7.3 Impact Testing and Hardness Testing
Key Takeaways
- Charpy V-Notch test measures impact energy (ft-lbs or J) at a specified temperature — evaluates notch toughness
- CVN is required for cold-environment structures, seismic applications, and when specified by contract
- Ductile fracture = fibrous/dull appearance (high energy absorbed); brittle = crystalline/shiny (low energy)
- Hardness testing measures resistance to indentation — high HAZ hardness indicates brittle martensite
- Common scales: Brinell (HB), Rockwell B (HRB), Rockwell C (HRC), Vickers (HV)
- AWS D1.1 does NOT require CVN unless specified by contract documents
7.3 Impact Testing and Hardness Testing
Charpy V-Notch (CVN) Impact Test
The Charpy V-Notch impact test measures a material's resistance to fracture under sudden loading (impact) at a specified temperature. It is the standard method for evaluating notch toughness — the ability to absorb energy before fracturing.
How It Works:
- A standard notched specimen (10 mm × 10 mm × 55 mm with a 2 mm deep V-notch) is placed in a fixture
- A heavy pendulum swings and strikes the specimen on the side opposite the notch
- The energy absorbed by the specimen during fracture is measured (in foot-pounds or joules)
- The specimen temperature is controlled (tested at specified temperature, often 0°F, -20°F, or -40°F)
Key Concepts:
| Term | Definition |
|---|---|
| Absorbed energy | Energy consumed by the specimen during fracture (ft-lbs or J) |
| Transition temperature | Temperature below which the material becomes brittle |
| Upper shelf energy | Energy absorbed at temperatures well above the transition zone (ductile fracture) |
| Lower shelf energy | Energy absorbed at temperatures well below the transition zone (brittle fracture) |
| Ductile fracture | Fibrous, dull appearance — material absorbed significant energy |
| Brittle fracture | Crystalline, shiny appearance — material absorbed little energy |
When Is CVN Testing Required?
- Structures in cold environments (bridges, outdoor storage tanks)
- Pressure vessels and boilers (ASME code)
- Seismic applications (AISC 341)
- When specified by contract documents or the Engineer
- AWS D1.1 does NOT require CVN unless specified in contract documents
Hardness Testing
Hardness testing measures a material's resistance to indentation. For welding, hardness is used to:
- Verify that the HAZ has not exceeded maximum allowable hardness (indicating brittle martensite)
- Check for proper heat treatment (PWHT effectiveness)
- Evaluate material properties across the weld cross-section
Common Hardness Scales:
| Scale | Method | Application |
|---|---|---|
| Brinell (HB) | Large ball indenter (10 mm) under heavy load | Forgings, castings, large specimens |
| Rockwell B (HRB) | 1/16" ball, 100 kgf | Softer materials (annealed steel, brass) |
| Rockwell C (HRC) | Diamond cone, 150 kgf | Harder materials (quenched steel, hard alloys) |
| Vickers (HV) | Diamond pyramid, variable load | Research, thin sections, micro-hardness |
| Knoop (HK) | Elongated diamond, low load | Micro-hardness of thin layers, coatings |
Hardness Traverse (Cross-Sectional Survey):
A hardness traverse takes readings across the weld cross-section:
- Base metal → HAZ → weld metal → HAZ → base metal
This reveals:
- HAZ hardening (high hardness values indicate martensite)
- Weld metal hardness relative to base metal
- Effectiveness of PWHT (hardness should be uniform and within limits)
Typical Maximum Hardness Limits:
- AWS D1.1: No specific maximum hardness requirement for most structural steels
- NACE MR0175/ISO 15156 (sour service): 22 HRC (248 HV) maximum for carbon steel
- API 1104: May specify hardness limits per project requirements
For the Exam: Know that CVN tests measure impact energy at a specific temperature and are required for cold-environment structures. Know the relationship between high HAZ hardness and brittle martensite. Know the Brinell, Rockwell, and Vickers hardness scales.
What does the Charpy V-Notch impact test measure?
High hardness values in the HAZ of a carbon steel weld most likely indicate:
Under AWS D1.1, Charpy V-Notch impact testing is required: