Furnace Atmosphere Conversion Creates Quality and Economic Benefits
Since the early 1950s, family-owned FMS Corporation (Minneapolis, Minn.) has been producing high-quality sintered-metal components for a variety of industries. Their products are used for many high-strength applications such as gears for ATV powertrains, sprockets, pumps, farming equipment and medical instruments. FMS’ products are made from a variety of alloy metal powders. These powders include low-alloy steels, stainless steels, brass, bronze, soft magnetic materials and exotic alloys. Once the component is formed into a net or near-net shape via mechanical compaction, the component is then sintered to achieve its strength.
Sintering is the process whereby the component is heated to a high temperature such that the individual powder particles fuse together. Each alloy combination reacts differently during the sintering process. Some powders are more sensitive to oxidation than others at high temperatures necessary to achieve diffusion. As a result, different powders require different levels of redox via the sintering atmosphere. In the case of low-alloy steels, hydrogen reduces any iron oxide that may be present and prevents the formation of additional iron oxide through the following reactions:
FeO + H2 = Fe + H2O
Fe3O4 + H2 = H2O + 3 FeO
At temperatures above 1292°F, hydrogen can also decarburize carbon steel through the following reaction:
C + 2H2 = CH4
In order to achieve these reactions during the sintering process, FMS previously relied upon dissociated ammonia (D/A) gas – 75% H2/25% N2 – as a blanketing atmosphere within its sintering furnaces. For some metal powders, D/A was further diluted with nitrogen. The use of D/A as a sintering atmosphere was becoming an increasing challenge for two reasons: reliability and cost. Ammonia dissociators must have their reactor retorts replaced regularly, which greatly increases both downtime and maintenance cost. In addition, continued expansion at FMS required the purchase of additional ammonia dissociators, increasing the capital outlay required for each new furnace.
FMS analyzed the options of continuing to use D/A or moving to H2/N2 as the source of its atmosphere gas. Robert Brooks recognized that many of the company’s alloys could be sintered in an atmosphere with a lower hydrogen concentration – lower than the 75% hydrogen produced by dissociated ammonia.
“We had already been diluting D/A with nitrogen on some of our lower-alloy powders. We knew there were additional alloys that could be sintered at even lower H2 concentrations, which would provide an economic benefit,” Brooks said.
It was also recognized that, in addition to flexibility, hydrogen/ nitrogen atmospheres are more precise, improving quality and eliminating the risk of nitride formation.
It was at this point that Praxair and FMS began to discuss replacing ammonia with H2/N2. The FMS team, headed by Robert Brooks and Darrin Stewart, worked with the Praxair team of Matt Coffman, Mario Hernandez and Robert Esper to identify the range of the company’s powder alloys and the appropriate hydrogen concentration for each alloy. They then set up a model for hydrogen and nitrogen flow based on the expected product mix. The model output provided a platform to assess the economics of ammonia versus H2/N2.
Esper, business development manager at Praxair, explained, “A specific cost-benefit analysis was prepared, comparing the costs of ammonia with H2/N2. The cost drivers favoring the change to H2/N2 included the elimination of retort replacement, elimination of energy cost to heat the dissociator, reduced maintenance time and, most significantly, increased furnace uptime.”
Also, expansion-related ammonia dissociators were no longer needed.
Based on the projected economic and quality benefits, ammonia was replaced with a liquid-hydrogen system while continuing to use the existing nitrogen system, both provided by Praxair. To facilitate the installation of the hydrogen system, the Praxair team offered design and material recommendations for the hydrogen distribution piping. The Praxair team also provided the foundation drawings and site resources necessary to ensure the installation met all NFPA and CGA safety codes. FMS worked directly with local contractors on the construction of the hydrogen tank foundation and the hydrogen distribution piping.
Once the installation was complete, FMS and Praxair collaborated on a safety orientation for operations personnel. The conversion was completed by replacing the atmosphere mixing meters supplying the hydrogen and nitrogen mixed gas to the sintering furnaces. The switch from an ammonia- to a H2/N2-based system proved to be smooth. The ammonia system was turned off, the H2/N2 valves were opened, and the furnace continued to produce the same high-quality parts.
“Since the atmosphere was essentially the same, our furnace operators did not need to change any of their procedures,” Brooks said.
Reduce Hydrogen Ratio
A systematic effort was undertaken to reduce the level of H2 required to sinter certain alloys. The optimum H2/N2 ratios were confirmed through the processing of pilot lots of components. The pilot lots were analyzed for hardness, strength, dimensional change, chemistry and metallurgical microstructure. Throughout the start-up of the H2/N2 system, the moisture content of the atmosphere was measured during the sintering cycles of multiple alloys. The data collected was used as a baseline for future comparisons.
“The project to systematically reduce hydrogen flow began with our low-alloy steels. Once the quality of those materials was confirmed, we began piloting our higher-alloy materials. This project has been a great success. On some alloys, required hydrogen flow has been reduced from 75% down to 25% with no reduction in properties or performance,” Brooks said. “There has been no furnace downtime due to atmosphere since we converted to the Praxair H2/N2 system.”
The changeover from dissociated ammonia to H2/N2 met all of FMS’s objectives for cost reduction and quality improvement.IH
For more information: Contact Patrick Diggins, Praxair Inc., 39 Old Ridgebury Rd., Danbury, CT 06810; tel: 205-329-4405; e-mail: Patrick_Diggins@Praxair.com; web: www.praxair.com or John Sweet, FMS Corporation, 8635 Harriet Avenue South, Minneapolis, MN 55420; tel: 952-888-7976; e-mail: John.Sweet@FMSCorporation.com; web: www.fmscorporation.com
1. American Gas Association, Lecture 35 Atmosphere in Furnace
Atmospheres for a Host of Applications
Praxair’s protective atmosphere systems offer measurable benefits over conventional, generated atmospheres. As reliable, safe and cost-effective alternatives to conventional endothermic, exothermic and D/A generators, these atmospheres can provide practical solutions for:
• Ferrous and nonferrous annealing
• Vacuum carburizing
• Quenching, backfill
• Carburizing, neutral hardening
• Inerting, purging
• Autoclave operations
• Powder production
• Hot isostatic pressing
• Ceramic metallizing
• Metal injection molding
• Glass-to-metal sealing