H. Jazaeri

Quantifying recrystallization by electron backscatter diffraction

The use of high‐resolution electron backscatter diffraction in the scanning electron microscope to quantify the volume fraction of recrystallization and the recrystallization kinetics is discussed. Monitoring the changes of high‐angle grain boundary (HAGB) content during annealing is shown to be a reliable method of determining the volume fraction of recrystallization during discontinuous recrystallization, where a large increase in the percentage of high‐angle boundaries occurs during annealing. The results are shown to be consistent with the standard methods of studying recrystallization, such as quantitative metallography and hardness testing. Application of the method to a highly deformed material has shown that it can be used to identify the transition from discontinuous to continuous recrystallization during which there is no significant change in the percentage of HAGB during annealing.

Multiscale 3D analysis of creep cavities in AISI type 316 stainless steel

Abstract A sample of AISI type 316 stainless steel from a power station steam header, showing reheat cracking, was removed from service and has been examined by a combination of microscale X-ray computed tomography (CT), nanoscale serial section focused ion beam–scanning electron microscopy (FIB-SEM), energy dispersive X-ray (EDX) spectrum imaging and transmission electron microscopy (TEM). Multiscale three-dimensional analysis using correlative tomography allowed key regions to be found and analysed with high resolution techniques. The grain boundary analysed was decorated with micrometre sized, facetted cavities, M23C6 carbides, ferrite and G phase but no σ phase. Smaller intragranular M23C6 particles were also observed, close to the grain boundaries. This intimate coexistence suggests that the secondary phases will control the nucleation and growth of the cavities. Current models of cavitation, based on isolated idealised grain boundary cavities, are oversimplified.

Static and Dynamic Grain Growth in Single-Phase Aluminium

Al-0.1Mg with a 3μm grain size was deformed in channel die plane strain compression at temperatures up to 200oC. It was found that the reduction in grain thickness was significantly less than that predicted from geometric considerations, and at larger strains, a minimum high angle grain spacing, which was equal to the crystallite size was achieved. The velocity of the high angle boundaries during this process is very many orders of magnitude larger than that predicted for curvature driven grain growth, and some possible explanations for this are discussed.

Application of small angle neutron scattering to study creep cavitation in stainless steel weldments

Abstract In the present paper, the use of small angle neutron scattering to study creep cavitation around reheat cracking in two 316H austenitic stainless steel header components is reported. The components had been removed after long time high temperature operation in nuclear power plants. Cavities up to 350 nm in size are quantified, and their distribution relative to the crack, near the crack origin and away from the crack tip along the crack direction, is measured. Cavitation increases significantly on approaching the crack near the crack mouth. Cavitation is less prominent near the crack tip with minimal variation away from the crack tip. More extensive cavitation near the crack is found in the component with the longer reheat crack.

Study of creep cavitation in stainless steel weldment

Abstract A study of creep cavities near reheat cracking in AISI Type 316H austenitic stainless steel headers, removed from prolonged high temperature operation in nuclear power plants, is reported. It is shown how application of scanning electron microscopy (SEM), cryogenic fractography and small angle neutron scattering (SANS) can be applied, in a complementary way, to observe and quantify creep cavitation. Creep cavities in the vicinity of the crack are found to be mainly surrounding inter-granular carbides. Trends in the size and area fraction of creep cavities relative to the crack path are quantified using optimised metallography. The SANS technique is found to be a very suitable method of quantifying creep cavitation within the size range up to 600 nm averaged over a larger gauge volume. It is shown that the cavity size distribution peaks in the region 100–300 nm, and this correlates closely with the quantitative SEM observations.

The Effect of Initial Grain Size on the Microstructures Developed during Cold Rolling of a Single-Phase Aluminium Alloy

High resolution Electron Backscatter Diffraction (EBSD) in a field emission gun scanning electron microscope (FEGSEM) was used to study substructural development during the deformation by cold rolling of a single-phase Al-0.1Mg alloy with initial grain sizes between ~3µm and 120µm. In the coarse-grained material, bands of elongated cells aligned at approximately 35º to the rolling direction were formed at low strains. However, as the grain size was reduced, fewer of these aligned microstructural features were formed, and at the smallest grain sizes, there was little evidence of significant substructure within the deformed grains. The alignment of low angle boundaries was analysed from EBSD data and shown to be a function of grain size, strain and boundary misorientation.

The effect of initial grain size on transition from discontinuous to continuous recrystallization in a highly cold rolled Al-Fe-Mn alloy

The deformation microstructures and the recrystallization behaviour of a highly deformed Al-Fe-Mn alloy has been investigated using high resolution EBSD. Of particular interest have been the effects of the initial grain size and the strain on the nature of the recrystallization process. The results show that conventional discontinuous recrystallization takes place for large initial grain sizes and low to moderate strains. However, small initial grain sizes and large strains promote a transition to continuous recystallization and a resultant micron-grained product.

Study of Creep Cavitation in a Stainless Steel Weldment Using Small Angle Neutron Scattering and Scanning Electron Microscopy

Strain-relief cracking, also referred to as reheat cracking, is a generic creep failure mode that has been observed in many welded stainless steel structures operating at high temperatures in UK nuclear power plant. One of the challenges currently faced by nuclear utility EDF Energy is the need to predict, with high certainty, the life-time of ageing engineering plant operating in the creep regime.A study of creep cavities near a reheat crack, ∼28 mm long, in an AISI Type 316H austenitic stainless steel header is reported. The cracked component was removed after 65,000 hours operation at a mean temperature of 525°C in a nuclear power plant. Creep cavities in the vicinity of the crack are observed using scanning electron microscopy (SEM) and found to be mainly surrounding inter-granular carbides. Optimized metallographic and image analysis procedures are used to quantify the size and area fraction of creep cavities as a function of distance from the crack along a line normal to the crack face. Small angle neutron scattering (SANS) is used to quantify creep cavitation, averaged over a large gauge volume, ∼20 mm3, at similar positions normal to the crack line. A systematic increase in the true fractional size distribution of cavities is observed approaching the crack. The cavity size distribution peaks between 100–300 nm diameter and this correlates closely with the quantitative metallography results of cavities in this sample. SANS has the advantage of being non-destructive and also the examined volume is several mm3, against an area of hundreds of μm2 in the case of SEM, therefore giving more statistically significant results. However the two techniques are complementary, as the detailed microscopic information from SEM is required to interpret the SANS data.Copyright