How is an obsolete spare reverse engineered and qualified?

Carbon is a very light element, and the X-ray fluorescence it produces has very low energy, which air and the sample itself absorb before it reaches a handheld detector. That means standard XRF cannot quantify carbon, even though carbon content decides a steel's grade and weldability. Spark optical emission spectroscopy, or combustion analysis, is used instead wherever carbon matters.

It depends on the alloy and whether the sample can be marked. XRF is non-destructive, portable and fast, and reliable for heavier elements such as chromium, nickel and molybdenum, so it suits stainless and nickel alloys. Spark OES leaves a small burn mark but measures carbon, sulphur and phosphorus directly, so it is the usual choice for carbon and low-alloy steels.

First article inspection is a full, documented check that a new or changed production process makes a part meeting every drawing and specification requirement, run on a representative first production unit before series production starts. Defence and aerospace buyers commonly write it into the purchase order as a requirement for a reverse engineered or newly qualified spare, not as an optional extra.

Form 1, Part Number Accountability, identifies the part and its drawing revision. Form 2, Product Accountability, records raw materials, special processes and functional tests. Form 3, Characteristic Accountability, lists every drawing characteristic with its requirement, the actual measured result and whether it conforms. Together the three forms are the documented evidence that the first article was checked completely.

A full first article inspection covers the first production run of a new part. A partial or full repeat is normal after a design change affecting form, fit or function, after a change of process, tooling, material, supplier or manufacturing location, or after a long gap in production, commonly around two years. A partial inspection then covers only the characteristics that actually changed.

Producing conforming drawings and passing first article inspection is not the final step. For naval and Coast Guard items, the inspecting authority named on the purchase order, typically a DQA(N) or DQA(WP) establishment, approves the drawings and the quality assurance plan before series production and inspects against that approved plan. The buyer's quality directorate, not the workshop, decides whether the spare is accepted.