Welding of Ultra-Fine Grained Steels
Weldability is an important application property of ultra-fine grained steel. The grain growth and microstructure transformation in the HAZ of ultra-fine grained steel are studied in the present chapter. Laser welding and arc welding processes of 400 MPa
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Welding of Ultra-Fine Grained Steels
Weldability is an important application property of ultra-fine grained steel. The grain growth and microstructure transformation in the HAZ of ultra-fine grained steel are studied in the present chapter. Laser welding and arc welding processes of 400 MPa grade ultra-fine grained carbon steel and 800 MPa grade ultra-fine microstructure Bainitic steel are discussed.
10.1
Introduction
As is shown in the previous chapters and many papers (Kang J H, 2007; Majta J, 2007; Mathis K, 2007; Murty S V S N, 2005 ) , both the strength and the toughness of steels can be dramatically enhanced by grain refinement. Whether the properties of a welded joint can match those of the base metal is the key issue for the application of these ultra-fine grained steels in engineering structures. The heat affected zone (HAZ) in the ultra-fine grained steel is the major concern in many studies (Hamatani H, 2007; Kawaguchi Y, 2002; Youn J G, 2002; Tian Z, 2002). In the HAZ, the microstructure of an ultra-fine grained steel will undergo severe transformations due to the effect of the welding thermal cycle. During heating, the base metal will start to transform into austenite when the temperature reaches the Acl point and accomplish at Ac3 point. The austenite grains will grow to form the coarse grained HAZ, and the size of the austenite grains is dependent on the time of grain growth at high temperature, which is related to the method and parameter of welding (wang et al,2002). During cooling, the austenite transforms into microstructure that is different from the base metal in the heat-affected zone (HAZ) (Lancaster J F, 1980). Welding performance is very important from the view point of joint strength and toughness, and both the softening and the deterioration of
O The chapter 10 was written by Zhiling Tian and Yun Peng.
10 Welding of Ultra-Fine Grained Steels 495 toughness are the major concerns in the stability of HAZ. Another concern is the weld metal. Whereas grain refining improves both the strength and the toughness of the base metal, the weld metal maintains largely a cast structure. With conventional welding materials, the toughness of the deposited metal can hardly match that of the base metal of ultra-fine grained steel when strength matching is required. In order to achieve integrate properties in a welded joint, it becomes necessary to develop new welding materials for the ultra-fine grained steels, especially for the high strength grades. In section 10.2, the grain growth and microstructure transformation in the HAZ of ultra-fine grained steel is studied. A computational simulation of HAZ grain growth based on the Monte Carlo model generated the grain size distribution across the HAZ, which coincides with to that of actual welded joints. In section 10.3, the welding of 400 MPa grade ultra-fine grained carbon steel is reported, including laser welding in 10.3.1 and arc welding in 10.3.2. The welding of ultra-fine grained anti-weathering steel and rebar is reported in 10.3.3 and 10.3.4, re
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