Considerations In Tungsten Carbide Selection

 

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

    

Cobalt %

4.5

6

10.5

14

20

Wear Longest

Wear out soonest

Diamond

CBN

ceramics

cermets

C-3

C-9

C-11

C-13

C-14

Co-CR alloys

Steel

Easiest to break

Hardest to break

 

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

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.

 

Here you can see 23 grades.  I graphed it so that the Cobalt slowly increases.  You can see where hardness seems to relate to grain size more than Co% especially in a couple places.   You can also see a lot of places where hardness and toughness don’t seem to relate to grain size and Co % much at all. 

 

   

                                                        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.  

History

 

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.