Investigation of Welding Parameters On Mechanical Properties of Different Welding Joints of Mild Steel

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ISSN (Print): 2321-5747, Volume-1, Issue-1, 2013

23

Investigation of Welding Parameters On Mechanical Properties of Different Welding Joints of Mild Steel

1Bhaskar Vishwakarma, ²Manish Verma & ³Tribhuwan Kishor Mishra

^1&2CEC, BILASPUR, ³GGITS JABALPUR E-mail : [email protected], [email protected], [email protected]

Abstract – The effect of welding current, electrode diameter, voltage and welding techniques on mechanical properties of mild steel weldments have been studied in this paper. Mild steel work pieces were welded under varying current electrode diameter, voltage and different welding techniques. The test pieces out of these welded work pieces were made for tensile, hardness and impact tests. Specimens for microstructure examination were also prepared. These test specimens were then subjected to hardness, impact, tensile tests and microstructure examination. It was observed from these tests that with increase in current, tensile strength and hardness decreased significantly while absorbed energy increased .Increasing electrode diameter improves tensile strength and hardness while absorbed energy has been decreased.

With increase in voltage, tensile strength and hardness decreased while absorbed energy increased. The different welding techniques i.e. forehand, normal and backhand show more or less similar effect on the mechanical properties of weldments. In microstructure examination it has been found that with increasing current and voltage the grains tends to be coarser, while with increasing electrode diameter it tends to be finer.

Index Terms—Hardness, Ultimate tensile strength, Welding joint.

I. INTRODUCTION

A wide variety of categorizing joining processes.

welding process wherein coalesce is produced by touching the tip of a coated electrode to the work piece and then with- drawing it quickly to a distance sufficiently to maintain the arc . Tensile strength, Effect of current, electrode diameter, number of passes, and voltage and electrode has enormous effect on mechanical properties of weld, HAZ, weld bead shape and size. The various affecting weld quality in welding are classified into the following three groups:

primary variables

The welding current, voltage and welding speed are most commonly used to change the characteristics of the weld metal and are called as primary variables.

Welding current is the most influential variable in welding process which controls the depth of fusion, the geometry of the weldment and depth of penetration.

Welding voltage is the electrical potential difference between the tip of the welding wire and the surface of the molten weld pool. It determines the shape of the fusion zone and weld reinforcement. High welding voltage produces wider, flatter, less deeply penetrating welds than low welding voltage. Depth of penetration is maximum at optimum voltage.

In the welding process increase in welding speed cause:

 Decrease in the heat input per unit length of the weld.

 Decrease in the weld reinforcement.

If the welding speed decreases beyond a certain point, the penetration also will decrease due to the presence of the presence of the large amount of weld pool beneath the electrode, which will cushion the penetrating force.

Secondary Variables

Secondary variables include the stick out, wire feed rate, torch angle etc. Electrode inclination is the angle which the electrode makes with the normal at the arc point in the longitudinal plane. If the torch is maintained perpendicular to the joint, it is called normal. When the electrode is pointed in opposite direction, it is said to be backhand welding.

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24 Distinct Level variables

There are specific welding situation, electrode size and type. The type of polarity of welding current fall under this category. So it is obvious that these variables have great influence on the mechanical properties, microstructure and quality of the weldments. Hence by proper control of these variables we can achieve high quality welds. In the present work the effects of these variables i.e. voltage, current, electrode diameter and different welding techniques on mechanical properties of weldments have been investigated.

II. METALLURGICAL PHENOMENA INVOLVED Since the advent of welding technology it has been extensively used as fabrication process. It involves many metallurgical phenomena. Weld metallurgy, is considerably affected by following phenomena.

(a) The base metal and the filler metal if used resolidify as an integrated mass under the equivalence of chill casting conditions. This cause redistribution of the micro constituents and the alloying elements in the weld metal zone.

(b) The base metal is subjected to complex heat treatment in the form of temperature gradient extending from high temperature to room temperature and followed by a cooling cycle induced by the neighboring cold metal and atmosphere.

(c) The temperature and the phase change that take place in and around the weld introduces volume change which result in the plastic flow, residual stresses and sometimes cracking too.

1.1 Parent Metal (Unaffected)

It is adjacent to heat affected zone in the parent metal that is not heat up sufficiently to change its microstructures.

1.2 Heat Affected Zone(HAZ)

Between parent metal and weld metal is the heat affected zone which is composed of parent metal that does not melt but is heated to a high temperature for sufficient time. This results in grain growth which changes mechanical properties and microstructure. Heat affected zone consists series of graded structures and contains variety of microstructures. In planes carbon steels there structure may range from hard martensite to coarse pearlite. This cause HAZ to be the weakest area in the weldment. Most of the weld failure occurs in this region.

1.3 Weld Metal Zone

Weld metal zone is formed as the weld metal solidifies from molten state. This mixture of parent metal and filler metal, the ratio depending upon the welding process used, the type of joint, the plate thickness etc. Welding zone microstructure changes with cooling rate. Depending upon the composition, a martensite structure in the weld indicates a very fast cooling, fine pearlite and coarse pearlite shows comparatively cooling rates.

Welds are small casting except that weld metal often cools much more rapidly. Microstructure variations due to input and thermal stress may lead to cracking. The possibility of entrapment of gases or imperfections,

Depend upon the welding process, welding conditions, qualification level; of welder etc.

1.4 Basic Solidification principle

The solidification of metals usually considered to be a nucleation and growth process i.e. transformation of liquid phase to a solid, normally occurs by a process of nucleation growth.

Nucleation phenomena are classified as homogeneous or heterogeneous depending up to whether the nucleation involves the creation of critical sized particles (i.e. Nuclei) of new (i.e. solid) phase and considerable super cooling usually required before the first solid nuclei is formed from which growth may proceed.

After nucleation or in the presence of a preexisting solid/liquid interface growth occurs by the addition of atoms to the solid.

Fig. 1 Basic electric welding operation III. EXPERIMENTAL PROGRAMME Most of the work has been done on welding and little work has been done on mild steel with weld electrode (A+B) so for experimentation, the most commonly used and available mild steel was used as

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25 work piece material .once other reason to select mild steel is that it is widely used material in industries.

The electrode materials available are:-

a) Electrode specification SIZE 2.85/2.54 X 345 Current 80-130

b) Electrode specification SIZE 3.15/4.25 X 450 Current 90-140

In AC welding as the electrode will be operating at a much higher current to very lower current depend upon electrode erosion. Are responsible for the quantity of heat that is imparted to the weld pool. This quantity of heat is specified by means of a specified term “heat input”. Heat input is the quantity of heat introduced per unit length of the weld from a traveling heat source.

This is given by the expression:

Where

H = Heat input (KJ/mm) V = Welding voltage (volts) I = welding current (Ampere) S = Welding speed (mm/sec) η = Heat transfer efficiency Results & Discussion

Different parameter are examined as followed

0 200

0 20 40

Hardness (VHN)

Voltage (volt)

Effect of Voltage on Hardness

specime n 1 specime n 2

0 100 200

0 5

Hardness

Electrode size

Effect of electrode size on Hardness specimen 1

specimen 2

specimen 3

0 100 200

Hardness

Current

Effect of current on hardness

specimen 1 specimen 2 specimen 3

0 50 100

0 100 200

Absorbed energy

Current

Effect of current on absorbed energy(charpy Impact test)

0 20 40 60

0 5

Absorbed energy

electrode diameter

Effect of electrode size on absorbed energy

0 20 40 60

0 10 20 30

Absorbed energy

voltage (volts)

Effect of voltage on absorbed energy

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26 440

445 450 455 460 465 470

0 100 200

UTM (MPa)

current

Effect of current on ultimate tensile strength

430 440 450 460 470

0 20 40

UTS

voltage (volts)

Effect of voltage on ultimate tensile strength

430 435 440 445 450 455 460

0 5

UTS

electrode diameter

Effect of electrode diameter on utlimate tensile strength

0 50 100 150 200

0 100 200

Hardness

Current

Effect of welding techniques on hardness

Backhand Normal Forehand

0 20 40 60

0 100 200

Absorbed energy

Current

Effect of welding techniques on absorbes energy

430 440 450 460 470

0 100 200

UTS (MPa)

Current

Effect of welding techniques on ultimate tensile strength

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27 IV. CONCLUSION

The following conclusion may be derived from the experimental investigations:

a) As current increase impact strength of weld metal increased while hardness decreased.

b) With increase in welding current ultimate tensile strength decreased.

c) As electrode diameter increases impact strength decreased while hardness increased.

d) With increase electrode diameter ultimate tensile strength increased.

e) As voltage increases impact strength while hardness decreased.

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