Last month, tooling manufacturer Dormer Pramet launched a new range of turn- ing, milling, and support tools for machining stainless steel, including larger diameter cutters and wiper inserts.
Dormer Pramet application specialist Ricky Payling explains that the exponential growth of stainless steel consumption has forced the cutting tool industry to evolve in order to machine the material.
Payling notes that, “according to the International Stainless Steel Forum, the global demand for stainless steel products has nearly doubled in the last 20 years and continues to grow 5% each year”. He adds that, in 2014, about 42-million tonnes of stainless steel were produced worldwide – a 70% increase from 2005 (24-million tonnes).
He says this rapid growth was not only to meet worldwide demand, but to support increased use within China, where stainless steel is consumed in household metal pro- ducts, building materials and machinery.
“This increase in production and consumption has coincided with advancements and new developments within the cutting tool industry,” he comments, adding that, owing to the difficult process of machining stainless steel, manufacturers have invested time and resources into finding new solutions to try to combat the numerous application challenges.
He says that some of these recently intro- duced solutions include changes to the cutting tool design, new geometries, improved coolant and developing stronger coatings to improve performance and tool life.
To that end, the company has developed a comprehensive assortment of indexable and rotary cutting tools to support machining of stainless steel.
Payling comments on its solid carbide burrs for applications up to 8 × D, adding that it has been specifically developed to overcome the problems typically associated with deep-hole drilling of stainless steel. “The new R459 broadens the existing MP-X range, which already consists of 3 × D and 5 × D solid carbide multi- material drills.”
Meanwhile, its versatile range of OEHT octagonal inserts for cutting depths between 4 mm and 10 mm, alongside REHT (round) and XEHT (wiper) inserts, are suitable for stainless steel.
“By using eight cutting edges – the highest number available for positive inserts – the new range offers an economical advantage to customers, while the wiper insert option boosts feed rate capabilities and surface finish quality,” he remarks.
Payling explains that, “drills with thin margins, for example, reduce the contact with the hole surface, minimising friction between the cylindrical land and the hole walls, decreasing work hardening and temperatures gene- rated”. Additionally, positive point geometries ensure excellent chip breaking and evacuation, while a specially designed web combines the benefits of maximum rigidity with space for efficient chip removal, reducing machine downtime.
He is enthusiastic about the developments relating to coolant, explaining that, owing to the significant heat generated during machining of stainless steel, which results in plastic deformation and severe crater wear, a large amount of coolant is necessary. “[A] generous flow of cutting fluid removes heat from the cutting edge, helps chip breaking and improves dimensional stability of the component.”
Meanwhile, he advises that a titanium aluminium nitride coating is ideally suited for machining stainless steel as it holds its hardness at higher temperatures, owing to a layer of aluminium oxide, helping to transfer the heat away from the cutting tool.
He explains that the strength of stainless steels combined with their ductility makes chip breaking more difficult. “This increases the risk of vibration, which can affect surface finish quality and reduce the service life of the tool or insert. He says that to address this, tooling and work pieces must be stable and well clamped, and tool overhang must be at its minimum, especially when drilling or internal turning.”
Payling points out that from a machinability perspective, in terms of tooling properties and applications, ferritic and martensitic stainless steels are classified as ISO P materials. Thus, machinability of ferritic and matensitic stainless steels is good and very similar to low alloyed steels.
He comments that ferritic stainless steels tend to be used in shafts for pumps, steam and water turbines, nuts, bolts, hot water heaters, and in the pulp and food processing industries. Martensitic stainless steels can be hardened and are, as a result, used for edges in cutlery steel, razor blades and surgical instruments.
He says that austenitic stainless steels are the most commonly used material classified as ISO M. Austenitic stainless steels account for 70% of all machined stainless steel. “This is primarily because of their very good resistance to corrosion.”
“Austenite stainless steel produces tough, long, continuous chips that are difficult to break. It is prone to work hardening during machining, producing hard surfaces and chips which lead to notch wear,” he explains. Payling adds that it also creates adhesion and produces built-up edges, resulting in poor surface finish and reducing tool life.
The mixed base structure containing both ferrite and austenite is known as a duplex stainless steel. Duplex stainless steels are used in the chemicals, food, construction, medical, cellulose and paper making industries, and in processes that include acids or chlorine. They are often used in equipment related to off- shore oil and gas applications.
Payling says that machinability of duplex is generally poor, owing to high yield point and high tensile strength. Although he says that duplex steels with a higher ferrite content – more than 60% – improves machinability.