Crack defects and countermeasures in the process of large forgings

In large forging, when raw material quality bad or not at that time, the forging process is often easy to produce forging crack. Here are a few due to poor materials forging crack. ( 1) Most ingot ingot defects caused by forging crack defects, when forging may cause cracking, 6 - picture 8 for 2 cr13 main shaft forgings center crack. This is because when the 6 t steel ingot solidification crystallization temperature scope narrow, linear shrinkage coefficient. Condensation feeding is insufficient, the internal and external temperature difference is big, axial tensile stress, the dendrite craze, form the axis of steel ingot intergranular crack, the crack in forging the further extension of main shaft forgings have crack. The defects can be eliminated by the following measures: (1) improve the refining of molten steel purity; (2) ingot casting slow cooling, reducing the thermal stress; (3) with good heating agent and insulation cap, increase the feeding capacity; (4) the center compaction forging process. ( 2) Harmful impurity in steel forging crack caused precipitate along grain boundary. Sulfur in steel is often in the form of FeS precipitate along grain boundary, its melting point 982 ℃, 1200 ℃ in the forging temperature, grain boundary on FeS melting will occur, and surrounded by grain size in the form of liquid membrane, heat resulting from the destruction of the combination of intergranular brittle, mild forging hammer will crack. Copper in steel in 1100 ~ 1200 ℃ temperature peroxide when heated, sexual atmosphere due to the selective oxidation, surface can form rich copper zone, when more than copper solubility in austenite, copper is distributed in the grain boundary in the form of liquid film, forming copper brittleness, forging forming. If there exist in steel tin, antimony, will seriously reduce the copper solubility in the austenite, intensifying the embrittlement tendency. Steel forgings reticular crack, because the copper content is too high, when heated, forging surface selective oxidation, the copper enrichment along grain boundary, rich copper forging cracks along the grain boundary formed by the combination of nuclear and expand. ( 3) Interphase ( The second phase) Caused by forging, the mechanical properties of the second phase in the crack of steel and metal substrate often have very big difference, thus the deformation flow can cause additional stress cause the overall process of plastic drop, once the local stress exceeds interphase and the matrix adhesion, there had separated form the hole. Such as steel of oxide, nitride, carbide and boride, sulfide, silicate, etc. If the phase is densely populated. Chain distribution, especially in the weak binding force along grain boundary exists, the high temperature forging will crack. 20 87 t simn steel ingot forging cracking caused by tiny AlN along the grain boundary precipitates, macro, its surface has oxidation, polyhedron columnar crystal. Microscopic analysis showed that the forging cracking and small granular AlN along a large crystal grain boundary precipitation. To prevent forging cracking caused by precipitation along the crystal aluminum nitride countermeasures are: 1) Limiting steel adding amount of aluminum, removal of nitrogen in steel titanium or use the add method of inhibition of AlN precipitation amount; 2) USES the hot ingot, too cold phase change process; 3) Elevating the temperature of the hot delivery ( ＞900℃) Direct heating forging; 4) Before forging adequately homogenizing annealing, the grain boundary precipitated phase diffusion.