CLADDING TECHNOLOGY SHANXI CO., LTDTensile and Impact Testing of Clad Plates and Weld Overlay Test Plates
Literature Overview
The mechanical performance of clad plates and weld overlay components is governed by the combined behaviour of the base metal, the overlay layer, and the interface between them. Tensile testing (per GB/T 228 or ASTM E8) and Charpy V-notch impact testing (per GB/T 229 or ASTME23) are the fundamental mechanical characterisation methods required by pressure vessel codes (GB/T 150, NB/T 47002, ASME VIII Div.1 and Div.2) to demonstrate that the composite material meets design requirements. This study note examines the specimen orientation, acceptance criteria, and practical challenges associated with these tests in the context of clad plate qualification and production verification.
Core Technical Principles
Specimen Orientation and Test Types
For clad plates, the tensile test specimens are typically prepared in three orientations:
| Test Type | Specimen Location | Purpose |
|---|---|---|
| Base metal tensile | Entirely in base metal | Verify base plate quality |
| Overlay tensile | Entirely in overlay layer | Verify overlay material quality |
| Transverse tensile | Through both layers (cross-section) | Evaluate bond strength and interface integrity |
| Longitudinal tensile | Along the weld/overlay direction | Evaluate through-thickness performance |
The transverse tensile test is particularly important because it loads the interface in tension, directly assessing the bond strength. If the interface is weak, the specimen will fail at the interface with a fracture surface showing no plastic deformation. If the interface is strong, the specimen will fail in the base metal or overlay layer with normal ductile fracture characteristics.
Impact Testing Considerations
Charpy V-notch impact testing of clad plates presents unique challenges because the notch can be placed at different positions relative to the interface:
| Notch Position | Distance from Interface | Purpose |
|---|---|---|
| Overlay side | 5 mm from overlay surface | Evaluate overlay toughness |
| Interface | Notch at interface | Evaluate interface toughness |
| Base metal side | 5 mm from base metal surface | Evaluate base metal toughness |
The standard practice (per NB/T 47014 and ASME IX) requires that the notch be placed with the root at a specific distance from the interface, typically 5 mm from the overlay surface for overlay-side testing. The impact energy values must meet the minimum requirements specified in the applicable code for the material and temperature.
Interpretation of Technical Points
Effect of Interface Dilution on Mechanical Properties
In weld overlay processes, the interface between the overlay layer and the base metal is subject to dilution, where base metal elements dissolve into the overlay melt pool. This dilution has profound effects on mechanical properties:
- For austenitic overlay on carbon steel: Dilution with iron reduces the nickel and chromium content, potentially shifting the microstructure from fully austenitic toward austenite-ferrite duplex. This can reduce ductility and increase yield strength in the dilution zone.
- For duplex overlay on carbon steel: Dilution introduces additional ferrite, potentially exceeding the optimal 50/50 phase balance and increasing the ferrite content to 70–80%, which can reduce ductility and increase susceptibility to intergranular corrosion.
- For nickel-based alloy overlay: Dilution with iron and carbon can precipitate brittle intermetallic phases (e.g., σ-phase, Laves phase) at the interface, severely reducing toughness.
Acceptance Criteria in Practice
The acceptance criteria for tensile tests of clad plates are typically:
| Property | Minimum Requirement |
|---|---|
| Yield strength | ≥ 90% of specified minimum for base metal or overlay (whichever is lower) |
| Tensile strength | ≥ specified minimum for base metal |
| Elongation | ≥ 20% (typical for austenitic overlay); ≥ 12% (typical for duplex overlay) |
| Reduction of area | ≥ 40% (austenitic); ≥ 25% (duplex) |
For impact tests, the minimum absorbed energy is typically 34 J at the design temperature for carbon steel base plates and 27 J for stainless steel overlay layers, though these values vary by code and application.
Common Defects and Their Impact on Test Results
| Defect Type | Effect on Tensile Test | Effect on Impact Test |
|---|---|---|
| Lack of fusion at interface | Premature fracture at interface | Low impact energy |
| Cracking in overlay layer | Reduced tensile strength | Brittle fracture |
| Inclusion stringers | Reduced elongation | Reduced ductility |
| Excessive dilution | Reduced yield strength in dilution zone | Reduced toughness |
| Segregation in overlay | Non-uniform properties | Variable impact energy |
Engineering Practice Cases
In a recent project involving a hydrogenation reactor with 316L overlay on 16MnR base plate, the transverse tensile test specimens consistently failed in the base metal with elongations of 25–30%, indicating excellent interface bonding. However, the impact tests at −20°C revealed a concerning trend: specimens with the notch at 5 mm from the overlay surface showed impact energies of 45–55 J, while specimens with the notch at the interface showed energies of only 18–25 J. This discrepancy was traced to the dilution zone, where the microstructure had shifted toward martensite due to the high cooling rate from the carbon steel substrate. The solution was to implement a post-weld heat treatment (PWHT) at 1050°C for 2 hours, which dissolved the martensite and restored the austenitic structure in the dilution zone.
Key Questions and Reflections
- How many specimens are required? The number of test specimens depends on the qualification scope. For a welding procedure qualification (WPQ) per NB/T 47014, typically 3 tensile and 3 impact specimens are required. For production verification, the frequency is typically one set per heat lot or per shift, depending on the quality plan.
- What if the interface fails in tensile testing? An interface failure in a transverse tensile test is a critical finding that indicates inadequate bond strength. This typically requires investigation of the welding parameters, particularly the heat input and the number of passes. In many cases, the root cause is insufficient preheating, which leads to excessive cooling rates and the formation of brittle phases at the interface.
- How to reconcile code requirements with engineering reality? Codes specify minimum impact energies that may not be achievable for certain overlay systems, particularly nickel-based alloys on carbon steel. In such cases, a deviation or alternative acceptance criterion must be negotiated with the design authority, supported by additional testing such as fracture mechanics evaluation.
Study Insights and Implications
The tensile and impact tests of clad plates and weld overlay components are not merely compliance exercises but essential tools for understanding the structural integrity of the composite material. The engineer must carefully consider the specimen orientation, notch position, and test temperature to ensure that the test results are representative of the actual service conditions. The interface region is always the weakest link, and the test results must be interpreted with full awareness of the dilution effects and microstructural evolution at the interface.
For bimetal pressure vessels, the mechanical performance of the overlay layer must be verified at multiple temperatures, particularly at the design minimum temperature. The impact energy values at low temperatures can be significantly lower than at room temperature, and the engineer must ensure that the overlay layer maintains adequate toughness throughout the operating temperature range. The application of fracture mechanics principles, including the J-integral and crack tip opening displacement (CTOD) methods, provides a more comprehensive assessment of interface integrity than the Charpy V-notch test alone.
In conclusion, tensile and impact testing of clad plates and weld overlay components requires careful specimen preparation, precise test execution, and thoughtful interpretation of results. The engineer must always consider the manufacturing history, the dilution effects, and the microstructural evolution at the interface when evaluating the test data. The ultimate goal is to ensure that the composite material provides adequate structural integrity throughout the design life of the pressure vessel, accounting for all expected service conditions and potential degradation mechanisms.