(1) The ingot defects caused forging crack

Most of ingot defects, forging may cause cracking, as shown in Figure 6-8 for of 2Cr13 spindle forging center crack. This is because the crystallization temperature range is narrow, the linear shrinkage coefficient The 6t ingot solidification. Condensation inadequate feeding, the temperature difference between inside and outside, axis tensile stress large crack along the dendrite, the formation of the the ingot axis of intergranular crack, the crack in the forging further extension to the spindle forging crack. The defect can be eliminated by the following measures: (1) improve smelting steel purity; ② the ingot slow cooling to reduce thermal stress; ③ good heating agent with insulation cap increased feeding capacity; ④ using central compacted forging process.

(2) Steel harmful impurities caused by forging cracks along the grain boundaries.

The sulfur in the steel is often FeS form along the grain boundaries, and a melting point of only 982 ° C, 1200 ° C forging temperature FeS will occur on the grain boundary melting and the form of a liquid film surrounds the grain, destruction of the binding between the grains generating hot brittleness, slight forging blow will crack.

Copper in the steel at a temperature of 1100 ~ 1200 ℃ peroxide atmosphere when heated, due to the selective oxidation, the surface layer will form a copper-rich zone, when more than copper in solubility in austenite, copper, by the form of a liquid film distribution is formed at the grain boundary the copper brittle and can not part forging. Steel in the presence of tin, antimony also seriously reduce the solubility of copper in austenite, exacerbate the embrittlement tendency. Steel forgings net cracks, due to the high copper content, forging heating the surface of the selective oxidation of copper along the grain boundary enrichment forging cracks along the grain boundary Cu-rich phase nucleation and expansion formed.

(3) Isostearic phase (second phase) caused by forging crack
The mechanical properties of the second phase in the steel is often and metal matrix are very different, and thus the deformation flow will cause additional stress causes the overall process plastic drop Once the local stress exceeds the binding force between the iso-phase and matrix, the separation occurs forming holes. For example, the steel of oxides, nitrides, carbides, borides, sulfides, silicates, etc.. If these phase was intensive. Distribution chain binding force, especially along the grain boundary is weak at present, the high temperature forging will crack. 87t ingot 20SiMn steel small AlN along the grain boundaries caused by forging cracking macroscopic morphology, its surface has been oxidized, showing a polyhedron columnar grain. The microscopic analysis showed that the precipitation of a large number of related forging cracking and fine particulate AlN along the grain boundaries.
Prevent forging cracking along the grain of the aluminum nitride precipitation response is:

1) limit steel plus aluminum content, remove nitrogen or titanium added method to inhibit the precipitation of AlN in steel;

2) hot delivery ingot, too cold phase change process;

3) Raise the heat to send the temperature (> 900 ℃) forging direct heating;

4) before forging sufficient homogenization annealing, the precipitation of grain boundary phase diffusion.



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