Case depth = √t x f (where t = time and f = multiplication factor)
To obtain a case depth of 0.040 inch at 1700°F, the approximate cycle time at temperature would be approximately 4.5 hours. If this temperature of 1700°F was raised to 1900°F, the process cycle time at temperature would be approximately 40-50 minutes to provide the total case (not effective case).
Traditional integral-quench furnace equipment is capable for limited excursions up to the temperature of 1800°F. The vacuum furnace is quite capable of reaching temperatures (depending on the element material analysis) up to 2400°F.
Significant and documented work has been accomplished and published to justify the strong case for high-temperature carburizing. The question is often raised regarding high-temperature carburizing, “What about grain growth?”
Grain growth is relevant to time and temperature. The higher the temperature and the longer the time at temperature, more grain growth will occur.
However, because we are now considering high-temperature carburizing at a suggested temperature of 1900°F, the process cycle time at 1900°F is approximately 50 minutes. Therefore, the grain growth that will occur will be considerably less than what would be observed by carburizing for a longer cycle time at a lower temperature.
Another hurdle that has been overcome in high-temperature carburizing is that of process enrichment gas. Traditional gas for enrichment has been city gas, which is comprised mainly of methane (CH4).
City gas is not sold as pure gas, so it is usually contaminated with other hydrocarbon products. The gas company mixes in other hydrocarbon gases to maintain a calorific value. City gas (methane) does not crack easily into its component parts at elevated temperatures. Therefore, alternative pure gases have been sought.
One gas that has attracted a great deal of attention is that of acetylene, which cracks very readily and easily at temperatures above 1700°F. If one considers the formula of acetylene (C2H2), it can be seen that there is a greater amount of carbon available for carburizing than there is with methane.
Once the carburizing has been completed, it is then necessary to follow the same principles of metallurgy in order to austenitize the newly formed case. This means selecting the appropriate austenitizing temperature and the appropriate quench medium in order to transform the newly formed carbon into martensite on rapid cooling. The rapid cooling can be accomplished either by blended gas mixtures or traditional oil-quench mediums.
In today’s age of improved process cost management, high-temperature carburizing offers the engineer a significant way of reducing process cycle time and accomplishing a uniform and repeatable surface metallurgy.