BALANCING KEY FACTORS IN STAINLESS STEEL MACHINING
Stainless steel is a versatile workpiece material that sees wide use where strength and resistance to heat and corrosion are essential. However, the same properties that make stainless steel alloys exceptional structural materials also complicate the processes employed to machine them into functional parts. A carefully considered combination of – or balance between – cutting tool properties and geometries and the application of aggressive cutting parameters can significantly boost productivity in stainless steel machining operations.INTRODUCTION
Stainless steel is a versatile workpiece material that sees wide use where strength and resistance to heat and corrosion are essential. However, the same properties that make stainless steel alloys exceptional structural materials also complicate the processes employed to machine them into functional parts. A carefully considered combination of – or balance between – cutting tool properties and geometries and the application of aggressive cutting parameters can significantly boost productivity in stainless steel machining operations.
ALLOY EVOLUTION
Basic stainless steel alloys are categorised as ferritic or martensitic. Ferritic alloys have 10-12 percent chromium and are not hardenable. Martensitic alloys have higher chromium and carbon content than ferritic stainless steels, as well as additions of manganese and silicon, producing an alloy that can be hardened via thermal treatment. Today, ferritic and martensitic stainless alloys are not generally used a lot in industrial environments but rather in household items such as kitchen or garden tools.
As the utilisation of stainless steel evolved, the alloys were frequently applied in situations that required mechanical strength as well as corrosion resistance. To improve the alloys’ strength, metallurgists added nickel to the alloys. Iron/chromium alloys became iron/chromium/nickel alloys. These materials are referred to as austenitic stainless steels, and they are common in industrial applications today where strength and resistance to corrosion and heat are needed. The alloys typically are used in petrochemical processing, in the food industry where hygiene standards require corrosion resistance and in general machinery intended for use in harsh environments.
Inevitably, increasing the performance capabilities of an alloy, such as stainless steel, also multiplies the challenges of machining it. The corrosion-resisting characteristics of ferritic and martensitic stainless steel alloys are basically chemical properties, and as a result these alloys are not significantly more difficult to machine than plain steels. However, the additions of nickel and other elements in austenitic stainless steels produce increased hardness, toughness, deformation resistance and thermal properties that decrease machinability.
UNDERSTANDING ALLOYS
Until recently, machining of austenitic stainless steel was not well understood. Machinists assumed that because the alloys were stronger, mechanical cutting forces would be higher and that it would be necessary to apply stronger, negative-geometry tools at reduced cutting parameters. However, that approach produced short tool life, long chips, frequent burrs, unsatisfactory surface roughness and unwanted vibrations.
In reality, the mechanical cutting forces involved in cutting austenitic stainless steel aren’t much higher than those typical when machining traditional steels. Most of the extra energy consumption required to machine austenitic stainless steels is the result of their thermal properties. Metal cutting is a deformation process, and when deformation-resistant austenitic stainless steel is machined, the operation generates excessive heat.
Evacuating that heat from the cutting zone is of primary importance. Unfortunately, in addition to being resistant to deformation, austenitic stainless steel also has low thermal conductivity. Chips created when machining plain steels absorb and carry away heat, but austenitic stainless steel chips absorb heat to only a limited extent. And because the workpiece itself has poor thermal conductivity, the excess heat goes into the cutting tool, leading to short tool life.
Toolmakers engineer carbide substrates to provide hot hardness sufficient to withstand the elevated temperatures generated when machining stainless steel. At the same time, at least equal in importance to the composition of the substrate is the sharpness of the tool’s cutting edge. A sharper tool cuts the stainless steel more than deforming it, and thereby reduces the generation of heat.
AGGRESSIVE CUTTING PARAMETERS
In the interest of evacuating heat from the cutting zone, the most effective way to machine stainless steel is to employ the largest depths of cut and feed rates possible. The goal is to maximize the amount of heat carried away in the chip. Because the poor thermal conductivity of stainless steel limits the amount of heat that can be absorbed by each cubic millimeter of chip material, creating larger chips with more cubic millimeters of volume will carry away more heat. Employing larger depths of cut will also reduce the number of cutting passes required to complete a part, an important consideration because austenitic stainless steel exhibits tendencies to strain- or work harden when machined.
There are practical limitations to these aggressive machining tactics. Surface finish requirements, for example, will limit the maximum feed rate. The power available from the machine tool, as well as the strength of the cutting tool and the workpiece, also impose limitations on the aggressiveness of the parameters that can be employed.
COOLANT STRATEGIES
The problematic thermal properties of austenitic stainless steel alloys suggest that application of coolant is nearly always crucial for success when machining them. The coolant must be of high quality, with at least eight or nine percent oil content in an oil/water emulsion, compared to the three or four percent oil content typical for many machining operations.
The manner in which the coolant is applied is important as well. The higher the pressure at which coolant is delivered to the cutting zone, the better it will do its job. Specialised delivery systems such as Seco Jetstream Tooling® that delivers a high-pressure stream of coolant directly to the cutting zone are even more effective.
TOOL COATINGS VERSUS WEAR PROCESSES
A hard coating deposited on the surface of the tool substrate reinforces hot hardness at the surface of the tool and improves tool life in high-temperature environments. However, a coating generally must be thick to insulate the tool substrate from heat, and a thick coating will not adhere well to a very sharp geometry. Cutting tool makers are working to engineer coatings that are thin but provide a good barrier against heat.
Austenitic stainless steels exhibit high ductility and a tendency to adhere to the cutting tool. Application of a coating can also inhibit adhesion wear, a condition that occurs when the cut material sticks to and builds up on the cutting edge. The adhered workpiece material may then pull away sections of the cutting edge, leading to poor surface finish and tool failure. The coating can provide lubricity that limits adhesion wear; higher cutting speeds also serve to minimize the adhesion wear mechanism.
Some austenitic stainless steel alloys contain abrasive hard inclusions, and increasing a cutting tool’s abrasion resistance with a hard coating can benefit tool life.
Notch wear results from an alloy’s tendencies toward strain or workhardening when machined. Notch wear can be described as very localised extreme friction wear, and it can be mitigated by application of appropriate coatings and other actions such as varying the depth of cut to spread the wear areas across the cutting edge.
TOOL DEVELOPMENT
Toolmakers focus ongoing cutting tool development efforts on finding a balance between tool properties that will provide optimum performance in specific workpiece materials. Carbide grade research seeks a balance between hardness and toughness so a tool is not so hard that it fractures but is hard enough to resist deformation. Similarly, a sharp cutting edge geometry is preferred but is not as mechanically strong as a rounded edge. Consequently, edge geometry development is aimed at creating tools that balance sharpness with as much strength as possible.
As part of the development process, toolmakers are revisiting their tool application guidelines. Current machining parameter recommendations are based, for the most part, on toughness and hardness characteristics of traditional steels, without consideration of the thermal factors that are so important when machining austenitic stainless steels and other high-performance alloys. Recently, toolmakers have begun working with academic institutions to revise tool testing procedures to take into account certain materials’ thermal characteristics.
The new guidelines reflect the creation of new reference materials. Traditionally, machinability standards were set according to one reference material, an alloyed steel, and based on mechanical loads produced during machining. Now there is a separate reference material for austenitic stainless steels for which baseline values for speed, feed and depth of cut have been established. Relative to that reference material, balancing or calibrating factors are applied to determine changes in the base values that will achieve optimum productivity in materials with different machining characteristics.
SPECIFIC GEOMETIRES FOR SPECIFIC MATERIALS
Many cutting tools provide very acceptable performance in a variety of materials under a wide range of cutting conditions and machining parameters. For one-time jobs with moderate productivity and quality requirements, these tools can be a cost-effective choice. To achieve maximum performance, however, toolmakers continually manipulate and balance a wide variety of tool elements to create cutting tools that provide top productivity and process reliability in specific workpiece materials.
The basic elements of a tool include its substrate, coating and geometry. Each is important, and in the best tools they operate as a system that produces results beyond the sum of the separate parts.
There are distinctions among the roles the tool’s parts play. The substrate and coating have passive roles; they are engineered to provide a balance of hardness and toughness, to withstand high temperatures, and to resist chemical, adhesion, and abrasive wear. The tool geometry, on the other hand, plays an active role because altering the geometry can change amount of metal that can be removed in a certain timeframe, the amount of heat that is generated, how chips form and what surface finish can be achieved.
Basic examples of performance-altering geometry differences include traditional turning geometry inserts from Seco called e.g. M3 and M5 that feature negative (0˚ clearance angle) cutting edge geometries and T-lands between the cutting edge and the tool rake face. The M3 geometry is a versatile medium-rough geometry that offers good tool life and chipbreaking in a wide range of workpiece materials. M5 geometries are aimed at demanding, high-feed roughing applications, combining high edge strength with comparatively low cutting forces.
Although versatile, the M3 and M5 geometries are strong, but not fully sharp, and generate a good deal of heat via deformation when machining austenitic stainless steel. In comparison, tool designs that can be more effective in stainless steel machining include the Seco MF4 and MF5 geometries that feature sharp, positive geometries with more narrow, positive T-lands that help maintain sharpness while providing support behind the sharp edge. The geometries are engineered to be open and free-cutting to facilitate for medium to finishing operations on steels and stainless steels. The MF5 geometry is especially effective in high-feed applications.
