Through repeated trial production and continuous research and development in recent years, ZHY casting has accumulated mature production experience, applied to mass production, greatly improved the qualification rate and won the praise of users at home and abroad. Key measures are as follows:
(1) Mold tooling design
C12a casting materials are cutting-edge products, especially valve castings, with complex structure, uneven wall thickness, thick connecting end, thin welding end and inner cavity; It has the characteristics of connection and transition between surface and plane, surface and surface; The internal and external surface roughness of the product is required to be high. Designing the mold and tooling is the basis for manufacturing good castings. According to the customer’s valve casting drawing, the casting engineer should consider the gradual transition of the thickness of the casting body, increase the valve casting fillet, divide the mold reasonably, and design the placement position of gating system, feeding block and cold iron, etc. The CAE casting simulation solidification computer analysis software is used to verify and eliminate the casting defects of the valve as far as possible in the design stage. After casting into valve castings, the casting process shall be evaluated, and mass production shall be carried out after passing the evaluation.
(2) Preparation of charge
The requirements for furnace charge are very few inclusions and P and s are very low. Materials with high purity and no impurities, oxides and rust must be selected. The main materials are industrial pure iron and return material. The return material shall pass through the steel crack melted in electric arc furnace, and the pure metal block shall be used for additional chemical elements. Other auxiliary materials shall also be selected in strict accordance with the specification requirements to ensure the chemical composition from the source of the furnace charge.
(3) Smelting method
The medium frequency induction furnace is used for roughing and AOD furnace refining. During roughing, the chemical composition of liquid steel is only analyzed for the first time. The composition is adjusted once according to the composition of c12a steel grade, and the liquid steel is poured into the mixing ladle for liquid steel transfer. Control the temperature of molten steel at 1590 ~ 1640 ℃, WC ＜ 0.5%, WP ≤ 0.025%, WS ≤ 0.01%, which is lower than the standard requirements. Before the molten steel enters the AOD furnace, open the cold argon gas control valve, and the cold argon pressure is ≥ 0.6MPa; According to the temperature and chemical composition of molten steel after entering the furnace, oxygen blowing and oxidation shall ensure that the maximum temperature of molten steel is controlled at (1740 ± 20) ℃. According to the heat release of C, Si and Mn in the furnace, the amount of ferrosilicon required to be added shall be calculated. Precious metals are not allowed to be added, and AOD batcher shall truly record. The inspector shall quickly conduct spectral analysis and print the analysis results to the proportioner of AOD furnace in front of the furnace. If there are elements in the chemical composition exceeding the standard, oxygen blowing oxidation shall be carried out again. At this time, the temperature of molten steel shall be strictly controlled and the composition shall meet the internal control standard (see Table 1).
| C | Mn | Si | S | P | Cr | Ni | N | Mo | Nb | V | Al |
| 0.09-0.11 | 0.40-0.60 | 0.3-0.5 | ≤0.01 | ≤0.020 | 8,50-9,50 | ≤0.30 | 0.037-0.70 | 0.90-1.00 | 0.070-0.10 | 0.20-0.25 | ≤0.10 |
Fill Fill argon into the reduction reaction, remove the scum and oxide, protect the ladle steel level with argon during tapping, and pour after standing. When the outer surface of the valve casting is ≤ 200 ℃, the sand shall be removed. After preheating the root of the pouring riser at 200 ~ 300 ℃, the oxyacetylene flame shall be used for gas cutting, and the residual height shall be ≥ 5mm.
(4) Heat treatment
After batch production verification, in order to ensAfter the verification of mass production, in order to ensure the metallographic structure and mechanical properties and prevent the cold crack of the product, the intelligent all fiber sealed heating electric furnace is adopted, the furnace temperature uniformity is good, the valve casting is heated evenly, the tempering is adopted at (1080 ± 14) ℃ normalizing + (1050 ± 14) ℃ normalizing + (760 ± 10) ℃, and the furnace is discharged and air cooled to room temperature. The interval between two normalizations and tempering shall be ≤ 2h, and the heating speed shall be ≤ 150 ℃ / h. The holding time shall be extended for 1H every 25.4mm of the maximum wall thickness of the valve casting, plus 1 ~ 2H. The tempering holding time shall be extended for 1H on the normalizing holding time, which shall be automatically controlled, and the technical parameters during heat treatment shall be recorded in the whole process. After heat treatment, the mechanical properties at room temperature meet the standard requirements (see Table 2), and the value fluctuates by ± 5%.
| Rm/MPa | Rp0.2/ MPa | AKV（-29℃）/ J | A（5d）（%） | HBW |
| 640 | 500 | 32 | 24 | 210 |
(5) Defect repair welding
When the c12a alloy steel valve casting needs repair welding if it has defects, the carbon equivalent of this material is 1.475%, exceeding the carbon equivalent limit value of cold crack by 0.4%. Under the action of welding thermal cycle, the grains near the weld and fusion line grow rapidly after high temperature heating, and it is very easy to form cold cracks under the action of welding residual stress. Before repair welding, remove the defects with carbon arc gouging, conduct P & T inspection, and repair welding after there are no defects. The electrical characteristics of welding are DC reverse connection method. The preheating temperature before welding is ≥ 205 ℃, and the interlayer temperature is ≤ 300 ℃. Immediately after welding, annealing at (730 ± 10) ℃ with holding time ≥ 4h, stress removal and dehydrogenation treatment.
The welding method adopts FCAW flux cored wire arc welding, and the specification of filler material: sfa-5.29; Level: AWS A5 29； Welding wire diameter: 1.2mm; Deposited metal: e91t1-b9m (Hobart brother); Welding material form: flux cored wire. Welding parameters: the welding current is 220 ~ 240A, the welding voltage is 25 ~ 30V, the extension length of welding wire is 15 ~ 20mm, the welding speed is 160 ~ 220mm / min, and the gas flow is 20 ~ 25L / min. Welding procedure qualification in accordance with ASME. It solves the problems of low efficiency, high labor intensity, many joints, difficult slag removal and high cost of electrode arc welding, and eliminates the defects such as cold crack, hydrogen embrittlement, air hole and slag inclusion in the repair welding area. Compared with electrode arc welding, it improves the working efficiency by 3 ~ 5 times, reduces the repair welding cost by more than 50%, and the hydrogen content of weld metal is less than 2.0 × 10-6. It has strong corrosion resistance and ensures that the physical and chemical properties meet and exceed the requirements of the base metal standard.
(6) Inspection and test
In batch production, 100% Pt, MT and RT tests are carried out one by one in strict accordance with ASME SA The physical and chemical properties of 217c12a alloy steel valve castings were evaluated according to the technical requirements, and the results were qualified.