Effect of Retained Austenite on Impact Toughness of the Multi-Phase Bainitic-Martensitic Steel

Abstract

The new class of bainitic steels can present toughness at room temperature greater than traditional quenched and tempered martensitic steel. This is because the microstructure of steel with high Si content (≈1.5wt%) submitted to bainitic transformation is compose of fine plates of bainitic ferrite separated by retained austenite. The inhibition of cementite precipitation leads to the improvement of toughness. The presence of cementite facilitates the nucleation of cracks. Moreover, the blocks of retained austenite are undesirable. This morphology is rather unstable and tends to transform into hard and brittle untempered martensite under the influence of small stress, contributing to a low toughness. However, it was observed in this work that the greater the volume fraction of retained austenite, the greater is the toughness (10-24 J) for multi-phase steel. The values of toughness were independent whether the retained austenite is present on film or block forms. The decrease of toughness values was observed by the tempered samples after the bainitic transformation (10-14 J). This occurred because the blocks of retained austenite decomposed into carbides, martensite and/or bainite.

Effect of cooling rate and composition on microstructure and mechanical properties of ultrahigh-strength steels

“…The same trend was observed by Kumar et al [20]. The decrease in toughness at the highest cooling rate (60°C/s) may be attributed to the very high strength values, and the presence of less retained austenite compared with the other cooling rates [35]. The lowest cooling rate (0.01°C/s) shows the lowest toughness even though it is associated with the lowest values of strength and highest retained austenite contents.…”

Corrosion behaviour of an industrial shot-peened and coated automotive spring steel AISI 9254

“…Specific alloying additions along with carbon, combined with tailored microstructure make them a special group of materials called 'high-strength low-alloy (HSLA) steels'. Fully martensitic or sometimes dual-phase microstructure [1][2][3][4][5][6][7][8], evolved during manufacturing and/or after appropriate heat treatment process to achieve the desired combination of properties, makes HSLA steels suitable candidate for a wide range of engineering applications. For specific applications, different alloying elements (commonly used Mn, Si, Cr, Mo, Nb, V, Ti etc.)…”

“…There have been increasing demands to improve the material performance with the aim to increase the fuel efficiency and reduce CO 2 emission by reducing automotive components' weight. As an attempt to satisfy such demands, the efforts have been focused to improve the spring material's mechanical performance (elasticity, strength, fatigue) by alloying additions and thermo-mechanical treatments [6][7][8][9]15,16]. Coil surface is also shot-peened which generates compressive residual stresses to enhance the fatigue properties and significantly affects the ultimate life of the spring [15,16].…”