- Published on Wednesday, 21 April 2010 01:00
- Written by Don Bush
Q: In the last Materials Q&A column, you discussed using forged bar to make hubbed-flange components such as bonnets. I noticed that ASTM A182 specifically states that forged bar cannot be used for flanges. Was your original statement in error?
A: Thank you for pointing this out. ASTM A182 paragraph 4.5 does include the statement:
“Except for flanges of any type, forged or rolled bar may be used without additional hot working for small cylindrically shaped parts within the limits defined by Specification A 234/A 234M for low alloy steels and martensitic stainless steels and Specification A 403/A 403M for austenitic and ferritic-austenitic stainless steels.”
This would seem to prohibit the use of forged bar to make flanges per ASTM A182.This same statement exists in the other “forging” specifications in various forms. ASTM A105 refers to ASTM A961 Section 6, which includes the following statement:
“Flanges, elbows, return bends, tees, and header tees shall not be machined directly from bar.”
Note that it does not specifically indicate the type of bar (i.e., hot-rolled or forged) that may not be used, so this would seem to imply that all bar, including forged bar, would be unacceptable to produce any type of flange.
Other specifications such as ASTM A181, A350, A522 and A727, include statements that specifically prohibit the use of forged bar for flanges of any type. The exact wording in these specifications varies slightly, but the requirements are essentially the same. The wording from ASTM A350 paragraph 5.3.3 is:
“Other parts, excluding flanges of all types, may be machined from hot-rolled or forged bar up to and including NPS 4.”
This would appear to be an open-and-shut case against the use of forged bar to make hubbed-flange components. However, consider the following definitions, which were developed by combining definitions and discussion points from both ASTM A788 and A961:
Forging: The product of a substantially compressive hot or cold plastic working operation that consolidates the material and produces the required shape. The plastic working must be performed by a hammer, press, forging machine, or ring rolling machine, and must deform the material to produce a wrought structure throughout the material cross section.
Forged Bar: A forging that has no bore, with a relatively constant cross section throughout its length and with an axial length greater than its maximum cross-sectional dimension, and a wrought microstructure throughout the material cross section. Note that radially-forged bar meets this definition.
Now consider the fact that the ASME Boiler and Pressure Vessel Code specifically allows the use of hubbed flanges machined from forged bar per Section VIII, Division 1, Mandatory Appendix 2, paragraph 2.2 (d) (1):
“Hubbed flanges may be machined from a hot rolled or forged billet or forged bar. The axis of the finished flange shall be parallel to the long axis of the original billet or bar.”
This situation creates quite a paradox. Let’s assume that a valve company or a subcontracted machine shop machines a hubbed-flange bonnet from a piece of forged bar (not hot-rolled bar) that is certified to ASTM A350 Grade LF2 Class 1. The $64,000 question then becomes: How should the component be marked and certified?
On the one hand, ASTM A350 explicitly prohibits the use of forged bar to make flanges of all types. Per paragraph 5.3.2, it states:
“The finished product shall be a forging as defined in the Terminology section of Specification A788.”
How can the bonnet be marked and certified as an “ASTM A350 LF2 Class 1” component when ASTM A350 does not allow the manufacture of any flanged component from forged bar?
On the other hand, the part has been machined from a bar shape that was produced by forging, which actually fits the definition of a “forging” as listed above, consistent with both ASTM A788 and A961. The forged bar meets the chemical and mechanical requirements of ASTM A350 Grade LF2 Class 1. ASME Section VIII specifically allows the hubbed-flange component to be machined from forged bar.
If the bonnet cannot be marked and certified as an “ASTM A350 LF2 Class 1” component, what then would be the proper marking and certification? It doesn’t fit under any other ASTM specifications, so that isn’t an option. Would one need to create a custom specification and certify the material to that specification? In that case, would customers accept the material for the bonnet?
At this point, common sense must prevail. The material meets the chemical and mechanical requirements of ASTM A350 Grade LF2 Class 1. This means the material itself can clearly be marked and certified as such. Section VIII of the ASME Boiler and Pressure Vessel Code specifically allows the use of forged bar to make hubbed flange components. Therefore, it only seems logical to mark and certify the finished components as ASTM A350 Grade LF2 Class 1. This same logic would apply to all of the “forging” specifications.
Since ASME also has jurisdiction over the ASME versions of each of these ASTM specifications in Section II of the Boiler and Pressure Vessel Code, it would be interesting to submit an official inquiry to ASME for clarification regarding these conflicting requirements.