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Commonly Used Heat Treatment Methods For Ductile Iron

According to different heat treatment purposes, the commonly used heat treatment methods for ductile cast iron are as follows:

1. High Temperature Annealing and Low Temperature Annealing.

The purpose of annealing is to obtain ferrite matrix ductile iron. After casting, different amounts of pearlite and cementite often appear in the structure of the casting, which makes the cutting process more difficult. In order to improve its workability and eliminate casting stress at the same time, annealing treatment is required.

When the as-cast structure is F+P+Fe3C+G (graphite), high temperature annealing is carried out, that is, the casting is heated to above the eutectoid temperature (about 900~950℃), kept for 2~5h, and then cooled to 600℃ with the furnace Air-cooled out of the oven.

When the as-cast structure is F+P+G (graphite), low-temperature annealing is carried out, that is, the casting is heated to near the eutectoid temperature (approximately 700~760℃), kept for 3~6h, and then cooled to 600℃ with the furnace and air-cooled. .

2. Normalizing

Normalizing can be divided into high temperature and low temperature normalizing. High-temperature normalizing is to heat the casting to above the eutectoid temperature, generally 880~920℃, hold for 1~3h, and then air-cool to make it cool quickly within the eutectoid temperature range to obtain pearlitic ductile iron. For thick-walled castings, air cooling or even spray cooling should be used to ensure that the pearlite matrix is obtained. If there is free cementite in the as-cast structure, the normalizing temperature should be increased to 950~980℃, so that all the free cementite is dissolved into austenite at high temperature.

Low-temperature normalizing is to heat the casting to 840~860℃, keep it for 1~4h, and then air-cool it out of the furnace. Nodular cast iron with pearlite + ferrite matrix is obtained by normalizing at low temperature.

The thermal conductivity of ductile iron is poor, and the internal stress of the castings after normalizing is relatively large. Therefore, a stress relief annealing should be carried out after normalizing, that is, heating to 550~600℃, heat preservation for 3~4h and air cooling.

3. Austempering

Austempering is suitable for ductile iron parts with complex shapes and easy deformation, and high comprehensive mechanical properties are required at the same time. The method is to heat the casting to 860~920℃, properly keep it warm and quickly put it into a salt bath furnace at 250~350℃ for 0.5~1h isothermal treatment, and then take it out for air cooling. After austempering, lower bainite + a small amount of retained austenite + spheroidal graphite are obtained. As the internal stress of austempering is not large, tempering is not necessary. After austempering, the tensile strength σb can reach 1100~1600MPa, the hardness is 38~50HRC, and the impact toughness αK is 30~100J/cm². It can be seen that austempering is an effective way to improve the comprehensive mechanical properties of ductile iron, but it is only suitable for parts with small structural dimensions, such as small-sized gears, rolling bearing rings, camshafts, etc.

4. Quenching and Tempering Treatment

For castings with complex forces and large cross-sectional dimensions, quenching and tempering treatment is generally used to meet the requirements of high comprehensive mechanical properties. During quenching and tempering, the casting is heated to 860~920℃, oil-cooled after heat preservation, and then tempered at a high temperature of 550~620℃ for 2~6h to obtain tempered sorbite and spheroidal graphite structure, with a hardness of 250~300HBW, which has a good Its comprehensive mechanical properties are often used to process diesel engine crankshafts, connecting rods and other parts.