of the advantages and disadvantages of residual austenite, and the study of the relationship between carbide morphology, quantity and size and strength and toughness. These provide a basis for further revising and improving various microstructure rating standards. However, it is also necessary to consider the actual situation of the product before using immature or one-sided test results as a basis for rating.
Heat treatment distortion is one of the most important indicators of heat treatment quality and the main content of heat treatment quality control. Designers should reasonably propose allowable deformation variables based on the characteristics and process of forgings. Although many factors affect heat treatment distortion, there are still regular patterns of distortion. Heat treatment workers should take specific measures based on the theory and practice of heat treatment distortion to ensure that the heat treatment distortion value does not exceed the technical requirements specified in the design.
When heat treatment is the final step in the workpiece processing process, the allowable value of heat treatment distortion is the workpiece size specified on the drawing, and the distortion variable should be determined based on the processing size of the previous step. For this reason, it is necessary to consult with the machining department and carry out pre-correction of dimensions before heat treatment according to the distortion pattern of the workpiece so that the heat treatment distortion is within the qualified range.
When heat treatment is an intermediate process, the machining allowance before heat treatment should be considered as the sum of the machining allowance and heat treatment deformation. Usually, the machining allowance is easy to determine, while the heat treatment deformation variable is complex due to multiple influencing factors. Therefore, sufficient machining allowance is reserved for mechanical processing, and the rest can be used as allowable deformation variables for heat treatment.
The structure, size, and shape of the workpiece have a significant impact on heat treatment distortion and cracking.
- 1) The cross-section of the parts should strive to be uniform to reduce stress concentration and distortion cracking tendency in the transition zone.
- 2) The workpiece should try to maintain symmetry between structure, material composition, and organization to reduce distortion caused by uneven cooling. If necessary, process holes can be opened to adjust the cooling speed of different parts.
- 3) The workpiece should avoid sharp edges, grooves, etc., as much as possible, and there should be rounded transitions at the steps.
- 4) Minimize the number of holes, grooves, and ribs on the workpiece, especially deep holes, grooves, and coarse ribs.