NB/T 47015 Post-Weld Heat Treatment Code for Pressure Vessels

Standard Overview and Scope

NB/T 47015, formally titled "Procedure for Post-Weld Heat Treatment of Pressure Vessels," serves as the primary domestic Chinese standard governing post-weld heat treatment (PWHT) operations for pressure vessels. In my more than twenty years of engineering practice, this standard has been the cornerstone document for determining whether a fabricated pressure vessel — particularly those incorporating cladding or weld-overlay layers — has been properly relieved of residual stresses. The standard applies to welded pressure vessels constructed from carbon steel, low-alloy steel, stainless steel, and nickel-based alloy materials, and it explicitly addresses the special considerations that arise when a base metal is metallurgically bonded to a dissimilar cladding layer.

The standard is organized around several critical pillars: the determination of whether PWHT is required, the selection of appropriate heat treatment parameters, the instrumentation and monitoring of temperature during the treatment cycle, and the documentation requirements for traceability. For engineers working on clad or overlay vessels, the standard provides the framework within which we must balance stress relief of the thick base metal against the risk of degrading the microstructure and corrosion resistance of the overlay layer.

Core Technical Requirements

Temperature Parameters and Holding Times

The most frequently referenced aspect of NB/T 47015 is the prescribed PWHT temperature ranges. The standard specifies minimum and maximum temperatures based on material classification, with the maximum temperature constrained by the material's upper critical temperature (Ac1) to avoid austenitization. The following table summarizes the typical PWHT temperature windows for common materials encountered in cladding applications:

The holding time is calculated based on the maximum thickness of the weld joint or the vessel wall, not the total vessel diameter. For clad vessels, the standard requires that the temperature be measured at the base metal, not at the cladding surface, because the base metal governs the stress relief requirement. This distinction is critical in practice — I have observed multiple cases where technicians incorrectly placed thermocouples on the cladding side, leading to inaccurate temperature readings and potentially inadequate stress relief.

Heating and Cooling Rate Control

One of the most technically demanding aspects of PWHT for clad vessels is the control of heating and cooling rates. NB/T 47015 specifies that the heating rate must not exceed 175°C/h for the first 100°C of temperature rise, and thereafter must be limited to a rate that does not exceed the value calculated by the formula:

Rate (°C/h) = 200 / maximum thickness (mm)

This rate limitation is particularly important for thick-walled clad vessels where the thermal gradient between the base metal and the overlay layer can generate significant thermal stresses. In my experience with hydrogenation reactors — which commonly feature 15CrMoR base plates with 304L or 316L cladding layers of 6–12 mm thickness — the heating rate must be carefully controlled to prevent delamination at the bond interface. A practical rule of thumb that I apply is to limit the heating rate to no more than 100°C/h for vessels with cladding layers thinner than 10 mm, and to reduce this further to 60–80°C/h when the cladding is nickel-based alloy, which has lower thermal conductivity than stainless steel.

Special Considerations for Clad and Overlay Vessels

The standard addresses several issues unique to bimetallic construction:

• The PWHT temperature must not exceed the maximum allowable temperature for the cladding material, which may be lower than the base metal's PWHT range.
• For weld-overlay layers applied by methods such as ESW, SAW, or PTA, the overlay welds themselves must be included in the PWHT cycle.
• The standard permits a reduced PWHT temperature when the full temperature range would risk sensitization of the stainless steel cladding, but this requires justification through additional mechanical testing.
• For vessels with nickel-based alloy cladding (e.g., Monel 400 or Hastelloy C276), the PWHT temperature must be carefully selected to avoid intergranular corrosion susceptibility in the overlay layer.

Engineering Practice and Defect Analysis

Common Defects and Their Relation to PWHT

Inadequate or improperly executed PWHT can lead to several failure modes in clad pressure vessels:

Case Study: 15CrMoR/316L Clad Hydrogenation Reactor

A representative case from my practice involves a 15CrMoR/316L clad hydrogenation reactor with a design pressure of 120 bar and a design temperature of 380°C. The base plate thickness was 45 mm, and the 316L cladding was 8 mm thick, applied by electroslag welding. The PWHT was performed at 720°C with a holding time of 2.5 hours (calculated as 45 mm × 0.5 h/25 mm + 1.0 h minimum). The heating rate was limited to 100°C/h, and the cooling rate was controlled at 50°C/h below 400°C.

During the PWHT cycle, thermocouples were placed at three locations: the base metal surface, the bond interface (via a welded thermocouple probe), and the cladding surface. The temperature differential between the base metal and the cladding surface was maintained below 50°C throughout the cycle. Post-PWHT, the overlay layer was tested for intergranular corrosion per ASTM A923 Practice E, and the bond strength was verified by a transverse tensile test per ASTM A263. All results were within acceptable limits.

The key lesson from this case is that the bond interface temperature, not the surface temperature, is the critical parameter for ensuring both adequate stress relief and preservation of the overlay microstructure.

Key Questions and Reflections

After studying NB/T 47015 in depth, several questions emerge that warrant further investigation:

1. How should the standard be interpreted when the base metal and cladding material have non-overlapping PWHT temperature ranges? For example, 12Cr1MoV requires 650–720°C, while 316L should not exceed 600°C to avoid sensitization. The standard provides guidance, but the boundary conditions are not always clear-cut.
2. What is the minimum PWHT temperature required for hydrogen diffusion from nickel-based alloy overlay layers? The standard addresses hydrogen diffusion for carbon steel, but the hydrogen trapping behavior in Ni-based alloys is fundamentally different.
3. How does the number of overlay weld passes affect the PWHT requirement? A PTA overlay with 3–5 passes may have a different residual stress distribution compared to a single-pass ESW overlay, yet the standard applies the same PWHT parameters.

These questions highlight areas where engineering judgment supplements the standard, and where additional testing or analysis may be required for critical applications.

Study Insights and Implications

NB/T 47015 is not merely a procedural document — it is a framework that requires deep understanding of metallurgy, thermodynamics, and welding mechanics to be applied effectively. The standard's provisions for PWHT temperature, time, and rate are derived from decades of empirical data, but they must be interpreted in the context of the specific material combination, vessel geometry, and service conditions. For engineers working on clad and overlay pressure vessels, the standard's value lies not only in its prescriptive requirements but also in its implicit recognition that the interaction between base metal and overlay material during thermal cycling is the primary source of potential failure.

In practice, I recommend that every PWHT procedure for a clad vessel include a pre-treatment review meeting where the base metal thickness, cladding material, overlay method, and service conditions are evaluated together. The procedure should explicitly state the thermocouple placement plan, the heating and cooling rate schedule, and the acceptance criteria for post-PWHT testing. This systematic approach, grounded in the standard's requirements but enhanced by engineering judgment, is the most reliable path to producing safe and durable bimetallic pressure vessels.