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Why add intermediate heat treatment processes in the processing of thin-walled or complex parts?

In mechanical manufacturing, a reasonable process is crucial for producing qualified products. Therefore, before production, we usually analyze the product to develop a reasonable process. This requires experienced engineers to consider materials, dimensions, deformation, stress, and other aspects. Today, we will discuss the machining techniques for thin-walled parts.

Why does the industry insist on "face first, hole later" in terms of processing sequence? The process flow is often not continuous, but is forcibly cut off by several "heat treatments". This seemingly disruptive approach to production rhythm is essentially aimed at eliminating the "internal stress memory" of metal materials, ensuring that the parts will not deform for several years after processing is completed.

1. The "release effect" of residual stress: After casting, forging, or rough machining, the internal structure of metal blanks is not static, but contains huge and unbalanced residual stresses. That's why many machine tool castings are placed outdoors for several years after casting before production. This is to completely release the stress of the metal through aging.

Physical mechanism: During rough machining, a large cutting amount will forcibly cut the metal fibers and inject huge cutting heat. It's like pulling countless small 'springs' inside the parts.

Deformation risk: If precision machining is directly carried out, as the surface material is cut off, the originally balanced stress will be broken. The parts will slowly twist like "rebound marks" on or off the machine tool.

Countermeasure: Increase intermediate heat treatment (such as stress relief annealing), which involves heating metal atoms to gain energy and rearrange them, releasing these "springs" in advance to lock the geometric shape of the part.

For thin-walled parts with a wall thickness of only 1-2 millimeters, the material's resistance to internal stress is extremely weak.

Section modulus: The structural rigidity of thin-walled parts is very low. Minor changes in internal stress can manifest macroscopically as significant warping or roundness deviation.

Process cycle: The typical process is "rough machining - stress relief treatment - semi precision machining - aging treatment - precision machining". Each heat treatment is clearing out the 'interference items' for the final step of precision machining.

Just like when we carry out stainless steel stretching process, we must continuously anneal the product to release stress, so as not to break the material.

3. Ensure "dimensional stability" during service. Some parts may be qualified when they first leave the factory, but after being placed for six months, their dimensions are found to have changed.

Creep phenomenon: If the processing stress is not completely eliminated in the process flow, the metal structure will slowly undergo displacement at room temperature (microscopic creep).

Consequence: This "delayed failure" is fatal for precision spindles or aerospace sensors. Intermediate heat treatment simulates long-term natural aging through manual means, allowing parts to reach a stable "retirement state" in advance during the processing stage.

4. Improve the machinability of materials. Some difficult to machine materials (such as high-temperature alloys and titanium alloys) may undergo "work hardening" after severe cutting.

Organizational evolution: The surface layer will become abnormally hard and brittle, leading to increased tool wear and even microcracks during subsequent precision machining.

Logic: Through intermediate heat treatment, the plasticity and toughness of the material can be restored, making the cutting force during precision machining smoother and achieving a mirror level surface finish.

Stress is a common problem encountered in mechanical manufacturing, and we must pay sufficient attention to it. Failure caused by unknown stress in many products will have serious consequences.