Due to the poor plasticity of superalloys, various cracks often occur during forging. In particular, ingots are more susceptible to cracking during forging due to the coarse columnar crystals.
1. The main causes of cracks are:
1) The content of harmful impurities is high. Lead, antimony, tin, antimony, arsenic and sulfur are all harmful impurities in high-temperature alloys. These elements have low melting point and are distributed on the grain boundaries in the alloy, which reduces the plasticity of the alloy;
2) Some elements in the alloy are relatively high in content, they form brittle compounds in the alloy and are distributed along the grain boundaries, which reduces the plasticity of the alloy;
3) The quality of the surface and interior of the ingot is poor, or there are some metallurgical defects in the bar (for example, inclusions, delamination, shrinkage, looseness, spot segregation, carbide accumulation, etc.), causing cracking during forging ;
4) When heating in a flame furnace, the sulfur content in the fuel and the furnace gas is too high, and the sulfur and nickel react to form a low-melting eutectic, which is distributed along the grain boundary and reduces the plasticity of the alloy;
5) The furnace temperature is too high, and the heating rate is too fast. Especially when heating the ingot and the blank with large cross-section size, the alloy has poor thermal conductivity and large temperature stress, which is easy to cause cracking;
6) The heating temperature is too high or the deformation temperature is too low;
7) The degree of deformation is too large or the deformation speed is too fast;
8) Improper deformation process, there is a large tensile stress and additional tensile stress.
2. To prevent cracking, the following countermeasures should be taken:
1) Raw materials should be inspected according to standards, and the content of harmful elements should be strictly controlled. When there are too many harmful elements (such as boron), the forging heating temperature can be appropriately reduced;
2) The ingot is heated by forging after being cleaned by suede or grinding wheel;
3) The furnace temperature and heating rate should be controlled during heating;
4) Avoid excessive sulphur in the fuel when heating in a flame furnace. At the same time, it should not be heated in a strong oxidizing medium to prevent oxygen from diffusing into the alloy and causing the plasticity of the alloy to decrease;
5) Pay attention to controlling the heating and deformation temperature;
6) When the ingot is lengthened, it should be lightly hit. When the as-cast microstructure is properly broken, the plasticity is increased, and then the deformation is increased. The total deformation per fire shall be controlled within the range of 30%-70% when lengthened. It shall not be combo in one place. It shall be spiral forging and shall be fed from the big head to the tail.
For alloy ingots and intermediate blanks with low plasticity, deformation processes such as plastic mats and upsets can be used.
7) The tooling should be preheated (preheating temperature is generally 150-350 °C), and good lubrication should be carried out for forging and die forging.