Pressure Vessel PWHT Records From China Tell You About the Oven, Not the Vessel
Quote from chief_editor on April 19, 2026, 4:21 amProcess plant buyers require post-weld heat treatment records for Chinese pressure vessels. The documentation practices around PWHT in Chinese fabrication shops create verification gaps that matter at inspection.
Three years into operation at a gas processing plant in Queensland, a routine fitness-for-service assessment on a Chinese-fabricated pressure vessel — a 2.4-meter diameter, 14-meter long separator, operating at 8.5 MPa in wet gas service — found hardness values at several weld heat-affected zones that were above the maximum allowed under NACE MR0175 for sour service. The hardness indicated that the weld HAZ had not been adequately softened by post-weld heat treatment.
The vessel had been delivered with a full PWHT record: a chart showing temperature versus time from a Chengdu fabricator's furnace, covering the specified hold temperature of 620°C for 2.5 hours, with temperature rising and falling within the required rate limits. The chart looked correct. The Australian inspection authority had accepted it.
What the chart recorded was the furnace atmosphere temperature — not the actual temperature at the weld HAZ in the vessel. The thermocouple that produced the chart had been attached to the furnace wall, not to the vessel surface or the weld itself. For a large vessel with thick shell sections, the temperature differential between furnace atmosphere and vessel metal during the heating and cooling phases can be significant — particularly if the vessel was loaded into the furnace before the furnace had reached uniform temperature, which accelerates the schedule but reduces the thermal soak uniformity.
A PWHT Chart Shows Where the Thermocouple Was. Not Where the Heat Went.
Post-weld heat treatment records in Chinese pressure vessel fabrication are commonly based on furnace atmosphere thermocouples rather than thermocouples attached to the vessel being treated. This practice is standard at many fabrication shops in Chengdu, Nanjing, and Tianjin, and is technically permitted under some PWHT specifications that do not explicitly require vessel-attached thermocouples. Under ASME VIII and AS 1210 — the relevant standard for the Queensland plant — vessel-attached thermocouples are required for complex geometries and thick sections. The Chengdu shop's practice was non-compliant with the applicable standard, but it had not been caught during documentation review because the review team had not checked whether the thermocouple location was the furnace or the vessel.
The hardness exceedance at three weld HAZ locations was consistent with localized areas that had not reached the hold temperature due to thermal non-uniformity during the treatment cycle. The furnace chart showed the specified cycle had been completed. The vessel's actual thermal history at those locations had not been verified.
The Remediation Was Weld Repair and Re-PWHT In Situ
Remediation of the above-specification HAZ hardness in the installed vessel — at 8.5 MPa gas service pressure in a production facility — required the vessel to be depressurized, isolated, hydrostatically tested after the weld repair, and re-certified. The weld repair itself required grinding out the affected HAZ zones, re-welding with a low-hydrogen procedure, and local PWHT using resistance heating elements attached directly to the vessel surface at the repair locations — the correct method.
The total remediation cost was $1.6 million, including an eight-day production outage. The vessel had been in service for three years and six months. It had passed Australian inspection authority acceptance on delivery based on a PWHT chart that documented the furnace, not the vessel.
A PWHT record that shows a furnace cycle tells you that a furnace ran a cycle. It tells you nothing about what happened to the metal inside.
Keywords: China pressure vessel PWHT documentation | post weld heat treatment China, Chinese pressure vessel certification, PWHT records fabrication China, pressure vessel procurement China quality
Words: 569 | Source: Industry pattern — pressure vessel PWHT documentation failure, gas processing plant, Queensland, 2024. Chengdu fabricator thermocouple location investigation, hardness survey data, in-situ repair records. | Created: 2025-02-01T09:00:00Z
Process plant buyers require post-weld heat treatment records for Chinese pressure vessels. The documentation practices around PWHT in Chinese fabrication shops create verification gaps that matter at inspection.
Three years into operation at a gas processing plant in Queensland, a routine fitness-for-service assessment on a Chinese-fabricated pressure vessel — a 2.4-meter diameter, 14-meter long separator, operating at 8.5 MPa in wet gas service — found hardness values at several weld heat-affected zones that were above the maximum allowed under NACE MR0175 for sour service. The hardness indicated that the weld HAZ had not been adequately softened by post-weld heat treatment.
The vessel had been delivered with a full PWHT record: a chart showing temperature versus time from a Chengdu fabricator's furnace, covering the specified hold temperature of 620°C for 2.5 hours, with temperature rising and falling within the required rate limits. The chart looked correct. The Australian inspection authority had accepted it.
What the chart recorded was the furnace atmosphere temperature — not the actual temperature at the weld HAZ in the vessel. The thermocouple that produced the chart had been attached to the furnace wall, not to the vessel surface or the weld itself. For a large vessel with thick shell sections, the temperature differential between furnace atmosphere and vessel metal during the heating and cooling phases can be significant — particularly if the vessel was loaded into the furnace before the furnace had reached uniform temperature, which accelerates the schedule but reduces the thermal soak uniformity.
A PWHT Chart Shows Where the Thermocouple Was. Not Where the Heat Went.
Post-weld heat treatment records in Chinese pressure vessel fabrication are commonly based on furnace atmosphere thermocouples rather than thermocouples attached to the vessel being treated. This practice is standard at many fabrication shops in Chengdu, Nanjing, and Tianjin, and is technically permitted under some PWHT specifications that do not explicitly require vessel-attached thermocouples. Under ASME VIII and AS 1210 — the relevant standard for the Queensland plant — vessel-attached thermocouples are required for complex geometries and thick sections. The Chengdu shop's practice was non-compliant with the applicable standard, but it had not been caught during documentation review because the review team had not checked whether the thermocouple location was the furnace or the vessel.
The hardness exceedance at three weld HAZ locations was consistent with localized areas that had not reached the hold temperature due to thermal non-uniformity during the treatment cycle. The furnace chart showed the specified cycle had been completed. The vessel's actual thermal history at those locations had not been verified.
The Remediation Was Weld Repair and Re-PWHT In Situ
Remediation of the above-specification HAZ hardness in the installed vessel — at 8.5 MPa gas service pressure in a production facility — required the vessel to be depressurized, isolated, hydrostatically tested after the weld repair, and re-certified. The weld repair itself required grinding out the affected HAZ zones, re-welding with a low-hydrogen procedure, and local PWHT using resistance heating elements attached directly to the vessel surface at the repair locations — the correct method.
The total remediation cost was $1.6 million, including an eight-day production outage. The vessel had been in service for three years and six months. It had passed Australian inspection authority acceptance on delivery based on a PWHT chart that documented the furnace, not the vessel.
A PWHT record that shows a furnace cycle tells you that a furnace ran a cycle. It tells you nothing about what happened to the metal inside.
Keywords: China pressure vessel PWHT documentation | post weld heat treatment China, Chinese pressure vessel certification, PWHT records fabrication China, pressure vessel procurement China quality
Words: 569 | Source: Industry pattern — pressure vessel PWHT documentation failure, gas processing plant, Queensland, 2024. Chengdu fabricator thermocouple location investigation, hardness survey data, in-situ repair records. | Created: 2025-02-01T09:00:00Z