Heat treatment process of cast iron

There are many kinds of heat treatment, such as cast iron heat treatment, carburizing and nitriding heat treatment, manganese sheet screw hardening and other metal heat treatment. Today, Dongguan Xinfei heat treatment plant introduces the heat treatment process of cast iron.

The heat treatment of cast iron aims at two aspects:

One is to change the matrix structure and improve the properties of cast iron,

The other is to eliminate casting stress.

It is worth noting that the heat treatment of cast iron can not change the original graphite morphology and distribution of the casting, that is, the graphite that was originally flake or ball is still flake or ball after heat treatment, and its size will not change, and its distribution will not change.

According to different technological purposes, heat treatment of cast iron can be divided into the following categories:

(1) Stress relief annealing heat treatment;

(2) Graphitization heat treatment;

(3) Change the heat treatment of matrix structure.

This chapter briefly introduces the theoretical basis and process characteristics of the above heat treatment process.

Section I stress relief heat treatment

Stress relief annealing is to keep the casting at a certain temperature, and then cool it slowly, so as to eliminate the forging residual stress in the casting. For gray cast iron, stress relief annealing can reduce the deformation after machining without disordering the geometry of the casting. For white cast iron, stress relief annealing can avoid deformation and even self cracking when the castings are subject to vibration or environment changes during storage, transportation and use.

1、 Generation of forging residual stress

In the process of solidification and subsequent cooling, the volume of castings shrinks or expands. This volume change is often constrained by the outside world and the various departments of the castings, and can not be carried out freely, so forging stress is produced. If the cause of stress is eliminated, the forging stress will be eliminated accordingly. This stress is called temporary forging stress. If the cause of stress is eliminated and the forging stress still exists, this stress is called forging residual stress.

In the process of solidification and subsequent cooling, because of the different wall thickness and cooling premise, the temperature and transformation degree of each part will be different, resulting in different volume changes of each part of the casting. If the forging alloy is already in the elastic state at this time, there will be mutual restriction among the casting departments. The residual stress in forging is often caused by different temperature and transformation degree.

2、 Theoretical basis of stress relief annealing

The results show that the forging residual stress is directly proportional to the temperature difference of each part and the elastic modulus of the forging alloy. For a long time in the past, it was thought that there was an elastic-plastic transition temperature in the cooling process of forging alloy, and that the elastic-plastic transition temperature of cast iron was about 400 ℃. Based on this familiarity, the heating temperature for stress relief annealing should be 400 ℃. However, the practice proves that the heating temperature is not ideal. Recent studies show that there is no elastic-plastic transition temperature in the alloy material, even in the state of solid-liquid coexistence, the alloy is still elastic.

Prescription

In the process of casting, the cooling rate of iron castings is different from the surface to the inside, forming the internal stress of casting. If it is not eliminated, it will make the parts deform or even crack in the process of cutting and using. In order to release stress, artificial aging and natural aging are often used. Heat the casting to about 500 ~ 560 ℃ for a certain period of time, and then take out the casting with furnace cooling for air cooling. This aging is artificial aging. Natural aging is to store the cast iron outside for 6 ~ 18 months, so that the stress can be released naturally. This aging can release the stress part, but because of the long time and low efficiency, it is not used very much.

2、 Heat treatment for improving the overall properties of iron castings

In order to improve the overall properties of iron castings, white anneal is often eliminated, ductile ductile iron annealed, normalized and quenched to improve the strength of ductile iron.

1. Eliminate white annealing

The surface or thin wall of ordinary grey cast iron or nodular cast iron is in the process of casting, because the cooling speed is too fast, white cast iron can not be cut. In order to eliminate the white cast iron and reduce the hardness, this kind of cast iron is often reheated to the eutectoid temperature (usually 880~900 ℃), and kept for 1 ~ 2H (if the Si content of cast iron is high, the time can be short) for annealing. The cementite is decomposed into graphite, and then the cast iron is slowly cooled to 400 ℃ -500 ℃ out of the furnace for air cooling. At 700-780 ℃, i.e. eutectoid temperature, the cooling rate should not be too slow, so that too much cementite can be transformed into graphite, which reduces the strength of iron castings.

2. Annealing of ductile iron to improve toughness

During the casting process of nodular cast iron, the white tendency of this common gray cast iron is large, and the internal stress is also large. It is very difficult to get pure ferrite or pearlite matrix for iron castings. In order to improve the ductility or toughness of iron castings, the iron castings are often reheated to 900-950 ℃ and kept for enough time for high temperature annealing, and then cooled to 600 ℃ and cooled out of the furnace. During the process, the cementite in the matrix decomposes into graphite, which precipitates from austenite. These graphite gather around the original spheroidal graphite, and the matrix transforms into ferrite.

If the as cast structure is composed of (ferrite + pearlite) matrix and spheroidal graphite, in order to improve the toughness, only the cementite in pearlite needs to be decomposed into ferrite and spheroidal graphite. Therefore, the cast iron is reheated to the eutectoid temperature of 700-760 ℃ and cooled to 600 ℃ after heat preservation.

3. Normalizing to improve the strength of nodular cast iron

The aim of normalizing ductile iron is to transform the matrix structure into fine pearlite structure. The process is to reheat the ductile iron with ferrite and pearlite as its matrix to 850-900 ℃, transform the original ferrite and pearlite to austenite, dissolve some spheroidal graphite in austenite, and transform the austenite to pearlite after heat preservation, so the strength of the casting is improved.

4. Quenching and tempering of ductile iron

As a bearing, ductile iron castings need higher hardness, so they are often quenched and tempered at low temperature. The process is as follows: the casting is heated to 860-900 ℃, the original matrix is austenitized by heat preservation, and then cooled in oil or molten salt to realize quenching. After heat preservation and tempering at 250-350 ℃, the original matrix is transformed into tempered martensite and retained austenite structure, and the original spherical graphite morphology is unchanged. The treated castings have high hardness and certain toughness, which preserve the lubrication property of graphite and improve the wear resistance.

As shaft parts, such as crankshaft and connecting rod of diesel engine, nodular cast iron is required to have comprehensive mechanical properties with high strength and good toughness. The process is as follows: the cast iron is heated to 860-900 ℃ for heat preservation to austenitize the matrix, then cooled in oil or molten salt to realize quenching, and then tempered at 500-600 ℃ for high temperature to obtain tempered sorbite structure (generally there is a small amount of pure massive ferrite), and the original spherical graphite morphology remains unchanged. After treatment, the strength and toughness match well, which is suitable for the working conditions of shaft parts.

5. Austempering treatment of nodular cast iron

The purpose of austempering treatment of nodular cast iron is to transform the matrix structure of cast iron into the lower bainite structure with the strength limit exceeding 1100mpa and the impact toughness AK ≥ 32j. The treatment process is: heat the ductile iron to 830-870 ℃ and austenitize the heat preservation matrix, then put it into 280-350 ℃ molten salt for heat preservation, so that the austenite part is transformed into lower bainite, and the original spheroidal graphite remains unchanged. High strength ductile iron was obtained.

The above heat treatment of cast iron shows that the heat treatment of cast iron can only change the matrix structure, not the shape and distribution of graphite, and the change of mechanical properties is caused by the change of matrix structure. The graphite sheet of common grey cast iron (including inoculated cast iron) has a great influence on the mechanical properties (strength and ductility), but the mechanical properties of grey cast iron improved by heat treatment are not significant. It should also be noted that the thermal conductivity of cast iron is worse than that of steel, and the existence of graphite leads to higher notch sensitivity than that of steel. Therefore, the cooling rate (especially quenching) in heat treatment of cast iron should be strictly controlled.