How to carry out quality control on seamless boiler tubes?

Boiler tubes refer to steel products that are open at both ends and have a hollow section. The ratio of its length to the surrounding area is large. According to the production method, it can be divided into seamless steel tubes and welded steel tubes. Wall thickness indicates a wide range of sizes, from capillary tubes with very small diameters to large-bore tubes up to several meters in diameter. Seamless steel pipes can be used in pipelines, thermal equipment, machinery industry, petroleum geological exploration, containers, chemical industry and special purposes.

Quality and Inspection

Specifications and Appearance

(1) GB3087-2008 “Seamless Steel Tubes for Low and Medium Pressure Boilers”. There are 43 types of steel pipe specifications for boilers with various structures, with an outer diameter of 10 to 426 mm. A total of 29 kinds of wall thickness 1.5 ~ 26mm. However, the outer diameter and wall thickness of superheated steam pipes, large smoke pipes, small smoke pipes and arch brick pipes used in locomotive boilers are otherwise regulated.
(2) GB5310-2008 “Seamless Steel Tubes for High Pressure Boilers” has an outer diameter of 22-530mm and a wall thickness of 20-70mm. Cold-drawn (cold-rolled) tubes have an outer diameter of 10-108 mm and a wall thickness of 2.0-13.0 mm.
(3) GB3087-2008 “Seamless Steel Tubes for Low and Medium Pressure Boilers” and GB5310-95 “Seamless Steel Tubes for High Pressure Boilers”. Appearance quality: cracks, folds, rolling folds, scars, delamination and hairlines are not allowed on the inner and outer surfaces of steel pipes. These defects should be completely removed. The cleaning depth shall not exceed the negative deviation of the nominal wall thickness, and the actual wall thickness of the cleaning place shall not be less than the minimum value allowed by the wall thickness.

chemical test

(1) GB3087-2008 “Seamless Steel Tubes for Low and Medium Pressure Boilers”. The chemical composition test method is in accordance with the relevant parts in GB222-84 and GB223 “Chemical Analysis Methods of Iron, Steel and Alloy”.
(2) GB5310-2008 “Seamless Steel Tubes for High Pressure Boilers” regulations. The chemical composition test method is in accordance with relevant parts in GB222-84 and “Chemical Analysis Methods of Iron and Steel and Alloys”, and GB223 “Chemical Analysis Methods of Iron, Steel and Alloys”.
(3) The chemical composition inspection of imported boiler steel pipes shall be carried out in accordance with the relevant standards stipulated in the contract.

steel number

(1) The steel grades of high-quality carbon structural steel are 20G, 20MnG, and 25MnG.
(2) Alloy structural steel Steel grades 15MoG, 20MoG, 12CrMoG, 15CrMoG, 12Cr2MoG, 12CrMoVG, 12Cr3MoVSiTiB, etc.
(3) The 1Cr18Ni9 and 1Cr18Ni11Nb boiler tubes commonly used in rusty and heat-resistant steels must be subjected to hydrostatic tests one by one in addition to ensuring chemical composition and mechanical properties, and flaring and flattening tests. Steel pipes are 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.

T22 Quality Control

(1) Tube billet control

The tube billet has a great influence 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 tube blank usually adopts the production process of “electric furnace + LF + VD”. 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 terms of smelting control technology of boiler steel, in addition to the chemical composition to hit the target, it is also required that the purity of boiler steel should be high, and the control of S and P harmful elements should be low, especially the S element should be strictly controlled. , mainly because S element is the main impurity element that causes the weakening of the original grain boundary, and is the main factor leading to the creep embrittlement of steel. For a long time, boiler tubes have been manufactured by forging and rolling billets. Forging and rolling billets can not only remove shrinkage cavities, slag inclusions and other pouring steel defects on steel ingots, but also eliminate residual defects through large rolling deformation.

In recent years, with the substantial improvement of the level of metallurgical process equipment, the production process of continuous casting and rolling of tube billet has been rapidly promoted due to its advantages of high efficiency, low consumption, high quality and low cost. Magnetic flaw detection, ultrasonic phased array flaw detection or infrared flaw detection and other detection methods, surface peeling and other related supporting means for defects found.

(2) Control points of heat treatment process

In the production process of seamless steel pipes for boilers, heat treatment is a key process. Heat treatment has an important impact on the internal quality and surface quality of seamless steel pipes, especially for the production of alloy seamless steel pipes, and has an impact on the durability of high-pressure boiler tubes for power plant boilers. Larger, 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 finally endows the steel pipe with excellent mechanical properties and internal organization. The ASME standard provides a wide range of process parameters. Manufacturers need to conduct field tests to find out the appropriate heat treatment process. The usual heat treatment process of T22: normalizing at 920°C-940°C, holding for more than 20 minutes; tempering at 720°C-740°C, holding for 2 hours.

(3) Control of non-destructive testing

Generally, boiler companies 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. It is required to conduct eddy current flaw detection for steel pipes one by one. The eddy current flaw detection method and acceptance standards are controlled by GB7735 higher level; The steel pipes are subjected to ultrasonic flaw detection one by one, and the flaw detection method is in accordance with GB5777 grade. The acceptance criteria for all specifications of steel pipes should meet the requirements of L2 grade; the non-destructive testing of outlet pipes or steel pipes delivered in accordance with ASME SA-213 standards is controlled in accordance with SA-450.