Failure and controls of metals

Wrought structure and workability in cold regions

Figure 25: critical curves, below we are safe, above we have failure. Notice in point (1) there’s a loss in ductility, this phenomenon will be described in detail in the following section.

Moreover, single-phase alloys generally feature the best hot-workability, which is decreased in the case of low-melting alloys and alloys containing insoluble phases. This is shown in the examples in Figure 97, from which we can notice the following:

• A rapid grain growth decreases the workability.
• Insoluble compounds decrease the workability, compared with soluble ones.
• Low-melting and brittle second phases decrease the workability, compared with ductile ones.

Hot brittleness in ferrous alloys

In several steels, marked loss of ductility is measured at intermediate temperatures, roughly within the temperature range 700-1100°C. In particular, the loss in ductility becomes very clear in micro-alloyed steels containing Ti, Nb.

V as well as N and Al. This is of greater importance (i) during straightening of continuouscasting billets and (ii) during hot plastic forming, because, in both cases, a low workability can induce crackingor other forms of damage to the material.

It was observed that in region II of the workability trend of wrought alloys fracture is always intergranularand it can occur by means of one of the following mechanisms:

1. Thin film of proeutectoid ferrite on grain boundaries. In the first mechanism, a thin film ofproeutectoid ferrite decorates the grain boundaries of the material. This promotes (i) strainlocalization in the ferrite thin film, which is softer than the grains and (ii) nucleation of voids at grainboundaries from micro-precipitates and second phases there located. This is shown in Figure.
2. single-phase

Austenitic precipitates at grain boundaries. Grain boundary cracking can occur even in the absence of a proeutectoid ferrite thin film. In this case, Nb (C,N and AlN particles precipitate at grain boundaries, causing a depletion of niobium adjacent to grain boundary, with the formation of a precipitate free zone (PFZ). The PFZ thus features a lower strength than the core of the grains, and this promotes the formation of microvoids.

This provides a justification for the workability gap in region II. Now, the trends in regions I and III have to be motivated as well. This can be done stating the following:

• Region I. At low temperature, the amount of ferrite is higher. This decreases the strain concentration in the film.
• Region III. Above the critical recrystallization temperature, microvoids are isolated from the other ones as soon as they nucleate, so that microvoid coalescence is prevented. The increasing temperature also motivates the increase in workability.

Effect of nitrogen

On hot brittleness. Nitrogen has a negative effect in hot brittleness of wrought alloys, due to the fact that titanium and aluminum nitrides increase the workability gap, making it deeper and wider. This is shown for aluminum in Figure below. In the case of titanium, things are more complicated, because titanium nitrides could also inhibit grain growth (thus with a positive effect) precipitating at grain boundaries. So, size and location of titanium nitrides has to be considered as well.

We can resume the issue of the workability gap in the following points:

• Region I. High amount of ferrite means low stress concentration.
• Region II. A) Low amount of ferrite produces thin films at grain boundaries, with a consequent stress concentration. B) In the absence of ferrite, precipitation at grain boundaries produces a precipitate-free zone with a depletion of niobium, thus reduced strength.
• Region III. Microvoids coalescence is prevented Region IB) Region IIA) Region III64Secondary

elements on grain size control

The grain size, resulting from hot working, strongly affects cracking susceptibility and ductility of alloys.

Several defects found in wrought alloys is therefore referred to uncontrolled or abnormal grain growth.

The grain size in a hot worked metal is ruled by:

• time spent at high temperature
• strain and strain rate imparted to the material
• temperature and strain gradients in the part volume

Secondary elements can be added to alloys in order to control the grain size. These secondary elements (such as Cr) form small precipitates, that control the grain size (i) stimulating recrystallization during deformation or (ii) controlling grain boundary migration. However, remarkable differences are observed if the composition is not carefully selected for this purpose. As a comparison, Figures below shows the effect of plastic deformation in an alloy with and without these secondary elements, respectively.

Figure 27: In the 2 figure: Al alloy

containing Cr after plastic deformation and annealing. The original fibrous structure is almost preserved and only few recrystallized grains are formed in the most heavily deformed regions. In the 3 figure: Al alloy without Cr after plastic deformation and annealing. The lack of Cr does not allow a proper control of grain growth. Effect of fibrous structure and texture Plastic deformation often stimulates the development of:

• texture, preferred crystallographic grain orientation (microscopic);
• fibrous structure is the alignment of different phases and secondary particles along plastic flow lines (macroscopic);
• banding phenomena (we will see more in depth below, similar to fibrous structure);
• The thermal treatment response is affected by the different hardenability achieved as a result of the local compositional variations in the banded structure.

The phenomenon of banding is a local enrichment in alloying elements along the plastic flow lines (we mentioned the

In Section 12, discussing lamellar tearing as a solidification defect). Banding is typically observed in two-phase alloys, such as ferritic-pearlitic steels. As a result of banding, different compositions are locally found. This affects thermal treatment response, and can possibly cause damage, because different hardenability properties are obtained for different local compositions.

On a macroscopic scale, the alignment of inclusions and constituents along the plastic flow lines generates a fibrous structure that is observed in hot forged and hot rolled parts. Such a structure emphasizes the strength properties along the fibers but also gives rise to marked depletion in toughness along the transverse-to-fiber direction, as already discussed in Paragraph 3 (see Figure 4, page 16, in particular).

The following figures supply some examples of the fibrous structure observed in engineering forged components and of a possible mechanism that can be adopted to mitigate the effects.

of the fibrous structure on surface of forged parts: straight-line flow pattern is negative from the point of view of the toughness, and can be broken up using corrugated surfaces, as in Figure.

Dead zones. Dead zones are regions of components where virtually no deformation is felt. This is a very negative condition, because differential accumulation of strain energy can cause differences in microstructural transformations (such as recrystallization, grain growth and phase precipitation), leading to possible damage. Possible locations of dead zones are shown in Figure below.

Oxidation products. Oxides formed on the surface can be embedded in the sub-surface metal leading to damage. This negative fact is reduced by means of descaling strands, which remove the surface oxide. This is shown in Figure. 66

Cold-forming defects. When dealing with cold deformation, attention should be paid to residual stresses. Indeed, much of the reasons for failures during cold forming accounts to uncontrolled build-up.

correct and if the material and manufacturing processes used were suitable for the intended application. It involves investigating the causes of failures and determining the appropriate corrective actions. Failure analysis can be conducted through various techniques, including visual inspection, non-destructive testing, and material testing. Visual inspection involves examining the failed component or structure to identify any visible signs of damage or deformation. Non-destructive testing methods, such as ultrasonic testing or X-ray inspection, can be used to detect internal defects or cracks. Material testing involves analyzing the mechanical properties of the failed material to determine its strength, ductility, and other relevant characteristics. The results of failure analysis can provide valuable insights into the design, manufacturing, and material selection processes. It can help identify any design flaws, manufacturing defects, or material deficiencies that may have contributed to the failure. Based on these findings, appropriate measures can be taken to prevent similar failures in the future, such as modifying the design, improving manufacturing processes, or selecting a different material. In conclusion, failure analysis is an important tool in understanding and preventing failures in plastic deformation processing. It helps ensure the reliability and safety of components and structures, and contributes to the continuous improvement of manufacturing processes.correct in termsof loading history, material properties, defects related to manufacturing processes and service environment.The ultimate activity is to establish the source of the failure and who is responsible for it. The general approach to failure analysis The general approach to failure analysis can be described by the following steps: 1. Measurement of materials properties on samples extracted from the failed structure. 2. Check of the actual loading conditions. 3. Identification of the theoretical failure conditions on the basis of the expected service load. 4. Comparison of the limiting stress with material properties. Very often, fracture occurs due to the presence of defects in the component. Limitation of load bearingability can be accounted to different types of defects: a. materials defects: coarse inclusions / brittle second phases / grain boundary precipitates; porosity /cracks; internal stresses; thermal cracks / decarburization; machining marks / other surface damage induced by

manufacturing cycles

b. design or service defects: improper consideration of notch effects; underestimation of service loads; improper evaluation of material stress condition

c. environmental effects: wear; corrosion; creep; high temperature

Macroscopic analysis

A preliminary visual inspection is usually used in order to identify the critical defect that led to failure. The detection of the main defect could be challenging if the structure is big, complicated and/or if failure occurred catastrophically or in an aggressive environment. Analytical methods and simulations are available to collect information about the actual stress state of the component.

Microscopic analysis

After the macroscopic inspection, samples are extracted which are believed to contain the nucleation sites. Optical and electron microscopy is used afterwards.

Extraction and handling of samples for failure analysis

Several rules must be respected when handling samples for failure analysis in order not to contaminate them. For instance,

fracture