One of the members of the China Foundry Industry Media Alliance, Thermal Processing Industry Forum ( ) - the home of online technical exchanges for personnel in the thermal processing industry. It is an industry technology website that mainly focuses on smelting, casting, forging, welding, heat treatment, physical and chemical testing, and other related knowledge, supplemented by knowledge of mechanical processing and computer design. Click the blue word "Heat Treatment Industry Forum" below the title to follow for free, and we will provide you with extended reading on casting, forging, heat treatment, and welding knowledge.

[Process example] Preventive measures for slag inclusion and coarse graphite defects in complex box parts of lost foam

The Z50E box is the main part of the 50E loader gearbox. There are oil holes, oil passages, and oil cylinders concentrated in it. The quality of its castings largely determines the performance of the loader. Since the production of cabinets with lost foam, the external quality has been greatly improved, but for some time, due to internal quality reasons, the scrap rate has reached about 30%, and the main defects are carbon slag and graphite coarse defects, which seriously affect the production cost and production efficiency. For this reason, the reasons for carbon deposits, slag inclusions, and pores in the 50E box were analyzed, and measures were found to improve the internal quality of castings and even the quality of the whole machine.

1. Casting and defect characteristics

The Z50E box structure adopts a three-cavity stepped box structure, with an overall size of 1035×570×607, a main wall thickness of 12, and a hot section wall thickness of 40-50. All important parts are scattered. Main casting quality requirements: each pump, valve surface, and reversing piston working surface must not have any defects.

Carbon slag defects are mainly concentrated in the oil cylinder, and will be exposed after processing 6-7mm, in the form of black spots, blocks, or flakes, we call it carbon slag.

Coarse graphite defects are mainly concentrated in the oil cylinder (working surface of the reverse gear piston) and the pump valve surface. The main manifestations are gray-black pinhole-like or flying-foot-like fragments during machining and thicker graphite flakes on the metallographic examination section.

2. Process analysis and process measures

Compared with other casting methods, lost foam casting has its characteristics. During the filling process of molten metal, a series of physical and chemical reactions different from other casting methods occur at the flow front interface. Heat transfer between liquid metal and coating, dry sand, and foam mold, the viscosity of the molten metal front increases, the fluidity decreases, and a certain amount of solid product foam pattern in the form of carbon black is produced during the decomposition and gasification process, and the casting solidifies and slow cooling.

According to the above characteristics, under the guidance of reducing the source of carbon slag and strengthening the discharge and collection of carbon slag, adjusting the temperature field distribution in the cavity, and using the equilibrium solidification theory to conduct an all-around analysis on-site. Process adjustment measures are as follows:

1. Pattern production: 50E box structure is relatively complex, local heat welds are scattered, and local heat welds are hollowed out at thick foam; dense structure and thin-walled easy-to-deform adopt different EPS densities, that is, dense structure mold adopts 0.020g/cm3, thin-walled easy-to-deform The deformation mold adopts 0.023g/cm3, which not only ensures the strength of the pattern but also prevents deformation due to rigidity, which is beneficial to reduce carbon residue defects.

2. Riser system: The gating system must ensure that the pattern is decomposed in an orderly manner, and the injection of molten iron is orderly and stable. The riser should not only collect the contaminated cold molten iron at the front end of the filling but also be able to implement shrinkage.

Due to the large and complex parts, there are many variable cross-section positions, stepped side injection, unilateral water inlet, open gating system, F straight: ΣF inner = 1: 1.4-1.5, scattered gates, middle gates, and lower gates. The area accounts for 70%-80% of ΣF. Considering the thick wall and thin wall and the filling speed of molten iron, two ingates are added at the lower end of the oil cylinder where carbon slag is likely to be generated, and two slag accumulation verticals with variable cross-sections at the top are installed. The front part of the pipe collection and filling model is polluted by cold iron, which reduces the cross-sectional area of the middle and upper valve faces and the upper pump face; distributes the cross-sectional areas of the middle and lower ends according to 20% and 80% of the internal area of ΣF, and balances the temperature of the entire casting field; conducive to the orderly filling of molten iron from bottom to top, and the formation of foam from bottom to top)

3. Use cold iron and chilled paint to adjust the temperature field of the hot section to prevent coarse graphite defects. To prevent carbon slag defects in important parts such as pumps, valve faces, and oil cylinders, an ingate is set in this part, but at the same time, this part is a thick-walled hot spot. Since the lost foam casting itself solidifies and cools slowly, it will undoubtedly increase the cost of this part. Hot spots in the part increase the tendency of the part to loosen.

To adjust the solidification and cooling speed of this part, a cooler is installed on the valve face of the pump near the gate. The chiller is required to be cast with vent holes to facilitate the extraction of foam gasification products. The cold water method is adopted at the oil tank where it is not convenient to install the chiller. coating.

4. Material control: Although the product structure is uneven in thickness, the thick wall position is mainly concentrated on the pump valve surface, and the oil cylinder is mainly subjected to a high-pressure position, coupled with slow solidification and negative pressure dry sand cooling, the target value of the chemical composition is as follows: C3. 1~3.3%Si1.6~1.8%Mn0.8~1.0% p≤0.12S≤0.1, C content in the molten iron is controlled at the lower limit, which is beneficial to prevent carbon slag defects and pores, and Mn is controlled at the upper limit, which is conducive to improving casting density.

The smelting process adopts cupola-intermediate frequency furnace double smelting, strictly controls the chemical composition, strengthens the inoculation before the furnace, improves its crystal structure, and weakens the sensitivity of the material to the wall thickness.

Increase the temperature of molten iron to ensure that the pouring temperature is controlled at 1450-1480°C, which is beneficial to reduce the generation of carbon slag defects.

5. Adjustment of negative pressure degree: Moderately increasing the negative pressure degree is beneficial to improve the filling speed of molten iron and the discharge of pattern decomposition products. Through the field test, the negative pressure is controlled at -, and the overall effect is good.

Through the improvement of the above process measures, the machining verification of mass-produced castings was carried out, and the loose defects of carbon slag were rarely found, and the internal quality of the castings was greatly improved, ensuring the working performance of the whole machine.

3. Conclusion

The filling molding, solidification, and cooling characteristics of lost foam are the basis for guiding the production process of lost foam products. Adopting a reasonable pouring system and controlling the EPS foam modulus and other technological measures are important technological measures for the production of products with strict intrinsic quality requirements.