High-pressure boiler seamless steel tubes are often under high temperature and pressure when used. The tubes will be oxidized and corroded under the action of high temperature flue gas and water vapor. It is required that the steel pipe has high durability, high resistance to oxidation and corrosion, and good structural stability. High-pressure boiler seamless 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 special-shaped seamless steel pipe is a general term for seamless steel pipes of other cross-sectional shapes other than round pipes. According to the different shapes and sizes of the steel pipe section, it can be divided into equal-wall thickness shaped seamless steel pipes (code D), unequal-wall thickness shaped seamless steel pipes (code BD), and variable-diameter shaped seamless steel pipes (code BJ). Shaped seamless steel tubes are widely used in various structural parts, tools and mechanical parts. Compared with the round pipe, the special-shaped pipe generally has a larger moment of inertia and a section modulus, and has a greater resistance to bending and torsion, which can greatly reduce the structural weight and save steel.
Production method of boiler seamless tube
Boiler seamless tube is a kind of seamless tube. The manufacturing method is the same as that of seamless pipes, but there are strict requirements on the steel grades used in the manufacture of steel pipes. According to the operating temperature, it can be divided into general boiler tubes and high-pressure boiler tubes.
(1) Manufacturing method:
① Generally, the service temperature of boiler seamless pipe is below 450℃. Domestic pipes are mainly made of No. 10 and No. 20 carbon steel hot-rolled pipes or cold-drawn pipes.
②High-pressure boiler tubes are often in high temperature and high pressure conditions when they are used. The tubes will be oxidized and corroded under the action of high-temperature flue gas and water vapor. The steel pipe is required to have high enduring strength, high oxidation and corrosion resistance, and good organizational stability.
Seamless steel tube production process:
a. Hot rolled (extruded) seamless steel tube: round tube billet → heating → piercing → three-roll cross rolling, continuous rolling or extrusion → tube removal → sizing (or reducing) → cooling → straightening →Hydrostatic test (or flaw detection) → marking → storage
b. Cold drawn (rolled) seamless steel tube: round tube blank→heating→perforation→heading→annealing→pickling→oiling (copper plating)→multi-pass cold drawing (cold rolling)→blank tube→heat treatment→Straightening→hydrostatic test (flaw detection)→marking→warehousing
Boiler seamless steel tube adopts steel grade:
(1) High-quality carbon structural steel grades are 20G, 20MnG, and 25MnG.
(2) Alloy structural steel grades 15MoG, 20MoG, 12CrMoG, 15CrMoG, 12Cr2MoG, 12CrMoVG, 12Cr3MoVSiTiB, etc.
(3) The commonly used 1Cr18Ni9 and 1Cr18Ni11Nb boiler tubes with rust and heat-resistant steels should be subjected to hydrostatic tests, flaring and flattening tests one by one in addition to ensuring chemical composition and mechanical properties. Steel pipes are delivered in heat-treated condition.
In addition, there are certain requirements for the microstructure, grain size and decarburization layer of the finished steel pipe.
The production process of high-pressure boiler tubes varies with different steel types.
Taking pearlitic chromium molybdenum vanadium steel as an example, the production process characteristics of boiler tubes are:
(1) The tube blank should be peeled, and the peeling amount is usually 5mm;
(2) Due to the hard steel, the tube blank is often cut or sawed with oxygen;
(3) Since the thermal conductivity of chrome-molybdenum-vanadium steel is lower than that of carbon steel, the heating rate should be slightly slower, the heating temperature is 1120-1180°C, and the piercing temperature is 1100-1160°C;
(4) This type of steel has good plasticity and low deformation resistance in the range of 1000-1100 ° C, so the perforation performance is better, and the deformation parameters can be selected according to medium carbon steel or alloy steel (such as 30CrMnSiA);
(5) After rolling, the steel pipe should be normalized and tempered. The normalizing temperature is 950-980°C, the tempering temperature is 730-750°C, and the holding time is 2-3 hours;
(6) The dimensional tolerance of the steel pipe is relatively strict to ensure the butt joint welding; the length of the pipe is as long as possible to reduce the number of welding joints.