1、 Raw material selection and quality control

The first step in the production of stainless steel castings is to ensure the high quality of raw materials, which is the fundamental guarantee of the entire production process.

Accurate control of alloy composition: The main components of stainless steel include iron, chromium (usually not less than 10.5%), nickel, molybdenum and other elements. It is necessary to strictly control the proportion of each element. Insufficient chromium content can lead to a decrease in corrosion resistance, while excessive carbon content can affect welding performance and corrosion resistance. 304 stainless steel requires a chromium content of 18-20% and a nickel content of 8-10.5%; 316 stainless steel also needs to contain 2-3% molybdenum.

Purity of raw materials: High purity raw materials must be used, and the content of harmful impurities such as sulfur and phosphorus must be strictly controlled. The sulfur content should be less than 0.03% and the phosphorus content should be less than 0.045%, otherwise it will reduce the toughness and corrosion resistance of the material. When raw materials enter the factory, spectral analysis should be conducted to ensure that the composition meets the standard.

Proportion of recycled materials used: Although using recycled stainless steel can reduce costs, the proportion must be controlled (usually not exceeding 30%), and different grades of recycled materials must be strictly classified and stored to avoid cross contamination. Recycled materials need to undergo melting testing before use to confirm their stable composition.

2、 Melting and pouring process control

Melting is the core process of stainless steel casting production, which directly affects the intrinsic quality of the final product.

Melting temperature control: The melting temperature of stainless steel is usually controlled between 1550-1650 ℃, depending on the steel grade. Excessive temperature can lead to element burnout (especially chromium oxidation loss), while insufficient temperature can result in poor fluidity. Infrared thermometers should be used for real-time monitoring, with a fluctuation range controlled within ± 15 ℃.

Deoxygenation and refining process: using a composite deoxidizer (a combination of silicon calcium, aluminum, etc.) to avoid the problem of inclusions caused by a single deoxidizer. AOD (argon oxygen decarburization) or VOD (vacuum oxygen decarburization) refining processes can effectively reduce carbon content and improve purity. The refining time should be ensured to be between 15-25 minutes to ensure sufficient reaction.

Pouring system design: The pouring system should ensure smooth filling of the metal liquid and avoid secondary oxidation caused by turbulence. The use of bottom pouring system can reduce oxidation and slag inclusion, and the sprue cup design should have slag blocking function. The pouring temperature is generally controlled at 50-80 ℃ above the liquidus line of the steel grade.

Protective measures: During the melting and pouring process, argon gas protection or flux coverage should be used to reduce the contact between the molten metal and air. The ladle should be preheated to above 800 ℃ to avoid sudden temperature drops. Large castings can be cast using vacuum or low-pressure casting techniques.

3、 Casting process design and mold making

Reasonable process design is the key to ensuring the dimensional accuracy and internal quality of castings.

Shrinkage rate calculation: The linear shrinkage rate of stainless steel is about 2.1-2.5%, which needs to be considered in advance in mold design. Different parts may require variable shrinkage design due to structural differences. Complex castings should be analyzed for shrinkage trends through simulation software.

Pouring riser system: Adopting the principle of "heat saving" to design the shrinkage system, the volume of the riser is generally 2-3 times that of the shrinkage area. Heating or insulation risers can extend the shrinkage time. For thick and large parts, cold iron can be set to control the solidification sequence.

Mold material selection: Stainless steel casting molds are usually made of chromium molybdenum alloy steel or high nickel cast iron, with a surface hardness of HRC40-50. The working surface of the mold needs to be polished to Ra0.8 or below to reduce surface defects on the casting. The preheating temperature of the mold is controlled at 150-300 ℃.

Design of parting surface: The parting surface should be selected as much as possible from the casting to the large section, reducing the draft angle (usually 1-3 °). The design of the live block should consider the convenience of demolding and avoid deformation of the casting. Ceramic core technology can be used for complex inner cavities.

4、 Heat treatment and post-treatment processes

Heat treatment is a necessary process for adjusting the microstructure and properties of stainless steel castings.

Solution treatment: Austenitic stainless steel (such as 304, 316) needs to undergo solution treatment at 1050-1150 ℃, with insulation time calculated based on wall thickness (usually 1.5-2.5 minutes/mm), followed by rapid water cooling. This can dissolve carbides and restore corrosion resistance. The uniformity of furnace temperature should be controlled within ± 10 ℃.

Stabilization treatment: For stabilized stainless steel containing titanium or niobium (such as 321, 347), it is necessary to maintain a temperature of 850-900 ℃ for 4-6 hours to allow carbon to fully combine with the stabilizing elements. Control the cooling rate at 30-50 ℃/hour to avoid generating new stress.

Stress relief: For martensitic stainless steel (such as 420), tempering at 650-750 ℃ for 2-4 hours is required. After heat treatment, shot blasting or vibration aging treatment should be carried out to further eliminate residual stress. Large castings require UT testing to confirm the absence of cracks.

Acid pickling and passivation: Use a mixture of 15-25% nitric acid and 2-5% hydrofluoric acid for acid pickling to remove the oxide scale. Then passivation treatment is carried out (usually using a 20-50% nitric acid solution) to form a dense oxide film on the surface. After treatment, it needs to be thoroughly rinsed with clean water, and the chloride ion content should be below 25ppm.

5、 Key points of quality inspection and control

A strict quality inspection system is the ultimate defense line to ensure the performance of stainless steel castings.

Chemical composition analysis: Each furnace of molten steel needs to be sampled for spectral analysis to ensure that the composition meets the standard. The deviation of key elements such as chromium and nickel should be controlled within ± 0.5%. Finished products should undergo ICP or OES testing and archiving.

Mechanical performance testing: Sampling and testing according to ASTM A351 and other standards, tensile strength, yield strength, and elongation must meet the requirements. Typical requirements for 316 stainless steel: tensile strength ≥ 485MPa, yield strength ≥ 170MPa, elongation ≥ 30%.

Non destructive testing: Important pressure bearing components require X-ray or ultrasonic testing in accordance with ASTM E446/E186 standards. Surface shall undergo penetration testing (PT) or magnetic particle testing (MT), and defects such as cracks and pores shall not exceed the allowable range of the standard.

Corrosion resistance verification: Conduct salt spray test (ASTM B117) or intergranular corrosion test (ASTM A262 Practice E), especially for welded parts that need to pass the corrosion test after sensitization treatment. Medical grade castings also require biocompatibility testing such as cytotoxicity.

Dimensional accuracy inspection: Use a coordinate measuring instrument to detect key dimensions. The general dimensional tolerance should reach CT7-8 level, and special requirements should reach CT5-6 level. The wall thickness deviation is controlled within ± 0.5mm.

6、 Common Problems and Solutions

Pore problem: mainly caused by incomplete degassing during melting or poor mold exhaust. The solutions include extending refining time, using dry molding materials, and increasing mold exhaust channels. Perform vacuum degassing treatment on the molten steel before pouring.

Hot cracks: often occur at the junction of thickness and thickness, due to hindered shrinkage. Optimization plan: Adjust the pouring system to solidify in sequence, add shrinkage feeders, and control the temperature gradient of the mold. For castings that have already developed cracks, they can be repaired by argon arc welding and then re heat treated.

Surface roughness: often caused by high mold temperature or improper coating. The countermeasures include optimizing the spraying process (coating thickness 0.1-0.3mm), controlling the mold temperature within a reasonable range, and using nanoscale casting coatings. For castings that have already encountered problems, sandblasting treatment can be carried out.

Composition segregation: Especially in high alloy steel, dendritic segregation is prone to occur. Preventive measures: Control the pouring temperature not to be too high, use electromagnetic stirring technology, and optimize the cooling speed. For castings that have already experienced segregation, it can be improved by homogenization annealing (holding at 1150-1200 ℃ for 10-15 hours).

The production of stainless steel castings is a systematic project that requires full process control from raw materials to finished products. With the advancement of technology, the application of new technologies such as computer simulation, 3D printing molds, and robot polishing is continuously improving the quality and production efficiency of stainless steel castings. Enterprises should establish a sound quality traceability system and continuously optimize process parameters in order to maintain a competitive advantage in the international market.