Cladding Process Qualification PQR and WPQR Technical Framework

Overview and Regulatory Context

Process qualification is the cornerstone of any cladding operation, providing the technical evidence that a specific welding procedure is capable of producing welds that meet the required mechanical, metallurgical, and corrosion resistance criteria. For cladding operations, the process qualification record (PQR) and the welding procedure qualification record (WPQR) serve as the documented proof that the procedure variables — including dilution rate, interpass temperature, preheat, and heat input — have been validated through testing and are capable of producing a conforming overlay layer. This study note examines the technical framework, testing requirements, and variable control strategies for cladding process qualification under the major international standards.

Standards Framework and Scope

Cladding process qualification is governed by several international and national standards, each with its own scope, variable definitions, and acceptance criteria. The following table summarises the key standards and their applicability:

The fundamental principle across all these standards is that the PQR/WPQR must demonstrate that the welding procedure is capable of producing a conforming overlay layer under the specified range of process variables. The qualification is not a one-time event but a living document that must be re-evaluated whenever a material change, process change, or variable change occurs that falls outside the qualified range.

Essential Variables and Control Strategies

The identification and control of essential variables is the core of process qualification. For cladding operations, the essential variables differ from those for fusion welding because the overlay layer's performance is primarily governed by its chemical composition, microstructure, and bond integrity with the substrate. The following table presents the key essential variables for cladding process qualification and their control strategies:

The dilution rate is perhaps the most critical variable in cladding process qualification. It represents the percentage of base metal that is melted and incorporated into the overlay layer, and it directly determines the chemical composition of the overlay. For example, when Inconel 625 is clad onto a carbon steel substrate, a dilution rate of 10% will produce an overlay with a nickel content of approximately 55%, while a dilution rate of 30% will reduce the nickel content to approximately 45%, potentially compromising the corrosion resistance of the overlay. The dilution rate must be measured through chemical analysis of the overlay layer — typically by sampling from the centre of the overlay cross-section — and must fall within the acceptance criteria specified in the applicable standard or specification.

Testing Requirements and Acceptance Criteria

The PQR/WPQR for cladding must include a comprehensive suite of tests to verify the metallurgical and mechanical quality of the overlay layer. The following table summarises the typical test requirements and acceptance criteria:

The acceptance criteria for these tests are not universal — they vary depending on the application, the material, and the governing specification. For example, in sour service applications (high H₂S partial pressure), the overlay layer must pass the NACE TM0177 HIC/SSC test, which requires the overlay to have a hardness of less than 22 HRC and a specific microstructure (ferrite content < 10% for stainless overlays). Failure to meet these criteria can result in catastrophic failure of the pressure vessel in service.

Requalification Triggers and Change Control

One of the most critical aspects of process qualification is the identification of requalification triggers — changes in materials, process variables, or equipment that require a new PQR/WPQR. The following table summarises the common requalification triggers:

The management of requalification triggers is a critical aspect of quality control in cladding operations. A common failure mode is the use of a qualified procedure outside its intended range — for example, applying a PQR qualified for 304 stainless overlay on carbon steel to a 316 stainless overlay on low-alloy steel without requalification. This practice is a violation of the applicable standard and can result in non-conforming overlay layers that fail to meet the required corrosion resistance or mechanical properties.

Engineering Practice and Case Study

A practical case illustrating the importance of rigorous process qualification involves the cladding of a hydrogenation reactor tube sheet with Inconel 625 overlay. The initial PQR was qualified with a dilution rate of 18%, an interpass temperature of 150°C, and a preheat of 100°C. During production, the welding team observed that the dilution rate was consistently exceeding 25%, leading to overlay layers that failed the intergranular corrosion test. Investigation revealed that the travel speed had been reduced to improve deposition rate, which increased the heat input and consequently the dilution rate. The corrective action was to re-qualify the procedure with a lower heat input range, to implement a mechanised cladding system with precise travel speed control, and to add dilution rate monitoring as a production hold point. This case demonstrates that process qualification is not a static document but a dynamic control system that must be actively managed throughout the production lifecycle.

Study Insights and Conclusions

The cladding process qualification framework, as defined by standards such as ASME IX QW-451, ISO 15614-7, and NB/T 47014, provides a rigorous and systematic approach to validating welding procedures for overlay applications. The key technical insights are: (1) the dilution rate is the most critical variable in cladding qualification and must be measured and controlled through chemical analysis; (2) the essential variables for cladding differ from those for fusion welding and must be specifically identified and controlled; (3) requalification is required whenever a material or essential variable change occurs, and this must be enforced through a formal change control process; and (4) the testing suite for cladding qualification must include not only mechanical and metallurgical tests but also corrosion resistance tests specific to the service environment. Engineers should approach process qualification not as a compliance exercise but as a technical investigation into the behaviour of the welding process and the overlay layer, with the goal of understanding and controlling the variables that determine overlay performance.