Study on the mechanism of influence of wear-resistant plate microstructure on toughness
Wear-resistant plates are annealed to obtain martensitic structure, but for large tools, bainite structure is produced due to the slow cooling rate. The formation of upper bainite leads to a decrease in toughness, which is caused by the preferential precipitation of carbides at the front of the austenite grain boundary. In order to meet the modern needs of long life and quality tools, there is a strong requirement to ensure toughness through micro-control. In order to clarify the effect of cooling rate after austenitizing on microstructure, especially on bainite grain size, carbide precipitation and diffusion, and toughness of wear-resistant plate, the researchers carried out research and analysis in this regard.
The chemical composition of AISI H13, H10 and H19 steels used in the current research is shown in Table 1. These steels are cast into ingot after electric arc furnace smelting, forged to the dimensions given in Table 1 at a specific forging heat greater than 6, and then annealed at 850 ° C. The sample is cut from the center of the hot forging material to the square Angle or surface, and its coordinates are parallel to the longitudinal.
The formation of bainite at different cooling rates after austenitizing H13 and H10 at 1200℃ and H19 at 1140℃ was studied by means of microstructure observation, size change and hardness measurement.
After quenching and tempering at different cooling rates, the plane strain fracture toughness K1c, fatigue crack diffusion rate, Charpy impact value, brittle transition temperature of V-notch Charpy test were measured for HRC44.
The plane strain fracture toughness test was carried out with a 30mm wide wear-resistant plate sample, and two types of fatigue tests were carried out:
(1) Fatigue crack growth test of the same type of specimen using plane strain fracture toughness test; The fatigue precrack is used, which is generated during cyclic stretching at a frequency of 550-20kg under changing load at a rate of 5 weeks per second.
(2) Rotary bending fatigue test, the smooth sample of Φ10mm is used to rotate at a speed of 3000 cycles /s, so as to obtain the S-N curve.
According to JISZ2202 No.3, Charpy impact test was carried out with U-shaped notched specimens. Charpy brittle transition tests were carried out using ASTM A307 samples in the temperature range of 20 ~ 300℃. The percentage of residual austenite was determined by X-ray diffraction. The content and size of residual carbides were determined by mapping analysis.
The test results show that the toughness of these wear plates can be improved by the following methods: improving the structure of martensite and bainite slat and improving the effective grain size, delaying the optimal precipitation of carbides along the grain boundary of the original austenite and bainite, inhibiting the dense distribution of MC and M2C type fine carbides in the matrix, reducing the percentage content of residual carbides and reducing their size. The decrease of chilling rate leads to the gradual increase of the width of bainite lath structure, which leads to the transformation of bainite from slaty to granular and leads to the deterioration of toughness. Among the tested steels, H13 steel has the highest toughness, which is closely related to the extremely low density of fine carbides in the matrix, the extremely small size of residual carbides and the extremely low percentage content.
