5.2 Manufacturing Processes & Heat Treatment
Key Takeaways
- Casting produces porosity, shrinkage cavities, hot tears (restrained contraction), and cold shuts, plus coarse grain that hampers UT.
- Forging causes laps and internal bursts, while rolling flattens inclusions into laminations parallel to the plate surface and creates seams.
- Every fusion weld has three zones: weld metal, the crack-prone heat-affected zone (HAZ), and unaffected base metal.
- Normalizing (air cool) refines grain, quench-and-temper hardens then restores toughness, and case hardening gives a hard surface over a tough core.
- Quench cracks come from severe rapid cooling, and tempering reduces the brittleness of as-quenched martensite.
From Raw Metal to Finished Part
Every discontinuity a Level III chases was either born in the material (inherent), created while the part was made (processing), or produced in service. This section covers the processing side: casting, deformation (forging, rolling, extrusion), welding, and machining, followed by the heat treatments that reset a part's microstructure. Knowing how each process works lets you predict the defect type, its likely orientation, and the method most likely to find it.
Casting
Casting pours molten metal into a mold and lets it solidify. Common routes include sand casting, investment (lost-wax) casting, die casting, and continuous casting. Because the metal solidifies from the mold wall inward and shrinks as it cools, castings carry a distinctive family of discontinuities:
- Gas porosity - rounded voids from dissolved gas coming out of solution.
- Shrinkage cavities - jagged voids where late-freezing metal was starved of feed.
- Hot tears - cracks that form when solidifying metal is restrained and cannot contract freely.
- Cold shut - a weak seam where two metal streams met without fusing.
- Inclusions and misruns - trapped slag or sand, or incompletely filled sections.
Castings also solidify with a coarse, dendritic grain that scatters ultrasound, so radiography is often preferred for internal casting volume while UT is applied with care.
Deformation Processing: Forging, Rolling, Extrusion
Forging shapes hot metal with compressive blows or pressure. Done well it closes internal voids and produces a favorable grain flow that follows the part contour, improving fatigue life. Its characteristic defects include laps (metal folded over onto itself and not welded, a surface-connected fold), internal bursts (from working a cold interior too fast), and flakes (hydrogen-related internal cracks).
Rolling squeezes ingot or slab between rolls into plate, sheet, bar, and structural shapes. Non-metallic inclusions and any residual pipe get flattened and stretched into laminations that lie parallel to the plate surface, which is the reason straight-beam UT is the standard plate check. Rolled bar and tube also show seams (elongated surface openings) and stringers (elongated inclusion trails).
Extrusion forces metal through a die to make a constant cross-section. Its discontinuities tend to be long and linear, following the working direction, much like seams.
Hot Working versus Cold Working
Deformation done above the recrystallization temperature is hot working; it refines grain and needs less force but leaves scale and looser tolerances. Deformation below that temperature is cold working (cold rolling, drawing, cold forging). Cold work raises strength and hardness while lowering ductility, because it multiplies dislocations and work-hardens the metal. That tradeoff matters for inspection: a cold-worked surface can be more crack-sensitive, and heavily worked material may need a stress-relief or full anneal to restore ductility before further forming.
Welding and Weld Zones
Welding fuses parts with a localized molten pool, usually with filler metal. Common processes include SMAW (shielded metal arc, or stick), GTAW (gas tungsten arc, or TIG), GMAW (gas metal arc, or MIG), and SAW (submerged arc). Every fusion weld has three metallurgical regions.
| Weld zone | Definition | Why it matters |
|---|---|---|
| Weld metal / fusion zone | Melted and re-solidified metal | Cast-like structure; porosity, slag, and cracks form here |
| Heat-affected zone (HAZ) | Base metal that did NOT melt but whose microstructure was altered by heat | Hardened and crack-prone; residual stress concentrates here |
| Base (parent) metal | Unaffected material | Reference for properties |
The HAZ is the classic trouble spot. Rapid weld thermal cycles can quench the HAZ of a hardenable steel into brittle martensite, and combined with residual stress and dissolved hydrogen this produces hydrogen-induced (cold) cracking. Typical weld discontinuities include lack of fusion, lack of penetration, slag inclusions (elongated and irregular), porosity (rounded), undercut, and hot or cold cracks. A Level III reviews base-metal hardenability, preheat, and heat input precisely because they govern HAZ risk.
Machining and Finishing
Machining removes metal by cutting, turning, milling, or grinding. It does not usually create bulk flaws, but it leaves directional tool marks that must not be mistaken for cracks; machining marks are uniform and repetitive along the tool path, while cracks are irregular. Aggressive grinding can overheat a hardened surface and produce fine grinding cracks, and any machining leaves residual stress that can influence later cracking. This is why finishing history is reviewed before surface NDT.
Heat Treatment
Heat treatment deliberately changes microstructure and properties.
| Process | What it does | Typical purpose |
|---|---|---|
| Annealing | Heat, then very slow (furnace) cool | Soften, relieve stress, improve machinability |
| Normalizing | Heat above critical, then air cool | Refine grain and produce a uniform structure after hot working |
| Quench and temper | Rapid quench to harden, then reheat lower to temper | High strength with restored toughness |
| Case hardening | Carburize, nitride, induction- or flame-harden the surface | Hard wear surface over a tough core |
| Stress relief | Moderate heat below transformation | Reduce residual and weld stress |
Quenching forms hard, brittle martensite but generates steep thermal and transformation stresses; if they exceed local strength, quench cracks result, favored by sharp corners, section changes, and severe quenchants. Tempering then reheats the quenched steel to trade a little hardness for a large gain in toughness and reduced cracking risk. Normalizing after hot working refines and homogenizes grain, improving both properties and inspectability. Case hardening creates a hard, wear- and fatigue-resistant surface while leaving a ductile core, but the hardened case is itself crack-sensitive, another reason process history guides method choice.
Common trap: annealing and normalizing both soften steel and improve its structure, but they are not interchangeable. Annealing uses a slow furnace cool for maximum softness and machinability, while normalizing uses an air cool to refine grain and leave the steel slightly stronger and harder than the fully annealed condition.
Which region of a fusion weld did NOT melt but had its microstructure altered by welding heat, making it especially prone to cracking?
A hot-worked steel part is heat treated specifically to refine its grain and produce a more uniform microstructure, using an air cool. Which process is this?
In rolled plate, laminations formed from flattened non-metallic inclusions are typically oriented in what way?