Abstract
During the production of forged metal components, the sequence of heat treatments that are carried out, as well as hot working, remarkably influences mechanical properties of the product, in particular impact toughness. It is possible to tailor impact toughness by varying tempering temperature and soaking time after hardening treatment, widening the application range of structural steels. In this work, we consider the effects of a second tempering treatment on the microstructural properties and impact toughness of a structural steel EN 10025-6 S690 (DIN StE690, W. n: 1.8931). The steel was first forged and quenched in water after austenitization at 890 °C for 4 h. After quenching different tempering treatments were performed, at 590 °C in single or multiple steps. The effect of these treatments was evaluated both in microstructural terms, by means of optical microscopy, scanning and transmission electron microscopy and X-ray diffraction, and in terms of impact toughness. The mechanical behavior was correlated with the microstructure and a remarkable increase in impact toughness was found after the second tempering treatment due to carbide shape change.
Highlights
S690 is a low-alloy high strength steel mainly employed in structural applications
The tempering treatments performed for long soaking times seem to significantly increase the impact toughness of different steel grades [7,8]
The results previously reported highlighted that for S690 steel a double tempering of 6 h and 4 after quenching permits to achieve increased impact toughness if compared to a single tempering h after quenching permits to achieve increased impact toughness if compared to a single tempering of 6 h or 10 h
Summary
S690 is a low-alloy high strength steel mainly employed in structural applications. One of the typical uses of this steel grade is offshore applications, where high impact toughness even at low temperatures is required. In these applications, the steel is supplied with a mainly bainitic microstructure. Considering the low carbon content this steel is characterized by a relatively good weldability [1,2]. In order to obtain the desired properties, high strength steels are formed by forging and thereafter undergo a quenching and tempering treatment [3,4,5]. One of the key mechanical properties of steels employed in structural applications is impact toughness
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