Precision Seamless Steel Tube Testing

Precision seamless steel tube inspection mainly conducts scientific testing and analysis on the chemical composition analysis, dimension and shape inspection, mechanical property testing, hardness testing, surface quality inspection and microstructural inspection, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Chemical composition analysis: Through the methods such as the spectroscopy analysis, the contents of the carbon, silicon, manganese, phosphorus, sulphur and the alloying elements such as the chromium, nickel and molybdenum in the steel tube are precisely determined. This is the basis for judging whether the material grade of the steel tube meets the standards (such as GB/T 699, GB/T 1591 and ASTM A106), and directly affects its mechanical properties and corrosion resistance.

Dimension and shape inspection: High-precision callipers, micrometers and laser diameter measuring instruments are used to comprehensively measure the outer diameter, inner diameter, wall thickness, ovality, wall thickness unevenness, length and straightness of the steel tube. For the precision seamless steel tubes, the control of the various dimensional tolerances is extremely strict, and is the key to ensuring their interchangeability and the precision of the subsequent processing.

Mechanical property testing: The tensile testing is carried out on a universal testing machine to determine the tensile strength, yield strength, elongation after fracture and reduction of area of the steel tube. These data are the important basis for evaluating the load-bearing capacity and plastic deformation ability of the steel tube, ensuring that it does not produce the excessive deformation or fracture under the service conditions.

Hardness testing: The Brinell (HBW), Rockwell (HRB/HRC) or Vickers (HV) hardness testers are commonly used for the measurement. The hardness values can indirectly reflect the strength, wear resistance and processing performance of the material, and are used to verify the effect of the heat treatment process. It is an important auxiliary indicator of the quality control.

Surface quality inspection: Through the visual inspection, endoscope or surface roughness tester, it is checked whether there are the defects such as the cracks, folds, scars, scratches, pitting and oxide scales on the inner and outer surfaces of the steel tube. The precision seamless steel tubes usually require the smooth surfaces, and the depth and length of the defects need to be strictly controlled within the ranges allowed by the standards.

Microstructural inspection: The metallographic specimens are prepared, and the microstructure of the steel tube, such as the morphology, size and distribution of the ferrite, pearlite, austenite and martensite, is observed under an optical or electron microscope. The microstructure analysis is the core link connecting the chemical composition, heat treatment process and the final performance, and can judge whether the structure is normal and whether there are the overheating, overburning or abnormal structures.

Testing Scope

Classification by material: Covering the precision seamless steel tubes of the various materials such as the carbon steels, alloy steels, stainless steels and high-temperature alloys. Such as the 20# and 45# carbon steels, 42CrMo and 15CrMo alloy steels, and 304 and 316L stainless steels. Each material corresponds to the different testing standards and performance requirements.

Classification by specification: The testing is applicable to the precision seamless steel tubes of the various specifications from the small diameters (such as the outer diameter Φ6 mm) to the large diameters (such as the outer diameter above Φ200 mm), and from the thin walls to the thick walls. For the special specifications such as the ultra-long tubes and extra-thick wall tubes, the adapted testing equipment and schemes need to be adopted.

Classification by application: For the different application fields such as the tubes for the hydraulic and pneumatic cylinder tubes, bearing tubes, machining tubes, automotive tubes, aerospace tubes, and boiler and heat exchanger tubes. The steel tubes of the different applications have the different testing items and acceptance criteria, for example, the hydraulic cylinder tubes emphasize the dimensional precision and surface roughness, while the boiler tubes focus on the high-temperature performance.

Classification by process stage: Including the incoming inspection of the raw materials (tube blanks), online inspection during the production process (such as the eddy current flaw detection), final inspection before the ex-factory of the finished products and the incoming re-inspection of the user side. The whole process quality control is implemented to ensure that the quality of each link reaches the standard.

Classification by standard system: The testing basis can cover the requirements of the different systems such as the national standards (GB/T), international standards (ISO), American Society for Testing and Materials standards (ASTM), European standards (EN), German standards (DIN), as well as the industry standards and enterprise internal control standards.

Testing Methods

Non-destructive testing (NDT): Under the premise of not damaging the inspected object, the physical methods are used to detect the internal and surface defects. Mainly including the ultrasonic testing (UT), used to find the defects such as the internal cracks and delaminations; eddy current testing (ET), sensitive to the surface and near-surface cracks and folds; magnetic particle testing (MT), applicable to the surface and shallow surface defects of the ferromagnetic materials; and penetrant testing (PT), used for the surface open defects of the non-ferromagnetic materials.

Physical and chemical analysis methods: The chemical analysis adopts the spark direct-reading optical emission spectrometer (OES) or inductively coupled plasma optical emission spectrometer (ICP-OES) for the fast and accurate quantitative element analysis. The metallographic analysis, after the sample preparation steps such as the cutting, mounting, grinding, polishing and etching, carries out the structure rating and defect observation under a metallographic microscope.

Mechanical testing methods: In accordance with the standards such as GB/T 228.1 or ASTM A370, the standard tensile specimens are prepared, and the tensile testing is carried out at the room temperature or the specific temperature, recording the stress-strain curves. The hardness testing needs to select the appropriate hardness scales and test forces according to the expected hardness of the material and the standard requirements.

Dimension measurement methods: The outer diameter and wall thickness are usually measured at multiple points with the outer diameter micrometers, wall thickness micrometers or online laser diameter/thickness measuring systems. The inner diameter can be measured with the inner diameter dial gauges or air gauges. The straightness (bend) can be measured with a platform combined with feeler gauges, or with a laser straightness measuring instrument.

Surface inspection methods: The macroscopic inspection is carried out under the sufficient lighting conditions, and a magnifying glass is used when necessary. The inner surface inspection needs to be carried out with the aid of an endoscope. The surface roughness is measured with a stylus-type or optical roughness tester along the axial and circumferential directions respectively, and the Ra or Rz values are evaluated.

Pressure resistance and sealing tests: For the steel tubes used for the fluid transportation, the hydrostatic testing or pneumatic testing needs to be carried out. Under the specified pressure, the pressure is maintained for a certain time, and it is checked whether there are the leakage, permanent deformation or fracture of the tube body. This is an important method for verifying the pressure-bearing integrity of the steel tube.

Testing Standards

Domestic Standards:

1. GB/T 3639-2009, Cold drawn or cold rolled precision seamless steel tubes

2. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

3. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

4. GB/T 232-2010, Metallic materials — Bend test method

5. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

6. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: Test method

International Standards:

1. EN 10305-1, Steel tubes for precision applications — Technical delivery conditions — Part 1: Seamless cold drawn tubes

2. ASTM A106, Standard specification for seamless carbon steel pipe for high-temperature service

3. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

4. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Spectrometer: The direct-reading optical emission spectrometer is the essential equipment for the rapid chemical composition analysis, which can complete the quantitative analysis of the multiple elements in the steel within tens of seconds, with the high precision and good repeatability. It is applicable to the rapid analysis before the furnace and the finished product inspection, and is the key tool for controlling the material composition consistency.

Universal material testing machine: An electronic or hydraulic testing machine used to carry out the mechanical property tests such as the tensile, compression and bending. Equipped with the high-precision sensors and extensometers, it can automatically collect the data and generate the reports, and is the core equipment for obtaining the core performance data such as the strength and plasticity of the material.

Hardness testers: Equipped with the Brinell, Rockwell, Vickers or Leeb hardness testers according to the testing requirements. The bench-type hardness testers have the high precision and are suitable for the laboratory; the portable hardness testers are convenient for the field and large workpiece testing. Some equipment can automatically rotate the turret and load, realizing the rapid measurement.

Non-destructive testing equipment: Including the multi-channel ultrasonic flaw detectors (with the automatic scanning devices), through-type or point-type eddy current flaw detectors, magnetic particle flaw detectors and fluorescent penetrant testing lines. These equipment can efficiently and comprehensively detect the internal and external defects of the products, and are the barriers for ensuring the safety and reliability of the steel tube.

Precision dimension measurement equipment: Such as the laser diameter measuring instruments, laser thickness measuring instruments, coordinate measuring machines (CMM), large vernier callipers, depth gauges and thread gauges. For the high-precision steel tubes, the coordinate measuring machines can realize the precise evaluation of the complex geometric tolerances.

Metallographic microscope and sample preparation equipment: The metallographic microscope (including the image analysis system) is used for the structure observation and quantitative analysis. The supporting cutting machines, mounting machines, grinding and polishing machines and etching devices are the basis for preparing the qualified metallographic specimens, ensuring the accuracy and representativeness of the structure observation.

Hydrostatic testing machine: A dedicated steel tube hydrostatic testing equipment, which can automatically complete the processes such as the loading, sealing, water filling, pressurizing, pressure holding, pressure releasing and unloading, and record the pressure-time curves. The test pressure is adjustable and the pressure holding time is controllable, and it is the key equipment for verifying the pressure-bearing capacity of the steel tube.