Materials & Design Materials and Design 28 (2007) 1898–1906 www.elsevier.com/locate/matdes
Effects of welding processes on the mechanical properties of HY 80 steel weldments P. Yayla *, E. Kaluc, K. Ural Mechanical Engineering Department, Engineering Faculty, Kocaeli University, 41040 Kocaeli, Turkey Received 25 July 2005; accepted 31 March 2006 Available online 23 June 2006
Abstract Different welding techniques are used in this study to evaluate the mechanical performance of weldments of HY-80 steel. Weldments are prepared using different welding processes such as shielded metal arc welding, gas metal arc welding, and submerged metal arc. The objective is to determine the optimum welding method for the steel. After welding, the effects of welding methods on weld metal microstructure and mechanical properties including weld metal tensile strength and Charpy V-notch impact toughness over the temperature range 20 to 20 C are investigated. Charpy impact and tensile tests are performed on standard notched specimens obtained from the welded and main sections of the material. The hardness distribution measurements on the differently welded specimens are conducted in order to gain a deep insight of different welding methods. 2006 Elsevier Ltd. All rights reserved. Keywords: High strength low alloy steel; HY 80 Steel; Heat affected zone; Weld toughness; Mechanical properties; Weldability
1. Introduction A group of low alloy steels designed to provide better mechanical properties and greater resistance to atmospheric corrosion than conventional carbon steels are known as ‘‘high strength low alloy’’ steels, or HSLA steels in short. This steel is also named as ‘‘Fine grained structural steels’’ in the European literature. Traditional welding design practices require the use of weld metal with higher yield strength than the base metal. For the steels with yield stress up to 350 N/mm2 the desired weld metal strength overmatch can be obtained without any special precautions. In the case of high strength steels with yield strength over 550 N/mm2, this high yield strength of weld metal often restricts the welding process by which an adequate weld metal toughness can be
*
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[email protected] (P. Yayla).
0261-3069/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.03.028
achieved. In such cases, a suitable welding method with a optimised welding procedure is likely to result in a weldment maintaining the structural integrity of welded structures. The HY-80 steel, getting its name from its minimum yield strength of 80 ksi or 550 N/mm2, is a high-strength low-alloy (HSLA) steel or fine grained structural steel used in quenched and tempered condition, with a combined tempered bainite an tempered martensite microstructure. Its many attractive properties, like good formability, weldability, and corrosion resistance, have made this steel a good selection for applications in many engineering and marine constructions, including submarine pressure hulls. Furthermore these steels are enticing: higher strength/weight ratio than conventional structural steels; and with modern mill processing, simplified lesscostly fabricating operations [1,2]. Despite these useful properties, the welding of this steel, when not critically controlled, has often posed problems, particularly in the shop floor conditions. According to their metallurgic
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Table 1 Steel composition used in this study Chemical composition
C
Ni
Cr
Mo
Si
Mn
Al
W
P
Cu
V
S
(%)
0.163
2.933
1.427
0.342
0.257
0.227
0.031
0.014
0.014
0.011
0.005
0.002
700
Heat input
600
577
500
20,7
623 25
20 400
Yield stress Yield strain
15
12,3
300
Yield Strain [%]
Weld voltage Filler material Filler mat. diam. Transition temp. Number of pass
: X-type V-type (the root distance 10 mm) 10–130 Amp MIL-E-10018 (Bo¨hler Fox U80N) 2.5/3.25 mm 150 C (Max) For X-type; 12 pass on the first, 15 pass on the second side For V-type 18 pass with a 4 mm thick backing plate 21,700 J/cm (Max)
Yield Stress [MPa]
Weld groove
30 658
636
Table 2 Welding parameter utilised in the preparation of the test samples
10 200
5,3
Submerged metal arc welding Weld groove X-type V-type (the root distance 10 mm) Weld voltage 450–500 Amp Filler material S3 Ni Mo 1 (UP-L 80Y) Filler mat. diam. 3 mm Transition temp. 150 C (Max) Weld speed 60–70 cm/min Shielding powder LW 330 Flux Number of pass For X-type; 6 pass on the first, 5 pass on the second side For V-type; 9 pass (first 2 SMAW) with 4 mm thick backing plate Heat input 21,700 J/cm (Max)
5
100
2,7
0
0 BASE METAL
SMAW
GMAW
SAW
Fig. 2. Yield stress and yield strain values for base metal and different weld types.
300
250
Impact energy [Joule]
Gas metal arc welding Weld groove X-type V-type (the root distance 10 mm) Weld voltage 180 –220 Amp Filler material AWS E 90 T5-G Filler mat. diam. 1.2/1.6 mm Transition temp. 150 C (Max) Weld speed 20–30 cm/min Shielding gas M 21 12–15 L/min Number of pass For X-type; 6 pass on the first, 6 pass on the second side For V-type; 18 pass with a 4 mm thick backing plate Heat input 21,700 J/cm (max)
200
150
100 SMAW GMAW SAW BASE METAL
50
0 -50
-40
-30
-20
-10
0
10
20
30
Temperature [˚C]
Fig. 3. Variation of Charpy impact test energy of the HAZ region with test temperature for the test samples taken 5 mm from the top surface.
Fig. 1. Extraction of Charpy impact samples from the weldments.
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characteristics, heat input of welding process significantly affects the heat affected zone (HAZ) mechanical properties.
300
Impact Energy [Joule]
250
200
2. Specimen preparation and welding applications
150
100 SMAW
50
GMAW SAW BASE META L
0 -25
-20
-15
-10
-5
0
5
10
15
20
25
o
Temperature [ C]
Fig. 4. Variation of Charpy impact test energy of the weld metal with test temperature for the test samples taken 3 mm from the top surface.
300
Impact Energy [Joule]
250
200
150
100
In this study, HY-80 steel is used as a main test material. The chemical composition of HY 80 steel used in this study is listed in Table 1. The steel has low carbon content to improve weldability and toughness properties. The test plate considered of 700 · 150 mm and 22 mm thickness, having X and V-type weld grooves are prepared for each conditions. These plates having the same type grooves are welded together using shielded metal arc welding (SMAW), gas metal arc welding (GMAW), and submerged metal arc (SAW) methods at flat position. Six samples are prepared and these samples are welded together with the aforementioned welding methods. As the effects of the welding parameters significantly affect weld mechanical properties [3], optimum welding parameters are used which are derived from industrial experience and the literature. These parameters are given in Table 2. Furthermore, for the preparation of the weld joints and welding procedure the MIL-STD-1688 [4] was taken as a reference.
SMAW
50
GMAW SAW
2.1. Tensile test samples
BASE META L
0 -25
-20
-15
-10
-5
0
5
10
15
20
25
o
Temperature [ C]
Fig. 5. Variation of Charpy impact test energy of the weld metal with test temperature for the test samples taken 6 mm from the top surface.
300
The tensile test samples having three rectangular dimensions of 5.0 · 12.5 mm are cut from the weldments. A special care is taken to have the weld zone at the middle of the tensile test samples and the weld section is kept vertical to longitudinal axis of the specimen. The samples are prepared and tested according to AWS B4.0 and ASTM E8M [5] standards. From at least three specimens for each test series, the average values for yield stress, rys, yield strain, ey, were deduced.
250
Impact Energy [Joule]
2.2. Charpy impact test samples 200
150
100
SMAW
50
GMAW SAW BASE META L
0 -25
-20
-15
-10
-5
0
5
10
15
20
25
o
Temperature [ C]
Fig. 6. Variation of Charpy impact test energy of the weld metal with test temperature for the test samples taken 9 mm from the top surface.
Test samples for Charpy V-notch (CVN) impact toughness evaluation are prepared according to the ASTM E 23 [6] standard. The samples were cut transversely to the weld, and 5 mm of the weld surface, with the notch normal to the weld surface. The extraction of the CVN tests samples from the weldments is done as indicated in Fig. 1. For one series of the tests, the notch was at the HAZ, and for the other the notch was at the middle of the weld metal. Carbon dioxide ice is used to obtain the minus temperatures for the impact tests. The CVN tests on base metal, weld metal and heat affected zone (HAZ) region are carried out at test temperatures of 20 C, 0 C and 20 C.
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2.3. Hardness measurements and microstructural examination For hardness measurements and microstructural examinations six samples of 10 · 20 · 80 mm dimensions are taken from the weldments. The sample preparation and hardness measurements were done according to the ASTM E92-82 [7] standard. The micro-hardness tests are con-
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ducted on cross-sections along a line 3 mm from the both surfaces of the plates at 0.5 mm intervals and at 9.81 N weight. Zwick 3212001 hardness testing machine was used. The hardness measurements are done in order to measure the degree of hardening along the base metal, HAZ and weld metal. For the microstructural examination, the surfaces of the samples are polished until the scratches on the
Fig. 7. Hardness profile across main material, HAZ and weld material regions for SMAW V test sample.
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cross-sections are eliminated well enough for the examination. The polished surfaces are then etched by 5% nital. The micrographs of the etched surfaces are utilised for the study of HAZ and the heat-treated zones between the weld passes. 3. Results and discussion 3.1. Tensile test The tensile tests are carried out on the samples using DARTEC Servo-hydraulic tensile testing machine. The tests are performed according to ASTM E8M [5] standard at ambient temperature. The yield stress and yield strain values obtained from these tests are given in Fig. 2. One of the significant outcomes of these tests is that in
all the tests the rupture occurred at the main material. As a result, the strength of both HAZ and weld material is not lower than that of the main material. The average yield stress and yield strain obtained from these welded samples are at the range of about 639 MPa and 6.8%, respectively. Regarding the measured yield strength and yield strain of the base HY 80 steel, 577 MPa and 20.7% values were obtained, respectively, which are compatible with the values given as 565–650 MPa (82.0–94.3 ksi) and %20 in MIL-STD-16216G [8]. 3.2. Charpy impact test At least three tests were carried out and the average values of tests were considered. The Charpy impact test consist of a pendulum, raised to a standard height, and
Fig. 8. Hardness profile across main material, HAZ and weld material regions for GMAW V test sample.
P. Yayla et al. / Materials and Design 28 (2007) 1898–1906
released to strike a standard specimen. The energy required to fracture the specimen is a measure of energy lost by the pendulum and named as Charpy impact energy. The Charpy impact tests results obtained from the main material showed rather good repeatability. In order to find out the Charpy impact energy of the HAZ, a number of tests samples for which the notch is on the HAZ, were carried out. These samples were taken 5 mm from the top surface. These results are given in Fig. 3, showing minimum impact energy for the GMAW and maximum for the SAW samples. These results are comparable with the results of Rittler and Dixon [1] who studied the Impact energy variation with a temperature ranging between 50 and 0 C for HY-80
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steel and observed almost a linearly increasing impact energy variation with temperature between 55 J and 125 J. The variations of Charpy impact energy with the test temperature for the samples extracted from the different sections of the weld plate are given Figs. 4–6. For all these tests, the Charpy notch is on the weld material. From these results it could be seen that the impact energy of weld metal varies significantly with the weld method, giving the minimum Charpy impact energy for the whole temperature range at the GMAW test samples. This is attributed to the elements reduction due to the oxidation effects of the gasses used in the GMAW welding. The highest Impact energy is observed on the SAW weld sections, mainly for
Fig. 9. Hardness profile across main material, HAZ and weld material regions for SAW V-type test sample.
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Fig. 10. Hardness profile across main material, HAZ and weld material regions for SMAW X-type test sample.
samples taken from 3 mm and 6 mm from the top surface of the weldments. Moreover, as shown in Fig. 6, higher impact energy is encountered in the SMAW joints taken from samples 9 mm from the top surface. This is attributed to the fact that every poses has a tempering affect in improving the mechanical properties of the previous passes [9,10]. 3.3. Hardness examination The hardness profiles across the welds and HAZ for different samples are shown in Figs. 7–12. Of all the test samples, the highest value of hardness was observed in the HAZ region. The main material had a hardness value of 235 HV. Regarding the hardness in weld region, in all the samples the region had hardness values higher than 275 HV. By contrast, the hardness values of the HAZ and weld regions were different for all the test samples. The maximum hardness up to a value of 425 HV was measured for SMAW and SAW weldments. The HAZ hardness of the GMAW sample was about %10 lover than the other two samples. These results are comparable with the results of Rittler and Dixon [1] who observed 350–400 HV hardness at the HAZ of SMAW of HY
80 steel. However, it has been known that for HAZ the same material can give the hardness values as high as 400 HV for GMAW and SAW weldments and this could still be acceptable. 4. Conclusions In this research, HY 80 steel of 22 mm thickness is used. X and V-type grooves are prepared for each condition. These plates are welded by using shielded metal arc welding (SMAW), gas metal arc welding (GMAW), and submerged metal arc welding (SAW) processes at flat position. Optimum welding parameters are used which derived from industrial experiences and the literature. Following welding, for each condition, tensile test specimens are extracted from the welded joint, Charpy-V test specimens and hardness test specimens are also prepared from the weld metal and base metal and also heat affected zone (HAZ) of the weldments. The present work has revealed that with the optimum welding parameters the HY80 steel could be welded effectively with the utilised welding methods without any post-weld heat treatment. However, the welding methods have remarkable effects on the fracture resistance and
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Fig. 11. Hardness profile across main material, HAZ and weld material regions for GMAW X-type test sample.
hardness of HAZ. In all the tensile tests carried out on the samples extracted from the weldments, the rupture occurred at the main material. These critical results were rather important, since the traditional welding design practices require the use of weld metal with higher yield strength than the base metal. The Charpy V-notch impact test results have shown that, due to higher heat input, the SAW and the SMAW specimens have given better HAZ toughness than the GMAW process. Moreover, the hardness test results have shown that the SMAW and SAW welding methods have given slightly higher hardness profile across welds metal and HAZ than the GMAW method on the section 3 mm below the top surface of the weldments. Particularly, the HAZ is transition zone on the welded joints and there is the risk of cracking along these
zones. The micro-hardness examination of the HAZ regions in all weldments reviled that The HAZ readings (390–430 HV) were consistently higher than both the base and weld metal readings. Although the hardness gradient varies from one method to another, the maximum hardness reaches up to the maximum values of 425 HV at the HAZ in all the methods. Regarding the weld metal, the similar trend is observed in the hardness profile, that is the hardness gradient varies from one method to another, the maximum hardness reaches up to the maximum values of 275 HV in the weld metal; which is well below than the HAZ hardness of 425 HV. In the roots of the weldments, the hardness distribution is lower than the upper surface of the weldments, which is mainly due to tempering effects of the filler passes.
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Fig. 12. Hardness profile across main material, HAZ and weld material regions for SAW X-type test sample.
Acknowledgements The authors thank Mr. O.N. Gungor and Mr. B. Cakici for their contributions to the experiments. Also the contributions of Dr. N. Sari and Dr. A. Arici on the sample preparation and hardness measurements have also influenced this work. The comments and critics of Dr. E. Engindeniz from Drahtwarenfabrik Drahtzug Stein GmbH & Co. of Germany is well appreciated. References [1] Ritter JC, Dixon BF. Improved properties in welded HY-80 steel for Australian warship. Weld J 1987;66(3):33–44. [2] Brosilow R. High-strength steels: a progress report. Weld Design Met Fabr 1991;64(11):40–4. [3] Sampath K, Civis DA, Kleinosky MJ. Effects of GMA welding conditions on high strength steel weld metal properties for ship
[4] [5] [6] [7] [8] [9]
[10]
structures. In: Proceedings of international symposium on low-carbon steels for the 90’s. Warrendale (PA, USA): Minerals, Metals & Materials Soc (TMS); 18–21 October 1993. p. 539–48. MIL-STD-1688 fabrication, welding and inspection of HY 80/100 submarine application. ASTM E8M-90a standard test methods for tension testing of metallic materials. ASTM E 23 standard test methods for notched bar impact testing of metallic materials. ASTM E92-82 standard test methods for vickers hardness of metallic materials. MIL-STD-16216G ship’s steel late, alloy, structural HY strength. Cakici B. Investigation of mechanical properties of HY 80 steel joints, welded by using arc welding methods, MSc Thesis. Kocaeli University, Graduate School of Natural and Applied Sciences; May 2002. Gungor ON. The effects of welding processes on the mechanical properties of the welded joint and HAZ for the quenched and tempered HY 80 high strength low alloy steel. MSc Thesis, Kocaeli University, Graduate School of Natural and Applied Sciences; October 1996.
