Throughout the development trend of engine technology at home and abroad, we are pursuing to reduce the wall thickness of the casting, so as to reduce the weight of the casting and the whole machine, and to reduce the fuel consumption. At present, the weight of the cylinder head of the engine per unit power reaches about 1.8gk/kw, and the corresponding main wall thickness of the casting is only about 3.5mm, which puts forward high requirements for the material properties of the casting
A. the tensile strength of dry type single cast test bar QB ≥ 250Mpa, and the tensile strength of specified body QB ≥ 250Mpa;
b. The hardness of the designated part of the casting is above 180hb, and the hardness difference of the thick and thin section of the casting is below 30hb;
C. the pearlite content of the main part of the casting body is more than 90%, the graphite type should be mostly A-type, the filling surface has a small amount of B-type and D-type, and the maximum length of graphite hydraulic pressure is less than 250um.
Although most professional engine casting manufacturers in our country have achieved modern production conditions through technical transformation and technology introduction, they often fail to meet the above material requirements.
1 Reasons
1.1 the melting temperature of molten iron is low and the undercooling is small, which makes the subsequent inoculation strengthening effect poor
1.2 the quality of charge (metal charge and non-metal charge) is poor, and the content of trace elements and non-metallic inclusions is high
1.3 improper alloying measures or (or improper selection of alloy elements, or improper addition of alloy, or improper alloying methods)
1.4 improper inoculation measures (composition, form, quantity, method, etc.)
1.5 improper disposal in the holding furnace (such as frequent and large-scale adjustment of chemical composition, so that the holding time of molten iron in the furnace is too long, and the element changes greatly), and the accuracy of composition control is poor
2 Countermeasures
2.1 improve the melting temperature, improve the stability of molten iron, increase its supercooling tendency, eliminate the “heredity” of raw materials, and ensure that the tapping temperature is greater than 1480 ° C, so as to ensure that the initial pouring temperature reaches 1450 ° C, and the final pouring temperature reaches 1400 ° C
2.2 strengthen cupola control to make the furnace condition stable, so as to ensure that the composition of molten iron entering the insulation furnace is stable (reduce the fluctuation of composition burning loss), which can reduce the time required for composition adjustment in the furnace, so as to avoid increasing the shrinkage tendency and white mouth tendency of molten iron
2.3 when the operation of increasing C is not necessary in the electric furnace, the carburizer with high absorption rate must be selected, and the time for full electromagnetic stirring and full absorption must be ensured, otherwise the iron sample taken cannot reflect the real C content of the whole melt, resulting in deviation of the actual carbon equivalent
2.4 reduce the fluctuation of carbon equivalent and improve the accuracy of composition control. It is required that △ CE ≤ 0.05% and △ Si ≤ 0.1%.
2.5 for the cylinder block with complex shape, thin wall and high strength, the molten iron of cylinder head castings should have both high strength and good casting performance. For this reason, its composition is usually designed as high strength equivalent (3.9-4.1%). To make it have good casting performance, and to achieve high mechanical performance, low alloying measures should be taken
A according to the resource situation of our country and the experiences and habits of most enterprises, Cr, Cu and other alloy elements are often used. It is beneficial to increase and refine pearlite, stabilize pearlite, improve graphite state, and obtain higher mechanical properties
The addition amount of B alloy must be controlled. Cr is an element that promotes the formation and stabilization of pearlite, and can refine pearlite, so it can significantly improve the strength of gray cast iron. However, Cr has a strong affinity with C, which is a strong carbide element, which will increase the tendency of white cast iron. At the same time, CR will also reduce the eutectic solidification temperature of cast iron, and expand the solidification temperature range of the iron, Therefore, the shrinkage porosity and tendency of gray cast iron are increased, and the compactness of castings is reduced, which may affect the strengthening effect of Cr on gray cast iron. When Cr is in the range of 0.2-0.3%, it can avoid harm and interest
Meanwhile, Cu is also an element to promote the stabilization and refinement of pearlite, and Cu is an element to promote graphitization, which can counteract the adverse effect of Cr increasing the tendency of white mouth
Therefore, the combination of Cr and Cu is recommended to achieve better results, which not only ensures good casting properties, but also improves the mechanical properties of castings
It should be pointed out that due to the effect of Cr and Cu elements, the pearlite can be increased and the lamellar structure with small spacing between pearlite sheets can be stabilized and refined, and the graphite state can be improved (A-shape), which is distributed in the size. Therefore, the resistance of cylinder block and cylinder head to thermal fatigue cracks under the action of thermal alternating stress is also proposed to be high (i.e. good thermal stability) [3]
2.6 proper inoculation treatment can improve the material strength of cylinder block and cylinder head casting, especially the uniformity of high hardness and microstructure, improve the sensitivity of thick and thin section, make the hardness difference within 30hb, and have good machinability. The appropriate inoculation treatment here includes:
A select the suitable inoculant. Among many inoculants, the inoculant containing ba.ca.sr (strontium) and other elements not only has a good anti inoculation effect, but also has a strong graphitization effect. It can significantly improve the section sensitivity of the casting, avoid the white mouth inclination of the casting at the minimum wall thickness, and the microstructure is more uniform.
B. suitable inoculation method. Among the methods of in bag inoculation, wire feeding inoculation, in mold inoculation and in stream inoculation, stream inoculation is the most convenient method, which is most suitable for mass flow production and has the best effect. The recommended particle size is 0.5-1.0mm, and the addition amount is 0.1-0.2%
C. It should be pointed out that the basi inoculant can make the hardness of the casting low, and can add trace Sn (0.04-0.06%) or sb (0.02%), which can be called the deficiency of low hardness
2.7 strictly control the charge, and the standard is (1) low trace elements; (2) clean; (3) it is strictly prohibited to mix alloy elements
