Microstructures and Properties of Cr26 High Chromium Cast iron and its heat treatment (2)

3.Mechanical properties test result

4.Metallographic structure

The microstructure of high chromium cast iron samples was observed by an optical microscope, which was typical hypoeutectic white cast iron, composed of low-temperature transformation microstructure and eutectic of primary austenite. Fig. 2 is a photograph of the Specimen 123. From Fig. 2A, it can be seen that the high-temperature austenite has a dendritic distribution. From the high-magnification photograph of Fig. 2B, it can be seen that fine secondary carbides have been precipitated from the primary austenite at high temperatures, the Carbide in the eutectic structure is in the shape of a broken net or block, which shows that the effect of remodification is better.

Under SEM, high-temperature austenite has changed into martensite at room temperature. Fig. 3 is a sem photo of Specimen 123. It can be seen that the eutectic austenite has transformed into needle-like martensite and a small amount of retained austenite without secondary carbide precipitation. A large number of secondary carbides have been precipitated before the transformation of austenite to Martensite at high temperatures. Fig. 4 is an SEM photograph of different quenched and tempered samples. It can be seen that the quenching temperature and the tempering temperature have an obvious effect on the precipitation of this secondary carbide. With high quenching temperature, the secondary carbide particles in primary austenite are larger and less, and with low quenching temperature, the secondary carbide particles are larger and smaller. A comparison of figures 4C and 4D shows that the quenching temperatures are the same. In figure 4C, after tempering at 440 °C, smaller particles than secondary carbides are distributed on the quenched Matrix, whereas in figure 4D, no such small particles are observed in the samples, this indicates that the martensite obtained by quenching is decomposed and precipitates fine carbides when tempered at 440 °C.

The metallographic observation shows that the microstructures obtained by four heat treatment processes at different temperatures are basically the same, and all are composed of martensite + carbide + retained austenite, and transformed into martensite and a small amount of retained austenite. The reason why the hardness is lower than 60HRC when the quenching temperature is lower than 1030 °C is considered to be related to the amount of retained austenite in the microstructure, the amount of retained Austenite is related to the precipitation behavior of Secondary Carbides in Matrix.

5.Results of wear test

ML210 PIN-ON-DISC WEAR Tester was used for the wear test. The specimen size was < 6mm 20mm. 6kg, rotating speed is 120r/min, after pre-grinding, the weight is measured on the Model DT2100 (precision 10,000 g) photoelectric balance as the weight before wear experiment, and then the new sandpaper is used to wear, experiment, measure the weight loss before and after wear. In order to make a comparison, the wear loss weights of CR15 high chromium cast iron and MN13 high manganese steel were measured at the same time. To ensure the accuracy of the experiment, two samples were used in the experiment. As can be seen from table 2, the wear resistance of high chromium cast iron of design composition is related to the heat treatment process; the key is the hardness and impact value after heat treatment; hardness plays a significant role in wear resistance. The best wear resistance is that the quenching temperature is 1050 °C, and the lowest weight loss is 0. 090985 G, its relative wear resistance is the Cr15 of 1. Thirty-two times as many as Mn13. 95 times.

6.Results

  • The CR26 high chromium cast iron has more than 60HRC after reasonable composition design and heat treatment, and the impact toughness is 10j/cm2.
  • the ideal heat treatment process for the designed Cr26 is: quenching temperature 1040 ± 10 °c, holding temperature 6H, using special quenching liquid, and tempering at 275 °C or 440 °C for 6H.
  • the microstructure of experimental Cr26 cast iron is composed of Martensite, carbide, and retained austenite. When the quenching temperature is high, the number of precipitated secondary carbide particles is large and small, and when the quenching temperature is low, the number and size of precipitated secondary carbide particles are large and small. Eutectic austenite transformation, for needle-like martensite and a small amount of retained AUSTENITE, no secondary carbide precipitation.
  • the wear resistance of CR26 cast iron is 1. 32 times that of Mn13 steel95 times.