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EXPERIMENTAL PROCESS AND INFLUENCE OF PROCESS PARAMETERS ON MECHANICAL PROPERTY OF STAINLESS STEEL 430 PLATE IN TIG WELDING PROCESSVIKARM1, PANKAJ SHARMA2

1RESEARCH SCHOLAR, 2ASSOCIATE PROFESSOR1,2DEPARTMENT OF MECHANICAL ENGINEERING1,2GURU JAMBHESHWAR UNIVERSITY OF SCIENCE AND TECHNOLOGY, HISAR

Keywords: TIG welding, welding current, stainless steel 430, tensile strength, hardness, microstructure.

A B S T R A C T

This experimental study is carried out to analyses the effect of various parameters, like welding current, voltage, speed, time, gas flow rate, electrode diameters etc. on weld quality and property of TIG welded stainless steel plates of SS430 grade. The main objective of industry reveals with production and manufacture better quality product at low cost and higher efficiency. The main aim of varying process variables is to achieve maximum metal deposition rate. TIG welding is the most commonly used technique because of its versatility and ease that can be maintained in almost all type of conditions. TIG welding is an operation which is used to join the similar and dissimilar metals and non-metals at less time and cost. This type of welding mainly focuses on increasing the depth of penetration and reduction in width of weld bead. Stainless steel-430 is a non-hardenable steel containing chromium and belongs to the ferritic group of steels. The fatigue properties of SS430 in the case of welded condition are poor and it is not used where applied impact and tensile loading will be experienced. In this present work, we used TIG welding experimental process to study the effects of welding process variables on metal deposition rate and also mechanical properties of stainless steel 430 plate. Main input parameters- welding current, voltage, gas flow rate, speed, electrode gap or length- were selected to determine their effect on mechanical properties. The plates of 8mm thickness have been used as base metal to prepare single pass butt joints. From the experimental results it was found that tensile strength decrease with increase in heat input rate and from SEM of tensile test fractured surfaces exhibited brittle and ductile failure. From micro hardness data value it was observed that hardness of materials increase with increase in heat input in weld zone and decrease in HAZ. SEM shows that smaller dendrite size and lesser inter-dendrite spacing were observed in the weld zone at low heat input. From this investigation, it is found that GTAW joints of stainless steel grade 430 should superior mechanical properties compare with other welding joints. This is due to the formation of very fine microstructure in the weld zone. The microstructure observation show that microstructure reveals excessive delta ferrite strength in the matrix of austenite

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1. INTRODUCTION Welding is a fabrication process that joins the similar and non-similar metals and non-metals by causing coalescence. In the welding process the coalescence of materials is produced by heating them to the recrystallization temperature with or without the use of pressure and filler materials. The process is done by melting the work-piece and adding a filler material to form a weld pool that cools to become a strong joint [1]. Welding is used for permanent joints of metals and non-metals. A welded joint is produced when two clean surfaces come into contact with each other and pressure or heat is applied to obtain a strong metallurgical bond. Most of the products could not even be made without the use of welding process, e.g. nuclear power plant, aircraft, guided missiles, pressure vessels, transport vehicle, chemical process equipment and many others. In the welding process many of the problems are inherent that can be avoided by proper consideration of the characteristics and requirements of the process [2].TIG welding is a type of welding process and it is widely used in modern industries for joining similar and non-similar materials. It is also called gas tungsten arc welding (GTAW). The main advantages of TIG welding process are that requires low heat affected zone and absence of slag. The quality and accuracy of joints mainly depends upon welding speed, current, voltage, type of shielding gas, power supply, gas flow rate etc. It is relatively a high strength welding technique. TIG welding process has been a most popular choice of welding process when a considerable precision welding operation or high level of weld quality is required. Laser welding process easily automated and produces area phase transformation area about ten times smaller than TIG welding process, hardness in the fusion zone is quite high for both processes, but it was reduced to about 200 HV for the laser welded steel and about 100 HV in TIG process after tempering [3]. The main problem comes in the TIG welding process is limited thickness of materials which can be welded in single pass, poor tolerance to some material composition and low productivity. TIG welding uses a separate filler metal and a non-consumable electrode with an inert shielding gas. It is a manual welding process in which welder uses both hands to weld, one hand is used for adding the filer metal to the weld joints and other hands for holding the torch that produce the arc. The setup of TIG welding is consists of a cylinder of argon gas, welding torch having connection of cable for current supply, a suitable power supply, tubing for shielding gas supply, and water tubing for cooling the torch. The ferritic stainless steel joints fabricated by the

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addition of 2 g Al in post-weld annealed condition resulted in better tensile properties compared to all other joints, there is a marginal improvement in the ductility of ferritic stainless steel weldments by the addition of 2 g Cu in post-weld annealed condition compared to all other joints and base metal, the FSS welds made by the addition of 3 g Cu in as-weld condition resulted in increased hardness [4]. The transverse shrinkage generated in GTAW weld joint is comparatively lower than GMAW weld joint and the tensile strength of GTAW weld joint is higher than that of the GMAW weld joint [5].The weld penetration is increased while the weld metal width decreased, among the fluxes SiO2 flux had a significant effect on enhancing the weld penetration in A-TIG, and TIG welding can increase ultimate tensile strength of weldment because of increasing the retained delta ferrite content of stainless steel weld [6]. The behavior of the welded joints at the optimum condition of process parameters is attributed to the higher dilution of the base metal into the weld, resulting in an increased amount of Mg2Si precipitates that are formed in the aluminum matrix, the metallographic analysis reveals a fine grain structure at the weld center, which results in higher mechanical properties [7]. The depth of penetration of weld bead decreases with increase in bevel height of V butt joint and tensile strength is higher with lower weld speed, the lower range of weld speed is suitable for achieving maximum tensile strength [8]. In the TIG welding process mainly three things are require, the first things is heat which is produced by electricity passing through the tungsten electrode by creating an arc to the metal. the second things is shielding gas comes from a cylinder of gas flow to the weld area to protect from air and the last things is filler metal is a metal wire or rod that is added by hand into the arc and melted.

2. MATERIALS SELECTION Base metal: Stainless steel 430 grade.Many stainless steels are consider to have good weld ability. Stainless steel may be welded by many welding techniques including the TIG, MIG, laser and electron beam, resistance welding etc.in the welding of stainless steel joint surface and filler metal must be clean. The coefficient of thermal expansion for the ferritic types is less than the carbon steel and this must be assumed to minimize distortion. Stainless steel 430 grade is a non-hardenable types of steel and containing the more quantity of straight chromium (16-18%) and belong to ferritic group of steels. This type of steel used in many chemical application because of its resistance to nitric acid. It is an iron based alloy which contain chromium and a thin layer of chromium oxide film which surface of a stainless steel provides good corrosion resistance and prevent further oxidation.

Table 1 Property of stainless steel 430

S.NO.

PROPERTY VALUE

1 Density in kg/m3 77502 Elastic modulus in

MPa200

3 Thermal conductivity in w/Kgk

26.1

4 Specific heat in j/Kgk 4605 Tensile strength in

MPa483

6 Yield strength in MPa 3107 Elongation in

percentage22

8 Hardness (Rockwell) B 85

Table 2 Chemical compositionGrade

C Mn

Si P S Cr

SS 430

Min.

Max.

-

0.12

-

1

-

1

-

0.04

-

0.030

16-

18

3. PROCESS PARAMETERS

In TIG welding process variables play an important role in the quality, bead geometry and weld penetration. Knowledge of process variables is important and necessary to produce weld of

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satisfactory quality. The process variables are changing from one range to other to produce a desired results and that are not completely independent.The following parameters affect the quality of the weld:Welding currentWhen the current is high, TIG welding leads to splatter and work piece gets damaged. When the current is low, TIG welding leads to sticking of the filler wire. Fixed current mode is used to the voltage to maintain a constant arc current. Larger heat affected zone (HAZ) can be found for lower welding current.Welding voltageWelding voltage may be fixed or adjusted. It depends upon the TIG welding equipment. A high initial voltage allows for easy arc initiation. Too high voltage, can lead to a large variable in welding quality.Shielding gasesThe selection of shielding gas depends on base metal and affects the welding temperature, welding speed, electrode life etc. Ar or He may be used for TIG welding applications. For very thin materials, pure argon is used. Helium is used for aluminium and copper. Argon helium and hydrogen mixture is used for welding of steels and nickel alloys.Welding speedWhen the welding speed is increased, heat input per unit length of weld decreases and penetration of weld decreases. Welding speed controls the bead size and penetration of weld. It does not depend on current. Excessive high welding speed causes the uneven bead shapes, increase the tendency to porosity.Gas flow rateGas flow rate is important factor which is affected the results and output. Flow rate range generally be 6-7 litre/min. In TIG uses a lot of shielding gas so it pays to set up the gas flow accuracy for obtain proper results.Electrode and filler rodElectrode used in TIG welding is made of tungsten metal because it is melting point and temperature of tungsten is very high that is 34220c. As a result the electrode is not melt and consumed during the welding process. Electrode used in the process have been clean finish that means chemically free, uniform size and polished surface. This is helps to better heat transfer. The diameter of electrode can varies between the range 5-6.4 mm and length in 75-610 mm.The selection of filler rod is depend upon the type of metal is used e.g. if we use grade SS430 as base metal then the filler rod is used ER430, ER309 and ER310.

In this experiment process ER430 used as filler rod.PolarityMainly two types of polarity is used in welding process: 1. DCSP (direct current straight polarity) 2. DCRP (direct current reverse polarity)In DCSP type polarity TIG welding used torch negative and the work piece positive. This type of polarity is also called DCEN (direct current electrode negative). The negative torch helps to avoid overheating the tungsten electrode. It is mostly used in DC type current and rate of penetration is good.

In DCRP the torch or tungsten electrode is connected to the positive terminal and work piece negative. This type of connection is not commonly used because in this case tungsten can easily overheat and burn away. It is produce a wide profile. Only one third of the heat used on the work piece and remaining two third on the tungsten electrode.

4. EXPERIMENTAL WORK

Experimental set-up

The experimental setup used in the TIG welding process include the welding machine, shielding gas cylinder with gas regulator and pressure gauge, welding torch and a motor which carries and guide the welding gun and travel with the desired constant speeds along the plate to be welded.

Fig. 1 GTAW setupSample preparation SS 430 plates with the dimensions of 100×50×8 mm

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were cut into the required dimension using lathe and cutting machine. TIG welding was carried out using a TIG AC/DC 3000 watt welding machine. Single v butt joint was selected to prepare the joints and plates are tapered at 600. The selected welding parameters for this study were: current, voltage, gas flow rate, speed, time, electrode gap, filler rod, electrode diameter. Stainless steel grade 430 plates of thickness 8mm was selected as materials for TIG welding process. SS430 plates with the dimensions of 100x50 mm (LxB) were cut into the required dimension using lathe and cutting machine and grinding machine is used to smooth the joining surfaces of plates. After that emery paper is used to remove the external materials on the surfaces of plates.

Fig. 3 Filler rod ER 430

4. RESULT AND DISCUSSIONS

Metal deposition rate

It is defined by the ratio of weight of metal before welding minus weight of plate after welding to the welding time.

Heat input calculation

Heat input rate=V ×I ×ƞ×60S×1000

Sample number

Heat input rate valueKg/mm

1 2.3852 2.4683 3.0304 3.721Where

V = voltage in voltsI = current in ampereȠ = arc efficiency (taking 0.75)S = speed in mm

Tensile test

The ultimate tensile strength of the specimen after welding is measure in a universal testing machine (UTM) which has capacity of 400 KN. This test is carried out a welding to the ASTM standard. Test is conducted at room temperature and prepared specimen dimension for tensile test shown in figure. The weld sample has exhibited lower tensile strength as compared to base metal strength. In the table

Table 3 Process parametersSr. no. Current

ampereVoltage

voltGas

pressurebar

Gas flow rate

lit/min.

Speedmm/sec.

Timesecond

Electrode gapmm

1 145 18 0.5 10 49.23 1.5 22 150 18 0.5 10 49.23 1.5 23 150 20 1 12 44.50 1.7 24 160 23 1 15 44.50 1.12 2

MDR=weight of plateafter welding−weight of platebeforeweldingwelding time

MDR=6.44−6.431.05

= 0.006945mm/s

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indicate the welding parameters and corresponding tensile property of the welded joint of stainless steel 430. When the welding current is too small, the root of the welded joint can be unweld and this lead to the low tensile strength and elongation of the welded joint. When the welding current is too high defect like undercutting and collapse appear in the layer. With the increment of welding current, the tensile strength, yield strength and elongation of the welded joints all go up first and then fall down.HardnessTIG welded sample were examined for the Rockwell hardness measurement across the weld zone with 100kgf load. The experimental study shows that higher hardness at base metal as compared to WZ and HAZ. The main reason of increase or decrease hardness in the weld zone hardness was attribute to the repeated thermal process experienced during the multipass procedure with melting and solidification of

the filler metal. The hardness values of all specimen

shown in table for both the sample at HAZ, BM and WZ. The HAZ and WZ were observed by microstructure study having fine grains with dendrite type. This is show that wide variation in the hardness measurement in the weld zone. When the heat input rate is higher hardness value is also high shown in the case of sample no.3.

Table 6 Hardness value at different zoneSample number Base metal Heat effected zone Weld zone

1 B88 B87 B822 B86 B84 B823 B93 B91 B86

Table 5 TIG welding mechanical results

Sample number Tensile strength

MPa

Elongation

%

yield strength

MPa

1 389 23 327

2 385 23.58 332

3 377 24.2 338

4 370 25.03 342

145 150 150 1600

50100150200250300350400450

MECHANICAL PROPERTIES

tensile strength yield strength elongation

CURRENT A

STRE

NGTH

MPa

145 150 16076

78

80

82

84

86

88

90

92

94

HARDNESS CHART

CURRENT

HARD

NESS

VAL

UE IN

B S

CALE

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5. MICROSTRUCTURE

Microstructure to enable a visualization of the total weld area of the specimen. It is allow to the detail of the fusion zone and heat effected zone. TIG process, it can be seen in dark colour region. In the TIG welding, there is a slight bending of the plate and the separation between heat effected and welding zone. In the figure show the difference of the grain size at the

fusion zone. This difference is caused by the different cooling times in the TIG welding process. TIG cooling is faster because this process has more precise energy and fast speed, so the grain do not have the possibility of growing. In the MIG process, on the other hand has a slower cooling speed, generating a large HAZ and several different grain size which also appear large in the figure show that both welding process structure.In the below figure shown that the microstructure reveals excessive delta ferrite stringers in the matrix of austenite and protocol is ASM handbook vol.9:2007 is used. The magnification value is 100X and etchant is 3% is used. Full penetration joints were produced by TIG welding it can be seen that in figure crack and porosities cannot be found. Both welding the width and fusion area are different with each other. In the figure shows the morphology of welding joints under the current of 145, 150 and 160 A

respectively. The amount of heat is high under high current and small at low current. When the current is small the width of the weld pool are small and the root of welding joint is probably unwelded. The increase of welding current bring about the rise of width and depth of the weld pool. When the value of current is large, the amount of heat input is overlarge and even much more melt metal penetrates the back face of the layer, which leads to the collapse of the front face of layer and to the uplift of its back face. This situation caused the degradation of welding joint and stress concentration.

6. CONCLUSION AND DISCUSSIONSIn the above TIG welding process study following conclusion found out

In the study found that the process parameters had varying effect on the tensile strength, voltage having the higher effects.

Current is also affects the deposition rate very significantly.

Hardness is weaker on weld zone When the weld speed is lower than the tensile

strength is high. Strength is increased with decreasing the heat

input rate. High penetration no any defect produced by

the TIG welding.

In tensile test, all the specimens failed in the weld region which means that the weld region is weaker as compared to other regions. The tensile properties of

Table 6 Hardness value at different zoneSample number Base metal Heat effected zone Weld zone

1 B88 B87 B822 B86 B84 B823 B93 B91 B86

Fig. 4 Microstructure view of base metal and weld zone

Magnification: 100% Etchant: 3X

Fig 5 Microstructure of weld zone

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the weld joints presented in table indicate that GTAW exhibits superior properties that of GMAW.The grain size of the weld region is affected by the heat input supplied during the welding process. Due to high arc temperature, the cooling rate is slow and leads to wide spacing in fusion zone.Alternating current (AC) polarity is used in GTAW process. So, polarity changes continuously. When electrode is positive, high heat is generated and when electrode is negative, minimum heat is generated. Polarity also changes on work piece. So, work piece also attains maximum and minimum heat. That is why, heat input in GTAW is lower than others. Due to this lower heat, arc temperature is also low and cooling rate is high. So, the tensile strength of GTAW joints are higher than that of others joints. Hence, future works will be carried out from experimental point of view to better understand how the welding parameters effect weld pool characteristics and shape in TIG welding process. We can also improve the various properties like hardness, tensile strength, elongation etc. by using proper and actual parameters.

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Magnification: 100% Etchant: 3X

Fig 5 Microstructure of weld zone

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