Hydraulic Steel Pipe Testing

Hydraulic steel pipe inspection mainly conducts scientific testing and analysis on the dimension and geometric precision testing, chemical composition analysis, mechanical property testing, surface quality and non-destructive testing, pressure resistance and sealing performance testing and metallographic structure and grain size examination, 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

Dimension and geometric precision testing: This is the most basic and critical testing item, including the precise measurement of the outer diameters, inner diameters, wall thicknesses, ovality and straightness of the steel pipe. The dimension deviations directly affect the connection sealing, pressure-bearing capacity and fluid resistance losses of the pipeline, and are the basic basis for judging whether the product meets the drawing and technical agreement requirements.

Chemical composition analysis: Through the methods such as the spectrometry analysis or chemical titration, the element contents of the steel pipe material, such as the carbon, silicon, manganese, phosphorus, sulphur, chromium and molybdenum, are precisely determined. The chemical composition is the foundation determining the mechanical properties, weldability, corrosion resistance and heat treatment processes of the steel, and ensuring that the material designations meet the standard provisions is the primary link of preventing the performance defects.

Mechanical property testing: Mainly including the tensile testing (determining the tensile strength, yield strength and elongation after fracture), hardness testing (Brinell, Rockwell or Vickers hardness) and the necessary impact toughness testing. These data directly reflect the anti-deformation and anti-fracture abilities of the steel pipe under the pressure loads, and are the core indicators of evaluating whether it can meet the safe operation requirements under the high-pressure and pulsating working conditions of the hydraulic systems.

Surface quality and non-destructive testing: Through the visual inspection, endoscopes and non-destructive testing methods, whether there are the defects such as the cracks, folds, scars, scratches and rust on the inner and outer surfaces of the steel pipe is checked. The common non-destructive testing technologies include the eddy current testing, ultrasonic flaw detection and magnetic particle flaw detection, used to find the micro cracks and discontinuities on and near the surfaces, preventing the defect propagation under the high pressures leading to the leakages or bursts.

Pressure resistance and sealing performance testing: Simulates the actual working conditions, and the hydrostatic testing (static pressure testing) or pneumatic testing is carried out on the steel pipe, verifying its pressure-bearing capacity and sealing under the specified test pressures. The test pressures are usually 1.5 times the nominal pressures or higher. After maintaining the pressure for a certain time, whether there are the leakages, permanent deformations or ruptures is checked. It is the mandatory safety verification before the ex-factory.

Metallographic structure and grain size examination: The microstructures (such as the morphologies and distributions of the ferrite, pearlite and martensite) of the steel pipe are observed through the metallographic microscopes and the grain size grades are determined. The metallographic structures directly affect the strength, toughness and corrosion resistance of the material, and can evaluate whether the heat treatment processes are appropriate, and check whether there are the process defects such as the overheating, overburning and decarburization.

Testing Scope

Seamless hydraulic steel pipes: Mainly testing the cold-drawn or hot-rolled seamless steel pipes. Due to the overall seamless, high strength and excellent pressure-bearing capacity, they are widely used in the high-pressure and ultra-high-pressure hydraulic systems. The testing emphasis is on the material uniformity within the full wall thickness ranges, internal defects (such as the inclusions and shrinkage cavities) and the high-precision geometric dimensions.

Welded hydraulic steel pipes: Including the longitudinally high-frequency welded pipes and submerged arc welded pipes. The testing core focuses on the weld quality. The strict non-destructive flaw detection, mechanical property testing and metallographic analysis need to be carried out on the welds and their heat-affected zones, ensuring that the weld strengths are not lower than the base material, and there are no welding defects such as the incomplete penetration, blowholes, slag inclusions and cracks.

Precision hydraulic steel pipes: Referring to the steel pipes with the high dimension precision and low surface roughness obtained through the processes such as the cold drawing, precision rolling or honing. In addition to the routine items, the testing scope particularly emphasizes the measurement of the inner hole surface roughness (Ra values), the integrity of the honing patterns, and the extremely high straightness and roundness requirements, to ensure the smoothness of the hydraulic cylinder movements and the long lives of the seals.

Plated or coated hydraulic steel pipes: To improve the corrosion resistance, some hydraulic steel pipes are subjected to the chromium plating, zinc plating, phosphating or special coating applications. The testing scope needs to be extended to the plating/coating thicknesses, adhesion (such as the cross-cut testing), hardness, porosity and salt spray corrosion resistance, ensuring that the protective layers are effective and do not fall off in the harsh environments.

Pipe fittings and connecting parts: The testing scope is not limited to the straight pipe sections, but also includes the pipe fittings used with the steel pipes such as the elbows, tees, joints and flanges. The thread precision, sealing surface quality, pressure-bearing performance and the matching with the steel pipes of these components need to be tested, ensuring the complete reliability of the whole pipeline system.

In-service hydraulic pipelines: For the equipment maintenance and periodic inspections, the testing scope points to the hydraulic system pipelines already in use. Focusing on the wall thickness reduction measurement (ultrasonic thickness measurement), corrosion condition evaluation, fatigue crack detection and seal aging inspection, to predict the lives and prevent the failures. It belongs to the condition monitoring and preventive maintenance categories.

Testing Methods

Dimension measurement method: The tools such as the digital callipers, micrometers, wall thickness micrometers and three-dimensional coordinate measuring machines (CMM) are used for the contact measurement; or the laser scanners and optical projectors are adopted for the non-contact measurement. For the inner diameters and deep holes, the pneumatic probes or inner diameter dial indicators are commonly used. The large-length straightness can be measured with the wire-pulling method or laser collimators.

Chemical analysis method: The spark direct-reading spectrometers (OES) are mainly adopted for the fast and multi-element simultaneous quantitative analysis, suitable for the incoming raw material and finished product testing. For the arbitration or more precise analysis, the inductively coupled plasma atomic emission spectrometers (ICP-OES) or carbon-sulphur analysers are used. The traditional chemical wet titration is used for the precise determination of the specific elements.

Mechanical testing method: The tensile testing is carried out on a universal material testing machine according to the GB/T 228.1 standard, preparing the standard round-bar or plate specimens. The hardness testing selects the Brinell (HBW), Rockwell (HRC/HRB) or Vickers (HV) hardness testers according to the material states and standard requirements. The impact testing is usually completed on the Charpy pendulum impact testing machines (V-type or U-type notches).

Non-destructive testing method: The eddy current testing (ET) is used for the fast screening of the surface and near-surface defects, especially for the precision pipes in the large-batch production. The ultrasonic testing (UT), including the longitudinal wave thickness measurement and transverse wave flaw detection, can effectively find the internal defects and precisely locate and quantify them. The magnetic particle testing (MT) is extremely sensitive to the surface cracks of the ferromagnetic materials. The penetrant testing (PT) is used for the surface open defect inspection of the non-ferromagnetic materials.

Pressure testing method: The hydrostatic testing is the most common method. Both ends of the steel pipe are sealed, filled with the liquid (usually water or oil), and a hydraulic pump is used to gradually pressurize to the specified test pressure, maintaining for the specified time (such as 10-15 seconds), observing whether the pressure gauges are stable and whether there are the leakages or permanent deformations of the pipe bodies. The pneumatic testing has the higher risks and requires the strict safety measures, commonly used for the special occasions not suitable for the liquid filling.

Metallographic examination method: The specimens are cut from the steel pipe, and a series of sample preparation processes such as the cutting, mounting, grinding, polishing and etching (such as using the 4% nitric acid alcohol solution) are carried out to prepare the observable metal surfaces. Subsequently, the microstructures are observed under the metallographic microscopes, and the grain size grades are rated according to the relevant standards (such as GB/T 6394), analysing the structure characteristics.

Testing Standards

Domestic Standards:

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

2. GB/T 6394-2017, Determination of estimating the average grain size of metal

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

2. 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

Universal material testing machine: The core equipment for the mechanical property tests such as the tensile, compression and bending. Through the precise loading of the servo control systems, the high-precision transducers measure the forces and displacements, and the computer software draws the stress-strain curves in real time and automatically calculates the key mechanical parameters such as the tensile strength, yield strength and elongation.

Direct-reading spectrometer and carbon-sulphur analyser: The direct-reading spectrometer is the main force of the chemical composition analysis. The samples are excited by the electric arcs or sparks to generate the characteristic spectra, and after the grating dispersion and photomultiplier tube detection, the percentage contents of the multiple elements are obtained within seconds. The carbon-sulphur analyser burns the samples in the high-frequency induction furnaces and precisely determines the contents of the two key elements of carbon and sulphur with the infrared absorption method.

Ultrasonic flaw detector and thickness gauge: The ultrasonic flaw detector transmits the high-frequency sound waves to the workpiece through the probes, receives and analyses the reflected echoes to detect the internal defects, with the multiple display modes such as the A-scan and B-scan. The digital ultrasonic thickness gauges are based on the pulse echo principle, specially used for the precise measurement of the remaining wall thicknesses, and are the important tools of evaluating the pipeline corrosion conditions.

Hydrostatic testing bench: The system specially used for the pressure resistance and sealing testing of the steel pipes. Mainly including the high-pressure pump sets, pressure vessels or sealing fixtures, high-precision pressure transducers and digital display instruments, safety valves, pipelines and control systems. The advanced testing benches can realize the functions of the automatic pressurization, pressure holding, pressure release, data recording and leakage alarming.

Metallographic microscope and sample preparation equipment: The metallographic microscope (including the optical microscope and digital imaging system) is the key to observing the microstructures. The supporting sample preparation equipment includes the cutting machines, mounting machines, pre-grinding machines, polishing machines and etching devices. A complete metallographic sample preparation and analysis system is the necessary condition for the material quality analysis and failure diagnosis.

Three-dimensional coordinate measuring machine and surface roughness tester: The coordinate measuring machine (CMM) utilizes the precision guide rails and probe systems, and can carry out the high-precision and digital measurement of the complex geometric dimensions and geometric tolerances (such as the straightness, roundness and coaxiality) of the steel pipe. The surface roughness tester moves the diamond styli across the surfaces, directly measuring and rating the roughness parameters such as the Ra and Rz of the inner holes or outer surfaces.