Continuous Cooling Transformation Diagram for Welding
A time-temperature-transformation (TTT) diagram shows how the microstructure of steel changes during the welding process. It helps predict microstructures and risks like martensite formation.
Physicochemical mechanism of action
The CCT diagram for welding processes shows which microstructure forms at which cooling rate. Time is on one axis, temperature on the other, and within it are the regions for ferrite, pearlite, bainite, and . If you overlay the cooling curve of the weld, you can read what ends up in the material. During welding, the steel cools from the melt in one go, so a dedicated diagram applies to weld seams, not the one from heat treatment.
Metallurgical relevance & risks in welding
If the cooling time t8/5 is too short, the curve drops into the martensite region, and the weld turns glass-hard and crack-prone. If it's too long, the grain grows, and the material gets softer and less tough. The real benefit of the diagram is that you can see both before you even strike the arc.
Shop practice & instructions
Before you start, check the material's diagram and derive the heat management from it, not the other way around. For plain and low-alloy structural steels, the usable t8/5 range is usually between 10 and 20 seconds. For higher-strength fine-grain structural steels, the window is much narrower and set by the manufacturer, so guessing gets expensive. You control it via , , and the layer sequence.