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416 Stainless Steel Heat Treatment Question

416 Stainless Steel Heat Treatment Question

416 Stainless Steel Heat Treatment Question

(OP)
I have three studs made from 416 stainless steel, consistent with ASTM A582. Two of them have a hardness of ~RB 90 while the third has a hardness of RB 99. Similarly, tensile specimens machined from these three studs have a tensile strength of 87 ksi vs 105 ksi. The two lower strength bolts are just over the minimum limit for A582 Condition A, but the higher hardness/strength stud is not high enough in tensile strength to meet the "T" condition. These studs are from a main boiler feed pump, so the service temperatures should be between 250°F and 350°F, so I believe 400-500°C embrittlement mechanism can be ruled out. Are there any thoughts that might explain the elevated hardness (other than a(n) [expletive]'d up heat treatment)?

RE: 416 Stainless Steel Heat Treatment Question

Different lots, chemistry, heat treatment, and properties.
RB99 is still within range for annealed material isn't it? I thought that this was RB100max.

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P.E. Metallurgy, Plymouth Tube

RE: 416 Stainless Steel Heat Treatment Question

(OP)
A582 lists HB 285 max which is ~RC 29 if my conversion table is correct.

RE: 416 Stainless Steel Heat Treatment Question

I could accept improper heat treatment. Why not use SA 193 B7 or B16 stud material? These are studs commonly used in BWCP and BFP's. I would stay away from the 416 stud material.

RE: 416 Stainless Steel Heat Treatment Question

(OP)
Metengr,
I'm leaning towards improper heat treatment, with the harder stud coming from a different heat/manufacturer. As for why this material was used, this is a failure analysis project - three studs, all broken, with two fatigue failures at one end and one overload (microvoid coalescence) on the third stud at the other end relative to the fatigue failures - the higher hardness stud is the overload fracture.

RE: 416 Stainless Steel Heat Treatment Question

SMF1964;
Interesting. As part of your RCA, I would avoid this material specification for studs in this type of service.

RE: 416 Stainless Steel Heat Treatment Question

1. I doubt that there is anything wrong with the material or the heat treatment
2. 416 is a free machining grade and is deliberately loaded with sulfide inclusions, it is not an appropriate fastener grade. The B16 would be amuch better choice.

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P.E. Metallurgy, Plymouth Tube

RE: 416 Stainless Steel Heat Treatment Question

This goes with my rule of thumb that whenever I get a failure of a free-machining resulfurized steel part, the root cause almost always turns out to be that they used the free-machining steel instead of a structural steel. Pay attention to what Ed said: These steels should NEVER be used for fastener applications.

RE: 416 Stainless Steel Heat Treatment Question

Are you using this grade in the annealed condition which will be ferritic structure? Or, was it hardened and tempered at very high temperature with a martensitic structure?
When hardened at 1800F + >1200F tempered, this alloy will yield a similar hardness as solely annealed condition, but will have a totally different structure.

RE: 416 Stainless Steel Heat Treatment Question

(OP)
The studs appear to be in the annealed condition.

RE: 416 Stainless Steel Heat Treatment Question

The Q&T structure would be the result of martensite formation, and if like most 416, this material has low carbon then the material would not Q&T to a higher hardness. I have seen 416 that would only reach 20RC (100RB) in the as quenched condition.

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P.E. Metallurgy, Plymouth Tube

RE: 416 Stainless Steel Heat Treatment Question

Typical properties for 416 supplied by Cartech below for Q&T. I doubt the carbon content would be low enough to provide an as-quenched hardness of only 100 Rb scale.

https://cartech.ides.com/ImageDisplay.aspx?E=79&am...

Annealing yes. With values near 100 Rb scale.

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