Primary carbide
Primary carbides form directly from the liquid melt during solidification and consist of Fe-C complexes like cementite (Fe3C). They increase the hardness, wear resistance, and toughness of steel, but oversized primary carbides can lead to hot cracking.
Physical-Chemical Mechanism of Action
Primary carbides form during solidification directly from the molten pool and consist mainly of cementite (Fe3C). They are difficult to dissolve and grow with increasing carbon content.
Metallurgical Relevance & Welding Risks
Primary carbides increase hardness, wear resistance, and toughness of the steel. However, if they grow too large, they can lead to hot cracking since they enlarge into glass-hard martensite during fast cooling. This can impair the mechanical strength of the weld and cause cracks.
Workshop Practice & Action Guidelines
To control primary carbides in steel, it is important to precisely regulate the carbon content and cooling steps during welding. For high-carbon steels (e.g., HSS), slow cooling should be ensured to avoid large primary carbide formation. Recommended cooling times vary by steel type, but generally should be between 10 and 30 seconds.