Microstructures and Properties of Cr26 High Chromium Cast iron and its heat treatment (1)
Mao Shuang Liang, Shui Heng Yong, Chen Shan shan, Zhao Ai min
( 1. Shanxi Baiyi Machinery Co., Ltd., Taiyuan 030003, China; 2. Research Institute of Metallurgy Engineering, Beijing University of Science and Technical, Beijing 100083, China)
Abstract: The chemical composition of Cr26 high chromium cast iron and the effects of heat treatment on its hardness, impact toughness were studied in this paper. And its microstructures of this kind of high chromium cast iron treated by different heat treatment technology were studied as well. Their results show that the hardness of the experimental high chromium cast iron is upward of 60 HRC when the high chromium cast iron is quenched in a special agent after heating to 1040 ℃and hold at this temperature for six hours and then tempered at 275 ℃or 440 ℃for six hours. And its impact toughness is up to 10 J/ cm2. The wear-resisting property is 1. 32 times s higher than that of the Cr15 high chromium cast iron and 1. 95 times higher than the high manganese steel ZGMn13.
Keywords : High chromium cast iron; Heat treatment; Hardness; Impact toughness; Wear resistance
The content of CR in high chromium cast iron is 10% ~ 30%, the carbide in its microstructure is mainly (Fe, CR)7C 3 type, the microhardness is as high as 1500 ~ 1800hv, and the shape of high chromium cast iron is discontinuous or Bar, which improves the hardness and toughness significantly. GB/T8263 can be divided into CR12, CR15MO, CR20MO, and CR26 according to the chromium content, among which CR26 high chromium cast iron CR content is 23% ~ 28%, and according to the production of different wear-resistant parts, can contain a certain amount of Mo, Ni, and Cu. The British Standard BS48844 divides CR26 into two grades according to the Carbon Content: high carbon and low carbon. Because CR26 high chromium cast iron contains more chromium than CR12 and CR15MO, it will be beneficial to improve the alloy’s hardenability. In this paper, a kind of CR26 high chromium cast iron with high hardness has been developed according to the working conditions of wear-resistant parts such as grinding rollers, lining plates, and grinding balls used by the fossil-fuel power station, the mechanical properties were measured, and the metallographic structure was observed.
The main idea of composition design is 1. increase the carbon content to obtain enough high hardness eutectic carbides. However, if the carbon content is too high, the carbide will form a network, and the Brittleness will be increased. 2 The chromium content is suitable to ensure that there will still be enough eutectic carbide at low addition level, and when the CR/C ratio is within this range, the matrix structure is mainly martensite after quenching In order to reduce the tendency of crack, the low-temperature tempering was carried out to eliminate the quenching stress
C has the most significant influence on the hardness of the material, and the hardness increases with the increase of C content. Carbon is the main element to produce eutectic carbide (CR, FE)7C 3. EUTECTIC carbide plays an essential role in wear resistance. When the content of C is high, the amount of carbide in the structure increases, the primary carbide is coarser, and the hardness further increases. Still, the degree of cleavage to the Matrix increases, the toughness and strength of the material decrease, and the carbon content are controlled at 2. 4% ~ 2. 9%.
The wear resistance of high chromium cast iron mainly depends on carbide content in Matrix; the more carbide, the higher hardness, the better wear resistance. Marafray formula for calculating the amount of eutectic carbides in high chrome cast iron: m 7 C 3(%) = 12. 33(C%) + 0. 55(CR%)-15. 2, it is clear that the amount of carbide in CR26 is significantly higher than that in CR15 when the carbon content is the same. With the increase of CR/C, the hardness of eutectic carbide also increases, but the toughness decrease, and it is easy to be broken. This is one of the important reasons why it is more difficult to produce CR26 high chromium cast iron than CR15 high chromium cast iron. Therefore, the chromium content was determined to be between 23% and 28%.
Mn has the effect of improving hardenability and stabilizing austenite, reducing the initial temperature of martensite transformation. When the content of Mn is high, some Mn is carbonized and exists in eutectic carbide. Mn is not only an alloying element but also a permanent element in high chromium cast iron, but Cr/C is more elevated in Cr26 high chromium cast iron, and austenite has been stabilized. 5% ~ 1. 0%, it is also avoided to produce coarse acicular martensite and increase the amount of retained austenite when the martensite transformation occurs.
The silicon content in high chromium cast iron is not easy to be too high. The silicon is mainly dissolved in the matrix, and the silicon concentration around the eutectic carbide is enriched. Still, the chromium concentration is decreased, hard and brittle martensite and micro-cracks are easily formed, and the silicon expands along with the interface, it is the contributing factor of material micro-spalling. Silicon as a high-chromium cast iron of the constant element, the element is generally controlled at 0. 4% ~ 1. 0%.
1.5 Molybdenum and nickel
The molybdenum part is dissolved in the Matrix. Molybdenum can increase the eutectoid transformation temperature, delay the incubation period of austenite transformation, shift the continuous cooling curve to the right, and decrease the critical cooling rate; it is an effective element to improve hardenability. But because the price of molybdenum is higher, general control is in 1. The continuous cooling curve of austenite is shifted to the right, the austenite phase zone is enlarged, the critical cooling rate is decreased, and the starting temperature of the martensite transformation is also decreased; it is the main alloy element to stabilize austenite. The amount of nickel used in high chromium cast iron is no more than 1. 5%. Nickel can improve the impact toughness and high temperature wear resistance of high chromium cast iron
1.6 Sulfur and phosphorus
Sulfur and phosphorus are harmful elements in high chromium cast iron, brought into the cast iron by raw materials, the less, the better, generally controlled at s ≤0.06%, p ≤0.08%.
1.7 Rare earth alloy
Rare Earth alloys are mainly used for inoculation treatment, including 0. 3% ~ 1. 0%.
2.Heat treatment process
Adopt 0. 5 T medium frequency furnace melting, pouring into < 20mm 160mm heat treatment test stick with a metal mould. The heat treatment process is as shown in Fig. 1. The quenching process is heated in two stages, I. E. 680 °C for 2 hours, then heated to 990,1010,1030,1050 °C for 6 hours, and tempered at 440 °C and 275 °C for 6 hours