Gray cast iron is a low-cost material. It has good thermal conductivity, good mechanical properties and wear resistance due to its flake graphite network. It is widely used in various fields. However, the traditional gray cast iron needs further alloying treatment to improve its properties to meet the needs of industry. There are many kinds of alloy elements used in gray cast iron, and their functions and dosage are also different. For example, silicon can improve the carbon activity in gray cast iron and promote graphitization. Molybdenum, copper, titanium and rare earth elements can change the morphology of graphite sheet and matrix, and change the material properties by changing the structure of gray cast iron.
There are dozens of alloy elements commonly used in gray cast iron. Each alloy element has its own behavior and influence mode of microstructure change in liquid phase and solid phase. Next, we discuss the effects of different alloy elements on the microstructure and properties of gray cast iron.
1) Nitrogen
Generally, the content of nitrogen in gray cast iron is low and belongs to trace elements. Therefore, there are few studies on the effect of nitrogen on the microstructure and properties of gray cast iron for a long time. However, after decades of research, a lot of progress has been made. It has been recognized that nitrogen has a significant impact on the quality of gray cast iron. Increasing the nitrogen content in an appropriate range can improve the tensile strength, but when the nitrogen content is too high, the gray cast iron castings will produce white structure and form pores and other defects. The reason for the above phenomenon is that the diffusion rate of nitrogen in molten iron is slow. It is also found that when nitrogen and hydrogen are together, pores will be formed even if the nitrogen content is within its allowable range.
The effect of nitrogen on the structure of gray cast iron mainly includes the effect on graphite morphology and matrix structure. Studies have shown that nitrogen can shorten the length of graphite sheet, blunt the end and increase the bending degree. The effect of nitrogen on the matrix structure of gray cast iron is mainly to stabilize pearlite. When the nitrogen content in gray cast iron is 0.005% – 0.0175%, it can effectively inhibit the formation of ferrite. Therefore, increasing the nitrogen content in an appropriate range can improve the tensile strength.
2) Vanadium
In the solidification process of gray cast iron, vanadium is a strong carbide forming element. Compared with chromium, vanadium has a stronger ability to promote the formation of carbides. Vanadium has a strong affinity with carbon, nitrogen and oxygen. In gray cast iron, vanadium is mostly combined with carbon and exists in vanadium carbides, with more v4c3. Even if a small amount of vanadium is added, the corresponding carbides will be precipitated in the form of fine precipitated phases, Therefore, the wear resistance of gray cast iron can be improved. Vanadium can also promote the formation of pearlite, refine pearlite and improve tensile strength and hardness. It was determined that every 0.1wt.% was added V average tensile strength can be increased by 15-30mpa, and 0.1wt.% can be added without eutectic cementite V can increase the hardness of gray cast iron by 8-10 Brinell hardness.
3) Chromium
Adding a small amount of chromium can inhibit ferrite nucleation, reduce the amount of ferrite, promote the formation of pearlite, prolong the incubation period of pearlite, improve the undercooling of eutectoid transformation, refine pearlite, and effectively improve the tensile strength and hardness of gray cast iron. Liao Qiqiang et al. Found that when the carbon equivalent is 3.85%, after 0.3wt.% After chromium alloying, the strength of the sample can reach the level of HT250, which is much better than that of the non alloyed sample, but it will increase the hardness difference of the section and increase the sensitivity of the section. When the carbon equivalent is 4.07wt.% Add 0.3wt.% Although the tensile strength of chromium does not reach 250Mpa, it is still better than the unalloyed sample. Although the hardness value of the section increases at this time, the section sensitivity is improved.
4) Phosphorous
Phosphorus segregates in the liquid phase during solidification of gray cast iron, and solidifies into phosphorus eutectic after eutectic transformation. Because phosphorus eutectic is hard and brittle, it will reduce the toughness and compactness of gray cast iron, but it can be used as anti-wear skeleton phase when it exists at the eutectic boundary, so as to improve the wear resistance of gray cast iron. Under normal circumstances, the mass fraction of phosphorus should be less than 0.2%, and the phosphorus content for engine cylinder liner and machine tool wear parts can be appropriately increased to more than 0.3%. H. R. Abbasi et al. Studied the effects of phosphorus as an alloy element on the microstructure, eutectic temperature, hardness and mechanical properties of gray cast iron. The addition amounts of five different phosphorus were 0.5%, 1%, 1.5%, 2% and 2.5% respectively. The results showed that when the addition amount of phosphorus increased from 0.45% to 2.58%, the eutectic phosphide increased from 4.7% to 17.81%, the mechanical properties decreased from 297.5mpa to 184.1mpa, the impact strength decreased from 4.3 to 2.7J, the hardness increased from 215.43hb to 249.38hb, and the eutectic temperature decreased from 1139.7 ℃ to 1102.5 ℃.
5) Copper
Copper is a graphitization element, which can reduce the critical temperature of austenite, strengthen the matrix, and refine the structure of graphite and pearlite. The maximum dissolution of Cu in gray cast iron matrix is 0.9wt.%, The graphite type of blank sample (without Cu) is flake type A. in the sample with Cu wire, the graphite becomes type B, D or e with the change of experimental conditions.
6) Nickel
Nickel can promote graphitization and inhibit carbide formation. For as cast gray cast iron with pearlite matrix, the higher the Ni content, the lower the strength. Under the austenitizing condition, the strength increases obviously with the increase of Ni content. The effects of Ni on pearlite matrix and austenite matrix are completely different. Under conventional conditions, Ni forms a solid solution in the ferrite phase, but the brittle cementite phase does not absorb Ni. In austenitized cast iron, Ni will exist in austenite and ferrite phase. Although the content is different, it will improve the matrix strength. The addition of nickel can also refine the flake graphite structure of gray cast iron and improve the wear resistance of gray cast iron.