Mechanical properties and quality control of boiler tubes
Posted by zora li on October 24th, 2023
Boiler tube is a type of seamless pipe. The manufacturing method is the same as that of seamless pipes, but there are strict requirements for the type of steel used in manufacturing steel pipes. According to the operating temperature, it is divided into two types: ordinary boiler tubes and high-pressure boiler tubes.
The mechanical properties of boiler tubes are important indicators to ensure the final performance (mechanical properties) of steel. It depends on the chemical composition and heat treatment system of the steel. In the steel pipe standards, according to different usage requirements, the tensile properties (tensile strength, yield strength or yield point, elongation), hardness and toughness indicators required by users are specified, as well as the high and low temperature properties required by users.
1) Boiler tubes generally use hot-rolled or cold-drawn tubes with a temperature below 350°C.
2) High-pressure boiler tubes are often used under high-temperature and high-pressure conditions. Under the action of high-temperature flue gas and water vapor, the tubes will oxidize and corrode. Steel pipes are required to have high lasting strength, high resistance to oxidation and corrosion, and good structural stability.
Folding specifications and appearance
1) Steel pipe specifications for structural boilers, outer diameter 10~426mm, 43 types in total. There are 29 types of wall thickness from 1.5 mm to 26 mm. However, there are other regulations on the outer diameter and wall thickness of locomotive boiler superheated steam pipes, large smoke pipes, small smoke pipes, and arch brick pipes.
2) The outer diameter of hot-rolled pipe is 22~530mm, and the wall thickness is 20~70mm. The outer diameter of cold drawn (cold rolled) pipe is 10~108mm, and the wall thickness is 2.0~13.0mm.
3) Appearance quality: There are no cracks, wrinkles, folds, scars, delaminations and hair lines on the inner and outer surfaces of the steel pipe. These defects should be completely eliminated. The cleaning depth shall not exceed the negative deviation of the nominal wall thickness, and the actual wall thickness at the cleaning location shall not be less than the minimum allowable wall thickness.
Boiler tube quality control
1) Tube blank control
The tube blank has a great influence on the production of seamless boiler tubes, mainly in terms of the stability of the chemical composition of the steel and the purity of the steel. In recent years, with the substantial improvement in the level of metallurgical technology and equipment, the continuous casting and rolling production process of tube blanks has been rapidly promoted due to its advantages of high efficiency, low consumption, high quality, and low cost. In order to ensure surface quality, tube blank factories are generally equipped with detection methods such as steel bar magnetic flux leakage detection, ultrasonic phased array flaw detection or infrared flaw detection, as well as surface peeling and other related supporting means when defects are found. .
2) Key points of heat treatment process control
In the production process of seamless steel pipes for boilers, heat treatment is a key process.
Heat treatment has an important impact on the intrinsic quality and surface quality of seamless steel pipes, especially for the production of alloy seamless steel pipes, and has a great impact on the durable performance of high-pressure boiler pipes for power station boilers.
Stable metallographic structure and good internal and external surface quality represent the manufacturing level of steel pipe companies.
3) Control of non-destructive testing
Generally, non-destructive testing of high-pressure boiler tubes generally uses eddy current and ultrasonic automatic flaw detection, and it is necessary to conduct eddy current flaw detection on the steel pipes one by one.
