M. Militzer The Centre for Metallurgical Process Engineering
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University of British Columbia
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Vancouver
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BC
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Canada V6T 1Z4
金属学报(英文版)
The microstructure evolution has been investigated for hot rolling of advanced low carbon steels containing Nb, Ti, V and Cu. The critical processing step to develop the properties of hot rolled steels is cooling after rolling when the austenite-to-ferrite transformation as well as precipitation takes place thereby determining the final mi- crostructure. Thus, the modelling work emphasizes the kinetics of errite formation. Ferrite growth rates can adequately be described by taking into account a solute-drag- like effect of Mn and Nb. The emphasis of the model is to predict the phase transfor- mation kinetics for the industrial practice of accelerated cooling. The ferrite grain size is essentially determined at the early stages of transformation and can be correlated to the trunsformation start temperature. Pcarbides and nitrides of Nb, Ti and V is controlled by Ostwald ripening of these particles. The ageing behaviour can then be described based on the Shercliff-Ashby model for precipitation hardening. The situa- tion is more complex for Cu precipitation where the ageing response is also related to a sequence of different precipitation types.
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low carbon steal
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A. Mitchell Advanced Materials Research Laboratory
,
University of British Columbia
,
Vancouver BC
,
Canada
金属学报(英文版)
In the pastfew years,severalnew melting processeshavebeenindustrialised forthe produc tion ofsuperalloys,titanium alloysand high qualitysteelsfor useintheaero engineindustry.Theseincludeelectron beam , plasma,inductionskull,andthe”triple melt”process( VIM + ESR+ VAR) . These developments have allbeen instituted in responsetothe major per ceived problem oftheindustry ;that oftheincidence of random defectsin the alloys whichcause problemsinregardto predictablelifing ofthefinished partinservice. Thedirectconse quenceofthe uncertaintyislossof revenue due to premature retirement of parts which mayhavecompleted on a fraction oftheir actualservicelife; a conservatism on the partof design parameters whichleadsto uneccesary weightin thepart;and ariskofeitherservicefailuresor partrecalls whichinterruptengine performance. Thebenefitsoftheprocesschangesinrespectoftheproducts’absolute propertiesand alsoonthereproducibility and inspectability ofthose properties have been substantial. Itisclearthattheprocesses offer the industry a solution to the present dilem ma of how to treatthe ”rare”defectsfrom the pointof view of fracture mechanicslifing methodology. The use ofprocesscontrols which guarantee”zero defect”productisdevelopingintoacriticaltoolfortheextension oftherangeof a given alloy’s use. Itislikelyto permit very substantial gainsinboth componentlife and intheallowablestressin rotating parts withoutthe needtointroduce new materials carrying new problems of user confidence, production reliability and data base. Weconcludethatthenew processesareatastageof development wherethey arereadyforwideindustry usein production. They will not add significantly tothecomponentcost andthey willgive a renwed lifespan to the more familiar materials and methodsof aero engineconstruction.
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