Heat treatment process of medium frequency induction heating furnace and its impact on forging quality

Effect of medium frequency induction heating furnace on the microstructure and properties of forgings

Ferritic and austenitic heat-resistant stainless steels, high-temperature alloys, aluminum alloys, magnesium alloys, etc., do not undergo allotropic transformation during the heating and cooling process of the medium frequency induction heating furnace, as well as some copper alloys and titanium alloys, etc. The structural defects produced during the forging process cannot be completely improved by heat treatment.

Materials that undergo allotropic transformation during the heating and cooling process of the medium frequency induction heating furnace, such as structural steel and martensitic stainless steel, etc., may have certain structural defects caused by improper forging processes or defects left over from the raw materials after heat treatment. The quality of forgings has a great impact. Here are some examples:

1) The structural defects of some forgings can be improved during post-forging heat treatment, and satisfactory structure and performance can still be obtained after the final heat treatment of the forgings. For example, coarse grains and Widmanstatten structures in generally overheated structural steel forgings, slight network carbides in hypereutectoid steel and bearing steel due to improper cooling, etc.

2) The structural defects of some forgings are difficult to eliminate by normal heat treatment. They require high-temperature normalizing, repeated normalizing, low-temperature decomposition, high-temperature diffusion annealing and other measures to improve them. For example, low-magnification coarse grains, twin carbides of 9Cr18 stainless steel, etc.

3) The structural defects of some forgings cannot be eliminated by ordinary heat treatment processes. As a result, the performance of the forgings after final heat treatment will be reduced or even unqualified. For example, severe stone fractures and edge fractures, overburning, ferrite bands in stainless steel, carbide networks and bands in ledeburite high-alloy tool steels, etc.

4) The structural defects of some forgings will further expand during the final heat treatment and even cause cracking. For example, if the coarse grain structure in alloy structural steel forgings is not improved during post-forging heat treatment, it will often cause coarse martensite needles and unqualified performance after carbonization, nitriding and quenching; coarse band carbonization in high-speed steel For materials, quenching in medium frequency induction heating furnace often causes cracking.

Prevention of heat treatment deformation

The following methods can be used to prevent heat treatment deformation of precision and complex molds.

(1) Reasonable selection of materials. For precision and complex molds, micro-deformation mold steels with good materials (such as air-hardened steel) should be selected. Mold steels with severe carbide segregation should be reasonably forged and subjected to quenching and tempering heat treatment. For larger and unforgeable mold steels, solid solution double Refinement heat treatment.

(2) The mold structure design must be reasonable, the thickness should not be too disparate, and the shape must be symmetrical. For molds with large deformations, the deformation rules must be mastered and machining allowances reserved. For large, precise and complex molds, a combined structure can be used.

(3) Precision and complex molds must be preheated to eliminate residual stress generated during machining.

(4) Reasonably select the heating temperature and control the heating speed. For precision and complex molds, slow heating, preheating and other balanced heating methods can be used to reduce the deformation of the mold heat treatment equipment.

(5) On the premise of ensuring the hardness of the mold, try to use pre-cooling, graded cooling quenching or warm quenching processes.

(6) For precision and complex molds, if conditions permit, try to use vacuum heating quenching and cryogenic treatment after quenching.

(7) For some sophisticated and complex molds, pre-heat treatment, aging heat treatment, quenching, tempering and nitriding heat treatment can be used to control the accuracy of the mold.

(8) When repairing mold blisters, pores, wear and other defects, use repair equipment with small thermal impact such as cold welders to avoid deformation during the repair process.

Several common heat treatment methods to improve the overall performance of cast iron parts

1. Eliminate white hole annealing

White spots appear on the surface or thin walls of ordinary gray cast iron or ductile graphite castings due to excessive cooling during the casting process, and the cast iron parts cannot be machined. In order to eliminate white spots and reduce the hardness, such cast iron parts are often re-induction heated to a temperature above the eutectoid temperature (usually 880-900°C), and kept for 1 to 2 hours (if the Si content of the cast iron is high, the time can be shorter) for annealing and carburizing The body is decomposed into graphite, and then the cast iron is slowly cooled to 400℃-500℃ and air cooled out of the furnace. The cooling rate should not be too slow near the temperature of 700-780°C, that is, the eutectoid temperature, so that excessive cementite will transform into graphite and reduce the strength of the cast iron.

2. Annealing of ductile iron to improve toughness

During the casting process of ductile iron, ordinary gray cast iron has a large whitening tendency and large internal stress. It is difficult to obtain a pure ferrite or pearlite matrix for cast iron parts. In order to improve the ductility or toughness of cast iron parts, cast iron is often The parts are reheated to 900-950°C and kept warm for sufficient time to perform high-temperature annealing, and then cooled to 600°C and cooled out of the furnace. During the process, the cementite in the matrix decomposes into graphite, and graphite is precipitated from austenite. These graphites gather around the original spherical graphite, and the matrix is completely converted into ferrite. If the as-cast structure is composed of (ferrite + pearlite) matrix and spherical graphite, in order to improve the toughness, the cementite in the pearlite only needs to be decomposed and converted into ferrite and spherical graphite. For this purpose, the cast iron part must be reheated. After being insulated above and below the eutectoid temperature of 700-760°C, the intermediate frequency furnace is cooled to 600°C and cooled out of the furnace.

3. Normalizing to improve the strength of ductile iron

The purpose of normalizing ductile iron is to convert the matrix structure into fine pearlite structure. The process is to reheat the ductile iron casting with a matrix of ferrite and pearlite to a temperature of 850-900°C. The original ferrite and pearlite are converted into austenite, and some spherical graphite is dissolved in the austenite. After heat preservation, the air-cooled austenite transforms into fine pearlite, so the strength of the casting increases.