CLADDING TECHNOLOGY SHANXI CO., LTDHardness Testing of Cladding Layers and Interface Regions
Literature Overview
Hardness testing is one of the most widely used non-destructive or minimally destructive methods for evaluating the mechanical properties of cladding layers, weld overlay deposits, and their interfaces with base metals. Whether assessing the wear resistance of a Stellite overlay on a sealing surface, verifying the hardness gradient in a multi-pass weld overlay, or examining the microhardness distribution across an interface, hardness testing provides rapid, cost-effective, and highly informative data. This study note examines the selection of hardness methods, test parameters, acceptance criteria, and the interpretation of hardness profiles in the context of cladding and bimetal manufacturing.
Core Technical Principles
Selection of Hardness Method
The choice of hardness method depends on the material hardness range, the thickness of the layer being tested, and the required spatial resolution:
| Method | Indentation Size | Hardness Range | Application |
|---|---|---|---|
| Brinell (HBW) | 2.5–10 mm diameter | 10–650 HBW | Base metal, thick overlay layers |
| Rockwell C (HRC) | ~0.2 mm depth | 20–70 HRC | Hardened overlay, Stellite, tool steels |
| Rockwell B (HRB) | ~0.25 mm depth | 20–100 HRB | Softer overlay layers |
| Vickers (HV) | 0.05–0.5 mm diagonal | 5–1500 HV | Thin layers, interface microhardness |
| Knoop (HK) | 0.05–0.3 mm long diagonal | 5–1500 HK | Thin films, brittle materials |
| Micro-Vickers (HV0.05) | ~0.05 mm diagonal | 5–2000 HV | Microstructural features, very thin layers |
For weld overlay applications, the Rockwell C scale is most commonly used for surface hardness verification of the overlay layer, while Vickers or micro-Vickers is used for interface characterisation and hardness profiling.
Test Parameters and Standards
| Parameter | Rockwell C | Vickers | Micro-Vickers |
|---|---|---|---|
| Load | 150 kgf | 10, 30, 50, 100 kgf | 0.05–1 kgf |
| Indentation depth | ~0.2 mm | 0.05–0.5 mm | 0.05–0.1 mm |
| Minimum layer thickness | 10× indentation depth | 3× indentation depth | 1.5× indentation depth |
| Standard | ASTM E18, ISO 6508 | ASTM E92, ISO 6507 | ASTM E384, ISO 6507 |
| Typical test time | 2–4 s dwell | 10–15 s dwell | 5–10 s dwell |
The minimum layer thickness requirement is critical: if the overlay layer is too thin, the indentation will penetrate into the base metal, giving artificially high or low hardness values that do not represent the overlay material. For a Stellite overlay layer of 3 mm thickness, Rockwell C testing is appropriate (indentation depth ~0.2 mm, ratio 15:1). For an interface dilution zone of 0.5 mm thickness, micro-Vickers at 10 gf (indentation depth ~0.05 mm, ratio 10:1) is required.
Interpretation of Technical Points
Hardness Gradient in Multi-Pass Weld Overlay
In multi-pass weld overlay applications, the hardness profile across the overlay thickness is rarely uniform. The first pass, deposited directly on the base metal, typically exhibits the highest hardness due to the highest dilution with base metal elements. Subsequent passes show progressively lower hardness as the dilution decreases. The final pass, deposited on the previous overlay layers, exhibits hardness closest to the nominal filler metal value.
A typical hardness profile for a 316L weld overlay on 16MnR base plate might be:
| Depth from Surface | Hardness (HV) | Notes |
|---|---|---|
| 0–0.5 mm | 220–250 HV | Top pass, low dilution |
| 0.5–1.5 mm | 200–230 HV | Intermediate pass |
| 1.5–2.5 mm | 250–280 HV | Lower pass, moderate dilution |
| 2.5–3.5 mm | 280–320 HV | First pass, high dilution |
| Interface | 300–350 HV | Dilution zone, possible martensite |
This gradient is normal and expected, but the engineer must verify that the hardness at all depths falls within the acceptable range for the specified overlay material. Excessive hardness at the interface may indicate the formation of brittle martensite or intermetallic phases, which can compromise toughness and corrosion resistance.
Stellite Overlay Hardness and Phase Transformation
Stellite alloys (e.g., Stellite 6, Stellite 6B) are cobalt-chromium-tungsten alloys that exhibit a unique hardness behaviour. As-cast Stellite typically has a hardness of 38–42 HRC, but this can increase to 45–48 HRC after solution heat treatment at 1050–1100°C followed by air cooling, due to the precipitation of carbides from the solid solution. The hardness of Stellite overlay layers is critical for wear resistance, and the specification typically requires a minimum hardness of 38 HRC and a maximum of 45 HRC to ensure adequate ductility.
The hardness of Stellite overlay can be affected by:
- Heat input: High heat input during welding can dissolve carbides, reducing as-deposited hardness.
- Cooling rate: Rapid cooling can retain carbides in solution, leading to lower hardness.
- Post-weld heat treatment: Solution treatment followed by controlled cooling can optimise the hardness and wear resistance.
Interface Microhardness Line Scanning
Microhardness line scanning across the interface between the overlay layer and the base metal provides detailed information about the dilution zone and the microstructural evolution. A typical line scan for a 316L overlay on 16MnR base plate might show:
| Distance from Interface (µm) | Hardness (HV0.1) | Microstructure |
|---|---|---|
| −200 (base metal) | 200 | Ferrite + pearlite |
| −100 | 220 | Ferrite + pearlite + some austenite |
| −50 | 280 | Austenite + some ferrite |
| 0 (interface) | 320 | Austenite + martensite |
| +50 | 280 | Austenite + some martensite |
| +100 | 250 | Austenite |
| +200 (overlay) | 230 | Austenite |
The peak hardness at the interface (320 HV in this example) indicates the dilution zone, where the microstructure has shifted toward martensite due to the lower nickel and chromium content. This zone is typically 50–150 µm wide and represents the most vulnerable region for corrosion and mechanical failure.
Engineering Practice Cases
In a project involving a high-pressure pump impeller with Stellite 6 overlay on a duplex stainless steel base, the surface hardness was measured at 42 HRC, well within the specification of 38–45 HRC. However, the microhardness line scan across the interface revealed a hardness peak of 850 HV at the interface, indicating the formation of brittle intermetallic phases (σ-phase and Laves phase) due to the high chromium and molybdenum content of the Stellite alloy reacting with the iron in the base metal. This finding was critical because the intermetallic phases are susceptible to cracking under thermal cycling and mechanical loading. The solution was to implement a post-weld heat treatment at 950°C for 2 hours, which dissolved the intermetallic phases and restored the ductility of the interface region.
Key Questions and Reflections
- How many hardness points are required per test location? For surface hardness verification of a weld overlay, typically 3–5 points are measured across the overlay width, with additional points at the edges to detect any hardness variation due to edge effects. For interface microhardness line scanning, 10–20 points are typically measured at 10–50 µm intervals, depending on the width of the dilution zone.
- What is the effect of surface preparation on hardness results? Surface preparation is critical for accurate hardness measurement. The surface must be flat, free of oxide scale, and polished to a mirror finish for microhardness testing. Any surface roughness or oxide layer will affect the indentation size and give erroneous hardness values. For Rockwell testing of weld overlay surfaces, the surface must be ground flat with a coarse grit (60–80) and then finished with a fine grit (120–180) to ensure a flat, oxide-free surface.
- How to handle hardness values outside the specification? If the surface hardness of a Stellite overlay is below 38 HRC, the overlay may not provide adequate wear resistance. If the hardness is above 45 HRC, the overlay may be too brittle and susceptible to cracking. In either case, the root cause must be investigated, and corrective action must be taken. For low hardness, the heat input during welding may have been too high, dissolving the carbides. For high hardness, the cooling rate may have been too rapid, or the post-weld heat treatment may have been inadequate.
Study Insights and Implications
Hardness testing of cladding layers and interface regions is a powerful tool for evaluating the mechanical properties and microstructural integrity of weld overlay components. The engineer must carefully select the appropriate hardness method, test parameters, and specimen preparation to ensure that the results are representative of the actual material properties. The hardness profile across the overlay thickness and the interface provides critical information about the dilution effects, microstructural evolution, and potential failure mechanisms.
For bimetal pressure vessels and critical components, hardness testing must be performed at multiple locations and depths, with particular attention to the interface region where the microstructural heterogeneity is greatest. The engineer must always interpret the hardness results in the context of the manufacturing process, the service conditions, and the applicable code requirements. The ultimate goal is to ensure that the cladding layer provides adequate wear resistance, corrosion resistance, and mechanical integrity throughout the design life of the component.
In conclusion, hardness testing is an indispensable tool in the quality assurance of cladding and weld overlay components. The engineer must approach hardness testing with a systematic methodology, careful attention to test parameters, and a deep understanding of the microstructural effects that influence hardness values. By integrating hardness data with other non-destructive testing methods, metallographic examination, and mechanical property testing, the engineer can build a comprehensive picture of the component's integrity and make informed decisions about its suitability for service.