How Much Distortion to Expect in Heat Treating

Distortion is an inevitable truth in heat treating—one that can cause significant challenges when preparing parts for service. Maintaining dimensional accuracy is essential, especially for mission-critical and safety-critical components with the tightest tolerances. At Paulo, accurately predicting the causes and ways in which parts distort has become an important part of our process. 

The number one question we get from customers is, “How much distortion can I expect in my parts?” Predicting and limiting distortion is vital for ensuring the proper functionality of your parts and reducing the risk of cracking. 

To help shine light on this topic, we’ve created this quick guide on how distortion is calculated and predicted, along with key adjustments that can be made if you’re seeing too much distortion in your parts.

Understanding Distortion

Before we get into how distortion is calculated, it’s important to understand how distortion works and why it occurs. Distortion in heat treatment is a byproduct of materials’ physical and metallurgical transformations when exposed to controlled heating and cooling cycles.

Increased heat causes a part to expand. Reduced temperature will slow that expansion rate and eventually cause contraction. A part’s surface will cool much faster than the core, causing an uneven cooling rate. That also means the surface transforms into martensite first and hardens out while the inside of the part is still at a higher temperature where no transformation has occurred yet. This delayed internal transformation causes what’s referred to as a “balloon effect.” The core expands and pushes out on the hardened external surface, creating deformation.

Predicting Change in Size and Shape

Every part you heat treat will have some degree of “ballooning” and distortion, but there are ways to predict how much your parts will deform. Knowing how your parts will change during heat treatment allows you to account for that change in the design of the part and, in a perfect world, avoid an additional round of machining after heat treatment.

The charts below demonstrate how specific geometries will react to heat treatment, and how material selection, part geometry, heat treating process, and quench severity all play a role in how much your parts can change.

The Role of Quenching: Predicting How Much Movement Will Occur

Quench severity is the primary indicator of how much your part will deform since it’s the process that cools your part and causes the internal and external transformation.

The different quenching media—water, oil, and air, or gas—each have their own typical distortion profile. As seen in the chart below, oil is a more severe quenching solution and can cause the most distortion (up to 0.003 in/in), and vacuum is a less severe, highly controlled solution with a small window of variance (0.001–0.0015 in/in)

Parts made from highly hardenable materials can be quenched with a less severe quench media. Whereas parts using a lower hardenable material need a more severe quenchant. As a general rule, the lower quench severities result in lower overall distortion.

Addressing Distortion Concerns in Current Parts

For parts already experiencing unacceptable levels of distortion, a reevaluation is necessary. This may involve tweaking the part design, altering the material, adjusting the heat treatment cycle, or implementing post-heat treatment corrective measures.

The goal in heat treating should be to come out with a shape that’s as near net as possible, in other words, a part that won’t require additional machining to function properly. Working with an experienced heat treater is key to making product distortion predictable, so part design can be compensated for the movement that is likely to occur during heat treatment.

Partners in Minimizing Distortion

At Paulo, minimizing distortion is a collaborative effort between our metallurgy team and your engineering and product teams. Through datagineering, we employ data-driven insights and the latest metallurgical knowledge to help you refine your parts and prepare them for heat treatment.Ready to get started on your next project? Connect with a heat treating expert.