Hi, you had asked how quench speed was measured. here are two chunks of source data. here is a link to a short version:
http://www.machinerylubrication.com/Read/430/quench-oils
i found my copy of Dr. Verhoeven's
Metallurgy for the Bladesmith and copied this chunk. if you haven't read the paper, it is full of goood stuff. PM me if anyone wants a copy and i will email it to you. i have triple checked, there are no copyright issues doing this.
There has been considerable research on this problem in the
latter decades of the 20th century and a method
developed in England has now been adopted as an
international standard for characterizing quench
fluids, ISO 9950. The test utilizes an Inconel 600
alloy cylinder (basically the same Ni-Cr alloy as used for
the heating elements of your electric stove), 12.5 mm in
diameter by 60 mm long. A metal clad type K
thermocouple (see Appendix A) is fitted into a hole
along its center and the output of this thermocouple is monitored during the quench. The
output produces a cooling curve such as that shown for an oil bath in Fig. 12.10. The
heat transfer during the quench can be partitioned into 3 stages, conventionally called A,
B and C. Initially, during the A stage, heat transfer is relatively slow as the heat must
pass through the vapor blanket that initially surrounds the immersed sample. Notice that
the B stage begins with a rapid increase in the rate at which the temperature drops. When
the oil (or water) begins to penetrate through the vapor blanket it contacts the hot steel and
immediately boils. The heat required to boil the liquid is removed from the steel and this
mode of heat transfer is extremely efficient. It is called
"nucleate boiling heat transfer", hence the name of stage
B shown on Fig. 12.10. When the boiling stops heat is
transferred directly to the liquid touching the steel
causing its temperature to increase which, in turn, drops
its density. Hence, this liquid rises and is replaced by
colder liquid contacting the steel piece. The motion of
the liquid is called convection and hence the name of
stage C shown on Fig. 12.10.
For evaluating quenching intensity we are mainly
interested in how fast the temperature is falling and,
hence, the most useful parameter is the cooling rate,
which will have units of oC/sec. (or oF/sec.). Therefore, it
has become common to characterize the quenching
power of a quenchant with a plot of cooling rate versus
temperature of the Inconel rod, and Fig. 12.11 is presented
to show you how such curves are related to the simple
cooling curve of Fig. 12.10. In order to obtain the cooling
rate at 400 oC, one constructs a tangent line to the cooling
curve at this temperature as shown. Then, moving from
the bottom of this line to the top one measures the rise,
ΔT, and the run, Δt. The ratio of the rise divided by the
run, ΔT/Δt (called the slope in geometry classes) will have units
of oC/sec. and will be the cooling rate when the Inconel
center is at 400 oC. With computer software it is a simple matter to determine the cooling
rate at each temperature and the solid line is the corresponding cooling curve with the
cooling rates in oC/sec. given along the top of the diagram. Notice that the maximum
cooling rate is 70 oC/sec. and it occurs at around 650 oC for this oil. There is an excellent
discussion of water, oil and polymer quenchants in reference [12.8], and you will see on
pages 78 to 81 that these cooling rate curves are now widely used to characterize the
quenching power of the various quenching oils and polymer quenchants that are
commercially available. Figure 12.12 presents cooling rate curves for several common
quenching fluids. Notice that the maximum cooling rate occurs at different temperatures
and varies from a high of 285 oC/sec. for salt water to a low of 65 oC/sec. for a normal
oil.
For a given quenchant, the speed of the quenching process will depend on the
temperature of the bath as well as any agitation used on the work piece during the
quenching operation.
