Man was carburizing even before the pyramids were built. Of course, he did not realize that he was actually carburizing, but as he was heating the iron for forging, he really was carburizing the iron. In a crude way, he was making steel by diffusing carbon into the surface of the iron that was being forged.
During the past 100 years or so (and really since the advent of the microscope and the development of basic metallurgical principles by Adolph Martens and Sorby’s contribution with phase diagrams during the late 1800s) carburizing has developed into a science.
In the early part of the last century, carburizing was typically accomplished by using the old pack-carburizing method. But it was “hit and miss” as far as the carbon potential was concerned. In the mid-1920s, it was discovered that one could carburize using hydrocarbons. This was accomplished by simple early oil-drip-feed furnaces. This was a top-loading pit furnace with a stop-cock-equipped pipe connected from an oil can. The carbon potential of the furnace atmosphere was simply controlled by the number of drips per minute into the process chamber. Crude as it was, it really did work and was the forerunner of gas carburizing.
In the mid-1930s, it was observed that natural gas, propane and butane are hydrocarbon gases. These gases were able to provide controlled atmospheres for carburizing (and other applications). This was the start of the gas-carburizing era. It was also found that the propane and butane gases are of an almost pure form with little or no contamination at all.
The control of the atmosphere is critical for the carburizing process for many reasons. The first reason is to control carbon potential within the “limits of solubility of carbon in austenite.” This means that if the carbon percentage drifts into that region, then sooting will begin to occur from the process-enrichment gas. This can lead to soft spots on the surface of the carburized component if soot is being deposited onto the surface. Further, it can also create maintenance problems. If too much carbon is introduced and is present in the surface of the steel for austenitize and quench, there is a strong probability of retained austenite being untransformed to martensite within the formed case.
The boost-diffuse method is performed and is safe for the atmosphere to peak out at 1.30-1.40%, for example, but then the enrichment gas must be either turned off or turned down. If the gas is turned off, the atmosphere CP must be monitored down to, say, 0.80-0.90% before quenching at 1500ºF (815ºC).
Control of the furnace carbon potential can be accomplished by one of the following or combinations:
- Furnace and generator dew-point measurement
- Furnace shim analysis of the process atmosphere
- Furnace CO/CO2 measurement
- Furnace infrared gas analysis
- Furnace oxygen-probe analysis
Control of the furnace atmosphere (not just for carburizing) is mandatory. Perhaps the most popular and current method of process-gas analysis is that of the use of the oxygen probe. The probe readings can be displayed either by the millivolt reading or by the more common practice of a direct readout of carbon potential. (Remember also that the oxygen probe will require a burnout every eight hours or so to keep the probe sample port clean and clear of any soot buildup.)
Control of the furnace atmosphere is perhaps one of the major controlling factors that will determine the surface metallurgy quality being accomplished for carburizing. Another controlling factor of the accomplished surface metallurgy is the final austenitize and quench temperature. If the case austenitize temperature is too high and the surface carbon content is at the appropriate level, then there is a strong probability that retained austenite will be left in the surface case metallurgy.
By carburizing the processed steel, a new surface steel has (in effect) been created and must therefore be treated as such. One can then determine the austenitize temperature by simply referring to then iron-carbon equilibrium diagram (ICE Diagram) for the appropriate surface carbon content and noting the austenitizing temperature on the vertical axis, plus 50°F (10°C) followed by the quench.