1、 What steel is stainless steel?

Stainless steel is a kind of steel. Steel refers to steel with less than 2% carbon (C), and iron with more than 2% carbon (C). The addition of chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), molybdenum (Mo) and other alloy elements in the steel smelting process improves the performance of the steel, so that the steel has corrosion resistance (that is, no rust), which is what we often call stainless steel.

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

Steel: materials with iron as the main element, carbon content generally below 2%, and other elements—— GB/T 13304-91 Classification of Steel

Iron: Metallic element, atomic number 26. Ferrous materials have strong ferromagnetism, good plasticity and thermal conductivity.

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

2、 Performance introduction

304 stainless steel

304 stainless steel is the most common steel. As a widely used steel, it has good corrosion resistance, heat resistance, low temperature strength and mechanical properties; Stamping, bending and other hot workability are good, and there is no heat treatment hardening phenomenon (non-magnetic, use temperature - 196 ℃~800 ℃).

Scope of application: household articles (Class 1 and 2 tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs); Auto parts (windshield wiper, muffler, molded products); Medical appliances, building materials, chemicals, food industry, agriculture, ship parts

304L stainless steel

As a low carbon 304 steel, its corrosion resistance is similar to that of 304 steel in general state, but its grain boundary corrosion resistance is excellent after welding or stress relief; It can also maintain good corrosion resistance without heat treatment, and the service temperature is - 196 ℃～ 800 ℃.

Scope of application: It is applied to outdoor machines in chemical, coal and petroleum industries with high requirements for grain boundary corrosion resistance, heat-resistant parts of building materials and parts with difficulties in heat treatment.

316 stainless steel

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).

Scope of application: sea water equipment, chemical, dye, paper making, oxalic acid, fertilizer and other production equipment; Photography, food industry, coastal facilities, ropes, CD rods, bolts, nuts.

316L stainless steel

As a low carbon series of 316 steel, except for the same characteristics as 316 steel, it has excellent resistance to grain boundary corrosion.

Scope of application: products with special requirements for resistance to grain boundary corrosion.

3、 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 the steel is superior to 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 uses.

316 stainless steel also has good chloride corrosion performance, so it is usually used in marine environment. The maximum carbon content of 316L stainless steel is 0.03, which can be used in applications that cannot be annealed after welding and require maximum corrosion resistance.

Corrosion resistance

The corrosion resistance of 316 stainless steel is superior to 304 stainless steel, and it has good corrosion resistance in the production process of pulp and paper. In addition, 316 stainless steel is also resistant to marine and corrosive industrial atmosphere. Generally speaking, 304 stainless steel and 316 stainless steel have little difference in chemical corrosion resistance, but they are different in some specific media.

The first developed stainless steel is 304. Under certain circumstances, this material is sensitive to pitting corrosion. An additional 2-3% of molybdenum can reduce this sensitivity, thus giving birth to 316. In addition, these additional molybdenum can also reduce the corrosion of some thermal organic acids.

316 stainless steel has almost become the standard material in the food and beverage industry. The price of 316 stainless steel is more expensive than 304 stainless steel due to the worldwide shortage of molybdenum element and more nickel content in 316 stainless steel. Spot 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 chromium oxide protective layer. Especially in small valves, the possibility of deposition on the valve plate is very small, so spot corrosion rarely occurs.

In various types of water media (distilled water, drinking water, river water, boiler water, sea water, etc.), the corrosion resistance of 304 stainless steel and 316 stainless steel is almost the same. Unless the content of chloride ion in the media is very high, 316 stainless steel is more appropriate. In most cases, the corrosion resistance of 304 stainless steel is not much different from that of 316 stainless steel, but in some cases, it may also be very different, which requires specific analysis.

Heat resistance

In intermittent use below 1600 ℃ and continuous use below 1700 ℃, 316 stainless steel has good oxidation resistance.

In the range of 800-1575 degrees, it is better not to continuously act on 316 stainless steel. However, when 316 stainless steel is continuously used outside this temperature range, it 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 treatmentAnnealing is carried out in the temperature range of 1850-2050 ° C, then rapid annealing, and then rapid cooling. 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. 316Cb, 316L or 309Cb stainless steel filler rods or electrodes can be used for welding according to the purpose. In order to obtain the best corrosion resistance, the welding section of 316 stainless steel needs to be annealed after welding. If 316L stainless steel is used, post weld annealing is not required.

Mechanical properties

Among all steels, austenitic stainless steel has the lowest yield point. Therefore, considering the 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.

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 crystal edge.

In this way, the chromium oxide protective layer cannot be formed at the edge of the crystal, thus reducing the corrosion resistance. This corrosion is called "intergranular corrosion". Therefore, 304L stainless steel and 316L stainless steel have been developed to resist this corrosion. The carbon content of 304L stainless steel and 316L stainless steel is low. Because the carbon content is reduced, chromium carbide will not be generated, and intergranular corrosion will not be generated

It should be noted that higher susceptibility to intergranular corrosion does not mean that non low carbon materials are more susceptible to corrosion. In high chlorine environment, the sensitivity is also higher. Please note that this phenomenon is due to high temperature (450 ℃ - 900 ℃). Usually welding is the direct cause of reaching this temperature. For conventional butterfly valves with soft seats, since we do not weld on the valve plate, it is not meaningful to use low-carbon stainless steel. However, most specifications require 304L stainless steel or 316L stainless steel. Why does stainless steel rust? When there are brown rust spots (spots) on the surface of the stainless steel pipe, people are surprised: "stainless steel is not rusty, rust is not stainless steel, it may be a problem with the steel".

In fact, this is a one-sided wrong view of the lack of understanding of stainless steel. Stainless steel will rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation, that is, it is not rusty, but also has the ability to corrode in the medium containing acid, alkali and salt, that is, corrosion resistance. However, the corrosion resistance of the steel varies with its chemical composition, protection status, service conditions and types of environmental media. For example, 304 steel pipe has absolutely excellent corrosion resistance in a dry and clean atmosphere, but if it is moved to the coastal area, it will rust soon in the sea fog containing a lot of salt; The 316 steel pipe performs well. Therefore, not any kind of stainless steel can resist corrosion and rust under any environment.

Source: Welding technology