On 2018-08-08 14:48:54
7 methods to reduce heat treatment deformation
Heat treatment technology is an important means to make all kinds of metal materials obtain excellent properties. In many practical applications, the reasonable selection of materials and various forming processes cannot meet the mechanical, physical and chemical properties of the metal workpiece, so the heat treatment process is essential.
But heat treatment equipment vacuum furnace process in addition to outside, it can play a positive role in the process also inevitably produce more or less deformation, and this is the mechanical processing, must be avoided in the relationship between the two is coexist and need to avoid, can only adopt the appropriate method to control the deformation within the scope of the small as far as possible.
1. Temperature measurement and control
There are many forms of heat treatment technology applied in industry, but their basic process is the process of heat action, which is composed of heating, heat preservation and cooling. The whole process can be described by several parameters such as heating speed, heating temperature, holding time, cooling speed and heat treatment period. In the heat treatment process, to use a variety of reheating furnace, metal heat treatment in these reheating furnace (such as the basic heat treatment of annealing, quenching, tempering, chemical heat treatment of carburizing, ammoniating, aluminizing, chromizing or dehydrogenation, deoxygenation, etc.). Therefore, the temperature measurement in the furnace has become an important process parameter measurement for heat treatment. In each heat treatment process specification, temperature is a very important content. If the temperature measurement is not accurate, the heat treatment process specification will not be properly implemented, resulting in product quality degradation or even scrap. The measurement and control of temperature is the key factor of heat treatment process and deformation.
2. Temperature control normalizing or isothermal annealing
Too high normalizing hardness, mixed crystal and large amount of Soxhlet or Weissner microstructure will increase the deformation of inner hole, so temperature-controlled normalizing or isothermal annealing should be used to treat forgings. Normalizing, annealing and tempering before quenching all have certain influence on the final deformation of the metal, and directly affect the change of metal structure. It has been proved by practice that the isothermal quenching can effectively make the metal structure more uniform and reduce its deformation.
3. Reasonable cooling method
The influence of cooling process on deformation after quenching is also an important reason for deformation. The quenching deformation of hot oil is smaller than that of cold oil and is generally controlled at 100℃±20℃. The cooling capacity of the oil is also critical to deformation. The mixing mode and speed of quenching affect the deformation. The faster the metal heat treatment cooling rate, the more uneven the cooling, the greater the stress, the greater the mold deformation. Precooling can be adopted as far as possible on the premise of ensuring mold hardness requirements; The use of fractional cooling and quenching can significantly reduce the thermal stress and microstructure stress, which is an effective method to reduce the deformation of some complex workpiece. For some particularly complex or high precision workpiece, the use of isothermal quenching can significantly reduce the deformation.
4. Reasonable part structure
After the metal heat treatment equipment processing in the cooling process, always the thin part of the cold faster, the thick part of the cold slower. In the case of meeting the actual production needs, the workpiece thickness disparity should be reduced as far as possible, and the section of parts should be uniform to reduce the distortion and cracking tendency of the transition zone due to stress concentration; The symmetry of structure and material composition and organization should be kept as far as possible to reduce distortion caused by uneven cooling. The workpiece should be as far as possible to avoid sharp edges and corners, grooves, etc., in the thickness of the workpiece junction, steps to have a rounded corner transition; To minimize the holes and grooves on the workpiece structure asymmetry; Parts with uneven thickness are reserved for processing amount.
5. Reasonable clamping method and fixture
The purpose of heat-resistant steel casting is to make the workpiece heated and cooled uniformly to reduce the uneven thermal stress and the uneven structure stress and reduce the deformation. The clamping method can be changed. Disc parts are vertical to the oil surface, shaft parts are vertical, compensation washer, support washer, superposition washer, etc. Spline hole parts can be carburized mandles, etc.
6, mechanical processing
When heat treatment is the next process of the workpiece processing, the allowable value of heat treatment distortion should meet the size of the workpiece specified in the drawing, and the distortion should be determined according to the size of the previous process. Therefore, according to the distortion law of the workpiece, the size should be pre-corrected before heat treatment, so that the distortion of heat treatment is just within the qualified range. When heat treatment is an intermediate process, the machining allowance before heat treatment should be regarded as the sum of machining allowance and heat treatment distortion. In general, machining allowance is easy to be determined, while heat treatment is more complicated due to the influence factors, so enough machining allowance is set aside for machining, and the rest can be regarded as allowable distortion of heat treatment. After heat treatment, it can be processed again. According to the deformation rule of the workpiece, reverse deformation and pre-expansion hole of shrinkage end can be applied to improve the qualified rate of deformation after quenching.
7. Use the right medium
Under the premise of ensuring the same hardness requirements, as far as possible to use oily media, experiments and practices have shown that under the premise of no difference in other conditions, the cooling rate of oily media is slower, and the cooling rate of water-based media is relatively faster. Moreover, compared with oil-based media, water temperature changes have a greater impact on the cooling characteristics of water-based media. Under the same heat treatment conditions, the deformation of oil-based media after quenching is relatively small compared with water-based media.