Defects in Thermo Mechanical Processing of Metals


Defects in Thermo Mechanical Processing of Metals 

Thermo mechanical processing of materials is a technique designed to improve the mechanical properties by controlling the hot-deformation process. This was originally designed to produce the required external shape of the product. Controlled rolling, controlled-cooling and direct-quenching are typical examples of thermo mechanical processing. Such processing saves energy in the manufacture of steel by minimizing or even eliminating the heat treatment after hot-deformation, thus increasing the productivity for high grade steels. It normally requires a change in alloy design and often reduces the productivity of the hot deformation process itself, but at the same time makes it possible to reduce the total amount of alloying additions and to improve weldability, while sometimes producing new and beneficial characteristics in the steel.

Thermo mechanical processing is the sophisticated combination of well-defined deformation operations and well-defined heat treatment in a single production stage to control the microstructure of the material being formed. It produces materials with the desired external qualities (dimensions, shape and surface quality) and acceptable mechanical properties. The process is normally considered as the final stage in the production of steels.

Thermo mechanical process defects are usually focused on individual forming technique. The defects generally range from mostly macroscopic ‘form and fracture’ related defects to defects related to strain localizations, as well as imperfections related to microstructure.  The defects in case of thermo mechanical processing have two possible origins namely (i) process related, and/or (ii) metallurgical.

The first one is usually related fully to the practices of the thermo mechanical processes including the forming techniques and the heat treatment, while the metallurgical origin defects can range from the starting solidification structure to structural developments during thermo mechanical process. It is difficult to establish a clear demarcation between the two origins. As an example, the defect of so called roll marks (a surface defect) can include work roll marks, work roll pin holes, chatter marks or any other repetitive structures and can be of both technological or process related, and of metallurgical origin.

Basic classification of common defects in thermo mechanical process is given below. It includes the nature (macroscopic or microscopic) and possible origin (process related or metallurgical) of the defects.

  • Form defects – They are incomplete/improper macroscopic metal flow leading to improper shape/size of the defects. They are mostly macroscopic with both of process related origin and metallurgical origin.
  • Surface defects – These defects are restricted to surface or near surface regions. They are both of process related origin and metallurgical origin.
  • Fracture-related defects – These defects constitute total or partial fracture. They are macroscopic with both of process related origin and metallurgical origin.
  • Strain localizations – This is a generic name used for any plastic instability. These defects are both of process related origin and metallurgical origin.
  • Structural defects – These defects are due to inappropriate structure causing inferior properties. These defects are mostly of metallurgical origin.

Some thermo mechanical defects of process related origin are given below.

  • Machine related – These defects are due to inappropriate forces either too small or too large.
  • Process parameter related – These defects are due to inappropriate strain, strain rate, or/and working temperature etc.
  • Die tool related – These defects include die tool marks, stress raisers etc.
  • Tribology related – These defects are due to inappropriate lubrication and friction experienced during processing.
  • Environment related – These defects are due to scaling, internal oxidation, and decarburization etc.

Some thermo mechanical defects of metallurgical origins are given below.

  • Starting structure – Theses defects include segregation, piping, and porosity of solidification structure.
  • Structural developments during processing – These defects are due to development of inappropriate structure during processing (strain localizations etc.)
  • Final structure – These defects are due to inappropriate property of the final structure.

Form defects

During thermo-mechanical processing, various types of form defects in size and shape of the product occurs.  The types of these defects, the reasons for their occurrence and the remedial measures are given in Tab 1.

Tab 1 Types of form defects, their process-origin and corrective measures
Sl.No. Defect type  Description Reason for defect Remedial measures
1 Bow, side and longitudinal bend Sidewise/longitudinal bending Inaccurate guiding or alignment in rolling/slitting Use of leveling or straightening rolling
2 Buckling Bending under compressive stresses Inaccurate stress distribution during forming process Straightening
3 Camber Curvature in rolled products Due to roll bending at the middle of the rolls during process of rolling Compensated by varying the roll diameter across the roll length
4 Cobbling Accidental distortion in shape through re-rolling Front end being caught and rolled again during rolling Reduced rolling speed
5 Exfoliation Disjoining during fabrication Pipes, unwelded blowholes, severe inclusion content during any forming process Control of solidification structure
6 Overfill/Underfill Inappropriate (over or under) metal flow Over or under feed of metal during Extrusion/drawing Control of feed rate
7 Warping Distortion or warpage Fast heating/cooling during any warm or hot forming Control of thermal history

A modern plant normally has level of automation which frequently incorporates machine/process-related corrective measures. Freedom from major form defects is mainly ensured by proper process optimization which is achieved either through numerical or physical modeling/simulation.

Surface defects

The surface defects are usually characterized as (i) deformation or forming process induced, (ii) environment induced, (iii) coating related, and (iv) induced by solidification structure. Some of the surface defects are described below.

Deformation or forming process-induced surface defects

These defects include seams and slivers caused by surface inclusions, snake skin or shark skin caused by tribological factors, wrinkling, and a range of plastic instabilities or strain localizations. Typical examples of the surface plastic instabilities are Lüder bands, orange peel and roping.

Lüder bands – They are also known as Lüder lines or stretcher strain marks. They become visible after press forming of a sheet and have a typical ‘flame-like’ pattern. This surface pattern in most applications is unacceptable for visible parts. The phenomenon is well known in most steels and in certain aluminum alloys. Lüder bands can easily be detected with a simple tensile test. Materials which are prone to form Lüder bands during press forming always show a ‘yield point elongation’ or ‘Lüder strain’ in the stress–strain curves obtained in a tensile test. The reason for these Lüder strains is the pinning of dislocations by carbon atoms in steel or by substitutional atoms in aluminum. In steel, the effect can be eliminated by giving the skin pass rolling or by roll leveling (bending/unbending). This liberates the dislocations from their pinning points. In aluminum, the effect can be avoided by grain size control (a grain size of above 10 micro meter to15 micro meter is needed).

Repetitive plastic yielding is observed under certain deformation conditions such as conditions with negative strain rate sensitivity. This is also called as dynamic strain aging which means dynamic interactions between mobile dislocations and solute atoms. This is also known as Portevin–LeChatelier (PLC) effect. This has been originally reported as ‘blue brittleness’ in mild steel and the effect is usually felt as visible bands on the product surface and also as serrated flow in a typical stress–strain diagram. .

The selection of exact alloy system and appropriate conditions of thermo mechanical process are often decided based on the severity and the nature of Lüder bands and PLC. There still exist controversies over the Lüder band and PLC terminology and on their mechanisms of formation.

effects-of-strain-aging

 Fig 1 (a) Generalizing the effects of strain aging, Lüder bands and PLC on a stress–strain diagram (b) Distinct patterns of the jerky flow during PLC (c) Lüder band progressing along the surface of a tensile sample

In Fig1 (a), serrations after the first yield point elongation is called Lüder band /strain or stretcher strain marks or type ‘A’ bands, while serrations after Lüder bands are called PLC or type B bands. On the other hand, in Fig 1 b serrated or jerky flow, associated with both Lüder bands and PLC, are generalized into following three categories namely

  • Type A – It appears as regular and equidistant ‘drops’ in a stress–strain diagram. The mechanism is bands nucleating and propagating as solitary plastic waves. The differentiation is mainly phenomenological.
  • Type B – It appears as C and A superimposed on each other. The mechanism is oscillatory or intermittent propagation.
  • Type C- It appears as heavily serrated flow with abrupt oscillations. The mechanism is stochastic and random nucleation without propagation.

Orange peel – Plastic deformation, such as stretching, bending and drawing, can develop rough surface which is usually termed as ‘orange peel’. Coarse surface grains are usually considered to have lesser constraints to plastic deformation. This, on the other hand, can cause severe non-uniform deformation between the surface grains, leading to an apparent roughening or an ‘orange peel’ condition. The development of surface roughness is dependent on the surface grain size with the effect being negligible at finer grain sizes in aluminum. Though cluster of small grains of the similar crystallographic orientations can act as individual coarse grain(s) and create ‘orange peel’ effects, there is no clear relationship between ‘orange peel’ and the crystallographic orientations of the surface grains.

Roping – Roping or looper lines, is also a phenomenon of surface roughening, witnessed during deep drawing. The name roping is linked to characteristic ‘roping’ marks on drawn shapes. The roping is also caused by non-uniform deformation, non-uniformity linked to irregularities or heterogeneities in the structure. As an example, macro-segregation from the cast structure can produce striated structure or surface banding or roping marks. Alternatively, the roping can also develop from deformed grains or bands of fine grains of similar orientations. In both the cases roping can originate from earlier coarse grains.

Wrinkling is the formation of surface roughness. Wrinkles are caused by internal compressive stresses, plastic as well as elastic. It is usually observed in the flange, but is also reported in the free-forming zone between die and tool, when the minor stress of sheet metal forming is compressive in nature. Generally, wrinkling is more severe in metals with lower normal anisotropy (values). Wrinkling also depends on tooling, elastic modulus and sheet thickness and is normally considered as a complicated phenomenon, exact initiation and growth of wrinkling limit being difficult to predict theoretically.

To avoid wrinkling, usually the blank-holder pressure is increased, which, on the other hand, introduces additional strain in the sheet metal and hence can provide a narrow processing window between wrinkling limit and failure limit. An alternative exists by forming at elevated temperatures. Thermal shrinking of sheet metal is attributed to reduce compressive stresses and the resulting wrinkling, while elevated temperature forming also improves drawability by reducing the yield strength. Alternate forming technologies, e.g. hydroforming and electro-magnetic forming, are often ‘recommended’ as techniques with lesser ‘wrinkling’.

Environment-induced surface defects

Environment-induced surface defects are frequently related to heat-treatment practices. These defects can range from formation of oxide scale to depletion of alloying elements (e.g. decarburization in steel) and internal oxidation.

Oxide scale is almost always associated with the hot working. Except for highly reactive and expensive metallic systems, where jacketing (e.g. Zr alloys) or protective coating (e.g. glass coating) is used, the oxide scale is typically a part of any thermo mechanical process practice of warm/hot working. The oxide scale is often complex. For example, in hot steel, the scale consists of three layers namely (i) inner wüstite (FeO), (ii) intermediate magnetite (Fe3O4), and (iii) outer hematite (Fe2O3), with interesting surface characteristics. The scale has strong implications to die-tool and metal interactions and to heat transfer. But the more important technologically is the control and removal of oxide scale. High-pressure water jets are an integral part of a modern hot rolling mill where water jets are used to control the severity of oxide scale. Even then an entire pickling process and the technology of pickling are usually associated with the thermo mechanical process technology. Improper pickling leads to surface defects (such as pickling blisters etc.) and even to embrittlement.

Decarburization in special steels during the thermo mechanical process practices lead to subsequent loss of hardenability, especially in surface/sub-surface layers. Preventive measures can range from reduced time/temperature of heat treatment to use of protective atmosphere and coating the stock with suitable (which may reduce oxygen potential at the surface) refractory compound.

Internal oxidation – Though internal oxidation which is formation of relatively fine sub-surface oxide inclusions, is mainly talked about in steel. Similar phenomena have also been reported in silver-aluminum, copper-aluminum and silver- indium alloys. Such mechanism has also been attributed to the formation of nitrogen, sulphur, selenium and tellurium bearing internal inclusions. The mechanism and kinetics of internal oxidation is fairly well studied. The stability of internally oxidized zone can best be described in terms of relative fluxes of oxygen and metal and the number of oxygen atoms per metal atom of the oxide compound.

Usually the internal oxidation is harmful and can be classified as a surface/subsurface defect strongly affecting the property. As an example, in non-grain-oriented electrical steel, a substantial decrease in magnetic properties can result from internal oxidation of laminations. Similarly, internal oxidation in low-carbon steel for packaging/decorative applications can have disastrous effects. Though there have also been discussions on employing internal oxidation in synthesizing oxide-dispersed steels for high-temperature applications, yet a clear technology remains to be formulated.

Surface defects related to coating – Coatings of metals and alloys are very common. The coatings can range from organic and inorganic coatings used for specialized applications to zinc and zinc-aluminum coatings (e.g. galvanizing etc.). The coatings can relate to surface defects such as defects during coating / handling and defects originating from subsequent (i.e. after coating) forming. The former can include scratches, fingerprints and crevice/pitting corrosion, while the latter can frequently be associated with ‘galling’. Forming operations subsequent to coating can remove the coating at some locations. The origin of this defect or ‘galling’ is due to adherence of coating to the die-tool surface. This is primarily a tribological problem and usual remedies involve appropriate lubrication and/or application of suitable low friction wear-resistant coating(s) to the tools.

Fracture related defects

The science of fracture mechanics has evolved significantly over the past few decades, still it is difficult to relate macroscopic fracture with exact microstructural origin. . Different yield criteria, isotropic as well as anisotropic, have been reasonably successful in predicting macroscopic necking and fracture behaviour in sheet metal forming. Relating such microstructural features with exact microstructural causes is, however, far more difficult.

On the other hand, the fracture related defects generally mark the limit for any forming operation. The typical example is the forming limit diagram schematically shown in Fig 2. The A forming limit diagram indicates how much deformation a material can take before it fails. As an example, when a material is bi-axially stretched along path ‘a’ till strain state ‘A’, no failure is predicted. However, following strain path ‘b’ till point ‘B’ causes failure as soon as the strain state reaches the forming limit curve. Failure is considered here as the point where a local neck develops. The real fracture of the material occurs immediately after the development of a local neck at a slightly higher strain. In the region where thickening of the sheet occurs, the sheet fails not only by local necking, but wrinkling is also likely to occur. This is not indicated in most forming limit diagrams. Along certain strain paths, e.g. path ‘c’, fracture (in this case a so called ‘shear fracture’) can occur before the actual forming limit diagram is reached. Also this line is often not indicated in a simple forming limit diagram.

forming-limiting-diagram

Fig 2 Schematic illustration of a forming limit diagram (the thick black line)

Categorizing of all the fracture-related thermo mechanical process defects is a big job. However the four common fracture-related defects namely (i) edge cracking, (ii) alligatoring, (iii) centre burst, and (iv) wire drawing split are described below.

Edge cracking is a common difficulty in compressive bulk-forming processes, such as rolling and forging. Though the severity of edge cracking does not cause complete fracture, but these phenomena can turn the thermo mechanically process product unusable or can result into expensive material removal. Typically, edge cracking involves appearance or initiation of small surface cracks at the edge and subsequent propagation of these cracks, often along the transverse direction, into the bulk of the material. The causes of edge cracking namely metallurgical or process related (uneven deformation and corresponding variations in stresses) are described below.

  • Limited ductility or workability – The limited ductility is inherent to the material or caused by some microstructural feature. Examples are large prior transformation grain size, presence of embrittling phases or elements and inclusions etc.
  • Camber – Heavily cambered rolls in wide-strip rolling can create edge cracking,
  • Spread – Deformation along the roll-axis or transverse direction is called spread. It depends on workability and width to thickness ratio of rolling/forging and frictional conditions. Excessive spread can cause edge cracking.
  • Edge shape – Inappropriate prior-deformation edge shape can also lead to edge cracking.
  • Friction and pass sequence – Inappropriate friction and pass sequence leading severe inhomogeneity in deformation can also cause edge cracking.

Alligatoring initiates as a crack along the centre plane of the rolled material and can vary in severity i.e. from partial separation of the upper and lower halves of the rolled material to a complete separation and even tangling on the rolls. Origin of alligatoring is inhomogeneous deformation i.e. severe forms are reported for a combination of inhomogeneous deformation and non-uniform recrystallization during primary rolling in aluminum magnesium alloys, zinc and copper based alloys.

Central burst or chevroning is the formation of internal voids at the centre. This defect can occur in rolling, extrusion and wire drawing. A similar phenomenon has also been reported in forging. Poor die design and structural heterogeneity are the primary causes. The same alloy with slight difference in composition and a correspondingly higher strain hardening can avoid central burst. Small reductions and large die angle also reduce this phenomenon.

Wire drawing split during high-speed wire drawing can be disastrous to productivity. The origin of split can be both process related and metallurgical i.e. contributions from different metallurgical and process parameters being possible. Remedial measures through changing process parameters, such as drawing speed, are possible. The split can often be subjective to a particular batch. Examples of the apparent causes for split in wire drawn metals are (i) split in patented wire through localized shear bands, (ii) split in copper wire through a combination of back-tension and relative inclusion presence, split in ferrite–pearlite–martensitic steel wire through crack nucleation at the ferrite-martensite interface, and (iv) split in tungsten wire through possible crack nucleation at the grain boundaries along transverse direction.

Strain localization

Strain localizations can be both microscopic as well as macroscopic. These can be identified through clear patterns of localized plastic flow and/or as distinct dislocation substructure. These are often responsible for the initiation of micro-cracks, ultimately leading to fracture.  These are considered as deformation heterogeneities or plastic instabilities and the instability criteria can be used to describe the formation of strain localizations. For example, a suitable instability criterion is also used in identifying the safe process regimes in a processing map. The criterion can be formulated either from localized deformation or from softening. A much cited example of the latter is the so-called Dillamore’s criteria. The criteria are simple in its approach and have been successfully used in describing the macroscopic angles of shear bands and also in describing their preferred appearance at certain orientation(s).

Structural defects

The objective of thermo mechanical processing is two-fold namely (i) to achieve defect-free desired size and shape, and (ii) to achieve a desired property. Defects relevant to the second objective typically relate to the structural defects. They are more specifically microstructure-related defects. In other words, the inability to achieve a desired microstructure and hence required microstructure related property can also be considered as a thermo mechanical process defect.  The defects as per defect structures are as follows.

  • Composition – This is the segregation or difference in composition. Depending on the scale, this can be termed as macro or micro segregation.
  • Point – These are vacancies both of interstitial and substitutional atoms
  • Line – These are dislocations. For thermo mechanical processing, of interest is dislocation sub-structure.
  • Planar – These are stacking faults, solid–vapour interface, grain and phase boundaries.
  • Volume – These are voids (includes gas bubbles and cavities) and inclusions

Segregation is the variations of composition over distances comparable to the dendrite arm spacing happen as a direct effect of ‘solute partitioning’. This is the basis of the so-called micro-segregation. Macro segregation, on the other hand, can be formed by combination of factors, ranging from shrinkage to convection currents and density differences.

Non-metallic or ceramic inclusions are often a by-product of a casting process originating from furnace / casting refractories and/or through reactions in the melt. The nature and the severity of such inclusions are usually identified by comparing unetched microstructures with standard charts. It is, however, important to point out that modern liquid metal processing routes ensure much better inclusion control than was feasible a few decades ago.

Gas porosity is related to the solubilities of gasses in the liquid metal.  The solubilities of gasses in liquid metals are orders of magnitude more than in the solid castings. The dissolved gasses in the melt can leave gas porosities and also inclusions, the latter in the form of reaction products. Various vacuum and inert gas melting and degassing techniques are primarily applied to control the content of harmful gasses in the melt.