6. EXPERIMENTAL INVESTIGATIONS
6.6. Additional material test
To confirm that standard SFS-EN ISO 15164-1 test has shown realistic results, an additional material test had to be conducted. CTOD tests and microstructure analysis, like analysing different faces and micro hardness, were done. HAZs were also calculated and CTOD tests were done to all HSS steels. Additional microstructure tests were performed on QT HSS steel (steel E) and TMCP HSS steel (steel C).
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In order to check if the impact toughness values were correct, a CTOD test was made on all welded structures. Fig. 24 shows the construction of the welded pieces with dimensions of 8 x 15 x 50 mm. The 8 mm in thick, 50 mm length and 15 mm lateral pieces were cut from the whole plate. Using tack welds these pieces were welded together and the fusion faces were machined. The gap was about 1.5 mm and root edge was about 1.0 mm, while a single-bevel (½-V) groove was used with a flank angle of 45 degrees.
Fig. 25 illustrates the welded pieces before they were separated by saw. The beginning and end of the groove were made with assisting pieces. The test pieces with dimension 5 mm width, 10 mm high and 50 mm long were made by machining after cutting.
Figure 24. Used one side single be-vel (½-V) groove in CTOD tests.
Figure 25. CTOD test pieces after welding.
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In fig. 26 there is an etched CTOD test piece where the red line indicates the fusion line, the blue line indicates the start notch, the yellow line indicates the fatigue notch, and the green line indicates the test area. The groove was welded with three or four beads according to the heat input. The same three heat inputs (1.0; 1.3 and 1.7 kJ/mm) were used as in previous tests.
Figure 26. Etched CTOD test piece.
The CTOD test equipment was made by the Welding Technology Laboratory at Lappeenranta University of Technology. Fig. 27 illustrates the pusher and its counterpart, while fig. 28 is a picture of the actual machine used for the testing.
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Figure 27. CTOD test components and test piece.
COMPUTER AND SOFTWARE PUSHER AND ITS
COUNTERPART
COOLING UNIT
FATIGUE TEST MACHINE
Figure 28. CTOD test machine.
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The testing temperature was -40°C. Ethanol was used to guarantee the con-stancy of the temperature, while and temperature adjustments were made with the application or removal of dry ice. Fig. 29 illustrates the equipment at the -40
°C test temperature.
Figure 29. Isolated equipment at -40 °C and liquid intermediate test agent.
As the size of the CGHAZ is quite small, a study of this region is particularly difficult in real welds. Therefore, a thermal simulation was used to generate a relatively large region of CGHAZ, which allowed the notch to be reliably located in the correct microstructure. The steels were subjected to a welding thermal simulation. Thermal simulation test blanks were cut from the surface position of each plate, with the test piece axis transverse to the rolling direction, in T-L direction. Fig. 30 shows the test blanks, 8 x 17 mm in size. After the thermal simulation, these blanks were machined down to a 5 x 10 mm size appropriate for CTOD test pieces. The weld HAZ thermal simulations were performed on a Gleeble 3800 simulator, as the one shared in fig. 31, which is owned by the StPSPU.
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Figure 30. Test pieces proportion to rolling direction.
DIGITAL CONTROL SYSTEM
Figure 31. The Gleeble 3800 machine used in StPSPU laboratory.
Table 12.Welding parameters and cooling time.
Thermal cycle
The thermal cycles were calculated depending on the welding conditions (Table 12). When calculating of the temperature field, the following assumptions were made: a point heat source on the plate surface moves along the x-axis with
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constant speed v, the origin of coordinates is fixed to the source, the plate surfaces are heat impermeable, and the plate is infinitely wide and long. Then the steady state of the temperature field T(x,y,z) in the moving reference frame is expressed by the following formula:
2 through the plate thickness and changes from coordinate x to time t is made according to the equation: t = - x/v. Then the thermal cycle of any point y, z at any time t can be calculated:
2
This formula was used to calculate the thermal cycle of the point having peak temperature Tmax = 1350°C at the top surface (z = 0). Three cycles are shown in
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The first heat input was 1.0 kJ/mm and was applied to an 8 mm thick plate. This involved heating to a peak temperature (Tp1) of 1350 °C at a rate of approximately 450 °C/s and holding the peak temperature for less than 2 s, followed by a cooling time from 1350 °C to 800 °C for 10 seconds, between 800
°C to 500 °C (∆t8/5) in 20s, and from 500 °C to ambient temperature in 40 seconds.
The second heat input was 1.3 kJ/mm and was applied to an 8 mm thick plate.
This involved heating to a peak temperature (Tp1) of 1350 °C at a rate of approximately 450 °C/s and holding at the peak temperature for less than 2 s, followed by a cooling time from 1350 °C to 800 °C in 15 seconds, between 800
°C to 500 °C (∆t8/5) in 35 s and from 500 °C to ambient temperature in 65 seconds.
Finally, the third heat input was 1.7 kJ/mm and was applied to an 8 mm thick plate. This involved heating to a peak temperature (Tp1) of 1350 °C at a rate of approximately 450 °C/s and holding at the peak temperature for less than 2 s, followed by cooling time from 1350 °C to 800 °C in 20 seconds, between 800 °C to 500 °C (∆t8/5) in 55 s and from 500 °C to ambient temperature in 80 seconds.
In simulation, which occurred in a Gleeble 3800 machine between watercooled copper made grip jaws, the non-standard Gleeble specimen has been heated and cooled, as seen in figs. 33 a and b.
a) b)
Figure 33. The 5x10 grips jaws (a) and non-standard Gleeble specimen (b).
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CTOD test pieces were produced from the thermal simulated test blanks with a 2.5 mm deep through-thickness notch cut in the sample. The position of the notch was in the center of the etched HAZ. The notch orientation was such that the crack propagation direction was parallel to the plate rolling direction, as seen in fig. 26, T-L direction. A fatigue crack of 2.5 mm nominal depth was then grown into the specimen, giving a nominal a/W (overall crack depth/ specimen width) value of 0.5. The CTOD samples were then tested at -40 °C, following ASTM E 1290-02 standard, to produce impact toughness.
The equation in standard ASTM E 1290-02 for CTOD value δ is given as:
𝛿=𝑚𝜎1
where σY = effective yield strength at the temperature of interest
σYS = yield or 0.2 % offset yield strength at the temperature of interest σTS = tensile strength at the temperature of interest
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𝐾=𝐵√𝑊𝑌𝑃 (13)
where K= stress intensity factor
P = force corresponding to Pc, Pu or Pm (See fig. 34) Y= Stress Intensity coefficient
𝑌=6�
𝑎0𝑊×�1.99−𝑎0𝑊�1−𝑎0𝑊��×�2.15−3.93𝑎0𝑊+2.7�𝑎0𝑊�2�
�1+2𝑎0𝑊�×��1−𝑎0𝑊�3
(14)
Constraint m in equation 11:
𝑚= 1.221 + 0.793𝑎𝑊0+ 2.751(𝑛)−1.418�𝑎𝑊0�(𝑛) (15) where
𝑛= 1.724−6.098𝑅 +8.326𝑅2 −3.965𝑅3 (16)
where
𝑅=𝜎𝜎𝑇𝑆
𝑌𝑆 (17)
𝑎
𝑊 function η in equation 11:
𝜂= 3.785−3.101𝑎𝑊0+ 2.018�𝑎𝑊0�2 (18)
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Figure 34. Types of Force versus Clip Gage Displacements Records (ASTM E 1290-02).
6.6.2. Compared microstructure examination
An additional test on the microstructure was conducted using a high-resolution microscope. The test results illustrate the microstructure differences between QT and TMCP steels. These tests also show the HAZ microstructure and the zone difference. QT HSS steel, steel E, has been investigated as a typical QT HSS steel and steel C has been investigated as a typical TMCP HSS steel. This test was conducted at StPSPU.
Specimen preparation included following techniques: sectioning, mounting, grinding, polishing, etching. Abrasive cut-off machine Buehler Powermet 3000 was used for sectioning. Mounting was performed on Buehler Simplimet 1000 mounting press in Epomet and Transoptic mounting resins.
Buehler Phoenix 4000 was used for grinding and polishing of the specimens.
Grinding was undertaken with a set of SiC abrasive papers starting out with the roughest (P180) and gradually introducing the finest (P400). Polishing materials were the diamond suspensions with particles ranging from 9 to 1 μm, alumina suspension 0.01 μm.
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Revealing of microstructure was conducted by etching of the specimens in nital e.g. 4% solution of HNO3 in ethanol.
The examination of microstructure was made using the light metallographic microscope LEICA DMI5000M with magnification up to x1000. Acquisition of images was performed by digital camera LEICA DFC320 attached to the microscope, which has 3 MPix image sensor. LEICA Application Suite software was used for enhancement and analysis of captured images. Image analysis provided accurate means for determining grain size according to ASTM E112.
Stereomicroscope LEICA Mz12.5 was used for examination of macrostructures of welded joints.
Hardness measurement was conducted on Vickers hardness tester Wilson Wolpert 452SVD according to ISO6507. Microhardness of single phases or tiny constituents were measured by microhardness tester Wilson Wolpert 402MVD with diamond pyramid indenter under load of 0.0025 N.
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