APPLICATIONS OF TURNING TOOL BITS

DESIGNATIONS OF TURNING TOOL BITS ACCORDING TO ISO, PN, DIN, F, GOST

 CEMENTED CARBIDE GRADES - APPLICATIONS AND TECHNICAL INFORMATION

Cemented carbides are manufactured by cementing fine particles of carbide into a composite by a binder metal. Most commonly used materials in cemented carbides are:

• tungsten carbide WC (basic component),
• titanium carbide TiC,
• tantalum carbide TaC,
• niobium carbide NbC,
• cobalt (binding material).

Cemented carbides, due to their structure, are classified as metal matrix composites, where carbide particles are bound together by metallic matrix. They constitute hard metal sinters of high-alloyed metals: WC, TiC, TaC, NbC, VC and metallic matrix (most commonly cobalt, less frequently used are nickel and iron). High hardness of these carbides (within the range of 1900 ? 2500 HV) determines wearing quality of the composite. Thier brittleness, however, limits significantly their application as materials subject to dynamic effects of external stress which occurs during machining. In order to utilize their assets and ensure acceptable malleability they are bound together through metallic phase. The following groups of cemented carbides may be distinguished according to their chemical composition: WC-Co with possible addition of TaC, NbC, VC: used mainly in tools for machining cast iron and non-metallic materials (2 ? 8% Co), rocks (10 ? 15% Co) and in tools for forming processes (20 ? 30% Co), WC-TiC-Co: used mainly in tools for machining steel, WC-TiC-TaC, NbC-Co: used mainly in tools for machining steel.

Types of carbide, application and designation:

PN-81/H-89500 standard divides carbide grades into three basic groups:

• for machining (S, U, H grades),
• for forming processes (G),
• for mine drilling (B, G),
• carbide grade designation consists of letters and numbers.

Initial letters of the designation suggest application of the carbide:

• S - according to ISO: P (blue colour) - grades suitable for machining materials producing long chips (mainly steel, cast steel, malleable cast iron),

• SM - used for milling steel,

• U - according to ISO: M (yellow colour) - grades suitable for machining materials producing both long and short chips (stainless steel, heat-resistant alloys),

• H - according to ISO: K (red colour) - used for machining materials producing short chips (mainly cast iron and aluminium alloys)

Numbers given in carbide's designation are conventional symbols. As the number increases, so does the malleability of carbide grade, while its wearing quality decreases.

Final letter of the designation points to:

• S - grades containing tantalum and niobium carbides,
• X - grades used mainly in milling of cast iron.

Carbide properties depend on their chemical composition, e.g. amount of tungsten, titanium, tantalum and niobium carbides, material which binds cobalt, as well as grain size of the particles and production methods.

Cemented carbides:

• have high hardness (depending on the chemical composition of approx. 90HRC),
• have high wearing quality,
• maintain machining properties in temperature up to approx. 1000 °C,
• are sensitive to temperature changes during machining (eg. intermittent machining),
• are sensitive to impact loading, impact value of carbides is 2-3 times lower than that of hardened steel,
• their cross-breaking strength is lower than compressive strength.

Application of cemented carbides used in machining (accoring to ISO 513):

Coatings were intruduced in order to raise wearing quality of multiedged blades:

• titanium carbide (TiC) - ensures high wearing quality and high adherence to foundation (which is the parent material of the blade),
• titanium nitride (TiN) - protects edges against built-up and lowers friction factor between the edge and machined material (chip); it results in significantly lower machining force,
• titanium nitride carbide (TiCN) - mainly used in intermittent machining as well as in heavy duty operations, e.g.: milling and threading,
• aluminium oxide (AkOs) - gives the edge resistance to high temperatures.

Coated blades are not recommended for machining of aluminium, titanium, zinc, tin and nickel, and their alloys due to high affinity of carbides and titanium nitrides to those metals.

Note: Proper selection of cemented carbide grade depends on its chemical composition, structure, physical and mechanical properties, propeties and quality of the workpiece, machining conditions as well as dynamic properties of the machine.

RECOMMENDED MACHINING PROPERTIES FOR TURNING

Recommended speeds and feeds for turning steel and cast steel castings:

Recommended speeds and feeds for turning cast iron and non-ferrous metals: