Study on Dissimilar Metals Welding of 15CDV6 and SAE 4130 Steels by Inter Pulse Gas Tungsten Arc Welding Academic research paper on "Materials engineering"

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Procedía Materials Science 5 (2014) 2382 - 2391

International Conference on Advances in Manufacturing and Materials Engineering,

AMME 2014

Study on dissimilar metals welding of 15CDV6 and SAE 4130 steels by Inter pulse gas tungsten arc welding

P. Naveen Kumara'*, Y. Bhaskara, P. Mastanaiaha, CVS Murthya

a Defence Research and Development Laboratory, Hyderabad 500 058, India

Abstract

In aerospace scenario, ultra high strength steels like 15CDV6 steels and SAE 4130 steels are widely used because of their outstanding high strength to weight ratio. These steels differ from each other in their alloy composition. 15CDV6 steel contains lower bainate phase in hardened and tempered condition whereas SAE 4130 steel possess tempered martensite in hardened and tempered condition. 15CDV6 steel obtain their strength by austenizing at 975°C ± 5°C followed by Oil/Water/Forced air quenching and tempering at 650°C ± 5°C, while the strength of SAE 4130 steels is obtained by austenizing at 870°C ± 5°C followed by Oil/Water quenching and tempering at 260°C ± 5°C.

In the present work, 3mm thick 15CDV6 alloy steel sheets were joined to SAE 4130 low alloy steel by Inter Pulse Gas Tungsten Arc welding using 8CD12 alloy filler wire, by taking both the base materials in different prior and post heat treatment conditions. Different welding parameters and heat- treatment cycles are employed. These weld coupons are evaluated by non-destructive tests like Dye penetrate test and X-ray radiography. The joints are evaluated for mechanical properties such as ultimate tensile strength, yield strength, percentage of elongation, and microhardness across weldment and measurement of residual stresses, both in as welded and post weld heat treated conditions. Evaluated mechanical properties were correlated to their weldment microstructures.

©2014ElsevierLtd.Thisisanopenaccessarticleunder the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of Organizing Committee of AMME 2014

Keywords: Inter pulse Gas Tungsten Arc welding; 15CDV6 steel; SAE 4130 steel; dissimilar metal welding.

Corresponding author. Tel.: 09666676734.

E-mail address: naveenkumarl809@gmail.com

2211-8128 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of Organizing Committee of AMME 2014 doi: 10.1016/j.mspro.2014.07.483

1. Introduction

In many engineering applications, welding is often performed between dissimilar metals in order to meet the end functional requirements. A successful weld joint between dissimilar metals should possess sufficient tensile strength and ductility so thatjoint will not fail when it is under service conditions. Such weld joints can be produced through different kind of welding processes depending upon the dissimilar metals under consideration. The adaption of dissimilar metal combinations provides possibilities for the flexible design of the product by using each material efficiency i.e., benefiting from the specific properties of each material in a functional way.

15CDV6 steel is a high strength low alloy steel. The name 15CDV6 is a French designation in which the first number is equal to 100 times the concentration of carbon. The letters which follow indicate the other elements present, in the decreasing order of concentration. The last digit is equal to four times the concentration of chromium. In the French notation, C stands for chromium, D for molybdenum, and V for vanadium. It therefore that, in 15CDV6 steel, the concentration of carbon is 0.15%, while that of chromium is 1-5%, and the concentrations of molybdenum and vanadium are less than 1.5% each. Hence, it is a low-alloy steel, in which the proportion by weight of all the alloying elements combined is less than 5 %. Bandyopadhyay [1] and Ravikumar [5] et al., stated that 15CDV6 steel is high strength and high toughness steel extensively used in rocket-motor hardware in the space programme. This steel has very high strength to weight ratios along with good toughness and weldability. Ding and Han [2] et al., reported that 15CDV6 steel contains bainatic structure, upon increasing temperature up to 650oC the material undergoes a secondary hardening effect where increase in strength and hardness of a material by precipitating the alloy carbides namely Mo2C, VC. Indumathi [7] et al., discovered long-term performance evaluation studies of a eco-friendly coating scheme which were carried out on 15CDV6 steel, an ultra-high- strength steel used in the aerospace industry.

Lang and N. Kenyon [3] et al., stated that SAE 4130 steel is an ultra-high strength low alloy heat treatable steel widely used in various air frame structure and components. 4130 steels are classified in the category of high strength medium carbon low alloy steels. It is one of the most widely used steels in air craft construction because of its combination of moderate strength and reasonable ductility in the quenched and tempered conditions.

SAE 4130 is a low alloy steel containing molybdenum and chromium as strengthening agents. This alloy is readily machined in conventional methods. Machinability is the best with the in normalized and tempered condition (18-21HRc). Although the alloy may be machined in the fully heat treated condition, Brown [4] and Dabezies [6] et al, state that Machinability becomes more difficult with increasing strength (hardness 46-48HRc) of the alloy. It has relatively low hardenability, because of which, uniform mechanical properties can be obtained only in section thickness that are small enough (3mm) through harden when quenched, after tempering at 260°C it contains tempered martensite further increasing in temperature this tempered martensite is transforming to coarse martensite, 15CDV6 and SAE 4130 steels are dissimilar with respect to chemical composition and heat treatment processes.

Considering the extensive usage of the materials in aerospace industries, welding of these two dissimilar materials are attempted. Table 2 shows type of weld and material heat treatment conditions. All the six types of weld coupons are NDE tested and mechanical properties such as ultimate tensile strength, yield strength, percentage of elongation, and across the weld bead hardness and residual stress are measured. Micro structures across the weld bead are analyzed and mechanical properties in relation to the metallurgical phases are studied.

2. Objective

The prime objective of the work is to realize a dissimilar weld joint between these two high strength steels, with radiography quality and to evaluate the microhardness and mechanical properties of dissimilar weldment along with the analysis of fusion zone microstructures.

3. Experimental Details

3.1. Parent Materials

The materials selected for this study are SAE 4130 steel and 15CDV6 steel. The chemical compositions of these materials are given Table 1. The details of heat treatment conditions of parent material are given in Table 2.

Table 1 : Chemical composition of the base material in wt%

Material C Si Mn Cr Mo V S P

15CDV6 ÔTÏ6 ÔÔ9 Ô82 L27 Ô87 Ô25 0.002 0.009

SAE 4130 0.3 0.26 0.48 0.86 0.25 - 0.02 0.019

Table 2: Material heat-treatment condition

Type of weld joint

Pre-weld condition

Post Weld treatment

Identification No. of weld coupon

Similar (SAE 4130)

Similar ( 15CDV6) Dissimilar (SAE 4130

15CDV6) Dissimilar (SAE 4130 15CDV6)

Dissimilar (SAE 4130 15CDV6)

Annealed Annealed Annealed

975 C ± 5 C followed by forced air quenching and tempering at 650°C ± 5°C,

870 C ± 5°C followed by

oil quenching and tempering at 260 C ± 5 C.

Al A2 A3 HT-1

Dissimilar (SAE 4130 &15CDV6)

Hardened & tempered as per base materials

3.2. Welding

• Pre welding cleaning methods: Before welding, the surfaces of the sheets were degreased with the thorough application of acetone using the lint free cotton cloth. Further a zone 25 mm from each side of weld joint was scrupulously cleaned with stainless steel rotary wire brush.

• Tack welding & Materia! Selection of filler-. The sheets were tack welded at both the ends and centre of the joint before full welding to avoid mismatch between the plates. For any combination of dissimilar welding, the selection of filler material plays a key role on the final weldment properties. For the present work, 8CD12 alloy filler wire used, this alloy is slightly modified chemical composition of 15CDV6 steel is having C-0.1, Mn-1.1, Cr-3.0, Mo-1.0, Si-0.9, S-0.009, P-0.015

• Weld: The sheets of two dissimilar materials SAE 4130 & 15CDV6 alloy steels with sizes of 100mm X 160mm X 3mm are welded by interpulse TIG welding process. The principle of interpulse TIG welding process is based on constriction of welding arc by magnetic field around the arc. The strength of the electric field is directly proportional to the rate of change of magnetic field. The interpulse generates high frequency pulses, the relationships of which are programmable to alter the magnetic field of the arc, thus enabling the control of the constriction of the arc, and leading to less heat input and small HAZ, compare to normal conventional TIG welding processes and consequently improvement in mechanical properties of the weldment are achieved by interpulse TIG welding processes. The welding parameters used in this present investigation are given in Table 3. The weld coupons were subjected to X-Ray radiography test and dye penetrate test as per acceptance criteria of ASME BPVC section VIII Div.2 and ASTM E 165 respectively.

Table 3 : Inter pulse TIG welding parameters

Welding Input Parameters First Pass Details Second Pass Details

Main Current (Amp) 175 165

Background Current (Amp) 140 130-140

Delta Current (Amp) 135 130

Delta Frequency (KHz) 20 20

Pre-Purging (Sec) 5 5

Post-Purging (Sec) 20 20

welding speed(mm/min) 33.3 38.75

3.3. Post weld Heat treatment (PWHT) & Metallography:

In order to investigate the influence of PWHT on microstructures and mechanical properties, two types of post weld heat treatment were employed. The details PWHT are as follows, 975 C±5C (for lmm/4min soaking) followed by forced air quenching and tempering at 650°C/40 min soaking followed by air cooling that is suitable for 15CDV6 alloy steel. 870 C ±5 C(for lmm/8min soaking}followed by oil quenching and tempering at 260°C/1 hour soaking followed by air cooling that is suitable for of SAE 4130 steel.

Metallographic studies consisting of micro structural examination of the weld zone, HAZ and base materials. For this purpose Olympus make optical microscope was employed. Differential etching was carried out using 2% nital for SAE 4130 & 5% nital solution for 15CDV6 steel.

4. Results and discussions

4.1. Tensile properties

The room temperature tensile properties of both the base materials SAE 4130& 15CDV6 steel in annealed condition and in post weld heat treated condition are shown in Table 4.

Table 4: Mechanical properties of similar and dissimilar weldments between Q&T SAE 4130 and 15CDV6

S.No Type of weld joint UTS(MPa) 0.2%YS (MPa) %EL Failure location

1 Al 612 467 16.00 HAZ of 4130

2 A2 541 412 13.36 HAZofl5CDV6

3 A3 534 402 12.32 HAZ of 4130

4 HT-1 735 588 9.72 HAZ of 4130

5 HT-2 1384 1234 5.84 HAZ of 15CDV6

6 HT-3 1009 853 10.76 HAZ of 4130

4.2. Microstructures

To understand the integrity dissimilar welds, microstructures of fusion zone (FZ) and HAZ are studied. The microstructure of dissimilar weld between SAE 4130 and 15CDV6 steel at annealed and various heat-treated conditions as shown in figures (1,2,3,4,5 and 6).

• A-l : The microstructure of A-l(similar 4130) consists of bi-phase coarse lamellar pearlite (dark areas) in a matrix of ferrite (white) with an average ferrite concentration in the base-metal region as shown in fig.l(a). Un-tempered martensite observed in the fusion zone as shown in fig 1(b), the HAZ shows a mixture of fine acicular products as shown in fig 1 (c)

Figure 1: A-l Microstrnctures; a) Base material ofSAE 4130, b) Weld Zone, c) HAZ ofSAE 4130

• A-2 : The micro structure of A-2(similar 15CDV6), the base material consists of lamellar pearlitic structure in a ferrite matrix as shown in 2(a). The fusion zone consists of bainitic features (fine) as shown in fig 2(c). The HAZ experiences of grain coarsening as shown in fig 2(b).

Figure 2: A-2 Microstrnctures; a) Base material of 15CDV6, b) HAZ of 15CDV6, c) Weld Zone

• A-3: The micro structure of A-3(dissimilar SAE 4130 & 15CDV6 steel), from 15CDV6 steel side the base material consists of lamellar pearlitic structure in a ferrite matrix as shown in fig 2(a) and SAE 4130 also consists of similar structure as mentioned above as shown in fig 1(a). HAZ of 15CDV6 steel side, it experiences grain coarsening due to low and high thermal cycles as shown in fig 2(b), because of high heat affect compare to parent material where as in HAZ of SAE 4130 side lamellar pearlite with high concentration of ferrite matrix as shown in fig 1(c). Weld zone of A-3 is found to be similar to that of the weld zone of A-2 (similar 15CDV6 weld) as shown in fig2(c).

• HT-1\ The micro structure of HT-1 from SAE 4130 steel side after post weld heat treatment the base material consists of fine acicular and tempered martensite as shown in fig 3(a), from 15CDV6 steel side the base material structure shows completely bainate structure as shown in fig 3(b). HAZ of 4130 consists of randomly oriented fine acicular martensite as shown in fig 3(e). HAZ of 15CDV6 consists of coarse martensite structure as compared to base material of 15CDV6 alloy shown in fig 3(d). The weld zone micro structure consists of martensite needles

crystallize along cleavage planes and show's three directions of the widmanstaten structure as shown in fig 3(c).

Figure 3: HT-1 Microstructures; a) Base material of SAE 4130 ,b) Base material ofl5CDV6 , c)Weld Zone, d) HAZ of 15CDV6, e) HAZ of

SAE 4130

• HT-2: The microstructure of HT-2, in the base material of SAE 4130 steel consists of tempered lath martensite as shown in fig 4(a), the HAZ on 4130 shows a mixture of fine acicular products as shown in fig 4(d). While in the base material of 15CDV6 steel consists of low carbon martensite in a ferrite matrix as shown in fig 4(b), the HAZ on 15CDV6 steel side showed the low carbon martensitic microstructure experiences coarsening as shown in fig 4(e). In the weld zone it is shown that completely bainitic structure shown in 4(c).

Figure 4: HT-2 Microstructures; a) Base material of SAE 4130 ,b) Base material ofl5CDV6 , c)Weld Zone, d) HAZ of SAE

4130, e)HAZof 15CDV6

• HT-3: The microstructure of HT-3, In the base material of SAE 4130 steel consists of tempered lath martensite as shown in fig 5(a), the HAZ on 4130 experiences martensitic coarsening due to exposing of high temperatures during welding as shown in fig 5(d), while in base material of 15CDV6 steel shows tempered bainate structure as shown in fig 5(b), in the HAZ of 15CDV6 steel shows a upper bainitic structure as shown in fig 5(e), and the weld zone consists of bainitic structure fig 5(c).

Figure 5: HT-3 Microstructures; a) Base material of SAE 4130 ,b) Base material of 15CDV6, c) Weld Zone, d) HAZ of SAE

4130, e)HAZof 15CDV6

4.3. Hardness:

Vickers- microhardness, with a load of 100gf, is measured on each specimen across the weld bead, say 15mm on each side of the weld bead. The distance between each indentation is maintained about 0.5mm. Prior to testing, each

specimen is prepared to have mirror finish, free from surface cracks, no water marks to identify the clear vision of indentation.

In A-l weld zone exhibited higher hardness (45HRc) than HAZ (29HRc) and base material (16HRc) an increased hardness in fusion zone probably due to martensite as shown in fig 7(a). A-2; In weld zone it is observed higher hardness (46HRc) than in HAZ (18HRc) and base material side (17HRc) as shown in fig 7(b). A-3; In weld zone it is observed higher hardness (45HRc) than in HAZ (17HRc) and base material of 15CDV6 side (17HRc), SAE 4130 side (15HRc), as shown in fig 7(c). HT-1; The base material of 15CDV6 exhibited higher hardness (32HRc) than in fusion zone (28HRc) and in base material of SAE 4130 (21HRc), the decreased hardness in SAE 4130 and in fusion zone probably due to the softening of these regions by high tempering temperature, as shown in fig 7(d). HT-2; The base material of SAE 4130 exhibited higher hardness (47HRc) as the increase in tempering temperatures for HT-2 condition there is hardness drooping trend will occur due to the formation of coarse tempered martensite. From 15CDV6 base material side the hardness is (41HRc), the HAZ of SAE 4130 is shown slight increase in hardness (44HRc) compare to HAZ of 15CDV6 (40HRc), as shown in fig 7(e). HT-3; in fusion zone it is exhibited a higher hardness (46HRc), in the HAZ of 15CDV6 side observed higher hardness than base material probably due to grain refinement occurred in HAZ. At SAE 4130 side drooping hardness is due to formation of high concentration of ferrite structure surrounded by martensite matrix, as shown in fig 7(f).

4.4. ResidualStress Measurement:

Before going to measure the residual stresss, the material surface should be clean from corrosion, dirt, oil and grease. In our present work the welded samples were electro- polished to avoid oil and grease. The electro polishing dissolution liquid composition consists of, Water - 140ml, Glycerin- 100ml, H3P04 - 430ml, H2S04 - 330ml. The experiment details are- Dissolution time (min): 1-5; Currents(Amp): 10-15; Temperature(°c): 38-40; Voltage(v): 10-15.

The residual stress measurement was carried out with PROTO make Portable X-ray residual stress analyzer employing Cr Ka radiation. From the works of Hilley [8] and Noyan and Cohen [9] et al., we can infer that X-ray techniques measure stress indirectly by measuring the surface strain, which is indicated by the position of a diffracted peak 0 for a crystal plane oriented at various angle to the surface of a specimen. The instrument uses a pair of solid state detector located on each side of the main beam. Diffraction peaks are captured by the individual pixels in the detector, giving rapid data captures without mechanical movement. Residual stresses were evaluated in this analyzer using multiple exposure sin2T technique based on the diffraction from hkl plane of (211) as shown in fig 8. The residual stress measurement comprises of at least 30 measurements of lattice spacing over a range of ¥ orientations (-45° to +45°) to the surface of the specimen. Residual stress measurements were carried out across the weldment (i.e. perpendicular to welding direction) for annealed and PWHT conditions as shown in fig 8, (a, b, c, d, e, f). The accuracy of stress measurements is approximately ± 20 MPa. When some anisotropy was encountered, more trials were performed. In weld/HAZ/parent metal where the grain size influenced the measurements, the X-ray goniometer was oscillated by ± 2°. For computation of stresses from measurement of strain data, appropriate X-ray elastic constants were used.

"SAE 4130 I *V SAE 4130

f*** ,*************** ***************

-15 -10 -5 0

i b : HAZ i HAZ

i 4 \ \ V

: ___________ - 15CDV6 f /! : /; WELC

15CDV6

-15 -10 -5

HT-1 HARDNESS |

| —*^3,HARDNESS~|

c -- WELD CENTER

**** V

15CDV6 HAZ HAZ SAE 4130

i i . i \ ******** *******

-15 -10 -5 0 5 10 15

HT-2 HARDNESS

e rjL ...

' s, * Y ¡t ■ i i i i

15CDV6 HAZ HAZ SAE 4130

-15 -10 -5 0 5 10 15

Distance in mm

d rA/' -► Weld center

• \ ' \ A • A

\ 1 '' «

■ % /

15CDV6 HAZ i ■ I i ■ i i I i i i i I i HAZ SAE 4130

-15 -10 -5 0 5 10 15

■HT-3 HARDNESS

15CDV6 HAZ

-15 -10 -5 0 5 10 15

Distance in mm

Figure 6: Hardness distribution across the similar and dissimilar weldments of SAE 4130 &15CDV6 steel: a)A-l,

b)A-2, c)A-3, d)HT-l, e)HT-2, f)HT-3.

| A1-RSM |

A2,RSM |

-140 -160 -180 -200 -220 -240 -260 -280 -300

a -"-Weld ce

/V .. j v V V s. \ \

SAE 4130 SAE 4130 \

-15 -10 -5 0 5 10 15

| —A3,RsjjT|

-200 -220 -240 -260 £ -280 1 -300 | -320

-as -340

If, "36° 0)

^ -380 -400 -420 -440

C ■Ah- V \ ! V\ (M / n

-15 -10 -5 0 5 10 15

Distance in mm

-150-,

-15 -10 -5

| —HT-1.RSM |

| -100

15 "125 ■o

•55 -150 a:

-175 -200 -225 -250

SAE 4130 15CDV6

-15 -10 -5 0 5 10 15

■ 'aA/v

-15 -10 -5 0 5 10 15

Distance in mm

—■—HT-3,RSM |

f . fi

: WELD

SAE 4130 15CDV6

-15 -10 -5 0 5 10 15

Distance in mm

Figure 7: Residual stress measurement across the similar and dissimilar weldments of SAE 4130 &15CDV6 steel: a)A-l, b)A-2, c)A-3, d)HT-l, e)HT-2, f)HT-3.

Conclusions

• Interpulse TIG Welding process have been performed sucessfully to join the low alloy steels SAE 4130 & 15CDV6 by using 8CD12 filler wire.

• In the fabrication of a High pressure vessels, made up of 15CDV6 steel welded joints are inevitable. In the case of large diameter pressure vessels with less thickness, controlled welding and low heat input is very difficult to achieve. Interpulse TIG welding process can achieve less heat input and small HAZ, and also minimize the thermal stresses near the weld joints.

• The weldments of HT-1 condition have failed in HAZ of SAE 4130 side because the austenizing and tempering temperatures of 15CDV6 is more compare to SAE 4130 i.e, Acl 765°C, Ac3 975°C, is for 15CDV6, Acl 745°C and Ac3 is 875°C for SAE 4130. The temprering temperature is 650°C, at high temperature tempering 15CDV6 steel undergoes a secondary hardening effect, where increase in strength and hardness of a material by precipitating the alloy carbides namely Mo2C, VC. At high temperature tempering, from other side SAE 4130 steel undergoes a coares martensite structure.

• The weldments of HT-2 condition have demonstrated adequate tensile strength of 1384 MPa among the other conditions and the microstructure predominently consists of feathery bainate structure in fusion zone .The samples have failed at 15CDV6 steel side because Acl and Ac3 are 765°C, 975°C therefore there is no sufficient temperature to austenizing the steel.

Acknowledgements

The authors would like to express their sincere gratitude to Shri. Sibnath Som, The Director, DRDL,

Hyderabad for his constant support to carry out this experiment. The authors also thank the Head Material

Development Division (MDD) for his constant support to carry out the characterization part as in metallography,

mechanical testing, microhardness and measurement of residual stresses.

References

[1] T.R Bandyopadhyay, P. Krishna Rao and Prabhu N 2006, Development of ultra-high strength Steels through eletroslag remelting and inoculation, Iron making and Steelmaking, Vol. 33, No. 4, pp. 331-336.

[2] Ding and Weixin Han., 2010, "Secondary hardening behaviour of ISCrMnMoV thin rolled tube". Acta Metall vol.23No.4 pp266

[3] F.H. Lang and N. Kenyon, Welding ofMarriaging Steels, WRC Bulletin, 2004, 159, p.p 1-43.

[4] K.R. Brown, Dec 1985,, "Ferrous Alloys-SAE 4130", Aerospace structural metals Handbook, Code 1201, p.p. 1-48

[5] B.V.R. Ravikumar and JS Soni,June 2009, "Microstructure and properties of 15CDV6 alloysteel", codel0906

[6] B. Dabezies and J. M. Signamarcheix., Mar 1989 "Laser Beam Welding of 15 CDV6 Steel Specimens" Number: PB90-151598.

[7] Indumathi S, Vasudevan, T.; Sundarrajan, S.; Subba Rao, B.V.; Murthy, C.V.S.; Yadav., D.R. 3 April - May, 2011. Cadmium- and chromate-free coating schemesfor corrosion protection of 15CDV6 steel. S.N, I.; P. 15-21, Metal Finishing vol. 109 issue

[8] Hilley M E,editor., 1971, Residual stress measurement by X-ray diffraction, SAE j784a, Warrandale, PA 15096: Soc of Auto Eng.;1971

[9] Noyan I C and Cohen J B., 1987, Residual stress measurement by diffraction and interpretation. New York: Springer-Verlag;