[Kislok]

Does anybody have experience with the shim stock (foil) test method? Is there any other process/equipment that can be used to determine the carbon content of a carburizing atmosphere? We use a dew point tester and are thinking of purchasing a shim stock tester. Any help would be greatly appreciated.

[HSRick]

There a numerous ways to confirm the carbon content in the furnace, in addition to the use of a dew pointer and carbon probe. I have used shim stock for years as a reliable method of confirming the carbon content in the furnaces I am running, and I use shim stock to dial in my carbon probe.

A good place to start learning about using shim stock is the ASM Heat Treat Handbook, in the chapter on Carbon Control (start on page 573 of the green ASM Handbook on Heat Treating).

To get started, all you need is:
- a port into your furnace for your sample equipment
- a suitable sampler (reference the ASM Handbook for some details on how to build one)
- some low carbon (ie 1010) shim stock, or thin gauge wire
- something to analyse your results.

If running shim you will need a LECO Carbon Analyzer, or a Spectrometer capable of determining carbon levels on shim (both are expensive pieces of equipment, so you may want to start off trying to find someone locally who can measure the carbon levels on your samples), OR, an accurate balance scale (one that can read to at least 0.0001 grams)

Once you have all this, start running your samples and seeing what you have vs what you think you might have.

If you are looking for an analyitical source, let me know and I may be able to assist you.

Rick Clift
H&S Heat Treating

[Kislok]

Thank you for your input. As I understand it, the shims stock tester is a pretty reliable and pretty reasonable solution (not very expensive~5000EUR). Do you know if it meets the common aerospace and automotive industry quality requirements?
Shlomi Kisluk
Chromat LTD

[jim]

At the Heat Treat show last year they had a foil shim stock tester. I have never used one or talked to anyone that has tried it. Are you using a carbon probe it seems not to many people use dew point alone.

[HSRick]

In answer to your question about shim stock analysis meeting the various quality requirements for automtive and aerospace, per the AIAG CQI 9 Heat Treat Requirements, Shim Stock Analysis is listed as one of the recognized (and accepted) methods for verifying the carbon content in the furnaces. However, as with any analytical method, results are only as good as your calibration and verification systems so it is very important to ensure your verification protocols are accurate and up to date.

[Richard Howell]

I have used shim to determine and set the carbon probes for over 25years, and have found it to be reliable. You would need to ensure that the proceedure that you use is sound and accurate to eliminate or reduce induced error.

1st-use thin shim stock 0.003 to 0.005 thick. Be sure you have sufficient weight to obtain three burns with a carbon analyzer (3 grams min.).

2nd, ensure that the time in the furnace is sufficient to through carburize the shim. Usally, for 0.003 to 0.005, 30 mins. is acceptable.

3rd-Cool the shim down under protective atmosphere such as pulling the stack back into the housing to cool leaving the gate valve open for potection. Or, you may quench it in oil, and clean the scale off with 50-50HCl and water.

4-If you decide to use the weight method, I would suggest that your original sample be approximately 1 gram, and if there was any question then you could burn it.

Use the following formula:
CP=(Wf-Wo)/Wfx100%=%C

Where Wf=weight final
Wo=Weight original
%C inital carbon of the shim (Anaylzed)

For the weight method, you may want to run a correlation study between the weight methord and the burn method.

I personally like the cabon analysis proceedure the best.

[Kislok]

Thank you All for your Help
I would like to use your experience one more time.

We have a process that is running for many years (Time Temp Cp are constant) and the tempering is constant, But there is a variation in hardness and depth between each lot of material. How much does the initial chemical composition (except Carbon) of the material affect the results? We also tried to elevate the tempering temperature +50C degrees, but the hardness did not budge. Why?

[DrOrpheus]

There's a million and a half variables that you didn't post. The other half million could be, furnace pyrometry, ie hot/cold spots in both carburizing and tempering furnaces, your atmosphere could be changing, ie time for new catalyst, dew point on generator, furnace leaks, burner tube leaks,etc; material lot differences, some stuff can vary greatly between two different lots depending on the type of material. With no mention of material, type of furnace, type of carburizing, type of atmosphere used, you don't leave a whole lot of information for people to work with.

But like you say, if it's the same way you've always done it, think of what has changed recently, or sometimes more importantly, what hasn't been changed. Assuming you're using endo, how is your output of endo been and have you been keeping a good eye on it to ensure you have good repeatability? Are your flow meters clean and sealed tight? Have you done any lab work on the work you've been doing? Any retained austenite or checking case depth? How often do you get your furnaces surveyed? And I've heard that some materials (ie. 8620) has a possibility of varying in properties after heat treatment from heat lot to heat lot, which is why we always use a part from that heat lot to measure case depth, rather than a generic sample, but I don't think it will be a huge difference that would require such extreme tempering temperature changes. I'd be sure both furnaces are working correctly and go from there. Run a thermocouple with your next tempering load and make sure your furnace is doing what it says it's doing. Check the inside of your carburizing furnace for excessive soot build up, make sure the flow meters aren't leaking, your burner tubes or heating elements are all in good shape.

[heattreater]

Kislok,
I agree with Dr. O, it is a very complicated process with many variables, some of which are difficult to control. You may want to look at the ASM Metals Handbook. Volume 4 has a ton of information on variation/repeatability of case depth. I think you will find enough information there to make you dizzy.

[josephmarkgreene]

With regard to shim stock testing, make sure that your shim stock port is airtight. Because shim stock is so thin, it is quick to both carburize and decarburize. Exposure to a small amount of air during cooling can add a lot of scatter to the data.

For routine redundant confirmation of furnace atmospheres, I've heard that a three-gas analyzer is a super tool.