High Alloy Steel Pipe Testing

High-alloy steel pipe inspection mainly conducts scientific testing and analysis on the chemical composition analysis, tensile property testing, hardness testing, impact toughness testing, corrosion resistance testing and macro and micro metallographic 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

Chemical composition analysis: The contents of the various alloying elements (such as Cr, Ni, Mo, Ti and Nb) and residual elements (such as S and P) in the high-alloy steel pipe are precisely determined. This is the basis for determining the material designations, evaluating the corrosion resistance, high-temperature properties and processing properties. Usually carried out in accordance with the standards such as ASTM A751 and GB/T 223, it is crucial for the smelting process control and quality consistency.

Tensile property testing: The yield strength, tensile strength, elongation after fracture and reduction of area of the steel pipe are determined at the room temperature or high temperature. These indicators directly reflect the load-bearing capacity and plastic deformation ability of the material under the static loads, and are the key basis of the design and material selection. The tests need to comply with the standards such as ASTM A370 and GB/T 228.1.

Hardness testing: The Brinell (HBW), Rockwell (HRC/HRB) or Vickers (HV) methods are adopted to measure the surface or cross-section hardness of the steel pipe. The hardness values can indirectly reflect the strength, wear resistance and heat treatment states of the material, commonly used for the on-site fast inspection and product consistency monitoring. The multi-point measurements at the positions specified in the standards need to be carried out and the average values are taken.

Impact toughness testing: Through the Charpy V-notch impact testing, the impact absorbed energy of the steel pipe at the specified temperature is determined. This indicator evaluates the ability of the material to resist the brittle fracture at the low temperatures or under the dynamic loads, especially crucial for the austenitic stainless steel pipes and duplex steel pipes serving in the low-temperature environments, complying with the ASTM A370 or GB/T 229 standards.

Corrosion resistance testing: For the high-alloy pipes such as the stainless steels and nickel-based alloys, the intergranular corrosion testing (such as ASTM A262 Practice E), pitting potential measurement, salt spray testing or stress corrosion cracking (SCC) testing are carried out. These tests simulate the harsh working conditions, evaluating the stability of the material in the specific corrosive media, and are the core of ensuring its safe applications in the fields such as the chemical and marine industries.

Macro and micro metallographic examination: The macro examination observes the low-power structure defects of the steel pipe, such as the shrinkage cavities, segregation and cracks. The micro examination (micro-structure analysis) observes the grain sizes, non-metallic inclusion grades, phase compositions (such as the austenite, ferrite and σ phases) and precipitated phases under the high-power microscopes, rated according to the standards such as ASTM E112 and ASTM E45, used to judge whether the heat treatment processes are appropriate and the internal quality of the material.

Testing Scope

Pipes for the oil and natural gas exploitation and conveying: Covering the oil well pipes (casing and tubing), conveying pipes and gathering and transmission pipelines. The testing emphasis is on the high strength, high corrosion resistance (such as the resistance to the H2S and CO2 corrosion) and collapse resistance, involving the comprehensive chemical compositions, mechanical properties, hardness, impact toughness and strict corrosion resistance tests.

Pipes for the petrochemical and coal chemical plants: Including the furnace tubes and process pipelines in the reactors, heat exchangers, catalytic cracking and ethylene cracking plants. The testing needs to focus on the long-term properties of the material under the coupled effects of the high temperature, high pressure, hydrogen-containing environments and the various corrosive media, such as the creep rupture strength, creep properties, high-temperature oxidation and carburization tendencies.

Pipes for the power station boilers and nuclear power: Involving the high-temperature superheater and reheater tubes of the ultra-supercritical power station boilers, as well as the nuclear power steam generator heat transfer tubes. The testing core is the high-temperature creep rupture strength, creep rupture life, micro-structure aging evaluation, intergranular corrosion sensitivity, as well as the extremely high dimension precision and surface quality requirements.

Pipes for the ships and marine engineering: Used for the seawater cooling systems, ballast tanks and chemical tanker cargo holds. The testing emphasizes the resistance to the pitting, crevice corrosion and stress corrosion cracking in the chloride environments. The accelerated corrosion tests simulating the seawater environments and the cathodic protection compatibility evaluations often need to be carried out.

Pipes for the food, pharmaceutical and bioengineering equipment: Requiring the extremely high cleanliness and corrosion resistance to prevent the product pollution. The testing focuses on the purity of the material (low impurity elements), excellent surface finish (inner surface roughness Ra values), sterility and the ability to withstand the cleaning with the various acids, alkalis and organic solvents.

Pipes for the aerospace and military industry: Applied to the hydraulic systems, fuel systems and engine components. The testing standards are extremely strict. In addition to the routine properties, the fatigue properties, fracture toughness, extremely high sensitivity of the non-destructive flaw detection and the whole-process requirements for the material traceability are particularly focused on.

Testing Methods

Spectrometry analysis method: The spark direct-reading spectrometers (OES) or inductively coupled plasma atomic emission spectrometers (ICP-OES) are adopted for the quantitative chemical composition analysis. The OES is suitable for the on-site fast screening, while the ICP-OES has the higher precision, especially good at the trace element analysis. Both methods need to be calibrated with the standard samples matching the materials to be tested.

Mechanical testing method: The tensile testing is carried out on a universal material testing machine, and the extensometers are used to precisely measure the deformations. The impact testing is carried out on a pendulum impact testing machine, and the test temperatures need to be precisely controlled. The hardness testing selects the suitable hardness testers and test forces according to the material states and standard requirements, ensuring that the indentation sizes meet the provisions.

Corrosion testing method: The intergranular corrosion commonly adopts the sulphuric acid-copper sulfate corrosion testing (copper chip method) or nitric acid method, rated through the bending method or metallographic method. The electrochemical tests such as the potentiodynamic polarization method are used to measure the pitting potentials and re-passivation potentials. The stress corrosion testing is carried out in the specific media (such as the MgCl2 solution) under the constant-load or constant-deformation conditions.

Metallographic analysis method: The specimens are cut from the steel pipe, and the metallographic samples are prepared after the mounting, grinding, polishing and etching (chemical or electrolytic etching). The optical microscopes or scanning electron microscopes (SEM) are used to observe the structures, and combined with the energy dispersive spectrometers (EDS) for the micro-area composition analysis, the abnormal structures are qualitatively and quantitatively determined.

Non-destructive testing method: Including the ultrasonic testing (UT, for the internal defects and thickness measurement), eddy current testing (ET, for the surface and near-surface defects, especially suitable for the austenitic steel pipes), penetrant testing (PT, for the surface open defects) and radiographic testing (RT, for the volumetric defects). These methods do not damage the workpieces, used for the full inspection or sampling inspection.

Dimension and geometric quantity measurement method: The measuring tools such as the callipers, micrometers, ring gauges and plug gauges are used to measure the outer diameters, inner diameters, wall thicknesses, lengths, ovality and wall thickness unevenness of the steel pipe. For the high-precision pipes, the laser diameter measuring instruments, ultrasonic thickness gauges or coordinate measuring machines (CMM) are adopted for the automated high-precision measurement.

Testing Standards

Domestic Standards:

1. GB/T 223, Methods for chemical analysis of iron, steel and alloy

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. ASTM A751, Standard test methods, practices, and terminology for chemical analysis of steel products

2. ASTM A370, Standard test methods and definitions for mechanical testing of steel products

3. ASTM A262, Standard practices for detecting susceptibility to intergranular attack in austenitic stainless steels

4. ASTM E112, Standard test methods for determining average grain size

5. ASTM E45, Standard test methods for determining the inclusion content of steel

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

7. 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

Direct-reading spectrometer and ICP spectrometer: The direct-reading spectrometer has the fast analysis speed, suitable for the furnace-front analysis and fast identification of the finished products, but the analysis methods need to be established for the different substrates. The ICP spectrometer has the low detection limits and wide linear ranges, can analyse the various elements from the major to trace amounts, and is the ideal equipment for the arbitration analysis and precise control.

Universal material testing machine and impact testing machine: The microcomputer-controlled electronic universal testing machine can realize the various tests such as the tensile, compression and bending, with the precise data acquisition. The instrumented impact testing machine can not only record the total impact energy, but also decompose the crack initiation energy and crack propagation energy, providing the more in-depth data for the fracture behaviour research of the material.

Hardness tester series: The desktop Brinell, Rockwell and Vickers hardness testers are used for the precise laboratory measurement. The portable Leeb hardness testers or ultrasonic hardness testers are suitable for the hardness sampling inspection of the on-site, large-diameter or installed pipes. The micro-Vickers hardness testers are used to measure the hardness of the specific phases or micro-areas.

Metallographic microscope and scanning electron microscope: The upright or inverted metallographic microscopes with the camera systems are used for the routine structure observation and rating. The scanning electron microscopes (SEM) have the higher depth of field and resolution, used to observe the fracture morphologies (tensile and impact fractures), fine precipitated phases and corrosion morphologies, combined with the EDS for the composition analysis.

Corrosion testing equipment: Including the constant-temperature water baths, electrochemical workstations (three-electrode systems), salt spray test chambers and high-temperature high-pressure autoclaves. The electrochemical workstations can carry out the tests such as the polarization curves and electrochemical impedance spectroscopy, used to study the corrosion mechanisms. The high-temperature high-pressure autoclaves can simulate the actual working conditions for the stress corrosion or uniform corrosion testing.

Non-destructive testing equipment: The multi-channel ultrasonic flaw detectors equipped with the probes of the multiple angles can realize the automated scanning. The eddy current flaw detectors equipped with the through-type or point-type probes are particularly suitable for the on-line high-speed testing of the pipes. The X-ray real-time imaging systems (DR) can realize the instant imaging and digital storage of the testing results, improving the detection efficiency and traceability.