2. Materials and Methods
AISI/SAE 304L (S30403) austenitic and AISI/SAE 420 martensitic (S42000) stainless steel plates with 3 mm thicknesses are prepared for TIG welding. Welding operation is applied within two passes.
The chemical composition of base metals from spectral analysis that applied in laboratories by AMETEK Spectromax Optical Argon Emission Spectrometer is given in
Table 1. Both of the base metals chemical compositions are within the required limit values as indicated in ASTM A240/A240M Standard [
12].
The effects of chemical compositions on weld metals microstructural and mechanical properties are intended to be investigated. Three different (ER312, ER316L and ER 2209) TIG welding rod compositions from manufacturers production analysis are listed in
Table 2.
The specimen couples are given in
Figure 1. All of the 420 martensitic stainless steel plates are heat treated at 300 °C in 45 min just before welding operation in order to decrease the cooling rates of weld region uniformly. As the cooling rate decreases in the weld region just after welding, hard and brittle phases like martensite can have less opportunity to form. Authors indicated that if the amount of martensite increases in the weld region, hardness increases but also failures and cracks occur at the same time in their research [
15]. 304L stainless steel plates were not heat treated before welding so that these alloys cannot be hardened by transformation of austenite to martensite phases as a result of their low carbon contents [
1,
2,
3,
4].
Argon with purity of 99.998% is used as shielding gas in welding operations. The weldments are also shielded by argon gas from the root sides. TIG welding is applied by ESAB TIG 4300i-AC/DC.
In TIG welding operation, the net heat input is estimated for both of the weld passes by
equation [
3,
4].
The symbols: ‘η’ indicates welding efficiency; ‘E’ is the welding voltage (volts); ‘I’ indicates welding current (Amperes); ‘V’ is the welding speed (mm/seconds).
‘η’ value is 70% (0.7) in TIG welding application for DC(−) current type [
4].
As the welding operation is applied within two passes; H
net is estimated for both root and final passes separately.
TIG welding parameters are listed in
Table 3.
The front and back side views of joined samples are given in
Figure 2.
After welding operation all of the weldments are post-weld heat treated for thermal stress relieving at 360 °C in 45 min inside a furnace as indicated in literature [
2].
After post-weld heat treatment, joints are machined for microstructural inspections, microhardness surveys and impact tests according to ISO 15614-1 standard as scheduled in
Figure 3.
Three impact test samples, one microstructural investigation with micro-hardness test specimen are prepared as given in
Figure 3.
Microstructural investigations were applied by Leica Brand optical microscope that can magnify up to 1000× capacity. The etchant is prepared by 20% NaOH solution as indicated in literature [
2,
3,
6]. Samples were electrolytically etched in NaOH solution by applying 2.5 volts and 1.6 amperes of current values.
The microvickers hardness test is applied to both 304L and 420 base metals and also with three individual places on weld metals and heat affected zones separately according to the EN ISO 9015-2 standard [
11] by 0.3 kg loading for 15 s at 22 °C constant laboratory temperature.
Welded martensitic stainless steels exhibit 4 distinct regions in their heat affected zones according to the distances to fusion zone from HAZ. These four regions exhibit different microstructures and hardness values in consequence of temperature distribution due to welding. The first region, just adjacent to the fusion boundary consists of mainly austenite and some ferrite at elevated temperatures during welding. Upon cooling as soon as welding finishes, dominant austenite phase transforms into martensite and delta ferrite remains in minor quantities. Hence, the maximum hardness values are determined in first region. As getting far away from the fusion zone to the base metal the temperature decreases. So the other next three zones in HAZ consists decreasing amounts of austenite that should be transformed into martensite upon cooling [
2].
In this study, the micro hardnesses on each HAZ sample is recorded by scanning the maximum values of micro hardnesses throughout the measuring line according to EN ISO 9015-2 Standard. The first indentation in HAZ is applied at a distance of minimum 0.3 mm away from fusion line and the following each other indentations are applied with minimum 0.3 mm intervals. Therefore, the maximum three values of microhardnesses are recorded in HAZ as shown in
Figure 4.
The width of weld metals and HAZ are approximately determined by macro imaging like shown in the middle macrograph of
Figure 4 as example. The width of the heat affected zones are measured approximately between 2 and 3 millimetres depending on the locations of weld regions. The approximate width value of HAZ and weld metal is estimated as 2.5 mm and 8.5 mm respectively.
Impact energy tests are applied on each two base metals and also on all welded specimens including weld metals according to the ASTM A370 [
9] and ASTM E23-12c [
10] standards at 22 °C laboratory constant temperature. Charpy impact test specimen is machined to 2.5 × 10 × 55 mm of sub-sized with a V-notch as given in
Figure 5.
Author Contributions
Conceptualization, A.B.B; methodology, A.B.B.; validation, A.B.B; formal analysis, A.B.B. and M.G.M.; investigation, A.B.B. and M.G.M.; resources, A.B.B. and M.G.M.; data curation, A.B.B. and M.G.M.; writing—original draft preparation, A.B.B and M.G.M.; writing—review and editing, A.B.B; visualization, A.B.B. and M.G.M.; supervision, A.B.B.; project administration, A.B.B.
Funding
This research received no external funding.
Acknowledgments
The authors express their thanks to Gazi University and Kırıkkale University, Metallurgical and Materials Engineering Departments Laboratories for their precious supports about testing instruments.
Conflicts of Interest
The authors declare no conflict of interest.
References
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Figure 1.
The dimensions of specimen couples for welding (mm).
Figure 2.
Welded specimens (a) front sides; (b) back sides.
Figure 3.
Test schedule on weldments according to ISO 15614-1 Standard.
Figure 4.
Microhardness test points
Figure 5.
Impact test specimen.
Figure 6.
304L base metal microstructure.
Figure 7.
420 base metal microstructure.
Figure 8.
Microstructures of sample welded by ER312 TIG Rod (a) 420 side-HAZ (Heat Affected Zone) and weld metal; (b) weld metal; (c) weld metal and HAZ-304L side (scale: 100 µm).
Figure 9.
Microstructures of sample welded by ER316L TIG Rod (a) weld metal HAZ-420 side; (b) weld metal; (c) 304L side-HAZ and weld metal (scale: 50 µm).
Figure 10.
Microstructures of sample welded by ER2209 TIG Rod (a) 420 side, HAZ and weld metal; (b) weld metal; (c) weld metal, HAZ and 304L side (scale: 100 µm).
Figure 11.
Microvickers test screen views of base metals (a) 420; (b) 304L (scale: 30 µm).
Figure 12.
The effect of TIG rod type on microhardness values of weld zones. Samples welded by (a) ER312; (b) ER316L; (c) ER2209 TIG Rods.
Table 1.
Spectral analysis of 304L and 420 stainless steel plates.
Material | Elements (weight %) | |
---|
C | Si | Mn | P | S | Cr | Mo | Ni | V | N | Fe | Others |
---|
304L | 0.0264 | 0.379 | 1.19 | 0.0211 | 0.0030 | 18.23 | 0.0542 | 8.01 | 0.102 | 0.0695 | 71.6 | 0.9968 |
420 | 0.235 | 0.506 | 0.628 | 0.0133 | 0.0021 | 13.36 | 0.0067 | 0.141 | 0.0418 | 0.0228 | 84.9 | 0.1433 |
Table 2.
Elemental production analysis values of TIG welding rods.
AWS A5.9 [13], EN ISO 14343-A [14] TIG Rods | Elements (weight %) |
---|
C | Mn | Si | Ni | Cr | Mo | Cu | N |
---|
ER312 | 0.15 | 1.6 | 0.4 | 8.8 | 30.7 | 0.2 | 0.14 | - |
ER316L | 0.01 | 1.7 | 0.4 | 12 | 18.2 | 2.6 | 0.10 | 0.04 |
ER2209 | 0.01 | 1.5 | 0.5 | 8.5 | 22.7 | 3.2 | 0.01 | 0.17 |
Table 3.
TIG welding parameters.
TIG Welding Rod | Welding Current DC(−) (Amperes) | Welding Voltage (Volts) | Pure Argon Shielding Gas Flow (l/min) | Welding Speed (mm/sec.) | Welding Heat Input (Joule/mm) | TIG Welding Electrode Type |
---|
(Ø2.0 mm) | Root Pass | 2nd. Pass | Root Pass | 2nd. Pass | Root Pass | 2nd. Pass | Root Pass | 2nd. Pass | Root Pass | 2nd. Pass | |
ER312 | 65–70 | 90–95 | 9 | 11 | 10 | 6 | 2.33 | 2.16 | 185.70 | 326.72 | WT 20 (red) (2% Thoriated) Ø2.4 mm |
ER316L | 2.29 | 2.18 |
ER2209 | 2.25 | 2.20 |
Table 4.
Micro-hardness values of weldments.
| Microvickers Hardness (HV0.3) |
---|
Test No. | Specimen welded by ER312 TIG Rod |
420 HAZ | Weld Metal | 304L HAZ |
1 | 480 | 269 | 222 |
2 | 478 | 261 | 218 |
3 | 482 | 265 | 220 |
Mean value | 480 | 265 | 220 |
Standard Deviation | 2 | 4 | 2 |
Test No. | Specimen welded by ER316L TIG Rod |
420 HAZ | Weld Metal | 304L HAZ |
1 | 485 | 204 | 191 |
2 | 471 | 185 | 196 |
3 | 483 | 206 | 207 |
Mean value | 480 | 198 | 198 |
Standard Deviation | 7.57 | 11.59 | 8.18 |
Test No. | Specimen welded by ER2209 TIG Rod |
420 HAZ | Weld Metal | 304L HAZ |
1 | 376 | 183 | 216 |
2 | 380 | 200 | 205 |
3 | 372 | 200 | 210 |
Mean value | 376 | 194 | 210 |
Standard Deviation | 4 | 9.81 | 5.51 |
Table 5.
Charpy impact energy values of base metal specimens.
Base Metal | Specimen No. | kgm | JOULE (kg·m2/s2) |
---|
304L | 1 | 3.80 | 37.278 |
2 | 4.20 | 41.202 |
3 | 4.10 | 40,221 |
4 | 4.00 | 39.240 |
Average Value | 4.03 | 39.485 |
Standard Deviation | 0.17 | 1.68 |
420 | 1 | 2.50 | 24.525 |
2 | 2.50 | 24.525 |
3 | 2.50 | 24.525 |
4 | 2.40 | 23.544 |
Average Value | 2.48 | 24.279 |
Standard Deviation | 0.05 | 0.49 |
Table 6.
Charpy impact energy values of welded specimens.
TIG Welding Rod Type |
---|
ER312 | ER316L | ER2209 |
---|
Specimen No. | kgm | JOULE (kg·m2/s2) | Specimen No. | kgm | JOULE (kg·m2/s2) | Specimen No. | kgm | JOULE (kg·m2/s2) |
---|
1 | 2.80 | 27.47 | 6 | 3.20 | 31.39 | 11 | 4.60 | 45.13 |
3 | 2.40 | 23.54 | 6 | 3.20 | 31.39 | 11 | 4.50 | 44.15 |
3 | 2.50 | 24.53 | 6 | 3.20 | 31.39 | 12 | 4.40 | 43.16 |
3 | 2.60 | 25.51 | 7 | 3.50 | 34.34 | 12 | 4.50 | 44.15 |
4 | 2.90 | 28.45 | 7 | 3.40 | 33.35 | 13 | 3.30 | 32.37 |
4 | 2.80 | 27.47 | 8 | 3.20 | 31.39 | 14 | 4.20 | 41.20 |
5 | 2.70 | 26.49 | 8 | 3.40 | 33.35 | 14 | 4.50 | 44.15 |
5 | 2.80 | 27.47 | 10 | 3.60 | 35.32 | 15 | 4.50 | 44.15 |
Mean Value | 2.69 | 26.36 | Mean Value | 3.3375 | 32.74 | Mean Value | 4.3125 | 42.31 |
Standard Deviation | 0.17 | 1.69 | Standard Deviation | 0.16 | 1.57 | Standard Deviation | 0.43 | 4.18 |
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