Abstract
The suppression effects of the fine precipitate dispersions of micro-alloy elements, such as Ti and Nb, to the grain growth in carbon steel weld zone have been investigated using both methods of the isothermal heating experiment and the metallographic observations of TIG welds. The results obtained are as follows.
1) Austenite grain diameter, Dx(T, t), in steel, which contains X% of micro-alloy element, isothermally heated at T°C for t sec is expressed by following equation
Dx(T, t)=Dx=0(T, t)exp(-mX)
where Dx=0(T, t) is austenite grain diameter of steel without micro-element. Designating exp(-mX) by P, parameter P would express quantitatively suppression effect of microalloy elements to austenite grain growth. Additions of Zr and Ti make P value considerably small comparing with Nb addition.
2) Time exponent, n, in isothermal grain growth, D=ktn, becomes remarkably small value as increasing Ti and Zr contents; at Ti 0.08 % and Zr 0.05 %, n value would be less than 0.1 and comes to show very weak temperature dependence. Also, addition of small amount of Ti or Zr considerably increases the activation energy for austenite grain growth.
3) Both sizes of celluler substructure and maximum grain in HAZ rapidly decrease as adding of Ti or Zr; linear relationship exists between both sizes.
