Causes and Effects of Distortion in Heat Treatment

15 September 2020

Heat treatment is a type of metalworking process that is utilised to change and alter the physical and even the chemical properties of a specific metal material. This process typically deals with heating or chilling material to extreme temperatures so that the same material can be either hardened or softened. Numerous factors such as strength, hardness, toughness, machinability, formability, ductility, and elasticity can all be affected by the heat treatment process.

To date, there are various techniques that industries maximise to get their desired products. These techniques include annealing, hardening, case hardening, normalising, tempering, precipitation strengthening, and quenching. While these techniques often promote the transformation of material, there are instances where the heat treatment process can cause distortion.

Types of Distortion

Heat treatment can cause two types of distortion. One type of distortion is dimensional distortion. This type of distortion occurs whenever the metal material changes volume whenever its crystal structure changes. Whenever the heated parts are subjected under one heat treatment technique, their internal crystal structure changes. Subsequently, the processed parts will be tempered, resulting in a change of volume that is insufficient to offset all the changes before heating and transformations.

Another type of distortion is shape distortion or warpage. This type of distortion is primarily caused by either processing or design issues. Some of the reasons why a metal material warp during heat treatment include rapid heating, overheating, non-uniform heating, cooling, or agitation, oil contamination, huge mass and section change, and asymmetric characteristics.

Causes of Distortion

Distortion generally can be caused by various factors. A material that is placed under a heat treatment process may have surface tearing or burnishing, which then creates stress risers on the part. Excessive stress from machining, particularly from drilling and milling, can likewise distort the material. If there are holes, slots, or varying sections on a part, then these specific areas will most likely quench faster and cause differential part cooling.

Overheating of a material due to heat treatment can lower its mechanical properties, which promotes the sagging or creeping of parts depending on the furnace orientation. Additionally, failure to normalise the material and improper annealing can both lead to its distortion. Excessive case depth of case hardening can also cause material distortion since deeper case depths increase the chance of causing the warpage.

There are three major elements that are considered to be the defining factor for distortion and all the causes mentioned above. These elements are part design or geometry, metal production method, and the overall quality of the material.

Effects of Distortion

Distortion of materials can cause numerous effects. The outside or inner diameter of the material grows, which requires it to be subjected under finishing operation. The length of a material may also shrink or grow, depending on its part geometry. Additionally, materials with long parts that are distorted will bend unevenly. And as the distortion occurs, materials with cross holes and slots may shrink and cause stress riser. The concentricity of the material likewise gets lost as its thin tubing adopts and oval shape.

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