Effect of austenitizing temperatures on the microstructure and mechanical properties of AISI 9254 steel

Abstract

In this study, the effect of austenitizing temperatures and low-temperature isothermal heat treatment (below martensite start temperature) on the microstructure and mechanical properties of AISI 9254 high silicon spring steel has been investigated. Experimental studies show that ultra-fine carbide-free bainite, tempered martensite and carbon enriched retained austenite could be observed in isothermally heat-treated samples where the as-received sample consisted of fine pearlite. A high tensile strength of ~2060 MPa, a total elongation of ~8 %, and absorbed energy of 105 J were achieved in a commercial high-Si steel by austempering below the Ms temperature. A good combination of strength and ductility has been obtained in prolonged austempering below the martensite start temperature (225 °C) from an austenitizing temperature of 870 °C.

Influence of the heat treatment on the quantitative features of the fracture surfaces and the mechanical properties of AISI 4340 steel sheets

“…The fracture surfaces of the specimens austenitized at 850°C and 900°C were composed of transgranular dimples, which were mostly uniform in size (Figure 5(a,b)). The presence of dimples on the fracture surfaces of these specimens is a conventional characteristic of ductile fracture behaviour of martensite microstructures with low austenitizing temperatures [19,46]. The uniformity and similarity of the morphology of dimples at the fracture surface of the specimens austenitized at 850°C and 900°C means that the mechanism of the nucleation and growth of voids was mostly similar, and it was an important factor in controlling and dominating the expansion of the voids.…”

Volume fraction of retained austenite in 1.2842 tool steel as a function of tempering temperature

“…RA volume dropped suddenly above the tempering temperature of 200ºC due to the thermal destabilization of this phase [21]. As stated by Dong et al [9], Kokosza and Pacyna [22] and Talebi et al [23], after the tempering temperature of 200°C, untransforfed austenite within the structure could be decomposed by a diffusion mechanism and causes precipitation of more carbides. This causes reduction in the austenite stability and results the transformation of austenite to a hard martensitic structure during the cooling process.…”

“…These studies generally were concentrated on the evaluation of the effect of retained austenite formed at various heat-treating conditions on the mechanical properties and to find the optimum heat treating conditions and/or composition lead to a better service life for a given application [8][9][10].…”

Influence of two-step austempering at different temperatures on mechanical and microstructural properties of AISI 9254 high silicon steel

“…Cruz et.al. [5] studied the influence of different austempering temperatures for various austempering times between 0.5 and 48 h. In another study, Murathan et al [17] reported a tensile strength of 2060 MPa and 8% total elongation with AISI 9254 steel after an austempering process below Ms temperature (225°C) for an extended time of 168 h. In order to reduce the cost of the heat treatment and for timesaving purposes, exploring a stepped austempering (twostep austempering) process thus seems worthwhile. As the term suggests, a stepped austempering process includes two stages of isothermal holding at different temperatures to exploit the advantages of both low-temperature and high-temperature austempering [18][19][20].…”