The process of heat treating is often the most overlooked step in producing quality metal components for automotive applications, but it is usually the first process suspected if there are any deficiencies in quality or final product performance. Indeed, the heat-treatment process can amplify dimensional and metallurgical issues. For the most part, however, these can be compensated for, or at least minimized by, proper heat-treatment process control.

 

As the name suggests, heat treatment involves the heating and cooling of components. However, it is also a chemical process where the metal or alloy is reacting and interacting with gas species in the furnace atmosphere at elevated temperatures. This article explains some of the practical applications and technologies to improve the control of these interactions between the metal and the atmosphere. The Linde technologies discussed have been successfully implemented globally and are helping manufacturers and others involved in heat-treatment operations for the automotive industry.

Atmosphere Generation and Control

Furnace atmosphere generation goes hand-in-hand with atmosphere control. The control of flow rates of the gas species going into the furnace does not necessarily mean the furnace atmosphere is controlled, since the reactions taking place in the atmosphere are not considered during the adjustment of flow rates. Only when flow control and atmosphere analysis are integrated does furnace atmosphere control begin.

Sampling and gas mixing are also important factors in generating and controlling the atmosphere, and they are important variables that may impact the associated cost and quality of the heat-treatment operation.

To achieve a final mixture – inside or outside the furnace – necessary to deliver the final metallurgical requirements of the particular metal being heat treated, atmosphere generation may mean: endothermically or exothermically generated gas mixtures; synthetically blended atmospheres produced from a mixture of industrial gases; or a combination of these (generated and synthetically blended).

Flow Control and Analysis

Flow control can be very complex or simple, depending on the material being treated and the metallurgical demands of the process.

Considering the strict quality standards of the automotive industry, it is critical to accurately monitor and tightly control the furnace atmosphere to ensure that the heat-treated parts consistently meet quality standards and customer requirements. The manufacture and heat treatment of virtually any metal automotive component (including for example, high-quality tube) must adhere to such standards and requirements.

Figure 1 shows a CARBOFLEX™ atmosphere-control system for a large roller-hearth furnace used in a tube annealing operation. The furnace has various inlets, and the atmosphere for the furnace is generated by mixing an endothermic gas and nitrogen. The system controls the generator by making sure the gas ratios are controlled when produced by the generator and mixed in correct proportion with the nitrogen for various zones.

One of the main challenges in furnace atmosphere control is the sampling of the atmosphere so gas flow levels can be accurately adjusted in a timely manner. The location and uniformity of the atmosphere sample, in addition to the response time, play a major role in determining the quality of atmosphere control. Therefore, it is critical to maintain uniformity within the atmosphere so that the sample basepoint serves as an accurate reference for the overall furnace atmosphere.

Atmosphere Circulation

Traditionally, circulation fans have been used to improve the uniformity and efficiency of the atmosphere by increasing convection.

Linde developed CARBOJET™ gas-injection technology, a new technique for improving the circulation of the atmosphere. This technology uses patented high-speed injection nozzles. By employing the pressure of the gas species, the atmosphere mixing and circulation can be accomplished much more effectively. The gas-injection technology is ideal for upgrading a variety of furnaces, including roller hearth, pusher, rotary-retort and pit.

Using computational fluid dynamics (CFD) modeling, Figure 2 compares the circulation of the atmosphere using a fan versus the new technology in a pit furnace for carburizing automotive gears. The model demonstrates the inhomogeneity of the atmosphere produced with the fan. These differences in convection lead to different carburizing coefficients, causing variations in carburizing case depth that produced out-of-spec parts.

With CARBOJET mixing technology, the uniformity of the furnace atmosphere is greatly improved, resulting in meeting extremely tight part specifications.

An additional benefit is realized with gas-injection technology. In addition to improving the metallurgical quality of parts, the furnace no longer requires circulation fans, which can accordingly be removed from operation. Fans are a major source of maintenance issues, including leakage of air or water due to vibration. Fan maintenance is costly and time consuming, and eliminating this aspect of operations will benefit the bottom line and reduce scrap. Figure 3 shows a typical fan after less than six months in a carburizing atmosphere. It is common for furnace fans to malfunction and break in a short period under these aggressive conditions.

The benefits of installing CARBOJET technology for use with pit furnaces include:

  • High uniformity of furnace atmosphere
  • Reduced carbon variation
  • Elimination of retorts
  • Shorter cycle times due to increased heating and cooling efficiency
  • Increased carburizing diffusion coefficient as a result of increased convection
  • Extended furnace lid life up to 300% due to elimination of vibrations

Challenges in Brazing and Sintering Atmosphere Control

Brazing, especially sintering, processes are carried out at relatively higher temperatures than other heat-treatment processes. Higher temperatures, typically in excess of 1100°C (2012°F) for sintering steel, mean that thermochemical reactions are faster and more unpredictable. In addition, the transition from these high temperatures to lower temperatures requires more demanding furnace atmosphere requirements as well as greater control of same.

The Ellingham diagram in Figure 4 shows the requirements of H2 content in an atmosphere for different alloys at different temperatures in order to achieve, for example, oxide reduction for complete wetting of brazing compounds, strength improvement of the sintered bodies or for merely brightening surfaces after these processes.

This complexity calls for proper atmosphere control in these hydrogen-containing atmospheres. Linde developed the HYDROFLEX™ atmosphere-control system to address these complex requirements. Figure 4 shows that as the temperature changes, the hydrogen content required to maintain an oxide-free surface would also change. The basic principle of control is to sample the atmosphere and then alter the flow rates of the incoming gases to create the conditions that enable the necessary reactions. The atmosphere-control system goes further by sampling at different temperatures for different gas species and then calculates changes in flow rates to optimize the use of reactive furnace-atmosphere gases. In the brazing process, the optimization of hydrogen use can improve safety as well as reduce consumption for lower operating costs.

Beyond the complexity of atmosphere control in the brazing process, sintering steel involves additional complexity that must be addressed. Figure 5 summarizes the challenging requirements of the atmosphere at each stage of the sintering process as well as the types of technologies available to address them.

The pressed green bodies to be sintered contain lubricants when they arrive at the entry zone. These lubricants help keep the compressed powder bodies intact when handled by the operator, but these hydrocarbon compounds need to be removed before the sintering stage. Any residual lubricant that enters the sintering zone will dissociate, causing weaker sintered bonds between powder particles.

There are a number of other issues that lubricant residues can cause in the sintered matrix, and it is therefore imperative to oxidize these lubricants as early as possible. Accordingly, the sintering process starts with an oxidizing atmosphere in the preheating zone, where the powder and the lubricants are oxidized, and the lubricants vaporize and are easily removed with the normal flow of gas from the zone atmosphere.

The lubricant-free bodies then enter a reducing atmosphere, where sintering begins. During sintering, both oxidation as well as carbon activity must be controlled. Linde developed SINTERFLEX® control and gas-mixing systems technology, which complements HYDROFLEX atmosphere control, to achieve more precise carbon control in the sintering furnace. This advanced system also optimizes carbon input to the furnace to avoid sooting and excessive carbon deposition, which can degrade sintering performance and furnace components.

The optimization of carbon, by adjusting the natural gas or propane flow rates, helps reduce excessive carburization of furnace belts. Carburization leads to metal dusting and, if left unchecked, to the eventual breakage of expensive furnace retorts and belts in the sintering process. Figure 6 shows typical metal dusting on a furnace retort made from highly alloyed temperature-resistant steel.

Optimization of carbon input in long-term operation with SINTERFLEX control has demonstrated that excessive metal dusting can be avoided or minimized, leading to improved lifetime of expensive furnace components as well as less downtime for replacement.

Cryogenic Treatment of Metals

Cold is also considered a heat-treatment process, and it has a direct effect on the microstructure of steel components. Subzero treatment to transform retained austenite to stable martensitic structures is particularly important for many high-quality automotive components for a variety of reasons.

When retained austenite is transformed into martensite by simply cold working, it becomes extremely brittle. Therefore, these austenitic structures should be transformed under controlled conditions and then tempered to avoid residual stress created during the transformation process. For instance, if an automotive component with retained austenite higher than metallurgical specifications goes into service, these components will likely have a short life span, provided the metal does not break during the first few cycles of stress conditions.

Cryogenic treatment of components is a well-known practice. Linde developed CRYOFLEX® technology and a range of cryogenic freezers to address this practice. Figure 7 shows a tunnel freezer that is designed to integrate to a continuous furnace so that parts coming from the furnace can move to the cryogenic treatment process without delay. Some alloys need to be cryogenically treated within two hours to avoid transformation. Batch freezers can also be tailor-built to match the size of baskets for integrated-quench furnaces so that the furnace charge can be moved directly to the cryogenic units (Fig. 8).

Control of the cooling and heating rates during the treatment process is critical. Uniformity of temperatures can also be controlled and documented to meet QC/QA requirements for certain automotive and aerospace standards.

Conclusion

Documented and repeatable process quality is key for suppliers of heat-treated metal components to the global automotive industry. Many quality issues can be avoided with a proactive scrutiny of existing heat-treatment processes and a robust implementation of measurement and control technologies.

The most important aspect of atmosphere control is furnace analysis and thermodynamic calculations to understand the thermochemical reactions taking place. This is because in a heat-treatment furnace, there are multiple interactions between the metal and the atmosphere and between the gases forming the atmosphere. Only with this understanding is it possible to accurately control the flow rates to meet the requirements that will enable these reactions.

The challenge is to achieve a high rate of continuous uniformity and to identify the right sample points so that atmosphere sampling is as accurate as possible. There are technologies and expertise available to reach such a high level of accuracy and ample opportunity to improve atmosphere measurement and control.

When implemented as part of quality control and assurance programs, these methods can help elevate suppliers to Tier 1 status and help Tier 1 suppliers become even more competitive in a global environment.

 

For more information:  Grzegorz Moroz, program manager, Heat Treatment and Atmospheres, Linde LLC, 100 Mountain Ave., Murray Hill, New Jersey 07974; tel: 800-755-9277; e-mail: grzegorz.moroz@linde.com; website: www.lindeus.com

SINTERFLEX is a registered trademark of Linde AG.

References

  1. Mr Gerd Waning, Standzeitverlängerung von Muffeln und Bändern durch optimierte Schutzgasauswahl
  2. Nayar, H.S., “Production sintering atmospheres,” Metals Handbook Vol. 7, Powder Metallurgy, ASM International, Metals Park, 1984
  3. Malas, A., Dionne, B.G., “A Case Study of Implementation of Carbon Control in Metal Injection Molding (MIM) Sintering Furnaces,” Proceedings of the Conference on Injection Molding of Metals, Ceramics and Carbides, Orlando, March 14-16, 2011
  4. Malas A., Holm T., Wiberg S., Furnace Atmospheres No. 7, “Sintering of Steel Linde Gas Special Edition,” Linde Gas, Munich, 2013