February 2018

Special Focus: Materials Manufacturing

Improve welding technology for heavy-wall pressure vessels in creep-resistant steel

The standard practice recommended for high-pressure vessels with heavy wall thickness requires the implementation of weld joint preparation with a narrow gap technique. This generally calls for a two-beads-per-layer sequence, along with the use of the submerged arc welding (SAW) process.

The standard practice recommended for high-pressure vessels with heavy wall thickness requires the implementation of weld joint preparation with a narrow gap technique. This generally calls for a two-beads-per-layer sequence, along with the use of the submerged arc welding (SAW) process. The SAW process provides a high-quality and uniform weld joint while reducing the residual stresses after welding. In refinery equipment that is subjected to high pressures and exposed to hydrogen (H2), high-strength materials, such as 2 ¼ Cr 1 Mo ¼ V, are commonly adopted.

To remain at the forefront of welding technology, a research program has been launched to adopt a narrower weld joint, with respect to the standard narrow gap, to achieve a monoweld welding sequencea with one bead per layer, as shown in FIG. 1. Another goal has been to match a normal customer target: making a bevel shape that minimizes the volume of weld material to reduce the potential risk of weld defects.

Fig. 1. Bevel transition from two weld beads per layer to one weld bead per layer.
Fig. 1. Bevel transition from two weld beads per layer to one weld bead per layer.

Vanadium-modified 2 ¼ Cr 1 Mo alloy used for the fabrication of high-pressure hydroprocessing reactors is known to offer several advantages over conventional 2 ¼ Cr 1 Mo alloys, including:

  • Higher design stress intensity levels, as shown in FIG. 2, with inherently thinner walls and lighter reactors
  • Increased tensile strength
  • Improved resistance to H2 attack
  • Greater resistance to temper embrittlement susceptibility
  • Better resistance to weld overlay disbonding.
Fig. 2. ASME allowable stresses for different materials.
Fig. 2. ASME allowable stresses for different materials.

The primary components involved in the development of the proposed technique were:

  • A weld bevel design for the one-bead-per-layer welding sequence
  • Inherent and suitable welding parameters, using proven welding consumables that are available on the market, to avoid the loss of past reliability
  • New weld torch design.

The base material and the welding consumables used for the tests were those that are normally employed and are recognized in the industry. This was done so as not to lose the experience and reliability granted by more than 70 reactors delivered with these materials. The chemical analysis is indicated in TABLE 1.

After numerous tests, a final welding procedure qualification was successfully completed, as shown in FIGS. 3 and 4.

Fig. 3. Test coupon under welding.
Fig. 3. Test coupon under welding.
Fig. 4. Test coupon after welding.
Fig. 4. Test coupon after welding.

TEST EXECUTION AND RESULTS

A recent study conducted on Grade 22V material, and the process of submerged arc welding with the narrow gap technique of two weld beads per layer, has identified a potential need for improvement in complying with the American Society of Mechanical Engineers (ASME) code-specified creep resistance properties. In another setting, with regard to the properties of toughness in weld joints, other possible inconsistencies were found in the narrow gap weld joint between the weld center line and center bead. Consequently, both  mechanical properties have been investigated.

Weld appearance and macrostructure

A macro-section of the new welding technology with the one-bead-per-layer welding sequence is shown in FIG. 5. The weld beads’ shape and dimensions are uniform and regularly distributed, eliminating large, concentrated, fine-grained zones (FIG. 6), as well as notable differences in the subdivisions of coarse-grained zones, both in the thickness and the cross-section of the weld joint.

Fig. 5. Weld macro-section showing one-weld-bead-per-layer welding sequence.<sup>a</sup>
Fig. 5. Weld macro-section showing one-weld-bead-per-layer welding sequence.a
Fig. 6. Weld solidification structure comparison for standard, narrow-gap, two-beads-per-layer sequence and new, one-bead-per-layer technology.
Fig. 6. Weld solidification structure comparison for standard, narrow-gap, two-beads-per-layer sequence and new, one-bead-per-layer technology.

Toughness test

Slightly better results are usually achieved with notch-in weld center line techniques than with notch-in bead center techniques. To counteract this, the welding parameters and inherent weld width were adjusted to achieve a thinner weld bead height, granting an optimized ratio of coarse-grained and refined-grain zones. FIG. 7 shows the weld toughness test results obtained among the different weld technologies and corresponding welding sequences.

Fig. 7. Weld toughness comparison chart for narrow-gap, two-beads-per-layer technology and new, one-bead-per-layer technology.
Fig. 7. Weld toughness comparison chart for narrow-gap, two-beads-per-layer technology and new, one-bead-per-layer technology.

Stress rupture test

In research conducted on the use of these materials,1 a potential critical zone in the two-beads-per-layer sequence weld joints was identified, in particular, when obtaining the required characteristics for creep resistance. As shown in FIG. 8, the combined effect of tempering, produced by successive beads (multipass) and the arrangement of beads in the two-beads-per-layer sequence, results in a localized central zone with a potentially large fine-grained structure that is undesirable for creep resistance.

Fig. 8. Typical standard, narrow-gap welding sequence.
Fig. 8. Typical standard, narrow-gap welding sequence.

A proposed solution to this potential problem was to adopt a larger bevel that would allow the execution of three or more beads per layer, as illustrated in FIG. 9. However, such an increase of weld beads carries the potential consequences of increasing weld defects occurrence and residual weld stresses.

Fig. 9. Different weld joint types.
Fig. 9. Different weld joint types.

Specific investigations were conducted on creep behavior through the execution of several stress rupture tests that were carried out in different post-weld heat treatment (PWHT) conditions. As shown in FIG. 10, the stress rupture test results, carried out on the test coupon welded with the new one-bead-per-layer technology sequence, exceeded the ASME requirements, even after a longer PWHT holding time.

Fig. 10. Stress rupture test at 540°C, 210 MPa—ASME Section VIII Division 2 Paragraph 3.4.4.5.
Fig. 10. Stress rupture test at 540°C, 210 MPa—ASME Section VIII Division 2 Paragraph 3.4.4.5.

Such improvement can be important in case of higher stress rupture test requirements (i.e., exceeding those required by ASME code) and/or in case of severe PWHT conditions.

The stress rupture test results seem to validate the investigation1 where, for a narrow-gap weld with two beads per layer, the larger portion of fine-grained microstructure in the weld centerline was indicated as a potential cause that could have led to a faster creep rate.

Shop floor validation

To validate such technology in production, further tasks were undertaken, including the design of a dedicated automatic, self-adjusting anti-drift system, and the design of a suitable slag removal device. FIG. 11 shows the entire weld arrangement that was adopted for production welds, several of which were carried out to validate the new one-bead-per-layer welding sequence technology—in particular, 284-mm thick circumferential weld joints and 140-mm thick longitudinal weld joints, as shown in FIGS. 12, 13 and 14. These welds were subjected to the required inspections—including ultrasonic examination by both manual and mechanized time of flight diffraction (TOFD), according to the ASME Code Section VIII Division 2, paragraph 7.5.5—with fully satisfactory results. A further examination was carried out according to API RP 934 A, Annex A, with no defects detected.

Fig. 11. Proprietary one-bead-per-layer welding system.
Fig. 11. Proprietary one-bead-per-layer welding system.
Fig. 12. Circumferential weld seam (284-mm thickness) before welding.
Fig. 12. Circumferential weld seam (284-mm thickness) before welding.
Fig. 13. Circumferential weld seam (284-mm thickness) during welding.
Fig. 13. Circumferential weld seam (284-mm thickness) during welding.
Fig. 14. Longitudinal weld seam (140-mm thickness) after welding.
Fig. 14. Longitudinal weld seam (140-mm thickness) after welding.

Takeaways

The new welding technology based on a submerged arc welding process and a one-weld-bead-per-layer welding technique was investigated and showed mechanical test results comparable to the standard, narrow-gap, two-weld-beads-per-layer welding technique. Creep resistance behavior can be improved over the two-weld-beads-per-layer welding technique, particularly in the case of severe requirements and heavy PWHT conditions (i.e., multiple cycles).

The adoption of the one-weld-bead-per-layer welding technology eliminates the need for weld torch repositioning (right/left side wall) for each weld pass execution, with inherently less occurrence of potential incorrect bead placement. However, stringent bevel preparation accuracy and inherent tolerances must be considered for the adoption of this technology. With the new technology, the weld amount has been significantly reduced by approximately 30%. The associated potential beneficial impacts on the level of weld residual stresses can be expected. HP

NOTES

a MONOWELD, patent pending

LITERATURE CITED

  1. Lundin, C., M. Prager and D. Osage, “Property and microstructural changes associated with long-term service of pressure vessel and piping steels at elevated temperatures, and their detection and effects on remaining life,” ESOPE 2016, Paris, France, 2016.

The Authors

Related Articles

From the Archive

Comments