5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,

IIT Guwahati, Assam, India

836-1

EXPERIMENTAL INVESTIGATIONS ON PLASMA ARC WELLDING

OF LEAN SUPERMARTENSITIC STAINLESS STEEL

1Birendra Kumar Barik, 1*P. Sathiya and 2S.Aravindan

1Department of Production Engineering, National Institute of Technology Tiruchirappalli-620015, Tamilnadu, India. 2Department of Mechanical Engineering,

Indian Institute of Technology Delhi, New Delhi-110016, India *Corresponding author: psathiya @ nitt.edu; Tel.:+91 431 2503510; Fax: +91 431

2500133

Abstract Nowadays the lean super martensitic stainless steel (LSMSS) becomes an economical alternative to the

traditional carbon and/or austenitic-ferritic (duplex) stainless steel for the construction of pipelines in transport

of gas and corrosive oils. Lean super martensitic stainless steel exhibits higher toughness, corrosion resistance

and weldability properties when compared to conventional martensitic stainless steel. The main purpose of this

study is to investigate the mechanical and metallurgical properties of welds made by the keyhole mode of

plasma arc butt welded joints. The macrostructure and microstructure were evaluated through optical

microscope. The mechanical properties such as tensile and impact tests were carried out at room temperature

and their fractured surfaces were also analysed through scanning electron microscope (SEM). The corrosion

resistance of the weld is determined by electro chemical analysis using Tafel plot and the same is correlated

with their microstructures.

Keywords:Lean super martensitic stainless steel, Mechanical and Metallurgical characterization, Corrosion resistance.

1 Introduction

The super martensitic stainless steels (SMSS) are

also known as weldable 12%Cr steels, weldable

13%Cr steels, low carbon martensitic stainless

steels and soft martensitic stainless steels. These

steels have carbon contents of the order of 0.01%

and Nickel addition in the range 1-6% to stabilize

the martensitic microstructures and chromium is

typically between 10-13%. Due to different

alloying and processing requirements, compared

with 22%Cr duplex stainless steels, these super

martensitic steels are substantially cheaper than the

competing duplex grades for pipeline and flow line

applications (Goldschmitz et al., 2004, Lauro et al.,

2003, Gooch et al.,1999, Carrouge 2002, Lippold et

al.,2005, van-der-Winden et al.,2002). Two classes

of SMSS may be identified: ′high′ and 'lean' grades.

Lean super martensitic stainless steel (LSMSS)

typically contain approximately 12%Cr, <4%Ni, no

molybdenum and about 0.01%C. The

microstructure of such steels is predominantly

tempered martensite with some austenite but delta

ferrite and untempered martensite may form in

weld HAZs (Woollin et al.,2002, Marshall et

al.,2001, Gooch et al.,1999, Folkhard 1984)

This alloy was developed as a practical and

economical alternative to carbon steel and

corrosion inhibitors, replacing some of the duplex

stainless steels used in offshore pipelines (Woollin

et al., 2006). Crack free welds with good HAZ

toughness can be processed without pre and post

weld heat treatment (Dufrane et al., 1999,

Enerhaug et al., 1999). In supermartensitic alloys,

the Cr and Ni ratio equivalents promote the

formation of a martensite and retained austenite

microstructure. The retained austenite phase may

represent a volume of only a few percent

(characteristic of low alloy) up to 40%

(characteristic of high alloy), but it is nevertheless

very difficult to identify because it is dispersed in

the martensitic structure (Toussaint et al., 2002).

Super martensitic stainless steels combine good

mechanical properties, weldability, toughness and

corrosion resistance to CO2 and H2S. Corrosion in

supermartensitic welds is clearly dependent on the

metallurgical phases present, like retained austenite

and δ ferrite. It was demonstrated that pitting

EXPERIMENTAL INVESTIGATIONS ON PLASMA ARC WELLDING OF LEAN SUPERMARTENSITIC STAINLESS STEEL

836-2

potential becomes nobler with the retained

austenite content. Moreover, δ-ferrite increases

corrosion susceptibility because it promotes the

precipitation of chromium nitrides or carbides and

the formation of chromium depleted zones (Bilmes

et al., 2006, Aquino et al.,2008). Also, micro

segregation, welding thermal cycles and inclusions

are important factors in the resulting microstructure

of a weld and its corresponding corrosion

susceptibility.

The plasma arc welding (PAW) closely

resembles the tungsten-inert gas (TIG) welding

process in that it also uses a non-consumable

tungsten electrode and a shielding gas such as

Argon. The keyhole welding is generally obtained

by using a stiff and constricted arc. With increased

plasma gas flow rate and electrode setback, a hole

known as the “keyhole” is pierced through the

entire metal thickness at the leading edge of the

weld pool, where the forces of plasma column

displaces the molten metal(Wu et al., 2011, Wu et

al., 2013).

The keyhole is a positive indication of full

penetration, and it allows higher welding speeds to

be used in PAW. LSMSS have high strengths,

around 480MPa proof strength and above

consequently, high strength filler metals are

required to provide matching weld metal strength.

Matching 12/13%Cr consumables have been

developed in solid wire and metal cored wire forms

but have not been successfully used for industrial

applications. Therefore, keyhole mode of PAW is

great alternative to weld LSMSS without using the

filler material. The preferred welding processes to

date for LSMSS have been automatic pulsed gas

metal arc (PGMA) and gas tungsten arc welding

(GTAW) welding. There is no metallurgical reason

why other conventional arc welding processes such

as manual metal arc (MMA) and flux cored arc

welding (FCAW) may not be used for girth welds.

At present, there is little information regarding

welding of LSMSS using high energy density

welding processes such as plasma arc welding

(PAW), laser beam welding (LBM), ion beam

welding (IBM).

In the present work, 410S lean super

martensitic stainless steel plates were welded using

keyhole mode of PAW process and the welded

joints were evaluated by mechanical and

metallurgical and corrosion tests.

2 Methodology

The flow chart of methodology is presented below:

Figure 1 Flowchart of methodology

2.1 Material

AISI 410S plates of 6 mm thickness, hot

rolled shot blasted plate in solution annealed &

pickled condition were used as the base material.

The base material has the yield strength of 348

MPa and ultimate strength of 480 MPa. The

specimens were prepared in dimension of 100 x

150 x 6 mm3.The chemical composition of the base

material is presented in Table 1.

Table 1 Chemical composition of base material

2.2 Welding Trials

Through preliminary trials welding parameters

were identified. Initially bead on plate experiments

were carried out and suitable parameters and their

range were obtained. The identified parameters

such as welding current (I), welding speed (S),

plasma gas flow rate (FR), nozzle standoff distance

(d) were used to conduct the keyhole mode of

plasma arc welding (PAW) on the material in flat

position. Using bead on plate tests, the full

penetration was achieved as shown in the Table 2.

The plasma arc welding (Fronius Magic Wave 440)

set up was used for welding is shown in Fig.2.

Figure 2 Experimental set up for PAW

Elements C Cr Ni Mn Si P S

In % 0.028 11.9 0.21 0.40 0.41 0.03 0.005

Welding Trials

PAW LSMSS

LSMSS Keyhole Plasma arc weldment

Mechanical

Invastigation Metallurgical

Invastigation

Corrosion

Studies

Results &

Discussions

Conclusions

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12

IIT Guwahati, Assam, India

Table 2 Parameters used for PAW p

S.

No.

d

mm

I

amp

S

mm/min

FR

l/min

1 4 140 375 1.5

2 4 160 375 1.5

3 4 160 250 2.5

4 4 170 125 2.5

5 5 160 375 1.5

6 5 160 250 1.5

7 5 160 125 2.5

8 5 170 125 2.5

9 6 170 125 2.5

10 6 170 375 1.5

11 6 160 375 2.5

12 6 160 250 1.5

13 6 160 375 1.5

14 6 160 250 2.5

but undercut

15 6 160 125 2.5

penetration

The cut blanks were machined with perpendicular

to each other than surface grounded to make air

tight gap between two joining plates. Just prior to

welding, plates were cleaned with fresh stainless

steel wire brush followed by acetone swabbing.

The welding process was performed under

as shielding gas and plasma gas. The photographic

views of the weld samples are presented in Figure

3. After welding, the weld surfaces were cleaned

with steel wire brush followed by acetone

swabbing. The weld was then subjected to liquid

penetrant test and radiographic tests.

confirmed that there is no micro cracks and

discontinuities.

Figure 3 Photographic view of weld sample

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12

Table 2 Parameters used for PAW process

Remarks

Not welded

Not welded

Not welded

Not welded

Not welded

Not welded

Partial

penetration

Not welded

Not welded

Not welded

Partial

penetration

Welded but

porosity

Partial

penetration

Full

penetration

but undercut

Full

penetration

without

defect

The cut blanks were machined with perpendicular

surface grounded to make air

es. Just prior to

were cleaned with fresh stainless

steel wire brush followed by acetone swabbing.

under Argon

as shielding gas and plasma gas. The photographic

views of the weld samples are presented in Figure

. After welding, the weld surfaces were cleaned

with steel wire brush followed by acetone

bjected to liquid

and radiographic tests. It is

no micro cracks and

Photographic view of weld sample

2.3 Mechanical Testing In order to evaluate the variation in the mechani

properties of the material tensile

temperature (RT), Charpy impact at RT and

and hardness tests were carried out.

specimens for all the mechanical testing were

prepared as per the guidelines of ASTM A370

standard. The fractured surfaces of tensile and

impact tested specimens were examined through

the scanning electron microscope (SEM).

2.4 Metallurgical Study

An optical microscope was used to identify the

microstructural changes in the base material, HAZ

and weld region. The specimens were prepared,

mechanically polished and etched with

reagent( 5 ml HCl + 1gm Picric acid + 100 ml

Ethanol) until the phases were identified.

2.5 Corrosion test details

The electrochemical measurements were perfo

using a conventional three electrode cell. It

contained a platinum grid, a saturated calomel

reference electrode (SCE) and a plate of the lean

supermartensitic stainless steel, of 1 cm² cross

sectional area, used as working elect

corrosive medium was prepared from di

water by adding 3.5% NaCl. Polarisation curves

were plotted under potentiodynamic

using a core running 1287 electrochemical interface

(ACM-Grill). The cathodic and anodic branches

were plotted in corrosion potential (Ecorr) verses

corrosion current (Icorr). The SEM images of base

and welded corroded surface were studied to

analyse the corrosion properties.

3 Results and Discussion

3.1 Macrostructure Examination

The weld bead was cross

correspondence of its normal symmetrical p

shown in the Figure 4, in order to investig

bead characteristics. Cross-section was

observation with optical microscop

geometry was studied by measurin

parameters, bead width, bead height and

penetration using image J software.

Figure 4 Weld bead profile

From Figure 4, it is observed that

achieved full penetration without any defects

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,

836-3

In order to evaluate the variation in the mechanical

ensile test at room

temperature (RT), Charpy impact at RT and -1960C

carried out. Standard

specimens for all the mechanical testing were

prepared as per the guidelines of ASTM A370

surfaces of tensile and

impact tested specimens were examined through

nning electron microscope (SEM).

An optical microscope was used to identify the

microstructural changes in the base material, HAZ

ecimens were prepared,

mechanically polished and etched with Villella’s

Cl + 1gm Picric acid + 100 ml

thanol) until the phases were identified.

The electrochemical measurements were performed

electrode cell. It

a saturated calomel

reference electrode (SCE) and a plate of the lean

supermartensitic stainless steel, of 1 cm² cross-

sectional area, used as working electrode. The

corrosive medium was prepared from distilled

Polarisation curves

were plotted under potentiodynamic regulation

ore running 1287 electrochemical interface

. The cathodic and anodic branches

were plotted in corrosion potential (Ecorr) verses

The SEM images of base

and welded corroded surface were studied to

Macrostructure Examination

cross-sectioned in

correspondence of its normal symmetrical plane, as

, in order to investigate the

was polished for

observation with optical microscope and the bead

by measuring the following

eters, bead width, bead height and depth of

Weld bead profile

it is observed that the weld has

full penetration without any defects with

EXPERIMENTAL INVESTIGATIONS ON PLASMA ARC WELLDING OF LEAN SUPERMARTENSITIC STAINLESS STEEL

836-4

wine cup shaped bead profile. The full penetration

in the weld zone due to the effect of keyhole mode

of PAW. The bead width and height are 12.71,

1.67 mm respectively.

3.2 Microstructure Characterization

The microstructure of the base material, 410S

LSMSS is shown in Figure 5. The steels typically

contain several percent of stable revert austenite

together with tempered martensite containing a

population of carbides. Published literature

indicates that it is possible to attain 20-30% stable

austenite in 13%Cr steel with 2-4%Ni, if tempering

is performed at about 100⁰C above Ac₁.[9-10]

Martensite, is a metallic magnetic or a

supersaturated solid solution of carbon trapped in a

body-centred tetragonal structure. It appears

microscopically as a white needle like or acicular

structure sometimes described as a pile of straw.

From Figure 5, it is observed that the dark phase

martensite is present in the matrix of bright phase

ferrite.

Figure 5 Microstructure of base metal

The HAZ microstructures of the LSMSS

are presented in Figure 6 with different

magnification. The banded structures of ferrite and

dark phase are martensites along with precipitated

globular particles of Cr2C3were observed. Less

amount of martensite than base metal

microstructure is observed. Some martensite

converted into ferrite phase.

Figure 6 Microstructure of HAZ

The weld metal microstructures are presented in

Figure 7. It is clearly seen that the two different

distinguished structures like dark phase and light

phase. Dark phase is called as martensite with leaf

structures and light phase is called as a ferrite

phase. More amount of leaf martensite was

observed as compared to HAZ but less as compared

to the base material. Some martensite got converted

into ferrite phase during welding process.

Figure 7 Microstructure of weld zone

3.3 Mechanical Evaluation

The welded specimens were tested for their tensile

strength. The obtained values for the percentage of

elongation, yield and tensile strength of the welded

joints are presented in Table 3.

Table 3 Tensile test results

Yield

Strength

(MPa)

Tensile

Strength

(MPa)

% of

Elongation

Point of

Fracture

378 512 14.20 Parent

Metal

The weld specimen exhibited higher yield strength

and tensile strength than the base material. The

fractured tensile test sample is presented in Figure

8, fractured at base material.

Figure 8 Tested tensile specimen

The base material got mean impact energy

of 190 J at room temperature (RT). For

measurement of toughness, the dominant parameter

is the ferrite grain structure and orientation. The

results of Charpy impact tests are given in Table

4.The results shows that weld specimen exhibited

more impact strength than base material at room

temperature and the impact energy is reducing

drastically as the temperature reduced to cryogenic

temperature (-196⁰C). Impact fractured surfaces is

shown in Figure 9.

50µm

50µm

50µm

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12

IIT Guwahati, Assam, India

(a) RT (b)-196⁰C Figure 9 Impact fracture specimens

Table 4 Impact energy values

Serial

No.

Testing

temperature

Impact value in

1 RT

2 -196⁰C

The fracture surface of tensile and impact

specimens were examined by Scanning Electron

Microscopy (SEM) to identify the mode of fracture.

The fractographic analysis of the tensile specimen

tested in air showed a ductile fracture by micro

void coalescence (dimples) as observed in

10. This typical dimple appearance may be also

attributed to the existence of internal interfaces due

to austenite particles and precipitates, which may

act as void nucleation sites. It is observed that

impact fracture at RT exhibited ductile fracture

at -1960C, it is cleavage and trans granular

completely brittle behaviour of fracture.

Tensile Test Impact RT Impact

Figure 10 SEM fractograph

The microhardness values for the welded

joint are shown in Figure 11. The parent metal

hardness was around 306 HV0.5 due to fine grain of

martensite and carbides are distributed in the

matrix. The hardness value dropped in the

grained weld zone to about 278 HV0.5

higher heat concentration in this region, leading to

coarse martensite and marginally higher ferrite

levels. In the HAZ region, a relatively

hardness value than weld zone was noticed (

HV0.5) in regions of a much finer martensitic

structure than that was observed in the weld metal

region. However from the statistical analysis

(p<0.05 is considered to be statistically significant)

there is a significant difference in hardness

observed between base and weld zones.

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12

Impact fracture specimens

values

Impact value in

joule

182

7

The fracture surface of tensile and impact

specimens were examined by Scanning Electron

the mode of fracture.

ographic analysis of the tensile specimen

tested in air showed a ductile fracture by micro

(dimples) as observed in Figure

This typical dimple appearance may be also

attributed to the existence of internal interfaces due

austenite particles and precipitates, which may

t is observed that

d ductile fracture and

trans granular fracture.

completely brittle behaviour of fracture.

Impact -1960C

SEM fractograph

The microhardness values for the welded

The parent metal

due to fine grain of

are distributed in the

The hardness value dropped in the coarse

0.5 due to the

higher heat concentration in this region, leading to

coarse martensite and marginally higher ferrite

HAZ region, a relatively higher

was noticed (279

) in regions of a much finer martensitic

ucture than that was observed in the weld metal

However from the statistical analysis

(p<0.05 is considered to be statistically significant)

in hardness was

base and weld zones.

Figure 11 Average micro hardness values at

different regions of the sample (*p=0.031

indicates the difference in hardness of base and

weld zones is significant)

3.4 Corrosion test results

Figure 12 shows the typical anodic polarization

curves for both welded and base metal specimens

and Table 5 lists their Ecorr and Icorr

the polarisation curve it reveals

specimen is more resistant to pitting

the base metal as the corrosion current (Icorr) value

is less. Tafel curve of welded sample is shifted

towards left, so more corrosion resistance as

compared to base material. The two phase

microstructure of LSMSS consisting of significant

amount of δ-ferrite.

Table 5 Corrosion test results

Sample Rp

mv

bA

mv

bC

mv

Ecorr

mv

1 Base -464.0 55.493 47.163 -522.72

2 Welded -455.0 40.793 35.864 -535.04

Figure 12 Tafel plots of base & welded specimen

4 Conclusions

Based on the experimental results, the following

conclusions were drawn.

i. The welds were produced without any defects.

ii. The microstructure consists of leaf martensite

and ferrite phase.

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,

836-5

hardness values at

different regions of the sample (*p=0.031

indicates the difference in hardness of base and

weld zones is significant)

shows the typical anodic polarization

se metal specimens

Icorr values. From

the polarisation curve it reveals that welded

pitting corrosion than

the base metal as the corrosion current (Icorr) value

sample is shifted

towards left, so more corrosion resistance as

The two phase

consisting of significant

Corrosion test results

Ecorr

mv

Icorr

mA/cm²

522.72 1.976X10¯ ⁴

535.04 7.54X10¯5

of base & welded specimen

Based on the experimental results, the following

produced without any defects.

consists of leaf martensite

EXPERIMENTAL INVESTIGATIONS ON PLASMA ARC WELLDING OF LEAN SUPERMARTENSITIC STAINLESS STEEL

836-6

iii. The higher tensile properties were obtained

than the base metal.

iv. The ductile mode of fractured was observed in

tensile fractured surface.

v. The impact energy value for welded joints is

more than base metal at room temperature and

drastically reduced at cryogenic temperature

(-196⁰C).

vi. The ductile mode of fractured was observed in

impact fractured surface at room temperature.

Cleavage and brittle mode of fractured was

observed in impact fractured surface at

cryogenic temperature (-196⁰C).

vii. The hardness of weldment is higher than the

HAZ hardness value.

viii. The corrosion resistance of welded sample is

more than the base metal sample.

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