Cemented carbide is a hard material that is used extensively in cutting tools that are intended for machining. Within an industrial context, references to carbide or tungsten carbide usually refer to this cemented composite.
Carbide cutters deliver many advantages. In the vast majority of cases, they provide a better surface finish on the machined part, and allow faster machining when compared to the use of high-speed steel (HSS) cutters. In addition, carbide tools are able to withstand higher temperatures at the cutter-workpiece interface than standard high-speed steel tools, which is the principal reason for their faster machining capability. Carbide usually provides superior performance for the cutting of tough materials such as high alloyed steel or stainless steel, as well as in situations where other cutting tools would wear away faster, such as when performing high-quantity, extended production runs.
Industries use of cemented carbide for cutting metals began in the 1930s. Since that time carbide has become by far the most popular material for the production of cutting tools. While some tools that feature relatively small sizes are wholly produced from carbide; others use carbide in the cutting area only. Originally the cutting area consisted of a carbide tip that was brazed or soldered to a tool body. However, in the 1940s cutting tool manufacturers began to produce cutting tools with the advantage of replaceable carbide segments that were mechanically mounted on to the tools body.
This clever innovation and the use of mechanical clamping, which provides much greater strength compared with the previously brazed connections, are now recognised as memorable milestones, not only in the area of tool manufacturing, but also in advancing the efficiency of all metalworking industries.
This major development led to impressive improvements in productivity within the area of machining operations. It was immediately possible to increase the load on the tool and to intensify operational metal removal rates. In addition to this cost effective method ensuring the simple and economical replacement of the cutting element when worn or in case of breakage, it allowed the manufacturing of cutting segment and the tool bodies to be divided. Depending on the shape of the inserts used, they could be quickly indexed ensuring the rapid change of a worn cutting corner by several methods, such as rotating the insert on its axis or by flipping it upside down. Initially the new cutting segments were known by several names, such as throwaway tips, interchangeable inserts, replaceable inserts, however, today the more widespread, generic term indexable inserts is used.
The technology used in the manufacturing of the indexable inserts is based on powder metallurgy, comprising of several manufacturing processes as follows:
- - preparing carbide powder (mixing)
- - pressing the powder (compacting)
- - sintering compact
- - post-sintering processing
- - coating
In principal these stages have remained unchanged over many decades. Although, at the same time, progress in science and technology has significantly impacted on the manufacturing process of inserts.
In the past, inserts were produced by the use of manual machines. Hence, the application of various complex powder metallurgical processes was very difficult or even impossible to perform. The introduction of more progressive industrial equipment, featuring advanced automation and computer control, made the technological processes more stable, controllable and reliable.Consequently, the mechanical properties of manufactured inserts became more uniform, predictable and repeatable; these factors allowed dramatic improvements in terms of the accuracy of sintered inserts by reducing production tolerances.
Fig.1. The milling insert H690 TNKX 1005 features
marked difference in height of the inserts' corners
Today, a typical insert production press is a highly engineered device that is computer controlled. A moveable punch can be made from several “sub punches”, each operated separately. Some press designs encompass multi-axial pressing options. The remarkable progress in press technology enables the production of complex shaped inserts that are characterised by variable corner heights (Fig. 1). This capability enables the realisation of optimal cutting geometry, which guarantees not only smooth and stable machining but also the increased accuracy of a machined surface.
Additionally, the advantages provided by the use of modern CAD/CAM systems make it possible to improve the design and the shaping parts of pressing die sets. Also, the ability to simulate the pressing processes related to new sintered products, when they are at the beginning of their design stages, allows further design amendments and enhancements to be made.
Advanced new techniques, related to sintering insert masters, improves process quality. Gradient sintering of multi-carbide substrate ensures a thin upper layer that feature high cobalt content. This gradient layer produces an excellent barrier against the development of cracks and guarantees increased resistance to brittleness and fracturing. Today, substrates of this type are commonly used in tools intended for turning operations.
Until the 1980s carbide grades were uncoated. In order to make grades more universal and applicable to machining various engineering materials, tool manufacturers invented grades that contained various additives. The adoption of coating technologies has dramatically changed the world of machining; now the vast majority of carbide grades are coated. The addition of this new technology permitted the grades to focus on cutting specific material groups. The substrates contained fewer additives; therefore their structures became more uniform and stable, which further improved control during production.
The introducing of coated carbides and on-going developments in this area by companies like Iscar, has enabled significant increase in cutting speeds. For example 30 years ago, when turning gray cast iron the cutting speed used was approximately 100 m/min for inserts which were made from Iscar’s IC20 (uncoated carbide grade). Today, the coated IC5005 allows speed values of up to 600 m/min. In another case, the milling of martensitic stainless steel during the same years was performed at about 80 m/min for IC50M (uncoated carbide grade), now 300 m/min is the acceptable value when using IC5500 (coated grade). These impressive numbers provide an excellent illustration of how coated carbides have allowed leaps in progress to be made in the area of cutting speeds.
Coating technology continues to develop in two principal directions - Chemical Vapor Deposition (CVD) and Physical Vapour Deposition (PVD). The main result of progress within the area of CVD was the introduction of Alumina ceramic coatings. This allows machining at elevated speeds due to its excellent temperature isolation properties, high hardness and chemical stability at high temperatures.
Fig.2. The nanolayer of structure of the
coating of the IC807 carbide grade
(SEM image)
PVD coatings were introduced during the late 1980s. PVD coatings performed a gigantic step in overcoming the complex problems that prevented progress within the field of nanotechnology. PVD coatings brought a new class of wear-resistant nano layered coatings. Such coatings (Fig. 2) are a combination of layers having a thickness of up to 50 nm (nanometers) and demonstrate significant increases in the strength of the coating compared to conventional methods.
Modern technology allows both methods – CVD and PVD – to be combined for insert coatings, as a means of controlling coating properties. In particular, ISCAR’s carbide grade DT7150 features a tough substrate and a dual MT CVD and TiAlN PVD coating. This was originally developed to improve the productive machining of special-purpose hard cast iron.
Fig.3.
Fig.4.
Another major advancement in insert technology relates to post-coating treatments. For instance, Iscar developed Sumotec, a treatment method for the already coated surface of an insert. The advanced post-coating technology delivers improved strength and wear resistance to carbide grades, enabling higher productivity.
In CVD coatings, due to the difference in thermal expansion coefficients between the substrate and the coating layers, internal tensile stresses are produced. Also, PVD coatings feature surface droplets. These factors negatively affect a coating and therefore shorten insert tool life. Applying Sumotec post-coating technologies considerably reduces and even removes these unwanted defects and results in increasing the tool life of the grade, also greater productivity (Fig.3 and 4).
Continuous developments in carbide insert technology have initiated several areas of development. Advanced methods of pressing and sintering, coating processes and post-coating treatments, new options for surface treatment and optimisation of cutting geometry are intended for manufacturing indexable inserts to meet the requirements of efficient machining dictated by modern metalworking industries.
The structure of Iscar grade IC6025 is designed
specially for turning ISO M materials
Iscar’s recently developed grade IC6025 is intended specifically for turning materials related to ISO M group (austenitic and duplex stainless steel). The grade coating is a multi-layered coating that features post-coated treatment. The grade enabled significant improvements in productivity related to turning materials in the aerospace industry (Fig.5).
Among the very latest advanced carbide grades, Iscar has developed the IC806 grade for turning and grooving high temperature alloys. The IC806 grade is a new complementary Sumotec PVD coated grade for machining high temperature alloys, especially Inconel 718.
This new grade for Inconel 718 machining belongs to a family of nickel based super alloys that are used extensively for applications where the ability to withstand high temperatures and high corrosion resistance properties are required. The material is widely used in the aerospace industry, in components that are placed in the hot section of the engine, and is also used in various oil industry sectors.
The microstructure of Inconel 718 consists of an austenitic structure possessing high tensile and yield strength. The major problems encountered when machining Inconel 718 are characterised by very high temperatures on the cutting edge of the insert, this is due to the abrasive elements in the materials composition (high nickel content of 50-55 percent and chrome 17-21 percent), which can cause high wear rates, chipping, notching and insert breakage. These factors contribute to reduced tool life and high deformation of the cutting edge even at low cutting speeds.
Another complexity associated with Inconel is its tendency to become malformed; this is due to its metallurgical sensitivity to residual stresses and self-hardening effects during cutting operations. The aim is to effectively machine this unique material, and has therefore successfully developed the IC806. This is a submicron grade with TiAlN and Iscar’s Sumotec coating resulting in superior wear resistant properties. IC806 has a hard submicron substrate with PVD coating and a special post coating treatment that provides substantially improved tool life and better reliability.
