Investigation of the Weldability of Austanitic Stainless Steel

This work concerns with the study of weldability of austenitic stainless steel 316 by using automatic tungsten gas shielded arc welding under various welding conditions under which it is designed to weld the samples. Results have been studied using impact and tensile strength tastings of the prepared welding joints using statistical approach. Results obtained showed that as gas flow rate of (CO2) increased the impact energy is increased, while increasing of welding current caused increasing of impact energy up to (120 ampere) then decreased. The tensile strength test results showed that as welding current is increased the tensile fracture load is decreased while increasing gas flow rate caused an increase in tensile fracture load up to 12 L/min then reduced. Microstructure examination of the weld zones did support the explanation of the variation of weld joint mechanical properties.


Introduction
The austenitic stainless steels, because of their high chromium and nickel content, are the most corrosion resistant of the stainless group providing unusually fine mechanical properties.Austenitic is the most widely used type of stainless steel.It has a nickel content of at least of 7%, which makes the steel structure fully austenitic and gives it ductility, a large scale of service temperature, non-magnetic properties and good weld ability.The range of applications of austenitic stainless steel includes house wares, containers, industrial piping and vessels, architectural facades and constructional structures [1].
Murat et al. [2] have studied on the resistance spot weldability of galvanized interstitial free steel sheets with austenitic stainless steel sheets.In microhardness measurements, the maximum hardness values were in the middle of the weld nugget.Emin Bayraktar et al. [3] have contributed their research on the selection of optimal welding conditions and developed new grade steels for automotive applications.The study based on impact tensile testing to spot welded sheets.The effect of nucleus size on mechanical properties in electrical resistance spot welding of chromide micro alloyed steel sheets was investigated by Aslanlar [4].Effects of Laser Welding Conditions on Toughness of Dissimilar ( austenitic stainless steel 316 with low carbon steel F/A) Welded Components was investigated by E. M. Anawa and A.G. Olabi [5] using Taguchi approach to optimize the dissimilar F/A joints in terms of its mechanical properties.
Bouyousfi et al. [6] have studied the effect of process parameters (arc intensity, welding duration and applied load) on the mechanical characteristics of the weld joint of austenitic stainless steel 304L.The results showed that the applied load seems to be the control factor of the mechanical characteristics of weld joint compared to the welding duration and the current intensity.Nizamettin K [7] has focused his study on the influence of welding parameters on the joint strength of resistance spot-welded titanium sheets.The results indicated that increasing current time and electrode force increased the tensile shear strength and the joint obtained under the argon atmosphere gave better strength.Hardness measurement results showed that welding nugget gave the highest hardness.L. Suresh Kumar et.Al [1] were experimental investigated welding aspects of AISI 304 and 316 by taguchi technique for the process of TIG & MIG welding.they used the TIG and MIG process to find out the characteristics of the metal after it is welded .The voltage is taken constant and various characteristics such as strength, hardness, ductility, grain structure, modulus of elasticity, tensile strength breaking point, HAZ are observed in two processes and analyzed and finally concluded.V Shankar et, al. [8], were investigated the solidification cracking which s a significant problem during the welding of austenitic stainless steels, particularly in fully austenitic and stabilized compositions.
They were study the solidification cracking in austenitic stainless steels with particular emphasis on nitrogen-alloyed and stabilized stainless steels.A statistical design of experiment (DOE) was used to optimise selected laser beam welding parameters (laser power, welding speed, and focus length).
Optimization of tensile strength of ferritic / austenitic laser welded components was studied by E.M. Anawa and A.G. Olabi [9].The experimental results indicate that the F/A laser welded joints are improved effectively by optimizing the input parameters using the Taguchi approach.The aim was to optimize the maximum ultimate tensile strength of F/A welded components, by minimizing the laser power and maximizing welding speed in order to optimize the cost and increase the production rate.
This work concerns with the study of weldability of austenitic stainless steel 316 by using automatic tungsten gas shielded arc welding under various welding conditions under which designed to welded the samples.The mechanical properties such tensile strength and notch impact strength are tested and investigated.

Base Metal Selection
The base metal used in the present work was stainless steel type ( Austenitic 316 ) the dimensions are: (1250 x 1000 x 2 mm (l x w x h)), the chemical compositions and the mechanical properties are shown in Tables 1 and 2 respectively.

Welding Process
The welding process was carried out in (General Pipe Company) by using TIG technique for welding process, this technique is widely used for different kinds of welding processes.The welding machine type is (PHOENIX 500) this machine can be used for multi welding processes such as: (TIG, MIG) technique, TIG welding technique was selected for this study to produce the joint for selected welding parameters.The welding process was carried out without filler metal.

Samples Preparation
The selected material was cut by using an electrical shear to (20) samples, which are exhibited in Fig 1 .Than the welding process was applied to join samples at different welding conditions which are presented in Table 3, using butt joint design, exhibited in Fig 2. *GFR : Gas Flow Rate, C: Current

Non Destructive Testing (visual inspection)
In an attempt to find any flout in the welded joints such as (cracks, pores …etc) and the welding penetration.The applied visual inspection was shows that acceptable quality of joints.

Tensile Samples Preparations
After finished the welding process the samples intended to be used for the tensile test were prepared, they were cut by using a hacksaw and no high temperature was generated that keeps the metal properties away from phase transformations, then the (Shaping Machine) was used to obtain a (V Groove) at the welded area, to prepare a Notched Tensile Strength (NTS) as shown in Fig. 4, [10] .For each welding condition five samples were prepared for tensile test

Microstructure Testing
Microstructure study was carried out in Mechanical Engineering Department.A small sample was cut from each welded plate to study the microstructure.Emery papers grades (80, 500, and 800) were used for grinding the samples before polishing .For etching stainless steel, a (1g) meta sulphate by meta sodium and (20 ml) of HCL and (100 ml) of distilled water was applied as an etchant.Microscopic examination was carried out using the ZIESS type optical microscope in Mechanical Department at Material laboratory at Benghazi University.

Results and Analysis
following welding process and preparing of samples, the testing process were performed and the results obtained are exhibited in Tables 4-7 and represents in Fig 7 -10.
The result analysis for the effect of gas flow and welding current on the mechanical properties are as follows:

Analysis of Impact Energy Results for Fixed Gas Flow Rate and variable Electrical Welding
Current.The welded samples were tested by charpy impact.An average of five impact specimens values were determined and tabulated in Table 4.These results are represented in a plot of impact energy with the variable welding current as shown in Fig. 7.
Table 4 shows that the maximum value of absorbed energy achieved was 26.8J at 120 A and the minimum value was 19 J at 80 A. The range between the maximum and minimum absorbed energy was 7.8 J.The results represented in Fig 7 shows that the effect of changing of welding current at a fixed gas flow rate, it is clear that increasing of welding current results to increasing in absorbed impact energy and toughness of the welded joint of 316 austenitic stainless steel.

Analysis Result of the Fixed Welding Current and Changing of Gas Flow Rate
The effect of gas flow rate on the impact energy of the welded joints of studied stainless steel was also investigated.Table 5 presents the results for the impact tests for the various shielding gas flow rates at a fixed welding current of 120 Amperes.The average of five values of impact strength specimens were plotted against the various gas flow rates as presented in Fig 8.The Y axis is presenting the mean of readings of impact tests.

Table 5 Impact Testing Result
From Table 5 the maximum value was achieved for impact test was 27.2J at 14 L/min and the minimum value was 24J at 7 L/min.The range between the maximum and minimum value was 3.2J.Fig 8 shows the effect of changing of gas flow rate at fixe welding current, and it is clear that increasing of gas flow rate caused increasing of absorbed energy which is reflecting increase in toughness of 316 stainless steel welded joints.The increasing of welding current and gas flow rate results an increasing of absorbed impact energy.

Analysis of Fixed Gas Flow Rate and Changing of Welding Electrical Current
Tensile testing was performed also for the study the effects of gas flow rates and welding currents on the tensile fracture load.For gas flow rate of 10L/min, welding currents of 80, 100,120, 160, and 180 amps were applied and five tensile samples were tested for each welding condition.

Analysis of the Fixed Electrical Current and Changing of Gas Flow Rate
The effect of gas flow rate was also studied on the tensile strength behavior of stainless steel.Table 7 presents the results for tensile strength at a fixed welding current of 120 A but for various shielding gas flow rates.The average of five tensile testing samples was plotted against the shielding gas flow rate in Fig10.From the Table 7 the maximum tensile strength test value achieved was 948 MPa at 12 L/min and the minimum value was 876 MPa at 8 L/min, the range between the maximum and minimum tensile strength value is 72 MPa.Where: The X axis is presents the changing in the gas flow.The Y axis is the mean of tensile strength.
Fig 10 shows the effect of changing of gas flow rate at fixe welding current and increasing of gas flow rate results an increase of tensile strength and hence the ductility of welded joints.Increasing of welding current, the tensile strength and ductility will be decreased were the increasing of gas flow rate caused increasing of tensile strength and ductility of welded joints.The increasing of electrical current was resulted in increasing of the impact energy and toughness but also cause decreasing for tensile strength and ductility.But the increasing of gas flow rate was resulted in increasing of impact and tensile strength which were an indication of increasing of toughness and ductility properties.

Solidification in the fusion zone
A review of solidification cracking in austenitic stainless steel welds shows that the problem is more prevalent in fully austenitic and stabilized stainless steels.Solidification mode is a major determinant of cracking susceptibility; ensuring an FA or F mode ensures the best resistance to cracking.An important aspect of weld solidification is the effect of solidification kinetics on the phases formed.The eutectic reaction L ↔ ϭ + γ in stainless steels is not typical of the classical eutectic in the sense that the composition difference between ϭ and γ phases is minor.The relatively minor difference in thermodynamic stability of the two phases in the vicinity of the eutectic permits non-equilibrium solidification to a metastable phase obtained by extrapolation of the equilibrium phase boundaries [11].Thus, the weld microstructure in stainless steels depends significantly on kinetic factors such as cooling rate and epitaxy, apart from equilibirum stability considerations.Under the rapid cooling and fast growth rates aided by epitaxy, the weld structure could solidify far away from equilibrium [12].
As Pb is insoluble in molten steel, and austenite / ferrite have a low capacity for dissolving S and P, all of these elements are vigorously segregated in the liquid during solidification.The resulting high impurity concentrations in the last liquid to solidify in the interdendritic regions have much lower melting points than those of the primary solidifying phase.The melting point of Pb is only 327-502°C and the melting point of the sulphides (MnS, FeS, CrS) is about 1100 -1200°C, i.e. much lower than that of Fe (1538°C).If sufficiently high stresses are generated before final solidification, the boundaries with segregated Pb and sulfides may separate to form solidification cracks in the fusion zone, which providentially was not observed in this experimental study.Figs.11(a, b) shows the base metal (BM) / HAZ of AISI316.

Conclusions
Referring to the previous discussion and the results obtained the following are concluded: 1.The welding parameters which studied in this manuscript were had high effect in the quality of welding and mechanical properties of the joints.

2.
The electrical current welding parameter had a great effect in the weld quality.It is inversely proportional with tensile strength and directly proportional with impact strength.

3.
The gas flow rate had strong effective on weld quality of (austenitic stainless steel 316).It is directly proportional with impact and tensile strength.

Fig 1 :Fig
Fig 1: Material after cutting process Testing:  Tensile Test Tensile tests were carried out in (General Pipe Company) at room temperature (25ºC), for all the prepared tensile samples.The machine test, and the diagram for tensile samples are shown in Fig 3 and Fig 4.

Fig 6 :
Fig 6: Schematic diagram of impact testing sample

Fig 8 :
Fig 8: A plot of Impact Test Fig 7: Plot of impact test the minimum value was 820 MPa achieved at 180 A. The range between the maximum and minimum tensile strength is 100 MPa.Fig 9 represents the effect of changing of welding current on tensile strength.It is clear that increasing of electrical current caused decreasing tensile strength and hence the ductility of the welded joints.

Fig 9 :
Fig 9: A plot of tensile test result Fig 10: A plot of tensile test result Effect of Welding Conditions, Welding Current, Gas Flow Rate and theirInteraction on Weld QualityInFig 7,  an increasing welding current (as it is increased to 120 A) the absorbed impact energy is increased while at low current (80 A) the impact energy was decreased.In Fig8where at increased gas flow rate (as it is increased up to14 L/min) the absorbed impact energy is increased while at low gas flow rate (7 L/min) the impact energy was decreased.In Fig9where for increased welding current as it is increased (100 A) the tensile strength is increased while at low welding current (80 A) the tensile strength was decreased.In Fig10whereas the gas flow is increased (12 L/min), the absorbed tensile strength is increased while at low gas flow (8 L/min) the tensile strength was decreased.

Fig. 12 :
Fig.12: The redistribution of elements in the fusion zone of a butt weld joining AISI316.

Table 1
Chemical Composition of Base Metal

Table 2 Mechanical
Properties of Base Metal

Table 6
presents the received testing results.To demonstrate the tensile strength behavior, the results were plotted as shown inFig 9.

Table 6
Readings of Tensile Test

Table 6
exhibits that the maximum tensile strength value was 920 MPa achieved at 100 A and

Table 7
Readings of Tensile Test