Nucor Steel has an extensive special bar quality (SBQ) steel program that dates back to the conversion of its plants in South Carolina, Tennessee and Nebraska to handle the product decades ago.

Many components and assemblies have internal features that are difficult to inspect, none more so than additively manufactured (AM) parts. Conventional quality control requires samples to be sectioned and subsequently scrapped. However, the advent in the 1980s of X-ray computed tomography (CT) for industrial material analysis and nondestructive testing heralded a new era for inspection of such parts, both internally and externally.

HTHA occurs in low-alloy carbon steels that are subjected to high pressure and high temperature, typically above 205°C (400°F). In this environment, hydrogen can dissociate and react with carbides in the steel to form methane.

A similar incident two years prior prompted the engine’s manufacturer, CFM International, to recommend inspections of fan blades on CFM56-7B engines with 20,000 cycles. All told, the FAA and CFM bulletins covered approximately 3,160 engines, or 76,320 separate fan blades.