Casting heat treatment
For foundry engineers and mechanical structure design engineers, heat treatment is a very meaningful and valuable method to improve the quality of materials. Heat treatment can change or affect the structure and properties of cast iron, while achieving higher Strength, hardness, and improve its abrasion resistance and so on.
Since there are so many types of heat treatment for different purposes, they can be basically divided into two categories. The first type is those whose tissue structure will not change or should not be changed by heat treatment, and the second is those whose basic structure changes. The first heat treatment procedure is mainly used to eliminate internal stress, which is caused by different cooling conditions and conditions during the casting process. The structure, strength and other mechanical properties will not change significantly due to heat treatment. For the second type of heat treatment, al Qaeda has undergone significant changes, which can be roughly divided into five categories: (1) Softening annealing: its purpose is mainly to decompose carbides, reduce their hardness, and improve processing performance. For grinding cast iron, the purpose is to obtain a very high fertility iron structure. (2) Normalization treatment: mainly used to improve or obtain uniform distribution of mechanical properties for castings that are entirely corrugated iron structure. (3) Quenching: mainly to obtain higher hardness or abrasion strength, and at the same time to very high surface wear resistance. (4) Surface hardening treatment: mainly to obtain a hardened surface layer, while obtaining very high surface wear resistance. (5) Precipitation hardening treatment: mainly to obtain high strength without drastic changes in elongation.
(1) Types of cast iron:
Cast iron can be regarded as a kind of steel with high silicon and high carbon content on the base. At the same time, there are graphite and Xueming carbon iron scattered inside the base. This graphite and Xueming carbon iron system crystallize out during the solidification process. Such precipitates can be referred to as eutectic graphite or eutectic carbide. The base mainly containing graphite is called gray cast iron, and the main content of Xueming carbon iron is called white cast iron. If the base contains both graphite and Xueming carbon iron, it is generally called mixed cast iron. Because of its high hardness and brittleness, white cast iron has fewer uses and is mainly used for wear-resistant materials. The main properties of gray cast iron are determined by the shape and size of eutectic graphite, which cannot be improved by heat treatment. In fact, it has very low strength and hardness. The shape of graphite precipitation cannot be changed by heat treatment. At most, during the heat treatment process, the carbides are decomposed and carbon is freed to form graphite.
Generally speaking, the graphite in gray cast iron mainly precipitates in layers, which can be called layered graphite cast iron. If the molten iron is mixed with magnesium and rare earth metals to make graphite precipitate in spheroids during the solidification process, this type of cast iron is called spheroidal graphite cast iron. With the same amount of graphite, this kind of base structure is very important for the material properties. Influence, the properties of spheroidal graphite cast iron are close to those of the same base steel, so its strength can be increased to 380~1000N/, the modulus of elasticity is also much higher than gray cast iron, and the elongation can be increased to 30%. If the amount of magnesium alloy is insufficient, no spheroidal graphite will be formed. The cumulative graphite is called vermicular graphite, and its characteristics are roughly between gray cast iron and spheroidal cast iron.
(2) Normalization of cast iron
The purpose of the normalization treatment is to make the polished iron structure formed on the base regardless of the casting structure. In addition, the polished iron can be refined after the normalization treatment. Compared with the casting structure, the toughness is not reduced and the high strength can be obtained. In addition, the normalization treatment can achieve the purpose of homogenization of the material. For alloyed cast iron, its strength and toughness can be improved at the same time. The first stage of the normalization treatment is in fact graphitization annealing, so after normalization After treatment, in principle, there should be no eutectic carbides, because the al Qaeda of the casting will be completely bolly iron after the normalization treatment.