3 arc cracks
It often occurs in sudden changes in the shape of mold corners, gaps, cavities, and die wiring flash. This is because the stress generated at the corners during quenching is 10 times the average stress of a smooth surface.
(1) The higher the carbon (C) content and alloying element content in steel, the lower the Ms point of steel. If the Ms point decreases by 2℃, the quenching cracking tendency increases by 1.2 times, the Ms point decreases by 8℃, and the quenching cracking tendency increases by 8 Times;
(2) The transformation of different structures in steel and the transformation of the same structure are not at the same time. Due to the different structure specific tolerances, huge structure stress is caused, which leads to the formation of arc-shaped cracks at the boundary of the structure;
(3) After quenching, it is not tempered in time, or the tempering is not sufficient, the retained austenite in the steel is not fully transformed, and it remains in the service state to promote the redistribution of stress, or the retained austenite becomes martensite when the mold is in service The change produces new internal stress, and when the comprehensive stress is greater than the strength limit of the steel, an arc-shaped crack is formed;
(4) It has the second type of tempered brittle steel. After quenching, high temperature tempering and slow cooling will cause harmful impurity compounds such as P and s in the steel to precipitate along the grain boundary, which will greatly reduce the grain boundary bonding force and strength and toughness, and increase the brittleness. Arc-shaped cracks are formed under the action of external force.
(1) Improve the design, try to make the shape symmetrical, reduce shape mutations, increase process holes and reinforcing ribs, or use combined assembly;
(2) Filled corners instead of right angles and sharp edges, and through holes instead of blind holes, improve processing accuracy and surface finish, reduce stress concentration sources, and generally have low hardness requirements for unavoidable right angles, sharp edges, blind holes, etc. , It can be bandaged or stuffed with iron wire, asbestos rope, refractory mud, etc., artificially create a cooling barrier, so that it can be slowly cooled and quenched to avoid stress concentration and prevent arc-shaped cracks from forming during quenching;
(3) The quenched steel should be tempered in time to eliminate part of the quenching internal stress and prevent the quenching stress from expanding;
(4) Tempering for a longer time to improve the fracture toughness of the mold;
(5) Fully tempered to obtain stable tissue performance;
(6) Multiple tempering can fully transform the retained austenite and eliminate new stress;
(7) Reasonable tempering to improve the fatigue resistance and comprehensive mechanical properties of steel parts; for mold steels with the second type of temper brittleness after high temperature tempering, they should be cooled quickly (water cooling or oil cooling) to eliminate the second type of temper brittleness. Prevent and avoid arc crack formation during quenching.