The research on high-strength thick-wall artillery barrel materials shows that both PCrNi3MoVA wear-resistant plate, PCrNi4MoV wear-resistant plate and PCrNi4WMoV wear-resistant plate have encountered the technical problem of temper brittleness. The reversible tempering brittleness of low alloy steel has been paid attention to and a lot of research work has been done. Some metallurgical workers mainly use step cooling and isothermal brittleness method to determine the FATT of tempered brittle steel. The fracture morphology of the steel was observed by SEM and the concentration of impurities and alloying elements on the grain boundary was observed by AES. The results show that the higher the FATT temperature is, the larger the ΔFATT temperature increment is, the tempering brittle fracture morphology of steel is intergranular fracture, and the impurity elements are precipitated on the grain boundary.
The main factors causing tempering brittleness are the embrittlement elements such As Si and Mn, which are used as alloying elements such as P, Sb, Sn and As, and the reversible segregation of the original austenite grain boundary of Ni, Cr and C, which increase the embrittlement of the embrittlement impurities. In the process of tempering brittleness, the precipitation of alloying elements and embrittlement impurity elements to austenite grain boundaries is not an isolated behavior, but a co-existing behavior that affects and restricts each other. Some metallurgical workers pointed out that austenite grain size and microstructure are also the main elements that control the tempering brittleness of steel.
In view of the above research results, how to make the PCrNi3MoVA wear plate in Rp≥1029MPa(105kg/mm2), its temper brittleness tendency to reduce to the minimum degree, its various performance indicators are up to 1029MPa strength grade, which is the main topic of the current research. In order to further improve the toughness of the steel and reduce its tempering brittleness tendency, it is considered that some elements must be reasonably adjusted within the specified range of chemical composition of the PCrNi3MoVA wear plate and be strictly controlled during the smelting process. As far as the actual situation is concerned, there are no good control measures for the impurity elements Sb, Sn and AS from the perspective of technical management, but the elements such as P, Si, Mn, Cr, Ni, Mo and C can be reasonably adjusted and strictly controlled, and the grain size of the steel can be refined through heat treatment and satisfactory microstructure can be obtained. To this end, the PCrNi3MoVA wear plate put forward the following views.
Firstly, the austenite grain size has a certain influence on the tempering brittleness tendency. With the increase of austenite grain size, the brittleness transition temperature of the pure steel with low tempering brittleness sensitivity in different states increases slightly. In the impure state of steel containing impurity P, the brittleness transition temperature increases sharply with the increase of austenite size. This indicates that coarse grains will lead to more severe tempering brittleness, but this effect is only shown in steels with high tempering brittleness sensitivity. In order to reduce the tempering tendency of the steel, it is necessary to use rapid heating, high temperature normalizing and low temperature quenching in the tempering process to make its grain evenly refined.
Secondly, the microstructure has a certain influence on the sensitivity of temper brittleness. When the wear plate obtains M or M+B, the temper brittleness sensitivity is small, and vice versa. The tempering brittleness increases with uniform low temperature bainite structure, especially when P content is high. Therefore, it is necessary to reduce the water temperature and increase the water cooling time during the quenching of the wear-resistant plate on the basis of the above adjustment, so that a large number of martensitic tissues can be obtained in the tube part of the gun body, which is also the main measure to reduce the tempering brittleness.
