Steel Castings

Steel castings

Category: Shaft and rudder system finishing

Steel castings refer to parts made of cast steel. The following will introduce them from the perspectives of classification, advantages and disadvantages, applications, and production processes:

Classification

• By chemical composition: they can be divided into two categories: carbon cast steel and alloy cast steel. Carbon cast steel can be divided into low carbon steel ZG15, medium carbon steel ZG25-ZG45, and high carbon steel ZG55; alloy cast steel can be divided into low alloy cast steel and high alloy cast steel according to the total amount of alloy elements.

• By use characteristics: they can be divided into cast tool steel, cast special steel, cast steel for engineering and structure, and cast alloy steel. Cast tool steel is divided into cast tool steel and cast mold steel; cast special steel includes cast stainless steel, cast heat-resistant steel, cast wear-resistant steel, cast nickel-based alloy, etc.

Advantages

• High design flexibility: designers have greater freedom of design choice for the shape and size of castings, and can manufacture parts with complex shapes and hollow sections. The conversion speed from drawings to finished products is fast, which is conducive to rapid quotation response and shortened delivery time.

• Strong metallurgical adaptability: different chemical compositions and organizational controls can be selected to meet the requirements of various projects; different heat treatment processes can be used to select mechanical properties and performance in a wide range, and have good welding and processing performance.

• Good overall structural properties: The isotropy of cast steel materials and the strong overall structural properties of cast steel parts improve the stability of the project. Coupled with the advantages of weight-reducing design and short delivery time, it has competitive advantages in terms of price and economy.

• Wide weight range: The weight of cast steel parts can vary in a wide range, from only tens of grams of investment castings to large cast steel parts of several tons, tens of tons or even hundreds of tons.

Disadvantages

• Uneven organization: After the liquid metal is injected into the mold, a layer of liquid metal in contact with the mold wall solidifies into finer grains. As the distance from the mold wall increases, the crystal growth mode changes, and an equiaxed crystal zone is formed in the center of the casting, resulting in uneven organization in the casting and relatively coarse grains.

• Loose organization: Liquid metal crystallization is carried out in the form of tree branch growth. It is difficult for the branches to be completely filled with metal liquid, resulting in the general lack of density of castings. In addition, the volume shrinkage of liquid metal during cooling and solidification is not sufficiently supplemented, which can also form loose or even shrinkage holes.

• Rough surface: The surface of steel castings is generally rough, which cannot be compared with the machined surface, and the shape is also more complex.

Application field

• Power station equipment field: Many parts in thermal power station and nuclear power station equipment need to withstand the corrosion of high temperature and high pressure steam. Steel castings can meet these requirements to the greatest extent. For example, turbine cylinders and generator stator housings are all steel castings.

• Railway transportation field: Some key components of locomotives and vehicles, such as wheels, side frames, bolsters, couplers, etc., are traditional steel castings. The switch used for railway switching is also a steel casting. It is subjected to strong impact and friction, and the working conditions are extremely harsh.

• Construction and engineering machinery field: Most parts of construction machinery and engineering machinery are subject to high loads or need to withstand impact wear, and a large part of them are cast steel parts, such as driving wheels, load-bearing wheels, rocker arms, track plates, etc. in the motion system.

Production process

• Smelting of cast steel: Generally, electric furnaces are used for smelting, mainly arc furnaces and induction furnaces. According to the different lining materials and slag systems used, they can be divided into acid furnaces and alkaline furnaces. Carbon steel and low-alloy steel can be smelted in any furnace, but high-alloy steel can only be smelted in alkaline furnaces.

• Casting process: The molding sand used to produce steel castings should have high refractoriness, anti-sand adhesion, strength, air permeability and yield. In the design of the pouring system and riser, the sequential solidification principle is often used to set the pouring system and riser to prevent the occurrence of shrinkage cavities and shrinkage. Chilled iron is also widely used. In addition, a bottom pouring system with a simple shape and a large cross-sectional area should be used as much as possible to allow the molten steel to fill the mold quickly and smoothly.

• Heat treatment: The heat treatment of cast steel is usually annealing or normalizing. Annealing is mainly used for steel castings with w (C) ≥ 0.35% or with particularly complex structures; normalizing is mainly used for steel castings with w (C) ≤ 0.35%. Such steel castings have low carbon content, good plasticity, and are not easy to crack when cooled.