22'1 Passivation treatment of SS

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22.1 Passivation treatment of SS

• A treatment after fabrication
• Surface contamination and damage by machining and
  welding, leading to corrosion
• Passivation removal of contamination with acid
  solutions.
• For SS304 20 Nitric acid water

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22.2 Sensitization

• Cr should be uniformly distributed for
  protection.
• Sensitization Cr forms carbides, leading to
  local depletion of Cr. This occurs after exposure
  to high temperature (e.g. welding or high
  temperature use).
• Loss of corrosion resistance and increasing
  brittleness.
• Cured by annealing treatment followed by
  quenching.

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22.3 Use temperatures of SS

• Continuous use without forming oxide scales

K.G. Budinski and M.K. Budinski (1999)
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Use of SS at high temperatures

• Oxidation resistance increases with Cr and Ni
  concentration.
• Sensitization formation of Cr carbides with
  prolonged use at high temperature embrittlement
  and loss of corrosion and oxidation resistance
• Creep time-dependent plastic deformation.
  Austenitic grades are generally better against
  creep.

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22.4 What attacks stainless steels?

• Hydrofloric and hydrochloric acids (even low
  concentration)
• Bleach (even low concentration)
• Nitric acid in high concentrations
• Crevice corrosion
• Corrosion of sensitized material
• Stress corrosion cracking
• Corrosion of two-phase (FA) alloys

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22.5 Heat treatment of stainless steels
Ferritic

• Factory treatment
• Annealing at 1000 C to uniformly dissolve Cr.
• Air cool to 600 C.
• Water quench to RT to avoid carbide at 470 C
• Stress relief treatment
• Given after a part is cold-worked or welded
• Same treatment as factory treatment

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Austenitic

• Factory treatment
• Annealing at 1000 C to uniformly dissolve Cr.
• Water quench to keep uniform Cr.
• Stress relief treatment
• Given after a part is cold-worked or welded
• Stress relief at 1000 C to relieve stress
• Water quench

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Martensitic

• Factory treatment
• Annealing at 1000 C to uniformly dissolve Cr.
• Slow cooled to ensure machinability/formability.
• Hardening treatment (post machining/forming)
• Annealing at 1000 C
• Oil quench or air cool for hardness
• High thermal stress and poor impact toughness
• Stress relief for toughness (after quench)
• 400 C
• Air cool

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Age hardenable

• Factory treatment
• Annealing at 1000 C to uniformly dissolve Cr.
• Slow cooled to ensure machinability/formability.
• Hardening treatment (post machining/forming)
• Annealing at 1000 C
• Air cool
• Aging treatment at 500 C
• Treatment recipe must be followed.

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Methods for controlling yield strength Summary

• Ferritic none
• Martensitic annealingquenchtempering
• Austenitic work-hardeningtempering
• Age-hardenable
• annealingquenchaging

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22.6 Comparison of stainless steels
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22.7 Age hardening the phenomenon
History of AH

• Discovered by German Alfred Wilm during WW-I.
• Strong Al alloys for machine gun cartridges.
• Quenched Al-3.5Cu with hope of quench hardening
• Strength increased after holding at RT.

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22.7 Age hardening the phenomenon
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Steps in age hardening treatment

• Homogenization at TH.
• Fast cool (e.g. quench) to room temperature
• Aging treatment at TA for controlled length of
  time

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Yield strength vs aging time