Zhuojin Pipe Fitting Co., LtdStudy Note on Cladding Isolation Layer Application in Pressure Vessel Manufacturing
Literature Overview
This paper by Bao Wenhong from Gansu Special Equipment Inspection and Testing Research Institute, together with Chen Hongwei, Wang Cen, Wu Jingwei, Liang Ruifeng, and Zhang Jianxiao from Lanzhou Lanchi Heavy Equipment Co., Ltd. (Welding, 2023, No. 5), documents practical engineering cases of using cladding isolation layers to solve dissimilar metal welding challenges in pressure vessel manufacturing. The paper addresses three specific welding scenarios involving low-alloy heat-resistant steels, low-alloy high-strength steels, and austenitic stainless steels, demonstrating how proper selection of isolation layer welding materials and welding procedures ensures joint quality under different heat treatment regimes.
Core Technical Challenge
Dissimilar metal welding in pressure vessel manufacturing presents unique challenges due to:
- Differences in thermal expansion coefficients between base materials
- Mismatched heat treatment requirements for the two joined materials
- Potential for intermetallic compound formation at the weld interface
- Different mechanical property requirements for each material
- Regulatory requirements for pressure-containing equipment
The isolation layer approach addresses these challenges by introducing an intermediate weld metal layer that acts as a metallurgical buffer between the dissimilar base materials, reducing direct interaction between incompatible alloys.
Case Studies and Technical Solutions
Case 1: Low-Alloy Heat-Resistant Steel to Low-Alloy High-Strength Steel
| Parameter | Material 1 (Heat-Resistant) | Material 2 (High-Strength) |
|---|---|---|
| Typical grade | 15CrMo / 12Cr1MoV | 16MnR / Q345R |
| Heat treatment | Normalized + tempered (920–940 °C + 620–680 °C) | No post-weld heat treatment required, or stress relief only |
| Key challenge | Different PWHT requirements | Isolation layer must accommodate both regimes |
| Isolation material | Low-alloy steel electrode (E7018 equivalent) | Compatible with both PWHT and no-PWHT conditions |
The critical issue in this case is that the heat-resistant steel component requires a full normalizing and tempering cycle to achieve its design mechanical properties, while the high-strength steel component may only require stress relief or no post-weld heat treatment. The isolation layer must therefore be designed to withstand the full PWHT cycle without degradation while maintaining adequate mechanical properties.
Case 2: Low-Alloy Heat-Resistant Steel to Austenitic Stainless Steel
| Parameter | Material 1 (Heat-Resistant) | Material 2 (Stainless) |
|---|---|---|
| Typical grade | 15CrMo / 12Cr1MoV | 06Cr19Ni10 (304) / 06Cr17Ni12Mo2 (316) |
| Heat treatment | Full normalizing + tempering required | No PWHT (austenitic SS) |
| Key challenge | Iron carbide formation, chromium depletion at interface | Isolation layer must prevent sensitization |
| Isolation material | Nickel-based alloy (309L, 312, or Inconel 625) | High Ni content to prevent Cr carbide formation |
This is the most challenging of the three cases. When low-alloy steel is welded directly to austenitic stainless steel, several problems arise:
- Chromium diffusion from the stainless steel into the low-alloy side creates a chromium-depleted, corrosion-susceptible zone
- Iron carbides form at the interface, creating brittle phases susceptible to cracking
- Thermal expansion mismatch creates residual stresses during PWHT
- The austenitic stainless steel cannot undergo normalizing heat treatment
The nickel-based isolation layer solves these problems by:
- Providing a high-nickel zone that prevents chromium diffusion into the iron base
- Acting as a thermal expansion buffer between the two materials
- Maintaining austenitic structure that is compatible with both welding and heat treatment
- Preventing direct contact between the low-alloy steel and stainless steel weld metals
Case 3: Low-Alloy Heat-Resistant Steel to Low-Alloy Heat-Resistant Steel (Same PWHT)
| Parameter | Material 1 | Material 2 |
|---|---|---|
| Typical grade | 15CrMo | 12Cr1MoV or similar |
| Heat treatment | Same normalizing + tempering cycle | Same cycle |
| Key challenge | Slight compositional differences | Matching weld metal chemistry |
| Isolation material | Low-alloy electrode matching the higher-alloy component | Ensures adequate alloy content in HAZ |
Welding Procedure Qualification and Quality Control
Welding Procedure Specification (WPS) Development
Each case requires a qualified welding procedure specification that addresses:
| WPS Element | Requirements |
|---|---|
| Process selection | SMAW for preparation and isolation; GTAW or SMAW for cover |
| Preheat temperature | Based on carbon equivalent of base materials (typically 100–250 °C) |
| Interpass temperature | Maximum 250 °C to prevent grain growth in HAZ |
| Heat input | Controlled to prevent excessive dilution or insufficient penetration |
| Welding sequence | Designed to minimize restraint and residual stress |
| Post-weld treatment | As specified for the heat-resistant component |
Non-Destructive Testing Requirements
For pressure vessel applications, the following NDT methods are typically required:
| NDT Method | Application | Acceptance Criteria |
|---|---|---|
| Radiographic Testing (RT) | Volume defects in weld metal | ASME Section V, Article 2 |
| Ultrasonic Testing (UT) | Planar defects, especially in isolation layer | ASME Section V, Article 4 |
| Magnetic Particle Testing (MT) | Surface and near-surface cracks | ASME Section V, Article 7 |
| Visual Testing (VT) | Surface appearance, geometric dimensions | ASME Section V, Article 9 |
Engineering Practice Insights
Material Selection Strategy for Isolation Layers
The selection of isolation layer material follows a hierarchy based on the dissimilarity of the base materials:
- Same family, different grades: Use a low-alloy steel electrode with alloy content matching the higher-alloy base material. This ensures adequate alloy content in the dilution zone.
- Iron to austenitic stainless: Use a nickel-based alloy (309L, 312, Inconel 625, or similar) with high nickel content (25–35% Ni) to prevent iron carbide formation and chromium depletion.
- Iron to nickel-based alloy: Use the same nickel-based alloy as the isolation material, ensuring compatibility with both sides.
- Stainless to nickel alloy: Use a 309L or 312 grade to bridge the composition gap.
Heat Treatment Compatibility
One of the most critical aspects of isolation layer design is ensuring that the isolation layer material maintains its mechanical properties and microstructure after the post-weld heat treatment required by the other base material. For example:
- A 309L isolation layer will maintain its austenitic structure through normalizing and tempering of the low-alloy steel
- A low-alloy isolation layer must be tempered to avoid brittle martensite after the austenitic stainless side is welded without PWHT
- Nickel-based alloys (Inconel 625, Monel) are inherently heat-treatment stable across a wide range of temperatures
Regulatory Compliance
Pressure vessel manufacturing is governed by strict regulatory frameworks. In China, these include:
- TSG 21-2016 (Supervision and Inspection Regulation for Stationary Pressure Vessels)
- GB/T 150 (Pressure Vessel Design and Fabrication)
- NB/T 47014 (Welding Procedure Qualification for Pressure Vessels)
- ASME Boiler and Pressure Vessel Code Section VIII (for international projects)
The welding procedure qualification must demonstrate that the isolation layer approach produces joints meeting all applicable acceptance criteria for the specific service conditions.
Key Reflections and Implications
This paper provides valuable practical documentation of isolation layer welding in pressure vessel manufacturing, an area where theoretical knowledge must be translated into field-proven procedures. The three cases presented represent common scenarios encountered in power generation, petrochemical, and heavy equipment manufacturing.
The most important engineering lesson is that the isolation layer is not merely a "buffer" but a functional component of the weld joint that must be designed with the same rigor as the base materials. Its composition, thickness, and welding parameters must be optimized for the specific dissimilar metal combination and heat treatment regime.
For pipe manufacturing engineers, these principles directly apply to dissimilar metal welds in piping systems, such as:
- Carbon steel to stainless steel transitions in heat exchanger bundles
- Low-alloy to austenitic welds in high-temperature service piping
- Alloy steel to nickel alloy welds in corrosion service
The isolation layer approach is particularly valuable when the two base materials have fundamentally different post-weld heat treatment requirements. In such cases, the isolation layer must be selected from materials that maintain their properties across the full range of heat treatment temperatures that will be applied.
This work reinforces the principle that successful dissimilar metal welding is as much about materials selection and process design as it is about welding technique. The welder's skill is necessary but insufficient without proper engineering design of the joint configuration, material selection, and heat treatment sequence.