4130 hardness --will test direction make a big difference?

[katrinali]

Hi Guys,

I came a cross an issue with hardness value on the 4130( 75/80K) bars:

we did some lab hardness test(from 3rd party, quite qualified) on 20mm thick slice,( milled surface) the test location is on the cut surface it self, 2 test around the edge, 2 test around mid radius , and 2 test around core, the hardness value we got is lower than 207, most of them are around 195-205 .
Another test was done by our supplie, sample cut from the same bar, 20mm thick and test done on the cut surface, while they got 210-220 surface hardness. they also did the test on the side surface of the bar, which is way higher, reach 240 or more.

My question is will the direction of the test influrence test value a lot ?( i know charp test does, but how abt hardness test? )

Coule anyone please give some hint on this one, thank you .

Cheers
Kat

 

[metengr]

First, how was the hardness measured? What scale? How were the surfaces prepared for hardness testing?

I think you should review the attached information before you start looking at material issues.

http://www.tensiletest.com/products_services/dwnlds/NIST Booklet.pdf

 

[katrinali]

Hi metengr,

Thanks for the information.

The 3rd party hardness test was done as per ASTM A370 and E10, and test equiment is Adelaid CR-300 KN , with a serial number.. surface was milled. Test method is 3000 kg brinell test . ATM universal tester and birnell adapter with 10mm diameter tungsten ball.

The other test with a higher hardness result was done using testing machine from : WOLPERD GROUP ,as per ASTM A370 AND E10, also milled surface. The test result near the edge of diameter are around 20BH diffence betweenthe cutting surface and the side surface.

This is really confusing ...

Thanks,
k

 

[metengr]

Kat;
Thanks for the follow-up. I have seen this type of variability in Brinell hardness numbers between labs and between material samples of the same heat. In general, the orientation for testing is not critical for macrohardness testing using Brinell or Rockwell testing machines, as with tensile or impact test specimen orientation. I have seen more spread in hardness with microhardness testing results regarding test orientation - mostly related to grain texture from forming.

I would place the spread in macrohardness based on using two different Brinell hardness testing machines and technique.

 

[CoryPad]

If this is cold formed bar, then the outer surface can be harder than the core. This could explain the difference between 195 HBW and 240 HBW.

 

[katrinali]

Hi metengr,

Do you think we can rely on the test result, if the machine has the calibration done regularly?
Actually we did another test which is just for reference, portable hardness test. which also shows a lower result, aroun 185-190. surface has been slight removed to obtain a smooth surface.

Thanks
Kat

 

[katrinali]

Hi Cory pad,

Thanks for the reply, i do understand the surface should be harder around the edge of the bar, but the problem is ...

Both test location is around the edge of the bar diameter.
few test was done on cutting surface<--which is away from the core and close to the surface.
few test was done on the bar surface<--longitudinal surface( have a radius, but milled smooth surface).

The test result is around 20BH difference...

Thanks,
Kat

 

[redpicker]

What size bar are we talking about?

Round bar or rectangular bar?

Where is the test location?

What is the heat treatment?

These are all issues that, while may not have merit with respect to your original question, could affect the hardness, particlularly when performed at different labs.

It is good practice to grind a smooth surface if you want an accurate result. I am not surprised that readings taken on a milled surface can vary by 10%.

rp

 

[EdStainless]

Look at the precision and bias statements in the hardness test spec.
Hardness is an approximation.
If they were not done with the same surface finish, same location, same diameter of ball, and same load, then the difference is to be expected.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[metengr]

kat;

Actually we did another test which is just for reference, portable hardness test. which also shows a lower result, aroun 185-190. surface has been slight removed to obtain a smooth surface.

Now you have introduced another variable by virtue of the fact that portable hardness testing is its own animal. Stick with the program and submit the second test bar to the first lab you mentioned and see what the results are before you go any further. Now you have the same machine with proper calibration with the second bar sample for a true hardness comparison.

 

[OGMetEngr]

I agree with most comments. Brinell varies from lab to lab, machine to machine. We see this all the time where I work. Portable testers also often give results quite different than fixed testers.

 

[katrinali]

Hi redpicker,
Thanks for the reply,the bar is larger than 10 ''round bar, Q&T, 80K, sample cut from the middle of the bar ( avoided the end of bar) test was done on the cut surface and longitudinal surface both, all milled properly .
The thing is 10% make a big difference ....

 

[katrinali]

Hi, EdStainless

Thanks for the reply, Here is the tricky part, customer have specs..and for 75/80k material , specs already give a range...which allow you go to the minimum..

Thanks.
k

 

[katrinali]

Hi metengr

tHANK YOU~!
I think we will do the " swich and test again", since it is hard to tell now .both lab test are qualified...
The portable test is our first quick reaction, i know usually it was not that accurate...


Thanks,
k

 

[redpicker]

katrinali
A 10 inch diameter bar is rather large for 4130, and you will find the hardness to be different at different depths below the OD surface, with the hardness being harder at the surface and dropping off towards the center.

When you say "sample cut from the middle of the bar", I assume you mean mid-length. You should be specifying and recording the location of the hardness test as a depth below the surface. If you expect to be able to compare hardnesses, you will need to be sure they are at the same depth.

If the most accurate hardness is required, it is recommended that the milled surface be ground to remove machine marks prior to testing. Personally, I prefer Brinell testers that actually read the diameter of the impression over those that provide a direct reading. I cannot tell from the models you posted which type your tests were performed on (I think you may have the make/model wrong). Regardless, making sure the tester is verified with a certified test bar prior to testing the part will remove a lot of error.

rp