Abstract
The properties of a microalloyed steel, with Nb and V in its composition, were studied, after different intercritical thermal treatments and at different austenitizing and tempering temperatures. The mechanical properties of the specimens were measured in a Vickers hardness tester, and their microstructures were analyzed by optical microscopy, with the aid of a digital image processor. After austenitizing at 1100 °C and tempering at 625 °C, the samples showed significantly higher tempering resistance, reflected by their retention of high hardness, which may be associated with a secondary hardening precipitation of Nb carbon nitrides. In the sample with dualphase microstructure, the martensite volume fraction varied from 18.2 to 26.3% and the ferrite grain size remained unchanged, upon the variation of the time length of the intercritical treatments. Tempered samples showed Vickers hardness (HVN) varying from 327 to 399, and dualphase samples showed HVN from 362 to 429.
Highlights
The high strength and high toughness of tempered steels promote their widespread use in machine structural parts
This paper presents the effect of the austenitizing and
Micrographs 2b-2d reveal martensite thin layers thickening on the ferrite grain boundaries with time, until it forms a continuous network around the ferrite grains
Summary
The high strength and high toughness of tempered steels promote their widespread use in machine structural parts. Dual-phase steels, presenting a microstructure composed of martensite and/or bainite in a ferrite matrix, are basically employed by the automobile industry. Direct-quench through fast cooling after controlled or hot rolling is employed to produce tempered steel plates. Mintz[4], Foley[5] and Weiss[6] compared the mechanical properties and the microstructures of direct-quenched and conventionally tempered steels, to determine the influences of V, B and Ti and of the process variables. Tempering temperatures, as well as the effect of the intercritical thermal treatment parameters, on the microstructure and mechanical properties of a microalloyed steel containing Nb and V. The influence of the austenitizing temperature over tempering resistance was evaluated
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