Last July, a customer’s incoming inspection flagged a lot of A-286 bolts for low hardness. Our metallurgy team pulled the furnace chart. The chart showed the load thermocouple had read 702°C during the aging cycle, not the specified 720°C. The furnace controller setpoint was 720°C. The actual metal temperature, measured at the load, was 18°C lower.
The bolts passed tensile testing at room temperature. They would have shipped. They would have been installed. And they would have under-performed in creep at 650°C: not failing outright, but relaxing faster than designed and needing re-torque at every inspection interval for the life of the machine.
The customer’s incoming hardness check caught the 18°C because they didn’t rely on the MTC alone. Here’s what they did differently.
What “trust but verify” looks like for heat-treated bolts
Most customers accept the mill test certificate and move on. This customer’s incoming QA procedure includes:
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Hardness test 3-5 bolts per lot. For A-286 per AMS 5732, that’s 24-35 HRC. Values near the bottom of the range are acceptable but warrant a closer look at the furnace chart.
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Read the furnace chart, not just the MTC. The MTC says “aged at 720°C for 16 hours.” The furnace chart shows whether the load thermocouple actually reached 720°C, for how long, and at what point in the cycle. An MTC that says 720°C is a statement. A furnace chart that shows 702°C is the truth.
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Cross-check the hardness against the tensile cert. A lot with 24-26 HRC hardness that certifies at 1000 MPa tensile is physically possible but statistically unusual for properly aged A-286. It warrants a question to the supplier before the bolts enter production.
In this case, step 2 caught the 18°C offset. Our furnace control thermocouple had drifted, and because the customer asked to see the load thermocouple data rather than just the MTC summary, the problem was caught before the bolts entered their assembly line.
What we fixed on our side
The incident triggered two changes in our heat-treat department:
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Load thermocouple data is now attached to every MTC as standard. Not just for customers who ask. If you order aged superalloy bolts from us, your cert package includes the furnace chart with load TC data (setpoint, actual, and deviation) for the full aging cycle.
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Furnace uniformity surveys moved from quarterly to monthly for aging furnaces. AMS 2750F requires quarterly surveys for Class 2 furnaces. We now survey aging furnaces monthly. The 18°C drift happened in the six weeks between quarterly surveys. Monthly surveys catch drift within two weeks.
Frequently Asked Questions
Why did the furnace chart matter more than the MTC?
The MTC stated aging at 720°C for 16 hours, but the load thermocouple logged 702°C. An 18°C shortfall leaves 4,000 A-286 bolts under-aged, so they relax faster in 650°C creep service even though the room-temperature tensile test still passes.
What hardness should properly aged A-286 show?
Per AMS 5732 the acceptance band is 24-35 HRC. A reading of 24-26 HRC that arrives with a 1000 MPa tensile cert is physically possible but statistically unusual, and it is worth pulling the furnace chart before that lot enters production.
How often should aging furnaces be surveyed for uniformity?
AMS 2750F calls for quarterly surveys on Class 2 furnaces. We moved our aging furnaces to monthly surveys after the 18°C drift, because the offset appeared inside a six-week gap between quarterly checks.
Is load thermocouple data part of the standard certificate package?
Yes, on every aged superalloy lot we ship. The package carries the furnace chart with load TC data for the full cycle, covering setpoint, actual and deviation, and it does not depend on the customer asking first.
What does a 50-piece trial prove on a heat treatment lot?
A 50-piece trial lets you section, test hardness and run elevated-temperature tensile on real parts before a production release. It is the fastest way to confirm the aging cycle behaves the way the certificate claims.
Bottom line: An MTC that says 720°C for 16 hours is a promise. A furnace chart with load thermocouple data is proof. When you receive aged superalloy bolts, ask for the chart. If the supplier will not provide it, ask why.