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ASME Section VIII UHA-51 Impact Test: Changes Affecting Cryogenic Vessels Made from Austenitic Stainless Steel
new edition of the ASME Boiler and Pressure Vessel Code is issued every three years. In the current 2010 edition there are a number of changes from the 2007 edition. We were especially interested in a change to Part UHA-51 paragraph (a)(4)(a) that affects the manufacture of cryogenic pressure vessels with minimum design metal temperatures (MDMT) below -320F(77K).
Part UHA of Section VIII of the ASME Boiler and Pressure Vessel Code addresses requirements for vessels fabricated from “high alloys steels”, which includes the 304 and 316 stainless steels generally used to fabricate vessels which operate at temperatures of -320F (liquid nitrogen) or below. Paragraph UHA-51 deals with impact testing, . This is an important issue for materials used in low temperature service because many steel alloys undergo a phase transition from a ductile state to a brittle state at temperatures well above -320F. Impact testing is one means of assessing the extent to which a material is brittle or ductile. In addition to insuring that the base material of a vessel remains ductile down to the MDMT, welded joints are also a concern. The heat introduced by welding can cause changes in the structure of the material. If the composition of the weld filler metal differs from the base metal then variations in composition will be created when the base metal and filler metal combine. Interaction between the hot metal and the surrounding atmosphere may also result in changes in composition and properties. For these reasons when impact testing is required, UHA-51 also requires a sample of the base metal as well as a sample weld and a sample Heat Affected Zone (HAZ) in the vicinity of the weld.
In general, UHA-51 requires impact testing at or below the MDMT for all materials. Fortunately it also provides some exceptions to this requirement. A requirement to impact test at liquid helium temperatures would be a serious impediment to vessel manufacturers. While impact testing has been performed at liquid helium temperatures, doubts have been expressed over the validity of impact testing at very low temperatures because the impact causes the sample to warm considerably. An exception to this requirement is provided by paragraph UHA-51(a)(4)(a), which allows the impact testing to be performed at -320F even though the MDMT is below -320F under certain conditions. Those conditions are:
New here is the addition of 308L filler metal. Previously this exception was only permitted if 316L filler metal with ferrite number no greater than 5 was used. This is a very important exemption for helium vessel manufacturers which bear an ASME code stamp so we thought that we would take a closer look. It is worth noting that the ASME B31.1 Process Piping Code does not include such an exception. Requiring that helium piping be tested to the requirements of B31.3 imposes a nearly impossible condition.
For many years only 316L filler could be used to qualify for the exemption from impact testing at the MDMT for temperatures below -320F and many still regard 316L as the filler metal of choice for low temperature applications because it consistently produces strong, tough welds. The 316L filler can be used to weld 316 or 304 stainless steels. In non-cryogenic applications the 316L filler metal is generally used to join 316 stainless steel while 308L filler is used for welding 304 stainless steel. Just as 304 tends to be less expensive than 316 stainless steel, the 308L filler metal is also less expensive than 316L filler metal. By carefully adjusting the composition of the filler metal alloys suppliers are now able to offer 316L and 308L fillers with very similar properties in terms of strength and toughness of the resulting weld.
Why is a ferrite number specified and why would it differ for the two fillers metals? Stainless steel exists in austenitic, martensitic and ferritic forms, each having a different crystal structure. Depending on its chemical composition, an alloy may be purely austenitic, purely martensitic, purely ferritic, or a mixture of two or all three forms. The relative quantity of each form will depend on the concentrations of Ni and Cr as well as the concentrations of other elements such as C, Mn and Mo. The ferrite number is a measure of the concentration of ferrite. Because the ferrite is ferromagnetic, the ferrite number can be measured magnetically. Ferrite tends to prevent hot cracking in welds and its presence is generally considered beneficial. However, like carbon steels and martensitic stainless steel, ferrite becomes brittle at low temperatures and it is desirable to limit the ferrite concentration in welds for low temperature service. Ideally, one would like to have just enough ferrite to prevent hot cracking but no more than that. By carefully controlling the chemical make-up of the alloys, suppliers are able to provide weld fillers for low temperature service which have a carefully controlled range of ferrite numbers and which are resistant to hot cracking. However, during welding some of the base metal will mix with the filler. The tendency to form cracks as the metal cools will depend on the quantities of sulfur and phosphorous present and these will vary from one heat (batch) to the next of the base metal. It is not possible to specify or control ferrite number with great precision. Both 308L and 304 stainless steels tend to have higher Cr and lower Ni content than 316, so it is not surprising that their ferrite numbers would tend to be higher for 308L and 304L alloys. The choice of the specific range of 4 to 14 seems reasonable, but may have been based on the test conditions for available data rather than detailed quantitative considerations.
This change to paragraph UHA-51 will give fabricators greater flexibility in the choice of weld filler metal for applications below -320F which require an ASME Code stamp. At Meyer Tool, keeping abreast of changes in the ASME Boiler and Pressure Vessel Code is part of the requirement for maintaining our Certificate of Authorization to issue U and R code stamps and another way we Reduce Project Risk to help you achieve lowest total cost of ownership.
Part UHA of Section VIII of the ASME Boiler and Pressure Vessel Code addresses requirements for vessels fabricated from “high alloys steels”, which includes the 304 and 316 stainless steels generally used to fabricate vessels which operate at temperatures of -320F (liquid nitrogen) or below. Paragraph UHA-51 deals with impact testing, . This is an important issue for materials used in low temperature service because many steel alloys undergo a phase transition from a ductile state to a brittle state at temperatures well above -320F. Impact testing is one means of assessing the extent to which a material is brittle or ductile. In addition to insuring that the base material of a vessel remains ductile down to the MDMT, welded joints are also a concern. The heat introduced by welding can cause changes in the structure of the material. If the composition of the weld filler metal differs from the base metal then variations in composition will be created when the base metal and filler metal combine. Interaction between the hot metal and the surrounding atmosphere may also result in changes in composition and properties. For these reasons when impact testing is required, UHA-51 also requires a sample of the base metal as well as a sample weld and a sample Heat Affected Zone (HAZ) in the vicinity of the weld.
In general, UHA-51 requires impact testing at or below the MDMT for all materials. Fortunately it also provides some exceptions to this requirement. A requirement to impact test at liquid helium temperatures would be a serious impediment to vessel manufacturers. While impact testing has been performed at liquid helium temperatures, doubts have been expressed over the validity of impact testing at very low temperatures because the impact causes the sample to warm considerably. An exception to this requirement is provided by paragraph UHA-51(a)(4)(a), which allows the impact testing to be performed at -320F even though the MDMT is below -320F under certain conditions. Those conditions are:
- type 316L or type 308L weld filler metals are to be used
- the welding process is to be GTAW or GMAW
- if type 316L filler metal is used then it shall not have a ferrite number greater than 5 while if type 308L filler metal is used it shall have a ferrite number between 4 and 14
New here is the addition of 308L filler metal. Previously this exception was only permitted if 316L filler metal with ferrite number no greater than 5 was used. This is a very important exemption for helium vessel manufacturers which bear an ASME code stamp so we thought that we would take a closer look. It is worth noting that the ASME B31.1 Process Piping Code does not include such an exception. Requiring that helium piping be tested to the requirements of B31.3 imposes a nearly impossible condition.
For many years only 316L filler could be used to qualify for the exemption from impact testing at the MDMT for temperatures below -320F and many still regard 316L as the filler metal of choice for low temperature applications because it consistently produces strong, tough welds. The 316L filler can be used to weld 316 or 304 stainless steels. In non-cryogenic applications the 316L filler metal is generally used to join 316 stainless steel while 308L filler is used for welding 304 stainless steel. Just as 304 tends to be less expensive than 316 stainless steel, the 308L filler metal is also less expensive than 316L filler metal. By carefully adjusting the composition of the filler metal alloys suppliers are now able to offer 316L and 308L fillers with very similar properties in terms of strength and toughness of the resulting weld.
Why is a ferrite number specified and why would it differ for the two fillers metals? Stainless steel exists in austenitic, martensitic and ferritic forms, each having a different crystal structure. Depending on its chemical composition, an alloy may be purely austenitic, purely martensitic, purely ferritic, or a mixture of two or all three forms. The relative quantity of each form will depend on the concentrations of Ni and Cr as well as the concentrations of other elements such as C, Mn and Mo. The ferrite number is a measure of the concentration of ferrite. Because the ferrite is ferromagnetic, the ferrite number can be measured magnetically. Ferrite tends to prevent hot cracking in welds and its presence is generally considered beneficial. However, like carbon steels and martensitic stainless steel, ferrite becomes brittle at low temperatures and it is desirable to limit the ferrite concentration in welds for low temperature service. Ideally, one would like to have just enough ferrite to prevent hot cracking but no more than that. By carefully controlling the chemical make-up of the alloys, suppliers are able to provide weld fillers for low temperature service which have a carefully controlled range of ferrite numbers and which are resistant to hot cracking. However, during welding some of the base metal will mix with the filler. The tendency to form cracks as the metal cools will depend on the quantities of sulfur and phosphorous present and these will vary from one heat (batch) to the next of the base metal. It is not possible to specify or control ferrite number with great precision. Both 308L and 304 stainless steels tend to have higher Cr and lower Ni content than 316, so it is not surprising that their ferrite numbers would tend to be higher for 308L and 304L alloys. The choice of the specific range of 4 to 14 seems reasonable, but may have been based on the test conditions for available data rather than detailed quantitative considerations.
This change to paragraph UHA-51 will give fabricators greater flexibility in the choice of weld filler metal for applications below -320F which require an ASME Code stamp. At Meyer Tool, keeping abreast of changes in the ASME Boiler and Pressure Vessel Code is part of the requirement for maintaining our Certificate of Authorization to issue U and R code stamps and another way we Reduce Project Risk to help you achieve lowest total cost of ownership.
