It’s pretty likely that if you’re working with sheet metal machines, you’re going to come across stainless steel. Stainless steel is commonly used to construct a variety of structures/vehicles across a range of industries such as construction, aeronautical and automobile manufacturing. It’s not all about the large products though – stainless steel is used for other applications such as mobile phones and cutlery. We feel it’s important for sheet metal fabricators to understand the fundamentals of the types of metal they’re working with which is why we’ll be exploring stainless steel and its characteristics in today’s article.
Putting the stainless in steel
Stainless steel is a corrosion-resistant steel alloy made up of various elements such as iron, chromium and carbon. This resistance is one of stainless steel’s defining properties and greatly contributes to its popularity – chromium being the secret ingredient. When combined with oxygen, it creates a thin layer of oxide film that coats the stainless steel – giving it its resistive properties.
Stainless steel materials must contain at least 10.5% chromium. Additionally, any damage caused by machining, cutting or abrasions is of little concern as the film repairs itself quite promptly thanks to the constant supply of oxygen. The oxide film itself is actually quite thin but works very effectively. Some of the other properties of stainless steel include:
- Fantastic durability;
- Easy formability – making it great for fabrication;
- High tensile strength;
- Long-lasting; and
- Temperature resistance.
The five different types of stainless steel
Stainless steel is actually more of an umbrella term as there are various types and grades of the alloy. The primary five types are:
- Austenitic: The most commonly used type of stainless steel (being used in a range of industrial applications as well as cookware and food processing equipment), Austenitic stainless steel is characterised by containing 18% chromium and 8% nickel.
- Ferritic: This type tends to have between 12% – 18% chromium and has poor weldability and formability compared to austenitic stainless steel. Some examples of uses are automotive exhaust systems and hot water tanks.
- Martensitic: Containing between 12% – 18% chromium and 0.1% – 1.2% carbon – giving it a higher carbon content than other stainless-steel types – martensitic stainless steel is magnetic and has great strength/hardness when it’s been heat-treated. Cookware, cutlery and dental/surgical instruments are a few of the many purposes martensitic stainless steel is used for.
- Precipitation hardening: This type of stainless steel has a very high strength-to-weight ratio and can be classified as either martensitic or semi-austenitic. Pump and valve shafts as well as aerospace components are just a couple of the possible applications for precipitation hardening stainless steel.
- Duplex: Combining both austenite and ferrite, Duplex stainless steel is the best of both worlds with higher strength and ductility. With a fairly high chromium component of anywhere between 18% – 28% and modest amounts of nickel ranging between 4.5% – 8%, duplex stainless steel is used in desalination plants and for marine applications. It possesses fantastic weldability and formability properties.
Recycling stainless steel
Stainless steel is 100% recyclable. What this means is that it can be smelted and reformed to be used in a new application and still retain all of its properties. And, since the metal recycling process expends significantly less energy compared to mining for fresh ore, it actually makes stainless steel a fairly eco-friendly material. What’s more, is that the elements that make up stainless steel are fairly easy to separate during the recycling process.
Stainless steel grades
Stainless steel grades are categorised by five series.
- 200 Series – Austenitic chromium-nickel-manganese alloys.
- 300 series – Austenitic chromium-nickel alloys.
- Type 301 – Good weldability and highly ductile.
- Type 302 – High corrosion resistance and strength thanks to an additional amount of carbon.
- Type 303 – Essentially the same as Type 304 with easier machinability due to added sulphur and phosphorus.
- Type 304 – The most common grade of stainless steel. The same corrosion resistance as Type 302 but not as strong. Also known as A2 per the International Organisation for Standardisation ISO 3506.
- Type 309 – higher temperature resistance than Type 304.
- Type 316 – Second most common grade – regularly used for food and surgical applications.
- Type 321 – Similar to Type 304 but with added titanium for a lower risk of weld decay.
- 400 Series – Ferritic and martensitic chromium alloys.
- Type 408 – Good heat resistance yet poor corrosion resistance.
- Type 409 – Ferritic stainless steel used for automobile exhausts.
- Type 410 – Martensitic stainless steel that has good wear resistance but poor corrosion resistance.
- Type 416 – Contains sulphur making it easy to machine.
- Type 420 – Also known as “cutlery grade” or “surgical grade” that is very easy to polish.
- Type 430 – Decorative stainless steel with reduced temperature and corrosion resistance.
- Type 440 – High-end cutlery steel that contains carbon for better edge retention (when the steel is appropriately heat-treated).
- 500 Series – Heat-resisting chromium alloys.
- 600 Series – Martensitic precipitation hardening alloys.
- Type 630 – The most common precipitation hardening stainless steel.
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