CLADDING TECHNOLOGY SHANXI CO., LTDWelding Procedure Specification for Cladding Operations
Overview and Functional Role
The Welding Procedure Specification (WPS) is the operational document that translates the qualified process parameters from the PQR/WPQR into actionable instructions for the welding operator. For cladding operations, the WPS serves as the primary reference for the welder, specifying the exact parameters, techniques, and sequence of operations required to produce a conforming overlay layer. Unlike fusion welding procedures, cladding WPS documents must address additional considerations such as dilution control, transition layer design, and overlay layer geometry, making them more complex and more critical to the quality of the final product. This study note examines the content, structure, and practical implementation of cladding WPS documents.
WPS Content and Structure
A comprehensive cladding WPS must include all the information necessary for the welding operator to execute the procedure correctly and consistently. The following table summarises the key elements of a cladding WPS and their technical significance:
| WPS Element | Content | Technical Significance |
|---|---|---|
| Process identification | Process name (e.g., MAG, GTAW, SAW) | Determines equipment and consumable type |
| Base metal specification | Grade, specification, thickness | Determines preheat and dilution behaviour |
| Overlay material specification | Grade, specification, wire type | Determines overlay composition and properties |
| Preheat temperature | Specified range (e.g., 100–150°C) | Controls dilution and cracking risk |
| Interpass temperature | Maximum value (e.g., ≤ 200°C) | Controls dilution and microstructure |
| Current and voltage | Specific values or range | Controls heat input and dilution |
| Travel speed | Specific value or range | Controls heat input and dilution |
| Wire feed speed | Specific value or range | Controls deposition rate and bead geometry |
| Shielding gas | Composition and flow rate | Controls arc stability and oxide content |
| Stick-out | Specific value (e.g., 12–15 mm) | Controls arc characteristics and bead profile |
| Pass sequence | Number of passes, sequence, overlap | Controls dilution, bond strength, geometry |
| Overlap ratio | 1/3 to 1/2 of bead width | Ensures complete fusion between passes |
| Transition layer design | Material, thickness, number of passes | Controls dilution and metallurgical compatibility |
| Post-weld treatment | Cooling rate, PWHT, pickling | Controls residual stress and microstructure |
The overlap ratio is a particularly important parameter in cladding WPS documents. A minimum overlap of 1/3 to 1/2 of the bead width is required to ensure complete fusion between adjacent passes and to prevent lack of fusion defects. Insufficient overlap results in a "stepped" overlay surface with potential lack of fusion at the pass boundaries, which can lead to delamination of the overlay layer during service. Excessive overlap, on the other hand, can increase the dilution rate and reduce the deposition efficiency. The WPS must specify the overlap ratio and the welder must be trained to achieve it consistently.
Transition Layer Design
For cladding operations involving dissimilar material combinations — such as stainless steel overlay on carbon steel or nickel-based alloy overlay on low-alloy steel — the design of a transition layer is often required to manage the dilution rate and prevent cracking. The transition layer is an intermediate layer of material that is metallurgically compatible with both the base metal and the final overlay material, and it serves to reduce the dilution rate of the final overlay layer. The following table summarises typical transition layer designs for common cladding combinations:
| Base Metal | Overlay Material | Transition Layer | Number of Passes | Purpose |
|---|---|---|---|---|
| Carbon steel | 304/316 stainless | 309L stainless (high Ni, Cr) | 1–2 passes | Reduce dilution; prevent cracking |
| Carbon steel | Inconel 625 | 309L or Inconel 82 | 1–2 passes | Reduce dilution; improve bond |
| Low-alloy steel | 316 stainless | 309L stainless | 1–2 passes | Reduce dilution; prevent cracking |
| Carbon steel | Hastelloy C276 | Inconel 625 | 1–2 passes | Reduce dilution; improve compatibility |
| Carbon steel | Monel 400 | 309L or Monel 400 | 1–2 passes | Reduce dilution; improve bond |
The transition layer design must be specified in the WPS and must be consistent with the qualified PQR/WPQR. The number of transition layer passes, the material specification, and the overlap ratio must all be specified to ensure that the dilution rate of the final overlay layer is within the acceptable range. For example, a two-pass transition layer of 309L stainless steel on a carbon steel substrate can reduce the dilution rate of the subsequent 316L overlay layer from approximately 35% to approximately 15%, which is within the acceptable range for most corrosion resistance requirements.
Pass Sequence and Bead Geometry Control
The pass sequence and bead geometry are critical parameters in cladding WPS documents because they directly affect the dilution rate, the bond strength, and the surface quality of the overlay layer. The following table summarises the typical pass sequence and bead geometry parameters for MAG and GTAW cladding:
| Parameter | MAG Cladding | GTAW Cladding |
|---|---|---|
| Typical number of passes | 3–6 passes | 2–4 passes |
| Bead width | 15–25 mm | 8–15 mm |
| Bead height | 1.5–3 mm | 1–2 mm |
| Overlap ratio | 1/3 to 1/2 | 1/3 to 1/2 |
| Pass sequence | Centre-out or side-to-side | Centre-out preferred |
| Travel speed | 200–400 mm/min | 100–300 mm/min |
| Heat input per pass | 0.5–2.0 kJ/mm | 0.3–1.5 kJ/mm |
The pass sequence is particularly important for controlling the dilution rate and the residual stress distribution in the overlay layer. A centre-out sequence — where the first pass is placed at the centre of the cladding area and subsequent passes are placed on either side — tends to produce a more uniform dilution rate and a more symmetric residual stress distribution compared to a side-to-side sequence. The WPS must specify the pass sequence and the welder must be trained to follow it consistently.
WPS-PQR Correspondence and Compliance
A fundamental principle of welding procedure management is that the WPS must correspond one-to-one with the qualified PQR/WPQR. The WPS is derived from the PQR and must specify all essential variables within the qualified range. Any deviation from the PQR-qualified range requires either a new PQR or a formal engineering assessment and approval. The following table summarises the correspondence between PQR and WPS elements:
| PQR Element | WPS Element | Correspondence |
|---|---|---|
| Qualified process | Specified process | Must match exactly |
| Qualified material range | Specified material | Must be within qualified range |
| Qualified variable ranges | Specified values | Must be within qualified range |
| Qualified test results | Acceptance criteria | Must meet or exceed qualified results |
| Qualified equipment | Specified equipment | Must be equivalent or better |
The management of WPS-PQR correspondence is a critical quality control function. In practice, this requires a formal document control system that tracks the version of each WPS, its corresponding PQR, and any amendments or revisions. The WPS must be clearly marked with the PQR number, the date of qualification, and the applicable material and variable ranges. Any change to the WPS — even a minor one — must be evaluated for its impact on the qualified range and either approved as a non-essential variable change or accompanied by a new PQR.
Engineering Practice and Common Errors
In practice, the most common errors in cladding WPS documents are: (1) the specification of parameter ranges that are wider than the qualified range, leading to operators selecting parameters outside the qualified range; (2) the omission of the transition layer design, leading to excessive dilution and non-conforming overlay composition; (3) the failure to specify the overlap ratio, leading to lack of fusion defects; and (4) the use of a WPS that has not been updated to reflect the current PQR, leading to non-conforming procedures being used in production. These errors can be mitigated through rigorous document review, operator training, and periodic audits of WPS compliance.
Study Insights and Conclusions
The Welding Procedure Specification for cladding operations is a critical technical document that bridges the gap between process qualification and production execution. Its effectiveness depends on the completeness of its content, the accuracy of its parameter specifications, and the consistency of its implementation by the welding operator. The key insights from this study are: (1) the WPS must include all essential variables within the qualified range and must not specify ranges wider than the PQR; (2) the transition layer design is a critical element of the WPS for dissimilar material cladding and must be specified with the same rigour as the final overlay layer; (3) the overlap ratio and pass sequence must be specified to ensure complete fusion and uniform dilution; and (4) the WPS-PQR correspondence must be maintained through a formal document control system. Engineers should approach the WPS not as a static document but as a living technical tool that must be continuously reviewed, updated, and enforced through operator training and quality audits.