Stainless steel is a marvel of modern metallurgy, prized for its exceptional corrosion resistance. We categorize it into four main types based on its internal structure: martensitic, ferritic, austenitic, and austenitic-ferritic. However, this robust material comes with a trade-off: poor machinability. Its relative machinability index typically falls between 0.3 and 0.5. Austenitic stainless steel is particularly challenging to work with. While each type of stainless steel presents unique machining characteristics, they all share common difficulties. Let’s delve into these challenges and explore effective strategies to improve their machinability.
Machinability Challenges of Stainless Steel
Working with stainless steel presents several common hurdles:
Severe Work Hardening
Austenitic and austenitic-ferritic stainless steels are prone to significant work hardening during machining. This phenomenon dramatically increases their yield limit. This, in turn, boosts the cutting force required. The result is accelerated tool wear, increased friction between the tool and workpiece, and a noticeable decline in the machined surface quality. Furthermore, the hardened layer penetrates deeper than in regular steel. This complicates subsequent cuts and increases tool wear.
High Cutting Forces
Stainless steel, especially austenitic types, requires a significantly higher cutting force. This is due to its high plasticity, severe work hardening, and high thermal strength. For instance, turning ICr18Ni9Ti stainless steel demands about 25% more unit cutting force than 45 steel.
Excessive Heat Generation and Poor Dissipation
Machining stainless steel generates a substantial amount of cutting heat. This happens because of large plastic deformation and high friction with the tool. Compounding this issue, stainless steel has low thermal conductivity (e.g., ICr18Ni9Ti is only one-third that of 45 steel). This leads to poor heat dissipation. As a result, cutting temperatures are very high, about 200-300°C hotter than when machining 45 steel under similar conditions.
Rapid Tool Wear
Hard carbide spots (like TC) in stainless steel intensify the mechanical wear on cutting tools. Moreover, the combination of high cutting temperatures and intense pressure between chips and the tool promotes diffusion wear and adhesive wear of the tool.
Adhesion and Built-Up Edge Formation
Stainless steel has high plasticity and strong adhesion. This is especially true for low-carbon variants like austenitic 1Cr18Ni9Ti and martensitic 1Cr13. These properties make it prone to forming a built-up edge on the tool. This negatively impacts the machined surface quality. It also makes it difficult to achieve a smooth finish.
Difficult Chip Control
Stainless steel chips are not easily broken during cutting. This is because of its high plasticity, toughness, and elevated high-temperature strength. Effective chip breaking and removal is a critical challenge for smooth stainless steel machining.
Dimensional Instability
Stainless steel has a large coefficient of linear expansion. During cutting, the workpiece can easily deform due to the influence of the cutting temperature. This makes dimensional accuracy difficult to maintain.
Strategies to Improve Stainless Steel Machinability
Overcoming these challenges requires a comprehensive approach. We combine the right tools, techniques, and preparation:
Optimize Tool Selection
Tool Material: For high-speed steel tools, opt for high-performance grades containing cobalt or aluminum. When using cemented carbide, YG cemented carbide (like YG8, YG6) is preferred. It offers enhanced toughness and thermal conductivity, which also helps reduce adhesion. While YT cemented carbide can be suitable for specific finishing operations, we generally do not recommend it for titanium-containing stainless steel.
Reasonable Selection of Tool Geometry Parameters
Rake Angle (γ): Use a larger rake angle, generally between 12° and 30°. This depends on the tool type and cutting conditions. For low-hardness martensitic stainless steel (2Cr13), a larger rake angle is beneficial. For austenitic-ferritic stainless steel, we advise a smaller rake angle.
Negative Chamfer: The negative chamfer (b) should not be too wide. We typically set it between 0.5 and 1.0 times the feed rate (f).
Relief Angle (α): The size of the relief angle primarily relates to the cutting depth. For small cutting thicknesses, choose a larger relief angle. For stainless steel turning tools, the relief angle is generally between 6° and 12°.
To enhance the strength of the cutter head, the blade inclination angle should be between -2° and -6°. For intermittent cutting, this angle can range from -5° to -15°.
Tool Blunting Standard: The allowable tool wear standard (VB) is half that of general tools.
Surface Roughness: Both the front and rear cutting edges of the tool require a lower surface roughness.
Chip Breaker Groove: To improve chip curling and breaking, we generally recommend an outward-inclined full-arc chip breaker groove. Its parameters are crucial. You should carefully select them based on specific data.
Select Appropriate Cutting Parameters
Cutting Speed (v): To reduce cutting temperature, the turning speed for stainless steel should be 40% to 60% of that used for ordinary carbon steel. The precise adjustment will depend on the specific type of stainless steel. For boring and cutting operations, the cutting speed may need further reduction due to specific conditions.
Feed Rate (f): The feed rate should not be excessively large. This avoids an overload on the cutting tool. Since residual area height and built-up edge height increase with feed rate, you should select a smaller feed rate during finish turning to improve surface quality.
Cutting Depth
The cutting depth should not be too small. This prevents cutting into the work-hardened layer or the previous process’s outer skin of the blank.
Optimize Cutting Fluid and Cooling Methods
Cutting Fluid Selection: When machining stainless steel, you need superior cooling, lubrication, and penetration from the cutting fluid. This is due to poor thermal conductivity, large cutting layer deformation, high cutting temperatures, and easy tool adhesion. Therefore, select cutting fluids like emulsions containing extreme pressure additives (such as S and Cl), sulfurized oil, carbon tetrachloride, or mixtures of kerosene and oleic acid.
Cooling Method: You should precisely aim the cutting fluid nozzle at the cutting area. Alternatively, consider using high-pressure cooling, spray cooling, or other advanced cooling techniques.
Appropriate Workpiece Heat Treatment
Martensitic Stainless Steel: Tempering treatment can significantly improve its machinability. When you control the hardness between 28 and 35 HRC, it exhibits good machinability.
Austenitic Stainless Steel: High-temperature annealing before machining can make the chips more brittle, thereby improving its machinability.
Conclusion
Stainless steel’s unique microstructure, particularly in austenitic types, makes it inherently difficult to machine. This is due to severe work hardening, high cutting forces, and poor heat dissipation. However, by strategically optimizing tool materials and geometry, precisely controlling cutting parameters, employing efficient cutting fluids, and performing appropriate heat treatments, we can significantly enhance its machinability. This multi-faceted approach is key to achieving successful and efficient stainless steel machining.