Strategies for Preventing Part Distortion in Heat Treatment

Distortion: it’s an inevitable evil. Metallurgical transformation combined with thermal expansion, both natural consequences of heat treating, results in some level of distortion. If you work with parts that require heat treatment, you probably know that a lot of time and energy goes into preventing, minimizing, and even reversing distortion—and that having a collaborative partnership with your heat treating team (whether they’re in-house or outsourced) is key to reducing the impact of distortion on the functionality and performance of your parts. 

In a recent article, we discussed how you can calculate and predict the amount of distortion and what type of movement you’ll likely see in your parts based on geometry and quenching methods. In this article, we’ll dive a little deeper into all of the controllable factors behind distortion and the best strategies to mitigate and minimize it.

Design Considerations

Combatting distortion begins at the engineer’s desk when designing the part. Certain design characteristics are more prone to dramatic distortion that can cause cracking and compromise the integrity of the part. Understanding these factors is critical for any engineering team, however there’s typically very little from a geometry standpoint that an engineer can change because the application requires a specific shape or function. 

Here are ways to help minimize distortion when designing your part:

• Balance cross sections
• Make a part as symmetrical as possible
• Bridge features to help minimize movement

Heat Treating Process Adjustments:

When design elements can’t be changed, certain measures need to be taken in heat treating to help keep the part as proportional as possible, true to its intended design. The following are factors that our heat treater should be considering to limit distortion. 

Orientation and Loading

Strategic Orientation
The orientation of parts within the furnace can affect how uniform the heat distribution is across the part. It’s crucial to position parts so that they experience even heating and cooling to prevent warping or bending. For example, if you place two components flush against each other, the surfaces that are touching will experience slower heating and cooling rates than the exposed surfaces. This uneven temperature gradient can cause warping. It’s also important to load position parts in a way that makes them less likely to warp under their own weight during heat treatment while also limiting the amount of surface area that’s resting on heat treating fixtures.

Cycle Design

The recipe for your part is another factor that can be fine-tuned to minimize distortion. In our previous article on calculating distortion, we talk about how distortion is predictable based on the severity of the quenchant type. Each quenchant has its own distortion profile, and the severity of quench you need is largely determined by your material type (and how hardenable the material is). In general, parts made from highly hardenable materials need a less severe quenchant (such as vacuum) which will result in less distortion, and those with lower hardenability will require a more severe quenchant.

Oil Quench Temperature

Low hardenability materials indicate a higher speed quench, such as oil. But not all oils are created equal. Higher temperature oils will be slightly less severe and may result in less distortion. 

Pressure Quench

How high does the pressure need to be in a vacuum quench? The beauty of quenching in a vacuum is that pressure is applied equally from all directions which can equate to a more uniform distortion. You have to balance pressure along with gas velocity to create a more uniform quench.

Advanced Technology

Tapping into the most advanced technology and data is also critical. At Paulo, our furnaces are custom fit with additional thermal sensors for more accurate heat treating, ensuring that the furnace is holding at the desired temperature throughout the heating chamber. Maintaining an even temperature gradient in the atmosphere creates conditions that minimize distortion. 

Reversing Distortion

So your parts are distorted during heat treatment and are out of spec, now what? There are two ways to counteract the distortion that occurred during heat treatment. 

The Clamp & Temper Method
Following heat treatment, some components can be clamped and tempered to reverse some of the distortion that occurred. This works best for relatively thin, flat components. Cam phasers are a great example of this. Coming out of heat treatment, it’s not uncommon for there to be 0.025” of variance due to distortion. By using the clamp and temper method, it’s possible to bring them back into spec (often within 0.008”). 

Components that may be good candidates for clamping are flat parts under ¼“ thick and 10” in diameter (although these dimensions can vary). Some examples include:

• Cam phasers
• Automatic and manual transmission clutch components
• Tooling 
• Brackets
• Spacers

Additional Machining
Other parts that have extremely tight tolerances may need additional machining after heat treating—either hard turning or grinding. For parts where this is determined to be the best approach, It’s common for up to 0.08” per inch of material stock to be left on the part for heat treatment and is machined off after. This tends to be a very expensive strategy and is usually reserved for tooling, dies, and parts with extremely tight tolerances. 

Heat Treating You Can Trust

Minimizing distortion in heat treatment is a multifaceted challenge that involves a deep understanding of material science, part design, manufacturing processes, and thermal processing techniques. At Paulo, we combine this knowledge with a collaborative approach with your engineering team and cutting-edge technology to ensure that your parts meet the highest standards of quality and performance. 

By partnering with us, you leverage a team of experts dedicated to the precision and integrity of your critical components. Our commitment is not just to process parts but to deliver a level of assurance that comes from a legacy of excellence in thermal processing solutions.