Time delay before applying NDT in welding

The following standards stipulate delay time before inspection:

BS EN 1011-2:2001 Welding – Recommendations for welding of metallic materials – Part 2: Arc welding of ferritic steels (with amendment AMD 14926 Feb 2004)

Paragraph 18 states: ‘Due to the risk of delayed cracking, a period of at least 16 hours is generally required before the final inspection is made of as-welded fabrications. The minimum time may be reduced for thin materials below 500N/mm2 yield strength or increased for materials of a thickness greater than 50mm or of yield strength over 500 N/mm2 … Welds that have been heat-treated to reduce hydrogen content or which have been stress relieved, need no additional time interval following the heat treatment before final inspection is made’

EEMUA 158 ‘Construction specification for fixed offshore structures in the North Sea’, 1994 revision, 2005 reprint (with amendment 4)

Paragraph 7.2.2 states: ‘… NDT shall not be carried out until at least 48 hours have elapsed following completion of welding …’

British Constructional Steelwork Association (BCSA) and Steel Construction Institute (SCI) ‘National Structural Steelwork Specification for Building Construction’ 5th edition 2007 Section 5: Workmanship – Welding

Paragraph 5.5.4 ‘Hold Times before final NDT’ states: ‘If there is a risk of delayed cracking, a period may be needed before the final inspection is made of as-welded fabrications. Recommended minimum hold times are given in Annex A.’

Table A Recommended minimum hold times

Table A Recommended minimum hold times
Weld size (mm Heat input (kJ/mm) Hold time (hours)
a or s ≤ 6 All Cooling period only
6 < a or s ≤ 12 ≤ 3 8
> 3 16
a or s > 12 ≤ 3 16
> 3 40
  1. Size applies to the nominal throat thickness (a) of a fillet weld, the nominal weld depth (s) of a partial penetration butt weld, or the nominal material thickness (s) of a full penetration weld.
  2. If two fillet welds are separated an unfused root face of less than 10mm then the governing weld size (a) shall be taken as the sum of their individual weld sizes.
  3. Heat input to be calculated in accordance with clause 19 of BS EN 1011-1.
  4. The time between weld completion and commencement of NDT shall be stated in the NDT report. In the case of ‘cooling period only’ this will last until the weld is cool enough for NDT to commence.

Reproduced by permission British Constructional Steelwork Association.

AWS D1.1/D1.1M:2010 ‘Structural Welding Code – Steel’

Paragraph 6.11 states: ‘…Acceptance criteria for ASTM A 514, A517, and A709 Grade 100 and 100W steels shall be based on NDT performed not less than 48 hours after completion of the welds…’. (These are all 690MPa yield grades, and for other grades, inspection immediately after completed welds have cooled to ambient temperature is permitted.)

Offshore standard DNV-OS-C401, October 2010

Chapter 2 Section 3 paragraph B105 states that ‘Final inspection and NDT of structural steel welds shall not be carried out before 48 hours after completion, except where PWHT is required. The time delay may upon agreement be reduced for NV36 grades or lower and for NV420 grades or lower for plate thicknesses less than 40mm, if the consistent low failure rate of delayed cracking has been documented for the materials and welding consumables in question’. (NV36 and NV420 have minimum yield stresses of 355N/mm2 and 420N/mm2 respectively.)

Recommendations have been made for minimum delays which should be imposed between completion of welding and inspection for hydrogen cracking in butt welds of up to 50mm thickness, as summarised in the table below:

Material Welding Process Arc Energy
(kJ/mm)
Heat Input
(kJ/mm)
Delay time before inspection (at an ambient temperature of 20°C)
Observed
Greatest delay time for crack initiation (hrs)
Proposed
Ultrasonic inspection (hrs)
C-Mn steel, up to 450 MPa yield SMAW ≤3 ≤2.4 4.7 12
SAW ≤3.5 ≤3.5 12.3 24
SAW 3.5-5 3.5-5 16.5 36
High Strength steels, such as HY100 (690MPa yield) SMAW ≤1.5 ≤1.2 64 72

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