5.3 Product Forms & Service Conditions

Product Forms and Their Discontinuity Signatures

A Level III selects methods partly by knowing the product form, meaning how the part was made, because each form carries a predictable discontinuity signature and lends itself to different NDT. The detailed catalog of discontinuities belongs to the next chapter; here the goal is to link form and service condition to inspection strategy.

The Four Major Product Forms

• Castings have the most internal volume risk and coarse grain, so they favor radiography and surface methods.
• Forgings are usually sound internally but hide tight surface laps, so surface methods plus angle-beam UT are standard.
• Wrought products carry planar laminations parallel to the surface, a textbook straight-beam UT target, plus surface seams.
• Weldments combine a cast-like fusion zone with a heat-altered HAZ, so they usually need both volumetric (RT/UT) and surface (MT/PT) coverage.

Understanding the form also tells you the orientation of the expected flaw, which drives method choice: RT struggles with tight cracks aligned to the beam, while angle-beam UT is chosen to intercept planar flaws such as lack of fusion.

Service-Related Degradation

Parts that passed initial inspection still degrade in service. In-service NDT targets four mechanisms the Level III must recognize.

Fatigue

Fatigue is progressive cracking under repeated (cyclic) loading, even at stresses well below the yield strength. Cracks initiate at stress concentrations such as notches, tool marks, weld toes, and corrosion pits, and grow a little each cycle, often leaving beach marks. Because a fatigue crack is a tight, surface-initiating planar flaw, surface methods (MT, PT, ET) catch initiation while UT sizes established cracks. Fatigue is why surface finish and geometry matter so much.

Fatigue proceeds in three stages: crack initiation at a stress raiser, slow propagation cycle by cycle (leaving beach marks), and sudden final fracture when the remaining section can no longer carry the load. Many steels show an endurance limit, a stress below which fatigue essentially never occurs, while aluminum and other nonferrous alloys have no true limit and can fail after enough cycles even at low stress. A Level III therefore treats every notch, weld toe, and corrosion pit as a potential fatigue origin.

Corrosion

Corrosion is the chemical or electrochemical loss of metal. Forms include general (uniform) thinning, localized pitting, stress-corrosion cracking (SCC) where corrosion plus tensile stress produces branching cracks, erosion-corrosion, and intergranular attack. Wall-loss corrosion is tracked with UT thickness gauging and, on tank floors and tubes, with magnetic flux leakage (MFL) and eddy current. SCC produces fine cracks best found by PT, ET, or UT.

Creep

Creep is slow, time-dependent deformation under sustained load at high temperature, a concern for boilers, turbines, and reformer tubes. It progresses through cavitation and microcracking to bulging and rupture. Early creep damage is monitored with replication metallography, ultrasonic and dimensional checks, and other advanced techniques, because the initial creep voids are too small for routine methods.

Hydrogen Damage

Hydrogen absorbed during welding, plating, pickling, or corrosive service embrittles steel. Hydrogen embrittlement causes delayed cracking under stress; hydrogen-induced cracking (HIC) and blistering appear in wet sour service; and hydrogen combined with a hardened HAZ produces weld cold cracking. Preheat, low-hydrogen consumables, and post-weld baking are the defenses, and delayed cracking means some inspections are deliberately timed hours after welding.

Other Mechanisms

Wear, thermal fatigue (cyclic heating and cooling), and overload round out the service picture. The unifying idea for the exam is to match the degradation mechanism to the flaw it produces and then to the method that finds that flaw.

Tying It Together for the Level III

The exam rewards linked reasoning: identify the product form, predict the likely discontinuity origin and orientation, factor in the material (grain size, magnetism, conductivity), consider access and surface condition, and only then choose the method. A cast austenitic pump housing, for example, argues for RT and PT rather than MT (nonmagnetic) or high-frequency UT (coarse grain). A cyclically loaded steel shaft argues for surface MT or PT at the fillets where fatigue starts. This service-and-form lens is exactly what separates a Level III's judgment from Level II execution.

Common trap: do not confuse the degradation mechanism with the flaw shape when selecting a method. Fatigue and stress-corrosion cracking both create tight planar cracks best found by surface methods or angle-beam UT, whereas general corrosion and creep produce volume loss or bulging better tracked by UT thickness gauging and dimensional checks.