Low alloying treatment of gray cast iron and the role of alloys
In the production of gray cast iron of low alloying treatment, people have been committed to producing high-strength and white-free castings. To obtain high tensile strength, the first thing is to select an appropriate carbon equivalent so that the casting iron liquid has developed austenite primary crystals when solidifying; then obtain fine and uniform A-type graphite through inoculation and other treatments, and try to maintain the continuity of the matrix; finally, ensure that all austenite dendrites transform into pearlite during eutectoid transformation. The higher the pearlite content in the matrix and the finer the pearlite, the higher the strength of the casting. To achieve this, it is necessary to strive to have enough austenite when cooling to the A1 phase line. The existence of austenite depends on its carbon content. Any measures that can prevent carbon from diffusing from austenite to graphite can increase the amount of austenite.
For ordinary gray cast iron, to obtain a completely pearlite matrix, it is necessary to quickly cool the casting while obtaining uniformly distributed A-type graphite. Some companies use high-temperature sand dropping to accelerate cooling to obtain pearlite. A small amount of alloy can delay the eutectoid transformation time and reduce the temperature, thereby ensuring the acquisition of complete pearlite and fine pearlite, which is also one of the mechanisms for adding a small amount of alloy to improve strength.
Low alloying treatment of gray cast iron is an important way to improve the mechanical properties and performance of gray cast iron and save materials. In production practice, a combination of adding a small amount of alloying elements in front of the furnace and inoculation technology is often used to produce different compositions of molten iron from the same base iron (original iron liquid) to meet the requirements of castings of different grades or different wall thicknesses of the same grade. This is also the most commonly used measure in production to improve the mechanical properties of castings, optimize production processes, and save costs.
The following lists the role of some alloying elements in gray cast iron for reference only.
1. Aluminum
1. It has strong graphitization ability to prevent the appearance of white cast iron;
2. Stabilize ferrite, increase the amount of graphite, and reduce hardness;
3. It is easy to produce oxide inclusions and pore defects, and is rarely used in general cast iron. However, there is a certain amount of aluminum in the inoculant, spheroidizer and scrap steel, so strict attention should be paid when using them;
4. Aluminum has active chemical properties and is easy to produce aluminum oxide film to prevent the oxidation of the matrix. It is generally used in special fields such as heat-resistant cast iron. It is not described in detail here.
However, under normal circumstances, after the aluminum content exceeds the standard, it is easy to produce oxidation reaction pores in the casting. This must be given enough attention by enterprises that use a large amount of iron cuttings and scrap iron!
2. Chromium
1. Strong carbide stabilizing element, increasing the tendency of white cast iron;
2. Stabilize cementite, reduce non-refined graphite, and increase hardness;
3. Improve wear resistance.
3. Copper
1. It has a weak graphitization ability and reduces the tendency of white cast iron;
2. Stabilize pearlite.
4. Molybdenum
1. Weak carbide stabilizing element;
2. Strongly promotes pearlitization and can refine graphite and pearlite.
5. Nickel
1. Graphitizing element, reduces the tendency of white cast iron;
2. Has a weak pearlite-forming effect, stabilizes austenite, and can refine graphite and pearlite;
3. Improves the density and toughness of castings;
4. Improves the hardness uniformity of different cross sections.
6. Titanium
1. Graphitizing element, reduces the tendency of white cast iron;
2. Refines graphite;
3. Improves the fluidity of molten iron;
4. Titanium has good denitrification and dehydrogenation capabilities.
However, excessive titanium has a great influence on the supercooling of molten iron and is prone to produce supercooled graphite.
7. Tin
1. Strong carbide stabilizing element, increases pearlite content and improves cast iron strength;
2. Tin-iron type segregation compounds are easily formed at the boundary of eutectic groups, affecting toughness;
3. When the matrix is all pearlite, adding tin will no longer have any effect.
8. Antimony
1. Increase primary austenite dendrites and strongly promote the formation of pearlite;
2. It has a slight effect of reducing the tendency of white cast iron.
However, antimony is very easy to segregate at grain boundaries, resulting in hard and brittle properties and reduced strength, so the dosage must be strictly controlled. By the way, antimony in ductile iron or vermicular iron will strongly interfere with spherical/vermicular graphite, causing graphite to change from spherical/vermicular to blocky.
9. Lead
A small amount of lead can cause Widmanstatten graphite (commonly known as: graphite hair) to appear in gray cast iron, seriously reducing strength.
10. Zinc
1. Adding less than 0.3% zinc to gray cast iron can remove oxygen;
2. Refining graphite, increasing the amount of combined carbon, increasing the tendency of white cast iron, and increasing the strength and hardness. The addition amount is 0.1~0.3%.
However, it is possible to form iron-zinc-carbon composite carbides.
At the same time, zinc-containing furnace charges have a great impact on environmental pollution, corrosion of furnace linings, and the physical and mental health of foundry production personnel.
To improve the performance of gray cast iron with alloy elements, it is necessary to select appropriate alloy elements according to the production status and requirements of the factory. According to the different properties of alloy elements, composite addition is often adopted in production, and two or more elements are used in combination.
The original iron liquid used for low alloying treatment should have a higher carbon equivalent, so that it has a small tendency to white cast iron, good casting performance, and is not easy to produce defects such as shrinkage and shrinkage. In addition, when the carbon equivalent is higher, high carbon and low silicon are selected, so that the best strength and cross-sectional uniformity can be obtained after adding alloys, preventing the increase of silicon from increasing ferrite, coarsening pearlite, and neutralizing the harmful effects of alloying elements.
In gray cast iron, it is also necessary to pay attention to the harmful effects of some trace elements (such as aluminum, zinc, lead, etc.), and their content should be strictly controlled.
