What is stainless steel? How it is made? What is the best grade of stainless steel for my project? Here’s a primer on what you need to know when it comes to selecting and pricing the right stainless steel.
What is stainless steel?
Stainless steel is an alloy that is made from two or more separate elements alloyed together. Like with all steel, the primary alloying element of stainless steel is the metal iron (with a small amount of added carbon). The addition of metal chromium to stainless steel is what gives it a high level of rust-resisting properties, This helps stainless steel become highly resistant to corrosion; an important factor due in large part to the varying types of environments in which the end products are used.
The chromium also increases tensile strength, hardness, hardenability, toughness, and resistance to wear.
How is stainless steel made?
All stainless steels are iron-based alloys containing at least 10.5% chromium. Beyond chromium, the rest of the makeup of each grade of stainless steel is defined by various alloying elements, which control the microstructure of the alloy.
What are the different types of stainless steel available?
Every grade of stainless steel falls under one of five families:
5. Precipitation Hardening
Austenitic stainless steel represents roughly 50% of the stainless steel used today. In fact, two of the most widely used stainless steel grades fall within this family: 304 stainless steel and 316 stainless steel. This family of stainless steel exhibits high levels of corrosion resistance, excellent weldability, and good formability.
Martensitic stainless steel grades contain moderate levels of corrosion resistance, high hardenability and machinability, poor formability and can be welded with caution. They can be heat-treated to provide a range of mechanical properties.
Ferritic stainless steel grades are high in chromium and are known for good ductility, resistance to corrosion and stress corrosion cracking. Ferritic stainless steel is commonly used in automotive applications, kitchenware, and industrial equipment.
Precipitation hardening is a heat treatment technique used to increase the yield strength of malleable materials, so it is extremely strong and resistant to corrosion. Duplex stainless steel is extremely corrosion resistant and work-hardenable alloys.
What is the difference between stainless steel 304 and 316?
This question is the common question when it comes to choosing a grade of stainless steel. Among the most commonly used stainless steel across plate, sheet, bar, and tube, these two grades are similar, but with slight variations:
- 304 stainless steel is the most widely used grade, most notably present in industrial applications and kitchen equipment. It is a highly heat-resistant stainless steel grade and offers good corrosion resistance to many chemical corrodents, as well as industrial atmospheres. With good formability, 304 can be readily welded by all common methods.
- 304L is an extra low carbon variation that avoids harmful carbide precipitation due to welding. This variation offers the same corrosion resistance as 304, but with slightly lower mechanical properties.
- When it comes to 316 stainless steel, you can expect better corrosion and pitting resistance to chlorides than 304 stainless steel and higher levels of strength at elevated temperatures. This is due in part to the addition of molybdenum. This is what makes it an ideal stainless steel grade for such applications as pumps, valves, textile and chemical equipment, pulp & paper and marine applications. 316L is the extra-low carbon version of 316 stainless steel that helps avoid carbide precipitation due to welding.
Now to the question of 304 vs. 316; while both are good for corrosion resistance, strength, welding, and heat resistance, 316 gets the slight edge in all these categories due to variation in alloying elements.
What are the stainless steel grades?
Stainless steel comes in a variety of shapes:
- Tread plate
- Expanded metal
Beyond 304 and 316 stainless steel, other more commonly used grades include:
- 410 stainless steel is a heat-treatable grade is ideally suited for use in environments where corrosion is not severe. This includes air, fresh water and with some chemicals and food acids. A quenched and double tempered variation is used for parts in hydrogen sulfide service.
- 409 stainless steel exhibits good resistance to oxidation and corrosion and creates opportunities to economically improve the performance of a wide range of parts where surface appearance is not important. Great formability and weldability allow this to be used in many applications, such as automotive exhaust systems.
- 17-4/Type 630 stainless steel is a grade that is commonly used on helicopter deck platforms, the paper industry, turbine blades, and nuclear waste casks. 17-4 stainless steel falls under the precipitation hardening family, combining high strength and hardness with corrosion resistance. Low-temperature heat treatment eliminates scaling and prevents excessive warpage.
- 17-4 Double Aged H1150 stainless steel, which exhibits high strength and excellent corrosion resistance makes this alloy extremely versatile. You will most commonly find this grade used in air spray guns, golf club heads, propeller shafts, suppressors, and silencers.
- 201 stainless steel was originally developed for sub-zero temperature applications. Through the years, it has also become a fit for a range of structural applications at ambient temperatures. This includes truck trailers, railroad freight cars, coal handling and other transportation equipment where good corrosion resistance, strength, and tolerance are necessary. 201 stainless steel sheet is formulated to have lower and more stable cost due to the substitution of lower-cost manganese and nitrogen for a portion of the nickel found in the 300 series alloys. This grade possesses a desirable combination of economy, plus good mechanical and corrosion properties.
- 430 stainless steel is part of the ferritic family of stainless steel and used in industrial and consumer products like fridge panels, chimney liners, dishwasher linings, and automotive trim. This grade combines good corrosion and heat resistance with good mechanical properties. It also has excellent stress corrosion cracking resistance as well as resistance to organic acids and nitric acid.
What factors affect the price of stainless steel?
Two factors affect the price of stainless steel: the mill and the surcharge.
The calculation of the base price for stainless steel is as follows:
- The price the mill charges to convert raw material to coil or plate plus the mill margin = the base price
It is important to note that multiple factors are taken into account including melting and refining times, casting, rolling, reductions, polishing, annealing, pickling, and more.
Overall, the price of stainless steel grades can fluctuate based on the ebb and flow of the raw material cost. To help offset these fluctuations, surcharges are added to the base price, typically per pound and varied by the mill. The price of these surcharges can vary daily, taking a collection of factors into consideration:
What are the different finishes of stainless steel?
From mill finishes to applied finishes, the stainless steel finishes that are applied to the surface of your stainless steel can play a bigger role than you may realize. While some products may only require standard stainless steel finishes, others may need something extra in order to enhance surface appearances, improve environmental performance or even reduce safety concerns.
Whether we are talking hot rolled or cold rolled, all stainless steel flat products are treated with a standard mill finish. Let’s review two of the more common mill finishes:
• 2B: This is achieved by cold rolling, heat treating and pickling, along with the application of a light rolling at the end in order to achieve a smooth and reflective sheen. Considered the most widely used surface finish, 2B is the basis for most polished and brushed finishes. Such common stainless steel grades as 201, 304, 304L and 316L come in a standard 2B finish.
• 2D This finish is also achieved by cold rolling, heat treating and pickling, but, unlike 2B, does not receive the light rolling. This achieves a low reflective matte surface appearance that is ideal for industrial and engineering applications yet has different aesthetics than 2B.
Applied finishes may further enhance the surface appearance and environmental performance of your product. This requires physical alteration of the metal, most notably through the use of abrasive material that removes portions from the surface.
The higher the abrasive grit used, the finer and more reflective the look. For example, one common applied finish is #4, which you will find on your average restaurant or kitchen equipment, food processing or dairy equipment, and elsewhere. This finish uses between 120 and 320 abrasive grit, depending on the application. For comparison, applied finish #3 uses between 100-120. The level of the grit impacts the surface roughness, which is typically measured in terms of micro-inches or micro meters, represented as Ra (roughness average). The higher the grit numbers, the finer the polishing lines and more reflective the finish.
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