By David A. Sinquefield, President, MPD Welding-GR Inc.

It shouldn't be that way, but stress relieving is one of those functions that often gets set aside until a later date, or not at all in some shops. From A-2 die steels to welded fabrications, the effects of machining, grinding and welding cause residual stresses that are left in the steel.
 

Stress-relief treating can reduce distortion and high stresses from welding that can affect service performance. The presence of residual stresses can lead to stress-corrosion cracking near welds and in regions of components that have been cold strained during processing.
 

Various manufacturing processes such as forming, machining, heat treating, and plating leave their characteristic residual stress pattern implanted in the steel. These residual stresses can lead to undesirable dimensional changes, during the production process.
 

For example, about 20 years ago, when I was a young foreman, the company I worked for was called in to look at a large compression mold we had done some welding on. The customer was having trouble spotting the mold. They would have the spot perfect, and then run some parts for tryout. After the 4th or 5th part produced from this mold, the parts would start to flash. Somehow the customer was the thinking the welding was the cause of these dimensional changes taking place.
 

Upon investigation, we discovered the tool was never stress relieved. Stress relief heat-treating was recommended. After stress relief, the tool had twisted the entire length about .050". After re-squaring, re-machining, welding and reheat stress relieving, the block was now stable and producing consistently good parts.
 

Although this is the worst case I can recall, this same condition keeps appearing on a regular basis. All the rework could have been avoided if the tool was stress relieved while in a rough-cut block state.
The most common means of relieving these stresses is thermal stress relief, vibration stress relief and cryogenic stress relief. As an example, carbon steel heat stress relieving is accomplished by holding a temperature of 1000-1250 degrees F for one h/in. of thickness, which is a temperature below that of the steel's transition temperature. The steel is then cooled slowly.
 

The difference between stress-relief heat-treating and post-weld heat treating is that the goal of post-weld heat treating is to provide, in addition to the relief of residual stresses, some preferred metallurgical structure or properties.

Next we have vibration stress relief, which uses sub-harmonic frequencies to reduce any residual stresses. This technology induces a mechanical energy into the work piece by sending the frequencies through the metal and eliminating the stresses from within.

Different levels of induced energy will have different effects on metal. As with heat, vibration will produce an optimum energy level that will cause stress relief. When the energy reaches its maximum point it stabilizes with the natural harmonic frequency of the steel.
 

Cold treating of steel is widely accepted within the metallurgical profession as a supplemental treatment that can be used to enhance the transformation of austenite to martensite and to improve stress relief of castings and machined parts. In most instances, cold treating is not done before tempering. In several types of industrial applications, tempering is followed by deep freezing and re-tempering without delay. For example, such parts as gages, arbors, mandrills, and roller bearings are treated in this manner for dimensional stability. Such materials as tool steels are done to improve wear resistance.
 

The use of cold treating is widely used in the tool and die industries but is mainly restricted in its uses to smaller parts for economical reasons.
 

It has been claimed that all three methods are equally effective. In order to evaluate these claims, a technical committee was formed to design and make specimens, apply the stress treatments, measure the changes in residual stress state, and report the results. A project review was conducted during the November 1999 Heat Treat Society Conference and Expo. Results and information on the future of heat- treating can be viewed at the Heat Treating Society's website.
 

Whatever conclusions this study reveals, I believe all three have their own unique purpose in certain applications. One question I have is, if they are all equally effective, then why do P-20 mold blocks show signs of movement when heat stress relief is applied after vibration stress relief has already been applied? This movement is non-existent, when initial stress relief is thermally applied.
 

Hopefully, future tests and studies will provide more definite answers to the effects of the various heat treatment methods on mold and tool steels.