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.
81
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
83
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%
84
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
85
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.
86
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
87
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
Welding current (I), A
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
Welding current (I), A
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
92
Figure 4.9 Response surface showing interaction effect of welding
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
Figure 4.11 Response surface showing interaction effect of welding
speed and oscillation on macrohardness
94
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
Figure 4.13 Response surface showing interaction effect of welding
speed and stand off on macrohardness
95
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
96
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.
97
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
98
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
99
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
Figure 4.20 Microhardness distributions in weld metal deposited
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
Figure 4.21 Microhardness distributions in weld metal deposited
at optimum welding conditions
101
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
input condition (500X)
103
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
Figures 4.28 and 4.29 shows the microstructure of the specimen
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
input condition (100X)
106
Figures 4.30 and 4.31 shows the microstructure of the specimen
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
input condition (100X)
Figure 4.30 Microstructure of the specimen hardfaced at medium heat
input condition (100X)
107
Figure 4.31 Microstructure of the specimen hardfaced at high heat
input condition (200X)
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)
108
Figure 4.33 Microstructure of unaffected base metal (500X)
Figure 4.34 Microstructure of unaffected base metal (1000X)