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| Author(s): |
Julian David Escobar Atehortua
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | Campinas, SP. |
| Institution: | Universidade Estadual de Campinas (UNICAMP). Faculdade de Engenharia Mecânica |
| Defense date: | 2018-04-05 |
| Examining board members: |
Paulo Roberto Mei;
Andre Paulo Tschiptschin;
Haroldo Cavalcanti Pinto;
Rubens Caram Junior;
João Batista Fogagnolo
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| Advisor: | Paulo Roberto Mei; Antonio Jose Ramirez Londono |
| Abstract | |
This work presents the study of the kinetic and compositional aspects behind the mechanism of austenite reversion and stabilization during the inter-critical tempering of a supermatensitic stainless steel. The experimental methodology used in this work consisted in: 1) Use of correlative synchrotron x-ray diffraction and laser dilatometry to characterize the martensite - austenite transformation kinetics during simple and multiple inter-critical tempering cycles. 2) X-ray Energy Dispersive Spectroscopy in Transmission Electron Microscopy and Atom Probe Tomography compositional characterization of the morphology and composition of the phases present after tempering. The mechanism of austenite reversion and stabilization was studied for two well-known starting microstructural conditions: 1) Compositionally homogeneous martensitic matrix, austenitized at 950 °C during 20 minutes, resulting in 99,7 % fresh martensite and 0,3 % stable titanium carbo-nitrides. 2) Microstructurally and compositionally non-homogeneous matrix, obtained after inter-critical tempering at 625 °C during 2,5 h, resulting in a microstructure composed by Ni-rich reverted austenite, Ni-rich fresh martensite, Ni-depleted tempered martensite. For the homogeneous matrix case, the diffusive and displacive reversion mechanisms were differenced by the use of slow and ultra-fast heating rates. The slow heating rate allowed to obtain the quasi-equilibrium transformation temperatures. The ultra-fast heating rate allowed the isolation of the isothermal austenite reversion, through the suppression of the possible contributions of phase transformations and diffusion during the heating stage. The austenite nucleation occurred mainly at heterogeneous sites, such as the martensitic lath/lath interfaces and the Ti (C, N) precipitates, having higher concentrations of austenite-stabilizing elements (nickel, manganese, copper, etc). The austenite growth was strongly influenced by the migration of austenite-stabilizing elements from the martensite to the moving interface, according to the local equilibrium partitioning of nickel. By increasing the isothermal reversion temperature, the kinetics of the transformation was accelerated, favoring fast austenite nucleation and decreasing the nickel partitioning necessary to the austenite lath growth. However, this also affected the thermal stability of the reverted austenite, since it was completely unstable upon cooling from the isothermal reversion temperatures above 670 °C; partially stable upon cooling from reversion temperatures between 625 and 650 °C; and completely stable upon cooling from reversion temperatures below 610 °C. The resulting microstructure after tempering at 625 °C during 2,5 hours presented the lowest hardness values due to the carbon extraction from the solid solution and to the maximization of the stable reverted austenite volume fraction at room temperature. Later on, the mechanisms of austenite reversion and stabilization were studied for the non-homogeneous matrix case. The mechanism behind the increment in the stable reverted austenite volume fraction after multiple-step inter-critical tempering was related to the generation of a transition metastable microstructure, imposed by the first inter-critical tempering cycle at 625 °C. The preferential reversion and stabilization of the Ni-rich fresh martensite laths, coming from the cooling stages, resulted in the stabilization of volume fractions of austenite above the thermodynamic equilibrium for the subsequent tempering temperatures below 625 °C. On the other hand, when the subsequent tempering temperature was above 625 °C, destabilization of the reverted austenite, previously obtained after the first tempering cycle, was observed. After single inter-critical tempering, the maximum hardness reduction was observed for the condition at 625 °C during 2,5 h, which also maximized the volume fraction of reverted austenite at room temperature. However, after multiple-step inter-critical tempering, no representative hardness reduction was observed even after completely suppressing the presence of fresh martensite, by maximizing the presence of stable reverted austenite. This was explained by the complete carbon arrest from the solid solution after the first inter-critical tempering cycle. The strong martensite/austenite interface segregation of ferrite-stabilizing elements resulted in the precipitation of 'qui' phase during the subsequent multiple tempering stages (AU) | |
| FAPESP's process: | 14/20844-1 - In situ study on kinetics of phase transformations and thermal stability of retained austenite in TRIP-assisted advanced high strength steel |
| Grantee: | Julian David Escobar Atehortua |
| Support Opportunities: | Scholarships in Brazil - Doctorate |