Creep-resistant-steels:how to select and weld.SOLUTIONS with Effective, Practical Advice
According to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, normal carbon steels are considered suitable for service at temperatures not exceeding 345 0C (650 0F). At this maximum temperature, per the above Code, the maximum safe design stress shall not exceed 25% of the minimum specified Ultimate Tensile Strength of the steel selected. (Sponsored Links) ASME Specifications define the types and the properties of several steels, like the following SA 515, SA 516, SA 302, SA 202, and SA 537 for plates, each of them further defined according to Grades or Classes. It is well known that the mechanical properties of materials are modified as a function of temperature. Specifically the Ultimate Tensile Stress and the elastic limit decrease in various degree with increasing temperature depending on the type of steel. For service at elevated temperature the above properties are not the governing factors. In fact steels may be subject to excessive plastic deformation for stresses well below those limits if applied for long enough time. For use at elevated temperatures, besides the long time stability and properties of Creep-resistant-steels, one has to consider also the resistance to oxidation and scaling at the service atmosphere as well as the resistance to corrosive attacks. Furthermore, before finalizing the selection, one has to take into account fabricability, weldability, maximum run-off temperature, expected life of the construction and cost. Particular attention requires service in presence of hydrogen at elevated temperature and pressure. A short introduction to Heat Resistance was published in Section 2 of Issue 55 of our Practical Welding Letter for March 2008. Click on In practice to improve the properties of Creep-resistant-steels one modifies the metals by alloying them with specified small additions of elements like Chromium, Molybdenum, Vanadium, and other materials having definite influence on favorable modifications of metallurgical microstructure. The accepted criterion for evaluating different Creep-resistant-steels is the stress that, if applied continuously on specified specimens at the given temperatures for one hundred thousand hours (corresponding to slightly more than eleven years), will cause in that time period an elongation Creep-resistant-steels for high temperature service are those suitable for applications that require exposures between 345 and 815 0C (650 and 1500 0F). For each one of the approved materials the usual Code establishes the allowable design stress at service temperature. The following Table lists a few Creep-resistant-steels usually considered for use in the specified temperature ranges.
(1) + Ni, V, Nb, N, Al Excluding austenitic stainless steels the other Creep-resistant-steels listed above, in particular those containing Chromium and Molybdenum are all martensitic, that is they will harden upon quick cooling from elevated temperatures like those found in welding. The usual precaution, to avoid cracking as a consequence of shrinking stresses, is to apply preheating before welding to decrease the cooling rate, and then to relieve the stresses as soon as possible after welding especially for thick materials and/or for constrained structures. Filler metals are generally similar in composition to the base Creep-resistant-steels except if post weld heat treatment cannot be performed. In that case austenitic filler metals may be preferred to provide more ductility in the weld. While base metals Creep-resistant-steels may be quite acceptable for elevated temperature service, it must be noted that welded joints are a potential source of weaker behavior in creep due to their intrinsic lack of homogeneity and possibly to the presence of weld discontinuities. The usual practice, as established in codes for nuclear plant components, is to reduce even more the stresses admissible in weldments. A related subject, Risk Based Inspection, concerning the continuing assessment of safe working conditions of plants designed to operate for decades without failures, was treated in Section 2 of Issue 57 of Practical Welding Letter for May 2008. Click on Any questions or comments or feedback? Write them down and send them to us by e-mail.Click on the Contact Us button in the NavBar at top left of every page.
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