The Assessment of Satoh Test as a Means to Understand Stress Relaxation Cracking Susceptibility
Industry needs an inexpensive means to understand the susceptibility of alloys to stress relaxation cracking (SRC) across all major alloy families (austenitic, ferritic and nickel-base) in the generation industry. SRC has been documented across a wide range of large power plant components such as thick-section circumferential welds, high-restraint attachment welds, and in bend sections of austenitic stainless steels. There is no standardized test or current industry practice to assess the SRC susceptibility of a given material. Attempts to simulate SRC are normally very expensive and invlove complicated test setups. A major driving force behind SRC is the accumulation of residual stresses during welding and/or fabrication techniques. Since residual stresses play a key role in the onset of SRC, it may be possible to screen alloys for susceptibility to SRC by assessing their accumulation of residual stresses during heating and cooling cycles.
One test, called the “Satoh Test”, involves the cooling of a rigidly constrained tensile specimen while monitoring the stress that accumulates. The specimen is free to expand during heating untila desired peak temperature is reached. It is then rigidly locked in place, and allowed to cool. Thermal contraction leads to the accumulation of stress, and depending on the material, stress is relieved ue to a phase transformation.
The Satoh Test has not been utilized in industry as a screening test, but may quickly and cost-effectively provide critical information on a given alloy’s susceptibility to SRC. This is extremely important as the duplication of SRC in simple tests has not been readily demonstrated to date.
Preliminary tests have been conducted with various alloys, and the results can be seen in the figure to the right. This graph shows the stress that accumulates, as a function of temperature, during the cooling cycle of a tested specimen for various alloys. These measurements were acquired in the Gleeble thermo-mechanical simulator. For alloys P91, T24, and Grade 12, the reduction of stress during the test is attributed to transformation of austenite to martensite/bainite and the associated increase in volume. After the transformation is complete, the stress begins to accumulate until ambient temperature is reached.
Work is in progress to determine if this test technique can assess alloys for susceptibility to SRC prior to implementation. This would have a rather profound impact in the selection of alloys for thick-section pressure vessels, high restraint applications and when determining the appropriate approach for weld repair of existing plant assets. The information obtained from the residual stress measurements will also be useful in examining a material’s behavior to fabrication processes like welding or forming.
Sponsor: Electrical Power Research Institute
Graduate Student: Jonathan Galler (PhD)
Collaborators: Special Metals Corporation