What are the differences between 304, 304L, 316 and 316L?
What exactly are "steel" and "iron", what are their properties, and what is the relationship between them? How did the terms "304", "304L", "316", and "316L" that we usually talk about come about, and what are the differences among them?
Steel: A material with iron as the main element, a carbon content generally below 2%, and containing other elements. -- GB/T 13304-91 "Steel Classification"
Iron: A metallic element with atomic number 26. Iron materials have strong ferromagnetism and good plasticity and thermal conductivity.
Stainless steel: A type of steel that is resistant to weak corrosive media such as air, steam, and water, or has the property of not rusting. The commonly used steel grades are 304, 304L, 316 and 316L, which belong to the 300 series of austenitic stainless steel.
Introduction to the properties of 304 Stainless steel
304 stainless steel is the most common type of steel. As a widely used steel, it features excellent corrosion resistance, heat resistance, low-temperature strength and mechanical properties. It has good hot workability such as stamping and bending, and no hardening phenomenon after heat treatment (non-magnetic, applicable temperature range: -196℃ to 800℃).
Introduction to the Performance of 304L Stainless Steel (L stands for low carbon)
As a low-carbon 304 steel, under normal conditions, its corrosion resistance is similar to that of 304 steel. However, after welding or stress relief, its resistance to intergranular corrosion is excellent. It can maintain good corrosion resistance without heat treatment and can be used at temperatures ranging from -196℃ to 800℃.
Scope of application
It is applied to outdoor machinery in the chemical, coal and petroleum industries with high requirements for resistance to intergranular corrosion, heat-resistant parts in building materials and parts that are difficult to heat-treat.
Introduction to the Properties of 316 Stainless steel
316 stainless steel, due to the addition of molybdenum, has particularly good corrosion resistance, atmospheric corrosion resistance and high-temperature strength, and can be used under harsh conditions. Excellent work hardening property (non-magnetic).
Introduction to the Performance of 316L Stainless Steel (L stands for Low carbon)
As a low-carbon series of the 316 steel grade, in addition to having the same properties as 316 steel, it has excellent resistance to intergranular corrosion.
316 and 316L stainless steels are molybdenum-containing stainless steel grades. The molybdenum content in 316L stainless steel is slightly higher than that in 316 stainless steel. Due to the molybdenum in the steel, the overall performance of this steel grade is superior to that of 310 and 304 stainless steels. 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 applications. 316 stainless steel also has good resistance to chloride erosion, so it is usually used in Marine environments. The maximum carbon content of 316L stainless steel is 0.03, making it suitable for applications where annealing is not possible after welding and where maximum corrosion resistance is required.
Corrosion resistance
The corrosion resistance of 316 stainless steel is superior to that of 304 stainless steel, and it has excellent corrosion resistance in the production processes of pulp and paper. Moreover, 316 stainless steel is also resistant to erosion by Marine and corrosive industrial atmospheres.
Generally speaking, there is not much difference between 304 stainless steel and 316 stainless steel in terms of chemical corrosion resistance, but there are some distinctions in certain specific media.
The first stainless steel developed was 304. Under certain circumstances, this material is relatively sensitive to Pitting Corrosion. Adding an extra 2-3% of molybdenum can reduce this sensitivity, and thus 316 was born. In addition, these extra molybdenum can also reduce the corrosion of certain 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 in 316 stainless steel, the price of 316 stainless steel is higher than that of 304 stainless steel.
Pitting corrosion is a phenomenon mainly caused by the deposition corrosion on the surface of stainless steel, 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 deposition on the valve plate is very small, and thus pitting corrosion rarely occurs.
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, in which case 316 stainless steel is more suitable.
In most cases, there is not much difference in corrosion resistance between 304 stainless steel and 316 stainless steel, but in some cases, the difference may be significant, and specific analysis is required on a case-by-case basis. Generally speaking, valve users should be well aware of this, as they will choose the materials of containers and pipelines based on the conditions of the medium. It is not recommended to recommend materials to users.
Heat resistance
316 stainless steel has excellent oxidation resistance in intermittent use below 1600 degrees and continuous use below 1700 degrees. Within the range of 800 to 1575 degrees, it is not advisable to use 316 stainless steel continuously. However, when used continuously outside this temperature range, this stainless steel has good heat resistance. The carbide precipitation resistance of 316L stainless steel is better than that of 316 stainless steel and can be used within the above-mentioned temperature range.
Heat treatment
Annealing is carried out within the temperature range of 1850 to 2050 degrees, followed by rapid annealing and then rapid cooling. 316 stainless steel cannot be hardened by heat treatment.
Welding
316 stainless steel has excellent welding performance. All standard welding methods can be adopted for welding. When welding, depending on the application, 316Cb, 316L or 309Cb stainless steel filler rods or electrodes can be used for welding respectively. To achieve the best corrosion resistance, the welded sections of 316 stainless steel need to undergo post-weld annealing treatment. If 316L stainless steel is used, post-weld annealing treatment is not required.
Mechanical properties
Among all types of steel, austenitic stainless steel has the lowest yield point. Therefore, considering mechanical properties, austenitic stainless steel is not the best material for valve stems, as to ensure a certain strength, the diameter of the valve stem would have to be increased. The yield point cannot be raised by heat treatment, but it can be increased by cold forming.
Magnetic
Due to the wide application of austenitic stainless steel, it has given people the wrong impression that all stainless steels are non-magnetic. For austenitic stainless steel, it can basically be understood as non-magnetic, and this is indeed the case for quenched forged steel. However, 304 that has undergone cold forming treatment will have a certain degree of magnetism. For cast steel, if it is 100% austenitic stainless steel, it has no magnetism.
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 high temperature of 450℃ to 900℃, its structure will change and chromium carbide will form along the crystal edges. In this way, a chromium oxide protective layer cannot be formed at the edge of the crystal, which leads to a decrease in corrosion resistance. This kind of corrosion is called "intergranular corrosion".
From this, 304L stainless steel and 316L stainless steel were developed to combat this kind of corrosion. Both 304L stainless steel and 316L stainless steel have a relatively low carbon content. Due to the reduced carbon content, chromium carbide will not form, and thus intergranular corrosion will not occur.
It should be noted that a higher intergranular corrosion sensitivity does not mean that non-low-carbon materials are more prone to corrosion. This sensitivity is also higher in a high-chlorine environment.
Please note that this phenomenon is due to high temperatures (450℃-900℃). Usually, welding is the direct cause of reaching this temperature. For conventional soft-seat butterfly valves, since we do not perform welding operations on the valve plate, it makes little sense to use low-carbon stainless steel. However, most specification sheets will require 304L stainless steel or 316L stainless steel.