High-temperature furnace-atmosphere processing is an important technology in fabricating metal parts. Whether the specific technique used is heat treating, brazing, annealing, sintering or hermetic sealing, high-temperature atmosphere processing enables manufacturers to provide the exact product characteristics required by their customers.
|Fig. 4. HOGEN units at H. Cross Co|
High temperatures cause metals to go through phase transformations that change their characteristics. By skillful use of various heat-treating techniques, manufacturers can tailor metal properties such as ductility and hardness to the specific requirements of the intended application. In the case of sintering and hermetic sealing, manufacturers can create products with entirely novel capabilities that may not be possible with other fabrication approaches.
Because high temperatures speed up most reactions and because oxygen is present in the atmosphere surrounding us, high-temperature processing is normally done in a manner that excludes atmospheric oxygen. If metals are heated in an oxygen-containing atmosphere, oxidation is unavoidable. Because oxidation is generally not a desirable process condition, heat-treat conditions are generally adjusted to exclude oxygen, either by use of vacuum heat treating or by using an atmosphere furnace that contains an atmosphere designed to eliminate oxygen.
The Need for Hydrogen
For atmosphere furnaces, it is generally not sufficient to use a nitrogen-only atmosphere for heat treating. While the use of nitrogen blanketing can reduce oxygen levels sufficiently to prevent undesirable reactions under many conditions, the high temperatures and open design of most heat-treat furnaces make it necessary to go beyond a simple nitrogen atmosphere. The use of hydrogen to act as an oxygen “scrubber” has long been best practice for high-temperature metals processing.
At the extremely high temperatures common in many types of thermal processing, atmospheric moisture will break down to oxygen and hydrogen constituents. Liberated oxygen can then react with the material being processed or with heated areas of the furnace (furnace belts and hot zones are often affected). The metal oxides created are very undesirable qualities. In the case of sintering, the resulting parts may be entirely unusable. It is critical that thermal-processing atmospheres have controlled humidity. In most cases, processors want the driest atmosphere possible, and they will humidify to controlled levels if they want to achieve specific processing results.
Synthetic (custom-blended) atmospheres comprised of blended, dry inert gases – such as argon or nitrogen plus hydrogen for oxygen removal – provide a flexible, reliable, high-performance atmosphere for thermal processing. Processors tailor the atmosphere composition to provide the performance desired, for the processes performed, at the lowest atmosphere cost possible.
HOGEN on-site hydrogen generators offer a convenient, reliable, cost-effective approach to providing small- to medium-sized hydrogen supplies with virtually zero hydrogen inventory. Because of hydrogen’s extremely wide flammability envelope, low initiation energy and absence of odor or other indication that the gas is present, important safeguards are required when storing and using hydrogen. By eliminating hydrogen storage, on-site, zero-inventory hydrogen generation eases the challenge of employing hydrogen in thermal processing.
|Fig. 1. HOGEN hydrogen generator at Loos & Co.|
Loos & Co., Inc. (www.loosco.com) was founded in 1958 in Pomfret, Conn. Gus Loos grew his cable and extrusion business into a vertically integrated manufacturer of wire, cable, extruded cable, cable assemblies and associated tools and fittings. Since 1971, the garage-sized business has developed into a 220,000-square-foot manufacturing plant. Today, the company can draw wire, strand cable, extrude plastics, design and manufacture hardware and tools for mechanical cable, custom design and manufacture cable assemblies, and test and package all of the above. Loos manufactures a wide variety of wire, aircraft cable and wire rope. These products are used in aerospace, military and commercial applications, including aircraft flight controls, elevators, fitness equipment, and rigging and scaffolding operations. They supply products to companies such as Boeing, General Dynamics and Bombardier.
Loos uses a pure-hydrogen atmosphere for the annealing of stainless and nickel-alloy cables and components. Because of the large volume of hydrogen that they use and their large facility size, the company has traditionally rented a tank relying on periodic liquid hydrogen deliveries from an industrial gas supplier.
All went well for many years, until Hurricane Katrina hit in 2008 and temporarily crippled domestic liquid-hydrogen supply. This potentially threatened hydrogen supply to Loos and other users of liquid hydrogen because the limited supply of hydrogen gas was reserved for critical uses such as electric power-plant generator cooling. Without hydrogen, production at Loos was forced to shut down. They could have changed their process to use disassociated ammonia. However, the company preferred to continue using hydrogen for the best possible surface-finish results. So, Loos purchased a HOGEN hydrogen generator to ensure that their production would never again be at risk through loss of hydrogen (Fig. 1).
The HOGEN hydrogen generator allowed Loos to back up their hydrogen supply and minimize the risk of liquid-hydrogen interruption by providing critical hydrogen for wire annealing. The hydrogen-generator installation is configured to allow an alternate supply from the generator or the tank source. Loos can therefore manage their two-supply approach for best supply reliability. Like a bank data center with two power sources, Loos has invested to ensure that their customers have the most reliable source of product possible.
|Fig. 2. Strand-annealing tube furnaces at H. Cross Co.|
H. Cross Company (www.hcrosscompany.com) was formed in 1939 to process refractory-metal ribbon, strip and wire (Fig. 2). Through steady growth over 70 years, the family-owned company has earned an important niche providing fabricated rolled and shaped goods from metals such as tungsten, molybdenum, rhenium and tantalum ribbon. These metals require annealing in a pure-hydrogen atmosphere at extremely high temperatures in order to become ductile enough to be rolled and drawn to custom specifications (Fig. 3). Many products are then custom tempered to achieve precise properties.
H. Cross has operated with a highly skilled workforce of experienced, long-term employees in eastern New Jersey just outside of New York City since the company was founded. H. Cross has used hydrogen for annealing since the company started, beginning with hydrogen cylinders delivered daily. In the early 1970s, they switched to electrolysis generation of hydrogen gas to avoid the disruption of daily hydrogen deliveries.
For H. Cross, electrolysis hydrogen generation for the last nine years using dual HOGEN hydrogen generators has provided several critical advantages, including:
- Zero-inventory hydrogen enables safe hydrogen use in densely settled NYC metroplex
- Highly pure, extremely dry hydrogen for consistent, predictable processing results
- No-delivery hydrogen means that employees can focus on production rather than interruptions
- Dual generators ensure a constant supply of gas even if a unit is shut down for maintenance
|Fig. 3. Push furnace (1000°C) for annealing refractory metals at H. Cross Co.|
H. Cross uses two compact hydrogen generators (Fig. 4), each capable of up to 40 scf/hour of hydrogen production, providing 99.9995+% pure, -85°F dew-point hydrogen up to 24/7. Each generator can supply most of the hydrogen requirements. The generators follow the factory hydrogen requirements, automatically matching hydrogen production rate to hydrogen requirements and using only the electricity required to produce the amount of gas actually used, resulting in no wasted power.
In 2011, H. Cross replaced their oldest HOGEN hydrogen generator with a new model after about nine years of use nearly 24/7, or about 80,000 hours of operation. Their other HOGEN hydrogen generator, also with nearly 80,000 hours of operation, has been upgraded and will be operated for several more years before replacement is planned.
|Fig. 5. High-temperature, controlled-atmosphere belt furnaces at Mini-Systems Inc.|
Mini-Systems Inc. (www.mini-systemsinc.com) is a worldwide leader in manufacturing precision thick- and thin-film chip resistors along with capacitors, metal/glass sidewall packages, custom thick-film hybrid circuits and multi-chip modules. Formed in 1968, Mini-Systems is called upon when high reliability, responsiveness and high performance are required in a broad range of applications including medical, spacecraft and military. A critical part of Mini-Systems’ product line is sealed packaged assemblies. Its factory is a compact facility situated near downtown Plainville, Mass.
Mini-Systems employs forming gas (5% hydrogen, 95% nitrogen) as a reducing gas atmosphere in their belt furnaces used to anneal and braze package assemblies (Fig. 5). For many years, the company used cylinder hydrogen for the forming-gas component to be mixed with the nitrogen being supplied via vaporized liquid nitrogen from a tank on the facility site. Upon learning in 2003 of the availability of HOGEN on-site hydrogen generation, they adopted hydrogen generation in place of cylinders to improve site safety, personnel productivity, process control and hydrogen economics. The company’s furnaces are kept hot 24/7, but they are generally used for processing parts only on the day and afternoon shifts.
Mini-Systems employs a mixing panel to blend 5% hydrogen with nitrogen for a reducing-gas atmosphere during the day. At night, they simply shut down the hydrogen generators to reduce electrical consumption, and they utilize a pure nitrogen atmosphere overnight into the belt furnaces. In the morning, the units take less than six minutes to produce ultrahigh purity hydrogen at 200 psig. The company employs two HOGEN 40 hydrogen generators (Fig. 6) to supply all of the hydrogen needed for their furnaces in a load-following manner. The hydrogen generators follow the plant load, making exactly the amount of hydrogen required at each moment, all with less than 4 grams of total hydrogen inventory at any time.
|Fig. 6. HOGEN 40 units at Mini-Systems Inc.|
HOGEN hydrogen generation allows these manufacturers to access all of the hydrogen that their processes require while entirely eliminating hydrogen deliveries, hydrogen handling and hydrogen storage. These hydrogen generators are available in capacities from under 10 scf/hour of hydrogen to over 1,100 scf/hour of hydrogen supply. IH
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