Why do castings need secondary tempering?
2025-04-08
The secondary tempering of castings can be understood as giving metal parts a "deep massage" to make them more durable and more stable. We can use examples from daily life to understand this process:
1. What is secondary tempering?
Metaphor: Imagine you baked a cake and found that the middle was a little wet after the first baking, so you baked it again to make it more uniform. Similarly, castings (such as engine parts) may be "uneven" inside after the initial casting and quenching. Secondary tempering is to heat it again to adjust its performance.
2. Why do we need to reduce hardness?
Question: After the initial quenching, the casting may be as hard and brittle as glass and easy to crack.
Solution: Secondary tempering is like "relaxing muscles". It softens the metal slightly by heating, but still maintains sufficient strength. For example, if a kitchen knife is too hard, it is easy to break and break. After tempering, it is sharp and not easy to break.
Popular summary: hard but not brittle, hard and soft.
3. Why do we need to eliminate internal stress?
Source of internal stress: During casting, the metal cools and shrinks unevenly, and the inside pulls each other like a "tug of war", forming invisible "internal injuries".
Risk: If not handled, the parts may suddenly deform or even crack during subsequent processing or use, just like ice suddenly cracking.
Solution: Secondary tempering allows the metal to "breathe a sigh of relief" through heating and slow cooling, eliminating internal pulling and becoming more stable.
4. Why is "secondary" instead of one necessary?
The first tempering may not be enough: the initial tempering may only solve part of the problem, and the residual stress or hardness may still be unsatisfactory.
The role of secondary tempering: like checking the corners for the second time when tidying up the room, to ensure that the problem is thoroughly solved and the material performance is more balanced.
Benefits in practical applications
Durability: Parts are more impact-resistant (such as car chassis).
Safety: Avoid accidental breakage due to internal stress (such as bridge components).
Convenient processing: It is easier to cut or drill after the hardness is moderate.
Summary Metaphor
Imagine a casting as a hard cookie: the first baking (quenching) makes it brittle, but the inside may be burnt (internal stress); the second tempering is like lowering the temperature and baking it again, making the cookie uniform inside and outside, crispy but not broken, and safer to bite.
In this way, the second tempering in the industry ensures that metal parts are both strong and stable, avoiding the risks of "invisible internal injuries".
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