NEW EQUIPMENT FOR LIQUID METAL EMBRITTLEMENT
CHARACTERISATION FOR VERTICAL AXIS TESTING MACHINES
Carpio, J., Álvarez, J.A., Casado, J.A., Gutiérrez-Solana, F.
División de Ciencia e Ingeniería del Terreno y de los Materiales (LADICIM)
E.T.S.I. Caminos, Canales y Puertos
Avda. de los Castros, s/n
University of Cantabria
39005 Santander (Cantabria)
SPAIN
ABSTRACT
This paper shows the design, validation and test results of a device for the performance of mechanical tests on specimens
disposed in horizontal position when the available testing machines apply vertical loads. The device in this paper solves the
specific problem of developing tensile and fracture toughness tests of specimens in liquid Zn at 450ºC when there is only a
vertical load machine over a flat surface. The paper begins with the explanation of the problem and the needed equipment.
Secondly, the proper solution and the design of the test setup are explained, including dimensional calculation and strength
distribution for this specific case. After that, room temperature validation tensile tests were performed, in order to know
correction factors to apply. Finally, three different tests were conducted on CT specimens and notched tensile specimens to
obtain toughness and metallurgical information.
Introduction
Hot dip galvanization is a very common technique used worldwide to protect structural steels versus corrosion. It consists of
the immersion of the steel piece into a liquid zinc bath, usually at 450ºC. The corrosion protection is achieved by three
mechanisms:
1.- The zinc and its oxides act as a barrier protection against oxidation.
2.- The zinc acts as a sacrificial anode that avoids iron oxidation when the barrier is broken.
3.- If the zinc barrier has little defects, the zinc oxide can cover them, avoiding the corrosion cell.
During immersion in liquid Zn, some reactions between iron and zinc occur, and Zn-Fe compound layers appear on the steel
base. So the steel base and zinc are metallurgically joined, not joined by adherence [1].
Occasionally, cracking during galvanizing of large structural steels occurs. Cracks appear in points with high residual stresses
from fabrication process. These cracks can be very big, and they affect the structural integrity of the components. Figure 1
shows a clear example of a big crack.
1,80 m
Figure 1. Example of failure during galvanizing.
Those kind of failures were very occasionally, but there became more frequent when low melting point elements, such as Sn
and Bi, were added to traditional liquid Zn baths, which contained Zn and ≈ 1% of Pb. Research projects have been launched
in Japan, United States and Europe to find the solution to this problem [2], but not clear conclusions have been obtained. One
of the last research projects about this topic is FAMEGA (Failure Mechanisms during Galvanizing), supported by the European
Commission, and which main goal is to know the failure mechanisms during galvanizing and to give recommendations to the
industry in order to avoid them.
One of the main tasks of this work is to establish the cracking micromechanisms during galvanization. In order to get this
achievement, tensile and toughness tests have to be conducted in liquid Zn, at 450ºC, the galvanization temperature. Usual
mechanical testing machines apply vertical loads. There are horizontal testing machines, but generally their design and size
are not appropriate to place under them a furnace with liquid Zn at 450ºC. Other possible solution is to use little crucibles
(usually made of graphite) which can be adapted to the specimen. The setup specimen-crucible is introduced in a chamber
that can control the temperature. See an example in Figure 2.
Figure 2. Example of crucible surrounding a specimen for liquid metal embrittlement tests [3].
This solution is not very appropriate when there are different kinds of specimens, and it reduces the possibilities of introducing
extensometers and other devices to measure strain. Thereby, it was thought that a device that allows test specimens in a
horizontal position using vertical axis testing machines was necessary. This device should protect the testing machine against
the hot and the contact to liquid Zn.
Solution: Crucible furnace and device for liquid metal embrittlement tests
To solve the explained technical problem, the first step was to choose the testing machine, and to determine the necessary
space to place the furnace with the liquid Zn and the device to transmit the load from the testing machine to the immersed
specimen.
The chosen apparatus was an INSTRON static and dynamic testing machine, H1730 model, with a capacity of 100 kN, placed
over a flat surface. See Figure 3.
Figure 3. INSTRON static and dynamic testing machine, H1730 model.
The available space under the machine is 108 cm height, 93 cm width and the length was totally free. The furnace is a crucible
one for low melting point metals (until 600ºC). It has got a cylindrical shape, its external dimensions are 55 cm height and 42
cm diameter, and the internal dimensions are 42 cm height and 24 cm diameter. The capacity of the used stainless steel
crucible is 48-50 kg of liquid Zn. Figure 4 shows a photograph of the furnace.
Figure 4. Crucible furnace for Zn melting.
The height between the furnace and the testing machine was 53 cm. Other conditions that had to be accomplished by the
testing device were [4]:
1.- Transmit the load to the specimen without excessive rubbing.
2.- The device material must have appropriate mechanical properties at 450ºC, galvanization temperature.
3.- Much more tough than the specimen.
4.- Load on the specimen has to be easily calculated.
5.- Space and conditions for COD or extensometer placing have to be permitted.
6.- Flexibility refers to different specimens, test conditions or even liquid metal.
The solution obtained was a metallic device similar to a “compass”, with two arms hold to an upper pin hole. In the lower part
there are two U-shaped beams, joined to the specimen and to the lower part of the device arms. This device design
guarantees that the only part which is immersed in the liquid Zn is the specimen and a part of the U-shaped beams, but not the
testing machine. See the design in Figure 5.
Figure 5. Drawings of the experimental testing device.
The experimental device is put on tracks, with bearings that roll on a surface covered with Teflon® (polytetrafluoroethylene), to
minimize rubbing. The joining between the tensile specimens and the device is direct, screwing the specimen into a hole in the
proper place of the U-shaped beams. The joining of the CT and other toughness specimens and their clevis is got with proper
screws and threads. Observe the final result in Figure 6.
Figure 6. Experimental testing device mounted with a notched tensile and a compact tensile (CT) specimen.
The material used for the experimental device was the EN 10028:2 X10CrMoVNb 9-1 +NT (1.4903) steel, with a yield stress of
349 MPa at 450ºC.
Engineering design
This section pretends not to exactly size the experimental device, but to develop the strength calculation required to know the
load applied to the specimen in function of the load applied by the testing machine, and we also pretend to give criterions to
evaluate if the device will not suffer plastic deformation in its critical points. Strength distribution is presented in Figure 7.
Figure 7. Strength distribution in the experimental device for testing in liquid Zn.
Due to the symmetry of the device, we get R = R1 = R2 and H = H1 = H2. These strengths are calculated by Eq. (1) and (2):
where
R=
F
2·senα
(1)
H=
F
2·tan α
(2)
α= angle between the device arms and the horizontal plane
There are two critical sections in this device:
a)
The lower pin hole that joins together the arm with the U-shaped beam, and the remained section of the arm and Ushaped beam closed to this pin hole (Figure 8).
b)
The corners of the U-shaped beams (Figure 9).
Figure 8. Connection point between the arm, the U-shaped beam and the lower pin hole of the device.
d
Figure 9. Scheme of a corner of the U-shaped beam.
The criterions to obtain the applied stress in the critical sections are extracted from the Eurocode 3 [5]. Those criterions are the
following:
- In the lower pin hole: Figure 8.
1) The shear strength Q and the bending moment M in the section of the lower pin hole fit the following equation:
⎛ M
⎜
⎜M
⎝ u ,b
where
2
⎞ ⎛ Q
⎟ +⎜
⎟ ⎜Q
⎠ ⎝ u ,b
2
⎞
⎟ ≤1
⎟
⎠
(3)
Q = 0,5·N
M = N· (t1+4c+2t2)/8
Mu,b = pin hole ultimate bending moment = 0,8·Wel·fy/γMp,
Qu,b =pin hole ultimate shear moment = 0,6·A·fu,b/γMp,
Wel = pin hole plastic section modulus
A = Cross-sectional area
γMp = Safety coefficient = 1,25
fy = Yield stress
fu,b = ultimate strength
2) There is no flattening between the lower pin hole and the remaining sections of the arm and U-shaped beam.
N<
where
1,5·t ·d · f y
γ Mp
(4)
t = minimum between t1 and 2·t2 in Figure 8.
- In the corner of the U-shaped beam: Figure 9.
1) There is no bending of the U-shaped beam:
σ=
where
M
·y ≤ σ y
I
σ = stress; σy = yield stress
I = Moment of inertia
y = Extreme fiber distance
M = Beam bending moment = H·d
(5)
Device validation
After the design of the testing device for immersed specimens, the next step is its validation. The tests performed to validate
the device were tensile tests. The experimental setup is shown in Figure 10, where the furnace is substituted by a horizontal
beam. The results obtained with the device were compared with those obtained in a universal testing machine. Two different
steels were tested:
-
EN 10025:2 S460N steel: 3 specimens tested with universal testing machine and 3 specimens tested with the device.
-
EN 10025:2 S355ML steel: 3 specimens tested with universal testing machine and 2 more tested with the device.
Figure 10. Experimental setup for the validation of the device for liquid Zn testing.
The results obtained for both of the steels are displayed in Figure 11 and in tables 1 and 2. A little loose of load by rubbing is
detected, but is proportionally constant, and a factor of 0.96 can correct effectively the different load obtained.
700
600
600
Universal
testing
machine
500
Device
Device
500
Universal testing
machine
400
400
300
300
SpecimenS460N-B1
Specimen S460N-B2
Specimen S460N-B3
Specimen S460N-B4
Specimen S460N-B5
Specimen S460N-B6
200
Specimen S355ml-b1
Specimen s355ml-b2
Specimen s355ml-b3
Specimen s355ml-B4
Specimen s355ml-B5
200
100
100
0
0
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
0,16
0
0,05
ε (basis: 25 mm)
0,1
0,15
0,2
0,25
ε (basis: 25 mm)
Figure 11. Stress-Strain graph of EN 10025:2 S460N and S355ML steel specimens tested in a universal testing machine and
the device for liquid Zn testing.
Table 1. Results for S460N steel validation tests.
S460N steel
U.T.
tests
Machine
Yield stress
512 ± 4
(Average, MPa)
Ultimate
strength
648 ± 6
(Average, MPa)
Device
Correction
factor
532 ± 3
0,96 ± 0,02
672 ± 1
0,96± 0,01
Table 2. Results
Table 2.for
Results
S355ML
for steel
S355ML
validation
steel validati
tests. on tests
S355ML steel
tests
U.T.
Machine
Yield stress
441 ± 10
(Average, MPa)
Ultimate
strength
519 ± 15
(Average, MPa)
Device
Correction
factor
460 ± 2
0,96 ± 0,03
544 ± 3
0,95 ± 0,04
Tests performed with the new device
The different tests have been performed for the FAMEGA project with the device described in this paper:
1) Toughness tests in liquid Zn with CT specimens of EN 10025:2 S450J0 steel.
2) Toughness tests in liquid Zn with notched tensile specimens made of EN 10025:2 S450J0 steel.
3) Interrupted tests in liquid Zn with CT specimens EN 10025:2 S460M steel.
The experimental setup is very similar for the three tests, and is shown in Figure 12. In Figure 13 appears a scheme of the CT
and notched tensile (axilsymmetric) specimens. These specimens have to pass the usual surface treatments previous to
galvanizing (washing, pickling and fluxing) [6].
Figure 12. Experimental setup for tests in liquid Zn.
Figure 13. Scheme of CT and notched tensile specimen to be tested in liquid Zn.
The objective of tests number 1 and 2 were to obtain the different material behaviour in liquid Zn, compared with the behaviour
in air at room temperature and 450ºC. The results of that comparison are displayed in Figure 14. There is a clear
embrittlement effect in liquid Zn.
The objective of test number 3 was to apply stress to a CT specimen without breaking it (F = 6.93 kN Æ H = 3.4 kN, with a
correction factor of 0.96) and study if there were cracking initiation. The observation by means of an electronic microscope in a
longitudinal section of the CT sample confirmed that there were cracking initiation, and the galvanizing layer composition in the
crack tip was very much richer in Sn than the original Zn bath. See the micrographs in Figure 15.
12
25
10
20
8
Air
15
Air
6
10
450ºC
4
450ºC
5
2
Liquid Zn
Liquid Zn
0
0
0
0,2
0,4
0,6
COD(mm)
0,8
1
0
1,2
0,02
0,04
0,06
0,08
0,1
0,12
0,14
Δφ (mm)
a)
b)
Figure 14. a) Toughness tests (J) of EN 10025:2 S450J0 steel in air at 20 and 450ºC and liquid Zn; b) Tensile tests at slow
strain rate of EN 10025:2 S450J0 steel in air at 20 and 450ºC and liquid Zn.
300 μm
Sn
Zn
Figure 15. Micrographs of a crack tip promoted by embrittlement in a CT specimen of EN10025:2 S460M steel.
Conclusions
This paper has shown the design, validation and an example of use of a device for mechanical tests of horizontally disposed
specimens when the available testing machines only apply vertical loads. This paper’s designed, validated and used device
had the main goal of testing steel specimens that are immersed in liquid Zn at 450ºC, but the sizing, the device material and
some aspects of the design can be easily adapted to each particular case. Validation tests are always necessary in order to
obtain the correction factor (0.96 in this paper). The designed device can be used for toughness, tensile and even fatigue tests
in aggressive environments, with different kinds of specimens. Finally, the device can be improved to avoid loads in not
desired directions introducing flexible articulations in the joints of the U-shaped beams with the specimens (it was not
necessary in the device of this paper).
Acknowledgments
This research is included in the FAMEGA project, sponsored by the European Commission, the Spanish Science and
Education Ministry and Cantabria Government. The authors are grateful to Boris Donnay (ARCELOR Inc.) and Bill Rudd
(Corus UK) for the material supply.
References
1.
2.
3.
4.
5.
6.
Marder, A.R. “The Metallurgy of Zinc-Coated Steel”, Prog. Mat. Science, 45. pp. 191-271.
Kinstler, T.J. “Current Knowledge of the Cracking of Steels during Galvanizing”
http://www.aisc.org/Content/ContentGroups/Engineering_and_Research/Research1/Final5906.pdf. Galvascience LLC, 79
pag. (2005)
Clegg, R.E., Jones, D.R.H. “Liquid metal embrittlement of tensile specimens of En19 steel by tin”. Eng. Fail. Anal., 10,
119-130 (2003).
Carpio, J., Álvarez, J.A., Casado, J.A., Gutiérrez-Solana, F. “Diseño y Validación de un utensilio para ensayos de
tracción y fractura de probetas sumergidas en Zn líquido.” Anales de Mecánica de la Fractura, 23-I, 45-50 (2006).
“Steel Structures Project. Part 1.1” Eurocode 3. CEN (1996).
Wetzel, D. “Batch Hot Dip Galvanized Coatings”, ASM Handbook. Volume 5: Surface Engineering. ASM International,
360-371 (1994)