Casting technology of nodular cast iron valve

The quality requirements for nodular cast iron valve castings are very strict, the body spheroidization rate is above grade 3, and the surface roughness is required to reach 140s according to jisb0601 standard; The dimensional tolerance shall be accepted according to JISB0407 standard.

  1. Modeling

The ductile iron castings of valve body and valve plate are molded with resin sand. The ductile iron casting of the valve body is divided in the middle, poured vertically and cast without riser. The 75mm hole adopts steel rod core to increase the cooling speed and prevent shrinkage porosity and shrinkage casting defects in the hole. At the same time, the overflow riser technology is adopted above the 75mm hole to reduce the slag collection casting defects on the upper surface of the 130mm hole.

The valve plate ductile iron casting is divided into boxes with the plane (upstream surface) facing downward, and the inner cavity core is combined and hung on the upper box. The bottom plane is poured, and a control pressure riser is set on the thick wall of holes on both sides.

  1. Calculation of gating system

Total weight of molten iron consumption G: g = gross weight of ductile iron castings + weight of gating and riser = 300 + 300 × 18% = 354kg, g = 360kg.

Valve ductile iron pouring time:

Take 16S after correction.

Average pressure head:

The minimum total cross-sectional area of valve ductile iron gating system is as follows:

Unit ratio of valve ductile iron gating system (corrected):

  1. Chemical composition

Reasonable chemical composition has an important influence on the mechanical properties and casting properties of nodular cast iron. Through the test, high carbon and low silicon, manganese, phosphorus and sulfur are adopted. The target chemical components are: WC = 3.5% ~ 3.7%, WSI = 2.4% ~ 2.8%, WMN < 0.4%, WP < 0.06%, WS < 0.02%, wre < 0.05%, WMG = 0.03% ~ 0.06%.

  1. Molten iron smelting process

(1) The smelting equipment adopts 1.5T power frequency induction furnace and 1t special dam ductile iron ladle.

(2) Metallic pig iron: special pig iron for Benxi nodular cast iron, its composition is WC = 4.3%, WSI = 1.2%, WMN = 0.5%, WP = 0.04%, WS = 0.032%; Scrap: ordinary low carbon steel; Carburizing agent: graphite electrode; Desulfurizer: Na2CO3 powder; Spheroidizing agent: T-1 spheroidizing agent, its composition is WMG = 8.2%, wre = 5%, WSI = 40.1%; Inoculant: 75sife, Al powder < 1.5%.

(3) For liquid iron refining, 80% pig iron and 20% scrap steel shall be added into the furnace to melt successively, and the temperature in the furnace shall be raised to 1500 ~ 1510 ℃ for short-time refining, so as to achieve the purpose of removing gas and impurities.

(4) The desulfurization treatment is carried out by industrial soda ash Na2CO3 in bag flushing method, which is carried out in two times, one flushing is 0.5%, and it is carried out when scalding, with the temperature of 1440 ~ 1460 ℃; The secondary injection of 0.5% was carried out simultaneously with spheroidizing inoculation.

(5) The addition amount of spheroidizing agent for spheroidizing and inoculation treatment is 1.6% ~ 1.8%; Inoculant 1.0% ~ 1.2%, followed by 0.3% in the bag, 0.5% with flow, 0.2% with floating silicon and 0.1% with instantaneous inoculation.

Firstly, 2 / 3 of the molten iron shall be flushed for spheroidization and inoculation. After the spheroidization reaction for 4 ~ 5min, 1 / 3 of the molten iron shall be flushed, and the flow shall be inoculated, slag raking, triangular test piece inspection, and then floating silicon shall be added for inoculation to prepare for pouring. Instantaneous inoculation adopts the method of gate box instantaneous inoculation, which is an effective measure to prevent graphite decay. After instantaneous inoculation, the spheroidization rate of valve nodular cast iron body can be increased by one level compared with that without instantaneous inoculation, and the ball diameter is small and uniform, and the toughness of valve nodular cast iron is also significantly improved. The treatment temperature is 1420 ~ 14500c and the pouring temperature is 1350 ~ 1370 ℃.

(6) Trace element alloying in actual production, to eliminate the spheroidization recession of valve ductile iron body (especially thick and large ductile iron), alloying measures are also an effective way. Therefore, we add a certain amount of bismuth to the molten iron, and the effect is obvious.