Final answer:
A butt joint is used to join two flat steel plates at a 90-degree angle when one plate is lying flat on the workbench and the other is centered upright against it.
Explanation:
Butt joint would be the type of joint used to join two flat steel plates at a 90-degree angle, with one plate lying flat on the workbench and the other centered upright against it. In a butt joint, the two ends of the plates are butted together and welded to form a right angle.
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