What Is a Furnace Duct?

Boiler tube refers to steel that is open at both ends and has a hollow section. The length of the tube is larger than that of the surrounding steel. According to the production method, it can be divided into seamless steel pipes and welded steel pipes. Wall thickness indicates a wide range of sizes, from a capillary with a small diameter to a large-diameter tube with a diameter of several meters. Steel pipes can be used in pipelines, thermal equipment, machinery industry, petroleum geological exploration, containers, chemical industry and special applications.

Steel pipe points

Boiler tube is a kind of seamless tube. The manufacturing method is the same as that of the seamless pipe, but

General boiler tubes are mainly used to manufacture water-cooled wall tubes, boiling water tubes, superheated steam tubes, superheated steam tubes for locomotive boilers, large and small smoke pipes, and arch brick pipes.

High-pressure boiler tubes are mainly used to manufacture superheater tubes, reheater tubes, air ducts, and main steam tubes for high-pressure and ultra-high-pressure boilers. The supply and demand trend of the high-pressure boiler tube industry is generally stable, but the supply and demand situation of specific sub-sectors will further differentiate. Insiders pointed out that the most critical link is the use and promotion of new energy-saving and thermal insulation 20g high-pressure boiler tube equipment.

New energy-saving 20g high-pressure boiler tube products are gradually increasing in the market, such as green environmental protection coatings, energy-saving water-saving bathroom products, environmental protection stone, environmental protection cement foam insulation boards, etc. Energy-saving and environmental protection products are in the broad market of 20g high-pressure boiler tubes. China's prospects are promising. ^[1]

(1) The grades of high-quality carbon structural steel are 20G, 20MnG, and 25MnG.

(2) Alloy structural steel grades 15MoG, 20MoG, 12CrMoG, 15CrMoG, 12Cr2MoG, 12CrMoVG, 12Cr3MoVSiTiB, etc.

(3) 1Cr18Ni9, 1Cr18Ni11Nb boiler tubes commonly used for rust-resistant and heat-resistant steels, in addition to ensuring chemical composition and mechanical properties, must be subjected to hydraulic tests one by one, and flaring and flattening tests must be performed. The steel pipe is delivered in a heat treated state.

In addition, there are certain requirements for the microstructure, grain size, and decarburization layer of the finished steel pipe. ^[2]

(1) GB3087-2008 "Seamless Steel Pipe for Low and Medium Pressure Boilers". Tensile test according to GB / T228-87, hydraulic pressure test according to GB / T241-90, flattening test according to GB / T246-97, flaring test according to GB / T242-97, cold bending test according to GB244-97.

(2) GB5310-2008 "Seamless Steel Tubes for High Pressure Boilers". Tensile test, hydraulic test and flattening test are the same as GB3087-82; impact test is GB229-94, flaring test is GB / T242-97, grain size test is YB / T5148-93; microstructure inspection is GB13298-91, decarburized layer inspection according to GB224-87, ultrasonic inspection according to GB / T5777-96.

(3) The physical performance inspection and indicators of imported boiler tubes shall be carried out in accordance with the relevant standards stipulated in the contract. ^[2]

tensile strength

The maximum force (Fb) that the sample is subjected to during the tensile breaking process is the stress () obtained from the original cross-sectional area (So) of the sample, which is called the tensile strength (b). The unit is N / mm ² (MPa). It indicates the maximum ability of a metal material to resist damage under tensile forces. The calculation formula is:

In the formula: Fb--the maximum force that the specimen withstands, N (Newton); So--the original cross-sectional area of the specimen, mm ²

Yield point

For a metallic material with a yield phenomenon, the stress at which the specimen can continue to stretch without increasing the force during tension (keep constant) is called the yield point. If the force decreases, the upper and lower yield points should be distinguished. The unit of yield point is N / mm ² (MPa).

Upper yield point (su): The maximum stress before the first drop in the specimen yield force; Lower yield point (sl): The minimum stress in the yield phase when the initial transient effect is not taken into account.

The formula for the yield point is:

In the formula: Fs--yield force (constant) in the tensile process of the sample, N (Newton) So--the original cross section of the sample

Deviation level	Standardized outside diameter tolerance
D1	± 1.5% , minimum ± 0.75 mm
D2	± 1.0%, minimum ± 0.50 mm
D3	± 0.75%, minimum ± 0.30 mm
D4	± 0.50%, minimum ± 0.10 mm

1 tube blank control

The billet has a greater impact on the production of seamless steel pipes, mainly in terms of the stability of the chemical composition of the steel and the purity of the steel. Generally, the high-pressure pipe billet uses the "electric furnace + LF + VD" production process. The electric arc furnace is mainly used to smelt qualified crude molten steel. In addition to the deoxidation, desulfurization and temperature control of the LF furnace, the most important task is to adjust the molten steel Chemical composition, VD furnace is mainly to purify molten steel to ensure the purity of molten steel. According to the application characteristics of high-pressure boiler tubes, in addition to the requirements of the chemical composition of the boiler steel for the smelting control technology, the purity of the boiler steel is also required, and the control of S and P harmful elements is low. In particular, the S element must be strictly controlled. , Mainly because the S element is the main impurity element that causes the weakening of the original grain boundary, and is the main factor that causes the creep embrittlement of steel. For a long time, boiler tubes have been manufactured from forged and rolled tube blanks. Forged and rolled tube blanks can not only cut steel defects such as shrinkage holes, slag inclusions, etc., but also eliminate residual defects through large rolling deformations.

In recent years, with the substantial improvement of metallurgical process equipment, the continuous casting and rolling production process of tube billets has been rapidly promoted due to its high efficiency, low consumption, high quality, and low cost. Tube billets are generally equipped with steel rod leakage Magnetic flaw detection, ultrasonic phased array flaw detection, or infrared flaw detection are used to detect defects, and the relevant supporting means are to be used for surface peeling.

2 Control Points of Heat Treatment Process

In the production process of seamless steel tubes for boilers, heat treatment is a key process. Heat treatment has an important impact on the internal quality and surface quality of seamless steel tubes, and is particularly important for the production of alloy seamless steel tubes. It also affects the long-term performance of high-pressure boiler tubes for power station boilers. Large, stable metallographic structure and good internal and external surface quality represent the manufacturing level of steel pipe enterprises. After the chemical composition of the steel pipe is determined, it is the heat treatment process that ultimately gives the steel pipe excellent mechanical properties and internal organization. The process parameters given by the ASME standard are relatively wide. Manufacturers need field tests to find the appropriate heat treatment process. T22's usual heat treatment process: normalizing at 920 -940 , holding for more than 20 minutes; tempering at 720 -740 , holding for 2h.

3 Control of nondestructive testing

Generally domestic boiler enterprises purchase high-pressure boiler tubes in accordance with GB5310 or enterprise procurement regulations (ordering technical conditions). Non-destructive testing generally uses eddy current and ultrasonic automatic flaw detection, requiring eddy current flaw detection of steel pipes one by one. The steel pipe is subjected to ultrasonic flaw detection one by one. The flaw detection method is according to GB5777. The acceptance standards of all specifications of steel pipes shall meet the requirements of L2. Ultrasonic thickness measurement and oblique flaw detection functions have begun to be deployed in domestic automatic flaw detection equipment, which will better improve the performance of domestic equipment and effectively detect layered defects in steel pipes. ^[3]