in Vol. 11 - September Issue - Year 2010
Eliminating Stress Corrosion Cracking in Stainless Steels
Schematic of LPB process
Fluorescent dye penetrant of a TIG weld on 316L stainless steel shows SCC arrested by LPB
LPB tool processing a nuclear waste closure weld
Stress corrosion cracking (SCC) is one of the most serious maintenance problems facing the power generation industry today. Material degradation due to SCC has cost the U.S. nuclear industry alone, over 10 billion dollars in the last thirty years and can drastically shorten the design life of an otherwise fully capable facility. Studies have revealed that materials commonly used in power plants and waste containment vessels such as austenitic stainless steels, are susceptible to SCC given the right environment and conditions.
SCC is the result of a combination of three factors – a susceptible material, exposure to a corrosive environment, and tensile stresses above a threshold. Welding, fit-up, and machining of components often leave them in a state of high magnitude residual tension, creating an ideal environment for SCC initiation. This type of damage is a direct cause of increased inspection requirements and expensive component repairs and replacements.
Removing any one of the three factors responsible for SCC eliminates the possibility of SCC initiation. The conventional approach to controlling the problem has been to develop new alloys that are more resistant to corrosion. This is a costly process and requires a significant time investment while often achieving only marginal success.
Surface enhancement techniques such as needle peening, surface abrasion, and cavitation peening are proposed to impede SCC by inducing compressive residual stresses into the surface of materials. However, the depth of compression achieved is typically shallow and SCC still occurs. These operations also cause a considerable amount of cold working that can exceed 50%. High levels of cold work further increase the risk of SCC initiation and produce a thermally unstable residual stress state. Retention of beneficial residual compression is critical in power generation applications at the operating temperatures in boiling water reactors, steam turbines and pressurized water reactor systems.
Low Plasticity Burnishing
Lambda Technologies in Cincinnati, OH has developed a surface enhancement process called Low Plasticity Burnishing (LPB®) that can completely eliminate SCC. The process uses a single pass of a free rolling ball under a hydraulic normal force to plastically deform a thin layer of material at the surface of the component, imparting a deep, stable layer of compressive residual stress with controlled, low cold work. The compressive surface layer becomes resistant to a host of damage mechanisms, including foreign object damage, fretting, fatigue, pitting and SCC.
LPB uses a hydrostatic tool to float the ball during operation, keeping it out of contact with the retaining socket. This enables the ball to roll freely across components, even with changing topography, such as weld seams. It also ensures that the ball cannot scrape or drag the surface. LPB tools are run using basic CNC machines or robots and can easily be integrated into existing manufacturing and maintenance operations.
The compression derived from LPB is much deeper than shot peening and delivers a magnitude of compression up to the yield strength of the material. The depth of compression extends well beyond the surface where SCC initiates. The material is held in residual compression after processing. This removes tensile stress from the SCC equation and effectively eliminates the problem.
LPB has the added benefit of creating only a very small amount of cold work, nominally 3-5%, so that its mechanical properties and composition are not altered. This allows material to retain the beneficial residual compression even in high temperature applications. LPB also leaves the processed area with a smooth finish that facilitates NDT and reduces overall inspection time.
Testing was performed to evaluate the benefits of LPB processing in preventing SCC initiation in welded austenitic stainless steels commonly used in power plants. Each specimen was welded and then subsequently LPB processed on half of the welded surface. Weld simulation specimens were made using both 304L and 316L stainless steels. X-ray diffraction residual stress measurements confirmed that the LPB treated half of the specimens were in a state of deep residual compression while the untreated halves of the specimens were in a state of residual tension from the welding operation.
The weld simulation specimens were tested in boiling MgCl2 in accordance with ASTM standards. After exposure, the specimens were examined for cracking using optical microscopy and fluorescent dye penetrant. The as-welded sides of all specimens exhibited severe hoop and radial SCC cracks that completely penetrated through the ½ inch thickness. The LPB treated side of the specimens contained no SCC and retained 100% of the initial compression induced by the LPB process. The SCC cracks terminate at the LPB treated boundary. This is due to the deep layer of compressive stress from LPB.
After rigorous testing, LPB was selected by the U.S. Department of Energy as the preferred surface treatment to prevent SCC in the closure welds of nuclear waste containers at the Idaho National Labs facility. Spent fuel containers have a design life of 50,000 years. To achieve this, they need to be able to resist a corrosive, underground storage environment without leaking. LPB provided a permanent, inexpensive and easy to implement solution for treatment of the closure welds.
LPB applications are not limited to the power industry. The successful elimination of SCC in critical components has been shown to save millions in repair and replacement in turbines, aircraft structures, pipes and more. LPB is in production use for numerous aerospace, nuclear and medical applications, with new applications being developed constantly.
Through innovative technologies like LPB, it is possible to greatly extend the life of stainless steel components. Even in the face of the harshest working environments, like those found in power turbines, nuclear reactors, refineries, chemical plants, waste treatment centers and other corrosive locations, LPB processing can provide the needed improvement in stainless steel integrity to ensure safe, clean, and efficient operation of both new and aging facilities for many years to come.