Materials Today: Proceedings 2 ( 2015 ) 2532 – 2537

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2214-7853 © 2015 Published by Elsevier Ltd. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization.doi: 10.1016/j.matpr.2015.07.198

4th International Conference on Materials Processing and Characterization

Structure – Properties Evaluation In Laser Beam Welds Of High Strength Low Alloy Steel

M.V.L Ramesha, P. Srinivasa Raob, V. Venkateswara Raoc & K. V. Phani Prabhakar da*

a,c Advanced System laboratory, Hyderabad - 500 058, India b GVPCOE, JNTU, Kakinada – 533 003, India

d ARCI, Hyderabad – 500 058, India

Abstract

The present investigation reports on a study that has been taken up to develop an 9elseunderstanding of the Laser beam welding characteristics of similar combination of high strength low alloy steel which is in the hardened condition .i.e in a quenched and tempered condition before welding. The joint characterization studies include microstrutural examination, micro hardness measurement across the weldment and evaluation of tensile and impact toughness properties. The fusion zone exhibits a epitaxial grain growth. The microstrutural features of heat affected zone and fusion zone vary, due to the different thermal cycles for which they are subjected during welding. The observed hardness distributions across the welds are correlated with the observed microstructures. The welds exhibited lower toughness data compared to parent metal and the tensile strength values of the welded specimens are close to that obtained for sheet specimens © 2014 The Authors. Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization.

Keywords:High Strength low alloy steel, Laser beam welding, Microstructure, Mechanical Properties.

1.Introduction:

* Corresponding author. Tel.: +91 7867-201222; fax: +91 7867 - 289466. E-mail address: r_mvl@yahoo.co.in.

© 2015 Published by Elsevier Ltd. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization.

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The 15CDV6 steel is high strength bainitic steel containing low concentrations of chromium, molybdenum and vanadium as alloying elements. It possesses good strength - ductility combination and excellent weldability1. 15CDV6 alloy steel lends itself remarkably well to all welding processes: oxyacetylene, electric arc, resistance, electron beam or laser beam welding.Welding can be carried out without the need for subsequent heat treatment and there is only negligible loss of properties during welding. No preheating is required up to 10mm thick. After welding, there is no excessive hardening of the bead. The annealing effect in the vicinity of the bead (which usually affects the strength ofwelded steel) is practically imperceptible with 15CDV6 due to the Presence of molybdenum and vanadium. The strength of the treated metal is maintained intact in the bead and its vicinity. Assemblies with strength greater than 1,000 Mpa can be obtained without any requirement for heat treatment2, 3, 4. The alloy finds much application in the aerospace and motor sports industries in such components as roll cages, Pressure vessels, suspensions, rocket motor casing, and sub frames5.

Laser beam welding (LBW) is a high energy density fusion welding process that is extensively employed in the aerospace industries. The principle advantage is the opportunity to achieve deeper, narrower welds than can be manufactured with an arc welding process6.

The present investigation reports on a study that has been taken up to develop an understanding of the Laser

beam welding characteristics of similar combination of high strength low alloy steel which is in the hardened condition .i.e in a quenched and tempered condition before welding. Detailed microstructural examination in the different regions of the welds and a correlation between the microstructure and mechanical properties forms the goal of the study and therefore assumes special significance, since such detailed studies are not so far reported. 2. Experimental Procedures:

High strength low alloy steel plates of 500 mm x 150 mm x 3.7mm size are used in this work. The plates are taken in the fully heat treated condition before welding. The plates are hardened at 980°C for 30 min. followed by forced air cooling and then tempered at 640°C for 30 min. followed by forced air cooling. The chemical composition is shown in table 1. Table – 1. Chemical composition of High strength low alloy steel.

Element Chromium Manganese Molybdenum Vanadium Carbon Silicon Fe Min 1.25 0.8 0.8 0.2 0.12 - Balance Max 1.5 1.1 1.0 0.3 0.18 0.13

Welding is carried out along 500mm length to realise three tensile and three impact test specimens for each set of

weldment in the longitudinal direction (direction of rolling). Specimens for metallography and hardness traverse are also obtained after discarding 15-20 mm from edge of the weld seam.Tentative welding parameters are arrested through the bead - on plate trails. Based on the feedback from the bead-on plate trails, sample plates (500 mm x150mm x 5.2mm) are welded with Laser power:3.5 KW, and travel speed: 2 mm/min.

After welding of coupons, three specimens each for tensile and impact testing, specimen for metallographic and hardness traverse evaluations are machined. The welds are subjected to standard metallographic sample preparation to examine the microstructure under optical microscope. To reveal the microstructure, the low alloy steel part of the sample is etched using 2% Nital (2 ml HNO3 and 98 ml methanol).Microhardness survey was conducted across the weld beads at mid thickness on cross-section of all the welded coupons employing a Vickers micro hardness tester (Future-Tech FM-700). All of the hardness readings are obtained at a load of 100gf load for 15 S. Measurements have been done with 0.15 mm between indentations.Tensile testing is carried out on samples of 12.5 MM width and 100 mm parallel length with longer grip as per ASTM A- 370 using a 30 tonnes machine equipped with hydraulic grips. Impact testing is carried out on Standard sub size specimens as per ASTM standard. 3. Results and Discussions 3.1Microstructure:

2534 M.V.L. Ramesh et al. / Materials Today: Proceedings 2 ( 2015 ) 2532 – 2537

Coarse HAZ Coarse to weld interface Weld Center

A view of the laser beam welded joint of similar metal combination of high strength low alloy steel is shown in Figure 1. In the as welded condition of 15CDV6 similar welds, there are five distinct regions in the weld and HAZ as labelled in Figure-2 .Region A represents the unaffected parent metal during the welding, which exhibits a lower banitic structure with feather like feature. Region C is the coarse HAZ region. During welding, the parent material adjacent to the weld interface is heated to a high temperature. If the cooling rate is slow it transforms to banitic structure if the cooling rate is fast it leads to martensite structure. Region B is the fine HAZ region. Region away from the fusion zone experience lower peak temperature and the martensite decomposes into austenite, ferrite and carbide constituents. Region D is the interface of the weld region to the coarse HAZ. Region E is the fusion zone. It shows a martensite structure with epitaxial grain growth.

Fig 1: Laser beam welded joint of high strength low alloy steel

Base Material 15CDV6 Fine HAZ

15CDV6

MDN-250

15CDV6

A

A B

B

C

D

E

C D E

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Fig 2: Optical microstructure of weld and HAZS in High strength low alloy steel

3.2Hardness:

Figure 3: Hardness traverse across the Weldment (as welded)

Micro hardness measurement across the weld in the as welded condition shows four distinct regions as shown in

figure (3). These regions are: A: weld, B: Coarse-grained HAZ, C: fine HAZ and D: unaffected parent metal. During welding as the different regions exposed to various temperatures, the hardness also varies accordingly. It should be noted that the average weld metal hardness is 450 Hv. Hardness measurements crossing into HAZ region revealed a significant downwards trend in hardness (Region ‘B’) and inclining trend in region ‘C’ of the HAZ. Region ‘D” corresponds to unaffected parent metal.

The HAZ comprises the region of the base metal, which has been affected by the heat of welding. Since heat diffuses away through the plate, the thermal cycle becomes less severe with increasing distance from the fusion boundary. The microstructure formed in the HAZ is a function of the composition of the steel and of the weld thermal cycle. The regions of the HAZ near the fusion zone experiences a high peak temperature, exceeding the AC3 and also is subjected to fast cooling rates, which would result in coarse grained HAZ. The peak temperature controls the austenite grain size, while the cooling rate controls the intra-granular microstructure formed in the austenite. Both of these are in turn a function of the composition of base metal and the heat-input of the welding process. For the heat input and plate thickness used in this investigation, the microstructure of the coarse grained HAZ is characterized by being predominantly martensite7. Regions away from the fusion zone experience lower peak temperature and probably these regions are heated to within the inter-critical zone. The microstructure can be a mixture of ferrite and high-carbon austenite, which transforms to martensite on cooling. The HAZ nearer to the base metal would be exposed to temperatures in the vicinity of eutectoid transformation. This would therefore lead to decomposition of martensite to its equilibrium constituents, namelyferrite and cementite. The extent of decomposition depends on the time of exposure at the temperature above Ms. 3.3Tensile Properties: Table 2 shows the results of ultimate tensile strength, yield strength and total elongation for the welds. The properties of the parent metals are also included for comparison purpose. The UTS, 0.2% YS and % elongation in the as welded condition match with those of the parent metal in the as received condition. Table 2: Tensile properties of laser beam weld joint of High strength low alloy steel.

A

B C D

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Coarse HAZ Coarse to weld interface Weld Center

Material Condition UTS (Mpa)

0.2%YS (Mpa)

% Elongation

As received 1080 970 12 As welded 1020 918 12

The fracture occurred away from the fusion boundary. This may be due to presence of low hardness region as a result of over tempering of this region due to exposure at high temperatures during the welding process. 3.4IMPACT TOUGHNESS:

The impact toughness data of normalised sub size specimens as per ASTM standard for laser beam welds of HSLA steel is presented in table- 3. The parent metal data also presented for comparison.

Table 3: Impact Properties of laser beam weld joint of High strength low alloy steel.

Material Condition Impact Energy (joules) As received Parent metal 55 As welded Weld metal 50

The welds exhibited lower toughness data compared to parent metal. The low toughness of as welded HSLA steel may be due to the presence of untemperedmartensite.

4. Conclusions: The following are the conclusions that arise out of the present study:

The tensile strength values of the welded specimens were close to that obtained for sheet specimens, demonstrating that the alloy studied in this work is successfully welded by the Co2 laser source.

The fusion zone exhibits a epitaxial grain growth.

Similar metal weld of low alloy consists of acicular martensite near fusion boundary and just away towards the base metal martensite is seen to decompose into austenite, carbides and ferrite constituents. The hardness distribution shows that weld is having maximum hardness and parent metal is having minimum hardness.

Lower toughness is noticed in the weld due to the presence of tempered martensite.

Acknowledgements The authors would like to thank Dr.Tessy Thomas, Director, ASL, Dr. KJayaraman, Director, DRDL and Shri J. Ram Mohan GM, SFC for the continuous encouragement and permission to publish this work. The support received from MDD Group of DRDL is duly acknowledged.

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