Technical requirements for forging high-precision steel castings
Using advanced water glass lost wax casting technology, precision steel castings made of ordinary carbon steel and low alloy steel are produced, with dimensional accuracy of CT4 and roughness tolerance of 3.2 microns. Precision steel casting refers to a kind of precision steel casting produced by casting.
Precision castings process steel castings with high precision, generally with a linear dimension tolerance of 1%, a angular dimension tolerance of ±1/2 degree, and a surface roughness of Ra1.6-3.2, which can save material and machining costs. It can produce parts with complex shapes, or build several parts into one, eliminating assembly or welding work. It can cast exquisite text or LOGO patterns on the surface of the product to enhance the product brand image. The forging material has a wide range of properties, high temperature resistance, wear resistance, corrosion resistance, and material strength of up to HRC15-60 to meet various working conditions. Castings are easy to manufacture in batches, ranging from small quantities to mass production, with high efficiency and good repeatability; low mold cost and low initial investment in product development.
Most large and medium-sized steel castings are key load-bearing and supporting parts in heavy and large technical equipment. The castings are extremely large in size, complex in shape, and have large cross-sectional thickness gaps. These castings are often in high temperature and high pressure operation, and their performance and quality requirements are very strict, especially the high temperature performance requirements are more stringent.
Steel castings should be used after heat treatment. Due to the presence of vents, cracks, shrinkage and shrinkage, coarse crystals, uneven structure and residual thermal stress in the cast steel parts, the compressive strength, especially plastic deformation and ductility of the steel castings are greatly reduced.
Precision steel castings are mainly used to manufacture some parts with complex shapes, difficult to forge or cut, but requiring high strength and plasticity.
The heat treatment of precision steel castings consists of three stages: heating, insulation and cooling. The determination of its process parameters is based on ensuring product quality and saving costs.
1. Heating
Heating is the process with the highest energy consumption in the heat treatment process. The main technology of the heating process is to select appropriate heating methods, heating rates and charging methods.
(1) Heating rate: For general small and medium-sized precision steel castings, the heating rate can be unlimited and the maximum power of the furnace can be used for heating. The use of hot furnace charging can greatly shorten the heating time and production cycle. In fact, under rapid heating conditions, there is no significant temperature lag between the surface and the core of the casting, and slow heating will lead to reduced production efficiency, increased energy consumption, and serious oxidation and decarburization of the casting surface. However, for some castings with complex shapes and structures, thick walls, and easy to produce large thermal stresses during heating, resulting in deformation or cracking, the heating rate should be controlled. Generally, low-temperature slow heating (below 600.C) or staying in the low or medium temperature zone for one or two times can be adopted. Rapid heating can still be used in the high temperature zone.
(2) Heating method: The heating methods for steel castings include radiation heating, salt bath heating and induction heating. The principle of selecting the heating method is fast and uniform, easy to control, high efficiency and low cost. Generally, the structural dimensions, chemical composition, heat treatment process and quality requirements of the casting should be considered.
(3) Loading method: The stacking method of the castings in the furnace should be given enough attention. The basic principle is to make full use of the effective space, ensure uniform heating conditions and prevent deformation of the castings.
2. Insulation
The determination of the heat treatment insulation time of steel castings should consider two factors: one is to make the surface and core temperature of the castings uniform; the other is to homogenize the structure. Therefore, the insulation residence time mainly depends on the thermal conductivity, cross-sectional wall thickness and alloy elements of the castings. Generally, alloy steel castings require longer insulation time than carbon steel castings. The wall thickness of the casting is usually the main basis for estimating the insulation time. According to experience, insulation for 1 hour for every 25mm wall thickness. For those greater than 25ram, the insulation time is extended by 1 hour for every additional 25ram. For the holding time during tempering and aging treatment, the purpose of heat treatment, holding temperature and element diffusion rate should be considered. The holding time should be longer than that during normalizing or quenching, generally 1.5 to 2 times of the holding time during normalizing or quenching.
3. Cooling
Precision steel castings can be cooled at different rates after holding to complete the phase transformation in the steel, so as to obtain the required metallographic structure and achieve the specified performance indicators. Generally speaking, increasing the cooling rate is conducive to obtaining a good organizational state and refining the grains, thereby improving the mechanical properties of the steel. However, too high a cooling rate can easily cause greater stress in the casting, which may cause deformation or cracking for castings with complex structures. The cooling method after tempering and holding is generally not particularly strict. It is only for some low-alloy cast steels that are sensitive to temper brittleness that the cooling after tempering and holding is particularly important. It is advisable to use a fast cooling method so as to pass through the temper brittle zone as soon as possible to avoid reducing the toughness of the cast steel. Common cooling media for heat treatment of steel castings include air, oil, water, brine and molten salt.
When choosing a custom precision investment casting manufacturer, you need to consider its process capabilities, material range, quality control and service level. Ningbo Suijin Machinery Technology Co., Ltd. can provide custom precision investment casting services. We focus on providing customers with high-precision, complex-shaped metal parts manufacturing services. Investment casting (also known as lost wax casting) is a precision casting process that can produce parts with precise dimensions and high surface finish. It is widely used in aerospace, automotive, medical, energy and other fields.