This has been discussed at length, and recently I made a long post on it.
Short answer:
It is the time starting when the blade enters the quench tank that starts the one second clock. The time from oven to tank should be smooth and quick, but does not need to be rushed. roof 50 degrees will not matter in the several seconds that it takes to move the blade from oven to tank. Once the blade drops below 1350F, the time matters.
Here is an excerpt from one post I made;
"....When you austenitize the steel there is a certain temperature that you want( target temp) . This temp is calculated to allow the carbon and alloy ingredients to be distributed, but not to allow for the steel to grow oversize grains. The hold time ( soak) at the temperature gives the ingredients time to diffuse.
Once the soak time is up, the diffusion is complete ( for our purposes), and we quench the steel to lock in the configuration of the structure. The steel is cooled in the quench at a speed that prevents the formation of pearlite, and ends up as martensite. The critical spot on this cooling curve is around 900-1000F. We call this the "pearlite nose". If we cool fast enough to supercool the austenite, we miss the nose and go to martensite....too slow and we get soft pearlite and a useless blade.
Many worry that the blade needs to be plunged in less than one second from 1500F to below 900F ( specs for 1095). This is not so. Once in the quench tank, the quenchant needs to be capable of that drop, but before the blade enters the quenchant, the steel only needs to stay above 1350-1400F, at which point it begins to convert to pearlite if allowed a slow cooling.
The steel will not cool down in air to any degree that will be a problem......."
