Cast Steel Pipe Testing

Cast steel pipe inspection mainly conducts scientific testing and analysis on the chemical composition analysis, tensile property testing, hardness testing, metallographic structure inspection, non-destructive flaw detection and dimension and geometric tolerance 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: The contents of the main elements such as the carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulfur (S), as well as the alloying elements such as the chromium (Cr), nickel (Ni) and molybdenum (Mo) in the cast steel pipes are determined through the spectrometers. This is the basis of judging whether the material designations of the cast steel pipes are compliant, directly affecting their mechanical properties, corrosion resistance and weldability. The strict control of the harmful elements such as the sulfur and phosphorus according to the standards is crucial for guaranteeing the product qualities.

Tensile property testing: The standard specimens are stretched on the universal material testing machines, determining the yield strength, tensile strength, elongation after fracture and reduction of area of the cast steel pipes. These data are the key indicators of evaluating the load-bearing capacities and plastic deformation abilities of the cast steel pipes, directly related to the safety and reliability of the pipes under the pressure loads.

Hardness testing: Usually adopting the Brinell hardness (HBW) or Rockwell hardness (HRB/HRC) methods, the hardness testers are used for the multi-point measurement on the cast steel pipe bodies or specimens. The hardness values can indirectly reflect the strengths, wear resistance and processing properties of the material, and have the important reference significance for evaluating the uniformity of the castings and heat treatment effects.

Metallographic structure inspection: Through the cutting, mounting, grinding, polishing and etching, the cast steel pipe specimens are prepared, and their micro-structures are observed with the optical microscopes or electron microscopes, such as the grain sizes, phase compositions (ferrite and pearlite, etc.), graphite morphologies and non-metallic inclusion distributions. This testing is the core means of evaluating the casting processes, heat treatment qualities and internal defects.

Non-destructive flaw detection: Under the premise of not destroying the cast steel pipes, the ultrasonic testing (UT) is used to detect the internal shrinkage cavities, porosity and cracks; the magnetic particle (MT) or penetrant (PT) testing is used to detect the surface and near-surface open defects; the radiographic testing (RT) is used to detect the internal volumetric defects. This is the key link of guaranteeing the structural integrity of the cast steel pipes, especially suitable for the pipes bearing the high pressures or under the important working conditions.

Dimension and geometric tolerance testing: The tools such as the callipers, micrometers, pi tapes and coordinate measuring machines are used to precisely measure the wall thicknesses, outer diameters, ellipticity, straightness, flange dimensions and bolt hole positions of the cast steel pipes. Ensuring that their dimensions meet the drawings and installation requirements is the basis of guaranteeing the docking and sealing properties of the pipeline systems.

Pressure testing: Usually referring to the hydrostatic testing or pneumatic testing. The cast steel pipes are sealed and filled with the liquid or gas, pressurized to the specified pressures (usually 1.5 times the working pressures) and maintained for a certain time, checking whether the leakage, deformation or rupture occurs. This is the final and most direct verification test of the pressure-bearing capacities and sealing integrity of the cast steel pipes.

Testing Scope

Carbon steel and low-alloy steel cast pipes: Covering the common designation cast pipes such as ASTM A216 WCB/WCC and GB/T 12229 WCB, widely applied to the conveying pipeline systems of the non-corrosive media such as the water, steam and oil products. The testing emphases are the routine mechanical properties, chemical compositions and macro defects.

Stainless steel and corrosion-resistant alloy cast pipes: Including the cast pipes of the materials such as the austenitic stainless steels (such as CF8/CF8M) and duplex stainless steels, used in the corrosive environments such as the chemical, marine and food industries. In addition to the routine items, the special corrosion resistance testing such as the intergranular corrosion testing and pitting potential determination needs to be added.

High-temperature cast steel pipes: Such as the materials of ASTM A217 WC6/WC9, used in the high-temperature and high-pressure environments such as the power plant boilers and thermal systems. The testing needs to particularly pay attention to the high-temperature tensile properties, creep rupture strengths, creep properties and structure stability after the long-term service.

Low-temperature cast steel pipes: Suitable for the cryogenic environments such as the liquefied natural gas (LNG) and liquid nitrogen, with the materials such as the low-carbon austenitic stainless steels or low-temperature carbon steels. The testing core is the low-temperature Charpy impact testing, to verify the anti-brittle-fracture abilities of the material at the low temperatures.

Wear-resistant cast steel pipes: Such as the materials of the high-chromium cast iron, used for the abrasive wear occasions in the mining and power industries. The testing focuses on the macro hardnesses, micro hardnesses of the material, as well as the types, morphologies and distributions of the carbides in the metallographic structures, to evaluate their wear resistance.

Castings for the pipeline valves and fittings: The cast steel tees, elbows, flanges and valve bodies are the key connection and control components of the pipeline systems. In addition to the body properties, the testing scope also needs to pay attention to the flow passage dimensions, sealing surface qualities and non-destructive testing of the pressure-bearing weak areas.

Repaired and remanufactured castings: After the repair of the in-service or retired cast steel pipes, the comprehensive chemical composition, mechanical property, metallographic structure and non-destructive testing re-inspection needs to be carried out on their repaired areas (such as the repair welding areas), ensuring that their properties recover to the qualified levels.

Testing Methods

Spectroscopic analysis method: Mainly using the spark direct-reading spectrometers (OES) or X-ray fluorescence spectrometers (XRF) for the fast quantitative analysis of the chemical compositions. The OES has the high precision, suitable for the fast analysis before the furnaces and finished product inspection; the XRF is suitable for the non-destructive or micro-destructive analysis, convenient for the on-site or large-size workpiece testing.

Mechanical property testing method: According to the standards such as GB/T 228.1 or ASTM A370, the room-temperature tensile testing is carried out on the universal testing machines. The impact testing is carried out on the pendulum impact testing machines according to GB/T 229 or ASTM A370, especially for the pipes for the low-temperature or specific working conditions, the impact absorption energies need to be tested at the specified temperatures.

Hardness testing method: The Brinell hardness (HBW) testing has the large indentation areas and can reflect the average properties of the material, often used for the castings; the Rockwell hardness (HRC/HRB) testing is simple to operate, used for the harder or thinner areas; the Leeb hardness (HL) testing is convenient for the on-site or large workpiece testing, but attention needs to be paid to its conversion relationships with the standard methods and applicability.

Macro and micro metallographic method: The macro metallographic inspection displays the low-power structure defects of the castings such as the porosity and segregation through the acid etching or sulfur printing and phosphorus printing methods. The micro metallographic inspection observes the specimens prepared through the standard processes with the optical microscopes or scanning electron microscopes (SEM), analysing the structure morphologies, grain sizes (rated according to GB/T 6394) and inclusions (rated according to GB/T 10561).

Ultrasonic testing (UT): Using the principle that the high-frequency sound waves reflect when encountering the defects during the propagation in the material, the single crystal or dual crystal probes are used, combined with the ultrasonic flaw detectors, which can effectively detect the positions, equivalent sizes and depths of the defects such as the shrinkage cavities, cracks and slag inclusions inside the cast steel pipes. For the coarse-grain cast steel materials, the lower frequency probes need to be selected to overcome the attenuation.

Radiographic testing (RT): The X-ray or γ-ray is used to penetrate the workpieces, forming the images on the films or digital imaging plates, intuitively displaying the shapes, sizes and distributions of the internal three-dimensional defects of the castings. This method is sensitive to the volumetric defects (such as the blowholes and shrinkage porosity), but has the high directionality requirements for the planar defects (such as the cracks).

Magnetic particle and penetrant testing (MT/PT): The MT is suitable for the ferromagnetic material cast steel pipes. After the magnetization, the magnetic particles are spread on the surfaces, displaying the magnetic marks of the surface and near-surface defects. The PT is suitable for all the non-porous materials. Through the application of the penetrants, cleaning and developers, the surface open defects are displayed. Both are the effective means of the surface defect testing.

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 229-2020, Metallic materials — Charpy pendulum impact test method

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

4. GB/T 10561-2023, Steel — Determination of content of nonmetallic inclusions — Micrographic method using standard diagrams

5. GB/T 12229, Carbon steel castings for general purpose valves

International Standards:

1. ASTM A216, Standard specification for steel castings, carbon, suitable for fusion welding, for high-temperature service

2. ASTM A217, Standard specification for steel castings, martensitic stainless and alloy, for pressure-containing parts, suitable for high-temperature service

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

4. ASTM A703, Standard specification for steel castings, general requirements, for pressure-containing parts

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

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 for the fast and precise chemical composition analysis. Through the excitation of the samples to generate the characteristic spectra, the spectra are dispersed by the gratings and detected by the photomultiplier tubes, and the element contents are obtained by comparing with the standard curves. The modern instruments have the high automation degrees, can simultaneously analyse dozens of elements, and have the designation identification functions.

Universal material testing machine: The key equipment of carrying out the mechanical property testing such as the tensile, compression and bending. Driven by the servo motors or hydraulics, equipped with the high-precision force transducers and extensometers, the computer control systems realize the precise control and data acquisition of the loads and deformations, outputting the complete stress-strain curves.

Metallographic microscope system: Including the inverted or upright optical microscopes, image acquisition cards, computers and professional analysis software. Used to observe and record the micro-structures of the cast steel pipes, and can carry out the quantitative metallographic work such as the grain size rating, phase area percentage measurement and inclusion analysis.

Ultrasonic flaw detector: The portable or bench-type equipment, with the core components including the pulse generators, receiving amplifiers, displays and probes. The digital instruments have the functions such as the A-scan waveform display, defect automatic alarm, data storage and playback, and DAC/TCG curve making, greatly improving the testing efficiency and reliability.

X-ray real-time imaging system: Composed of the X-ray machines, high-resolution flat panel detectors (or image intensifiers), mechanical control systems and image processing workstations. It can realize the dynamic and real-time testing of the cast steel pipes, with the fast imaging speeds, convenient for the multi-angle observation and three-dimensional positioning of the defects.

Brinell/Rockwell/Leeb hardness testers: The Brinell hardness testers press the cemented carbide balls into the specimens under the fixed loads, measuring the indentation diameters to calculate the hardness values. The Rockwell hardness testers directly read the values by measuring the indentation depth differences. The Leeb hardness testers are portable, converting the hardness through the ratio of the impact body rebound speeds and impact speeds.

Coordinate measuring machine (CMM): Used for the high-precision testing of the complex geometric dimensions and geometric tolerances of the cast steel pipes. Through the probe contact with the workpiece surfaces, the computer systems collect the spatial point coordinates, and the parameters such as the diameters, distances, angles, roundness, cylindricity and position degrees are calculated through the software, realizing the digital testing.