Lithoidal fracture of high-carbon steels

Abstract

1. The effect of carbon on the susceptibility of steel to the formation of primary and resistant lithoidal fracture is not the same in all cases. The directional effect and the intensity of the effect depend on the quantity and composition of the carbide phase and its solubility in austenite. Increasing amounts of carbon in the steel favor local melting during overheating, especially in the grain boundaries, intersections of grains, and in places rich in low-melting impurities (phosphorus) and elements lowering the melting point of iron, and also around nonmetallic inclusions of the MS type. With satisfactory fracture (fine crystalline, cryptolithoidal) of such steel the most important properties are irreversibly low. 2. It was found that the large enrichment in phosphorus (50–60 times) of energetically weak zones of the matrix (boundaries and joints of grains, boundaries between the matrix and nonmetallic inclusions) occurs only with their melting. 3. Overheating of alloy steel Kh2N4V to temperatures leading to the first portions of liquid steel induces substantial enrichment in phosphorus, chromium, and tungsten. Cooling leads to precipitation of excess phase of complex composition (white phase). The precipitation of this phase predetermines the resistance of the pseudograins to rectification. 4. Stable lithoidal fracture is due to precipitation in boundary areas on the surface of prior austenite grains of compounds difficult to dissolve in ordinary heat treatments that include nitrides, special carbides, carbonitrides, intermetallic compounds, and other excess phases.