Failures in stainless steel welds – examples
and causes
Briony Holmes
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Content
Overview of a few examples:
1. Solidification cracking
2. HAZ Liquation cracking
3. AISCC (Atmospherically-induced stress corrosion
cracking)
4. Heat tint
5. Sigma phase embrittlement
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1. Solidification Cracking
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Solidification crack
 Butt weld
 Repair weld
 Solidification crack (arrowed)
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Solidification crack surface
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Causes of failure
 Duplex stainless steel

Less common than austenitic
 Bend test failure
 Factors involved in solidification cracking



Tensile stress
Delta ferrite content
Sulphur
 Cause - Unusually wide weld bead due to
weaving?
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2. HAZ Liquation cracking
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HAZ Liquation Cracking
 Short intergranular cracks:


In high-temperature zone of the HAZ; or
In previously deposited weld metal, during a subsequent
run.
 Due to formation of grain boundary liquid films at
temperatures below the alloy melting temperature.
 On cooling, this liquid is unable to accommodate
tensile strains, caused by contraction, and cracks
may form.
http://www.twi.co.uk/technical-knowledge/knowledge-summaries/liquation-cracking/
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HAZ Liquation Cracking
A. Photomacrograph of HAZ liquation
cracking in austenitic stainless steel
cladded with a nickel alloy weld
metal. Location of liquation cracking
indicated by arrows;
B. Liquation cracking in AISI 316
austenitic stainless steel weld metal,
reheated by subsequent weld bead.
Location of liquation cracking
indicated by arrows;
C. HAZ liquation cracking in AISI 310
austenitic stainless steel;
D. Scanning electron micrograph of
liquated film on liquation crack
surface in an AISI 316 austenitic
stainless steel weld metal.
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Factors affecting liquation cracking
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3. Atmospherically-induced stress
corrosion cracking (AISCC)
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AISCC of Nuclear Waste Storage
Vessels
 Airborne particles of salt deposit
on metal surface and remain
there for a long time;
Austenitic
stainless steel
waste storage
vessel
 Particles absorb moisture from
air, forming a thin layer of
concentrated
salt
solution,
causing pitting corrosion;
 If sufficient residual stress is
present, this can lead to AISCC;
 AISCC does not require full
immersion or saturated solution

it may occur in ambient
environments and at
temperatures less than 60°C.
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Stress corrosion cracking
 Austenitic stainless steel
 Branched, transgranular cracks
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AISCC testing*
 Different surface densities
of salts deposited
 Strain gauges attached to
monitor changes in strain
during test period (months)
 Different relative humidity
levels
*research performed at TWI
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AISCC testing
 Samples kept in an
environmental chamber for
months at different
humidities, salts and stresses;
 Regularly inspected for pitting
corrosion and cracking;
 Residual stresses associated
with the container welds were
also modelled and measured.
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Summary of test results
 AISCC can occur due to residual or applied
stress:

400MPa (0.2% strain) or more if the other necessary
conditions are present.
 Deliquescence of salt particles is dependent on
relative humidity:

but independent of the amount of salt present.
 The time to initiate cracking may be sensitive to
temperature.
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Implications
 Minimise residual stresses generated by welding;
 Avoid particles settling on container surfaces
before and during the storage period;
 Rinse off the residues of dust and aerosol
particulates to minimise any contamination
caused by either an airborne mechanism or
manual handling;
 Waste storage should be equipped with effective
ventilation and a filter system to prevent any
entry of particulates into the storage space.
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4. Heat tint
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Heat tint
Pitting and crevice corrosion
 Lowered corrosion resistance
 Corrosion pits in heat tint
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Heat tint
 304L stainless steel welded pipe
 Pitting corrosion much larger under the surface
than visible on the surface
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Causes of failure
 Oxidation of the root bead + adjacent HAZ during welding
of SS;
 Cr-rich scale is formed, the surface becomes Cr-depleted
which impairs corrosion resistance;
 Avoid heat tint:
 Need good shielding and backing (purging) gas;


Shield: Ar, Ar/He, Ar/N mixtures.
Purge: Ar or N2.
 Use several volumes of purge gas before welding;
 Maintain purge for 3 or 4 weld passes in multi-pass
weld;
 May also pickle and passivate.
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5. Sigma phase
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Duplex stainless steel
 Dye penetrant showing weld metal/HAZ crack
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sigma (σ) phase formation
 Sigma (σ) phase: high chromium brittle intermetallic phase.
 Precipitates between 500 and 1000ºC over time.
 Forms more readily in δ ferrite than in austenite.
 Affects toughness and corrosion resistance.
 Grades containing Mo require less time for σ phase
precipitation.
 Control the amount of δ ferrite in austenitic SS welds and
the thermal cycle.
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Fracture face
 Brittle fracture due to sigma phase
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sigma (σ) phase formation
σ phase on δ/γ grain boundary
γ
δ
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Duplex stainless steel
Microstructure
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Causes of failure
 Duplex stainless steel
 Sigma phase precipitation
 Incorrect heat treatment of forging
 Tensile stress


Residual stress
Applied stress during hydrotest
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Summary
 Welding (and heat treatment) can introduce or promote
various failure mechanisms
 Best practice for welding should be followed
 For further guidance:
http://www.twi-global.com/technical-knowledge/
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Contact
Briony Holmes
Stainless Steels and Non-Ferrous Alloys Section
Materials Group
TWI Ltd
Tel: +44 (0)1223 899 000
E-mail: [email protected]
Web: www.twi-global.com
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Copyright
Presentation prepared for the
Scottish Enterprise – Technology Transfer Project
by TWI
TWI holds copyright on this presentation.
All rights reserved. No part of this presentation may be
reproduced or transmitted in any form or by any means,
electronic or mechanical, including photocopy, recording,
or any information storage and retrieval system, without
the express permission of TWI.
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Composition of phases
EDX spectrum
Sigma phase (white)
Rich in Chromium and
Molybdenum
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