Effect of the Austenite Interface on Pearlite Transformation Behavior

2018 ◽  
Vol 941 ◽  
pp. 639-644
Author(s):  
Hiroshi Hasegawa ◽  
Tatsuya Nakagaito ◽  
Yoshimasa Funakawa
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Intercritical Annealing ◽  
Transformation Behavior ◽  
Transformation Behaviour ◽  
Austenite Grain ◽  
Austenite Grains ◽  
Austenite Grain Boundary

The effect of the characteristics of austenite interface with ferrite on the pearlite transformation behaviour after intercritical annealing was investigated. Most austenite grains were situated mainly on ferrite grain boundaries and had the Kurdjumv-Sachs (K-S) or near K-S relationship to one of the neighbor ferrite grains before pearlite transformation. The pearlite transformation started mainly from the austenite grain boundary faced to ferrite. The pearlite transformation showed stasis. This indicates that some austenite is stabilized thermally against the pearlite transformation. The fraction of austenite having only the K-S or near K-S interface to neighbor ferrite grains was correspond to the fraction of austenite grains which does not include pearlite. The pearlite transformation was difficult to start from austenite interface having the K-S relationship to ferrite since the interface between austenite grains and ferrite grains was stabilized energetically in the case of their interface having the K-S relationship.

Cavitation control in steels of high residual element content

1980 ◽  
Vol 295 (1413) ◽  
pp. 305-305 ◽  
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Prior Austenite ◽  
Low Alloy Steels ◽  
Boundary Zone ◽  
Austenite Grain ◽  
Alloy Steels ◽  
Austenite Grain Boundary ◽  
Prior Austenite Grain ◽  
Transverse Boundary

The cavitational mode of failure of prior austenite grain boundaries in bainitic creep-resisting low alloy steels is now well established as a principal factor in the high incidence of cracking problems which has developed on modern power plant in recent years. The microstructural features dominating the cavitation process at the reheat temperature in a ½CMV bainitic steel of high classical residual level have been determined. The prior austenite grain boundaries become zones of comparative weakness ca . 1 pm thick at 700 °C and are incapable of sustaining significant shear loads. Deformation is therefore initiated by a relaxation of load, through a process of prior austenite grain boundary zone shear, from inclined to transverse boundaries such that a concentration of normal stress develops across the latter. The overall deformation is thereafter determined by cavitation of the transverse boundary zones, the necessary inclined boundary displacements being accommodated by further grain boundary zone shear. Transverse boundary cavitation is shown to be an essentially time-independent process of localized ductile microvoid coalescence resulting from the plastic deformation of the boundary zone.

In situ observation on microstructure evolution of 22MnB5 in hot stamping process

2019 ◽  
Vol 116 (2) ◽  
pp. 209
Author(s):  
Ling Kong ◽  
Yan Peng
Keyword(s):  
Martensitic Transformation ◽  
Grain Boundary ◽  
Cooling Rate ◽  
Microstructure Evolution ◽  
Hot Stamping ◽  
High Strength ◽  
Continuous Cooling ◽  
Austenite Grain ◽  
Austenite Grains ◽  
Austenite Grain Boundary

High temperature confocal laser scanning microscopy (CLSM) was used to investigate the microstructure evolution of high-strength boron steel 22MnB5 during hot stamping. The experimental results show that it is complete austenitized at temperatures about 810 °C during the heating process. Most of the initial austenite grain size is small and locally coarse. At 920 °C to 1000 °C, the phenomenon of B remelted and solidified was observed which played a very good pinning role at the austenite grain boundary, preventing coarsening of austenite grains. The segregation of B and the addition of Mn result in a significant reduction in both the minimum boron reverse melt content and the final solidification temperature. In the continuous cooling stage, martensitic transformation occurs at the cooling rate of 60 °C/s, and the martensite start point is 400 °C and martensite finish point is 280 °C. A large number of bursts are concentrated from 380 °C to 330 °C. There are two main forms of martensitic transformation: first, martensite begins to appear at the coarse austenite grain boundary, and grows transgranularly. Second, the new martensite laths starts from the previously formed laths and grows at a certain angle into the austenite grains. The main factor in the increase of martensite in continuous cooling is the formation of variable temperature martensite rather than the growth of martensite laths. At the cooling rate of 20 °C/s, the bainite and ferrite transformation appeared and the conversion temperature of bainite was about 600 °C. The cooling speed has a great influence on the performance of the 22MnB5 hot stamping component. The room temperature microhardness at cooling rates of 5 °C/s, 20 °C/s, and 60 °C/s was 194 HV, 243 HV, and 430 HV, respectively. Therefore, ensuring sufficient cooling rate is a key condition for obtaining ultra-high strength hot stamping components.

Temperature Dependence of Austenite Nucleation Site on Reversion of Lath Martensite

2010 ◽  
Vol 638-642 ◽  
pp. 3424-3429 ◽  
Author(s):  
Nobuo Nakada ◽  
Toshihiro Tsuchiyama ◽  
Setsuo Takaki ◽  
Naoki Miyano
Keyword(s):  
Grain Boundary ◽  
Temperature Dependence ◽  
Elastic Strain ◽  
Prior Austenite ◽  
Nucleation Site ◽  
Lath Boundary ◽  
Austenite Grain ◽  
Austenite Grains ◽  
Austenite Grain Boundary ◽  
Prior Austenite Grain

The temperature dependence of austenite nucleation behavior within lath martensitic structure was investigated in an ultralow carbon 13%Cr-6%Ni martensitic stainless steel partially reversed at (austenite + ferrite) two phase region. The shape and nucleation site of the reversed austenite grains were varied depending on the reversion temperature; fine acicular austenite grains frequently formed along the lath boundaries at a temperature lower than 915 K, while the granular ones tended to nucleate mainly on the prior austenite grain boundaries at a higher temperature. In order to explain the temperature dependence of nucleation site transition, the difference in energetics of austenite nucleation between the lath boundary and the prior austenite grain boundary was discussed on the basis of the classical nucleation theory and FEM analysis. The calculation of the changes in interfacial energy and elastic strain for austenite nucleation suggested that the lath boundary acts as more preferential nucleation sites for austenite rather than the prior austenite grain boundary to reduce the increment of elastic strain when the reversion temperature is low.

scholarly journals Reconstruction and Size Prediction of Prior Austenite Grain Boundary (PAGB) using Artificial Neural Networks

2020 ◽  
Vol 58 (12) ◽  
pp. 822-829
Author(s):  
Bong-Kyu Kim ◽  
Nam Hoon Goo ◽  
Jong Hyuk Lee ◽  
Jun Hyun Han
Keyword(s):  
Neural Networks ◽  
Grain Size ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Detection Rate ◽  
Prior Austenite ◽  
Data Sets ◽  
Austenite Grain ◽  
Austenite Grains ◽  
Prior Austenite Grain

To automatically reconstruct the prior austenite grains from as-quenched martensitic structure, we applied a deep learning algorithm to recognize the prior austenite grains boundaries hidden in the martensitic matrix. The FC-DenseNet architecture based on FCN (fully convolutional networks) was used to train the martensite and ground truth label of the prior austenite grain boundaries. The original martensite structures and prior austenite grain boundaries were prepared using different chemical etching solutions. The initial PAGS detection rate was as low as 37.1%, which is not suitable for quantifying the basic properties of the microstructure such as grain size or grain boundary area. By changing the weight factor of the neural net loss function and increasing the size of the data set, the detection rate was improved up to 56.1%. However, even when the detection rate reached 50% or more, the quality of the reconstructed PAGS was not comparable to the analytically calculated results based on EBSD measurements and crystallographic orientation relationships. The prior austenite grain size data sets were obtained from martensite samples via the FCDenseNet method, and had a linear correlation with the mechanical properties measured in the same samples. In order to improve the accuracy of the detection rate using neural networks, it is necessary to increase the number of neural networks and data sets.

Crystallography of Ferrite Nucleation at Austenite Grain Boundary in a Low Carbon Steel

2010 ◽  
Vol 654-656 ◽  
pp. 7-10 ◽  
Author(s):  
Tadashi Furuhara ◽  
Hayato Saito ◽  
Goro Miyamoto ◽  
Tadashi Maki
Keyword(s):  
Grain Boundary ◽  
Transformation Temperature ◽  
Electron Backscatter Diffraction ◽  
Low Carbon Steel ◽  
Low Carbon ◽  
Orientation Relationships ◽  
Austenite Grain ◽  
Austenite Grains ◽  
Austenite Grain Boundary ◽  
Ferrite Nucleation

Orientation relationships of proeutectoid ferrite formed at an austenite grain boundary with respect to adjacent austenite grains were investigated by means of electron backscatter diffraction in an isothermally transformed Fe-1.5Mn-0.2C (mass%) alloy. A grain bounadry ferrite holds nearly the Kurdjumov-Sachs (K-S) orientation relationship with some small misorientation against at least one of the adjacent austenite grains. There is strong variant selection by the austenite grain boundary for ferrite nucleation. At a higher transformation temperature, the fraction of ferrites holding a near K-S relationship with respect to the opposite austenite grain is lower. As the transformation temperature becomes lower, the misorientation from the K-S relationship becomes smaller against the near K-S related austenite grain whereas the misorientation against the irrationally oriented austenite grain becomes larger.

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