What is the difference between 304, 304L, 316, and 316L?

Stainless steel is a type of steel. Steel refers to steel containing less than 2% of carbon (C), and more than 2% is iron. The addition of chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), molybdenum (Mo) and other alloying elements during the smelting process improves the performance of the steel and makes the steel more resistant to corrosion. (that is, not rusted) is what we often call stainless steel.

- What exactly are "steel" and "iron", what are their characteristics, and what is their relationship? How did the 304, 304L, 316, and 316L we usually call come from, and what are the differences between them?
 

Steel: A material with iron as the main element, carbon content generally below 2%, and other elements. 
 

Iron: A metallic element with atomic number 26. Iron materials have strong ferromagnetism, good plasticity and thermal conductivity.
 

Stainless steel: resistant to weak corrosive media such as air, steam, and water, or stainless steel. The commonly used steel grades are 304, 304L, 316, 316L, which are 300 series steels of austenitic stainless steel.

2、Why do stainless steels have different steel grades?
 

In the smelting process of stainless steel, due to the different varieties of alloying elements added, the amount of addition of different varieties is different. Their characteristics are also different. In order to distinguish them, they are crowned with different steel grades. The following is a table of "alloying elements" content of different steel grades of common decorative stainless steels for reference only:

 

3、Performance Introduction

- 304：304 stainless steel is the most common type of steel. As a widely used steel, it has good corrosion resistance, heat resistance, low temperature strength and mechanical properties; good hot workability such as stamping and bending, and no heat treatment hardening (non-magnetic, then use the temperature -196 ℃ ~ 800 ℃).
 

- 304L：As a low-carbon 304 steel, its corrosion resistance is similar to that of 304 steel under normal conditions, but after welding or after stress relief, its resistance to grain boundary corrosion is excellent; it can also remain good without heat treatment. High corrosion resistance, operating temperature -196℃～800℃.
 

- 316：316 stainless steel has excellent corrosion resistance, atmospheric corrosion resistance and high temperature strength due to the addition of molybdenum, and can be used under harsh conditions; excellent work hardening (non-magnetic).
 

- 316L：As a low carbon series of 316 steel, in addition to having the same characteristics as 316 steel, its resistance to grain boundary corrosion is excellent.

4、Scope of application
 

304：Household goods (1 and 2 tableware, cabinets, indoor pipes, water heaters, boilers, bathtubs). Auto parts (windshield wipers, mufflers, moldings). Medical equipment, building materials, chemicals, food industry, agriculture, ship parts.

304L：It is used in field open-air machines in chemical, coal and petroleum industries that require high resistance to grain boundary corrosion, heat-resistant parts of building materials and parts with difficult heat treatment.

316：Seawater equipment, chemical, dye, paper, oxalic acid, fertilizer and other production equipment; photography, food industry, coastal facilities, ropes, CD rods, bolts, nuts.

316L：Products with special requirements for resistance to grain boundary corrosion.

5、Performance Comparison

- Chemical Composition

316 and 316L stainless steels are molybdenum-containing stainless steels. The molybdenum content in 316L stainless steel is slightly higher than that of 316 stainless steel. Due to the molybdenum in the steel, the overall performance of this steel is better than that of 310 and 304 stainless steel. Under high temperature conditions, when the concentration of sulfuric acid is lower than 15% and higher than 85%, 316 Stainless steel has a wide range of uses. 316 stainless steel also has good resistance to chloride attack, so it is often used in marine environments. 316L stainless steel has a maximum carbon content of 0.03 and can be used in applications where annealing after welding cannot be performed and where maximum corrosion resistance is required.
 

- Mechanical Properties
 

Austenitic stainless steel has the lowest yield point of all steels. Therefore, from the perspective of mechanical properties, austenitic stainless steel is not the best material for the valve stem, because to ensure a certain strength, the diameter of the valve stem will increase. The yield point cannot be increased by heat treatment, but can be increased by cold forming.
 

- Corrosion Resistance
 

The corrosion resistance of 316 stainless steel is better than that of 304 stainless steel, and it has good corrosion resistance in the production process of pulp and paper. And 316 stainless steel is also resistant to erosion by marine and aggressive industrial atmospheres. Generally speaking, 304 stainless steel and 316 stainless steel have little difference in chemical corrosion resistance, but there are differences in some specific media.
 

The stainless steel originally developed was 304, which is sensitive to pitting corrosion under certain circumstances. Adding an additional 2-3% molybdenum reduced this sensitivity, thus giving birth to 316. In addition, these additional molybdenum can also reduce the corrosion of some hot organic acids.
 

316 stainless steel has almost become the standard material in the food and beverage industry. Due to the worldwide shortage of molybdenum and the higher nickel content of 316 stainless steel, the price of 316 stainless steel is more expensive than that of 304 stainless steel.
 

Pitting corrosion is a phenomenon mainly caused by the deposition corrosion of stainless steel surface, which is due to the lack of oxygen and the inability to form a protective layer of chromium oxide. Especially in small valves, the possibility of deposits on the valve plate is very small, so pitting corrosion is also rare. In various types of water media (distilled water, drinking water, river water, boiler water, seawater, etc.), the corrosion resistance of 304 stainless steel and 316 stainless steel is almost the same, unless the content of chloride ions in the medium is very high, then 316 stainless steel more suitable.
 

In most cases, the corrosion resistance of 304 stainless steel and 316 stainless steel is not much different, but in some cases it may be very different and needs to be analyzed on a case-by-case basis.

- Heat Resistance
 

In intermittent use below 1600 degrees and continuous use below 1700 degrees, 316 stainless steel has good oxidation resistance. In the range of 800-1575 degrees, it is best not to use 316 stainless steel continuously, but when 316 stainless steel is used continuously outside this temperature range, the stainless steel has good heat resistance. The carbide precipitation resistance of 316L stainless steel is better than that of 316 stainless steel, and the above temperature range can be used.

- Heat Treatment
 

Annealed in the temperature range of 1850-2050 degrees, then rapidly annealed, then rapidly cooled. 316 stainless steel cannot be hardened by overheating.

- Welding
 

316 stainless steel has good weldability. All standard welding methods can be used for welding. According to the application, 316Cb, 316L or 309Cb stainless steel filler rods or electrodes can be used for welding. In order to obtain the best corrosion resistance, the welded section of 316 stainless steel needs to be post-weld annealed. Post weld annealing is not required if 316L stainless steel is used.

- Magnetic
 

Due to the widespread use of austenitic stainless steels, people have the false impression that all stainless steels are not magnetic. For austenitic stainless steel, it can be basically understood as non-magnetic, and this is true of quenched forged steel. But the 304 processed by cold forming will be somewhat magnetic. For cast steel, it is not magnetic if it is 100% austenitic stainless steel.

6、Low Carbon Type Stainless Steel
 

The corrosion resistance of austenitic stainless steel comes from the protective layer of chromium oxide formed on the metal surface. If the material is heated to a temperature of 450°C to 900°C, the structure of the material changes and chromium carbides form along the crystal edges. This prevents the formation of a protective chromium oxide layer at the crystal edges, resulting in reduced corrosion resistance. This corrosion is called "intergranular corrosion".

As a result, 304L stainless steel and 316L stainless steel were developed to combat this corrosion. Both 304L stainless steel and 316L stainless steel have lower carbon content, because the carbon content is reduced, so there is no chromium carbide and no intergranular corrosion. It should be noted that higher susceptibility to intergranular corrosion does not mean that non-low carbon materials are more susceptible to corrosion. This sensitivity is also higher in high chloride environments.

It should be taken note that this phenomenon is due to high temperature (450℃-900℃). Usually soldering is the direct cause of reaching this temperature. For soft seat conventional butterfly valves, since we do not weld on the valve plate, it doesn't make much sense to use low carbon stainless steel, but most specifications will call for 304L stainless steel or 316L stainless steel

7、Why does stainless steel rust
 

When brown rust spots (spots) appear on the surface of stainless steel pipes, people are amazed: "Stainless steel does not rust, and if it rusts, it is not stainless steel, and there may be a problem with the steel." In fact, this is a one-sided misconception about the lack of understanding of stainless steel. Stainless steel will also rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation—that is, rust resistance, and also has the ability to corrode in media containing acids, alkalis, and salts—that is, corrosion resistance. However, the size of its anti-corrosion ability is changed with the chemical composition of the steel itself, the protection state, the use conditions and the type of environmental media. For example, 304 steel pipe has absolutely excellent anti-corrosion ability in a dry and clean atmosphere, but if it is moved to a seaside area, it will rust soon in the sea fog containing a lot of salt; and 316 steel pipe shows good. Therefore, it is not any kind of stainless steel that is resistant to corrosion and rust in any environment.