Inoculated high-speed steel
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Transkript: Englisch(automatisch erzeugt)
00:00
I think we're going to make a slightly early start.
00:33
Let me introduce the first speaker of the afternoon, second session. Professor Alexander Krauss from Slovak University of Technology.
00:42
And the topic is, you know, cumulated high speed steel. Thank you very much. Ladies and gentlemen, dear colleagues, first of all, I would like to express my gratitude to all the organizers of this exciting conference and particularly
01:02
to Henry Badishya for giving me such wonderful possibility to introduce to you my adventures in the field of high speed steels. High speed steels are iron based alloys with high content of both carbide formers and carbon and with a long history.
01:23
Despite the strong competition with cemented carbides, cutting ceramics and the super hard tool materials, high speed steels have persisted to this day as a key tool material in the manufacture of certain types of cutting tools.
01:46
This can be primarily attributed to the highest values of toughness and strength compared to other tool materials. With respect to high speed steels, cutting tools are commonly manufactured
02:01
of root material and are shaped to the required geometries by its machining. The metallurgical process with respect to the road high speed steel involves high reduction rate for plastic deformation of cast ingots
02:24
into round rods that results in high production cost of root materials. In contrast, precision casting, allowing near net shape workpieces can reduce the volume of the roughing operation and the large depths of cut.
02:48
And as a result, the material consumption in the manufacture of cutting tools. As for service properties, cast high speed steels due to the presence
03:03
of the eutectic carbide networks at the primary matrix grind boundaries have considerably higher wear resistance but lower toughness compared to commercial road high speed steels of a similar chemical composition.
03:24
Therefore, in order to provide good all-round performance, the toughness of cast high speed steels needs to be required. In the case of cast high speed steels, toughness improvement can be achieved
03:46
using a small addition of different elements to the melt, which can act there as inoculants, thus resulting in microstructure refinement.
04:00
With this in mind, the main purpose of the present work was to investigate the inoculation effect of powder additions of tungsten and tungsten carbide in cast steel of AISIM2 grade
04:23
Before casting, the inoculating treatment of the parent steel was carried out using powder additions which were added to the melt in the ranges as shown in this slide.
04:43
The molten steel was poured into ceramic molds and cast ingots were 1.2 kg in mass. Heat treatment of the steel involved annealing, austenitizing, quenching and tempering.
05:02
The conditions for certain heat treatment operations are also shown in this slide with respect to annealing, austenitizing, quenching and triple tempering which completed the heat treatment of the studied steels.
05:22
In this figure, micrographs for the parent and the inoculated steel show that inoculating additions favor the refinement of the primary grains of the matrix and simultaneously a transition from the extremely slender columnar dendrites
05:47
in the parent steel to the equiaxed morphology in the inoculated steels. This figure shows that strong refinement effect in the matrix
06:00
has been achieved in all the inoculated steels. The above mentioned structural changes with respect to the matrix can be referred first of all to an increase in the number of nuclei in the melt
06:21
under the effect of inoculating additions. In particular, EDS analysis indicated the presence of the tungsten-rich carbide phase in the bulk of the primary matrix grains.
06:42
As for carbides, according to x-ray diffraction analysis, three carbide types were detected in the s-caste structure of the studied steels. As shown in this figure, for example, for the parent steel, it had M6C, M2C and MC.
07:06
The typical morphology of the M6C eutectic carbide is fish bone, which is known to be unaffected by chemical composition or cooling rate.
07:24
Morphology of M2C eutectic carbide in M2 high-speed steels can vary widely depending on minor additions of inoculants. As a reflection of these, two types of morphology for M2C carbide
07:44
have been revealed in the s-caste microstructure of the studied steels, namely rod-like and lamellar. Pay attention, please, that vanadium and tungsten,
08:01
like other carbide formers in this type of steel, are uniformly distributed in all the above-mentioned types of eutectic carbides. You can see here, and you can see here also.
08:25
The typical chemical composition of eutectic carbides are shown in this table, which also illustrates the main differences in the contents of the principal carbide formers with respect to the certain type of eutectic carbide.
08:43
You can see, for example, that M6C carbide has the lowest content of both vanadium and chromium, and on the other hand, the largest content of iron. M3 eutectic carbide is featured by the highest content of vanadium,
09:07
and M2C carbide compared to M6C carbide has evidently higher content of both vanadium and chromium,
09:21
and lower content of iron. It's very important because the origin of eutectic carbides and specific chemical composition of these eutectic carbides control behavior of this carbide during heat treatment.
09:42
This figure shows how the inoculating additions have affected the formation of different types of eutectic at solidification, as well as their volume fractions in the S-cast microstructure of the studded steels.
10:02
It is seen from this figure that compared to the parent steel, both inoculating additives have completely suppressed the formation of M3 eutectic and at the same time
10:21
favored the formation of the M6C eutectic at the expense of the M2C eutectic, primarily with rod-like morphology. This tendency is more evident in the case of pure tungsten because the dissolution of tungsten carbide
10:44
simultaneously leads to enrichment of the melt in carbon, which in contrast to tungsten is known to favor the formation of M2C eutectic.
11:01
In the microstructure of the steel, inoculated with 0.6% tungsten, the individual particles of the primary M26 carbide have been also revealed in the S-cast microstructure of this steel.
11:27
With respect to thermal stability, M6C and MC carbides are considered as stable phases, whereas M2C carbide is a metastable one.
11:43
As can be seen from this figure, the typical fish-borne morphology of this carbide and uniform distribution of vanadium in it have not changed after austenitizing. That is not the case for the M2C carbide.
12:05
During austenitizing, vanadium diffuses out of the M2C carbide and interacting with surrounding austenitic matrix forms vanadium-rich carbide MC.
12:23
You can see simultaneously the precipitation of MC carbide causes transition of the initial M2C carbide into M6C carbide, and both result in the formation of such mixture of M6C and MC carbides.
12:46
You can see that in this mixture, the distribution of vanadium is extremely, extremely heterogeneous. That reflects its key role in the decomposition process.
13:03
After M2C carbide decomposition, two types of eutectic carbides can be observed in the microstructure of the studied steels after heat treatment, as shown in this figure.
13:22
Besides eutectic carbides, secondary carbides are also present in the microstructure of high-speed steel after heat treatment. For example, TEM investigations indicated the presence of two types of the secondary carbides
13:43
in the microstructure of the steel, inoculated with 0.6% tungsten, large one, and small one. The large carbides are observed as a role
14:00
in the bulk of the metric grains, while small carbides preferentially precipitate as the prior austenite grain boundaries. Both carbides were identified as M6C phase.
14:24
In general, the cutting ability of high-speed steels depends on a combination of properties, among which the four most important ones are hardness, red hardness, toughness, and wear resistance.
14:43
The typical values for the hardness and for red hardness are shown in these figures. And from these figures, it is seen that inoculation didn't affect significantly either the hardness and the red hardness.
15:06
In contrast, this figure shows that all the inoculated steels have considerably greater toughness compared to the current steel. That can be referred to as the following
15:22
changes in their microstructure. First of all, the decrease in matrix grain size, then transitions from calamnidine rights to equiaxed grains,
15:41
refinement of the eutectic colonies and carbides, and finally the formation of more interrupted and thin eutectic carbide network. On the other hand, the formation of more
16:03
continuous and close eutectic carbide leads to the deterioration in toughness as shown for example for the steels inoculated with the largest amount of both additives.
16:23
This confirms the key role of eutectic carbide in cast high speed fracture in the heat treated condition. Eutectic carbide act as initiation sites
16:43
for cracks which are formed as a rule at the carbide matrix interface by its decohesion. Later, these micro cracks initiate
17:01
fracture that preferentially occurs by adiabonding mechanism as a carbide matrix interface or by cracking of eutectic carbides.
17:23
The general refinement of the microstructure in all the inoculated steels benefits enhanced wear resistance that in turn reduces the wear intensity as shown in this figure.
17:45
For example, in this figure it is seen that one surface of aware tracks for the parent steel evidently differs from that for the inoculated steel
18:02
still inoculated with 0.6% tungsten carbide by stronger surface grooving and severe and deeper adhesive failure of the oxide scales.
18:26
The ability of eutectic carbide network to resist wear can be probably associated not only with its continuity and the thickness
18:41
but also with morphology of the eutectic carbides which form this network. Conclusions are as the following. Thank you very much.
19:01
Thank you very much. Thank you very much, Professor Karls. And then we are open for questions.
19:21
Can I speak? Thank you for your interesting talk and for going into direction of solving a very, very big problem in the field of any kind of a high-speed steel. I wonder whether you tested several powder grain sizes
19:42
you know that as for tungsten or tungsten carbide you can have powders, commercial powders as fine as 0.2 micrometers which is 200 nanometers actually for instance I think Hermann Stark or the like or coarser powders.
20:02
I believe but I have not any element that there should be an influence also of the particle size you are addicting. Yes of course it's a very important thing because the size of carbides even in a scast state or for example in root material after deformation
20:23
hot plastic deformation are very important. Is very important size. And for example it's possible to improve for example carbide heterogeneity by using powder metallurgy. But in this case the price
20:43
is very, very high and for example compared to the common root high-speed steels the application of powder metallurgy high-speed steel is very, very low. As first of all as for cutting tools
21:02
maybe in another fields the application of powder metallurgy steels are high is high but as for cutting tools no. Thank you. Eric.
21:20
Hi. I don't know if I understood well you put WC powders and two questions one is do they dissolve completely when you put them in? Do they? One small please. Does WC dissolve in the molten metal? Yes. It's also in some cases a problem.
21:45
It's very important to introduce these particles in the melt in proper way. I think that Professor Badesha knows it because he introduced for example ceramic particles and it's the main problem.
22:00
And we use special technology for introducing these powder particles. They were preheated in vacuum furnace and then were covered with thin film of pure iron and then introduced in the melt.
22:23
And you were able to avoid the dissolution of the tungsten carbide? Yes. Yes. Because you see if we try to dissolve for example only pure carbides for example WC carbide it's very difficult because
22:43
the temperature is very high. But if you immerse them into the melt the conditions for the solutions are quite enough.
23:02
Very interesting. You made a comparison. You have very good results. You showed all the inoculated variants were really a better improvement compared to the parent material. Have you also made comparisons to material produced by sort of traditional metallurgy? Not as cost structures but
23:22
what would the comparison be then? We compared for example cutting properties of our cast tools with the tool made of commercial route high speed stills. And the results were very good.
23:41
But it's necessary to stress that it's impossible to use cast metal as universal metal. It's necessary to search special conditions for this tool. For example, geometry of the tool is very, very important.
24:01
And cutting conditions are very, very important. It's necessary to use this cast metal only in the conditions where for example strong abrasion where it's dominated
24:21
and for example where the level of dynamic loss is very, very low. And then such tool is very, very good for in order to improve in general durability in this case.
24:47
My question is regarding the shape of what you were casting because if you don't dissolve the carbides the effect of the carbide will depend on the distribution. The distribution will depend on the flow conditions and the degree of refinement also.
25:03
So these results should be linked with the conditions of processing. I understand. You see, we use these additions in order to get special specimens
25:20
of different shapes. But in general, when we cast the cast tool it's possible to get for example the shape practically which is ready for using. It's necessary only to grind for example
25:42
because really it's near net shape. Cutters for example, different. In the first, I will show you maybe you didn't see it here.
26:04
It's cast tool for example. Yes, I understand. My question is the distribution of the particles inside that piece will be different than the distribution of the particles in a rod. Yes, yes. It depends for example on the weight
26:21
of these cutters for example. Yes, of course. Because the geometrical parameters of the cast tool is very, very important also. Thank you. The lady at the middle. Could you pass?
26:42
For words about structure after casting and after tempering. Yes. Because you're speaking only about carbides but it seems to me that martensite and radiodostenoid Thank you very much. It's very valuable. No, thank you very much.
27:01
So not only carbides change their Yes, yes. Well, it's very important to have extra one question. Yes. When you're speaking about the grain size, what do you mix? Okay. In our case that was grains, so-called primary grains.
27:22
Primary grains. Because you see for example there are grains of prior primary grains. Yes, austenite and so on. As for structure, when we cast into ceramic molds, the structure after casting is
27:41
bainite and retained austenite. If to chill mold, certainly martensite plus retained austenite. And after annealing,
28:00
certainly a mixture of special carbides and ferrite. And after quenching martensite and plus for example it's possible from 10 to 30% of retained austenite.
28:22
And for this reason, high speed steels are temperate using multi tempering cycle. In order to effectively reduce the volume of retained austenite.
28:43
Okay. There's a question online. Yes. A question from AK Steel Research. Ferrite grain inoculate or carbide inoculate? That's the question. Yes, of course. First of all, for inoculating ferrite, because I told it in my presentation
29:02
that it's very important to have this nucleation sites for ferrite. Because ferrite, in the case of high speed steels, is the primary solid solution which precipitate from the melt.
29:20
Thank you. Do we have any other questions? Okay, if not, let's thank again the speaker. Thank you.