Cutting tool materials

Introduction

The selection of cutting tool material and grade is an important factor to consider when planning a successful metal cutting operation.

A basic knowledge of each cutting tool material and its performance is important when making the correct selection. Considerations include the workpiece material to be machined, the component type and shape, machining conditions and the level of surface quality required for each operation.

This section provides information on each cutting tool material, advantages and recommendations for best use.

Cutting tool materials have different combinations of hardness, toughness and wear resistance, and are divided into numerous grades with specific properties. Generally, a cutting tool material that is successful in its application should be:

• Hard, to resist flank wear and deformation
• Tough, to resist bulk breakage
• Non-reactive with the workpiece material
• Chemically stable, to resist oxidation and diffusion
• Resistant to sudden thermal changes

Coated cemented carbide cutting tool material

• Coating – CVD
• Coating – PVD
• Cemented carbide

What is coated cemented carbide cutting tool material?

Coated cemented carbide currently represents 80-90% of all cutting tool inserts. Its success as a cutting tool material is due to its unique combination of wear resistance and toughness, and its ability to be formed into complex shapes.

Coated cemented carbide combines cemented carbide with a coating. Together they form a grade which is customized for its application.

Coated cemented carbide grades are the first choice for a wide variety of cutting tools and applications.

Coating - CVD

Definition and properties

CVD stands for Chemical Vapor Deposition. The CVD coating is generated by chemical reactions at temperatures of 700-1050°C.

CVD coatings have high wear resistance and excellent adhesion to cemented carbide.

The first CVD coated cemented carbide was the single layer titanium carbide coating (TiC). Alumina coatings (Al2O3) and titanium nitride (TiN) coatings were introduced later. More recently, the modern titanium carbonitride coatings (MT-Ti(C,N) or MT-TiCN, also called MT-CVD) were developed to improve grade properties through their ability to keep the cemented carbide interface intact.

Modern CVD coatings combine MT-Ti(C,N), Al2O3 and TiN. The coating properties have been continuously improved for adhesion, toughness and wear through microstructural optimizations and post-treatments. See Inveio™ technology.

MT-Ti(C,N) - Its hardness provides abrasive wear resistance, resulting in reduced flank wear.

CVD-Al2O3 - Chemically inert with low thermal conductivity, making it resistant to crater wear. It also acts as a thermal barrier to improve plastic deformation resistance.

CVD-TiN - Improves wear resistance and is used for wear detection.

Post-treatments - Improve cutting edge toughness in interrupted cuts and reduce smearing tendencies.

Applications

CVD coated grades are the first choice in a wide range of applications where wear resistance is important. Such applications are found in general turning and boring of steel, with crater wear resistance offered by the thick CVD coatings; general turning of stainless steels and for milling grades in ISO P, ISO M, ISO K. For drilling, CVD grades are usually used in the peripheral insert.

Coating – PVD

Definition and properties

Physical Vapor Deposition (PVD) coatings are formed at relatively low temperatures (400-600°C). The process involves the evaporation of a metal which reacts with, for example, nitrogen to form a hard nitride coating on the cutting tool surface.

PVD coatings add wear resistance to a grade due to their hardness. Their compressive stresses also add edge toughness and comb crack resistance. See Zertivo™ technology.

The main PVD-coating constituents are described below. Modern coatings are combinations of these constituents in sequenced layers and/or lamellar coatings. Lamellar coatings have numerous thin layers, in the nanometer range, which make the coating even harder.

PVD-TiN - Titanium nitride was the first PVD coating. It has all-round properties and a golden colour.

PVD-Ti(C,N) - Titanium carbonitride is harder than TiN and adds flank wear resistance.

PVD-(Ti,Al)N - Titanium aluminium nitride has high hardness in combination with oxidation resistance, which improves overall wear resistance.

PVD-oxide - Is used for its chemical inertness and enhanced crater wear resistance.

Applications

PVD coated grades are recommended for tough, yet sharp, cutting edges, as well as in smearing materials. Such applications are widespread and include all solid end mills and drills, and a majority of grades for grooving, threading and milling. PVD-coated grades are also extensively used for finishing applications and as the central insert grade in drilling.

Cemented carbide

Definition and properties

Cemented carbide is a powdery metallurgical material; a composite of tungsten carbide (WC) particles and a binder rich in metallic cobalt (Co). Cemented carbides for metal cutting applications consist of more than 80% of hard phase WC. Additional cubic carbonitrides are other important components, especially in gradient sintered grades. The cemented carbide body is formed, either through powder pressing or injection moulding techniques, into a body, which is then sintered to full density.

WC grain size is one of the most important parameters for adjusting the hardness/toughness relationship of a grade; the finer grain size means higher hardness at a given binder phase content.

The amount and composition of the Co-rich binder controls the grade’s toughness and resistance to plastic deformation. At equal WC grain size, an increased amount of binder will result in a tougher grade, which is more prone to plastic deformation wear. A binder content that is too low may result in a brittle material.

Cubic carbonitrides, also referred to as γ-phase, are generally added to increase hot hardness and to form gradients.

Gradients are used to combine improved plastic deformation resistance with edge toughness. Cubic carbonitrides concentrated in the cutting edge improve the hot hardness where it is needed. Beyond the cutting edge, a binder rich in tungsten carbide structure inhibits cracks and chip hammering fractures.

Applications

Medium to coarse WC grain size

Medium to coarse WC grain sizes provide the cemented carbides with a superior combination of high hot hardness and toughness. These are used in combination with CVD or PVD coatings in grades for all areas.

Fine or submicron WC grain size

Fine or submicron WC grain sizes are used for sharp cutting edges with a PVD coating to further improve the strength of the sharp edge. They also benefit from a superior resistance to thermal and mechanical cyclic loads. Typical applications are solid carbide drills, solid carbide end mills, parting off and grooving inserts, milling and grades for finishing.

Cemented carbide with gradient

The beneficial dual property of gradients is successfully applied in combination with CVD coatings in many first choice grades for turning and parting and grooving in steels and stainless steels.

Uncoated cemented carbide cutting tool material

What is uncoated cemented carbide cutting tool material?

Uncoated cemented carbide grades represent a very small proportion of the total cutting tool assortment. These grades are either straight WC/Co or have a high volume of cubic carbonitrides.

Applications

Typical applications of this cutting tool material are machining of HRSA (heat resistant super alloys) or titanium alloys and turning hardened materials at low speed.

The wear rate of uncoated cemented carbide grades is rapid yet controlled, with a self-sharpening action.

Cermet cutting tool material

What is cermet cutting tool material?

Cermet is a cemented carbide with titanium-based hard particles. The name cermet combines the words ceramic and metal. Originally, cermet was a composite of TiC and nickel. Modern cermets are nickel-free and have a designed structure of titanium carbonitride Ti(C,N) core particles, a second hard phase of (Ti,Nb,W)(C,N) and a W-rich cobalt binder.

Ti(C,N) adds wear resistance to the grade, the second hard phase increases the plastic deformation resistance, and the amount of cobalt controls the toughness.

In comparison to cemented carbide, cermet has improved wear resistance and reduced smearing tendencies. On the other hand, it also has lower compressive strength and inferior thermal shock resistance. Cermets can also be PVD coated for improved wear resistance.

Applications

Cermet grades are used in smearing applications where built-up edge is a problem. Its self-sharpening wear pattern keeps cutting forces low even after long periods in cut. In finishing operations, this enables a long tool life and close tolerances, and results in shiny surfaces.

Typical applications are finishing in stainless steels, nodular cast irons, low carbon steels and ferritic steels. Cermets can also be applied for trouble shooting in all ferrous materials.

Hints:

• Use low feed and depth of cut
• Change the insert edge when flank wear reaches 0.3 mm
• Avoid thermal cracks and fractures by machining without coolant

Ceramic cutting tool material

What is ceramic cutting tool material?

All ceramic cutting tools have excellent wear resistance at high cutting speeds.

There are a range of ceramic grades available for a variety of applications.

Oxide ceramics are aluminium oxide based (Al2O3), with added zirconia (ZrO2) for crack inhibition. This generates a material that is chemically very stable, but which lacks thermal shock resistance.

(1) Mixed ceramics are particle reinforced through the addition of cubic carbides or carbonitrides (TiC, Ti(C,N)). This improves toughness and thermal conductivity.

(2) Whisker-reinforced ceramics use silicon carbide whiskers (SiCw) to dramatically increase toughness and enable the use of coolant. Whisker-reinforced ceramics are ideal for machining Ni-based alloys.

(3) Silicon nitride ceramics (Si3N4) represent another group of ceramic materials. Their elongated crystals form a self-reinforced material with high toughness. Silicon nitride grades are successful in grey cast iron, but a lack of chemical stability limits their use in other workpiece materials.

Sialon (SiAlON) grades combine the strength of a self-reinforced silicon nitride network with enhanced chemical stability. Sialon grades are ideal for machining heat resistant super alloys (HRSA).

(1) Mixed ceramics

(2) Whisker-reinforced ceramics

(3) Silicon nitride ceramics

Applications

Ceramic grades can be applied in a broad range of applications and materials, most often in high speed turning operations but also in grooving and milling operations. The specific properties of each ceramic grade enable high productivity when applied correctly. Knowledge of when and how to use ceramic grades is important for success.

General limitations of ceramics include their thermal shock resistance and fracture toughness.

Polycrystalline cubic boron nitride cutting tool material

What is polycrystalline cubic boron nitride cutting tool material?

Polycrystalline cubic boron nitride, CBN, is a cutting tool material with excellent hot hardness that can be used at very high cutting speeds. It also exhibits good toughness and thermal shock resistance.

Modern CBN grades are ceramic composites with a CBN content of 40-65%. The ceramic binder adds wear resistance to the CBN, which is otherwise prone to chemical wear. Another group of grades are the high content CBN grades, with 85% to almost 100% CBN. These grades may have a metallic binder to improve their toughness.

CBN is brazed onto a cemented carbide carrier to form an insert. The Safe-Lok™ technology further enhances the bondage of CBN cutting tips on negative inserts.

Applications

CBN grades are largely used for finish turning of hardened steels, with a hardness over 45 HRc. Above 55 HRc, CBN is the only cutting tool which can replace traditionally used grinding methods. Softer steels, below 45 HRc, contain a higher amount of ferrite, which has a negative effect on the wear resistance of CBN.

CBN can also be used for high speed roughing of grey cast irons in both turning and milling operations.

Polycrystalline diamond cutting tool material

What is polycrystalline diamond cutting tool material?

PCD is a composite of diamond particles sintered together with a metallic binder. Diamond is the hardest, and therefore the most abrasion resistant, of all materials. As a cutting tool material, it has good wear resistance but it lacks chemical stability at high temperatures and dissolves easily in iron.

Applications

PCD cutting tools are limited to non-ferrous materials, such as high-silicon aluminium, metal matrix composites (MMC) and carbon fibre reinforced plastics (CFRP). PCD with flood coolant can also be used in titanium super-finishing applications.

Sandvik Coromant grades

Information regarding the Sandvik Coromant insert and grade assortment can be found here.

With this information you can choose your insert or grade based on cutting material or application area.