Table of Contents
- Properties of Stainless Steel
- Types of Stainless Steel Used in Industry
- Problems in Welding Austenitic SS
- Carbide Precipitation or Inter Granular Corrosion
- Remedies: Do’s for Welding Austenitic Stainless Steel
Stainless steel is the type of high alloy steel with at least 11.5% Chromium. Iron content exceeds that of any other element. Carbon is generally less than 1.5%.
Properties of Stainless Steel:
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- Mechanical Properties: Compared to other materials, stainless steel has strong mechanical properties at ambient temperatures, In particular, it combines ductility, elasticity, and hardness, In addition, it offers good mechanical behavior at both low and high temperatures. So widely used in all Industries.
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- Oxidation Resistance: Stainless steel has the best resistance of all metallic materials when used in structural applications, having a critical temperature above 800°C. Grades of Stainless steel can be used for Sub-zero temperatures.
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- Corrosion Resistance: With a minimum chromium content of 10.5%, stainless steel is continuously protected by a passive chromium oxide layer, This special feature gives stainless steel its resistance to corrosion.
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- Versatility: Stainless steel has a wide variety of finishes, from matte to bright, including brushed and engraved. It is widely used by architects for building envelopes, interior design, and street furniture.
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- Easy Maintenance: Stainless steel objects are easy to clean, and common cleaning agents.
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- Environment friendly: Stainless steel is the “green material” and is infinitely recyclable. It is environmentally neutral and inert when in contact with elements like water, and it does not release compounds that could change their composition.
Types of Stainless Steel Used in Industry:
- Austenitic Non-Magnetic & work Hardening
- Ferritic Soft & Magnetic
- Martensitic Magnetic & Hard
- Duplex Magnetic & Wok Hardening
- Precipitation Hardening
Problems in Welding Austenitic SS:
- Carbide Precipitation or IGC
- Heat of Welding
- Porosity
- Contamination
Carbide Precipitation or Inter Granular Corrosion:
The major problem encountered in welding austenitic stainless steel is intergranular corrosion or carbide precipitation.
- When welding Austenitic SS between 420 to 880 deg. C base metal temperature also known as “Sensitisation Temperature” a large volume of Cr is picked in the grain Boundaries of the HAZ area
- This forms Chromium Carbide which precipitates and forms at grain boundaries –the area adjacent to the HAZ area
- So on working condition or in service, the HAZ area starts corroding at a faster rate as this area cannot form Cr2O3 due to Cr depletion
- This phenomenon is called Inter Granular Corrosion or Carbide Precipitation
Remedies:
- Controlling the carbon content (0.03% or below)
- Addition of Carbide stabilizers like Ti, Nb.
- Heat Treatment (Solution annealing).
- Controlled welding below 450 Degrees
The heat of welding:
- Cracking from the HAZ area
- Loss of Corrosion Resistance
- Warping or Distortion of Material
- Loss of Mechanical Properties
Porosity :
- This is caused by Dirt, Grease & marking material
- Poor Quality of Flux coating
Contamination:
- Contaminants in SS like Sulphur, Carbon, Iron, Copper & lead is the root Couse of failure of welded joints and also poor corrosion resistance
Remedies: Do’s for Welding Austenitic Stainless Steel:
- Rigid Fixturing with more tack welds
- Sequence welding to control heat
- Baking of electrode 200 deg –one hour before welding.
- Proper Cleaning of weld area before starting the job
- Use short arc & low heat Input Welding Electrodes
- Use Correct or optimum diameter electrode during welding
- Clean slag after every pass / between passes.
Hence with these simple strategies, we can weld all applications of austenitic stainless steel.
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