316 stainless steel is an austenitic stainless steel alloy that contains molybdenum. The addition of molybdenum gives 316 stainless steel excellent corrosion resistance and strength, making it suitable for use in harsh environments. 316 stainless is commonly used in applications like marine components, pharmaceutical and chemical equipment, valves and pumps, and food processing equipment. When it comes to machining 316 stainless steel, it can present some challenges due to its toughness and work hardening behavior. However, with the right techniques and process considerations, 316 stainless steel can be machined effectively. In this article, we’ll take a look at the best practices for machining 316 stainless steel.
Properties of 316 Stainless Steel
Let’s first review some of the key properties of 316 stainless steel: High corrosion resistance – With the addition of 2-3% molybdenum, 316 stainless has excellent resistance to pitting and crevice corrosion in chloride environments. Good mechanical properties – 316 stainless has high tensile and yield strength, good ductility and impact toughness even in sub-zero temperatures. Austenitic structure – The austenitic structure of 316 stainless gives it good formability and weldability. However, it also contributes to work hardening during machining. Non-magnetic – The austenitic structure makes 316 stainless non-magnetic. This can be advantageous for applications where paramagnetism is undesirable. Heat treatable – 316 stainless can be hardened through cold working. However, it cannot be hardened through heat treatment.
Challenges in Machining 316 Stainless Steel
The properties of 316 stainless steel that give it excellent corrosion resistance also pose some difficulties during machining. Here are some of the major challenges: Work hardening – The austenitic structure promotes work hardening, causing the material to strengthen and become harder during machining. This accelerates tool wear. Toughness – The high tensile strength of 316 makes it difficult to cut and machine smoothly. Thicker sections are especially problematic. Built-up edges – The ductility of 316 can cause built-up edges to form on the cutting tool, affecting surface finish and dimensional accuracy. Long stringy chips – 316 produces long stringy chips during machining, which are hazardous and hard to control. High heat generation – Frictional heat generated during machining can be very high, accelerating tool wear.
Tool Selection
Choosing the right cutting tools is critical for effective machining of 316 stainless steel: Coated carbide tools – Carbide cutting tools with titanium carbon nitride (TiCN) or titanium aluminum nitride (TiAlN) coatings perform best. The coating reduces heat buildup and improves lubricity. Positive rake cutting geometry – Using tools with a positive rake angle like 5-15° reduces cutting forces and built-up edge formation. High helix angle end mills – End mills with a high helix angle like 45-60° promote efficient chip ejection for better surface finish. Polished flutes – For roughing cuts, end mills with polished flutes help minimize chip adhesion and reduce chatter.
316 Stainless Steel Sheet Metal Fabrication tools – For sheet metal cutting and bending, use carbide-tipped shears and tool steel punch/die sets with the appropriate V-angle and edge radius.
Turning 316 Stainless Steel
For turning operations like facing, boring, grooving and threading 316 stainless steel, the following tips can help: Rigid setup – Use a rigid machine setup with sturdy chucks to minimize vibration. Precision-ground chuck jaws help gripping. High pressure coolant – High pressure coolant (1000 psi or higher) is extremely beneficial for 316 turning to break chips, improve surface finishes and reduce heat buildup. Slower speeds and feeds – Compared to carbon steel, start with 10-30% lower cutting speeds and feeds when turning 316. This reduces forces and work hardening. Shallow depths of cut – Take light 0.010″-0.020″ roughing cuts and 0.005″ or lower finishing cuts to control work hardening. Positive cutting geometry inserts- Use coated carbide inserts with positive cutting geometries like CNGG, DNMG, TNMG. A 5-15° positive rake angle helps curl chips. Low friction toolholder coatings – Toolholders coated with titanium nitride or diamond-like carbon minimize built-up edge and reduce cutting forces.
Milling 316 Stainless Steel
When milling parts made of 316 stainless steel, pay attention to the following recommendations: Climb milling – Utilize climb milling with a small amount of lead angle. This produces the best surface finish by reducing work hardening. Slow feed rates – Use lower feed rates between 0.004-0.012″ per tooth, taking light 2-5% radial depth cuts and axial depths less than tool diameter. carbide end mills – Carbide end mills with TiAlN coating enable high cutting speeds like 100-130 m/min and offer longevity. For slotting, use coated solid carbide end mills. High tension toolholding – Heat-shrink toolholders provide maximum rigidity and reduce tool chatter. For removing large amounts of material, choose hydraulic or shrink-fit holders. Copious coolant – High pressure, high volume coolant is vital for milling 316 stainless. Use 1000 psi pressure through the spindle if possible, and flood the work area to reduce heat and chip welding.
Drilling 316 Stainless Steel
Drilling holes in 316 stainless steel presents difficulties like rapid tool wear, hole chatter and poor surface finish. Here are some tips for drilling 316 stainless steel: Reduced feed rates – Start with feed rates around 0.001-0.002″ per revolution of drill diameter. Increase conservatively up to 0.004″ per revolution as drill diameter increases. High helix drills – For less thrust force and torque, choose drills with a high helix angle over 30°. Combined with high pressure coolant, this enables faster hole making without chatter. Extreme point drills – Extreme point angle drills like the Split Point produce smaller chisel edges for lower thrust and torque. This results in smoother, high quality holes. Coolant-through capability – For drills larger than 6mm diameter, use coolant-through drills to get internal cooling and chip flushing when deep drilling. Peck drilling – Peck drilling helps break chips and reduce work hardening when drilling deep holes. A peck depth around 0.020″ works well. Dwell time should be 1-2 seconds between pecks.
Grinding 316 Stainless Steel
While grinding is not commonly used on 316 stainless steel, here are some recommendations if needed: Silicon carbide wheels – For grinding 316 stainless, use harder silicon carbide grinding wheels. Softer aluminum oxide wheels wear rapidly. Coarse grit wheels – Wheels with coarse grit sizes like 36-60 are most suitable for rapid stock removal from 316 stainless steel. Shallow cuts – Make light grinding passes of 0.001″-0.002″ depth to control heat generation and minimize surface burn. Coolant direction – Direct copious amounts of grinding coolant into the grinding contact zone for maximum chip flushing and cooling. Dressing periodically – Dress grinding wheels periodically to expose fresh cutting grains. This promotes cutting efficiency and reduces loading.
Finishing 316 Stainless Steel
Finally, properly finishing the machined 316 stainless components is vital for looks and function: Passivation – Passivation removes free iron and improves corrosion resistance. Citric or nitric acid passivation is common for 316 stainless parts. Electropolishing – Electropolishing is an excellent final finishing method to smooth surfaces and deburr parts. It also enhances corrosion resistance. Media blasting – For a uniform satin sheen, media blast components with clean stainless steel media. Alternatively, glass beads provide a brighter finish.
