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Effect of microstructure on fatigue behaviour of advanced high strength ductile cast iron produced by quenching and partitioning process

Effect of microstructure on fatigue behaviour of advanced high strength ductile cast iron produced by quenching and partitioning process
Effect of microstructure on fatigue behaviour of advanced high strength ductile cast iron produced by quenching and partitioning process
The fatigue behaviour of high strength ductile cast iron produced by a quenching and partitioning process (Q&P) is evaluated. The Q&P process is receiving increased attention as a new way to produce ultra-high strength steels with multiphase microstructures composed of a martensite matrix and substantial amounts of carbon-stabilized retained austenite. Currently, the mechanical properties arising from applying the Q&P process in ductile cast irons have not been investigated. Thermal treatments consisted of heating the material to 880°C for a 2 hour soaking time followed by quenching in oil at 140°C and 170°C, intermediate temperatures between Ms and Mf allowing the formation of a controlled amount of athermal martensite. The material was reheated to 300°C and 375°C (the partition treatment) for different times between 15 and 120 minutes and subsequently air-cooled to room temperature. Microstructural evolution and carbon partitioning was investigated by in situ synchrotron X-ray diffraction. With a microstructure composed of tempered martensite, bainitic ferrite and carbon-enriched stabilized austenite, a new class of properties was obtained, with an enhanced strength when compared to ADI (austempered ductile iron) while still maintaining reasonable elongation. Fatigue testing was undertaken using polished plain bend bars (no stress concentration features) assessed under four-point bending. Uninterrupted tests at varying loads show that the higher partitioning temperature is beneficial for fatigue life. Fracture initiation sites are primarily from pores and a number of decohered graphite nodules. A strong influence of the microstructure on subsequent fatigue crack growth is observed from interrupted testing with replica records and SEM examination of tested samples, with cracks exhibiting significant tortuosity, at times even appearing to grow approximately parallel to the tensile axis. Using these results, the effects of the Q&P treatment, particularly the role of the partitioning time and temperature, on the fatigue properties of ductile cast iron are assessed.
fatigue, ductile iron heat treatment, retained austenite, quenching and partitioning, austenite stability
0142-1123
Caetano Melado, André
ed785592-8fa2-4eeb-bcca-3e744d4e3c0c
Nishikawa, Arthur Seiji
31c64798-3c53-4a1d-b0c1-56d54c994e4c
Goldenstein, Hélio
ae2f4971-979b-4007-8e83-23dfda3a3eb4
Giles, Michael A.
aac8af59-de52-4f82-9968-f2a5d9d1f5ee
Reed, Philippa A.S.
8b79d87f-3288-4167-bcfc-c1de4b93ce17
Caetano Melado, André
ed785592-8fa2-4eeb-bcca-3e744d4e3c0c
Nishikawa, Arthur Seiji
31c64798-3c53-4a1d-b0c1-56d54c994e4c
Goldenstein, Hélio
ae2f4971-979b-4007-8e83-23dfda3a3eb4
Giles, Michael A.
aac8af59-de52-4f82-9968-f2a5d9d1f5ee
Reed, Philippa A.S.
8b79d87f-3288-4167-bcfc-c1de4b93ce17

Caetano Melado, André, Nishikawa, Arthur Seiji, Goldenstein, Hélio, Giles, Michael A. and Reed, Philippa A.S. (2017) Effect of microstructure on fatigue behaviour of advanced high strength ductile cast iron produced by quenching and partitioning process. International Journal of Fatigue. (doi:10.1016/j.ijfatigue.2017.07.009).

Record type: Article

Abstract

The fatigue behaviour of high strength ductile cast iron produced by a quenching and partitioning process (Q&P) is evaluated. The Q&P process is receiving increased attention as a new way to produce ultra-high strength steels with multiphase microstructures composed of a martensite matrix and substantial amounts of carbon-stabilized retained austenite. Currently, the mechanical properties arising from applying the Q&P process in ductile cast irons have not been investigated. Thermal treatments consisted of heating the material to 880°C for a 2 hour soaking time followed by quenching in oil at 140°C and 170°C, intermediate temperatures between Ms and Mf allowing the formation of a controlled amount of athermal martensite. The material was reheated to 300°C and 375°C (the partition treatment) for different times between 15 and 120 minutes and subsequently air-cooled to room temperature. Microstructural evolution and carbon partitioning was investigated by in situ synchrotron X-ray diffraction. With a microstructure composed of tempered martensite, bainitic ferrite and carbon-enriched stabilized austenite, a new class of properties was obtained, with an enhanced strength when compared to ADI (austempered ductile iron) while still maintaining reasonable elongation. Fatigue testing was undertaken using polished plain bend bars (no stress concentration features) assessed under four-point bending. Uninterrupted tests at varying loads show that the higher partitioning temperature is beneficial for fatigue life. Fracture initiation sites are primarily from pores and a number of decohered graphite nodules. A strong influence of the microstructure on subsequent fatigue crack growth is observed from interrupted testing with replica records and SEM examination of tested samples, with cracks exhibiting significant tortuosity, at times even appearing to grow approximately parallel to the tensile axis. Using these results, the effects of the Q&P treatment, particularly the role of the partitioning time and temperature, on the fatigue properties of ductile cast iron are assessed.

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Melado et al Final Corrected - Accepted Manuscript
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Accepted/In Press date: 11 July 2017
e-pub ahead of print date: 13 July 2017
Keywords: fatigue, ductile iron heat treatment, retained austenite, quenching and partitioning, austenite stability

Identifiers

Local EPrints ID: 415478
URI: http://eprints.soton.ac.uk/id/eprint/415478
ISSN: 0142-1123
PURE UUID: a0f6fe72-ea21-474b-a949-d0214fc735f7
ORCID for Philippa A.S. Reed: ORCID iD orcid.org/0000-0002-2258-0347

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Date deposited: 10 Nov 2017 17:31
Last modified: 17 Dec 2019 05:58

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