Effect of RE on the Structure and Properties of Mg Modified 35Cr2Ni2Mo Cast Steel

Cast steel 35Cr2Ni2Mo exhibits excellent stability and safety under different harsh conditions due to its high strength, toughness, wear resistance, fatigue resistance, and ease of processing. It has been widely used in the manufacturing of some large mechanical components in industries such as mining and metallurgy. With the rapid development of modern industry, manufacturing equipment is gradually transitioning towards large-scale, efficient, and rigorous production. Faced with harsh working conditions, wear-resistant cast steel needs to have better comprehensive performance in order to provide solid support for the long-term and stable operation of various machinery.

In the original cast structure of steel, grain size has an important impact on the performance of wear-resistant cast steel. A small solidification structure can effectively improve the comprehensive performance of wear-resistant cast steel, reduce the generation of cracks and fatigue, and extend its service life. Under existing technological conditions, the more particles that can serve as the crystallization core during the solidification process of steel, the stronger the heterogeneous nucleation ability. The columnar crystal particles are small, and the equiaxed crystal area is relatively larger, resulting in a more obvious grain refinement effect. In addition, studies have shown that if the size of non-metallic inclusions formed in steel during smelting and solidification is greater than 5 μ m. It is easy to become a source of microcracks, thereby reducing the strength, toughness, fatigue resistance, and corrosion resistance of the steel. Kuroshim et al. discussed the relationship between the size and distribution of inclusions that cause actual fatigue failure, and the results showed that the scatter of fatigue life is closely related to the size distribution of inclusions in the critical volume of fatigue failure. In the study of the influence of inclusions on the wear resistance of Cr13 cast steel, Wu Nianqiang et al. also verified the correlation between wear mechanism and inclusions. Large sized inclusions increase the probability of cracking, leading to a decrease in the wear resistance of the steel. Over the years, although numerous scholars at home and abroad have proposed various methods to refine the cast microstructure, there are still many difficulties in effectively controlling the size of precipitates under existing conditions.

Alloying is one of the effective means to improve the comprehensive mechanical properties of materials. Rare earth elements, as a popular alloying element for controlling the size of inclusions, can undergo chemical reactions with impurity elements when added to molten steel, promoting the refinement of inclusions, changing their morphology and distribution, and purifying the molten steel.

In the previous research work, ZHY Casting improved the microstructure and properties of cast steel by adding Mg element to 35Cr2Ni2Mo. Based on the previous research, this article further added RE element to the steel to explore the influence of RE on the microstructure, inclusions, and properties of cast steel.

The actual chemical composition of the cast steel 35Cr2Ni2Mo Mg used in the research institute and the control group 35Cr2Ni2Mo Mg RE are shown in the table, and the two cast steels were subjected to the same heat treatment process. For the convenience of discussion, 35Cr2Ni2Mo Mg will be referred to as No. 1 steel and 35Cr2Ni2Mo Mg RE will be referred to as No. 2 steel.

20.321.060.0030.0040.731.58 0.00392.030.560.0071

Cut 10 mm x 10mm x 10 mm metallographic samples from the same part of two types of test steels, polish them sequentially with 200-2000 grit sandpaper, and then perform mechanical polishing. After polishing, the surface of the sample is subjected to corrosion treatment using nitric acid alcohol with a concentration of 3%. The microstructure and inclusions of the sample were observed and analyzed using optical metallographic microscopy (OM, Nikon MA200) and scanning electron microscopy (SEM, TESCAN MIRA3).

The hardness characterization test was conducted using a microhardness tester (HV-1000) with a load weight of 0.1 kg and a loading time of 15 seconds. The average value of 10 different parts was taken.

The tensile performance test adopts WDW-100G microcomputer controlled high-temperature electronic universal testing machine, and the sample size is φ 10 x 80 mm, with a stretching speed of 2.00 mm/min.

The size of the wear resistance test sample is 57 mm x 25.7 mm x 6mm. It was tested on an MLG-130 dry sand rubber wheel friction and wear testing machine, using 16-26 mesh quartz sand as the abrasive. The frequency of the testing machine is 80Hz, and the wear time is set to 40 minutes. The load applied to the sample is 100 N. After the wear test is completed, the sample is placed in an ultrasonic shaker for 10 minutes to ensure that all debris on the worn surface is completely removed. Subsequently, spray the sample with alcohol and blow dry to prevent oxidation. The wear rate is determined by the ratio between the amount of wear and the wear time. Relative wear resistance is the ratio obtained by dividing the weight loss of other materials after the experiment by the weight loss of the standard material, using the selected material’s wear loss as a reference, while ensuring the same experimental parameters.

The effect of adding RE element to cast steel 35Cr2Ni2Mo Mg on the microstructure and properties of cast steel is as follows:

1) The microstructure of the steel is lath tempered sorbite. Compared to No.1 cast steel, No.2 cast steel has a finer carbide size. The average size of inclusions in No.1 steel is 1.78 μ m. Inclusion size ≤ 5 μ The proportion of m is 98.55%; The average inclusion size of No. 2 steel is 1.47 μ m. Inclusion size ≤ 5 μ The proportion of m is 99.94%;

2) The hardness value of cast steel increased from 48.67 HRC to 52.90 HRC, the tensile strength increased from 1254 MPa to 1274 MPa, the yield strength increased from 940 MPa to 1198 MPa, but the elongation decreased from 5.76% to 4.97%;

3) The wear resistance of No. 2 steel is better than that of No. 1 steel. After 40 minutes of wear with an applied load of 100 N, the wear weight loss of No. 1 steel was 2.579 3 g, and that of No. 2 steel was 2.503 2 g. 1. The wear mechanism of No. 2 steel is mainly micro cutting, but No. 1 steel is accompanied by slight fatigue failure.

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