API 650 material groups

What is the underlying factor or factors that differentiate the API 650 material groups. This question was raised when I observed that a flush-type clean out fitting’s maximum opening dimensions are dependent on material group number. I cannot find a simple definition of material group other than the API 650 table that simply lists the various acceptable material specification by group number.
I know the API Standard 650 material groups are differentiated by the average values of longitudinal and transverse notch toughness impact values.


The API material Groups are only used for non-impact tested materials to determine if they’re permitted by the Figure 4.1 curves. Thus the Groups reflect what temperature / thickness combinations the Code feels the materials can be assumed to meet the Table 4.5 impact test energies.

You would expect the Code would group materials with similar chemistries, mechanical properties and production methods so it’s reasonable to make a common assumption for them. Some of this can be seen in the Group descriptions. Group I is semi-killed whereas Group II and above are fully killed.

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