Best welding prac0ce for high
performance SS and High Nickel Alloys
Dr Graham Sussex
Presented by webinar for SWS
July 2022
316
2507
800H
Content
• Basics about carbon steels, stainless steels and high nickel alloys
• Differences in proper0es that affect welding behaviour
• The similari0es and differences between welding the various
families and grades of stainless steel and nickel alloys
• What welding can do to mechanical and metallurgical proper0es
and par0cularly corrosion resistance
• Preparing to weld: effects of fabrica0on techniques and cleanliness,
gaps, angles, fillers, welding method and heat input
• Welding workmanship, quality control and defect limits for
different materials
• Inspec0on and post weld cleanup
2
Carbon steel, stainless steel and nickel alloys
Steel is an Fe & C alloy:
• Residual elements such as S, P, Mn, Si, …
• Variable Mo, Cr, V, etc
• Range from carbon (mild) steel through low alloy to tool
steels.
• Readily weldable if “Carbon equivalent: CE” low AND metal not too
thick. Otherwise heat treatment to slow cooling and limit cracking.
• Low CE typical 0.41 for CE = C%+(Cr%+Mo%+V%)/5 + (Ni% + Cu%)/
15
• Not resistant to corrosion unless alloyed or protected.
3
What is a stainless steel?
A corrosion resistant alloy (CRA) - ohen resistant to high
temperature oxida0on
Stainless steels >10.5% Cr, typical C <0.1% and low impuri0es: P & S.
• Austeni0c*: readily weldable - strengthened by cold working –
sohened by hea0ng [as are duplex grades]. NO hard spots from
welding – except strength increase from excess reinforcement.
• Duplex*: stronger, slightly less duc0le & slightly less easy to weld
• Martensi0c and Precipita0on Hardening alloys require heat
treatment aher welding. Heat treatment to strengthen or sohen
• Ferri0c*: always low carbon but (except for 3/5Cr12) hard to weld.
Even modern stabilised alloys require extreme cleanliness, good
gas shielding to prevent cracking and low heat input to avoid grain
growth
* All alloys with high Cr and/or Mo require careful heat control
because of intermetallic phase forma0on
4
Nickel alloys: the four families
Two basic types:
corrosion resistant or resistant to high temperature oxida0on
I.
II.
Nickel and nickel-copper solid solid solu0on alloys
Chromium bearing solid solu0on alloys for corrosion and/or heat
resistance
III. Nickel and molybdenum alloys for reducing environments (low
hydrogen weld consumables) – not if there are oxidisers
IV. Precipita0on hardening alloys used for corrosive service aher medium
temperature aging to form fine strengthening precipitates. [NiCuAlTi,
NiCrAlTi, NiCrFeNbAlTi]
• Like stainless steels, these grades require care in processing to avoid
contamina0on for welding to be successful.
• Surface oxide is of par0cular concern and must be removed before
welding (usually mechanically) as it melts at ~1900oC and may be
incorporated in solidifying welds.
5
Specific Alloys in the chromium and nonchromium alloy groups
Chromium-containing
Family
Ni-Cr-Fe
Ni-Cr
Ni-Cr-FeMo
Ni-Cr-Mo
Example
Alloy 800
Alloy 600
Alloy 825
Alloy 625,
C-types
Cr-containing grades form
a passive oxide film and
are suitable for oxidizing
to moderately reducing
corrosion environments.
Non-Chromium-containing
Family
Ni-Mo
Ni-Cu
C.P. Nickel
(Commercially
Pure)
Example
Alloy B-2
Alloy 400
Alloy 200
Non-Cr-containing grades
depend on the bulk
composi0on of the alloy to
provide corrosion resistance,
suitable for moderately to
strongly reducing
environments.
6
SS is steel with >10.5% chromium
(& iron >50%)
• The chromium rapidly forms an
oxide based passive film which is
extremely thin – a few millionths of
a mm – and self repairing star0ng
in milliseconds. It grows to its old
thickness in 4 to 24 hours. It needs
O2 and H2O – moist air is enough.
Precau+ons?
• Enough chromium for the
environment, i.e. [Cr] vs [Cl-]
• No steel contamina0on or heat 0nt
• Smooth surface to avoid
concentra0on of “nas0es” in
dimples, grooves or burrs
7
Weld microstructures – different between SS
families and from wrought product
• Welds are different
from wrought
alloys as they
solidify rapidly with
irregular
microstructure.
• Welds have a cast
structure &
generally are less
corrosion resistant
which is why fillers
are overalloyed.
Outokumpu Welding Handbook
8
SS Corrosion Resistance when oxygen is present
Pirng Resistance Equivalent
PREn = %Cr + 3.3 (%Mo + 0.5%W) + 16 %N
• For ferri0cs, the N term is usually omised.
• The term ‘super’ is used ferri0c, duplex and austeni0c grades with
PRE ≥ 40 or more to withstand seawater. Group II [Cr containing] Ni
alloys can be assessed by PREn.
• Nickel does not appear as it does not affect corrosion ini#a#on
• [Nickel does reduce corrosion in the reducing environment of a pit
or crevice ]
• The PRE is only an empirical ranking from correla0on of
temperature required to cause pirng with the PREn number.
• Higher PRE means greater corrosion resistance and correlates with
the temperature to cause pirng (CPT) and crevice (CCT)
temperature in TEST solu0ons – NOT service temperatures.
9
There is a con0nuous transi0on in alloys from
pure iron to pure nickel
316
2205
304
0 5 8 10
Fe
steel
310
20
601
625
718
400 [Ni, Cu]
C-276
32 42 53 58 60 65
100%Ni
800
25
825
Oxidising - more Cr, Moç
Ni
Nickel 200
èReducing – more Mo, Ni
Austeni0c Ni alloys can be cold worked for strength – like SS.
Other strengthening means? Solu0on strengthening – different
sizes of big atoms “lock” together [Cr, Mo, Ni] Group II
Or – precipita0on hardening [Ti, Nb(Cb), Al, …] Group IV
10
Localised corrosion: comparing SS and Ni alloys
Crevice and pirng temperatures vs PRE(N)
SS & Ni-alloy ranking by Cri0cal Pirng Temperature (CPT, ASTM G-48C)
and Cri0cal Crevice Corrosion Temperature (CCT, ASTM G-48D)
Alloy
316
825
6%Mo SS
625
C-276
%Mo
2.2
3
6
9
16
CPT, ºC
20
30
75
>85
>85
CCT, ºC PREN
<0
25.3
5
33.6
35
48.4
35
51.7
45
67.1
Austeni0c SS
Ni group II
Hi Mo austeni0c SS
Ni group II
Ni group II
Some other high alloy stainless steels have beser pirng and
crevice corrosion resistance than some of the nickel alloys.
11
Lets look at welding and physical proper0es
Material Resis+vity
Melt
Magne+c Thermal Xpn
Thermal
nohm.m midrange
µm/mK
Conduc+vity
Mild steel
120
1510oC
Yes
12.5
49
430
316
600
750
1475oC
1390oC
Yes
No
10.5
17
25
15
254
2205
C276
825
850
800
1300
1210
1355oC
1415oC
1347oC
1385oC
No
Yes
No
No
17
14
12
15
13
15
10
15
• Higher resis0vity may cause overhea0ng for covered electrodes
• Lower mel0ng point means less power to achieve temperature
• Low thermal conduc0vity means hea0ng is localised so given power
gives a higher local temperature
• Higher thermal expansion risks distor0on if thermal conduc0vity low,
i.e. austeni0c 316 and 254 most at risk in above
12
• Could only use magne0c par0cle NDT/NDE on magne0c alloys
Low thermal conduc0vity +
high thermal expansion " risk of distor0on
• Minimum tack welds required for fitup depending on strength of alloy
• Tacks must be consumed by final weld
or removed [Super austeni0cs
recommend start and end of tack is
ground to minimise crack risk!]
• Best prac0ce if tacks are gas shielded
and not heat 0nted
• For austeni0cs & Ni alloys use about
half the spacing between tacks in
stainless steel compared to tack
welding carbon steel or ferri0c SS.
Duplex spacing intermediate.
13
Welding Corrosion Resistant Alloys:
general characteris0cs
• ALL are affected by contamina0on – cleanliness is
cri0cally important
• Thermal conduc0vity is low compared to carbon steel
• Thermal expansion of austeni0cs is rela0vely high
• Molten weld pool is viscous – depending on nickel
content
• Weld penetra0on reduces with as higher nickel
• Premium product so need welders qualified for the
material and, usually, weld procedures
14
Weld penetra0on?
Reduces with increasing nickel content
Mild steel – 0% Ni
304 SS - 8% Ni
Alloy 600 - 72% Ni
12% Cr SS – 0.5% Ni
2507 – 7% Ni
Monel – 70% Ni
Higher nickel also makes for a more viscous weld pool – so
increasing the current does not speed up the welding – but will
increase kJ/mm, possibly interpass temperatures and may
cause distor0on or seed intermetallic phases
15
What more nickel means for penetra0on
16
Design of the weld joint
High nickel metals (including austeni0c SS) different to
carbon and low alloy steels because of viscous Ni metal
Bevel angle: more open
Root face: reduced
Root opening: larger
Carbon and low alloy steels
Stainless steels
504
17
NI and IMOA publica0ons
Guidelines for fabrica0on duplex SS (3rd Edn 2014)
Welding nickel alloys NI #11012 (2nd Edn 2018)
Fabrica0on austeni0c SS (2020)
18
Why clean surfaces are cri0cal for welding
• Impuri0es are pushed to centreline of weld during solidifica0on
and may cause a band suscep0ble to corrosion or fracture
• Sulphur can form low mel0ng point nickel compounds and cause
cracking
• Zinc (from galvanising or zinc rich paint) can cause liquid metal
embrislement in austeni0c phases
• Moisture in flux [high RH?] may cause cluster porosity
• Dirt can cause carbide sensi0sa0on. Rare for sensi0sa0on to occur
otherwise in modern low carbon alloys
19
Sulphur from EP oil cracked nickel 200 weld with low
mel0ng point sulphur on grain boundaries
Sulphur and phosphorous can cause HAZ and weld cracking.
Carbon from curng fluids, grease, etc may also cause
intergranular cracking
Similar problems with low mel0ng point metals
ALL similar for nickel alloys and austeni0c stainless steels
20
Liquid Metal Embrislement [LME] by hea0ng
Zinc contaminated austeni0c
• All zinc/zinc-iron intermetallic must be removed from galvanised item.
• Beser for this to be removed chemically rather than by abrasion.
• Copper “rubbings” are also likely to cause Liquid Metal Embrislement
[LME] during welding BUT sta0c contact is usually OK.
• Copper backing bars can be a problem if they are fused
21
Cluster porosity
Usually from moisture in flux; dry in furnace (up to
340oC for 30 minutes [longer and it may start to flake]
depending on electrode type) or keep dry in oven
(~110oC)
22
Weld decay of 316 at high kJ/mm site weld
note the good factory spiral weld at leh
High Nickel alloys usually
very low carbon
Metal grains with chromium carbide
precipitates along grain boundaries – either
too much heat or carbon on the surface.
Lower corrosion resistance and will corrode 23
High Nickel alloys and contamina0on
• Mul0ple studies agree that preven0ng (or removing) carbon steel
contamina0on is cri0cal to prevent possible pirng of high nickel alloys
in corrosive service
• It is less clear that heat 0nt is a significant issue par0cularly in hot
service but in corrosive service, mechanical (fine grinding) or chemical
means are generally used to remove it.
24
Cleaning Ni alloys or stainless steels: 1
[Austeni0c fabrica0on of SS and Ni alloys]
If the cleaner contains sodium (hydroxide, silicate, …)
it must be fully removed - especially for nickel alloys
25
Cleaning Ni alloys or stainless steels: 2
[Austeni0c fabrica0on of SS and Ni alloys]
26
General welding issues for CRA
• Welding itself is not generally# the problem
• Oxides on nickel removed abrasively –NOT wire brush burnished
• The main issue is post-weld cleaning
- remove heat 0nt especially for stainless steels
- remove underlying layer with lower Cr – also cri0cal for
stainless steel
- ensure there is no embedded steel
All without damaging the surface
# high heat and temperature may cause distor0on
# gas shielding and purging can be significant issues
# however, it is beser to purge and clean rather than asempt to
clean an unpurged, badly oxidised weld
27
Typical heat 0nt & its effect on stainless steel
316Ti
304
Tint colour starts pale straw and
moves through the rainbow to black
as the oxide layer thickens due to
higher and longer 0me at
temperature.
Heat 0nt reduces corrosion
resistance – as measured by
Boulton & Avery, Euro-Inox
pirng poten0al
28
Effect of heat input
The HAZ is all around the weld pool including the back face.
For austeni0cs aim for 20% less heat input than carbon steel.
Larger welds give more heat input and larger HAZ
29
Exterior or mechanical hea0ng
Rough grinding of SS pipe for
weld prepara0on giving
overheated (blue) area
suscep0ble to asack. PLUS,
Ra>0.5μm severely decreases
corrosion resistance
SS oƒake welded to pipe externally with
inadequate internal gas purging of the
30
stainless steel
Why we use the specified consumables
• Molten weld looses chromium and nitrogen: PRE decreases
• Recommended filler materials deliver slightly greater corrosion
resistance so that (small) weld is cathodic to (large) parent material
• In duplex stainless steels, the filler material usually has higher
nickel to maintain microstructure balance.
• N2 in shield gas likely to increase austenite and risk chloride SCC in
duplex SS grades.
• Nickel alloys evolve because they are used in more aggressive
environments – hence correct consumable most important, e.g B2
to B3 to B4 changes for reducing environments (Group III).
• Welding to Code (generally for strength) so use alloy P classifica0on
and appropriate F for consumable.
• NI 11012 has consumable recommenda0ons for nickel alloys and is
a free download.
31
High end austeni0c SS: 904L and >6% Molybdenum
• Prior to widespread acceptance of duplex grades, 904L
(with 4.5% Mo and 26% nickel content) was used in the
pulp and paper industry for its resistance to stress corrosion
cracking. Its copper and molybdenum also gave a market in
resis0ng a wide range of concentra0ons of sulphuric acid.
• Super austeni0c alloys (254 et al) have >6% Mo plus higher
Ni and N2 to ensure corrosion resistance, austeni0c
structure and proper0es respec0vely.
• Similarly to the 904L micrograph, the
ini0al solidifica0on of the alloy’s
dendrites have <6% Mo and this
segrega0on allows selec0ve corrosion.
Hence fillers for 6% Mo alloys have 9%
Mo to ensure all phases have >6% Mo
corrosion resistance.
32
Interpass temperature & arc energy for
different families of SS
Interpass / preheat temperature, 0C
300
250
200
Stabilised ferri0cs? Clean and keep heat input down
150
Austeni+c SS
Duplex SS
Super Duplex SS
100
0.5
1
1.5
Arc energy input , kJ/mm
2
2.5
Must have sufficient heat for fusion, low interpass to avoid distor0on and
low heat input to avoid intermetallics in high alloys. Ni alloys slightly less.
175oC ohen accepted as interpass for Ni alloy – but some require 95oC 33
A detour to duplex SS materials
•
•
•
•
•
•
Combines benefits of ferri0c and austeni0c alloy proper0es
Mechanical proper0es – much stronger than austeni0cs
Duc0lity – but less than austeni0cs
High corrosion resistance – select the level required
Chloride (pirng, crevice, cracking, etc.) – especially good resistance
to chloride stress cracking
Oxidizing environments – like all stainless steels
Product specifica0ons for duplex provide composi0on ranges and, if
nitrogen is not at the upper end, welded fabrica0ons may not comply
with test requirements.
ASTM A923 for super and standard duplex and A1084 for lean duplex
provide tests although ISO 17781 is more comprehensive in corrosion
resistance, impact and ferrite/austenite ra0o.
34
TTT curves for duplex: 50% loss toughness 475oC
Weld repairs
add 0me at
temperature:
α’ near 475oC
σ near 800oC
35
Duplex cooling rates
The issue:
• Under prac0cal welding condi0ons, the cooling rate is ohen
too fast for sufficient austenite to form
• Welds can contain over 80% ferrite especially for thin
materials
Result:
• Low duc0lity - can fail bend tests
• Poor corrosion resistance
Cool more slowly to promote austenite?
• Slow cooling will allow intermetallic
precipitates to form which reduce
mechanical proper0es
Austenite
Ferrite
Intermetallics 36
The narrow “thermal” road in welding duplex
37
Duplex stainless steels – NOT only chemistry
• Fast cooling aher welding? A duplex structure with too
much ferrite – aim is 50:50 austenite:ferrite
• High ferrite in duplex has poor mechanical proper0es
and can have poor corrosion proper0es
Austenite
Ferrite
AND other dark other phases in the Heat Affected Zone (HAZ)
38
Harmful phases even in duplex rolled plate
Austenite
Ferrite
Black spots of σ (intermetallic) phase along ferrite boundaries.
Brisle at ambient temperature and has lower corrosion resistance
39
A lisle bit of metallurgy:
intermetallic sigma forma0on
•
•
•
•
•
40
Prolonged heating in range 550-950oC (e.g., multi-run
welds, thick material, slow travel, high heat input) can
transform delta ferrite to sigma – brittle and low corrosion
resistance intermetallic when at ambient temperature
If δ ferrite <5%, no issue
If >15%, then a significant issue
About 10% it may be an accepted δ ferrite level
A potential problem with super austenitic or super duplex
with high levels of Cr and Mo
40
BUT low sigma in low alloy austeni0cs at 700oC
Long term exposures with low alloy austenitcs showed about 1%
sigma aher ~3 years at 700C, i.e., not possible in welding. High
chromium, molybdenum and niobium facilitate sigma forma0on. 41
Welding ferri0c stainless steels
• The structural 12% chromium grades of stainless steel are
available with various levels of 0tanium and nickel.
• A substan0al use is as rail cars for minerals and also boiler
exhaust ducts with no SCC risk. Not for decora0ve use.
• Tough & the balanced microstructure allows welding to
plate thickness without heat treatment
• The transforma0on loop in the phase diagram means grain
growth is not the issue that it is with other ferri0c grades.
ALSO corrosion fa0gue failures aher welding showed that, for
the 0tanium stabilised formula0ons, the composi0on had to
sa0sfy:
• Ti > 4(C + N) & Ferrite Factor (FF)<8.3 where
• FF = (Cr+3Si+16Ti+Mo+2Al+4Nb) – (2Mn+4Ni+40(C+N)+4Cu)
42
42
Second genera0on 12% Cr ferri0cs - U0lity Grades
for ductwork and rail wagons
43
43
Current genera0on of new ferri0c alloys
• Extremely Low Carbon [ELC] (0.002~0.008%)
• Compare with <0.03% for austeni0c “L”
• Stabilised with 0tanium &/or niobium
• Also very low nitrogen content
• Chromium up to 30%
• Excellent corrosion resistance
• No stress corrosion cracking in chloride solu0ons
• Free from sensi0sa0on aher welding*
• Good weldability*
• BUT restricted to 2 ~ 3 mm thickness
Brewery hot water tank
*Must have clean weld prepara0on including zero residue solvents,
excellent gas shielding, low s0ckout, no drahs and zero moisture to
prevent nitride precipitates and possible hydrogen cracking.
44
Excessive heat input will lead to grain growth and poor duc0lity,
e.g. a 1.5mm sheet with 0.2mm diameter grains in the weld zone
44
Precau0ons for TIG Welding of new ferri0c SS
• Thoroughly degrease including the filler
• Avoid air aspiration into gas shield possibly including the use of a
gas lens to provide a larger volume of laminar flow.
• Change any contaminated gas lens screen
• Minimise weaving to avoid contamination
• High purity Ar or He for shield gas and pre/post purge
• Minimise heat input and cool fast: inter-pass below 95oC, or else
grain growth and precipitates are probable
45
45
Welding high nickel alloys
• TIG can be accurate with low heat input and less heat 0nt but is slow.
Weld bead should be slightly convex provided sufficient filler metal is
melted. Tube is regularly orbital TIG welded.
• MIG is faster, has higher heat input and generally has darker scale.
• Shielded metal arc (s0ck) used for small quan00es or odd shapes
where automa0c welding not possible. Molten flux helps to shape the
bead. Dry storage essen0al as moisture causes hydrogen and porosity
or, with type III (low hydrogen) nickel, poten0al cracking.
• For thicker than about 6mm or overlay welding, submerged arc is more
operator friendly but is rarely used mainly because of weld metal
dilu0on by base metal. SAW cannot be used for group IV.
46
Specific welding precau0ons
• Nickel welding is slower than SS for the same process:
because of the weld pool sluggishness.
• Material welded in annealed condi0on.
• Cold worked items solu0on annealed to reduce risks of weld metal
solidifica0on cracking – if impuri0es get through the cleaning process
• Porosity a risk if N2, O2 or water (H2) penetrate gas shield – possibly use
diffuser/trailing shield.
• Normal leak checks on gas leads – flow will aspirate air.
• Consider consumables with Ti and Al to mop up O2 and N2
• Back shielding un0l root passes completed and preferably later passes as well
• For mul0ple passes, ensure removal of oxides and slag especially if high
temperature use is proposed
• Spaser should not occur as it is controllable by the welder - caused by excess
amps, long arc, wrong polarity or moisture in the flux – both for stainless and
nickel alloys
47
Workmanship in welding: Ni and SS alloys
• Transport, storage and handling must avoid carbon steel, sulphur
(oils, markers, ..), grease, chlorides (from pens or the environment),
low mel0ng point metals (Zn, Cu, Pb, ..)
• Areas affected by curng must be dressed especially if heated – eg,
plasma curng and even laser curng although to a lesser extent
because of the lower heat input
• Dedicated tooling and working spaces are required for cleanliness
• Heat input and interpass temperatures as shown previously
• Tack welds do maser and arc strikes should not be leh
• Hydrotest water has chloride limits especially for stainless steels
• Adverse weather (wind,..) warnings about loss of gas shielding
• Etc, etc, ….
48
Inspec0on aher welding
1. 100% visual welder inspec0on aher removal of weld scale
2. Confirm that all welds have been completed and record results as set
out by weld procedure.
3. Depending on the specific code, inspec0on of required percentage to
verify conformance by visual and dye penetrant as required
4. Dye penetrant only shows loca0on; size of the developed stain can be
used to es0mate the depth of a defect especially in cas0ngs.
5. For structural components, undertake the required level of
radiography and ultrasonic inspec0on BUT radiography detects bulk
defects and only finds cracks aligned with the beam path.
6. Ultrasonic examina0on can detect inhomogenie0es including cracks.
7. Following the removal of heat 0nt either by mechanical or
(electro)chemical method(s) confirm that any surface roughness Ra
requirements have been sa0sfied
49
Visual surface imperfections (A/FA, B and C)
CORROSION & STRUCTURAL
Permissible surface imperfections AS/NZS1554.6
50
Permissible defects food process welds: AS/NZS 2980
Not too different from 3A and EHEDG
51
51
Welding pipe and tube
The outside is easy – mechanical or chemical clean and inspect
The inside: need to ensure the heat 0nt is low enough for sanitary
or corrosion resistance purposes
• There are 3 sources for results showing similar results in terms of
weld colour and oxygen in the purge gas:
• AWS
• EHEDG
• Industry tes0ng
Corrosion tests have also shown that provided the heat 0nt is no
more than pale straw, the corrosion resistance is not degraded.
•
•
52
Purge welding of tube
•
•
•
•
•
53
If it cannot be cleaned post welding, then heated areas must
not exceed pale straw - <50ppm oxygen until the temperature
<250oC.
Paper dams can be dissolved or foam or rubber disc dams
can be removed afterwards to save on gas use.
Simple backing bars not sufficient to control heat tint.
And is the purge gas argon (heavier than air- so vent at top)
or nitrogen (about the same as air) or a lighter mix with
helium or hydrogen (so vent at bosom)?
And do NOT cause turbulent flow as it mixes air into the
purge gas
53
Need good tube prepara0on
For good
orbital
welding, the
face must also
be smooth and
NOT bevelled
[EHEDG]
54
Sugaring: welding 321 tube with no purging
White cauliflower oxides formed in air.
Images of metallurgical cross section
with yellow oxygen image by EDAX –
no carbides. TIG weld 321 tube
Koetecki “WeldingSS – Q&A, Section
1.40”
55
55
Heat 0nt varia0on with oxygen level
The weld and heat-affected zone surface may be permised to have light
straw colour oxide samples 1 through 3 below. It can be used as a guide.
P
!
Weld discolora0on levels on inside of austeni0c stainless steel tube
according to AWS D18.1:2009 with 50ppm oxygen – almost iden0cal to
EHEDG recommenda0on of 40ppm oxygen
56
As-welded good surfaces
Welded with <25ppm O2
Super duplex: manual TIG
EHEDG guide
316 – orbital weld
57
57
Welding defects: incomplete root
penetra0on on a pipe
This creates several problems:
• The full cross sec0on is not available to
carry service loads
• The lack of penetra0on is a:
• Notch which can ini0ate fa0gue
cracking
• Crevice which can ini0ate crevice
corrosion – this is the main concern
inside pipelines
• If there is inside access, root welds may be
back ground and given a sealing weld pass
• This is not possible with small diameter pipes
and great care is needed to avoid lack of
penetra0on
58
Inspec0on and witnessing – for tube
• Permissible defect sizes follow tube spec in AWS D18.1.
• Intent is that poor external finish is the trigger for internal
inspec0on
• Internal weld inspec0on by clever systems or by monitoring weld
process.
• Borescopes provide magnified images - generally look worse than
simple visual examina0on
• Must have cost process agreed before inspec0ons.
Outokumpu
59
59
Inspec0on of inaccessible tube
Bead wander/width: criteria for internal quality
Only used when interior inspection not possible
Minimum width >50% (maximum width)
Weave <25% of average width
AWS D18.1/EHEDG 35
60
Cleaning, pickling & passiva0on
Before the welding
• Cleaning: removing soil, grease, oil, etc to allow free access for
water and oxygen to grow the passive film
ARer the welding
• Pickling: controlled asack of the stainless steel; removes heat 0nt
& Cr depleted layer, embedded iron, inclusions; dulls the surface;
provides a passive surface immediately on rinsing
• High nickel alloys? Fine grinding or a pickling chemical process if
required
Possible addi+onal step for very aggressive environments
• Passiva+on: clean moist air will passivate stainless steel;
appearance not changed; chemical passiva0on strengthens the
passive film and is rapid; air passiva0on adequate unless the
environment is very aggressive for the grade
61
Chemical and mechanical cleaning of CRA
Austeni0c fabrica0on
? Only if
conduc0ve
Debris?
Next slide
62
Wire brushing?
• With a STAINLESS STEEL wire brush!
• Good to remove surface debris when the weld is s0ll warm.
• Heat 0nt? No. Usually it just burnishes and brightens the
surface without improving its corrosion resistance.
• Chromium depleted layer under the heat 0nt? No. You
need metal removal either by abrasion or by pickling.
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Some common pickling treatments
from ASTM A 380 (for annealed material)
• Normally use HNO3/HF mixes – but ASTM A380 has one H2SO4
mixture followed by an HF/HNO3 treatment.
• If HF >3%, then pickling faster.
• Time and concentra0on depends on temperature, s0rring, severity
of heat 0nt – sugges0ons include:
• A: 15-25% HNO3/1-8%HF at 21-60oC for 5-30 minutes.
• B: 10-15%HNO3/0.5-1.5%HF at 21oC for 5-30 minutes.
• Range of treatment condi0ons is due to range of heat 0nt and
specific alloy – more corrosion resistant alloys require longer
exposure or stronger pickling formula0ons
• AMS 2700 has many passiva0on treatments (now in ASTM A967)
including dichromate addi0ons for lower Cr alloys AND one with
anodic polarisa0on, i.e. mild electropolishing
64
Tank, paste or spray [with excellent PPE]?
• Both paste and spray at ambient
temperature so poten0ally slower
• Paste, unless s0rred, exhausts base
acid and can dry out causing stains
• Paste tends to be smaller scale
Effect of s0rring
Grade
304, 316
904L
2205
254 SMO
Increase (%)
30-40
25-30
12-20
65
5-10
Chemical surface treatment
Electropolishing
ASTM B912 and/or ISO 15730; may be used as
an alterna0ve to pickling; ensure complete
removal of both the dark oxide and underlying
chromium-depleted layer – slight smoothing
Electrocleaning
Ohen on site and usually performed using carbon fibre wands and mild
acids. Can effec0vely remove weld oxida0on, but chromium depleted
layer may not be fully removed on thick weldments. Very operator
dependent especially on dura0on aRer ini0al 0nt removed.
66
The liquid heats and may “spit” requiring PPE.
67
Effec0veness of cleaning processes
Austeni0c fabrica0on
Embeds
fragments and
causes micro
crevices
68
68
L to R: as welded, mechanically polished
and electropolished Austeni0c fabrica0on
69
Acceptance of surface by sample comparison?
TIG & pickled
Abraded
Electropolished
Surfaces to be
electropolished are
ohen pickled to
remove weld scale
first – and then all
the clean surface is
electropolished
70
Is it clean? – ASTM A380 tests
Organic contaminants - grease and oils
• Pure water break by dipping - moderately sensi0ve
• Atomiser test with pure water film - X100 more sensi0ve
• Evapora0on of pure solvent will leave a ring
Free iron or steel – poten+al corrosion sites
§ Gross indica0ons along folds, edges or rubbed surfaces
§ Wet and dry with an atomiser 6 to 8 0mes with 2:1 cycle of wet:dry.
Mist (tap water, no coalescence) and look for rust.
§ Chemical spot tests are much more sensi0ve if applied to oil free
surfaces but residue must be removed for food service.
§ Acidfied copper sulphate spray – pink within 6 min means iron
§ Ferroxyl (must be fresh) goes deep blue in 15 sec or lighter blue
more slowly with rust
71
Thank you for your +me
Are there any [further] ques+ons?
h[ps://inquiries.nickelins+tute.org/
hsps://www.nickelins0tute.org/ : hsps://www.imoa.info/index.php
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