A Comparative Study Into The Fracture Toughness Properties of Duplex Stainless Steels
A Comparative Study Into The Fracture Toughness Properties of Duplex Stainless Steels
A Comparative Study Into The Fracture Toughness Properties of Duplex Stainless Steels
A R T I C L E I N F O A B S T R A C T
Keywords: Beside their high mechanical strength, duplex stainless steels are a suitable choice in highly corrosive envi
Duplex stainless steel ronments. These types of steels are used in steel bridges more and more frequently exposed to low temperatures
Fracture toughness and fatigue loads. However, for low temperature applications, it must be guaranteed that brittle fracture is
Weldment
avoided since duplex stainless steel shows a toughness-temperature relationship similar to that of carbon steel.
SENB fracture toughness
Charpy-V impact toughness
For this reason, in the frame of the German national FOSTA research project “P 1390”, comprehensive in
vestigations have been started into the material selection of duplex stainless steel to avoid brittle fracture
considering the fracture mechanic based background of EN 1993-1-10. For this purpose, Charpy-V impact tests
and fracture toughness tests have been systematically carried out for various duplex stainless steels in order to
create the basis for the development of toughness requirements for new duplex classes. The validity of the
already existing Master Curve concept and the applicability of the transition temperature correlation for duplex
stainless steels based on experimental fracture toughness and Charpy-V impact tests have been investigated. The
aim of this contribution is to present first results of these investigations.
1. Introduction insufficient for duplex stainless steels. For this reason, the rules have
recently been updated in the revised version of the draft prEN 1993-1-4
In the last twenty years, the use of duplex stainless steel has become [4] in such a way that maximum permissible element thicknesses
more and more an interesting alternative for steel bridges instead of depending on the stainless steel strength, its toughness quality, the
using carbon steel [1]. Duplex stainless steels offer an attractive com applied stress level and the reference temperature are given in relation
bination of properties, including high mechanical strength, good to the procedure used for EN 1993-1-10. Before the draft will be pub
corrosion resistance in highly corrosive environments, low maintenance lished, the given values of element thicknesses, which, in a first step,
costs and good weldability. Furthermore, duplex stainless steels obtain have been derived on the basis of Langenberg et al. [5], shall be checked
high toughness properties at low temperatures from the austenite crys with results from the presented investigations.
tallographic phase, while the ferrite content contributes to an Different influencing parameters like weldments and its sub param
improvement of the mechanical strength of the material. eters like the welding process and type of filler material may have an
Besides all these advantages, the prevention of brittle fracture is very influence on the fracture behaviour of the material [6]. Fracture
important when steel bridges are exposed to low temperatures. The toughness of both the base metal and weldments of duplex stainless steel
choice of material to avoid brittle fracture is covered in EN 1993-1-10 were already investigated in different studies in the past [7–14]. The
[2] for carbon steel material S235 to S690 while the currently pub results show promising results at sub-zero temperatures. However, there
lished version of EN 1993-1-4 [3] refers to different types of stainless are still some other parameters to be considered like the influence of the
steel including duplex stainless steels (1.4062, 1.4162, 1.4482, 1.4662, plate thickness on the fracture behaviour or the degree of cold forming
1.4362 and 1.4462) to be used only down to − 40 ◦ C service tempera on the transition temperature in order to develop a fracture mechanics
ture. As it can be seen, EN 1993-1-4 covers some different types of based concept for duplex stainless steels to avoid brittle fracture. For this
duplex stainless steel, however the stated toughness requirements are reason, in the frame of the German national FOSTA research project “P
* Corresponding author.
E-mail addresses: nariman.afzali@uni-due.de (N. Afzali), georjina.jabour@uni-due.de (G. Jabour), natalie.stranghoener@uni-due.de (N. Stranghöner), p.
langenberg@iwt-ag.de (P. Langenberg).
https://doi.org/10.1016/j.jcsr.2023.108283
Received 28 February 2023; Received in revised form 2 October 2023; Accepted 13 October 2023
Available online 25 October 2023
0143-974X/© 2023 Elsevier Ltd. All rights reserved.
N. Afzali et al. Journal of Constructional Steel Research 212 (2024) 108283
1390”, a comprehensive investigation has been started to close these of these duplex stainless steels.
gaps. In the frame of the presented investigations, Charpy-V impact tests
were performed for these three different types of duplex stainless steels
2. Transition temperature correlation and master curve concept with varying plate thicknesses: 1.4462 - thicknesses 25 mm and 80 mm,
1.4162 - thicknesses 25 mm and 50 mm as well as 1.4662 - thickness 25
One of the main objectives of this study is to verify the validity of the mm. All different tested types of duplex stainless steels were commer
Master Curve concept and the applicability of the transition temperature cially produced by Outokumpu. The chemical compositions of the
correlation for duplex stainless steels as both were developed based on investigated stainless steels are given in Table 1.
experimental test results for carbon steels. Based on EN 1993-1-10, the In this study, the influence of the weldment on the impact toughness
fracture mechanics concept relies on a transition temperature correla behaviour of the material has been investigated as well. For this case, K-
tion combined with a Master Curve approach for temperature dependent joint weldments were carried out using the Metal Active Gas (MAG)
fracture toughness [15,16]. welding process using stainless steel plates 1.4462 (25 mm and 80 mm)
Herein, the so-called modified Sanz-Correlation describes the tran and 1.4162 (25 mm and 50 mm). In order to have an increased tough
sition temperature correlation between the temperature at a Charpy-V ness of the weldment, the welding consumable 22 9 3 N L was chosen
impact toughness of 27 J (T27J) and the temperature at a fracture which provides a higher nickel content for an improved austenite for
toughness of 100 MPa√m (T100), see Eq. (1), [16–18]: mation, see Table 1 and [6]. The selected welding gas was a combination
of 98% argon (Ar) and 2% carbon dioxide (CO2). The welding direction
T100 = T27J − 18 ± 2 • σ (1) was parallel to the rolling direction. No post-weld treatment was applied
on the welded plates. The welding parameters are presented in Table 2.
where σ is the standard deviation (σ = 13 ◦ C).
The influence of the degree of cold forming on the impact toughness
The Master Curve is given by Eq. (2) according to [19–21] to describe
of duplex 1.4462 and lean duplex 1.4162 with each a thickness of 25 mm
the fracture toughness of materials and weldments with beff = 5 ad for
was another parameter to be investigated. Cold forming was applied on
plates with a semielliptical surface crack, ad is the design crack depth at
25 mm thick duplex strips in a 1600 kN servo hydraulic universal testing
which brittle fracture occurs and Pf describes the failure probability:
machine whereby the degree of cold forming was measured by means of
[ (
T − T100
) ] ( )0.25 (
25 1
)0.25 a video extensometer. After unloading the duplex strips, the remaining
Kmat = 20 + 77 • exp + 11 • • ln (2) amount of cold forming was recorded. Three different strips with
52 beff 1 − Pf
different degrees of cold forming (4%, 9% and 11%) were prepared.
It is important to acknowledge that the Master Curve method is Afterwards, Charpy-V impact test specimens were fabricated from these
primarily intended for materials displaying cleavage initiation. Cleavage strips.
within the ferrite phase, also present in the austenite-ferrite duplex Furthermore, a total of 22 fracture toughness tests were carried out
microstructure, is mostly described by the known weakest link mecha for 1.4462, 1.4162 and 1.4662 steels in different thicknesses.
nism [22]. The austenite phase in the duplex stainless steel, however,
tends to inhibit cleavage crack growth in the ferritic phase [23]. This
mechanism does not lead to a fully cleaved fracture at the lowest tem 3.2. Charpy-V impact tests
peratures, it does indicate fracture initiation and the corresponding
fracture toughness level. Despite this known facts, different studies show In total, 371 Charpy-V impact tests were performed for 1.4462,
that the Master Curve method as such is applicable as a practical tool 1.4162 and 1.4662 duplex stainless steel base material, weldments and
[5,12,22,24,25]. The following described investigation is based on this cold formed base material. Herein, the influence of the degree of cold
and own studies because it was shown that the method can effectively forming was investigated for both 25 mm thick duplex 1.4462 and lean
describe the fracture toughness behaviour of duplex stainless steel, duplex 1.4162 stainless steel plates.
particularly in the ductile-to-brittle transition temperature region [22]. The Charpy-V impact tests were conducted according to ISO 148-1
[33] on a pendulum motion “Charpy 450 J" from Zwick/Roell at Uni
3. Experimental investigation versity of Duisburg-Essen, Institute for Metal and Lightweight Struc
tures, Germany as shown in Fig. 1 (a) and (b). Charpy-V impacts tests
3.1. Material and preparation of the specimens offer a practical and worldwide accepted method to qualitatively assess
the toughness behaviour of a material as a function of the temperature.
Duplex stainless steels benefit from the properties of both austenitic The V-notch of the ISO 148-1 Charpy-V test specimens had a notch angle
and ferritic stainless steels. They are formed by a two-phase micro of 45◦ , a notch radius of 0.25 mm and a notch depth of 2 mm, see Fig. 1
structure with a phase balance of approximately 50% ferrite and 50% (c). For the welded test specimens, the test specimens were fabricated
austenite. The austenite contributes to a high corrosion resistance and with the notch placed in the heat affected zone (HAZ). For each test
high toughness at low temperatures and the ferrite provides high series, at least 21 tests were performed at different temperatures be
strength and resistance to stress corrosion cracking [12,26]. tween room temperature down to about − 196 ◦ C. Fig. 2 shows some
The more highly alloyed duplex stainless steels, as exemplary sample photos of fractured surfaces of duplex 1.4462 (25 mm) test
1.4462, display a very great corrosion resistance, especially to stress specimens at different temperatures.
corrosion cracking. Beside chrome, nickel is one of the most important Based on the experimental results, hyperbolic tangent curve fittings
elements in the composition of such duplex stainless steels which is used have been conducted based on the Oldfield regression model [34] to
as an austenite stabilizing element to increase the toughness [26,27–30]. achieve impact toughness transition curves, see Eq. (3). Herein, CVE is
However, high nickel prices have more recently led to a demand for the Charpy V-notch energy in J, T is the temperature in degrees Celsius
replacing the costly nickel by more cost-effective manganese and ni and A, B, and C are fitting constants. The applicable transition temper
trogen. Due to this cost factor, lean duplex stainless steels with lower atures criteria, like T27J or T40J, have been determined by means of the
nickel content started to receive a great deal of attention in recent de achieved fitting curves.
cades in the industry [31]. For this reason, it is very important to verify a [ ( )]
T− B
satisfactory fracture toughness for duplex stainless steels with low nickel CVE(T) = A • 1 + tanh (3)
C
content. Finally, in the presented study, 1.4462 and the lean duplex
stainless steels 1.4162 and 1.4662 were included in order to investigate The impact toughness transition curves for all different test series are
the influence of the nickel content on the impact and fracture toughness presented in Fig. 3 in different groups in order to show the influence of
2
N. Afzali et al. Journal of Constructional Steel Research 212 (2024) 108283
Table 1
Chemical composition (wt%) of base material and welding consumable.
Element
Cr Ni Mo Mn Si N P C
1.4462 (25 mm) 22.25 5.67 3.14 1.34 0.39 0.17 0.025 0.014
1.4462 (80 mm) 22.41 5.65 3.18 1.40 0.41 0.17 0.260 0.017
1.4162 (25 mm) 21.30 1.60 0.46 4.35 0.57 0.25 0.026 0.023
1.4162 (50 mm) 21.40 1.55 0.33 4.25 0.56 0.23 0.025 0.029
1.4662 (25 mm) 23.90 3.70 1.59 2.86 0.41 0.29 0.025 0.023
22 9 3 N L1 21.00–24.00 7.00–10.00 2.50–4.00 2.50 1.00 0.10–0.20 0.030 0.030
1
Welding consumable according to ISO 14343 [32].
Fig. 1. Pendulum impact testing “Charpy 450 J" machine and geometry of the Charpy-V specimen.
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N. Afzali et al. Journal of Constructional Steel Research 212 (2024) 108283
Fig. 2. Sample photos of fractured surfaces of Charpy-V test specimens of duplex 1.4462, 25 mm.
performed in a test rig containing a three-point loading fixture in a results and a cubic polynomial fitted equation based on experimental
cooling box. Using liquid nitrogen, the cooling box can be cooled down data presented by Ericsson et al. [24]. JIC was determined by fitting a
to a testing environment temperature of minimum − 150 ◦ C, see Fig. 4 power law, considering the data points between 0.15 mm and 1.5 mm
(b). offset lines parallel to the construction line, see Fig. 5 (c) and (d) and Eq.
The construction line has been calculated based on Eq. (5) and Eq. (5), with C1 and C2 are the power law coefficients. As it is shown in
(6) with σYS is the yield strength and σTS is the ultimate strength of the Fig. 5 (c) and (d), JIC was defined as the intersection point between the
material in MPa. fitted curve and 0.2 mm crack growth offset to the blunting line (con
The testing temperature could be held constant with ±0.2 ◦ C during struction line). Regarding the applicability of the Master Curve method
the entire testing. The SENB fracture toughness tests were carried out (ASTM E1921) to duplex stainless steels, it must be pointed out that
with a 100 kN hydraulic testing machine. The load was applied defining the critical initiation remains a challenge. The Master Curve
displacement controlled at a constant rate of 0.01 mm/s, while the crack method is traditionally designed for materials displaying a clear cleav
mouth opening displacement (CMOD) was measured with a clip-gauge. age initiation, posing an inherent difficulty when applied to materials
The compliance method was used to carry out the fracture toughness like duplex stainless steels, which do not undergo a fully cleaved fracture
tests. The SENB specimens were subjected to different loading/unload mechanism when the temperature decreases. For this reason, it is a
ing cycles whereby the unloading ratio was set to 50% of the actual challenge to define a precise critical initiation criterion for these mate
maximum load, see Fig. 5 (a). Up to now, fracture toughness tests were rials. Having SEM (Scanning Electron Microscope) images would help to
performed for 1.4462 stainless steel material of plate thicknesses 25 mm visually demonstrate semi brittle fracture despite the stable J-R curve,
and 80 mm at different temperatures. Fig. 5 (b) shows exemplary the particularly in the context of the 0.2 mm crack extension offset criterion.
load-CMOD diagram for duplex 1.4462 (25 mm) at − 150 ◦ C. This analysis is scheduled to be conducted in the near future. However,
Fig. 5 (c) and (d) exemplary show the J-Δa curves for 1.4462, 25 mm some studies (like [22]) show that the 0.2 mm crack growth offset to the
at − 150 ◦ C and 1.4462, 80 mm at − 80 ◦ C. Herein, J is the fracture en blunting line, although not synonymous with the critical initiation,
ergy per unit fracture surface area and Δa is the crack extension in mm. proves to be a suitable and consistent criterion. ESIS P1–92 [38], which
pertains to the assessment of fracture resistance in ductile materials, also
J = 2σY Δa (5)
endorses this criterion. This criterion correlates well with prior fracture
σYS + σ TS events, effectively capturing the fracture behaviour in duplex stainless
σY = (6) steels. By considering the discussed points and arguments, KJC was
2
calculated based on Eq. (6), where E is Young's modulus of duplex
To achieve the material properties, yield and tensile strength, tensile stainless steel at test temperature and ν = 0.3 is Poisson's ratio.
tests were carried out for 1.4462, 25 mm with standard tensile tests √̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
specimens at room temperature. All tensile tests were carried out in a EJ IC
200 kN servo hydraulic universal testing machine with a video exten KJC = (8)
(1 − ϑ2 )
someter to measure the specimen elongation according to ISO 6892-1
[36] requirements. Table 4 provides the material properties for All fracture toughness test results are presented in Table 5. The re
1.4462, 25 mm and 80 mm, 1.4162, 25 mm and 50 mm, and 1.4662, 25 sults show the direct influence of the temperature on the fracture
mm as achieved from own tensile tests or as given in the inspection toughness of the material, as with decreasing testing temperature, the
certificates 3.1 acc. to EN 10204 [37]. However, since the tensile fracture toughness decreases as well, see Table 5. Furthermore, from this
properties at low temperatures are essential for the evaluation of frac table, it can be observed that the plate thickness has a noticeable in
ture toughness test results, the Young's modulus as well as the yield and fluence on the fracture toughness of the material, too. The fracture
tensile strength of the material at each fracture toughness testing tem toughness results for the higher plate thickness are lower.
perature were obtained by considering the room temperature tensile test The results also show that the highest fracture toughness values were
4
N. Afzali et al. Journal of Constructional Steel Research 212 (2024) 108283
achieved for duplex 1.4462 base material with a thickness of 25 mm investigated test series. The results are added to the transition temper
because of its high nickel content in comparison to the other lean duplex ature correlation diagram according to EN 1993-1-10 in the range
materials. The specimens made from the thicker plates show lower limited by the lower and upper bounds considering a two-time standard
fracture toughness values for both duplex 1.4462 and lean duplex deviation (σ = 13 ◦ C), see Eq. (1). By adding the evaluated results, it
1.4162. The results obtained for the welded duplex material 1.4662 becomes obvious that for all test series, these values vary in the pre
show significantly lower fracture toughness in comparison to the base sented range. By calculating the standard deviation based on the actual
material. results for the base material of the different duplex stainless steel grades,
One of the objectives of this contribution was to prove the applica the standard deviation shows a rather small value of σ = 12 ◦ C. Based on
bility of the existing correlation between T27J and T100,exp and the val this observation, it can be concluded that the transition temperature
idity of the Master Curve concept for duplex stainless steels. Different correlation according to Eq. (1) is applicable to the investigated duplex
studies started to check the applicability of these concepts, see stainless steels.
[5,24,39]. As the experimental investigations in this project are ongoing In the frame of this investigation, the results were evaluated ac
and not finalized yet, in the frame of this study, the available results up cording to ASTM E1921 [40] and analyzed with regard to the Master
to the point of preparing this contribution were used for this validity Curve concept. The Master Curve concept was originally developed for
check. The T100,exp-temperature was calculated based on the fracture carbon steels [21] and the validity of this concept shall be checked for
toughness tests for all test series, see Table 5. The relationship between duplex stainless steels. The aim of this analysis was to observe the scatter
the transition temperature T27J resulting from Charpy-V impact tough of the actual individual fracture toughness test results (KJC) to check the
ness tests and T100,exp is presented in Fig. 6 (a) for the different validity of the Master Curve expressed by Eq. (2) for all fracture
5
N. Afzali et al. Journal of Constructional Steel Research 212 (2024) 108283
Fig. 4. SENB fracture toughness test specimen geometry according to ASTM E1820 and test setup.
6
N. Afzali et al. Journal of Constructional Steel Research 212 (2024) 108283
Fig. 5. Exemplary loading programme and J-Δa diagram for duplex 1.4462.
for the specimens fabricated from thicker plates for both duplex 1.4462 duplex stainless steels. Furthermore, the Master Curve approach pre
and lean duplex 1.4162. The results obtained from welded 1.4462 sented in ASTM E1921 which was developed for carbon steel material is
stainless steel plates show lower fracture toughness values for the heat also valid for the tested duplex stainless steels.
affected zone in compared to the base material.
The achieved test results show that the transition temperature cor
relation according to EN 1993-1-10 is applicable to the investigated
7
N. Afzali et al. Journal of Constructional Steel Research 212 (2024) 108283
Table 5
Fracture toughness test result.
1
Material Plate thickness Base material/HAZ Temperature JIC KJC T100,exp
[mm] [◦ C] [kJ/mm2] [MPa√m] [◦ C]
8
N. Afzali et al. Journal of Constructional Steel Research 212 (2024) 108283
Data availability Analysis, PVP-Vol. 170, American Society of Mechanical Engineers, ASME, New
York, USA, 1989, pp. 93–100.
[19] K. Wallin, Methodology for Selecting Charpy Toughness Criteria for Thin High
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research project "P 1390". Forskning, No. 4013/89, TO40–05, VTT Manufacturing Technology, Finland,
August 1990.
[20] K. Wallin, Methodology for Selecting Charpy Toughness Criteria for Thin High
Acknowledgement Strength Steels, part 1, 2 and 3, Jernkontorets Forskning, No. 4013/89, TO40–05,
VTT Manufacturing Technology, Espoo, Finland, 1994.
The authors thank Stiftung Stahlanwendungsforschung, Essen and [21] K. Wallin, Fracture Toughness of Engineering Materials: Estimation and
Application, EMAS Publishing, 2011. ISBN: 978-0-955299-4-6-9.
Forschungsvereinigung Stahlanwendung e. V. (FOSTA), Düsseldorf, [22] M. Faccoli, R. Roberti, Probabilistic Fracture Toughness of a Duplex Stainless Steel
both Germany, for the financial support of this project. A special thank in the transition Range, Eng. Fract. Mech. 97 (January 2013) 207–215.
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environment under applied cathodic potentials, J. Mater. Eng. Perform. 17 (2008)
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