High-temperature sintering has been proven to provide achievable increases in material properties when applied to conventional powder-metallurgy (PM) applications. As material properties and strength requirements increase, some thermal processors are transitioning from traditional sintering furnaces to high-temperature models. However, not all high-temperature sintering furnaces are created equal. Continuous vacuum furnaces provide many advantages when it comes to efficiently producing top-of-the-line PM parts, products and components at elevated temperatures.

Traditional sintering, the heating process that causes adjacent metal particles to bond together, typically takes place at temperatures below 2150°F (1175°C), while high-temperature sintering occurs at temperatures between 2150°F (1175°C) and 2800°F (1538°C). Enhanced mechanical properties are achievable when sintering at these elevated temperatures. For example, higher sintering temperatures have been found to increase diffusion bonding and cause pores to become more rounded. Additionally, the necks (where adjacent particles of metal begin to share molecules, locking them into a rigid structure) between the particles grow larger, causing the pores to shrink and significantly increasing the strength of the material.

Thanks to these property improvements, the resulting PM parts may find use in high-performance applications where only more-advanced materials were previously employed. High-temperature sintering applications include additive-manufactured components; high-end corrosion-resistant materials, such as stainless steels, 17-4PH and Inconel; and materials requiring extremely high sintering temperatures, including tool steels and refractory metals, as well as highly reactive materials like titanium.


Commonly Used Sintering Furnaces

Sintering furnaces are key players in the PM process. These furnaces typically apply solid-state sintering methods to convert compacted powders into products with excellent material properties and strengths used in many applications. To achieve this result, sintering furnaces maintain temperatures below the powder’s melting point to bond, rather than liquefy, the powder materials. This temperature causes the substance’s powder particles to bond and form a compact crystallized mass, increasing its strength and structural integrity. Common equipment for sintering includes batch vacuum furnaces, continuous mesh-belt furnaces, pusher furnaces and continuous vacuum furnaces.

  • Generally used for low- to medium-volume sintering of materials with operating temperatures up to 3000°F (1649°C), batch vacuum furnaces (Fig. 1) are typically used for sintering stainless steels and higher-end alloys.
  • With a typical temperature range of up to 2150°F (1175°C), continuous mesh-belt furnaces (Fig. 2) are used for continuous high-volume sintering of iron, stainless steel, brass, bronze and copper components.
  • Capable of achieving temperatures of up to 3000°F (1,649°C), pusher furnaces (Fig. 3) are often used for high-volume sintering of iron and stainless steels.
  • Offering temperatures up to 3000°F (1649°C), continuous vacuum furnaces (Fig. 4) provide higher-volume sintering of materials, including stainless steels and higher-end alloys.

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Fig. 2. With a typical temperature range of up to 2150°F (1175°C), continuous mesh-belt furnaces are used for continuous high-volume sintering of iron, stainless steel, brass, bronze and copper components.

 

Sinter Figure 3

Fig. 3. Capable of achieving temperatures of up to 3000°F (1649°C), pusher furnaces are often used for high-volume sintering of iron and stainless steels.

 

Sinter Figure 4

Fig. 4. Continuous vacuum furnaces provide many advantages when it comes to efficiently producing top-of-the-line PM parts, products and components at elevated temperatures.

 

Vacuum Sintering Furnaces for High-Temperature Sintering

The use of a vacuum sintering furnace, in which sintering is accomplished in a vessel where the pressure is maintained below atmosphere for either a portion of or for the entire sintering cycle, is recommended for high-temperature sintering applications because the provided vacuum delivers several advantages over other available methods. Because sintering via a vacuum furnace occurs at a reduced gas density within a sealed chamber and below atmospheric pressure, gas, moisture and contaminants are removed by a pump, while impurities such as oxides, moisture, hydrocarbons, air and nitrogen are evaporated. The resulting environment is extremely clean because the vacuum method removes nearly all impurities, allowing for a purer sinter.

Typical maximum temperatures for vacuum furnaces can reach above 3000°F (1649°C), making the equipment suitable for ceramics and exotic materials, which often require higher-temperature sintering to achieve desirable properties.

There are several types of vacuum furnaces available for high-temperature sintering.

  • Batch vacuum furnaces: After product is loaded into a chamber of a batch vacuum furnace, the temperature ramps up, sintering takes place and then the oven cools down. The door is opened to remove the product. This type of furnace is best suited for lower to mid-volume production.
  • Continuous vacuum furnaces: These furnaces offer a load chamber, a heating chamber and a cooling chamber. This method allows the heat chamber to always be under vacuum and at temperature. As a result, cycle times using continuous vacuum furnaces are faster than batch vacuum furnaces. Additionally, the product is loaded in one end, and the finished product exits from the other end.
  • Modular vacuum furnaces: Also called multi-chamber vacuum furnaces, modular vacuum furnaces offer continuous processing similar to that of continuous vacuum furnaces, but they can be equipped with any number of chambers and can be expanded. Also, configuration of the furnace can be changed as needed. The expandability and versatility make this furnace a suitable option for thermal processors who anticipate future growth. Like continuous vacuum furnaces, modular vacuum furnaces offer a load chamber, a heating chamber and a cooling chamber, allowing the heat chamber to always be under vacuum and at temperature. As a result, cycle times using a modular vacuum furnace for continuous processing are faster than batch vacuum furnaces. In addition, this type of furnace can incorporate multiple steps: a vacuum or atmosphere debind section that feeds into a vacuum sintering section with an added vacuum oil-quench section, as well as a wash and temper stage, so it produces a finished product rather than sintering the product in the furnace and then sending the product out to a separate heat-treat operation.

 

Because of the continuous process and the vacuum provided in the heat chamber, continuous vacuum sintering furnaces offer several advantages, including positive effects on material properties, energy efficiency, environmental friendliness and cost effectiveness.

 

Mechanical Property Improvements

Because high-temperature continuous vacuum sintering provides a pure sintering environment, it produces a positive effect on the microstructure of the material, including reduced pore size, improved pore rounding and improved pore-size distribution. Faster sintering reactions are also possible because the process occurs closer to the melting point of the material. Additionally, alloys of a higher melting point are broken down and impurities, which tend to slow down sintering reactions, are removed. This allows for faster sintering reactions.

 

Energy-Efficiency Gains

Continuous vacuum furnaces also tend to be more energy efficient when compared to continuous belt furnaces or high-temperature pusher furnaces because they can be turned off when not needed and only minimal furnace conditioning is required when they are restarted. In addition, they only heat products and their supports; they do not need to heat flowing gases.

 

Sustainability Increases

While continuous belt furnaces and high-temperature pusher furnaces use atmosphere and then release that into the environment, atmosphere-free continuous vacuum furnaces tend to be more environmentally friendly options because there are little to no emissions, no explosive/reactive gases are used and no burn-off flames are required. In addition, vacuum furnaces are “cold walled,” meaning that no heat is given off. There is less heat loss with this method, which also translates into reduced energy use and waste.

 

Lower Costs

In addition to creating a pure sintering environment, the lack of atmosphere also provides cost benefits over continuous belt and pusher furnaces. While traditional sintering furnaces flow atmosphere gases (e.g., hydrogen) 24 hours a day, seven days a week to create the required atmosphere, continuous vacuum furnaces do not involve the constant use of expensive atmosphere gases. As a matter of fact, it is possible to save an estimated $150,000 a year in atmosphere gas when using a continuous vacuum furnace in place of a pusher furnace. The reduced operational time, thanks to the efficiency of the continuous furnace operation, can save as much as an estimated $31,000 each year.

 

Conclusion

When you compare the cost and performance of continuous vacuum furnaces to other models when applied to high-temperature PM sintering, it is easy to see why this type of furnace is well suited to these applications. Continuous vacuum furnaces are more energy efficient thanks to faster sintering reactions, offer a lower total annual cost due to the minimal use of atmosphere gas, achieve improved material properties as a result of the purer sintering environment, offer the flexibility to process exotic and standard PM materials at elevated temperatures and are more sustainable as a result of the vacuum atmosphere.


For more information: Mark Saline is president of Gasbarre Thermal Processing Systems. He can be reached at 814-834-2200 or msaline@gasbarre.com.

 

All images supplied by Gasbarre Thermal Processing Systems, except where noted.