A Unique Carburizing System Proves its Worth
Beginning in 1997, George Barbour of Heavy Carbon Co. began testing an in situ endothermic generator in a Surface Combustion Super Allcase batch IQ furnace at Euclid Heat Treating Co.
The generator unit sits atop the furnace and produces endothermic gas, which is injected directly into the furnace (Fig. 1). It has taken many redesigns, valving changes, control modifications and endless testing to prove its worth as a viable system to the commercial heat treater.
The Endocarb Concept
With conventional carburizing, it is important to establish a carbon potential low enough to maintain the necessary reaction of cracking an air/gas mixture into a carburizing atmosphere. If the carbon potential (CP) is operated too rich, soot will form and slow down the reaction. With this weakened reaction, the CP will start to decline and cause the control instrument to add more gas in order to maintain the set CP. This, in turn, will cause more soot to form. As the amount of soot in the furnace increases, the reaction will slow down even more and possibly stop. When this happens, the soot must be removed in order for a suitable reaction to take place and create the required carburizing atmosphere.
There are three recognized methods for removing the soot from the furnace.
1. Raise the dew point in the atmosphere. This is a simple and safe matter of operating a leaner CP in the atmosphere, but it is very time consuming, easily taking 24 hours or more.
2. The doors to the furnace and the quench area can be opened and the atmosphere removed to allow outside air to burn out the soot in the furnace. This is very tricky and should be attempted by experienced personnel only because the furnace and the quench area can be overheated and cause damage to the equipment.
3. Using a wand to supply air to heavily sooted areas will burn off the soot, but this also can cause overheating and damage to the equipment.
Heavy Carbon Co. has invented a fourth method (method #4), which utilizes the Endocarb system. With this method, soot is not able to build up an accumulation and is always under control. This method is based on the idea that when a clean furnace is first brought up to temperature, the reaction that takes place with the air/gas mixture is very positive while creating an endothermic atmosphere consisting of about 40% nitrogen, 40% hydrogen and 20% CO. These numbers will vary a little with other gases created in very low numbers and with a dew point that is usually above 30 with an air/gas ratio of about 2.7 to 1.0. With method #4, the furnace is always clean, and the reaction is always positive.
A clean, positive reaction is the most important part of a carburizing atmosphere. To enrich the carburizing atmosphere, a hydrocarbon gas is added to bring down the dew point and raise the CP. Normally, a carburizing atmosphere is operated with a CP of about 0.9% C to a maximum of about 1.15% C.
If the CP is operated above this setpoint for very long, soot will start to form and slow down or even stop the reaction. The soot range is about 1.25-1.30 CP. This is the basis for conventional carburizing, and the Harris equation can be used to predict the time required to produce a carbon case depth.
Heavy Carbon Co.’s Endocarb system will produce a case depth in less time than conventional carburizing. For this reason, the Harris equation must be adjusted to accurately predict the time necessary to produce a case depth using the Endocarb system.
With method #4, the CP is operated with 1.4% C using the Endocarb system. This is possible because the CH4 is maintained below 2%. If the CH4 is allowed to rise above 2% for an extended period of time, soot will form and slow the reaction. This, in turn, will cause the controller to add more gas and start a downhill slide.
The whole system of carburizing is dependent on the reaction that takes place with the air/gas mixture. When the reaction starts to slow, the data logger will show the CP line declining. As this happens, the CH4 line will start to climb. When the proper reaction takes place, the CH4 line and the CP line will be level. A carbon-potential-controlling instrument cannot force carbon into a steel surface or speed up the process, but it can quite accurately control the air/gas ratio necessary to produce an atmosphere that will meet the standards set by the system.
New Method Reduces Carburizing Time, Soot Buildup
The Endocarb system allows Euclid to produce a rich, reactive atmosphere using only natural gas and air. The carbon in this atmosphere penetrates the part surface 30-40% faster than traditional cycles using a remote endothermic gas generator. The Endocarb system produces a fine grain structure and a sharp drop-off at the desired effective case depth. This also provides favorable ductile-core properties.
Endothermic gas is produced at a high temperature in the Endocarb unit, within the furnace, just before the gas enters the furnace chamber, providing a better reactive atmosphere with less sooting (Fig. 2).
Data from the Shop Floor
All heat treaters know that furnace cycle time is the most influential variable affecting efficiency, productivity and profitability. This is especially true when running deep-case carburizing cycles. We have found, through statistically significant repeat cycles, that we have been able to decrease cycle times by one-third on all double-temperature carburize loads in the 0.030- to 0.120-inch effective case range. When you consider conventional endothermic cycles that require up to 30 hours to achieve the specified case depths, saving one-third of total furnace time is important.
There is nothing new about the necessity of running boost-diffuse cycles in order to efficiently meet deep-case requirements. Only so much carbon can be absorbed at a constant carbon potential and thus the need to diffuse the case. What sets the proprietary method #4 apart from conventional processing is the ability to process at much higher carbon potentials without the detrimental effects of network carbides and/or retained austenite.
Another positive for method #4 is the ability to transition from high-carbon processing to mid-range or low-carbon processing without a burnout cycle. Conventional boost/diffuse long-cycle carburizing causes a soot buildup not experienced with method #4. The lean carburizing atmosphere provided by this system is in stark contrast to conventional carburizing methods. We have run cycle after cycle of clean work that conforms to all metallurgical requirements.
In addition to the very real and dramatic savings attributed to reduced cycle times, method #4 has proven itself to be an invaluable tool relative to meeting effective case-depth requirements on 1000- and 1200-series plain-carbon steels. Meeting the effective case-depth requirements on plain-carbon steels can be problematic for heat treaters. Few alternatives exist other than increased cycle times (essentially applying more diffused total case) or dramatically increasing the severity of the quench. Quench severity – being diametrically opposed to dimensional stability – is not a viable option for many parts. Method #4, with its ability to diffuse a high-quality carbon case, dramatically narrows the disparity between the total applied case depth and effective case depth.
In essence, the Heavy Carbon system (method #4) allows the effective case-depth requirements to be met on plain-carbon steels in a reduced time/temperature relationship without sacrificing critical dimensional stability.
Data from the Metallurgical Lab
Due to the higher carbon concentration in the case structure, the martensite structure is much finer and transformation products such as bainite are less likely to form. Another benefit from the higher case carbon profile is that the effective case depth becomes a larger percentage of the total case depth, therefore reducing carburizing times considerably. Shorter cycles allow for more available furnace time, and the profit gained easily pays for the cost of the unit.
The Heavy Carbon unit is energy efficient in that there is no heat loss from using an external endothermic generator. Since the unit is internal, thermal energy from the unit helps heat the furnace chamber, and there is no loss of heat/energy due to the transfer and reheating of cooled endothermic gas from the generator. The Heavy Carbon unit does not affect the temperature uniformity of the furnace.
The structure in the microphotograph (Fig. 3) shows a more coarse grain structure and unwanted soft transformation products that develop from a traditional carburizing cycle.
The structure in the microphotograph (Fig. 4) shows a finer martensitic grain structure obtained by method #4. This is due to the increase in carbon content throughout the case.
The Endocarb system allows the controller to regulate the air/gas mixture that will keep the atmosphere clean regardless of the length of the carburizing time. This means that there is never a need to use methods #1, #2 or #3 if method #4 is used as instructed. Method #4 is user-friendly and easy to operate.
The benefits of this system are producing a fine-grain and a low-oxide deep case in less time while using less fuel and less power. This system also makes it possible to carbonitride with a high-temperature reaction while adding ammonia at a lower temperature. These features are in addition to the elimination of downtime for furnace burnout because it is no longer needed. IH
For more information: Contact George Barbour, Heavy Carbon Co., Inc., PO Box 146, Pittsford, MI 49271; tel: 517-523-3685; fax: 517-523-9019; e-mail: email@example.com; web: www.heavycarbon.com