WRC 302

Part 1: Postweld Heat Treatment of Pressure Vessels

Postweld heat treatment produces both mechanical and metallurgical effects in pressure vessel steels that will vary widely with the composition of the steel, its past thermal and mechanical history including welding, the temperature and duration of the postheat, and the heating and cooling rates accompanying postheat. When the majority of welded pressure vessels were fabricated from carbon steels, postweld heat treatment (PWHT) was performed to relieve the locked-up stresses resulting from fabrication, and the process was identified as "stress relief." As higher-strength alloy steels were applied to pressure vessels, it was realized that there were other effects involved, some of which were beneficial and others detrimental to the performance of the vessels. This review is an attempt to clarify the present status of PWHT as a fabrication tool and to provide a background for defining the conditions under which postheating is necessary and those in which it may be detrimental.

The discussion of PWHT will be limited to treatments performed in the temperature range of 400°C to just below the transformation temperature of the steel and to steels with minimum yield strengths up to 700 MPa (100 ksi). Temperatures below this range largely fail to accomplish the desired effects, while treatments into or above the transformation range evoke an entirely different set of responses in the steels that are accompanied by an equally different set of procedures and problems.

Part 2: Relaxation Stresses in Pressure Vessels

Creep-relaxation stresses were measured in A737 Grade B, A543 Type B Class 2 and A387 Grade 22 Class 2 steel for times of 8 or 9 hrs in the 538°C (1000°F) to 677°C (1250°F) temperature range. The test procedure used was a fixed grip tension test type. The data values were to be intermediate between standard creep test and stress-relaxation test results and simulate the load maintaining behavior of a complex component during post weld heat treatment.

Creep-relaxed stresses below 69 MPa (10 ksi) were achieved after 8 hrs at 538°C (1000°F) for A737 Grade B, and after 8 hrs at 593°C (1100°F) for A543 Type B. Stresses below 69 MPa (10 ksi) were not achieved for A387 Grade 22 for times up to 8 hrs at 649°C (1200°F) but were achieved after 2 hrs at 677°C (1250°F). The data curves had a nonlinear region in the yield point stress range at temperature after which creep-relaxation occurred in a logarithmic manner with time.

The room temperature properties of the steels decreased with stress-relief out to 8 or 9 hrs. For treatments producing creep-relaxed stresses below 69 MPa (10 ksi) the strength decrease was about 5% for A737 Grade B, about 10% for A543 Type B Class 2 and about 20% for A387 Grade 22 Class 2.

Comparison of the data with true creep and true stress-relaxation data show the values measured in the investigation to be intermediate between these two limits, a result of the nature of the test. A semi-empirical computational method was used to construct approximate stress-relaxation curves from the data. It is proposed that the data developed here represent a realistic approximation of those found in heavy weldments of these steels.

Part 3: A Study of Residual Stress in Pressure Vessel Steels

This investigation determined the levels of residual stress found in three pressure vessel steel weldments using the blind hole drilling and Battelle Chip Removal techniques. The purpose of the program was to study residual stress levels in a high strength steel weldment, the effect of stress relief in two microalloyed weldments, and to compare the results from the two measurement techniques.

Surface residual stresses both longitudinal and transverse to a multipass submerged arc weld seam were measured in a heavy section A543 Class 1 weldment made with a high strength welding electrode. Top surface (weld face) stresses both in the center of and at a cut face were determined. Stresses through the thickness of the weldment on a cut face were also measured. Two techniques were used in this study.

It was found that the residual stresses perpendicular and parallel to the weld seam were high and tensile at the weld surface in the center of the weldment length, but these stresses decline sharply as the cut face is approached. The maximum tensile stresses were less than the yield strength of the steel. Large residual compressive stresses were found at the center thickness of the weld on the cut face. Both measurement techniques gave essentially equivalent results.

The two A737 microalloyed steel weldments were also made using the submerged arc process and a matching strength electrode. These were cut into 75 mm blocks prior to testing, so some of the as-welded stresses were changed. The stress relief experiments showed that nearly full stress relief for these steels could be achieved after a treatment of 2 hrs at 550°C.