MIC AND THE WELDING INDUSTRY

• Wood pulp digesters
• De-aerators
• Liquid petroleum gas vessels
• MEA tanks
• Ammonia spheres
• Steam generator tubes
• Bridges and welded structures generally
• Offshore structures

Go to: Fatigue of structures | Stress corrosion cracking | Controlled shot peening - the process

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Cracks can form in welded structures where the combination of applied load and tensile residual stress are greater than the ultimate tensile stress of the material. The induced residual compressive stress will be beneficial in reducing the mean stress at the structure’s surface, hence the advantages gained beyond just stress relieving a structure.

One characteristic of fatigue is the random or unpredictable nature of its occurrence. This is often the result of variability in the welding process and stress concentrations at the weld toe and heat affected zone. Significant improvements in fatigue life can be obtained by modifying the residual stress levels in the material at these vulnerable areas through the application of controlled shot peening or heat treatment, although the best performance is a combination of both.

The figure below show the advantage of using controlled shot peening and heat treatment, also the combination of both:

The grinding of welds and toes in particular to improve profile and remove weld debris needs to be carefully controlled as tensile stresses induced by grinding can negate any benefits of stress relief if performed after heat treatment.

Go to: Applications | Stress corrosion cracking | Controlled shot peening - the process

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Stress corrosion cracking (SCC) is the unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment. SCC is chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments but the combination of tensile stress is key. The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal otherwise. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than pure metals. The specific environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and unexpected failure.^[1]

The critical combination of tensile stresses is necessary for this failure mechanism to occur and can be the result of the crevice loads due to a stress concentration (weld toe), or can be caused by the type of assembly or residual stresses from fabrication (e.g. grinding).

Where stress corrosion cracking is concerned, subsequent controlled shot peening should be considered to overcome those tensile stresses from residual or applied situations.

Go to: Applications | Fatigue of structures | Controlled shot peening - the process

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Controlled Peening is the bombardment of a surface with high quality spherical media called shot in a technically defined and controlled way.

Each piece of shot striking the metal acts as a peening hammer imparting an indentation or dimple into the surface.  The action of impinging the surface yields the material in tension and shear, further movement is restrained by the core material and a surface residual compressive stress results. The magnitude of the compressive stress is directly related to the yield strength of the base material and is approximately equal to 80% of that value in compression.

The media used can be steel, stainless steel, ceramic or glass and can vary in size from 50 micron to 3mm in diameter. A range of process controls have been developed over the years to ensure that the depth and magnitude of stress is fully repeatable. Indeed it has been proven many times that it is critical that any peening technique is applied in a fully mechanised manner removing the inconsistencies introduce by manual processing. This is particularly critical when applied to large welded structures as the surface and/or weld profile can influence the benefits gained.

COVERAGE
Poor or partial coverage of the area, particularly in manual processing may result in areas of lower magnitude compressive stress;perhaps none at all or as the original surface in high tension. We have developed fluorescent traces or dyes specifically formulated to aid visual inspection of the surface, to ensure the correct coverage has been achieved.

Go to: Applications | Fatigue of structures | Stress corrosion cracking

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