Hot Working Characteristics of AISI 321 in Comparison to AISI 304 Austenitic Stainless Steels

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dc.contributor.advisor Stumpf, Waldo E. en
dc.contributor.coadvisor Stumpf, Waldo E.
dc.contributor.postgraduate Chimkonda Nkhoma, R.K. (Richard Kasanalowe) en
dc.date.accessioned 2015-01-19T12:13:26Z
dc.date.available 2015-01-19T12:13:26Z
dc.date.created 2014/12/12 en
dc.date.issued 2014 en
dc.description Thesis (PhD)--University of Pretoria, 2014. en
dc.description.abstract Although the austenitic stainless steels 304 and 321 are often treated nominally as equivalents in their hot rolling characteristics, the question remains whether any subtle differences between the two allow further optimisation of their respective hot rolling schedules. The hot workability of these two types of austenitic stainless steels was compared through single-hit Gleeble simulated thermomechanical processing between 800℃ and 􀀄􀀅00℃ while the strain rate was varied between 0.00􀀄s􀀈􀀉 and 5s􀀈􀀉. It was found that the constants for the hyperbolic sinh equation for hot working of AISI 321 steel are Q = 465 kJ/mol, 􀀖􀀗 = 􀀘.􀀙6 􀀚 􀀄0􀀉􀀛 􀀜􀀝􀀞􀀈􀀉􀀟􀀈􀀉, 􀀠 = 0.00􀀘 􀀜􀀝􀀞􀀈􀀉 and 􀀡 = 6.􀀄 while for 304 steel the constants are Q = 446 kJ/mol, 􀀖􀀗 = 􀀅.􀀄4 􀀚 􀀄0􀀉􀀛 􀀜􀀝􀀞􀀈􀀉􀀟􀀈􀀉, 􀀠 = 0.008 􀀜􀀝􀀞􀀈􀀉and 􀀡 = 6.􀀄. It is shown that the occurrence of dynamic recrystallisation starts when the Zener-Hollomon parameter 􀀢 􀀣 6.4 􀀚 􀀄0􀀉􀀛s􀀈􀀉 for both steels but that the differences in the values of Q and A3 (the structure factor) between the two steels does lead to consistently lower steady state stresses for the steel 321 than is found in the steel 304 at the same Z values. This may, therefore, offer some scope for further optimisation of the hot rolling schedules and in particular in the mill loads of these two respective steels. A modelled constitutive equation derived from hot working tests to predict hot rolling mill loads is proposed and validated against industrial hot rolling data for AISI 321 stainless steel. Good correlation is found between the predicted Mean Flow Stress, the Zener-Hollomon Z parameter and actual industrial mill load values from mill logs if allowances are made for differences in Von Mises plane strain conversion, friction and front or back end tension. The multipass hot working behaviour of this steel was simulated through Gleeble thermomechanical compression testing with the deformation temperature varying between 1200℃ down to 800℃ and the strain rate between 0.001s-1 and 5s-1. At strain rates greater than 0.05s-1, dynamic recovery as a softening mechanism was dominant, increasing the dynamic recrystallisation to dynamic recovery transition temperature DRTT to higher temperatures. This implies that through extrapolation to typical industrial strain rates of about 60s-1,most likely no dynamic recrystallisation in plant hot rolling occurs in this steel but only dynamic recovery. Grain refinement by DRX is, therefore, unlikely in this steel under plant hot rolling conditions. Finally, mill load modelling using the hot working constitutive constants of the near-equivalent 304 instead of those specifically determined for 321, introduces measurable differences in the predicted mill loads. The use of alloy-specific hot working constants even for near-equivalent steels is, therefore, recommended. en
dc.description.availability unrestricted en
dc.description.degree PhD en
dc.description.department Materials Science and Metallurgical Engineering en
dc.description.librarian lk2014 en
dc.identifier.citation Chimkonda Nkhoma, R(K 2014, Hot Working Characteristics of AISI 321 in Comparison to AISI 304 Austenitic Stainless Steels, PhD Thesis, University of Pretoria, Pretoria, viewed yymmdd <http://hdl.handle.net/2263/43302> en
dc.identifier.other D14/9/83 en
dc.identifier.uri http://hdl.handle.net/2263/43302
dc.language.iso en en
dc.publisher University of Pretoria en_ZA
dc.rights Β© 2014 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. en
dc.subject Dynamic recrystallisation (DRX) en
dc.subject Dynamic recovery (DRV) en
dc.subject AISI 321 en
dc.subject Constitutive equation en
dc.subject Mean flow stress (MFS) en
dc.subject UCTD en
dc.subject Dynamic recrystallisation to dynamic recovery transition temperature (DRTT)
dc.subject Hot working,
dc.subject Modelling
dc.title Hot Working Characteristics of AISI 321 in Comparison to AISI 304 Austenitic Stainless Steels en
dc.type Thesis en
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