Stainless Steel Brazing

The term stainless steel describes a chromium containing iron base alloys in a wide variety basically used in distinct applications requiring heat or corrosion resistance. Stainless steels are easily brazed by using any process though demanding a bit tighter process controls not so much necessary while brazing carbon steels.

This particular material seeks lot of rigorous demands & are vital since they are imposed by the steel's inherent characteristics & daunting service environments. A sound knowledge of varied properties of stainless steel & appropriate process controls leads to a successful brazing of the steel components.

Base Metals
There are certain basic categories which re essential for the fine categorization of a stainless steel & these are :

• Austenitic (nonhardenable) steels
• Ferritic (nonhardenable) steels
• Martensitic (hardenable) steels
• Preipitation hardening steels
• Duplex stainless steels

The aforesaid alloys consists of chromium which imparts corrosion resistance & are iron based. Varying from one alloy to another, this corrosion resistance of the stainless steels, these as well vary from one corrosive medium to another for any given alloy.

• Austenitic (Nonhardenable) Stainless Steels
  Stainless steels of this nature actually consist of nickel or nickel with manganese additions & possess extreme heat & is corrosion resistant. This is required for :
  • Stabilizing austenite down to room temperatures
  • Through heat treatment, forcing the alloys to be nonmagnetic & nonhardenable
  
  Torch & furnace brazing are extensively used for brazing this particular series of stainless steels.
  
  
• Ferritic (Nonhardenable) Stainless Steels
  Low on carbon alloys of iron & chromium, sufficient quantity of chromium is added to iron which stabilizes the low temperature phase in steels, ferrite in case of a wide temperature range. Corrosion takes place with the application of electrochemical action which ultimately destroys the bond between the base & the filler metals.
  
  
• Martensitic (Hardenable) Stainless Steels
  This particular stainless steel consists of iron-carbon-chromium alloys combination & these have two types which are :
  • Low chromium & low carbon grades
  • High chromium & high carbon grades
  
  Comparatively expensive than the ferritic nonhardenable grades, these have balanced alloy compositions which even gets hardened merely by heat treatment. It even requires the usage of brazing thermal cycle well compatible with the heat treatment essential for the alloys.
  
  
• Precipitation Hardening Stainless Steels
  Developed especially for applications demanding high strength coupled with heat & corrosion resistance qualities, these alloys are actually austenitic stainless steels enhanced with the addition of aluminum, titanium, copper & molybdenum. With the application of special heat treatments, these additional elements makes it possible to precipitation-harden the alloys & this process isn't completely austenitic as the hardening reactions are quiet complex & even at times involve martensite formation.
  
  
• Duplex Stainless Steels
  These steels are a duplex of austenite & ferrite characterized by approx 28% of chromium content. They have limited brazing applications than the other steel types. The properties such as thermal expansion rates, response to heat treatment, precipitation of different phases & chemical compositions are determined by reviewing the technical data of a specific alloy.

Filler Metals
Brazing for stainless steels require certain specific filler metals which are commercially available & the ones based with alloys mostly preferred are :

• Copper
• Silver
• Nickel
• Cobalt
• Platinum
• Palladium
• Gold

Stainless steel has no dearth of filler metals variety commercially when it comes to the brazing part. Such filler metals have certain factors vital to be considered before their final selection for a specific application. These are majorly :

• The service criteria including services like operating temperatures, stresses & environment
• Requirement of heat for martensitic or precipitation hardening steels
• Brazing procedures
• Cost
• Application of special precautions like sensitization of unstabilized austenitic steels at certain temperatures