Exploration • Drilling • Field Services
Typical conditions with reference to API RP 934 A1 and API TR 934 B2
are:
l API RP 934 A, 3.1.6, 7.6.1 (May 2008): l SA 387-22-23
accordance with ASME BPVC4 l SA 542-D-4a5
less than 8h l Not less than three cycles in each case
Multiple cycles result from the necessity of repair welding; of these, two cycles are available to production, and one to two to the operator. Tere are diverse reasons for PWHT; the aim is tempering for the achievement of the necessary microstructure in the weld metal, the HAZ and the parent material. Another aim is that of reducing welding residual stresses. Te API 934 series draws attention to the dangers that excessively “stringent” heat treatment may cause. Mechanical properties may then drop below the minimum requirements.
Influence of PWHT conditions on the weld metal’s toughness A single-V butt weld (without full penetration) was welded as shown in the macrosection. Te welding-test parameters are summarised
below:
l Grade of steel: SA-542-D4a (2¼Cr1Mo¼V) l Plate thickness: 105mm l Tempering conditions: 720°C/140 min. l Intermediate Stress Relief (ISR): 660°C/180 min. l Heat input: 18 to 24kJ/cm in every case l Welding process: 121 (single-electrode SAW
process) l PWHT: One cycle in each case (HP value):
Table. 1. Six different case studies PWHT Holding temp. [°C]
Case Study 1 Case Study 2
Case Study 3 Case Study 4
Case Study 5 Case Study 6
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712 710 690 720 690 720 690 720 705
Holding time [min.]
480 60
420 120 360 120 360 214 510
HP value
20.64 20.34 20.51 20.51
20.51 20.51
24 18 24 18
24 24
Energy [kJ/cm] : 690+/-14° C, holding time in : 705+/-14° C, holding time not
Testing was performed only on the weld metal/in the weld seam, using the tensile test (transverse, at room temperature) and the Charpy V-notch impact test (four samples across weld thickness, transverse, at -29° C).
Properties of CrMoV steels as a function of HP value Te critical variable for the weld metal is toughness in PWHT min. state, i.e. the condition of the reactor during commissioning. Two influencing factors, heat input and PWHT, were examined during the test. Te fabricators of pressure vessels generally use a specific heat input of between 18 and 24kJ/cm.
20.34 / 20.51 / 20.64
Fig. 2. The influence of HP value on the mechanical strength values of a CrMoV plate in the 130 to 160mm thickness range.
PWHT is, on the one hand, a parameter originating from API RP 934 A, but also results, on the other hand, from the steel users’ empirical data and their procedure qualification tests. Still more players are now involved in the discussion during definition of
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