Effect of welding parameters on mechanical properties of ... · PDF fileEffect of welding parameters on mechanical properties of GTAW ... Duplex Stainless Steel ... effect of welding

Effect of welding parameters on mechanical properties of GTAWof UNS S31803 and UNS S32750 weldments

Prabhu Paulraj* and Rajnish Garg

University of Petroleum & Energy Studies, Dehradun 248007, Uttarakhand, India

Received 13 October 2015 / Accepted 13 November 2015

Abstract Duplex Stainless Steel (DSS) and Super Duplex Stainless Steel (SDSS) pipes were welded by GasTungsten Arc Welding (GTAW) process. The effect of welding parameters such as heat input, cooling rate,shielding/purging gas composition and interpass temperature on tensile strength, hardness and impact toughness werestudied. The microstructure analysis revealed presence of intermetallic phases at root region of the weldments. Allmechanical properties were improved at lower heat input and high cooling rate due to grain refinement and balancedmicrostructure [ferrite and austenite]. All weldments exhibited higher strength than base materials. Weld root regionwas harder than centre and cap region. SDSS is more susceptible to sigma phase formation due to higher alloyingelements and weld thermal cycles, which lead to considerable loss of toughness. Higher nitrogen contents in shieldingand purging gas resulted strengthening of austenite phase and restriction of dislocations, which ultimately improvedmechanical properties. Higher interpass temperature caused reduction in strength and toughness because of graincoarsening and secondary phase precipitation.

Key words: DSS, SDSS, Tensile strength, Hardness, Impact toughness, Heat input, Shielding gas, Interpasstemperature

1. Introduction

Duplex stainless steels (DSS) and Super Duplex StainlessSteels (SDSS) are dual phased steels comprising ferrite andaustenite theoretically in equal proportions. They have excep-tional mechanical and corrosion properties. Super duplexstainless steels have high chromium and molybdenum contentwhich makes them highly corrosion resistive and highermechanical strength. DSS and SDSS have wide rangeof applications in offshore, chemical, paper and pulpindustries [13].

Welding is a key fabrication technique in applications ofduplex and super duplex stainless steels. Welding of DSSand SDSS grades are challenging operation due to their com-plex microstructure. Improper welding cycles may lead todestroy material properties such as drastic reduction in tough-ness, strength and corrosion resistance [4].

It is important to obtain balanced microstructure (50%Austenite and 50% Ferrite) after welding. The high arc energyprocesses (high heat input) such as Submerged Arc Welding(SAW), Gas Metal Arc Welding (GMAW) and Gas TungstenArc Welding (GTAW) induce slower cooling rates. This leads

to formation of higher amount of austenite as well as wide heataffected zones (HAZ). On the other hand, low arc energy pro-cesses like Electron Beam Welding (EBW), Laser Beam Weld-ing (LBW) cause faster cooling and hence very less austenitereformation. The faster cooling may even lead to precipitationof chromium nitrides [5].

The intermetallic phase precipitation is a major issue inDSS/SDSS fabrication. It has been found that toughness isthe most sensitive property which gets affected due to sec-ondary phase formation [6]. Hence prolonged exposurebetween 600 and 1000 C is not favourable for DSS/SDSSapplications [7].

During welding of duplex stainless steels and super duplexstainless steel, it is important to prevent oxidation of the weld-ments in order to avoid loss of corrosion resistance. Hence,inert gas such as argon or helium gas purging technique arecommonly used to prevent oxidation. Two to five percent nitro-gen addition in argon gas [8] improves and assist phasebalance.

From the literature review, it was found that there has notbeen a thorough research work done by considering all thewelding parameters of GTAW of DSS and SDSS. The mainaim of this work is to characterize mechanical properties of*e-mail: paulrajprabhu1@gmail.com

Manufacturing Rev. 2015, 2, 29 P. Paulraj and R. Garg, Published by EDP Sciences, 2015DOI: 10.1051/mfreview/2015032

Available online at:http://mfr.edp-open.org

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

OPEN ACCESSRESEARCH ARTICLE

http://www.edpsciences.org/http://dx.doi.org/10.1051/mfreview/2015032http://mfr.edp-open.orghttp://mfr.edp-open.orghttp://creativecommons.org/licenses/by/4.0/

the DSS and SDSS materials welded by GTAW technique,which is most commonly used welding process for DSS andSDSS. This paper focuses on effect of welding parameterson mechanical properties of DSS and SDSS weld. We havestudied various mechanical properties such as Tensile Strength,Ductility, Impact toughness and Hardness with respect to thewelding variables heat input, cooling rate, shielding gas, purg-ing gas and inter-pass temperature.

2. Experimental details

2.1. Research materials

DSS and SDSS pipes with different PREN were used forthis study. Table 1 shows the chemical composition and PRENvalue of base materials. The pipes were of 2diameter,5.54 mm thickness (Sch 80) and 150 mm long. Table 2 showsmechanical properties of base materials. Figure 1 shows typicalmicrostructure of base material comprising dark ferrite phaseand bright austenite phase.

The filler metal used for this work was a Sandvik manufac-tured filler metal of 2 mm size. The chemical composition offiller metal is given in Table 3.

2.2. Welding

DSS and SDSS pipes were welded with Gas Tungsten ArcWelding (GTAW) process. Ar+2%N and Ar+5%N gas mixturewas used as shielding gas and back purging gas (refer Tables 5and 6). Welding experiments were carried out by varying mate-rial grade, welding heat input, cooling rate and by varyingshielding gas, purging gas and inter-pass temperature in orderto study the effect of welding parameters on mechanical prop-erties of DSS and SDSS welds. The welding parameters rangesare tabulated in Table 4.

During welding experiments, one of the above parameterwas varied and other parameters were kept constant. Tables5 and 6 show experimental details of this section.

After welding, welded pipes were liquid penetrant exam-ined to ensure no surface defects and followed by radiographic

examined to ensure no defects though out the thickness. Sub-sequently, mechanical test samples were prepared as perASME Sec-IX.

2.3. Metallography

Metallographic studies were performed using opticalmicroscopy. First the specimen were polished on emery sheetsup to 1200 grit fineness. Then cloth polishing was done usingalumina powder of 0.05 lm size. The weldments etched with20% NaOH solution and they were examined under opticalmicroscopes. The ferrite content measurements were done bypoint count method in accordance with E562 standard.

2.4. Mechanical testing

After successful completion of welding, the specimen weresubjected to mechanical testing. The weldments were charac-terised with tensile test, hardness test and impact test. For ten-sile test, ASME IX standard followed. The material thicknessand width were 5.30 mm and 13.20 mm respectively. Threesets of tests were performed and average value was taken tothis study. Fractured surfaces were examined with SEMimages. Charpy V-notch impact tests were performed on a pen-dulum type impact tester as per ASTM A370 standard. Thespecimen dimensions used for impact tests was5 10 55 mm. All the impact tests were carried out at46 C.

Hardness measurements were taken on transverse sectionof weldments where hardness values were measured at weldmetal, HAZ and base metal. Also hardness was measuredalong thickness of weldments. The ASTM E92 standard wasfollowed with 10 kg of test load.

Table 1. Base metal chemical composition.

Material grade Cr Mo Ni N C PREN Remarks

UNS S31803 22.9 3.03 7.92 0.15 0.017 35.15 DSS-Low PRENUNS S31803 22.9 3.04 7.63 0.17 0.019 36.30 DSS-High PRENUNS S32750 25.1 3.71 8.9 0.2 0.016 40.36 SDSS-Low PRENUNS S32750 25.1 3.75 8.86 0.21 0.028 41.40 SDSS-High PREN

Table 2. Base material properties.

Grade Tensile strength(MPa)

Hardness(VHN)

Impact toughness[J] at 46 C

DSS-Low PREN 750 255 127DSS-High PREN 762 260 109SDSS-Low PREN 806 301 140SDSS-High PREN 830 310 119

Figure 1. Typical base material microstructure.

2 P. Paulraj and R. Garg: Manufacturing Rev. 2015, 2, 29

3. Results and discussion

The results of mechanical tests for first part of this work(i.e. to study the effect of heat input on weldments) are tabu-lated in Tables 7 and 8. The Experiment no. 14 were doneon low PREN DSS/SDSS grades and Experiment no. 58are done on high PREN DSS/SDSS grades.

Effect of shielding/purging gas composition and interpasstemperature on mechanical properties on DSS and SDSS weldsare shown in Tables 9 and 10.

3.1. Microstructure of weldments

A typical microstructure of weldments is shown in Figures2 and 3. The weld region microstructure differed across thethickness of the pipe. It can be divided into cap and root

regions. Due to constant reheating effect in the root region,secondary austenite was found to be precipitated at theroot region. The microstructure at cap region cosist of

Table 7. Results of mechanical tests for DSS joints.

Exp.no.

Specimen Heatinput

(kJ/mm)

Tensilestrength(MPa)

Impacttoughness at46 C (J)

Hardness(VHN)

1 DSS- LowPREN

1.05 795 168 2662 1.10 780 140 2643 1.15 767 123 2594 1.20 755 85 2555 DSS- High

PREN1.0 805 115 275

6 1.05 802 104 2707 1.1 785 97 2668 1.15 777 94 261

Table 8. Results of mechanical tests for SDSS joints.

Exp.no.

Specim