Effect of Welding Processes on Tensile and Impact Properties, Hardness and Microstructure of AISI 409M Ferritic Stainless Joints Fabricated by Duplex Stainless Steel Filler Metal

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ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2009, 16(5): 66-72

Effect of Welding Processes on Tensile and Impact Properties, Hardness and Microstructure of AISI 409M Ferritic Stainless Joints Fabricated by Duplex Stainless Steel Filler Metal A K Lakshminarayanan ,

K Shanmugam ,

V Balasubramanian

[Centre for Materials Joining and Research (CEMAJOR). Department of Manufacturing Engineering, Annamalai University. Annamalai Nagar-608 002, Tamil Nadu , India]

Abstract: The effect of welding processes such as shielded metal arc welding. gas metal arc welding and gas tungsten arc welding on tensile and impact properties of the ferritic stainless steel conforming to AISI 409M grade is studied. Rolled plates of 4 mm thickness were used as the base material for preparing single pass butt welded joints. Tensile and impact properties, rnicrohardness , microstructure and fracture surface morphology of the welded joints have been evaluated and the results are compared. From this investigation. it is found that gas tungsten arc welded joints of ferritic stainless steel have superior tensile and impact properties compared with shielded metal arc and gas metal arc welded joints and this is mainly due to the presence of finer grains in fusion zone and heat affected zone. Key words: ferritic stainless steel; shielded metal arc welding; gas metal arc welding; gas tungsten arc welding; tensile; impact property Ferritic Stainless Steels (FSS) constitute approximately one-half of the AISI type 400 series stainless steels. These steels contain 10.5% to 30% of Cr along with other alloying elements, notably molybdenum. FSS are noted for their excellent stress corrosion cracking (SeC) resistance and good resistance to pitting and crevice corrosion in chloride environments!' J. While these alloys have useful properties in the wrought condition, welding is known to reduce toughness, ductility, and corrosion resistance because of grain coarsening and formation of martensite. For these reasons, the application of this group alloys is Iimited-'". The welding arc heats a zone in the base metal above a critical temperature (955 C) and causes rapid grain growth of the ferrite. This coarse grain zone lacks ductility and toughness and a small amount of martensite may be present, which leads to increase in hardness. Excessive grain growth can be avoided, of course, by using lower welding heat inputs. It has also been suggested that nitride and carbide formers such as B, AI, V and Zr can be added to FSS Biography: A K Lakshrninarayanant 1980-). Male. Doctor Candidate:

to suppress grain growth during weldingl'", Villafuerte and Ken!!] attempted to weld ferritic stainless steel by gas tungsten arc welding (GTAW ) process and observed that approximately for constant values of heat input per unit distance, the equiaxed fraction increased with welding speed, as long as sufficient titanium and aluminum were present to form nucleate for the second phase. In a later study by Villafuerte et a1C5] , the tin quenching of GTA welds of commercial ferritic stainless steels had shown direct evidence of heterogeneous nucleation of equiaxed grains on tin particles ahead of advancing columnar interfaces. However, the addition of titanium alone did not lead to the formation of equiaxed grains, despi te an increased tendency for branching and the protrusion of some primary dendrite stalks ahead of others. Mohandas et al[6] made a comparative evaluation of gas tungsten and shielded metal arc welds of AISI 430 ferritic stainless steel and found that the greater ductility and strength of gas tungsten arc welds as compared to those of shielded metal arc welds can be E-mail: akln2k2@yahoo. rom:

Revised Date: April 27. 2009

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Effect of Welding Processes on AISI 409M Ferritic Stainless Joints

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attributed to the equi-axed morphology of the fusion-zone grains in the gas tungsten arc welds, and also to inert gas shielding. Meyers and Toit[7] investigated the impact properties of 11 %-12 % of chromium steels and found that carbon and nitrogen affect the impact properties of the heat-affected zone in these steels. Silva et al[8] , investigated the microstructural characteristics of the HAZ in AISI 444 ferritic stainless steel and reported that needle-like Laves phase precipitation occurred in the HAZ, near the partially-melted zone and other secondary phases such as chi and sigma were also observed, as well as nitride, carbide and carbonitride precipitates. Even though, the ferritic stainless steels have lot of potential to become a cheaper alternative to austenitic stainless steels, the fabrication processes, especially arc welding processes are not conducive to this material due to above discussed problems. Traditionally, these steels are welded using austenitic stainless steel fillers to avoid grain coarsening and also the formation of martensite in the weld region. However, recent studies[9] proved that duplex stainless steel consumables can be used effectively to weld ferritic stainless steels which can also yield higher strength compared to austenitic stainless steel consumables. Keeping this in mind, an investigation was carried out to study the effect of welding processes such as shielded metal arc welding (SMAW), gas metal arc welding (GMA W) and gas tungsten arc welding (GTAW) on tensile and impact properties of the ferritic stainless steel (AISI 409 M Table 1

grade) joints fabricated using duplex stainless steel consumables and the results are revealed in this paper.

1

Experimental Work

The rolled plates of 4 mm thickness AISI 409M grade ferritic stainless steel were cut into the required dimension (300 mm X 150 mm ) by oxy-fuel cutting and grinding. The initial joint configuration was obtained by securing the plates in position using tack welding. Square butt joints were fabricated using welding processes such as shield metal arc welding, gas metal arc welding and gas tungsten arc welding. All necessary care was taken to avoid joint distortion and the joints were made with applying clamping devices. Duplex stainless steel (DSS) consumables were used to fabricate the joints. The welding conditions and process parameters used to fabricate the joints are given in Table 1. The soundness of all the welded plates was checked using ultrasonic testing. The chemical composition of the base metal and all weld metals were obtained using a vacuum spectrometer (ARL-Model: 3460). Sparks were ignited at various locations of the samples and their spectrum was analysed for the estimation of alloying elements. The chemical composition of the base metal and all weld metals in weight percent is given in Table 2. The welded joints were sliced using power hacksaw and then machined to the required dimensions for preparing tensile and impact test specimens. Two different tensile specimens were prepared as shown in Fig. 1 (a) and (b). The unnotched smooth tensile

Welding conditions and process parameters Process

Parameters SMAW

GTAW

GMAW

Welding machine

Lincoln. USA

Lincoln. USA

Polarity

DCRP

AC

DCRP

Arc voltage/V

25

22

30

Welding current/ A

Lincoln. USA

120

90

150

3

2.5

4

Heat input/O· mm- I)

1 000

800

1 125

Electrode diameter/mm

4.0

2. 0

1.6

Welding speed/j mrn •

S-I)

Shielding gas

Argon (99.99%)

Argon (99.99%)

Shielding gas flow rate/Ot· min-I)

14

14

Table 2

Chemical composition of base metal and filler metal

Material

(mass percent.

C

Mn

P

S

Si

Cr

Ni

Ti

Base Metal (AISI 409M)

0.028

1. 10

0.030

0.010

0.40

10.90

0.39

0.004

DSS (AISI 2209)

0.030

0.80

0.018

0.016

0.80

22.00

9.00

Mo

Cu

%) Fe

Balance 3.00

Balance

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Journal of Iron and Steel Research, International

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Microstructural examination was carried out using a light optical microscope (Make: MEJ 1, Japan; Model: MIL-7100) incorporated with an image analyzing software (Metal vision). The specimens for rnerallographic examination were sectioned to the required size from the joint comprising weld metal, HAZ and base metal regions and were polished using different grades of emery papers. Final polishing was done using the diamond compound (1 fJ-m particle size) in the disc polishing machine. The specimens were etched with 5 mL hydrochloric acid, 1 g picric acid and 100 mL methanol applied for 10 - 15 s. The fractured surface of the tensile and impact tested specimens was analysed using Scanning Electron Microscope (Make: JEOL, Japan; Model: 6410LV) at higher magnification to study the fracture morphology to establish the nature of the fracture.

(h)

I.

(c)

1:"

2

I.

.1

(a) Unnorched tensile specimen, (c)

Fig. 1

Cb) Notched tensile specimen;

Sub-size impact specimen

Dimensions of tensile and impact specimens

specimens were prepared to evaluate transverse tensile properties of the joints such as yield strength, tensile strength and elongation. The notched specimens were prepared to evaluate notch tensile strength and notch strength ratio of the joints. Tensile test was conducted in 100 kN, electro-mechanical controlled Universal Testing Machine (Make: FIE-BLUE STAR, India; Model: UNITEK-94100 ), ASTM E8M-04 guidelines were followed for preparing and testing the tensile specimens. Charpy Impact specimens were prepared to the dimensions shown in Fig. 1 (c) to evaluate the impact toughness of the weld metal. Since the plate thickness is small, subsize specimens were prepared. Impact test was conducted at room temperature using pendulum type impact testing machine (Make: ENKAY, India) with a maximum capacity of 30 J. The amount of energy absorbed in fracture was recorded and the absorbed energy is defined as the impact toughness of the material. ASTM E23-04 specifications were strictly followed for preparing and testing the specimens. Vicker' s microhardness testing machine (Make: Shimadzu , Japan; Model: HMV-2T) was employed for measuring the hardness of the weld with 0.5 kg load.

2. 1

Results

Tensile properties The transverse tensile properties such as yield strength, tensile strength and percentage of elongation, notch tensile strength and notch strength ratio of SMA, GMA and GTA welded joints were evaluated. In each condition, three specimens were tes: od. and the average of three results is presented in Table 3. The yield strength and tensile strength of unwelded base metal are 359 MPa and 480 MPa respectively. But the yield strength and tensile strength of SMA W joints are 230 MPa and 285 MPa, respectively. This indicates that there is a reduction of 41 % in strength compared to base metal. Similarly, the yield strength and tensile strength of GMAW joints are 215 MPa and 270 MPa, respectively which are 44% lower compared to base metal. However, the yield strength and tensile strength of GTA W joints are 310 MPa and 390 MPa, respectively. Of the three welded joints, the joints fabricated by GTA W process exhibited higher strength values, and the enhancement in strength value is approximately 27 % compared to SMAW joints and 31 % compared to GMA W joints. The elongation of unwelded base metal is 12%. But the elongation of SMAW joints is 8. 35 %. This suggests that there is a 30 % reduction in ductility due to SMA welding. Similarly, the elongation of GMAW joints is 7. 25 %, which are 39 % lower compared to the base metal. However, the elongation of GT AW joints is 9. 56 %. Of the three welded joints, the joints fabricated by GTA W exhibited higher due-

Table 3 Joint

Transverse tensile and impact properties of base metal and welded joints

Yield strength/MPa

Tensile strength/MPa

Elongation/

%

Notch tensile strength/MPa

Notch strength ratio/NSR

Impact toughness/J

Base metal

359

480

12. 0

577

J. 20

20

SMAW

285 270

8.35

325

J. 14

18

GMAW

230 215

7.25

286

J. 06

16

GTAW

310

390

9.56

465

J. 19

22

tility values. and the improvement in ductility is approximately 13 % compared to SMAW joints and 24 % compared to GMAW joints. Notch tensile strength (NTS) of unwelded base metal is 577 MPa. But the notch tensile strength of a SMAW joint is 325 MPa. This reveals that there a decrease of 44 % in NTS due to SMA W welding. Similarly, the NTS of GMA W is 286 MPa and the NTS of GTA W is 465 MPa. Of the three welded joints. the joints fabricated by GTAW exhibited higher NTS values. and the enhancement is 30 % compared to SMAWand 38% compared to GMAW. Another notch tensile parameter. NSR. is found to be greater than unity C> 1) for all the joints. This suggests that the joints are insensitive to notches, and they fall under "notch ductile materials" category. The NSR is 1. 20 for unwelded base metal. but it is 1. 14 and 1. 06 for SMAWand GMAW joints respectively. Of the three welded joints, the joints fabricated by GTAW exhibited relatively higher NSR (1. 19). and the improvement in NSR is 2. 6 % compared to SMAW and 11 % compared to GMAW process. Joint efficiency is the ratio between tensile strength of welded joint and tensile strength of the unwelded parent metal. The joint efficiency of GMA W joints is approximately 56 % and the joint efficiency of SMAW joints is 59 %. Of the three types of welded joints. the joints fabricated by GTA W exhibited a relatively higher joint efficiency (81 %) • and the joint efficiency is 31 % higher compared to the GMAW joints and 27 % higher compared to SMA W joints.

2. 2

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Effect of Welding Processes on AISI 409M Ferritic Stainless Joints

Issue 5

impact toughness of GTAW joints is 20 J. Of the three welded joints. the joints fabricated by GTAW process exhi bi ted higher impact toughness val ues , and the enhancement in toughness value is approximately 10% compared to SMAW joints and 20 % compared to GMAW joints.

2. 3

Hardness and microstructure

The hardness across the weld cross section was measured using a Vickers Micro-hardness testing machine. and the results are presented in graphical form as shown in Fig. 2. The hardness of weld region is greater than the heat affected zone (HAZ) region and the base metal region. irrespective of welding processes. The hardness of the SMAWand GMA W joints in the weld metal region is 262 VHN and 275 VHN respectively. However, the hardness of the GTAW joints in the weld metal region is 306 VHN. which is relatively higher compared to SMAWand GMA W joints. Microstructure of all the joints was examined at different locations and the optical micrographs taken at weld metal (WM) region. heat affected zone. WM-HAZ are displayed in Fig. 3. The joints fabricated by SMAW. GMAWand GTA W processes primarily contain both solidified dendritic austenite and ferrite matrix. Heat affected zone primarily contains coarse grained ferrite microstructure irrespective of welding processes. 400 r - - - - - - - - - - - - - - - - ,

Impact toughness

Charpy impact toughness values of all the joints were evaluated and they are presented in Table 3. The impact toughness of unwelded base metal is 22 J. But the impact toughness of SMAW joints is 18 J. This indicates that there is a decrease of 18 % in toughness value due to SMAW welding. Similarly. the impact toughness of GMAW joints is 16 J. which is 27 % lower compared to base metal. However. the

200 '--_ _--'--''--_ _--' o 4.00 8.00 12.00 16.00 Distance from weld centre/mm

Fig. 2

Hardness across weld

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A: Austenite; F: Ferrite (a) SMAW-interlace, (b) S'v1AW-weld centre, (c) SMAW-HAZ; (d) GTAW-interface, (e) CTAW-weld centre, (f) GTAW-HAZ, (g) GMAW-interlace, (h) GMAW-weld centre: (i) (;MAW-HAZ

Fig. 3

2. 4

Optical micrographs of welded joints

Fracture surface The fractured surface of tensile specimens of welded joints of ferritic stainless steel was analyzed using SEM to reveal the fracture surface morphology. Fig. 4 displays the fractographs of tensile specimens and impact specimens. The tensile and impact fracture surface of DSS weld metal joints show ductile fracture irrespective of welding processes. However dimple size is much finer in GT A welded joints com-

pared to SMAW and GMAW welded joints. Elongated dimples are seen in GMA W joints. Large numbers of fine dimples are seen in GTA W joints Fig. 4 (c) compared to SMA Wand GMAW joints. Since fine dimples are a characteristic feature of ductile fracture. the GTA W joints have shown higher ductility compared to all other joints (Table 3). Impact fracture surfaces showed ductile fracture [Figs. 4 (d). (e) and (0]. Finer dimples are seen in GTAW com-

(a)-(c) Tensile specimen; (a)

SMAW;

(b) GTAW;

Fig. 4

(c)

GMAW;

(d)-(f) Impact specimen

(d) SMAW;

(e)

GTAW;

(£)

GMAW

SEM fractographs of tensile and impact specimens

pared to SMAWand GMAW joints.

3

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Effect of Welding Processes on AISI 409M Ferritic Stainless Joints

Issue 5

Discussion

Transverse tensile properties of the welded joints presented in Table 3 indicate that the GTA W joints are exhibiting superior tensile properties and impact toughness compared to SMAWand GMAW joints. During tensile test, all the specimens invariably failed at the heat affected zone (HAZ) irrespective of welding processes. This implies that the fusion zone region is much stronger than the HAZ. This is also evident from the hardness profile shown in Fig. 2. The region very adjacent to the fusion zone region undergoes softening. The softening characteristics is influenced by the weld thermal cycles or in other words by heat input supplied by the welding process. HAZ primarily contains coarse grained ferrite microstructure irrespective of welding processes, but, there is an appreciable difference in the grain size. Weld metal regions of all joints primarily contain solidified dendritic structure of austenite and ferrite matrix. Of the three welding processes used, GMAW process produces coarser grains compared to SMAWand GTA W processes. The heat input supplied by the GMAW process is relatively higher than SMAW and GTAW processes (Table 1). The variations in heat input of the welding processes influence the weld thermal cycle and subsequently causes variations in microstructural features and hardness char-

acteristics of fusion zone and HAZ. Of the three welding processes used in this investigation to fabricate the joints, the GTA W process has lower heat input (800 J/ mm) compared to the SMAW and GMAW processes. Since GMAW is a consumable electrode process, the filler metal is always connected to positive (reverse) polarity of the direct current (DCRP). This leads to a large amount of heat generation (approximately twothirds of total heat generation) at the filler metal end. Further, a current of 150 A is passing through a small diameter of filler metal (1. 6 mm), and the current density is very high in the GMAW process. Much heat generation and very high current density combine to enhance the arc temperature and arc forces[IO]. Very high arc temperature increases the peak temperature of the molten weld pool and adjacent HAZ causing a slow cooling rate. This slow cooling rate, in turn, causes relatively coarser grains in the fusion zone and HAZ. On the other hand, very high arc force increases the depth of penetration and enhances the welding speed correspondingly (Table 1). This may be the one of the reasons for the formation of coarse grains Fig. 3 (a) in HAZ of GMAW joint. In GTA W, the alternating current (AC) polarity is used, and the high heat generation end is continuously changing (50 times in one second). Whenever, the electrode becomes positive, more heat is genera-

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Journal of Iron and Steel Research, International

ted (two-thirds of total heat) at this end, Similarly. whenever the workpiece becomes positive. more heat is generated at this end, In one half of a cycle. electrode attains maximum heat and in the other half of a cycle. the workpiece attains minimum heat. and this will change in thenext cycle I!] , So. while using alternating current. the maximum heat generation end is not fixed as in the case of GMA Wand SMAW. Whatever. it may be the process. the heat energy from the arc is utilized to melt the filler metal as well as to melt the base metal, However in GTA W. the filler rod is melted in the plasma region of the arc (midway between positive and negative polarity) and not in the positive polarity as in the case of GMA Wand SMA W processes. Due to this reason. heat input of GT A W process is lower than for SMA Wand GMAW processes. Lower heat input and lower current density reduces the arc temperature and arc forces in GTA W' 12], Lower arc temperature reduces the peak temperature of the molten weld pool and adjacent HAZ causing a fast cooling rate. This fast cooling rate. in turn. causes relatively narrower dendritic spacing in the fusion zone, These microstructures generally offer improved resistance to indentation and deformation[U i and this may be one of the reasons for higher hardness and superior tensile and impact properties of GT AW joints compared to SMA Wand GMA W joints,

Annama lairragar , Tamil Nadu . India for extending the facilities of Metal Joining Laboratory and Materials Testing Laboratory to carryout this investigation, The authors are very grateful to Dr. G. Madhusudhan Reddy. Scientist-F. Defence Metall urgical Research Laboratory (DMRL). Hyderabad for his valuable suggestions. guidance and discussion. References: [IJ

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4

Conclusions

[8J

Cleiton C Silva • .Ie"ualdo P Faria", Helio C Miranda. et al. Microst ruct ural Characterization of the HAZ in AISI 4..]4 Ferru ic

Of the three welded joints. the joints fabricated by GTAW process exhibited higher tensile strength values and the enhancement in strength value is approximately 27 % compared to SMAW joints. and 31 % compared to GMAW joints, (2) Of the three welded joints. the joints fabricated by GTA W process exhibited higher impact strength values and the enhancement in strength value is approximately 10 % compared to SMA W joints. and 20 % compared to GMA W joints. (3) Hardness is lower in the HAZ region compared to the weld metal and base metal regions irrespective of welding technique. Very low hardness is recorded in the GMA W joints (262 VHN) and the maximum hardness is recorded in the GT AW joints (306 VHN), (1)

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The authors are grateful to the Department of Manufacturing Engineering. Annamalai University.

Aluminium Alloy Joints [J]. International Journal Advanced Manufact uring technology. 2009. ·10: 286.