Boiler Container Plate Testing

Boiler and pressure vessel plate inspection mainly conducts scientific testing and analysis on the chemical composition analysis, tensile property testing, impact toughness testing, hardness testing, bending property testing and ultrasonic flaw detection, 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: To ensure that the boiler and pressure vessel plates have the required mechanical properties and weldability, their chemical compositions must be strictly tested. The main analysed elements include the carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), molybdenum (Mo) and nickel (Ni). As the harmful elements, the contents of the phosphorus and sulfur need to be strictly controlled, usually requiring P ≤ 0.025% and S ≤ 0.015%, to reduce the hot brittleness and cold brittleness risks of the material.

Tensile property testing: This testing is used to evaluate the strengths and plasticity of the material under the static loads, and is the key of judging whether the material can withstand the working pressures. The yield strengths (ReH/Rp0.2), tensile strengths (Rm), elongations after fracture (A) and reduction of area (Z) are mainly determined. For the boiler and pressure vessel plates, the higher yield ratios and good plasticity are usually required, to ensure that the sudden brittle fracture does not occur under the pressure.

Impact toughness testing: Used to evaluate the capability of the material to resist the impact loads at the low temperatures or with the notches, and is the important indicator of preventing the low-stress brittle fracture. The Charpy V-notch impact testing is usually adopted, determining the impact absorption energies (KV2) at the temperatures specified by the standards (such as 0°C, -20°C and -40°C). For the steel plates of the different thicknesses and service temperatures, the standards have the clear minimum requirements for the impact energies.

Hardness testing: The hardness is the indicator of the capability of the material surfaces to resist the local plastic deformation, related to the strengths, wear resistance and processing properties of the material. The Brinell hardness (HBW) or Vickers hardness (HV) is commonly used for the testing. Through the hardness testing, the strength uniformity of the material can be indirectly evaluated, and it is helpful to judge the property changes of the welding heat-affected zones, preventing the failures caused by the local hardening or softening.

Bending property testing: The bending testing is used to evaluate the plastic deformation capability of the material and its surface and internal defect conditions. The 180° cold bending testing is usually carried out, and the mandrel diameters are determined according to the plate thicknesses and standard requirements. After the testing, the outer surfaces of the bends of the specimens are checked, and the cracks, lamination or fracture should not appear, proving that the material has the good forming and processing properties and welding adaptability.

Ultrasonic flaw detection (UT): This is the main non-destructive testing method of detecting the internal macro defects (such as the lamination, white spots, inclusions and internal cracks) of the steel plates. The principle that the high-frequency ultrasonic waves generate the reflections when meeting the defects during the propagation in the material is used for the detection. According to the relevant standards (such as NB/T 47013.3), the 100% area scanning is carried out on the boiler and pressure vessel plates, and the quality grading is carried out according to the equivalent sizes, indicated lengths and distributions of the defects.

Testing Scope

Manufacturing process inspection: Covering the whole process monitoring from the steelmaking, continuous casting, rolling to heat treatment. Including the furnace-front analysis of the steel melt purity, low-power structure inspection of the cast slabs, recording and control of the rolling process parameters (such as the finishing rolling temperatures and reduction ratios), as well as the verification of the heat treatment (normalizing, quenching and tempering, etc.) process curves. The purpose is to ensure the process stability and provide the guarantee for the final product performance.

Raw material incoming inspection: Referring to the first comprehensive inspection after the steel plates are delivered to the boiler and pressure vessel manufacturers. In addition to the checking of the material test certificates (MTC), the appearance dimensions, markings and surface qualities of each plate or each batch of plates need to be inspected, and the specimens are taken according to the proportions or standard requirements for the re-inspection of the chemical compositions, mechanical properties and non-destructive testing, confirming that they meet the procurement technical agreement requirements.

Welded joint and heat-affected zone inspection: The manufacturing of the boiler and pressure vessels heavily relies on the welding, so the testing of the welded joints is crucial. The testing scope includes the weld metals, fusion lines and the heat-affected zones on both sides. The tensile, bending, impact (including the sampling at the different positions of the welds, fusion lines and heat-affected zones) and hardness testing need to be carried out, to evaluate the rationality of the welding processes and the reliability of the joint properties.

Periodic inspection of the in-service equipment: For the boilers and pressure vessels already in operation, the periodic inspection is carried out according to the regulations such as the Supervision Regulation on Safety Technology for Stationary Pressure Vessels (TSG 21). The testing scope emphatically focuses on the stress concentration areas, corrosion-thinned parts, expansion conditions of the original defects and welds, etc. The methods such as the ultrasonic thickness measurement, magnetic particle/penetrant flaw detection and ultrasonic flaw detection are usually adopted, evaluating the remaining safe service lives.

Additional inspection for the specific service environments: For the boiler and pressure vessel plates serving under the special conditions such as the high temperatures, high pressures, hydrogen environments and corrosive media (such as H2S and alkaline environments), the corresponding special inspection items need to be added. For example, the high-temperature tensile testing, hydrogen-induced cracking (HIC) testing, stress corrosion cracking (SSC) testing and intergranular corrosion testing are carried out, to evaluate the applicability of the material in the specific environments.

Plate dimension and appearance quality inspection: The tools such as the callipers, micrometers and tapes are used to measure the thicknesses, widths, lengths and flatness of the steel plates. The appearance inspection checks whether the plate surfaces have the defects such as the cracks, scars, laps, bubbles, inclusions and pressed-in oxide scales through the visual inspection or with the help of the low-power magnifying glasses, as well as whether the edge states meet the cutting or processing requirements.

Testing Methods

Spectroscopic analysis method: Used for the fast quantitative analysis of the chemical compositions. The common methods include the spark discharge atomic emission spectrometry (OES) and X-ray fluorescence spectrometry (XRF). The OES method has the high analysis precision and is the main means of the furnace-front analysis and laboratory re-inspection; the XRF method is often used for the on-site fast screening. Before the analysis, the flat and smooth specimen surfaces need to be prepared, and the corresponding standard substances need to be used for the calibration.

Mechanical testing method according to the standards: All the mechanical property testing must be strictly carried out according to the national standards or industry standards. For example, the tensile testing is according to GB/T 228.1, using the universal material testing machines; the impact testing is according to GB/T 229, using the pendulum impact testing machines; the bending testing is according to GB/T 232. The sampling positions, directions, dimension machining and testing rates of the specimens all need to comply with the standard provisions, to ensure the comparability and reliability of the data.

Non-destructive testing (NDT) methods: Mainly including the ultrasonic testing (UT), radiographic testing (RT), magnetic particle testing (MT) and penetrant testing (PT). The UT is used for the internal defect testing; the RT (such as the X-ray and γ-ray) can provide the intuitive images of the defects, often used for the important welds; the MT is suitable for the surface and near-surface defect testing of the ferromagnetic materials; the PT is suitable for the surface open defect testing of the non-porous metal materials. The method selection depends on the defect types, positions and testing standard requirements.

Metallographic inspection method: The micro-structures of the material are observed through the optical microscopes or scanning electron microscopes (SEM). The inspection contents include the grain size rating, non-metallic inclusion rating, micro-structure analysis (such as the ferrite, pearlite and bainite ratios), decarburized layer depth determination, as well as the micro-structure observation of the various areas of the welded joints. The metallographic inspection is the important bridge connecting the material processes, structures and properties.

Hardness testing method: According to the material thicknesses, expected hardness values and standard requirements, the appropriate hardness testing methods are selected. The Brinell hardness (HBW) has the large indentations and good representativeness, often used for the raw material inspection; the Vickers hardness (HV) has the wide testing force ranges, and can be used for the thin plates or surface hardened layer testing; the Rockwell hardness (HRC/HRB) is simple to operate. Before the testing, the specimen surfaces need to be appropriately ground to ensure the accurate testing results.

Macro (low-power) inspection method: The macro structures and defects of the material are displayed through the methods such as the acid etching or sulfur printing. The hot acid etching method is commonly used to inspect the dendrites, segregation, porosity, residual shrinkage cavities and white spots of the steel plates. This method can quickly find the material non-uniformity and macro defects over the large areas, and is the effective means of evaluating the metallurgical qualities.

Testing Standards

Domestic Standards:

1. GB/T 713, Steel plates and strips for pressure equipments

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

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

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

5. GB/T 3808, Verification of pendulum-type impact testing machines

6. NB/T 47013.3, Nondestructive testing of pressure equipments — Part 3: Ultrasonic testing

7. TSG 21, Supervision regulation on safety technology for stationary pressure vessels

International Standards:

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

2. ISO 148-1, Metallic materials — Charpy pendulum impact test — Part 1: Test method

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 of carrying out the mechanical property testing such as the tensile, compression and bending. The modern testing machines are usually the microcomputer-controlled electro-hydraulic servo type or electronic universal type, with the high-precision load transducers and displacement measurement systems. They can precisely control the loading rates, automatically record the force-displacement curves, calculate the various strength and plasticity indicators, and the data can be directly output and stored.

Pendulum impact testing machine: The dedicated equipment used to determine the Charpy V-notch impact absorption energies. The testing machines need to meet the requirements of GB/T 3808, and the pendulum energies are usually 300J or 150J. The equipment is equipped with the precise angle indicators and energy display devices. During the testing, the center line of the specimen notches and the center of the support span need to be aligned, there should be no initial kinetic energy loss at the moment of the pendulum release, and the environmental temperatures are strictly controlled.

Direct-reading spectrometer: The key instrument of the fast quantitative chemical composition analysis. The instrument consists of the light sources (spark generators), dispersion systems, detection systems and computer control systems. During the use, the working curves need to be regularly calibrated with the standard samples, and the environmental temperature and humidity and argon purity need to be controlled. It has the fast analysis speeds and high precision, and can simultaneously determine dozens of elements.

Ultrasonic flaw detector: The main equipment of the non-destructive testing of the internal defects. The digital ultrasonic flaw detectors have the A-scan display functions, and can set the DAC/TCG curves for the quantitative evaluation. The straight-beam probes, angle-beam probes and dual crystal probes of the various angles are used in combination. The equipment needs to be regularly calibrated with the standard test blocks (such as the CSK-ⅠA and RB series test blocks) for the sound velocities, incidence points and K values (refraction angles).

Metallographic microscope and sample preparation equipment: The basic equipment system of the metallographic analysis. Including the cutting machines, mounting machines, pre-grinding machines, polishing machines, etching devices and optical microscopes. The advanced research may also use the image analysis systems and scanning electron microscopes (SEM). The quality of the sample preparation process (sampling, mounting, grinding, polishing and etching) directly affects the accuracy of the observation results.

Hardness tester series: The various hardness testers are equipped according to the different testing needs. The Brinell hardness testers need to be equipped with the reading microscopes to measure the indentation diameters; the Rockwell hardness testers are the simplest to operate; the Vickers hardness testers and micro-Vickers hardness testers are suitable for the more refined testing. All the hardness testers need to be regularly verified and calibrated with the standard hardness blocks, ensuring the traceability of the measurement results.