Manufacturers continuously face the challenge of complying with the impact testing provisions of ASME Section VIII, Division 1.
As Code impact testing rules are quite complex, instances of misconception and/or oversight are not unusual, hence the 1enigma. The material provided in this document is not intended to replace the Code rules. The intent of this review of impact testing rules and the decision charts provided is to help manufacturers meet Code requirements.
The presentation material consists of a series of impact testing decision charts, and a selection of the most common problems, misconceptions and oversights with regard to impact testing as it applies to pressure vessel construction to the ASME Code, Section VIII, Division 1, CSA B51 Part 1, and the Alberta Safety Codes Act and Pressure Equipment Safety Regulation. The primary focus of the document is on Subsection C, Part UCS impact testing requirements.
2.0 IMPACT TESTING DECISION CHARTS
2.1 Impact Testing Requirements (Materials)
2.2 Impact Testing Requirements of Welding Procedures with Filler Metal
2.3 Impact Testing Requirements of Welding Procedures without Filler Metal
2.4 Impact Testing Requirements Vessel [Production] Impact Tests
2.5 Impact Testing Requirements Vessel [Production] Impact Tests
3.0 MATERIALS & DESIGN
3.1 CSA B51-14, Part 1, Clause 7.1.3
Carbon and low alloy steel used for the construction of pressure vessels at a minimum design metal temperature below –50 °F shall be impact tested at the MDMT or lower temperature with the test results meeting UG-84 of ASME Section VIII Division 1.
To qualify for exemption under UG-20(f) the material shall be limited to P Number 1, Group Number 1 or 2, and the thickness as defined in UCS-66(a) [see also, Note(1) in Fig.UCS-66.2] is limited to ½ inch for curve A materials and 1 inch for curve B, C, or D materials of Figure UCS-66.
Under the provisions of UCS-67(a)(3), for MDMTs that are colder than –20 °F but not colder than –55 °F, for materials exempt from impact testing by UCS-66(g) or Figure UCS-66, Curves C or D from Figure UCS-66 must be made with welding procedures qualified by impact testing. Qualification of welding procedures with impact testing is not required when no individual weld pass in the production weld exceeds ¼” in thickness and the welding consumables, for each heat and/or lot of filler metal or combination of heat/or lot of filler metal and batch of flux used to join these base metals has been classified by the consumable manufacturer through impact tests per the applicable SFA specification at a temperature not warmer than the MDMT. The welding procedure qualification shall include impact tests of welds and heat affected zones, as specified by the first paragraph of UCS-67.
This paragraph requires that welded joints be postweld heat treated when
required by other rules of Section VIII-1 or when the MDMT is colder than –55
°F, and the coincident ratio as defined in Fig. UCS-66.1 is 0.35 or greater, except
for P-No. 1 materials that are impact tested per UG-84 with energy values
specified in UCS-68(b), and Category A & B joints plus fillet weld requirements
as per UCS-68(b)(1) & (2) are met.
3.5 Exemption Combinations
3.5.1 Interpretation VIII-1-89-138R
If a vessel is constructed of a combination of P-No. 1 Group No. 1 or 2 materials and other materials listed in Subsection C, the rules of UG-20(f) may be applied to the portion constructed of P-No. 1 Group No. 1 or 2.
3.5.2 Interpretation VIII-1-95-15
The allowable temperature reduction determined from Fig. UCS-66.1 (coincident ratio less than 1) and the temperature reduction permitted by UCS-68(c) (postweld heat treatment when not otherwise a requirement per Code) may be combined.
3.5.3 Interpretation VIII-1-95-160
The additional temperature reduction provided by UCS-68(c) may be applied to the provision of UCS-66(c), which allows ANSI B16.5 and B16.47 flanges and split loose flanges as specified within the Code paragraph, exemption from impact testing when the MDMT is no colder than –20 °F.
3.5.4 UG-20(f) + UCS-66(b)
A temperature reduction determined from Fig. UCS-66.1(coincident ratio less than 1) may not be applied in addition to an exemption from impact testing under UG-20(f).
3.5.5 UG-20(f) + UCS-68(c)
A temperature reduction permitted by UCS-68(c) may not be applied in addition to an exemption from impact testing under UG-20(f).
3.5.6 Table UCS-56-1 Note (b)(2) + UCS-68(c)
A temperature reduction permitted by UCS-68(c) may not be applied when the provisions of table UCS-56-1, General Note (b)(2) is used to avoid the requirement to postweld heat treat (200 °F preheat for P-No. 1 materials over 1¼ in. nominal thickness through 1½ in. nominal thickness).
4.0 WELDING PROCEDURES
This Supplementary Essential Variable requires that the procedure qualification test be subjected to PWHT essentially equivalent to that encountered in the fabrication of production welds, including at least 80% of the aggregate time(s) at temperature(s). For example, to remain within the WPS requirements, the maximum post weld heat treatment time(s) at temperature(s) for the production weld or production test coupons is 1.25 times the time(s) at temperature(s) qualified by the PQR coupon.
This Supplementary Essential Variable specifies that the minimum base metal thickness qualified is the thickness of the test coupon T or 5/8” inch, whichever is less. However where T is less than ¼ inch, the minimum thickness qualified is ½T.
4.3 UG-84(h)(2)(-c) and UG-84(g)(5)
Paragraph UG-84(h)(2)(-c) requires that the base material for the weld test coupon meet the minimum notch toughness requirement for the thickest material of the range of base material to be qualified by the procedure. Paragraph UG-84(g)(5) then requires that the weld metal impact test values shall be at least as high as those for the base material. This paragraph may place an additional limit on the qualification thickness of a WPS, for instance when QW-403.7 allows the WPS to qualify to 8 inches. For example, assume a WPS is qualified on a P-No. 1 Group No. 1 base material which has a minimum specified yield strength of 55 ksi. If the test is made on a 1½ inch thick plate and the provisions of QW-403.7 are utilized, the test specimen would normally qualify the WPS for thicknesses up to 8 inches. Fig. UG-84.1 requires that the impact test results must average 30 ft-lbs. or more to qualify for a thickness of 3 inches or more.
5.5 UG-84(f)(2) Impact Testing of Welds
All test plates (PQR and Production, when not exempted) shall be subjected to heat treatment, including cooling rates and aggregate time(s) at temperature(s) as established by the Manufacturer for use in actual manufacture.
This paragraph can have significant consequences when an exemption under the referenced paragraphs is not available, for instance P-No. 3, Gr. Numbers 1 and 2 materials, reheat treatment of the vessel could lead to unacceptable WPS and production impact tests. Material impact test results may also be unacceptable under reheat treatment as provided for in Subsection C. Particular attention should be paid to this paragraph when dealing with UHT materials.
One must remember that irrespective of the Subsection C exemptions to UG-84(f)(2), the WPS(s) may become unacceptable if the vessel itself, or production impact test coupons are subjected to heat treatments that are outside of QW-407.2 requirements for the welding procedure [at least 80% of aggregate time(s) at temperature(s)].
5.6 Location, Orientation, Temperature, and values of Weld Impact tests UG-84(g).
5.7 The 2013 Edition of ASME Section VIII, Division 1, has included requirements for additional HAZ coupons which is dependent on the base metal thickness and joint type.
UG-84(g)(2) illustrates through Figure UG-84.5 and Table UG-84.6 the location of where impact specimens must be taken. The significant change is depending on the type of joint (either a single or two sided welded joint), additional heat affected zone (HAZ) specimens are required. Prior to the change, material under 1 ½” only required one set of impacts in the material and one set in the HAZ, and for materials over 1 ½” two sets were required in the material and one in the HAZ. The table below illustrates the total number of impact test sets and specimens required.
6.1 UG-84(j) Rejection
If the vessel test plate fails to meet the impact requirements, the welds represented by the plate shall be unacceptable. Reheat treatment and retesting or retesting only are permitted.
Under the provision for reheat treatment it is necessary to consider the applicability of the welding procedure under QW-407.2, as well as UG-84(f)(2) implications.
Note: This document has been revised to include changes to ASME Section VIII, Division 1, Subsection C, Part UCS up until and including the 2015 Edition and CSA-B51, 2014 Edition.
1 In the Webster’s Ninth New Collegiate Dictionary, Enigma is defined as “something hard to understand or explain”.