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 Corrosión
La corrosión es el proceso en el que la superficie del producto reacciona química o electroquímicamente con el medio circundante, formando una película en la superficie.
Después de la corrosión, la superficie pierde su brillo metálico y desarrolla manchas de corrosión de diversos colores.
La corrosión se forma cuando la superficie del producto no está limpia, dejando agua residual, emulsión u otros líquidos, o cuando las condiciones de almacenamiento son inadecuadas, como climas húmedos o gotas de agua en contacto con la superficie. Además, una atmósfera corrosiva en el ambiente puede desencadenar corrosión.
08 Oxidación
La ruptura localizada y el levantamiento de la superficie del producto se denomina piel superficial. La aparición de filamentos metálicos en forma de aguja deformados en la superficie se conoce como sistema de arranque.
La piel superficial o el sistema de arranque generalmente se distribuye de forma continua o intermitente a lo largo de la dirección de laminación. Las áreas afectadas a menudo contienen cascarilla de óxido u otras impurezas centrales. En tuberías y barras, la piel superficial aparece como deformación en forma de lengua o escama de pescado a lo largo de la dirección de procesamiento, que puede agravarse en todo el producto, formando nudos y llagas. Los defectos espinosos son más comunes en productos laminados posteriores.
Las principales causas de la piel superficial incluyen: defectos superficiales como grietas o irregularidades en el palanquín; temperaturas de procesamiento demasiado altas, que causan oxidación superficial, o pasos de proceso previos que dejan trampas de rendimiento; contaminación por emulsión, agua o aceite en el contenedor de extrusión o la aguja de perforación; y defectos de herramientas o daños mecánicos durante el procesamiento.
Las principales causas del sistema de arranque son: grietas en los rodillos y bordes irregulares del palanquín con depresiones irregulares.
09 Arrugas
Las arrugas son ligeras depresiones corrugadas y áreas elevadas en la superficie del producto, lo que la hace desigual.
Las arrugas generalmente se alinean con la dirección de laminación o aparecen en diagonal, como protuberancias únicas o múltiples que se sienten redondeadas y cubren un área más grande. Las causas principales incluyen laminación desigual, rectitud de flexión unidireccional, tensión desigual de los rodillos o manguitos, plasticidad deficiente o excentricidad significativa en palanquines de tubería, deformación no uniforme en tuberías de pared gruesa, deformación excesiva y ajuste deficiente de los rodillos.
10 Superficie Picada
Una superficie picada muestra pequeñas depresiones puntiformes que crean una textura desigual y rugosa. El picado puede ser localizado o continuo; los pozos individuales se llaman marcas de viruela, y los casos graves se denominan picados. El picado inducido por grano grueso se conoce comúnmente como piel de naranja. Causas principales: temperatura de recocido demasiado alta o duración demasiado larga, lo que lleva a grano grueso; decapado excesivo; tasa de procesamiento insuficiente que deja la superficie rugosa; dureza no uniforme de la superficie del rodillo, desgaste excesivo, corrosión u oxidación de las herramientas de laminación, o contaminación por metal extraño.
11 Daño Mecánico
Las fuerzas externas causan desalineación entre las capas del producto o lámina y hoja, resultando en cicatrices como tiras, haces, cráteres o ranuras afiladas en la superficie. Los tipos comunes incluyen abrasiones, rayones, magulladuras y heridas por presión.
13 Composición Desigual, Organización Desigual
Después de la laminación o el recocido, las diferencias significativas en la composición química en diferentes partes del producto se denominan composición desigual. Esto puede causar variaciones en el color o las características de deformación, segregación visible o zonas de segregación en la macroinspección, diferencias en la microestructura y el rendimiento, e incluso agrietamiento.