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Titel.doc6. Welding High Alloy Steels 71
Basically stainless steels are characterised by a chromium content of at least 12%. Figure
6.1 shows a classification
scale-resistant and
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.
phase diagram (left figure),
fects of two different
groups of alloying elements
on the equilibrium diagram.
portant element cause a
Classification of Corrosion-Resistant Steels
Chromium Vanadium Molybdenum Aluminium Silicon
Nickel Manganese Cobalt
T
A4
A3
T
A4
A3
T
A4
A3
6. Welding High Alloy Steels 72
tenite area, partly with downward equilibrium line according to Figure 6.2 (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 6.2 (left figure)
and form a purely austenitic and transforma-
tion-free steel.
steels.
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
500°C this system contains the intermetallic
σ-phase, which decomposes in the lower
temperature range into a low-chromium α-solid solution and a chromium-rich α’-solid solution.
Both, the development of the σ-phase and of the unary α-α’-decomposition cause a strong
Effects of Some Elements in Cr-Ni Steel
Element Steel type, no. Effect
Carbon
l
l
l
critical cooling rate
oxidation- and corrosion-resistance
toughness at low temperature, grain-refining
Oxygen
(20 to 30 times stronger than Nickel)
Niobium
l
1.4511,1.4550,
intergranular corrosion
crack tendency by formation of manganese
sulphide
Molybdenum
l
l
1.4401,1.4404,
developer
Phosphorus,
developer, all types are alloyed with small
contents for desoxidation
intergranular corrosion, acts as a grain refiner
and as ferrite developer
used as heat ageing additive
Copper
l
l
l
corrosion cracking, improves ageing
30Fe 10 20 40 50 60 70 80 90 Cr%
Chromium
6. Welding High Alloy Steels 73
embrittlement. With higher alloy steels, the diffusion speed is greatly reduced, therefore both
processes require a relatively long dwell time. In case of technical cooling, such embrittle-
ment processes are suppressed by an increased cooling speed.
Nickel is a strong austenite developer, see Figure 6.5 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
ternary system Fe-Cr-Ni,
important phases which
Cr and 10% Ni (left figure)
forms at first δ-ferrite. δ-
ferrite is, analogous to the
Fe-C diagram, the primary
from the melt solidifying
from the crystallographic
6.4.
30Fe 10 20 40 50 60 70 80 90 Ni
0
200
400
600
800
1000
1200
1400
1600
700
800
900
1000
1100
1200
1300
1400
1500
1600
% Cr30 25 20 15 10
70 % Fe
800
900
1000
1100
1200
1300
1400
1500
1600
% Ni
% Cr
S+d
d+ g+
6. Welding High Alloy Steels 74
During an ongoing cooling, the binary area ferrite + austenite passes through and a transfor-
mation into austenite takes place. If the cool-
ing is close to the equilibrium, a partial trans-
formation of austenite into the brittle α-phase
takes place in the temperature range below
800°C. Primary ferritic solidifying alloys show
a reduced tendency to hot cracking, because
δ-ferrite can absorb hot-crack promoting ele-
ments like S and P. However primary austen-
itic solidifying alloys show, starting at a certain
alloy content, no transformations during cool-
ing (14% Ni, 16% Cr, left figure). Primary aus-
tenitic solidifying alloys are much more
susceptible to hot cracking than primary fer-
ritic solidifying alloys, a transformation into the
σ-phase normally does not take place with
these alloys.
of certain groups of high alloy steels.
The diagram of Strauß and Maurer in Figure 6.8 shows the influence on the microstructure
formation of steels with a C-content of 0,2%. The classification of high-alloy steels in Figure
6.1 is based on this dia-
gram. If a steel only con-
tains C, Cr and Ni, the
lowest austenite corner will
And also other elements
austenite or ferrite devel-
oper. The influence of
these elements is de-
scribed by the so-called
4. Aus
te ni
tic -fe
rri tic
s te
el s
3. Aus
te ni
tic s
te el
+
+
+
+
+
+
+
+
+
+
+ indicates that the alloy elements can be added in a defined content to achieve various characteristics
© ISF 2002br-er06-07e.cdr
Figure 6.7
Maurer - Diagram
%
0 2 4 6 8 10 12 14 16 18 20 22 24 26%
ferrite / perlite
equivalents. The Schaeffler diagram reflects additional alloy elements, Figure 6.9. It repre-
sents molten weld metal of high alloy steels and determines the developed microstructures
after cooling down from very high temperatures. The diagram was always prepared consider-
ing identical cooling conditions, the influence of different cooling speeds is here disregarded.
The areas 1 to 4 in this diagram limit the chemical compositions of steels, where specific de-
fects may occur during welding.
Depending on the composition, purely ferritic chromium steels have a tendency to embrittle-
ment by martensite and therefore to hot cracking (area 2) or to embrittlement due to strong
grain growth (area 1).
growth during welding is the
greatly increased diffusion
pared with austenite. After
weldability.
embrittlement of the material
σ-phase. As explained in
creased ferrite contents,
increased chromium con-
cooling speed.
Schaeffler Diagram With Border Lines of Weld Metal Properties to Bystram
0% Ferri
austenite
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
sigma embrittlement between 500-900°C
hot cracking susceptibility above 1250°C grain growth above 1150°C
Chromium-equivalent = %Cr + %Mo + 1,5x%Si + 0,5x%Nb
N ic
a le
n t =
1000
2000
3000
4000
5000
6000
6. Welding High Alloy Steels 76
Finally, area 4 marks the strongly increased tendency to hot cracking in the austenite. Rea-
son is, that critical elements responsible for hot cracking like e.g. sulphur and phosphorous
have only very limited solubility in the austenite. During welding, they enrich the melt residue,
promoting hot crack formation (see also chapter 9 - Welding Defects).
There is a Z-shaped area in the centre of the diagram which does not belong to any other
endangered area. This area of chemical composition represents the minimum risk of welding
defects, therefore such a composition should be adjusted in the weld metal. Especially when
welding austenitic steels one tries to aim at a low content of δ-ferrite, because it has a much
greater solubility of S and P, thus minimising the risk of hot cracking.
The Schaeffler diagram is not only used for determining the microstructure with known
chemical composition. It is also possible to estimate the developing microstructures when
welding different materials with or without filler metal. Figures 6.11 and 6.12 show two exam-
ples for a determination of the weld metal microstructures of so-called 'black and white' joints.
Application Example of Schaeffler - Diagram
0 4 8 12 16 20 24 28 32 36 0
4
8
12
16
20
24
28
Chromium-equivalent
² ·
1
2
3
Application Example of Schaeffler - Diagram
0 4 8 12 16 20 24 28 32 36 0
4
8
12
16
20
24
28
Chromium-equivalent
a le
n t
² ·
123
6. Welding High Alloy Steels 77
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 6.13. 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
corrosion resistance start-
12%. In addition to the
problems during welding
sistance caused by the
iron. In such a system a
potential difference is a
precondition for the devel-
a cathode. To develop
De Long Diagram
16 17 18 19 20 21 22 23 24 25 26 27
Chromium-equivalent = %Cr + %Mo + 1,5 x %Si + 0,5 x %Nb
N ic
air
water
Fe(OH)3
iron
2 ®
- - ®
6. Welding High Alloy Steels 78
such a local element, a different orientation of grains in the steel is sufficient. If a potential
difference under a drop of water is present, the chemically less noble part reacts as an an-
ode, i.e. iron is oxidised here and is dissolved as Fe2+-ion together with an electron emission.
Caused by oxygen access through the air, a further oxidation to Fe3+ takes place. The ca-
thodic, chemically nobler area develops OH- ions, absorbing oxygen and the electrons. Fe3+-
and OH--ions compose into the water-insoluble Fe(OH)3 which deposits as rust on the sur-
face (note: the processes here described should serve as a principal explanation of electro-
chemical corrosion mechanisms, they are, at best, a fraction of all possible reactions).
If the steel is passivated by chromium, the corrosion protection is provided by the develop-
ment of a very thin chromium oxide layer which separates the material from the corrosive
medium. Mechanical surface damages of this layer are completely cured in a very short time.
The examples in Figure 6.15 are more critical, since a complete recovery of the passive layer
is not possible from various reasons.
passive layerpassive layer
passive layerpassive layer
active dissolution of the gap
crevice corrosion
If crevice corrosion is pre-
sent, corrosion products built
oxygen has no access to
restore the passive layer.
corrosive medium can ac-
sign, Figure 6.16.
With pitting corrosion, the
chemical composition of the
attacking medium causes a
local break-up of the passive layer. Especially salts, preferably Cl—ions, 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 6.17.
Stress-corrosion cracking occurs when the material displaces under stress and the passive
layer tears, Figure 6.18. Now the unprotected area is subjected to corrosion, metal is dis-
solved and the passive
layer redevelops (figures 1-
3). The repeated displace-
Steel
br-er-06-17e.cdr
1 2 3 4 5 6
121110987
br-er-06-18e.cdr
Figure 6.19 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 6.20 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 6.21.
Influence of Elongation Speed on Sensitivity to Stress Corrosion Cracking
SpRK
S e n s it iv
it y t o s
tr e s s c
o rr
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 6.22. 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 6.22) the steel has become sus-
ceptible to corrosion.
sufficiently long heat treatment, chromium will
diffuse to the grain boundary and increase the
C concentration along the
Figure 6.22). In this way, the
complete corrosion resis-
in Figure 6.22).
IC is also described as in-
tergranular disintegration.
boundary, complete grains
Carbon content
C h ro
n it e
resistance limit
1 - homogenuous starting condition 2 - start of carbide formation 3 - start of concentration balance 4 - regeneration of resistance limit
1
2
3
4
Figure 6.22
The precipitation and re-
controlled process, the
stant temperature, the
regained by diffusion of
chromium.
Figure 6.25 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 6.23
¬ R
unsaturated austenite
Heat treatment time (lgt)
1 incubation time 2 regeneration of resistance limit 3 saturation limit for chromium carbide
1
2
3
to intergranular disintegration 23 6 sensitive
© ISF 2002br-er-06-24e.cdr
Figure 6.24
6. Welding High Alloy Steels 83
As carbon is the element that triggers the intergranular corrosion, the intergranular corrosion
diagram is relevantly influenced by the c con-
tent, Figure 6.26.
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)
than 0,03%
carbon is also important for the selection of
the shielding gas, Figure 6.27. 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.
avoid intergranular corro-
steel by alloy elements like
niobium and titanium, Fig-
significantly higher than
Tempering time
T e
m p
10 1
10 2
10 3
10 4
10 5
10 6
Time s
6. Welding High Alloy Steels 84
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 6.28 shows
the effects of a stabilisation in the intergranular corrosion diagram. If both steels are sub-
jected to the same heat treatment (1050°C/W means heating to 1050°C 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 1300°C, 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 600°C 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 Gas on Intergranular Disintegration
Shield ing gas Ar [% ] C O2 O2
S 1 99 / 1
M 2 82 18 /
C omposition
0,2 0,5 1 2,5 5 10 25 50 100 250 h 1000 400
450
500
550
600
°C
700
0.030 % C 0.51 % Nb Nb/C = 17 0.018 % C
0.57 % Nb Nb/C = 32M2
e a tm
800
700
650
600
550
500
450
°C
e a tm
tu re
0,3 1 3 10 30 100 300 1000 h 10000 Time
1050°C/W
X5CrNi18-10 unstabilized
e a tm
tu re
0,3 1 3 10 30 100 300 1000 h 10000 Time
1300°C/W
1050°C/W
X5CrNiTi18-10 stabilized
W.-No.:4301 (0,06%)
6. Welding High Alloy Steels 85
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 6.29.
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
comes to components
cial conditions: high tem-
ing of nickel-base-alloys.
Materials listed there are selected examples, the total number of available materials is many
times higher.
alloys. These alloys are
are quite gummy in the an-
nealed or hot-worked con-
Group A Group D1
Nickel 200 Ni 99.6, C 0.08 Duranickel 301 Ni 94.0, Al 4.4, W 0.6
Nickel 212 Ni 97.0, C 0.05, Mn 2.0 Incoloy 925 Ni 42.0, Fe 32.0, Cr 21.0, Mo 3.0, W 2.1, Cu 2.2, Al 0.3
Nickel 222 Ni 99.5, Mg 0.075 Ni-Span-C 902 Y2O3 0.5, Ni 42.5, Fe 49.0, Cr 5.3, W 2.4, Al 0.5
Group B Group D2
Monel 400 Ni 66.5, Cu 31.5 Monel K-500 Ni 65.5, Cu 29.5, Al 2.7, Fe 1.0, W 0.6
Monel 450 Ni 30.0, Cu 68.0, Fe 0.7, Mn 0.7 Inconel 718 Ni 52.0, Cr 22.0, Mo 9.0, Co 12.5, Fe 1.5, Al 1.2
Ferry Ni 45.0, Cu 55.0 Inconel X-750 Ni 61.0, Cr 21.5, Mo 9.0, Nb 3.6, Fe 2.5
Group C Nimonic 90 Ni 77.5, Cr 20.0, Fe 1.0, W 0.5, Al 0.3, Y2O3 0.6
Inconel 600 Ni 76.0, Cr 15.5, Fe 8.0 Nimonic 105 Ni 76.0, Cr 19.5, Fe 112.4, Al 1.4
Nimonic 75 Ni 80.0, Cr 19.5 Incoloy 903 Ni 39.0, Fe 34.0, Cr 18.0, Mo 5.2, W 2.3, Al 0.8
Nimonic 86 Ni 64.0, Cr 25.0, Mo 10.0, Ce 0.03 Incoloy 909 Ni 58.0, Cr 19.5, Co 13.5, Mo 4.25, W 3.0, Al 1.4
Incoloy 800 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, Al 0.03, Si 0.15
Incoloy 825 Ni 42.0, Fe 30.0, Cr 21.5, Mo 3.0, Cu 2.2, Ti 1.0 Inco C-276 Ni 38.4, Fe 42.0, Cu 13.0, Nb 4.7, W 1.5, Al 0.03, Si 0.4
Inco 330 Ni 35.5, Fe 44.0, Cr 18.5, Si 1.1 Group E
Monel R-405 Ni 66.5, Cu 31.5, Fe 1.2, Mn 1.1, S 0.04
Typical Classification of Ni-Base Alloys
Figure 6.30
6. Welding High Alloy Steels 86
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…

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