The Effect of Different Annealing Strategies on the Microstructure Development and Mechanical Response of Austempered Steels

Abstract

This study focuses on the effect of non-conventional annealing strategies on the microstructure and related mechanical properties of austempered steels. Multistep thermo-cycling (TC) and ultrafast heating (UFH) annealing were carried out and compared with the outcome obtained from a conventionally annealed (CA) 0.3C-2Mn-1.5Si steel. After the annealing path, steel samples were fast cooled and isothermally treated at 400 °C employing the same parameters. It was found that TC and UFH strategies produce an equivalent level of microstructural refinement. Nevertheless, the obtained microstructure via TC has not led to an improvement in the mechanical properties in comparison with the CA steel. On the other hand, the steel grade produced via a combination of ultrafast heating annealing and austempering exhibits enhanced ductility without decreasing the strength level with respect to TC and CA, giving the best strength–ductility balance among the studied steels. The outstanding mechanical response exhibited by the UFH steel is related to the formation of heterogeneous distribution of ferrite, bainite and retained austenite in proportions 0.09–0.78–0.14. The microstructural formation after UFH is discussed in terms of chemical heterogeneities in the parent austenite.

Viewpoints on Technological Aspects of Advanced High-Strength Bainitic Steels

“…Only in the case of rapid annealing, the mechanical properties of the microstructure formed through isothermal treatment at 400 • C improved with respect to conventional annealing. Particularly, similar strength values were achieved with an increase of 40% in total elongation and 50% in energy absorbed, as measured under the uniaxial strain to fracture [57].…”

Experimental investigation and optimization of machining parameters of deep cryogenically treated and tempered steels in electrical discharge machining process

“…Points with large values will take values close to "0". In the case of "higher is better", normalization is as in Equation (5).…”

“…3,4 With this method, the transformation of microstructure of austenite phase into martensite phase steels and their alloys, and form of fine carbide in cementite are provided more homogeneous distribution. 5 Thus, serious improvements are obtained in the mechanical properties of the steel materials such as wear resistance and macro-hardness. 6,7 The effects of cryogenic processing on machining performance are comprehensively studied in traditional manufacturing methods such as drilling, milling, and turning process etc.…”

Heat Treatment of Steels

“…Hernandez-Duran et al [17] report on the effect of different annealing strategies on the microstructure development and the mechanical response of bainitic steels. Their study focuses on the effect of non-conventional annealing strategies on the microstructure and mechanical properties of austempered steels.…”