Hardness testing on part casted with IC 8620 3

[SeasonLee]

We have a OEM part casted with IC 8620, hardness requirement is annealed Rockwell 82~90 Rb.

The problem encounted is :

Shall we remove the scale (or decarburization) before testing the hardness on a casted part ?
We tested the hardness on a flat surface and the hardness reading seems unstable, some of them < 80 Rb and some of them > 90 Rb. But the heat treater tested it on the ground gate area with all data within the specified range. The heat treater insists that's the right place to test the hardness since the gate already ground. Why we have to test it on the ground area ? Any one who can give me the reasons.

Thanks in advance for any inputs.

 

[CoryPad]

You need to remove scale and any decarburized layer when performing hardness testing. The inherent base metal hardness is what is important, as this correlates with the material strength.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SeasonLee]

Thanks for your input, Cory. Further questions arose :

1. Is this a must preparation to remove the scale and any decarburized layer before hardness testing ? Any documents or ASTM test spec cover this surface preparation ?

2. Can we identify the scale or decarburized layer with naked eyes ?

3. What is the reason caused a decarburized layer ? Is it caused by the annealing process during heat treating ?

Thanks again

Season

 

[CoryPad]

1. ASTM E 18 Standard Test Methods for Rockwell Hardness and Rockwell Superficial Hardness of Metallic Materials states:
[tt]
6.1 The test shall be carried out on a smooth, even surface that is free from oxide scale, foreign matter, and, in particular, completely free from lubricants.[/tt]

2. Scale can be detected with an unaided eye - it is dull instead of shiny. Decarburization is detected on a sectioned, polished, and etched specimen. There are files of varying hardness that could be used to determine if there is a decarburized layer, but that usually is for harder steel (> 50 HRC).

3. Decarburization occurs when the carbon concentration of the heat treating atmosphere is lower than that of the workpiece.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SeasonLee]

Thanks again, Cory.
According to the heat treater's comment, we made a test comparison before and after surface grinding, please ref to the photo attached for details, here is the summary :

Sample # HRb data Before grinding HRb data after grinding
5 78.6 87.3
7 81.0 84.0
8 72.4 87.4

1. I am anxious to know what is the reason to make the test data higher after surface grinding ( by belt sanding ).

2. As you can see from the photo attached,are there any oxided scales or decarburized layer on the samples before grinding.

3. For this case, do you think is it correct to grind the surface before hardness testing.

Have a great Thanksgiving holiday

Season

 

[CoryPad]

1. There is a soft layer on the surface that is removed.

2. No obvious scale or other surface irregularities. Decarburization is not detectable via external viewing.

3. Yes, I think it is correct since you want to know the bulk properties, not just a surface-localized value. This is assuming your design does not require a hard surface (for wear resistance or fatigue crack initiation).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SeasonLee]

Thanks for your prompt reply, Cory

We are an OEM supplier and we are not quite sure its application, but we know the customer use as is after some machining in house.

The parts casted and annealed in China, but we noticed some hardness discrepancies on our incoming check, and decided to send them to a local heat treater to anneal it again in order to meet the hardness spec HRB 82~90.

I was surprised on the heat treater to test the hardness on the gate area when I visited, they told me that’s the right place to check the hardness, that is the reason why I was talking about the surface grinding earlier since the heat treater told me the ground gate area is just like surface ground. But, the fact is our customer clearly specified on the drawing the flat surface is the place to check the hardness, I haven’t told the heat treater till now.Do you have any idea about the hardness check on a certain area.

The test result is all parts failed (below HRB 82) on checking at the flat surface will pass (over HRB 82) the test on the gate area. I am not quite sure what makes this difference.

We are told that the parts must be over 82 HRB and our customer will not agree to test the hardness on a ground surface. It’s really a headache for us now since the heat treater insists they are right both on their heat treating and the test data, I trust what the heat treater said.

Can you tell me more about the soft layer on the casting part, how can I convince our customer to accept the surface grinding concept.

Thanks lots

Season

 

[CoryPad]

Your most recent post is quite informative. Your customer has a requirement for location and condition (unground). Thus, you need to change the processing to bring the parts into conformance. The soft layer likely is decarburization. Your heater treater needs to adjust the atmosphere to match the carbon concentration of the part to avoid decarburization. You should not trust the heat treater.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Helicopterjunky]

SeasonLee,

Parts were most likely decarbed from your China supplier.
The local heat treater should not be at fault because they are staring out with decarbed product. If you had known it was decarbed before you had sent it to the local heat treater, they could have carbon restored the surface but how are they to know unless you told them. I worked in commercial heat treating and this kind of thing happened often. Customers who did not know the condition of what they had wanting the heat treater to work some magic. Go back to your Chinese supplier and make them stand behind the stuff they sell. You also need to specify on your drawings the final surface condition, "No Decarb allowed".

 

[SeasonLee]

Thanks for all inputs.
Will request our supplier to pay more attention on the atmospherets carbon content during annealing process.

Best Regards

Season

 

[SeasonLee]

I am still thinking are there any possibilities to cause a decarbed part except decarburization during annealing heat treating process.

In order to comply with the alloying elements of IC 8620, the caster is requested to furnish a chemical composition analysis from the molten metal on each production lot. This will rule out the problem caused by the raw material.

We provide this part as casted without any plating, what I can think of the last and the only one possibility is the casting process, is it possible to lose the carbon during casting ?

In case decarburization happened, what is the process of carbon restoration? Just like the case hardening by carburizing ?

I am trying to learn more from this case, thanks again for your inputs

Season

 

[TVP]

Yes, it is possible to lose carbon at the surface during the casting process. Carbon can segregate towards the center (away from the surface) during solidification. Carbon can be removed from the surface due to interactions with the mold sand. Carbon removal from the surface can occur due to oxidation (scale formation) due to excessive time at high temperatures.

Carbon restoration is essentially the same as case hardening by carburizing: the carbon content of the furnace atmosphere is made higher than that of the part, and therefore carbon will diffuse from the atmosphere into the part.

It is very easy to determine where the decarburization is occurring. You need to obtain parts during the various phases of the manufacturing process:

1. Parts immediately after casting (gates and risers still attached, no machining, etc.)
2. Parts after any machining operations (gate removal, surface grinding, etc.)
3. Parts after thermal treatment (annealing or whatever is being performed in China)

Take these parts and cross-section them transversely. These sections should be ground and polished, then etched with ~ 2-5% nital (2-5 mL nitric acid, 95-98 mL ethanol). The etching will reveal any decarburization that is present. The following links explain the process for fasteners specifically, but it works the same for castings:

http://www.struers.com/resources/elements/12/79844/Application_Notes_Fasteners_English.pdf

http://www.buehler-asia.com/brochure/apps_support-tech-notes_vol1_issue1.pdf

 

[Helicopterjunky]

SeasonLee,

I do not think the decarbed surface is related to casting. Go to a very reputable heat treater. Explain to them what you have and ask them if they would carbon restore the parts for you. They will most likely want to cut up several parts and measure depth and extent of decarb before attempting carbon restore. If they know what they are doing they will likely insist on this. They will try to put approximately .2% carbon back on the surface of the part thru the depth of the decarb layer. They need to do this fairly accurately in order to avoid actually carbon enriching the surface beyond the core carbon content. They will also most likely want to abrasive blast the parts before putting them in their furnace. The carbon restore process will utilize differnt dew points and control setting at the generator and furnace. This will require a little one on one with yourself and the heat treater. Good Luck, hopefully you will be able to salvage these.

 

[arunmrao]

An interesting discussion has developed and many valuable inputs are presented. I shall offer a few comments and some of these may be repetetive.

8620 cast steel under normal cast conditions of processing will not exhibit remarkable decarburization.

Annealing the castings in an unprotected atmosphere shall again not be a major contributor. If there has been an extended time or higher temperature exposure,this should be clearly visible by scaling marks and coarse grains.

Removal of gates and feeder heads by flame torch and followed by grinding can result in some decarburization at the surface. Normally I do not offer these areas for any hardness testing. I always offer the opposite faces. Also there could be micro porosity in these regions,which might result in showing lower hardness.

A good heat treater can easily notice on visual inspection incidence of scaling. This can be further supported by sectioning the casting and performing a microstructural study.

An experienced heat treater never takes up the complete lot for case hardening . He picks up a few samples randomly and based on the inputs provided by the customer performs a sample test. The results of these are thenmatched with the expected results. If any corrections are needed they may perform one more trial run before taking up for production.

In this case I find that this step might have been missed .It may be difficult now to trace the source of error,unless a few stray pieces not case hardened are available some where.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[Helicopterjunky]

Unfortunately this is sometimes what you can expect when going to China to save pennies. Best bet might be just to send them back to your Chinese supplier for refund or replacement or better yet let them restore the surface. Why we as engineers work to aid our low cost structure competition puzzles me.

 

[redpicker]

This is an interesting thread, but I am confused about a few points. Can you either confirm these or point out where I am wrong.

SeasonLee's customer has

specified the part to be cast from 8620 material

specified the part to be annealed to 82-90 HRB, and

specified the part to be hardness tested on an as-cast surface, away from any gating or risering, and

will reject the parts for being too soft on the surface although the bulk harness is within specification

I'm sorry, but this does not make a lot of sense.

Annealing treatments are, by definition, softening treatments. The intent of the treatment is to soften the material because of hardening that occurs during processing (in this case, hardening from cooling from casting temperatures and from cutting of the gates and risers). I would tend to agree with the heat treater that, in this instance, the correct place to check the hardness would be on the ground area where the riser had been removed as this would be expected to be the hardest area and the purpose of the check to to insure the annealing treatment has been effective.

Cast steels will have a slightly lower hardness on the surface. If this is not accpetable to the customer, they really should specify other processing or otherwise make allowance for this fact.

Finally, I have a problem with the posted photo. It appears that the hardness tests were performed direclty on the shot-blast surface with no grinding whatsoever. With Rockwell hardenss testing, surface preparation is critical to obtaining an accurate reading. Both the tested surface and the surface contacting the anvil need to be flat, parallel, and smooth. I've seen testing showing that grinding with 80 grit paper is generally adequate, but I would like to see at least the finish obtained with 240 grit or finer.

rp

 

[arunmrao]

Redpicker,I concur with your last remark on method of testing. I missed seeing the photographs earlier. Sorry about that.
You need to grind the surface before measuring hardness. The sample has to be flat,indenter perfectly perpendicular to the sample and when the load is applied the sample should be firmly held. As per the photographs these requirements are not met and hence results can be suspect.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[SeasonLee]

redpicker, you spoke of all questions that were exactly in my mind, thanks for your input.

Now, I know the soft top layer is an inherent fact on the casting part, if this is not acceptable to our customer, may I ask what is the allowance for this fact as you mentioned : they really should specify other processing or otherwise make allowance for this fact.

I am glad to know the proper way to make surface preparation on hardness testing, but I still have a little confusion on the surface preparation. Cory mentioned earlier that ASTM E18 states :
6.1 The test shall be carried out on a smooth, even surface that is free from oxide scale, foreign matter, and, in particular, completely free from lubricants.

What is the definition of a smooth surface ? The surface roughness of this part is around 110 RMS as you can see on my photo posted earlier, I believe this is the most surface condition on the casted part, so I am wondering shall we grind all casted parts before performing hardness test. Are there any other documents deal with surface preparation before hardness testing?

Thanks for all inputs

Season

 

[redpicker]

There are several difficulties in using the Rockwell Hardness test method and you have identified one of them. The Rockwell test method is based on depth of penetration and anything that could affect the measured depth of the penetrator into the material (other than the material's hardness) is going to introduce error. A rough surface, movement of the part, scale, dirt, oil, etc..., are all going to cause problems. The standard is specifically vague since there are many reasons for performing the test and as many ways of preparing a surface; the group that writes the specification does not want to exclude an acceptable use or preparation method. I saw some research a few years ago that tried to define "an acceptable surface" (as opposed to "a required surface"), but as far as I know, this work was never published. This work was also performed with Rockwell "C" testing, not Rockwell "B", so the different loads and penetrator would affect the conclusions.

If you think of the surface needed for a Vickers test or a Brinell test, however, you will get closer to the intent of the standard (perhaps you'll want to look at the ASTM standards for those tests). While one of the advantages of the Rockwell test is that since you don't have to measure the size of the indentation, it helps to have the same quality of surface.

rp