Heat treatment process and properties of high-wear-resistant cast iron

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1. Stress-relief annealing: In wear-resistant cast irons and alloy cast irons, uneven wall thicknesses of the castings lead to residual stresses and structural stresses during heating, cooling, and phase transformation processes. Furthermore, large parts tend to have residual stresses inside them after machining, and all of these internal stresses must be eliminated. The typical heating temperature for stress-relief annealing is 500–550°C, with a holding time of 2–8 hours; followed by furnace cooling (for gray cast iron) or air cooling (for ductile cast iron). This process can eliminate 90–95% of the internal stresses in castings, but the microstructure of the cast iron remains unchanged. If the temperature exceeds 550°C or the holding time is too long, graphitization will occur instead, reducing the strength and hardness of the casting. 2. High-temperature graphitization annealing to eliminate white cast iron When castings cool down, white cast iron often forms in the surface layer and thin sections. White cast structure is hard and brittle, has poor machinability, and is prone to flaking. Therefore, annealing (or normalizing) must be used to eliminate the white cast iron structure. The annealing process involves heating to 550–950°C and holding at that temperature for 2–5 hours, followed by cooling in the furnace to 500–550°C before being taken out for air cooling. During the high-temperature holding period, metastable cementite and eutectic cementite decompose into graphite and A; during the subsequent cooling process, secondary cementite and eutectoid cementite also decompose, leading to a graphitization process. The decomposition of cementite leads to a decrease in hardness, thereby improving machinability. 3. Normalizing of ductile iron The purpose of normalizing ductile iron is to obtain a pearlitic matrix structure, refine the grain size, and uniformize the microstructure in order to improve the mechanical properties of the castings. Sometimes normalizing also serves as a preparatory step for surface quenching of ductile iron in terms of microstructure; normalizing is divided into high-temperature normalizing and low-temperature normalizing. The temperature for high-temperature normalizing generally does not exceed 950–980°C, while low-temperature normalizing usually involves heating to the intercritical temperature range of 820–860°C. After normalizing, further treatment is usually required to eliminate the internal stresses generated during normalization. 4. Quenching and tempering of ductile iron To improve the mechanical properties of ductile iron, the casting is generally heated to 30–50°C above Afc1 (where Afc1 represents the temperature at which phase A ceases to form during heating), held at that temperature before being quenched in oil, resulting in a martensitic structure. To appropriately reduce the residual stress after quenching, tempering is generally performed thereafter; the microstructure resulting from low-temperature tempering consists of tempered martensite, residual bainite, and spherical graphite. This type of material has good wear resistance and is used for parts that require high wear resistance and high strength. The medium-temperature tempering temperature is 350–500°C; after tempering, the microstructure consists of tempered martensite along with spherical graphite. This treatment is suitable for thick parts that require good wear resistance as well as certain levels of strength and elasticity. The high-temperature tempering temperature is 500–600°C; after tempering, the microstructure consists of tempered sorbite with spherical graphite, granting it excellent comprehensive properties combining toughness and strength.