How to measure wear resistance of sintered carbides for cutting tools

[Viktor]

I have a problem of selection the best carbide for a particular application. I would like to measure its wear resistance in order to select a particular grade (chemical composition, grain size, etc) before I make any tooling for producing the cutting inserts of specific shape. Although I could probably carry out a simple cutting test, what is the regime for this test I should select. As I understand, the cutting regime should be different for different grades so this type of test is not really of any help. I am wondering if someone knows a simple way to solve this problem?
Thank you.
Viktor

http://viktorastakhov.tripod.com

 

[tomwalz]

Sorry, the only simple way is to try it and see if you like the results. Carbide wears, or the cobalt matrix leaches out or it is attacked by acid or it is pulled apart by an affinity for the material it is cutting. At high temperatures with friction you can also get tribolitic effects adding to the situation. There is also the trade off between wear and impact resistance. Carbide can just shatter, can macro-crack and come apart or can micro-crack and come apart. Crack propagation properties are also important.

The best bet is to find a good rep that has worked with the material you want to cut and already has a solution.

tom

 

[scottmaha]

i really do not understand why you have to make something so simple so difficult.

with todays carbide and coating technology, selecting the right insert for the job has become easy. we machine several different types of material (inconel 625, 4140, 4340, hasteloy-x, 6061 aluminum, 1045, etc..)and i have always found it simple to select an insert to effectively machine these materials. i choose to use kennametal, but the other big manufacturers should suffice also.

for example viktor. if you need to turn inconel, you should select an insert who's carbide possess a high transverse rupture strength, ie. the carbide is tough. to help deal with the heat and shear forces that can be encountered when machining inconel, you should select an insert that has a TiALN, PVD coating. the edges of the insert should be sharp and the chip breaker should be positive.

see viktor. simple.

 

[Viktor]

Dear Scottmaha
Thank you for your reply and your opinion.

I don’t think it as simple as you think. Leading carbide manufactures have a number of different carbide grade which differ in their chemical composition, grain size, coating, shape and many other “external” parameters. If you compare the information they give you : TRS, compressive strength and (if you ask insistently) fracture toughness, you will see great differences. Moreover, if you try to ask them about correlations of these parameters with cutting parameters, you will have their answer in the format you give me: more TRS, higher hardness, etc. Try to ask them about wear resistance of their tool materials and at best you will be provided with meaningless data they collected from the so-called Filex machine test which has nothing to do with cutting, not even close.

Now, you can ask me a logical question: What you try to prove? Or: Why do you need overcomplicate the issue? I can provide you with a detailed and well-supported answer. However, at this point, I just provide you with the following info:

A recent CIRP working paper states, "A recent survey by a leading tool manufacturer indicates that in the USA the correct cutting tool is selected less than 50% of the time, the tool is used at the rated cutting speed only 58% of the time, and only 38% of the tools are used up to their full tool-life capability. One of the reasons for this poor performance is the lack of the predictive models for machining." The same was found in an earlier survey of cutting regime selection on CNC machine tools in the American aircraft industry. If we recall that the United States now spends more than $125 billion annually to perform its metal removal tasks using conventional machining technology, the price of the lack of knowledge on machining and carbide selection is self-evident.
Viktor

http://viktorastakhov.tripod.com

 

[tomwalz]

We use the figures of 5,000 grades from 1,500 suppliers. In truth we think those figures are only very roughly accurate. Many distributors will buy carbide and market it under their own grade names.

There is just not a lot of good machining data available free.

We have been brazing cermets and ceramics to replace carbide on saws and other applications for several years. We have been looking for the kind of data you want and especially the reason behind it so we can translate metal cutting data into wood and plastic.

There is a perceived advantage in the industry to keeping performance data secret. It is sort of a “Prisoner’s Dilemma” Game theory problem. I have more on this at our web site:

http://www.carbideprocessors.com/Brazing/book/05.htm

The following is an excellent source also
World Directory and Handbook of Hardmetals and Hard Materials - sixth Edition
Kenneth J A Brookes
Published by:
International Carbide Data
33 Oakhurst Avenue
East Barnet Hertfordshire
EN4 8DN United Kingdom
Telephone & Fax international (+44) 181-368-4997

 

[Viktor]

Dear tomwalz

Thank you for you answer. Your website is really interesting so I should spend some time to appreciate its content.

I would like to ask you a question:

DO YOU REALLY BELIVE THAT CARBIDE PRODUCERS HAVE RELEVANT CUTTING DATA?

My experience of working with many carbide producers in USA and some from Europe does not provide me with the positive answer to this question. Normally, carbide producers share with you various metallurgical data including SEM images of their carbides. These images look very cool. However, I try to explain that these look for me as the other side of the Moon. Moreover, these images have not much stronger correlation with the cutting performance that the mentioned celestial body. If you annoy them well enough, the may show you some cutting data (do not mix with the data shown by trade reps – a few questions are enough to nock them out so a white towel flies right away – done without even biting an ear), you find that they do not have proper test methodology and equipment. Their explanation to tool wear is rather strange. They use the friction coefficient in cutting data. When you explain then that the shear and normal stress change along the tool chip interface in very different manner (so thus the friction coefficient), they invent the average friction coefficient that has the meaning of average body temperature of patients in the Detroit Central Hospital….They could not make distinction between abrasion, adhesion, diffusion, chemical (oxidation), etc types of tool wear. When you try to explain them about plastic lowering of the cutting edge due to its creep, the communication breaks as a telephone line when using a calling card. End so on...
Viktor

http://viktorastakhov.tripod.com

 

[tomwalz]

I am very much afraid I must agree with you.

Yesterday I tried to buy some Alumina parts for saw tips and the salesman told that me that he didn’t know how much performance data he was allowed to give me since much of the information is proprietary.

I think the Scottmaha approach is the best one if you can find a knowledgeable salesperson and you are doing something common. In all fairness some, such as NTK, do have much better literature than others.

Following is a list of different values I have found for different materials. No one gives more than few values. There is a bit of redundancy because I am often talking to non-technical and I don’t really care how they give me a value as long as they do.

Breaking strength
Compressive strength
Density
Electrical Resistivity
Fracture toughness
Grain size
Hardness
Lattice Parameters
Magnetic Susceptibility
Mean thermal expansion coefficient
Melting Point
Poisson's ratio
Rigidity
Shear Modulus
Specific Heat
Structural Properties
Thermal Conductivity
Thermal Expansion
Thermal shock resistance
Tool Life above 780 m / min.
Tool Life below 780 m / min.
Transverse Rupture Strength
Volume elasticity
Wear Resistance
Young's modulus



Have a good weekend.
Tom

 

[tomwalz]

P.S.

We do a lot of recommendation on materials for wear uses, cutting, etc. When we are asked about impact resistance, bond strength, or anything similar my answer is that the materials work on saw blades in steel cutting and saw mills so it should work in their application. This is not the answer I would like to give but it is the best information I have. The other side of this is that when I explain that transverse rupture strength can fall off as the tool heats no one seems to understand that cermets and ceramics can truly be tougher than tungsten carbide.

Still have a nice weekend.
Tom

 

[Viktor]

Dear Tom

Thank you for your detailed answer. I do appreciate the list of properties of carbides you provided. Thank you also for the interesting discussion and sharing you knowledge.

However, “Huston, we have a problem!”

The problem is that among multiple useful properties you listed, only three have something to do with the cutting process. Lets consider them first:
1, 2 Tool life above and below 780 m/min. I would like to remind you that tool life besides the cutting speed, depends on many parameters of the cutting process including tool geometry (at least 5 major parameters), cutting feed, depth of cut, the work material metallurgical state (for a given work material we can have at lest 3-5 entirely different situations), stiffness and other conditions of the machine, coolant (type, delivery, temperature, etc). Moreover, tool life is the worst criterion and, in my opinion (sorry) I should not be listed in your list because it not objective criterion. Even tool wear is not objective criterion either. For example, for different flank angles, the same material may have very different flank wear – this difference however, has nothing to do with its cutting properties. Because carbide companies do not provide the test conditions, their data on tool life is almost useless for end users and can only be used for advertisement, not for technical merit. Moreover, the absence of the proper tool life test methodology (besides childish one defined by the American National Standard ANSI B46.1-1978 – it was out of date 50 years ago as based on the Taylor equation) just adds uncertainness.

3. Wear resistance. The standard wear resistance test has nothing to do with cutting so the data gained from this test can hardly be applied in cutting.

Now, I would really appreciate if you can tell me how to correlate say Magnetic Susceptibility and other physical and mechanical data on your list with cutting properties of carbide. OK, let me even simplify you assignment. Fracture toughness – what it has to do with cutting (accounting for the standard ASTM method of its determination) cutting properties of a carbide? Everybody knows that it is better (or, at least, it seems to be better) to have higher fracture toughness. Right? What is exact number you are looking for a particular cutting application and tool design? Provide quantitative explanation or correlation, please.

I can go over every and each item on your list showing that there is no direct correlation between a particular item and the cutting process. If you want, I could show you a number of NUMERICAL AND PRACTICAL examples to support my statements. Besides, I could provide a number of references (available in public domain) to support my point. I would be very thankful if you can do the same.
I really count on the continuation of our discussion.

With the best regards
Viktor

http://viktorastakhov.tripod.com

 

[tomwalz]

You are absolutely correct, of course. I tend to use industrial language and an industrial approach when talking about this. This is often inaccurate e.g. “silver solder” is actually a braze alloy”.

Some of those values are there because different manufacturers and others use them for whatever reason. We did compile the list but did not add anything of our own. Also they may give insight into other properties of the materials.

We use an electrochemical process to clean and surface treat carbides and ceramics before we braze them, which explains some of the properties. Magnetic susceptibility lets us know if we can handle the parts with a magnet in production.

Transverse rupture strength is there because it is commonly used. We do think the data on transverse rupture strength and elevated temperatures has some value explaining how the three react in cutting.

You are correct about tool life. It is all too often subjective. However we are seeing some good results using ammeters to measure required cutting force as the tool dulls.

It appears that fracturing and micro-fracturing can contribute greatly to tool wear defined as how long and well a tool will cut.

In a brazed application we treat the tip, braze alloy and holder as a composite rather than three separate materials. This approach has allowed us to greatly improve both bond strength and impact resistance.

With brazed tools magnetic susceptibility can be important for induction heating and thermal conductivity can be important for torch heating.

Some of the values may be there to explain to an end user when to use which grade.

The tools get heated three times; when brazed, when ground and in use. In addition you have to allow for the differences in coefficients of expansion between the carbide and the holder when brazing. The diamond wheel that grinds the tips puts considerable side pressure on them. Some of the values appear that they might be helpful in determining how hard the part will be to grind and which diamond, bond, etc. to use.

A highly regarded person for wear analysis is:
Margaret Ziomek-Moroz
Albany Research Center
U.S. Department of Energy
1450 Queen Ave. SW
Albany, OR 97321-2198
USA

PH: 541-967-5892
FAX: 541-967-5936
Direct 541 967-5943
mailto:moroz@alrc.doe.gov

Again, I agree with you fully and wish I had your knowledge and understanding. Our approach is definitely from the sawmill, cabinet shop side. We do spend a fair amount of time trying to figure out how various published data applies in the real world.

At the risk of being presumptuous, we do use consultants occasionally if you would be interested. Currently we are trying to find someone who can tell us how transverse rupture strength relates to a susceptibility to edge chipping in a carbide part. WE are also looking for help in bringing our ceramic brazing technology to market. We can prepare certain cermets and ceramics so they can be brazed and used instead of tungsten carbide. We are looking for specific industrial applications.

Finally, I would very much appreciate any information you cared to provide.

Tom

 

[gunsmith]

I am not permanently involved in manufacturing processes but the thumb rule is soft carbide for hard material and hard carbide for soft materials. The coatings like TiAlN will cause longer lifetimes of the cutting tool but the tools should be deburred perfectly before coating. If not the coating won´t increase the tool´s lifetime but it´s price. The burr brakes of at the first cutting operation and the cutting edge is unprotected.

Most tool manufacturers do not control the cutting edges for remaning burrs of the sharpening process.

Victor, if You want to have a concrete message for the advantage o coating, You should order the uncoated tool and do inspection of the cutting edges by yourself and deburring and coating by a reliable supplier.

Andreas

 

[Viktor]

Dear Tom

I do appreciate the information you shared with me and with other specialists in this forum. Believe me, I met a lot of people from industry but no one sounded so convincing as you do. It gives me a hope that there are some knowledgeable people around so what I am doing every and single day is not just waste of my time – it will be implemented by specialists like yourself.

As for me, I am working on the theory of metal cutting that works, that could be implemented in the shop flour floor to make fast and intelligent selections of proper tool materials, tool design, cutting regime accruing great economy of cutting operations. I know that any theory works if and only if it can be implemented by any operator in the shop so I try do develop very simple and practical matrixes and indexes for the cutting process. A milestone in this development is resource of the cutting tool. It seems that I found a relatively simple and reliable way to calculate such a resource and thus to compare different tool materials for a given application. This is a part of my book in preparation “Practical Physics of Metal Cutting”.

As for correlation between TRS and chipping. What could I say – everything correlatives in this world. However, in my opinion, you have to determine the reason for chipping by observing the fractography of chipped edges. Often, chipping occur due to insufficient fracture toughness of the tool material (unfortunately, the method used today to measure fracture toughness of carbides is not adequate) rather than due to insufficient TRS. High local contact temperatures lead to the so-called plastic lowering of the cutting edge due to its creep. It changes the geometry of the cutting edge increasing contact stress on the rake face in the region adjacent to the cutting edge. Often this is the cause for chipping.

Specially for Andreas

I did what you suggested many-many times with different cutting inserts produces by different manufactures starting with Sandvik, Kennametal, ISKAR down to Valenite, Carboloy, etc. I know what is the difference in substrate preparations is. Besides, my prime attention is always paid to the radius of the cutting edge. For roughing, when the uncut chip thickness is great, there is not problem. For finishing, however, we have a huge problem when the uncut chip thickness becomes less than 8 times radius of the cutting edge (which varies from one producer to another, from coated to uncoated inserts). Residual stresses due to machining go up so fact and tool life goes down without any apparent reason. It gets worse for finishing tool like broaching tool, shave cutters, etc. – unfortunately not many people pay attention to the edge preparation even for these tools particularly on re-sharpening of these tools.

Coating are generally good. However, it still a matter of luck to select proper coating for a given application. Nowadays, we have thousands of them but no clear guidance to steer us in the ocean of available coatings. Unfortunately, there is no such thing as “universal” coating so a once found good coating for a certain steel may not be good at all if the work material differs for say 1% of Si or any other component (carbon, for example). Moreover, a particular application of a coating is not only a function of the work material but also depends on tool geometry and design, machining regime, coolant, etc. As you see, we have toooooooo many variables – how to solve this puzzle? I know a common answer – try, try, try…..
Viktor

http://viktorastakhov.tripod.com

 

[tomwalz]

Your research is incredibly important. Would you be interested in submitting a paper for a conference?

There is a symposium planned as a tribute to Dr. Erwin Rudy to be part of the ASM conference in Pittsburgh in 2003.

Subject: Cemented Carbides And Related Hard Materials, Past, Present And Future

The ASM and this symposium are really looking for input from all sectors. I, personally, am particularly interested in what is being done on plant floors. This symposium is not about the mechanisms of how hardmetals work as much as it is about what is going on now and what is coming or needs to come. As an example of the range it will also include talking about tool holding materials such as the advantages of stainless steel saw bodies. This connection is not immediately obvious unless you know how imporastn the tool holder is to the life and performance of the hardmetal tip.

The ASM has approved the symposium and we are calling for papers. I am the representative on the committee from industry. (Something like that.) You may contact me or Margaret Ziomek-Moroz.

Thanks,
Tom


Margaret Ziomek-Moroz
Albany Research Center
U.S. Department of Energy
1450 Queen Ave. SW
Albany, OR 97321-2198
USA

PH: 541-967-5892
FAX: 541-967-5936
Direct 541 967-5943
mailto:moroz@alrc.doe.gov

Tom Walz
Northwest Research Institute, Inc.
Carbide Processors, Inc.
3847 S. Union Ave.
Tacoma, WA. USA 98409
800 346-8274
tomwalz@email.msn.com

http://www.carbideprocessors.com