Hardness value verifcation 1

[metboss]

Hi Gents,

I came across an issue with hardness value variation on the 4130 round bars. The details are as follow:
Material: AISI 4130 Round bar, Heat treament condition: Quenched & tempered, Hardness tester used: Brinnel, Hardness values mmeasured: 283-312 BHN, Acceptable limit as per API 6A: 197-235 BHN.

Now our contractor has come up with following statement:

Hardness readings taken previously were not accurate since material was exposed to environmental aging and hence, surface hardness found more.

Now, they had skim the face of raw material and conducted hardness test again and harness values are coming within the acceptable range of API 6A.


Can we accept this variation in hardness before and after surface skimming(slight machining)? Please advise.

Regards

 

[kumkumvijay]

I share my opinion as below:
1. There is nothing like environmental aging for steel grade 4130.
2. High hardness can be possible due to following reason:
1. quenched with high severity instead of required: wrong HT cycle followed or part wrongly mixed with other part HT cycle for making full load in the furnace.
2. Improperly heating and quenching causing mixed microstructure and high hardness.
3. Wrong BHN measurement on curved surface with out flat surface preparation or curve compensation set up not available in the Brinell Hardness tester.
4. Sever banding issue in the steel causing poor mixing of micrstructure and high hardness on carbon band.
3. Now hardness within spec may be due to:
1. Supplier heat treated the part again.
2. Improper heating and quenching cycle caused mixed microstructure and resulting different hardness when measured at different location.
3. BHN properly measured with flat surface preparation or curvic surface measurement with compensation setting.
4. How to accept the part:
I hope with the following reports, you can accept the parts:
1. Surface and core microstructure picture at 100X magnification and % martensite more than 90% at 5mm depth range.
2. Core microstructure at 100X and % martensite and other phase.
3. No mixed microstructure on the surface.
4. No Banding or banding with in the hardness specification.

 

[TVP]

There is no such phenomenon of environmental aging that can be used to explain elevated surface hardness for quenched and tempered steels. You should have the bars properly evaluated by a third party for surface hardness, core hardness, and microstructure.

 

[metengr]

I concur with TVP for validation and acceptance of this material.

 

[weldstan]

I have observed Brinnel hardness checks improperly made over mill scaled surfaces, with resulting hardness always greater than actual when properly performed. The initially recorded hardness values may have resulted from similar improper performance. Having said that, I would not take the supplier's word and would require further evaluation as per TVP and metengr.

 

[metboss]

What is the way forward? Can we accept or reject? I need to take decision immediately. What is core hardnes and microstructure evalution test? How much raw will it consume for this test? If we go for this test, after cutting sample, can we use remaning raw material for manaufacturing desired component (wellhead equipment)?

Thanks in advance for your advice.

 

[metengr]

You can remove a 1/2" thick cross section from the end of one of the bars and send to a local met lab for the above testing. If the test results are acceptable, you can use the material. If not, material is rejected.

 

[EdStainless]

Don't you require them to cut a slice off anyway?
How else can they give you center and mid-radius hardness values.
And to do that the piece cut off needs to be at least as thick as 1/2D.

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