7.3 Impact Testing and Hardness Testing

Charpy V-Notch (CVN) Impact Test

The Charpy V-notch impact test measures notch toughness — the energy a notched specimen absorbs when fractured by a single sudden blow at a controlled temperature. It is the standard way to judge a weld's resistance to brittle fracture, and it is governed by ASTM E23. A swinging pendulum strikes the specimen on the face opposite the notch; the energy lost from the pendulum's swing equals the energy absorbed, read in foot-pounds (ft-lbf) or joules (J).

Standard specimen: a bar 55 mm long x 10 mm x 10 mm with a 2 mm-deep V-notch machined at mid-length. The notch has a 45-degree included angle and a 0.25 mm (0.010 in) root radius. Sub-size specimens (7.5, 5, or 2.5 mm thick) are used when the material is too thin for full size, and their acceptance energy is reduced proportionally. The notch concentrates stress so the test probes the material's worst-case behavior; even small changes in notch geometry shift the result, which is why machining tolerances are tight.

Fracture appearance: a ductile break is fibrous, dull, and gray and absorbs high energy; a brittle break is shiny, crystalline, and flat and absorbs little energy. The CWI uses percent shear (fibrous) fracture and lateral expansion alongside the energy value.

When required: CVN testing is specified for cold-service structures (bridges, outdoor tanks), pressure vessels and boilers (ASME), and seismic applications (AISC 341). AWS D1.1 does not require CVN testing by default — it is performed only when the contract documents or the Engineer call for it, typically with a stated minimum energy at a stated temperature (for example, 20 ft-lbf at minus 20 degrees F).

Hardness Testing

Hardness testing measures resistance to indentation and is the inspector's quick window into the heat-affected zone (HAZ). In welding it is used to (1) confirm the HAZ has not become excessively hard — high hardness signals brittle martensite from rapid cooling and elevates the risk of hydrogen-induced (cold) cracking; (2) verify a post-weld heat treatment (PWHT) softened the joint; and (3) survey property variation across the weld.

A hardness traverse takes a row of readings across the polished section: base metal -> HAZ -> weld metal -> HAZ -> base metal. A spike in the HAZ flags martensite; uniform low values confirm an effective PWHT. A widely cited limit is NACE MR0175 / ISO 15156 sour service: 22 HRC (about 248 HV) maximum for carbon-steel welds. AWS D1.1 sets no general structural hardness ceiling.

Macroetch and Microetch Examination

Etch tests chemically attack a polished cross-section so the weld geometry and structure become visible. The macroetch (macro) test is examined at low magnification (up to ~10x) and is the CWI's tool for measuring fillet-weld leg size, effective throat, depth of root penetration, sidewall fusion, and number of passes, and for spotting incomplete fusion, undercut, or cracks. Common etchants are iodine tincture, a 1:1 hydrochloric-acid/water solution, ammonium persulfate, or nital (nitric acid in alcohol), applied for roughly 15 to 60 minutes.

The microetch is examined under a microscope (50x and higher) to reveal grain structure, martensite, and the fine HAZ.

AWS D1.1 macroetch acceptance for fillet welds requires the etched face to show fusion to the root of the joint but not necessarily beyond, leg sizes equal to or greater than the specified leg, and no cracks. Macroetch is the companion to the fillet break test in welder qualification: the break test proves internal soundness on a fracture surface, while the macroetch measures geometry and fusion on a sectioned face.

Exam trap: match the test to the property — CVN = toughness/energy, hardness = indentation/martensite, macroetch = fusion and weld dimensions. High HAZ hardness equals brittle martensite, not good ductility.

Reading the Transition Curve and Notch Location

Because toughness depends strongly on temperature, CVN results are often plotted as an energy-versus-temperature transition curve. The curve has a low lower shelf (brittle), a steep transition region, and a high upper shelf (ductile). The transition temperature is the point where the metal shifts from tough to brittle behavior — defined either by a target energy (for example, the temperature at which the specimen first reaches the required ft-lbf) or by the 50% fibrous (shear) fracture appearance.

The inspector should know that a weld can be perfectly strong in a tensile test yet fail catastrophically by brittle fracture if it is used below its transition temperature, which is exactly why cold-service and seismic codes call for CVN data.

For a welded joint, three notch locations are usually tested because each samples a different microstructure: the weld metal centerline, the fusion line, and the HAZ a set distance from the fusion line. Each location can have very different toughness, so a single value never tells the whole story.

Hardness, PWHT, and the Inspector's Documentation

Hardness ties directly to cracking risk and heat treatment. When the carbon equivalent (CE) of the steel is high, the HAZ cools into hard martensite unless preheat and controlled heat input slow the cooling rate; a post-weld hardness survey verifies the result. After a PWHT (stress relief), the inspector expects the traverse to show reduced and more uniform hardness — confirmation the treatment tempered the martensite and relieved residual stress. A persistently high reading after PWHT suggests the cycle was too short, too cool, or the wrong material was treated.

The CWI's job with these tests is again to witness, verify, and record rather than operate the equipment:

• Confirm the specimen temperature at the moment of impact (CVN must be struck within a few seconds of leaving the bath) and that the pendulum machine is calibrated.
• Record all three CVN values at each set (codes commonly average three specimens with a minimum single value), the test temperature, and percent shear.
• Document hardness scale and load (HRC versus HV are not interchangeable) and the traverse map across base, HAZ, and weld.
• For macroetch, confirm the etchant and section preparation and measure leg, throat, and root penetration against the WPS.

Exam trap: CVN reports a set of energies at a stated temperature, not a single pass/fail number; and Rockwell C and Vickers values must never be casually swapped, though conversion tables exist.