01 Overheating and burning
When metal is heated too hot or held at temperature too long during processing, the grain structure coarsens — that's overheating. If you push it close to the melting point or really overshoot, low-melting-point constituents at the grain boundaries can actually melt and weaken the structure; that's burning (overcooking).
Overheating shows up as a rough, pitted surface, orange-peel texture, and coarse grains. Alloy strength doesn't drop much, but room-temperature impact toughness and ductility take a serious hit. The material turns brittle, and fractures show coarse crystalline facets. Under high magnification you'll see coarse equiaxed grains, maybe a coarse second phase or Widmanstätten structure. Overheating isn't necessarily a scrap condition — sometimes you can salvage it with re-deformation or re-processing, or downgrade it for less demanding use.
Burning is worse: the surface gets rough, grain boundaries coarsen, straighten, and look hairy, and you may see cracks plus an oxidized film that's prone to melting. Bonding strength drops sharply. Microstructurally, you'll find coarse grain boundaries, melted cavities or eutectic spheres, a liquid-phase network, and irregular voids where several grains meet. After rolling or extrusion, expect grain-boundary cracks, edge cracks on plate, blooming or open cracks on rod ends, or pieces that crack apart — the cracked areas show coarse dendrites and melting traces.
The usual culprits: heating temperature too high, hold time too long, or sitting too long near a high-temperature source. Also, finishing hot-working at too high a temperature or lingering in the hot zone. And if the alloy has low-melting-point elements or excessive low-melting inclusions, you're asking for trouble.
02 Cracks and cracking
(1) Processing cracks: These come from improper process parameters or tooling defects. They split into hot cracks and cold cracks — the difference is in the surrounding microstructure: hot cracks show no obvious change, cold cracks show heavy deformation. Morphologically, you'll see longitudinal, transverse, edge, open-mouth, spiral, periodic transverse, fracture, 45° diagonal, and irregular cracks. By location: local, head, surface, center, and intergranular.
To diagnose processing cracks, look at both the metal itself and the process conditions.
Metal-side factors: alloy composition and impurity levels; ingot defects like cold shuts, scars, and exudations on the surface, plus shrinkage, porosity, inclusions, and segregation internally — these make cracking likely during processing. A well-developed columnar structure with a weak surface is prone to cracking along grain boundaries or the weak plane. Also, if the alloy picked up cracks, burning, overheating, or poor second-phase distribution in earlier steps — or has uneven structure or micro-cracks from phase transformation — you're set up for failure.
(2) Heat treatment cracks
① If the alloy has high residual stress and the thermal stress from heating lines up with it, the combined load can exceed the metal's strength and cause cracking.
② During heating, if a second phase precipitates along grain boundaries, it creates additional stress — or phase transformation itself can cause obvious
03 Inclusions
Inclusions are foreign metal or non-metallic materials embedded in the product surface or matrix, with a clear interface and different properties from the base metal.
In processed products, inclusions typically come from unmelted metal particles in the ingot, large primary crystals, or foreign metal that got mixed in — or from metal compounds, silicates, and covering agents.
Classification and characteristics are basically the same as for cast products. In wrought products, inclusions are usually granular, elongated along the processing direction, with a distinct form and color, and a clear interface. You can spot them under a metallurgical microscope without etching.
04 Foreign matter indentation
When metal or non-metal particles get pressed into the product surface, that's foreign matter indentation.
Metal indentations have a clear interface with the substrate, sharp contours, and a different metallic luster. They appear as points or blocks, and when they peel off, they leave pits. Non-metallic indentations vary in form and color, are more brittle, lack metallic luster, and distribute as points, sheets, or strips along the processing direction — they're hard to peel off.
Where does the foreign matter come from? Oxide scale from heating, lubricant stuck to dies and parts, die fragments and adhered material, metal chips, burrs and fringes from cracked edges, furnace debris and oil, contaminants in the coolant emulsion, and foreign material in the rolling pass or on guide plates.
05 Bubble
After processing or annealing, the surface shows strip-like or bubble-shaped bulges along the processing direction. When cut open, these bulges reveal a cavity, which is referred to as bubbling.
Bubbles typically appear as smooth, elongated strips along the processing direction. When sectioned, the inner walls show a bright metallic color, sometimes with oxides or other inclusions. They are often symmetrically distributed on both sides and are more pronounced in thinner sheets, strips, or thin-walled tubes. Primary causes include ingot porosity, shrinkage voids, improper furnace atmosphere control during billet annealing, excessive furnace temperature, poor fit between the extrusion billet and container or pad, over-lubrication of the piercing needle, inadequate container cleanup, or cracked piercing needles.
06 Lamination
A gap appears in the thickness direction of the product, which separates into layers along the processing direction.
The bonding between layers is incomplete, with larger areas sometimes containing oxides or central impurities. This is often observed in thin strips or after welding thin strips. Fracture inspection of pipes or bars may also reveal delamination in the fracture surface.
The main causes of delamination are: trapped air holes, shrinkage, or central impurities in the ingot that persist through processing and form layers; improper distribution of reduction in hot rolling of plate and strip, with excessive reduction per pass; uneven heating of the billet, with temperatures too high or too low; excessive lubrication on the extrusion billet or piercing needle, cracked piercing needles, or incomplete removal of the extrusion shrinkage tail, which leads to delamination during subsequent stretching or rolling.
07 Corrosion
Corrosion is the process where the product surface reacts chemically or electrochemically with the surrounding medium, forming a film on the surface.
After corrosion, the surface loses its metallic luster and develops corrosion stains of various colors.
Corrosion forms when the product surface is not clean, leaving residual water, emulsion, or other liquids, or when storage conditions are improper—such as humid climates or water droplets contacting the surface. Additionally, a corrosive atmosphere in the environment can trigger corrosion.
08 Oxidation
Localized rupture and turning up of the product surface is called super skin. The appearance of needle-like metal filaments warping on the surface is known as starting system.
Super skin or starting system typically distributes continuously or intermittently along the rolling direction. Affected areas often contain oxide scale or other central impurities. In pipes and bars, super skin appears as tongue-like or fish-scale warping along the processing direction, which can become severe across the entire product, forming knots and sores. Thorny defects are more common in post-rolled products.
The main causes of super skin include: surface defects like cracks or unevenness on the billet; processing temperatures too high, causing surface oxidation, or previous process steps leaving performance traps; contamination from emulsion, water, or oil on the extrusion container or piercing needle; and tooling defects or mechanical damage during processing.
The main causes of starting system are: roll cracks and uneven billet edges with irregular depressions.
09 Wrinkles
Wrinkles are slight corrugated depressions and raised areas on the product surface, making it uneven.
Wrinkles generally align with the rolling direction or appear diagonally, as single or multiple bulges that feel rounded and cover a larger area. Main causes include uneven rolling, one-way bending straightness, uneven tension from rolls or sleeves, poor plasticity or significant eccentricity in pipe billets, non-uniform deformation in thick-walled pipes, excessive deformation, and poor roll adjustment.
10 Pitted Surface
A pitted surface shows tiny point-like depressions that create an uneven, rough texture. Pitting can be localized or continuous; individual pits are called pockmarks, and severe cases are termed pockmarked. Coarse grain-induced pitting is commonly known as orange peel. Main causes: annealing temperature too high or duration too long, leading to coarse grain; over-pickling; insufficient processing rate leaving the surface rough; non-uniform roll surface hardness, excessive wear, corrosion or oxidation of rolling tools, or contamination from foreign metal.
11 Mechanical Damage
External forces cause misalignment between layers of the product or sheet and foil, resulting in scars such as strips, bundles, craters, or sharp grooves on the surface. Common types include abrasions, scratches, bruises, and pressure wounds.
13 Uneven Composition, Uneven Organization
After rolling or annealing, significant differences in chemical composition across different parts of the product are called uneven composition. This can cause variations in color or deformation characteristics, visible segregation or segregation zones in macro-inspection, differences in microstructure and performance, and even cracking.