PWHT For Lot Qualification Coupon Per NB 2431.1<\/strong><\/h3>\n\n\n\nNB 2431.1 (c) also specifies that the coupon must be tested in the as-welded condition or in the applicable<\/em> post weld heat treated condition. This means that if the production weld is envisaged to remain as-welded, the lot qualification coupon must also be tested in the as-welded condition. If the production weld is envisaged to be subjected to a PWHT, the lot qualification coupon must also be subject to PWHT before removing samples for mechanical testing.<\/p>\n\n\n\nThe term \u2018applicable\u2019 here indicates that the PWHT given to coupon must be of similar nature (that is same time and temperatures). Specifically, the holding time for PWHT of coupon must be at least 80% of the cumulative holding time to be applied to the weld metal in production application.<\/p>\n\n\n\n
If the production joint undergoes multiple heat treatments in the fabrication, the total time of all heat treatments must be reckoned to calculate the holding time for PWHT of the coupon. This holding time can be applied to the coupon in one cycle.<\/p>\n\n\n\n
The cooling rate is an important thing in the PWHT. This is why; the NB 2431.1 (c) specifies that \u201ccooling rate from the post weld heat treatment temperature shall be of the same order as that applicable to the weld metal in the component.\u201d<\/p>\n\n\n\n
In my work place, many SMAW electrodes are used in both as-welded joints, as well as joints with PWHT. Such electrodes are qualified through two coupons: first, in which samples for mechanical testing are to be removed in as-welded condition; second \u2013 in which samples for mechanical testing are to be removed after a relevant PWHT.<\/p>\n\n\n\n
How To Prepare Tensile And Impact Specimens For Lot Qualification<\/strong><\/h3>\n\n\n\nNB 2431.1 (d) \u2013 specifies that tensile and impact specimens shall be located, oriented and prepared in accordance with the requirements of applicable SFA specification. Also, requirements of NB 2321.1 and 2321.2 are applicable \u2013 these paragraphs define the size of specimens and number of samples and testing standard to be followed.<\/p>\n\n\n\n
This raises a question in mind. The SFA specifications generally require testing of sets of five impact specimens, among which the highest and lowest values of energy must be discarded, and the remaining three must be averaged for determining acceptance. However, NB 2331\/2332 specify only three specimens.<\/p>\n\n\n\n
For lot qualification testing of NB 2431, is it required to do five impact tests to meet the requirement of NB 2331\/2332, or is three enough?<\/p>\n\n\n\n
The Interpretations III-1-79-29, III-82-60 point us in the direction that the practice of taking five specimens, and discarding the highest and lowest values is the right practice. However, the Interpretation III-1-04-40 again changes the direction, and indicates that it is enough to take three specimens.<\/p>\n\n\n\n
So, all in all \u2013 three specimens should be enough.<\/p>\n\n\n\n
NB 2431.1 (d) also specifies that the longitudinal axis of the impact and drop weight test specimens shall be located at least \u00bc t from the surface, where \u2018t\u2019 is the thickness of test coupon.<\/p>\n\n\n\n
Acceptance Criteria For Tensile Test<\/strong><\/h3>\n\n\n\nNB 2431.1 (e) \u2013 specifies that \u201cOne all weld metal tensile specimen shall be tested and shall meet the specified minimum tensile strength requirements of the base material specification.\u201d<\/p>\n\n\n\n
These are a couple of interesting things to note here.<\/p>\n\n\n\n
Firstly, this line only asks \u2018minimum tensile strength requirements\u2019, and does not ask yield strength, and % elongation. This firmly establishes that the lot qualification asked by NB 2400 does not<\/em> require YS and % elongation to be reported (even though the relevant SFA specification might require it). This also finds confirmation in Interpretation III-1-04-35. The Interpretation III-1-78-151 further hammers home this point with absolute clarity.<\/p>\n\n\n\nSecondly, the acceptance criterion for tensile strength obtained in this lot qualification is the strength of the base material<\/em> to be used in the production joint, and not the strength specified in the relevant SFA specification. This has been made repeatedly clear by ASME through multiple Interpretations. One such Interpretation is III-1-78-48. Another example is III-81-103.<\/p>\n\n\n\nAlright. What if the production weld is between two dissimilar base metals that have different tensile strengths? What then would be the acceptance criteria for the tensile strength during the testing of lot qualification per NB 2431.1?<\/p>\n\n\n\n
This question has been addressed in the last line of NB 2431.1 (e), which states that \u201cwhen base materials of different specifications are to be welded, the tensile strength requirements shall conform to the specified minimum tensile strength requirements of either of the base material specifications.\u201d<\/p>\n\n\n\n
Acceptance Criteria For Impact Test<\/strong><\/h3>\n\n\n\nNB 2431.1 (f) \u2013 specifies when<\/em> impact testing is required, and the acceptance criteria for impact tests. Whenever the base material requires impact test per NB 2331 and NB 2332, the welding material needs to undergo impact testing too. The acceptance criteria for the weld would be same as that for the base material. If two different base metals are used, \u201cthe weld metal may conform to either of the two requirements.\u201d<\/p>\n\n\n\nMechanical Testing As Per NB 2431.2: Standard Test Requirements<\/strong><\/h2>\n\n\n\nAs an alternative to NB 2431.1, the NB 2431.2 has been provided as the procedure (required tests, method of preparing the coupon, and acceptance criteria) for preparing the test coupon for lot qualification. This alternative is only applicable to carbon steel and low alloy steel electrodes classified under SFA 5.1 or SFA 5.5 respectively.<\/p>\n\n\n\n
Most requirements specified in NB 2431.1 remain applicable to NB 2431.2 too. Fundamentally, the focus is directed at the test assembly specified in SFA 5.1\/SFA 5.5, with the size suitably increased to accommodate impact and drop weight samples. The welding of the test coupon follows all methodology outlined in the relevant SFA specification.<\/p>\n\n\n\n
The main point of difference lies in NB 2431.2 (e). As per this paragraph, one all weld tensile is to be tested, and it must meet the specified minimum tensile strength requirement of the SFA specification for the applicable electrode classification. Note that NB 2431.2(e) says \u201crequirement of the SFA specification\u201d, while NB 2431.1(e) had specified \u201crequirements of the base material specification\u201d.<\/p>\n\n\n\n
In essence, the standard test requirements of NB 2431.2 require compliance to SFA specification, while the general test requirements of NB 2431.1 require compliance to base material specification, which is generally found in the relevant SA specification of Section II Part A. This understanding has been further solidified by ASME through an Interpretation, III-1-78-48.<\/p>\n\n\n\n