DMR 249 A
DMR 249 A
DMR 249 A
Abstract.DMR249A Medium strength (low carbon) Low-alloy steels are used as structural
components in naval applications due to its low cost and high availability. An attempt has been
made to weld the DMR 249A steel plates of 8mm thickness using shielded metal arc welding
(SMAW) and gas tungsten arc welding (GTAW). Welds were characterized for metallography
to carry out the microstructural changes, mechanical properties were evaluated using vickers
hardness tester and universal testing machine. Potentio-dynamic polarization tests were carried
out to determine the pitting corrosion behaviour. Constant load type Stress corrosion cracking
(SCC) testing was done to observe the cracking tendency of the joints in a 3.5%NaCl solution.
Results of the present study established that SMA welds resulted in formation of relatively
higher amount of martensite in ferrite matrix when compared to gas tungsten arc welding
(GTAW). It is attributed to faster cooling rates achieved due to high thermal efficiency.
Improved mechanical properties were observed for the SMA welds and are due to higher
amount of martensite. Pitting corrosion and stress corrosion cracking resistance of SMA welds
were poor when compared to GTA welds.
1. Introduction
DMR 249A steel is one of the prestigious grade of steel and developed indigenously for structural
applications in hull and body of warships and submarines [1,2]. Medium strength (low carbon) Low-
alloy steels exhibit excellent mechanical properties and are extensively used in offshore applications
and construction/repair of naval ships, where corrosion resistance against marine environment is
significantly required. Poor corrosion resistance of low alloy steel fails the entire ship during the
service. DMR 249A steels have high strength and are easy to weld [3,4]. Conventional fusion welding
processes i.e., gas tungsten arc welding (GTAW), submerged arc welding (SAW) are used for
construction and fabrication of naval ships and bridges [5]. For onsite repairs of ship body and hulls
are manually welded using shielded metal arc welding process[6]. Studies on the stress corrosion
cracking (SCC) behavior of HSLA steel showed high susceptibility to stress corrosion cracking and
due to high strength [7,8]. The welding process may lead to change in the original microstructure of
the alloy due to welding thermal cycles which can affect the localized corrosion behavior of the alloy.
During welding, it resulted in some amount of welding defects during welding and residual stresses in
welded components [9]. Thus a risk of stress corrosion cracking (SCC) will occur during service. The
main hazardous risk is that SCC always causes unexpected brittle failures without any externally
visible indication, which will significantly restrict its application in the marine environment. Cathodic
protection system has been used to protect the HSLA steel from corrosion and improve service life in
the offshore and naval ships. In view of the above problems, an attempt has been made to study on
DMR249A steel welds and to compare and correlate the microstructural changes with observed
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ICRAMMCE 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 330 (2018) 012018 doi:10.1088/1757-899X/330/1/012018
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mechanical properties and corrosion resistance for the DMR 249A steel welds made with shielded
metal arc welding (SMAW) and gas tungsten arc welding (GTAW) process.
2. Experimental Details
DMR 249A Low alloy steel plates of 300X150X8mm3 were used in the present study. Chemical
composition of the base metal and filler wire/electrode are given in Table 1. Welds made with
Shielded metal arc welding (SMAW) and Gas Tungsten arc welding (GTAW) are shown in Fig.1.
Microstructure studies were characterized at various zones of the welds using optical microscopy.
Micro-hardness measurements were carried out with a load of 0.5Kgf for 20 seconds along the
longitudinal directions of the weld as per ASTM E384-09. Tensile testing is carried out using a
universal testing machine at room temperature as per ASTM-E8. Pitting corrosion resistance of base
metal and welds were determined using potentio-dynamic polarization testing in 3.5%NaCl solution
using a basic electrochemical system. Constant load Stress corrosion cracking (SCC) testing was
carried out with applied stress of 50% yield strength and in 3.5% NaClsolution.
Fig. 1 DMR 249A steel welds made with (a). SMAW and (b). GTAW
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ICRAMMCE 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 330 (2018) 012018 doi:10.1088/1757-899X/330/1/012018
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cooling rate of welding processes and more amount of martensite is formed in SMAW welds when
compared to that of GTA welds.
Fig 2 Optical Microstructures of different regions of DMR 249A steel and its GTA welds
Fig 3OpticalMicrostructures of different regions of DMR 249A steel and its SMA welds
3.2. Hardness Studies
Hardness testing (VHN) results of the welds are given in Table2. In the weld region hardness
of GTAW is slightly lower than that of SMAW. In the HAZ region, hardness in SMAW is slightly
higher than that of GTAW. In both welds, hardness of weld region is much higher when compared to
base metal. This may be attributed to the observed martensite formation in the weld region. HAZ
hardness was found to be lower than weld region mainly because of bainite and tempered martensite.
Relatively higher hardness of SMAW welds is due to more amount of martensite which forms because
of faster cooling rate.
Table 2 Average Vickers Hardness values of various zones of DMR 249A welds.
Zone/Region SMAW GTAW
Base Metal 167 VHN 167 VHN
HAZ 201 VHN 204 VHN
Weld Zone 216 VHN 210 VHN
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ICRAMMCE 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 330 (2018) 012018 doi:10.1088/1757-899X/330/1/012018
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Table 3 Tensile test data of DMR 249A low alloy steel welds
Tensile Yield
S.No Material % Elongation
Strength (MPa) Strength(MPa)
1 BASE METAL 605.49 400 39.06
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ICRAMMCE 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 330 (2018) 012018 doi:10.1088/1757-899X/330/1/012018
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ICRAMMCE 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 330 (2018) 012018 doi:10.1088/1757-899X/330/1/012018
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ICRAMMCE 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 330 (2018) 012018 doi:10.1088/1757-899X/330/1/012018
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Table 4 SCC test data for Base metal, GTAW and SMAW specimens
Material Constant Load (MPa) Time to SCC Failure
(50% Y.S) (Hours)
Base Metal 200 1050 (44 Days)
SMAW 215 860 (36 Days)
GTAW 205 1224 (51 Days)
4. Conclusion
1. DMR 249A steels are successfully welded using shielded metal arc welding and gas tungsten arc
welding process and obtained defect free weld joints.
2. In both the welds, formation of martensite is observed in the weld region and is due to faster cooling
rates whereas partly bainite and martensite is observed in heat affected zone.
3. Relatively coarse and dendritic martensite was observed for the welds made with SMAW and is due
to high heat input and thermal efficiency.
4. Improved mechanical properties are observed due to the presence of high amount of martensite in
SMA welds when compared to GTA welds.
5. Pitting corrosion and SCC resistance of DMR 249 steel welds was found to be sensitive to welding
process. Cooling rate of weld depends on Heat input and heat efficiency of welding process.
6. GTA welding due to its low heat efficiency results in slow cooling rate and less amount of
martensite. Better pitting corrosion and SCC resistance of DMR 249 welds is attributed to the less
number of ferrite/martensite interfaces which are sources of corrosion initiation.
References
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