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Welding of Stainless Steel |
Welding of Stainless Steel - When 11.5% or more chromium is added to iron, a fine film of chromium oxide forms spontaneously on the surfaces exposed to air. The film acts as a barrier to retard further oxidation, rust or corrosion. As this steel cannot be stained easily, it is called stainless steel.
All stainless steels can be grouped into three metallurgical classes, i.e.
(a) Austenitic
(b) Ferritic
(c) Martensitic., based on their microstructures. Each of the classes has different welding requirements.
Austenitic stainless steels
(i) They possess austenitic structure at room temperature.
(ii) They possess the highest corrosion resistance of all the stainless steels.
(iii) They possess greatest strength and scale resistance at high temperatures.
(iv) They retain ductility at temperatures approaching absolute zero.
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(v) They are non-magnetic so that they can be easily identified with a magnet.
(vi)They have the following composition
C 0.03 to 0.25%
Si 1 to 2%
Ni 3.5 to 22 %*
Mo and Ti in some cases.
(vii) They may find uses in
Aircraft industry (engine parts)
Chemical processing (heat exchangers)
Food processing (kettles, tanks)
Household items (cooking utensils)
Dairy industry (milk cans)
Transportation industry (Trailers and railway cars),.etc.
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Ferritic stainless steels
(i) They possess a microstructure which is primarily Ferritic.
(ii) Ferritic stainless steels have a low, carbon to chromium ratio.
This eliminates the effects of thermal transformation and prevents hardening by heat treatment.
(iii) These steels are magnetic and have good ductility.
(iv) Such steels do not work harden to any appreciable degree.
(v) Ferritic steels are more corrosion resistant than martensitic steels.
(vi) Ferritic steels develop their maximum softness, ductility and corrosion resistance in the annealed condition.
(vii) Ferritic stainless steels have the following chemical composition:
(viii) Ferritic stainless steels have the following uses:
Lining for petroleum industry
Heating elements for furnaces
Interior decorative work
Screws and fittings
Oil burner parts
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Martensitic stainless steels
(i) Martensitic stainless steels are identified by their martensitic microstructure in the hardened condition.
(ii) Because of the higher carbon to chromium ratio, martensitic stainless steels are the only types hardenable by heat treatment.
(iii) These steels are magnetic in all conditions and possess the best thermal conductivity of the stainless types.
(iv) Hardness, ductility and ability to hold an edge are characteristics of martensitic steels.
(v) Martensitic stainless steels can be cold worked without difficulty, especially with low carbon content, can be machined satisfactorily, have good toughness, show good corrosion resistance to weather and to some chemicals and are easily hot worked.
(vi) Martensitic stainless steels have the following composition:
(vii) A few typical uses of martensitic stainless steels are as follows:
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Pumps and valve parts
Rules and tapes
Turbine buckets
Surgical instruments, etc.
The structural condition known as austenite is favorable to the production of a tough and ductile weld, capable of withstanding considerable stress without fracture.
Hence, for corrosion resistance and for a high degree of heat resistance Austenitic stainless steels are used ill welded assemblies ill reference to Ferritic or Martensitic stainless steels (Austenitic stainless steels, except for the free machining grades, are more weldable than the ferritic and martensitic stainless steels).
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