Thoriated Tungsten Electrode Rod Testing

Thorium-tungsten electrode rod inspection mainly conducts scientific testing and analysis on the chemical composition analysis, physical property testing, micro-structure and structure testing, surface quality testing, radioactivity testing, and dimension and 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: This is the basic core item of the thorium-tungsten electrode rod testing, aiming to precisely determine its main components and trace additive elements. The content of the tungsten (W) is mainly analysed, usually above 97%, as well as the content of the key additive thorium oxide (ThO₂), with the common specifications such as 2% or lower. At the same time, the limits of the impurity elements such as the iron (Fe), copper (Cu) and carbon (C) need to be strictly tested. These impurities will directly affect the electrical conductivity, thermal stability of the electrode and the stability of the welding arcs.

Physical property testing: This testing item focuses on the macro physical characteristics of the electrode rods. Mainly including the density determination, and the high density is the basis of ensuring the high melting points and high current carrying capabilities of the electrode. The mechanical properties such as the tensile strengths and elongations also need to be tested, evaluating their fracture resistance during the clamping, transportation and use. The straightness or camber of the electrode is also a key indicator, directly affecting their centering in the welding torches and the stability of the arcs.

Micro-structure and structure testing: The micro-structures of the thorium-tungsten electrode rods are observed through the metallographic microscopes or scanning electron microscopes (SEM). The grain sizes, shapes and distribution uniformity of the tungsten, as well as the dispersion states of the thorium oxide particles in the tungsten matrices are emphatically analysed. The uniform and fine grains and the dispersed thorium oxide particles can significantly reduce the electron work functions and improve the electron emission capabilities, thereby improving the arc starting properties and arc stability.

Surface quality testing: The visual and instrumental inspection of the electrode rod surface states is carried out. The surfaces are required to be smooth and clean, without the defects such as the cracks, burrs, laps, pits, oxide scales and other surface defects affecting the use. For the ground electrodes, the surface smoothness needs to be checked; for the polished electrodes, the mirror effects are required. The surface qualities directly affect the arc concentration and weld qualities during the welding.

Radioactivity testing: Since the thorium oxide with the weak radioactivity is added, this testing is crucial. The dedicated instruments (such as the α and β surface contamination meters) need to be used to measure their surface contamination levels and air kerma rates, ensuring that they meet the exemption or management requirements of the national Basic Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources (GB 18871) and other relevant regulations, guaranteeing the safety of the production, storage and use personnel.

Dimension and tolerance testing: The precision measuring tools such as the micrometers and projectors are used for the precise measurement of the diameters and lengths of the thorium-tungsten electrode rods. The diameter tolerances are usually required to be controlled within ±0.05mm, and the length tolerances are determined according to the customer requirements. The strict dimension control ensures that the electrode can perfectly match with the welding torch collets of the different brands and models, avoiding the overheating or arc wandering caused by the poor contact.

Testing Scope

Raw material incoming inspection: The testing is carried out on the initial raw materials such as the purchased tungsten powders and thorium oxide powders. For the tungsten powders, the particle size distributions, bulk densities, Fisher particle sizes and impurity contents need to be tested; for the thorium oxide powders, the purities, radioactivity activity concentrations and physical characteristics need to be tested. Controlling the material quality from the sources is the premise of ensuring the stable final electrode performance.

Production process quality monitoring: Covering the whole process from the powder mixing, isostatic pressing, vertical melting or sintering, swaging, drawing to the final grinding and polishing. The testing points are set at the key processes, for example, checking the densities and micro-structures of the sintered blanks, and monitoring the diameter uniformity and surface defects of the drawn wires, ensuring that the intermediate product qualities meet the process design requirements.

Finished product factory delivery full-item inspection: For the finished thorium-tungsten electrode rods completing all the processing processes, the comprehensive testing is carried out according to the product standards or technical agreements. Including all the above testing items, ensuring that each batch of products reaches the specified requirements in the chemical compositions, physical properties, dimensions, appearances and safety, and the corresponding quality certificate documents are issued.

Verification testing for the specific application scenarios: For the special applications in the high-end industrial fields such as the aerospace, nuclear power and pressure vessels, the verification testing under the simulated working conditions needs to be carried out. For example, under the specific shielding gases (such as the argon), the long-time high-current arc starting and arc stability testing is carried out, evaluating their burn-off rates, arc patterns and anti-contamination capabilities under the extreme conditions.

Market circulating commodity supervision and sampling inspection: The quality supervision departments or third-party testing institutions carry out the random sampling inspection on the thorium-tungsten electrode rod products sold in the markets. The purpose is to supervise the product qualities of the manufacturers and sellers, crack down on the counterfeit and shoddy products, and maintain the market order and the rights and interests of the end users. The testing items usually focus on the safety and key performance indicators.

Pre-use and in-use inspection: At the welding operation sites, the operators need to carry out the simple appearance and dimension inspection before using the electrode. During the use, through the observation of the arc stability, electrode tip patterns (which should be hemispherical) and abnormal burn-off conditions, whether the electrode qualities are qualified is indirectly judged. This is a practical on-site fast evaluation method.

Testing Methods

Inductively coupled plasma atomic emission spectrometry (ICP-OES): Used for the precise determination of the main elements tungsten and thorium, as well as the multiple trace impurity elements in the thorium-tungsten electrode rods. After the dissolution of the samples, the high-temperature plasma is used to excite the atoms to generate the characteristic spectra, and the quantitative analysis is carried out through the spectral intensities. With the advantages of the high sensitivity, wide linear ranges and multi-element simultaneous determination, it is the mainstream method of the chemical composition analysis.

X-ray fluorescence spectrometry (XRF): As a fast and non-destructive testing method, often used for the online or fast composition analysis in the production processes. Through the measurement of the characteristic X-ray fluorescence generated after the samples are excited by the X-rays, the elemental compositions are qualitatively and quantitatively analysed. Although the analysis capability for the light elements is weak, it is fast and effective for the determination of the heavy elements such as the tungsten and thorium, suitable for the large-batch screening.

Metallographic microscope analysis method: Used to observe the micro-structure organizations of the thorium-tungsten electrode rods. The electrode samples need to be cut, mounted, ground, polished and etched (usually using the mixed solution of the hydrofluoric acid and nitric acid) to prepare the metallographic specimens. Under the microscopes, the grain sizes, shapes, second phase distributions and various internal defects such as the pores and inclusions are observed, providing the basis for the process improvements.

Archimedes drainage method for the density measurement: This is the classic physical method of measuring the densities of the thorium-tungsten electrode rods. First, the masses of the dry samples are weighed in the air, then they are immersed in the distilled water to weigh their apparent masses. According to the Archimedes principle, their volumes are calculated, thereby obtaining the actual densities of the samples. This method has the simple equipment and accurate and reliable results, and is the key means of evaluating the material densification degrees.

Universal material testing machine mechanical property testing: The universal material testing machines are used to carry out the tensile testing on the thorium-tungsten electrode rod specimens of the standard sizes. Through the recording of the load-displacement curves during the tensile processes, the key mechanical parameters such as the tensile strengths, yield strengths and elongations after fracture of the material can be precisely calculated, evaluating their fracture resistance and plastic deformation capabilities.

Surface contamination and dose rate measurement method: The α and β surface contamination monitors are used to wipe or directly measure the electrode surfaces, evaluating the radioactive contamination levels. The environmental dose rate meters are used to measure the surrounding dose equivalent rates at the specified distances (such as 5cm from the electrode surfaces). All the operations and result determinations need to strictly follow the national radioactive testing standards and safety regulations.

Testing Standards

Domestic Standards:

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

2. GB 18871, Basic standards for protection against ionizing radiation and for the safety of radiation sources

International Standards:

1. 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

Inductively coupled plasma optical emission spectrometer (ICP-OES): This instrument is the core equipment of the precise quantification of the trace and constant elements. Mainly consisting of the sampling systems, radio frequency generators, plasma torches, dispersion systems and detectors. The clean laboratory environments are required, equipped with the ultrapure water machines and high-purity argon, and the calibration with the standard solutions is regularly carried out, to ensure the accuracy and repeatability of the testing data.

X-ray fluorescence spectrometer (XRF): Divided into the wavelength dispersive type (WDXRF) and energy dispersive type (EDXRF). Used for the fast composition analysis of the thorium-tungsten electrode rods, usually equipped with the rhodium (Rh) target X-ray tubes. The equipment needs to be placed in the rooms with the stable temperature and humidity, and equipped with a series of standard samples matching the matrices of the samples to be tested, used to establish the calibration curves and realize the accurate quantification.

Metallographic microscope and image analysis system: Including the inverted or upright metallographic microscopes, equipped with the observation modes such as the bright field, dark field and polarized light, as well as the high-resolution digital camera systems. Combined with the professional image analysis software, the quantitative analysis such as the automatic grain size rating and second phase particle statistics can be carried out on the photographed metallographic images, making the structure evaluation more objective and precise.

Precision electronic balance and density determination device: The electronic balances used for the density measurement need to have the high precision (usually required to be 0.1mg) and good stability. The density determination devices usually include a weighing support, beakers and the fine wire suspension devices which can immerse the samples in the liquids. The whole system needs to be regularly calibrated with the standard weights and standard blocks of the known densities.

Universal material testing machine: Used for the mechanical property testing, the models with the appropriate measuring ranges should be selected, and equipped with the dedicated fixtures suitable for the brittle materials, such as the clamping blocks with the V-shaped grooves, to prevent the sample slipping or crushing. The equipment needs to be regularly verified by the metrology departments, ensuring the accuracy of the measurement parameters such as the force values and displacements.

Radioactivity testing instruments: Mainly including the α and β surface contamination meters and environmental dose rate meters. The surface contamination meters need to be equipped with the different types of probes (such as the ZnS scintillation probes for the α measurement and plastic scintillation probes for the β measurement). All the radioactivity testing equipment must be regularly sent to the qualified institutions for the verification or calibration according to the specified cycles, ensuring the validity and legal effect of their measurement results.