MFN Trainer Column
in Vol. 9 - January Issue - Year 2008
The Importance Of A Uniform Stress Profile
MFN Trainer Dan Spinner
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When most people think of peening to extend a component life, they think in terms of an arc height. The prevalent thought process says that an arc height translates into some level of surface compression, which in term translates into some predetermined life endpoint extension which is directly attributed to the depth of compressive stresses left in the component. Not much discussion or thought is ever heard about what has happened to any tensile stresses that may exist at or near the surface of the component that is to be peened. Again, conventional thought says that the tensile stresses are driven into the component beneath the layer of compression, and the compressive stresses will off-set any deleterious effects of these tensile stresses.
Recently, much more attention is being paid to the residual tensile stresses, particularly on welded components, as Stress Corrosion Cracking (SCC) is becoming an ever increasing problem. Peening of welds, either with needle guns or air hammers, has been the commonly accepted practice to "treat" welds; the belief being that a level of compression will offset the uneven tensile stresses that are formed in the heat affected zone (HAZ) due to non-uniform cooling of the weld. More progressive operations do use shot peening.
The problem occurs when corrosive agents are introduced to the welded component (salt water, CO2, chlorine, H2S). As the component wears, even minimally, the surface can experience an interruption (micro crack or tear) that allows the corrosive agent to start to migrate in an intergranular fashion and does not take long to approach or exceed the depth of compressive stresses, typically < 0.5mm. When the granular structure of the base metal is weakened in this manner, the underlying tensile stresses can easily over come the cohesive forces and causes a crack to initiate and then propagation is aided by the presence of these tensile stresses. This phenomenon is the reason that having a uniform stress profile is important.
One way to address this is the heat treat process used by people who manufacture thickwall pressure vessels. After welding, the weld is subjected to high heat for several hours to relieve the tensile stresses. Since these vessels are static in operation, fatigue is not typically a concern. When addressing transporting pipes in pipelines, process pipes,
down-hole instruments, and large storage tanks, compression in addition to stress relief is important due to the constant motion experienced in these operations.
A newer technology, Ultrasonic Impact Treatment (UIT) addresses the need for a uniform stress profile in one application process. The technology harnesses acoustical energy in conjunction with very low amplitude (10-30?) mechanical impact; the mechanical impact provides the requisite plastic deformation of the surface to impart compression, while the acoustical energy travels deep within the base metal relieving tensile stresses. This technology has the capabilities to relieve tensile stresses to depths of 15mm while imparting compressive stresses to depths of 2.5-3.0mm. There now exists a uniform stress profile of deep compressive stresses over a “neutral stress zone” instead of tensile stresses. This phenomenon created by UIT has shown increased resistance to SCC by factors of +4 X life when compared to shot peening and needle peening.
The most important fact to remember from this data is that simply putting compressive stresses on the surface of a component is not always the only factor to consider.
Author: Dan Spinner