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Open AccessArticle

The Effect of Tempering on the Microstructure and Mechanical Properties of a Novel 0.4C Press-Hardening Steel

1
Materials and Mechanical Engineering, Centre for Advanced Steels Research, University of Oulu, 90014 Oulu, Finland
2
Raahe Works, SSAB Europe, 92100 Raahe, Finland
*
Author to whom correspondence should be addressed.
Appl. Sci. 2019, 9(20), 4231; https://doi.org/10.3390/app9204231
Received: 12 September 2019 / Revised: 2 October 2019 / Accepted: 4 October 2019 / Published: 10 October 2019
(This article belongs to the Special Issue Recent Trends in Advanced High-Strength Steels)
In this paper, the effects of different tempering temperatures on a recently developed ultrahigh-strength steel with 0.4 wt.% carbon content were studied. The steel is designed to be used in press-hardening for different wear applications, which require high surface hardness (650 HV/58 HRC). Hot-rolled steel sheet from a hot strip mill was austenitized, water quenched and subjected to 2-h tempering at different temperatures ranging from 150 °C to 400 °C. Mechanical properties, microstructure, dislocation densities, and fracture surfaces of the steels were characterized. Tensile strength greater than 2200 MPa and hardness above 650 HV/58 HRC were measured for the as-quenched variant. Tempering decreased the tensile strength and hardness, but yield strength increased with low-temperature tempering (150 °C and 200 °C). Charpy-V impact toughness improved with low-temperature tempering, but tempered martensite embrittlement at 300 °C and 400 °C decreased the impact toughness at −40 °C. Dislocation densities as estimated using X-ray diffraction showed a linear decrease with increasing tempering temperature. Retained austenite was present in the water quenched and low-temperature tempered samples, but no retained austenite was found in samples subjected to tempering at 300 °C or higher. The substantial changes in the microstructure of the steels caused by the tempering are discussed. View Full-Text
Keywords: steel; martensite; tempering; press-hardening steel; martensite; tempering; press-hardening
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MDPI and ACS Style

Haiko, O.; Kaijalainen, A.; Pallaspuro, S.; Hannula, J.; Porter, D.; Liimatainen, T.; Kömi, J. The Effect of Tempering on the Microstructure and Mechanical Properties of a Novel 0.4C Press-Hardening Steel. Appl. Sci. 2019, 9, 4231.

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