Ratio of Shape Factor to Cooling Rate
The ratio between the weld bead shape factor and its cooling rate affects the microstructure and risk of cracking. A high shape factor (slow cooling) can lead to hard martensite, while a low shape factor (fast cooling) gives soft austenite.
Physical-Chemical Mechanism of Action
The shape factor of a weld describes the ratio between the length and diameter of the weld. A higher shape factor means the weld is longer than it is thick. The cooling rate depends on this shape: a longer weld cools slower than a short one.
Metallurgical Relevance & Welding Risks
A high shape factor (slow cooling) can lead to a microstructure with lots of martensite, making the weld glass-hard and prone to cracking. A low shape factor (fast cooling) can lead to more austenite, making the weld softer and less prone to cracking. However, there is also a risk of hot cracks with fast cooling.
Workshop Practice & Action Guidelines
To achieve a good balance between shape factor and cooling rate, pay attention to the length of the weld compared to its diameter. For multi-pass welds, it is important that each layer cools evenly. Use a low gas flow for slow cooling and a high gas flow for fast cooling. The optimal cooling time should be between 5 and 25 seconds to get a balanced microstructure.