Over the years, ESR Technology’s Forensic Engineering and Materials team have examined a wide range of metallic structures. Many of these items (bridges, pillars, columns, panels, beams and even cast statues) are of historic interest and measures must be taken to preserve them.
When repairs to historical structures are necessary (weld repairs, patches, re-painting), the main objective is to determine the material type. This can be carried out using techniques that are minimally invasive. A popular method is to use on-site metallurgical replication of the surface, which involves polishing a small area to a mirror finish, carrying out a mild acid etch and taking a replica of the grain structure. This is very effective and causes no structural damage.
In order to determine the material composition, a small drilling or filing can be taken from an area of minor visual impact. An alternative method to determine the microstructure, hardness and material composition is by removal of a small section of material for laboratory analysis, this however is not always possible, particularly on heritage structures.
Cast iron is a common structural material, as it is low cost, versatile and long lasting. It has several forms, depending on the original composition of the iron and the rate of cooling. These variations in microstructure affect the properties in terms of strength, ductility and corrosion resistance.
Figure 1 shows a pearlite blackheart cast iron, which is a common material used in supporting pillars and columns in Victorian railway stations. The metallic structure contains coarse islands of graphite (black areas) within a pearlitic lamellar matrix (plates of ferrite and iron carbide). The black, craggy lumps of graphite create a strong, high strength material that has good impact properties, although is not as wear resistant.
The cast iron shown in Figure 2 has an entirely different structure, containing thin flakes of graphite randomly dispersed within a matrix containing iron carbide (white areas), pearlite (stripy lamellar areas) and phosphide eutectic (grey spotty area). This is typical of material used to form complex shapes, such as railings, large cheap castings and engine blocks due to its high fluidity and superior corrosion resistance.
Figure 1: Blackheart cast iron
Figure 2: Grey flake cast iron
Non-destructive replication can be performed to examine the steel or cast iron microstructure. More information relating to the process of the technique can be found here Metallurgical Replication – A Forgotten Non-destructive Art?
On-site replication can be carried out oat relatively short notice, causes minimal disruption and provides a permanent record of the microstructure, or metallurgical defects identified, which can be examined either immediately after replication or can be taken to the laboratory for more detailed examination. The technique has been used extensively by ESR Technology Ltd for Crossrail, GrowHow UK Ltd, Environment Agency and Hyder Consulting.
If you are interested in the replication process or require a non-destructive approach to material identification, defect examination or creep and thermal degradation assessments feel free to contact Mr Eirwyn Davies (email@example.com).