In our final IH Rx Podcast, Dan Herring and I covered some readers’ questions about carburizing. We transcribed and edited that podcast into a two-part article, which is seen here and will continue in the September issue.
RM: Thanks for joining me today, Dan. Let’s jump right in and answer some of our readers’ questions about carburizing. I have a number of questions here from our podcast listeners and readers, so let’s get started with number one.
Q: What is the best way to measure carbon potential in an atmosphere furnace?
DH: Well, I know that the de-facto standard for measuring carbon potential is to use an oxygen probe, or what we call a carbon probe. They have become such an integral part of our atmosphere carburizing processes. I think of carbon probes like I do thermocouples – a necessity in an atmosphere furnace.
In addition to oxygen probes, I know that people are supporting oxygen probes by using three-gas analyzers, especially to establish the CO level in the furnace to help assist the control factors of the oxygen probes. Also, people are using more shim stock than ever before. That’s a technology that was used back in the day and lost popularity for a few years, but it is now back in favor. People are even supporting oxygen probes by using dew-point analyzers. These technologies have become so reliable that they are an entire system of measuring carbon potential.
Q: What about vacuum furnaces? Can you use shim stock in these furnaces?
A: I have actually seen a design for a vacuum shim puller, but no one is using them in vacuum furnaces. What they are doing is running discs inside with the loads and actually measuring surface carbon by spectroanalysis of those discs after the load comes out of the furnace. As you know, Reed, there are no in-situ devices in vacuum carburizing currently available to control the carbon potential. Basically, what you are doing is carburizing at the saturation limit of carbon in austenite and then diffusing that carbon away from the surface. Then a simulator (provided by the furnace manufacturers) uses a carburization simulation program to predict the case profile and the hardness profile of that case.
Q: What does “case depth” mean when I see it on a drawing?
A: It’s important for engineers and designers to call out the correct terms. There are three terms: total case depth, effective case depth and finished effective case depth (after machining). Total case depth is defined as base carbon plus 0.04%. The best way to verify the case depth is to section the part. We can see the case-core interface that way, often by etching in nital and checking the etch depth on the cross section.
Effective case depth, which is a function of the steel’s alloy content, is about 0.40% C or about 50 HRC. We can determine this using a microhardness traverse. There are different definitions in the U.S. and international specifications, so these specifications are even a variable for our terminology.
Macroscopically, we can also get an approximate total case depth. A loop or low-power microscope can help identify total case depth on an etched, and sometimes even an unetched, cross section. A rule of thumb says that total case depth multiplied by 2/3 is the effective case depth. Fracture testing is another quick way to identify approximate case depth.
Turn bars can also be used to accurately determine the depth of carbon penetration, particularly for process development. Taper bars are typically used for deeper case depths (>0.100 inch). Similarly, a 1018 or 8620 disc is used, as mentioned previously. These are 1.25 inches in diameter, ground flat on both sides, often with a hole in it to wire it to a basket. The disc is carburized and quenched and then lightly polished with some fine sandpaper followed by a spectrographic burn. This gives you a surface carbon of your parts. Shim stock will also give you this number.
Be sure to use the correct hardness scale if you are testing surface hardness. Depending on the depth of carburization, a Rockwell C test should only be used if the case depth is greater than 0.030 inch. If the case depth is 0.020 inch, you should switch to the “A” scale. For a case depth of 0.015 inch, use the 30N scale. For a 0.010-inch case depth, the 15N scale should be used.
Q: Where does soot come from?
A: If your process is sooting, you are not in control of it. As we all know, disastrous consequences can occur if you lose control of your process. First of all, soot can deposit on the surface of parts and actually cause carburization in unwanted areas at times. It can also act to prevent carburizing because it creates a solid barrier on the surface you want to carburize.
Soot is responsible, many times, for the formation of carbides and carbide necklaces on samples. Basically, your atmosphere inside the furnace is unstable – too high in carbon potential or it is not stable. As a result of that, carbon in the form of a soot will deposit on the surface of parts.
One other comment I should make is that sometimes you don’t even realize the furnace is sooting because the oil actually washes the soot off after you oil quench the parts. But where does that soot go? It goes into the oil, and it degrades the performance of the oil.
So, soot is never a good thing in your furnace. If it is occurring, you have to look at whether the furnace is leaky, if there is too high an amount of gas additions going into the furnace or if a nitrogen/methanol process is out of control. In the case of methanol, if your sparger or your injector is dispersing larger droplets, they will crack and form soot on the parts. If your endothermic generator produces unstable gas, you will create a CO/CO2 reaction and form carbon. Soot is never a good thing.
Daniel Herring, The Heat Treat Doctor, was a regular contributor of columns, blogs and articles to Industrial Heating for much of 20 years. Dan retired almost two years ago and is pursuing educational interests. Our website contains numerous resources, including these podcasts and two e-books from The Heat Treat Doctor. Additionally, our website bookstore is a source for several comprehensive Herring book volumes on atmosphere and vacuum heat treating.
Listen to the following podcast for the complete discussion, and come back next month for the balance of our transcribed and edited article.