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Title: Cold work embrittlement in strip steels
Author: Lewis, S. G.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 1999
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Interstitial Free (IF) steels are highly formable, ultra low carbon/nitrogen strip grades containing titanium and/or niobium to remove interstitial elements from solution. They are typically used for severe forming operations such as oil sumps in cars. However, these steels typically have proof stresses of 150-170MPa. With the addition of solid solution strengthening elements, they can be used to form complex structural components for the automotive industry. Phosphorus is commonly used to provide solid solution strengthening, since small additions lead to a significant increase in strength. However, phosphorus additions have been identified as a potential cause of Cold Work Embrittlement (CWE). CWE is a phenomenon whereby a formed component fails in a brittle manner during a secondary working operation (e.g. impact or redraw). The problem has been attributed to the segregation of phosphorus to grain boundaries and has mainly been observed in laboratory impact tests on high phosphorus IF steels. The potential for CWE to occur has attracted world-wide concern from steelmakers and steelusers, but no in-service failures have been reported and the commercial significance of this phenomenon is still not clear. The work detailed in this thesis involves study of the techniques used to measure embrittlement and developing an understanding of the fracture mechanisms involved. The literature review details the metallurgy, production and processing of IF steels as well as a discussion of CWE. The experimental work includes a detailed evaluation of the current test techniques, including the effects of varying key test parameters on the susceptibility to brittle fracture. Work is also presented on compositional and processing effects, including the use of Atom Probe Field Ion Microscopy to study grain boundary segregation. Finally, a small sample test, developed for use in the power industries, is used to study CWE susceptibility.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (D.Eng.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available