1、 What is stainless steel?

Stainless steel is a type of steel, which refers to steel containing less than 2% carbon (C) and more than 2% iron. Adding alloying elements such as chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), molybdenum (Mo), etc. during the smelting process improves the properties of steel and gives it corrosion resistance (i.e. no rust), which is commonly known as stainless steel.

What are "steel" and "iron", what are their characteristics, and what is their relationship? How did we get what we usually call 304, 304L, 316, and 316L, and what are the differences between them?

Steel: A material primarily composed of iron, with a 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 type of steel that is resistant to weak corrosive media such as air, steam, water, or has non rusting properties. The commonly used steel grades are 304, 304L, 316, 316L, which are 300 series austenitic stainless steel.

2、 Why are there different grades of stainless steel?

During the smelting process of stainless steel, the amount of alloying elements added varies depending on the type of alloy element added. Their characteristics are also different, and in order to distinguish them, different steel grades are labeled. The following is a table of the "alloy element" content of different steel grades of commonly used decorative stainless steel for reference only:

Chemical composition (mass fraction,%)

steel grade	carbon	silicon	manganese	phosphorus	sulfur	chromium	nickel
304	≤0.08	≤1.00	≤2.00	≤0.045	≤0.03	18-20	8-10
301	≤0.15	≤1.00	≤2.00	≤0.045	≤0.03	16-18	6-8
202	≤0.15	≤1.00	7.5-10	≤0.05	≤0.03	17-19	4-6
201	≤0.15	≤1.00	5.5-7.5	≤0.05	≤0.03	16-18	3.5-5.5

- 304Stainless steel-

Performance Introduction

304Stainless steel is a common type of steel, which has good corrosion resistance, heat resistance, low temperature strength, and mechanical properties as a widely used steel; Good hot workability such as stamping and bending, no heat treatment hardening phenomenon (non-magnetic, suitable for use at a temperature of -196 ° C)℃～800℃）.

Scope of Application

Household items (Class 1 and 2 tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs)

Automotive parts (windshield wipers, mufflers, molded products)

Medical equipment, building materials, chemicals, food industry, agriculture, ship components

- 304LStainless steel-

(L is low-carbon)

Performance Introduction

As a low-carbon 304 steel, its corrosion resistance is generally similar to that of 304 steel, but after welding or stress relief, its resistance to grain boundary corrosion is good; It can maintain good corrosion resistance even without heat treatment, with a usage temperature of -196 ° C℃～800℃.

Scope of Application

Applied to outdoor machines in the chemical, coal, and petroleum industries that require high resistance to grain boundary corrosion, as well as heat-resistant building materials and parts that have difficulty in heat treatment.

- 316Stainless steel-

Performance Introduction

316Stainless 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).

Scope of Application

Equipment, chemicals, dyes, papermaking, oxalic acid, fertilizers and other production equipment used in seawater; Photography, food industry, coastal facilities, ropes, CD poles, bolts, nuts.

- 316LStainless steel-

(L is low-carbon)

Performance Introduction

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

Scope of Application

Products with special requirements for combating grain boundary corrosion.

Performance comparison

chemical composition

316316L stainless steel is a type of stainless steel containing molybdenum. The molybdenum content in 316L stainless steel is slightly higher than that in 316 stainless steel Due to the presence of molybdenum in the steel, its overall performance is superior to 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. 316L stainless steel has a high carbon content of 0.03 and can be used in applications where annealing is not possible after welding and high corrosion resistance is required.

corrosion resistance

316Stainless 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 the erosion of oceans 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 may be differences in certain specific media.

The initially developed stainless steel was 304, which is sensitive to pitting corrosion under specific conditions. Adding an additional 2-3% 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 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 higher than that of 304 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 sedimentation on the valve plate is very small, so point 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, there may be significant differences that need to be analyzed on a case by case basis. Generally speaking, valve users should have a clear understanding because they will choose the material of the container and pipeline based on the condition of 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. Do not continuously apply 316 stainless steel within the temperature range of 800-1575 degrees, but when continuously using 316 stainless steel outside of this temperature range, the stainless steel has good heat resistance. 316L stainless steel has better resistance to carbide precipitation than 316 stainless steel and can be used within the above temperature range.

heat treatment

Annealing is carried out within the temperature range of 1850-2050 degrees, followed by rapid annealing and then rapid cooling. 316 stainless steel cannot be hardened by heat treatment.

welding

316Stainless steel has good welding performance.All standard welding methods can be used for welding. During welding, 316Cb, 316L, or 309Cb stainless steel filler rods or welding rods can be used according to the purpose. To achieve better corrosion resistance, the welded 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 properties

Among all steels, austenitic stainless steel has a lower yield point.Therefore, from the perspective of mechanical performance, austenitic stainless steel is not the best material for use in valve stems 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.

magnetism

Due to the widespread use of austenitic stainless steel, people have the misconception that all stainless steels have no magnetism. For austenitic stainless steel, it can be basically understood as non-magnetic, and this is indeed true for quenched forged steel. But the 304 treated by cold forming will have some magnetism to some extent. For cast steel, if it is pure austenitic stainless steel, it has no magnetism.

Low carbon 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 450℃Up to 900℃At high temperatures, the structure of the material will change, and chromium carbide will form along the edges of the crystal. This prevents the formation of a chromium oxide protective layer at the edge of the crystal, resulting in a decrease in corrosion resistance. This type of corrosion is called 'intergranular corrosion'.

Thus, 304L stainless steel and 316L stainless steel were developed to combat this corrosion. The carbon content of 304L stainless steel and 316L stainless steel is relatively low, because the carbon content is reduced, 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 is also higher.

Please note that this phenomenon is caused by high temperatures (450 ° C)℃-900℃）. Welding is usually the direct cause of reaching this temperature. For conventional butterfly valves with soft seats, using low-carbon stainless steel does not make much sense as we do not perform welding operations on the valve plate. However, most specifications require 304L stainless steel or 316L stainless steel

Why does stainless steel also rust

Why does stainless steel also rust? When brown rust spots (dots) appear on the surface of stainless steel pipes, people are greatly surprised: they believe that "stainless steel does not rust, and if it rusts, it is not stainless steel anymore. It may be a problem with the steel. In fact, this is a one-sided misconception about the lack of understanding of stainless steel. Stainless steel can also rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation, that is, stainless steel, and also has the ability to corrode in media containing acid, alkali, and salt, that is, corrosion resistance. But the magnitude of its corrosion resistance varies with the chemical composition of the steel itself, the state of protection, the conditions of use, and the type of environmental medium. Like 304 steel pipe, it has excellent corrosion resistance in dry and clean atmosphere, but when moved to coastal areas, it will quickly rust in sea fog containing a large amount of salt; And 316 steel pipes perform well. Therefore, not any type of stainless steel can resist corrosion and rust in any environment.