Bearing Steel Pipe Testing

Bearing steel pipe inspection mainly conducts scientific testing and analysis on the chemical composition analysis, low-power structure examination, high-power micro-structure examination, mechanical property testing, surface and internal defect non-destructive testing and dimension and geometric precision testing, 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: As the basis of the material qualification of the bearing steel pipe, the contents of the elements such as the carbon, silicon, manganese, chromium and molybdenum need to be precisely determined. For example, the high-carbon chromium bearing steel (such as GCr15) has the strict requirements for the content ranges of the carbon and chromium, to ensure that the uniform spheroidized pearlite structures are obtained, thereby guaranteeing the quenching hardness, wear resistance and contact fatigue lives of the bearings.

Low-power structure examination: The macro defects of the steel pipe are checked through the acid etching or sulphur printing methods. The main testing items include the general porosity, centre porosity, ingot segregation, as well as whether there are the defects such as the residual shrinkage cavities, subcutaneous bubbles, white spots, laps and internal cracks. These macro defects will seriously affect the load-bearing uniformity and fatigue strength of the bearings, and are the important judgement basis of the internal quality of the material.

High-power micro-structure examination: The micro-structure states of the steel pipe are rated under the microscope. The core contents include the grade rating of the non-metallic inclusions (types A, B, C and D), the grade rating of the banded carbides and network carbides, as well as the spheroidization grade rating of the annealed structures. The advantages and disadvantages of these micro-structures directly determine the processing performance and service lives of the bearings.

Mechanical property testing: Evaluates the basic properties of the steel pipe under the loads. The main testing items include the Brinell hardness or Rockwell hardness, to ensure that the material has the sufficient strength and wear resistance basis. For some bearing steel pipes with the special requirements, the tensile testing may also be carried out, determining the indicators such as the tensile strength, yield strength and elongation.

Surface and internal defect non-destructive testing: Under the premise of not damaging the material, the inner and outer qualities of the steel pipe are comprehensively screened. This is the key link of ensuring that the bearing steel pipe has no harmful defects. The common methods include the ultrasonic flaw detection, eddy current flaw detection, magnetic flux leakage flaw detection and magnetic particle flaw detection, used to find the surface and near-surface defects such as the cracks, folds, inclusions and pits.

Dimension and geometric precision testing: Ensures that the steel pipe meets the dimension requirements of the subsequent finishing and assembly. The testing items cover the tolerances of the outer diameters, inner diameters and wall thicknesses, as well as the ovality, straightness (straightness) and wall thickness unevenness of the steel pipe. The high-precision bearings have the extremely high requirements for the dimension stability and consistency of the steel pipe.

Testing Scope

Material scope: The testing covers the various steels for the bearings, mainly including the high-carbon chromium bearing steels (such as GCr15 and GCr15SiMn in the GB/T 18254 standard), carburized bearing steels (such as G20CrMo and G20CrNi2Mo), stainless bearing steels (such as 9Cr18 and 9Cr18Mo) and high-temperature bearing steels. The testing emphases and standards of the different materials are different.

Product form scope: The testing objects include the hot-rolled seamless bearing steel pipes, cold-drawn (rolled) seamless bearing steel pipes and some precision cold-rolled bearing steel pipes. The steel pipes formed by the different processing processes have the different internal structures, surface states and residual stresses, and the attention points and judgement criteria during the testing are adjusted accordingly.

Defect type scope: The testing scope covers all the defect types from the macro to micro. The macro defects include the surface defects such as the cracks, folds, scars, scratches and pits; the micro defects include the various non-metallic inclusions, carbide inhomogeneities (network, banded and liquid segregation), decarburized layer depths and micro pores.

Process link scope: The testing runs through the whole-process quality control of the bearing steel pipe production. Including the incoming re-inspection of the continuous casting billets or steel ingots, intermediate product inspection after the rolling or forging, structure and property inspection after the heat treatment (such as the spheroidizing annealing), and the final comprehensive inspection before the ex-factory of the finished products.

Standard conformity scope: The testing needs to be based on and verify whether the products meet the relevant national, industrial or enterprise standards. Domestically, mainly based on GB/T 18254 "High carbon chromium bearing steel", and the international standards such as ISO 683-17, ASTM A295 and JIS G4805 are also referenced, meeting the needs of the different markets and customers.

Testing Methods

Spectrometry analysis method: Used for the fast and accurate determination of the chemical compositions. The direct-reading spectrometers (OES) or X-ray fluorescence spectrometers (XRF) are usually adopted. During the sample preparation, the tested surfaces need to be ground flat and smooth, and the contents of the various elements are quantitatively analysed through the intensities of the characteristic spectra generated by the excitation. This method is the necessary means of the furnace-front control and finished product acceptance.

Metallographic examination method: This is the core method of evaluating the internal quality of the bearing steel pipe. Through a series of sample preparation processes such as the sampling, mounting, grinding, polishing and etching, the specimens observable under the microscopes are prepared. Under the optical microscopes or scanning electron microscopes, according to the standard diagrams such as GB/T 18254 or ISO 4967, the non-metallic inclusions and microstructures are qualitatively and quantitatively rated.

Ultrasonic flaw detection method: Mainly used to detect the internal volumetric defects (such as the inclusions and shrinkage cavities) and planar defects (such as the delaminations and cracks) of the steel pipe. The pulse reflection method is adopted. The probes are coupled on the steel pipe surfaces, and the positions and equivalent sizes of the defects are judged by receiving the ultrasonic signals reflected from the internal defects. It is crucial for ensuring the internal integrity of the material.

Eddy current/magnetic flux leakage flaw detection method: These two electromagnetic testing methods are mainly used for the high-speed and automated detection of the surface and near-surface defects of the steel pipe. The eddy current flaw detection is sensitive to the surface cracks; the magnetic flux leakage flaw detection has the good detection effects for the deeper pits and fold-type defects. They are often used in combination with the ultrasonic flaw detection, forming the complementary detection of the internal and external defects of the steel pipe.

Mechanical property testing method: The hardness testing usually adopts the Brinell hardness testers (HBW) or Rockwell hardness testers (HRC). During the testing, the flat specimen surfaces need to be selected, ensuring that the indentation spacings meet the standards. The tensile testing is carried out on the universal material testing machines, obtaining the strength and plasticity indicators of the material by recording the load-displacement curves.

Dimension measurement method: The various measuring tools and instruments are comprehensively used. The outer diameters and inner diameters commonly use the digital callipers, micrometers or laser diameter measuring instruments; the wall thicknesses can be measured with the ultrasonic thickness gauges or dedicated micrometers; the straightness (straightness) is rated by placing the steel pipe on the platform and measuring the maximum bending clearances with the feeler gauges or using the laser straightness measuring instruments.

Testing Standards

Domestic Standards:

1. GB/T 18254, High carbon chromium bearing steel

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

International Standards:

1. ISO 683-17, Heat-treated steels, alloy steels and free-cutting steels — Part 17: Ball and roller bearing steels

2. ISO 4967, Steel — Determination of content of nonmetallic inclusions — Micrographic method using standards diagrams

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

4. ASTM A295, Standard specification for high-carbon anti-friction bearing steel

5. JIS G4805, High carbon chromium bearing steels

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

Direct-reading spectrometer: The core equipment of the chemical composition analysis. Its working principle is to excite the samples through the electric arcs or sparks, generate the atomic characteristic spectra, receive them with the photomultiplier tubes or CCD detectors after the grating dispersion and convert them into the electrical signals, and the computers calculate the contents of the various elements. It has the advantages of the fast analysis speed, high precision and simultaneous determination of the multiple elements.

Metallographic microscope and image analysis system: The professional instruments for the micro-structure observation and rating. Including the optical microscopes (usually equipped with the functions such as the bright field, dark field and polarization) and the more advanced scanning electron microscopes (SEM). Combined with the professional image analysis software, the automatic statistics and quantitative analysis of the inclusion quantities, sizes, shapes and grain sizes can be realized.

Ultrasonic flaw detector: The key equipment of the non-destructive testing of the internal defects. Mainly consisting of the ultrasonic generators, transducers (probes), signal amplifiers and displays. For the bearing steel pipe testing, the multi-channel automatic flaw detection systems are usually adopted, equipped with the focused probes or combined probes, which can realize the full-coverage scanning during the spiral advancement of the steel pipe, and automatically mark and sort the defective pipes.

Eddy current/magnetic flux leakage automatic flaw detection line: The automated equipment realizing the high-speed on-line non-destructive testing of the steel pipes. The eddy current equipment consists of the oscillators, detection coils and signal processing units; the magnetic flux leakage equipment consists of the magnetization devices, magnetic sensitive transducers and signal analysis units. They are integrated into the production lines, can detect and reject the steel pipes with the surface defects in real time, with the extremely high detection efficiency.

Hardness tester and universal material testing machine: The basic equipment of the mechanical property testing. The Brinell/Rockwell/Vickers hardness testers are used to measure the ability of the material to resist the local indentation deformations. The microcomputer-controlled electronic universal testing machines apply the loads through the servo motors driving the ball screws, equipped with the high-precision transducers and extensometers, can precisely complete the tests such as the tensile, compression and bending, and automatically generate the reports.

Precision dimension measuring instruments: The series of tools guaranteeing the geometric precision. Including the high-precision digital display micrometers, laser diameter measuring instruments (for the on-line dynamic measurement of the outer diameters), ultrasonic thickness gauges, as well as the large platforms, right-angle rulers and dial indicators for measuring the straightness and ovality. The coordinate measuring machines (CMM) are used for the precision testing of the three-dimensional spatial dimensions of the complex or high-precision samples.