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Considerations In Tungsten Carbide Selection

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Things to Consider:

• Hardness

• Toughness - whole body breakage

• Toughness - fracture initiation

• Toughness - fracture propagation

• Toughness - edge fracturing

• Wear resistance

• Corrosion resistance

• Temperature resistance

• Sharpness

• Edge retention

Factors effecting performance

• Kind of binder

• Amount of cobalt 

• Size of grains

• Mixture of grain sizes

• Mixture of material

• Manufacturing techniques

Amount of Cobalt

We are talking about cemented tungsten carbide.  This is tungsten carbide grains cemented together by cobalt.  Cobalt is the binder.  As you get more cobalt you get a softer grade that is more impact resistant.  If you have less cobalt you get better wear but the part will break more easily when hit.  Generally as you go from a minimum of 2% cobalt to a maximum of 20% cobalt you get a part that is harder to break but also a part that will wear out faster.

 Hardness vs. Wear Resistance

Rule of thumb:  More cobalt means it will be harder to break but it will also wear out faster.

 Grain size: Smaller grains give better wear and larger grains give better impact resistance.   Very fine grain tungsten carbides give very high hardness while extra coarse grains are best in extremely severe wear and impact applications such as rock drilling and mining applications. 

More cobalt generally makes a tungsten carbide harder to break

Tungsten carbide as used here means WC grains in a cobalt binder.  Cobalt is softer than the tungsten carbide grains so the more cobalt you have the softer the overall materials will be.  This may or may not relate to how hard the individual grains are. 

Grain size and Cobalt in combination

A good tungsten carbide manufacturer can change the characteristics of their tungsten carbide in a great number of ways.  

This is an example of good information from a tungsten carbide manufacturer

                                                            Rockwell          Density Transverse Rupture

Grade          Cobalt %   Grain Size         C          A         gms /cc Strength

OM3                4.5       Fine                  80.5     92.2      15.05                270,000             

OM2                6          Fine                  79.5     91.7      14.95                300,000

1M2                 6          Medium           78         91.0      14.95                320,000

2M2                 6          Coarse            76         90         14.95                320,000

3M2                6.5       Extra Coarse  73.5      88.8      14.90                290,000

OM1                9          Medium            76         90         14.65                360,000

1M12             10.5      Medium            75         89.5      14.50                400,000

2M12             10.5      Coarse             73         88.5      14.50                400,000

3M12             10.5      Extra Coarse   72         88         14.45                380,000

1M13             12         Medium            73         88.5      14.35                400,000
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2M13             12         Coarse             72.5      87.7      14.35                400,000

1M14             13         Medium            72         88         14.25                400,000

2M15             14         Coarse             71.3      87.3      14.15                400,000

1M20             20         Medium            66         84.5      13.55                380,000

Grain size alone does not determine strength

                                                Transverse Rupture

Grade               Grain Size         Strength

OM3               Fine                  270,000

OM2               Fine                  300,000

1M2                Medium            320,000

OM1               Medium            360,000

1M20              Medium            380,000

1M12              Medium            400,000

1M13              Medium            400,000

1M14              Medium            400,000

2M2                Coarse              320,000

2M12              Coarse              400,000

2M13              Coarse              400,000

2M15              Coarse              400,000 

3M2                Extra Coarse     290,000

3M12              Extra Coarse     380,000
 

Grain Size & Cobalt % Compared to Hardness & Toughness 

In the very early days of carbide you made carbide tougher or harder by changing the amount of Cobalt in the binder.  Cobalt is metal and softer than carbide grains so more cobalt made it tougher and less made it harder. Then people learned how to change the grain size. 

Bigger grains made carbide tougher and smaller grains made it harder. By varying grain size and cobalt % you can make carbide a lot tougher or a lot harder.

If you add more Cobalt to large grains then you get even more toughness.  However there is a limit to how tough you can make carbide or want to make carbide.  If you get it too “tough” then it is too soft.  Remember we are using the term ‘tough’ here as the opposite of hard.

If the grains are too large and there is too much Cobalt then the carbide will move and deform under pressure.  One of the major strengths of carbide is its ability to handle pressure or compressive force.   If it is too soft it loses that ability.

Toughness

Hardness

Co% bottom line

Grain size bottom line

Grain size & Cobalt %

What you can do is mix Cobalt % with grain sizes and get carbide that is both tough and hard so you get long wear without breakage.

This is a graph of 23 different grades of modern carbide. In the left graph you can see by the Co% line on the bottom that as co% goes up hardness drops and toughness stays sort of the same.  This is because grain size differs. In the middle graph we increase grain size and hardness drops while toughness sort of drops.   

Neither Cobalt percentage or grain size alone determines how a grade will perform. 

Electrochemical Effects

Electrical Conductivity - Tungsten carbide is in the same range as tool steel and carbon steel while Cermet II grades conduct more like glass. 

By the addition of titanium carbide and tantalum carbide, the high temperature wear resistance, the hot hardness and the oxidation stability of hardmetals have been considerably improved, and the WC-TiC-(Ta,Nb)C-Co hardmetals are excellent cutting tools for the machining of steel. Compared to high speed steel, the cutting speed increased from 25 to 50 m/min to 250 m/min for turning and milling of steel, which revolutionized productivity in many industries.

Specifying a large WC particle size and a high percentage of Cobalt will yield a highly shock resistant (and high impact strength) part.

The finer the WC grain size (and therefore the more WC surface area that has to be coated with Cobalt) and the less Cobalt used, the harder and more wear-resistant the resulting part will become. To get the best performance from carbide as a blade material, it is important to avoid premature edge failures caused by chipping or breakage, while simultaneously assuring optimum wear resistance.

 As a practical matter, the production of extremely sharp, acutely angled cutting edges dictates that a fine grained carbide be used in blade applications (in order to prevent large nicks and rough edges). Given the use of carbide which has an average grain size of 1 micron or less, carbide blade performance therefore becomes largely influenced by the % of Cobalt and the edge geometry specified. Cutting applications that involve moderate to high shock loads are best dealt with by specifying 12-15 percent Cobalt and edge geometry having an included edge angle of about 40º. Applications that involve lighter loads and place a premium on long blade life are good candidates for carbide that contains 6-9 percent cobalt and has an included edge angle in the range of 30-35º. 

Additives to WC

Grades C-5 to C-8 commonly have added tungsten carbides such as Tantalum tungsten carbide and Titanium tungsten carbide.  This is partly because of the problems cutting iron based materials such as steel and may not have any advantages in cutting other materials.  Adding titanium tungsten carbide gives better hardness at high temperature as well as greater wear resistance and resistance to cratering.  Adding 'tantalum tungsten carbide increases hardness while it lowers strength and wear resistance. 

Micrograins

Micrograin and nanograin tungsten carbides are becoming popular and rightly so.  They do work well.  The tighter grains can mean better wear and a tougher tungsten carbide.  Typically they wear longer, retain a better edge longer and polish better. 

HIP

HIP is hot isostatic pressure.  Ordinarily tungsten carbide is rammed in a mold with the pressure all coming from the direction of the ram.  HIP is a means of applying pressure evenly from all sides of the tungsten carbide. 

I have a mid size lathe (17") with enough speed (3000 RPM) and power (10 HP) to use carbide with real effciency but most of my work is done with HSS.

I would not encourage a noob or those with underpowered equipment to get into carbide tooling except in rare cases and certainly not for form tools like thread cutting, radii, or parting tools. When you consider one stick of HSS costs about the same as one carbide insert and with HSS you have a hundred or more sharpenings, you can grind and re-shape it to any configuration you want. Therefore, carbide would be the last thing I'd buy as a budget limited home shop machinist.

I strongly submit that anyone considering carbide as a means of avoiding sharpening tools or to emulate what the big dogs are doing are followng the wrong path. HSS and carbide have their place. In the home shop carbide can be effciently applied (machining hard materials for example were tool wear is a factor) but most of the time I would highly reccommend HSS.

Here's my prescription: Learn to free hand grind HSS tools on a bench grinder. Also learn how to touch up bench-ground tools with a slip stone. . There's only about a dozen basic configurations and the relief angles and rake angles once learned are almost automatic. Buy a box or two of good quality HSS tool bits that suit your applications and a parting tool or two that fit your parting set-up.

I see good HSS blanks in box quantities in eBay several times a month and it sells for about half the MSC price. I also see carbide inserts for budget prices but you have to bird-dog opportunities and, naturally, be careful you don't purchase the wrong or inferior stuff. HSS is one of the few things I almost insist be of US brand name origin. My personal preferences are Cleveland MoMax, Rex XXX, and similar from Latrobe and Butterfield.

You are gong to make mistakes. So what? Every mistake is a learning opportunity. However, it makes sense to make mistakes on practice pieces and not the irreplacable casting or the workpiece you already have 40 hours in.

I recall a fellow who had never cut a dovetail until he made a jewel box of cocobolo - a hardwood sold by the pound stead of the board foot. I suggested he make the box first of pine and use it for tools after it was done. That way he could practice all the joints and techniques on low cost material before he commtted to the high priced (whew!!) stuff. But no, he made the box of cocobolo and screwed up the dovetails. Only then did he fnd someone to show him how to make them. So. Like I said. Practice on the cheap stuff.

Getting back to carbide Vs HS there is no Vs. Commercial shops use caride because there is an economic incentive when 20 HP 5000 RPM spindles, rigid tooling, and rigid machine tools are the rule.

In the home shop with less than 1 HP and small tooling budgets, HSS makes a LOT of sense.

As for C2 vs C5. It really doesn't matter much in alumnum or copper alloys what grade of carbide you use for small volume work. It does on steel particularly alloy steels at max feeds and speeds. There you will want C5 for roughing and maybe C6 for finishing. So if you had to have a one size fits all carbide inert I would recommend C5.

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