Boiler Steel Plate Testing

Boiler steel plate inspection mainly conducts scientific testing and analysis on the chemical composition analysis, room-temperature tensile testing, high-temperature tensile testing, impact toughness testing, hardness testing and bending 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: This is the basic testing item of the boiler steel plate, aiming to precisely determine the contents of the five elements carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulphur (S) in the steel plate. For the alloy steel boiler plates, the contents of the alloying elements such as the chromium (Cr), molybdenum (Mo), nickel (Ni) and vanadium (V) also need to be analysed, to ensure that they meet the standard requirements of the specific designations (such as Q245R, Q345R and 15CrMoR), thereby guaranteeing the smelting quality and the basic mechanical properties of the steel.

Room-temperature tensile testing: This testing is used to determine the key mechanical property indicators of the boiler steel plate at the room temperature, including the tensile strength (Rm), yield strength (ReL or Rp0.2), elongation after fracture (A) and reduction of area (Z). These data are the core basis for evaluating the ability of the steel plate to resist the deformation and fracture under the pressure loads, and are the basis for the design calculations of the wall thicknesses and safety factors of the boiler pressure-bearing components.

High-temperature tensile testing: The boiler steel plate works under the high-temperature and high-pressure environments for a long time, and its high-temperature performance is crucial. This testing is usually carried out at the design temperatures or higher temperatures, determining the high-temperature yield strength and tensile strength of the material. The results are the key input parameters for determining the allowable stresses of the steel plate under the high-temperature conditions, evaluating its creep resistance and carrying out the boiler life assessment.

Impact toughness testing: The Charpy V-notch impact testing is mainly adopted, carried out at the specified temperatures (such as 0 ℃, -20 ℃ or lower). By determining the impact absorbed energy (KV2), the ability of the steel plate to resist the brittle fracture under the low temperatures or with the stress concentrations is evaluated. For the low-temperature environments or large thick-wall boilers, the impact toughness is an important safety indicator for preventing the catastrophic brittle fractures.

Hardness testing: The Brinell hardness (HBW) or Rockwell hardness (HRB/HRC) methods are commonly used for the testing. The hardness value indirectly reflects the strength, wear resistance and processing performance of the material. By detecting the hardness at the different positions of the steel plate, the uniformity of its structures and whether there are the local performance deviations caused by the improper heat treatments or surface decarburization can be evaluated.

Bending testing: By bending the specimen to the specified angle, it is checked whether there are the defects such as the cracks, peeling or delaminations on the tensile surface. This testing aims to evaluate the plastic deformation ability and process adaptability of the steel plate, verifying whether it has the sufficient deformation ability without cracking during the subsequent manufacturing processes such as the plate rolling and welding.

Testing Scope

Steel plates for the boiler drums: Mainly referring to the steel plates used to manufacture the core pressure-bearing component of the boiler — the drum (steam drum), usually the high-quality carbon steels or low-alloy high-strength steels, such as Q245R, Q345R and 13MnNiMoR. These steel plates have the highest requirements for the purity, structure uniformity, mechanical properties and welding performance, and are the key objects of the testing.

Steel plates for the boiler furnace tubes and tube plates: Used to manufacture the furnace tubes, tube plates and other components bearing the high-temperature flame radiation and flue gas erosion. In addition to the routine mechanical properties, the high-temperature strength, oxidation resistance and thermal fatigue resistance are more emphasized in the testing. The heat-resistant steels are often selected for the materials, such as 15CrMoR.

Steel plates for the boiler headers and piping: Used to manufacture the various headers and connecting pipelines. This type of steel plates usually has the higher requirements for the welding performance and impact toughness, and the testing needs to ensure that they have the good weldability, to bear the complex welded structures and alternating stresses.

Steel plates for the boiler structural parts: Referring to the structural steel plates used to manufacture the non-directly-fired pressure-bearing parts such as the boiler steel frames, platforms and shields, such as Q235B and Q355B. The testing items are relatively simplified, but there are still the basic requirements for their dimensional tolerances, surface quality and weldability.

Incoming acceptance testing of the steel plate raw materials: The comprehensive testing carried out before the steel plates enter the warehouse, covering the chemical composition, mechanical properties, dimensions, shapes and surface quality. It is the first key gate for ensuring the qualified quality of the source materials of the boiler manufacturing.

Re-inspection and verification testing of the steel plates: The re-testing carried out after the steel plates are stored for a certain period, or when there are doubts about the quality certificates, or before manufacturing the important parts. It aims to verify the stability of the material properties, preventing the performance changes caused by the reasons such as the aging and improper storage.

Testing Methods

Spectrometry analysis method (OES): The direct-reading spectrometer or spark source atomic emission spectrometer is adopted for the fast, non-destructive or micro-destructive analysis of the chemical composition of the steel plate. This method has the high precision and fast analysis speed, and is currently the main method of the chemical composition testing in the raw material acceptance and production process control.

Tensile testing method: In accordance with the standards such as GB/T 228.1 or ASTM A370, the standard tensile specimens are prepared and tested on a universal material testing machine. During the testing, the load-displacement curves are recorded, and the key mechanical property parameters such as the yield strength, tensile strength and elongation are finally calculated.

Charpy impact testing method: In accordance with the standards such as GB/T 229 or ISO 148, the standard V-notch impact specimens are prepared and tested on a pendulum impact testing machine. By measuring the energy consumed by the pendulum to break the specimen, the impact absorbed energy is obtained, evaluating the toughness of the material.

Ultrasonic flaw detection (UT): Utilizes the principle that the high-frequency sound waves produce the reflections when encountering the defects (such as the delaminations, inclusions and flakes) during the propagation inside the steel plate. This method can effectively find the planar defects parallel to the detection surface inside the steel plate, and is the preferred method of the non-destructive testing for ensuring the internal quality of the steel plate.

Magnetic particle flaw detection (MT): Applicable to the detection of the surface and near-surface defects of the ferromagnetic boiler steel plates. By magnetizing the workpiece and applying the magnetic particles on the surface, the leakage magnetic fields at the defects will attract the magnetic particles to form the magnetic indications. This method has the extremely high sensitivity to the linear surface defects such as the cracks and folds.

Metallographic examination method: The metallographic specimens are prepared through the processes such as the sampling, mounting, grinding, polishing and etching, and the microstructures of the steel plate (such as the grain sizes, banded structures, non-metallic inclusion grades and decarburization layer depths) are observed under an optical microscope or scanning electron microscope, thereby evaluating its metallurgical quality and heat treatment state.

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 4338, Metallic materials — Tensile testing at elevated 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 A370, Standard test methods and definitions for mechanical testing of steel products

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

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

4. 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 carrying out the room-temperature and high-temperature tensile testing and bending testing. It has the precise load and displacement control systems, can automatically draw the stress-strain curves, and calculate and output the various mechanical property parameters. The range and precision need to meet the requirements of the corresponding standards.

Pendulum impact testing machine: The equipment specially used for the Charpy impact testing. Its pendulum is lifted to the specified height and then released to break the notched specimen placed on the supports, and the impact absorbed energy is read through the pointer or digital display system. The testing machine needs to be equipped with the low-temperature baths meeting the different temperature requirements.

Direct-reading optical emission spectrometer: The key equipment of the chemical composition analysis. By exciting the steel plate sample to generate the characteristic spectra, the spectra are received by the photomultiplier tubes or CCD detectors through the grating dispersion, and by comparing with the standard samples, the contents of the multiple elements are quickly and quantitatively analysed, with the analysis precision up to the ppm level.

Ultrasonic flaw detector: Composed of a pulse generator, probes, receiving amplifier and display. The probes convert the electrical pulses into the ultrasonic waves transmitted into the steel plate, and receive the signals reflected from the defects, displaying the depths and equivalent sizes of the defects on the screen. The multi-channel or automatic scanning systems are commonly used to improve the detection efficiency.

Brinell/Rockwell hardness tester: The dedicated instruments for measuring the hardness of the steel plate. The Brinell hardness tester calculates the hardness value by measuring the indentation diameters of the steel balls of the specified diameters, suitable for the softer or medium-hardness materials; the Rockwell hardness tester measures the indentation depths, suitable for the wider hardness ranges, with the faster operation.

Metallographic microscope system: Including the inverted or upright optical microscopes, image acquisition systems and analysis software. It is used to observe and rate the microstructures of the steel plate, and can carry out the grain size rating, inclusion analysis, phase composition identification and decarburization layer measurement, etc. It is an important analysis tool connecting the material process and performance.