Specifying 316L And Other Medical-Grade Stainless Steels25th Sep 2020
Do You Need 316L? Would Another Grade Be Better?
316L is usually the first choice for medical device manufacturing but it is not the only stainless steel that is available for healthcare applications.
Other grades of medical stainless steel may be better suited for your purposes and/or more cost-effective.
But let’s start with 316L. Here is why it is the ‘go-to’ stainless steel for most medical applications…
Specify 316L Surgical Steel For Body-Contact Applications
You should consider specifying 316L surgical steel if your product is going to touch any part of the human body (though other grades may be possible too). Its metallurgical composition makes it a low-allergy steel. The ‘L’ refers to its low carbon content, making it extra resistant to corrosion.
AISI 316L (WNR 1.4404/UNS S31603) is an Austenitic stainless steel; it is non-magnetic so it will not interfere with sensitive instrumentation. The chemical composition is:
- Carbon (C) – 0 to 0.035% (for hardness – though it will make the steel less ductile)
- Chromium (Cr) – 16-18% (toughens the steel and increases corrosion resistance, especially at higher temperatures)
- Manganese (Mn) – 0 to 2% (improves strength, hardenability and hot working properties)
- Molybdenum (Mo) – 2-3% (adds corrosion resistance and strength at high temperatures)
- Nickel (Ni) – 10-15% (for superior formability, weldability and ductility)
- Phosphorus (P) – 0-0.045% (improves machinability and strength)
- Silicon (Si) – 0-1% (a ferrite stabiliser that improves oxidation resistance)
- Sulphur (S) – 0-0.03% (improves machinability but lowers transverse ductility and notched impact toughness; has little effect on longitudinal mechanical properties).
316L offers excellent formability (making tight bends possible) and – despite its excellent properties for medical manufacturing – it is not the most expensive stainless steel.
But it is significantly more expensive than other grades such as 303 or 304. So you may be able to reduce costs by specifying another grade if your medical application will not involve bodily contact.
Here’s how the prices compare for a 25 mm x 1,000 mm section of stainless steel round bar:
- 17-4PH/AMS5659/H1025 – £67.00
- 15-5PH/AMS5643/H1150 – £65.00
- 440 – £63.00
- 316/316L – £23.00
- 304 – £14.50
- 303 – £13.50.
316L is also more difficult to machine – more with regard to milling than turning. That means more time spent on each component, increasing labour costs. For guidance, 316L can take slightly longer to machine than 303 or 304 and at least twice as long to machine as aluminium.
But 316L is a key speciality for Penta. Customers come to us for stainless steel machining when they have encountered quality, consistency and delivery difficulties with their existing suppliers.
316/316L are readily available in round bar, sheet and plate. Some sizes are available in square or rectangular wrought bar, along with tube, angle and channel sections.
303 vs 304/304L Stainless Steel
AISA 303 (WNR 1.4305/UNS S30300) and AISI 304 (WNR 1.4301/UNS S30400) are also Austenitic and so offer the same advantages as 316L but have a hint of magnetism.
You would not specify them for body-contact applications but they would be suitable for components such as frames, bases, brackets, bodies or guards. They offer excellent corrosion resistance for indoor applications. But they may – over time – show signs of tarnish or pitting in outdoor applications or in environments with aggressive cleaning regimes.
303 has a higher carbon (0-0.15%) content than 316L (C: 0 to 0.035%) and less molybdenum (0-1%), making it less resistant to corrosion than 316L. It is also likely to have less nickel (8.00-10.00%) which is added to stainless steels to make them more ductile and formable.
303 remains the most readily machinable stainless steel because of its sulphur content. It is excellent for moving parts. 303 is available at Penta only in round bar diameters.
304 is the most utilised stainless steel in the world. It has a minimum 18% chromium content and a minimum 8% nickel content. This is the standard ‘18/8’ that you find in kitchen sinks, cookery pans, utensils and cutlery because it can withstand the acids in fruit and other foods.
AISI 304L (WNR 1.4306 or 1.4307/UNS S30403) offers better corrosion resistance than 304 because it has a lower maximum carbon content:
- 304 – 0.08%
- 304L – 0.03%.
Like 316 grade, 304/304L are readily available in round bar, sheet and plate. Some limited sizes are available in square or rectangular wrought bar (along with tube, angle and channel sections).
440B vs 440C Stainless Steel
AISI 440B (WNR 1.4112/UNS S44003) and AISI 440C (WNR 1.4125/UNS S44004) are Martensitic so they can be heat treated and hardened but have less resistance to chemicals than Austenitic stainless steels.
440 is often referred to as ‘razor blade steel’. It is used for dental and surgical instruments, heavier duty cutting blades, punches, dies and other tooling applications. Its high resistance to heat and corrosion also make it a good choice for applications such as bearings and valve seats.
But 440 has a high carbon content compared with 316L (0 to 0.035%). The B and C in 440’s AISI number refer to the amount of carbon in each steel:
• 440B – 0.75 to 0.95%
• 440C – 0.95 to 1.20%.
This high carbon content increases hardness (but reduces corrosion resistance). And this toughness can make 440 more difficult to machine (though it can be good to machine in its annealed form).
Heat-treatable steels such as 440 take longer to machine because of their hardening abilities. When heat treatment and machining to fine tolerances are part of the application, care must be taken with shrinkage allowances to ensure limits are maintained with consistency.
440B and 440C are commonly stocked in round bar and plate.
420 Stainless Steel
AISI 420 (WNR 1.4034/UNS S42000) is a medium-carbon Martensitic stainless steel widely used for medical, surgical and dental instruments, knives, cutlery, hand tools, pump shafts and plastic moulds. It is adapted for mirror polishing.
420 provides good strength and reasonable impact resistance. Its corrosion resistance is lower than the common Austenitic grades. 420’s (anti-corrosion) chromium content is 13% – not as high as 316L (16-18%). 420 it is magnetic in both its annealed and hardened conditions.
It can be supplied pre-machined to reduce machining time. 420 is a good stainless steel to machine thanks to the addition of sulphur (up to 0.03%). It is usually stocked in round bar, sheet and plate.
17-4/17-4 PH vs 15-5/15-5 PH Stainless Steel
Both types of stainless steel are annealed and then age-hardened at a single low temperature to reach different temper conditions. As this treatment is carried out at a low temperature, no distortion occurs and there is only superficial discolouration. Several temper conditions are stocked to suit the required properties for differing component applications. The grades are readily available only in solid round bar format.
17-4 and 17-4 PH (AISI 630/WNR 1.4542/UNS S17400) are Martensitic stainless steels. They are hardenable, offer high wear resistance and good anti-corrosion properties. However, they are magnetic: 17-4 PH more so than 17-4 because it contains more ferrite.
17-4 PH is the most widely used of all the stainless steels that have been precipitation-hardened to increase their strength.
This stainless steel has a higher tensile strength and impact toughness, making it a key alloy for a wide number of aerospace, nuclear, oil and gas industry applications. It is a good choice for critical components that have to support heavy loads or withstand other strong forces.
15-5/15-5 PH (WNR 1.4545/UNS S15500) is suitable for intricate parts (requiring machining) where distortion in conventional heat treatment presents challenges.
Like 17-4 grade, 15-5’s high strength and corrosion resistance make it the stainless steel of choice for aerospace applications such as landing gear components. Demands in the nuclear, oil and gas sectors also make it a common choice. Other applications include valve parts, gears, shafts, fittings and fasteners.
15-5 is precipitation-hardened but is tougher than 17-4. Specify 15-5 for applications that require higher corrosion resistance and better transverse strength properties than other Martensitic stainless steels.
Machining Stainless Steels For Medical Equipment
Machining stainless steels to the tolerances, quality levels and consistency required by medical device OEMs requires high levels of expertise and experience. This is usually why customers call Penta – to solve issues that have arisen with other suppliers.
Tool wear is usually higher when machining stainless steels (especially the hardened grades). Milling tends to be more challenging than turning – and tool ‘push off’ is a greater factor in the machining process. Good inspection practices are needed to ensure consistent quality outcomes.
Get Penta Involved
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- We communicate well
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