karkaistettavuuskäyrä

Karkaistettavuuskäyrä, often translated as hardenability curve, is a graphical representation used in metallurgy to illustrate the depth to which a steel or other ferrous alloy can be hardened by a heat treatment process, typically quenching. This curve plots the hardness achieved at different depths from the quenched surface of a standard test specimen. The specimen, often a Jominy bar, is heated to its austenitizing temperature and then quenched from one end, creating a gradient of cooling rates across its length.

The x-axis of the karkaistettavuuskäyrä typically represents the distance from the quenched end, while the y-axis represents the Rockwell hardness (HRC) measurement. The curve starts with the highest hardness at the quenched end and gradually decreases as the distance from the end increases, indicating slower cooling rates and thus lower achievable hardness. Factors influencing the shape and position of the karkaistettavuuskäyrä include the chemical composition of the alloy, particularly the presence of alloying elements like manganese, molybdenum, and chromium, which tend to increase hardenability. Grain size and the presence of non-martensitic transformation products also play a role.

Understanding the karkaistettavuuskäyrä is crucial for selecting the appropriate steel grade and heat treatment parameters to achieve desired mechanical properties in a component. It allows engineers and metallurgists to predict the hardness profile throughout a part of a given cross-section and cooling rate, ensuring that critical areas meet performance requirements. A steel with high hardenability will maintain a significant hardness at greater depths from the quenched surface compared to a steel with low hardenability.