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Influence of raw materials on forging quality

2019-10-10
The influence of raw materials on the quality of forgings. Good quality of raw materials is the prerequisite to ensure the quality of forgings. If there are defects in raw materials, it will affect the forming process of forgings and the final quality of forgings.
 
First, the chemical elements of the raw materials exceed the specified range or the content of impurity elements is too high, which will have a great impact on the forming and quality of the raw materials' forgings
 
Phosphorus (P) can be dissolved in ferrite, and its solid solution strengthening ability is very strong. When it is integrated into ferrite, the strength and hardness of steel are significantly increased, while the plasticity and toughness are significantly reduced.Phosphorus also has a great tendency to segregation.
 
The solubility of sulfur (S) in steel is very small. During the solidification process of steel ingot, sulfur gathers at the final solidification place, forming sulfide inclusion, which seriously affects the plasticity of steel.When steel is forged at 800-1200℃, due to the low plasticity of the eutectic crystal of iron sulfide at the grain boundary or melting, resulting in the forging cracking, this phenomenon is called thermal brittleness.This steel must be annealed and diffused for a long time above 1000℃.
 
Nitrogen (N) when the steel with higher nitrogen content cools rapidly at high temperature, the nitrogen in the ferrite is in a state of supraturation. Then, at room temperature or slightly higher temperature, the nitrogen will gradually precipitate out in the form of Fe4N, which makes the strength and hardness of the steel increase and the plasticity and toughness decrease greatly. This phenomenon is called aging brittleness.
 
The solubility of hydrogen (H) in steel decreases with the decreasing of temperature, when the hydrogen content is higher of ingot, fast cooling after forging, rolling, the precipitation from the solid solution hydrogen atoms too late to billet surface diffusion, and focused on the defect in steel form H2, produce considerable pressure, the pressure in the organization under the joint action of stress and thermal stress, cause hydrogen embrittlement, and appear tiny cracks, the so-called white dots.
 
Oxygen (O) has very little solubility in ferrite.The high melting point, hardness and brittleness of oxide will seriously affect the plasticity of steel and reduce the fatigue strength.Iron oxide also forms fusible communities with other inclusions and distributes at grain boundaries, resulting in hot brittleness of steel
 
Second, the raw material has shrinkage residual, subcutaneous foaming, serious carbide segregation, thick non-metallic inclusions and other defects, forging easy to cause cracks.The dendrite, serious loose, non-metallic inclusion, white point, oxide film, segregation zone and mixed metal in the raw material are easy to cause the performance of the forging to decline.The surface crack, folding, scarring and coarse crystal ring of the raw material are easy to cause the surface crack of the forging.