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Particle erosion behaviour of boiler tube materials at elevated temperature
281
Weer, 103 (1985) 281 - 296
PARTICLE EROSION BEHAVIOUR OF BOILER TUBE MATERIALS AT ELEVATED TEMPERATURE Y. SHIDA and H. FUJIKAWA
~umitomo Metaf industries Ltd., Central Research ~~h~nag~u bander, Amag~ahi (Japan) (Received September 18, 1984;accepted
Loboratories,
No. 3 I-Chome,
April 26, 1985)
Summary The particle erosion behaviour of typical boiler tube materials, including carbon steel, low alloy steels and austenitic steels, at elevated temperatures up to 650 ‘C was studied using irregularly shaped silica particles. Using 304 steel, the influence of various factors, namely particle concentration, velocity and impingement angle, was examined. The erosion behaviour did not seem to differ significantly from that obtained at room temperature. The erosion rate was a linear function of the particle concentration. The velocity exponents obtained at 300 and 650 “C were both approximately 2.8. The peak impingement angle was at acute angles of 20” - 30”, with a tendency for the peak angle to be slightly higher at 300 “C than at 650 “C. However, the temperature effect was clearly observed in that the erosion rate at acute impingement angles increased signific~tly with the temperature su~esting that the steel tends to show a behaviour more typical of ductile materials as the temperature is increased. The erosion morphologies at low angles indicated cutting for every temperature used and the lengths of the cutting tracks obtained at 20” also increased with temperature. The erosion rate varied significantly between materials, e.g. the alloy (Incoloy) 800 eroded the most and the 12Cr-lMo-V steel eroded the least at every temperature used, although every material showed an increase in the erosion rate with temperature. From an attempt to compare the erosion rate data obtained at 20” for every material at every temperature with the tensile properties of the steels, it was found that the yield strength of materials correlates reasonably well with the erosion rate. The erosion rate was apparently proportional to the reciprocal of the yield strength, suggesting that the flow stress included in Finnie’s cutting theory may be conveniently substituted by the yield strength multiplied by a constant.
1. ~troductio~ In the past, extensive studies of solid particle erosion have been made, mainly at room temperature. Many parameters are now known to influence erosion behaviour (see for example ref. 1). The velocity of impinging particles 0043-1648/85/$3.30
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282
influences the erosion rate considerably, with the measured velocity exponent being usually between 2.0 and 2.5 [ 11. The impingement angle is another important factor, with the maximum erosion rate occurring for ductile materials at sharp impingement angles of about 20” - 30” and for brittle materials at normal impingement [l]. Particle properties such as size, hardness and shape and particle concentration are also influencing factors [ 11. The erosion rate, of course, depends on the target materials. Temperature may also be an influencing factor. However, so far, study of the temperature effect has been limited. Tilly [2] reported test results of various materials up to 600 “C and observed varying tendencies depending on the materials. Recently the development of coal conversion and utilization technology has accelerated the need for greater elucidation of the particle erosion behaviour, particularly at elevated temperatures. In coal technology energy plants, the prevention of erosion damage of high temperature components may be very important. From this point of view, the particle erosion behaviour at high temperatures has become a topic of extensive interest. Finnie et al. [3] observed an increase in the erosion rate with temperature for a 310 stainless steel. Tabakoff [4] also observed an increase in the erosion rate and a tendency for the velocity exponent to decrease with temperature using a 304 steel, a titanium alloy (Ti-6Al-4V) and a nickel-base superalloy (Inconel 718) and mentioned that the erosion rates of the Ti-GAl-4V and Inconel 718 alloys increase abruptly as the specimen temperature approaches the annealing temperature of the materials. Hansen [5] compared the erosion resistance of various materials at room temperature and 700 “C and at 90” impingement and observed that most metals had erosion resistances that were similar and that were smaller than those of several ceramics or cermet materials and coatings. In his study, the temperature effect was not clearly observed. Those studies were mainly related to the erosion behaviour at elevated temperatures, excluding the corrosion effect. On a more practical basis, several erosion-corrosion studies have also been reported [ 6 - 81. Wright et al. describe the presence of a limiting particle velocity above which erosion predominates and below which corrosion predominates. However, the amount of study so far is so small that the high temperature erosion behaviour is not understood in a generalized form. Presumably, the high temperature erosion behaviour may be complex owing to the variations in materials properties, degree of oxidation etc. In the present work the high temperature erosion behaviour of boiler tube materials was studied. Previous studies cited above were mainly concerned with turbine materials. There has been no systematic study of the erosion resistance of boiler tube materials. In this study, first, the effect of important parameters such as the velocity and impingement angle was examined at elevated temperatures. Secondly, the effect of temperature on the erosion resistance of various boiler tube steels was studied keeping other conditions constant and an attempt was made to correlate the erosion resistance with a material property to determine the controlling factor for the erosion resistance of boiler tube materials.
283
2. Experimental
details
2.1. Test materials Frequently used materials were selected for the experiment, namely carbon steel, low alloy steels (1.25Cr-lMo-V, 2.25Cr-lMo), 12Cr-lMo-V steel, 304 steel and the alloy (Incoloy) 800. The chemical compositions of the materials used are shown in Table 1. The test materials were tubes of boiler grade fabricated in the factory. Specimens were cut from the tubes to a shape of 20 mm wide X 30 mm long X 3 mm thick and were prepared by abrading the test surface on Sic paper (up to 320 mesh), degreasing and weighing before inserting into the specimen holder of the erosion test rig. In Table 1 the typical tensile properties of the steels are also shown. They will be used in the discussion to evaluate the relationship with the erosion rate results. .?.2. Experimental equipment and test conditions For the study of high temperature erosion, a blasting-type test equipment as shown schematically in Fig. 1 was constructed. Accelerating gas and solid palticles were supplied separately to the heating furnace, where they were heatea rip to the test temperature, mixed at the inlet of the acceleration nozzle and then blasted onto a specimen. The temperature of the specimen was kept constant by the test furnace, and the gas and particle mixture was
r-l
Ash Tank
Vibrator Constriction
Gas&Ash
Heating Furnace
Test
Furnace
Ash Receiver
Fig. 1. Schematic diagram of the high temperature blast erosion tester.
0.33
0.62
0.51
0.19
0.08
0.07
12Cr-lMo-V
304
Alloy
mmm2)
MO
1.13
proof
strength;
32.85
1.68
or the 0.2%
18.50
10.25
0.59
20.85
0.98 0.87
2.20
11.40
0.44
0.95
Cr
1.20
Ni
0.55
0.65
MII
strength
0.34
0.10
2.25Cr-1Mo
is the yield
0.25
0.13
oY (kgf
0.28
0.22
1.25Cr-lMo-V
800
tensile property
(wt.%) of the following metals
and typical
c steel
Amount
composition
Si
1
c
Steel
Chemical
TABLE
0,
Al,
(Ti,
0.28
0.30
V
(kgf
54 15
18
45
29
39
22 22 22
56 65 48
is the
44
48
23
48
33
56 59
“C
fracture
31 23 40 15 14
18
mm-*)
elongation.
35 43 47 53 45 47
mm-*)
Ef
38 25 29 25 45 48
;IITgfgggf (a)
500
temperatures
ef (%)
following
strength;
at the
tensile
75 48
65
ii
is the ultimate
35 22
54 78
56
25
65
33
35
51
30 48
@)
P&f mm?)
;!TgkYgf
ff
value
mm-2)
mm -*)
0.29)
0.31;
property
Room temperature
Tensile
values of test materials
15 21 14 13
650 “C
23 27 33 31
67 40 54 50
285
heated up to the same temperature as the specimen by the heating furnace. Thus isothermal high temperature erosion testing was carried out. The velocity of the particles was controlled by the flow rate of the gas. The inner diameter of the blasting nozzle was 4 mm and the distance between the tip of the nozzle and the centre of the specimen was kept constant at 9 mm. Argon gas (99.999%) was used as the accelerating gas to minimize the influence of oxidation at elevated temperatures on the erosion behaviour. The gas velocity was varied from 40 to 120 m s-l. However, when the effect of factors other than velocity was examined, a velocity of nominally 120 m s-’ was used. The solid particles used were silica particles (mostly quartz) of purity 98%, made by crushing natural silica rock, of 120 pm average particle size. The particles were irregularly shaped as shown in Fig. 2. The particles were supplied at a rate of 660 g h-’ and the duration of the test was usually 1 h, i.e. 660 g of particles were blasted onto a specimen. The impingement angle was varied from 15” to 90”. The test temperatures used were from room temperature to 650 “C.
(a) Fig. 2. (a) Size distribution
(b) and (b) microscopic
view of the silica (quartz)
particles
used.
2.3. Examination of the test specimens After the erosion test was finished, the mass change of the specimen and the maximum thickness loss of the eroded surface (measured using a surface profilometer) were determined to evaluate the degree of erosion damage. Also, the eroded surface was examined by scanning electron microscopy for the study of erosion morphology and its cross section was examined by optical microscopy to observe the damaged surface zone.
3. Results A series of experiments were conducted using mainly the 304 steel firstly to examine the influence of typical factors, namely particle concentration, velocity and impingement angle, on the erosion rate at 300 and 650 “C.
286
Secondly, the effect of the temperature was examined for a representative condition (fixed particle concentration, velocity and impingement angle) using the materials shown in Table 1. Thirdly, the erosion morphology was examined to study the relationship between the morphology and the erosion behaviour observed. 3.1. Effect of influencing parameters at high temperatures 3.1.1. Concentration of particles It is known that the particle concentration influences the erosion rate of materials [l]. Tilly and Sage [ 91 observed that erosion decreases with increasing particle concentration, a typical reduction being 50% for a fortyfold increase in concentration, using a result for an 11% Cr steel and a titanium alloy at room temperature. In the present study the effect of particle concentration was examined using the 304 steel at 300 “C, a 20” impingement angle and a velocity of 120 m s-l. Particle concentration values were varied from 30 to 120 g mp3. As shown in Fig. 3, a linear relationship between the concentration and the erosion mass loss was observed. Thus, within the particle concentration range used, it may be concluded that the total mass of particles determines the rate of erosion. In the present case, the concentration range used was relatively small (only fourfold), so it is difficult to discuss the effect at elevated temperature in comparison with the results of previous work [ 1, 91. 3.1.2. Velocity It is well known that velocity is one of the most important parameters influencing erosion. Here, the effect of velocity was examined at 300 and 650 “C. Using the 304 steel, tests were conducted under conditions of a 20” impingement angle and 120 g mm3 concentration. As shown in Fig. 4, log-log plots for the erosion mass loss uersus the velocity exhibited a linear relationship. The velocity exponents obtained were approximately 2.8 for both temperatures, although the erosion rate was greater at 650 than at 300 “C. It has been reported that the exponents for various erosion data at room temperature usually range between 2.0 and 2.5 [l, 10, 111. The exponent obtained in the present study was slightly greater than 2.5. Tabakoff [4] reported that the velocity exponents, obtained for Ti-GAl-4V and Inconel 718 alloys at a 25” impingement angle and 60 - 1300 OF, tended to decrease with temperature. In particular, the decrease for the Ti-6Al-4V alloy was marked at greater than 1000 OF, above which the yield strength of the material significantly decreases. However, in the present study, a significant change in the velocity exponent due to temperature elevation was not observed. The 304 steel used exhibits a decrease in the yield strength from to 14 kgf mmd2 at 650 “C. approximately 27 kgf mme2 at room temperature Thus, although the material strength decreases, the velocity exponent does not seem to change significantly. Probably, at least under the experimental conditions used in the present study, the velocity exponent may not be a
l
40
-5
JOI
2c
1C 2
8 -1
96 9 s
4
3
-0
r”
I
2
1
-a
L
-0
20
40
60
80
Partkle concentration
100
(olm3)
120
20
40
00
Velochy
80100
200
: V (m/e)
Fig, 3. Effect of particle concentration on the erosion rate of the 304 steel (temperature, 300 “C; impingement angle, 20’; velocity, 120 m s-r). Fig. 4. Effect of velocity obtained for the 304 steel at 300 “C!(0) and 650 “C (0) at an impingement angle of 20’ and a particle concentration of 120 g mm3. The mass loss rate Am = AVn where n - 2.8 for both temperatures.
strong function of the target material strength particle properties and particle motion.
but may be a function
of the
3.1.3. Impingement angle The impingement angle is another important parameter influencing erosion. An attempt was made to study any difference in the effect of impingement angle as the temperature was increased. Erosion experiments were carried out using the 304 steel at 300 and 650 “C, the impingement angle varying from 15” to 90”. The results shown in Fig. 5 indicate that the maximum erosion occurred at a low angle of about 20” as expected from various room temperature studies in the past. Also, it was observed that the erosion rates at low angles were greater at 650 “C than at 300 ‘C, with no obvious difference being observed at higher angles. Tabakoff [4] observed a similar result for a 304 steel. This may be a characteristic feature which occurs because of the temperature elevation. In other words, the 304 steel exhibited behaviour more typical of a ductile material as the temperature was raised.
288
0
1
..--IL
20
j
---J...l
I
10
0
50 30 40 Angle of tmwy?ment
60 70 (degree)
80
2
90
Fig. 5, Effect of impingement angle on the erosion mass loss rate obtained for the 304 steel at 300 “C (of and 650 “C (0) (velocity, 120 m s-l; particle concentration, 120 g me3 f.
The measure of erosion used so far has been the mass loss of the specimen, However, when the thickness loss was obtained as an erosion measure, the dependence apparently shifted towards larger angles of approximately 30” (Fig. 6). Figure 7 shows the results for several steels. The carbon steel, the 1,25Cr-lMo-V steel and the 304 steel all exhibited a maximum erosion thickness loss at 30”. However, it is interesting to compare the thickness loss curves in Figs. 6 and 7. The thickness losses at 20” in comparison with those at 30” appear to be smaller at 300 “C than at 650 “C. Therefore, there is a possibility that the exact peak angle may vary slightly with temperature. In fact, the mass loss curves for the 304 steel as shown in Fig. 5 suggest that the peak angle may be at a slightly higher angle at 300 “C than at 650 “C. This result also suggests that the steel exhibits behaviour more typical of a ductile material as the temperature is increased.
/
o,
0
t 15)
I
20
I
30
/
40
1
I
/
/
I
50
60
70
80
so
lo
Angie of Impingement (degree)
Fig. 6. Comparison of impingement angle dependence plotted as the mass loss rate (0) and the thickness loss rate (0) obtained for the 304 steel at 650 “C!(velocity, 120 m s-l; particle concentration, 120 g mP3).
289
1
0’
0
1
,o
20
30
1 40
’
50
Angle of Impingement
60
1
1
70
80
1 90
’
(degree)
Fig. 7. Effect of impingement angle on the erosion thickness loss rates of several steels at 300 “C (velocity, 120 m s-l; particle concentration, 120 g mP3): 0, 304 steel; 0, carbon steel; l, 1.25Cr-lMo-0.3V steel.
3.2. Effect of temperature on the erosion rate of various steels As described in the previous sections, a greater erosion rate was observed for the 304 steel at low impingement angles and at higher temperatures. Therefore temperature is one of the very important factors which determine the erosion rate. To obtain the temperature effect on erosion for various steels, tests were conducted at room temperature and at 300, 500 and 650 “C keeping other conditions constant at a particle concentration of 120 g mm3 and an impingement angle of 20”. The results are shown in Fig. 8. A tendency
1
01
RT
300 Temperature
500
850
(‘C)
Fig. 8. Effect of temperature on the erosion thickness loss rate of various steels (impingement angle, 20”; velocity, 120 m s-l; particle concentration, 120 g mm3; RT, room temperature): q, alloy 800; a, carbon steel; 0, 304steel;=, 2,25Cr-1Mosteel; A, 12Cr-lMo-V steel.
290
for the erosion rate to increase with temperature was obtained for every steel. In particular, the increase was obvious above 500 “C. Thus it may be concluded that the erosion rate of steels impacted at low angles definitely increases as the temperature is increased. Also, it was observed that the rates of erosion were significantly different depending on the type of steel. The alloy 800 exhibited the greatest erosion rate at all temperatures while the 12Cr-lMo-V steel exhibited the smallest. The carbon steel, the 2.25Cr-1Mo steel and the 304 steel showed similar erosion rates. 3.3. Examination of the eroded surface 3.3.1. Observation from the surface To obtain the effect of the temperature on the erosion morphology the surfaces of eroded specimens were examined by scanning electron microscopy. As shown in Fig. 9 for the 304 steel, the morphology was apparently of the cutting type. Interestingly, the length of the cutting track became
(a)
(b)
(cl
(d)
Fig. 9. Variation in the morphology of the eroded surface as a function of the temperature (304 steel; impingement angle, 20”; velocity, 120 m s-l; particle concentration, 120 g me3): (a) room temperature; (b) 300 “C; (c) 500 “C; (d) 650 “C.
291
larger as the temperature was raised. As a measure of the track length, the lengths 1 of 30 - 40 tracks for each specimen were measured and the average length f was recorded. The average lengths determined for specimens eroded at 20’ are plotted against temperature in Fig. 10 together with the thickness loss measured. Obviously, the curve is quite similar for both the average track length and the thickness loss. Thus it is evident that the erosion mechanism may be of a cutting type throughout the temperature range up to 650 “C and the cutting length per particle increases with temperature, probably as a result of the softening of the steel.
RT
500
300 Temperature
W
550
1
Fig. 10. Variation in the average length of cutting tracks with temperature showing a similar trend to that of the thickness loss (304 steel; impingement angle, 20”; velocity, 120 m s-1; particle concentration, 120 g mA3): --, thickness loss; 0, f.
3.3.2. Observations from cross sections Cross sections were also observed in an attempt to examine changes in the surface zone of the steels due to erosion damage. Figure 11 depicts examples eroded at 300 “C. For a 20” impingement the grain boundary of the alloy 800 and the carbide zones of the 1.25Cr-lMo-0.3V steel were deflected towards the direction of particle flow. The photographs indicate that the thickness of the worked surface zone was several micrometres. For 90” impingement (normal incidence) it appeared that the surface was more rippled and the thickness of the worked surface zone was 5 - 10 pm, which was greater than for the acute angle, suggesting that more energy was stored in the material at a higher impingement angle. 4. Discussion 4.1. Application of Finnie’s cutting theory As described in Section 3.1.3, the angle at which the peak erosion loss occurred was greater when the maximum thickness loss was used as an erosion measure than when the mass loss was used as a measure. Since the
292
>
”
*
‘,.
:.^,, ‘_ , :? / -.. ._’
I
‘:
, h
(a)
(1,)
.‘,‘9/
/
:
-
/,’
<’
Fig. 11. Typical examples of cross sections of the eroded surface obtained at 300 ‘C, 120 m s-l and 120 g m-3: 1.25Cr-lMo-0.3V alloy at (a) 20” and (b) 90” impingement angles; (c) alloy 800 at 20” impingement angle.
erosion morphology was of a cutting type, an attempt was made to discuss the results on the basis of Finnie’s classical cutting theory [ 121. This result may be expected simply from Finnie’s cutting analysis. According to the analysis, there are two expressions depending on the cutting situation. When a particle loses velocity during cutting (case l), the volume AW of material removed may be expressed as mV2
AW=
2p(l
+ mr2/I)
cos2e
(1)
When a particle leaves the metal with some velocity mV2
AW=
2p(l+
where p= 1
K +mr211
mr2/I)
z sin(28) - 2P 1
sin26 P
(case 2),
AW may be (2)
293
AW is the erosion volume loss, m is the mass of the particle, p is the flow stress during cutting, V is the velocity of the particle, B is the angle of impingement, I is the moment of inertia of the particle and Ii is the ratio of the vertical to the horizontal forces. The maximum mass loss may appear in case 2. The peak angle must be given by tan(28) = P. Usually, 8 is about 20”. However, when the erosion is estimated by the thickness loss, eqns. (1) and (2) may simply be multiplied by sin 8. Then the peak angle appears in case 1. This gives, using the thickness loss scheme, AW’ a: cos% sin 8 Therefore, d(AW’) dtl
(3)
the peak angle must be obtained
by differentiating
eqn. (3):
a cos8 (cos28 - 2 sin28) = 0
Thus tan20 = 0.5, i.e. 8 = 35”, may be obtained for the peak erosion angle. The experimental result for the 304 steel at 650 “C (Fig. 6), i.e. that the peak angle for the mass loss was around 20” and that for the thickness loss was around 30”, is in good agreement with the theoretical expectation. However, as mentioned in Section 3.1.3, the peak angle could be shifted slightly depending on the temperature. 4.2. Differences
in the erosion rate depending on the materials and the
temperatures The erosion rate obtained for low angle impingement increased significantly with temperature. The rate was also dependent on the target steel. An attempt was made to describe those differences by a parameter. In room temperature studies made in the past many researchers have attempted to explain differences in erosion resistance between different materials. There have been two major streams of approach: one approach is to attempt correlation with mechanical parameters and the other approach seeks correlation with thermal parameters. Finnie and coworkers [3,13] suggested that the indentation hardness is a good parameter to explain the relative erosion resistance of pure metals but that this would not explain the behaviour of steels hardened by heat treatment. Rickerby [14] showed that the mechanical energy density AE = *(u, + o,)ef (where o, is the yield strength; o, is the ultimate tensile strength and ef is the fracture elongation) is a good parameter for the relative erosion resistance of pure metals. However, many researchers, on the basis of the considerable heat generation at the surface during erosion, have proposed thermal parameters such as the melting temperature [ 151 and C, AT (where C, is the heat capacity and AT is the temperature difference between the test temperature and the melting temperature) [IS]. Those thermal parameters have been described to correlate well with the erosion resistance of pure metals. As such, erosion data for pure metals have been explained by various parameters. However, a similar attempt for practically important alloys has not been successful.
294
The materials used in the present study are steels often used for high temperature components. The thermal properties such as the heat capacity and melting temperature of these materials do not vary so much. For example, for ferritic steels (carbon to 12% Cr steels), differences in the heat capacity, the thermal expansion coefficient, the melting temperature etc. are only within 10% at the most. Thus the thermal parameters could not explain the observed difference (e.g. 200% at room temperature) in the erosion resistance. Therefore mechanical parameters were tested for correlation using the data shown in Fig. 8. The mechanical energy density was firstly tested using the values shown in Table 1. However, the correlation was very poor. Secondly, the yield strength, the tensile strength and the elongation tabulated in Table 1 were examined separately. As a result, the yield strength showed a reasonable correlation as shown in Fig. 12, while the tensile strength and elongation did not correlate well. While there is some scatter, the data for different temperatures and steels fell in a reasonably narrow band around a slope of -1 in a log-log plot of the maximum thickness loss versus the yield strength. Thus yield strength could be an indicator of the erosion resistance of those high temperature steels. According to Finnie’s cutting analysis, target materials are characterized by their flow stress. As shown in eqns. (1) and (2), the volume AW of material removed is reciprocally proportional to the flow stress p. As a matter of fact, the present results, as shown in Fig. 12, show that the amount of erosion is reasonably proportional to the reciprocal yield strength of the material at the temperature of interest. Thus the present results
Yield
or
Proof
Strength
(kgflmm2)
Fig. 12. Relationship between the erosion thickness loss rate and the yield strength at test temperatures (120 m s-l, 120 g m- 3, 20’): l, 650 “C; A, 500 “C; 0, 300 ‘C; A, room temperature. The numerals beside the symbols indicate the following steels: 1, carbon steel; 2, 1.25Cr-lMo-V steel; 3, 2.25Cr-1Mo steel; 4,12Cr-lMo-V steel; 5, 304 steel; 6, alloy 800.
295
suggest that the flow stress may be substituted by cru, (where CY is a constant), at least within the range of materials and test conditions used. Finnie [12] suggests that the flow stress may be substituted by an indentation hardness. In fact, he found a good correlation for pure metals. Unfortunately, the present authors have not measured the hardness at high temperatures. It is not clear whether the yield strength is a better parameter than the hardness at present. However, hardness could be related to the yield strength [ 171. Thus it is not unreasonable that the yield strength may represent the flow stress. In the actual cutting situation, the material around the tip of the cutting particle may be heated up and very high strain rate deformation may occur. If the temperature is increased, the yield strength may be decreased. The material under deformation may be work hardened. Also, there is a possibility that, owing to the heat generated, the work-hardened surface may be stress relieved to some extent. The situation may be as such complex. Therefore the actual flow stress must be somewhat modified from the yield strength obtained by slow strain rate tensile testing. In fact, in Fig. 12, there appears to be a tendency for the slope representing the plots for data at room temperature only to be smaller than -1 and that for 500 and 650 “C to be greater than -1. However, the present empirical finding that the yield strength obtained by a slow strain tensile test may be a good parameter to correlate with the erosion resistance may be very useful, since there has been no such finding for practical steels and elevated temperature erosion behaviour.
5. Conclusions The particle erosion behaviour of typical boiler tube materials at elevated temperatures has been studied using irregularly shaped silica particles. As a result of the study, the following conclusions were obtained. (1) The effect of particle concentration, velocity exponent and peak impingement angle was studied mainly using 304 steel at 300 and 650 ‘C. The effect of those parameters did not seem to differ significantly from the tendency qualitatively expected from the well-established room temperature behaviour. (a) The erosion rate was a linear function of the particle concentration in the concentration range between 30 and 120 g mm3. (b) The velocity exponent obtained for the 304 steel was approximately 2.8 for both 300 and 650 “C. (c) The peak erosion was obtained at low impingement angles, namely at about 20°, for the maximum mass loss and at about 30” for the maximum thickness loss. However, there was a tendency for the peak angle to shift slightly to lower angles as the temperature was raised. (2) A temperature effect was observed such that, as the temperature was increased, the erosion rate at low impingement angles increased significantly but at high impingement angles it did not change significantly. Thus
296
the steels may exhibit behaviour more typical of ductile materials as the temperature is increased, (3) All materials tested showed an increase in erosion rate with temperature. However, the erosion rates of the materials differed considerably, the alloy 800 being the least resistant and the 12Cr-lMo-V steel being the most resistant. (4) The morphologies of eroded surfaces indicate that the mode of damage may be by cutting at every temperature tested. The length of the cutting track observed at low impingement angles increased with temperature. This tendency agreed well with that of the thickness loss. (5) The erosion data obtained at 20” for different materials and test temperatures were compared with the tensile properties. As a result, it was found that the yield strength of materials is a reasonably good parameter to correlate with the erosion rate. The erosion rate was app~ently proportional to the reciprocal yield strength, suggesting that the flow stress included in Finnie’s cutting theory may be conveniently substituted by the yield strength multiplied by a constant.
References 1 G. P. Tilly, Treatise Muter. Sci. Technol., 13 (1979) 287. 2 G. P. Tilly, Wear, 14 (1969) 63. 3 I. Finnie, A. Levy and D. H. McFadden, in W. Adler (ed.), Erosion: Prevention and Useful Application, ASTM Spec. Tech. Publ. 664, 1979, p. 36 (ASTM, Philadelphia, PA). 4 W. Tabakoff, in A. V. Levy (ed.), Proc. Conf. on Corrosion/Eros~on of Coal Conuersion System Materials, Berkeley, CA, January 24 - 26, 1979, National Association of Corrosion Engineers, Houston, TX, 1979, p. 700. 5 J. Hansen, in W. Adler (ed.), Erosion: Prevention and Useful Application, ASTM Spec. Tech. Publ. 664, 1979, p. 148 (ASTM, Philadelphia, PA). 6 R. H. Barkalow, J. A. Goebel and F. S. Pettit, in W. Adler (ed.), Erosion: Preuention and Useful Application, ASTM Spec. Tech. Publ. 664, 1979, p. 163 (ASTM, Philadelphia, PA). 7 I. G. Wright, in A. V. Levy (ed.), Proc. Conf. on Corrosion/Erosion of Cool Conuersion System Materials, Berkeley, CA, January 24 - 26, 1979, National Association of Corrosion Engineers, Houston, TX, 1979, p. 103. 8 I. G. Wright, V. Nagarajan and W. E. Merz, Znt. Corrosion Forum, Toronto, April 6 10, 1981, National Association of Corrosion Engineers, Houston, TX, 1981, Paper 3. 9 G. P. Tilly and W. Sage, Wear, 16 (1970) 447. 10 I. Finnie and D. H. McFadden, Wear, 48 (1978) 181. 11 I. M. Hutchings, in A. V. Levy (ed.), Proc. Conf. on Corrosion/Erosion of Coal Conversion System Materials, Berkeley, CA, January 24 - 26, 1979, National Association of Corrosion Engineers, Houston, TX, 1979, p. 393. 12 I. Finnie, Erosion and cavitation, in ASTM Spec. Tech. Publ. 307, 1962, p. 70 (ASTM, Philadelphia, PA). 13 I. Finnie, J. Wolak and Y. Kabil, J. Mater., 2 (1967) 682. 14 D. G. Rickerby, Wear, 84 (1983) 393. 15 C. E. Smeltzer, M. E. Gulden and W, A. Compton, J. BUS~C En& 92 (1970) 639. 16 I. M. Hutchings, Wear, 36 (1975) 371. 17 H. O’Neill, Hardness Measurement of Metals and Alloys, Chapman and Hall, London, 2nd edn., 1967.
Weer, 103 (1985) 281 - 296
PARTICLE EROSION BEHAVIOUR OF BOILER TUBE MATERIALS AT ELEVATED TEMPERATURE Y. SHIDA and H. FUJIKAWA
~umitomo Metaf industries Ltd., Central Research ~~h~nag~u bander, Amag~ahi (Japan) (Received September 18, 1984;accepted
Loboratories,
No. 3 I-Chome,
April 26, 1985)
Summary The particle erosion behaviour of typical boiler tube materials, including carbon steel, low alloy steels and austenitic steels, at elevated temperatures up to 650 ‘C was studied using irregularly shaped silica particles. Using 304 steel, the influence of various factors, namely particle concentration, velocity and impingement angle, was examined. The erosion behaviour did not seem to differ significantly from that obtained at room temperature. The erosion rate was a linear function of the particle concentration. The velocity exponents obtained at 300 and 650 “C were both approximately 2.8. The peak impingement angle was at acute angles of 20” - 30”, with a tendency for the peak angle to be slightly higher at 300 “C than at 650 “C. However, the temperature effect was clearly observed in that the erosion rate at acute impingement angles increased signific~tly with the temperature su~esting that the steel tends to show a behaviour more typical of ductile materials as the temperature is increased. The erosion morphologies at low angles indicated cutting for every temperature used and the lengths of the cutting tracks obtained at 20” also increased with temperature. The erosion rate varied significantly between materials, e.g. the alloy (Incoloy) 800 eroded the most and the 12Cr-lMo-V steel eroded the least at every temperature used, although every material showed an increase in the erosion rate with temperature. From an attempt to compare the erosion rate data obtained at 20” for every material at every temperature with the tensile properties of the steels, it was found that the yield strength of materials correlates reasonably well with the erosion rate. The erosion rate was apparently proportional to the reciprocal of the yield strength, suggesting that the flow stress included in Finnie’s cutting theory may be conveniently substituted by the yield strength multiplied by a constant.
1. ~troductio~ In the past, extensive studies of solid particle erosion have been made, mainly at room temperature. Many parameters are now known to influence erosion behaviour (see for example ref. 1). The velocity of impinging particles 0043-1648/85/$3.30
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282
influences the erosion rate considerably, with the measured velocity exponent being usually between 2.0 and 2.5 [ 11. The impingement angle is another important factor, with the maximum erosion rate occurring for ductile materials at sharp impingement angles of about 20” - 30” and for brittle materials at normal impingement [l]. Particle properties such as size, hardness and shape and particle concentration are also influencing factors [ 11. The erosion rate, of course, depends on the target materials. Temperature may also be an influencing factor. However, so far, study of the temperature effect has been limited. Tilly [2] reported test results of various materials up to 600 “C and observed varying tendencies depending on the materials. Recently the development of coal conversion and utilization technology has accelerated the need for greater elucidation of the particle erosion behaviour, particularly at elevated temperatures. In coal technology energy plants, the prevention of erosion damage of high temperature components may be very important. From this point of view, the particle erosion behaviour at high temperatures has become a topic of extensive interest. Finnie et al. [3] observed an increase in the erosion rate with temperature for a 310 stainless steel. Tabakoff [4] also observed an increase in the erosion rate and a tendency for the velocity exponent to decrease with temperature using a 304 steel, a titanium alloy (Ti-6Al-4V) and a nickel-base superalloy (Inconel 718) and mentioned that the erosion rates of the Ti-GAl-4V and Inconel 718 alloys increase abruptly as the specimen temperature approaches the annealing temperature of the materials. Hansen [5] compared the erosion resistance of various materials at room temperature and 700 “C and at 90” impingement and observed that most metals had erosion resistances that were similar and that were smaller than those of several ceramics or cermet materials and coatings. In his study, the temperature effect was not clearly observed. Those studies were mainly related to the erosion behaviour at elevated temperatures, excluding the corrosion effect. On a more practical basis, several erosion-corrosion studies have also been reported [ 6 - 81. Wright et al. describe the presence of a limiting particle velocity above which erosion predominates and below which corrosion predominates. However, the amount of study so far is so small that the high temperature erosion behaviour is not understood in a generalized form. Presumably, the high temperature erosion behaviour may be complex owing to the variations in materials properties, degree of oxidation etc. In the present work the high temperature erosion behaviour of boiler tube materials was studied. Previous studies cited above were mainly concerned with turbine materials. There has been no systematic study of the erosion resistance of boiler tube materials. In this study, first, the effect of important parameters such as the velocity and impingement angle was examined at elevated temperatures. Secondly, the effect of temperature on the erosion resistance of various boiler tube steels was studied keeping other conditions constant and an attempt was made to correlate the erosion resistance with a material property to determine the controlling factor for the erosion resistance of boiler tube materials.
283
2. Experimental
details
2.1. Test materials Frequently used materials were selected for the experiment, namely carbon steel, low alloy steels (1.25Cr-lMo-V, 2.25Cr-lMo), 12Cr-lMo-V steel, 304 steel and the alloy (Incoloy) 800. The chemical compositions of the materials used are shown in Table 1. The test materials were tubes of boiler grade fabricated in the factory. Specimens were cut from the tubes to a shape of 20 mm wide X 30 mm long X 3 mm thick and were prepared by abrading the test surface on Sic paper (up to 320 mesh), degreasing and weighing before inserting into the specimen holder of the erosion test rig. In Table 1 the typical tensile properties of the steels are also shown. They will be used in the discussion to evaluate the relationship with the erosion rate results. .?.2. Experimental equipment and test conditions For the study of high temperature erosion, a blasting-type test equipment as shown schematically in Fig. 1 was constructed. Accelerating gas and solid palticles were supplied separately to the heating furnace, where they were heatea rip to the test temperature, mixed at the inlet of the acceleration nozzle and then blasted onto a specimen. The temperature of the specimen was kept constant by the test furnace, and the gas and particle mixture was
r-l
Ash Tank
Vibrator Constriction
Gas&Ash
Heating Furnace
Test
Furnace
Ash Receiver
Fig. 1. Schematic diagram of the high temperature blast erosion tester.
0.33
0.62
0.51
0.19
0.08
0.07
12Cr-lMo-V
304
Alloy
mmm2)
MO
1.13
proof
strength;
32.85
1.68
or the 0.2%
18.50
10.25
0.59
20.85
0.98 0.87
2.20
11.40
0.44
0.95
Cr
1.20
Ni
0.55
0.65
MII
strength
0.34
0.10
2.25Cr-1Mo
is the yield
0.25
0.13
oY (kgf
0.28
0.22
1.25Cr-lMo-V
800
tensile property
(wt.%) of the following metals
and typical
c steel
Amount
composition
Si
1
c
Steel
Chemical
TABLE
0,
Al,
(Ti,
0.28
0.30
V
(kgf
54 15
18
45
29
39
22 22 22
56 65 48
is the
44
48
23
48
33
56 59
“C
fracture
31 23 40 15 14
18
mm-*)
elongation.
35 43 47 53 45 47
mm-*)
Ef
38 25 29 25 45 48
;IITgfgggf (a)
500
temperatures
ef (%)
following
strength;
at the
tensile
75 48
65
ii
is the ultimate
35 22
54 78
56
25
65
33
35
51
30 48
@)
P&f mm?)
;!TgkYgf
ff
value
mm-2)
mm -*)
0.29)
0.31;
property
Room temperature
Tensile
values of test materials
15 21 14 13
650 “C
23 27 33 31
67 40 54 50
285
heated up to the same temperature as the specimen by the heating furnace. Thus isothermal high temperature erosion testing was carried out. The velocity of the particles was controlled by the flow rate of the gas. The inner diameter of the blasting nozzle was 4 mm and the distance between the tip of the nozzle and the centre of the specimen was kept constant at 9 mm. Argon gas (99.999%) was used as the accelerating gas to minimize the influence of oxidation at elevated temperatures on the erosion behaviour. The gas velocity was varied from 40 to 120 m s-l. However, when the effect of factors other than velocity was examined, a velocity of nominally 120 m s-’ was used. The solid particles used were silica particles (mostly quartz) of purity 98%, made by crushing natural silica rock, of 120 pm average particle size. The particles were irregularly shaped as shown in Fig. 2. The particles were supplied at a rate of 660 g h-’ and the duration of the test was usually 1 h, i.e. 660 g of particles were blasted onto a specimen. The impingement angle was varied from 15” to 90”. The test temperatures used were from room temperature to 650 “C.
(a) Fig. 2. (a) Size distribution
(b) and (b) microscopic
view of the silica (quartz)
particles
used.
2.3. Examination of the test specimens After the erosion test was finished, the mass change of the specimen and the maximum thickness loss of the eroded surface (measured using a surface profilometer) were determined to evaluate the degree of erosion damage. Also, the eroded surface was examined by scanning electron microscopy for the study of erosion morphology and its cross section was examined by optical microscopy to observe the damaged surface zone.
3. Results A series of experiments were conducted using mainly the 304 steel firstly to examine the influence of typical factors, namely particle concentration, velocity and impingement angle, on the erosion rate at 300 and 650 “C.
286
Secondly, the effect of the temperature was examined for a representative condition (fixed particle concentration, velocity and impingement angle) using the materials shown in Table 1. Thirdly, the erosion morphology was examined to study the relationship between the morphology and the erosion behaviour observed. 3.1. Effect of influencing parameters at high temperatures 3.1.1. Concentration of particles It is known that the particle concentration influences the erosion rate of materials [l]. Tilly and Sage [ 91 observed that erosion decreases with increasing particle concentration, a typical reduction being 50% for a fortyfold increase in concentration, using a result for an 11% Cr steel and a titanium alloy at room temperature. In the present study the effect of particle concentration was examined using the 304 steel at 300 “C, a 20” impingement angle and a velocity of 120 m s-l. Particle concentration values were varied from 30 to 120 g mp3. As shown in Fig. 3, a linear relationship between the concentration and the erosion mass loss was observed. Thus, within the particle concentration range used, it may be concluded that the total mass of particles determines the rate of erosion. In the present case, the concentration range used was relatively small (only fourfold), so it is difficult to discuss the effect at elevated temperature in comparison with the results of previous work [ 1, 91. 3.1.2. Velocity It is well known that velocity is one of the most important parameters influencing erosion. Here, the effect of velocity was examined at 300 and 650 “C. Using the 304 steel, tests were conducted under conditions of a 20” impingement angle and 120 g mm3 concentration. As shown in Fig. 4, log-log plots for the erosion mass loss uersus the velocity exhibited a linear relationship. The velocity exponents obtained were approximately 2.8 for both temperatures, although the erosion rate was greater at 650 than at 300 “C. It has been reported that the exponents for various erosion data at room temperature usually range between 2.0 and 2.5 [l, 10, 111. The exponent obtained in the present study was slightly greater than 2.5. Tabakoff [4] reported that the velocity exponents, obtained for Ti-GAl-4V and Inconel 718 alloys at a 25” impingement angle and 60 - 1300 OF, tended to decrease with temperature. In particular, the decrease for the Ti-6Al-4V alloy was marked at greater than 1000 OF, above which the yield strength of the material significantly decreases. However, in the present study, a significant change in the velocity exponent due to temperature elevation was not observed. The 304 steel used exhibits a decrease in the yield strength from to 14 kgf mmd2 at 650 “C. approximately 27 kgf mme2 at room temperature Thus, although the material strength decreases, the velocity exponent does not seem to change significantly. Probably, at least under the experimental conditions used in the present study, the velocity exponent may not be a
l
40
-5
JOI
2c
1C 2
8 -1
96 9 s
4
3
-0
r”
I
2
1
-a
L
-0
20
40
60
80
Partkle concentration
100
(olm3)
120
20
40
00
Velochy
80100
200
: V (m/e)
Fig, 3. Effect of particle concentration on the erosion rate of the 304 steel (temperature, 300 “C; impingement angle, 20’; velocity, 120 m s-r). Fig. 4. Effect of velocity obtained for the 304 steel at 300 “C!(0) and 650 “C (0) at an impingement angle of 20’ and a particle concentration of 120 g mm3. The mass loss rate Am = AVn where n - 2.8 for both temperatures.
strong function of the target material strength particle properties and particle motion.
but may be a function
of the
3.1.3. Impingement angle The impingement angle is another important parameter influencing erosion. An attempt was made to study any difference in the effect of impingement angle as the temperature was increased. Erosion experiments were carried out using the 304 steel at 300 and 650 “C, the impingement angle varying from 15” to 90”. The results shown in Fig. 5 indicate that the maximum erosion occurred at a low angle of about 20” as expected from various room temperature studies in the past. Also, it was observed that the erosion rates at low angles were greater at 650 “C than at 300 ‘C, with no obvious difference being observed at higher angles. Tabakoff [4] observed a similar result for a 304 steel. This may be a characteristic feature which occurs because of the temperature elevation. In other words, the 304 steel exhibited behaviour more typical of a ductile material as the temperature was raised.
288
0
1
..--IL
20
j
---J...l
I
10
0
50 30 40 Angle of tmwy?ment
60 70 (degree)
80
2
90
Fig. 5, Effect of impingement angle on the erosion mass loss rate obtained for the 304 steel at 300 “C (of and 650 “C (0) (velocity, 120 m s-l; particle concentration, 120 g me3 f.
The measure of erosion used so far has been the mass loss of the specimen, However, when the thickness loss was obtained as an erosion measure, the dependence apparently shifted towards larger angles of approximately 30” (Fig. 6). Figure 7 shows the results for several steels. The carbon steel, the 1,25Cr-lMo-V steel and the 304 steel all exhibited a maximum erosion thickness loss at 30”. However, it is interesting to compare the thickness loss curves in Figs. 6 and 7. The thickness losses at 20” in comparison with those at 30” appear to be smaller at 300 “C than at 650 “C. Therefore, there is a possibility that the exact peak angle may vary slightly with temperature. In fact, the mass loss curves for the 304 steel as shown in Fig. 5 suggest that the peak angle may be at a slightly higher angle at 300 “C than at 650 “C. This result also suggests that the steel exhibits behaviour more typical of a ductile material as the temperature is increased.
/
o,
0
t 15)
I
20
I
30
/
40
1
I
/
/
I
50
60
70
80
so
lo
Angie of Impingement (degree)
Fig. 6. Comparison of impingement angle dependence plotted as the mass loss rate (0) and the thickness loss rate (0) obtained for the 304 steel at 650 “C!(velocity, 120 m s-l; particle concentration, 120 g mP3).
289
1
0’
0
1
,o
20
30
1 40
’
50
Angle of Impingement
60
1
1
70
80
1 90
’
(degree)
Fig. 7. Effect of impingement angle on the erosion thickness loss rates of several steels at 300 “C (velocity, 120 m s-l; particle concentration, 120 g mP3): 0, 304 steel; 0, carbon steel; l, 1.25Cr-lMo-0.3V steel.
3.2. Effect of temperature on the erosion rate of various steels As described in the previous sections, a greater erosion rate was observed for the 304 steel at low impingement angles and at higher temperatures. Therefore temperature is one of the very important factors which determine the erosion rate. To obtain the temperature effect on erosion for various steels, tests were conducted at room temperature and at 300, 500 and 650 “C keeping other conditions constant at a particle concentration of 120 g mm3 and an impingement angle of 20”. The results are shown in Fig. 8. A tendency
1
01
RT
300 Temperature
500
850
(‘C)
Fig. 8. Effect of temperature on the erosion thickness loss rate of various steels (impingement angle, 20”; velocity, 120 m s-l; particle concentration, 120 g mm3; RT, room temperature): q, alloy 800; a, carbon steel; 0, 304steel;=, 2,25Cr-1Mosteel; A, 12Cr-lMo-V steel.
290
for the erosion rate to increase with temperature was obtained for every steel. In particular, the increase was obvious above 500 “C. Thus it may be concluded that the erosion rate of steels impacted at low angles definitely increases as the temperature is increased. Also, it was observed that the rates of erosion were significantly different depending on the type of steel. The alloy 800 exhibited the greatest erosion rate at all temperatures while the 12Cr-lMo-V steel exhibited the smallest. The carbon steel, the 2.25Cr-1Mo steel and the 304 steel showed similar erosion rates. 3.3. Examination of the eroded surface 3.3.1. Observation from the surface To obtain the effect of the temperature on the erosion morphology the surfaces of eroded specimens were examined by scanning electron microscopy. As shown in Fig. 9 for the 304 steel, the morphology was apparently of the cutting type. Interestingly, the length of the cutting track became
(a)
(b)
(cl
(d)
Fig. 9. Variation in the morphology of the eroded surface as a function of the temperature (304 steel; impingement angle, 20”; velocity, 120 m s-l; particle concentration, 120 g me3): (a) room temperature; (b) 300 “C; (c) 500 “C; (d) 650 “C.
291
larger as the temperature was raised. As a measure of the track length, the lengths 1 of 30 - 40 tracks for each specimen were measured and the average length f was recorded. The average lengths determined for specimens eroded at 20’ are plotted against temperature in Fig. 10 together with the thickness loss measured. Obviously, the curve is quite similar for both the average track length and the thickness loss. Thus it is evident that the erosion mechanism may be of a cutting type throughout the temperature range up to 650 “C and the cutting length per particle increases with temperature, probably as a result of the softening of the steel.
RT
500
300 Temperature
W
550
1
Fig. 10. Variation in the average length of cutting tracks with temperature showing a similar trend to that of the thickness loss (304 steel; impingement angle, 20”; velocity, 120 m s-1; particle concentration, 120 g mA3): --, thickness loss; 0, f.
3.3.2. Observations from cross sections Cross sections were also observed in an attempt to examine changes in the surface zone of the steels due to erosion damage. Figure 11 depicts examples eroded at 300 “C. For a 20” impingement the grain boundary of the alloy 800 and the carbide zones of the 1.25Cr-lMo-0.3V steel were deflected towards the direction of particle flow. The photographs indicate that the thickness of the worked surface zone was several micrometres. For 90” impingement (normal incidence) it appeared that the surface was more rippled and the thickness of the worked surface zone was 5 - 10 pm, which was greater than for the acute angle, suggesting that more energy was stored in the material at a higher impingement angle. 4. Discussion 4.1. Application of Finnie’s cutting theory As described in Section 3.1.3, the angle at which the peak erosion loss occurred was greater when the maximum thickness loss was used as an erosion measure than when the mass loss was used as a measure. Since the
292
>
”
*
‘,.
:.^,, ‘_ , :? / -.. ._’
I
‘:
, h
(a)
(1,)
.‘,‘9/
/
:
-
/,’
<’
Fig. 11. Typical examples of cross sections of the eroded surface obtained at 300 ‘C, 120 m s-l and 120 g m-3: 1.25Cr-lMo-0.3V alloy at (a) 20” and (b) 90” impingement angles; (c) alloy 800 at 20” impingement angle.
erosion morphology was of a cutting type, an attempt was made to discuss the results on the basis of Finnie’s classical cutting theory [ 121. This result may be expected simply from Finnie’s cutting analysis. According to the analysis, there are two expressions depending on the cutting situation. When a particle loses velocity during cutting (case l), the volume AW of material removed may be expressed as mV2
AW=
2p(l
+ mr2/I)
cos2e
(1)
When a particle leaves the metal with some velocity mV2
AW=
2p(l+
where p= 1
K +mr211
mr2/I)
z sin(28) - 2P 1
sin26 P
(case 2),
AW may be (2)
293
AW is the erosion volume loss, m is the mass of the particle, p is the flow stress during cutting, V is the velocity of the particle, B is the angle of impingement, I is the moment of inertia of the particle and Ii is the ratio of the vertical to the horizontal forces. The maximum mass loss may appear in case 2. The peak angle must be given by tan(28) = P. Usually, 8 is about 20”. However, when the erosion is estimated by the thickness loss, eqns. (1) and (2) may simply be multiplied by sin 8. Then the peak angle appears in case 1. This gives, using the thickness loss scheme, AW’ a: cos% sin 8 Therefore, d(AW’) dtl
(3)
the peak angle must be obtained
by differentiating
eqn. (3):
a cos8 (cos28 - 2 sin28) = 0
Thus tan20 = 0.5, i.e. 8 = 35”, may be obtained for the peak erosion angle. The experimental result for the 304 steel at 650 “C (Fig. 6), i.e. that the peak angle for the mass loss was around 20” and that for the thickness loss was around 30”, is in good agreement with the theoretical expectation. However, as mentioned in Section 3.1.3, the peak angle could be shifted slightly depending on the temperature. 4.2. Differences
in the erosion rate depending on the materials and the
temperatures The erosion rate obtained for low angle impingement increased significantly with temperature. The rate was also dependent on the target steel. An attempt was made to describe those differences by a parameter. In room temperature studies made in the past many researchers have attempted to explain differences in erosion resistance between different materials. There have been two major streams of approach: one approach is to attempt correlation with mechanical parameters and the other approach seeks correlation with thermal parameters. Finnie and coworkers [3,13] suggested that the indentation hardness is a good parameter to explain the relative erosion resistance of pure metals but that this would not explain the behaviour of steels hardened by heat treatment. Rickerby [14] showed that the mechanical energy density AE = *(u, + o,)ef (where o, is the yield strength; o, is the ultimate tensile strength and ef is the fracture elongation) is a good parameter for the relative erosion resistance of pure metals. However, many researchers, on the basis of the considerable heat generation at the surface during erosion, have proposed thermal parameters such as the melting temperature [ 151 and C, AT (where C, is the heat capacity and AT is the temperature difference between the test temperature and the melting temperature) [IS]. Those thermal parameters have been described to correlate well with the erosion resistance of pure metals. As such, erosion data for pure metals have been explained by various parameters. However, a similar attempt for practically important alloys has not been successful.
294
The materials used in the present study are steels often used for high temperature components. The thermal properties such as the heat capacity and melting temperature of these materials do not vary so much. For example, for ferritic steels (carbon to 12% Cr steels), differences in the heat capacity, the thermal expansion coefficient, the melting temperature etc. are only within 10% at the most. Thus the thermal parameters could not explain the observed difference (e.g. 200% at room temperature) in the erosion resistance. Therefore mechanical parameters were tested for correlation using the data shown in Fig. 8. The mechanical energy density was firstly tested using the values shown in Table 1. However, the correlation was very poor. Secondly, the yield strength, the tensile strength and the elongation tabulated in Table 1 were examined separately. As a result, the yield strength showed a reasonable correlation as shown in Fig. 12, while the tensile strength and elongation did not correlate well. While there is some scatter, the data for different temperatures and steels fell in a reasonably narrow band around a slope of -1 in a log-log plot of the maximum thickness loss versus the yield strength. Thus yield strength could be an indicator of the erosion resistance of those high temperature steels. According to Finnie’s cutting analysis, target materials are characterized by their flow stress. As shown in eqns. (1) and (2), the volume AW of material removed is reciprocally proportional to the flow stress p. As a matter of fact, the present results, as shown in Fig. 12, show that the amount of erosion is reasonably proportional to the reciprocal yield strength of the material at the temperature of interest. Thus the present results
Yield
or
Proof
Strength
(kgflmm2)
Fig. 12. Relationship between the erosion thickness loss rate and the yield strength at test temperatures (120 m s-l, 120 g m- 3, 20’): l, 650 “C; A, 500 “C; 0, 300 ‘C; A, room temperature. The numerals beside the symbols indicate the following steels: 1, carbon steel; 2, 1.25Cr-lMo-V steel; 3, 2.25Cr-1Mo steel; 4,12Cr-lMo-V steel; 5, 304 steel; 6, alloy 800.
295
suggest that the flow stress may be substituted by cru, (where CY is a constant), at least within the range of materials and test conditions used. Finnie [12] suggests that the flow stress may be substituted by an indentation hardness. In fact, he found a good correlation for pure metals. Unfortunately, the present authors have not measured the hardness at high temperatures. It is not clear whether the yield strength is a better parameter than the hardness at present. However, hardness could be related to the yield strength [ 171. Thus it is not unreasonable that the yield strength may represent the flow stress. In the actual cutting situation, the material around the tip of the cutting particle may be heated up and very high strain rate deformation may occur. If the temperature is increased, the yield strength may be decreased. The material under deformation may be work hardened. Also, there is a possibility that, owing to the heat generated, the work-hardened surface may be stress relieved to some extent. The situation may be as such complex. Therefore the actual flow stress must be somewhat modified from the yield strength obtained by slow strain rate tensile testing. In fact, in Fig. 12, there appears to be a tendency for the slope representing the plots for data at room temperature only to be smaller than -1 and that for 500 and 650 “C to be greater than -1. However, the present empirical finding that the yield strength obtained by a slow strain tensile test may be a good parameter to correlate with the erosion resistance may be very useful, since there has been no such finding for practical steels and elevated temperature erosion behaviour.
5. Conclusions The particle erosion behaviour of typical boiler tube materials at elevated temperatures has been studied using irregularly shaped silica particles. As a result of the study, the following conclusions were obtained. (1) The effect of particle concentration, velocity exponent and peak impingement angle was studied mainly using 304 steel at 300 and 650 ‘C. The effect of those parameters did not seem to differ significantly from the tendency qualitatively expected from the well-established room temperature behaviour. (a) The erosion rate was a linear function of the particle concentration in the concentration range between 30 and 120 g mm3. (b) The velocity exponent obtained for the 304 steel was approximately 2.8 for both 300 and 650 “C. (c) The peak erosion was obtained at low impingement angles, namely at about 20°, for the maximum mass loss and at about 30” for the maximum thickness loss. However, there was a tendency for the peak angle to shift slightly to lower angles as the temperature was raised. (2) A temperature effect was observed such that, as the temperature was increased, the erosion rate at low impingement angles increased significantly but at high impingement angles it did not change significantly. Thus
296
the steels may exhibit behaviour more typical of ductile materials as the temperature is increased, (3) All materials tested showed an increase in erosion rate with temperature. However, the erosion rates of the materials differed considerably, the alloy 800 being the least resistant and the 12Cr-lMo-V steel being the most resistant. (4) The morphologies of eroded surfaces indicate that the mode of damage may be by cutting at every temperature tested. The length of the cutting track observed at low impingement angles increased with temperature. This tendency agreed well with that of the thickness loss. (5) The erosion data obtained at 20” for different materials and test temperatures were compared with the tensile properties. As a result, it was found that the yield strength of materials is a reasonably good parameter to correlate with the erosion rate. The erosion rate was app~ently proportional to the reciprocal yield strength, suggesting that the flow stress included in Finnie’s cutting theory may be conveniently substituted by the yield strength multiplied by a constant.
References 1 G. P. Tilly, Treatise Muter. Sci. Technol., 13 (1979) 287. 2 G. P. Tilly, Wear, 14 (1969) 63. 3 I. Finnie, A. Levy and D. H. McFadden, in W. Adler (ed.), Erosion: Prevention and Useful Application, ASTM Spec. Tech. Publ. 664, 1979, p. 36 (ASTM, Philadelphia, PA). 4 W. Tabakoff, in A. V. Levy (ed.), Proc. Conf. on Corrosion/Eros~on of Coal Conuersion System Materials, Berkeley, CA, January 24 - 26, 1979, National Association of Corrosion Engineers, Houston, TX, 1979, p. 700. 5 J. Hansen, in W. Adler (ed.), Erosion: Prevention and Useful Application, ASTM Spec. Tech. Publ. 664, 1979, p. 148 (ASTM, Philadelphia, PA). 6 R. H. Barkalow, J. A. Goebel and F. S. Pettit, in W. Adler (ed.), Erosion: Preuention and Useful Application, ASTM Spec. Tech. Publ. 664, 1979, p. 163 (ASTM, Philadelphia, PA). 7 I. G. Wright, in A. V. Levy (ed.), Proc. Conf. on Corrosion/Erosion of Cool Conuersion System Materials, Berkeley, CA, January 24 - 26, 1979, National Association of Corrosion Engineers, Houston, TX, 1979, p. 103. 8 I. G. Wright, V. Nagarajan and W. E. Merz, Znt. Corrosion Forum, Toronto, April 6 10, 1981, National Association of Corrosion Engineers, Houston, TX, 1981, Paper 3. 9 G. P. Tilly and W. Sage, Wear, 16 (1970) 447. 10 I. Finnie and D. H. McFadden, Wear, 48 (1978) 181. 11 I. M. Hutchings, in A. V. Levy (ed.), Proc. Conf. on Corrosion/Erosion of Coal Conversion System Materials, Berkeley, CA, January 24 - 26, 1979, National Association of Corrosion Engineers, Houston, TX, 1979, p. 393. 12 I. Finnie, Erosion and cavitation, in ASTM Spec. Tech. Publ. 307, 1962, p. 70 (ASTM, Philadelphia, PA). 13 I. Finnie, J. Wolak and Y. Kabil, J. Mater., 2 (1967) 682. 14 D. G. Rickerby, Wear, 84 (1983) 393. 15 C. E. Smeltzer, M. E. Gulden and W, A. Compton, J. BUS~C En& 92 (1970) 639. 16 I. M. Hutchings, Wear, 36 (1975) 371. 17 H. O’Neill, Hardness Measurement of Metals and Alloys, Chapman and Hall, London, 2nd edn., 1967.