Refinement of grain structure of martensitic stainless steel

Among the many strengthening mechanisms, the method of grain refinement is the best way to control the strength and toughness of the material. Other methods do not improve the strength and toughness at the same time. We mainly improve the strength and toughness of martensitic stainless steel from the aspects of grain refinement and control of microstructure and phase structure.

L.D. barlowand M. Dutoit respectively carried out austenitizing heat treatment experiments on two groups of AISI420 martensitic stainless steel (with carbon content of about 0.47%), in which Mo was introduced into group 1. The dissolution of carbides in the austenitizing process also affects the austenite grain size. As shown in Figure 1, when the temperature is less than 1075 ° C, the ASTM average size is above 9 and keeps stable. When the temperature is greater than 1075 ° C, the ASTM rapidly decreases. The increase of grain size is related to the increase of temperature (thus providing a higher driving force for grain growth during heat treatment), which is caused by the dissolution of grain fixed carbides. The grain size increases with the increase of temperature and the dissolution of grain carbides. Due to the high stability of carbides in this steel, the grain growth under group 1 is inhibited at temperatures below 1120 ° C. When all alloying elements are in solid solution state due to the dissolution of carbides at higher temperatures, the austenite grain size of the two groups of hot furnaces is similar. The average carbide diameter of the two kinds of heating is shown in Fig. 2, and the average carbide diameter measured in group 1 is about 1.3 at 1000 ° C μ m. About 0.8 at 1100 ° C μ m. This reduction in the average carbide diameter was less pronounced when group 2 was heated, from about 0.75% at 1000 ° C μ M is less than 0.6 at 1100 ° C μ m。

When Zhou Beibei studied 00crl3ni7co5mo4w steel, she found that after solid solution treatment (1100 ° C, 1H), followed by aging strengthening (510 ° C, 10h), it was shown in the transmission electron microscope (TEM) that the spacing between the slats was about 0.2 μ M U.M. The martensite structure with fine structure and a small amount of austenite (reverse transformation) are produced after aging treatment. This structure is very helpful to the improvement of mechanical properties of materials and is a manifestation of strengthening the pearlization.

The tensile strength and yield strength of martensitic stainless steel obtained from the experiment are 1316mpa and 1450mpa respectively; The elongation reached 10.8%.

Chunfang Wang et al. Studied the relationship between the structure and properties of martensitic stainless steel. The grain size of martensitic stainless steel is closely related to the toughness of the material. Lath martensitic stainless steel is an important characterization of the strength and initial property. OM, SEM and EBSD analysis revealed the structure of lath martensitic stainless steel, The boundary orientation of the cleavage crack propagation structure is observed to understand the role of the binding body in the cleavage crack. The results show that the size of martensitic stainless steel lath decreases with the refinement of austenite, and the average lath width is about 0.3 μ m。

FIG. 4A shows that the average number and size of slats are 75, 45, 19 and 4 at 77-373k μ M U.M. from these four curves, it can be seen that with the decrease of the test temperature, it gradually transits from the high absorption energy region (use) to the low absorption energy region (LSE). The LSE decreases with the increase of the group size, and the trend of subdividing the group is more obvious, when the size of the slat bundle is refined from 129 to 4 μ M U.M, the LSE increased more than 9 times, from 5 to 47j, indicating that the refinement of microstructure contributes greatly to the toughness of martensitic stainless steel. Fig. 4B shows the relationship between 50% fat (brittle transition temperature) and the size of the martensitic stainless steel sheet bundle. 50% fat has a strong dependence on the size of the slat bundle, and there is a linear relationship between the reciprocal of the square root of the slat bundle size and 50% fat.