Stress Relieving

The "Quench" Period

These residual stresses are created during the quench period of solution heat treating. The quench period is required, and rapidly removes heat from the part to "freeze" the molecules in position — done by plunging the very hot metal into a colder water bath. This has the side effect of inducing locked-in residual stresses. Not much can be done to avoid these stresses in the first place without reducing the part's strength, so we're basically left with one method of stress relieving for hand forgings: compression stress relieving.

Elastic vs. Plastic Behavior

Without introducing too many additional concepts, it's worth mentioning two metallic behaviors — elastic and plastic — that are easy to confuse. We use both words in everyday life, but they mean something very specific in metallic behavior. Elastic behavior means that when a part is subjected to a force (push, pull, or squeeze) and the force is removed, it returns exactly to its original shape. Plastic behavior differs — the material does not return to its original shape after the force is applied.

Springs are another good example of elastic behavior — they always return to the same position unless overloaded. When you apply a force to a material, it first undergoes elastic (spring-like) deformation; go beyond that point, and it turns into plastic, permanently deformed behavior.

How the Stresses Cause Distortion

From the heat-treat operation, the residual stresses locked into the part exist because they were created in the elastic stress state — like thousands of tiny, invisible springs locked into the surface layers. When the part is machined and material is removed, a lot of the elastic stress is removed along with it, but the problem comes from the remaining material that isn't removed. Because the part isn't symmetrical, machining removes more of these "springs" from one side than the other.

The unbalanced elastic forces act like a wedge and physically cause the part to move once it's released from the holding fixtures. The part, wanting to return to a balanced stress state, moves elastically to a new shape — what's called distortion.

Compression Stress Relieving

To eliminate the offending elastic behavior and reduce residual stresses, a plastic operation must be applied to the metal soon after solution heat treating. This is what we do at American Handforge: compression stress relieving is that plastic operation. The room-temperature hand forging is returned to the forge press after solution heat treating and placed between two flat, narrow dies. The press plastically reduces the thickness slightly — by 1 to 5 percent — in increments along the entire length of the part. The compression effectively "breaks up" all the little springs, and they simply disappear.

At American Handforge, we carefully control every aspect of this operation to yield a consistent, low-residual-stress forging suitable for precision machining and dimensional stability. We compress the forging just enough to achieve the required plastic metal flow, but not so much that it introduces other undesirable effects. It sounds simple, but as with any metal process, control and precision are essential.

Stress-Relieved Temper Codes

Following compression stress relieving, the hand forging is artificially aged to its final temper. The Aluminum Association has designated the compression stress-relieved tempers by adding "52" to the end of the basic temper — -T652, -T7452, and -T352 are examples of this temper code.