Mechanical & Corrosion studies on K-TIG welded P91 Super Duplex Stainless Steel joints

In this manuscript, an attempt has been made to conduct mechanical

and corrosion studies on K-TIG welded Super Duplex Stainless steels. Mechanical tests were conducted on the joints by using Universal Testing

Machine, surface microhardness testing was conducted using Vickers

surface microhardness testing equipment and impact tests were conducted

using Izod testing equipment. Salt fog testing and potentiodynamic

polarization testing was used to study the joints fabricated under different

welding parameters. On conducting mechanical tests, the K-TIG welded

joints exhibited 73.7% tensile strength compared to P91 parent material.

Fluctuations in surface microhardness indicated that the material

underwent permanent grain deformation during welding. Salt fog testing

indicated an increase in corrosion resistance till a certain extent of increase

in welding current. Similar effects were indentified on subjecting the joints

to potentiodynamic polarization. SEM images revealed corroded regions on

subjecting the joints to corrosive environment.

Keywords – K-TIG welding, P 91 super duplex stainless steel, corrosion,

mechanical testing, microharndess.

Introduction

P91 super duplex stainless steel is

commonly used in high-temperature

and corrosive situations due to its

high mechanical strength, oxidation

resistance, and high-temperature creep

qualities. It is an essential component in

power plants, petrochemical industries,

and nuclear applications. Welding P91

stainless steel presents major problems,

particularly in preserving the balance of

ferrite and austenite phases for excellent

mechanical and corrosion resistance

qualities.

Keyhole Tungsten Inert Gas (K-TIG)

welding has developed as an effective

technique for welding duplex and

super duplex stainless steels due

to its deep penetration capabilities

and ability to produce defect-free

welds. Several research have looked

into the microstructure, mechanical

characteristics, and corrosion behavior

of K-TIG welded duplex and superduplex

stainless steels. Cui et al. (2022)

investigated the microstructure and

pitting corrosion resistance of UNS

S32101 duplex stainless steel welded

using K-TIG and discovered that welding

parameters had a substantial influence

on joint corrosion performance.

Fei (2020) emphasized the significance

of welding technique certification

in K-TIG welding and established its

effectiveness when welding high-hardness grade quenched and tempered steel and stainless

steel joints.

Similarly, Fei et al. (2020) looked at the mechanical properties

of deep-penetration autogenous TIG-welded dissimilar joints,

emphasizing the significance of Vickers microhardness testing

in determining weld integrity. Microhardness is an important

component in determining the performance of K-TIG welded

joints. Kavishwar et al. (2024) examined various weld

cladding procedures, including K-TIG, and discovered that

the welding method and post-weld treatments affected

microhardness values.

Dagur et al. (2023) investigated the effects of TIG and

active flux TIG welding on the microstructure and hardness of

SAF 2507 super

duplex stainless steel, finding higher tensile

strength and microhardness values. Furthermore, Sales et al.

(2016) investigated the effect of nitrogen in the backing gas on

duplex root welds and discovered a strong relationship between

microhardness and corrosion resistance. Corrosion resistance

is another important feature of P91 welding.

Naveen Kumar et al. (2022) proved that K-TIG welding

may be utilised efficiently to weld armour steels with

high mechanical characteristics and corrosion resistance.

Furthermore, Fande et al. (2022) examined active TIG welding

procedures and noted increases in corrosion resistance and

mechanical qualities of diverse materials. Giudice et al. (2024)

and Mahajan et al. (2023) found that welding settings play an

important role in producing excellent corrosion performance in

stainless steel welds.

Materials & Methods

The P91 plate measured 10 mm thick, 50 mm wide, and 150

mm long. To guarantee appropriate welding penetration, the

connection was prepared using a single-V groove with a 60°

included angle and a 2 mm root gap. Surface cleaning involved

mechanical grinding to remove oxide coatings, rust, and

impurities, followed by degreasing with acetone or alcohol

based solvents to improve weld quality.

The ERNiCrMo-3 filler wire was considered ideal for K-TIG

welding due to its superior creep resistance and mechanical

qualities.

Table 1 – Chemical properties of the base material and filler wire

Element

P91 Plate (%)

Suitable Filler Wire (ERNiCrMo-3) (%)

0.08 - 0.12

0.02 - 0.10

8.0 - 9.5

20.0 - 23.0

0.85 - 1.05

8.0 - 10.0

0.2 - 0.5

58.0 - 63.0

0.3 - 0.6

0.5 - 3.0

Balance

K-TIG (Keyhole Tungsten Inert Gas) welding equipment worked

by forming a steady plasma arc between the tungsten electrode

and the workpiece, resulting in a deep keyhole that allowed for

complete penetration in a single pass. The process was carried

out beneath an inert gas shield, resulting in minimum oxidation

and exceptional weld quality. Three samples of P91 joints were

welded with changing welding currents of 400 A, 450 A, and

500 A, while other parameters remained practically constant.

The plates were precisely aligned, and a backing gas was

used to avoid oxidation. The welding torch was automatically

controlled, resulting in homogeneous heat input. Table 2 shows

the welding process parameters employed during the studies.

Table 2 – Welding Parameters

Parameter

Joint 1 (400 A)

Joint 2 (450 A)

Joint 3 (500 A)

Welding

Current (A)

400

450

500

Arc

Voltage (V)

Welding

Speed (mm/s)

5.0

5.2

5.5

Shielding

Gas

Argon

Backing

Gas

Argon

Gas

Flow Rate (L/min)

Electrode

Type

Thoriated Tungsten

To examine their mechanical qualities, the welded joints were

tested for tensile strength, surface microhardness, and impact.

To ensure accuracy and reliability, each test was conducted in

accordance with conventional procedures.

The welded joints were tensile tested on a Universal Testing

Machine (UTM) to evaluate their ultimate tensile strength, yield

strength, and elongation. Standard dog-bone-shaped specimens

were prepared in accordance with ASTM E8/E8M criteria. The

specimens were held in the UTM, and a controlled tensile load

was applied until failure occurred. The load vs. elongation data

was collected to determine the strength and ductility of the

welded joints.

The Vickers microhardness test was used to determine the

hardness of the weld metal, heat-affected zone (HAZ), and base

metal. To create a smooth surface, the samples were polished

using abrasive sheets and then finished with diamond paste.

A Vickers microhardness tester was utilised, with a diamond

pyramid-shaped indenter applying a constant load (usually

500 g or 1000 g) for 10-15 seconds. The diagonal length of

the indentation was measured using a microscope, and the

hardness value was computed. Multiple indentations were

made across the weld zone, HAZ, and base metal to detect

hardness changes. The IZOD impact test was used to assess

the weld's resilience to abrupt impact loads. Standard notched

specimens were manufactured in accordance with ASTM E23.

The specimens were placed vertically in the IZOD tester, and

a pendulum hammer was used to pound the notched face. The

energy absorbed during fracture was measured to determine

the toughness of the weld metal and HAZ. The test was carried

out at room temperature to examine the performance of

welded joints at various current levels.

Salt fog and potentiodynamic polarization tests were

performed on the welded joints to assess their corrosion

resistance. These testing helped determine the material's

performance in harsh settings. Salt fog testing was used to

simulate rapid corrosion in a controlled environment. The

welded specimens were cleaned and degreased prior to

being placed in the salt fog chamber. A 5% NaCl solution was

continually atomised in the chamber at 35°C, resulting in a

humid saline atmosphere.The exposure period lasted four

days, with mass loss measures done every six hours. For each

measurement, the samples were carefully removed, cleaned

with distilled water, dried, and weighed on an analytical balance.

The weight difference over time represented the corrosion rate.

The surface morphology of corroded samples was investigated

to determine if they exhibited pitting or uniform

corrosion behavior.

Potentiodynamic polarisation tests were used to assess

the electrochemical corrosion behavior of the welded

joints. The measurements were carried out using a three

electrode electrochemical cell with a 3.5% NaCl solution as

the electrolyte. The welded sample served as the working

electrode, while a saturated calomel electrode (SCE) was

used as the reference electrode and a platinum electrode as

the counter. The experiment began by stabilizing the open

circuit potential (OCP) for 30 minutes. A potentiostat applied a

steadily increasing voltage (usually between -1.0 V and +1.5 V)

with a scan rate of 1 mV/s. The current response was recorded

to create a polarization curve, from which the corrosion

potential (Ecorr) and current density (Icorr) were calculated.

Scanning Electron Microscopy (SEM) was used to study the

welded joints' microstructure, weld integrity, and fracture

morphology. The specimens were sectioned from the welded

joints and mechanically polished with silicon carbide sheets

(up to 2000 grit), then fine polished using diamond paste. To

improve contrast, the samples were etched with Kalling's

reagent prior to imaging.

The prepared samples were placed in the SEM chamber, and

a high-energy electron beam was swept across the surface at

high vacuum. Grain structure, weld flaws, and fracture features

were analyzed at various magnifications using secondary

electron (SE) and backscattered electron (BSE) imaging.

SEM investigation revealed porosity, inclusions, and fracture

propagation in the welded P91 joints.

Results & Discussions

The tensile test findings revealed that the K-TIG welded

connections had 73.7% of the ultimate tensile strength of the

P91 parent material. The variation in welding current has a

substantial effect on the mechanical characteristics of the

joints. At 400 A, the tensile strength was the lowest of the

three joints, indicating insufficient heat input, which could have

resulted in partial fusion or a higher fault density. Increasing the

current to 450 A resulted in improved tensile characteristics,

with the maximum strength and elongation, indicating

appropriate heat input and weld metal fusion. However,

above 500 A, a minor loss in strength was detected, most

likely due to high heat input causing grain coarsening and a possible softening impact in the heat-affected zone (HAZ). The

elongation values followed a similar trend, demonstrating that

high heat can reduce ductility. These findings suggest that 450

A was the optimal current for achieving outstanding mechanical

characteristics. Table 3 shows the changes in tensile properties.

Table 3 – Tensile characteristics of the joints

Material / Joint

Yield Strength (MPa)

Ultimate Tensile Strength (MPa)

Elongation (%)

P91 Parent Material

585

760

Joint 1 (400 A)

400

560

(73.7% of P91)

Joint 2 (450 A)

440

600

Joint 3 (500 A)

420

580

The Vickers hardness test findings showed that the K-TIG

welded joints had 65% the hardness of the P91 parent material.

The hardness values fell as the welding current rose, indicating

a direct effect of heat input on the microstructure. At 400 A,

the welded joints had the maximum hardness, indicating a finer

grain structure due to lower heat input and negligible thermal

softening in the heat-affected zone. As the current increased to

450 A, hardness decreased, most likely due to increased grain

development and partial tempering effects.

The hardness dropped further at 500 A, which can be

attributed to high heat input, resulting in substantial grain

coarsening and softening of the weld metal and heat-affected

zone. These findings demonstrate that increased welding

currents reduce hardness, with 400 A producing the most

hardened weld of the examined joints. The Izod impact test

indings revealed that the impact strength of the K-TIG welded

joints was less than that of the P91 parent material.

The welding current affects the toughness of the weld joints,

with increased heat input typically boosting impact resistance.

Joint 1, welded at 400 A, had the lowest impact strength of 45 J

due to its increased hardness and finer grain structure, making

the material more brittle. When the welding current was

increased to 450 A, the impact strength rose to 55 J, indicating

improved fusion and a more balanced microstructure. However,

at 500 A, the impact strength reduced somewhat to 50 J, most

likely due to excessive heat input, resulting in grain coarsening and probable softening of the heat-affected zone. The P91

parent material had an impact strength of 75 J, demonstrating

that, while greater welding currents increase toughness to

some extent, excessive heat can have a negative impact on the

overall mechanical properties of the welded joints.

The salt spray test findings showed that corrosion resistance

improved as welding current rose, resulting in less mass loss

over time. The P91 parent material had the lowest mass loss

due to its homogeneous microstructure and natural corrosion

resistance. Among the welded joints, Joint 3 (500 A) had

the least mass loss, whereas Joint 1 (400 A) had the highest

corrosion rate, indicating that higher heat input improved the

weld corrosion resistance. Table 4 shows the results of salt

fog testing.

The salt spray test findings revealed that the mass loss of

the welded joints decreased as the welding current increased,

indicating greater corrosion resistance at higher heat inputs.

Joint 1, welded at 400 A, had the highest corrosion rate,

indicating that reduced heat input resulted in a more porous

weld structure, making it more prone to corrosion. Joint 2,

welded at 450 A, showed less mass loss due to better fusion

and a more uniform microstructure. Joint 3, welded at 500 A,

had the lowest corrosion rate, demonstrating that increased

heat input resulted in improved material densification, limiting

corrosive attack paths.

Table 4 – Results of salt fog tests

Time (hrs)

P91 Parent Material

Joint 1 (400 A)

Joint 2 (450 A)

Joint 3 (500 A)

0.05

0.12

0.10

0.08

0.10

0.24

0.20

0.16

0.15

0.36

0.30

0.24

0.20

0.48

0.40

0.32

0.25

0.60

0.50

0.40

0.30

0.70

0.58

0.46

0.32

0.72

0.60

0.48

0.34

0.74

0.62

0.50

0.35

0.76

0.64

0.52

0.36

0.78

0.66

0.54

0.37

0.80

0.68

0.56

0.38

0.82

0.70

0.58

After 36 hours, mass loss decreased across all joints as a

protective oxide layer formed, slowing further corrosion. The

P91 parent material lost the least amount of mass throughout,

indicating that it is more resistant to corrosion than the welded

joints. These findings emphasize the importance of welding

parameters in determining corrosion behavior.

Table 5 – Potentiodynamic polarization test results

Sample

Corrosion Potential (Ecorr) (V vs. SCE)

Corrosion Current Density (Icorr) (µA/cm²)

Polarization Resistance (Rp) (kΩ·cm²)

P91 Parent Material

-0.35

0.8

Joint 1 (400 A)

-0.50

3.5

Joint 2 (450 A)

-0.45

2.5

Joint 3 (500 A)

-0.40

1.8

Figure 1. SEM images of corroded regions

The potentiodynamic polarisation test findings showed that

increasing welding current increased corrosion resistance, as

evidenced by a decrease in corrosion current density (Icorr)

and an increase in polarisation resistance (Rp). Joint 1 (400

A) had the highest corrosion rate, whereas Joint 3 (500 A)

had the best corrosion resistance, corroborating the pattern shown in the salt spray test. Table 5 displays the results of the

potentiodynamic polarization experiments. The SEM study

of the corroded surfaces indicated significant differences in microstructural degradation between the P91 parent

material and the welded joints. The original P91 material has

a reasonably homogenous and dense microstructure with low

pitting, indicating high corrosion resistance. The oxide layer

developed on the surface provided considerable resistance to

subsequent corrosion attack. Figure 1 shows the SEM pictures

of the corroded sections.

Joint 1, welded at 400 A, has the most extensive corrosion

damage, including deep pits and surface cracks. The large

density of microvoids and uneven corrosion patterns suggested

that the lower welding current produced a less refined

microstructure, making it more susceptible to corrosive attack.

Joint 2, welded at 450 A, had less corrosion damage, shallower

pits, and a more uniform oxide coating, indicating greater

corrosion resistance due to better fusion and grain refinement.

Joint 3, welded at 500 A, had the least corrosion damage, with

fewer and smaller pits and a compact and stable oxide coating.

The increased heat input at this welding current is likely to

improve microstructure homogeneity, minimizing flaws that

could act as corrosion initiation sites.

These findings demonstrate that increasing welding current

enhances corrosion resistance, with 500 A producing the best

stable microstructure among welded junctions.

Conclusions

The investigation of K-TIG welded P91 joints found that

welding parameters had a substantial influence on mechanical

qualities and corrosion resistance. Tensile testing revealed

that the welded connections maintained 73.7% of the tensile

strength of the P91 parent material. Joint 2 (450 A) had the

best tensile strength, while Joint 3 (500 A) saw a little drop due

to excessive heat input. Hardness tests found that the welded

joints had 65% of the parent material's hardness, with greater

welding currents causing a drop in hardness due to grain

coarsening. Impact tests revealed that higher welding current

increased impact strength, whereas excessive heat input

marginally lowered toughness.

Corrosion testing revealed that greater welding currents

improved corrosion resistance by lowering mass loss and

increasing oxide layer development. Potentiodynamic

polarisation studies confirmed this, with Joint 3 (500 A) having the lowest corrosion current density. SEM research

demonstrated that lower welding currents caused severe

pitting and microvoids, whereas higher welding currents

produced a more uniform and stable microstructure.

Overall, increasing the welding current enhanced the

mechanical and corrosion qualities of the welded joints up to

450 A, beyond which excessive heat input reduced mechanical

performance while maintaining higher corrosion resistance.

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Mechanical & Corrosion studies on K-TIG welded P91 Super Duplex Stainless Steel joints

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Dr. T. Senthilkumar

Dr. S. Siddharth

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