Classification of forging grades
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I have a question for everyone. While studying standard *NB/T47008, I found that for forgings of a certain material grade, their mechanical properties are related to the nominal thickness, not to the forging grade (I, II, III, IV). The forging grade only affects certain factors, such as the inspection items and the number of inspections required. I always thought that the higher the forging grade, the better the performance, but it seems this idea is flawed after all. According to the definition of nominal thickness for long-neck flange forgings in 3.5 of NB/T47008, the flange thickness C shall be the nominal thickness. If it is a large-diameter flange with a nominal thickness in the range of “>200–300”, but the mechanical properties (tensile strength, yield strength) within this range are not sufficient for strength calculations, then this situation has nothing to do with the forging grade; in such cases, it is necessary to use a material grade with better properties. Then why is it necessary to assign a forging grade to forgings? I have also had brief exposure to ASME materials; within this material standard system, there are no forging grades – a single material grade corresponds only to one set of mechanical property data. For example, for A105 forgings, the GBT12228 domestic standard specifies requirements regarding the forging grade, whereas the SA105/SA105M standards do not include such requirements. However, when comparing the data on chemical elements and mechanical properties in the two standard systems, it is found that they are essentially consistent. So what is the purpose of specifying a forging grade then? I would also appreciate some advice from the experts; thank you very much!
The forging grade classification of forgings is based on different mechanical properties and requirements for inspection tests. Generally speaking, the higher the forging grade, the stricter the required mechanical properties and inspection criteria. However, in some cases, the mechanical properties of forgings are not entirely related to their forging grade. For example, in the case you mentioned, when the nominal thickness reaches a certain level, even with the highest grade of forging available, it is not possible to meet the strength requirements; in such cases, it is necessary to use a different material grade to satisfy the requirements. Furthermore, in different ** or standard systems, the meaning and requirements of forging grades may vary. Some standards may not specify a forging grade, but rather list the mechanical properties and other criteria of the forgings directly. The setting of forging grades may also be intended to facilitate the classified management and inspection of forgings of different specifications and sizes. In short, the forging grade is a general classification method, and its specific application depends on the particular circumstances. -
The sampling methods and sampling rates vary for different grades of forgings; the higher the grade, the more inspection items there are and the higher the sampling rate, which in turn results in a higher reliability of performance.
Well, taking NB/T47008 as an example, it can be determined from Tables 2 and 4 that the higher the forging grade, the more inspection items there are, the greater the number of inspections required, and the higher the ultrasonic inspection standards. The forging grade simply indicates a lower defect rate.
I used to think that the higher the forging grade, the better the performance would be. However, based on the standard requirements for different forging grades, this is not the case. The forging grade does not affect the required mechanical properties, and the requirements regarding the forging ratio are also the same. This means that forging factories may use the same forging processes when producing Grade I and Grade III forgings; those that pass the ultrasonic testing and mechanical property tests are classified as Grade III or IV forgings, while those that fail are classified as Grade I or II forgings. Additionally, there may be requirements in the manufacturing standards for containers regarding forgings of different grades, but there are many container standards, and I am still unable to verify this.
Yes, the forging grade does not necessarily indicate better mechanical properties; rather, it is a way to classify and manage forgings of different specifications and uses. Generally, although forgings of different grades have the same chemical composition and mechanical property requirements, there may be some minor differences, such as the number and requirements of inspection items, size ranges, surface quality, and so on. By distinguishing between forgings of different grades, some confusion or errors can be avoided. Furthermore, in container manufacturing standards, there may also be requirements for forgings of different forging grades, which are related to the application scenario of the container and its pressure rating, among other factors. -
I understand what you mean, but I feel your answer isn’t very accurate. I haven’t had any contact with people who work in forging processes, but when I visited a flange factory, I specifically asked about the forging techniques. I was also very curious as to how a single blank can be forged to produce forgings of different grades. However, most of the manufacturers replied that it was “confidential and not available for disclosure.” I also tried to find out more indirectly, and heard that during the forging process a forging master oversees everything – determining the direction, the amount of force to be applied, and the number of blows needed; basically, the forging is carried out based on the experience of this master. At the same time, the forging standards regarding the forging ratio do not specify different ratios for various forging grades; therefore, I believe that forging manufacturers should forge parts without distinguishing between different grades, following the experience of a single craftsman. It is then through mechanical testing and ultrasonic inspection that the forging grades are determined – those with fewer defects are considered of higher quality. So I don’t think it’s the case that “the higher the forging grade, the more likely defects will occur.”
If you are a designer, I would also like to ask: if for a container device the manufacturing standards do not specify any requirements regarding the forging grade, how should one choose the forging grade? If meeting the strength calculations is sufficient, can Grade I or Grade II forgings be chosen? Is the choice of forging grade also related to the design pressure? Perhaps once the design pressure is high, the manufacturing standards will require a forged grade. In other words, if the issue of assuming safety responsibilities is not taken into account, can forgings of a lower forging grade be chosen?
It is related to the degree of danger and importance of the pressure vessel; refer to Chapter 4 of HG/T20581
For applications involving highly hazardous media as specified in the fixed volume regulations, requirements are also placed on forgings
The fighter jets of the former Soviet Union had superior speed. The United States wants to develop a research vehicle. Later, one was obtained; when it was taken apart, it was seen that the parts affecting air resistance were extremely precise, and the length of the rivets protruding from many compartments was utterly appalling. This incident shows that flexibility can be applied in less important areas, while strictness is required in important ones. Forgings I-IV have strict inspection levels; the higher the forging number, the stricter the requirements, as it is used in demanding applications where failures are not acceptable.