Significant micro structural alterations due to long time expositions at high temperatures that can result in imperfections of metallurgic origin, include: 1-Graphitisation, sigma phase, annealing embrittlement. 2-Grain growth, Graphitisation and mechanical fatigue. 3-Hardening, cavitations and embrittlement. 4-Sensitisation, graphitisation and erosion Which would be the correct answer for the above statement? Forum contributions would be appreciated Luis Marques

Luis High temperature related effects or damages in materials exhibit different micro-structural changes. Each material exhibits its own inherent metallurgical combined with mechanical effect transformation depending on the service temperature, service duty and hours of exposure in the particular environment. 1. Answering to your question 1: Graphitization is related to materials like C..S or C.I grades. Sigma - is related to both ferrtic and austenitic grade materials. Embrittlement - is exhibited from low temperature to high temperature. For Ex: Carbon steel will exhibit embrittlement due to hydrogen entry even at room temperature. The same mateirial exhibits high temperature hydrogen embrittelment when exposed to H2-N2 mixed synthesis gas at higher pressure and temperature. Temper embrittlment is another phenomenon exhibited by Cr-Mo alloy steels when exposed at moderate temperature and moderate pressure service duties (including refinery service or ammonia plant operations). Hardening - happens for different materials depending on the service duty and service temp / pressure. Hardening due to nitriding effect in Cr-Mo steel or even in Austenitic S.S materials at specific temperature range and material becoming brittle is a known cause. Cavity is a creep related damage in high temperature materials . Mechanical fatigue - can be both ambient temperature related or service exposure related. This depends on cyclic operation duty (for ex: pumps, compressor) or even for static high pressure equipment (like static equipment like high pressure Reactors) where nozzles in high temperature / high pressure static equipment like Converters / Reactos are subject to low cycle fatigue (which is both temperature and mechanical related) Embrittlement: can happen due to liquid-metal, chagnes in micro structure due to hardening (internal bore carburization, nitriding, oxidation), penetration due to hydrogen in hard zonesand making the material brittle etc Sensitization is related to both exposure or even without exposure for a long time. Even normal air cooling can make the material become sensitized due to transformation taking plac while cooling (For EX: S.S 304 weld getting sensitized even while cooling after welding is performed). Erosion : is due to fluid or gas velocity related effect . If you have specific questions on the particular material, it would help to understand your question and answer. For a general question on a vast material usage in a refierny or petro chemical complex plat and the vast changes that can happen to the same material under different exposure and environmetn conditions the forum cannot give specific answer without knwing details. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd India Jan 10, 2006 E-mail:nishi@vsnl.com >Significant micro structural alterations due to long time >expositions at high temperatures that can result in >imperfections of metallurgic origin, include: > >1-Graphitisation, sigma phase, annealing embrittlement. > >2-Grain growth, Graphitisation and mechanical fatigue. > >3-Hardening, cavitations and embrittlement. > >4-Sensitisation, graphitisation and erosion > >Which would be the correct answer for the above statement? > >Forum contributions would be appreciated > >Luis Marques