Table of Contents

Materials and Structure





When materials are joined by a welding process they are locally melted, re-solidify, and cool down to room temperature. The joining of two steel plates (pearlite) by welding is shown in the top diagram together with temperatures in the weld zone and the regions of the two plates altered by the process.

The lower diagram shows the microstructure in the "heat affected zone" (HAZ). In (a) the system is shown with a central molten region and austenite produced by local heating of the pearlite. In (b) the weld has cooled to room temperature slowly enough that pearlite is formed in the weld region. This may have a different grain size than the starting material, but will have similar properties. In (c) the cooling rate was too fast to permit the equilibrium phase transition to occur and martensite was formed from both the liquid zone and the austenite. The weld is now hard and brittle, undesirable properties for a mechanical joint. In order to avoid this situation, the cooling rate should be reduced or the weld should be heat treated (tempered).

From: Ashby and Jones, "Engineering Materials 2," Pergamon (1986), and Askeland, "The Science of Engineering Materials," PWS (1994)