The importance of control of thermal cracks of steel castings
Introduction
Thermal cracking occurs as a result of the uneven cooling of steel during the casting process. It is a common problem in the production of steel castings and requires special attention to its prevention. Properly controlling the cooling rate of the steel during casting can prevent thermal cracking and enhance the quality of the casting.
Definition
Thermal cracking occurs when one area of the steel cools faster than other areas of the same casting during the casting process. Thermal gradients created due to differences in the cooling rates lead to stresses that cause micro-fractures in the steel, leading to cracks in the resulting parts. In severe cases, the hardness of the steels can be reduced, leading to an increased risk of breakage.
Causes
The cooling process of the steel during casting can contribute to thermal cracking if not precisely controlled. This is due to the fact that the thermal gradients created in the steel result in compaction and shrinking of the material in the frozen (solidified) area, causing stress that can lead to thermal cracking.
Common Causes of Thermal Cracking in Steel Casting
1. Improper material density: A higher material density can cause internal heat gradients that can lead to thermal cracking. The higher the material density, the more extreme the thermal gradient and the more likely the steel is to crack.
2. Poor gating design and machining: Poorly designed and machined gating systems can cause uneven cooling of the steel and create internal heat gradients that can cause thermal cracking. Poorly machined gates can also cause the steel to flow in an unpredictable manner, resulting in an uneven cooling of the steel.
3. Poor castability and poor mixing of the alloy: Poorly designed and manufactured alloys can cause the steel to cool unevenly. Additionally, poor mixing of the alloy can lead to inconsistency in the melt and create areas of the casting that are not completely solidified.
4. Improper pouring and cooling rate: If the pouring rate is too fast or the cooling rate is too slow, it can create a thermal gradient within the steel that can cause thermal cracking.
Control of Thermal Cracking
When controlling the cooling rate, it is important to consider that the alloys used for casting can behave differently in terms of cooling rates. Certain materials may be more prone to thermal cracking than others due to their inherent properties. The casting process can be adjusted to control the cooling rate, such as increasing the wall thickness of the casting, using a higher pouring temperature, and using a slower pouring rate. Other techniques to prevent thermal cracking include increasing the size of the risers and expanding the feeders to ensure adequate flow.
The design of the gating system and the machining of the gates and runners is also crucial in controlling thermal cracking. A well-designed and properly machined gating system can ensure adequate and uniform cooling of the steel. The inclusion of air vents can also be beneficial in controlling thermal cracks as they allow air to escape and reduce the pressure within the hot spots of the steel.
Conclusion
Thermal cracking is a common phenomenon that can lead to weakened parts and can reduce the overall strength and quality of the steel casting. It is important to take precautions to prevent the occurrence of thermal cracking and the best way to do this is to carefully and deliberately control the cooling rate of the steel during casting. Additionally, proper gating and machining of the gates and runners should also be taken into account to reduce the risk of thermal cracking. Following these steps can ensure the production of high-quality steel castings with increased longevity and safety.
