Tubular Welding Electrode Testing

Tubular welding electrode inspection mainly conducts scientific testing and analysis on the appearance quality inspection, dimension deviation inspection, chemical composition analysis, mechanical property testing, process performance evaluation, and diffusible hydrogen content determination of the deposited metal, 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

Appearance quality inspection: Checks whether there are the defects such as the cracks, pits, burrs, oxide scales and oil stains on the surface of the welding electrode. The uniformity and density of the flux covering need to be emphatically observed, ensuring that there are no eccentricity, exposed core, damage and excessive moisture absorption phenomena. These defects will directly affect the arc stability and weld forming quality.

Dimension deviation inspection: The vernier callipers and other measuring tools are used to precisely measure the diameter, length and their allowable tolerances of the welding electrode. The testing contents include the core wire diameter, flux thickness, total length and the exposed length of the gripping end, ensuring that they meet the requirements of the corresponding product standards (such as GB/T 5117 and AWS A5.1), to guarantee the stability of the welding process and the smooth wire feeding.

Chemical composition analysis: The core wire metal is analysed through the spectrometry analysis, wet chemical analysis and other means, determining the contents of the key elements such as the carbon, manganese, silicon, sulphur, phosphorus, chromium, nickel and molybdenum. This item is the basis for judging the metallurgical performance of the welding electrode, and directly affects the strength, toughness, corrosion resistance and crack resistance of the weld metal.

Mechanical property testing: By welding the test plates and processing them into the standard tensile, bending and impact specimens, the yield strength, tensile strength, elongation, reduction of area and impact toughness (often testing the Charpy V-notch impact energy at 0 ℃, -20 ℃ or -40 ℃) of the weld metal are tested. This is the key indicator for evaluating the load-bearing capacity and brittle fracture resistance of the welding electrode.

Process performance evaluation: Under the actual or simulated welding conditions, the arc starting ease, arc stability, spatter amount, slag detachability, weld forming (including the slag coverage and ripple patterns) and all-position welding adaptability of the welding electrode are evaluated. This testing comprehensively reflects the operating experience and the final weld appearance quality of the welding electrode.

Diffusible hydrogen content determination of the deposited metal: The standard methods such as the glycerin method, gas chromatography method or mercury method are adopted to determine the diffusible hydrogen content in the deposited metal. The low-hydrogen or ultra-low-hydrogen welding electrodes have the strict requirements for this indicator. It is a crucial control item for preventing the cold cracks (delayed cracks) of the weld, especially suitable for the structural welding of the high-strength steels, thick plates and the structures with the high restraint.

Flux moisture content and moisture absorption resistance testing: For the moisture-sensitive welding electrodes such as the low-hydrogen type, the moisture content of the flux needs to be detected. Usually, after the welding electrode is exposed in a certain temperature and humidity environment for the specified time, its weight changes are measured or the flux moisture content is directly tested, to evaluate its moisture absorption resistance, ensuring that the blowholes and hydrogen-induced cracks will not be caused by the excessive moisture during the welding.

Testing Scope

Carbon steel and low alloy steel welding electrodes: Applicable to the welding electrodes for welding the structural steels such as the low-carbon steels and low-alloy high-strength steels, such as the E4303, E5015 and E7018 types. The testing needs to emphatically focus on their mechanical properties (strength and toughness), diffusible hydrogen content and all-position welding process performance.

Stainless steel welding electrodes: The welding electrodes used for welding the austenitic, martensitic, ferritic and duplex stainless steels, such as the E308-16 and E316L-16 types. The testing core lies in the chemical composition (especially the contents of the alloying elements such as the chromium, nickel and molybdenum), intergranular corrosion resistance, ferrite number control and weld appearance forming.

Surfacing welding electrodes: The welding electrodes used for the surfacing welding with the special performances such as the surface wear resistance, corrosion resistance and high temperature resistance. In addition to the basic performances, the testing scope focuses more on the hardness, wear resistance (such as the wear testing), corrosion resistance (such as the salt spray testing) and metallographic structure analysis of the surfacing layers.

Cast iron welding electrodes: The welding electrodes used for the repair welding and connection of the cast iron parts, such as the nickel-based and iron-based welding electrodes. The testing needs to evaluate the colour matching degree of the weld metal and the base metal, machinability, crack resistance (especially the white cast iron tendency) and the matching of the linear expansion coefficients.

Non-ferrous metal welding electrodes: Including the copper and copper alloy, aluminium and aluminium alloy, nickel and nickel alloy welding electrodes, etc. The testing difficulties lie in their special chemical activities (such as the oxidation of the aluminium), sensitivity to the blowholes, process challenges caused by the differences in the electrical and thermal conductivities, as well as the corresponding chemical composition and mechanical property tests.

Special function welding electrodes: Such as the underwater welding electrodes, vertical-down dedicated welding electrodes and high-efficiency iron powder welding electrodes. The testing scope needs to be targeted at their special application scenarios, such as the waterproofness, arc stability and hydrogen control of the weld metal of the underwater welding electrodes; and the deposition efficiency and arc force of the high-efficiency welding electrodes.

Testing Methods

Visual inspection and dimension measurement method: In accordance with the standards such as GB/T 25775 "Technical delivery conditions for welding consumables", the systematic visual inspection of the welding electrode appearance is carried out. The dimension measurement uses the measuring tools with the precision not lower than 0.02 mm, carried out at at least three positions at the two ends and the middle of the welding electrode, and the average value is compared with the tolerance band for the judgement.

Spectrometry analysis and chemical titration method: For the chemical composition analysis of the core wire, the spark direct-reading optical emission spectrometer or inductively coupled plasma optical emission spectrometer is the first choice for the rapid multi-element determination. For the arbitration or the value fixing of the reference materials, the classic chemical titration method and gravimetric method are adopted for the precise analysis, ensuring the accurate and reliable results.

Welding procedure evaluation and mechanical property testing method: In accordance with the standards such as GB/T 25774 "Testing methods of welding consumables" or AWS B4.0, the test plates of the standard dimensions are prepared. After the welding, after the non-destructive testing is qualified, the test plates are processed into the standard tensile, bending and impact specimens, and tested on the universal testing machines and impact testing machines according to the corresponding standards (such as GB/T 2650-2654).

Diffusible hydrogen determination method: The glycerin method or gas chromatography method specified in GB/T 3965 "Determination of diffusible hydrogen in deposited metal" is commonly used domestically. The welding electrode is deposited on the test block under the strictly controlled process, rapidly placed in the collection device, and the hydrogen released is collected at the constant temperature of 45 ℃, and the hydrogen content per 100 g of the deposited metal is converted through the volume or pressure.

Flux moisture content determination method: Usually, the heating weight loss method in the annex of GB/T 3965 is adopted. A certain amount of flux powder is taken, baked in a high-temperature oven (such as 350 ℃ ± 10 ℃ for the low-hydrogen welding electrodes) to the constant mass, and the moisture content is calculated through the mass difference before and after baking. This method is direct and effective, and is the basis for the incoming re-inspection of the welding electrodes and the pre-welding drying.

Metallographic structure analysis method: The weld metal, fusion zone and heat-affected zone are sampled, mounted, ground, polished and etched, and then the microstructure is observed under an optical microscope or scanning electron microscope. It is used to evaluate the grain size, inclusion morphology and distribution, and whether the unfavourable microstructures (such as the martensite and network ferrite) exist, correlating with their mechanical properties and crack resistance.

Testing Standards

Domestic Standards:

1. GB/T 5117, Non-alloy and fine grain steels covered electrodes

2. GB/T 25775, Technical delivery conditions for welding consumables

3. GB/T 25774, Testing methods of welding consumables

4. GB/T 3965, Determination of diffusible hydrogen in deposited metal

5. GB/T 2650, Destructive tests on welds in metallic materials — Impact tests — Test specimen location, notch orientation and examination

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

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

International Standards:

1. AWS A5.1, Specification for carbon steel electrodes for shielded metal arc welding

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

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: Including the spark direct-reading optical emission spectrometers and inductively coupled plasma optical emission spectrometers, used for the fast and accurate qualitative and quantitative chemical composition analysis of the core wire metal. They can simultaneously determine the multiple elements, and are the key equipment for the raw material acceptance and finished product quality control of the welding electrodes.

Universal material testing machine: Used for the tensile testing of the deposited metal and the bending testing of the welded joints. It can precisely measure the mechanical parameters such as the tensile strength, yield strength and elongation of the specimens. The equipment is equipped with the various fixtures and extensometers to meet the testing needs of the different standards and specimen types.

Impact testing machine: Usually a pendulum impact testing machine, used to determine the Charpy V-notch impact absorbed energy of the weld metal or heat-affected zone at the specific low temperatures (such as -20 ℃ and -40 ℃). The equipment needs to be equipped with a low-temperature bath to precisely control the testing temperature of the specimen, evaluating the brittle fracture resistance of the material.

Diffusible hydrogen determination instrument: Depending on the selected method, the equipment includes the glycerin method collection device (with the constant temperature water bath and gas measuring tube) or gas chromatograph. The latter has the higher sensitivity, can distinguish the different forms of hydrogen, and can simultaneously determine the multiple gas components. It is an important tool for the precise hydrogen content research and quality control.

Precision oven and balance: Used for the flux moisture content testing. The oven needs to be able to precisely control the temperature (such as the range from the room temperature to 400 ℃ with the precision ±5 ℃), and have the good temperature uniformity. The precision of the analytical balance needs to reach 0.0001 g, to ensure the accuracy of the moisture content calculation results.

Metallographic specimen preparation system and microscope: The system includes the cutting machines, mounting machines and grinding and polishing machines. The microscopes include the optical microscopes and scanning electron microscopes, used to observe the microstructure and fracture morphology of the weld, and carry out the energy dispersive spectroscopy micro-area composition analysis. They are the essential equipment for the in-depth research on the welding metallurgical defects and the causes of the performances.

Welding power source and auxiliary tooling: For the process performance evaluation and the welding of the mechanical property test plates, the welding power sources with the stable performance and adjustable parameters (such as the DC arc welding machines) are needed. At the same time, the dedicated welding fixtures, test plate positioning devices and temperature and humidity monitoring equipment need to be equipped, to ensure the standardization and repeatability of the welding process parameters and environmental conditions.