Stainless steel has long been one of the main engineering materials.
Stainless steel products are common everywhere: in turbomachinery, cookware, aerospace engineering, surgery, oil and gas, and food industries. Such a wide use of stainless steel is explained by the most important property of this material – its resistance to corrosion.
"Stainless " and "corrosion " steel are generally used synonymously, along with other terms such as rust-resistant steel, inox steel, and non-corrosive steel.
Stainless, originally meaning "free of red rust stains", was developed as a result of iron oxidizing. Corrosion is caused by chemical and physical-chemical reactions. There are different types of corrosion.
Stainless steel can be divided into the following groups, according to their main functional features:
• Corrosion-resistant steel - resistant to corrosion under normal conditions
• Oxidation- or rust-resistant steel - resistant to corrosion under high temperatures in aggressive environments
• Heat-resistant or high-temperature steel that does not change its strength under high temperature stress
Therefore, corrosion-resistant steel can be considered a type of stainless steel.
The functional features of these stainless-steel groups determine the choice of a steel grade as a material for cutting tool design. For a manufacturer involved in machining stainless steel, the key parameter is machinability. The machinability of stainless steel relates to a steel content and structure. Accordingly, stainless steel is classified by the following types:
• Chrome stainless steel
- Martensitic and ferritic-martensitic
• Chrome-nickel stainless steel
- Austenitic and super austenitic
- Duplex (ferritic-austenitic)
In addition, there is a separate category of precipitation-hardening (PH) stainless steel, which can be both martensitic and semi-austenitic.
Stainless-steel belongs to one type or another, which enables estimating machinability and selecting the required cutting tool. In cutting tool design, the type of stainless steel is a major factor for forming cutting geometry, choosing tool material (particularly carbide grade), and deciding about coolant supply.
From a machinability point of view, ferritic and martensitic stainless steel is very similar to high alloy steel. Therefore, standard ISO 513, which specifies the main groups of cutting tool applications, classifies ferritic and martensitic stainless steel as ISO P. However, the situation with austenitic, duplex, and PH steel is totally different. These types of stainless-steel feature poor machinability and their machining is specified as ISO M.
Machinability is also influenced by heat treatment. When cutting ISO M stainless steel, due to the low machinability of this material, a cutting tool edge works under heavy load conditions.
• When beginning with poor heat conduction of such steel – a high temperature causes thermal cracking.
• Self-hardening, due to material deformation during machining, leads to plastic deformation and a fracture.
• High-material strength results in intensive fatigue wear and breakage.
• The tendency of material sticking contributes to a built-up-edge formation.
The ISO S group of applications considers machining high-temperature super alloys (HTSA), titanium, and titanium alloys. Among a wide range of HTSA, there are iron-based alloys that, in fact, are heat-resistant stainless steels and their machinability leaves much to be desired.
Hence, stainless steel is a whole class of engineering materials, which differs by content, property and machinability. Machining stainless steel requires tools, varied in cutting geometry and tool material, that cover three main groups of application: ISO P, M, and S.
The metalworking industry needs more and more effective tools for stainless steel machining and cutting tool producers are in constant search of an appropriate answer to the growing industrial demands. Despite a long history of stainless steel machining, cutting tool manufacturers contrive to find new sources for innovative developments and improved existing solutions. The latest products of ISCAR, one of the cutting tool leaders in the world, clearly supports this conclusion.
Based on accumulated experience, ISCAR has developed three new chipformers for ISO turning inserts. The chipformers, which determine the profile of an insert rake face, are designated R3M, M3M and F3 and specify the main fields of application: rough, medium-duty, and finish turning stainless steel. They are used in both negative and positive turning insert designs. Typical features of the chipformers are a specially shaped deflector for better chip control and a wavy surface to prevent chip hammering. The 3D modeling of the chip flow greatly contributed to forming the deflector during the design stage. To help identify the chipformer, there is an engraved contour around the hole of the insert and each chipformer is characterized by the number of contour curves.
According to ISCAR experts, in combination with the most advanced carbide grades that feature applying ISCAR’s post-coating treatment technology SUMOTECH, the new chipformers provide higher performance and increase tool life.
In the metalworking industry, Swiss-type machine tools are very popular for manufacturing small-sized stainless steel parts, particularly for aerospace and medicine products. The recently introduced inserts for grooving and turning with a new NX chipformer, which enriched the ISCAR SWISSCUT INNOVAL Line, were developed specifically for these machining tools. The new chipformer considerably improves chip control in machining stainless steel. As in the previous case, 3D modelling of a chip formation facilitated in finding the optimal chipformer shape.
In parting and grooving, ISCAR launched two new carbide grades with PVD TiAlN coating for machining stainless steel: IC1010 for medium to high cutting speeds and IC1030 for low to medium cutting speeds.
The technical capabilities of a cutting tool are largely determined by its properties. In drilling stainless steel, many of the latest ISCAR developments have been aimed at creating better tool material, which has resulted in two new carbide grades: IC806 and IC5500. IC806 with PVD coating is dedicated mainly for deep drilling difficult-to-cut heat resistant stainless steel (ISO S and ISO M groups), while IC5500, which features both new a substrate and a CVD coating, ensures high performance in drilling ferritic and martensitic stainless steel (ISO P group).
The impressive results of the carbide grade IC5500 in drilling had a direct impact on expanding the application range of the grade, and was later adopted by the indexable milling product line. The family of milling cutters with round inserts has recently expanded with new tools intended for machining profile surfaces, especially the working surfaces of blades in turbomachinery. The new inserts are available in two design versions. The first version, produced from grade IC5820, is directed to machining austenitic, duplex, and precipitation-hardening stainless steel (ISO M and ISO S groups). The second version, made from carbide grade IC5500, is intended for milling ferritic and martensitic steel (ISO P group). Using grade IC5500, well-proven in drilling, as the material for the round insert, provides considerably increased cutting speeds.
An interesting solution of improving performance in machining austenitic, duplex, and precipitation-hardening stainless steel (ISO M and ISO S groups) is represented by the recently developed family of five-flute solid carbide endmills. The endmill concept utilizes the vibration-proof principle and gained successful recognition of ISCAR's CHATTERFREE products - variable angular pitch and different flute helix portions. A new element is a sub-flute wear control mechanism. The endmills are made from a new PVD coated hard submicron carbide grade IC608. The combination of the chatter-free approach, the sub-flutes, a reinforced taper core, and the advanced wear-resistant grade in the endmill design brought about a noticeable increase of tool life.
Effective Cooling – A Steppingstone to Success
When machining difficult-to-cut stainless steel, in many cases, effective coolant supply is a factor in performance optimization. The TANGSLIT slitting cutters with coolant channels directed to each cutting edge, that were introduced over the past few years, provide a reasonable solution to the customer. The cutters, suitable for either low (up to 10 bar) or high (up to 340 bar) coolant pressure, reach increasing cutting speeds of up to 50% and ensure efficient chip disposal and improved surface finish.
Following the growing demands for machining with high pressure coolant supply, ISCAR added new adapters with inner coolant channels to the threading tools. The adapters carry highly economical inserts with 10 cutting corners.
Demands for more and more efficient tools for machining stainless steel require an appropriate response from the cutting tool manufacturer. High competence and expertise, multiplied by an innovative initiative, is the decisive point here and the way to progress. ISCAR remains determined and will continue following this practice in future developments.