Advanced Metal Manufacturing: Precision Heating and Cooling (part 1)
Manufacturers of automotive and aerospace parts, medical devices, robots and machine components depend on advances in metals processing to address a range of challenges, often simultaneously. This includes the need for lighter-weight and higher-strength components, performance at temperature extremes, rapid prototyping and manufacturing, superior surface quality and durability and, of course, controlling production costs.
This article reviews industrial-gas technologies that the metals industry can use to respond, improve and even help transform advanced metal-parts manufacturing and processing. Specifically, it covers technologies for aluminum remelting, furnace atmosphere-control systems, deep cold treatment, powder-metal sintering and additive manufacturing.
Oxyfuel combustion technology is already well-known and proven successful in aluminum remelting operations. As a further development of conventional oxyfuel, the low-temperature oxyfuel method – based on the principles of “flameless combustion” – has today become an established technology. New low-temperature oxyfuel combustion systems have proven to deliver even higher melt rates with reduced oxidation, lower fuel consumption and ultra-low nitrous oxide (NOx) emissions. Simply stated, oxyfuel flames are diluted with furnace gases to reduce flame temperature and promote an effective heat distribution (Fig. 1). The actual diluted flame is nearly invisible, hence the term “flameless combustion.”
The main benefits of low-temperature oxyfuel are:
- More uniform heating and melting, avoiding hot spots and dross
- Higher furnace thermal efficiency, saving fuel and increasing production
- Ultra-low levels of NOx emissions – reduced up to 90%
- Integrated flame monitoring by UV cell for safe operation in all process steps
- Low maintenance costs, and no need for a recuperator or regenerative solutions
No Hot Spots
For more than 20 years, Linde has pioneered the use of oxyfuel applications in the aluminum industry. This includes investing extensively in R&D efforts in oxyfuel for aluminum melting and introducing a variety of solutions such as the tiltable rotary furnace (TRF), the WASTOX® combustion process and the AIROX® combustion process.
Although oxyfuel is accepted as state-of-the-art for rotary furnaces, the aluminum industry has been more cautious about adopting the technology for reverberatory furnaces. One concern has been the risk of overheating the aluminum surface and creating hot spots due to the high flame temperature. However, the application of flameless-combustion principles to the development of a low-temperature oxyfuel burner specifically for aluminum melting has enabled more efficient remelting in reverberatory furnaces.
First developed for larger steel-reheating furnaces, most new installations have employed flameless oxyfuel combustion since 2003. It provides excellent temperature uniformity, low flame temperatures and reduced NOx emissions. These features are perfect for aluminum melting conditions, leading to higher melt rates, fewer hot spots and reduced dross formation.
A conventional flameless burner relies on the fact that industry standards allow for the absence of a stable UV signal at process temperatures above the ignition temperature of 750°C (1400°F). Aluminum remelting furnaces partly operate close to this temperature, and variations between the cold batch and the roof may appear. Therefore, the low-temperature oxyfuel burner was developed and designed to use permanent supervision by a UV-cell flame safeguard. This design is unique and patented by Linde.
To date, low-temperature oxyfuel has been successfully installed in 31 furnaces at 15 plants in 10 countries, and the interest from the industry remains high.
Mechanical fans are commonly used in heat-treatment processes to circulate the atmosphere gases and to maintain furnace atmosphere and temperature homogeneity. Fans are inefficient for this task, however, and also require an inner liner and special lid, which take up valuable loading space. Inside the furnace, the gas velocities frequently are too low or variable, which can lead to an irregular carburizing case profile.
Moreover, fans are expensive to maintain and prone to frequent failures. CARBOJET® injection-mixing technology, developed and patented by Linde, uses the free energy available from the injection of nitrogen to stir the atmosphere and maintain its uniformity. The mixing requires no moving parts. High-pressure nitrogen flows through specially designed gas injectors to circulate and maintain the atmosphere inside the furnace. This results in improved circulation, atmosphere uniformity and time savings in surface reactions like carburizing, carbonitriding, etc.
The system, which uses patented high-speed injection nozzles, can be used in virtually any type of heat-treatment furnace – pit, continuous, roller-hearth, walking-beam, etc. The technology can also be used with any protective furnace atmosphere. It is suitable for use either with pure nitrogen or with a mixture of endogas, exogas or monogas, and hydrogen.
The atmosphere mixing technology was optimized with computational fluid dynamics (CFD) modeling. CFD calculations showed that the nozzle circulates 28 times more atmosphere than the volume of gas used in the nozzle. Figure 2 models atmosphere circulation in a pit furnace equipped with a fan (left) versus with the new gas injection mixing technology (right).
The results of operation with CARBOJET in a pit furnace for carburizing advanced parts showed that the uniformity of both the temperature and the carbon profile were improved using the high-speed gas injection system compared with the fan. Moreover, the furnace reaches a uniform temperature at a faster rate.
The carbon-potential distribution at various depths was also more homogeneous using the fanless injection-mixing system. As shown in Table 1, the overall scatter in carbon content was reduced by about one-third, indicating that the high-speed injectors resulted in better furnace homogeneity and more uniform case depth.
Table 1. Standard Deviation %C
Improved atmosphere mixing with the high-speed gas injection system promotes convection as well as faster and more thorough carburization processes. The following are characteristic process benefits that are helping heat-treatment operations meet demanding standards while controlling operating costs.
- Reduced soot formation
- Higher utilization of carburizing gases
- Increased carbon transfer on material surfaces
- Homogeneous product quality
More Capacity, Better Cooling
Superior results have been achieved in continuous furnaces including roller-hearth furnaces (Fig. 3), rotary-hearth furnaces, walking-beam furnaces, etc. Furnace fans can be eliminated.
A series of CARBOJET high-speed nozzles are installed through the furnace wall (Fig. 4), roof, cover or bottom – not only in high-temperature zones but also in the furnace cooling zone. Little maintenance is needed since the nozzles are made from durable alloys, and there are no moving parts.
The furnace atmosphere gas flows for the new system remain the same as with conventional operation. Significant increases in capacity are achieved thanks to the markedly higher cooling effect. This means, specifically, either the cooling process can run considerably faster up to attainment of the previous temperature at the exit or the exit temperature of the treated parts can be lower for the same cycle time.
Linde’s experience indicates average base increases in productivity of 10%. This improvement can be doubled by means of simple modifications if heating capacity allows. In some cases, Linde has even achieved 30% improvements, depending on component geometry and the alloy. In addition, new continuous roller-hearth furnaces can be more compact or sized accordingly, thus requiring less space and less material.
Deep Cold Treatment
The hardness, wear resistance and stability of tool steels and components depend on the underlying composition and molecular structure of the alloy. Subzero treatment with N2 or CO2 can alter the phase structure to help improve quality and finished-parts performance.
Hardened steel alloys and parts that resist mechanical wear and thermal cycling are in high demand for diverse industries, from automotive and aerospace to metalworking and many other manufacturing processes. Extended service life can be a critical design factor in transportation applications, for example, and it can reduce maintenance cycles and downtime in manufacturing. Extended wear is also important for the tool-and-die industry, where a single die may be used to form tens of thousands of metal parts or tens of millions of injection-molded parts.
The need for subzero treatment grows with the demand for high-performance materials for precision parts, tools and a growing number of metal parts and assemblies. In some cases, this treatment can also help simplify the production of existing high-quality, tempered-steel components to make manufacturers more competitive. In addition to high-alloy steels, subzero treatment can also be used to improve the strength and life of nonferrous metal parts and components, including age-hardened aluminum alloys.
While cryogenic treatment of components is a well-known practice, processes require high repeatability. In addition, moving batches of components between production stages can impede operations. CRYOFLEX® technology and range of cryogenic freezers were developed to address the emerging needs of deep-cold treatment. Figure 5 shows a tunnel freezer that can be integrated with a continuous furnace to smoothly move parts coming from the furnace to the cryogenic-treatment process without the need for unloading.
Similarly, batch freezers (Fig. 6) can be tailor-built to match the size of baskets for integral-quench furnaces. Some alloys need to be cryogenically treated within two hours to avoid transformation. With this customization, the furnace charge can be moved directly into the cryogenic units without waiting for parts to completely cool.
Control of the cooling and heating rates during the treatment process is critical. New cryogenic freezing systems are designed to tightly control the uniformity of temperature within the box and can automatically document that control to meet QC/QA requirements for certain automotive and aerospace standards.
Industrial gases and related process-control technology play a critical role in forming, treating and finishing advanced metal parts. Responding to challenges is an ongoing process that can yield significant cost, quality and performance advantages. For advanced metal manufacturing, solutions can literally shape the future.
Part 2 of this article can be found here.
For more information: Contact Linde LLC, 200 Somerset Corporate Blvd., Suite 7000, Bridgewater, NJ 08807; tel: 800-755-9277; web: www.lindeus.com. Author Grzegorz Moroz is program manager, metals; Akin Malas is head of applications technology, metals; and Johannes Lodin is sr. expert combustion technology, metals.