It is widely used in mining and power plant due to its excellent wear resistance. For example, high chromium cast iron is applied to crusher hammer head, lining plate and impeller in mining industry, impeller, sheath and guard plate of impurity pump in cement and power plant industry. Alloy elements, carbide types, heat treatment process and matrix structure will affect the properties of high chromium cast iron.
(1) Element C: the higher the hardness value of carbide and alloy in the structure, and the wear resistance increases with the increase of carbon content. However, when the carbon content is too high, the impact toughness of high chromium cast iron will be reduced. In high chromium cast iron, part of the C element is dissolved into the austenite matrix to increase the matrix hardenability, so that the martensite matrix obtained by quenching is in the state of supersaturation of carbon content. C concentration has an important effect on the wear properties of high chromium cast iron. There is a critical value of carbon content between 2 wt.% and 3 wt.% of C content. Below this critical value, high chromium cast iron shows hypoeutectic form and has low wear performance. The wear resistance increases with the increase of carbon content; When it is higher than the critical value, the high chromium cast iron is hypereutectic, and its wear resistance tends to be stable, or the change of carbon content has limited influence on the wear performance. Another part of C element is dissolved into carbide, and the amount of carbide will increase with the increase of carbon content. Too many carbides will reduce the toughness of wear-resistant cast iron and increase the brittleness of wear-resistant cast iron, resulting in the first cracking and spalling of carbides in the wear process, which will have an adverse impact on the wear resistance of cast iron. The ratio of Cr atom to C atom will affect the morphology and quantity of carbides.
When the ratio of Cr atom to C atom is relatively high, M7C3 is mainly dispersed block and hexagonal, which has little splitting effect on the matrix and high toughness; When the ratio of Cr atom to C atom is relatively low, M7C3 is mainly network, which has serious splitting effect on the matrix and low toughness; When the ratio of Cr atom to C atom is relatively high, high chromium cast iron has high toughness, which can alleviate the phenomenon of stress concentration in the wear process, delay the peeling phenomenon of carbide, and make the cast iron have higher wear resistance. On the other hand, the solid solution of Cr in austenite matrix can improve the stability of austenite, shift the continuous cooling curve to the right and increase the hardenability.
(2) Mn element: part of Mn atoms exist in carbides in high chromium cast iron, and the other part is dissolved in austenite to increase the hardenability of the alloy. Manganese can stabilize austenite and expand austenite zone. And Mn atom can sharply reduce the martensitic transformation temperature. Therefore, when the content of Mn atom is high, there is a high content of retained austenite in the microstructure of high chromium cast iron after quenching.
(3) Ti element: Ti improves the wear resistance of cast iron through the following action mechanism: (a) forming high hardness carbide TiC (3200-3800 HV); (b) TiC particles can be used as the nucleation core of M7C3, improve the nucleation rate of M7C3 carbides, and further refine the microstructure and M7C3 carbides; (c) Improve the microhardness of the matrix, so as to improve the wear resistance of cast iron.
(4) NB element: NB improves the wear resistance of cast iron through the following action mechanism: (a) NB can form high hardness carbide NBC (2400 HV); (b) NB can dissolve in the matrix and increase the microhardness of the matrix; (c) NB can dissolve in M7C3 carbides and increase the microhardness of M7C3 carbides; (d) NBC has good morphology, such as block and hook, which can firmly hold carbide particles in the matrix; (e) With the increase of Nb element, the coarse rod-shaped M7C3 carbide is gradually refined and becomes a fine and dispersed block, that is, the isolation of carbide is enhanced. For 18.7% Cr / 2.7% C high chromium cast iron, the simultaneous addition of Nb and Mo (1nb and 1Mo) can promote the formation of NBC.
(5) Element V: the action mechanism of strong carbide forming element V is similar to that of Ti and Nb. It can increase the wear performance of cast iron by forming high hardness carbide VC (2400-3000 HV), refining the structure, and changing the continuous network and long strip eutectic carbide into block and hexagonal distribution.
When strong carbide forming elements such as Ti and C form tic, it will reduce the carbon concentration in the matrix, inhibit the formation of eutectic carbide M7C3 and adversely affect the wear resistance. When the amount of Alloying Elements Nb, V and Ti is small, the amount of NBC, VC and tic generated is small, and the refinement effect on the structure is limited. Therefore, the beneficial effect on the wear resistance is far less than the adverse effect caused by the reduction of the amount of carbide M7C3. When the content of three alloying elements is moderate, the number of high hardness carbides (NBC, VC, TIC) increases, which can refine and round the whole block M7C3 carbides and strengthen the matrix. When the addition amount of the three is too high, NBC, VC and tic gather and grow up, greatly reduce the number of M7C3 carbides and reduce the matrix strength, so as to reduce the wear performance.
(6) W element: when the W content is less than 4 wt.%, it can dissolve in the matrix and eutectic carbide M7C3, so as to increase the strength of matrix and carbide and the hardness of cast iron; When the W content is about 4 wt.%, M2C carbide can be formed; As shown in Figure 1, when the W content is greater than 4 wt.%, fishbone w6c carbide is formed. As shown in Fig. 1 (b), there are microcracks in the eutectic carbide M7C3, and w6c, the alloy element W, increases the number of carbides to form fishbone w6c carbides, which improves the wear resistance of high chromium cast iron of cast iron.
(7) Element B: Cao HaiYe et al. Found that when the mass fraction of B is in the range of 0.09% ~ 0.67%, with the increase of B content, the wear resistance of Cr20 white cast iron after heat treatment first increases and then decreases, while the wear resistance of as cast cast cast cast iron changes little. Strong carbide forming element B can dissolve and increase the number of carbides as cast. When the content of B is appropriate, the carbides in Cr20 white cast iron change from network to short rod and block, which are evenly distributed in the matrix, which are conducive to improve the wear properties of cast iron. When the content of B is too high, the continuity of carbide can be increased and the carbide can be further coarsened. Finally, the cutting effect on the matrix is increased, which has an adverse impact on the wear resistance of cast iron.
(8) Rare earth elements: for high chromium cast iron, rare earth is used as nucleating agent to refine carbide and structure, so as to improve wear resistance. Liu Xu and others found that La2O3 can be used as the nucleation core of eutectic carbide M7C3, so as to refine eutectic carbide. Furrows and carbide spalling pits will appear in the wear process, and the addition of La can alleviate this phenomenon.