Hydrogen diffusion and the percolation of austenite in nanostructured bainitic steel

The diffusion of hydrogen in austenite is slower than in ferrite. Experiments have been conducted to study the behaviour of hydrogen in a nanostructured steel sample consisting of a mixture of thin plates of bainitic ferrite and intervening films of retained austenite, with the latter phase present in a quantity larger than the percolation threshold, i.e. it has three-dimensional connectivity. The structure was then heat treated to control the fraction of austenite, and hence to study the role of hydrogen when the austenite decomposes below the value required to sustain percolation. The experiments have involved both thermal desorption analysis and permeation, and when combined with theoretical analysis, indicate a significant influence of percolating austenite in hindering the passage of hydrogen into the steel during hydrogen charging, and its permeation through the composite nanostructure. The effect is not as large as might be expected from a simple comparison of independent data on the diffusivities of hydrogen in the two lattices, because the effective diffusivity in ferrite is found to be much smaller than in the defect-free ferrite, owing to trapping effects. The morphology of the austenite is demonstrated to play a role by comparing with a sample containing a larger volume fraction of austenite but present as isolated grains which are ineffective to the permeation of hydrogen.X1199Ysciescopu

Synchrotron analysis of toughness anomalies in nanostructured bainite

High-resolution synchrotron X-ray diffraction has been used to characterise the notch root regions of Charpy impact test specimens of a superbainitic steel, both before and after loading. The changes in the volume fraction of austenite induced by the application of a three-point-bending load were quantified. Analysis of diffraction peak shifts revealed the extent of residual tensile and compressive strains present due to both machining and an applied load. The results lend support to the hypothesis that the comparatively low energies absorbed during Charpy impact testing of superbainitic steels, < 7 J, are due to the formation of stress-induced martensite at the notch root, prior to crack initiation.The authors are grateful to Prakash Srirangam Venkata and Bernard Ennis for their valuable contribution to the synchrotron work; to Diamond Light Source (Oxford) for access to the synchrotron facilities (under experiment EE8564); to Tata Steel UK and the Engineering and Physical Sciences Research Council of the UK for financial support (under EP/H500375/1 and EP/I02249X/1).This is the final version of the article. It was first available from Elsevier via http://dx.doi.org/10.1016/j.actamat.2015.11.02

Strength and toughness of clean nanostructured bainite

A nanostructured steel has been produced using a clean steel-making technique. The mechanical properties have been comprehensively characterised. The maximum strength of the material recorded was 2.2 GPa at yield, with an ultimate tensile strength of 2.5 GPa, accompanied by a Charpy impact energy of 5 J, achieved by heat treatment to refine the prior austenite grain size from 145 to 20 µm. This increased the strength by 40% and the Charpy V-notch energy more than doubled. In terms of resistance of the hardness to tempering, the behaviour observed was similar to previous alloys. Despite reducing the hardness and strength, tempering was observed to reduce the plane-strain fracture toughness.This work was supported by Rolls-Royce