- Email: [email protected]
Effect of laser processing on the cavitation erosion of Cr-Mo-Cu alloy cast iron
Wear, 119 (1987)
13 - 27
13
EFFECT OF LASER PROCESSING ON THE CAVITATION EROSION OF Cr-MO-Cu ALLOY CAST IRON LI ZHIZHONG and ZOU JIN
Metals Department, Beijing (China)
Institute of Metals and Chemistry, Academy of Railway Sciences,
XU ZHONGBIN
Chemistry Department, Beijing (China)
Institute of Metals and Chemistry, Academy of Railway Sciences,
(Received October 21,1986;
revised February 25.1987;
accepted March 2,1987)
Summary The cavitation behaviour of CI-MO-Cu alloy cast iron (cylinder liner material of diesel engines) samples processed by a continuous wave CO* laser was investigated with a magnetostriction apparatus. Untreated samples manifested the lowest resistance to cavitation erosion. Laser surface-fused and phase transformation hardened samples came next, but the improvement was very small. Samples clad with a Co 50 alloy by means of a laser exhibited excellent cavitation resistance, so much so that no erosion hole was created on the surface of the alloy layer after a 600 min cavitation test. The main mechanism in the cavitation erosion process for each microstructure is described and discussed.
1. Introduction Cavitation erosion of wet cylinder liners in high power diesel engines of railway locomotives is a major problem confronting designers, manufacturers and users [l]. With the steady growth in the speed of locomotives and the loads carried, the need to improve the cavitation erosion resistance (CER) of wet cylinder liners is an urgent problem. Selecting a suitable surface treatment technology for the outer walls of cylinder liners in order to obtain a highly cavitation-resistant layer may provide an effective and economical solution. Laser heat treatment is a new technology developed since 1970. It has brought about many improvements in the wear, corrosion and fatigue properties of materials in various applications [2 - 41. We have studied cavitation erosion in cast iron and we believe that there is great potential for improving the CER of cast iron cylinder liners in the use of this new technology. 0043-1648/87/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
14
2. Experimental details 2.1. Samples The samples were made of Cr-Mo-Cu alloy cast iron, a material used commonly for wet cylinder liners in high powered engines. Its composition is listed in Table 1, and its microstructure contains type A + B graphite, pearlite and small amounts of iron phosphide-austenite eutectic. The shape and size of the samples are shown in Fig. 1. The area of the surface exposed to the cavitation test is 2.01 cm2. All the samples were cut from a centrifugally cast cylinder liner blank (180 mm in diameter). The samples were so cut that the outer surface of the cast blank was to be exposed to the cavitation test. TABLE 1 The composition of Cr-Mo-Cu Composition
alloy cast iron
(wt.%)
C
Si
S
P
Mn
Cr
MO
CU
Ni
3.24
2.59
0.043
0.397
0.70
0.52
0.34
0.36
0.32
I.
$16 Testing
_j
(mm’
Surface
Fig. 1. Shape and size of the samples.
2.2. Laser processing
of samples
Using the beam of a 1 kW continuous wave CO2 laser, two kinds of processing of the samples were carried out. 2.2.1. Laser surface treatment
fusion and phase transformation
hardening
The samples were prior phosphated to increase the absorption of light energy. The parameters for the processing were as follows: laser power, 1 kW, diameter of light spot, 3.25 t 0.25 mm; power density, 14.2 - 10.4 kW cmM2,scanning velocity, 40 mm s-‘. After one strip was scanned the samples were moved 2.75 f 0.25 mm crosswise and the successive strips were scanned with slight overlap. The centre portion of each strip was the fused zone and on both sides of it were phase transformation hardened zones. The micro-
15
structure of the fused zone was dendritic martensite, eutectic cementite and retained austenite. Graphite and iron phosphide-austenite eutectic phases were fully dissolved and thus vanished. Its surface hardness was 882 HV. In the phase tr~sformation hardened zones were twinned martensite, retained austenite and graphite, and a small amount of graphite was dissolved in the matrix. The minimum surface hardness was 662 HV. The hardness of the matrix was only about 300 HV. The maximum hardened depth was 0.36 + 0.01 mm below the surface (Fig. 2).
Fig. 2. Surface microstructure of the laser surface-fused and phase transformation hardened samples. (a) Fused zone. (b) Phase transformation hardened zone.
TABLE 2 The nominal composition of Co 50 alloy powder Compo6ition
(wt.%)
c
Ni
Cr
B
Si
Fe
MO
co
0.3 - 0.7
26 - 30
18-20
2.5 - 3.5
3.5 - 4.0
<12
4 -6
Balance
2.2.2. Laser cladding The parameters for the laser cladding were as follows: laser power, 1.2 kW; diameter of light spot, 2.0 mm; power density, 38 kW cmm2; scanning velocity, 4.0 mm s-l. -When one scan was completed the sample was moved 1 mm crosswise and then the next strip was scanned with definite overlap. The surface of each sample was coated with Co 50 alloy powder; its composition is listed in Table 2. The thickness of the alloy was 0.85 + 0.05 mm and its structure was very fine dentritic cobalt-rich solid solution in the eutectic mixture matrix. Many fine boride and carbide particles were dispersed in it (Figs. 3 and 4). The surface hardness was as high as 960 HV.
16
Fig. 3. Microstructure of the alloy layer of the laser-clad samples. Etchant: HzSOh 5 ml; HNO3,3 ml.
HCl,~ 92 di
Fig. 4. Distribution of compounds in the alloy layer of the laser-clad samples. Arrows: A, boride; B, Crfi3. Colouring agent: NaOH, 4g; KMnOa, 4g; HzO, 100 ml.
2.3. Cavitation test The laser-processed and untreated samples were exposed to cavitation in a magnetostriction apparatus (Fig. 5). The feature of this apparatus is its strong cavitation erosion power. Very severe testing conditions were selected (such as higher supersonic output power, larger vibrating amplitude, longer testing time, etc. than conventional), so as to reveal the effect of laser processing under unusually disadvantageous circumstances. The testing conditions employed in the present work were as follows. (1) Supersonic generator: output power, 250 W; frequency, 18 kHz; vibrating amplitude, 100 pm. (2) Cavitation testing media: salt water, 3% (this concentration causes the maximum corrosion rate for cast iron [5]); testing temperature, 50 “C (at about this temperature there occurs the maximum cavitation weight loss rate [6,7]). (3) Testing time: up to 600 min.
=7 II
Fig. 5. Magnetostriction
sample
Solution
Testing Surface vibratory cavitation apparatus.
17
(4) Number of samples: untreated, three; laser surface-fused and phase transformation hardened, three and three; laser-clad, three. Prior to testing, each sample was ground with number 400 emery paper to obtain the same surface roughness. During the cavitation test, the weight loss after different times and the maximum depth of erosion holes were measured. Cross-sections of the samples were examined with an optical microscope and by scanning electron microscopy (SEM). The main cavitation erosion process was analysed so as to study the effect of laser processing on it.
3. Test results and discussion Magnetostriction cavitation erosion testing is an important experimental facility for studying the cavitation erosion behaviour of materials. The damage to samples of cast iron, as seen both in the topographic view and in micrographs of cross-sections through the damaged region, is very similar to what occurs in actual cylinder liners [8]. Topographic views of three samples of different kinds before and after cavitation testing are shown in Fig. 6. The weight losses of the various samples during the cavitation test are plotted in Fig. 7. However, owing to the markedly localized nature of such damage, the maximum depth of the erosion holes is more significant, because, although there may be only a few erosion holes on the outer wall of a cylinder liner, if the depth of any one of them exceeds a certain limit (in many cases half
0
30
600
min
Fig. 6. Macrophotographs of samples before and after the cavitation test: (a) untreated; (b) laser surface fused and phase transformation hardened; (c) laser clad.
18 0.5,
Teat Time (minutes) Fig. 7. Weight loss us. test time. Curves: 1, untreated samples; 2 and 3, laser surfacefused and phase transformation hardened samples: 4, laser-clad samples.
TABLE 3 The maximum depth of the holes in the eroded sample Test time (min)
0
10 30 600
Maximum Untreated samples
0.074 0.098 0.91
depth of the holes (mm) Laser surface-fused and phase transformation hardened samples
Laser-clad samples
Fused zones
Hardened zones
Holes
Cracks
Overlap zones
0.014 0.018 0.78
0.044 0.066 0.78
0.145 0.175 0.78
0.175 0.78
0.028 0.78
0 0 0 0
the wall thickness), that cylinder liner can no longer operate and must be replaced. Therefore this is an important criterion. Our measurements of the maximum depth are listed in Table 3. We analyse and discuss these results in the following sections. 3.1. Untreated cast iron After the preliminary stage of cavitation it can be seen (Figs. 8(a) and 8(b)) that the erosion damage initiates at the graphite, which is the weakest
(4
0)
(cl Fig. 8. Micrographs of the untreated samples (arrows, cracks along the graphite): before test; (b) tested for 10 min; (c) tested for 30 min; (d) tested for 150 min.
(a)
phase. Owing to the notch and corrosion sensitivity of the pearlite matrix, damage then progresses by fatigue and cleavage to detach pieces of metal and form many erosion holes (Fig. 9). Eventually, the whole of the surface
(a)
(b)
Fig. 9. Surface scanning electron micrographs of the untreated samples after the cavitation test: (a) 10 min; (b) 30 min; A, fatigue cracks; B, fatigue streaks; C, cleavage.
20
is rough with many deep cavities (Fig. 10). After 600 min cavitation testing, the maximum hole depth can reach 0.91 mm. A schematic drawing of the propagation of the erosion damage is shown in Fig. 11. At first, because of the stress concentration and the action of cavitation at the pointed ends of the graphite particles, cracks are created and penetrate downwards from the graphite. Then fatigue cracks develop across the pearlite. Thus particles are detached from the matrix by cleavage. The tendency for erosion holes to develop along graphite particles is evident.
(4 Fig. 10. The honeycomb tion test.
appearance of the untreated samples after a 600 min cavita-
~~yT&qJPearlite Matrix
raphite
Fig. 11. Schematic diagram of the propagation of erosion damage in cast iron.
Accordingly, we can recognize that cavitation erosion of cast iron in testing is mainly due to the direct action of the cavitation. Although traces of corrosion can be observed (Fig. 12), corrosion seems to play only an auxiliary role (such as to accelerate the spread of fatigue cracks, etc.). During the cavitation test, although much stronger corrosion media and corrosive factors were employed than would occur in actual use, since otherwise the test time might have been too short for the effect of corrosion to be properly represented; nevertheless, the cavitation action predominated. All the same, such experimental conditions are very similar to the high load and long-term continuous operation of locomotives on the Chinese National Railways.
21
Fig, 12. Trace of corrosion in the erosion hole (arrow) (SEM).
3.2. Cast iron following laser surface-fusion and phase transformation hardening treatment In the fused zone the amount of damage during the first stage of cavitation was perceptibly reduced. For example after 10 and 30 min testing, the maximum erosion hole depths were 0.014 mm and 0.018 mm respectively (in untreated samples after the same time the maximum depths were 0.073 and 0.098 mm). Careful observation and analysis show that the erosion in the fused zone had its origin in the dendritic martensite. The dendritic martensite was fragmented by way of fatigue and cleavage and then detached from the matrix. This was because the dendritic martensite was weaker than the eutectic, its absorption of the cavitation energy was low and it was distributed in relatively isolated patches in the microstructure. When the dendritic martensite was all eroded, only the eutectic framework remained. Then the framework was eroded progressively by way of brittle rupture (Fig. 13). Small blow holes and cracks remained in the fused zone after laser processing. At these points there was little resistance to cavita-
(a)
WI
Fig. 13. Surface scanning electron micrographs of the laser surface-fused and phase evocation hardened samples (fused zone) after a 10 min test: (a) the remaining framework of the eutectic (axrows, the eroded dendritic martensite); (b) the brittle rupture of the framework.
(a)
(b)
Fig. 14. Surface scanning electron micrographs of the fused zone after the cavitation test: (a) 10 min; (b) 600 min; A, small blow hole; B, small crack; C, large crack.
(a)
(b)
Fig. 15. Micrographs of the phase transformation
hardened zone after the cavitation test.
tion erosion (Fig. 14), even at the first stage of cavitation, and the degree of erosion was relatively serious. In the phase transformation hardened zone, it is clear from careful observation and analysis that the cavitation erosion originated from the weakest phase, i.e. the graphite and then also propagated along it (Fig. 15). The twinned martensite also failed by way of fatigue and cleavage (see Fig. 16). Although the pearlite matrix was transformed to martensite, its ability to retard the downward development of the erosion holes was not increased significantly, so that after 10 and 30 min cavitation testing, the maximum erosion hole depths were 0.044 mm and 0.066 mm respectively. These regions were the weaker areas in the whole testing surface (Fig. 17). Their share in the total weight loss was greater than that of the fused zone. In the laser surface-fused and phase transformation hardened samples, though the total weight loss was lowered, the maximum hole depth after 600 min testing was 0.78 mm, whereas that of the untreated samples was 0.91 mm (see Table 3). This improvement is very small. Since the maximum
23
(4
0)
Fig. 16. Surface scanning electron micrographs of the phase transformation hardened zone after the cavitation test: (a) 10 min; (b) 30 min; A, corrosion trace; C, cleavage.
Fig. 17. Surface scanning electron micrograph zone (arrow) after a 600 min cavitation test.
of the phase transformation
hardened
hole depth is an important criterion for determining the likelihood of failure in liners, we can deduce that there will be only a minor increase in the cavitation erosion life of actual cylinder liners if the laser surface fusion and phase transformation hardening process is used. 3.3. Laser cladding of cast iron It is clear that if the CER of cast iron is to be significantly increased, the harmful effect of graphite should be eliminated, and the pearlite, martensite and eutectic structures, which are also unfavourable to CER, must be replaced. The most effective and economical method is to coat the surface of the cast iron with a fine-structured alloy layer of predetermined thickness which can provide sufficiently good absorption of cavitation energy and corrosion resistance. The Co 50 alloy layer obtained by laser processing in the present work fulfils to a large extent the above requirements. Its essential features are as follows.
24
The base metal does not have to withstand high temperatures. (2) The alloy layer on the surface of the cast iron has a uniform thickness and a very fine structure. (3) A metallurgical bond is formed between the alloy layer and the base metal, and the transition zone is very narrow (Fig. 18). (4) The base metal below the transition zone is also phase transformation hardened and so it will support the alloy layer and enhance its beneficial effect. (1)
(b)
(d) Fig. 18. Distribution of (a) cobalt, (b) nickel, (c) chromium layer of the laser-clad samples.
and (d) iron in the alloy
(5) There are many other advantages such as low deformation after processing, high processing rate, savings in alloy consumption, economies in the use of energy, etc. The work described in this paper shows that cast iron samples, clad with a Co 50 alloy layer processed by laser, exhibit excellent CER in tests. After 600 min testing no erosion hole was found on the surface of the alloy layer (Fig. 19). Observation by SEM showed that its surface was only slightly
25
a /’
Surface
alloy
of
layer
(a)
Surface of alloy layer
(b)
Fig. 19. Micrographs of the alloy layer: (a) before test; (b) after a 600 min cavitation test.
roughened (Fig. 20). This is because the Co 50 alloy layer is provided with a very high capacity for work hardening [9] and low stacking fault energy, so that it can absorb relatively more cavitation energy and thus be more resistant to cavitation erosion. Therefore it is very resistant to corrosion. The small weight loss from the samples was mainly due to a few small erosion holes which were created on the boundary line between the alloy layer and the base metal (Fig. 21). There were some small blow holes in the alloy layer of the samples after laser processing (Fig. 20). Some fine cracks were formed in the alloy layer of some of the samples during the cavitation test (none of the cracks penetrated into the transition layer). These might be expected to reduce the CER of the alloy layer but after 600 min cavitation testing no cavitation erosion had appeared. Further tests are needed to study the possible harmfulness of such imperfections.
4. Conclusions (1) Cavitation erosion can act so locally that the fine microstructure may be eroded selectively. Thus the weakest phases, e.g. the graphite in the matrix or the dendritic martensite in the eutectic, are eroded first. (2) Under the same cavitation conditions, the degree of erosion is dependent on the microstructure of the material, the absorption of cavitation energy, and the corrosion resistance and it is expected that the microfracture mechanical properties of each phase in the microstructure may affect the cavitation erosion behaviour considerably.
(4
(b)
(d)
(e) Fig. 20. Surface scanning electron micrographs of the laser-clad samples: (a) before test; (b) after a 600 min cavitation test; (c) higher magnification view of (b); (d) A, the central region of a scan strip; (e) B, the overlap region between the scan strips.
(4
(b)
Fig. 21. A few small holes created on the boundary line between the alloy layer and the base metal after a 600 min cavitation test.
27
(3) Untreated cast iron samples manifest the lowest cavitation erosion resistance. (4) Laser surface-fused and phase transformation hardened samples show only a small increase in CER. (5) Samples laser clad with a Co 50 alloy show a significant increase in CER.
Acknowledgment The many ways in which help with this work was given by the Machinery and Electricity Research Institute, Beijing, and the Locomotive and Car Factory, Shenyang, are gratefully acknowledged.
References 1 Y. K. Zhou, J. G. He and F. G. Hammitt, Cavitation erosion of diesel engine wet cylinder liners, Wear, 76 (1982) 321 - 328. 2 Li Zhizhong, Laser Heat Treatment, Academy of Railway Science, Institute of Metals and Chemistry, Defence Press, Beijing, China, 1978 (in Chinese). 3 Li Zhizhong, Current Situation and Perspectives in Laser Heat Treatment, Academy of Railway Science, Institute of Metals and Chemistry, Beijing, China, 1978 (in Chinese). 4 C. M. Preece and C. W. Draper, The effect of laser quenching the surface of steels on their cavitation erosion resistance, Wear, 67 (1981) 321 - 328. 5 Y. Y. Yang, Chemical Corrosion and Protection, Vol. 1, 1984, pp. 44 - 46 (in Chinese). 6 Standard vibration cavitation test, ASTM G32 - 77. 7 Xu Zhongbin, The Effect of Some Experimental Factors on Cavitation Erosion, Academy of Railway Science, Institute of Metals and Chemistry, Beijing, China, 1986 (in Chinese). 8 Y. K. Zhou, J. G. He and F. G. Hammitt, Cavitation erosion of cast iron diesel engine liners, Wear, 76 (1982) 329 - 335. 9 C. J. Heathock, A. Bell and B. E. Protheroe, Cavitation erosion of cobalt-based stellite alloys, cemented carbides and surface-treated low ahoy steels, Wear, 74 (1981- 1982) 11 - 26.
13 - 27
13
EFFECT OF LASER PROCESSING ON THE CAVITATION EROSION OF Cr-MO-Cu ALLOY CAST IRON LI ZHIZHONG and ZOU JIN
Metals Department, Beijing (China)
Institute of Metals and Chemistry, Academy of Railway Sciences,
XU ZHONGBIN
Chemistry Department, Beijing (China)
Institute of Metals and Chemistry, Academy of Railway Sciences,
(Received October 21,1986;
revised February 25.1987;
accepted March 2,1987)
Summary The cavitation behaviour of CI-MO-Cu alloy cast iron (cylinder liner material of diesel engines) samples processed by a continuous wave CO* laser was investigated with a magnetostriction apparatus. Untreated samples manifested the lowest resistance to cavitation erosion. Laser surface-fused and phase transformation hardened samples came next, but the improvement was very small. Samples clad with a Co 50 alloy by means of a laser exhibited excellent cavitation resistance, so much so that no erosion hole was created on the surface of the alloy layer after a 600 min cavitation test. The main mechanism in the cavitation erosion process for each microstructure is described and discussed.
1. Introduction Cavitation erosion of wet cylinder liners in high power diesel engines of railway locomotives is a major problem confronting designers, manufacturers and users [l]. With the steady growth in the speed of locomotives and the loads carried, the need to improve the cavitation erosion resistance (CER) of wet cylinder liners is an urgent problem. Selecting a suitable surface treatment technology for the outer walls of cylinder liners in order to obtain a highly cavitation-resistant layer may provide an effective and economical solution. Laser heat treatment is a new technology developed since 1970. It has brought about many improvements in the wear, corrosion and fatigue properties of materials in various applications [2 - 41. We have studied cavitation erosion in cast iron and we believe that there is great potential for improving the CER of cast iron cylinder liners in the use of this new technology. 0043-1648/87/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
14
2. Experimental details 2.1. Samples The samples were made of Cr-Mo-Cu alloy cast iron, a material used commonly for wet cylinder liners in high powered engines. Its composition is listed in Table 1, and its microstructure contains type A + B graphite, pearlite and small amounts of iron phosphide-austenite eutectic. The shape and size of the samples are shown in Fig. 1. The area of the surface exposed to the cavitation test is 2.01 cm2. All the samples were cut from a centrifugally cast cylinder liner blank (180 mm in diameter). The samples were so cut that the outer surface of the cast blank was to be exposed to the cavitation test. TABLE 1 The composition of Cr-Mo-Cu Composition
alloy cast iron
(wt.%)
C
Si
S
P
Mn
Cr
MO
CU
Ni
3.24
2.59
0.043
0.397
0.70
0.52
0.34
0.36
0.32
I.
$16 Testing
_j
(mm’
Surface
Fig. 1. Shape and size of the samples.
2.2. Laser processing
of samples
Using the beam of a 1 kW continuous wave CO2 laser, two kinds of processing of the samples were carried out. 2.2.1. Laser surface treatment
fusion and phase transformation
hardening
The samples were prior phosphated to increase the absorption of light energy. The parameters for the processing were as follows: laser power, 1 kW, diameter of light spot, 3.25 t 0.25 mm; power density, 14.2 - 10.4 kW cmM2,scanning velocity, 40 mm s-‘. After one strip was scanned the samples were moved 2.75 f 0.25 mm crosswise and the successive strips were scanned with slight overlap. The centre portion of each strip was the fused zone and on both sides of it were phase transformation hardened zones. The micro-
15
structure of the fused zone was dendritic martensite, eutectic cementite and retained austenite. Graphite and iron phosphide-austenite eutectic phases were fully dissolved and thus vanished. Its surface hardness was 882 HV. In the phase tr~sformation hardened zones were twinned martensite, retained austenite and graphite, and a small amount of graphite was dissolved in the matrix. The minimum surface hardness was 662 HV. The hardness of the matrix was only about 300 HV. The maximum hardened depth was 0.36 + 0.01 mm below the surface (Fig. 2).
Fig. 2. Surface microstructure of the laser surface-fused and phase transformation hardened samples. (a) Fused zone. (b) Phase transformation hardened zone.
TABLE 2 The nominal composition of Co 50 alloy powder Compo6ition
(wt.%)
c
Ni
Cr
B
Si
Fe
MO
co
0.3 - 0.7
26 - 30
18-20
2.5 - 3.5
3.5 - 4.0
<12
4 -6
Balance
2.2.2. Laser cladding The parameters for the laser cladding were as follows: laser power, 1.2 kW; diameter of light spot, 2.0 mm; power density, 38 kW cmm2; scanning velocity, 4.0 mm s-l. -When one scan was completed the sample was moved 1 mm crosswise and then the next strip was scanned with definite overlap. The surface of each sample was coated with Co 50 alloy powder; its composition is listed in Table 2. The thickness of the alloy was 0.85 + 0.05 mm and its structure was very fine dentritic cobalt-rich solid solution in the eutectic mixture matrix. Many fine boride and carbide particles were dispersed in it (Figs. 3 and 4). The surface hardness was as high as 960 HV.
16
Fig. 3. Microstructure of the alloy layer of the laser-clad samples. Etchant: HzSOh 5 ml; HNO3,3 ml.
HCl,~ 92 di
Fig. 4. Distribution of compounds in the alloy layer of the laser-clad samples. Arrows: A, boride; B, Crfi3. Colouring agent: NaOH, 4g; KMnOa, 4g; HzO, 100 ml.
2.3. Cavitation test The laser-processed and untreated samples were exposed to cavitation in a magnetostriction apparatus (Fig. 5). The feature of this apparatus is its strong cavitation erosion power. Very severe testing conditions were selected (such as higher supersonic output power, larger vibrating amplitude, longer testing time, etc. than conventional), so as to reveal the effect of laser processing under unusually disadvantageous circumstances. The testing conditions employed in the present work were as follows. (1) Supersonic generator: output power, 250 W; frequency, 18 kHz; vibrating amplitude, 100 pm. (2) Cavitation testing media: salt water, 3% (this concentration causes the maximum corrosion rate for cast iron [5]); testing temperature, 50 “C (at about this temperature there occurs the maximum cavitation weight loss rate [6,7]). (3) Testing time: up to 600 min.
=7 II
Fig. 5. Magnetostriction
sample
Solution
Testing Surface vibratory cavitation apparatus.
17
(4) Number of samples: untreated, three; laser surface-fused and phase transformation hardened, three and three; laser-clad, three. Prior to testing, each sample was ground with number 400 emery paper to obtain the same surface roughness. During the cavitation test, the weight loss after different times and the maximum depth of erosion holes were measured. Cross-sections of the samples were examined with an optical microscope and by scanning electron microscopy (SEM). The main cavitation erosion process was analysed so as to study the effect of laser processing on it.
3. Test results and discussion Magnetostriction cavitation erosion testing is an important experimental facility for studying the cavitation erosion behaviour of materials. The damage to samples of cast iron, as seen both in the topographic view and in micrographs of cross-sections through the damaged region, is very similar to what occurs in actual cylinder liners [8]. Topographic views of three samples of different kinds before and after cavitation testing are shown in Fig. 6. The weight losses of the various samples during the cavitation test are plotted in Fig. 7. However, owing to the markedly localized nature of such damage, the maximum depth of the erosion holes is more significant, because, although there may be only a few erosion holes on the outer wall of a cylinder liner, if the depth of any one of them exceeds a certain limit (in many cases half
0
30
600
min
Fig. 6. Macrophotographs of samples before and after the cavitation test: (a) untreated; (b) laser surface fused and phase transformation hardened; (c) laser clad.
18 0.5,
Teat Time (minutes) Fig. 7. Weight loss us. test time. Curves: 1, untreated samples; 2 and 3, laser surfacefused and phase transformation hardened samples: 4, laser-clad samples.
TABLE 3 The maximum depth of the holes in the eroded sample Test time (min)
0
10 30 600
Maximum Untreated samples
0.074 0.098 0.91
depth of the holes (mm) Laser surface-fused and phase transformation hardened samples
Laser-clad samples
Fused zones
Hardened zones
Holes
Cracks
Overlap zones
0.014 0.018 0.78
0.044 0.066 0.78
0.145 0.175 0.78
0.175 0.78
0.028 0.78
0 0 0 0
the wall thickness), that cylinder liner can no longer operate and must be replaced. Therefore this is an important criterion. Our measurements of the maximum depth are listed in Table 3. We analyse and discuss these results in the following sections. 3.1. Untreated cast iron After the preliminary stage of cavitation it can be seen (Figs. 8(a) and 8(b)) that the erosion damage initiates at the graphite, which is the weakest
(4
0)
(cl Fig. 8. Micrographs of the untreated samples (arrows, cracks along the graphite): before test; (b) tested for 10 min; (c) tested for 30 min; (d) tested for 150 min.
(a)
phase. Owing to the notch and corrosion sensitivity of the pearlite matrix, damage then progresses by fatigue and cleavage to detach pieces of metal and form many erosion holes (Fig. 9). Eventually, the whole of the surface
(a)
(b)
Fig. 9. Surface scanning electron micrographs of the untreated samples after the cavitation test: (a) 10 min; (b) 30 min; A, fatigue cracks; B, fatigue streaks; C, cleavage.
20
is rough with many deep cavities (Fig. 10). After 600 min cavitation testing, the maximum hole depth can reach 0.91 mm. A schematic drawing of the propagation of the erosion damage is shown in Fig. 11. At first, because of the stress concentration and the action of cavitation at the pointed ends of the graphite particles, cracks are created and penetrate downwards from the graphite. Then fatigue cracks develop across the pearlite. Thus particles are detached from the matrix by cleavage. The tendency for erosion holes to develop along graphite particles is evident.
(4 Fig. 10. The honeycomb tion test.
appearance of the untreated samples after a 600 min cavita-
~~yT&qJPearlite Matrix
raphite
Fig. 11. Schematic diagram of the propagation of erosion damage in cast iron.
Accordingly, we can recognize that cavitation erosion of cast iron in testing is mainly due to the direct action of the cavitation. Although traces of corrosion can be observed (Fig. 12), corrosion seems to play only an auxiliary role (such as to accelerate the spread of fatigue cracks, etc.). During the cavitation test, although much stronger corrosion media and corrosive factors were employed than would occur in actual use, since otherwise the test time might have been too short for the effect of corrosion to be properly represented; nevertheless, the cavitation action predominated. All the same, such experimental conditions are very similar to the high load and long-term continuous operation of locomotives on the Chinese National Railways.
21
Fig, 12. Trace of corrosion in the erosion hole (arrow) (SEM).
3.2. Cast iron following laser surface-fusion and phase transformation hardening treatment In the fused zone the amount of damage during the first stage of cavitation was perceptibly reduced. For example after 10 and 30 min testing, the maximum erosion hole depths were 0.014 mm and 0.018 mm respectively (in untreated samples after the same time the maximum depths were 0.073 and 0.098 mm). Careful observation and analysis show that the erosion in the fused zone had its origin in the dendritic martensite. The dendritic martensite was fragmented by way of fatigue and cleavage and then detached from the matrix. This was because the dendritic martensite was weaker than the eutectic, its absorption of the cavitation energy was low and it was distributed in relatively isolated patches in the microstructure. When the dendritic martensite was all eroded, only the eutectic framework remained. Then the framework was eroded progressively by way of brittle rupture (Fig. 13). Small blow holes and cracks remained in the fused zone after laser processing. At these points there was little resistance to cavita-
(a)
WI
Fig. 13. Surface scanning electron micrographs of the laser surface-fused and phase evocation hardened samples (fused zone) after a 10 min test: (a) the remaining framework of the eutectic (axrows, the eroded dendritic martensite); (b) the brittle rupture of the framework.
(a)
(b)
Fig. 14. Surface scanning electron micrographs of the fused zone after the cavitation test: (a) 10 min; (b) 600 min; A, small blow hole; B, small crack; C, large crack.
(a)
(b)
Fig. 15. Micrographs of the phase transformation
hardened zone after the cavitation test.
tion erosion (Fig. 14), even at the first stage of cavitation, and the degree of erosion was relatively serious. In the phase transformation hardened zone, it is clear from careful observation and analysis that the cavitation erosion originated from the weakest phase, i.e. the graphite and then also propagated along it (Fig. 15). The twinned martensite also failed by way of fatigue and cleavage (see Fig. 16). Although the pearlite matrix was transformed to martensite, its ability to retard the downward development of the erosion holes was not increased significantly, so that after 10 and 30 min cavitation testing, the maximum erosion hole depths were 0.044 mm and 0.066 mm respectively. These regions were the weaker areas in the whole testing surface (Fig. 17). Their share in the total weight loss was greater than that of the fused zone. In the laser surface-fused and phase transformation hardened samples, though the total weight loss was lowered, the maximum hole depth after 600 min testing was 0.78 mm, whereas that of the untreated samples was 0.91 mm (see Table 3). This improvement is very small. Since the maximum
23
(4
0)
Fig. 16. Surface scanning electron micrographs of the phase transformation hardened zone after the cavitation test: (a) 10 min; (b) 30 min; A, corrosion trace; C, cleavage.
Fig. 17. Surface scanning electron micrograph zone (arrow) after a 600 min cavitation test.
of the phase transformation
hardened
hole depth is an important criterion for determining the likelihood of failure in liners, we can deduce that there will be only a minor increase in the cavitation erosion life of actual cylinder liners if the laser surface fusion and phase transformation hardening process is used. 3.3. Laser cladding of cast iron It is clear that if the CER of cast iron is to be significantly increased, the harmful effect of graphite should be eliminated, and the pearlite, martensite and eutectic structures, which are also unfavourable to CER, must be replaced. The most effective and economical method is to coat the surface of the cast iron with a fine-structured alloy layer of predetermined thickness which can provide sufficiently good absorption of cavitation energy and corrosion resistance. The Co 50 alloy layer obtained by laser processing in the present work fulfils to a large extent the above requirements. Its essential features are as follows.
24
The base metal does not have to withstand high temperatures. (2) The alloy layer on the surface of the cast iron has a uniform thickness and a very fine structure. (3) A metallurgical bond is formed between the alloy layer and the base metal, and the transition zone is very narrow (Fig. 18). (4) The base metal below the transition zone is also phase transformation hardened and so it will support the alloy layer and enhance its beneficial effect. (1)
(b)
(d) Fig. 18. Distribution of (a) cobalt, (b) nickel, (c) chromium layer of the laser-clad samples.
and (d) iron in the alloy
(5) There are many other advantages such as low deformation after processing, high processing rate, savings in alloy consumption, economies in the use of energy, etc. The work described in this paper shows that cast iron samples, clad with a Co 50 alloy layer processed by laser, exhibit excellent CER in tests. After 600 min testing no erosion hole was found on the surface of the alloy layer (Fig. 19). Observation by SEM showed that its surface was only slightly
25
a /’
Surface
alloy
of
layer
(a)
Surface of alloy layer
(b)
Fig. 19. Micrographs of the alloy layer: (a) before test; (b) after a 600 min cavitation test.
roughened (Fig. 20). This is because the Co 50 alloy layer is provided with a very high capacity for work hardening [9] and low stacking fault energy, so that it can absorb relatively more cavitation energy and thus be more resistant to cavitation erosion. Therefore it is very resistant to corrosion. The small weight loss from the samples was mainly due to a few small erosion holes which were created on the boundary line between the alloy layer and the base metal (Fig. 21). There were some small blow holes in the alloy layer of the samples after laser processing (Fig. 20). Some fine cracks were formed in the alloy layer of some of the samples during the cavitation test (none of the cracks penetrated into the transition layer). These might be expected to reduce the CER of the alloy layer but after 600 min cavitation testing no cavitation erosion had appeared. Further tests are needed to study the possible harmfulness of such imperfections.
4. Conclusions (1) Cavitation erosion can act so locally that the fine microstructure may be eroded selectively. Thus the weakest phases, e.g. the graphite in the matrix or the dendritic martensite in the eutectic, are eroded first. (2) Under the same cavitation conditions, the degree of erosion is dependent on the microstructure of the material, the absorption of cavitation energy, and the corrosion resistance and it is expected that the microfracture mechanical properties of each phase in the microstructure may affect the cavitation erosion behaviour considerably.
(4
(b)
(d)
(e) Fig. 20. Surface scanning electron micrographs of the laser-clad samples: (a) before test; (b) after a 600 min cavitation test; (c) higher magnification view of (b); (d) A, the central region of a scan strip; (e) B, the overlap region between the scan strips.
(4
(b)
Fig. 21. A few small holes created on the boundary line between the alloy layer and the base metal after a 600 min cavitation test.
27
(3) Untreated cast iron samples manifest the lowest cavitation erosion resistance. (4) Laser surface-fused and phase transformation hardened samples show only a small increase in CER. (5) Samples laser clad with a Co 50 alloy show a significant increase in CER.
Acknowledgment The many ways in which help with this work was given by the Machinery and Electricity Research Institute, Beijing, and the Locomotive and Car Factory, Shenyang, are gratefully acknowledged.
References 1 Y. K. Zhou, J. G. He and F. G. Hammitt, Cavitation erosion of diesel engine wet cylinder liners, Wear, 76 (1982) 321 - 328. 2 Li Zhizhong, Laser Heat Treatment, Academy of Railway Science, Institute of Metals and Chemistry, Defence Press, Beijing, China, 1978 (in Chinese). 3 Li Zhizhong, Current Situation and Perspectives in Laser Heat Treatment, Academy of Railway Science, Institute of Metals and Chemistry, Beijing, China, 1978 (in Chinese). 4 C. M. Preece and C. W. Draper, The effect of laser quenching the surface of steels on their cavitation erosion resistance, Wear, 67 (1981) 321 - 328. 5 Y. Y. Yang, Chemical Corrosion and Protection, Vol. 1, 1984, pp. 44 - 46 (in Chinese). 6 Standard vibration cavitation test, ASTM G32 - 77. 7 Xu Zhongbin, The Effect of Some Experimental Factors on Cavitation Erosion, Academy of Railway Science, Institute of Metals and Chemistry, Beijing, China, 1986 (in Chinese). 8 Y. K. Zhou, J. G. He and F. G. Hammitt, Cavitation erosion of cast iron diesel engine liners, Wear, 76 (1982) 329 - 335. 9 C. J. Heathock, A. Bell and B. E. Protheroe, Cavitation erosion of cobalt-based stellite alloys, cemented carbides and surface-treated low ahoy steels, Wear, 74 (1981- 1982) 11 - 26.