1. Stress relief annealing
In the process of heating, cooling and phase change, the effect force and microstructure stress will be generated due to the uneven wall thickness of wear resistant cast iron and alloy cast iron. In addition, large parts are also prone to residual stress after machining, all these internal stresses must be eliminated. The usual heating temperature of de-stress annealing is 500 ~ 550℃ and the holding time is 2 ~ 8h, then the furnace is cold (gray iron) or air cold (ductile iron). This process can eliminate 90 ~ 95% of the internal stress in castings, but the cast iron microstructure does not change. If the temperature exceeds 550℃ or the holding time is too long, it will cause graphitization, which will reduce the strength and hardness of the casting.
2. High temperature graphitization annealing to eliminate the white mouth of casting
When the casting is cooled, the surface layer and the thin section often produce white mouth. White - mouth structure hard and brittle, poor processing performance, easy to flake. Therefore, the method of annealing (or normalizing) must be used to eliminate the white tissue. The annealing process is: heating to 550-950℃ 2 ~ 5 hours of insulation, followed by furnace cooling to 500-550℃ and then air cooling. During high temperature insulation, the hypercementite and eutectic cementite were decomposed into graphite and A, and the secondary cementite and eutectoid cementite were also decomposed in the subsequent cooling process, resulting in graphitization. As a result of the decomposition of cementite, resulting in a decrease in hardness, thereby improving the machinability.
3. Normalizing the ductile iron
The purpose of nodular iron normalizing is to obtain the pearlite matrix structure, refine the grain size, and improve the mechanical properties of the casting. Sometimes normalizing is also the preparation of ductile iron surface quenching in the tissue. High temperature normalizing temperature generally does not exceed 950 ~ 980℃, low temperature normalizing temperature generally heating to the total folding temperature range of 820 ~ 860℃. In order to eliminate the internal stress caused by normalizing, four people should be treated after normalizing.
4. Quenching and tempering of ductile iron
In order to improve the mechanical properties of ductile iron, generally the casting is heated to 30 ~ 50℃ above Afc1 (Afc1 represents the final forming temperature of A when heated), and the martensite structure is obtained by quenching into oil after the heat preservation. In order to reduce the residual stress after quenching, tempering should be carried out after quenching. The low temperature tempering structure is tempering martensite with residual bainite and spheroidal graphite. This structure has good wear resistance and is used for parts requiring high wear resistance and high strength. After tempering at the medium temperature of 350-500℃, the microstructure is tempered troostite plus spheroidal graphite, which is suitable for thick parts with good wear resistance, certain effective stability and elasticity. The tempering temperature at high temperature is 500-60d ℃. After tempering, the microstructure is tempered soxhlet and spheroidal graphite, which has a good combination of toughness and strength.
On 2018-08-08 11:45:46
