What is the meaning of “steel” and “iron”, what are the characteristics, and what is the relationship between us, 304, 304L, 316, 316L, which we usually say, and what is the difference between them?
Steel: A material containing iron as the main element and having a carbon content of generally less than 2% and containing other elements. ——GB/T 13304-91 “Steel Classification”
Iron: A metal element, atomic number 26. Iron materials have strong ferromagnetism and good plasticity and thermal conductivity.
Stainless steel: resistant to weak corrosive media such as air, steam, water or stainless steel. The commonly used steel grades are 304, 304L, 316, and 316L, which are 300 series steels of austenitic stainless steel.
304 stainless steel
304 stainless steel is the most common steel grade. As a widely used steel, it has good corrosion resistance, heat resistance, low temperature strength and mechanical properties. It has good hot workability such as stamping and bending, and has no heat treatment hardening phenomenon (non-magnetic , the temperature is -196 ° C ~ 800 ° C).
Scope of application
Household items (1, 2 types of tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs)
Auto parts (windshield wipers, mufflers, molded products)
Medical equipment, building materials, chemicals, food industry, agriculture, ship parts
304L stainless steel – (L is low carbon)
As a low-carbon 304 steel, its corrosion resistance is similar to that of 304 under normal conditions, but it has excellent resistance to intergranular corrosion after welding or after stress relief; it can also be maintained well without heat treatment. Corrosion resistance, the use temperature -196 ° C ~ 800 ° C.
Scope of application
It is used in field open-air machines for chemical, coal, and petroleum industries with high resistance to intergranular corrosion, heat-resistant parts for building materials, and parts with difficult heat treatment.
316 stainless steel
Due to the addition of molybdenum, 316 stainless steel has excellent corrosion resistance, atmospheric corrosion resistance and high temperature strength, and can be used under severe conditions; it has excellent work hardening property (non-magnetic).
Scope of application
Equipment for the use of equipment, chemicals, dyes, paper, oxalic acid, fertilizers, etc. in seawater; photo, food industry, coastal facilities, ropes, CD rods, bolts, nuts.
316L stainless steel – (L is low carbon)
As a low carbon series of 316 steel, it has excellent resistance to intergranular corrosion in addition to the same characteristics as 316 steel.
Scope of application
A product that has special requirements for resistance to grain boundary corrosion.
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 steel, the total 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 chloride attack properties and is therefore commonly used in marine environments. 316L stainless steel has a maximum carbon content of 0.03 and can be used in applications where annealing is not possible and maximum corrosion resistance is required.
316 stainless steel has better corrosion resistance than 304 stainless steel and has good corrosion resistance in the production of pulp and paper. Moreover, 316 stainless steel is also resistant to erosion by marine and aggressive industrial atmospheres.
In general, 304 stainless steel and 316 stainless steel have little difference in chemical resistance, but they differ in some specific media.
The stainless steel originally developed was 304. In certain cases, this material is sensitive to pitting Corrosion. An additional 2-3% increase in molybdenum can reduce this sensitivity, thus giving birth to 316. In addition, these additional 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 shortage of molybdenum in the world and the high nickel content in 316 stainless steel, the price of 316 stainless steel is more expensive than 304 stainless steel.
Pitting corrosion is a phenomenon mainly caused by deposition corrosion of stainless steel surfaces because oxygen deficiency does not form a chromium oxide protective layer.
Especially in small valves, the possibility of deposits on the valve plate is small, so pitting corrosion is rare.
In all types of water media (distilled water, drinking water, river water, boiler water, sea water, etc.), 304 stainless steel and 316 stainless steel have almost the same corrosion resistance, unless the chloride ion content 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 vary greatly, and specific analysis is required. In general, valve users should be aware of the fact that they will select the material of the container and the pipe according to the condition of the medium. It is not recommended to recommend the material to the user.
316 stainless steel has good oxidation resistance in intermittent use below 1600 °C and continuous use below 1700 °C. In the range of 800-1575 degrees, it is preferable not to continuously apply 316 stainless steel, but when 316 stainless steel is continuously used outside this temperature range, the stainless steel has good heat resistance. 316L stainless steel has better carbide precipitation resistance than 316 stainless steel and can be used in the above temperature range.
Annealing is carried out at a temperature ranging from 1850 to 2050 degrees, followed by rapid annealing and rapid cooling. 316 stainless steel cannot be hardened by heat treatment.
316 stainless steel has good welding properties. 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 for welding according to the application. For best corrosion resistance, the welded section of 316 stainless steel requires post-weld annealing. If 316L stainless steel is used, post-weld annealing is not required.
Among all steels, austenitic stainless steel has the lowest yield point. Therefore, from the viewpoint of mechanical properties, austenitic stainless steel is not the best material for the valve stem, because the diameter of the valve stem is increased to ensure a certain strength. The yield point cannot be increased by heat treatment, but can be improved by cold forming.
Due to the wide application of austenitic stainless steel, it gives people the wrong impression that all stainless steels are not magnetic. For austenitic stainless steels, it can be basically understood as non-magnetic, as is the case with quenched forged steel. However, the 304 processed by cold forming will be somewhat magnetic. For cast steel, if it is 100% austenitic stainless steel, it is not magnetic.
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 temperature between 450 ° C and 900 ° C, the structure of the material changes and chromium carbide forms along the edges of the crystal. Thus, a chromium oxide protective layer cannot be formed at the edge of the crystal, resulting in a decrease in corrosion resistance. This type of corrosion is called “intergranular corrosion.”
This resulted in the development of 304L stainless steel and 316L stainless steel to combat this corrosion. Both 304L stainless steel and 316L stainless steel have a lower carbon content. Because of the reduced carbon content, chromium carbide is not produced and no intergranular corrosion occurs.
It should be noted that higher intergranular corrosion sensitivity does not mean that non-low carbon materials are more susceptible to corrosion. This sensitivity is also higher in high chlorine environments.
Please note that this phenomenon is due to high temperatures (450 ° C – 900 ° C). Usually soldering is the direct cause of reaching this temperature. For soft seat conventional butterfly valves, the use of low carbon stainless steel does not make much sense since we do not weld on the valve plate, but most specifications require 304L stainless steel or 316L stainless steel.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)