Steel Space Frame Material Testing

Steel space frame material inspection mainly conducts scientific testing and analysis on the chemical composition testing, mechanical property testing, dimension and shape deviation testing, surface quality and defect testing, process property testing and non-destructive 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 testing: This is the basic link of the steel space frame material inspection, aiming to verify whether the chemical compositions of the steel meet the design requirements and relevant standards (such as GB/T 1591 and GB/T 699). Mainly analysing the contents of the five major elements carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulfur (S). For the low-alloy high-strength steels, the alloying elements such as the chromium (Cr), nickel (Ni), molybdenum (Mo) and vanadium (V) also need to be tested. The composition deviations will directly affect the mechanical properties and weldability of the material.

Mechanical property testing: Evaluates the behavior of the steel under the applied forces, and is the core of ensuring the structural safety. The key indicators include the yield strength, tensile strength, elongation after fracture and reduction of area, and the tensile testing is carried out according to the standards such as GB/T 228.1. For the space frames bearing the dynamic loads, the impact toughness testing (such as the Charpy V-notch impact testing) also needs to be carried out, to evaluate the anti-brittle fracture ability of the material under the low-temperature or fast-loading conditions.

Dimension and shape deviation testing: For the components such as the steel pipes and sections, their outer diameters, wall thicknesses, lengths, ovality, straightness and end face perpendicularity, etc. need to be precisely measured. These dimension deviations directly affect the machining precision, connection clearances and final installation quality of the components. The testing usually adopts the tools such as the ultrasonic thickness gauges, callipers, micrometers and laser range finders, and the judgement is carried out according to the product standards such as GB/T 3091 and GB/T 706.

Surface quality and defect testing: Checks whether the material surfaces have the defects such as the cracks, scars, folds, inclusions, delaminations and rust. The serious surface defects will become the stress concentration sources and significantly reduce the fatigue lives of the structures. The testing methods include the visual inspection, magnetic particle flaw detection (MT) and penetrant flaw detection (PT), used to find the discontinuity defects on the surfaces and near the surfaces.

Process property testing: Mainly evaluates the cold bending properties and weldability of the steel. The cold bending testing (GB/T 232) is used to inspect the ability of the steel to bear the specified bending angles without the cracks, reflecting its plasticity. The weldability evaluation carries out the preliminary judgement through the carbon equivalent (Ceq or Pcm) calculation, and may be supplemented with the welding process qualification testing, to determine the appropriate welding materials and process parameters.

Non-destructive testing: Under the premise of not damaging the material, the internal defects are probed. The common methods include the ultrasonic testing (UT), used to detect the internal cracks, delaminations and inclusions, etc.; the radiographic testing (RT), mainly used for the inspection of the internal defects of the welds. The non-destructive testing is the necessary means of ensuring the internal quality integrity of the key load-bearing components.

Testing Scope

Main structural steels: The testing scope covers the steels used for all the load-bearing components in the steel space frames, such as the seamless steel pipes, longitudinal welded pipes or sections (angle steels and H-beams) commonly used for the chords and web members. This includes the inspection of the incoming raw materials (steel plates, steel strips and pipe blanks), as well as the re-inspection of the finished or semi-finished products such as the processed and formed members, bolted balls and welded hollow balls.

Connection materials and fasteners: Including the high-strength bolts (such as the torshear type or large hexagon head bolts of GB/T 1228), bolted balls, welded hollow balls, cone heads, sealing plates and sleeves, etc. Their materials, hardnesses, proof loads, torque coefficients (high-strength bolts) and dimension precision need to be tested. The quality of the connection materials directly determines the safety and reliability of the nodes.

Welding consumables: The testing of the welding consumables such as the electrodes, welding wires and fluxes is crucial. Their types, specifications, chemical compositions, deposited metal mechanical properties (tensile strength and impact toughness) and diffusible hydrogen contents need to be checked, ensuring that they match the base materials and can meet the weld properties required by the design.

Anti-corrosion and fire-proofing coating materials: For the steel space frames adopting the coating protection, the anti-corrosion coatings (such as the zinc-rich primers, intermediate paints and topcoats) and fire-proofing coatings need to be tested. The testing items include the coating thicknesses (dry film and wet film), adhesion, impact resistance, salt spray resistance, as well as the fire resistance limits and thermal insulation properties of the fire-proofing coatings.

Space frame structure nodes and welds: This is the key area of the testing. The node testing includes the thread precision and tensile strength of the bolted balls, and the joint misalignment and weld quality of the welded balls, etc. The weld testing scope includes all the on-site installation welds and important factory welds, especially the groove forms, weld dimensions, appearances and internal qualities.

Material re-inspection and witnessed sampling: According to the provisions such as the "Code for acceptance of construction quality of steel structures" (GB 50205), for the steels belonging to one of the following situations, the sampling re-inspection should be carried out: the imported foreign steels; the steels of the mixed batches; the plates with the thickness ≥40mm and with the Z-direction performance requirements; the steels with the design re-inspection requirements; the steels with the quality doubts. The re-inspection must be sampled and sent for testing under the witness of the supervision engineers.

Testing Methods

Spectroscopic analysis method: Used for the fast and non-destructive qualitative and semi-quantitative analysis of the chemical compositions on site. The common equipment are the handheld X-ray fluorescence spectrometers (XRF) or laser induced breakdown spectrometers (LIBS). This method can obtain the contents of the main alloying elements of the material within seconds, suitable for the material designation identification and incoming fast screening, but the precision is usually lower than the laboratory chemical analysis methods.

Laboratory chemical analysis method: As the arbitration and precise quantitative analysis methods, including the carbon-sulfur analysers (infrared absorption method for determining C and S), inductively coupled plasma optical emission spectrometers (ICP-OES) or atomic absorption spectrometers (AAS) for determining the metal elements. This method is carried out according to the GB/T 223 series standards, with the precise and reliable results, and is the basis of issuing the formal material reports.

Universal material testing machine testing: This is the core equipment of the mechanical property testing. Through the configuration of the different fixtures and accessories, the tests such as the tensile, compression and bending can be completed. During the tensile testing, the testing machine automatically draws the stress-strain curves and calculates the key data such as the yield strength, tensile strength and elongation. The whole process needs to be strictly carried out according to the GB/T 228.1 standard.

Impact testing machine testing: Used to determine the impact toughness (impact absorbed energy KV2) of the material. The standard Charpy V-notch specimens are broken by the pendulums at one time at the specified temperatures (such as 20℃, 0℃, -20℃ and -40℃), and the absorbed energy is read. The testing is carried out according to GB/T 229, and is the important method of evaluating the ductile-brittle transition characteristics and anti-dynamic-fracture abilities of the steel.

Ultrasonic flaw detection method: The testing is carried out by utilizing the characteristics of the high-frequency sound waves that reflect and refract when they encounter the defects during the propagation inside the material. This method (UT) is sensitive to the planar defects (such as the cracks and delaminations), has the large detection depths, and is suitable for the internal defect detection of the thick-wall steel pipes, steel plates and large forgings (such as the bolted balls), with the implementation standards such as GB/T 2970 and GB/T 6402.

Magnetic particle flaw detection and penetrant flaw detection: The magnetic particle flaw detection (MT) is suitable for the surface and near-surface defect detection of the ferromagnetic materials (such as the carbon steels and low-alloy steels). The penetrant flaw detection (PT) is suitable for the surface open defect detection of all the non-porous materials. These two methods are relatively simple to operate and intuitively display the defect shapes, widely applied in the inspection of the weld surfaces, member ends and machined surfaces of the parts.

Testing Standards

Domestic Standards:

1. GB/T 1591, High strength low alloy structural steels

2. GB/T 699, Quality carbon structure steels

3. GB/T 1228, High strength bolts with large hexagon head for steel structures

4. GB 50205, Code for acceptance of construction quality of steel structures

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

6. GB/T 232-2010, Metallic materials — Bend test method

7. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

8. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

9. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: Test method

10. GB/T 2970, Ultrasonic testing method for thick steel plates

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

3. 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

Spectrometer: Represented by the handheld X-ray fluorescence spectrometers, with the characteristics of the portability, fastness and non-destructiveness. Through the X-rays exciting the sample atoms to generate the characteristic X-ray fluorescence, the element types and contents are determined by analysing the fluorescence spectra. The instrument has the built-in calibration curves, suitable for the fast identification of the steel designations and alloy element screening on the construction sites, and is the first gate of the material control.

Universal material testing machine: Usually the microcomputer-controlled electro-hydraulic servo or electronic testing machines, with the measuring ranges from tens of kN to thousands of kN. The core is composed of the loading frames, servo actuators, high-precision load transducers, displacement extensometers and computer control systems. It can precisely control the loading rates, collect the force and deformation data in real time, and automatically generate the test reports. It is the equipment for the mechanical property testing.

Impact testing machine: It is the pendulum impact testing machine, composed of the frames, pendulums, specimen supports, energy indicating mechanisms and safety protection devices. The key parameters are the initial potential energy and impact speed of the pendulums. The equipment needs to be regularly verified, ensuring that its base energy losses and impact speeds meet the requirements of GB/T 3808, to guarantee the accuracy and comparability of the impact absorbed energy testing results.

Ultrasonic flaw detector: The digital ultrasonic flaw detectors are the mainstream of the modern testing. The core components include the pulse generators, receiving amplifiers, digital signal processors and displays. Combined with the probes of the different frequencies and angles (straight-beam probes and angle-beam probes), the defects of the different depths and orientations can be probed. The advanced equipment has the functions such as the A-scan waveform display, B-scan imaging, automatic defect judgement and data storage, greatly improving the detection efficiency and reliability.

Hardness tester: Used to measure the hardnesses of the steel and fasteners, indirectly evaluating their strengths. The common types include the Brinell hardness testers (HBW, large indentations, representing the average hardnesses), Rockwell hardness testers (HRC and HRB, convenient operation) and Leeb hardness testers (portable, suitable for the on-site large components). The hardness testing is often used for the heat treatment effect verification of the material and the hardness sampling inspections of the high-strength bolts.

Coating thickness gauge: Divided into the magnetic thickness gauges and eddy current thickness gauges. The magnetic method is used to measure the thicknesses of the non-magnetic coatings (such as the paints and plastics) on the magnetic metal substrates (such as the steel); the eddy current method is used to measure the thicknesses of the insulating coatings on the non-magnetic metal substrates (such as the aluminium), or the non-conductive coatings on the magnetic substrates. The equipment is light and portable, and can carry out the mass point testing of the anti-corrosion and fire-proofing coatings on the construction sites, ensuring that the coating thicknesses meet the design specifications.