Round Link Chain Testing

Round link chain inspection mainly conducts scientific testing and analysis on the tensile property testing, dimension and geometric tolerance testing, hardness testing, surface quality and defect testing, fatigue life testing and metallographic structure analysis, 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

Tensile property testing: This is the core item of the round link chain inspection, aiming to evaluate the mechanical properties of the chains under the static loads. Usually including the breaking load testing of the whole chains and the tensile testing of the single links, requiring that the chains do not fracture or permanently deform under the specified loads, to verify whether the yield strengths and tensile strengths of their materials meet the design and standard requirements.

Dimension and geometric tolerance testing: The strict control of the geometric precision of the round link chains, mainly including the measurement of the key dimensions such as the internal lengths, internal widths, round steel diameters and weld position diameters of the links. At the same time, the flatness, twist and the fit clearances between the various links need to be checked, ensuring that the chains run smoothly without the interference during the operation or connection. It is the basis of guaranteeing their interchangeability and assemblability.

Hardness testing: Through the Brinell hardness (HB) or Rockwell hardness (HRC) testing, the hardness values of the link materials, especially the weld areas, are evaluated. The appropriate hardness is the key of the chains having both the strengths and toughness. The excessively high hardness is easy to cause the brittle fracture, and the excessively low hardness is easy to cause the plastic deformation or wear. The testing results need to comply with the material heat treatment process specifications.

Surface quality and defect testing: The visual inspection or non-destructive testing methods such as the magnetic particle flaw detection and penetrant flaw detection are used to check whether the link surfaces have the defects such as the cracks, laps, scars and rust. The special attention is paid to whether the weld areas have the welding defects such as the lack of penetration, slag inclusions, blowholes and undercuts. These defects are the stress concentration points, seriously affecting the fatigue lives and safety of the chains.

Fatigue life testing: Simulating the alternating load states of the chains under the actual working conditions, the cyclic loading testing is carried out on the fatigue testing machines. Through the recording of the number of cycles until the fracture of the chains under the specific stress amplitudes, their fatigue resistance is evaluated. This is crucial for the chains in the dynamic and high-frequency equipment (such as the scraper conveyors and hoists).

Metallographic structure analysis: Through the sampling, mounting, polishing and etching to prepare the metallographic specimens, the micro-structures of the link materials are observed under the microscopes. The grain sizes, phase compositions, non-metallic inclusion grades and the structure changes of the welding heat-affected zones are analysed, judging the material properties and the rationality of the heat treatment and welding processes from the micro level.

Impact toughness testing: Usually carried out on the Charpy notched impact testing machines, used to evaluate the capability of the chain materials to resist the brittle fracture under the low temperatures or dynamic loads. Through the measurement of the impact energies absorbed when the specimens fracture, the toughness reserves of the material are judged. It is particularly important for the round link chains in the cold environments or bearing the impact loads.

Testing Scope

High-strength round link chains for the mines: Mainly used in the equipment such as the scraper conveyors, stage loaders and coal plows in the underground coal mines, and are the key load-bearing components of the coal mining. The testing strictly follows the coal industry standards such as MT/T 929, with the extremely high requirements for the breaking loads, fatigue lives, dimension tolerances and surface defects, to ensure the safe operation under the harsh working conditions.

Round link chains for the lifting and hoisting: As the important components of the sling assemblies, used for the lifting operations in the ports, construction sites and factories. The testing is according to the national standards such as GB/T 20946, emphatically verifying their working load limits, deformations under the proof loads, as well as the connection reliability of the end fittings, directly related to the safety of the personnel and equipment.

Marine anchor chains and mooring chains: Used for the ship mooring and positioning, needing to withstand the huge static loads and dynamic wind and wave impacts. The testing scope includes the whole process from the raw materials to the finished chains, conforming to the international and national standards such as ISO 1704 and GB/T 549, with the special attention to the evaluation of the welding qualities, breaking loads and seawater corrosion resistance.

Transmission chains for the conveying machinery: Applied to the various apron conveyors and hoists as the traction components. The testing needs to pay attention to the pitch precision of the chains, wear properties of the hinge pairs, as well as the engagement smoothness with the sprockets, ensuring the smooth transmission, low noise and long service lives.

Chains for the metallurgical and engineering machinery: The special chains used in the high-temperature, high-dust and heavy-load metallurgical production lines or large engineering machinery. In addition to the routine items, the testing scope may extend to the high-temperature strength testing, wear-resistant coating evaluation and special anti-corrosion performance testing, to adapt to the extreme working conditions.

Samples for the certification and supervision sampling inspection: The chain samples sent for the testing to obtain the production licenses (such as the MA/KA safety marks for the mining products), classification society certifications (such as CCS, DNV and ABS) or to respond to the national market supervision sampling inspections. The testing scope covers all the mandatory items of the product standards, and is the important link of the market access and product quality supervision.

Testing Methods

Mechanical property testing method: The universal material testing machines are used to carry out the tensile testing on the whole chains or single links until the fracture. Through the transducer recording of the load-displacement curves, the parameters such as the maximum breaking loads and yield loads are precisely determined. The testing needs to ensure the constant loading speeds and firm clamping to prevent the slipping. It is the direct method of determining the load-bearing capabilities of the chains.

Geometric quantity measurement method: The comprehensive use of the tools such as the callipers, micrometers, ring gauges, plug gauges, projectors or coordinate measuring machines (CMM). The multiple-point measurements are taken on the key dimensions such as the internal lengths and internal widths to take the averages; the flatness and twist of the links are detected using the surface plates and dial indicators, ensuring the unified measurement datum and accurate and reliable data.

Non-destructive testing method: The magnetic particle flaw detection (MT) is suitable for the ferromagnetic materials, forming the magnetic indication displays at the defects after the magnetization; the penetrant flaw detection (PT) is suitable for the surface defect testing of all the non-porous materials; the ultrasonic flaw detection (UT) can be used to detect the internal defects. These methods effectively identify the hidden dangers such as the cracks and inclusions without destroying the chains.

Metallographic analysis method: According to the standards such as GB/T 13298, the specimens are taken at the cross-sections of the links (including the base materials, heat-affected zones and weld centers). After the grinding, polishing and etching, the observation and photographing are carried out under the metallographic microscopes or scanning electron microscopes. According to the relevant standard reference images, the structure types, grain sizes and defects are evaluated. It is the important means of the process verification.

Fatigue testing method: On the dedicated chain fatigue testing machines, the periodically changing tensile loads (usually the pulsating loads) are applied to the chains. The maximum and minimum loads and frequencies are set, and the operation continues until the chain fracture, recording the number of cycles. This method can truly simulate the actual working conditions, and is the key of evaluating the chain durability.

Hardness testing method: According to the chain sizes and expected hardness ranges, the Brinell, Rockwell or Vickers hardness testers are selected. Before the testing, the test points need to be ground until smooth. During the testing, the indenters are perpendicular to the specimen surfaces, and the values are read after the specified load holding times. The testing is usually carried out at the multiple parts of the links (such as the straight sides, arcs and weld positions) to evaluate the uniformity.

Testing Standards

Domestic Standards:

1. MT/T 929, High-tensile steel chains for mining

2. GB/T 20946, Short link chain for lifting purposes — General conditions of acceptance

3. GB/T 549, Electric welded anchor chains

4. GB/T 13298, Inspection methods of microstructure for metals

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

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

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 1704, Ships and marine technology — Stud link anchor chains

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 testing the tensile properties of the round link chains, and the measuring ranges need to cover the breaking loads of the chains to be tested. Equipped with the dedicated chain tensile fixtures, which can firmly clamp the two ends of the links, and equipped with the high-precision force transducers and deformation measuring devices, to record the load-displacement curves in real time and output the accurate mechanical property data.

Chain fatigue testing machine: The testing equipment specially used to simulate the dynamic loads, driven by the hydraulic or electric servo systems, which can precisely control the load amplitudes, frequencies and waveforms. Equipped with the cycle counters and automatic shutdown functions, which automatically stop when the chain fracture is detected or the preset number of cycles is reached, used to scientifically evaluate the lives and reliability of the chains.

Hardness tester series: Including the Brinell hardness testers (for the softer or medium-hard materials), Rockwell hardness testers (fast measurement) and portable Leeb hardness testers (for the on-site or large workpieces). Need to be equipped with the standard-conforming indenters and calibration blocks, ensuring the accuracy and comparability of the results when testing at the different parts.

Non-destructive testing equipment: The magnetic particle flaw detectors (including the magnetization devices, magnetic suspension spraying systems and UV lamps), penetrant testing reagent kits (penetrants, cleaners and developers), ultrasonic flaw detectors (with the appropriate probes). These equipment are used for the comprehensive troubleshooting of the material and process defects from the surfaces to the interiors, and are the important defense line of the quality control.

Geometric quantity measuring instruments: The high-precision digital callipers/micrometers, dedicated ring gauges and plug gauges, optical projectors and coordinate measuring machines (CMM). The CMM can non-contactingly obtain the three-dimensional coordinate points of the link surfaces through the probes, and carry out the fitting and calculation of the complex geometric tolerances through the software, realizing the high-precision and high-efficiency dimension testing.

Metallographic sample preparation and observation equipment: Including the metallographic specimen preparation equipment such as the cutting machines, mounting machines, pre-grinding machines and polishing machines, as well as the metallographic microscopes and image analysis systems. The microscopes need to have the objectives of the different magnifications and good illumination systems. The image analysis software can be used for the quantitative analysis of the grain sizes and phase ratios, providing the micro-structure basis.