Stainless steel is a type of steel that is widely used in various industries due to its unique properties and characteristics.
It is a steel that contains less than 2% carbon and more than 2% iron, and is made stronger by the addition of chromium, nickel, manganese, silicon, titanium, molybdenum and other alloying elements. This results in a steel with excellent corrosion resistance, meaning it does not rust.
However, not all stainless steel is created equal, as there are different grades of stainless steel, each with unique properties and characteristics. The commonly used steel grades are 304, 304L, 316, and 316L, which are 300 series steels of austenite.
In this article, we will explore the differences between these grades of stainless steel, their applications, and the factors to consider when choosing the most suitable grade for a particular application.
So, whether you are a professional in the chemical, coal, and petroleum industries, or simply a homeowner looking to upgrade your kitchen appliances, this article is for you.
I. What kind of steel is stainless steel?
Stainless steel” is a type of steel that contains less than 2% carbon and more than 2% iron, and is made stronger by the addition of chromium, nickel, manganese, silicon, titanium, molybdenum and other alloying elements. This results in a steel with a corrosion resistance, meaning it does not rust.
“Steel” and “Iron” are different terms, and their properties and characteristics vary. The terms “304,” “304L,” “316,” and “316L” are different grades of stainless steel, each with unique properties and characteristics. It is important to understand these differences to determine the most suitable grade for a particular application.
Steel:
A kind of material that use iron as the main element with carbon content (less than 2%) and other elements.
–GB/T 13304-91 Steel Classification
Iron:
A metallic element with atomic number 26.
Iron materials have strong ferromagnetic properties, excellent plasticity and thermal conductivity.
Stainless steel:
A kind of steel with resistance to air, steam, water and other weakly corrosive media or stainless steel properties.
The commonly used steel grades are 304, 304L, 316, and 316L, which are 300 series steels of austenite.
II. Why does stainless steel have different grades?
In the production of stainless steel, different alloys are added, leading to differences in their characteristics. To differentiate them, they are given different steel numbers. The following table of alloying elements is a common reference for different steel numbers in decorative stainless steel.
Chemical composition (mass fraction, %)
| Steel Grade | C | Si | Mn | P | S | Cr | Ni |
| 304 | ≤0.08 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.03 | 18-20 | 8-10 |
| 301 | ≤0.15 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.03 | 16-18 | 6-8 |
| 202 | ≤0.15 | ≤1.00 | 7.5-10 | ≤0.05 | ≤0.03 | 17-19 | 4-6 |
| 201 | ≤0.15 | ≤1.00 | 5.5-7.5 | ≤0.05 | ≤0.03 | 16-18 | 3.5-5.5 |
– 304 stainless steel
Performance introduction
304 stainless steel is a widely used and common type of steel that has good resistance to corrosion, heat, low temperature strength, and mechanical properties. It is ideal for stamping and bending processes, as it does not undergo heat treatment hardening and remains non-magnetic. It can be used in temperatures between -196°C and 800°C.
Applicable range
304 stainless steel is commonly used in household goods such as class I and II tableware, cupboards, indoor plumbing, water heaters, boilers, and bathtubs. It is also used in automotive parts, such as windshield wipers and mufflers, and in medical devices, building materials, the chemical industry, the food industry, agriculture, and ship components.
– 304L stainless steel – (L is low carbon)
Performance introduction
As a low-carbon steel, 304L has similar corrosion resistance to 304 in its general state. However, after welding or stress-relieving, it has excellent resistance to grain boundary corrosion. It can also maintain good corrosion resistance without heat treatment in a temperature range of 196°C to 800°C.
Applicable range
304L is commonly used in outdoor equipment in the chemical, coal, and petroleum industries that require high resistance to grain boundary corrosion, as well as in heat-resistant parts of building materials and components that are challenging to heat treat.
– 316 stainless steel
Performance introduction
Due to the addition of molybdenum, 316 stainless steel has excellent corrosion resistance, resistance to atmospheric corrosion, and high-temperature strength, making it suitable for use in harsh conditions. Its work hardening properties are also excellent (non-magnetic).
Applicable range
Marine equipment, chemical, dye, paper, oxalic acid, fertilizer and other production equipment; camera, food industry, coastal area facilities, ropes, CD rods, bolts, nuts.
– 316L stainless steel – (L is low carbon)
Performance introduction
As a low-carbon series of 316 stainless steel, it has the same characteristics as 316, but has excellent resistance to grain boundary corrosion.
Applicable range
Products with special requirements for resistance to grain boundary corrosion.
III. Performance Comparison
Chemical composition
Due to the presence of molybdenum, 316 and 316L stainless steels exhibit excellent corrosion resistance and high-temperature strength. With its superior performance compared to 310 and 304 stainless steels, 316 stainless steel is widely used in harsh conditions, including high temperatures and sulfuric acid concentrations between 15% and 85%.
Additionally, its resistance to chloride attack makes it a popular choice for marine environments. With a maximum carbon content of 0.03, 316L stainless steel is ideal for applications that require no post-weld annealing and maximum corrosion resistance.
Corrosion resistance
In the production of pulp and paper, 316 stainless steel has better corrosion resistance compared to 304 stainless steel. It is also resistant to marine and aggressive industrial atmospheres.
Generally, there is little difference in terms of chemical resistance between 304 and 316 stainless steel, although there are differences in certain specific media. 304 stainless steel, being the first developed stainless steel, is more susceptible to Pitting Corrosion (PC) in certain conditions. The addition of 2-3% molybdenum reduces this sensitivity, leading to the creation of 316. Moreover, extra molybdenum also decreases the corrosion of specific hot organic acids.
316 stainless steel has become the industry standard material for food and beverage industries, however, due to the worldwide scarcity of molybdenum and the higher nickel content in 316, it is more expensive than 304 stainless steel. Pitting corrosion occurs primarily due to the deposition of corrosion on the surface of stainless steel, resulting from the inability to form a protective layer of chromium oxide due to lack of oxygen.
In most types of aqueous media (distilled water, drinking water, river water, boiler water, seawater, etc.), the corrosion resistance of 304 and 316 stainless steel is almost the same, unless the chloride ion content in the medium is very high, in which case 316 stainless steel is more suitable. In most cases, the corrosion resistance performance of 304 and 316 stainless steel is not significantly different, but in some cases, there may be a big difference, which requires a specific analysis of each case.
Valve users should have a clear understanding of their requirements, as they will choose the material for their vessel and pipe based on the medium. Recommending materials to users is not recommended.
See also:
Heat resistance
316 stainless steel has good oxidation resistance when used intermittently below 1600°C and continuously below 1700°C. It is best not to use 316 stainless steel continuously within the temperature range of 800-1575°C, but it has good heat resistance when used continuously outside of this range. 316L stainless steel has better resistance to carbide precipitation than 316 stainless steel and can be used in higher temperature ranges.
Heat treatment
316 stainless steel need to anneal in the temperature range of 1850-2050oC, then anneal and cool rapidly because it cannot be hardened by overheating.
Welding
316 stainless steel has good welding properties and can be welded using all standard welding methods. Depending on the application, a filler rod or electrode of 316Cb, 316L, or 309Cb stainless steel can be used for welding. For optimal corrosion resistance, post-weld annealing is required for the welded cross-section of 316 stainless steel. However, post-weld annealing is not necessary if 316L stainless steel is used.
Mechanical properties
| Type | UTS N/mm | Yield N/mm | Elogation % | Hardness HRB | Comparable DIN number | |
|---|---|---|---|---|---|---|
| wrought | cast | |||||
| 304 | 600 | 210 | 60 | 80 | 1.4301 | 1.4308 |
| 304L | 530 | 200 | 50 | 70 | 1.4306 | 1.4552 |
| 316 | 560 | 210 | 60 | 78 | 1.4401 | 1.4408 |
| 316L | 530 | 200 | 50 | 75 | 1.4406 | 1.4581 |
In all types of steels, austenite has the lowest yield strength. Hence, in terms of mechanical properties, austenite is not the most suitable material for use in valve stems. This is because, to ensure a specific strength, the diameter of the stem must be enlarged. The yield strength cannot be increased through heat treatment, but can be increased through cold forming.
Magnetic properties
Due to the widespread use of austenite, a false perception has been created that all stainless steels are non-magnetic. It is generally understood that austenite is non-magnetic and hardened forged steels are indeed so. However, 304 that has undergone cold forming may be somewhat magnetic. On the other hand, 100% austenite cast steel is non-magnetic.
IV. Low-carbon stainless steel
The corrosion resistance of austenite comes from the protective layer of chromium oxide that forms on the surface of the metal. If the material is heated to high temperatures of 450°C to 900°C, the structure of the material changes, and chromium carbide is formed along the crystal edge, preventing the formation of a protective layer of chromium oxide at the edge of the crystal and resulting in a reduction in corrosion resistance.
This corrosion is called ‘intergranular corrosion.’ To combat this corrosion, 304L and 316L stainless steels were developed with a lower carbon content, meaning there is no chromium carbide and no intergranular corrosion.
It should be noted that a higher sensitivity to intergranular corrosion does not mean that non-low-carbon materials are more susceptible to corrosion, and this sensitivity is also higher in high chlorine environments. Note that this phenomenon is due to high temperatures (450°C – 900°C), often caused by welding.
For a conventional butterfly valve with a soft seat, there is no need to use low-carbon stainless steel as we do not weld on the valve plate, even though most specifications call for 304L or 316L stainless steel.
V. Why stainless steel rust?
Why does stainless steel still rust?
When brown rust spots appear on the surface of a stainless steel pipe, it can be surprising, as many people have the misconception that stainless steel does not rust.
However, stainless steel can corrode under certain conditions, as it has the ability to resist atmospheric oxidation, but also has the ability to corrode in mediums containing acid, alkali, and salt.
The corrosion resistance of stainless steel varies depending on its chemical composition, the state of protection, the conditions of use, and the type of environmental medium. For example, 304 steel pipes have excellent corrosion resistance in a dry and clean atmosphere, but they can quickly rust in a seaside area with sea spray containing large amounts of salt.
On the other hand, 316 steel pipes perform well in these conditions. It is important to note that not all stainless steel is resistant to corrosion and rust in all environments.
Are there any test made like a salt spry test according to a norm which shows the difference in corrosion resistance between grade 304 and 316 ?