ASNT NDT Level III cheat sheet

Magnetic particle testing requires ferromagnetic material: iron, nickel, cobalt and most carbon/low-alloy steels. Austenitic stainless steel, aluminum, copper and titanium are nonmagnetic and cannot be MT inspected.

Longitudinal velocity v_L = sqrt((K + 4G/3)/rho) and shear velocity v_S = sqrt(G/rho); in steel v_L is about 5.9 mm/microsecond and v_S about 3.23 mm/microsecond, roughly half.

Acoustic impedance Z = rho*v. At normal incidence the reflection coefficient is R = (Z2 - Z1)/(Z2 + Z1); a steel-to-air interface reflects nearly all the energy, which is why couplant is required.

Attenuation increases with frequency and with grain size. Coarse-grain castings and austenitic weld metal scatter ultrasound strongly, so lower frequencies and larger transducers are used.

IIW carbon equivalent: CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. Values above about 0.40 indicate high hardenability and increased cold-cracking risk in welded steel.

Precipitation-hardening alloys such as 7075 aluminum and 17-4PH stainless gain strength by solution treatment, quenching and aging; overaging reduces strength.

Alloy 2024 (Al-Cu) has high strength with lower corrosion resistance, 6061 (Al-Mg-Si) is weldable and corrosion resistant, and 7075 (Al-Zn) gives the highest strength of common aluminum alloys.

Reference values: steel density 7.8 g/cm3 with v_L 5900 m/s; aluminum 2.7 g/cm3 with v_L 6320 m/s; water 1.0 g/cm3 with 1480 m/s; air 343 m/s.

Common arc processes are SMAW, GMAW, FCAW, SAW and GTAW. SAW gives high deposition on thick plate; GTAW is preferred for thin sections, root passes and reactive metals.

Weld discontinuities include porosity (entrapped gas), slag inclusions (flux-based processes), lack of fusion, incomplete penetration, undercut, solidification (hot) cracks and hydrogen (cold) cracks.

Hydrogen-induced cold cracking requires three conditions simultaneously: diffusible hydrogen, a hard susceptible microstructure, and tensile restraint; cracking may appear hours after welding.

Hot cracking forms at the end of solidification along grain boundaries containing low-melting films; high sulfur, high restraint and deep narrow bead shapes increase susceptibility.

Casting discontinuities: shrinkage cavities (irregular, at thermal centers), gas porosity (rounded), sand inclusions, hot tears and cold shuts.

Wrought-product discontinuities such as laminations and stringers lie parallel to the rolled surface, so straight-beam ultrasonic scanning is performed normal to that surface.

Forging defects include laps, seams, bursts and flakes (hydrogen-induced internal cracks); inspection typically uses straight and angle beams from the as-forged surface.

The heat-affected zone is base metal thermally altered but not melted; grain coarsening next to the fusion line lowers toughness and can create a hardness peak.

The basic methods are visual (VT), liquid penetrant (PT), magnetic particle (MT), eddy current (ET), radiography (RT) and ultrasonic testing (UT); PT, MT, ET and VT address surface or near-surface flaws, RT and UT are volumetric.

PT relies on capillary action: penetrant dwell, removal of excess, then developer draw-out. Types include water-washable, post-emulsifiable, solvent-removable, visible dye and fluorescent.

MT applies only to ferromagnetic material. Wet fluorescent technique is more sensitive than dry powder; contact prods can cause arc burns and require demagnetization after use.

Near field length N = D^2/(4lambda) and beam spread to the first minimum follows sin(theta) = 1.22lambda/D; larger diameter and higher frequency produce a narrower, longer beam.

Snell's law: sin(theta1)/v1 = sin(theta2)/v2. For acrylic-to-steel the first critical angle is about 28 degrees and the second about 58 degrees; beyond the second critical angle only surface waves propagate.

Radiographic geometric unsharpness Ug = F*t/d, where F is source size, t the source-side object-to-film distance and d the source-to-object distance; smaller F and larger d sharpen the image.

Eddy current penetration depth decreases as frequency, conductivity or permeability increases; ET applies to electrically conductive materials and is usually limited to nonmagnetic or saturated parts.

Electromagnetic yoke lifting power: an AC yoke must lift at least 10 lb (4.5 kg) at maximum pole spacing and a DC yoke about 40 lb (18 kg); verify before each use.

Document hierarchy runs regulation or law, then code, then specification, then standard, then procedure, then work instruction; a code is a mandatory rule set adopted by reference.

Mandatory language convention: "shall" is a requirement, "should" a recommendation, "may" a permission; specific clauses override general text.

ASME BPVC Section V gives NDE method requirements (Article 2 RT, Article 4 UT, Article 6 PT, Article 7 MT, Article 8 ET); Section VIII Div. 1 gives vessel acceptance criteria; Section IX covers welding and brazing qualification.

Commonly referenced ASTM documents include E165 (penetrant), E709 (magnetic particle), E94 and E1742 (radiography), E317 (ultrasonic) and E543 (agencies performing NDT.

A code is a mandatory body of rules; a specification states the purchaser's detailed requirements for a product, material or service; a standard is a consensus technical document.

Acceptance criteria are numerical limits on indication type, size and distribution, set either by quality-level reference documents (e.g., RT reference radiographs) or by fracture mechanics.

Qualification of advanced ultrasonic techniques commonly requires a demonstration on a flawed specimen using a written, approved procedure before production use.

Retention of inspection reports, procedures, personnel certifications and calibration records is governed by the contract and the applicable code.

Discontinuity geometry drives method choice: planar flaws such as cracks and lack of fusion are best found by UT with the beam normal to the plane; volumetric flaws such as porosity and slag by RT.

Surface-breaking defects are addressed by VT, PT, MT (ferromagnetic) or ET (conductive); subsurface defects require UT or RT.

Selection inputs include material, discontinuity type and orientation, part geometry and access, surface condition, governing code requirements, inspection speed and cost.

UT is superior for crack-like defects and for thickness measurement; RT is superior for internal porosity and provides a permanent image record.

Acceptance limits derive from service loading, fracture mechanics, safety margin and the governing code; they are established before inspection and not adjusted to match results.

Probability of detection curves quantify reliability; POD depends on defect size and orientation, method, procedure, operator and equipment.

Technique qualification uses reference blocks containing known artificial or natural defects to demonstrate sensitivity, coverage and sizing accuracy.

For critical applications two complementary methods may be specified so that differently oriented discontinuities are both covered.

ASNT SNT-TC-1A is a recommended practice: the employer writes an NDT written practice that defines training, experience, examination and certification for Levels I, II and III.

ANSI/ASNT CP-189 is a standard with mandatory requirements, including that NDT Level III candidates be examined by an ASNT NDT Level III or ASNT-certified Level III.

ISO 9712 certification is issued by an independent national certifying body, is sector-specific, has three levels, and is valid for five years subject to recertification.

Level I performs specific calibrated tests under supervision; Level II sets up, calibrates, interprets to written criteria and may supervise; Level III selects methods, approves procedures, interprets results and certifies personnel.

Qualification is the demonstrated physical, mental and technical ability to perform a task; certification is the written attestation by employer or agency that qualification requirements are met.

Vision requirements: near vision of Jaeger 1 or equivalent at 12 in (30 cm) with at least one eye, verified annually, plus color contrast differentiation; far vision as specified in the written practice.

Recertification intervals do not exceed five years; Level III recertification is by examination or a documented points system defined in the written practice.

Training hours and experience requirements are method- and level-specific, tabulated in the written practice, and documented experience must be gained under qualified supervision.