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Report number: 10-OJKB
Three-dimensional investigation of mangan sulfides in steels by using
electrolytic extraction
Oscar Juneblad
Kristofer Bölke
Supervisor: Andrey Karasev
May, 2013
Dept. of Material Science and Engineering
Royal Institute of Technology
Stockholm, Sweden
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Abstract The mechanical properties in steel are highly dependent on the characteristics of non-‐metallic inclusions. To observe and analyze these inclusions are therefore of great importance. In this project, the three-‐dimensional method electrolytic extraction has been used together with a SEM (scanning electron microscope) to observe and determine the characteristics of mangan sulfide inclusions. Characteristics such as; size, quantity, composition and morphology of inclusions were determined by observing and analyzing printed SEM-‐pictures of inclusions in five different samples. Three samples from a cast ingot and two from hot rolled steel were investigated. The samples from the cast ingot were taken from different locations in the ingot and the two rolled samples were rolled with different pressure. The mangan sulfide inclusions in these five samples were investigated, analyzed and conclusions were made based on the results from this study.
Key words: Electrolytic Extraction, Mangan Sulfide, Inclusions, SEM, Carbon Steel
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Table of Contents Abstract ............................................................................................................................. 2
1. Introduction ................................................................................................................... 4
2. Experimental .................................................................................................................. 6 2.1. Sampling ............................................................................................................................... 6 2.2. Electrolytic extraction ........................................................................................................... 7 2.3. SEM investigation of mangan sulfide inclusions .................................................................... 8
3. Results and Discussion ................................................................................................... 9 3.1. Mangan sulfides in cast ingot (samples OK1-‐OK3) ................................................................. 9
3.1.1. Morphology of inclusions .................................................................................................... 10 3.1.2. Size and aspect ratio ........................................................................................................... 12 3.1.3. Number of inclusions per unit volume ................................................................................ 16
3.2. Mangan sulfides in rolled steel (samples OK4-‐OK5) ............................................................. 17 3.2.1. Damaged inclusions ............................................................................................................ 17 3.2.2. Morphology of inclusions .................................................................................................... 18 3.2.3. Size and aspect ratio ........................................................................................................... 20
4. Conclusions .................................................................................................................. 22
5. Recommendations for future work ............................................................................... 23
6. References ................................................................................................................... 24
Appendix ......................................................................................................................... 25 Equations .................................................................................................................................. 25
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1. Introduction The mechanical properties in steel are highly dependent on the characteristics of non-‐metallic inclusions such as size, quantity, composition and morphology. The characteristics of non-‐metallic inclusions therefore play a huge part in the final product. Depending on what type of steel that is produced and what kind of environment it will be exposed to, different demands on the mechanical properties will be set. To meet these demands and to get the mechanical properties required, the control of non-‐metallic inclusions and preventing their occurrence are of great importance. The types of inclusions that are investigated and analyzed in this project are mangan sulfides.
Mangan sulfides are shown in nearly all industrial steel and they exist as pure mangan sulfides or as a shell of mangan sulfide with an oxide kernel, where the oxide act nucleator in the mangan sulfide crystallization [1]. Mangan sulfides presence in steel affects the steel in several ways and got a bad impact to the most mechanical properties. It’s shown that an increased amount of mangan sulfides will decrease ductility, fracture toughness, impact strength, creep strength and the fatigue strength [2][3][4]. The polishability will also be reduced with mangan sulfide present on the surface and it will be much harder to get a shiny surface [2].
Mangan sulfide inclusions are softer and got a higher thermo-‐expansion coefficient than the surrounding metal matrix and when the material deform, like hot rolling, the sulfides will elongate and get stretched out. The elongated sulfides will get sharp edges with high stress concentrations, which will lead to mechanical anisotropy [1][5]. After deformation the material will get different properties depending on which way the stress are applied. If the stress is applied in the rolling direction, same direction as the elongated sulfides, the material will have a higher tensile strength, be more ductile, higher yield stress and have a higher fracture stress than if the stress was applied perpendicular to the rolling direction [5].
Materials with mangan sulfide inclusions that are deformed are more likely to fracture and start cracking than material without mangan sulfide inclusions. When a material that contains mangan sulfide inclusions is deformed, at a certain stress, the inclusions cannot maintain bonding with the matrix and will therefore debond, which leads to voids and cavities around the inclusions. Larger mangan sulfide inclusions are more dangerous to the material, increases the risk that the inclusion will debond from the matrix and also increases the risk of cracking [5]. Voids and cavities can be present in the boundary between the inclusion and the matrix, from the solidification or they could form because of the stress concentrations between the softer mangan sulfide and the harder metal matrix. These stress concentrations exists because of the different mechanical properties between the matrix and the non-‐metallic inclusions [4][3]. Voids and cavities act crack initiators and when a stress is applied void coalescence and void growth occurs and the crack will propagate [4].
The distance between the mangan sulfide inclusions also plays a major role to the mechanical properties and especially to the crack propagation. During deformation the voids and cavities around the debonded inclusions interlink and grow and strain bands will form between the inclusions. A short distance between the mangan sulfide inclusions will create strain bands at an early stage and the crack will propagate much faster than if the distance was longer [5].
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The micro-‐cracks, cavities and voids between the matrix and the mangan sulfide inclusions also increase the pitting and lower the local corrosion resistance in stainless steel. With mangan sulfide inclusions present the pitting occurs at a lower temperature than if there were no inclusions present [1].
Because of the negative impact the mangan sulfide inclusions has on the mechanical properties in steel and that they are almost always present in all steel types, the analyze of these inclusions are extremely important. Two major methods can be used to determine inclusion characteristics of different type of sulfides. These two methods vary both in execution as well as in the results obtained and it is important to have knowledge about these two types before investigating since they both have some serious advantages and disadvantages.
The 2D method is based on a polished cross-‐section of a sample where one observes the metal surface from above in a microscope. What is observed is highly dependent on the angle of the cutting section and will thereby not provide all information needed for classifying the characteristics of inclusions in that sample, figure 1 [6]. Other problems are the damaged and broken inclusions obtained by cutting of cross-‐section or in sample preparation. However the location of inclusions in the sample stays intact and provides information of how the inclusions are arranged in the matrix.
For 3D investigation of inclusions the electrolytic extraction (EE) process can be used. In this case the metal surface dissolves whereas most of the large size inclusions stay intact and one can observe inclusions on film filter after filtrating of solution as well as inclusions staying in the metal surface, figure 1. This method gives a more accurate determination of inclusion characteristics compared with the two-‐dimensional method since the whole morphology and size can be observed. A disadvantage in this case is that the locations of inclusions are unknown. Because of the dissolution, location order of inclusions on film filter does not match the location order in the sample before the extraction. Some inclusions may also be eliminated, damaged or even broken because of the current applied for the extraction to occur. For high-‐alloyed carbon steel it is difficult to apply the three-‐dimensional method since the metal-‐face in these types of steel is too hard to dissolve. Another problem are the carbides covering the inclusions making them impossible to analyze.
Since the samples that are analyzed in this project comes from low carbon steel, and that the main part of this project involves determination of large size inclusion characteristics, EE is the most advantageous method and can be used to investigate the sulfides in this project.
Figure 1.[1] 2-‐dimensional analysis of inclusions on polished cross-‐section and 3-‐dimensional analysis of inclusions on film filter of steel sample.
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The EE process is used together with a Scanning Electron Microscope (SEM), to analyze the characteristics of mangan sulfide inclusions in five different steel samples. Three of the samples are taken from a cast iron ingot and are located in different places of the ingot; one at the edge, one in the center of the ingot and one therein between. In these three samples the inclusions are investigated on film filter and on metal surface. The other two samples are taken from the same type of steel after hot rolling with different pressure resulting in a diameter difference. The rolled samples are only investigated on film filter. The steel of the samples that are observed and investigated is a free cutting steel with high sulfur content. The higher sulfur content makes the steel softer and therefore easier to cut.
In this study, five samples are investigated. The inclusions are analyzed and compared along with their number value and how often they appear in the steel. The purpose is to get a better understanding of mangan sulfide inclusions in steel and make conclusions based on the results obtained in the investigation.
2. Experimental
2.1. Sampling A total of five samples were given from an industrial company for analyzing sulfide inclusions. The steel type is 42CrMo4 and is a carbon steel with a sulphur content of 250ppm. Typical composition for the steel samples is shown in table 1. Table 2 shows the difference between the samples and how they are investigated. In the rolling procedure OK5 was exposed to higher rolling pressure than OK4 resulting in a smaller diameter.
Table 1.[1]. Chemical composition of steel type 42CrMo4.
Table 2. Sample information.
Steel C Si Mn Cr Mo S 42CrMo4 0,38-‐0,45% ≤0,40% 0,60-‐0,90% 0,90-‐1,20% 0,15-‐0,30% ≤0,035%
Sample Source Location Rolling pressure
EE Film filter Metal surface
OK1 Ingot Edge
× × ×
OK2 Ingot Middle
× ×
OK3 Ingot Center
× × ×
OK4 Rolled Random Lower × ×
OK5 Rolled Random Higher × ×
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2.2. Electrolytic extraction In the EE process inclusions are extracted from a metal surface and filtered through a film filter. Figure 2 shows a schematic illustration of the EE set up. A metal matrix is dissolved in an electrolyte with the presence of electric current. Mangan sulfides, which are not soluble in the electrolyte, remain in the solution and collects on the film filter after filtration of this solution. The film filter can then be observed in SEM for analysis of inclusions. The purpose of using this method is to reveal the mangan sulfide inclusions from inside the metal samples for observation in SEM.
The lab started off with five metal piece samples which all underwent similar procedure: The samples (15x10x5mm) surface was, before EE, grinded to remove impurities such as dust, oxides etc. The sample was then measured and was cleaned by acetone and then by petroleum benzene in an ultrasonic bath for about 2-‐3 minutes to remove more of the remaining impurities on the surface. The sample was weighed very carefully before entering the EE process. A plastic film was wrapped around the four sides of the sample that surrounded the upcoming dissolved surface. This was made to prevent these sides from getting dissolved in the process. All components in the EE process were carefully cleaned with methanol.
First off, a jar was filled with 250ml of a 10% AA (10% acetylacetone – 1% tetramethylammounium chloride – methanol) electrolyte solution. A metal pincer was then attached to the sample and both were cleaned with methanol. The sample was lowered into the jar of electrolyte until it was fully underneath the surface, figure 2. A wire was connected to the metal pincer and another to a metal circle surrounding the sample in the electrolyte. The two wires was then connected in to a “Potentiostat” where current (30-‐60mA), voltage (3-‐4V) and electric charge could be regulated. To stop the process, a timer was set to terminate the experiment when 500 coulombs was reached. During the extraction the process had to be overseen every 30 minutes to check if it was stable and if any parameters had to be changed. After three hours 500 coulombs was reached and the extraction stopped. The metal matrix had now been dissolved and the surface had turned black because of the carbide precipitation. The inclusions that were of interest had now either stayed in the metal surface of the sample or fallen out in the solution. The color of the solution turned red because of all the metal ions detached from the sample.
Next step was filtration of the solution. All parts in the filtration equipment were cleaned with methanol before built up. The metal pincer attached to the sample was removed from the jar with electrolyte and was cleaned with methanol before undergoing an ultrasonic bath, three times in a row. The jar of red solution was poured through the filtration system with the presence of a magnet in the bottom of the jar to avoid metallic particles coming along. The solution was filtered through a 1μm pore size Polycarbonide (PC) film filter. A lid consisting of glass was placed over the top jar in the filtration system to prevent dust and other things from ending up in the solution. To ease the flow of the solution through the film filter an aspirator was connected to generate low pressure in the bottom jar were the solution end up in, figure 3. When all red metallic solution had been filtrated some methanol was poured through the filtration system to make sure that all solution had passed the film filter. The film filter, now containing inclusions from the solution, was removed and ready for observation in SEM. The plastic film on the metal sample was removed and the sample was weighed again so that the dissolved weight from the extraction process could be calculated for each sample for usage in the calculation of number value.
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2.3. SEM investigation of mangan sulfide inclusions Pictures of mangan sulfide inclusions were taken in SEM for analyse after the EE process, figure 4. The characterization of inclusions in terms of size and quantity was determined by observing the printed pictures. To obtain the size of the inclusions, maximum length (L) and width (W) were measured, by hand, with a calliper. The real lengths of the inclusions were calculated by using each SEM pictures scale and magnification. All the inclusions of a measurable size was measured and counted and the frequency of the inclusion size was determined in every sample.
Number value of inclusions was calculated with equation (1) in appendix and shows number of inclusions per volume in the sample. This value has been calculated for samples taken from cast ingot observed on film filter. Total number value was calculated with equation (2) in appendix and is a formula to determine the average number value for an entire sample when observing film filter. Standard deviation has been used to show how much variation or dispersion there is from the average value and is used when size of the inclusions are analysed. Since this study involves a lot of measurements this is an appropriate method to get a better overall view of the size distribution. These three methods will be represented in some of the figures presented in the result part.
For observation of inclusions after the EE process, inclusions were observed both on metal surface and on film filter. The film filter observed was cut out from a circle and obtained the shape of a circular sector. As can be seen in
Figure 2.[2]. Schematic illustration of electrolytic extraction set up.
Figure 5. Location on film filter.
Figure 3.[2]. Filtration.
Figure 4. Measurements of some of the classified morphologies in cast ingot taken in SEM(OK1-‐OK3).
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figure 5, different areas of the filter were observed in the investigation; top, middle and bottom. In this study, most results obtained are based independenly of location of filmfilter. However it can be seen in figure 13(a) in results part that the observed location on film filter does matter but this phenomena needs a more detailed separate investigation.
3. Results and Discussion The results and discussion are divided into two separate parts that are compared separately. The first part includes samples taken from the cast ingot and the second part includes samples taken after rolling.
3.1. Mangan sulfides in cast ingot (samples OK1-‐OK3) Classification of morphologies was determined when observing mangan sulfides in SEM. The different morphologies observed in cast ingot samples can be seen in table 3.
Table 3. Classification of morphologies for OK1-‐OK3.
Name Regular (Re) Irregular (Ir) Rod (Ro) Dendrite (D) Spherical (S)
Morphology
Size range
(μm) 2 -‐ 41 4 – 41 4 – 54 13 – 36 3 – 5
Aspect ratio range (L/W)
1 – 3 1 – 12 2 – 29 2 -‐ 3 1
Frequency (%) 49% -‐ 80% 13% -‐ 23% 0% -‐ 21% 0% -‐ 3% 1% -‐ 2%
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3.1.1. Morphology of inclusions
Frequencies of different morphologies of mangan sulfide inclusions are shown. In 5(a), 50% of the inclusions got a regular shaped morphology and the rest of the morphologies are mainly irregulars and rods, except for a few percent of dendrites and spheres. In 5(b) there is a large increase in frequency of regular shaped and there are no rods or dendrites present. The frequency of irregular shaped has decreased from 5(a) to 5(c). In 5(c), the frequency of regular shaped inclusions has increased further compared to 5(b) while the frequency of irregulars has decreased.
The tendency in figure 5 shows that the amount of regular shaped morphology increases from 5(a) to 5(c) while all other morphologies shows a distinct decrease. This is a result of the decreasing of cooling rate towards the center of the ingot.
50%
24%
21%
3% 2%
(a)
Regular
Irregular
Rod
Dendrite
Sphere
79%
19%
2%
(b)
Regular
Irregular
Sphere 81%
13%
4% 2%
(c)
Regular
Irregular
Rod
Sphere
7%
49% 36%
8%
(a)
Regular
Irregular
Rod
Dendrite
65%
35%
(b)
Regular
Irregular 71%
22%
7%
(c)
regular
irregular
rod
Figure 5. Frequency of different morphologies of all inclusions in (a) OK1, (b) OK2 and (c) OK3.
Figure 6. Frequency of different morphologies of large size inclusions (≥10μm) in (a) OK1, (b) OK2 and (c) OK3.
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Since the larger size inclusions are known to have a worse effect on the mechanical properties it is of great interest to analyze this size more closely. Here a large size inclusion was set to ≥10μm. In 6(a) the frequency is dominated by irregular shaped, with rods to follow, and there is only a small amount of regulars and dendrite shaped. The frequency of regular shaped inclusions has increased a lot from 6(a) to 6(b) and decreased for irregular shaped inclusions. This pattern continues to 6(c). The tendency in figure 5 repeats in figure 6.
Frequency of inclusion morphologies on film filter and metal surface is observed. In 7(a) the regular inclusions on metal surface got higher frequency than the regular ones on film filter. The frequency of the irregular-‐and rod shaped are very similar and in both of these morphologies the inclusions on film filter got higher frequency than the inclusions on metal surface. The sphere and dendrites represents a very small amount of the total frequencies, both on metal surface and film filter. The frequency of dendrites is almost identical on film filter and on metal surface, while the frequency of spheres is higher on metal surface than on film filter.
In 7(c) the frequency of regular shaped inclusions has increased and are higher on film filter than on metal surface. The frequency of irregular shaped inclusions has also changed and the metal surface frequency is much higher than on film filter. The frequency of rods and dendrites on film filter has increased and there are no rods or dendrites present on metal surface in 7(c).
The tendency of morphology frequency on metal surface and film filter from 7(a) to 7(c) is that there is more regular shaped inclusions on film filter than on metal surface in 7(a) and inversely in 7(c), while the irregular shape frequency in 7(a) on film filter is higher than on metal surface and inverse in 7(c).
0 10 20 30 40 50 60 70 80 90
Freq
uency(%)
Morphology
(a)
0 10 20 30 40 50 60 70 80 90
Freq
uency(%)
Morphology
(b)
0 10 20 30 40 50 60 70 80 90
Freq
uency(%)
Morphology
(c)
Film filter
Metal surface
Figure 7. Frequency of morphologies on film filter and metal surface in (a) OK1, (b) OK2 and (c) OK3.
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These results are very much similar to the previous studies regarding comparison of film filter and metal surface and it is clear that there are some tendencies but it needs more investigation to be better construed.
3.1.2. Size and aspect ratio
It is clearly shown that the frequency of larger size inclusions increases from 8(a) to 8(c), where 8(c) got the highest amount of large size inclusions and the inclusions are also most scattered. This is to be expected since the cooling rate varies from the edge to the center of the ingot. The cooling rate is much faster at the edge of the ingot leading to smaller inclusions whereas the cooling rate decreases towards the center of the ingot, due to sulphur segregation. The solidified metal pushes the sulfur to the center. Moreover, the cooling rate of ingot center is slower [8].
The mangan sulfides precipitates when a critical amount of mangan and sulphur are present [1]. Due to the big amount of sulfur that is collected in the center, a higher frequency of larger inclusions is obtained.
0
5
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20
25
30
1 9 17 25 33 41
Freq
uency(%)
Length(µm)
(a)
0
5
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1 9 17 25 33 41
Freq
uency (%
)
Length(µm)
(b)
0
5
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30
1 9 17 25 33 41
Freq
uency(%)
Length(µm)
(c)
Figure 8. Size distribution of all inclusions alanyzed in (a) OK1, (b) OK2 and (c) OK3.
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The size distribution obtained on film filter and metal surface is compared to see where inclusions are most likely to end up. In 9(a) it is shown that the inclusions on metal surface got a higher frequency of smaller inclusions (1-‐9μm) and that the film filter got a higher frequency of larger inclusions (≥10μm). Size distribution for 9(b) shows that the amount of smaller size inclusions has increased on film filter compared with 9(a). Unfortunately there is no info about the metal surface in OK2. In 9(c) it is shown that the frequency of smaller size inclusions on film filter has increased additionally and the frequency of larger size inclusions has decreased compared with 9(a). Figure 9 also shows that the frequency of larger size inclusions on metal surface has increased. The overall tendency is that the frequency of larger inclusions on metal surface increases from 9(a) to 9(c), while the frequency of larger inclusions on filter decreases. The frequency of smaller inclusions on film filter increases from 9(a) to 9(c).
These results are very interesting but difficult to explain. Perhaps the tendency shown in figure 9 could be more explainable if data from metal surface in OK2 was obtained. To understand this behavior more investigation is needed in this area.
0 5 10 15 20 25 30 35 40
1 9 17 25 33 41
Freq
uency(%)
Length(µm)
(a)
0 5 10 15 20 25 30 35 40
1 9 17 25 33 41
Freq
uency(%)
Length(µm)
(b)
0
5
10
15
20
25
30
35
40
1 9 17 25 33 41
Freq
uency(%)
Length(µm)
(c)
Film filter
Metal surface
Figure 9. Size distribution of inclusions on film filter and metal surface for (a) OK1, (b) OK2 and (c) OK3.
Figure 10. Frequency of larg zise inclusions (>10μm) in the samples.
0
20
40
60
OK1 OK2 OK3
Freq
uency(%)
Samples
Frequency large size inclusions
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Figure 10 shows a clear tendency in frequency of large size inclusions (≥10 μm) between the three samples. The amount of large size inclusions increases from OK1 to OK2 to OK3 and is to be expected from theory of how the cooling rate changes.
The results above show what average size and standard deviation is obtained when analysing film filter and metal surface for different morphologies. Missing data limits the morphologies to three different ones in 11(a) and only two in 11(b) and 11(c).
The average size of irregular shaped inclusions on film filter decreases from 11(a) to 11(c) whereas the regular shaped ones stays almost the same. For metal surface the average size of both regular shaped and irregular shaped inclusions increases from 11(a) to 11(c) along with the standard deviation values. An interesting tendency is that the average size and standard deviation values on film filter and metal surface for regular and irregular shaped inclusions for 11(a) and 11(c) show opposite behaviour. This behaviour could not be explained in this study and needs more time for investigation.
0
4
8
12
16
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24
28
32
36
Length(µm)
Regular Irregular Rod
Morphology
0
4
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36 Length(µm)
Regular Irregular
Morphology
0
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12
16
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32
36
Length(µm)
Reguar Irregular
Morphology
Film filter average Film filter±STD Metal±STD Metal average
(a) (b) (c)
Figure 11. Average size of inclusions in classified morphologies for film filter and metal surface in (a) OK1, (b) OK2 and (c) OK3.
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Aspect ratio shows the ratio between length and width (=L/W) of an inclusion and provides an idea of how compact the inclusions structure is. 12(a) shows a dispersed aspect ratio where about 45% of the inclusions got an aspect ratio value close to 1. The results shown for 12(b) and 12(c) are quite similar and here around 70% of the inclusions got an aspect ratio close to 1. The tendency is that the amount of larger aspect ratio values decreases from 12(a) to 12(c). It has been shown in earlier studies that inclusions tend to get a smaller aspect ratio when heated during a certain amount of time [7]. In this study the inclusions aren’t heated, but the cooling rate is slower in the center of the ingot and therefore similar tendencies are shown. Higher frequency of L/W value closer to 1 is obtained in center of the ingot and the inclusions attend a more regular shape. This happens because of the slower cooling rate in the center of the ingot.
0
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0,25
1,75
3,25
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uency(%)
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(a)
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(b)
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1,75
3,25
4,75
6,25
7,75
9,25
10,75
12,25
Freq
uency(%)
L/W
(c)
Figure 12. Aspect ratio in (a) OK1, (b) OK2 and (c) OK3.
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3.1.3. Number of inclusions per unit volume
It is shown in 13(a) that the distribution of inclusions varies depending on what sample that is observed. The distribution is fairly homogenously in OK1 and OK2 whereas OK3 shows a big tendency of an increased amount of inclusions towards the top part of the film filter. It should be mentioned that the dotted lines represents a continued tendency based on known data and are therefore not necessarily true. Figure 14(b) can be observed to get an overall appreciation of how inclusions distribute. There is a slight increase of number of inclusions at the top location of the film filter. Due to powerful carbide precipitation, a lot of small size inclusions observed on film filter by SEM were covered and because of that to few inclusions were investigated to obtain trustworthy number value results.
(19) (15)
(13) (17)
(4)
(7)
0 500 1000 1500 2000 2500 3000 3500 4000
Booom Middle Top
Nv
Locabon on filmfilter
(a)
OK1
OK2
OK3
0 500
1000 1500 2000 2500 3000 3500 4000
Booom Top
Nv
Locabon on filmfilter
(b)
0
1000
2000
3000
4000
T.Nv
OK1 OK2 OK3 Samples
Total number value
T.Nv Film filter
T.Nv±STD
Figure 13. Number value (Nv) of different locations on film filter in (a) comparison between samples where data labels ansers to the number of inclusions and (b) average number value for the entire film filter studied.
Figure 14. Total number value (T.Nv) for the samples.
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Figure 14 show that OK1 contain the highest frequency of inclusions per volume. In OK2 the number of inclusions has decreased, to follow up with a slight increase again in OK3. The standard deviation for the samples varies showing that number of inclusions are most scattered on film filter in OK1 and least scattered on film filter in OK2. Even though there is an increase of total number value between OK2 and OK3 one can observe behaviour similar to an exponential decreased curve between the three samples.
3.2. Mangan sulfides in rolled steel (samples OK4-‐OK5)
3.2.1. Damaged inclusions Inclusions may have a tendency to be damaged during the manufacturing or lab process by various reasons. The damaged inclusions have a different look compared with the undamaged and this results in that the damaged inclusions real size can’t be observed. Figure 15 and 16 shows two types of different damaged inclusions that were observed in this investigation when observing samples after rolling: Broken-‐Unbroken (BU) and Broken-‐Broken (BB).
Figure 16. Broken-‐Broken(BB) damaged inclusion Figure 15. Broken-‐Unbroken(BU) damaged inclusion.
2%
88%
10%
(a)
BB
UU
BU
10%
51%
39%
(b)
BB
UU
BU
Figure 17. Frequency of deformation in (a) OK4 and (b) OK5.
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In rolled samples, a lot of deformed sulfide inclusions were observed. By observing figure 17 it is clear that more inclusions are BB or BU in 17(b). It is shown that there are a lot more broken inclusions in the rolled sample with a smaller diameter, which have been rolled at higher pressure. This could be explained because of the higher pressure; the pressure is so high that the inclusions tend to break. Another cause could be how the samples have been rolled and in what temperature interval. Maybe the reason why they break is that the sample has been rolled a wrong number of times at a temperature where the inclusions are more brittle. The information about how the samples were rolled and at which temperature couldn’t be obtained from the company and therefore it’s hard to tell if these factors matter or not. If OK4 and OK5 were rolled exactly the same way and under the same circumstances, except for rolling pressure, the reason why there are a big difference of broken inclusions in OK4 and OK5 is most likely because of the higher pressure in OK5. Inclusions may also break during the sample preparation but this could not be the reason why there is such a big difference in the amount of broken inclusions in the two samples since both samples have been prepared similar way.
3.2.2. Morphology of inclusions Classification of morphologies was determined when observing mangan sulfides in SEM. The different morphologies observed from rolled samples can be seen in table 4. OK4 was shown to include 13% thick-‐elongated and 87% elongated shaped inclusions whereas OK5 was shown to include 26% thick-‐elongated and 74% elongated shaped inclusions.
Table 4. Classification of morphologies for OK4 and OK5.
Name Elongated(E) Thick-‐elongated(T-‐E)
Morphology
Size range (μm)
9 – 339 8 -‐ 87
Aspect ratio range (L/W)
5 -‐ 66 3 -‐ 11
Frequency (%)
74% -‐ 87% 13% -‐ 26%
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The tendency from figure 17 repeats itself in figure 18. 18(a) shows a very small amount of deformed inclusions with 0% of broken thick-‐elongated shaped inclusions. In 18(b) the situation has changed with more broken sulfides in both morphologies. This could be explained due to that the thick-‐elongated shaped inclusions consist of a kernel of oxygen surrounded with manganese sulfide. This morphology is therefore more difficult to deform than the elongated ones why no broken inclusion of that type exists in 18(a) but in 18(b) a lot of thick elongated inclusions are broken.
Figure 18. Frequency of classification of deformation for different morphologies in (a) OK4 and (b) OK5.
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Elongated Thick-‐elongated
Freq
uency(%)
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(b)
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UU
BU
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Freq
uency(%)
Morphology
(a)
BB
UU
BU
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20
3.2.3. Size and aspect ratio
The size distribution in 19(a) is somewhat scattered and has some very large size inclusions. In 19(b) the size distribution is much more gathered at a smaller size, this can also be seen in figure 20. OK4 is rolled at a lower pressure and because of this the obvious results would be that 19(b) should have a higher amount of large size inclusions than 19(a), higher pressure will elongate the sulfides more. This is not the case in the results shown in figure 19 and 20. The reason could be that the higher pressure that was used during rolling in 19(b) broke more inclusions than in 19(a). This can be seen more in figure 17 and 18. Since both samples undergone the same procedure during the lab, the main reason for the differences of broken inclusion lays somewhere else. Probably the largest inclusions in 19(b) broke during rolling and therefore the inclusions total size could not be determined. Because of this phenomenon the inclusions are generally smaller in 19(b) than in 19(a).
0
5
10
15
20
25
5 45 85 125 165 205 245 285 325
Freq
uency(%)
Length(μm)
(a)
0
5
10
15
20
25
5 45 85 125 165 205 245 285 325
Freq
uency(%)
Length(µm)
(b)
0
50
100
150
200
Length(µm)
OK4 OK5 Samples
Average size
Average size
Average±STD
Figure 19. Size distribution in (a) OK4 and (b) OK5.
Figure 20. Average size of inclusions
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The aspect ratios in both samples are quite similar. The frequency of the aspect ratio in 21(a) is a bit more scattered than in 21(b). There are also frequencies of higher aspect ratio values in 21(a) than in 21(b). This could be because 21(b) got a larger amount of broken inclusions than 21(a). It should be mentioned that the results in figure 21(a) is more accurate than 21(b) because of the amount of broken inclusions. Since the amount of broken inclusions in OK5 is more than four times higher than OK4 , figure 17, the comparison of inclusions size is somewhat unfair.
0
2
4
6
8
10
12
14
16
18
20
1 7 13 19 25 31 37 43 49 55 61 67
Freq
uency(%)
L/W
(a)
0
2
4
6
8
10
12
14
16
18
20
1 7 13 19 25 31 37 43 49 55 61 67
Freq
uency(%)
L/W
(b)
Figure 21. Aspect ratio in (a) OK4 and (b) OK5.
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4. Conclusions
• Electrolytic extraction can successfully be applied for determination of inclusion characteristics in terms of size, morphology and quantity.
• Distribution of inclusions on film filter is inhomogeneous. It is therefore necessary to analyze all zones on the film filter to obtain trustworthy results about inclusion characteristics.
• Main tendencies in ingot samples:
-‐Inclusion size increases from the edge towards the center. -‐The frequency of large size inclusions increases from 41% to 54% from the edge to the center -‐Inclusions attend a more compact shape towards the center. -‐The frequency of regular shaped inclusions increase from the edge to the center. For large size inclusions the regular shaped morphology increases from 7% to 71%. -‐Highest amount of inclusions were found at the edge and decreased towards the center
• Main tendencies in rolled samples:
-‐Frequency of broken inclusions increases in the sample with highest rolling pressure. -‐Higher pressure is one of the reasons why more inclusions tend to break. Other reasons could be rolling temperature, manufacturing problems during rolling or process execution. -‐These reasons are possible reasons why inclusions break and perhaps with more information about the rolling process one could be surer. -‐Largest inclusions were found in the sample that has been rolled at lowest pressure. Not expected since higher pressure should elongate inclusions more. This is explained by the higher amount of broken inclusions.
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5. Recommendations for future work This study has shown the behavior of mangan sulfide inclusions from samples taken from cast ingot and hot-‐rolled steel. For future work it is necessary to study following phenomena more closely:
To get a clearer tendency regarding the number value of inclusions it is of great importance to analyze a higher amount of inclusions than that was done in this investigation.
Regarding the electrolytic extraction process, some interesting results were obtained when comparing the size distribution on metal surface and film filter. These results were unexplainable in this study and needs further investigation.
Rolling has shown to have a big impact on broken inclusions. More detailed information about the rolling procedure is very important to determine the causes of why the inclusions tend to break. This information was omitted by the industrial company in this investigation, which made analyze of broken inclusions more difficult.
To get a more accurate determination of size distribution in rolled samples, broken and unbroken inclusions should be analyzed separately. This was not the case in this study, which made the comparison between rolled samples somewhat unfair because of the big difference in amount of broken inclusions.
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6. References [1] I. I. Reformatskaya and L. I. Freiman; Precipitation of Sulfide Inclusions in Steel Structure and Their Effect on Local Corrosion Processes. PROTECTION OF METALS, 2001, Vol. 37, No. 5, p. 459-‐464, ISSN 0033-‐1732/01/3705-‐0459$25.00
[2] Lifeng Zhang and Brian G. Thomas; State of the art in the control of inclusions during steel ingot casting. METALLURGICAL AND MATERIALS TRANSACTIONS B, 2006, Vol. 37B, p. 733-‐761
[3] Hiroshi Yaguchi; Effect of MnS inclusions size on machinability of low carbon. leaded resulfurized, free-‐machining steel. J. APPLIED METALWORKING, 1986, Vol. 4, No. 3, p. 214-‐224
[4] P. A. Thornton; The Influence of Nonmetallic Inclusions on the Mechanical Properties of Steel: A Review, JOURNAL OF MATERIALS SCIENCE, 1971, No. 6, p. 347-‐356
[5] S. B. Hosseini, C. Temmel, B. Karlsson and N.-‐G. Ingesten; An IN-‐Situ Scanning Electron Microscopy Study of the Bonding between MnS inclusions and the Matrix during Tensile Deformation of Hot-‐rolled Steels. METALLURGICAL AND MATERIAL TRANSACTION A, 2007, Vol. 38A, p. 982-‐989, DOI: 10.1007/s11661-‐007-‐9122-‐9
[6] Yuichi Kanbe, Andrey Karasev, Hidekazu Todoroki and Pär G. Jönsson: Analysis of Largest Sulfide Inclusions in Low Carbon Steel by Using Statistics of Extreme Values. Steel research int. 82, 2011, No. 4, p. 313-‐322, DOI: 10.1002/srin.201000141
[7] W. H. McFarland and J. T. Cronn; Spheroidization of Type II Manganese Sulfides by Heat Treatment. METALLURGICAL TRANSACTIONS , 1981, Vol. 12A, p. 915-‐917, ISSN 0360-‐2133/81/0511-‐0915$00.75/0
[8] Discussion with Andrey Karasev
Figures
[1] Powerpoint presentation by Andrey Karasev
[2] Hamid Doostmohammadi: A Study of Slag/Metal Equilibrium and Inclusion Characteristics during Ladle Treatment and after Ingot Casting, 2009, ISBN 978-‐91-‐7415-‐520-‐4, p. 16.
Tables
[1]http://www.steel-‐grades.com/Steel-‐grades/Carbon-‐steel/42crmo4.html
2013-‐05-‐06
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Appendix
Equations
!! =!∗!!"#$%&!!"#
ƍ!"##$!"
(1)
!!"# = ! ∗!!"#$%&'!!"#$%
∗ ! ∗ !!"#$%&'!!"#$%
∗ !
!.!! =!! + !! + !!
!!"#(!)!!"#(!)!!"#(!)∗ !!"#$%& ∗
ƍ!"##$!"
(2)
NV: Number of inclusions (amount/mm3). n: Number of inclusions for pictures observed in different areas. Afilter: Area of filmfilter (mm2). AObs: Total observed area (mm2). ƍsteel: The density of steel (g/mm3). ΔW: Dissolved weight (g). a: number of screens were inclusions were found in SEM. Lpicture: Measured length of SEM-‐picture (mm). Lscale: Measured length of scale in SEM-‐picture (mm). Wpicture: Measured width of SEM-‐picture (mm). S: Length of scale in SEM-‐picture (mm).