What is a ‘lamination’?
When steel is manufactured, it goes through several processes, the last being the conversion of the raw steel ingot into plate that can be used for making a range of things, including locomotive boilers. To convert the ingot to plate, the white-hot ingot passes through several rollers in the rolling mill, to achieve the desired size and thickness.
During this process, impurities, known as ‘inclusions’, can find their way into the steel. These can be a range of unwanted but mainly unavoidable non-metallic compounds such as sulphides, oxides, phosphides or slag introduced during processing. As the ingot is successively rolled, the inclusion extends and flattens, becoming extremely thin and in cross-section, are not usually, but can be, visible to the naked eye. While laminations may not affect the performance of steel used to manufacture boilers and, after all, 76077’s boiler provided perfectly satisfactory service for 15 years, the problem comes when needing to carry out repairs.
The original plan for 76077’s boiler was to weld patches at the bottom of the steel firebox wrapper to replace plate that had thinned due to corrosion. Unfortunately, the non-destructive testing (NDT) revealed many more laminations than usual. Welding can open up a lamination and cause the steel to split. Hence, much more steel has had to be removed from the firebox to a point where there are fewer laminations that can be avoided, allowing the new steel to be welded in.
Words, picture and graphic: Ian Crowder