What are "steel" and "iron", what are their characteristics, and what is their relationship? How do we usually refer to 304, 304L, 316, and 316L, and what are the differences between them?
Steel: a material with iron as the main element, carbon content generally below 2%, and containing other elements—— GB/T 13304-91 Classification of Steel
Iron: a metallic element with atomic number 26. Iron materials have strong ferromagnetism and good plasticity and thermal conductivity.
Stainless steel: a steel grade that is resistant to weak corrosive media such as air, steam, water, or has rust resistance. The commonly used steel types are 304, 304L, 316, and 316L, which are 300 series steels of austenitic stainless steel.
Introduction to the Properties of 304 Stainless Steel
304 stainless steel is the most common type of steel, and as a widely used steel, it has good corrosion resistance, heat resistance, low-temperature strength, and mechanical properties; Good hot workability for stamping, bending, and other processes, with no hardening phenomenon during heat treatment (no magnetism, can be used at temperatures ranging from -196 ℃ to 800 ℃).
Introduction to the Performance of 304L Stainless Steel (L is Low Carbon)
As a low-carbon 304 steel, its corrosion resistance is generally similar to that of 304 steel, but its resistance to grain boundary corrosion is excellent after welding or stress relief; Even without heat treatment, it can maintain good corrosion resistance at temperatures ranging from -196 ℃ to 800 ℃.
Applicable scope
Applied to outdoor machinery in chemical, coal, and petroleum industries that require high resistance to grain boundary corrosion, heat-resistant parts in building materials, and parts with difficulty in heat treatment.
Introduction to 316 stainless steel performance
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 properties (non magnetic).
Introduction to the Performance of 316L Stainless Steel (L is Low Carbon)
As a low-carbon series of 316 steel, besides having the same characteristics as 316 steel, it has excellent resistance to grain boundary corrosion.
316 and 316L stainless steel are molybdenum containing stainless steel varieties. 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 is better than 310 and 304 stainless steel. Under high temperature conditions, 316 stainless steel has a wide range of applications when the concentration of sulfuric acid is below 15% and above 85%. 316 stainless steel also has good resistance to chloride corrosion, so it is commonly used in marine environments. The maximum carbon content of 316L stainless steel is 0.03, which can be used in applications where annealing is not possible after welding and maximum corrosion resistance is required.
Corrosion resistance
316 stainless steel has better corrosion resistance than 304 stainless steel, and has good corrosion resistance in the production process of pulp and paper. Moreover, 316 stainless steel is resistant to marine and corrosive industrial atmospheres.
Generally speaking, there is not much difference in chemical corrosion resistance between 304 stainless steel and 316 stainless steel, but there are differences in certain specific media.
The initially developed stainless steel was 304, which is sensitive to pitting corrosion in specific situations. Adding an additional 2-3% molybdenum can reduce this sensitivity, giving rise to 316. In addition, these additional molybdenum can also reduce the corrosion of certain thermal organic acids.
316 stainless steel has almost become a standard material in the food and beverage industry. Due to the global shortage of molybdenum and the higher nickel content in 316 stainless steel,the price of 316 stainless steel is more expensive than that of 304 stainless steel.
Spot corrosion is a phenomenon mainly caused by deposition corrosion on the surface of stainless steel, which is due to lack of oxygen and cannot form a chromium oxide protective layer.
Especially in small valves, the possibility of sedimentation on the valve plate is very low, 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, 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, there may also be significant differences, which require specific analysis. Generally speaking, valve users should have a clear idea, as they will choose the materials of containers and pipelines based on the medium, and it is not recommended to recommend materials to users.
Heat resistance
316 stainless steel has good oxidation resistance in intermittent use below 1600 degrees and continuous use below 1700 degrees. It is best not to continuously use 316 stainless steel within the temperature range of 800-1575 degrees, but when using 316 stainless steel continuously outside this temperature range, it has good heat resistance. The resistance to carbide precipitation of 316L stainless steel is better than that of 316 stainless steel, and can be used within the above temperature range.
heat treatment
Anneal within the temperature range of 1850-2050 degrees, then rapidly anneal and rapidly cool. 316 stainless steel cannot be hardened by overheating treatment.
welding
316 stainless steel has good welding performance. All standard welding methods can be used for welding. When welding, 316Cb, 316L, or 309Cb stainless steel filler rods or welding rods can be used according to the purpose. To achieve the best corrosion resistance, the welding section of 316 stainless steel needs to undergo post weld annealing treatment. If 316L stainless steel is used, post weld annealing treatment is not required.
Mechanical performance
Among all steels, austenitic stainless steel has the lowest yield point. Therefore, from a mechanical performance perspective, austenitic stainless steel is not the optimal material for valve stems, as to ensure a certain level of strength, the diameter of the valve stem will increase. The yield point cannot be improved through heat treatment, but can be improved through cold forming.
magnetic
Due to the widespread application of austenitic stainless steel, people have the mistaken impression that all stainless steels lack magnetism. For austenitic stainless steel, it can be understood as non-magnetic, and this is true for quenched forged steel. But the 304 processed by cold forming will have some magnetism. For cast steel, if it is 100% austenitic stainless steel, there is no magnetism.
Low carbon type stainless steel
The corrosion resistance of austenitic stainless steel comes from the chromium oxide protective layer formed on the metal surface. If the material is heated to a high temperature of 450 ℃ to 900 ℃, the structure of the material will change, and chromium carbide will form along the edge of the crystal. This prevents the formation of a chromium oxide protective layer at the edge of the crystal, leading to a decrease in corrosion resistance. This type of corrosion is called intergranular corrosion.
304L stainless steel and 316L stainless steel were developed to resist this corrosion. The carbon content of 304L stainless steel and 316L stainless steel is relatively low because the carbon content is reduced, so chromium carbide will not be produced and intergranular corrosion will not occur.
It should be noted that a higher sensitivity to intergranular corrosion does not necessarily mean that non low-carbon materials are more susceptible to corrosion. In high chlorine environments, this sensitivity also increases.
Please note that this phenomenon is caused by high temperatures (450 ℃ -900 ℃). Welding is usually the direct cause of reaching this temperature. For conventional butterfly valves with soft seats, using low-carbon stainless steel is not very meaningful as we do not perform welding operations on the valve plate. However, most specification books require 304L stainless steel or 316L stainless steel.