A comprehensive quantitative characterisation of the multiphase microstructure of a thick-section high strength steel

Abstract

The through-thickness heterogeneous microstructure of thick-section high strength steels is responsible for the significant scatter of properties along the thickness. In this study, in order to identify the critical microstructural features in the fracture behaviour and allow for design optimisation and prediction of structural failure, the through-thickness microstructure of thick-section steels was extensively characterised and quantified. For this purpose, samples were extracted from the top quarter and middle thickness positions, and a combination of techniques including chemical composition analysis, dilatometry, and microscopy was used. The hardness variation through the thickness was analysed via micro-Vickers measurements and the local hardness variation in the middle section was studied via nanoindentation. The middle section presented larger prior austenite grain (PAG) sizes and larger sizes and area fraction of inclusions than the top section. Additionally, cubic inclusions were observed distributed as clusters in the middle, sometimes decorating PAG boundaries. Defects associated with the cubic inclusions or the interface between the matrix and the circular and cubic inclusions were observed in the mid-thickness. Moreover, the middle section presented long interfaces with the most significant hardness gradients due to the presence of hard centreline segregation bands. Hence, the microstructural and nanoindentation analyses indicated the middle section as the most likely area to have the lowest fracture toughness and, therefore, the most unfavourable section for fracture performance of the investigated S690QL high strength steel. The detrimental effect of the middle section was confirmed via CTOD tests where the middle presents lower fracture toughness than the top section.

Using an Internal State Variable Model Framework to Investigate the Influence of Microstructure and Mechanical Properties on Ballistic Performance of Steel Alloys

“…Several studies [82,[95][96][97][98][99][100] have quantified the distribution of second-phase particle number densities for several high-strength steels (Table 2), for second-phase particle number densities in the range of 10 2 -10 3 particles/mm 2 . The nominal upper and lower bounds for η were selected as 250 and 4000 particles/mm 2 , respectively, for the parameter sensitivity study.…”

“…The grain size sensitivity exponent, Z, has been calibrated for a variety of materials, including steel [112]. Grain sizes in steel have been shown to vary from 1 µm in diameter for high-strength steels (and even less than 1 µm in specially designed ultra-fine-grain materials) to greater than a 100 µm size for mild steels [98,105,113]. The bounds for the grain size term, GS, were selected as 1 µm and 100 µm, accordingly.…”

The Influence of Short-Term High Temperature Environment on the Non-Uniform Distribution of Ferrite Grain Size in 40 mm-thick Q345 Steel

“…Xie et al studied the influence of holding time on carbide size and distribution in 960 Mpa grade low alloy steel, and found that prolonging the holding time leads to the coarsening of carbides [19]. in terms of research on the models and characterization of material grain size distribution [20][21][22][23], Yu et al studied the non-uniformity of microstructure in both 2 and 3 mm-thick Dp600 thin plates, and established models for accurately describing the non-uniform distribution of microstructure in thin plate [20]. Kazakov et al investigated the microstructural non-uniformity of 25-70 mm hot-rolled thick plates and proposed a method to evaluate microstructural anisotropy [21].…”

“…Kazakov et al investigated the microstructural non-uniformity of 25-70 mm hot-rolled thick plates and proposed a method to evaluate microstructural anisotropy [21]. Yamashita et al analyzed the influence of friction stir welding on the non-uniformity of microstructure along the thickness direction in 2 mm dual-phase stainless steel, and revealed the variation of grain size in the α and γ phases along the thickness direction [22]. virgínia et al found the non-uniformity of microstructure in quarter and middle thicknesses of 80 mm-thick S690Ql high-strength steel and proposed a characterization method for the multiphase microstructure of quenched and tempered high-strength steel [23].…”