Abstract
The evolutions of microstructure and texture in 2205 duplex stainless steel (DSS) welds produced by two high energy density (HED) processes, CO2 laser beam welding (LBW) and electron beam welding (EBW) were investigated. A variety of analytical techniques were applied for the study on microstructure and texture of the welds. In which, optical microscopy and electron microscopy were used to evaluate the detailed microstructure. X-ray diffraction (XRD) was put to investigate the crystallographic textures among the base metal, heat affected zone and fusion zone. Particular attention was focused on the determination of microtexture in HED welds by using electron backscatter diffraction (EBSD) technique. After that, an effort was put to compare the results by both of X-ray macro-texture and EBSD-microtexture.
The recorded micrographs illustrates that the HED welds are mainly composed of d-ferrite grained structure, which is further decorated with allotriomorphic and Widmanstätten austenite (g) at grain boundaries. With preheating treatment, the volume fraction of austenite in LB weld is gradually increased, and then leading to a completely different morphology. An apparent amount of transformation twins are found in g phase under TEM observations. No matter that they are Widmanstätten austenite in nonpreheated welds or blocky austenite in preheated welds, all of the transformation twins have the same {111} twin boundary. Furthermore, modulated fringes composed of ferrite, secondary austenite and amorphous phase are also found in the nonpreheated LB weld. It is ascribed to the rapid cooling effect occurred in the nonpreheated LB weld. Two chromium nitrides (CrN and Cr2N) are also identified and attributed to their different driving forces.
A remarkable texture gradient is found in the base metal along the thickness direction for both of austenite and ferrite phases in 2205 duplex stainless steel. The texture is governed separately by the {001}//ND-fibre, a-fibre, Goss and rotated cube components. Despite the analogous local texture evolutions revealing in both LB and EB welds, the global solidification textures in the two processes are considerably different. For which, the texture of LB weld is predominantly evolved with the Goss component. However, the texture of EB weld is mainly composed of the pronounced cube {001}<100>, while the Goss {011}<100> and rotated cube {001}<110> are weakened. The microtexture analysis shows that the centre region of the weld is dominated by oriented nucleation mechanism. Whereas, regions near the fusion boundaries are governed by oriented growth mechanism. The texture feature from EBSD does consist well with the XRD measured result. Moreover, the measurement of local texture from EB weld clearly indicates that a high percentage of high angle grain boundaries distributed in the crown. By contrary, a high percentage of low angle grain boundaries distributed in the root. Both of them again reflect the cooling effect of weld on the solidification mechanism. Throughout this study, the key factors to be responsible for the evolution of solidification texture of HED welded DSS are summarized. Those are thermal conductivity of the weld, turbulent flow in the molten pool, parent textures and the orientation relationship between ferrite and austenite.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0116101-131708 |
| Date | 16 January 2001 |
| Creators | Chen, Chih-Peng |
| Contributors | N. J. Ho, P. Shen, W. T. Tsai, J. C. Huang, H. L. Tsai, W. T. Wu |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-0116101-131708 |
| Rights | off_campus_withheld, Copyright information available at source archive |
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