Microstructure-Property Correlations in Fiber Laser Welded Nb-Ti Microalloyed C-Mn Steel
- PDF / 4,181,818 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 76 Downloads / 155 Views
ÓASM International 1059-9495/$19.00
Microstructure-Property Correlations in Fiber Laser Welded Nb-Ti Microalloyed C-Mn Steel Qian Sun, Xiao-Kang Nie, Yang Li, and Hong-Shuang Di (Submitted July 4, 2017; in revised form December 14, 2017; published online January 17, 2018) Mechanical Performance of traditional gas-shielded arc welded joints of 700 MPa grade microalloyed CMn steel cannot meet service requirements. Laser welding, with its characteristic high energy density, is known to improve the welding performance of experimental steels. In the present study, Nb-Ti microalloyed steel with a thickness of 4.5 mm was welded using a 4 kW fiber laser. The microstructure, precipitation, and mechanical properties of the welded joints were studied. The hardness and tensile strength of the welded joints were higher than those of the base metal (BM). The microstructure of the fusion zone (FZ) and coarse grain heat affected zone (CGHAZ) was lath martensite (LM), while the microstructure of the fine grain HAZ and mixed grain HAZ consisted of ferrite and martensite/austenite islands. Although LM was observed in both the FZ and CGHAZ, the hardness and calculated tensile strength of the FZ were lower than those of the CGHAZ, due to a reduction in solid solution strengthening by element loss and the dissolution of high-hardness precipitates in FZ. Most precipitates such as [(Nb,Ti)C and (Nb,Ti)(C,N)] that were present in the BM were dissolved, which led to an increase in C and N in solid solution in the FZ. Thus, the elastic modulus of the FZ was higher than that of the BM. Similarly, the elastic modulus of the CGHAZ was higher than that of the BM due to the segregation of C and N atoms during the welding process. The toughness of the FZ was superior to that of the BM, and the toughness of the HAZ approached 91% of that of the BM. The change in toughness primarily depended on the microstructural refinement, the increase in the fraction of grains with high misorientation, the residual austenite in the FZ and CGHAZ, and the dissolution of coarse precipitates. Keywords
fiber laser welding, impact toughness, microalloyed steel, microstructure, nanoindentation, precipitates
1. Introduction Advances in the application of high-strength steels has enabled the fabrication of increasingly light-weight structures. Welding is the main joining method for such steels and the service life of a steel structure depends on the mechanical properties of these joints. Fusion welding techniques that have been used to join manufacturing components include arc welding, gas-shielded arc welding, inert gas tungsten arc welding, submerged arc welding, laser welding, resistance welding, electron beam welding, etc. In recent years, the focus has been on laser welding due to the characteristics of high power, flexible beam delivery, low maintenance costs, high efficiency, and compact size (Ref 1). A number of studies have been carried out on laser welding of steels. For instance, Miranda et al. (Ref 2) studied the effect of heat input on the microstructure and hard
Data Loading...
