Higher request for the strength, plasticity and toughness of the machine parts, need to use steel. The output of steel is second only to iron, accounts for about 15% of the total output of castings.
A, according to the chemical composition of cast steel can be divided into two categories, carbon steel and alloy steel. With carbon steel casting the most widely used, more than 80% of the total steel casting production.
1, carbon steel casting
In general, high melting point, low carbon steel ZG15 casting performance is poor, only used in the manufacture of machine parts or carburized parts; Medium carbon steel ZG25 ~ ZG45, is higher than that of the comprehensive performance of various types of cast iron, namely, high strength, good plasticity and toughness, therefore is suitable for manufacturing complex shape, high strength and toughness requirement of parts, such as the train wheels, forging hammer frame and the anvil block, roll and high pressure valves, etc., is the most used carbon cast steel category; The low melting point of high carbon steel ZG55, its casting properties of a medium carbon steel is good, but its low plasticity and toughness, used to make a few wear parts.
2, alloy steel casting
Based on what amount of alloy elements, low alloy cast steel can be divided into two high alloy steel and alloy steel.
1) the low alloy steel, the main application of manganese, silicon manganese, chrome, etc. Such as ZG40Mn, ZG30MnSi1, ZG30Cr1MnSi1, etc. Used to make gear and hydraulic press working cylinder and turbine rotor parts, such as the ZG40Cr1 are commonly used to manufacture high strength gear and shaft high strength and other important mechanical parts.
2) high alloy steel, wear-resisting, high temperature or special performance, such as corrosion resistance. Such as high manganese steel ZGMn13, is a kind of wear-resistant steel, mainly used in the manufacture of parts are used for dry friction conditions, such as the anterior wall of the excavator grab and grab bucket tooth, tractor and the tracks of the tank; Chromium and chromium nickel stainless steel ZG1Cr18Ni9 stainless steel ZG1Cr13 and ZGCr28 of nitric acid corrosion resistance is very high, mainly used in the manufacture of chemical, petroleum, chemical and food equipment parts.
Second, the cast steel casting process characteristics
The mechanical properties of the cast steel is higher than cast iron, but its worse than cast iron casting performance. Because the melting point of cast steel is higher, easy oxidation of liquid steel, steel illiquid, contraction, its volume shrinkage rate is 10-14%, linear contraction was 1.8 ~ 2.5%. To prevent steel misrun and cold shut, shrinkage cavity and shrinkage, cracks and defects such as adhering sand, more complicated than the cast iron craft measures must be taken:
1, due to poor liquidity of liquid steel, to prevent steel cold insulation and misrun, steel wall thickness not less than 8 mm; Gating system to simple structure, and section size is bigger than cast iron; Use dry mold or hot mould; Properly increasing pouring temperature, average of 1520 ° ~ 1600 ℃, because of the high pouring temperature, liquid steel superheat, keep long liquid, liquid can be improved. But the water temperature is too high, can cause coarse grains, defects such as hot cracking, porosity, and adhering sand. So generally small, thin wall and complex shape casting, the pouring temperature is about steel melting point temperature of + 150 ℃; Large, thick wall castings pouring temperature about 100 ℃ higher than the melting point.
2, due to the shrinkage of casting steel more than iron, to prevent the casting shrinkage cavity, shrinkage porosity defect, adopts the riser and mostly on the casting process, cold iron and subsidies and other measures, in order to realize the solidification sequence.
In addition, in order to prevent the steel shrinkage cavity, shrinkage, porosity and crack defects, should make its uniform thickness, avoid sharp corners and rectangular structure, saw foam are added into the mould with sand, add coke to the core, and adopts hollow core and sand core and so on to improve the sand mold or core of breathable and back down.
