Revisiting Temperature and Magnetic Effects on the Fe-30 Wt Pct Ni Martensite Transformation Curve
- PDF / 803,773 Bytes
- 6 Pages / 593.972 x 792 pts Page_size
- 91 Downloads / 250 Views
ON
THE martensite transformation curve, which expresses the volume fraction transformed vs an external variable, is an expeditious tool to select a steel and its treatment to fit an engineering purpose. Historically, the martensite transformation curves have been described by empirical equations with coefficients that referred to the steel composition.[1,2] Moreover, such equations assumed athermal (time-independent, nonthermally activated) kinetics. The isothermal (thermally activated and time-dependent) transformation and the mechanically induced ones were acknowledged considerably later. Meantime, microstructure and thermodynamic aspects were formally introduced in descriptions of the transformation curve,[3–11] and athermal and isothermal aspects have been acknowledged in steels transformed by continuously cooling.[12–14] The renewed interest in the description of the martensite transformation curves (the topic of the present work) stems from the fact that
J.R.C. GUIMARA˜ES is with the Universidade Federal Fluminense, Escola de Engenharia Industrial Metalu´rgica de Volta Redonda, Volta Redonda, RJ 27255-125, Brazil and also with the Mal.Moura 338H/22C, Sa˜o Paulo, SP 05641-000, Brazil. P.R. RIOS is with the Universidade Federal Fluminense, Escola de Engenharia Industrial Metalu´rgica de Volta Redonda. Contact e-mail: [email protected]ff.br Manuscript submitted May 1, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
martensite has become a means to optimize engineering steels.
II.
BACKGROUND
In a previous paper,[15] we introduced a formalism to describe the martensite transformation curve observed in steels, considering generally accepted aspects of such transformation (diffusionless, autocatalytic, nucleation-controlled, and displacive) with progress that is also influenced by the environment where the units form.[16] The formalism has been validated with library data pertaining to a bursting FeNiC alloy,[17] low-carbon steels,[7] and in a FeNiMn alloy that transforms isothermally.[18] Further to describing transformation curves with very high fitting correlations, the parameters of the model expeditiously relate to kinetic aspects of the transformation. In this study, we extend the validation by considering the martensite transformation in Fe-30wt pctNi, which exhibits microstructural diversity.[19,20]
III.
FORMALISM
The early work by Cech and Turnbull[21] demonstrated that in particulate materials, the martensite transformation initiates heterogeneously at limited and randomly distributed particles such that the probability that a particle contains at least one nucleation locus at temperature T, which may be expressed as[22]
Pp ¼ 1 exp qp nTV ;
where nTV is the number density of nucleation sites at temperature T and qp is the mean particle volume. Of course, any grain that has at least one operational nucleation locus at temperature T will be partially transformed as a primary plate arises at each nucleation site. Therefore, the above probability is equivalent to the volume fraction of partially transfor
Data Loading...
