Nothing Special   »   [go: up one dir, main page]

DMR 249 A

Download as pdf or txt
Download as pdf or txt
You are on page 1of 8

IOP Conference Series: Materials Science and Engineering

PAPER • OPEN ACCESS Related content


- Effect of Welding Process on
Comparative Studies on microstructure, Microstructure, Mechanical and Pitting
Corrosion Behaviour of 2205 Duplex
mechanical and corrosion behaviour of DMR 249A Stainless Steel Welds
Raffi Mohammed, G Madhusudhan Reddy
and K Srinivasa Rao
Steel and its welds
- Studies on microstructure, mechanical and
pitting corrosion behaviour of similar and
To cite this article: Raffi Mohammed et al 2018 IOP Conf. Ser.: Mater. Sci. Eng. 330 012018 dissimilar stainless steel gas tungsten arc
welds
Raffi Mohammed, Dilkush, K Srinivasa
Rao et al.

- Comparative Studies on Microstructure,


View the article online for updates and enhancements. Mechanical and Pitting Corrosion of Post
Weld Heat Treated IN718 Superalloy GTA
and EB Welds
Dilkush, Raffi Mohammed, G
Madhusudhan Reddy et al.

This content was downloaded from IP address 185.89.100.231 on 27/10/2018 at 14:02


ICRAMMCE 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 330 (2018) 012018 doi:10.1088/1757-899X/330/1/012018
1234567890‘’“”

Comparative Studies on microstructure, mechanical and


corrosion behaviour of DMR 249A Steel and its welds

Raffi Mohammed1*, Dilkush2, Madhusudhan Reddy G4, Srinivasa Rao K3


1
Department of Metallurgical & Materials Engineering, NIT - Andhra Pradesh, India
2
Department of Metallurgical & Materials Engineering, RGUKT - Nuzvid, India
3
Department of Metallurgical Engineering, Andhra University, Visakhapatnam, India
4
Defence Metallurgical Research Laboratory, Hyderabad, India

*Corresponding author E-mail: raffia.u@gmail.com

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

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Published under licence by IOP Publishing Ltd 1
ICRAMMCE 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 330 (2018) 012018 doi:10.1088/1757-899X/330/1/012018
1234567890‘’“”

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

Table.1 Chemical composition of DMR 249A Steel and Filler/Electrode


Material C Cr Ni Mn Si S P Mo Cu V Al
DMR 249A 0.095 0.30 1.05 1.5 0.29 0.007 0.01 - 0.45 0.05 0.04
Filler 0.039 - 2.15 0.91 0.22 0.028 0.015 - - - -
wire/Electrode

3. Results and Discussions


3.1. Microstructural Studies
Optical microstructure of the different regions of the base metal and its welds made with SMAW and
GTAW are shown in Figs.2& 3. Base metal microstructure consists of ferrite and pearlite whereas
weld region was found to have martensite in both the welds which may be due to fast cooling rates
during welding. However HAZ region consisted of partly bainite and martensite in both the welds.
Base metal has a fine grain structure whereas weld region and HAZ region are coarse dendritic grain
structure. So weld and HAZ regions resulted in having more tendencies to crack formation. SMAW
welding process has a high heat input and thermal efficiency than GTAW welding process. High
thermal efficiency of SMAW results in faster cooling rate during welding and solidification when
compared to GTAW process. Formation of martensite in the weld region is mainly due to faster

2
ICRAMMCE 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 330 (2018) 012018 doi:10.1088/1757-899X/330/1/012018
1234567890‘’“”

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

3.3. Tensile studies


Failed tensile specimens are shown in the Figs. 4-6. Both the welds failed in HAZ region. Stress-strain
curves were shown in Fig. 7. Tensile testing data is given in Table 3. Tensile strength of the SMAW is
observed to be higher than that of base metal and GTA Welds. Improvement in strength may be
correlated with observed microstructure. Similarly yield strength value of SMAW is higher than that
of base metal and GTAW.

3
ICRAMMCE 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 330 (2018) 012018 doi:10.1088/1757-899X/330/1/012018
1234567890‘’“”

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

2 SMAW 652.67 430 45.93

3 GTAW 550.67 410 31.33

Fig. 4 Failed tensile specimen of Base metal (DMR 249A)

Fig. 5 Failed tensile specimen of DMR 249A GTA weld

Fig. 6 Failed tensile specimen of DMR 249A SMA weld

4
ICRAMMCE 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 330 (2018) 012018 doi:10.1088/1757-899X/330/1/012018
1234567890‘’“”

Fig. 7 Stress-strain curves of DMR 249A and its welds

3.4. Pitting Corrosion Studies


Potentio-dynamic polarization curves of the different regions of the welds made with SMAW and
GTAW are shown in Figs. 8-9. Corrosion potential values (Ecorr) are recorded to compare the pitting
corrosion resistance of various regions of the welds. More the positive value of Ecorr, better will be
the corrosion resistance. Results clearly revealed that HAZ is having poor pitting corrosion resistance
than weld zone and base metal in both the welds. Crack initiates in HAZ region, it is due to relatively
lower corrosion potential which acts as anodic site when compared to base metal and weld region. It
can also be observed that weld region of GTAW is having relatively better pitting resistance than that
of SMAW. This may be correlated to the observed microstructure of martensite formation in the weld
zone. Generally it is believed that interface between the microstructure phases ferrite, pearlite and
martensite acts as source of nucleating pits. Relatively less amount of martensite in the GTA welds
may be the reason for better pitting corrosion resistance.

Fig. 8Potentio-dynamic polarization curves of SMAW

5
ICRAMMCE 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 330 (2018) 012018 doi:10.1088/1757-899X/330/1/012018
1234567890‘’“”

Fig. 9 Potentio-dynamic polarization curves of GTAW


3.5. Stress Corrosion Cracking Studies
Base metal, Gas Tungsten Arc Welding (GTAW) and Shielded Metal Arc Welding (SMAW)
specimens were tested in sodium chloride environment in the constant load type machine. Time to
failure at constant load of 50% yield strength is the criteria for assessing stress cracking corrosion
resistance. Failed SCC test specimens are shown in Fig. 10. SCC test data is given in Table 4. Base
metal which has a combination of ferrite and pearlite fine grained microstructures was observed to be
less susceptible to stress corrosion cracking (SCC). Investigation results clearly revealed that the time
to failure of GTA Welds is higher when compared to base metal and SMA Weld samples. SMA welds
were found to be susceptible to stress corrosion cracking and are attributed to the observed
microstructure changes that occur during welding. Interface between martensite and ferrite will act as
source of crack initiation and propagation for stress corrosion process. Relatively more number of
interfaces of ferrite / martensite in SMA welds might have caused the less SCC resistance when
compared to GTAW welds of steel. Lower heat efficiency of GTAW process decreases the cooling
rate and hence less amount of martensite formation. This reduces the number of favourable sites of
crack initiation of SCC in GTAW welds and improves the SCC resistance.

Fig. 10Failed SCC specimens of DMR 249A and its welds

6
ICRAMMCE 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 330 (2018) 012018 doi:10.1088/1757-899X/330/1/012018
1234567890‘’“”

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
1. Malakondaiah G, in International Conference on Metals and Alloys: Past, Present and Future
(2007), p 17.
2. S. Mallike, B.S. Minz, B. Mishra, Material Science Forum., 710(2012), 149.
3. R. Pamnani, M. Vasudevan, T. Jayakumar, P. Vasantharaja, Trans Indian Inst Metals., 70
(2017), 49.
4. TWI GSP No. 5663, An Evaluation of the A-TIG Welding Process (1994).
5. J. Cwiek, K. Nikiforov, Corros. Sci., 40(2004), 831.
6. H.Y. Liou, R. I. Shieh, F. I. Wei, S. C. Wang, Corrosion, 49 (1993), 389.
7. H.Y. Liou, R. I. Shieh, F. I. Wei, S. C. Wang, Corrosion, 49 (1993), 98.
8. L. Vehovar, Mater. Corros., 45 (1994), 354.
9. L. P. Borrego, J. T. B. Pires, J. M. Costa, J. M. Ferreira, Eng. Failure Anal., 14 (2007) 1586.

You might also like