This month, we pick up or discussion where we left off in August. If you missed that part of the discussion, check it out in the August issue (View Part 1).
Q: Why is it important to understand the water-gas reaction in carburizing?
A: There are 183 chemical reactions taking place inside a carburizing furnace. Of all those chemical reactions, the water-gas reaction is the most important.
CO + H2O = CO2 + 2H
Carbon monoxide (CO) plus water going to carbon dioxide plus hydrogen. That reaction is reversible, so it can go to carbon monoxide and water vapor. Actually, if you are measuring with an oxygen probe or a three-gas analyzer, you are looking at the CO/CO2 ratio of that water-gas reaction. If you are using dew point, you are looking at the H/H20 ratio in that equation. So, even though it is an equilibrium reaction, the water-gas reaction is the one that we are taking a snapshot of every time we have a device controlling the gas carburizing process.
Q: Do you always have to use the boost-diffuse carburizing method?
A: The simple answer is no, you don’t. Boost-diffuse is very commonly used, but so is carburizing at a constant carbon potential. Let me give you an example of both. In the boost-diffuse carburizing realm, let’s say we are processing at 925-930°C (~1700°F). The limit of the saturation of carbon in austenite at that temperature is about 1.3%. Therefore, when you are doing a boost-diffuse carburize at 1-1.1% carbon (even as high as 1.2% carbon), you then need to diffuse that high-carbon surface into the steel. Typically, you diffuse to 0.9-0.80% carbon in a classic boost-diffuse process.
The other option is to carburize at constant carbon potential at 0.8-0.85% at your final carbon content. The disadvantage of that is you are going to find the carbon is not driven into the surface of the steel quite as quickly. Therefore, your carburizing cycles tend to be somewhat longer. On the flipside, your carburizing tends to be more uniform. If you are carburizing steels that have carbide formers in them, you will reduce the amount of carbides formed in the case. Both techniques are used, but some find a speed advantage with the boost-diffuse process and others desire the microstructure advantage that the constant-carbon process offers.
Modular multi-retort endothermic gas (Rx®) generator
(courtesy of Surface Combustion, Inc.)
Q: The next few questions sort of go together. These questions relate to the temperature for carburizing. What is the highest practical temperature for gas carburizing and for vacuum carburizing? How can I minimize grain growth during carburizing because grain growth is going to be a function of our temperature? Let’s talk a little bit about the highest practical temperatures for each of those processes and then how we can avoid grain growth, particularly for higher-temperature processes.
A: Maybe one of the things we should start with is the fact that you may not want to use the highest carburizing temperature where carburizing uniformity is important. In fact, you may want to use the lowest practical temperature possible. This is slower, but it tends to give you a more uniform case depth. I have seen gas carburizing ranges that vary from 1475°F all the way up to 2000°F in gas carburizing and 1525-2200°F for vacuum carburizing.
There is quite a range of temperatures, but I think that a practical limit for gas carburizing is somewhere between 1750 and 1800°F. I say that because of the fact that you start to get into equipment limitations and start to develop more and more equipment-related maintenance and breakdowns as the temperature gets higher. Vacuum carburizing doesn’t have that particular limitation. Most vacuum carburizers are capable of 2400°F, but the carburizing temperature is limited by the type of steel that you are running.
That leads into your comment about grain growth. The higher your carburizing temperature is, the more we tend to grow grains. The larger the grain size, ultimately the lower the mechanical properties of the material. As a result of that, the practice is to limit the carburizing temperature (1700-1750°F) to avoid grain growth. The other option is to use steels with specific alloying additions that pin the grain boundaries and prevent grain growth. For example, niobium additions or nitrogen additions to the steel are useful for this purpose.
In vacuum carburizing, there is a technology available for putting nitrogen into the surface of the steel prior to carburizing. Many processors that are exceeding the conventional 1750°F and going to 1900-1925°F are using these types of additions.
Endoinjector® fuel-injection gas mixing system mounted on an endothermic gas generator
(courtesy of Atmosphere Engineering Company)
Q: What carburizing uniformity throughout the load should we expect and why?
A: This is a good question because many people think that vacuum carburizing can give you a more uniform case depth than atmosphere carburizing. That statement is not totally true. What happens in vacuum carburizing is that we start carburizing when we reach temperature. With atmosphere carburizing and its longer cycle times, we tend to begin the carburizing process a little bit early before the load is uniformly at temperature, which leads to greater case-depth variation.
Although it is practical to consider a +/-0.0015 to +/-0.002 inch as being good carburizing uniformity throughout the load for vacuum carburizing, it is more like +/-0.0025-0.005 inch, depending on how the atmosphere carburizing process is run. If you want more uniform processing, be sure to run a carburizing uniformity test. If you don’t like the results, delay the onset of carburizing until the load is more uniformly at temperature.
Q: What variation in carbon potential can I expect?
A: If you look at the theoretical variation in carbon potential of an oxygen probe, it is somewhere in the range of +/-0.010-0.015% carbon, so the device itself is very uniform in its ability to hold carbon potential. I would expect a variation of no more than 0.02% carbon.
Q: Do I need to be at process temperature before beginning my carburizing cycle?
A: That is the number-one mistake that people make. They begin the carburizing process too soon. One possibility is that they begin the cycle after temperature stabilizes on the chart, which results in sooting due to too low of a temperature. The other is that carburizing is initiated while parts are still nonuniform in temperature, which results in nonuniform case depths.
This is a very good question. I would encourage everyone to run tests with embedded thermocouples to identify when the core of that part reaches temperature and when the load at temperature is and then not start the carburizing before that time.
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.