Basic information for STAINLESS STEEL

1. Stainless Steels
Presented by
Welding department

2. What are stainless steels ?
“ stain less”
 Steels containing 10.5 - 30%
Chromium
 The chromium oxide forms a thin
passive layer on the surface when
exposed to atmosphere.
 This prevents corrosive attack and
gives the steel its property.
 Minimum 10.5% Cr needed to protect
against atmospheric corrosion.
 Elements like Ni, Mo, Cu, Nb, Ti etc
added to improve mechanical
properties and corrosion resistance

4. Natural Passivation

5. Carbon steel VS Stainless steel

6.  Corrosion resistant
 Very durable
 Temperature resistant
 Easy formability and
fabrication
 Low-maintenance (long lasting)
 Attractive appearance
 Environmentally friendly
(recyclable)
General properties of Stainless Steel

7. Types of stainless steels
• Martensitic
• Ferritic
• Austenitic
• Duplex
(Austenite +Ferrite)
• Precipitation hardening

9. FERRITE FORMER
 Chromium
 Titanium
 Niobium/Columbium
 Silicon
 Molybdenum

10. AUSTENITE FORMER
 Nickel
 Nitrogen
 Copper
 Carbon
 Manganese

11. Effect of Alloying Elements and their Purpose
Chromium (Cr)
 Oxidation & Corrosion Resistance
 Formation of ferrite
Nickel (Ni)
 Increases resistance to mineral acids
 Produces tightly adhering high temperature oxides
 Increases Toughness properties

12. Effect of Alloying Elements and their Purpose
Molybdenum (Mo)
 Increases resistance to chlorides.
 Resistance to pitting corrosion
Copper (Cu)
 Provides resistance to sulfuric acid.
Manganese (Mn)
 Increases the solubility of nitrogen

13. Effect of Alloying Elements and their Purpose
Sulfur (S)
Improves resistance to chlorides.
 Improves weldability (Penetration) of certain
austenitic SS.
 Improves the machinability of certain austenitic SS.
Titanium (Ti)
 Stabilizes carbides to prevent formation of Cr carbide.
 Precipitation hardener

14. Effect of Alloying Elements and their Purpose
Niobium (Nb) & Columbium (Cb)
 Carbide stabilizer.
 Precipitation hardener
Aluminum (Al)
 Deoxidizer – Precipitation hardener
Carbon (C)
 Carbide former and Strengthener

15. Welding of stainless steels
All standard welding processes is SMAW, GMAW,
FCAW, GTAW, PAW and SAW maybe used depending
on the application.
Generally corresponding grades of filler metal
composition are used to match corrosion and / or
heat resistance properties along with strength.
Weldability problems are different for different
types of stainless steels eg martensitic, Ferritic,
austenitic, duplex and precipitation hardening.
Weldability considerations are similar for wrought
and cast alloys

16. Martensitic stainless steels
ASME: P.No-6
Martensitic SS – 4xx
Martensitic SS
High Carbon
Low Chromium
 Cr – 11.5-17.0 %
 Ni – 1.25-2.50 %
 Mo – 0.50-2.0%
 C – 0.05-0.20%

17. Martensitic stainless steels
 AISI 403, 410, 416, 420, 431 & 440
A/B/C grades
 Can be hardened by heat treatment
 Martensitic structure - higher carbon
grades used in tempered condition.
 Used for cutlery, surgical instruments,
steam, gas & hydel turbine blades, ball
bearings and races.

18. Welding of Martensitic Steels
 Higher carbon grades used in the quenched and
tempered condition
 Problem of Hydrogen induced cracking in HAZ.
 Pre-heat and post-weld heat treatment required if
welded with matching composition martensitic SS
electrodes.
 Austenitic SS electrodes generally used which
avoids cracking problems without pre and post
heating

19. Application of Martensitic SS

20. Ferritic stainless steels
ASME: P.No-7
Ferritic SS
Low Carbon
High Chromium
Ferritic SS – 4xx
 Cr – 11.5-30.0 %
 Ni – 0.50-2.50 %
 Mo – 0.75-4.2%
 C – 0.07-0.12%

21.  AISI 405, 409, 430, 446 grades
 Ferritic structure - higher ductility and resistance
to SCC & pitting corrosion.
 Used as thin sheet for corrosion, oxidation & heat
resisting applications and decorative purposes eg.
Automobile exhausts, Solar water heater, catalytic
converters and automobile trim.
Ferritic stainless steels

22. Welding of Ferritic steels
 Softer and more ductile than martensite steels
 Cannot be hardened by heat treatment
 Ferrite phase does not transform to martensite but
susceptible to 475 deg embrittlement and sigma phase
formation in higher chromium grades.
 Problem of grain growth during welding leading to
brittle structure in HAZ.
Generally welded with austenitic SS electrodes

23. Applications of Ferritic SS
Automobile exhausts
Solar water heater

24. Duplex stainless steels
ASME: P.No-10H
60% austenite
+
40% ferrite structure
 Cr – 19.5-30.0 %
 Ni – 1.0-8.0 %
 Mo – 0.10-5.0%
 C – 0.025-0.040%
 Mn – 1.0-6.0%

25. Duplex stainless steels
 AISI 2205, 2304, 2507
 Almost twice the strength of austenitic steels
 Excellent pitting + SCC resistance
 Used for plant and piping in oil and gas production,
corrosive applications to resist chloride ion media. Higher
strength structurals

26. Weldability of Duplex steels
Duplex stainless steels have fairly good weldability.
All standard welding processes can be used.
Not quite as easily welded as the austenitic grades but low
thermal expansion in duplex grades reduces distortion and
residual stresses after welding.
All grades
 Solidify as ferrite, austenite formation during cooling
 Austenite/Ferrite ratio dependent on 2 primary variables
Alloying effects – Cr & Ni equivalents
Heat input/cooling rate

27. Welding metallurgy of Duplex steels
 Faster cooling rate produces higher Ferrite which leads to
reduced low temperature impact strength and corrosion
resistance
 Slow cooling through 1050 – 550 C produces carbides,
nitrides, sigma etc which affect corrosion resistance and cause
embrittlement
Fast cooling Correct cooling Slow cooling
Too high Ferrite Between 30 -40 %
ferrite
Nitrides, Carbides
& intermetallics

28. Welding of Duplex Stainless Steels
–Overmatch nickel in filler metal
–Control heat input and cooling rates carefully
–Use N2 in shielding gas
–Ensure low impurities in base and filler

29. Application of Duplex SS

30. Precipitation Hardening stainless
steels
Cr 12 –18%
C 0.05 – 0.15%
Ni 3.0 – 27.0% +
Mo, Cu, Al, V, Cb, Ti
 Can be hardened by heat treatment
 This sub-group provides a combination
of austenitic and martensitic properties.
 Hardening is achieved by adding one or
more elements such as aluminium,
molybdenum, niobium, titanium, and
copper.
 It is capable of developing high tensile
strength through heat treatment
 Available as forgings, castings, bar and
plate and used for compressor blades,
pumps, gears for metering chemicals etc

31. Stainless Steel Casting grades
Alloy Type Wrought grade Casting grade
Martensitic 12Cr 410 CA15
Ferritic 21Cr 442 CB30
Austenitic 19Cr 9Ni 304 CF8
Austenitic 19Cr 9Ni Nb 347 CF8C
Austenitic 19Cr 12Ni 2.5Mo 316 CF8M
Austenitic 25Cr 20Ni 310 CK20
do Heat Resistant - HK40
Duplex 25Cr 5Ni 2.5Mo 3
Cu
329Cu CD4MCu
Duplex 22Cr 5Ni 3Mo 0.15N Alloy 2205 Alloy 2205

32. Wrought Grade
C – Corrosion service
H – High – temperature service
Example:
CA15
C - Corrosion service
A - Refer Fig
15 – 0.15%Carbon (max)

33. Austenitic stainless steels
ASME: P.No-8
 Cr – 18.0-29.0 %
 Ni – 8.0-22.0 %
 Mo – 0.75-6.0%
 C – 0.03-0.24%

34.  AISI 304, 310, 316, 321 & 347 grades
 Austenitic structure gives good weldability with
excellent ductility and toughness down to
cryogenic temperatures.
 Nickel improves general corrosion resistance in
high temperature
 Widely used for Boiler components, chemical,
petrochemical, fertilizer plant and food
processing. Also used for nuclear and cryogenic
plant
Austenitic stainless steels

35. •Has 50% higher coefficient of linear expansion, than carbon
steels
•Has poor thermal conductivity, 30% less than carbon steels
•Results in much higher distortion after welding
•Steps to prevent distortion
- closer tacking
- greater use of jigs and fixtures
- use of balanced and skip welding
techniques
- limit heat input by use of low currents and
stringer beads
Physical properties of austenitic stainless
steels

36. Welding of Austenitic steels
Generally good weldability as there is no martensitic
transformation but following problems encountered:
 Sensitization leading to inter-granular corrosion –IGC
 Hot cracking
 Stress corrosion cracking –SCC
 Sigma phase formation leading to embrittlement
 Higher distortion during welding

37. Sensitization and inter-granular
corrosion
 Sensitization refers to the
precipitation of carbides at grain
boundaries in a stainless steel or
alloy, causing the alloy to be
susceptible to intergranular
corrosion.
 austenitic stainless steel may become
sensitised if they are heat treated or
used at temperature in range 450-
900c. the heat affected zones of welds
may also be sensitised in some
circumstance

38. Sensitization and inter-granular
corrosion

39. Steels to prevent IGC
 Steels with elements having higher affinity for carbon
eg. Ti , Nb/Cb – stabilised steels. Form carbides in
preference to Cr.
321 grade – Ti stabilised
347 grade - Nb stabilised
 Steels with low carbon 304L & 316L grades( 0.03% C
max)

40. Hot cracking in austenitic welds
 Hot cracking or solidification cracking is
caused due to low melting eutectics (S, P, Nb,
Ti, N) formed at the grain boundary.
 Due to presence of 5 – 10 % ferrite phase in
weld deposit as the weld solidifies, in
combination with shrinkage stresses, leads to
cracks in fully austenitic welds
 Promoted by S, P, Nb, Ti, N etc.
 Prevented by adjusting weld metal
composition to give 5 – 10 % ferrite phase in
the deposit.
 Also prevented by reducing heat input and
controlling design stress.

41. THE BASICS OF COLD WORKING AND ANNEALING
 Cold working makes alloys stronger and
harder while reducing the material’s ductility.
 annealing reduces strength and hardness
while restoring ductility.
r – nominal outside radius of pipe or tube
R – nominal bending radius to centerline of
pipe or tube

44. Before welding of stainless steel
 Preserve the materials in clean (carbon-free) environment
 Provide wood support / insulation material support
 Avoid contact with carbon steel, Low alloy steel (to avoid formation of
Iron oxide)
 Removing the oils, Dust, contaminant
 Edge preparation - Cobalt based tool
 Grinding/ polishing – Aluminum oxide wheels
 Cleaning – acetone
 use fixture

45. Before welding of stainless steel

46. During welding of stainless steel
 Argon purging (root & hot) - water soluble dam – to avoid oxidation (pitting)
in root pass
 Ensure shielding gas flow
 Avoid arc strike
 Inter pass temperature – 150 deg.
 Inter pass cleaning – SS wire brush
 Avoid Sudden stoppage of arcing

48. After welding of stainless steel
 Cleaning – Removal of heat tint
 preserve the weldment in clean environment

49. Types of weld Cleaning
 Mechanical Weld Cleaning
 Chemical Weld Cleaning
 Electrochemical Weld Cleaning

50. Mechanical Weld Cleaning
 Mechanical weld cleaning is a common and low cost method used for cleaning
stainless steel
 involves grinding machines and abrasives to clean the top layer of metal surfaces
where rust and other slag particles can form

51. Chemical Weld Cleaning
 chemical pickling paste for cleaning after a welding job. The paste is
applied to the affected areas using a brush or spray and left on the surface
for some time to interact with the metal
 chemical pickling paste contains a variety of toxic acids, including
hydrofluoric, nitric, and sulfur acids. These chemicals are quite dangerous
for the human body and they can cause serious, long-term damage to the
skin and internal organs if they are consumed or breathed in.

52. Electrochemical Weld Cleaning
 Electrochemical weld cleaning, also known as electropolishing, is
considered the most effective method of cleaning stainless steel
 It is faster, safer and preferred by welders, compared to the other two
methods. It doesn’t pose any major health risks for the welder.

54. Thank You