Simulation of diffusion controlled precipitation in heat treatable steels and its correlation with thermophysical properties

Transcription

1 Thermo-Calc user meeting, RWTH Aachen, Simulation of diffusion controlled precipitation in heat treatable steels and its correlation with thermophysical properties S. Weber1, L. Mujica2, M. Walter2, S. Klein2 1 2 Lehrstuhl für Neue Fertigungstechnologien und Werkstoffe, Bergische Universität Wuppertal Lehrstuhl Werkstofftechnik, Ruhr-Universität Bochum

2 Outline A) Introduction B) Experimental C) Results - Microstructure - Thermal conductivity (lph and lel) - Cementite precipitation D) Summary & Conclusions slide 2 of 18

3 A: Introduction slide 3 of 18 Thermal and electric conductivity of alloy steels depend on thermomechanical history and alloy composition Certain technical applications require high or low lth values: - polymer processing - die casting - hot sheet metal forming ( press hardening ) Scientific question: How does heat treatment influence lth und sel (lel)? source: Wikipedia source: VDEh / Volkswagen

4 A: Introduction slide 4 of 18 Focus on non-, low- and medium-alloy heat treatable steels Quenched condition: almost all alloying elements in solid solution (a ) Quenched & tempered : depleted a matrix + carbides Heat conduction by electron and phonons: a martensite: lel ( ) lph ( ) => major impact carbides: lel ( ) lph ( ) => minor impact Significant influence of heat treatment is expected Upper threshold: 82 W m-1k-1 of pure a-fe at room temperature

5 B: Experimental Heat treatment and measurements Chemical composition of C45E steel (values in mass-%) measured by OES C45E C Cr Cu Mn Ni Si P S Mo Fe bal. TH=900 C, 20 min // H2O + LN2 => fully a-martensitic microstructure Tempering at T=700 C from 1 s to s (200 h) => a + cem Measurement of electric and thermal conductivity ( 4PP / dynamic method)

6 B: Experimental - Dictra TC-S, Dictra 2.7 // TCFe7, MOBFe2 Single cell system, circular, r=30µm Nucleus radius of 2 nm BCC_A2 matrix; supers. condition Isothermal calculations at 700 C Only Fe-C-Cr-Mn-Ni-Si considered P, S, Cu, Mo and N neglected Chemical composition of C45E steel and cementite nucleus (values in at.-%) for Dictra calculation C Cr Mn Ni Si Fe C45E (supers. condition) bal. cementite (nucleus) e-11 bal

7 C: Results - Microstructure

8 C: Results - Electric and thermal conductivity Isothermal tempering at 700 C; RT values

9 C: Results - Electric and thermal conductivity 2h / 20h tempering at different T; RT values Reference: J. Wilzer, PhD Thesis, 2014, Ruhr-Uni Bochum

10 C: Results - Cementite precipitation 2.5 T= K T= K Fast precipitation and growth of cementite LENP growth of cementite finished after a few seconds Carbon depletion of ferritic/martensitic matrix

11 C: Results - Cementite precipitation Diffusion controlled enrichment of cementite by Cr (and Mn, not shown) Purification of ferritic/martensitic matrix Enrichment of Ni/Si in the matrix due to low/no solubility in cementite

12 C: Results - Discussion depletion of matrix from subst. el. cementite formation and a relaxation Isothermal tempering at 700 C; RT values

13 D: Summary & Conclusions 1. Thermal conductivity of C45E changes significantly by tempering 2. Contribution of a matrix to lth dominates => electronic part 3. General function of carbides: age hardening and matrix depletion 4. Dictra simulation on C45E steel shows: - LENP growth of cementite with early impact on lth - Diffusion controlled Cr/Mn depletion of matrix, delayed impact on lth - Enrichment of matrix with Si/Ni important for alloy design 5. Consequences for alloy development (for high lth): - carbon level does not have to be reduced - technical, metastable conditions to be considered - elements with low solubility in carbides to be avoided reference S. Klein et al., Diffusion processes during cementite precipitation and their impact on electrical and thermal conductivity of a heat-treatable steel, Journal of Materials Science, accepted (open access)

14 Thank you for your attention and your questions! Prof. Dr.-Ing. Sebastian Weber Lehrstuhl für Neue Fertigungstechnologien und Werkstoffe Bahnhofstr. 15, Solingen, Germany phone: +49 (0)