Overheating occurs when metal is held at too high a temperature for too long during heating or processing, causing the structure and grains to coarsen. Overburning occurs when metal is heated close to its melting temperature — or overheated severely — so that low-melting-point constituents melt locally at the grain boundaries and the boundaries weaken.
Overheated metal shows a rough, pitted surface, an orange-peel effect, and coarse grains. Overheating does not lower alloy strength much, but it sharply cuts room-temperature impact toughness and ductility. The material turns brittle, and fractures show coarse crystalline facets. Under high magnification the structure may show coarse equiaxed grains, a coarse second phase, or a coarse Widmanstätten structure. Overheating is not always scrap — the material can sometimes be reworked or downgraded through further deformation or reprocessing.
Overburned surfaces are rough, with coarser, straighter grain boundaries that look hairline-like, and may show cracks and an easily melted, oxidized film. Overburning greatly reduces the metal's cohesion. Microscopically, grain boundaries coarsen and show melted cavities or eutectic spheres, a network of melted liquid phase, and irregular cavities where several grains meet. After rolling or extrusion, cracks appear along grain boundaries — plate edge cracks, rod ends that 'blossom' open, open-mouth cracks, or pieces cracked completely apart — and the cracked surfaces reveal coarse dendrites and melting traces.
The main causes of overheating and overburning are: heating temperature too high or heating time too long, or local exposure to a high-temperature source for too long; finishing temperature too high, or the metal staying too long in the high-temperature zone; and alloys containing low-melting-point elements or large amounts of low-melting-point inclusions.
02 Cracks and Cracking
(1) Processing cracks. Poor processing practice or process defects can cause processing cracks. They fall into two groups — hot cracks and cold cracks — distinguished by the microstructure around the crack: hot cracks show no obvious change around the fracture, while cold cracks show severe deformation. By appearance, cracks can be longitudinal, transverse, side, open-mouth, spiral, periodically transverse, 45° diagonal, or irregular; by location, they can be local, head-end, surface, center, or intergranular.
Processing cracks must be analyzed from two angles: the metal itself and the processing technology.
Factors related to the metal itself include: alloy composition and impurity content; ingot surface defects such as cold shuts, scabs, and segregation spots, plus internal defects such as shrinkage, porosity, inclusions, and segregation — all of which tend to develop into cracks during processing; a strongly developed columnar structure that creates weak planes prone to cracking along grain boundaries; and defects carried over from previous processing, such as existing cracks, overburning, overheating, poorly distributed second phases, non-uniform structure, and micro-cracks caused by phase transformation.
(2) Heat treatment cracks
(1) When the alloy carries large residual stress — for example, when heating-induced thermal stress and residual stress act in the same direction and together exceed the strength of the metal — cracking results.
(2) When a second phase precipitates along grain boundaries during heating and generates additional stress, or when phase transformation produces pronounced structural stress, cracking can occur if the combined stress exceeds the alloy's strength.
03 Inclusions
Inclusions are metal or non-metal materials that appear on the surface or inside the matrix of processed products, with a clear interface and different properties from the base metal.
Inclusions in processed products come mainly from unmelted metal particles in the ingot that grow into large primary crystals, from foreign metal entering the melt, and from metal compounds, silicates, and covering agents.
Inclusions in wrought products are basically the same in classification and characteristics as those in castings. They are usually granular, elongated along the processing direction, with a definite form and color and a clear interface, and can be found under a metallurgical microscope without etching.
04 Foreign-Matter Indentation
Foreign-matter indentation is metal or non-metal material pressed into the surface of a product.
Metal indentations have a clear interface with the base metal, distinct contours, and a different metallic luster; they are point- or block-shaped, and leave pits when they peel off. Non-metal indentations vary in form and color, are more brittle, have no metallic luster, and are distributed as points, sheets, or strips along the processing direction; they are harder to peel off.
Sources of foreign matter include: oxide scale formed by heating; lubricant sticking to the surfaces of dies and products; fragments of dies and adhered material; metal chips, burrs, and fringes from cracked edges; furnace debris and oil; debris in the emulsion; and foreign matter in the rolling pass and on the guide plate.
05 Bubbling
Bubbling appears after processing or annealing as strip-shaped or bubble-shaped bulges distributed along the processing direction; when cut open, the bulge is a cavity.
Blisters are mostly strip-shaped with a smooth surface, elongated along the processing direction; the inner wall of the opened blister has a bright metallic color, sometimes with oxides or other inclusions. Blisters usually appear symmetrically on both sides and are most visible in thin sheet, strip, and thin-walled tube. They are typically caused by porosity or shrinkage cavities in the ingot; improper furnace atmosphere control or excessive temperature during billet annealing; poor fit between the ingot, extrusion container, and extrusion pads; excessive lubrication on the container or piercing mandrel; an unclean extrusion container; or a cracked piercing mandrel.
06 Lamination
Lamination is a separation into layers along the processing direction, appearing as gaps in the thickness of the product.
The separation runs between layers that were never fully bonded; the affected area is usually large and may contain oxide or centerline impurities. It often shows up in thin strip, or after thin strip is welded. Fracture testing of pipe and bar stock also reveals delamination.
Lamination is usually caused by: porosity, shrinkage cavities, or centerline impurities in the ingot that persist into the worked product and open into laminations; improper pass distribution or excessive reduction in hot rolling of plate and strip; non-uniform billet heating, or heating temperature too high or too low; excessive lubrication on the billet or piercing mandrel in tube and bar extrusion; a cracked piercing mandrel; or an extrusion shrinkage tail that is not fully cropped before further drawing and rolling.
07 Corrosion
Corrosion is the chemical or electrochemical reaction between the product surface and the surrounding medium, which forms a film on the surface.
After corrosion, the surface loses its metallic luster and develops corrosion stains of different colors.
Corrosion is caused by: a product surface that is not clean, with residual water, emulsion, or other liquids, or improper storage where a humid climate or water droplets reach the surface; and a corrosive atmosphere in the environment.
08 Peeling and Splinters
Local rupture that turns up on the product surface is called peeling. Needle-like metal filaments that curl up on the surface are called splinters.
Peeling and splinters are distributed continuously or intermittently along the rolling direction. Peeled areas often carry oxide scale or other foreign matter and centerline impurities. On pipe and bar, peeling forms tongue- or fish-scale-like warping along the processing direction; in severe cases it runs through the product or forms local knots and sores. Splinters appear mainly in rolled products during post-processing.
Peeling is usually caused by: a surface defect such as a hairline crack or an uneven surface on the billet; processing temperatures so high that the product surface oxidizes, or surface defects carried over from a previous process; water, oil, and other dirt on the rolls, extrusion container, or piercing mandrel; and surface defects or mechanical damage from the processing tools.
Splinters are usually caused by: cracks in the rolls; and billet edges that are not flat or have irregular depressions.
09 Wrinkles
Wrinkles are slight corrugated depressions and raised areas that leave the product surface uneven.
Wrinkles generally run along the rolling direction or diagonally, as single or multiple bulges with a rounded feel and a larger area. They are usually caused by: uneven rolling, or one-way bending without proper straightening; uneven tension (uneven tension rolls or tension sleeves); poor plasticity or severe eccentricity in the tube blank; non-uniform or excessive deformation in thick-walled tube; and process issues such as poor roll adjustment.
10 Pitting
Small, point-like depressions that leave the surface uneven and rough are called pitting. Pitting can be local or continuous; individual pits are called pockmarks, and a heavily pitted surface is described as pockmarked. When coarse grains are the cause, the condition is commonly called orange peel. Pitting is usually caused by: annealing temperature too high or time too long, producing coarse grains; over-pickling; too small a processing reduction or a surface that is not smooth; and rolls with non-uniform surface hardness, serious wear, corrosion, oxidation, or foreign metal on the rolls.
11 Mechanical Damage
Mechanical damage occurs when external force displaces layers of the product or of sheet and foil stock, leaving scars in the form of strips, clusters, craters, and sharp grooves. Common examples are abrasions, scratches, bruises, and pressure marks.
13 Uneven Composition and Uneven Structure
When different areas of a rolled or annealed product show large differences in chemical composition, the condition is called uneven composition. It can cause differences in color or deformation behavior; macro inspection may reveal segregated bands or zones, differences in microstructure and properties, and in severe cases, cracking.
Products after rolling or annealing, different parts of the chemical composition of the phenomenon of large differences is called uneven composition. Uneven composition can cause differences in colour or deformation characteristics, macro-inspection can be seen in the composition of segregation or segregation area, microstructure and performance differences, and even cracking.
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