Method for Assessing Grain Boundary Density in High-Strength, High-Toughness Ferritic Weld Metal
- PDF / 5,167,402 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 102 Downloads / 252 Views
I.
INTRODUCTION
THE high strength achieved by developed welding methods, such as shield metal-arc welding,[1,2] friction stir welding,[3] laser beam welding,[4,5] et al., is always a key performance indicator for weld metal of steels. For the nature of the interlocking structure and large grain boundaries, acicular ferrite (AF) has been known to be the most desirable microstructural constituent.[6,7] When AF volume fraction reaches 0.6 to 0.8, the strength grade of weld metal is usually more than 600 to 700 MPa[8,9] and with excellent low-temperature toughness.[10] Besides AF volume fraction, the average grain size or effective grain size is an important parameter in evaluating the quality of AF-dominated weld metal. But it is hard to measure an effective average grain size for great impingements happen in the AF grain growth process[11,12] and it needs a complex process to detect the effective grain size. So some researchers have attempted to measure the morphological grain size instead of the average grain size or effective grain size to describe the weld metal,[13] but data fluctuate. In the present work, a method of measuring specific parameters with morphological photos for assessing grain boundary density in high-strength, high-toughness AF-dominated weld metals is raised. This method detects the average grain size or effective grain size and provides a reference for judging the size of average grain boundary density or effective grain boundary density. The feasibility of this method is also discussed.
XUANWEI LEI, Ph.D. Candidate, JIHUA HUANG, Professor, and SHUHAI CHEN and XINGKE ZHAO, Associate Professors, are with the University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted May 27, 2016. Article published online November 8, 2016 198—VOLUME 48A, JANUARY 2017
II. ASSESSING GRAIN REFINEMENT IN HIGH-STRENGTH, HIGH-TOUGHNESS FERRITIC WELD METAL A. Background The two-dimensional (2D) morphology of AF presents a long and narrow needle, and in some early research, three-dimensional (3D) interlocked thin-plate morphology seemed to present interlinking features.[7,14] As the development of the microstructure characterization technique progressed, serial sectioning and computer-aided 3D reconstruction and visualization allowed for better observation of the 3D morphology. Wu et al.[15,16] and Wang et al.[17] reconstructed the 3D morphology of AF, and they found the morphology of AF varied from laths to plates rather than being needle-like. The length and the width of AF ranged from several to a few tens of microns, and the thickness was normally less than several microns. At the same time, the in situ observation, which was carried out using a high-temperature laser scanning confocal microscopy (LSCM), was adopted on AF transformation. LSCM gives continuous live pictures on 2D AF nucleation and growing. The results of the researches by Zhang et al.,[18] Wang et al.,[11] and Terasaki et al.[12] showed AF grew synchronously in length direction and thick
Data Loading...
