Why was the joint factor of 0.6 canceled when non-destructive testing cannot be performed on Class B circumferential joints?
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Dear teachers, I would like to ask why the rule stating that \"the joint coefficient for closed welded joints that cannot be inspected should be set at 0.6\" was removed from GB/T150? There is a theory that some people might use the fact that flaw detection is not possible as an excuse to reduce the welding joint coefficient to 0.6, thereby avoiding non-destructive testing. To prevent such exploitation of this loophole to avoid testing, the value of 0.6 was removed. Then change it to “When non-destructive testing is not possible, GTAW should be used for the root pass”; does that mean that in cases of extremely high hazards where non-destructive testing is not feasible, using GTAW for the root pass allows the coefficient to be set at 1? However, a coefficient of 1 is applied only when non-destructive testing shows that the defect level of the weld is at grade II, indicating that the weld does not weaken the base material (here, weakening is considered solely in terms of the presence of defects; weakening caused by differences in the microstructure between the HAZ and the base material is not taken into account). The results of non-destructive testing are the basis for determining the joint coefficient. But since no non-destructive testing is conducted at present, there is no such basis; only gas-shielded welding is used. Is it risky to assign a value of 1 when the condition of the weld defects is unknown? From a welding perspective, methods such as GTAW with argon or PAW with plasma are used for single- or double-sided welding; however, it is not possible to guarantee perfect formation on the opposite side. Especially given the common situation where the misalignment gap during on-site assembly often exceeds acceptable limits. What’s more serious is that, if no non-destructive testing is carried out, people dare to weld joints that require argon filling without actually filling them with argon. Without argon shielding, oxidation occurs at the root area; the melting point of the oxides is higher than that of the normal molten iron, preventing the molten iron from reaching the back side. As a result, the root area is not fully welded and the back side cannot be properly formed. Additionally, in the absence of argon, alloying elements such as Cr are lost, which reduces the corrosion resistance of the back side of the root area. This also leads to brittle cracking in the metal at the root of the weld. Currently, welders’ wages are determined solely based on the quality of the X-ray inspections; no flaw detection is carried out, and as a result, it is not possible to prevent welders from working in a reckless manner from the perspective of actual quality control in production. Therefore, in practical welding operations, no non-destructive testing is carried out; only gas shielded welding is used for the root pass, assuming that full penetration on the back side has been achieved and that there are no defects. Using a value of 1 in such cases seems a bit like ignoring other factors. In my opinion, it is more reasonable to use a coefficient of 0.6 when no non-destructive testing is available, as this provides a greater safety margin. However, there is a tendency to eliminate this requirement nowadays. GB/T150 has already removed it, and although GB/T151-2014 for tube sheets and shell-side cylinders still retains the value of 0.6, it’s uncertain whether it will be removed in future versions. So why is the requirement to use a coefficient of 0.6 for closed welded joints that cannot be inspected being eliminated now?
The provision in the GB/T 150 standard stating that “the joint factor for closed welded joints that cannot be inspected shall be 0.6” has been abolished, primarily to enhance the safety and reliability of welded joints. Originally, this provision was intended to set a lower safety factor for welded joints that cannot undergo non-destructive testing, thereby enhancing their safety. However, in practice, this may lead some production units to exploit this provision to avoid conducting more stringent non-destructive testing; they simply reduce the joint factor to meet inspection requirements, thereby potentially neglecting any real improvement in weld quality. After this regulation was abolished, the standards shifted to emphasize the need to employ more effective welding techniques and inspection methods to ensure the quality of welded joints. For example, it is suggested that when non-destructive testing is not possible, more reliable welding techniques such as gas shielded welding should be used to ensure the integrity of the welds. Furthermore, it is emphasized that even when traditional non-destructive testing is not possible, the welding quality should still be ensured through other means, such as improving the welding process and providing additional training for welders. In short, this change is intended to prevent manufacturing units from relying solely on reducing the joint coefficient to meet compliance requirements, rather than encouraging the use of more advanced welding techniques and comprehensive quality control measures to fundamentally improve the safety and reliability of welded joints. .
I checked GB150-1998, and this requirement is no longer included in it; I searched further in the earlier version GB150-89, which means that this requirement was removed in 1998. This clause is: 1.8.3 Ring welds with single-sided welding where flaw detection is not possible, without gussets: Φ=0.6. This coefficient is applicable only to ring welds on shells with a thickness of no more than 16 mm and a diameter of no more than 600 mm. There is one condition: if it’s an empty canister with no internal components, it should be possible to perform radiographic testing using double-wall single-image or double-wall double-image techniques ； For circumferential butt welds that are affected by internal components or are too thick to be penetrated, ultrasonic testing can also be used for inspection. At the very least, local non-destructive testing can be carried out. Therefore, I personally believe that the cancellation of this requirement is a result of both the development of non-destructive testing technologies and the increasing inspection standards for container manufacturing. Since there are already means for testing, container manufacturers cannot be allowed to use this as an excuse to avoid conducting tests. Additionally, Article 1.9 of TSG 21-2016 stipulates special handling measures when compliance with these regulations cannot be achieved. It also provides a pathway through which you can propose welding continuity coefficients that do not conform to the standards; such coefficients can be reviewed and used if approved. Of course, the cost involved here is definitely higher than that of conducting a test.
This clause is still retained in the GB/T 151 \"Heat Exchangers\" standard