Alternative Atmospheres for the Sintering Process
Nitrogen gas is used primarily as a carrier gas that protects powder-metal components from oxidation and excessive decarburization during the sintering process. Most atmospheres currently used in sintering parts in continuous belt furnaces contain nitrogen as a primary constituent of the atmosphere, which offers stability, safety and reliability in the process. It is important to understand how different nitrogen-bearing atmospheres compare with respect to cost and other qualitative benefits.
Atmosphere chemistry (Table 1) and temperature are important parameters as parts are processed through the furnace preheat and hot zones. Also, it is essential to prevent oxidation during the sintering process and provide an atmosphere that reduces surface oxides. Primary active species in furnace atmospheres that can cause carburizing, decarburizing and oxidation during sintering process are shown below as: Carburizing: CO, CH4; Decarburizing: H2O, CO2, O2, H2
Pourtalet and Renowden  published data and explained the effect of endothermic gas on sintering carbon steel parts. Under an endothermic atmosphere, a part containing 0.8% carbon could be decarburized in the furnace hot zone, where the higher temperature brings the carbon potential down to approximately 0.2%, and then recarburized (albeit inconsistently) in the transition zone, where the carbon potential increases. The amount of recarb/decarb that occurs varies depending on the original carbon content of the part, quality of the atmosphere and the condition of the furnace. Often, natural gas is added to the transition zone to increase the carbon potential. Under these conditions, it is common to see a difference in carbon content within the part of 0.15% from core to surface due to this recarb/decarb process.
Although synthetic nitrogen/hydrogen atmospheres of various compositions are generally less reactive than endothermic gas (with respect to carbon exchange to the part), the dew point and hydrogen concentration have a significant impact on preventing decarburization of the part. The dew point in the hot zone must be about -40 F to prevent decarburization of high-carbon parts. Since continuous sintering furnaces are open ended, air ingress can cause the dew point to fluctuate, and it is rare to find a furnace that consistently maintains the required dew point. Many companies add natural gas as a carbon boost to compensate for dew point fluctuation. Soot can form, parts can be carburized in the cooling zone and carbon control can become more difficult if natural gas is not accurately controlled and too much is injected. In addition, an increase in hydrogen concentration has a positive impact on effectiveness of natural gas (methane) to transfer carbon to the part.
If lubricant vapors are prevented from entering the hot zone, the only reactive species contained in the synthetic atmosphere of the hot zone are H2O, trace O2, H2, and CH4 (if natural gas is added). Therefore, if the hydrogen concentration is known, control of synthetic sintering atmosphere can be achieved by monitoring and controlling the dew point and residual methane concentration. Figure 1 shows an example of a closed-loop dew-point system.
With Fe-C and Fe-C-P alloys, time, temperature, contaminant species and sintering atmosphere are crucial for successful sintering. Improvements (primarily consistency) occurred when the atmosphere was changed from pure endothermic gas to a more neutral, low reactivity nitrogen/hydrogen synthetic atmosphere. Due to endothermic gas variability both from a temperature and compositional standpoint, carbon content up to 0.2% can be introduced in processed parts, which not only can decrease shrinkage, but also can cause dramatic carbon variations resulting in a deterioration of magnetic properties. Using higher percentages of hydrogen actually improves magnetic properties by reacting with the carbon in the part and forming CH4.
It may be concluded that nitrogen/ hydrogen and low-reactivity atmospheres help maintain consistent part geometry and mechanical properties. There is much less concern about changes in part quality due to variations in the composition (peak CH4 shaving), dew point and other variations in generated atmosphere as with pure generated endothermic atmospheres.
Low-reactivity atmosphere technologies
In most cases, the major component of low-reactivity atmospheres is N2 as shown in Table 1. Depending on volume economies and other factors, synthetic nitrogen can be supplied either as bulk cryogenic (liquid) or high purity generated on-site. Two preferred methods to add small amounts of reactive gases, such as hydrogen and traces of CO, to produce sintering atmospheres (depending on economics) are: 1) mixing with synthetic H2 (cryogenic liquid or gas storage) plus additions of CH4 (< 0.8 %), and 2) diluting endothermic gas (e.g., 85% N + 15% endo) to obtain N + 5.4% H + 2.9% CO + <0.01% CO2 + <0.8% CH4. (NOTE: CH4 additions lower the CO2 content and the dew point. It has no direct carbon control function; only CO can perform that function.)
Selection of one of these atmospheres is often dictated by the local situation, or more commonly in the U.S., the economic frame conditions including cost of gases, environmental regulations, handling of hydrogen (gaseous or liquid storage), availability and cost of natural gas, cost of electricity and availability of endothermic gas generators.
Comparing costs of generated atmospheres is difficult due to the different generation process factors including centralized and decentralized generators, energy consumption, capital investment, maintenance downtime, etc. However, due to the recent volatility of natural gas prices (which have been as high as $8-10/MM Btu), the economics of synthetic, nitrogen-based mixed-gas atmospheres have become very attractive compared with traditional generated endothermic gas (Fig. 2). At today's natural-gas prices, the benefits of a consistent, controllable atmosphere chemistry and dew point can be realized using a significantly lower cost atmosphere. In addition, elimination of keeping spare parts on hand and reducing the need endothermic generator maintenance by using nitrogen-based atmospheres mixed with endothermic or hydrogen gas can result in substantial savings along with more consistent sintered parts.
Figure 3 shows the cost of endo gas as function of different flow rates based on relative endothermic generator size. The price of natural gas has a strong influence on the cost of straight endothermic gas. Prior to the latest natural gas price increase, the U.S. average natural gas cost was $5.25/MMBtu. The applied model is somewhat generic based on a cross section of several regions; it has to be tailored individually for each location and equipment structure. (If you assume no significant investment, then you can also assume a relative reduction in cost shown in dark blue on graph.)
Cost structure of protective sintering atmospheres
A comprehensive positioning of gas cost based on the costs reactive endothermic atmosphere, low reactivity diluted endo (with nitrogen) and synthetic liquid nitrogen/hydrogen is shown in Fig. 4. The following conclusions can be drawn from comparing the information presented in Figs. 2, 3 and 4:
- Gas from bulk (liquid) nitrogen/ hydrogen (for higher flow rates) is typically much more expensive than high purity, on-site generated gas
- On-site generated gas is more economical than endo gas as natural gas pricing approaches $6/MMBtu at a minimum consumption of ~10,000+ scfh
- Diluted endo in combination with high-purity, on-site nitrogen reduces on-site costs by another 5% from straight on-site nitrogen/hydrogen
The quality and reliability of mixed low-reactivity industrial atmospheres are generally superior and more reproducible than generated endothermic sintering atmospheres. Only low reactivity gases (mixed synthetic H2/N2 gases and diluted endo) allow robust processes (e.g., N2, 5H2, 2CO, 1CH4). Parallel sintering flexibility of different carbon levels in one furnace run is now possible using low reactivity atmospheres plus a possible addition of small amounts of CH4. At a given gas consumption, low reactivity gas is price competitive or less expensive than conventional atmospheres (e.g., endo) considering dramatic NG price fluctuations. High-purity (99.999%) industrial gas can now be easily produced on-site for lower flow rates (e.g., APSA). Specific processes (e.g., diluted endo) allow for an "over-the-fence" on-site generation of the sintering atmospheres (no mixing of synthetic gases, no hydrogen storage and reduced generator maintenance).