Mechanical properties and microstructure of boron-alloyed steel 19 MnB 4

Microalloying elements are useful to achieve maximum improvements of the mechanical properties in boron alloyed steels supposedly because they form precipitates with nitrogen and carbon to protect boron from precipitation. On the other hand, they are most useful for austenitic grain refinement and for the generation of the required minimum austenitic grain size (MAGS). A second heat treatment (700°C 9h/air) before quenching the Jominy samples is only efficient in a certain range of austenitic grain size. No increase of hardenability is observed above a certain plateau austenitic grain size (PAGS) for each steel group. A minimum austenitic grain size (MAGS) is required to achieve maximum strength. Boron can only improve the hardenability and tensile properties, if the austenitic grain size before martensitic transformation is optimal. Titanium appears to be the most effective element to prevent boron precipitation. The hardenability, yield and impact strength of the steel group B–AI–Ti is good, especially for sample 44.