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In this way refining of ferritic grain is a very attractive processing route.
There are a large number of welding processes but, among these, the fusion welding processes are the most used in the industry.
Grain growth might affect directly the mechanical strength of the ultra-fine grained steel.
It's possible to identify the recrystallized ferrite grains on figure 3(b).
Weld speed (cm/min) Heat input (kJ/cm) Number of tests ∆t800/500 (s) Average Standard deviation 60 2.4 4 0.6 0.13 40 3.6 2 1.3 0.02 20 7.2 3 4.3 1.80 10 14.4 6 17.4 3.32 Table 2.

In terms of the microstructural evolution, lower density of defects on the grain boundary were observed and the nano-grains can be maintained about 100 nm even when annealed for 30 h at 700 oC, which suggests high thermal stability at this temperature.
Introduction Bulk nano-grained (NG) materials, which are structurally characterized by nano-grains with a large number of grain boundaries, have been found to exhibit many exceptional physical, chemical and mechanical properties relative to their coarse-grained counterparts[1-3].
In the CRed Ni-based alloy, the large diffusion coefficient and large number of nucleation sites resulting from the severe cold rolling allow for rapid precipitation of a fine dispersion.
From the Fig. 3, it can be seen that most grains are equiaxed shapes and grain boundaries in the annealed-CRed alloys become more distinct, which ascribes to the decrease in the number of defects in near-boundary regions and some extent of stress relaxation.
NG Ni-based alloy with average grain size of about 50 nm was prepared via severe cold-rolling at room temperature.

Fine grains can be formed by breakdown of elongated grains as well as dynamic recrystallization.
However, there is a similar point, that is, that the grains in ASB are refined grains.
Large numbers of refined grains are formed in localization deforming area due to multifactor interact.
The pattern of these grains shows that the refined grains were formed by breakdown of elongated grains in the center of the ASB due to shearing stress.
So, huge stress concentration will be generated at the position of piling up of dislocations due to swarming into of large numbers of dislocations, leading to breakdown of elongated grains and forming the refined grains with dimension of 0.1~0.3μm.

Al-5Ti-C alloy has a good grain refining capacity for commercially pure aluminum.
The grain refinement experiment was made in the well type resistance furnace.
As we can see from Figure 2 (a), on the Al substrate of Al-5Ti-C alloy are uniformly distributed a large number of strip-like or lump-like substances with the size of about 20 ~ 55μm in length and 8 ~ 12μm in width and small black particles.
After a heat preservation time of 120 minutes, there is a significant difference of the grain size between the top and bottom of the sample, the grains at the top being large, while those at the bottom small, and the closer to the bottom, the finer the grains(See Figure 3 (e)).
When the heat preservation time continues to increase to 120 minutes, a large number of precipitates appear at the bottom of the sample (as shown in Figure 4 (e)).

Gallium spots in the unstressed (left) and stressed (right) fine-grained aluminium foil.
Coarse-grained (grain size of 2 to 3 mm) disks of a radius R of 9 mm and a thickness b of 2 mm were prepared by hot pressing of chemical grade NaCl single crystals in a cylindrical mould at 650ºC and annealing at 450ºC during 6 hrs.
The number of wet GB's in each specimen was counted using an optical microscope.
Results and Discussion In the first set of experiments, the number of wet GB's in each specimen was found to increase with increasing load.
The 0,0 0,3 0,6 0,9 0 25 50 75 100 Number of wet GB's (%) Shear stress ττττ , MPa a -0,2 0,0 0,2 0,4 0,6 0,8 0 20 40 60 Number of wet GB's (%) Normal stress σσσσ, MPa b effect of normal stresses manifests itself only at two extreme parts of the histogram (Fig. 3b) where shear stresses are small.

Grain size and shape evaluation.
Grain boundary characteristics.
As a consequence, the number of grain boundaries between grains <111> and <100>, which have a misorientation angle of about 60 o , increases.
In order to prove this hypothesis the mean misorientation angle of three types of grain boundaries was determined: (i) between two <100> oriented grains, (ii) between two <111> oriented grains and (iii) between <100> and <111> oriented grains.
This confirms the hypothesis of grain rotation resulting from the absorption of dislocations in the grain boundaries. 0 10 20 30 40 50 60 1.39 2.77 3.79 true strain mean misorientation angle boundary between two <100> grains boundary between two <111> grains boundary between <100> and <111> grains Fig. 6.

It is assumed that the grain boundaries act as sources and sinks for dislocations, while in the grain interior no, or only few dislocations will be stored.
However, there is a strong dependency of the character of the grain boundaries, which is mainly determined by the number of ECAP passes, on the mechanical behaviour of the UFG materials.
As shown by Cabibbo et al. [22] the degree of misorientation changes from mainly low angle boundaries to high angle boundaries with increasing number of ECAP-passes.
The results obtained clearly reveal a strong influence of the number of ECAP-passes on the SRS.
As known from [22], with increasing number of ECAP-passes the misorientation between adjacent grains increase and consequently annihilation processes of lattice dislocations deposited at the grain boundaries will take place much more easily.

The high elongation of ferrite grains facilitated simultaneous homogeneous nucleation of austenite grains throughout the matrix upon heating; and, therefore, promoted the development of ultrafine grained structure with the size of structural elements well below 1 micron.
An average grain size in the annealed state was about 10 mm.
The relatively coarse grained microstructure consisting of equiaxed grains (the grain size is 2.3 mm) that contains many annealing twins evolve in the sample annealed at 900°C (Fig. 4d).
However, the large number of annealed grains appears simultaneously throughout the deformation microstructure due to high density of grain/subgrain boundaries evolved by severe deformation.
The annealing behaviour of the rolled steel was characterised by the appearance of a large number of new austenite grains, leading to the formation of equiaxed submicrocrystalline structure with a average grain size of about 100-200 nm upon heating to 500°C < T < 800°C.

The average grain size (dAve) is related to recrystallized grains and sub-grains, and their relationship is as follows: dAve= drex*φrex+ dsub*(1-φrex)
Low deformation temperature will increase the number of recrystallization, thereby increasing the brittle fracture characteristics along the recrystallized grain boundaries.
When the crack propagates in the deformed grains containing sub-grains, due to the low strength of the sub-grain boundary, the crack preferentially propagates to the sub-grain boundary.
Therefore, when the grain size decreases, the grain cross point increases with the decrease of grain size, the number of cross points through the crack propagation increases, and the probability of crack propagation increases.
The mathematical relationship between deformation temperature and sub-grain size, recrystallized grain size and recrystallized grain fraction was established.

As a result, the elongation of alloy is decreased with pass number increasing.
In Fig. 2(b), a number of new fine grains appear along the initial grain boundaries.
The sub-grain boundaries consist of low-angle grain boundaries (LAGBs).
This is why elongation is decreased with pass number increasing.
The elongation is decreased with pass number increasing.