The influence on all heat treatment processes with fluid media like quenching oil is shown here: you see the process scheme: we measured the weight of the 3 CFC CF222 samples (see part 1, 6.8% open porosity in average) before Carburizing m0 (1), after Oil quenching m1 (1b), after washing in alkaline solution m2 (2) and after annealing in air m3 (3).

Next follows in general a total oil absorption of the samples mentioned before of about 5 to 6 % (black columns) after the first about 5 process cycles.

In detail the analysis diagram above shows: after carburizing and oil quenching (green columns) an oil absorption of 6.0 % in cycle 1. Over the time, with stable conditions, the absorption reduces to about 0 % (see cycle 7 and 8), because the pores were filled with oil from the cycles before.

But why wasn’t the oil washed out in step 2 in the washing step (blue columns)?
One reason is that oil is hydrophobic, another that it was some solidified and cracked after annealing (step 3, red columns). Note: the first annealing cycles with 170 ^oC didn’t show much oil staying in the pores or cracked oil, see desorption of high 2.8 and 2.5 % (red columns), because the temperature was higher than the beginning oil boiling temperature range for Petrofer ISORAPID 221 at about 150 ^oC.

And, note, if the annealing temperature was increased again, here up to 180 ^oC in cycle 8 (red arrow), again higher 2.3 % or about 50 % of the oil was emitted.

Note: an oil absorption during Carburizing couldn’t be measured because it took place in one chamber, but it occurs! Evidence by an increased C-level in the furnace chamber for a Rack application for Annealing of Allen keys with oil quenching see the next figures.
The furnace control system turned down the higher C-levels. And, note, the net load of the furnace was increased with the CFC-Rack by about 70 %. In comparison to that the C-level caused by oil is shortly about 50 % higher than with the steel basket.

Some stable process is possible if the annealing temperatures are not higher than the beginning oil boiling temperature range and the temperature cycles are stable.

The following Reference shows the former steel basket on the left and the new CFC Trays on the right. The advantages we achieved were obvious. From a load increase of 70 % over less leveling or rework of the Allen keys and CTQ values like hardness or microstructure in a smaller range to around 50 % reduced handling times during loading and unloading of the furnace carriers. Lastly the life time cycles could be increased dramatically. Conclusion: outside the box thinking and some efforts brought the Wiha company a short pay back time incl. high efficiency and a competitive advantage.

The solutions avoiding oil-absorption and desorption start with understanding the processes in detail. Based on this deep understanding there are some possibilities starting with some design considerations (high open porosity on the edges vertical to the fibers and low parallel) over some little process changes up to the usage of antimony, silicon impregnated or highly densified CFC or C/C-SiC with only 1 to 3 % open porosities. Ask us for the difference!

Sources: Dr. Jorg Demmel, 2002-2019

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