Appearance Inspection Method of Ductile Cast Iron
Ductile cast iron is a high-strength cast iron material whose comprehensive properties are close to steel. Based on its excellent properties, it has been successfully used to cast some parts with complex forces and high requirements for strength, toughness and wear resistance. Ductile cast iron has rapidly developed into a cast iron material second only to gray cast iron and widely used. The so-called “substituting iron for steel” mainly refers to ductile iron.
Generally, the technical requirements of ductile iron castings mainly include: mechanical properties, geometric shapes and dimensions, surface quality and defects.
The appearance of ductile iron parts is not allowed to exist the following defects:
1. Macroscopic defects such as pores.
2. Sand inclusion.
3. Slag inclusion.
4. Porosity or shrinkage.
5. Microscopic defects such as poor spheroidization and excessive crystal grains.
Ductile cast iron parts should not be too dry or too wet. Too dry will cause the mold to fail to fall off. And too wet to easily fall off. After the molding sand of ductile iron parts is molded, the internal surface should be dried with a coating agent. So as to prevent the molten iron from impacting the sand mold and causing the sand, and mud to enter the inside of the cast iron. The method of drying the coating agent for ductile iron parts is generally igniting and drying. Because the coating agent is containing alcohol, cold iron is added to some sand molds. The role of cold iron is to accelerate cooling and reduce shrinkage.
The inspection of ductile iron parts includes appearance inspection, magnetic particle inspection and ultrasonic inspection.The appearance inspection is a heavy work in ductile iron parts, which requires a lot of grinding cooperation.
Generally speaking, the appearance inspection requirements of ductile iron parts should meet the following points:
1. No cracks, no welding, no surface non-metallic inclusions and sanding.
2. Surface cleanliness reaches Sa 2.5.
3. Surface roughness reaches A2.
4. Pores reaches C2.
5. Cold barrier reaches D1.
6. No mechanical scratches.
In the appearance inspection of ductile iron parts, we should pay special attention to the difference between surface pores and surface blisters.
Generally, surface pores are smooth and regular on the inner wall. While the surface blisters are relatively irregular and contain more dust or sand and other nonmetallic impurity. If the surface recesses of ductile iron castings are pores, it can be compared with the SCRATA test block. If it is judged to be the surface blisters, they generally need to be polished and repaired, because the roots of most blisters will extend into the metal. When the shot peening effect of ductile iron parts is not good, the roughness will not meet the requirements. If there are oxide scales on the surface of ductile iron parts and large-area surface defects that are difficult to polish, such as smears, re-shot blasting or polish should be considered. Because this type of defect will affect the appearance inspection, and it is particularly easy to miss the inspection of the surface sand.
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