Artie:
Literally "boiler plate" means a material that is listed in the Boiler Code (officially known in the USA as the ASME Code for Boilers and Pressure Vessels). Obviously there are many such materials ranging from carbon steels to alloy steels containing many other elements. See for example:
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Figuratively it means "the usual stuff", typically a plain carbon steel consisting of iron with less than 0.3% carbon and 1.5% manganese which can easily be fabricated by welding. See also:
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Pittsburgh Pete
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