bhadeshia123 | Sigma-phase in alloys of iron: crystal and microstructure @bhadeshia123 | Uploaded January 2020 | Updated October 2024, 1 hour ago.
Sigma-phase is an intermetallic compound. It takes many forms: FeCr, FeMo, FeW, (Fe,Ni)(W,Mo) etc. with the ratio of iron to the other solute not exactly stoichiometric. It is very difficult to deform and so embrittles the steel when present in a coarse form. However, as fine particles, it does not embrittle, rather, it strengthens the alloy, especially when the service temperature is above ambient. Sigma-phase often nucleates at austenite-ferrite boundaries in duplex stainless steels.
phase-trans.msm.cam.ac.uk/2005/Fealloys.html

$Stress corrosion cracking and hydrogen embrittlement Dr Clayton Thomas presents at the Warwick Manufacturing Group Seminar organised by Prakash Srirangam. Stress corrosion cracking (SCC) and hydrogen embrittlement (HE) are part of a phenomena called environmentally assisted fracture, which causes the catastrophic failure of metals, at lower than expected loads. Hence over the years it has caused many engineering failures. Some of these failures will be discussed in this webinar, to illustrate how important it is to consider the phenomena when designing components and structures. The methods and standards used to prevent the problem will discussed, and show how they affect material selection and manufacturing. The similarities and differences between SCC and HE are considered and although many mechanisms of the phenomena are well understood, many are not and some examples of these will be discussed. SCC occurs in a range of materials and environments and some of the more common examples will be covered. Also, Hydrogen embrittlement covers a range of failures processes and some of these will be discussed. Hydrogen sulphide causes sulphide stress corrosion cracking, which is a major problem in the oil and gas industry. Hence it must be considered when designing equipment and it strongly influences both the material selection and fabrication and this will also be covered. About Clayton Thomas: Originally from South Wales and currently living in Barnsley, South Yorkshire. Clayton gained a degree in metallurgy at Imperial College, London, and then went to the University of Sheffield and attained a Masters degree in metallurgy (subject of masters thesis was the environmentally assisted fracture in a martensitic stainless steel) and a PhD, investigating offshore corrosion fatigue. He worked for three years at British Steel Technical (in Rotherham) as a metallurgical investigator and then corrosion test laboratory manager, and then joined Cameron (now called One Subsea) as corrosion engineer in 1990, who are a major supplier of oil and gas completion equipment. He eventually became senior metallurgist and worked on major topside and subsea projects, where he was responsible for the material selection and corrosion prevention for all topside and subsea projects and for the metallurgy and welding of equipment. Since 2000, he has have been the director of his own company, Lloyd-Thomas Consultancy Ltd, providing metallurgical support and training. He has continued to work in oil and gas as well as other engineering sectors, such as renewable energy, power engineering, aerospace and biomedical. He has carried out numerous failure investigations and have acted as an expert witness in litigation cases. He has also run over 300 metallurgy courses for companies and colleges on various aspects of metallurgy and an approved trainer for the AMRC, which is part of the University of Sheffield, as well as providing metallurgy training courses for the Institute of Materials, Minerals and Mining and the Engineering Training Institute of Australia.$

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Sigma-phase in alloys of iron: crystal and microstructure @bhadeshia123