Grain Refining Effect of Zr in Carbon Steel Welds

Abstract

The effect of Zirconium on the austenite grain growth in a coarsend-grain region in the weld heat affected zone of a plane carbon steel was investigated using the constant temperature annealing method.
Also, the welding experiment was carried to study the effect of Zirconium on the solidification mode and microstructure. The results obtained are as follows.
(1) The austenite grain grows according to the equation, D=k₀tⁿ on the higher temperature than 1150°C. The temperature dependence of the time constant, n is weak and its values are less than 0.3. While, the temperature dependence of k₀ can be expressed by Arrhenius type. The apparent activation energy for the austenite grain growth obtained from the experiments exhibits twice with 0.04% Zr as large as without Zr.
(2) The suppression effect of Zr to the austenite grain growth is remarkable with Zr content over 0.04%. It will be depended on the existence of ZrC particles which are nearly as large as 1.0μ, in diameter and distributed in the austenite grains.
(3) The influence of Zr upon the solidification mode and structure of weld metal can be negligible primarily because so little Zr is used, that is, the cellular substructure near fusion line and the cellular dendritic structure around bead centre, regardless of the presence or absence of Zr. The cell size, however, tends to fine away as Zr content is increased.
(4) As increasing Zr content, the temperature range between Ar₁ and Ar₃ becomes narrow because Ar1 point rises, and the tendencies to decrease the needle-like ferrite and to increase the granular ferrite are recognized. The addition of Zr does not contribute to increase the hardenability of the specimens, but tends to low the maximum hardness in the welding heat affected zone.