Evaluation of potential of high Si high C steel nanostructured bainite for wear and fatigue applications

Abstract

The present study is concerned with the potential of high carbon, high silicon steel grades isothermally transformed to bainite at low temperature (< 300 °C). A first part gives an overview of design principles allowing very high strength and ductility to be achieved, while minimising transformation duration. Wear and fatigue properties are then investigated for over ten variants of such material, manufactured in the laboratory or industrially. The results are discussed against published data. Tensile strength above 2 GPa are routinely achieved, with, in one case, an exceptional and unprecedented total elongation of over 20%. Bainite plate thickness and retained austenite content are shown to be important factors in controlling the yield strength, though additional, non negligible parameters remain to be quantified. Rolling-sliding wear performances are found to be exceptional, with as little as 1% of the specific wear rate of conventional bainitised 100Cr6. It is suggested that this results from the decomposition of retained austenite in the worn layer, which considerably increases hardness and presumaby introduces compressive residual stresses. Fatigue performance were slightly improved over 100Cr6 for one of the two industrially produced material, but significantly lower otherwise. Factors controlling fatigue resistance require further investigations.

Tensile Ductility of Nanostructured Bainitic Steels: Influence of Retained Austenite Stability

“…They consist of ultrafine bainitic laths (typical width under 50 nm) surrounded by retained austenite [3,11]. Interestingly, the initial mechanical properties [2] were at best on par with those of quenched and tempered high strength spring steels [16], but were later improved to reach an unprecedented 21% elongation for over 2.1 GPa in tensile strength [10,11].…”

“…These microstructures are obtained in relatively high carbon steels (0.6-1.2 wt %, although the concept can be extended to lower carbon contents) through isothermal transformation over durations ranging from 10 to over 100 h [5,11,15]. They consist of ultrafine bainitic laths (typical width under 50 nm) surrounded by retained austenite [3,11].…”

“…These microstructures are obtained in relatively high carbon steels (0.6-1.2 wt %, although the concept can be extended to lower carbon contents) through isothermal transformation over durations ranging from 10 to over 100 h [5,11,15]. They consist of ultrafine bainitic laths (typical width under 50 nm) surrounded by retained austenite [3,11]. Interestingly, the initial mechanical properties [2] were at best on par with those of quenched and tempered high strength spring steels [16], but were later improved to reach an unprecedented 21% elongation for over 2.1 GPa in tensile strength [10,11].…”

“…Bainitic microstructures formed at low temperatures (350 • C or less) have received a considerable amount of attention in the recent years [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. These microstructures are obtained in relatively high carbon steels (0.6-1.2 wt %, although the concept can be extended to lower carbon contents) through isothermal transformation over durations ranging from 10 to over 100 h [5,11,15].…”

Martensite and bainite in nanocrystalline steels: understanding, design and applications

“…The transformation was significantly accelerated to complete the processing within hours (as opposed to days), by increasing the magnitude of the driving force for transformation making controlled additions of aluminium and cobalt (Table 1). A further rate increment was possible by reducing carbon, manganese, chromium, and molybdenum contents and by refining the prior austenite grain size with the help of niobium additions [10].…”