In recent years, although nodular iron castings, vermicular iron castings and various light alloy castings have been developed rapidly, but due to the unique advantages and relatively cheap price of gray iron, its consumption is still large, and its application is still very wide. Therefore, further strengthening the research on gray iron and giving full play to its potential functions are still important issues for the foundry industry to continue to study.
1. Cylinder block and cylinder head casting
For a long time, the cylinder block and cylinder head are made of gray cast iron. Due to the demand of energy saving and lightweight, some enterprises use Al alloy to manufacture the cylinder block and cylinder head, which can greatly reduce the casting quality and fuel consumption of the engine. But in recent years, due to the improvement of engine power and the rise of working temperature of the cylinder block and cylinder head, Al alloy is difficult to meet the requirements, Some enterprises also use vermicular graphite cast iron to produce cylinder block and cylinder head castings. At present, many factories at home and abroad have used vermicular graphite cast iron to produce cylinder block and cylinder head with good results. However, due to the unique excellent properties of gray cast iron and the continuous development of high CE and high-strength gray cast iron materials, the gray cast iron cylinder block and cylinder head castings are still dominant. In the 1990s, our company cooperated with the Research Institute of internal combustion engine affiliated to a European company to design 6DL type high-power vehicle diesel engine. The other side disagreed with our proposal of using vermicular cast iron, and claimed that the cylinder head should adopt high-grade gray iron, because the hardness, dimensional stability and thermal conductivity of vermicular cast iron are not as good as that of gray iron.
ZHY casting studied the production method of HT300 cylinder block and cylinder head castings under the condition of high CE
(1) In this process, the carburizer must be graphitized at high temperature, which has the following functions: the amount of W (s) is generally less than 0.05%, or even less than 0.03%; in addition, the amount of W (n) in the carburizer is greatly reduced after high temperature graphitization, because only in the high temperature graphitization stage, s, N, h can be volatilized, which is the key to prevent N2 porosity in castings.
(2) The amount of W (c) is controlled at 3.25% – 3.35%, and the amount of W (SI) is controlled at 2.10% – 2.40%. This high CE is very beneficial to reduce the shrinkage of molten iron.
(3) When w (s) is less than 0.05%, we must increase s.
(4) The amount of W (MN) should be strictly controlled. It is found that with the increase of W (MN) content, the properties of the material will decrease. But it is not to say that the lower the content of W (MN), the better. The lower the content of W (MN), the higher the oxidation tendency, the worse the fluidity and the higher the shrinkage tendency.
(5) The range of alloying is: w (CR) 0.25% ~ 0.30%, w (Cu) 0.5% ~ 0.6%, w (MO) 0.2%, w (SN) 0.02%.
(6) To choose a good inoculant, it is because of the common SIFE inoculant, some supercooled D-type graphite often appears in the graphite morphology. In the production of high grade gray cast iron, flow inoculation is a very common process. The inoculant used with flow must have uniform particle size, no powder, uniform melting point and rapid melting.
(7) The casting can be cooled slowly in the sand box by delaying the opening time, so as to reduce the casting stress and improve the casting quality.
Through the above measures, ZHY casting has steadily produced the cylinder block and cylinder head castings of high-power diesel engines for trucks. The material grade reaches HT300, with high performance and small shrinkage tendency.
2. Machine tool casting
At present, China’s high-grade CNC machine tools needed in the important fields of automobile, aviation, railway, national defense and so on have been imported for a long time. The cast iron bed is the key basic component of the machine tool, and its dimensional accuracy and accuracy retention are the key factors affecting the machining accuracy and accuracy retention of the machine tool. The machine tool and precision mechanical casting working under low stress state usually fail not because of fracture damage but because of poor dimensional stability of the casting [14-16].
ZHY casting is a horizontal machining center bed casting exported to Japan. The material grade is HT300, the casting structure is complex, the mass is 11940 kg, the size is 4060 mm × 2970 mm × 1440 mm, and the main wall thickness is 20-110 mm.
The chemical composition of molten iron melted in 10 t cupola is: w (c) 3.1% ~ 3.2%, w (SI) 1.6% ~ 1.8%, w (MN) 0.8% ~ 1.0%, w (P) ≤ 0.1%, w (s) 0.06% ~ 0.1%. Two 10 t pours were used for casting. The temperature of pouring was 1 490 ~ 1 510 ℃, the pouring temperature was 1 370 ~ 1 400 ℃, and the pouring time was 70 ~ 100 s. The inoculant is 75sife, the amount of inoculant is 0.5% ~ 0.6%, and the time of packing is more than 120 h.
The results show that the metallographic structure of the guide rail surface of the lathe bed is A-type graphite, the volume fraction of pearlite is more than 98%, the tensile strength is 310-340 MPa, and the hardness is 180-200 Hb.