BY ANDREW CULLISON
Structural designers must be aware of residual stress in weldments and common methods used to relieve it
Stress -- the sources are numerous, but fortunately there are a variety of ways to relieve it. No, I'm not talking about the hectic pace of our personal lives, although it certainly can apply, but instead I'm referring to the fabrication of metals.
Residual stress is an internal stress that is not a result of externally applied loads. If stress buildup in the weldment is excessive, the fatigue life of the metal is reduced.
Importance of Stress Relief
Cold working, hot rolling, grinding, quenching treatments, welding, and thermal cutting all can induce residual stress into metal. The nature of residual stress, its distribution, and prediction of the level within a metal is a complex and not completely understood phenomenon, but you can be sure it is present.
Welding, in particular, because of the rapid thermal expansion and contraction created along a very localized area, is a prime source of residual stress. A very high heat source is applied to a small area relative to the cooler surrounding area. That point where the arc is directed is rapidly heated from ambient temperature to temperatures that can be in excess of 3000°F. The metal expands as it is brought to a molten state. As the molten weld pool solidifies along the joint, there is resistance to its shrinkage by the already solidified weld metal and the unmelted base metal adjacent to the weld. This resistance creates a tensile strain in the longitudinal and transverse directions of the weld. Distortion is often the result, and if the stress is excessive, buckling, stress corrosion cracking, and shortened fatigue life are possible.
All welds will have some residual stress, and it will never be totally reduced to zero strain. But the level of stress can be very high depending on certain conditions. Heat input, base metal thickness, cooling rate, restraint of the weldment, and welding process play roles in the level of residual stress induced into a weldment.
Thermal or Nonthermal
There are two major approaches to stress relieving: thermal and mechanical. A major difference between the two is thermal treatment, which in addition to relieving stress, will also effect metallugical changes in the metal. A postweld heat treatment entails uniform heating of the weldment, holding at temperature, and then a controlled cooling.
As metal becomes hotter, it becomes weaker. Once a certain temperature is reached, there is a reduction in yield strength from the rated property of the steel. The residual stress decreases to that of the lower yield strength, and it is thereby relieved. The effect sometimes visibly manifests itself in the straightening of a distorted material.
For carbon and low-alloy steels, stress relieving is commonly performed in the range of 1100° to 1350°F. The time at which the weldment is held at temperature is dependent on the thickness of the workpiece and its chemical composition.
The most commonly used method of stress relieving weldments is by postweld heat treatment. Its effectiveness is dependent on the control exercised in bringing the component to temperature and then its subsequent cooling. Therefore, it should be performed by those knowledgeable in its application.
Shot peening is a cold working method that reduces stress. Small, round metal balls, or shot, are projected onto the surface of the weldment. The shot imparts small indentations into the surface, which induces compressive stress. The tensile residual stresses at the surface of the weldment must "overcome" the compressive stress for a fatigue crack to initiate. If properly applied, the compression works to counteract the tensile stresses. Fatigue cracks have a low probability of developing in the shot-peened area.
Caution must be taken to ensure the shot-peening operation is performed with knowledge of its variables. There are three important variables to control in its application: surface compressive stress, maximum compressive stress, and depth of compressive stress. The velocity of the shot is another controlling factor. If the impingement of the shot is too deep, detrimental stresses may be induced, negating the desired results.
Vibratory Stress Relief
Another mechanical means of stress relief is by vibration. A mechanical vibrating device is attached to the weldment. The vibration's resonant frequency can be controlled by specially designed machines. The amount of time the weldment is subjected to the vibration is usually dependent on its weight. The vibration can be applied during or immediately after welding. The vibration seems to even the stress distribution within the weldment by means of plastic deformation of the metal's grains, reducing sharp peaks of stress.
This process is known to be effective in bringing geometric stability to the workpiece. Presently, the process is not endorsed by codes, but neither is it rejected.