On 2018-08-08 14:48:54
Causes of material deformation after heat treatment
The deformation of materials after heating, heat preservation and cooling is mainly caused by the change of the internal structure and structure of the metal under the action of chemical and mechanical action, so that the metal alloy can obtain the technology to improve the function of the material. Medium frequency induction furnace heat treatment process is to make all kinds of metal materials for the important means of superior function, heat treatment process, besides have positive role process will produce inevitable deformation, and this is must be avoid in the mechanical processing, under the coexistence and avoid between the two, can only adopt the appropriate method to control the deformation within the scope of the small as far as possible.
Temperature is the pivotal factor of deformation
There are many types of heat treatment technology of if induction furnace applied in industry, but their basic processes are all thermal processes, which are composed of three stages: 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 medium frequency induction furnace equipment, metal heat treatment is carried out in these equipment (such as the basic heat treatment of annealing, quenching, tempering, chemical heat treatment of carburizing, ammonia, aluminum, chromium or hydrogen, deoxidizing, etc.). Therefore, the measurement of the temperature in the medium frequency induction furnace becomes an important process parameter measurement of heat treatment. In each heat treatment process specification, temperature is very important content. If the temperature measurement is not correct, the heat treatment process specification is not accurate implementation, as a result of product quality decline or even scrap. Temperature measurement and control is the hinge of heat treatment process and the hinge factor affecting deformation.
(1) the high temperature strength loss of the workpiece is relatively reduced and the plastic resistance is enhanced after the process temperature is reduced. In this way, the comprehensive ability of the workpiece to resist stress deformation, quenching deformation and high-temperature creep is enhanced, and the deformation will be reduced.
(2) after the process temperature decreases, the temperature range of heating and cooling of the workpiece decreases, and the temperature inconsistency of various parts caused by this will also decrease, and the thermal stress and tissue stress caused by this will also relatively reduce, so that the deformation will be reduced;
(3) if the process temperature decreases and the heat treatment process time is shortened, the high-temperature creep time of the workpiece will decrease and the deformation will also decrease.
Other influencing factors of deformation and reduction measures
(1) prepare for heat treatment
Normalizing through high, mixed crystal, a large number of sostenite or weiermann structure will make the deformation of the inner hole increase, so we should use temperate-controlled normalizing or isothermal annealing treatment forgings. The normalizing, annealing and tempering of metal before quenching all have a certain influence on the ultimate deformation of metal, which directly affects the change of metal structure. It has been proved that isothermal quenching during normalizing can effectively average the metal structure and reduce its deformation.
(2) apply proper cooling methods
The influence of metal cooling process on deformation after quenching is also an important reason for deformation. The deformation of hot oil quenching in medium frequency induction furnace is smaller than that of cold oil quenching, which 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 speed, the more uneven the cooling, the greater the stress, the greater the deformation of the mold. Can ensure the mold hardness requirements, as far as possible to use precooling; It is an effective method to reduce the deformation of some workpiece with complicated shape by adopting the graded cooling and quenching. For some particularly complex or high precision workpiece, the use of isothermal quenching can significantly reduce deformation.
(3) part structure
In the cooling process of metal after the medium frequency induction furnace heat treatment, the always thin part of the cooling fast, the thick part of the cooling slow. In the case of the actual production demand, the thickness of the workpiece should be reduced as far as possible, the workpiece section to achieve the average, in order to reduce the temperature change transition zone; The workpiece should try to maintain the symmetry of structure and material composition and organization to reduce the distortion caused by uneven cooling. The workpiece should try to avoid sharp edges and corners, grooves, etc., in the profile radiator workpiece thickness interface, steps to have rounded corner transition; Minimize the structure of holes and grooves on the workpiece; The method of reserving machining quantity can be used for parts with unequal thickness.
(4) adopt proper clamping method and fixture
The purpose is to make the workpiece heating and cooling average, in order to reduce the thermal stress uneven, uneven organizational stress, to reduce the deformation, can change the way of clamping, disc parts and oil vertical, shaft parts vertical assembly, using compensation washer, support washer, stack washer, spline hole parts can be carburized mandels.
(5) use appropriate media
Under the premise of ensuring the same hardness requirements, try to use oily medium, experiment and practice proved that under the condition of no difference between other premises, the cooling speed of oily medium is slower, and the cooling speed of water-based medium is relatively faster. In addition, compared with the oil medium, the change of water temperature has a greater impact on the cooling characteristics of the water medium. Under the same premise of heat treatment, the deformation of the oil medium after quenching is relatively small compared with the water medium.