Heat Treat Distortion

Preventative practices should be considered when designing stamped/flat stock parts in order to reduce distortion caused by heat treating and austempering.

First, let's review these two processes. Heat treating is a process that hardens annealed steel materials, giving the material temper. Based on the material thickness, hardness is usually specified using the 15N, 30N, and Rockwell C (RC) scales. Heat treated parts are usually quenched in oil that is slightly higher than room temperature.

Austempering is a process that utilizes a salt bath at a higher temperature (typically in the range of 600 degrees F depending on desired hardness) to quench parts. The advantage of the austempering process is that distortion is usually reduced due to the substantially higher salt bath quenching temperature. Both of these heat treat processes use time and temperature to alter the microstructure, and hence the mechanical properties of the steel.

There are three major considerations to remember when designing flat stock parts that require heat-treating or Austempering:

1. Die Cut Minimization - The layout of any cutouts and holes across the section should be as even as possible to equalize cooling rates. Also, avoid sharp edged slots, stamp marks or rough surface finishes which will act as stress concentrators and crack initiation sites.
2. Use Stiffening Ribs Whenever Possible - Stiffening ribs are additions to part geometry that help keep the part flat during the change of the microstructure of the material. Stiffening ribs should go with the grain of the material whenever possible.
3. Minimize the Slenderness Ratio - A part that is .032 x .850 x 6.00 will sag much more than a part that is .032 x .850 x 3.00. Sagging usually results in twisting or bowing in the part. There are additional measures that can be taken such as fixturing and jigging the part during these processes, however, usually it proves to be cost prohibitive.

By keeping these practices in mind during the design stages of flat metal parts, your processes will be much more consistent, and the distortion of your parts from heat treating will be minimized.

Please contact the Newcomb Spring facility nearest you for more information on heat treating and austempering distortion as well as other considerations of spring design. Newcomb Spring also offers a special technical guide on CD that includes an animated glossary and information on spring characteristics. To request one of our technical cds visit our Quick Contact page - please write that you are requesting a CD in the Notes field.