I. What kind of steel is stainless steel?
Stainless steel is a kind of steel that refers to the amount of carbon (C) containing less than 2%, and more than 2% of that is iron.
In the steel smelting process, adding chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), molybdenum (Mo) and other alloying elements improve the performance of steel so that the steel has a corrosion resistance (i.e. not rust), which called stainless steel.
What is “Steel” and “Iron”, what are the characteristics, what is the relationship, how 304, 304L, 316, 316L come from, and what are the differences between them?
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 stainless steels melting process, due to the addition of different varieties of alloying elements, different varieties and different amounts, their characteristics are also different.
In order to distinguish them, different steel numbers are named.
The following composition table is a common decorative stainless steel of different steel number “alloying elements” (for reference only).
Chemical composition (mass fraction, %)
– 304 stainless steel –
304 stainless steel is the most common steel, as a widely used steel, has good corrosion resistance, heat resistance, low temperature strength and mechanical properties; good thermal processing for stamping and bending, no heat treatment hardening phenomenon (non-magnetic, easy to use temperature between -196 oC and 800 oC).
Household goods (I, II class tableware, cupboards, indoor plumbing, water heaters, boiler, bathtubs);
Auto parts (windshield wipers, mufflers, moldings);
Medical devices, building materials, chemicals, food industry, agriculture, ship components.
– 304L stainless steel – (L is low carbon)
As low-carbon steel, in the general state, its corrosion resistance is similar to 304.
But after welding or eliminating stress, 304L has excellent resistance to grain boundary corrosion.
It can also maintain good corrosion resistance without heat treatment at a temperature of 196oC ~ 800oC.
304L is used for field open-air equipment in the chemical, coal and petroleum industries that require high resistance to grain boundary corrosion, and for heat-resistant parts of building materials and parts that are difficult to heat treatment.
– 316 stainless steel –
Due to the addition of molybdenum, 316 stainless steel has excellent corrosion resistance, atmospheric corrosion resistance and high-temperature intensity, which can be used under harsh conditions; its work hardening properties are also excellent (non-magnetic).
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)
As a low-carbon series of 316 stainless steel, it has the same characteristics as 316, but has excellent resistance to grain boundary corrosion.
Products with special requirements for resistance to grain boundary corrosion.
III. Performance Comparison
316 and 316L stainless steels are molybdenum-containing stainless steels.
The molybdenum content of 316L stainless steel is slightly higher than that of 316 stainless steel.
Due to the molybdenum in the steel, the overall performance of this grade is superior to 310 and 304 stainless steels, and 316 stainless steel has a wide range of uses at high temperatures, with sulfuric acid concentrations below 15% and above 85%.
316 stainless steel also has good resistance to chloride attack, so it is commonly used in marine environments.
With a maximum carbon content of 0.03, 316L stainless steel can be used in applications where no annealing after welding and where maximum corrosion resistance is required.
316 stainless steel has better corrosion resistance than 304 stainless steel in the production of pulp and paper. 316 stainless steel is also resistant to marine and aggressive industrial atmospheres.
In general, there is little difference between 304 and 316 stainless steel in terms of chemical resistance, although there are differences in certain specific media.
The original developed stainless steel was 304, which is more sensitive to Pitting Corrosion (PC) under certain circumstances. An additional 2-3% of molybdenum could reduce this sensitivity, thus 316 was born.
In addition, extra molybdenum also reduces the corrosion of certain hot organic acids.
316 stainless steel has almost become the industry standard material for food and beverage industries.
Due to the worldwide shortage of molybdenum and the higher nickel content of 316 stainless steel, 316 stainless steel is more expensive than 304 stainless steel.
Pitting corrosion is a phenomenon caused primarily by the deposition of corrosion on the surface of stainless steel, which is because of the inability to form a protective layer of chromium oxide due to lack of oxygen.
Particularly in small valves, the likelihood of deposition on the valve plate is small, so pitting corrosion is rare.
In all types of aqueous media (distilled water, drinking water, river water, boiler water and 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 is more suitable.
In most cases, the corrosion resistance performance of 304 stainless steel and 316 stainless steel is not much difference, but in some cases may be very different, which needs for a specific analysis of specific cases.
Generally valve users should know what they are looking for, because they will choose the material of the vessel and pipe according to the medium. Recommending material to users is not recommended.
316 stainless steel has good oxidation resistance in intermittent use below 1600oC and in continuous use below 1700oC.
It is best not to act continuously with 316 stainless steel in the temperature range of 800-1575oC, but the stainless steel has good heat resistance when used continuously outside this temperature range.
316L stainless steel has better resistance to carbide precipitation than 316 stainless steel and it can be used in the above temperature range.
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.
316 stainless steel has good welding properties. It can be welded using all standard welding methods.
Depending on the application, 316Cb, 316L or 309Cb stainless steel filler rod or electrode can be used for welding.
For optimum corrosion resistance, the welded cross-section of 316 stainless steel requires post-weld annealing. If 316L stainless steel is used, post-weld annealing is not required.
|Comparable DIN number|
In all kinds of steels, austenite has the lowest yield point.
Therefore, in terms of mechanical properties, austenite is not the best material for use in valve stems. This is because in order to ensure a certain strength, the diameter of the stem is increased. The yield point cannot be increased by heat treatment, but can be increased by cold forming.
Because of the widespread use of austenite, it has created the false impression that all stainless steels are non-magnetic.
For austenite, it is basically understood that they are non-magnetic, and hardened forged steels are indeed so. However, 304 treated by cold forming will be somewhat magnetic. For cast steel, 100% austenite 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 450oC to 900oC, the structure of the material changes and chromium carbide is formed along the crystal edge.
This prevents the formation of a protective layer of chromium oxide at the edge of the crystal, resulting in a reduction in corrosion resistance. This corrosion is called “intergranular corrosion”.
304L stainless steel and 316L stainless steel were developed to combat this corrosion. 304L stainless steel and 316L stainless steel have a lower carbon content, and because the carbon content is reduced, there is no chromium carbide and no intergranular corrosion.
It should be noted that higher sensitivity to intergranular corrosion does not mean that non-low carbon materials are more susceptible to corrosion. This sensitivity is also higher in a high chlorine environment.
Note that this phenomenon is due to high temperatures (450°C – 900°C). Often welding is the direct cause of reaching this temperature.
For a conventional butterfly valve with a soft seat, as we do not weld on the valve plate, it is meaningless in using low carbon stainless steel, although most specifications will call for 304L or 316L stainless steel.
V. Why stainless steel rust
Why does stainless steel still rust?
When brown rust spots (spots) appear on the surface of stainless steel pipe, people are greatly surprised: think “stainless steel is not rusty, rust is not stainless steel, maybe 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 – stainless, and also has the ability to corrode in the medium containing acid, alkali and salt -corrosion resistance.
However, its corrosion resistance varies with the chemical composition of the steel, the state of protection, the conditions of use and the type of environmental medium.
Such as 304 steel pipe, in a dry and clean atmosphere, has absolutely excellent corrosion resistance.
But if move it to the seaside area, it will soon rust in the sea spray containing large amounts of salt while 316 steel pipe performs excellently.
Therefore, not any kind of stainless steel can be resistant to corrosion and not rust in any environment.