Furnace Brazing

Furnace brazing defines a process of joining metallic or ceramic materials using a molten filler metal which are drawn into the joint by capillary action or through preform it is preplaced. The filler metal is actually a lower melting point material which becomes molten when it is below the melting point of the materials that are being joined. This kind of brazing is considered best for mass production of brazing components of 1 to 1.5 kg each.

Requirements for a Furnace Brazing
The requirements for a successful furnace brazing are some real essential ones. It demands a suitable furnace necessary for heating. It also requires an absolutely perfect atmosphere which can protect the steel assemblies during brazing & cooling against oxidation, or oxidation and decarburization. This is accomplished in chambers adjacent to the brazing furnace. An apt atmospheric condition is even required to generate proper wetting of the joint surfaces by the molten copper filler metal by using a brazing flux.

Preference of Furnace Brazing over other Brazing Methods
There are varied reasons which makes furnace brazing preferable over other brazing methods. These are :

• It allows the simultaneous brazing of a number of joints
• Can easily preassemble the components to be brazed
• Prior to brazing, filler metal can be preplaced in contact with the joint
• It can braze similar assemblies

Types of furnace brazing
Furnace brazing generally demand certain types of furnaces for its application which are :

• Box-type
• Wire mesh belt type
• Roller hearth type.

Furnace types are essential for furnace brazing which employ either a gaseous atmosphere or vacuum. All this is vital, since, clean atmosphere without any inclusion of fluxes eliminates post braze cleaning .


Heating Requirements
Brazing furnaces are heated by using varied types of heating systems. These are categorized into :

• For high temperature, electric-resistor type heating elements required
• For low temperature, gas-fired radiant-tube heaters required

Materials
The materials which are brazed should be properly cleaned before they are inserted in the furnace. They are made rid of surface scale, grease and other contaminants. The most widely used fillers for furnace brazing are based on :

• Silver
• Copper
• Nickel
• Gold

Nickel & gold are popularly applied to stainless steels, and heat and corrosion resistant alloys.

Furnace Brazing with Fusion Paste Alloys
The furnace brazing pastes are supplied without flux which only consists of atomized filler metal and a neutral binder. A complete line of controlled atmosphere brazing paste is offered due to fusion which can be used for various base metals and filler metals. As we know, copper and copper based alloys, including copper/phosphorous based systems, silver brazing alloys, and aluminum figure in the filler metals.

Furnace Paste Selection Guide
Type of Atmosphere	Filler Metal	Base Metals Joined	Recommended Binders
Exothermic & Endothermic Atmospheres	Bronzes (Various Cu-Sn Ratios)	Carbon Steels	EXO
	Copper (AWS BCu – 1a)	Carbon Steels	EXO CCL CNG CAP CBL CDW
Mixtures including Dissociated Ammonia and Pure Hydrogen	Bronzes (Various Cu-Sn Ratios)	Stainless Steels	CDW
	Copper (AWS BCu – 1a)	Stainless Steels	CNG CDW CCL CBL CAP

Features of Binders
The recommended binders for furnace paste have certain essential features which have been dealt with extensively.

• EXO (Exothermic & Endothermic)
  • The most popular choice for copper brazing in exo/endo atmospheres
  • No hot or cold slump; non-spattering
  • Minimal to no residue under broad range of part cleanliness and atmosphere conditions
  • Not recommended for H2:N2 atmospheres
  • It is easily dispensed through "squeeze bottles"
  • Compatible with wide range of metal concentrations
  
  
  
• CCL
  • Apt for exo/endo & H2:N2 atmospheres, and vacuum
  • Leaves minimal to no residue
  • Non-spattering; no hot or cold slump
  • Can be easily dispensed through "squeeze bottles"
  • Once dried, it adheres to part
  • Compatible with wide range of metal concentrations
  
  
  
• CDW
  • Almost like a CBL, has improved/reduced stringiness and fewer hot/cold slump
  • Perfect for extremely low dew point hydrogen atmosphere
  • Is better than CBL in terms of cleaning properties with water, though, not as water washable as EXO and CCL
  
  
  
• CNT, CNG, CKW
  • Has low ash value
  • Perfect for both atmosphere or vacuum brazing
  • Maintains low dew points (minimum 20° or below) and high temperatures (1800° or above) & leaves minimal to no residue
  
  
  
• CBL, CBC, CFW
  • Non-drying & are neutral binders for atmosphere brazing
  • Depending upon the application, flow properties may be modified with additives
  • Restrictive flow and slump properties permit use on most vertical joints
  
  
  
• PNP
  • Due to pronounced slump in both hot & cold stages, it is not recommended for use on vertical joints
  • Highly recommended at marginal dew points & lower furnace temperatures (in silver brazing) range
  
  
  
• CAP
  • Suitable for vacuum furnace usage
  • Leaves no residue under these conditions
  • Suitable for use in H2:N2 and dissociated ammonia atmospheres with with low dew points (0 to -30°F)

Advantages
There are some real plus points of furnace brazing which makes it quite popular. Let us take a look at some of these salient features.

• Furnace can perfectly control the brazing temperature.
• The furnace brazed materials are low on residual stresses & haver uniform heating.
• Heightened temperature imparts additional strength to the joints.
• Uniform distribution of filler metal in the joints relies upon for gas tightness.
• Obtainment of bright surfaces free of scale.
• The furnace can as well execute the brazing of silver, phosphorus copper & copper brazing.

The most significant advantage of furnace brazing is its controlled heating cycle. It even allows the processing of clean parts without requiring the need for post-braze cleaning.

Limitations
Apart from possessing such advantageous features, there are certain limitations of a furnace brazing which needs to be known as well.

• Adjustment of joint clearances is a must when different coefficients of expansion or masses are heated differently in the furnace land. (Such a problem might be solved by employing a filler metal which has a long range between liquidus and solidus. It should even exhibits a sluggish flow to make it possible to deal with the problem.)
  
  
• When compared to other brazing equipment types, furnace brazing is quite expensive in terms of initial cost of the furnace and atmosphere generator.
  
  

Applications
Simply because of high capital outlay & less productive duty cycles, furnace brazing is extremely applicable to value added components when compared with other 'lower cost' brazing alternatives. Let us take a look at the varied applications of furnace brazing.

Furnace brazing can braze varied steel components which are :

• Machine parts
• Deep drawn sheet metal parts
• Light stampings
• Castings
• Small forgings

Apart from these, there are other applications of this particular brazing which are namely :

• Aero engine components
• Power generation components (nuclear & gas turbine)
• Marine & engineering applications