6. Tool Tipping Materials

Considerations in material selection

1.  The big question:  How much material can you cut for how little cost?

2.  The longer you run between tool service the better.

3.  Go for as much wear as you can get.

4.  Settle for as much toughness as you absolutely have to have. 

5.  Welded materials are cheaper to install with expensive, automatic machines. 

6.  Harder materials run longer and have to be brazed. 

7.  Harder tungsten carbides and cermets cannot be brazed successfully without special processes.

8.  The kind of material is maybe one -third of the success

9.  The people and the equipment are two-thirds of the success

10.  The right braze alloy allows you to move up to a higher grade without breakage. 

11.  No two tungsten carbide grades, hardfacing alloys or anything else are exactly alike.

12.  The salesman is not really an expert in your operation.  That is your job.  

13.  Test until you find what works for you then keep testing for what works better. 

Historical progression - Historically the progression went somewhat as follows

·         Steel

·         Tool Steel (High speed steel)

·         Cutting alloys - Co-Cr-W-Fe-Si-C (Haynes alloys, Stellite®, Talonite®)

·         Tungsten carbides (mostly tungsten carbides)

·         Cermets & Ceramics

·         Cubic Boron Nitride

·         Diamond

The major focus has always been machining steel.   The desire has been to do as much work as fast as possible.  As you work faster in machining you generate more heat.  I attended a lecture where the following values were given.  The point was to show how the development of newer materials effected machining operations.   The example is based on a certain amount of work taking 100 minute using tool steel.  The same amount of work can be done more rapidly using other materials.  This example is certainly interesting but is very narrow and overlooks a huge range of variables.   A big part of the difference was feed and speeds.  A bigger part must have been changing the tools as they wore out from heat, wear, corrosion, etc.     

Comparative times to cut steel including tool changes and tool servicing

·         Tool Steel - 100 minutes

·         Cutting alloys - 50 minutes

·         Tungsten carbides  15 minutes

·         Cermets & Ceramics - 5 minutes

·         Diamond - 1 minute

Run times - in typical wood sawing applications

Steel                                         2 - 4 hours

Talonite® (Stellite®)                  4 - 12 hours

Tungsten carbide                       8 - 40 hours

Cermets                                    8 - 120 hours

Knoop Hardness Ratings

Diamond                                               6,000 - 6,500

Silicon tungsten carbide (solid)                2,130 - 2,140

Aluminum oxide (corundum)                  1,635 - 1,680

Tungsten carbide (Co binder)                 1,000 - 1,500

Hardened steel                                          400 -   800

Steel

Steel is Iron with a very little bit of carbon in it. (Iron with .1 - .3% carbon with a maximum of 2.5%).  Part of the difference between iron and steel is the iron tungsten carbides in steel.

Tool Steel

There are basically two kinds depending on how it is made.  These are ingot cast and powder metal.  Tool steels can be hardened to at least Rockwell C63 and will retain Rockwell C52 at 1,000 F.  T-15 is generally considered to be best in the widest number of applications

Seven major kinds of tool steel

High speed

Hot work

Cold work

Shock resisting

Mold steels

Special purpose

Water hardening

Cutting Alloys (also hardfacing alloys)

Co-Cr-W-Fe-Si-C (Haynes alloys, Stellite®, Talonite®)

These alloys are Cobalt, Chromium, Tungsten, Iron, Silicon and Carbon alloys.   A Rockwell of C68, tensile above 100,000 lb/sq.in.  Extremely acid resistant.  They were widely used for cutting and machining tools but have been replaced by balanced high-speed steels and cermet type cutting tools.  They are currently used in hard-facing and high heat corrosion applications.  They have excellent high heat, wear and corrosion resistance.  They are more impact resistant than many grades of tungsten carbide but not all. 

Typical hardfacing alloy chemical composition

Co        balance

Ni         3 max

Si         2 max

Fe        3 max

Mn       2 max

Cr        28 - 32

Mo       1.50 max

W         3.5 - 5.5

C          .9 - 1.4

They are popular in automatic tool tipping applications.  Generally the performance is not as good as the correct grade of tungsten carbide but they can be welded on and ground automatically more readily than shaped tungsten carbide.  The labor savings are considered to offset the additional expense of the material and the reduced wear.

Hardfacing alloys Such as Talonite or Stellite® form tungsten carbides which give them a lot of their strength and wear resistance.  These are Cobalt -Chromium alloys.  When it is welded on the Chromium and Molybdenum combine chemically with the carbon to form Chromium tungsten carbide and Molybdenum tungsten carbide.  This gives Talonite superior wear resistance.  The cobalt forms a soft and strong matrix that holds the tungsten carbide grains in place.

Tungsten carbide (Mostly tungsten carbides - also titanium, chromium, tantalum)

These materials were developed in Germany and popularized during World War II because tungsten was scarce.  You could machine more metal if you made tungsten carbide than if you used it for High speed steel.  You can typically cut three to 10 times faster with tungsten carbide than you can with high-speed steel.

Tungsten carbide is actually grains of tungsten carbide in a matrix.  Commonly this matrix is cobalt.  This is pretty handy because you can mix carbon, tungsten and cobalt together and sinter them.  The tungsten and the carbon form tungsten carbides and the cobalt does not.  You get very hard grains for wear resistance and the cobalt stays relatively soft for impact resistance.  These are sometimes called cemented materials and cemented tungsten carbide because the tungsten carbide grains are cemented together with cobalt or other materials such as nickel and nickel-chrome alloys.

Tungsten carbide is fairly yielding compared to the ceramics.  You can take tungsten carbide, heat it and bend it into spirals and curves for cutters, which you cannot do with ceramics.   

Cermets & Ceramics 

These are solid materials.  Instead of individual grains they are solid pieces of something.    Cermet technically means a metal-based ceramic.  Now it most commonly means Titanium Carbonitride. 

Ceramics

This usually includes cermets, which are metallic based ceramics.  Cermets can be Aluminum Oxide, Silicon Nitride, Tungsten carbide and Titanium Carbonitride.   Usually cermets mean Tungsten carbide and Titanium Carbonitride.  If cermet is used alone it most likely (but not certainly) refers to titanium Carbonitride.  The story is given that this is because of a problem with translation from English to Japanese.  

Ceramics as a class have low tensile strength and are relatively brittle.  They are extremely strong under compression.  Ceramics are extremely hard, very wear resistant, and typically have melting points well above the highest common metals.  In addition they have excellent resistance to chemical corrosion.  Organic solvents do not affect them. 

Cermets- Titanium based cermets have high rigidity, compressive strength, hardness and abrasion resistance.  They also have high strength at elevated temperatures and excellent resistance to chemical attack.

Cubic Boron Nitride

CBN can come close to equaling diamond in hardness with a rating of 5,000 kg./mm² vs. diamond at 8,000 kg./mm².  It has an advantage over diamond in that it is more heat resistant.

Diamond

This is still the hardest substance known.  It is available as PCD (polycrystalline diamond) which is lots of little diamonds in a matrix.  This is a very good cutting tool tip material except that it is very heat sensitive.  It is hard to braze because the common tool brazing alloys have a range of 1200 - 1350 F, which is the range at which the matrix breaks down.  Diamond tipped tools are very expensive.  They are generally regarded as being worth the additional expense if they do not break.  They are very fragile compared to other tipping materials.  Even though they may make sense economically the high initial investment required severely limits their use.

Saw tipped with Talonite ® Alloy	Saw tipped with tungsten carbide

List of materials

Steel

Tool Steel (High-speed steel)

Cutting / hardfacing alloys with WC (Talonite +WC)

Tungsten carbides – standard & premium WC grades

Cermets

Ceramics

Cubic Boron Nitride

Diamond

Best materials

Wear               Toughness      Corrosion        Cutting speeds sfm

Diamond           Steel                 Ceramics          Diamond           3,000 - 4,000

PCD                 Tool Steel         Cermets            CBN                3,000 - 4,000    

CBN                Hardfacing        Hardfacing        Ceramics          3,000 - 4,000

Ceramics          Talonite            Special WC       Cermets            1,100

Cermets            Talonite +        WC                   Coated WC           800

Tough WC        Wear WC         Steel                 WC                      500

WC    C-1         WC   C-14                                Steel                     150 - 175

WC    C-14       WC   C-1

Wear WC         Tough WC

Talonite +         Cermets

Talonite            Ceramics

Hardfacing        CBN

Tool Steel         PCD

Steel                 Diamond

Worst Materials

Most expensive

Costs   to buy                          to install & grind                                to run

Diamond                                   3.  Diamond - brazed                              Steel                            

PCD                                         3.  PCD - brazed                                   Hardfacing                               

CBN                                        3.  CBN - brazed                                   Tungsten carbides

Ceramics                                  3.  Ceramics - brazed                            Special tungsten carbides

Cermets                                    2.  Cermets - brazed                              Cermets

Special tungsten carbides           2.  Special tungsten carbides                  Ceramics         

Tungsten carbides                     2.  Tungsten carbides                             CBN

Hardfacing                                1.  Hardfacing - welded                         PCD

Steel                                         1.  Steel - welded                                  Diamond

Least expensive

Tungsten carbide        Typical values

                        C-1                   C-4      

Toughness        290,000            230,000  (Transverse Rupture Strength)

Wear                91.3                  92.8                  Rockwell

Warning: This is a complex issue.  1.  Successful cutting in terms of wear and breakage is perhaps one-third material used, one-third operator skill and one third equipment condition.  2.  There are over 5,000 grades of tungsten carbide so this is over simplified.  3.  Tungsten carbide can overlap hardfacing alloys in values.  Tungsten carbides and cermets can overlap.  There are many ways to make and use diamonds.  Anyone disagreeing with this chard might indeed be right.  Each application is unique.  You need to experiment with different grades and different materials from different sources until you find the best for you.

Gross Physical Breakage 

Steel                                         Hardest to break                                                          

Talonite (Stellites®)                                                                 

Tungsten carbide                                              

Cermets                                   

Ceramics

Diamond                                   Easiest to break

Note:  there can be considerable overlap here.   Cermets are generally easier to break than tungsten carbide but some cermets are much tougher than some tungsten carbides.  Some tungsten carbides also outperform Talonite depending on the respective materials and the testing.  

Wear 

Hardness - Rockwell C    Wear factor

Steel                                         42 -44                           1                      Worst for wear

Talonite (Stellites®)                   48-55                            6 - 8                            

Tungsten carbide                       66-80                            10 - 25

Cermets                                    92                                 20 - 50

Ceramics

Diamond                                                                                               Best for wear

As rule of thumb, hard materials wear better but break easier.    Tungsten carbide wears better than Talonite and cermets wear better than tungsten carbide.

Chemical wear

Solid diamond (perfect diamond coatings)            Most chemically resistant - Best for wear

Ceramics

Cermets

Talonite (Stellites®)      

Tungsten carbide

Diamond (polycrystalline diamond in a matrix - PCD)

Special steels

Steel     (ordinary grades)                                                                                  

Note:  The situation is not nearly this clear.  To a great extent it depends on what steel, tungsten carbide, etc and what chemical in what conditions.  

Microfracturing 

Solid diamond (perfect diamond coatings)                                    Most likely to fracture

Ceramics

Cermets

Tungsten carbide - Just the exposed WC grains and not the whole part

Diamond (polycrystalline diamond in a matrix - PCD) - just the exposed grains

(Probably no microfracturing)    

Talonite (Stellites®) -

Special steels

Steel     (ordinary grades)

Note:  It is possible but pretty unlikely to have Microfracturing in the last three.    These materials all take a very sharp edge and that edge is susceptible to nicking but that is a slightly different condition although it has similar effects.

Talonite is harder to break than tungsten carbide.   Tungsten carbide wears better than Talonite.  If you are cutting high acid materials such as green cedar then the tungsten carbide grains still wear better than Talonite but the cedar acids dissolve the tungsten carbide binder so the tungsten carbide grains fall out and the tip gets dull.   You can use a cermet tip, which is more acid resistant than Talonite or standard tungsten carbide.  This is great on relatively clear green lumber for example.  If you get some very knotty boards or start mixing dry lumber with the green then the constant change in impacts can cause micro-fractures to form in the edges of the cermets and they will get dull faster than tungsten carbide.

Using the materials

Talonite and the similar Stellite® alloys have advantages over tungsten carbide in high acid applications where the problem is not wear but is actually chemical erosion.  Talonite can be welded on with automatic machines, which can be a significant labor saver.  It is also much easier to run an automatic tipper than to braze tungsten carbide with a torch.    Some people just never quite catch onto consistent torch brazing although most people pick it up readily.   

Talonite has the advantages over tungsten carbide of being harder to break, possibly having less drag (lower coefficient of friction) and Talonite can be ground with less expensive wheels.  Tungsten carbide requires diamond wheels and Talonite can be ground with CBN (cubic boron nitride) wheels.

Cutting speeds

High speed steel                           150 - 175 surface feet per minute

Tungsten carbide                          500 sfm

Coated tungsten carbide                800 sfm

Cermets                                    1,100 sfm

Ceramics, CBN & diamond       3,000 - 4,000 sfm

Which Material Takes and Keeps the Sharpest Edge?

Sharpness is critical and obvious in terms of the quality of the finished product.  It is less obvious but still important other ways because sharper tips use up to 20% less energy and will successfully handle higher feeds and speeds up to as much as 30 to 35%. 

Cermets will take and hold a sharper edge than tungsten carbides and they will keep this edge if used in proper applications such as clear cedar, paper covered materials and other consistent applications.  If they are used in rougher sawing applications they will lose their edge due to Microfracturing. 

Quite often all the materials are sharpened to the same configurations.  This is a mistake. 

Talonite, steel and cermets can be run with steeper angles but they are generally run the same way tungsten carbides are run.