Smelting technology of high-strength HT250 gray cast iron

Since its inception, gray cast iron has been widely used in the machinery industry due to its comprehensive performance superior to other materials. In particular, alloyed HT250 is mostly used in engine blocks, cylinder heads, machine tools there are special requirements. But how to smelt high-quality molten iron is a problem that has plagued foundry workers for many years. The LJ465Q crankcase produced by our factory is the main component of the mini-car engine. The material is HT250. It has special requirements for physical chemical properties. Due to raw materials smelting factors, the quality of molten iron used to be good bad, which seriously affected the production. After years of tackling key problems, various problems are now basically solved. The author of this article summarizes the experience gained in quality research in recent years, proposes corresponding solutions for the characteristics of the product in smelting.

1 Determination of chemical composition charge ratio

1.1 Determination of chemical composition

The chemical composition of gray cast iron must be reasonably selected according to the wall thickness technical requirements of the casting.

(1) The choice of carbon equivalent.

The traditional method believes that increasing the strength of gray cast iron requires reducing the carbon equivalent, increasing the amount of manganese to increase the proportion of pearlite in gray cast iron, thereby increasing the strength. However, the method of reducing the carbon equivalent to increase the strength of gray cast iron will bring many unfavorable factors, such as poor process performance, increased white mouth tendency, difficulty in processing, so it has been widely adopted. Currently, the commonly used method is to use the highest possible carbon equivalent under the premise of achieving higher strength. The increase of carbon equivalent can greatly improve the casting performance of gray cast iron, reduce shrinkage cavities porosity, reduce the casting waste rate. The LJ465Q crankcase produced by our factory is a typical thin-walled part, the thinnest part of the wall thickness is 315-4mm, the carbon equivalent is controlled at 410%-411%. At the same time, in view of the core role of carbon in the chemical composition, referring to the previously produced 276Q 376Q crankcases, the carbon content is 313% to 314%, the silicon content is 210% to 211%. The composition is analyzed by a fast spectrometer in front of the furnace. control. So far, 130,000 castings have been produced. The tensile strength of the castings can reach more than 250MPa, the hardness can reach 190～240HBS, which fully meets the technical requirements of the product.

(2) Control of alloying elements

The role of alloying elements in gray cast iron is mainly to improve the strength of the casting, which is mainly manifested as: ①refining graphite eutectic clusters; ②increasing the content of pearlite in the matrix making the spacing of pearlite thin; ③improving the permeability The thermal stability of the carbon body; ④The formation of hardened phases such as carbides phosphorus eutectic containing alloying elements. However, the addition of alloying elements must consider its impact on performance on the one hand, economy on the other. Although some precious metals (such as nickel) can greatly help the improvement of the mechanical properties of castings, they are more expensive increase the casting cost, so they are used. The alloys added by our factory are copper, ferromanganese, ferrochromium. The added amounts are respectively 0170%～0175%, 0175%～0180%, 0125%～0130%.

1.2 Determination of charge ratio

In order to ensure higher mechanical properties, a large amount of scrap steel is used in the charge, the amount of pig iron used is correspondingly reduced to produce qualified cast iron. This has two meanings in actual production. ①The performance of castings is effectively improved; ②The cost of casting is well controlled due to the reduction in the amount of pig iron used. Since the smelting equipment used in this plant is a power frequency electric furnace, the addition of a large amount of scrap steel will inevitably require that the carbon content of the molten iron be increased to a reasonable range through an appropriate method. Therefore, the carbon content of molten iron is adjusted by adding rapid recarburizer in the furnace. At the same time, it should be noted that excessive addition of recarburizer will reduce the quality of molten iron, for which our factory has paid a painful price. The charge ratio directly determines the amount of recarburizer added, so the charge ratio used in the LJ465Q crankcase produced by our factory is pig iron: scrap steel: recharge = 2:5:3. At the same time, a rapid recarburizer is added during the smelting process to increase the carbon content in the molten iron.

2 choice of charge

The correct choice of charge is a necessary condition for obtaining high-quality molten iron. It only affects the chemical composition of the casting, but also has a greater impact on the structure. Due to the different trace elements contained in the charge, although the same chemical composition the same liquid iron treatment method, the results may be quite different, which is called the heredity of pig iron by foundry scholars. Our factory has strict requirements on pig iron. Through continuous trials explorations, we finally designated two manufacturers nationwide to purchase at designated locations, strengthened cooperation with suppliers to ensure the quality of pig iron. Due to the increase in the proportion of scrap steel used in the charge, it is required that the composition of the scrap is small, the content of impurities is low, the surface rust is small, the thickness of the scrap should be greater than 3mm. At the same time, the reheating materials should be stacked separately the surface is clean free of impurities. For LJ465Q crankcase, it should use its main body as much as possible, followed by castings of the same type (such as LJ462Q crankcase, 376Q, 276Q engine block), again ordinary furnace materials.

3 smelting operation

The inherent quality, appearance quality defects of castings are all related to molten iron. High-quality molten iron is the most basic most important prerequisite for obtaining high-quality castings. The molten iron must meet the following requirements: ①It has a high tapping temperature fluidity to ensure that the cast castings have a clear outline, no cold barriers, insufficient pouring, other defects. At the same time, the castings can be made uniform, thereby improving the castings Mechanical properties; ②The chemical composition should meet the predetermined requirements; ③Various harmful elements must be reduced to a certain level, to keep the molten iron pure, to reduce inclusions such as slag gas. a practical point of view, an appropriate increase in the tapping temperature is beneficial to the mechanical properties quality of the castings, but it is detrimental to the burning of the melting equipment alloy elements. Since the minimum wall thickness of the LJ465Q crankcase is only 6mm the shape is complex, the tapping temperature is 1520～1550℃, the pouring temperature after inoculation is 1440～1460℃, the temperature of the molten iron is controlled by a rapid thermometer.

4Strengthen inoculation treatment

4.1 Selection of inoculants

With high-quality molten iron coupled with effective inoculation, it is an effective method to obtain high-performance castings. Effective inoculation can only refine the matrix structure change the graphite shape, but also reduce the tendency of white mouth improve the processing performance. In recent years, my country has changed the situation of using ferrosilicon alone as an inoculant in the past, developed a variety of compound inoculants. The inoculant used by our factory is Foseke compound inoculant, which is combined with 75 ferrosilicon for inoculation. The effect is quite good. For many years, the controllable index of LJ465Q crankcase rarely fails to meet the requirements.

4.2 How to add inoculants

The inoculant is generally added at the tap hole, slowly enters the bag with the molten iron, is evenly washed stirred by the molten iron. In actual operation, when the molten iron in the ladle reaches about 20%, start adding the inoculant, when the molten iron in the ladle reaches 80%, all the addition is completed. Therefore, it is ensured that the inoculant can be fully evenly dissolved in the molten iron in the ladle, the composition of the molten iron in the whole ladle is uniform, so as to ensure the consistent structure of the casting.

5 Conclusion

(1) Determining a reasonable chemical composition charge ratio is a prerequisite for obtaining high-quality molten iron producing high-performance gray iron parts;

(2) Strengthen the management control of the charge to reduce the influence of trace elements on the quality of castings;

(3) By increasing the melting temperature, thereby reducing the adverse effects of non-metallic inclusions, it is an effective way to obtain high-quality castings;

(4) Strengthening the inoculation process can refine the matrix structure, change the graphite shape, reduce the tendency of white mouth, improve the processing performance.