Factors Causing Nut Deformation During Heat Treatment

There are numerous factors leading to nut deformation in heat treatment. Based on years of fastener production experience, the main causes can be summarized into three core categories:

1. Volume Change Caused by Solid-State Phase Transformation

During heating and cooling processes, steel nuts undergo solid-state phase transformation. Different metallographic structures inside the steel have different specific volumes, which lead to overall volume changes of nuts. This further causes dimensional expansion or shrinkage, resulting in out-of-tolerance of key dimensions such as thread bore diameter and across-flat dimension of nuts.

2. Thermal Stress

Thermal stress includes thermal stress generated by rapid heating and rapid cooling. Obvious temperature difference exists between the surface and core of nuts during sharp temperature rise or drop, producing uneven thermal stress. When the thermal stress exceeds the yield strength of nut material at the corresponding temperature, irreversible plastic deformation will occur, causing geometric distortion such as overall skew, end face warpage and hole offset.

3. Structural Stress

Metallographic structures inside steel transform sequentially in the cooling process. Asynchronous phase transformation produces uneven structural stress, which aggravates overall deformation and local distortion of nuts and damages the original geometric precision.

Accuracy Analysis of Nuts after Heat Treatment

Nut deformation caused by metallographic phase transformation is inevitable during heat treatment. Taking the commonly used 6H internal thread precision as an example: if nuts meet full thread precision requirements and pass go/no-go gauge inspection before heat treatment, some products with thread dimensions close to the tolerance limit will exceed the tolerance range after heat treatment.

The proportion of unqualified products varies depending on the tolerance bandwidth of threads, the distribution position of thread dimensions within the tolerance zone, as well as the stability of heat treatment specifications and process control.