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.