Vendor selection and QC sign-off — separating real nitriding houses from job shops

Ask these questions in this order. Each one filters out a class of vendor before the next one is worth asking.

1. "What atmosphere control do you run on gas cycles: open-loop ammonia bleed, dissociation pipette, or hydrogen-sensor closed-loop?" An open-loop shop is delivering whatever the retort dissociates that day. A pipette shop is delivering snapshots between readings. A hydrogen-sensor shop is delivering a setpoint. This question separates Kn-controlled from open-loop in one sentence.

2. "Do you work to AMS 2759/10 or /12, and what class?" AMS 2759/10 governs Kn-controlled gas nitriding; /12 governs Kn- and Kc-controlled FNC. A shop that runs to either has an instrumented atmosphere and a defined class structure. A shop that has never heard of the spec is not in the aerospace-grade tier.

3. "Are you Nadcap heat-treat certified, CQI-9 audited, or both?" Nadcap is the aerospace audit; CQI-9 is the AIAG automotive heat-treat assessment. Neither is required for commercial work, but their absence is information.

4. "What microhardness equipment do you have on-site (Vickers or Knoop), and what load range?" A shop with a Vickers or Knoop tester in-house can section a coupon, polish it, and run a traverse without shipping to an outside lab. A shop that ships to an outside lab will not catch a bad cycle until the certificate comes back two weeks later.

5. "What is your typical turnaround on an H13 die at 0.4 mm case depth?" Sets the expectation. A 60-hour cycle plus queue, metallography, and certificate prep runs three to five weeks at a serious house.

6. "Do you run a witness coupon program?" A coupon of the customer's substrate sectioned and traversed alongside the production load is the only way to prove what the load actually saw. A shop without a coupon program is asking you to trust the furnace log.

7. "What compound layer thickness and phase do you typically deliver on H13 at the case depth I quoted?" A shop running a controlled process answers with a range, for example "6-10 µm, γ' dominant at our standard Kn setpoint." A shop running open-loop gives a vague answer or quotes a number with no phase.

8. "Can you send me a redacted sample certificate from a similar part?" The most diagnostic question on the call. A serious house sends a certificate with a microhardness traverse plot, compound-layer thickness from a microsection, phase identification, Kn cycle log, and load traceability. A job-shop certificate is a single surface hardness number and a date.