chapter 8 metallurgical studies on stainless steel...

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CHAPTER 8

METALLURGICAL STUDIES ON STAINLESS STEEL

CLADDINGS

8.1 INTRODUCTION

Successful cladding requires thorough understanding of base metal

and the clad metal. In this investigation, AISI 316 L austenitic stainless steel

overlays deposited on carbon steel plates at low, medium, high and optimum

weld heat input conditions by the PTAW process are characterized with the

help of microstructural analysis. Due to the difference in melting temperature

range between the carbon steel substrate and the stainless steel clad metal the

dilution of the clad bead could be significantly affected. When dilution is

altered, it will influence the resulting microhardness and the microstructure of

the claddings. Therefore, a metallurgical study is carried out across the

various zones of the cladding. Microhardness survey was conducted to

confirm the different phases present and colour metallography was used to

reveal microstructures at different zones of the claddings. Since the weld

metal ferrite content can influence a wide range of properties which includes

corrosion resistance and resistance to hot cracking, the amount of delta ferrite

present in the claddings is measured and the effect of heat input on ferrite

content is presented.

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8.2 EXPERIMENTAL PROCEDURE

8.2.1 Measurement of microhardness in stainless steel claddings

Four test specimens were prepared from the overlay plate cladded at

different heat input conditions as well as at optimum dilution condition for

carrying out the metallographic characterisation of the claddings. Standard

metallurgical procedures were followed to prepare specimens obtained from

the claddings. The samples were etched with 10 % oxalic acid solution for

revealing the different zones of the weldments such as clad metal, fusion zone

and heat affected zone. The base metal zone is etched with a 2 % nital to

facilitate microhardness studies. The microhardness measurement was carried

out as per ASTM E 384 Standard Test Method by using a MITUTOYO

(Model: M11, Japan) microhardness tester, as shown in Figure 8.1.

Figure 8.1 Microhardness testing machine

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The microhardness is measured at very close distances across the

bead profile along its centreline perpendicular to the direction of welding.

This is carried out across the four different zones namely the Base Metal

(BM) zone, Heat Affected Zone (HAZ), Fusion Boundary Zone (FBZ) and

the Weld Metal (WM) zone. The measurements on each specimen was carried

out initially from the base metal and progressed towards various zones along

the centreline of the weld bead profile perpendicular to the direction of

welding. A Vickers indenter with a 100 gram load was used with a loading

time of 20 seconds to make indentation on the specimens. The measurements

are taken on the two sides of the specimen to assess the different phases

present in the microstructural constituents of the claddings.

8.2.2 Measurement of ferrite number in stainless steel claddings

Stainless steel cladding usually contain delta ferrite, which is

expressed in terms of FN. The Ferritescope utilizes the eddy current to

measure the magnetic properties which are calibrated in terms of ferrite

number. The ferrite number measurement is carried out using a Fischer

feritescope (Model: FS-311), as shown in Figure 8.2. The feritescope is

calibrated with standard specimens before measuring the ferrite number.

Figure 8.2 Feritescope used for Ferrite number measurements

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The four test specimens were prepared from the overlay plate

cladded at different heat input conditions as well as at optimum dilution

condition for carrying out the ferrite number measurement of the claddings.

The top surface of cladded specimen is ground and polished to a depth of

0.5mm for ferrite measurement. The ferrite number is measured on the top

surface of the ground cladded plates at different locations as shown in Figure-

8.3.

Figure 8.3 Different locations of Ferrite number measurement

Ferrite number is measured at the specified locations on the top

surface of the cladding at the centre of the clad bead, along the direction of

welding. The average of the five values was calculated for each specimen.

8.2.3 Microstructural analysis in stainless steel claddings

Four test specimens were prepared from the overlay plate cladded at

different heat input conditions as well as at optimum dilution condition for

carrying out the microstructural studies and standard metallurgical procedures

were employed to prepare the samples for this investigation. Colour

metallographic technique was also used to reveal the microstructure as both

primary and secondary structures are visible in this method. The etchant used

for this analysis along with the etching methods with time duration are given

in Table 8.1.

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The microstructures were taken by using a NEOPHOT

(Model: NEO-32), an optical metallurgical microscope with an inbuilt camera

to photograph the microstructures with different magnifications. The

micrographs were taken for the claddings deposited at low, medium, high and

optimum heat input conditions and are taken in the section transverse to the

welding direction.

Table 8.1 Details of etchants used for base and clad metals

Sl

noMaterial Etchant Description

Etching

duration,

seconds

1 Structural

steel

Etchant I 2% of Nital at room temperature

(applied on the surface by a cotton

swab)

30-35

2 Structural

steel

Etchant II 50 ml cold-saturated (in distilled water)

sodium thiosulphate solution and 1 gram

potassium metabisulphite

(specimen is immersed and steered)

30-90

3 Stainless

steel

Etchant A

(for Black &

White images)

10% Oxalic Acid (electro etched at

6 Volts and 1 Amps/cm2) 90 -120

4 Stainless

steel

Etchant B

(for Colour

images)

20 gram Ammonium bi-fluoride

and 0.5 gram Potassium meta-

bisulphite diluted in 100ml distilled

water (specimen is immersed and

steered)

30 -60

8.3 RESULTS AND DISCUSSION

8.3.1 Measurement of microhardness in stainless steel claddings

The microhardness survey was conducted on the claddings

deposited at different heat input conditions and at optimum heat input

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condition by the PTAW process. For each specimen, measurements were

taken on both sides and the mean value is calculated. The results of the

microhardness survey are presented from Figures 8.4 to 8.7.

There is no appreciable change in the micro hardness of the base

metal region in all the claddings which confirm that the properties of the base

metal are retained. It is found that the HAZ increased with increase in heat

input and this might be due to the formation of bainitic structure in the HAZ.

It is evident that the microhardness in the HAZ of the claddings deposited at

low and optimum heat input did not increase which gave an interpretation that

they possesses higher ductility. The increase in the microhardness of the

claddings deposited at medium heat conditions is moderate but in the case of

high heat input specimen it is fairly high.

In the Fusion boundary zone (FBZ), the increase in microhardness

of the claddings deposited at high heat input condition is the hardness

suddenly increased to a higher value and then drops marginally. This sudden

increase in hardness might be due to the formation of the martensitic structure

in the FBZ and the indentations are incorporated in Figure 8.6. In the overlay

region, the microhardness decreased gradually in all the claddings and an

appreciable reduction in microhardness is observed in the claddings deposited

at high heat input condition.

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Figure 8.4 Microhardness distribution at different regions at low heat

input condition

Figure 8.5 Microhardness distribution at different regions at

medium heat input condition

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Figure 8.6 Microhardness distribution at different regions at high heat

input condition

Figure 8.7 Microhardness distribution at different regions at

optimum heat input condition

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8.3.2 Measurement of Ferrite number in stainless steel claddings

The ferrite number is measured on the surface of the claddings

deposited at different heat inputs and optimum heat input conditions. These

measurements were taken on five different locations on each cladding and

presented in Table 8.2.

Table 8.2 Variation of ferrite number on the surface of the cladding

Sl

NoLocations

HEAT INPUT

Low

(4.10 KJ/mm)

Medium

(5.54 KJ/mm)

High

(6.81 KJ/mm)

Optimum

(4.31 KJ/mm)

1 1 11.8 6.8 4.2 5.2

2 2 10.3 6.3 5.1 6.4

3 3 10.5 7.3 4.8 5.8

4 4 12.8 8.8 4.9 6.3

5 5 11.5 8.5 5.3 6.1

Average 11.38 7.54 4.8 5.96

Higher values of ferrite number were observed in the cladding

deposited at low heat input condition. Also lower ferrite number was noticed

in the cladding deposited at high heat input condition which has enabled the

formation of more austenitic phases than the ferrite.

8.3.3 Microstructural analysis in stainless steel cladding

A large number of optical micrographs were obtained at different

magnifications on the cross sections of claddings deposited at different heat

inputs and at optimum heat input condition are presented in Figures 8.8 –

8.14. The photomicrographs are taken along the section which was

perpendicular to cladding direction.

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Figure 8.8 shows the microstructures of base metal etched with

Etchants I and II respectively for obtaining black and white and colour

photomicrographs at a magnification of X200. Both the microstructures

clearly reveal the presence of ferrite phases and pearlite phases. Presence of

blue coloured pearlite in brown coloured ferrite matrix is visible in the colour

photomicrograph.

ETCHANT-I / X200 ETCHANT-II / X 200

Figure 8.8 Microstructure of the base metal

The optical photomicrographs obtained at 200 X and 400 X from the

cladding deposited at low heat input condition and etched with the etchants A

and B are shown in Figure 8.9. It is evident from the figure that the

microstructure represents austenite matrix containing little amount of delta

ferrite. The coloured microstructure reveals the matrix of austenite (bluish and

brownish) and delta ferrite (white) phases. The presence of columnar dendrite

structure indicates faster cooling rate due to low heat input condition. The

primary structures of the austenitic regions are blue and the final solidification

regions are brown in colour.

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ETCHANT-A / X200 ETCHANT-B / X 400

Figure 8.9 Microstructure of the cladding deposited at low heat

input (4.10 KJ/mm) condition

Figure 8.10 depicts the photomicrograph obtained from the cladding

deposited at medium heat input condition and etched with the etchants A and

B. It is evident from the microstructure that the presence of vermicular

structure is visible in the clad layer. Also, the microstructure at a

magnification of 400 X, with etchant B reveals the matrix of austenite

(greenish blue / bluish) and delta ferrite (pale yellow). The primary structures

of the austenitic regions are greenish blue and the final solidification regions

are bluish in colour.

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Figure 8.10 Microstructure of the cladding deposited at medium heat


The photomicrographs of the cladding deposited at high heat input

condition and etched with the etchants A and B are shown in Figure 8.11.


Figure 8.11 Microstructure of the cladding deposited at high heat


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It is evident from the figure, that the presence of vermicular

morphology of ferrite in the austenite matrix is visible in the clad layer. The

colour microstructure obtained at a magnification of 400 X reveals the

presence of ferrite (white) and austenite (blue) matrix. Coarser austenite

grains are found in the microstructure which may be due to higher heat input

in the cladding. The blue color cellular crystals show a slightly wavy surface,

indicating the beginning of dendritic crystal formation and the presence of

delta ferrite is clearly seen in small white patches.

The optical photomicrograph obtained from the cladding deposited

at optimum heat input condition and etched with the etchants A and B are

presented in Figure 8.12. It resembles to the microstructures obtained from

the cladding deposited at low heat input condition with a slightly coarser

austenite grains.


Figure 8.12 Microstructure of the cladding deposited at optimum heat


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8.6 SUMMARY

The stainless steel claddings were characterised to evaluate the

properties influencing their corrosion and wear resistance. A significant

increase in the hardness is found in the cladding deposited at high heat input

condition. Higher ferrite content was found in the cladding deposited at low

heat input condition. The microstructure of the cladding produced at optimum

heat input condition has vermicular ferrite in the austenite matrix.

chapter 8 metallurgical studies on stainless steel...

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