What are common heat treatment defects?

Heat treatment processes are used to modify engineering properties so that structural components are able to withstand the specified operating conditions and result in the desired lifespan. Heat treatment involves the controlled heating and cooling of metals to bring about desired changes in the physical properties of the metal. Because of the extreme temperatures involved, the chemical and magnetic properties of the metal may also be altered in the process; for example, the shape could change, or materials such as glass could be produced. Heat treatment is a very important manufacturing process used to improve a product’s performance and characteristics in many different ways.

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A part is typically machined after heat treatment is complete. Care should be taken during heat treatment to achieve minimum distortion of the part geometry and characteristics, as this will help reduce machining costs. To achieve this outcome, proper care must be taken with heat treatment process parameters, including heating and cooling rates and carburization schedules. Such tune-ups are usually costly and time consuming, but necessary to avoid any defects.

Here are some common defects associated with the heat treatment process:

Decarburization

This term is typically used in metallurgy to describe the reduction of the content of carbon in metals (usually steel). Decarburization occurs when metal is heated to temperatures of 700°C (almost 1300°F) or above, when carbon in the metal reacts with gases containing oxygen or hydrogen. The removal of carbon removes hard carbide phases resulting in a softening of the metal. This occurs primarily at the surface, which is in contact with the decarburizing gas.

Decarburization can be either advantageous or detrimental, depending on the application for which the metal will be used. For this reason, it is something that can be done intentionally as a step in a manufacturing process, or something that happens as a side effect of a process (such as rolling) that must be either prevented or later reversed via a carburization step.

Uneven hardness

Uneven hardness refers to uneven hardness on the same surfaces. In this case, when machining is performed on harder surfaces, the machining will become more difficult. Harder areas can be caused by the formation of steam bubbles during cooling or as a result of an imperfect cleaning of the surface. To avoid this defect, stir the bath, use a more appropriate dipping method, or clean the parts thoroughly before heating.

Excessive surface hardness or brittleness

During hardening, a carburizing and embrittlement effect occurs and the surface cracks. Check the activity of the anti-decarburization medium (it must not be carburizing), the inoculants of the salt bath, and the protective atmospheres. Brittleness can be due to insufficient tempering or overheating. 

Deformations

Deformations can happen due to uneven heating, fast cooling, or when a part is incorrectly supported in the furnace. They can also occur due to incorrect dipping technique into the quenching bath or when there are stresses present before heat treatment. To avoid this before preheat, check the furnace capacity, make changes to step quenching, or reduce the hardening temperature. In addition, check the supports and, if possible, heat vertically. Review the method of dipping tools during cooling.

To learn more about heat treatment, check out our course on Heat Treatment Fundamentals.

 

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