Machining stainless steels subjects the cutting edge to higher levels of temperatures and cutting forces, smearing and work-hardening than associated with many other workpiece materials. These destructive forces join forces to keep cutting data down, shorten tool-lives and deteriorate component quality – making stainless steel turning an area that needs special consideration.

Cutting tool development has continually provided new opportunities for more efficient machining with cutting edges that are not as prone to the typical breakdown factors. Stainless steel machining has progressed a long way especially during the past decade and this development is now taking another step with a new-generation indexable insert grade, dedicated for a broad area of stainless steel turning. A grade that will in many cases cope with most, if not all, operations throughout a machine shop.

Stainless steel turning is very much a matter of eliminating uncertainties in machining and that is, of course, a task for production people throughout the metalworking industry.

Sandvik Coromant has developed cemented carbide grades for indexable inserts since the 1950s, introducing premium tungsten carbide grades during the 1960s along with coated cemented carbides; thermal barriers of aluminium oxide to inserts during the 1970s; toughness increase to functional gradient substrates during the 1980s; enhanced wear resistance to grades during the 1990s, involving medium-temperature chemical vapour deposition (MT-CVD).

Stainless steel is an area where there has been a continuous need for higher productivity levels balanced with process reliability.

In the today’s metal cutting process, the cutting edge geometry and tool material are developed to function together to provide the most suitable levels of such factors as cutting forces, heat generation and chip control. In stainless steel, reducing cutting forces is a high priority. This is because the heat generated in deforming the metal, combined with the limited amount of heat that leaves with the chip, substantially elevates the temperature in the cutting zone on the insert. The cutting zone temperature is already significantly higher when machining stainless steel (ISOM materials) than in steel (ISOP materials), especially at higher cutting speeds.

The deformation hardening and ductility of stainless steel, along with the relatively low thermal conductivity through the austenitic phase of the metal, govern the development of load on the cutting edge and consequently the type and rate of wear development and tool-life. By limiting the magnitude of cutting forces and providing suitable chip control, through carefully developed macro and micro cutting edge geometries adapted for stainless steel turning, indexable inserts can be designed to give broad capability throughout stainless steel material-grades. To suit various applications, material grades vary considerably as regards properties and machinability – hence the development of machinability-improved stainless steels, such as SANMAC, which helps to improve productivity.

From the machine shop perspective, the capability of a new cutting edge to overcome previous limitations may be several. As regards metal removal rate, for example, when machining large batches of flange-type components in machinability-improved austenitic bar material to forged parts in duplex material. Other examples include eliminating cutting edge fractures during unattended internal turning of tubes in other stainless steel grades or establishing the optimum cutting data for efficiently machining a variety of components in different stainless steels. To find the most suitable balance between productivity and process reliability is always the over-riding direction. To address such practical requirements, Sandvik Coromant has over the years developed various grades dedicated for the ISO M-area of workpiece materials.

Balanced tool material properties are important to take into consideration as not even half of the cutting edge breakdowns when machining stainless steel are related to the toughness behaviour of the tool material or insufficient resistance to heat generation in the cutting edge. Conclusively, then, these cases cause a hidden reliability problem, not a toughness problem.

More recently, the newly developed GC2025 has become a first-choice insert grade for stainless steel turning, completely re-engineered with greater capability, providing scope for higher productivity while retaining a high level of reliability.

The development of coated cemented carbide insert grades have provided remarkable increases in performance and this is no difference in GC2025. The aim has been to provide an insert grade having a broader application area – as far as possible, one grade for all operations – and with higher level of machining performance and machining security.

When developing the new insert grade consideration was taken to cutting edge geometry, insert substrate and insert coating with the aim of achieving suitable performance throughout the application area. The cutting edge geometry for stainless steel machining is quite sharp in comparison to those for steel, in order to minimise cutting forces.

The chip-breaking ability also needs to be harder than that for steel to ensure chip control in a wide cutting data area.

The insert substrate has to provide a satisfactory foundation for the coatings and ensure proper adhesion. When considering the basic properties of the cemented carbide substrate, where cobalt provides toughness and tungsten material carbide resistance to thermal load as well as deformation, it was established that the grain size, especially of the tungsten carbide, plays a role in defining the toughness behaviour of the insert – a large grain providing significantly better toughness than a small one.

Mastering the different material aspects and processes provides the basis for success for coated indexable insert performance. Today’s insert substrate development has its background in that it is very much a matter of finding an optimum combination to support the coating by being able to absorb cracks and to have sufficient resistance to being plastically deformed locally. It is also about process control and getting the insert substrates sufficiently clean in production for the coating to adhere and grow in a controlled manner.

It can also be a case of the relatively reactive gas atmosphere in the sintering furnace over and near the melting point of the susceptible Cobalt temperature or of the surface condition from handling inserts in the production chain.

Multi-layer coatings have been in use for many years now and teething-problems have been eliminated during the course of their development. The new GC2025 has been provided with improved performance as regards adhesive, insulative and wear resistant properties, leading to higher cutting data capability and improved tool-wear development, especially regarding those relating to smearing tendencies.

The top coating of GC2025 is a smooth, golden layer of titanium nitride (TiN), this acts mainly as a marker for used cutting edges on the insert, indicating wear clearly, as well as enhancing the frictional properties of the insert surface.

The next coating, under the TiN-coating, is made up of a newly developed aluminium oxide (Al2O3) coating. This is a multi-layer structure of alumina which has some interesting properties in machining. The mechanism of the multi-layer forces cracks to use more energy in penetrating the coating. This is done by crack deflection and energy absorption in each sub-layer – very similar to the function of sandwich constructions in the aerospace industry, etc.

The alumina-coating also acts as an effective barrier against chemical wear, and as such, against crater wear. Also, importantly for stainless steel machining, it acts against excessive thermal load. In this respect, the thermal shock resistance is important along with the resistance to mechanical impulse and creep.

As a foundation for the multi-layer insert-coating, a medium temperature chemical vapour deposition titanium carbonitride (MTCVD-TiCN) layer has been deposited on the substrate. The structure of this layer is also interesting in that its grains grow as columns. These provide exceptionally good wear resistance due to that all the grains support each other in the column structure. This means that wear has to be abrasion through a number of grains not initiating and consequently propagating cracks. The result is a substantial prolonging of tool-life in applications where predictable tool-life is critical.

In addition, the intrinsic properties of the separate coating layers provide a much improved ability to withstand the formation of built-up edges as well as other smearing tendencies.

Workpiece material adhesion to the insert results in alternation of material sticking and being torn away, leading to cutting edge damage. The new coating provides critical boundaries at each layer which influences the fundamental properties of the subsequent layer.

Having one grade suitable for most if not all applications throughout a machine shop may not be impossible any more with the development of GC2025. The grade has shown promising results in broad capability as regards turning different stainless steel grades. This, in combination with the ability to machine at considerably higher cutting data and at high levels of process reliability – creating the basis for an insert suitable for most if not all operations when turning stainless steel.

Finally, a few practical application hints that have become important to observe when preparing for un-disturbed production that includes stainless steel turning :

• When programming, remember that the surface of bars have higher hardness values than the centre and that smearing tendencies increase towards the centre of the bar (especially when the spindle speed limit of the machine has been reached).
• Change shims more often if different indexable insert geometries are used and cutting loads are used with the same tool-holder.
• For intermittent cuts, reduce or remove coolant supply.
• Establish collaboration with a cutting tool supplier who provides back-up service for applications, etc.
• Reduce machinability variations by limiting the number of workpiece material suppliers.

Characteristic demands on the cutting tools for stainless steel machining are high cutting forces, high temperatures, smearing tendency and work – hardening component surfaces.

The hardness of workpiece materials affects the tool-life of cutting tools. A harder material usually calls for a lowering of the cutting speed to maintain tool-life. Stainless steel is deformation-hardened when it is cold-drawn, with the austenitic hardening the most. Even bar stock that has been straightened is cold-drawn to some extent, with surface hardness values of 300 HB or more even though the inside of the material is about half the hardness as specified. Chromium has a relatively minor influence on machining compared to nickel and molybdenum.The latter especially leads to added deformation hardening. Also metal cutting itself involves some cold-working. The deformation-hardened layer is a lot thicker especially in austenitic stainless steel than in carbon steel. Usually it is most advantageous to select a depth of cut and feed rate to ensure that the actual cutting edge penetrates the material past the hard zone.

Thermal conductivity plays an important role in metal cutting, in that when machining steel, most of the heat from the cutting zone is removed with the chip. Stainless steel however has poorer thermal conductivity, leaving more heat in the zone. These higher cutting temperatures increase the tendency for more tool wear to take place. Plastic deformation is a higher risk factor as well. A positive geometry and an open chip-breaker with smoother, softer chip-flow mean less heat.

Ductility is a factor which makes considerable demands on the chip-breaking ability when it comes to low-carbon steels. However, the austenitic stainless steels are also highly ductile, requiring considerable energy to cut a chip, with more heat as a result.

Burr-formation is another characteristic of stainless steel machining usually promoted by excessively hard cutting action. This can be reduced or eliminated with a positive, sharper cutting edge more suited for this material range.

There are several factors which combine to make the machining of stainless steel, especially the austenitic types, more demanding. Deformation hardening, poor thermal conductivity and ductility mean that the selection and application of specially developed cutting tools for stainless steel should be considered and performed correctly. Also that the machining conditions and the state of the machine tools is up to a good standard.

A positive insert means more continuous cutting and chip flow process with smaller variations in cutting forces and lower temperatures, as well as less deformation-hardening of the material taking place.

A sharp cutting edge means softer cutting action with lower forces involved and less burr formation taking place on the workpieces. The sharper edge means less deformation of the workpiece material. A positive, sharp cutting edge, combined with a correctly balanced open chip-breaker, has been found to be the best solution for machining stainless steel.

However, these features need to be balanced against the need to achieve sufficient stability and edge strength in the machining process. The insert has to be well supported with sufficient support faces and have reinforced cutting edges to cope with the greater forces as well as intermittent cuts.

When it comes to work-hardening of the component surface, austenitic and super austenitic are most susceptible followed by the duplex types. Cutting forces are highest when it comes to machining super austenitic, duplex as well as martensitic types. As regards cutting action, chip-forming and breaking ability, super austenitic and duplex types are the most demanding followed by austenitic.

AB Sandvik Coromant is based in Sandviken, Sweden. www.sandvik.com