Preventive measures for arc-shaped cracks

Preventive measures for arc-shaped cracks
It often occurs at sudden changes in the shape of die corners, bosses, knife lines, sharp corners, right angles, notches, cavities, die connection flash, etc. This is because the stress generated at the corners during quenching is 10 times the average stress of the smooth surface.
(1) The higher the carbon (C) content and alloying element content in the steel, the lower the Ms point of the steel. The Ms point decreases by 2 ℃, and the tendency of quenching cracks increases by 1.2 times, the Ms point decreases by 8 ℃, and the tendency of quenching cracks increases by 8 times;
(2) The transformation of different microstructures and the transformation of the same microstructure in steel are different at the same time. Due to different specific tolerances of microstructures, huge structural stress is caused, which leads to the formation of arc-shaped cracks at the interface of microstructures;
(3) After quenching, the steel is not tempered in time, or the tempering is not sufficient, and the residual austenite in the steel is not fully transformed, which is retained in the service state, causing the stress to redistribute, or when the die is in service, the residual austenite undergoes martensitic transformation and generates new internal stress. When the comprehensive stress is greater than the strength of the steel, arc cracks are formed;
(4) Class II tempered brittle steel. After quenching, it is tempered slowly at high temperature, which causes harmful impurities such as P and S in the steel to precipitate along the grain boundary, greatly reducing the grain boundary adhesion and strength toughness, increasing brittleness, and forming arc-shaped cracks under the action of external forces during service.

Preventive measures:
(1) Improve the design, make the shape symmetrical as far as possible, reduce the sudden change of shape, increase the process hole and reinforcing rib, or adopt combined assembly;
(2) Round corners replace right angles and sharp corners and sharp edges, and through holes replace blind holes to improve processing accuracy and surface finish, reduce stress concentration sources. For places where it is impossible to avoid right angles, sharp corners and sharp edges, blind holes, etc., the general hardness requirements are not high, iron wire, asbestos rope, fire-resistant mud, etc. can be used for binding or filling, and artificial cooling barriers can be created to slow cooling and quenching, avoid stress concentration, and prevent the formation of arc cracks during quenching;
(3) Quenched steel shall be tempered in time to eliminate part of quenching internal stress and prevent quenching stress from expanding;
(4) Temper for a long time to improve the fracture toughness of the die:
(5) Fully tempered to obtain stable structure and properties;
(6) Repeated tempering can fully transform residual austenite and eliminate new stress;
(7) Reasonable tempering can improve the fatigue resistance and comprehensive mechanical properties of steel parts;
(8) For die steel with Class II temper brittleness, it shall be cooled quickly after high temperature tempering (water cooling or oil cooling) to eliminate Class II temper brittleness and prevent and avoid arc crack shape during quenching.