The traditional method of high strength gray iron is to reduce the eutectic degree, to increase the austenite content and the branching of austenite dendrite, and to reduce the amount of graphite. However, the increase in strength is at the expense of casting properties. The low eutectic degree reduces the fluidity and increases the shrinkage. The result is low yield, many casting defects and low qualified rate of castings. For cast iron with low eutectic degree, there are also greater wall thickness sensitivity and section sensitivity. At present, the process of increasing eutectic degree + multiple low alloying + molten iron purification + good inoculation is advocated at home and abroad. Here are some methods about how to obtain A-type graphite.
C-type graphite can be avoided by using hypoeutectic gray iron. However, when CE is low, and w (c) content is low, the cooling rate is high, and E-type graphite will be produced. If the amount of W (c) is not too low, D-type graphite will be formed. A small amount of D-type graphite is allowed in HT250, such as 5% – 15% of the total graphite (subject to the order contract). D-type graphite is surrounded by ferrite. At this time, D-type graphite has little effect on tensile strength. At higher CE, as long as the mold is rigid enough and expanded by graphitization, there will be no shrinkage porosity in the casting. HT300 can be used for machine tool castings with high strength, high modulus of elasticity and low stress. Its CE is 3.82% (eutectic degree is about 0.89). HT250 can be used for the thin and complex cylinder block, with CE of 3.9% ~ 4.1%. Ce of HT250 brake disc is 3.9% ~ 4.2%.
2. Low alloying and Si / C ratio
The tensile strength, hardness and modulus of elasticity decrease with the increase of CE alone, which must be compensated by low alloying and proper increase of Si / C ratio. The former aims to promote the growth of austenite dendrites, refine pearlite and graphite, while the latter can increase the number of austenite dendrites, reduce graphite and refine graphite by reducing C and increasing Si, and Si also strengthens the matrix. The commonly used low alloying elements are Cu, Cr, Mn, Mo and sb. A certain amount of n is also beneficial. Generally, the content is 0.2% – 0.4% of W (CR), 1.5-2 times of W (CR), 0.6% – 1.1% of W (MN), 0.1% – 0.35% of W (MO), 0.015% – 0.025% of W (sb), 60-120 ppm of W (n). Do not overdo sb and n. Ni and Sn are precious metals and are generally not used. In order to ensure the alloying effect of Mo, the amount of W (P) must be reduced, otherwise Mo will be broken due to the formation of mo-p-c-fe eutectic. In order to prevent the leakage of thin-walled parts, the amount of W (P) should also be limited.
Si / C ratio: 0.62-0.65 for machine parts, 0.60-0.62 for engine block and 0.59-0.68 for brake disc.
3. Incubation treatment
Inoculation is very important to increase graphite core and control graphite distribution. Cr, Mn and Mo in alloying elements are both carbide forming elements and positive segregation elements. If the inoculation is not good, D-type graphite, free cementite, white edge or white mouth will be easily produced. In order to obtain medium-sized A-type graphite and improve the homogeneity of cross section, inoculation treatment is necessary.
Most inoculants are Fe Si ba. Fe Si SR can be selected for the engine block to prevent leakage. In order to prevent porosity, Fe Si re, Fe Si Zr or Ti containing inoculant were used. The inoculant containing RE can neutralize the interfering elements in the charge and has a strong ability of deoxidizing. The inoculated grade Fe Si 75 containing about 1% Al and 1% CA respectively is widely used because of its low price, but it is easy to be inoculated and degenerated. The Si series inoculants containing Ba, Sr, Zr, re and Ca belong to long-term inoculants. When large package treatment or large casting or air pressure casting is used, long-term inoculants are often used.
For induction furnace smelting, graphite type carburizer and SiC have been used in recent 10 years in China. Pretreatment in the later stage of smelting and inoculation outside the furnace can significantly increase the graphitization core and ensure the A-type graphite. It is also effective to pretreat thin-walled gray iron castings with FeSi containing Al Zr ca.
The inoculation effect of low-S liquid iron is poor, and the graphite core formed is easy to dissolve. Therefore, it is necessary to raise the w (s) content of the molten iron to 0.07% – 0.11% before inoculation. The O-S inoculant containing O and s can produce more oxygen and sulfur compounds in the molten iron, increase graphite core, overcome supercooled graphite, and use the graphitization expansion at the later stage of solidification to remove the shrinkage porosity at the hot spot.
The factory is used to multiple inoculation. The author suggests that the emphasis should be put on delayed inoculation, which is more economical and effective than tapping inoculation. For small and medium-sized assembly line production, it is recommended to adopt flow inoculation or spray inoculation; for large resin sand production, it is recommended to adopt nozzle inoculation, sprue cup inoculation or wire feeding inoculation. Small and medium-sized castings are often used to place inoculation blocks at the bottom of the sprue. In a small foundry, the method of inoculating at the bottom of the ladle, when the ladle is raised or the end ladle is used to receive the molten iron, is also better than the method of inoculating at the bottom of the ladle.
The inoculant should be baked at 250 ~ 350 ℃ before use. In addition, the inoculant should be moisture-proof and the stock of inoculant should not be too large.
In recent years, some attempts have been made to modify cast iron with nano technology. Dalian Jiaotong University  used nano-sized SiC to do the impact test, and found that the graphite is refined and evenly distributed. It is a new trend that nanotechnology is grafted on cast iron.
When there are problems in production, people are used to find out the reasons from the aspects of chemical composition design and process conditions, but ignore the “invisible hand” in the charge. The reason for the quality fluctuation after the change of charge is the genetic factors such as charge structure, gas content and trace elements.
In order to save the amount of FeSi and reduce the burden cost, some factories use Z18 pig iron, but the pig iron of the supplier is biased to have C-type graphite, as a result, C-type graphite also appears in the casting. It is suggested that z14 and Q12 pig iron should be selected for the production of high strength gray cast iron.
Pig iron and return material have porosity or broken white slag, and castings are prone to such defects, so they are not suitable for use.
The results show that w (as) content is more than 0.02% and w (PB) content is more than 0.002%. In order to maintain good cutting performance, the amount of W (TI) should be less than 0.04%. In order to strengthen the matrix of N and promote the microalloying of graphite end passivation, it is forbidden to fix n with Fe Ti in front of furnace. Therefore, in the production of high-strength gray cast iron, it is necessary to master the content of as, Pb, Ti and other trace elements in the furnace burden, and carry out burden accounting.
In order to desalinate the influence of bad heredity in furnace burden, the purification work of degassing and impurity removal should be done well in induction furnace smelting; the superheat temperature of molten iron should be controlled to 1500-1550 ℃, but not too high. Some factories take measures to cut off the power at 1520 ℃, and use the inertia of 20 ~ 30 ℃ to raise the temperature to reach the set target temperature. This method can be used for reference.
In the induction furnace smelting workshop where chips are often used, it is necessary to do a good job in chip supervision. The chips shall not be polluted by water and oil, and the chips with different components shall not be confused.
In addition to the control of the above four aspects, other process details should also be noted, such as: dispersion of the internal sprue, overflow riser at the end of the iron flow, do not box too early, etc. More practices are expected to be summarized by readers for peer sharing.