cold cracking studies of high strength structural steel ... · these steels are susceptible to...

ADVANCES in NATURAL and APPLIED SCIENCES

ISSN: 1995-0772 Published BYAENSI Publication EISSN: 1998-1090 http://www.aensiweb.com/ANAS

2017 April 11(4): pages 410-417 Open Access Journal

ToCite ThisArticle: Dhanabal. M, Kailasam. R, Kiruba Shankar. T. S., Cold Cracking Studies of High Strength Structural Steel for Naval Application by SMAW Process. Advances in Natural and Applied Sciences. 11(4); Pages: 410-417

Cold Cracking Studies of High Strength Structural Steel for Naval Application by SMAW Process

1Dhanabal. M, 2Kailasam. R, 3Kiruba Shankar. T. S

1Assistant professor, VSB Engineering College, Karur, Taminadu, 2Assistant professor, VSB Engineering College, Karur, Tamilnadu, 3Assistant professor,VSB Engineering College,Karur, Tamilnadu,

Received 28 January 2017; Accepted 22 April 2017; Available online 1 May 2017

Address For Correspondence: Dhanabal. M, Assistant professor, VSB Engineering College, Karur, Taminadu, E-mail: [email protected].

Copyright © 2017 by authors and American-Eurasian Network for ScientificInformation (AENSI Publication). This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/

ABSTRACT The present work here studies from the Cold crack test carried out on welded joints of a High strength structural steel, using the SMAW process. The reason for the solidification cracks is, most likely, a combination of chemical composition, part restraint and joint configuration. The levels of carbon and nickel present in the material have been shown to enhance the risk of solidification cracking in ferritic steels. Also the joint configuration of the CTS test sample increases the restrain on the weld metal, hence increasing the risk of solidification cracking. KEYWORDS: Controlled thermal severity test, DMR 249B, Microstructure, Hardness profile.

INTRODUCTION

For highly-demanded steel structures like pressure vessels, load carrying vehicles, ships and offshore-

structures, low alloyed structural steels with yield strengths of 355 and 460 MPa have been used successfully

during the last years. For the optimum exploitation of these steels in high-loaded steel constructions good

weldability and high resistance against brittle fracture are required. New alloying concepts as well as new

rolling techniques and heat treatment processes, i.e. thermo mechanical rolling with accelerated cooling or direct

quenching, have led to the development of a new class of high-strength structural steels with low carbon

equivalents showing yield strengths up to 690 MPa, excellent toughness and component safety.

Steels used for less critical surface ship hull application are of C-Mn (e.g.BQuality steel) or low alloy

grades depending on the type of vessel constructed. But steels used for more critical submarine pressure hull

application have undergone many changes. They were of normalized C-Mn ferrite pearlite type high tensile

grades, in first half of this century. Later in 50’s tempered martensite (Quenched and Tempered – QT ) type of

steels were introduced. The typical grades of QT steels such as HY– 80 and HY-100 are extensively used even

today. These steels are susceptible to hydrogen induced cracking (HIC) which necessitates careful welding with

appropriate pre heat, adding to the fabrication costs. The weldability related problems of HY steels arise due to

relatively higher carbon content and higher carbon equivalent as can be understood from Graville welability

diagram in figure 1, which shows susceptibility of HAZ cracking of low alloy steels. The diagram also shows

that excellent weldability can be achieved even at high carbon equivalent, if carbon content is kept low. Based

on this low carbon containing HSLA steels were produced in mid 80’s to replace the HY grade steels.

411 Dhanabal. M et al., 2017/Advances in Natural and Applied Sciences. 11(4) April 2017, Pages: 410-417

Fig. 1: Graville Diagram

In order to satisfy present-day and future high demands in the construction of offshore structures, special

steel grades with up to 690 MPa minimum yield strength have been developed. In thermo mechanically rolled

and accelerated cooled steels, the ferritic transformation is favored by the lower carbon content and by the

rolling schedule. Accelerated cooling leads to a reduction of the pearlite fraction in favor of a fine bainite.

These steels show improved toughness behavior in comparison with the normalized variants. Quenching and

tempering is necessary in order to produce steels with minimum yield strength of 690 MPa with simultaneous

high toughness and good processing behavior.

An indigenization program held at NMRL, to develop a particular grade of steel for naval applications,

known as B-quality steel was taken up with bhilai steel plant (BSP). However, BSP had previously made a

microalloyed version of this steel using Nb as a grain refiner but, the studies carried out by NMRL showed that

if size, shape, and distribution of Nb containing precipitates are not properly controlled during rolling then the

impact transition temperature was adversely affected in the weld heat affected zone. A steel without containing

Nb was made at BSP this steel has the properties comparable to those of the imported B – quality steel and was

certified for use in the naval vessels.

DMR 249 B and DMR 249 A steels are usually used in naval applications. DMR 249 A steel is used for

surface vessel hulls and DMR 249 B is used for Submarine pressure hull applications. DMR 249 B is used in

present study. The mechanical properties and chemical composition of this steel is listed in following table 2&3

respectively.

Table 2: Mechanical properties of DMR 249 B

Properties Y.S. (Mpa) UTS (Mpa) Toughness (J)

DMR 249B 580 650 78@-60°C

Techno-economically fusion welding is the only method of joining for the structural applications. Thus the

primary aspect in any structural steel’s application is the weldability of the material. In fact, the weld joint,

comprising of base metal, heat-affected-zone (HAZ) and weld metal, is the most critical part of a fabricated

structure. Therefore, while designing any fabricated structure the weldability aspect of the steel needs to be

properly addressed. Thus, an in-depth study on the weldability of a structural steel is an important requirement

Table 3: Chemical composition of DMR 249 B Steels.

Elements

DMR-249B

%C

%Mn

%Si %S

%P

%Cr %Ni

%Cu

%V

0.08

0.3-0.6

0.17-0.37 0.01

0.015

0.3-0.7 1.8-2.2

0.4-0.7

0.03

249 A

249 B

249 A

249 B

ig.1. Graville Diagram for High

Strength Steels


%Al

%Mo

0.03

0.25-0.35

Experimental work:

Controlled Thermal severity test was carried out under three various conditions by varying the moisture

level in the electrode to vary the diffusible hydrogen level in the weld metal. They are,

1. Unbaked electrode

2. Moisturized electrode

3. Baked electrode

Baked electrode:

The Electrode was loaded in the electrical furnace at the temperature of 150°C. The furnace was heated up

to 480°C with the heating rate of 100°C per hour. The electrode was maintained at same temperature for 3 hours

then furnace was switched off and allowed to cool up to 150°C. Then the welding was carried out

Moisturized electrode:

Electrode was dipped into water for 30 mins. and kept in the atmosphere for 4 days. Then the welding was

carried out.

Controlled Thermal Severity Test:

Steel plates were machined and assembled as per the figure-3. Rolling direction was positioned parallel to

the test weld. Anchor multiple pass welds were made on both sides with the proper inter pass temperature. After

cooling and bolt tightening two test welds were made at room temperature with single pass welding in flat

position. The assembly was allowed to stand for a period of 72 hours after which the welds were sectioned to

prepare three test pieces from the transverse section for macrostructure and hardness profile study (figure-4).

Series of CTS test as shown in table-5 was designed to evaluate the relationship between test parameters such as

TSN, cooling rate and diffusible hydrogen. Commonly all parameters were kept constant and one was varied for

a test.

Fig. 3: CTS Test assembly

Fig. 4: Photograph of CTS sample


RESULT AND DISCUSION

Metallography study:

On the optical microscope all samples cut from the test specimen with bithermal and trithermal weld

metals are examined. It was concluded that in either case presence of cracks were not found. In figures (5.a,b,c)

shown are some typical structure of bithermal control test sample, and in figures(5.de,,f) shown are some typical

structures of the trithermal control test samples.

Fig. 5: microstructure of baked electrode weld metal in bithermal control

Fig. 6: microstructure of unbaked electrode weld metal in bithermal control

Fig. 7: microstructure of moisturized electrode weld metal in bithermal control


Fig. 8: Microstructure of baked electrode weld metal in trithermal control

Fig. 9: Microstructure of unbaked electrode weld metal in trithermal control

Fig. 10: Microstructure of moisturized electrode weld metal in trithermal control

Micro Hardness profile study:

On the microhardness machine the hardness of all samples cut from the test specimen with bithermal and

trithermal condition was measured along base metal, HAZ, weld metal, HAZ and base metal. It was concluded

that the maximum HAZ hardness in baked, unbaked and moisturized electrode is 320Hv, 323Hv and 368Hv

respectively. In figures (6.a,b,c) shown are hardness profile of bithermal control test sample, and in graph

(6.d,e,f) shown are hardness profile of trithermal test samples.


0

50

100

150

200

250

300

350

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76

DISTANCE

HA

RD

NE

SS

Fig. 11: Hardness profile of baked electrode in bithermal condition

0

50

100

150

200

250

300

350

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70

DISTANCE

HA

RD

NE

SS

Fig. 12: Hardness profile of unbaked electrode in bithermal condition

0

50

100

150

200

250

300

350

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61

DISTANCE

HA

RD

NE

SS

Fig. 13: Hardness profile of moisturised electrode in bithermal condition


0

50

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80

DISTANCE

HA

RD

NE

SS

Fig. 14: Hardness profile of baked electrode in trithermal condition

0

50

100

150

200

250

300

350

0 10 20 30 40 50 60 70 80

DISTANCE

HA

RD

NE

SS

Fig. 15: Hardness profile of unbaked electrode in trithermal condition

0

50

100

150

200

250

300

350

400

0 10 20 30 40 50 60 70 80

DISTANCE

HA

RD

NE

SS

Fig. 16: Hardness profile of moisturized electrode in trithermal condition


Conclusion:

The test result obtained by controlled thermal severity test revealed that DMR 249B steel is not prone to

hydrogen induced cold cracking.

In moisturized electrode test sample HAZ hardness is high when compare to another two condition

REFERENCES

1. Nikola Bajic, Marko Rakin, Dzafer Kudumovic, Zoran Radosavljevic, 2011. srdan Bajic “Testing of

cracking susceptible of high strength micro alloyed steel” DRUNPP, Sarajevo, 6: 1.

2. Martínez-Mateo, I. and O. Fernández-García “ SOLIDIFICATION CRACKS IN HSLA STEEL JOINTS

AFTER CONTROLLED THERMAL SEVERITY TESTS”

3. BS7363: 1990 “Methods for Controlled Thermal Severity (CTS) Test and Bead-on-plate (BOP) Test for

Welds.” British Standard Institution.

4. PARGETER, R.J. AND M.D. WRIGHT, 2010. “Welding of Hydrogen-Charged Steel for Modification or

Repair” 89.

5. Dr. Parmar, R.S., 2008. “welding engineering and technology” khanna puplishers.

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