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China ajxu.cumt@gmail.com, bfanyu2002@hotmail.com Keywords: Fracture Toughness; Coarse-grained Heat Affected Zone (CGHAZ); X80 Pipeline Steel; Weld Thermal Simulation; Finite Element Analysis (FEA).
A large number of fracture toughness (as denoted by CTOD) tests together with 3D finite element analysis are performed using single edge notched bending (SENB) and tension (SENT) specimens at different temperatures.
Coarse-grained heat-affected zone (CGHAZ) is considered as the material microstructure in preparation of the weld thermal simulated fracture mechanics specimens.
Weld thermal simulation has been used to produce the microstructural properties of coarse-grained HAZ (CGHAZ) in X80.
A large number of CTOD (crack tip opening displacement) tests are carried out at -90℃, -60℃ and -30℃. 3D FEA are employed to model the crack tip stress fields of the tested specimens.

According to the initial grain size of 25 µm these measurements show, therefore, that ABE is effective in reducing the grain size of this Mg-based alloy.
The recorded grain sizes, however, were achieved by ECAP process after eight passes, where un-processed material possessed a grain size of 15~22 µm [9].
As the increase of strain, the size of fine grain is less affected by ABE passes, while the coarse grains are refined continuously by dynamic recrystallization [3].
Fig. 3 a) True stress versus true strain at room temperature for as-received and processed experimental alloy, b) Variation of the yield stress, UTS and tensile elongation with the number of ABE passes.
The more the number of passes the more pronounced hardening effect may be seen in the material.

With increasing number of passes the mean grain size reduces by a factor of about four (Fig. 3a).
(thick lines: high angel grain boundaries; thin lines: low angel grain boundaries) (a) (b) Figure 3: Grain parameters obtained by EBSD grain reconstruction: (a) mean grain size and (b) mean aspect ratio determined from individual grains. 354 Texture.
(a) (b) Figure 5: (a) Deviation in ϕ1 from the ideal cube component and (b) texture strength with the number of passes In general, during ECAP the grains are sheared and thus become elongated.
The inclination of the grain long axis with the extrusion direction decreases with increasing number of passes.
The increase in inclination with increasing number of passes is correlated with the texture obliquity and grain refinement.

This algorithm can remove the dependence of computation on the number of order parameter.
The nucleation number in each grain can be expressed as follows: (8) Clearly the nucleation rate is determined by the average subgrain size and the matrix grain size of grain .
This implies that higher deformation temperature will trigger recrystallization easier and speedup the DRX kinetics due to combined effect of increased nucleation number and increased grain boundary migration rate.
The initial matrix grain size has no effect on the steady grain size.
Grain boundary curvature and grain growth kinetics with particle pinning.

On the other hand, the number of research works as to SPD of commercial medium carbon steels is still limited, because SPD processing is relatively difficult in steels with higher flow stress.
Grains of pearlite with size of ~ 50 μm are lined by the finer ferrite grains (~10 μm in diameter).
These pearlite grains are lined by finer ferrite grains, Fig. 1a.
Finishing ECAP deformation the corresponding effective strain in dependence of number of passes was εef = 2.7, 3.4 and 4 respectively for individual samples.
The deformed microstructure, which resulted from different ECAP straining of steel, related to different number of passes through die channel 500 nm (N= 4 and N= 6 passes) is presented in Fig. 4.

On the middle of guide roller, a large number of grid shaped cracks were observed only on the surface.
In addition, element content of Cr was a little higher, which easily leads to chromium carbide precipitating along grain boundary and weakens the binding force between grains.
Grain boundaries were evident along direction of cracks propagation.
Hard and brittle carbides distributed on the grain boundaries.
Therefore, the cracks extended along grain boundaries and became intergranular cracks.

The steel sheet having the texture of the second group shows high r-value and good formability, therefore a number of studies have been focused on the texture formation and its utilization [4,5].
Journal Title and Volume Number (to be inserted by the publisher) 3 Figure 3 Recrystallisation occurred at grain boundary of {100}<011> by-crystal after 70% cold rolling and its texture.
A {h,1,1}<1/h,1,2> recrystallisation grain was found.
The pole figure shows the orientations at the interior of the grains and at the close point to the grain boundary.
Within A grain, the orientation slightly shifts at the close position to the grain boundary and its rotational direction was opposite to the orientation of B grain.

All ferrite grains are equiaxed and the average grain size is significantly reduced.
Their initial grain sizes of ferrite are all about 50 µm .
Fig.7 Effect of strain on number of recrystallized grains of ferrite in per area(a) and average recrystallized grain size(b) in the three steels deformed at 700 oC at 0.001s -1.
Fig.8 Effect of strain on number of recrystallized grains of ferrite in per area(a) and average recrystallized grain size(b) in the three steels deformed at 700 oC at 0.1s -1.
Fig.9 TEM image of the carbon replica of the Nb-microalloyed steel Fig.10 Effect of strain on number of recrystallized grains of ferrite in per area(a) and average recrystallized grain size(b) in the two steels deformed at 700 oC at 0.001s -1.

It also revealed that the size of austenite grain and nonmetallic inclusions on borders of grains were reduced, and sulphide inclusions were dissolved partially.
By modifying became aware obtaining ingots and castings with fine-grained structure.
The microstructure of the unmodified sample number 200 Table 3.
The structure contains minor non-metallic inclusions on the boundaries and within grains (sample number 637 in Fig. 2).
Microstructure of modified sample number 637 Table 4.

(6) The driving force for grain growth is a reduction of grain boundary area.
If secondphase particles are present in the microstructure they exert a pinning effect on grain boundaries, which reduce the rate of grain growth.
The number fractions of precipitates are shown in Fig. 5.
The calculated number fraction is also compared to the measured.
The discrepancies obtained in the number fraction distributions are thought to be partly due to the small number of size groups used in the calculations.