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Tempering Response of Mixed Martensitic/Bainitic Microstructures in Quench and Tempered Plate Steels
Başlık:
Tempering Response of Mixed Martensitic/Bainitic Microstructures in Quench and Tempered Plate Steels
Yazar:
Vieira, Igor, author.
ISBN:
9780355974317
Yazar Ek Girişi:
Fiziksel Tanımlama:
1 electronic resource (249 pages)
Genel Not:
Source: Dissertation Abstracts International, Volume: 79-10(E), Section: B.
Advisors: Emmanuel De Moor Committee members: Amy Clarke; Alexander Gysi; John Speer.
Özet:
High strength heavy gauge plate steels develop different cooling rates throughout the thickness during quenching. Several grades do not fully harden to martensite, resulting in mixed microstructures of martensite and bainite, which are subsequently tempered to achieve required properties. Therefore, comprehension of the tempering of bainitic and mixed microstructures is essential to help optimize tempering conditions for plate steels.
The tempering response of martensitic, bainitic, and mixed microstructures was examined with a focus on alloying effects induced by molybdenum (Mo), vanadium (V), chromium (Cr) and silicon (Si). Hardenability characterization was conducted through Jominy end quench testing and construction of continuous cooling transformation (CCT) diagrams. Increased alloying additions, in particular Mo and Cr, improved hardenability and hence increased the allowable thickness of fully hardened plate steels. Characterization was performed using scanning electron microscopy (SEM), dilatometry, Mössbauer spectroscopy, X-ray diffraction (XRD), electrical resistivity, Vickers micro-hardness, tensile and Charpy V-notch testing. A dilatometric analysis of non-isothermal tempering was proposed, which was able to evaluate tempering stages I through III, as well as characterize secondary hardening and Mn segregation to cementite. Application of a rule of mixtures approach using results for fully martensitic and bainitic tempered microstructures was able to predict the hardness of the mixed conditions in most cases. The presence of bainite in the initial microstructure yielded an improved tempering resistance, which was related to the lower driving force and number of nucleation sites for cementite precipitation. Secondary hardening was affected by isothermal bainitic transformation temperature and Cr and Si additions. The secondary hardening peak shifted to higher temperatures for a steel isothermally transformed at higher temperatures. Peak secondary hardening was postponed and less pronounced with Cr addition in martensitic and bainitic steels, respectively, which is believed associated with the segregation of Cr to cementite, retarding its dissolution. Silicon is responsible for a change in the secondary hardening mechanism in bainitic steels by reducing the extent of cementite precipitation, permitting solute carbon to be readily available. Thus, peak secondary hardening shifted to lower tempering temperatures in martensitic steels, and greater softening rates were observed in the bainitic conditions.
Notlar:
School code: 0052
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Yer Numarası | Demirbaş Numarası | Shelf Location | Lokasyon / Statüsü / İade Tarihi |
---|---|---|---|
XX(679485.1) | 679485-1001 | Proquest E-Tez Koleksiyonu | Arıyor... |
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