Pipe, pipe fittings and common sense

First, the pipeline system overview:

In order to transport liquids or gases, various pipes must be used. In addition to the steel pipes for straight pipes, various pipe fittings are also required: When the pipe is bent, elbows must be used. When the pipe is tapered, the heads must be used. To use the tee, the flange of the pipe joint should be connected with the joint. In order to achieve the purpose of opening the conveying medium, various valves are also needed. In order to reduce the influence of thermal expansion or contraction or frequent vibration on the pipeline system, expansion is also required. Section. In addition, on the pipeline, there are various joints, plugs, etc. connected to various instruments. We are used to collectively refer to components other than straight pipes in pipe systems as pipe fittings.

Second, metal materials common sense:

Metal materials are the most widely used materials in our daily life and industry. It is mainly divided into steel, iron and non-ferrous metals, and the steel is the most common material used. The main component of steel is iron, and the rest is artificially added alloying elements and various impurities. It is precisely because of the variety of these added alloying elements. A variety of steels are formed in different quantities, such as ordinary carbon steel, stainless steel, alloy steel, etc. Among these added elements, carbon C plays a very important role.

2.1 Common chemical elements in steel:

The majority of the various steels are iron (Fe), which usually contains the following elements (commonly referred to as alloying elements in steel): C (carbon) Si (silicon) Mn (manganese) P (phosphorus) S (sulfur) and Cr (chromium) Ni (nickel) Mo (molybdenum) Ti (titanium) V (vanadium) and the like. Under normal circumstances, where P and S are impurity components, the lower the steel quality, the better.

2.2 Classification of steel:

According to the variety of alloying elements added in steel, we can simply divide steel into three categories: carbon steel. alloy steel. stainless steel.

(1) Carbon steel: Among them, there are only five kinds of alloying elements: C, Si, Mn, P, and S. According to the content of P and S impurities, they are divided into ordinary carbon steel (P, S is generally ≤0.040%) and excellent carbon steel (P , S is generally ≤0.03%)

Common steel grades are: plain carbon steel: Q215A.Q235BF. Excellent carbon steel: 20#.45#.16Mn and so on. This steel has the same strength and toughness, is not corrosion resistant, and can be used in applications where the requirements are not high, and the cost is the lowest.

Alloy steel: In addition to the five elements contained in carbon steel, 10% or less of elements such as Cr Mo V are added. Common steel types are: 15CrMo 12Cr1MoV 1Cr5Mo. Compared with carbon steel, alloy steel has higher strength and higher temperature resistance, but its corrosion resistance is still poor. Therefore, alloy steel is usually used in high temperature and high pressure applications where corrosion is not high, such as boiler steel, power plant hot steam transportation. Etc., the cost of use is at a medium level.

(3) Stainless steel: usually based on carbon steel. A high proportion of alloys such as Cr and Ni are added, and the content ratio can reach more than 20%. Common steel grades are: 304, 304L, 321, 316, 316L, 1Cr18Ni9Ti. The first few steel numbers expressed by numbers are Japanese and American steel numbers, and the last one (1Cr18Ni9Ti) is the domestic steel number representation method. The following is a numerical representation of the steel number as an example to illustrate the relationship between several stainless steel components: as a general understanding, and for the convenience of beginners, we can think of it (but not very accurate):

304-------Basic steel type, only containing (C<0.08%), Cr (~18%), Ni (~9%)

304L-----304 (C<0.05%) of ultra-low carbon C is called 304L

321------- 304 + Ti (~0.5%)

316------- 304 + Mo (~2.5%)

316L----- ultra-low carbon C 316 (C<0.05%) is called 316L

The strength and toughness index of stainless steel is the best among various steels. Its most special advantage is corrosion resistance. Stainless steel must be used in corrosive environments such as chemical papermaking. Of course, its cost is also the highest.

2.3. Performance of steel:

The performance of a steel is always reflected by some indicators. For steel, we usually use the chemical composition and mechanical properties of the steel to reflect its quality and performance. Mechanical properties usually have three indicators:

Tensile strength (σb, TS): The maximum external tensile force a material can withstand when it is broken.

Yield strength (σs, YS): The minimum external tensile force that a material can withstand plastic deformation.

Elongation (ψ, EL): The percentage of the material that extends in the length direction after breaking. The larger the value, the better the plasticity of the material. Sometimes, the hardness value, the impact toughness value, and the like are also used to reflect the material properties.

Our most common carbon steel 20#, Q235 and foreign SS400, SS41, St37, etc., their performance is very close, or basically can replace each other, their relevant parameters are as follows:

C: 0.18-0.21% Si: 0.30% Mn: 0.5% P, S 0.025%

TS: ~550Mpa YS: ~260Mpa EL: ~ 25%

2.4 Steel structure and its changes