HOW TO USE/MAINTAIN/STORE TUNGSTEN CARBIDE TOOLS

HOW TO USE/MAINTAIN/STORE TUNGSTEN CARBIDE TOOLS

Tungsten carbide, often called simply “carbide,” is a familiar material around the shop. This compound of tungsten and carbon has revolutionized the metal-cutting world over the decades, enabling increased speeds and feeds and providing longer tool life. 

Tungsten carbide (chemical formula: WC) is a chemical compound (specifically, a carbide) containing equal parts of tungsten and carbon atoms.

To fully understand the applications and importance of tungsten carbide, it is important to look at its properties. In its most basic form, tungsten carbide is a fine grey powder. However, it can be pressed, molded and formed into various shapes for use in cutting tools, industrial machinery, abrasives and jewelry.

The main application for carbide tungsten is machine tools. Carbide cutting edges are frequently used for machining strong and tough stainless steel or carbon steel. They are also used in instances where other tool types would wear away, for example, on top quality high production lines.

The machining applications and tools in which tungsten carbide tools are used are: turning, drilling, milling, facing, planning, threading, parting off, grooving and deep hole boring. These tools are used for cutting steel, nonferrous materials and cast iron in a wide range of industries including automobile, aerospace, earth moving, oil equipment and heavy engineering.

Blending: 

The first operation after reduction of the tungsten metal powder is the milling of tungsten and carbon prior to the carburising operation. Here, 94 parts by weight of tungsten and six parts by weight of carbon - usually added in the form of lampblack - are blended together in a rotating mixer or ball mill. This operation must be performed under carefully controlled conditions in order to insure optimum dispersion of the carbon in the tungsten.

Compacting:

 The most common compacting method for grade powders involves the use of a die, made to the shape of the eventual product desired. The size of the die must be greater than the final product size to allow for dimensional shrinkage that takes place in the final sintering operation. These dies are expensive, and usually made with tungsten carbide liners. Therefore, sufficient number of the final product (compacts) are required, to justify the expense involved in manufacturing a specific die. 

Sintering: 

A cobalt compact is heated in a hydrogen atmosphere or vacuum furnace in temperatures ranging from 2,500 to 2,900°F, depending on the composition. Both time and temperature are carefully adjusted in combination, to effect optimum control over properties and geometry. The compact will shrink approximately 16% on linear dimensions, or 40% in volume. The exact amount of shrinkage depends on several factors, including particle size of the powders, and the composition of the grade. Control of the size and shape is most important and is least predictable during the cooling cycle. This is particularly true with those grades of cemented carbides with higher cobalt contents.

Classification of Carbide Tools

Cemented carbide products are classified into three major categories:

• Wear Grades — used primarily in dies, machine and tool guides, and in everyday items such as line guides on fishing rods and reels. Used anywhere good wear resistance is required.

• Impact Grades — also used for dies, particularly for stamping and forming, and in tools such as mining drill heads.

• Cutting Tool Grades — the cutting tool grades of cemented carbides are divided into two groups, depending on their primary application. If the carbide is intended for use on cast iron that is a nonductile material, it is graded as a cast iron carbide. If it is to be used to cut steel, a ductile material, it is graded as a steel grade carbide.

Saw tips get dull for several reasons. Abrasion, adhesion, diffusion and fatigue are the 4 reasons for dullness.

1.   Abrasion

Abrasion or straight wear is countered by smaller, more consistent grain size.  What is called abrasion is often thought of a straight wear. However, a big part of it is actually pulling carbide grains out of the metal matrix.  Smaller grains have less surface area for wear and less surface area exposed so are also less likely to be pulled out. Grains can also be more tightly packed.  Finally, you can add elements to chemically lock the carbide tighter.  Instead of ordinary concrete this is much more like concrete reinforced with rebar. 

2. Adhesion

The materials used in tungsten carbide have an affinity to the materials being cut.  This functions two ways.  One way is adhesion where the material being cut actually sticks to the tungsten carbide in a sort of welding process.

3. Diffusion

The second way is where the material being cut dissolves one or more of the materials in the tungsten carbide.  Usually it is the cobalt binder, in the tungsten carbide.  This is very readily seem cutting high acid woods.  It is also important cutting metals.  The solid solubility of nickel in aluminum does not exceed 0.04% while cobalt can have a factor several times that.  In addition, nickel / chromium binder chemically locks the nickel to the chrome which makes it much less reactive than elemental cobalt. Right is representation of the electron configuration of cobalt.  ((Cobalt 27 (2:8:15:2)). With only two electrons in the outer shell it is highly reactive.   

4.  Fatigue

This is standard metal fatigue. On a large scale you see it by bending piece of metal repeatedly until it snaps or tears.   The binder in tungsten carbide work hardens and fails much like any other metal.  Its susceptibility to metal fatigue can be changed by minor changes in chemistry and processing.  

Additional wear factors:

1. Wear – the grains and the binder just plain wear down

2. Macrofracture – big chunks break off or the whole part breaks

3. Microfracture – edge chipping

4. Crack Initiation – How hard it is to start a crack

5. Crack propagation - how fast and how far the crack runs once started

6. Individual grains breaking

7. Individual grains pulling out

8. Chemical leaching that will dissolve the binder and let the grains fall out

9. Rubbing can also generate an electrical potential that will accelerate grain loss 10. Part deformation - If there is too much binder the part can deform

11. Friction Welding between the carbide and the material being cut

12. Physical Adhesion – the grains get physically pulled out. Think of sharp edges of the grains getting pulled by wood fibers.

13. Chemical adhesion – think of the grains as getting glued to the material being cut such as MDF, fibreboard, etc

14. Metal fatigue – The metal binder gets bent and fatigues like bending a piece of steel or other metal

15. Heat – adds to the whole thing especially as a saw goes in and out of a cut. The outside gets

The main advantage of using carbide tools is that it produces a better finish on the work piece being worked on. Faster machining can be achieved while using carbide tungsten tools. Ability to withstand high temperatures, in comparison to other cutting tools, is another important advantage of using carbide tungsten tools.

Tungsten Carbide is about three times stiffer than steel and it is denser than titanium and steel. In terms of hardness, tungsten carbide is equivalent to sapphire or corundum. Carbide has a high melting point and is very hard. In addition, carbide tools have high precision cutting capabilities.

  

How to make Tungsten Carbide Tool

How to make Tungsten Carbide Tools

Tungsten carbide, often simply “carbide,” is a familiar material around the industry. This compound of tungsten and carbon has revolutionized the metal-cutting world over the decades, enabling increased speeds and feeds and providing longer tool life.

It was first theorized as a tool for cutting material in 1925, by Dr. Samuel Hoyt, a scientist of the Research Laboratory of the General Electric Company at Schenectady. He also developed Carboloy, an alloy of tungsten, carbide, and cobalt which later GEC opened as the Carboloy division to produce tungsten carbide cutting tools. In the late 1930s, Philip M. McKenna, founder of Kennametal, found out that by adding a titanium compound to the mix, it made the tools work better at higher speeds. This began the forward march toward today’s lightning-fast cutting speeds.

Tungsten carbide (WC) is an inorganic non-natural compound composed of equal parts of tungsten (W) and carbon (C) atoms. Tungsten Carbide exhibits a hexagonal structure lattice made of a grid of those tungsten and carbon. It’s most striking properties, that make it useful for making tools, are a high density, a very high melting point of 2600 °C. It is also a very hard compound and also has the metal-like, high values of electric and thermal conductivity. Tungsten carbide is approximately twice as strong as steel.  

Prior to 1945, tungsten carbide was prepared by melting tungsten, carbon black and metal oxides at a temperature of 2000 °C but the tools made from this metal were found to be far too brittle for industrial use.

Making of Tungsten Carbide Tools

Mining & Processing

There are several tungsten ores that can be mined and refined into tungsten or made into tungsten carbide. Wolframite is the best known. Scheelite and/or wolframite are commonly located in narrow veins which are very slightly inclined and often widen with the depth.

The ore is then crushed, heated and treated with chemicals. The result: tungsten oxide. Then, the fine particles of tungsten oxide are carburized, turning them into tungsten carbide. In one method, the tungsten oxide is mixed with graphite (carbon). This mixture is then processed continuously and heated to over 1200˚ C (2200˚ F) which causes a chemical reaction that removes the oxygen from the oxide and combines the carbon with the tungsten to yield tungsten carbide.

 Grain Size and Grading

Once processing is done, the carbide grain sizes will determine the mechanical properties of the final product. This is key in its processing. The size of the tungsten oxide particles, and how long and at the temperature that the oxide/carbon mixture may have been processed decided or as such creates the size of the carbide grain.

So, as consequences go, if the size of the grain is too large and the percentage of binder used is much higher in comparison, the carbide deforms under this pressure. Now, one of the major advantages of carbide to be alloyed with tungsten is its proficiency to cope under pressure or compressive force. If the alloy now is too soft it loses that ability.

The tungsten carbide particles processed are received at a fraction of the size of a grain of sand. A series of sieves are used to sort out the different grain sizes: less than one micron, one and one half microns, and so forth.

Now, the tungsten carbide grains, after sieving, are ready for blending into “grade powder.” In the tungsten carbide industry, one speaks of grades rather than alloys, but they mean the same thing.

The story of Tungsten Carbide (WC) and its powder metallurgy, and particularly that of the hard metal industry, is depicted by a progressively widening range of available grain sizes; whilst simultaneously narrowing the grain size distribution for each grade of WC powder.

The tungsten carbide grains go into a mixing vessel with the other components of the grade. Powdered cobalt metal is made to act as the “glue” to hold all the components. The other materials, such as titanium carbide, tantalum carbide and niobium carbide are added to improve the properties of the material when cutting.

Pressing

Once the mixing process is completed, the liquid must now be removed. This generally happens in a spray dryer, that looks similar to a stainless steel silo. An inert, drying gas, one of nitrogen or argon, is blown into the spray dryer from the bottom up. Once all the liquid is removed, the sand like remaining dry material is the “grade powder”

The grade powder then goes into insert shaped molds that are specially designed and made to allow for the shrinkage that will happen later on in the process to create the cutter inserts. Just as pharmaceutical tablets are formed, in a process similar to that, the powder is compressed into the molds.

Heating

After pressing is completed, it is fairly delicate and the form looks like oversized inserts. These inserts are then removed from the like-shaped molds and are placed on graphite or molybdenum trays, and go into a sintering furnace where they are heated in a low-pressure hydrogen atmosphere to 1100-1300˚ C (about 2000- 2400˚ F). Any cobalt present melts. The insert in the furnace consolidates into a solid, smaller size.

The inserts are removed from the furnace and are now cooled. On cooling, they become dense and hard. A quality control check is performed. The inserts, after the check, are usually ground or honed to achieve the correct dimensions and cutting edge. Honing, done to a radius of 0.001″ is typical, though some of these parts receive a cutting-edge radius of half a thousandth or as large as 0.002″, and some of these are left extremely sharpened as sintered in the furnace. 

The next step is grinding and other such operations. Even still, some of the types and designs of inserts retrieved from the sintering furnace are in their final shape and have the correct edge as well. Hence, they do not need further grinding.

Introducing Tungsten

Tungsten occurs naturally in nature as Wolframite, FeMnW04, and Scheelite, Ca W04.

The ore is digested in hydrochloric acid to remove the calcium and to precipitate tungstic acid, H₂W0₄, which is then dissolved in ammonia and crystallized as ammonium tungstate, (NH₄)₂W0₄. Ammonium tungstate is evaporated by boiling to give ammonium paratungstate, or APT, 5(NH₄)₂O.12W0₃.5H₂0. At temperatures below 50⁰C, fine APT is obtained. By calcining APT or tungstic acid, tungsten oxide, W0₃, is obtained. The lower the temperature, finer the W0₃ produced. When W0₃ is reduced in hydrogen, tungsten, W, is produced: because oxygen forms a more stable.

Coating

Cutting conditions may not be optimal and hence the tools’ lives need to be prolonged so as to face these conditions. Many types and combinations of coatings have been developed. They can be applied in two ways: by chemical vapor deposition (CVD) or physical vapor deposition (PVD). Both types are applied in furnaces.

Chemical vapor deposition

For CVD, the coating is usually 5- 20 microns thick. Milling and drilling inserts usually receive 5–8  icrons, as these operations require better surface finish, and they encounter more impact, so they require greater edge toughness. For turning applications, the coatings tend to be in the range of 8–20 microns. In turning, heat and abrasion tend to be more of a concern.

In its simplest explanation, CVD involves flowing a precursor gas or gases into a chamber containing one or more heated objects to be coated. Chemical reactions occur on and near the hot surfaces, resulting in the deposition of a thin film on the surface.

Physical vapor deposition

PVD coatings are typically about 2-4 microns thick. Different manufacturers use different numbers of layers. These PVD coatings are well-suited to applications cutting high temperature, nickel-based, cobalt-based or titanium-based materials, and sometimes steel and stainless steel.

Tool manufacturers are meeting the pressures for ever increasing feeds and speeds, and the need for longer tool life and lower costs, by continually improving the designs of tungsten carbide cutting tools and developing better and better coating technologies.

Classification & Types of Cutting Tools

Classification & Types of Cutting Tools

What are Cutting Tools?

A Cutting Tool or Cutter is a tool used to create inserts or remove any remainder material of a workpiece by clearing out the excessive deformity. Cutting Tools are pointed and are mounted on various machines tools to be used in the process of cutting.

Cutting Tools are always harder than the material they are used to cutting. The Cutting Tools used for work should also withstand the heat released in the process.


Classification of Cutting Tools is done on the basis of various factors. The basic classification can be done measuring the contact distance between a tool and the metal.
Cutting Tools are of classified according to two types-

Single Point Tools

The tools which make use of a single sharp cutting edge to remove deformities are known as Single Point Tools. The act of turning, the exact opposite of boring, makes use of Single Point Tools as it works on the exterior diameter of a cast hole. The sharp tip of a Single Point Tool is usually a round, forming a nose radius.

Multiple Cutting Tools

Multiple Cutting Tools have more than one cutting edge to them and are used for various different purposes. A Multiple Cutting Tool is mounted on a machine and used by utilising the tool in a rotation motion. The activities of drilling and mining make use of Multiple Cutting Tools.

Cutting tools can be classified in the basis of following properties:

Wear Resistance-

Wear Resistance is the ability of a tool to resist wearing with continuous use. Wearing of a tool happens as friction is created when a cutting tool mounted on a machine gets in contact with the piece of metal it is being worked on. If a tool does not have the ability to resist this wear, then it is bound to break or not be usable because of the tool shape changing due to it.

To reduce the damage caused by friction, Cobalt is added in the tool composition. This increases the longevity of the tool 

Strength & Toughness-

It is the ability of a tool to withstand immense cutting pressure with working on complicated pieces or types of cuts required. A tool should withstand the vibrations created while cutting.

To improve a tool's toughness and strength, some manufactures add Nickel and/or Molybdenum to the composition. When a tool is tough, it won’t chip away or break down with prolonged use.

Hot Hardness-

Hot Hardness is the ability of a tool to withstand the heat and temperatures produced while trying to cut a piece of metal. The act of cutting produces significant heat on the tool. This heat can melt the cutting tool and break it off.

To increase a cutting tools hot hardness, metals like- Tungsten, Aluminium, Vanadium and Molybdenum are added. These metals are responsible for increasing the heating capacity of the tool. Generally, the tool’s hot hardness needs to be more than the material it is going to interact with.

( TO take reference of the table from http://www.e-codestar.com/rshuster/@http@zdaps@addr@rickspage/lib/mach/cuttoolclass.html for the above three properties)

Young’s Modulus-

It is the measure of how stiff a solid is. Stiffness isn’t to be confused with hardness, toughness, or even strength. Young's modulus E, can be calculated by dividing the tensile stress,  by the engineering extensional strain,  in the elastic (initial, linear) portion of the physical stress–strain curve:



where,

E is the Young's modulus (modulus of elasticity);
F is the force exerted on an object under tension;
A is the actual cross-sectional area, which equals the area of the cross-section perpendicular to the applied force;
ΔL is the amount by which the length of the object changes (ΔL is positive if the material is stretched, and negative when the material is compressed);
L0 is the original length of the object.

Tool Life-

The life of a tool is the time a tool will take before it wears out. The equation to calculate this is called the Taylor Tool Life equation. The Taylor tool life equation can be written as:

v(T)n = C

Where,

v is the cutting speed, m/min;
T is the tool life, in minutes;
C is the cutting speed for a tool life of 1 minute;
n is the Taylor exponent (Do not confuse this use of n with the cold working index n).

Cutting tools can be classified on the basis of their material composition:

Carbon Steel

A Carbon Steel is made up of plain carbon steel with additional composition mentioned below:

Silicon – 0.1 to 0.4 %
Carbon –   0.8 to 1.3 %
Manganese  – 0.1 to 0.4 %

High Speed Steel

These tools are made up of steel that is meant for speed cutting. There are different types of High Speed Steel to make cutting tools. The popular ones are:

Cobalt High Speed Steel

This type of High Speed Steel is composed of:

Cobalt= 12%
Tungsten= 20%
Chromium= 4%
Vanadium= 2 %

18 – 4 – 1 High Speed Steel

18 - 4 - 1 High Speed Steel is composed of:

Tungsten = 18%
Chromium= 4%
Vanadium= 1%
Carbon= 0.75%

Molybdenum High Speed Steel

This type of High Speed Steel is composed of:

Molybdenum = 6%
Tungsten= 5%
Chromium= 4%
Vanadium= 2%

Cast Alloys

These type of tools are now limited in use. The composition of Cast Alloys is:

Cobalt = 40% - 55%
Chromium = 30%
Tungsten = 10% - 20%

Carbides

These are made by mixing tungsten and carbon at a high temperature of 1500 Centigrade. Then Cobalt is added to this mixture and heated at a temperature of 1400 Centigrade. The composition of Carbides is:

Tungsten= 82%
Titanium carbide= 10%
Cobalt= 8%

Ceramics

The material composition of a ceramics tool is:

Aluminium oxide=  90%
Chromium oxide= 10%

Types of Cutting Tools

What is a Boring Tool?

Boring is the act of enlarging an already casted hole by a drill. A boring tool is a Multi Point Tool which works on the internal diameters of a cast hole. With the help of a boring tool, the diameter of the hole can be increased. A boring tool also gives the hole a clean and a smooth finish, getting the piece ready for the next step.

What is an Indexing Tool?

An Indexing tool is a specialised tool used on a workpiece to make it circularly indexed. Indexing is the process in which a particular piece moves or is being moved from one location to the other quickly and precisely. The piece of work is held in between an index tool in the same way as in a metalworking lathe. Indexing head tools are usually used on milling machines and many other drill tools

What is a Brazed Tip Tool?

Brazing is the process of joining two dissimilar materials with a third material. Brazing and Brazing Alloying are close concepts except Brazing is done on a higher temperature than Brazing Alloying.  Brazed tip tools are often a cheaper alternative available than other specialised tools tips. These tools are also tougher when it comes to resistance and wearing than other standard tool in use.

The types of tools are also based on their material properties. These are as follows

Carbon Steel

This material is amongst the lowest grade of tools and falls in the family of low grade alloys. A tool made from Carbon Steel has is hard, tough and has strength when it is hardened at a certain temperature.

Carbon Steel Tools are suitable to be cut with at a lower cutting speed as above a temperature of 180oC, the Carbon starts melting. This limits this type of tool to lower speed machines in turn, rendering them unable for metal cutting.

The materials it is composed of are cheap, easily available and comfortably forged. Carbon Steel tools have a hardness of about 62 Rc and usually opted for working with wood.

High Speed Steel

HSS has a higher resistance and hot hardness than Carbon Steel because of its material composition. Tools made from High Speed Steel can be used for Metal cutting at a speed of about 2 to 3 times faster than Carbon Steel Tools.

HSS tools can cut through a metal comfortably and can perform high speed cut with faster metal removal rate. The melting point of this steel is about 900oC. Cutting Tools made with High Speed Steel are suitable for interrupted cuts on metals using different machines and processes.

Cast Alloys

Cast Alloys were introduced in the early 1900’s and were in regular use then. They have a maximum hardness value of about 55 - 64 Rc. Cast Alloys have a better resistance than its toughness, lower comparatively to HSS.

Cast Alloys can be used at a slight higher speed than High Speed Steels. These tools maintain their hardness from up-to 760oC and they are highly alloyed, which makes them brittle and susceptible to damage. Tools made from Cast Alloys are now limited in use.

Carbides

Carbides or Cemented Carbides are materials that have a high Hot Hardness over various temperatures, high thermal conductivity and a high Young’s Modulus making them a suitable material for manufacturing carbide cutting tools. Usually Carbides are made of Tungsten powder and Carbon, mixed in a ratio of weight- 94 : 6. Then the next step involves sintering it with Cobalt at high temperatures.

There are a wide range of grade of Tungsten Carbide available in the Market. The addition of Cobalt gives increased hardness, depending on the amount present in the mixture. Tungsten Carbide has a higher wear resistance than Tungsten. Typical cutting speed is of- 30 - 150 mm; when coated it is about 100 - 250 mm.

There are three main types of Carbides- Straight Tungsten Carbide, Titanium Carbide or Tantalum Carbide & Composite Carbides. Straight Tungsten Carbide is strong and has a high resistance. Titanium Carbides help in reducing the chip rate of a tool and help in improving the hot hardness.

Carbide tools are also known as Tungsten Carbide Cutting Tools.

Ceramics

Developed in the 1960’s, Ceramics tools are made by sintering Boron Nitride in powder form and Aluminium Oxide at about a temperature of 1700oC. Because of Ceramics being usually refractory, they can withstand high temperatures of heat, up to 1200oC. Ceramics are brittle even more than Cast Alloys, which makes them sensitive to thermal shock.

Ceramics are costly to produce and are sensitive, this renders them usable for only high quality machinery with a high production rate thus lowering their use.

Which is the Hardest Cutting Tool Material?

The Hardest Cutting Tool is Diamond. Diamonds are the hardest substance known to man yet they are comparatively brittle. Traditionally, Single Diamond Tools used for the purpose of cutting. Nowadays, machines make use of Poly-crystalline Diamonds as a replacement. Though Diamonds are costly and are hence not used a lot to make tools, other than specific industries.

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