Metal Heat Treating: How to Avoid Cracks

Metal heat treating is a useful but extreme process for automobile, airplane and other parts that undertake a heavy load. Cracking is one significant issue that may arise when rapidly heating and cooling steel to change its chemical and mechanical properties. Cracks can render a part unusable.

The best time to plan for potential part cracking risks is not during the induction heat treating process itself – but when the part is being designed. The Zion team is often consulted during the part design phase so that part design, material selection and desired heat treatment depth all produce the optimal outcome.

Induction heat treating allows an area of a part to be heated by electric current with a copper coil. The heating is isolated to a specific part section, leaving other areas of the part unaffected. After the region is heated to the correct temperature, it is quenched to achieve a desired hardness.

Material

The first thing to consider when designing a part for heat treating is that some materials are more prone to cracking than others.

Certain steel grades, such as low carbons or alloys, will not get very hard but are also the least likely to crack. Materials with higher hardenability often exhibit a higher likelihood to initiate cracks.

At the design level, it is imperative to select a metal that can give you the hardness level necessary and provide a good degree of strength and ductility or flexibility to withstand the requirements for the application and the induction heat process without being compromised.

Crack Initiation Zones

During the design process, remember that certain areas of a part are more likely to lead to cracks than others. Part components like sharp edges, holes, grooves, slots, shoulders and corners are considered crack initiation zones because the heating and cooling rates are much higher or different than the surrounding areas. Cracking can occur because of the excess stress of uneven heating or quenching.

In the design stage, it is best to consider adding these part design elements after the heat treatment has already taken place to help lessen the chances of cracking. While machining the part after induction heat treatment can add to the project cost, evenly heating and quenching the part will be significantly simpler and will reduce the likelihood cracking and distortion.
If a design element must be added before treatment, proper engineering practices, like radiusing or chamfering of a sharp edge/corner, will work to mitigate cracking issues.

Other situations may call for heat sink material to be placed in the hole or slots. This material will help dampen the volatility of quenching, but the hardening will not be as uniform in those areas. Some parts must be induction heat treated in multiple zones and proper tempering between each treatment may be a necessity, especially if the heated areas are close together.

Treatment Depth

Treatment depth is the last aspect of part design that should be considered to avoid cracking.
A material should not be heated and hardened at a deeper depth than needed for an application. As the depth of desired hardening increases, so does the amount of heat introduced into the part and the likelihood of a cracking issue.
In general, treatment to increase wear will be a thinner case depth than a treatment to increase strength.
Do you have parts that you’ve struggled to heat treat properly? As specialists in induction heat treating, the team at Zion Industries can find the right solution for you. Contact us or call 330-483-4650 to learn more.