Hardening, 1.25

The mechanism of the hardening processes has been investigated by Zinke in Austria, von Euler in Sweden and Hultzsch in Germany using blocked methylol phenols so that only small isolable products would be obtained. 

As these condensation reactions can occur at the two ortho and the para positions in phenol, w-cresol and 3,5-xylenol, cross-linked structures will be formed. It has been pointed out by Megson that because of steric hindrance the amount of cross-linking that can take place is much less than would involve the three reactive groups of all the phenolic molecules. It is now generally considered that the amount of cross-linking that actually takes place is less than was at one time believed to be the case. 

These quinone methide structures are capable of polymerisation and of other chemical reactions. 

It is likely that the quinone methide and related structures formed at these temperatures account for the dark colour of phenolic comjjression mouldings. It is to be noted that cast phenol-formaldehyde resins, which are hardened at much 

As already stated, flow rules are a tool to determine the stresses from a given strain rate. They do not allow to calculate how the yield surface changes by hardening (see section 3.3.1). This task is performed by hardening laws, to be discussed now. 

The yield surface can change its shape, size, and position in stress space. Mathematically, this can be realised by modifying the yield criterion (3.23) with additional terms that describe the change of the yield surface (the modified yield surface is denoted as g) [115]  

is the current plastic deformation, and ki is a set of hardening parameters which may depend on the deformation history, the strain rate, or the temperature. As in the original yield criterion, the material deforms elastically if the stress state hes with the surface = 0 i. e., if 0 holds. 

To take hardening into account, we need to find a quantity that can describe the deformation history of the material. This quantity has to increase during plastic deformation, regardless of the deformation orientation, for, in general, any plastic deformation causes hardening. A frequently used quantity is the so-called equivalent plastic strain To define this strain, we need 

Since the equivalent plastic strain rate defined this way is positive for all plastic strain rates, the equivalent plastic strain increases for any plastic deformation, regardless of the deformation orientation. 

When ice cream leaves the factory freezer at about — 5 °C, its ice content is only about half that at a typical serving temperature of —18 °C, so it is very soft. As shown in Chapter 2, the microstructure of dispersed ice crystals and air bubbles is thermodynamically unstable -the system tends towards a state in which the phases are less dispersed. If the ice cream were simply stored at the factory freezer outlet temperature it would deteriorate very quickly. The ice crystals and air bubbles would coarsen their mean size would increase and their total number would decrease. Since it is not possible to stabilize the microstructure thermodynamically, the best that can be achieved is to trap it kinetically, i.e. to slow down the rate at which coarsening occurs, so that significant 

Ice cream is hardened in a hardening tunnel, an enclosed chamber into which the ice cream passes on a conveyor belt from the factory freezer. Inside, cold air (typically —30 °C to —45 °C) is blown over the ice cream. The lower the air temperature, and the faster the air flow, the faster heat is removed from the ice cream. Air turbulence also increases the rate of heat transfer. The chamber is enclosed to minimize exchange of cold air inside the system with warm ambient air, and so to reduce the build up of frost that would reduce the efficiency. Cold stores, which are typically about — 25°C, are not suitable for hardening because they are not cold enough and have still air, so they cannot cool the ice cream rapidly enough to minimize recrystallization. 

Electrochemically deposited composites containing particles of refractory compounds, like oxides,4 11 16 nitrides,15 17 18 carbides14 1519 23 or borides24 

Dispersion hardening or strengthening of a material means an increased resistance to deformation. The movement of dislocations in the metal facilitates metal deformation. Incorporated particles block the dislocation movement and thus strengthen the metal.4,11 12,21 Grain refinement of the metal due to the codeposition of particles has also been thought to contribute to the hardening effect, but this is not supported by experimental evidence. For several composites it was found that the grain structure of the metal matrix was not altered by the codeposition of particles. 

A heat treatment of as-plated composites is sometimes5,19 considered necessary to achieve maximum hardening. Reported investigations4,11,12,23 do not show a significant improvement in the mechanical properties of annealed composites, but composites do retain their strength up to higher 

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For concrete work in cold weather a specially rapid-hardening cement may be made by addition of calcium chloride to the cement. 

Photographic material containing gelatin can be hardened during manufacture the process involves cross-linking between the gelatine polypeptide chains induced by hardener. 

In case hardening C and N are introduced into the iron by heating in a Nj —NH3 atmosphere or by immersing the steel in molten NaCN/Na2C03 at about 900"C. 

Austenitic steels retain the ccp structure right down to room temperature. For this reason these steels cannot be hardened by quenching. 

Hardening per RTFOT (Rolling thin-film ouen test) NF T 66-032/ASTM D 2872... 

Once the cementation has been completed the rig will wait on cement (V OC), i.e. wait until the cement hardens prior to running in with a new assembly to drill out the plugs, float collar and shoe, all of which are made of easily drillable materials. 

The chemistry of cement slurries is complex. Additives will be used to ensure the slurry remains pumpable long enough at the prevailing downhole pressures and temperatures but sets (hardens) quickly enough to avoid unnecessary delays in the drilling of the next hole section. The cement also has to attain sufficient compressive strength to withstand the forces exerted by the formation over time. A spacer fluid is often pumped ahead of the slurry to clean the borehole of mudcake and thereby achieve a better cement bond between formation and cement. 

Fig. 1 Beginning stage of pitting origin (hardened structural steel)...

Correction for beam hardening artefacts in computerised tomography... 

Inspection of frame equipment and pipe - lines for the presence of the fatigue, hardening, and other cracks, corrosion. 

At the anode, a chemical oxidation reaction is bound to take place. In normal fixers, sulfite (SOj ) is oxidized and acid (H ) is released as a consequence of this oxidation. Due to the decrease of the sulfite concentration and the decrease in the pH, the fixing solution becomes unstable and sulfur precipitation starts to occur when the pH of the fixer decreases below 4.0. In the case of hardening fixers, there is also an upper limit to the pH, since aluminum-hydroxides starts to precipitate when the pH exceeds 5.0. 

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