Conventional vs. Low-Pressure Carburizing
Many developments have been carried out in carburizing technology, and many articles have been written. However, we feel it is important to remind new engineers of the basic rules that justify the strong development of this process.
Carburizing is simply defined as adding carbon to iron or steel by heating in the presence of carbon to harden the surface of a component. Depending on the metallurgical recipe, the hardness can vary with carbon content. This process can be achieved in either a conventional (atmosphere) furnace or a low-pressure (vacuum) carburizing furnace (LPC).
Conventional (Atmosphere) Furnace
Conventional carburizing is an atmosphere-controlled process with control of the carbon potential. Conventional carburizing is a popular method using older but reliable technology. This proven technology has been instrumental in pioneering the carburizing process for a number of industries.
Economical and cost-effective, this furnace remains stable due to its process, which provides constant carburizing case depth as a result of the precise control of the endothermic gas. This mixture of hydrocarbon gases and air provides the carbon potential (CO) necessary for the carburizing process. Due to the presence of oxygen in the atmosphere, there are risks of intergranular oxidation (IGO), decarburization and scaling on the surfaces of the treated parts. The constant control of the carbon potential and the ratio of hydrocarbon gas, hydrogen and nitrogen safeguard the surfaces from deterioration.
The main features of conventional carburizing are:
• Permanent injection of endothermic-gas mixture at atmospheric pressure in batch or conventional furnaces
• Even level of carbon content in the furnace
• Length of furnace (conventional furnace) or duration of cycle (batch furnace) is relative to the case-depth requirement
Low-Pressure Carburizing Furnace
Low-pressure carburizing (LPC) is run without any oxygen and with injections of carburizing gas (e.g., acetylene). The LPC process has been chosen for its tight process control, repeatability, optimized metallurgy and shorter cycle time at higher temperature versus atmospheric carburizing.
The development of a new carburizing technology for new applications requires an understanding of the standard processes. The LPC process is a thermal treatment using alternative boosts of carburizing gas and neutral gases without causing intergranular oxidation. This process also provides uniform homogeneity of carburizing case depths thanks to a precise control during the recipe development. The target is to develop carburizing recipes at higher temperatures and with a higher enrichment capability without affecting the material properties.
The main features of LPC are:
• Hydrocarbon thermal cracking at low pressures (7-13 mbar) in a vacuum cold-wall furnace
• Alternate injection of hydrocarbon gas boosts (carburizing) and of neutral gas (diffusion)
• Number and duration of the steps are determined by the case depth required
While older technology is more familiar, there are many benefits to new, advanced LPC technology. The newer technology of vacuum carburizing is based on the enrichment of carbon on the surface by the cracking of a single carburizing gas (typically propane or acetylene) at a low pressure. Thus, the vacuum has the advantage to improve the absorption of carbon. This absorption of carbon is simulated during the validation of the process and is tracked each time the recipe is run.
LPC Principle with Infracarb® and CBPWin®
ECM Technologies’ ICBP® LPC furnaces use the Infracarb® patented process. Infracarb consists of alternately injecting hydrocarbon (C2H2 or C3H8), which enriches the surface by breaking the molecule at high temperatures, and a neutral gas (N) for diffusion.
The objective is to maintain a high-level concentration of monoatomic carbon – obtained after the dissociation of the reactive gas on the surface of the parts – without reaching the carbon solubility limit in the steel (i.e., no soot accumulation). Thus, the reaction yield is greatly superior to that obtained in conventional atmospheric carburizing based on a CO/CO2 balance, while still being less prone to atmospheric changes. Not needing to consider carbon potential in low-pressure carburizing eliminates the need for in-situ control and makes the process easier.
The Infracarb process ensures precise control of simple physical parameters for optimal results: temperature, length of the gas injection phases, flows and pressures. The process temperature is generally higher than traditional carburizing – 880-1050°C (1616-1922°F). The cycle time is reduced up to 50% depending on the carburizing depth. Acetylene (C2H4) – used for LPC – and nitrogen (N) – used for diffusion – are the most common gases utilized in the process under standard conditions.
Acetylene is a synthesis gas, and its purity can be controlled, providing exceptional chemical reactivity (dissociation rate is over 60%). This allows very complex parts, such as diesel injection components, to be carburized. Also, it is not very sensitive to possible heterogeneity in washing before treatment and prevents the occurrence of soft areas. Large surface areas can be carburized, and it has a high enrichment effect. The number and length of the phases depend on the carburizing depth required. Simulations using Infracarb allow precise adjustments of recipe parameters to achieve perfect control of the carburizing depth.
Advantages of LPC over Conventional Furnaces
The LPC furnace uses a cold-wall technology so the exterior surface of the furnace stays at a low temperature, reducing the amount of heat emitted into the plant. Without any flames, or hot walls, the furnace can be located in the middle of the machining area within the plant. This allows for better part flow and eliminates the need for a fireproof wall enclosing the heat-treat area (Figs. 2 and 4).
With cold-wall technology, dangers specific to hot-wall technology such as open flames and hot surfaces are also eliminated. In addition, the plant does not need ventilation hoods to get rid of the burned gas. For these reasons, the LPC furnace occupies less plant floor space, especially as it allows a higher throughput.
Safety concerns regarding the use of acetylene are also easier to address. Acetylene suppliers now have supply solutions with reliable explosion-proof and fire-prevention systems. Since endothermic gas is not needed, there are no concerns regarding carbon monoxide.
The biggest advantage of an LPC furnace is the reduction of cycle time using higher carburizing temperatures. Some production parts are now carburized at 1030°C (1886°F), reducing the carburizing time approximately 75%, and new trials are taking place at even higher temperatures.
The modularity of the equipment also provides advantages. All chambers are independent, which means a different cycle can be run at different temperatures in each. It is easy to add additional chambers on an LPC furnace to increase the overall production volume, whereas a conventional furnace is limited to its original number of chambers. Moreover, it is easy to turn treatment chambers off in times of decreased production, while the conventional furnace has to run at full capacity with empty trays.
Efficiency and energy savings are further advantages of the LPC furnace. Vacuum technology is the most efficient with only 35 kW to maintain 2,200 pounds of steel at temperature. Gas usage is reduced tremendously because it is used in the most efficient way. LPC furnaces can be ready for production in as little as 90 minutes, and each chamber can be turned on or off at any time to be ready for production in 30 minutes.
Finally, the LPC furnace is so similar to a machining center that both can be operated and maintained by the same team. The entire process control is done by the equipment, without the need for controlling the carbon potential or the dew point. The system control time, temperature and flows are the only process variables. Maintenance of an LPC furnace is easier with less danger and less permitted confined-space requirements. With only electric heat, the system does not need a burner specialist, and standard maintenance tools can address equipment issues.
There are significant differences between a conventional furnace and an LPC furnace. Low-pressure carburizing carried out in vacuum furnaces with cold walls offers many advantages compared to traditional conventional atmospheric carburizing: no oxidation, better carburizing homogeneity, excellent reproducibility of load-to-load processing, tightening of metallurgical tolerances (such as carburizing depth) and, finally, core hardness.
The LPC process respects environmental standards since it significantly reduces CO2 emissions and harmful chemical products. In an ever-changing industry, becoming familiar with the newest technology will push your production to perform at its very best and meet your demands for increased productivity and higher-quality parts. IH
References available upon request
For more information: Dennis Beauchesne, general manager, ECM-USA, Inc., 8920-58th Pl Ste 100, Kenosha, W.I. 53144; tel: 262-925-6321; fax: 262-605-4814; e-mail: email@example.com; web: www.ecm-usa.com
Sidebar: Low-Pressure Carburizing of Sintered Parts
Powder-metal (PM) processing typically requires four steps: de-waxing, high-temperature sintering, carburizing and surface hardening. The conventional carburizing treatment is typically carried out in dedicated atmospheric furnaces for sintering and heat treatment respectively, which requires intermediate handling operations and repeated heating and cooling cycles.
Trials conducted by Höganäs AB in an industrial furnace (see figure) indicate that a multipurpose batch vacuum furnace is able to realize all these steps in one unique cycle. The furnace comprises two chambers – one heating cell and one gas-quenching cell – separated by an intermediate leak-tight and insulated door. The multiple benefits brought by this technology improve quality and save money. These include vacuum integrity along the entire cycle (avoiding oxidation), no limitation on sintering temperature, repeatability of the process, gas-quench flexibility, energy savings (no re-heating) and environmental benefits.
The main goal is to use this technology to manufacture high-load transmission gears in PM materials.
The absence of handling between operations in the multipurpose furnace guarantees that the parts will not be affected by contamination or damaged between sintering and heat treatment.
Frequently, batch furnaces are used for carburizing PM parts. Carbon enrichment is controlled by O2 sensors or CO/CO2 ratio. In the new furnace, the carburizing phase is completely controlled by Infracarb® (discussed in this article).
The case depth and the carbon profile are simulated and adapted for porous materials. Low-pressure carburizing processes can be achieved at any temperature up to 1050°C (1922°F). The carbon enrichment and the diffusion time can be controlled separately to achieve the required microstructure. The cycle is shortened and the diffusion is faster than in an atmospheric carburizing furnace. Moreover, there is no internal oxidation of the parts.
Gas quenching permits high flexibility and more repeatable results than oil quenching because there is no boiling or vapor formation around the parts. The parts exit the furnace clean, and a washing operation is not necessary.
The main saving factor comes from the fact that carburizing and gas-quench steps are carried out in-situ in the de-waxing/sintering equipment. The high modularity and restricted footprint of the furnace is also an advantage.
What does it mean for PM parts?
Studies have concluded that low-pressure carburizing is very suitable for control of process parameters without oxidation. Positive results have been achieved on the control of case profiles and control of core hardness with base carbon content and cooling-speed variation. The multipurpose furnace potentially offers an improvement at every stage of the PM production process. It will be the tool for further optimization to improve mechanical properties like fatigue strength, reduce distortion and validate the whole process for the production of high-performance PM gears.