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5.
Welding of High-Alloy Steels,
Corrosion
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5. Welding of High-Alloy Steels, Corrosion 58
Basically stainless steels are characterised by a chromium content of at least 12%. Figure
5.1 shows a classification
of corrosion resistant
steels. They can be sin-
gled out as heat- and
scale-resistant and
stainless steels, depend-
ing on service tempera-
ture. Stainless steels are
used at room temperature
conditions and for water-
based media, whilst heat-
and scale-resistant steels
are applied in elevated
temperatures and gaseous
media.
Depending on their microstructure, the alloys can be divided into perlitic-martensitic, ferritic,
and austenitic steels. Perlitic-martensitic steels have a high strength and a high wear resis-
tance, they are used e.g. as knife steels. Ferritic and corrosion resistant steels are mainly
used as plates for household appliances and other decorative purposes.
The most important group are austenitic steels, which can be used for very many applications
and which are corrosion resistant against most media. They have a very high low tempera-
ture impact resistance.
Based on the simple Fe-C
phase diagram (left figure),
Figure 5.2 shows the ef-
fects of two different
groups of alloying elements
on the equilibrium diagram.
Ferrite developers with
chromium as the most im-
portant element cause astrong reduction of the aus-
Classification of Corrosion-Resistant Steels
non-stabilized
(austenite withdelta-ferrite)X12CrNi18-8
stabilized
(austenite withoutdelta-ferrite)
X8CrNiNb16-13
ferritic austenitic
stainlesssteels scale- and heat-resistantsteels
corrosion-resistant steels
semi-ferritic ferritic-austenitic
X40Cr13 X10Cr13 X8Cr13 X20CrNiSi25-4
perliticmartensitic
ISF 2002br-er-06-01e.cdr
Figure 5.1
Modifications to the Fe-C Diagramby Alloy Elements
ChromiumVanadiumMolybdenumAluminiumSilicon
NickelManganeseCobalt
Alloy elements in %Alloy elements in % Alloy elements in %
T
A4
A3
T
A4
A3
T
A4
A3
ISF 2002br-er-06-02e.cdr
Figure 5.2
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5. Welding of High-Alloy Steels, Corrosion 59
tenite area, partly with downward equilibrium line according to Figure 5.2 (central figure).
With a certain content of the related element, there is a transformation-free, purely ferritic
steel.
An opposite effect provide austenite developers. In addition to carbon, the most typical mem-
ber of this group is nickel.
Austenite developers cause an extension of
the austenite area to Figure 5.2 (right figure)
and form a purely austenitic and transforma-
tion-free steel.
The table in Figure 5.3 summarises the ef-
fects of some selected elements on high alloy
steels.
The binary system Fe-Cr in Figure 5.4 shows
the influence of chromium on the iron lattice.
Starting with about 12% Cr, there is no more
transformation into the cubic face-centred
lattice, the steel solidifies purely as ferritic. In
the temperature range between 800 and
500C this system contains the intermetallic
-phase, which decomposes in the lower
temperature range into a low-chromium -
solid solution and a chromium-rich -solid
solution. Both, the development of the -phase and of the unary --decomposition cause a
Effects of Some Elementsin Cr-Ni Steel
Element Steel type, no. Effect
Carbon
l
l
l
All types
l
l
l
Increases the strength, supports development
of precipitants which reduce corrosion
resistance, increasing C content reduces
critical cooling rate
Chromium
l
All types
l
Works as ferrite developer, increases
oxidation- and corrosion-resistance
Nickel
l
l
All types Works as austenite developer, increases
toughness at low temperature, grain-refining
Oxygen
l Special types l
Works as strong austenite developer
(20 to 30 times stronger than Nickel)
Niobium
l
1.4511,1.4550,
1.4580 u.a.
Binds carbon and decreases tendency to
intergranular corrosion
Manganese
l
l
All types
l
l
Increases austenite stabilization, reduces hot
crack tendency by formation of manganese
sulphide
Molybdenum
l
l
1.4401,1.4404,
1.4435 and others.
l
Improves creep- and corrosion-resistance
against reducing media, acts as ferrite
developer
Phosphorus,
selenium, or
sulphur l
1.4005, 1.4104,
1.4305
l
l
Improve machinability, lower weldability,
reduce slightly corrosion resistance
Silicon l
l
All types
l
l
Improves scale resistance, acts as ferrite
developer, all types are alloyed with small
contents for desoxidation
Titanium
l
l
1.4510, 1.4541,
1.4571 and others
l
Binds carbon, decreases tendency to
intergranular corrosion, acts as a grain refiner
and as ferrite developer
Aluminium
l
Type 17-7 PH
l
Works as strong ferrite developer, mainly
used as heat ageing additive
Copperl
l
l
Type 17-7 PH,1.4505, 1.4506
l
l
Improves corrosion resistance against certain
media, decreases tendency to stress
corrosion cracking, improves ageing
ISF 2002br-er06-03e.cdr
Figure 5.3
Figure 5.4
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5. Welding of High-Alloy Steels, Corrosion 60
strong embrittlement. With higher alloy steels, the diffusion speed is greatly reduced, there-
fore both processes require a relatively long dwell time. In case of technical cooling, such
embrittlement processes are suppressed by an increased cooling speed.
Nickel is a strong austenite developer, see Figure 5.5 Nickel and iron develop in this system
under elevated temperature a complete series of face-centred cubic solid solutions. Also in
the binary system Fe-Ni
decomposition processes
in the lower temperature
range take place.
Along two cuts through the
ternary system Fe-Cr-Ni,
Figure 5.6 shows the most
important phases which
develop in high alloy steels.
A solidifying alloy with 20%
Cr and 10% Ni (left figure)
forms at first -ferrite. -
ferrite is, analogous to the
Fe-C diagram, the primary
from the melt solidifying
body-centred cubic solid
solution. However -ferrite
is developed by transfor-
mation of the austenite, but
is of the same structure
from the crystallographic
point of view, see Figure
5.4.
Binary System Fe - Ni
30Fe 10 20 40 50 60 70 80 90 Ni
0
200
400
600
800
1000
1200
1400
1600
Temp
erature
C
%
Nickel
Fe Ni3
FeNi 3
S+S+
ISF 2002br-er-06-05e.cdr
Figure 5.5
Sections of the Ternary System Fe-Cr-Ni
700
800
900
1000
1100
1200
1300
1400
1500
1600
0 5 10 15 20 % Ni
% Cr30 25 20 15 10
70 % Fe
0 5 10 15 20 25700
800
900
1000
1100
1200
1300
1400
1500
1600
40 35 30 25 20 15
% Ni
% Cr
60 % Fe
Temperature
C C SS
S+
SSS+
S+
S+
ISF 2002br-er-06-06e.cdr
Figure 5.6
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5. Welding of High-Alloy Steels, Corrosion 64
The ferrite content can only be measured with a relatively large dispersal, therefore DeLong
proposed to base a measurement procedure on standardized specimens. Such a system
makes it possible to measure comparable values which don't have to match the real ferrite
content. Based on these measurement values, the ferrite content is no longer given in per-
centage, but steels are grouped by ferrite numbers. In addition to ferrite numbers, DeLong
proposed a reworked Schaeffler diagram where the ferrite number can be determined by the
chemical composition, Figure 5.13. Moreover, DeLong has considered the influence of nitro-
gen as a strong austenite developer (effects are comparable with influence of carbon). Later
on, nitrogen was included into the nickel-equivalent of the Schaeffler diagram.
The most important feature
of high alloy steels is their
corrosion resistance start-
ing with a Cr content of
12%. In addition to the
problems during welding
described by the Schaeffler
diagram, these steels can
be negatively affected with
view to their corrosion re-
sistance caused by the
welding process. Figure
5.14 shows schematically
the processes of electro-
lytic corrosion under a
drop of water on a piece of
iron. In such a system a
potential difference is a
precondition for the devel-
opment of a local element
consisting of an anode and
a cathode. To develop
De Long Diagram
16 17 18 19 20 21 22 23 24 25 26 27Chromium-equivalent = %Cr + %Mo + 1,5 x %Si + 0,5 x %Nb
Nickel-equivalent=%Ni+30x%C+30x%N+0,5x
%Mn
21
20
19
18
17
16
15
14
13
12
11
10
austenite
Schaeffler-austenite-martensite-line
austenite + ferrite
form
erly
mag
netic
allym
easu
red
ferrite
conten
tsin
vol.-%
ferritenu
mbe
r
2%
4%
6%
7,6%
9,2%
10,7%
12,3%
13,8%
0%
0
2
4
68
1012141618
ISF 2002br-er-06-13e.cdr
Figure 5.13
Corrosion Under a Drop of Water
air
water
Fe(OH)3
iron
2Fe +O+H O 2Fe +2OH++ +++ -
2
H O2
O
OH-
cathode
anode
2Fe 2Fe +4e ++ -
4e-
O +2H O+4e 4OH2 2- -
O2 OH
Fe+++
2Fe++
ISF 2002br-er-06-14e.cdr
Figure 5.14
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5. Welding of High-Alloy Steels, Corrosion 66
If crevice corrosion is pre-
sent, corrosion products built
up in the root of the gap and
oxygen has no access to
restore the passive layer.
Thus narrow gaps where the
corrosive medium can ac-
cumulate are to be avoided
by introducing a suitable de-
sign, Figure 5.16.
With pitting corrosion, the
chemical composition of the
attacking medium causes a
local break-up of the passive layer. Especially salts, preferably Clions, show this behaviour.
This local attack causes a dissolution of the material on the damaged points, a depression
develops. Corrosion products accumulate in this depression, and the access of oxygen to the
bottom of the hole is obstructed. However, oxygen is required to develop the passive layer,
therefore this layer cannot be completely cured and pitting occurs, Figure 5.17.
Stress-corrosion cracking occurs when the material displaces under stress and the passive
layer tears, Figure 5.18. Now the unprotected area is subjected to corrosion, metal is dis-
solved and the passive
layer redevelops (figures 1-
3). The repeated displace-
ment and repassivation
causes a crack propaga-
tion. Stress corrosion
cracking takes mainly
place in chloride solutions.
The crack propagation is
transglobular, i.e. it does
not follow the grainboundaries.
Pitting Corrosion of a
Storage Container
Steel
br-er-06-17e.cdr
Figure 5.17
Model of Crack PropagationThrough Stress Corrosion Cracking
1 2 3 4 5 6
121110987
offset; passive layer; metal surface; dislocation
br-er-06-18e.cdr
Figure 5.18
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5. Welding of High-Alloy Steels, Corrosion 67
Figure 5.19 shows the expansion-rate dependence of stress corrosion cracking. With very
low expansion-rates, a curing of the passive layer is fast enough to arrest the crack. With
very high expansion-rates, the failure of the specimen originates from a ductile fracture. In
the intermediate range, the material damage is due to stress corrosion cracking.
Figure 5.20 shows an example of crack propagation at transglobular stress corrosion crack-
ing. A crack propagation speed is between 0,05 to 1 mm/h for steels with 18 - 20% Cr and 8 -
20% Ni. With view to welding it is important to know that already residual welding stresses
may release stress corrosion cracking.
The most important problem in the field of welding is intergranular corrosion (IC).
It is caused by precipitation of chromium carbides on grain boundaries.
Although a high solubility of carbon in the austenite can be expected, see Fe-C diagram, the
carbon content in high alloyed Cr-Ni steels is limited to approximately 0,02% at room tem-
perature, Figure 5.21.
TransgranularStress Corrosion Cracking
ISF 2002br-er06-20e.cdr
Figure 5.20
Influence of Elongation Speed onSensitivity to Stress Corrosion Cracking
SpRK
completecover layer tough fracture
Sensitivi
tyto
stresscorrosion
cracking
Elongation speed
2 1
T=RT
ISF 2002br-er06-19E.cdr
Figure 5.19
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5. Welding of High-Alloy Steels, Corrosion 68
The reason is the very high affinity of chro-
mium to carbon, which causes the precipita-
tion of chromium carbides Cr23C6 on grain
boundaries, Figure 5.22. Due to these precipi-
tations, the austenite grid is depleted of
chromium content along the grain boundaries
and the Cr content drops below the parting
limit. The diffusion speed of chromium in aus-
tenite is considerably lower than that of car-
bon, therefore the chromium reduction cannot
be compensated by late diffusion. In the de-
pleted areas along the grain boundaries (line
2 in Figure 5.22) the steel has become sus-
ceptible to corrosion.
Only after the steel has been subjected to
sufficiently long heat treatment, chromium will
diffuse to the grain boundary and increase the
C concentration along the
grain boundary (line 3 in
Figure 5.22). In this way, the
complete corrosion resis-
tance can be restored (line 4
in Figure 5.22).
Figure 5.23 explains why the
IC is also described as in-
tergranular disintegration.
Due to dissolution of de-
pleted areas along the grain
boundary, complete grains
break-out of the steel.
Carbon Solubility ofAustenitic Cr - Ni Steels
0 0.05 0.1 0.15 0.2 0.25 %0,3Carbon content
600
700
800
900
1000
1100
C
1200
A
Heattreatmenttemperature
to Bain and Aborn
ISF 2002br-er06-21e.cdr
Figure 5.21
Sensibility of a Cr - Steel
Chromium
contentofaustenite
resistance limit
1 - homogenuous starting condition2 - start of carbide formation3 - start of concentration balance4 - regeneration of resistance limit
1
2
3
4
Distance from grain boundary ISF 2002br-er-06-22e.cdr
Figure 5.22
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5. Welding of High-Alloy Steels, Corrosion 69
The precipitation and re-
passivation mechanisms
described in Figure 5.22
are covered by intergranu-
lar corrosion diagrams ac-
cording to Figure 5.24.
Above a certain tempera-
ture carbon remains dis-
solved in the austenite
(see also Figure 5.21).
Below this temperature, a
carbon precipitation takes
place. As it is a diffusion
controlled process, the
precipitation occurs after a
certain incubation time
which depends on tem-
perature (line 1, precipita-
tion characteristic curve).
During stoppage at a con-
stant temperature, the
parting limit of the steel is
regained by diffusion of
chromium.
Figure 5.25 depicts characteristic precipitation curves of a ferritic and of an austenitic steel.
Due to the highly increased diffusion speed of carbon in ferrite, shifts the curve of carbon
precipitation of this steel markedly towards shorter time. Consequently the danger of inter-
granular corrosion is significantly higher with ferritic steel than with austenite.
Grain Disintegration
ISF 2002br-er-06-23e.cdr
Figure 5.23
Area of Intergranular Disintegrationof Unstabilized Cr - Steels
Reciprocalofheattreatmenttemperature1/T
oversaturatedaustenite
austenite -chromium carbide (M C )
no intergranular disintegration23 6
unsaturated austenite
Heat treatment time (lgt)
1 incubation time2 regeneration of resistance limit3 saturation limit for chromium carbide
1
2
3
austenite + chromium caride (M C )
to intergranular disintegration23 6 sensitive
ISF 2002br-er-06-24e.cdr
Figure 5.24
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5. Welding of High-Alloy Steels, Corrosion 70
As carbon is the element that triggers the intergranular corrosion, the intergranular corrosion
diagram is relevantly influenced by the c con-
tent, Figure 5.26.
By decreasing the carbon content of steel,
the start of carbide precipitation and/or the
start of intergranular corrosion are shifted
towards lower temperatures and longer
times. This fact initiated the development of
so-called ELC-steels (Extra-Low-Carbon)
where the C content is decreased to less
than 0,03%
During welding, the considerable influence of
carbon is also important for the selection of
the shielding gas, Figure 5.27. The higher the
CO2-content of the shielding gas, the
stronger is its carburising effect. The C-
content of the weld metal increases and the
steel becomes more susceptible to inter-
granular corrosion.
An often used method to
avoid intergranular corro-
sion is a stabilisation of the
steel by alloy elements like
niobium and titanium, Fig-
ure 5.28. The affinity of
these elements to carbon is
significantly higher than
that of chromium, therefore
carbon is compounded into
Nb- and Ti-carbides. Now
carbon cannot cause anychromium depletion. The
Precipitation Curves of VariousAlloyed Cr Steels
Tempering time
Temp
eringtemperature
quenchtemperature
18-8-Cr-Ni steel17% Cr steel
precipitation curves for
cooling curve
ISF 2002br-er06-25e.cdr
Figure 5.25
Figure 5.26
Influence of C-Contenton Intergranular Disintegration
101
102
103
104
105
106
Times
400
500
600
700
800
900
1000
Temperatu
re
C
0.07%C0.05%C
0.03%C
0.025%C
ISF 2002br-er-06-26e.cdr
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5. Welding of High-Alloy Steels, Corrosion 71
proportion of these alloy elements depend on the carbon content and is at least 5 times
higher with titanium and 10 times higher with niobium than that of carbon. Figure 5.28 shows
the effects of a stabilisation in the intergranular corrosion diagram. If both steels are sub-
jected to the same heat treatment (1050C/W means heating to 1050C and subsequent wa-
ter quenching), then the area of intergranular corrosion will shift due to stabilisation to
significantly longer times. Only with a much higher heat treatment temperature the inter-
granular corrosion accelerates again. The cause is the dissolution of titanium carbides at suf-
ficiently high temperature. This carbide dissolution causes problems when welding stabilised
steels. During welding, a narrow area of the HAZ is heated above 1300C, carbides are dis-
solved. During the subsequent cooling and the high cooling rate, the carbon remains dis-
solved.
If a subsequent stress relief treatment around 600C is carried out, carbide precipitations on
grain boundaries take place again. Due to the large surplus of chromium compared with nio-
bium or titanium, a partial chromium carbide precipitation takes place, causing again inter-
Influence of Shielding Gason Intergranular Disintegration
S hie ld ing g as A r [% ] C O2 O2
S 1 99 / 1
M 1 90 5 5
M 2 82 18 /
Composit ion
0,2 0,5 1 2,5 5 10 25 50 100 250 h 1000400
450
500
550
600
C
700
0.058 % C0.53 % NbNb/C = 9
0.030 % C0.51 % NbNb/C = 17 0.018 % C
0.57 % NbNb/C = 32M2
M1
S1
Heat treatment time
Heattreatmenttemperature
ISF 2002br-er06-27e.cdr
Figure 5.27
Influence of Stabilizationon Intergranular Disintegration
800
700
650
600
550
500
450
C
Heattreatmenttemperature
0,3 1 3 10 30 100 300 1000 h 10000Time
1050C/W
X5CrNi18-10 unstabilized
800
700
650
600
550
500
450
C
Heattreatmenttemperature
0,3 1 3 10 30 100 300 1000 h 10000Time
1300C/W
1050C/W
X5CrNiTi18-10 stabilized
W.-No.:4301 (0,06%)
W.-No.:4541
ISF 2002br-er06-28e.cdr
Figure 5.28
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5. Welding of High-Alloy Steels, Corrosion 72
granular susceptibility. As this susceptibility is limited to very narrow areas along the welded
joint, it was called knife-line attack because of its appearance. Figure 5.29.
In stabilised steels, the chromium carbide represents an unstable phase, and with a suffi-
ciently long heat treatment to transform to NbC, the steel becomes stable again. The stronger
the steel is over-stabilised, the lower is the tendency to knife-line corrosion.
Nowadays the importance
of Nickel-Base-Alloys in-
creases constantly. They
are ideal materials when it
comes to components
which are exposed to spe-
cial conditions: high tem-
perature, corrosive attack,
low temperature, wear re-
sistance, or combinations
hereof. Figure 5.30 shows
one of the possible group-
ing of nickel-base-alloys.
Materials listed there are selected examples, the total number of available materials is many
times higher.
Group A consists of nickel
alloys. These alloys are
characterized by moderate
mechanical strength and
high degree of toughness.
They can be hardened only
by cold working. The alloys
are quite gummy in the an-
nealed or hot-worked con-
dition, and cold-drawn
material is recommended
for best machinability andsmoothest finish.
Knife-Line Corrosion
br-er-06-29e.cdr
Figure 5.29
ISF 2002br-er-06-30e.cdr
Alloy Chem. composition Alloy Chem. Composition
Group A Group D1
Nickel 2 00 Ni 99.6, C 0.08 Duranickel 3 01 Ni 94.0, A l 4.4, W 0.6
Nicke l212 Ni 97.0 ,C 0 .05,Mn2.0 Incoloy925 Ni 42.0 ,Fe 32.0 ,Cr 21.0 ,Mo 3 .0 ,W 2.1 ,Cu2.2 ,Al0 .3
Nickel 222 Ni 99.5, Mg 0.075 Ni-Span-C 902 Y2O30.5, Ni 42.5,Fe 49.0,Cr 5.3,W 2.4,Al 0.5
Group B Group D2
Monel 400 Ni 66.5, C u 31.5 Monel K -500 Ni 65.5, C u 29.5, A l 2.7, F e 1.0, W 0.6
Monel450 Ni 30.0,Cu68.0,Fe 0.7,Mn0.7 Inconel718 Ni 52.0,Cr 22.0,Mo 9.0,Co 12.5,Fe 1.5,Al1.2
Ferry Ni 45.0, C u 55.0 Inconel X -750 Ni 61.0, C r 21.5, Mo 9.0, Nb 3.6, F e 2.5
Group C Nimonic 90 Ni 77.5,Cr 20.0,Fe 1.0,W 0.5,Al 0.3,Y2O30.6
Incone l600 Ni 76.0 ,Cr 15.5 ,Fe 8 .0 Nimonic 105 Ni 76.0 ,Cr 19.5 ,Fe 112.4 ,Al1 .4
Ni moni c 75 Ni 80.0, C r 19.5 Incoloy 9 03 Ni 39.0, Fe 34.0, C r 18.0, M o 5.2, W 2.3, A l 0.8
Nimonic 86 Ni 64.0,Cr 25.0, Mo 10.0, Ce 0.03 Incoloy909 Ni 58.0, Cr 19.5,Co 13.5, Mo 4.25, W 3.0,Al 1.4
Incoloy800 Ni 32.5,Fe 46.0, Cr 21.0,C 0.05 Inco G-3 Ni 38.4, Fe 42.0,Cu 13.0,Nb 4.7,W 1.5,Al0.03,Si 0.15
Incoloy825 Ni 42.0,Fe 30.0, Cr 21.5,Mo 3.0, Cu2.2,Ti 1.0 Inco C-276 Ni 38.4,Fe 42.0,Cu13.0,Nb 4.7, W 1.5, Al0.03,Si 0.4
Inco 330 Ni 35.5,Fe 44.0,Cr 18.5,Si 1.1 Group E
MonelR-405 Ni 66.5,Cu 31.5,Fe 1.2, Mn1.1, S 0.04
Typical Classification of Ni-Base Alloys
Figure 5.30
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5. Welding of High-Alloy Steels, Corrosion 73
Group B consists mainly of those nickel-copper alloys that can be hardened only by cold
working. The alloys in this group have higher strength and slightly lower toughness than
those in Group A. Cold-drawn or cold-drawn and stress-relieved material is recommended for
best machinability and smoothest finish.
Group C consists largely of nickel-chromium and nickel-iron-chromium alloys. These alloys
are quite similar to the austenitic stainless steels. They can be hardened only by cold working
and are machined most readily in the cold-drawn or cold-drawn and stress-relieved condition.
Group D consists primary of age-hardening alloys. It is divided into two subgroups:
D 1 Alloys in the non-aged condition.
D 2 Aged Group D-1 alloys plus several other alloys in all conditions.
The alloys in Group D are characterized by high strength and hardness, particularly when
aged. Material which has been solution annealed and quenched or rapidly air cooled is in the
softest condition and does machine easily. Because of softness, the non-aged condition is
necessary for trouble free drilling, tapping and all threading operations. Heavy machining of
the age-hardening alloys is best accomplished when they are in one of the following condi-
tions:
1. Solution annealed
2. Hot worked and quenched or rapidly air cooled
Group E contains only one material: MONEL R-405. It was designed for mass production of
automatically machined screws.
Due to the high number of possible alloys with different properties, only one typical material
of group D2 is discussed here: Material No. 2.4669, also known as e.g. Inconel X-750.
The aluminium and titanium containing 2.4669 is age-hardening through the combination of
these elements with nickel during heat treatment: gamma-primary-phase (') develops which
is the intermetallic compound Ni3(Al, Ti).
During solution heat treatment of X-750 at 1150C, the number of flaws and dislocations in
the crystal is reduced and soluble carbides dissolve. To achieve best results, the material
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5. Welding of High-Alloy Steels, Corrosion 74
should be in intensely worked condition before heat treatment to permit a fast and complete
recrystallisation. After solution heat treatment, the material should not be cold worked, since
this would generate new dislocations and affect negatively the fracture properties.
The creep rupture resistance of X-750 is due to an even distribution of the intercrystalline 'phase. However, fracture properties depend more on the microstructure of the grain bounda-
ries. During an 840C stabilising heat treatment as part of the triple-heat treatment, the fine '
phase develops inside the grains and M23C6precipitates onto the grain boundaries. Adjacent
to the grain boundary, there is a ' depleted zone. During precipitation hardening (700C/20
h) ' phase develops in these depleted zones. ' particles arrest the movement of disloca-
tions, this leads to improved strength and creep resistance properties.
During the M23C6transformation, carbon is stabilised to a high degree without leaving chro-
mium depleted areas along the grain boundaries. This stabilisation improves the resistance
of this alloy against the attack of several corrosive media.
With a reduction of the precipitation temperature from 730 to 620C as required for some
special heat treatments additional ' phase is precipitated in smaller particles. This en-
hances the hardening effect and improves strength characteristics.
Further metallurgical discussions about X-750, can be taken from literature, especially with
view to the influence of heat treatment on fracture properties and corrosion behaviour.
The recommended processes for welding of X-750 are tungsten inert gas, plasma arc, elec-
tron beam, resistance, and pressure oxy arc welding.
During TIG welding of INCONEL X-750, INCONEL 718 is used as welding consumable. Joint
properties are almost 100% of base material at room temperature and about 80% at 700 -
820C. Figure 5.31 shows typical strength properties of a welded plate at a temperature
range between -423 and 1500F (-248 820C).
Before welding, X-750 should be in normalised or solution heat treated condition. However, it
is possible to weld it in a precipitation hardened condition, but after that neither the seam nor
the heat affected zone should be precipitation hardened or used in the temperature range of
precipitation hardening, because the base material may crack. If X-750 was precipitation
hardened and then welded, and if it is likely that the workpiece is used in the temperature
range of precipitation hardening, the weld should be normalised or once again precipitation
hardened. In any case it must be noted that heat stresses are minimised during assembly or
welding.
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5. Welding of High-Alloy Steels, Corrosion 75
X-750 welds should be solution heat treated before a precipitation hardening. Heating-up
speed during welding must be from the start fast and even touching the temperature range of
precipitation hardening only as briefly as possible. The best way for fast heating-up is to in-
sert the welded workpiece into a preheated furnace.
Sometimes a preheating before welding is advantageous if the component to be welded
has a poor accessibility, or the welding is complex, and especially if the assembly proves to
be too complicated for a post heat treatment. Two effective welding preparations are:
1. 1550F/16 h, air cooling
2. 1950F/1 h, furnace cooling with 25-100F/h up to 1200F, air
A repair welding of already fitted parts should be followed by a solution heat treatment (with a
fast heating-up through the temperature range of precipitation hardening) and a repeated
precipitation hardening.
A cleaning of intermediate layers must be carried out to remove the oxide layers which are
formed during welding. (A complete isolation
of the weld metal using gas shielded proc-
esses is hardly possible). If such films are not
removed on a regular basis, they can become
thick enough to cause material separations
together with a reduced strength. Brushing
with wire brushes only polishes the surface,
the layer surface must be sand-blasted or
ground with abrasive material. The frequency
of cleaning depends on the mass of the de-
veloped oxides. Any sand must be removed
before the next layer is welded.
X-750 can be joined also by spot-, projection-,
seam-, and flash butt welding. The welding
equipment must be of adequate performance.
X-750 is generally resistance welded in nor-
malized or solution heat treated condition.
Figure 5.31