Metal Stress and Fatigue Test Results Published
Phys.org recently published an article about the National Institute for Materials Science (NIMS) regarding their short- and long-term testing of a wide variety of structural materials manufactured in Japan to ensure they can withstand long-term stresses. NIMS scientists summarised the institute’s major findings in the journal Science and Technology of Advanced Materials.
According to the article, NIMS data revealed that scientists need to choose the type of analysis method for creep rupture data according to the type of material. How creep happens in materials during testing not only depends on the amount of stress applied, but also on the temperature conditions. Materials react differently to varying temperature depending on their chemical composition, the amounts of minor elements in them, and the crystal grain size. Ferritic heat-resistant steels, which are commonly used in thermal power plants, were found to have very long-term, inherent creep strength, depending on the amount of minor solutes present in the steel.
Fatigue limits, on the other hand, are affected by a metal’s tensile strength and hardness. NIMS scientists found that some metals can last for an incredibly long time without forming cracks as long as they are constantly exposed to room temperatures. These same metals, however, would eventually form cracks if exposed to the same stress but at high temperature.
Another important contributing factor to the mechanical integrity of steel products is corrosion. For example, sulfidic corrosion of steel piping is a major concern in the petroleum refining and petrochemical industry. When exposed to hydrocarbon containing sulphur compounds at elevated temperatures, carbon steels with low silicon content (<0.10%) can corrode at an accelerated rate. Sulfidation thins the pressure boundary wall and can result in a leak releasing highly hazardous chemicals to the atmosphere. API RP 939C Guidelines for Avoiding Sulfidation (Sulfidic) Corrosion Failures in Oil Refineries recognizes implementing Retrospective PMI into a Material Verification Program (per API RP 578) as an inspection method to detect and track sulfidation corrosion.
One study focusing on corrosion-related accidents in petroleum refineries in both the European Union (EU) and Organization for Economic Cooperation and Development (OECD) countries reports that the inadequacy of material composition was identified as the key component of failure in 9 of 99 significant refinery accidents. Other industry-reported data suggests a probability that as much as 3% of rogue material will make its way into the field as part of a final fabricated assembly, piping circuit, pressure vessel or other critical process equipment.
The addition of certain alloying elements is one way to improve corrosion resistance and prevent failure of finished steel products. X-ray fluorescence technology provides fast, accurate elemental analysis of the composition of steel during the manufacturing process to help ensure that the correct alloying elements are added in the correct amounts. Handheld XRF analysers are capable of distinguishing alloy grades that are nearly identical in composition to one another.
Furthermore, in-situ alloy steel material verification using handheld x-ray fluorescence (HHXRF) is an accurate, inexpensive, and non-destructive positive material identification (PMI) test method. HHXRF verifies that correctly purchased materials are received, confirms QA/QC for in-process fabrication as well as outgoing products, and helps ensure that the finished products match the engineering design and application for which they are intended.