416 Stainless Steel Heat Treatment Question 1

[SMF1964]

I have three studs made from 416 stainless steel, consistent with ASTM A582. Two of them have a hardness of ~R[sub]B[/sub] 90 while the third has a hardness of R[sub]B[/sub] 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 [expletive]'d up heat treatment)?

 

[EdStainless]

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

 

[SMF1964]

A582 lists HB 285 max which is ~R[sub]C[/sub] 29 if my conversion table is correct.

 

[metengr]

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.

 

[SMF1964]

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.

 

[metengr]

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

 

[EdStainless]

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

 

[mrfailure]

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.

 

[MagBen]

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.

 

[SMF1964]

The studs appear to be in the annealed condition.

 

[EdStainless]

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

 

[metengr]

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/ImageDispl...ypical+Room+Temperature+Mechanical+Properties

Annealing yes. With values near 100 Rb scale.