CHAPTER 4 HARDNESS AND MICROSTRUCTURE...

79

CHAPTER 4

HARDNESS AND MICROSTRUCTURE STUDIES

4.1 INTRODUCTION

The successful deposition of a wear-resistant tungsten carbide on a

stainless steel surface requires a through understanding of metallurgy of the

stainless steel and of the hardfacing material. The hardfacing process involves

a heterogeneous weld between the ferritic base plate and alloy weld metal. In

order to maintain the required wear resistance and mechanical properties of

the hardfaced components, controlling the deposit chemistry and

microstructure is essential. The composition of a weld metal is affected by the

composition of base and filler metal used, the amount of dilution of filler

metal and any material losses associated with the welding process used. In

general, the effect of process parameters on microstructure are due to the

compositional and thermal effect. The compositional effect are largely limited

to the fusion zone but thermal cycles affect both the fusion zone and HAZ.

Hardness of the base and weld metal is affected by its composition,

the metallurgical effect of welding process, cold working of metal, heat

treatment, preheat or interpass temperature, base plate thickness and many

other factors. In hardfacing, rapid cooling from high HAZ temperatures may

cause higher hardness than the base metal. In HAZ areas where the maximum

temperature is lower, however, the hardness may be lower due to tempering

effect.

80

The performance of the hardfaced components during its service

depends on the microstructure of the weld and the HAZ of the base metal. It is

necessary to identify and quantify the various structures in those zones to get

an idea about the mechanical and wear properties of the hardfaced

components.

Based on the various considerations given above, the following

detailed metallurgical studies were carried out.

1. Analysis of Macrohardness

2. Microhardness survey

3. Microstructural studies of hardfaced layer and substrate

4.2 EXPERIMENTAL INVESTIGATIONS

Three specimens representing low, medium, and high heat input

conditions and a specimen representing optimum dilution conditions were

used for microhardness survey and metallurgical studies. All the specimens

prepared for bead geometry study in as-hardfaced condition were used for

Macrohardness study.

4.2.1 Development of Regression Models to Predict Macrohardness

To measure the macrohardness of the weld metals, the top surface

of the specimens was ground flat. Three readings were taken on the top

surface and its average values were found which are presented in Table 4.1.

Macrohardness measurements were carried out using a Rockwell hardness

tester. A diamond indenter with 150 kg load was used to make indentations on

all the specimens.

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Table 4.1 Design matrix and Estimated Macrohardness of Hardfaced

components

Trial

No.

Design MatrixEstimated

Macrohardness, HRC

I S F H N 1 2 3 Average

01 -1 -1 -1 -1 1 53 51 53 52.33

02 1 -1 -1 -1 -1 46 45 46 45.66

03 -1 1 -1 -1 -1 46 41 46 44.33

04 1 1 -1 -1 1 45 44 42 43.66

05 -1 -1 1 -1 -1 42 51 55 49.33

06 1 -1 1 -1 1 50 50 47 49.00

07 -1 1 1 -1 1 49 46 49 48.00

08 1 1 1 -1 -1 41 39 43 41.00

09 -1 -1 -1 1 -1 49 45 51 48.33

10 1 -1 -1 1 1 42 42 43 42.33

11 -1 1 -1 1 1 51 49 49 52.33

12 1 1 -1 1 -1 42 43 43 45.66

13 -1 -1 1 1 1 41 39 40 40.00

14 1 -1 1 1 -1 44 46 49 46.33

15 -1 1 1 1 -1 39 40 43 40.66

16 1 1 1 1 1 44 42 43 43.00

17 -2 0 0 0 0 52 55 54 53.66

18 2 0 0 0 0 49 49 52 50.00

19 0 -2 0 0 0 47 45 41 44.3

20 0 2 0 0 0 45 45 45 45.00

21 0 0 -2 0 0 41 41 41 41.00

22 0 0 2 0 0 43 43 43 43.00

23 0 0 0 -2 0 46 44 44 44.66

24 0 0 0 2 0 48 48 45 47.00

25 0 0 0 0 -2 42 40 41 41.00

26 0 0 0 0 2 41 41 45 42.33

27 0 0 0 0 0 57 56 56 45.00

28 0 0 0 0 0 45 44 50 46.33

29 0 0 0 0 0 39 49 47 45.00

30 0 0 0 0 0 46 45 47 46.00

31 0 0 0 0 0 42 42 43 42.33

32 0 0 0 0 0 42 43 41 42.00

82

From these hardness values the regression model was developed

using the same procedure as explained in section 3.5.7 for bead geometry

study. The developed model is given below:

Macrohardness = 44.866 - 1.097I - 0.789S - 0.318F - 0.653H + 0.514N +

1.757I2

-0.038S2

- 0.700F2

+ 0.257H2

-0.784N2

- 0.354IS + 1.355IF +

0.646IH - 0.313IN -0.229SF + 1.145SH +1.354SN - 0.896FH - 0.270FN -

0.979HN (4.1)

To develop final regression model, the insignificant coefficients

were eliminated without affecting the accuracy of the developed models using

student t-test (Davies 1978). The final mathematical model constructed using

those significant coefficients are given below:

Macrohardness = 45.069 - 1.097I - 0.789S - 0.653H + 0.514N + 1.757I2

-

0.700F2

- 0.784N2

+ 1.355IF + 0.646IH + 1.145SH +1.354SN - 0.896FH -

0.979HN (4.2)

It is found that the reduced model is better than the full model

because the reduced model has higher values of R2 (adjusted) and lesser

values of standard error of estimates than that of the full model. The values of

R2 (adjusted) and standard error of estimates for full and reduced model is

given in Table 4.2. The adequacy of the developed model was tested using the

ANOVA technique (Montgomery 2003). The calculated values of F-ratio and

R- ratio are given in Table 4.3, which shows that the developed model is

adequate. Conformity tests were conducted using the same experimental

setup to confirm the results of the experiments. The results of the conformity

tests shown in Table 4.4 depict the accuracy of the developed model, which is

above 94%. The above developed model can be used to predict the hardness

by substituting the coded values (-2, -1, 0, +1, +2) of the respective process

parameters. The responses calculated using the developed regression model

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for each set of coded welding parameters are also represented in graphical

form in Figures 4.1 to 4.17.

Table 4.2 Comparison of R2

values and standard error of estimates for

full and reduced macrohardness model

Parameters

R2 values Standard error of estimates

Full

model

Reduced

model

Full

model

Reduced

model

Macrohardness 0.644 0.735 2.046 1.735

Table 4.3 Analysis of variance for testing adequacies of the

macrohardness model

Parameter

1st order

terms

2nd order

terms

Lack of

fit

Error

terms F-

ratio

R-

ratioRemarks

SS DF SS DF SS DF SS DF

Macrohardness 302.078 5 16.64 15 38.424 6 7.694 5 4.162 10.38 adequate

F-ratio (6, 5, 0.05) = 4.95, SS-Sum of squares R-ratio (20, 5, 0.05) = 4.56,

DF-Degrees of freedom

Table 4.4 Results of conformity tests for macrohardness

Process parameters in

coded form

Predicted

values of

hardness (using

models)

Actual values of

hardnessError, %

I S F H N

-0.11 -0.22 0.09 -0.3 1 44.41 45.29 1.98

-0.79 -0.35 0.94 1.02 0.5 42.95 41.67 -2.98

-0.66 0.03 0.90 1.03 1 42.02 42.44 1.00

100X valuePredicted

valuePredicted valueActualError%

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4.2.2 Microhardness Survey

Standard metallurgical procedures were used to prepare the surface

of the specimens to carry out microhardness survey along the different

metallurgical zones such as unaffected base metal, transition zone, HAZ,

fusion zone and deposited weld metal. The following etchant was used to

reveal the microstructure of the base and weld metals (Siva 2009).

Aquaregia solution (1 part of concentric nitric acid + 3 parts of

concentric hydrochloric acid +1 part of distilled water ).

Microhardness survey was carried out using a Mitutoyo

microhardness tester on various zones of the as-hardfaced specimens starting

from unaffected base metal to weld metal along the centre line of a single

bead. A Vickers indenter with 500 g load was used to make indentations on

specimens hardfaced at low, medium, and high heat input and optimum

dilution conditions. The microhardness values obtained were plotted against

the distance covered along different zones in graphical form and are shown in

Figures 4.18 to 4.21.

4.2.3 Microstructural Studies

Standard metallurgical procedures were employed to prepare the

specimens considered for this study as per ASTM standard E3-01. Color

metallography technique was used to reveal various phases present in all

zones of the base metal and weld metal. The etchant used for these studies is

given in section 4.2.2.

The etched specimens were subjected to an extensive microstructure

survey using large incident light camera microscope (zeiss Neophot 30) to

study the microstructure of unaffected base metal, HAZ, fusion line and clad

metal under different magnifications ranging from 100X to 1000X. A large

number of photomicrographs were taken to study the type and nature of

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phases present. Some typical photomicrographs revealing the microstructure

at different zones of the specimens have been presented in Figures 4.22 to

4.31.

4.3 RESULTS AND DISCUSSIONS

Results obtained from the above investigations are presented and

discussed in the same sequence as detailed in the previous section.

4.3.1 Direct Effect of Process Parameters on Macrohardness

Direct effect of process parameters such as welding current (I),

welding speed (S), powder feed rate (F), oscillation frequency (H) and stand

off (N) on macrohardness are obtained from the developed model equation

(4.2) and detailed below.

4.3.1.1 Direct effect of welding current on macrohardness

From Figure 4.1, it is found that macrohardness decreases with

increase in welding current. This may be due to an increased heat input.

4.3.1.2 Direct effect of welding speed on macrohardness

From Figure 4.2, it is evident that the macrohardness decrease with

an increase in welding speed. This may be due to increased dilution of base

metal in the pool with an increase in welding speed, since the weight of

deposited metal per unit length decreases while the cross section of the bead

decreases very little (Jean Cornu 1988). The speed, therefore, exerts an

influence on the composition of the weld bead analogous to that of current.

The effect of dilution is more dominant than the effect of heat input on the

macrohardness with increased welding speed, hence the macrohardness

decreases with increase in welding speed.

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130 135(-2) 142(-1) 149(0) 156(1) 163(2) 168

Welding current (I), A

30

35

40

45

50

55

60

S= 85 mm/min

F= 38 gram/min

H= 40 mm

N= 10 mm

Hard

ness,

HR

C

Figure 4.1 Direct effect of welding current on macrohardness

72 77(-2) 81(-1) 85(0) 89(1) 93(2) 98

Travel Speed (S), mm/min

25

28

31

34

37

40

43

46

49

Hard

ness, H

RC

I= 149 Amps

F= 38 gram/min

H= 40 mm

N= 10 mm

Figure 4.2 Direct effect of welding speed on macrohardness

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4.3.1.3 Direct effect of powder feed on macrohardness

From Figure 4.3, it is clear that the macrohardness increases with

increase in powder feed rate initially but decreases further with the increase in

powder feed rate.

4.3.1.4 Direct effect of torch oscillation frequency on macrohardness

Figure 4.4 shows that macrohardness decreases with increase in

oscillation frequency. An increase in oscillation frequency resulting in

increased heat input causing a larger volume of the base plate to melt and

hence decreased hardness.

32 34(-2) 36(-1) 38(0) 40(1) 42(2) 44

Powder Feed Rate (F), gram/min

30

35

40

45

Hard

ness, H

RC

I= 149 Amps

S= 85 mm/min

H= 40 mm

N= 10 mm

Figure 4.3 Direct effect of powder feed rate on macrohardness

88

34 36(-2) 38(-1) 40(0) 42(1) 46(2) 48

Oscillation Frequency (H), cycle/min

35

38

41

44

47

50

% D

ilution (

I= 149 Amps

S= 85 mm/min

F= 38 gram/min

N= 10 mm

Hard

ness,

HR

C

Figure 4.4 Direct effect of oscillation frequency on macrohardness

4.3.1.5 Direct effect of stand off distance on macrohardness

From Figure 4.5, it is evident that macrohardness decreases with

increase in stand off. An increase in stand off resulting in decreased heat input

causing faster cooling which results in increased hardness.

89

7 8(-2) 9(-1) 10(0) 11(1) 12(2) 13

Stand Off (N), mm

37

41

45

49

Hard

ne

ss,

HR

C

I= 149 Amps

S= 85 mm/min

F= 38 gram/min

H= 40 cycle/minHa

rdne

ss, H

RC

Figure 4.5 Direct effect of stand off on macrohardness

4.3.2 Interaction Effect of Process Parameters on Macrohardness

Interaction effects of process parameters such as welding current

(I), welding speed (S), powder feed rate (F), oscillation frequency (H) and

stand off (N) on macrohardness are discussed below.

4.3.2.1 Interaction effect of welding speed and powder feed rate on

macrohardness

Figure 4.6 shows that macrohardness decreases when welding

current increases from 140A to 150A for all levels of power feed rate. But

hardness increases with the further increase in welding current above 150A.

The rate of increase in hardness is higher at higher powder feed rate. The

corresponding response surface diagram is shown in Figure 4.7.

90

135 140(-2) 145(-1) 150(0) 155(1) 160(2) 165


35

40

45

50

55

60

Hard

ne

ss,

HR

C

N=10 mmS= 60 mm/minH=30 cycle/min

38 g

ram

/min

36 gram/m

in

42 gram/m

in

40 gram/m

in

34 gram/min

Figure 4.6 Interaction effect of welding speed and powder feed

rate on macrohardness

Figure 4.7 Response surface showing interaction effect of welding

speed and powder feed rate on macrohardness

91

4.3.2.2 Interaction effect of welding current and oscillation frequency

on macrohardness

From Figure 4.8 it is clear that macrohardness decreases with an

increase in welding current from 140A to 150A at all levels of oscillation

frequency. But hardness increases when welding current increases beyond

150A. The interaction effect is illustrated using response surface diagram as

shown in Figure 4.9.

130 135(-2) 142(-1) 149(0) 156(1) 163(2) 168


40

45

50

55

60

Hard

ness, H

RC

S=85 mm/minN= 10 mmF=38 gram/min

36 cycle

s/min

38 cycles/m

in

40 cycles/m

in

42cycle

s/min

44 cycles/m

in

Figure 4.8 Interaction effect of welding current and oscillation

frequency on macrohardness

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current and oscillation frequency on macrohardness

4.3.2.3 Interaction effect of welding speed and oscillation frequency on

macrohardness

Figure 4.10 shows that macrohardness decreases with the increase

in welding speed when oscillation frequency is from 36 cycles/min to 40

cycles/min. However, the reverse trend is seen when oscillation frequency is

above 40 cycles/min. The corresponding response surface diagram shown in

Figure 4.11 clearly illustrates the interaction effect.

4.3.2.4 Interaction effect of welding speed and stand off on

macrohardness

Figure 4.12 shows that macrohardness decreases with the increase

in welding speed when stand off ranges from 8 mm to 10 mm. However, the

reverse trend is seen when stand off ranges from 11 mm to 12 mm.

Figure 4.13 shows the corresponding response surface diagram.

93

54 56(-2) 58(-1) 60(0) 62(1) 64(2) 66

Welding speed (s), mm/min

35

40

45

50

55

60

Hard

ne

ss,

HR

C

N=10 mmI= 150 AF=16 gram/min

42cycle

s /m

in40 cycles/min

38 cycles/min

44 c

ycle

s/m

in

36 cycles/min

Figure 4.10 Interaction effect of welding speed and oscillation on

macrohardness


speed and oscillation on macrohardness

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75 77(-2) 81(-1) 85(0) 89(1) 93(2) 95

Welding speed (S), mm/min

30

35

40

45

50

Hard

ne

ss, H

RC

I=149 AmpsF= 38 gram/minH=40 cycle/min

8mm

10 mm

9 mm

11 mm

12 mm

Figure 4.12 Interaction effect of welding speed and stand off on

macrohardness


speed and stand off on macrohardness

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4.3.2.5 Interaction effect of oscillation frequency and powder feed rate

on macrohardness

Figure 4.14 shows that macrohardness increases with an increase in

oscillation frequency from 36 cycles/min to 38 cycles/min for all levels of

powder feed rate. But hardness decreases with increase in oscillation

frequency from 38 cycles/min to 44 cycles/min for all levels of powder feed

rate. The response surface diagram for the interaction effect is shown in

Figure 4.15.

34 36(-2) 38(-1) 40(0) 42(1) 44(2) 46

Oscillation frequency (H), cycle/min

30

35

40

45

50

Hard

ness,

HR

C

I=149 AmpsS= 85 mm/minN=10 mm

34 gram/min

42 gram/m

in

36 gram/min

38 gram/min

40gram/min

Figure 4.14 Interaction effect of oscillation frequency and powder

Feed rate on macrohardness

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Figure 4.15 Response surface showing interaction effect of oscillation

frequency and powder feed rate on macrohardness

4.3.2.6 Interaction effect of oscillation frequency and stand off

on macrohardness

Figure 4.16 shows that macrohardness increases with an increase in

oscillation frequency when stand off is 8 mm. Macrohardness decreases and

then increases with an increase in oscillation frequency when stand off is from

9 mm to 10mm. But hardness decreases with increase in oscillation frequency

when stand off is from 11 mm to 12 mm. The interaction effect is depicted

using response surface diagram in Figure 4.17.

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34 36(-2) 38(-1) 40(0) 42(1) 44(2) 46

Oscillation frequency (H), cycles/min

30

35

40

45

50

55

Hard

ne

ss,H

RC

I=149 AmpsS=85 mm/minF=38 gram/min

12 mm

11 mm

10 mm

9 mm

8 mm

Figure 4.16 Interaction effect of oscillation frequency and stand off

on macrohardness

Figure 4.17 Response surface showing interaction effect of oscillation

frequency and stand off on macrohardness

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4.3.3 Analysis of Microhardness Survey

The microhardness survey carried out in various zones of

specimens is presented in the form of hardness profiles in Figures 4.18 to

4.21. The survey was carried out over the specimens hardfaced at low,

medium, and high heat input and optimum dilution conditions. The

microhardness profile may be divided into four parts such as unaffected base

metal (BM), heat-affected zone (HAZ), fusion boundary zone (FBZ) and weld

metal (WM).

It is evident from these figures that the width of the HAZ is

increased with increase in heat input and the microhardness values of HAZ,

particularly the coarse grained region of HAZ, are higher than the unaffected

base metal.

0 3 4.3 4.75 4.9 5 5.05 5.2 5.5 6.5 8

Distance , mm

100

150

200

250

300

350

400

450

500

550

600

650

700

Mic

roh

ard

ne

ss, V

HN

( 5

00 g

m. )

BM HAZ FBZ OVERLAY

I = 163 AS = 85 mm/minF = 38 gram/minH = 40cycles/minN = 10 mm

F

S

Figure 4.18 Microhardness distributions in weld metal deposited at

high heat input conditions

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0 3 4.3 4.75 4.9 5 5.05 5.2 5.5 6.5

Distance , mm

100

150

200

250

300

350

400

450

500

550

600

650

700

Mic

roh

ard

ne

ss, V

HN

( 5

00 g

m. )

BM HAZ FBZ OVERLAY

I = 142 AS = 89 mm/minF = 40 gram/minH = 38cycles/minN = 11 mm

F

S

Figure 4.19 Microhardness distributions in weld metal deposited

at low heat input conditions

It is evident from these figures that hardness values of hard facing

are higher than that of all other zones (Gurumoorthy et al. 2007).

The microhardness of weld metal was found to be high for

optimum dilution specimen as compared to low, high and medium heat input

specimens. The maximum hardness value at weld metal region for optimum

dilution specimen was about 698 VHN.

100

0 2.5 3.5 3.8 3.95 4.01 4.05 4.2 4.5 5.5 8

Distance , mm

100

150

200

250

300

350

400

450

500

550

600

Mic

roh

ard

ne

ss, V

HN

(5

00

gm

. )

BM HAZ FBZ OVERLAY

I = 149 AS = 85 mm/minF = 42 gram/minH = 40 cycles/minN = 10 mm

F

S


at medium heat input conditions

0 2.5 3.5 3.8 3.95 4.01 4.05 4.2 4.5 5.5 8

Distance , MM

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

Mic

roh

ard

ne

ss, V

HN

(5

00

gm

. )

BM HAZ FBZ OVERLAY

I = 163 AS = 77 mm/minF = 42 g /minH = 44 cycles/minN = 8 mm

F

S


at optimum welding conditions

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4.3.4 Microstructural Analysis

Weld Metal Microstructure:A large numbers of photomicrographs

were taken at different magnifications for specimens hardfaced at low,

medium and high heat input and optimum dilution conditions. However, it is

not possible to include all those in this report and hence, only a few selected

photomicrographs are presented in this section. The following analysis is

made taking each photomicrograph into consideration. The color etchant

described in section 4.2.2 was used for etching the base metal.

The photomicrographs presented in Figures 4.22 to 4.31 were

enlarged to about six times their original magnification during printing and

magnification indicated in the figures corresponds to the original

magnification at which photomicrographs were taken. There is a very little

published information on factors affecting structure and properties of weld

metals deposited by PTAW. Hence, an investigation into the effect of heat

input on weld metal microstructure was carried out.

Optical color metallography revealed evidence of carbides in all

specimens. It shows characteristics of carbides as they appear in different

zones of a weld bead overlay with normal cooling in air.

Figures 4.22 and 4.23 shows the microstructure of the specimen

hardfaced at high heat input conditions at 200X and 500X magnifications

respectively. In the photomicrographs, the tungsten carbides are clearly seen.

102

Figure 4.22 Unetched structure of the specimen hardfaced at high heat

input condition (200X)

Figure 4.23 Unetched structure of the specimen hardfaced at high heat


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Figures. 4.24 and 4.25 show the microstructure of the specimen

hardfaced at high heat input conditions at 500X and 1000X magnifications

respectively. In the photomicrographs, the metal dendrites oriented

perpendicular to the base metal.

Figure 4.24 Microstructure of the specimen hardfaced at high heat input

condition (500X)

The microstructures of the specimen hardfaced at optimum dilution

conditions at 200X and 1000X magnifications are depicted in Figures. 4.26

and 4.27 respectively. In the photomicrographs, the solidified dendrites are

present (Shengfeng Zhou et al 2008).

104

Figure 4.25 Microstructure of the specimen hardfaced at high heat input

condition (1000X)

Figure 4.26 Microstructure of the specimen hardfaced at optimum

dilution condition (200X)

105


hardfaced at low and medium heat input conditions at 100X magnification

respectively. This photomicrograph shows the carbides distributions.

Figure 4.27 Microstructure of the specimen hardfaced at optimum

dilution condition (1000X)

Figure 4.28 Microstructure of the specimen hardfaced at low heat


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hardfaced at medium and high heat input conditions at 100X and 200X

magnifications. This photomicrograph shows interface between the weld

metal and base metal.

Figure 4.29 Microstructure of the specimen hardfaced at medium heat


Figure 4.30 Microstructure of the specimen hardfaced at medium heat


107

Figure 4.31 Microstructure of the specimen hardfaced at high heat


Base Metal Microstructure

Figures 4.32 to 4.34 show the microstructure of the base metal at

200X, 500X, and 1000X, magnifications respectively. Presence of austenite

is visible in all photomicrographs.

Figure 4.32 Microstructure of unaffected base metal (200X)

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