Modern Brazing of Stainle...

Modern Brazing of Stainless Steel

Brazing stainless steel becomes more economical and efficient with the use of advanced furnace design and control

BY STEPHEN L. FELDBAUER

Atmosphere integrity and control are key to the successful brazing of stainless steel components. Due to the very high affinity that stainless has for oxygen at high temperatures, the presence of oxygen or moisture in the furnace will oxidize the surfaces to be brazed and result in a defective braze. Vacuum furnaces and humpback furnaces have been the traditional systems used to braze stainless steel because of their ability to ensure a very low oxygen partial pressure in the atmosphere. However, both furnace technologies bring with them issues that result in higher operational costs, increased maintenance costs, and other process-related costs that can be avoided by using straight through, continuous belt furnace technology. Recent advances in furnace design and atmosphere control have made it possible for stainless steel to be brazed in straight through, continuous belt furnaces. This step forward in technology now permits continuous processing at lower operational costs, less maintenance, and higher yields than realized with traditional brazing systems. Oxygen levels of less than 10 ppm and dew points as low as 85F are common in a state-of-the-art straight through, continuous belt brazing furnace. The adoption of this technology by some of the leading producers of brazed components has allowed the industry to move forward in becoming more competitive.


Fig. 1 - Schematic of brazing process.
 
Surface Cleanliness
A brazed joint is formed by the filler metal melting and flowing via a capillary effect into the pores of the closely fitted surfaces of the joint to form an alloy of the metals upon solidification Fig. 1.

The key to successfully achieving a good brazed connection is surface preparation. The presence of contaminants or oxides prevents the filler metal from coming into contact with one of the surfaces to be brazed. In the case of minor oxidation, the pores of the surfaces to be brazed will be sealed by the oxide. As illustrated in Fig. 2, this prevents the capillary action and, ultimately, the brazing from occurring. Hence, the initial cleanliness of the surfaces to be brazed is extremely important, but it is equally important that the cleanliness of these surfaces be maintained during the brazing process.


Fig. 2 - Effect of oxidation on brazing.
 
Cleaning Stainless Steel
Achieving and maintaining the necessary level of cleanliness is much more difficult for brazing stainless steel components than brazing steel components. The chromium in the stainless steel forms a much more stable oxide at a much lower oxygen level than iron. The free energy of various oxides is shown as a function of temperature in Fig. 3. The lower the free energy, the more stable the oxide is and more difficult it is to reduce.

The oxides present on the surface must be reduced prior to the part reaching the melting temperature of the filler metal. In a vacuum system, the reduction is achieved by reducing the partial pressure of oxygen at an elevated temperature.

Cr2O3 (s) → 2Cr (s) + 3/2 O2 (g)

The partial pressure of oxygen must be below 1021 atmospheres for the oxide to be reduced at 1900F. In a humpback furnace and a straight through brazing furnace, the reduction is typically achieved through a reaction of hydrogen with the oxygen present in the oxide to form water vapor Fig. 4.

The presence of too much water vapor or oxygen in the system will prevent the reaction from proceeding. The dew point is used to determine the amount of water vapor in the system at given conditions. The dew point is the temperature at which an amount of water vapor in the system will saturate the atmosphere. Figure 5 shows the equilibrium dew point as a function of temperature for carbon steel and stainless steel. An atmosphere that has a dew point below the equilibrium dew point for a given temperature will result in the reduction of the oxide.


Fig. 3 - Oxide stability
 
The typical dew point required for brazing stainless steel joints in hydrogen is 50 to 55F. Although the temperature at which most stainless steel brazing furnaces are set is ~2070F, the time at which the oxides must be reduced is when the parts are approaching the maximum temperature. Hence, the dew point must be below the equilibrium value at ~2000F.

Equipment
The equipment used to braze stainless steel has changed over the years. These changes have been a result of technological advancements in the areas of atmosphere control and furnace design.

The traditional method of brazing was to use a vacuum furnace. The low oxygen level was achieved by removing the atmosphere from the furnace while heating up the components to be brazed. The lack of atmosphere eliminated the opportunity for oxidation of the stainless steel and reduced any oxides that were already present on the surface of the base metal.


Fig. 4 - Oxide reduction mechanism.
 
Although effective, this method of brazing has a number of disadvantages. The capital cost of a vacuum furnace is two to four times that of competing technologies. The process is a batch process. The furnace must be heated and cooled with each batch of product. The result is wasted time and energy due to the large mass of the vacuum furnace. The reduced volume of production and the significantly higher capital cost for such a system make it more applicable to high-value niche products.

To braze larger volumes of products, the continuous belt furnace is used with a reducing atmosphere. The most common gas used in these atmospheres is hydrogen. If the amount of oxygen that is entering the furnace is small, the hydrogen can react with it and prevent the oxidation of the base metal.

Furnace Types
There are two types of continuous belt furnaces: the humpback furnace and the straight through, continuous belt furnace.

In the past, the humpback furnace was the most common method of continuous stainless steel brazing because the arched form of the muffle took advantage of the very low density of the hydrogen in relation to that of oxygen. The stratification of the atmosphere inside the furnace provided a very low dew point for the control of the oxides and effective brazing Fig. 6.


Fig. 5 - Dew point requirements
 
The humpback furnace affords the producer the ability to braze larger volumes of product with equivalent quality, higher production rates, and higher thermal efficiency than the vacuum furnace.

The disadvantages of the humpback furnace are twofold. The height of the product must be restricted to ensure that the hump is effective in maintaining the seal through the stratification of the atmosphere. As well, the hump presents many problems with respect to product tipping and general furnace maintenance.

A straight through, continuous belt furnace (Fig. 7) avoids many of the issues presented by the hump in the humpback furnace. The belt rides on a horizontal surface throughout the furnace. Product stability and height issues are minimized along with the added maintenance that is seen in the humpback furnace.


Fig. 6 - Schematic of humpback furnace.
 
Until recent years, it was thought that the atmosphere and dew point could not be controlled to a level sufficient to braze stainless steel in a continuous belt furnace without the use of a flux. Due to advances in atmosphere technology and furnace design, this is now a very common approach to brazing stainless steel without a flux.

Successful Control of the Process
Controlling the time, temperature, and atmosphere relationship that is experienced by the product is the ultimate key to this process. Figure 8 illustrates the various zones and the function of each zone of the straight through furnace. The preheat section brings the product to the brazing temperature while removing any binders in the paste. The high heat section is where the oxides on the surface of the base metal are reduced and the filler metal melts to form the braze joint. Finally, the cooling must be done in such a way that there is no distortion or oxidation of the product.

Due to the many zones of control in this type of furnace, the temperature and the atmosphere may be varied throughout the furnace to optimize the brazing process. Each step of the brazing process is dealt with separately. The temperature and atmosphere needed for each of these steps are provided for the appropriate amount of time to reach completion.


Fig. 7 - SStraight through, continuous belt furnace from Abbott Furnace co.
 
Optimal atmosphere control is achieved through zoning, composition, multiple injection points throughout the furnace, flow rate, and overall directionality of the atmosphere flow in the furnace. Approximately 80 to 90% of the total atmosphere that is introduced into the furnace should flow toward the front of the furnace. Forward atmosphere flow provides optimal heating and a minimization of atmosphere usage to flush any volatiles from the product.

Understand the Variables
Important variables that must be considered in making such a system are quality of the construction, selection of the appropriate sealing materials, and incoming atmosphere integrity. Once a very pure atmosphere has been delivered to the furnace, the purity of the atmosphere must be maintained throughout its use. Mechanical additions to the furnace in the form of curtains and uniquely designed stacking aid in this control.


Fig. 8 - Schematic of straight through, continuous belt furnace.
 
Advancements in the area of muffle design and muffle composition have greatly improved the performance of these systems. No longer are designers restricted to steel as the primary material for muffles. Advanced ceramics, with their many desirable properties of thermal stability and wear, are now common materials for muffles.

Summary
Advancements in materials, manufacturing techniques, furnace designs, and a better understanding of the stainless steel brazing process have made it possible to take advantage of the lower cost and higher throughput of a straight through, continuous belt furnace, providing brazing houses with another tool to become more competitive.

STEPHEN L. FELDBAUER sfeldbauer@abbottfurnace.com is with Abbott Furnace Co., St. Marys, Pa.