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Super Duplex SS Hardness limit (minimum)

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metboss

Petroleum
Sep 12, 2012
152
Hi All,

Can anyone please advise me on below query ?

We received raw material (Super Duplex Steel Bars UNS S32760, size: 160mm OD, Heat treatment: Solution Annealed and quenched) and it was found that 19 HRC(216 BHN) in Material Test certificate.
But, when we measured, the actual hardness is found to be 255 BHN which is within the acceptable limits of hardness as required for Super Duplex (270 BHN Max).
When we contacted our principal supplier about the differences which we found in hardness values, they clarified that mill performs hardness at mid wall and that’s why, mill measured hardness value (216 BHN=19HRC)) is lower than surface hardness value (255 BHN) what we measured.
Can we accept this justification ? What is the minimum hardness value for Super Duplex ?
The intended material should have a greater abrasion resistance and it will be used for downhole completion (drilling) application.

Thanks and regards
 
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I don't believe there is a minimum hardness spec, so long as it meets yield and UTS. It's up to you to specify a minimum hardness for abrasion resistance.

Nathan Brink
 
When you think about it the min UTS and min yield set the low end, and the min elongation and max hardness set the upper end.
My hunch is that the hardness at mid-radius is being taken on the tensile sample from that location. This is fairly common.

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P.E. Metallurgy, Plymouth Tube
 
Thanks NBrink & Ed for your input.

Final question, is there any reason behind such drastic hardness variation between surface & mid-wall (from 255 to 216 BHN) ?

 
My guess is that either they were given a light draw pass for size control/surface finish improvement, or it could be from rotary straightening. Either of these would impart some cold work to the outer surface.

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P.E. Metallurgy, Plymouth Tube
 
Yeah, processing could account for a variation in hardness, but just to be on the safe side, I'd look into your hardness testing procedure.

I've seen huge variations in hardness between those little handheld hardness testers that a lot of end users and fabricators use. They're very dependent on proper surface prep, calibration, etc. Could be a real variation in hardness, but could absolutely be a measurement error...

Nathan Brink
 
And of course there is the correction to make for testing on a curved surface, deducting a bit from the readings.
And there is no conversion for duplex stainless between hardness scaled, don't try.
We had to build our own.

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P.E. Metallurgy, Plymouth Tube
 
Thank you all for your valuable feedback.

 
Testing mid radius is a route practice for big bars, that is more representative than that on surface. Any cold working tends to harden surface more, so surface could get a higher strength/hardness. Also, Rockwell scale normally gets you a lower converted HB compared with direct Brinell.
 
Thank you all for your answers.

Even, the supplier has also come up with the same explanation that outer surface of our raw material: hollow cylindrical bar (6" OD x 4" ID)was subject to skim machining/cold working which induced a slight increase in surface hardness. Again, other pieces are also tested using Rockwell testing unit (HRC scale)and results are as follow:

Hardness as per mill MTC: 21 HRC (material condition: 25Cr Super Duplex; solution annealed & quenched

Measured surface hardness after surface machining at fabrication workshop: 23 HRC



 
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