Impact toughness of 17/4PH versus thickness

[AnnyB]

I have a material data sheet for UNS S 17400 shows the following toughnesses at -196°C:
Condition H1150: 8.8J for test samples from 1" round bar
Condition H1150-M: 38J for test samples from 1" round bar
Condition H1150: 6.8J for test samples from 4" round bar
-Why is the toughness or thicker bar lower? Is it because of quench sensivity of the material?
-Where are these toughness measured? I thought test samples are always cut near the bar surface but then I don't understand the toughness reduction because the surface is always well quenched.
-Should a thick material always be imapct tested from samples taken at the surface and in the core?

Thanks

 

[EdStainless]

Traditionally such samples are either taken from the mid-radius or center, depending on the specifications.
On larger parts you never use surface samples.
On larger parts the grain size will tend to be coarser, which hurts impact toughness.
Also with this alloy the interior of a large section will tend to spend more time wile heating and cooling in the aging treatment in the 800-900F range which also lowers toughness.

If you need toughness look into 15-5PH (designed for thicker sections than 17-4) or even 13-8PH (which is available as remelted material and is super tough).

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P.E. Metallurgy, Plymouth Tube

 

[AnnyB]

Dear EdStainless,
Thank you very much for answering

 

[MagBen]

Note, 1'' and 4'' bar use a different cooling rate after hardening. H1150M has a lower solution treatment before aging to 1150F, so there are more reverted austenitic in H1150M than H1150, so considerably higher impact.
A 4'' bar can hardly get a very uniform structure from center to surface. For bar size >= .75'', the impact specimen is normally mid-radius. There are some difference in impact strength between big and small bars, especially at transverse direction.