General materials contain some alloy compound phases. The existence of alloy compound phases enhances the matrix strength, hinders the crack propagation, promotes the generation of secondary cracks and prolongs the fatigue life of materials. Fig. 1 shows the secondary crack morphology and compound phase EDS analysis of fatigue fracture of low alloy HT250.

It can be seen from Fig. 1 (a) that there is a small amount of alloy compound phase inside the low alloy HT250 material (shown by the solid line arrow), the alloy compound phase has regular shape, smooth edge and granular existence, and secondary cracks are generated at the edge of the alloy compound phase (shown by the dotted line arrow in the figure). EDS analysis of the alloy compound phase in Figure 1 (a) shows that it is mainly composed of Fe, Mn, Cr, Si, C and other elements. CR has a strong alloying effect. The addition of Trace Cr promotes the formation of alloy compounds in low alloy HT250. Fig. 2 shows the morphology of fatigue crack growth zone and EDS analysis of compound phase on the fatigue fracture surface of low alloy HT250.

It can be seen from Fig. 2 (a) that two fatigue cracks with different propagation directions continue to expand forward along the grain boundary (shown by the dotted arrow in the figure). The existence of alloy compound phase near the grain boundary hinders the convergence and further propagation of the two fatigue cracks (shown by the solid arrow in the figure) and changes the crack propagation direction. EDS analysis of the compound phase in 2 (a) shows that it is mainly composed of C, Cr, Fe, Mn, Si and other elements.

It can be seen from Fig. 1 and Fig. 2 that the trace alloy compound phase in low alloy HT250 has smooth edges and regular shapes, which weakens the stress concentration effect in the material. The alloy compound near the grain boundary has a strengthening effect relative to the grain boundary. When the fatigue crack propagates along the grain boundary to near the compound, it will hinder the continuous propagation of the fatigue crack. Due to the difference between the strength of the alloy compound phase and the grain, secondary fatigue cracks in different directions will be generated near the alloy compound phase under the action of continuous extrusion, which prolongs the fatigue life of the material. At the same time, the shedding of micro alloy compound phase will also hinder the movement of dislocation and further improve the fatigue life of the material.