Quench Severity, Biot Number, and the Agitation Screen

Which depends entirely on which regime the part is in. Everyone reaches for more agitation when a quench comes out soft, and on a thick section it does almost nothing -- because the heat is not surface-limited, it is conduction-limited. Grossmann's H-value is the standard ranking of quench severity, and the usual table runs from still oil near 0.25, through agitated oil near 0.4, still water near 1.0, agitated water near 1.5, up to agitated brine above 2.0. What that table does not say is when moving up it helps. The Biot number answers that. It is the ratio of the internal conduction resistance to the surface transfer resistance. When it is SMALL the surface is the bottleneck -- heat cannot get OFF the part fast enough, and anything that improves the surface condition, a more severe quenchant, more agitation, better fixturing so the vapor blanket breaks, directly improves the cooling rate. When it is LARGE the surface is already removing heat faster than the interior can supply it, the center cools at a rate set by the steel's own conductivity, and a more severe quench changes the surface and does essentially nothing at the core while adding distortion and cracking risk. On a 1 in bar, still oil gives a Biot of 0.25 and agitated oil 0.40 -- both surface-limited, so the move from still to agitated raises the cooling rate by roughly the H ratio of 1.6 times, and the quenchant is the whole story. On a 6 in bar, still water gives 6.0 and agitated brine 12.0 -- both conduction-limited, so doubling the severity barely moves the center cooling rate and mostly doubles the gradient. Those two sentences are the reason large sections are made from deep-hardening alloys and small ones are not. A REGIME SCREEN, not a hardness prediction. It says whether agitation is worth doing; it does not say what hardness results, which needs the steel's hardenability (its Jominy curve or its ideal diameter), its chemistry, the prior microstructure, and a CCT diagram. Grossmann H-values are themselves approximate rankings from a bar-quench correlation, not physical properties, and real quenching passes through vapor-blanket, boiling, and convection stages with wildly different heat-transfer coefficients rather than the single value the number implies. It does not address distortion, quench cracking, residual stress, part orientation and racking, quenchant temperature and contamination, or the temper that must follow. The steel's specification, its hardenability data, and the metallurgist govern.

radius = diameter / 2; Biot number Bi = h r / k = 2 H r with H the Grossmann severity; Bi below 0.5 is surface-limited, above 2 conduction-limited, between the two transitional.

Grossmann's quench severity H-value as the standard ranking (still oil about 0.25, agitated oil about 0.4, still water about 1.0, agitated water about 1.5, agitated brine above 2.0), cited by name and entered rather than looked up, with the Biot number and the conventional surface-limited and conduction-limited regimes. A REGIME SCREEN, not a hardness prediction. The metallurgist governs.

A dimensionless ratio computed from a user-entered severity ranking and the section size; no hardenability or CCT data is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: grossmann_h, section_diameter_in, second_grossmann_h, radius_in, biot, second_biot, h_ratio

• Regime, not hardness it says whether agitating helps; hardness needs hardenability and a CCT diagramheat treating practice
• H is a ranking Grossmann values come from a bar-quench correlation, not from a measured coefficientGrossmann quench severity
• Severity can hurt on a conduction-limited section a more severe quench adds distortion and cracking without core benefitheat treating practice