NB/T 47015 Heat Treatment Requirements for Weld Repairs in Pressure Vessels

Literature Overview and Context

NB/T 47015 is the Chinese national standard governing welding technical specifications for pressure vessels. It provides comprehensive requirements for preheat, interpass temperature control, post-weld heat treatment (PWHT), and post-heat procedures for both initial welding and repair operations. In the context of bimetal pressure vessel fabrication, where weld overlay layers are applied to achieve corrosion resistance, the heat treatment requirements for repairs become particularly critical because the overlay material and the base metal often have significantly different thermal properties and metallurgical sensitivities.

This study note focuses on the heat treatment parameters specified in NB/T 47015 for repair operations, with emphasis on the differences between initial welding and repair heat treatment, the interaction between repair heat treatment and the existing overlay layer, and the engineering implications of non-compliance.

Core Heat Treatment Parameters for Repairs

NB/T 47015 specifies heat treatment requirements based on the material group, wall thickness, and welding process. For repairs, the standard mandates that the heat treatment parameters must be at least as stringent as those for the original welding operation, with additional considerations for the repair geometry.

Parameter	NB/T 47015 Requirement	Typical Value for Carbon Steel (P-No.1)	Typical Value for Low-Alloy Steel (P-No.5)
Preheat Temperature	Based on material, thickness, and welding process	100–150°C for t > 25 mm	200–250°C for t > 25 mm
Interpass Temperature	Maximum limit specified	≤250°C	≤300°C
Post-Heat Temperature	For hydrogen embrittlement prevention	200–250°C	250–300°C
Post-Heat Dwell Time	Minimum 1 hour per 25 mm thickness	1 h per 25 mm	1 h per 25 mm
PWHT Temperature	Based on material and thickness	550–620°C	550–650°C
PWHT Dwell Time	Minimum 1 h per 25 mm thickness	1 h per 25 mm	1 h per 25 mm
PWHT Heating Rate	Limited to prevent thermal shock	≤100°C/h for t ≤ 50 mm	≤100°C/h for t ≤ 50 mm
PWHT Cooling Rate	Controlled to prevent cracking	≤100°C/h below 400°C	≤100°C/h below 400°C

Repair-Specific Heat Treatment Considerations

Preheat Requirements for Repairs

The preheat temperature for a repair must be determined based on the base metal composition at the repair location. For a bimetal pressure vessel with a carbon steel base and stainless steel overlay, the preheat is typically governed by the base metal requirements because the base metal has the higher susceptibility to hydrogen-induced cracking. However, if the repair involves welding into the overlay layer, the preheat must also consider the overlay material's sensitivity to thermal cracking.

A critical issue is the thermal gradient between the preheated repair zone and the surrounding unheated material. For localized repairs on a large vessel, achieving uniform preheat over the entire affected area can be challenging. NB/T 47015 requires that the preheat be applied to a zone extending at least 100 mm beyond the repair area, but in practice, the effective preheat zone may be larger due to thermal conductivity.

Post-Heat and PWHT Interaction with Overlay Layers

Post-heat treatment is applied immediately after welding to reduce hydrogen concentration in the weld metal and heat-affected zone (HAZ). For overlay repairs, post-heat must be applied carefully to avoid exceeding the maximum interpass temperature of the overlay material. For example, if the overlay is Inconel 625, the post-heat temperature should not exceed 250°C to prevent sensitization and potential intergranular corrosion.

PWHT after repair is required when the original welding operation required PWHT. However, the PWHT cycle must be carefully designed to avoid detrimental effects on the existing overlay layer. The following considerations are essential:

Temperature sensitivity of the overlay material: Austenitic stainless steels (304, 316) are generally tolerant of PWHT temperatures up to 650°C, but prolonged exposure above 500°C can lead to sensitization. Nickel-based alloys (Inconel 625, Hastelloy C276) have different sensitization thresholds.
Thermal expansion mismatch: The coefficient of thermal expansion of the overlay material may differ significantly from the base metal. During PWHT, differential expansion can generate residual stresses at the interface, potentially leading to delamination.
Multiple PWHT cycles: If multiple repairs are performed, each requiring PWHT, the cumulative thermal cycling can affect the metallurgical properties of the overlay layer. The number of PWHT cycles must be limited to prevent degradation.

Engineering Practice Cases

Case 1: Repair of a 316L Overlay Layer on a Carbon Steel Hydrogenation Reactor

A hydrogenation reactor with a carbon steel (16MnR) base and 316L overlay layer required repair of a lack of fusion defect detected in the overlay layer. The repair was performed using GTAW with E316L-16 electrode. The following heat treatment sequence was applied:

Preheat: 150°C applied over a 300 mm radius zone around the repair area
Interpass temperature: Controlled at 200°C maximum
Post-heat: 250°C for 2 hours immediately after welding
PWHT: 620°C for 4 hours, with heating rate of 80°C/h and cooling rate of 80°C/h below 400°C

The PWHT cycle was verified by thermocouples placed at the repair location, the overlay/base interface, and the vessel exterior. The post-PWHT NDE (UT and PT) confirmed complete fusion and no new defects.

Case 2: Repair of a Nickel-Based Alloy Overlay on a Low-Alloy Steel Vessel

A pressure vessel with a P91 base metal and Inconel 625 overlay layer required repair of a porosity cluster in the overlay. The repair was performed using PTA cladding with Inconel 625 powder. The heat treatment requirements were more stringent due to the P91 base metal:

Preheat: 250°C (governed by P91 requirements)
Interpass temperature: ≤300°C
Post-heat: 300°C for 2 hours
PWHT: 760°C for 2 hours (P91 tempering temperature)

The challenge was that the Inconel 625 overlay layer was exposed to 760°C during PWHT, which is above its typical sensitization threshold. To mitigate this, the PWHT was performed in a protective atmosphere (argon) to minimize oxidation, and the overlay layer was inspected for intergranular corrosion after PWHT.

Key Reflections and Recommendations

Thermocouple placement is critical: The heat treatment parameters are only effective if the temperatures are accurately measured at the critical locations. For bimetal vessels, thermocouples should be placed at the repair location, the overlay/base interface, and the vessel exterior to capture the thermal gradient.
Heat treatment cycle verification: A temperature-time record should be maintained for each heat treatment operation. This record serves as evidence of compliance with NB/T 47015 and provides data for future reference.
Post-heat is not optional: Many operators skip post-heat for "minor" repairs, but this can lead to delayed hydrogen cracking. Post-heat should be applied immediately after welding, even if the repair is small.
Coordinate PWHT with repair scheduling: If multiple repairs are planned, it is more efficient to perform all repairs first and then apply a single PWHT cycle, rather than PWHT after each repair. However, this approach must be evaluated against the risk of hydrogen cracking between repairs.
Material-specific considerations: The heat treatment parameters must be tailored to the specific materials involved. A generic "carbon steel PWHT cycle" may not be appropriate for a bimetal vessel with a nickel-based overlay.

In conclusion, NB/T 47015 provides a comprehensive framework for heat treatment of weld repairs in pressure vessels, but its application to bimetal components requires careful engineering judgment. The interaction between the base metal and overlay material during heat treatment must be thoroughly understood, and the heat treatment cycle must be designed to protect both materials. Engineers should always prioritize the preservation of the overlay layer's metallurgical integrity while ensuring that the repair achieves full fusion and meets the mechanical property requirements.