Spotlight – How to... effectively use microscopy in the metals industry
Dr. Roger Barnett, Applications Development Engineer, Carl Zeiss Microscopy Ltd, gives an overview of microscopy in the metals industry.
At its most fundamental level, microscopy is the use of tools to observe objects and features that cannot be viewed with the naked eye. With the right sample preparation and microscope, a user can see features in metals ranging from cracks and surface pits to individual grains, inclusions and even dislocations. This study, referred to as metallography, gives a clear picture of the microstructure of any metal component or material.
However, simply viewing the structure is not enough – industrial microscopy needs to yield information that a user, or automatic process, can act on to add value to the product. World production of steel alone runs to over a billion tonnes a year and incremental improvements to a process can yield massive savings.
Choosing the technique
The first step is to select the most effective microscopy techniques.
Optical microscopy is the classic metallographic tool, a widely available and mature technology that is ubiquitous throughout industry and academia. A high-end light microscope can resolve features of 0.25µm or less in two dimensions on properly prepared cross-sections. Multiple imaging modes, combined with different metallographic preparation methods, enable differentiation of regions of interest – grains, inclusions, cracks and different phases.
Confocal optics and laser scanning extend light microscopy into three dimensions, mapping surface topography with a height resolution in the nanometre range. For smaller features, a scanning electron microscope (SEM) can resolve objects down to the nanometre range, and different detectors show contrast by topography, elemental composition or even crystallographic orientation. X-ray microscopy or assessment of multiple tomographic slices prepared by focused ion beams or related methods allows full 3D mapping, making cracks, pores, changes in density or 3D crystallography visible.
How is it used?
Some of the most effective and important uses of microscopy in the steel industry are the most routine – process control and quality assurance along the entire production workflow. Light microscopy can determine the composition and phases present in metallurgical coal to improve efficiency of iron production, as can mineralogical analysis of iron ore. Non-metallic inclusions are present in all industrial steels and the type and amount of these inclusions affect the bulk properties – tensile strength, toughness and fatigue limit. Inclusions can potentially cause a critical failure of the entire component due to a reduction in properties or initiating a crack, in the case of unusually large inclusions.
Alloy 625 (a nickel alloy) overlay on 8630 steel, elemental composition mapped by energy-dispersive X-ray spectroscopy (EDS) on a scanning electron microscope. Sample provided by TWI Ltd.
3D Surface topography of structural steel after grit blasting with alumina. Sample provided by TWI Ltd.
These are therefore automatically mapped and quantified by light microscopy and individual inclusions assessed in detail by SEM. The yield strength of a metal is strongly affected by grain size. This parameter is straightforward to determine by light microscopy and ensures a product falls within specifications.
At the surface, roughness governs the mechanical behaviour at the interface between two materials and affects joining and contact processes. It can affect the bond strength of any applied coatings, susceptibility to corrosion, wear behaviour or the cosmetic appearance. Confocal and/or laser scanning light microscopy non-destructively builds a 3D map of a surface, which can then be analysed according to international standards to quantify the roughness, peaks, troughs and general surface texture. It is effective for quality assurance and ensuring materials meet specifications, or for procedure development and monitoring of surface-affecting processes. The effectiveness of this method is not limited solely to roughness – any feature visible at the surface can be measured and the data fed back into a process, be it corrosion pitting, localised deformation or even just the shape and size of a weld cap.
Light micrograph of 316 stainless steel, showing clearly visible grain structure. Supplied by TWI Ltd.
Microscopy is also effective in improving the manufacture of metal components, not just for routine assessment of components but in the development of new manufacturing and joining procedures. For example, welding two different materials together is often challenging, with many issues to be overcome. These include different coefficients of thermal expansion, varied melting temperatures and formation of brittle intermetallic compounds in the weld – an inherent weakness that may later lead to failure in service.
Alongside mechanical testing, advanced analytical microscopy and energy-dispersive X-ray spectroscopy (EDS) of trial joints allows the manufacturer to understand and modify its welding processes to avoid excessive dilution, cracking or any other danger signs. Here, niobium and molybdenum concentrate along grain boundaries indicating segregation and precipitate formation during welding, while iron is observed penetrating up to 20µm into the nickel alloy – often observed in joints of this type.
No system is perfect and there is always a risk of component failure during service. Another critical application of microscopy is the science of failure investigation to determine the root cause of a failure. Often, multiple microscopes are required to correlate features at multiple length scales and properly understand a complex failure. Scanning electron microscopy allows viewing of fracture faces to identify failure mode and initiation point, EDS identifies and locates corrosion products, light microscopy of cross-sections gives information on local microstructure and X-ray microscopy is useful to map pores, voids and cracks in three dimensions. This information is matched with in-service data to identify all contributing factors to a failure. Knowing the cause allows manufacturers to design against future failures, recall faulty batches and verify whether a failure was due to improper manufacture or service.
For more advanced alloys or metals research, other microscopic techniques provide actionable information. X-ray microscopy can generate 3D maps of density, phase and porosity with resolutions down to 50nm. Helium-ion microscopy allows resolution of features at the <0.5nm scale. SEM can be equipped with electron-backscattered diffraction systems that determine crystal orientation maps showing texture and local strain. This can be applied in 2D or in 3D by sequential analysis of metal slices prepared using focused ion beams. It is even possible to create 3D maps of grain orientation and texture over volumes as large as 1mm3 using diffraction contrast tomography on an X-ray microscope.
For routine jobs, an experienced supervisor should determine a standardised workflow for operators to ensure quality, but for expert areas such as failure investigation, more freedom is required. Microscopes are amazingly flexible tools, but there is no single imaging or analysis technique suitable for every job. The right microscope, analysis and sample preparation must be selected for the job to be done to give the right actionable information in the minimum time.