Material: basic material ZGMn13 alloyed with Mo Ni and composited with Cu.
The crack and fracture location of the parts are mainly concentrated on the edges or thinner parts which are easy to be impacted, and the cracks are zigzag to the matrix.
The characteristics of the intergranular expansion are shown. The fracture surface of two parts was selected for analysis, and the morphology was shown in Figures 1 and 2. It can be seen that there is no plastic deformation and thinning of wall thickness at the fracture site,No obvious fracture source can be observed under a 10 times magnifying glass.It is a typical brittle fracture. Fig. 1 shows that the fracture is uneven and coarser, showing a gray rocky luster, which is a typical overburnt fracture. The proportion of the occupied area appears on the fracture of Figure 2.
A larger and more developed columnar crystal area. The fracture often occurs when the casting temperature is too high, which is a typical fracture of the rod like crystal. In addition, the loose holes with concentrated distribution and distribution can be observed at the fracture surface.
The specimens were cut from the coarse-grained and columnar developed fracture sites, and the microstructure was observed by OLYMPUSGX51 microscope.
Enlarging 50 times can see serious intergranular porosity and loose porosity. The area of distribution is large, and is densely concentrated near the fracture surface, as shown in figures 3 and 4. The samples were etched with 4% nitric acid ethanol solution and observed under 100~500 microscope.
Microstructure is austenite and the eutectic carbides distributed along the austenite grain boundaries and the zigzag and star shaped eutectic carbides in the crystals conform to the overburning characteristics of high manganese steel. Further observation shows that many grain boundaries have melted, and there are triangular grains and holes formed by metal melting. See figures 5 and 6.
Scanning electron microscope analysis
Using scanning electron microscopy to observe metallographic samples, we can see that continuous grain holes have appeared in the grain boundaries where the overburning characteristics are not obvious under optical microscope.
The smooth free surface indicates that the grain boundary has melted, as shown in Fig. 7. observation
Coarse eutectic carbonization is found in the austenite grain boundaries with severe cracking.
See Figure 8.
Fig. 7 melting morphology of grain boundary Fig.7? Meltedgrainboundary
The inclusion of cracks in the grain boundary gap can be analyzed by EDAX energy spectrometer.
Fig. 8 eutectic carbides at the grain boundary
Quantity, as shown in Figure 9. The highest sulfur content is 1.61%, and the oxygen content reaches up to 28.9%, indicating that the grain boundary has been burned and oxidized. The silicon content is low near the grain boundary, which is 0.22%~0.31%.
Analysis and discussion：
Based on the above analysis, it can be concluded that the main reason for the brittle fracture of the track plate is the severe embrittlement of the grain boundaries due to overburning. In the process analysis, it is found that under normal conditions, when the treatment temperature of water toughening is slightly higher than that of the process, this serious phenomenon of overheating will not occur, so the high temperature of water toughening is not the root cause of overheating. The theoretical analysis shows that the solubility of sulfur in the austenite is very small, and the temperature range of the coexistence of the solid and liquid phase of the high manganese steel can reach hundreds of degrees Celsius, forming the segregation of the steel when the steel is solidified.
The higher the injection temperature, the longer the time required for complete solidification, and the segregation will be more serious. At the same time, sulfur will easily form a low melting point with iron (1190
The FeS and the eutectic with a lower melting point (988) are distributed in the grain boundary. When the temperature reaches a certain water treatment temperature, it first melts and causes overburning. The coarse columnar crystal at the fracture point also shows that the pouring temperature is too high. At the same time, the silicon element in the grain boundary is reduced greatly, which reduces the AC3 temperature of the material and increases the tendency of grain boundary melting. Therefore, the temperature of smelting casting is too high。
In addition, another reason for brittle fracture of the track plate is loose porosity in the material. The formation is due to volume shrinkage during solidification, insufficient metal solution and lack of timely replenishment.
It greatly destroys the continuity of the metal matrix.
The mechanical properties of the material decrease obviously, resulting in fracture when the part is forced.
(1) the pouring temperature is too high in the casting, which causes the sulfur element to be segregated on the grain boundary, and forms the compound with low melting point. It is one of the reasons for the high temperature and overheating when the water toughening treatment is heated.
(2) there is a serious porosity porosity in the material, which reduces the impact resistance and causes the brittle fracture of the track plate.