Hardness testing tubulars

[kevlar49]

When hardness testing tubulars, I have read that there is a certain thickness-to-diameter ratio required to get an accurate Rockwell test. Where can I find guidance on that? We have tested this carburized tubular 1020 with the Rockwell 15N scale and still get low readings, but I am not sure if it is my hardness testing method or the heat treat process that is the problem. Unfortunately, I cannot section the part to take a microhardness.

 

[TVP]

EdStainless should be along shortly with some helpful advice, but in the meantime you can check out this link, which is from Ed Tobolski of Instron/Wilson Instruments:

http://www.thefabricator.com/articl...hardness-and-brinell-testing-of-tube-and-pipe

 

[redpicker]

Of course, it is possible that your parts are just soft. What readings are you getting compared to what you were expecting. 1020 does not have much hardenability, so it can be tricky to carburize. What is the section size? What are your heat treatment parameters?

Are you sure you are getting them into the quench quickly enough? Years ago, I was heat treating some 1020 tubes that came out < 45 HRC on a mounted section. They were quenched from 1560F, which was standard for the shop for carburized parts. I raised the temperature to 1700F prior to quench and they jumped to >60 HRC. They were so small that just transferring from the furnace to the quench allowed them to cool below the Ar3.

And, you should be running a coupon with your parts if the parts cannot be sectioned, so you can section the coupon to verify the process.

rp

rp

 

[kevlar49]

1550F with no ammonia (purely carburizing)90min, 600F temper 3 hrs, section is 0.06" thick, 5/8" OD.

Good idea about the coupon. I included that on the re-write of the instructions.

We checked a part by reaustenitizing for 10 min at 1550 F and then oil quenching and found that the hardness jumped from HRC 29 to HRC 36.

I suspect that we should have used ammonia. I am specifying 5% of the endo gas but no lower than 4% to avoid void formation.

 

[EdStainless]

Since you are using a superficial Rockwell I suspect that you are OK when it comes to deformation under load.
What is the OD and wall for these parts?
Do you correct for the curvature of the parts? If they are large diameter it isn't a big deal. On sizes under 1.5" it has a noticeable effect. When you test on the OD of a tube or bar the portions of the intention to the sides are not fully supported and you get indentions that are a little too deep, showing lower hardness. It may only be 1 point. I had a correction table somewhere but I can't find it now.

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Plymouth Tube

 

[kevlar49]

OD 5/8" so <1.5" we might see an effect. Wall thickness is 0.060". Yes, we corrected for roundness. The HRC numbers reported earlier were converted from HR 15N.

 

[blacksmith37]

90 minutes @ 1550F should pretty much bring the full thickness (30 mils each side) to the carbon level potential of the furnace atmosphere.

 

[CoryPad]

Are you measuring directly on the OD surface? You need to check ASTM E 18 or similar to see about limiting wall thickness and curvature correction.

 

[kevlar49]

Corypad,

I am taking into account the roundness correction.

 

[EdStainless]

First, STOP converting. I have fired outside labs for not reporting the actual hardness test values. Unless you have conversions that have been done by multiple parties on you actual samples then don't trust published ones.

You have to cut up some pieces so that you can test on the OD while supporting it with a ball end anvil. It is the only way to know that you are safe with your method.

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Plymouth Tube