Abstract
The influence of cooling rate on the microstructure and mechanical properties of two new ultrahigh-strength steels (UHSSs) with different levels of C, Cr and Ni has been evaluated for the as-cooled and untempered condition. One UHSS had higher contents of C and Cr, while the other one had a higher Ni content. On the basis of dilatation curves, microstructures, macrohardness and microhardness, continuous cooling transformation diagrams were constructed as a guide to heat treatment possibilities. Cooling rates (CRs) of 60, 1 and 0.01 °C/s were selected for more detailed investigations. Microstructural characterization was made by laser scanning confocal microscopy, field emission scanning electron microscopy combined with electron backscatter diffraction, electron probe microanalysis and X-ray diffraction. Mechanical properties were characterized using macrohardness, tensile and Charpy V-notch impact tests. UHSS with the higher C and Cr contents showed lower transformation temperatures and slower bainite formation kinetics than that with the higher Ni content. Higher cooling rates led to lower volume fractions and carbon contents of retained austenite together with finer prior austenite grain size, as well as effective final grain size and lath size. These changes were accompanied by higher yield and tensile strengths. The best combinations of strength and toughness were obtained with martensitic microstructures and by avoiding the formation of granular bainite accompanied by proeutectoid carbides at low CR. For the cooling rates studied, UHSS with the higher C and Cr contents showed the higher hardness and strength but at the cost of toughness.
Highlights
Nowadays, ultrahigh-strength steels (UHSSs) are widely used in many applications such as automotive, locomotive and truck components, pressure vessels, offshore platforms, engineering machinery, the mining, military and aerospace industry, due to their good combinations of strength, toughness and ductility [1,2,3,4,5].Recently, different grades of steels have been developed as potential replacements for the high-cost ultrahighstrength steels containing high levels of Co and Ni such as AerMet100, HP 9-4-20, HP 9-4-30 and AF1410 to attain substantially reduced costs and processing time
We explore the phase transformations, together with the resultant microstructure and mechanical properties of the current lower-purity Eglin steels with traces of Ti and Nb for the wide range of cooling rates that can be encountered in practical processing
The effect of C content and Cr/Ni ratio is clearly observed from Fig. 2: Ac1 increased as a result of increasing Cr/Ni ratio and a small increase in C content, while Ac3, Ms and Bs decreased
Summary
Ultrahigh-strength steels (UHSSs) are widely used in many applications such as automotive, locomotive and truck components, pressure vessels, offshore platforms, engineering machinery, the mining, military and aerospace industry, due to their good combinations of strength, toughness and ductility [1,2,3,4,5]. Based on a medium-carbon V-microalloyed composition alloyed with moderate levels of Si, Ni, Cr, Mo and W, Dilmore and Ruhlman [6] developed the ultrahigh-strength Eglin steel that, after tempering at 260 °C, achieves a hardness of 458 HV, an ultimate tensile strength of 1685 MPa, a yield strength of 1392 MPa, elongation to fracture of 17.5% and a Charpy V-notch impact toughness of 37 J at room temperature. We explore the phase transformations, together with the resultant microstructure and mechanical properties of the current lower-purity Eglin steels with traces of Ti and Nb for the wide range of cooling rates that can be encountered in practical processing. Microstructures are characterized with regard to the fractions of martensite, bainite and retained austenite together with the sizes of prior austenite grains and the sizes of grains defined by low-angle and high-angle grain boundaries
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