1.1 Heat Treatment Of Steels Flashcards
Define CCR (Critical cooling rate)
the minimum cooling rate required to obtain 100% martensite in a steel on quenching from austenite.
Define Critical Diameter
the diameter of a long, cylindrical bar which can be quenched with a given severity of quench to give 50% martensite at its centre.
Define Hardenability
the ability of steel to form martensite on quenching. The higher the hardenability, the lower the critical cooling rate and hence the larger the component cross-section which can be quenched to produce a structure with 100% martensite.
Explain how alloying additions affect hardenability
Alloy additions such as Ni, Cr, Mo increase hardenability by slowing down the rate at which austenite transforms by diffusive transformations. This is because the elements have different solubilities in γ and α and must therefore redistribute during transformation – inherently slow for substitutional solid solutions.
What’s the effect of carbon content on hardenability
hardenability increases as C content increases
What’s the effect of grain size on hardenability
The larger the austenite grains, the higher the hardenability. The diffusive transformations (ferrite, pearlite, bainite) all nucleate on the austenite grain boundaries, so the larger the grain size the smaller the boundary area per unit volume, and the slower the start of the transformation. Increasing grain size therefore increases the time to complete diffusive transformations, so martensite formation is easier – the hardenability is greater.
Define Equivalent diameter De
The equivalent diameter is the diameter of a long cylindrical bar which, if subjected to the same cooling conditions as the component, would have a cooling rate at its axis equal to the position of slowest cooling in the component.
