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1. Overview: For non-destructive testing of everyday workpieces, standards are the most important basis for carrying out such tests. From the selection of inspection methods for workpieces and the precautions during the inspection process, to the final evaluation of the workpieces and the preparation of the reports with relevant parameters, it is often necessary to follow certain standard specifications recognized by both the supplier and the buyer. With the continuous strengthening of international cooperation, our exchanges with foreign countries are also becoming increasingly extensive. Among these, when it comes to product quality inspection, what standards should be followed and what level of inspection should be employed are often among the key topics of discussion between the supplier and the buyer. Therefore, it is also very beneficial for our work to conduct a certain comparison between the domestic non-destructive testing standards for welds in welded components and foreign as well as international standards, and to analyze their applications in daily life. 2. Non-destructive testing standards for domestic and foreign welds: There are established standards for the non-destructive testing of steel structure welds in China, with those used most frequently in the boiler and pressure vessel industry, such as GB/T 11345, NB/T 47013, GB/T 3323, and TB/T 1558. Recently, with the use of aluminum alloy lightweight materials, flaw detection of aluminum alloy welded structures has become increasingly important. Due to the late start in China, there are not yet many practical standards available. With the development of aerospace and high-speed rail technologies, there are an increasing number of aluminum alloy welded structural components. If high-speed trains are all equipped with aluminum alloy car bodies, and prefabricated profiles and sheets are used extensively and connected by welding – such as Alstom’s all-aluminum welded car bodies in France, or Knorr-Bremse’s aluminum alloy welded air tanks in Germany – then, due to the lack of mature inspection standards in China, it is necessary to follow international standards as required by foreign manufacturers. As a result, research is currently being conducted on non-destructive testing standards for aluminum alloy welded structural components. The main standards for non-destructive testing of welded structural components at home and abroad are as follows: GB/T 14693 – Symbols for non-destructive testing of welds; GB/T 3323 – Radiographic testing of metal fusion welding joints; GB/T 12605 – Radiographic testing procedures and quality grading for butt welds of steel pipes; GB/T 11345 – Manual ultrasonic testing methods for steel welds and grading of test results; GB/T 15830 – Ultrasonic testing methods and grading of inspection results for butt ring welds of steel pipes; JB/T 9212 – Ultrasonic testing of welds in atmospheric-pressure steel oil tanks; JB/T 6061 – Magnetic particle testing methods for welds and grading of defect indications; JB/T 6062 – Penetrant testing methods for welds and grading of defect indications; NB/T 47103 – Non-destructive testing of pressure equipment; TB/T 1558 – Ultrasonic testing of butt welds; ISO 5817 – Quality grading of imperfections in fusion welded joints of steel, nickel, titanium, and their alloys (excluding beam welding); ISO 10042 – Quality grading of imperfections in arc welded joints of aluminum and its alloys; ISO 17636-1 – Non-destructive testing of welds – Radiographic testing – X-ray and gamma-ray film techniques; ISO 17636-2 – Non-destructive testing of welds – Radiographic testing – X-ray and gamma-ray digital imaging techniques; ISO 10675-1 – Non-destructive testing of welds – Part 1: Evaluation of radiographic testing for products made of steel, nickel, titanium, and their alloys – Acceptable levels; ISO 10675-2 – Non-destructive testing of welds – Part 2: Evaluation of radiographic testing for aluminum alloy products – Acceptable levels; ISO 17640 – Non-destructive testing of welds – Ultrasonic testing – Testing techniques, acceptance levels, and result evaluation; ISO 11666 – Non-destructive testing of welds – Ultrasonic testing of welded joints – Acceptance levels; ISO 17638 – Non-destructive testing of welds – Magnetic particle testing of welded joints; ISO 23278 – Non-destructive testing of welds – Magnetic particle testing of welded joints – Acceptance levels; ISO 3452 – Non-destructive testing – Penetrant testing; ISO 23277 – Non-destructive testing of welds – Penetrant testing – Acceptance levels; JIS Z3105 – Radiographic testing methods for aluminum welds and methods for grading film images; JIS Z3080 – Methods for ultrasonic testing of aluminum welds at an oblique angle and methods for grading test results; JIS Z3081 – Methods for ultrasonic testing of steel pipe welds at an oblique angle and methods for grading test results; ASTM E1032 – Methods for radiographic testing of welded components; ASTM E390 – Standards for radiographic testing of steel fusion welds; ASTM E1648 – Reference films for radiographic testing in aluminum fusion welding. 3. Comparative analysis of domestic and international standards for weld testing: Since ASTM standards require the use of specific reference charts for weld testing, this approach is relatively difficult to apply when high precision is required, which is why it is rarely used in China unless there is a specific need. The domestic boiler and pressure vessel industry primarily uses NB/T 47013 for inspection. In the railway industry, since steel structures were primarily used in the past, GB/T 3323 and GB/T 11345 were generally employed for radiographic and ultrasonic testing. With the development of high-speed railways and exchanges and cooperation with foreign countries, as well as the successful creation of high-speed EMUs, many companies have now switched to using standards such as ISO 5817 and ISO 10042 for such inspections. At the same time, the standardized ISO 17636-1 is also widely used; GB/T 3323 is essentially equivalent to this standard, except for the acceptance criteria. With the widespread adoption of international standards, the requirements for welding inspection have also gradually increased accordingly. The International Welding System standards are a series of standards for radiographic, ultrasonic, magnetic particle, and penetrant testing, developed based on ISO 5817 and ISO 10042 as their core. These two standards only specify the acceptance levels and do not address specific testing methods; moreover, many of the provisions are not necessarily applicable to all testing methods. For example, in the case of root lack of fusion, the standards specify the thickness of the unfused portion, which may not be possible to determine using our conventional non-destructive testing methods. Therefore, it is up to the user to choose the appropriate method and relevant provisions based on the specific situation. In ISO 5817 and ISO 10042, they are classified into three grades—B, C, and D—based on the strictness of the testing requirements, with grade B having the highest demands. In terms of the standard requirements, there are more inspection items specified compared to current domestic standards; for example, provisions regarding worm-like pores have been added. The standards require control over the size of the projection of the weld bead in the vertical direction as well as its dimensions in the cross-sectional direction. In particular, the requirements for defect control under Class B are stricter than those set by domestic standards – for instance, individual pores are allowed to be no larger than 0.2s (where s is the thickness of the welded layer). Therefore, the requirements for thin-walled welds with a thickness of less than 6 mm are significantly higher than those specified by domestic standards. Higher requirements are placed on thin plates during radiographic inspection. The ISO standards specify image quality indices for different thicknesses under two image quality grades, A and B, as well as three exposure arrangements—single-wall single-shadow, double-wall double-shadow, and double-wall single-shadow. For the double-wall dual-shadow and double-wall single-shadow methods, image quality requirements are specified separately based on the placement of the image quality meter – either on the radiation source side or on the film side. ISO standards define the requirements for image quality; the single-wall single-shadow method is based on the thickness of a single wall, while the double-wall double-shadow and double-wall single-shadow methods are based on the thickness of both walls (national standards, on the other hand, are based on the thickness of a single wall). This ensures consistency with the thickness used to determine the exposure conditions. But in reality, in terms of the thickness of the weld being inspected, the IQI sensitivity obtained using the double-wall penetration method is always half that of the single-wall penetration method. The ISO standards relax the requirements regarding image quality for the use of 192 Ir sources within certain thickness ranges. For example, in Class A: for T = 10–24 mm, the requirement can be reduced by 2 indices; for T > 24–30 mm, it can be reduced by 1 index ； Grade B: T = 12～40 mm, can be 1 index lower (all values refer to the single-wall single-shadow method). This is because the smaller the source penetration thickness, the greater the image quality loss. Lowering the standards is an acknowledgment of the fact that a certain image quality level has declined. Previously, domestic linear sensitivity quality meters used 16 sets of filaments, while the ISO standard employs 19 sets of filaments, with three additional sets added to meet the stricter requirements for inspecting thin sheets. For thicknesses less than 2 mm, the ISO standard requires a quality index of 17–19 (with corresponding filament diameters of 0.080 mm, 0.063 mm, and 0.050 mm); accordingly, a suitable type of quality meter must be specified in accordance with the national standard. Regarding the assessment of weld defects and the conditions for quality acceptance, ISO 5817 and ISO 10042 are applicable to butt welds, fillet welds, and branch connections (such as T-joints); however, the selection of specific grades is determined by relevant regulations or product manufacturing specifications. The purpose of this general standard is to implement hierarchical control over the welding quality of industrial products in order to meet various requirements. It is a quality control standard based on long-term industrial production experience and certain theoretical foundations, rather than one based on fracture mechanics; it is not a so-called applicability standard that requires theoretical predictions of safe service life depending on the nature, location, size of defects, and operating conditions. The tolerance level of the former is clearly higher than that of the latter; the former serves as the foundation and guarantee for the latter. This standard is applicable to the quality assessment of fillet welds, which represents a gap that urgently needs to be filled in China’s railway, boiler, and pressure vessel industries; it should be promptly incorporated, absorbed, and adapted in light of national conditions. ISO standards can serve as a reference for revising and improving our own national and industry standards in various aspects of weld defect quality assessment, particularly regarding the evaluation of fillet weld defects, single-sided welds, and defects in small-diameter pipes. In Europe, the determination of whether welding defects are acceptable is based on a comprehensive assessment taking into account factors such as the nature, quantity, size, and location of the defects. In particular, it poses challenging requirements for the quantification of RT and UT defects, which is also an issue that needs to be addressed urgently in our country. 4. Conclusion As can be seen from the above, with the development of international trade, the exchange between domestic standards and advanced foreign standards is becoming increasingly frequent. And now, newly formulated or revised domestic standards, including industry standards, largely adopt various advanced foreign standards that are equivalent, functionally equivalent, or partially equivalent to them – this is also a common trend among domestic standards. Learning from the advanced experiences of other countries is also beneficial for the development of our standards and for the growth of the entire industry. In our work, we can also combine these elements to **improve our detection efficiency**. Furthermore, within the existing standard systems, ISO standards are highly consistent with and closely related to EN standards, whereas American standards such as ASTM form their own system and differ significantly from the former two in many aspects; each has its own advantages. In contrast, Japan’s JIS standard system is not used as widely, compared to the aforementioned standards which are becoming increasingly popular in the country. Therefore, in our work, since different standards often have certain differences, they cannot be completely equivalent. Therefore, when certain standards are used, choosing or recommending the same standard system whenever possible can **reduce the workload**. A review of the standards shows that foreign standards such as ASTM or EN are coordinated with one another, each covering different areas: some describe general testing methods, some address specific types of workpieces, some outline acceptance rules, and some cover the acceptance criteria for testing materials, thus forming a comprehensive system. Domestic standards still have a long way to go in this regard.