Heat treatment method of ductile iron

- Nov 29, 2019-

In addition to the annealing of malleable ductile iron to decompose cementite into agglomerated graphite, the heat treatment of cast iron is aimed at two aspects: one is to change the matrix structure to improve the performance of cast iron, and the other is to eliminate the stress of the casting. It is worth noting that the heat treatment of the casting cannot change the original graphite shape and distribution of the casting. That is, the original sheet or spherical graphite is still in the form of flakes or spheres after heat treatment, and its size does not change, and the distribution does not change.

First, the time limit

During the casting process, the casting speed of the cast iron parts is different from the surface and the inside, forming the internal stress of the casting. If it is not eliminated, it will deform or even crack the parts during the cutting and use. In order to release stress, artificial aging and natural aging are often used. The casting is heated to about 500~560 °C for a certain period of time, and then the casting is taken out to cool the casting. The aging is artificial aging; the natural aging is to store the cast iron parts outdoors for 6-18 months, so that the stress is naturally released. The aging can release the stress portion, but it is not used because of the long time and low efficiency.

Second, to improve the overall performance of cast iron parts for the purpose of heat treatment

In order to improve the overall performance of cast iron parts, it is often necessary to eliminate white-mouth annealing, improve the ductility of ductile iron, and improve the normalizing and quenching of the strength of nodular cast iron.

1. Eliminate white-mouth annealing

The surface or thin wall of ordinary gray cast iron or spheroidal castings has a white mouth due to excessive cooling rate during the casting process, and the cast iron parts cannot be machined. In order to eliminate the white mouth and reduce the hardness, such cast iron parts are often reheated to above the eutectoid temperature (usually 880-900 ° C), and kept for 1 to 2 h (if the content of Si in the cast iron is high, the time can be short), annealing, cementite decomposition For graphite, the cast iron parts are slowly cooled to 400 ° C -500 ° C to be air-cooled. At a temperature of 700-780 ° C, that is, near the eutectoid temperature, the cooling rate is not too slow, so that the excessive transformation of cementite into graphite reduces the strength of the cast iron.

2. Improve ductility of ductile iron

In the casting process, the ordinary gray cast iron tends to have a large white mouth and a large internal stress. It is difficult to obtain a pure ferrite or pearlite matrix for the cast iron. In order to improve the ductility or toughness of the cast iron, the cast iron is often used. The pieces are reheated to 900-950 ° C and kept for a sufficient time for high temperature annealing, and then cooled to 600 ° C to be cooled. During the process, the cementite in the matrix decomposes graphite, and graphite is precipitated from the austenite. These graphites are concentrated around the original spherical graphite, and the matrix is completely converted into ferrite.

If the as-cast structure consists of a (ferritic + pearlite) matrix and spheroidal graphite, in order to improve the toughness, it is only necessary to decompose the cementite in the pearlite into ferrite and spheroidal graphite, for which the cast iron piece is reheated. After the eutectoid temperature of 700-760 ° C is heated, the furnace is cooled to 600 ° C and cooled.

3. Improve the normalizing of the strength of nodular cast iron

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

4. Quenching and tempering of ductile iron

Ductile castings require higher hardness as bearings, and the cast iron parts are often quenched and tempered at low temperatures. The process is: the casting is heated to a temperature of 860-900 ° C, the original substrate is all austenitized and then cooled in oil or molten salt to achieve quenching, and then heated and tempered at 250-350 ° C, the original matrix is converted back to The fire martensite and the retained austenite structure have the same shape of the original spheroidal graphite. The treated casting has high hardness and certain toughness, retains the lubricating properties of the graphite, and improves the wear resistance.

Ductile iron parts are used as shaft parts, such as crankshafts and connecting rods of diesel engines, which require high-strength mechanical properties with high toughness and good toughness. The process is as follows: the cast iron piece is heated to a temperature of 860-900 ° C to keep the matrix austenitized, and then quenched by cooling in oil or molten salt, and then tempered at a high temperature of 500-600 ° C to obtain tempered sorbite structure. (Generally there is still a small amount of lumpy ferrite), and the original spheroidal graphite has the same shape. After treatment, the strength and toughness are well matched, and it is suitable for the working conditions of the shaft parts.

5. Austempering treatment of ductile iron

The purpose of the austempering treatment of ductile iron is to convert the matrix structure of the cast iron part into a tough lower bainite structure with a strength limit of more than 1100 MPa and an impact toughness of AK ≥ 32 J. The treatment process is: heating the spheroidal graphite iron piece to a temperature of 830-870 ° C to maintain the austenitizing of the substrate, and then injecting it into the molten salt of 280-350 ° C to keep the austenite part into lower bainite, and the original spheroidal graphite is not change. Get high strength ductile iron.

The heat treatment of the above cast iron shows that the heat treatment of the cast iron can only change the matrix structure, and can not change the morphology and distribution of the graphite. The change of mechanical properties is caused by the change of the matrix structure. Graphite sheets of ordinary gray cast iron (including cast iron) have great influence on mechanical properties (strength and ductility), and the mechanical properties of gray cast iron are not significantly improved by heat treatment. It should also be noted that the thermal conductivity of cast iron is worse than that of steel. The presence of graphite leads to a higher notch sensitivity than steel. Therefore, the cooling rate (especially quenching) in the heat treatment of cast iron is strictly controlled.

Third, the surface heat treatment of cast iron

The purpose of heat treatment on the surface of cast iron parts is to improve the wear resistance of cast iron parts. Induction heating quenching, laser heating quenching, soft nitriding and other processes in steel are suitable for cast iron. Diesel engine and internal combustion engine cylinder liners are currently treated by laser heating quenching or soft nitriding. The heating of the laser-heated cast iron parts is very fast, and the workpiece can form a hard layer of high-carbon martensite structure after air cooling, so the wear resistance is greatly enhanced. After the soft nitriding treatment of the cast iron, the surface layer forms a high-hardening layer of the e-phase compound (Fe2-3N), which not only has high hardness, but also has a small friction coefficient, so the anti-wear ability of the spheroidal graphite casting is greatly improved.