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Hardness test

We are manufacturing pressure vessel, and recently the customer want to do the Hardness test. The BHN# want the customer. Do we need the hardness test after build the vessel.

The vessel Design pressure is 150 FV psig.
and the design temperature is 300 F ( Degree Faranite)

Please let me know the answer?

Thanks.

Moe

Moe Hussain
- Louisville, Kentucky, USA
August 20, 2008

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August 22, 2008

I can not see why your customer would want BHN A brinell hardness number is a "micro hardness" number which means that it is a destructive test as you need a small piece to mount, polish and check. The equipment to do this test is not cheap when compared to a portable Rockwell hardness tester that is in the correct range.
You need to find out if you can use a piece of scrap or a "drop" that is not near any flame cut since it will destroy the tank to get the "coupon". Find out what a certified lab will charge you for this test and let the customer know that this test will be an extra charge for each time that they want it done.
I think that a portable Rockwell tester is the way to go since it does minimal damage to the tank and is not terribly expensive. It is operator sensitive and will give bad information if it is not done properly.

James Watts
- Navarre, Florida

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August 26, 2008

Hi Moe and James,

I must contradict you, James. You seem to be describing the Vickers hardness test, done on a polished surface with a diamond indenter and indeed requiring a large and expensive machine to do it.

The Brinell hardness test is a macro test which involves (for testing steels) pressing a 10 mm diameter tungsten carbide ball onto the surface of the steel with a 3000 kg force. This leaves an impression in the steel which (again for steels) is likely to be in the range 2 to 4 mm diameter. From the impression diameter and the ball diameter and the load, is calculated a number (the Brinell Hardness Number). In practice, a printed table of values is used to look up the Brinell number, rather than doing an involved calculation for every test.

There is a wide range of machines available for the test, ranging from permanent floor-standing machines, to portable "wrap-around" styles that are variously mechanical or hydraulic and weigh only a few kilograms but still apply the correct 3000 kg load.

There are also hand-held electronic testers that use a quite different test procedure then report the results of the test in whatever units (such as Brinell) that you want, but these devices must be calibrated against proper testers and are really only suitable for repeated testing of similar items - they have their place, but that place is not isolated tests on different components.

There is a straight-line relationship between the Brinell hardness number and the tensile strength of the steel.

The only possible reason for the test in the circumstances that you describe, is as a quick check on the strength of the steel plate that you are fabricating. It can easily be done without damaging the plate, on the plate stock before you cut and weld it.

The Brinell test cannot be used to monitor the localised hardening that might be caused by welding, because the diameter of its test impression is greater than the size of weld-affected zones, so it is quite impossible to distinguish between parent metal, heat-affected zone, and weld metal. For that sort of examination, you must indeed destroy the item so that a Vickers tester (See James' post) can be used.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.

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Hi Bill,
My bad, I confused it with knoop.

James Watts
- Navarre, Florida
August 27, 2008

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August 30, 2008

Hi James,

Yes, Knoop is a totally microscopic test, useful on areas of individual phases in the microstructure.

Vickers is, I guess, partly microscopic. At least you can see the impression with the unaided eye and it was made under macro conditions, but you need the microscope that's built into the testing machine in order to measure the actual size of the impression.

Brinell is close enough to totally macro. Often a graduated magnifier (around X3 or X5) is used to measure the impression but that hardly qualifies as a microscope.

Cheers,
Bill

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.