Case Hardening

I fully support John Fawcett’s comments about cyanide heat treatment. It can be full of hazzards for both professionals and amateurs, as the following example shows.

The `hardening shop’ I hinted at in my earlier posting on this subject, was a small room next door to our toolroom. It was accessed through a personal door in one corner of the toolroom. As a young apprentice, I could never comprehend that next door to the hardening shop was a fairly large work area know as the drug-store. It must have been the most significant fire risk area of the whole factory, since it was stocked with open bins of finely powdered coal, sulphur, and various other industrial chemicals. These chemicals were openly weighed into bins, which in turn were elevated along conveyors. There were bails of natural rubber too, ready to be cut up and fed to two-roll mills and large Banbury mixers.

Back in the hardening shop were a couple of gas-heated cyanide baths measuring some 18" deep by about 9" diameter. There was also a small electric furnace for hardening HSS. I think it was normally heated to about 1250-1300C. For quenching, there was a bath of hot water, and standing on the floor was a rather large tank of oil which measured about 4' high and 4' diameter. Allowing for the volume of any items to be quenched, the oil tank was kept topped up to within an inch or so of the rim.

It became necessary to harden several cylindrical mould inserts. At a guess, these steel inserts measured about 14" long and 6" diameter. The actual mould cavity was by way of a hole down the centre of the inserts. This through-hole measured perhaps 1.7" diameter. In order to lower these inserts into the cyanide bath, the wise ones in charge made up a hook and had it bolted to the platform of one of those hand pumped stacker-trucks. The idea being that the mould inserts could be lifted from the cyanide bath and then lowered into the oil.

Unfortunately, when it came time to quench the first heated insert, the stacker truck could not be pumped up high enough to lift the insert over the edge of the oil tank. It was therefore necessary to rapidly transfer the red-hot insert from the hook of the stacker truck to a steel wire hoist above the oil tank, the hoist in turn being secured to the steel roof truss. The hoist consisted of two pulleys around which were several turns of twisted wire. The transfer went well, and the insert was lifted up and over the edge of the bath of oil.

Then the real drama began. The hoist hadn’t been used for many years, and as the insert was being lowered into the oil bath, the hoist wire spun around and jammed. This was a perfect situation for a massive fire because, with the red-hot insert half in and half out of the oil, ignition was almost instantaneous. Burning oil began to pour over the sides of the tank, and there was panic. Somehow, they managed to free the hoist and totally submerge the insert, but flames began to leap towards the roof, as sand and all sorts of other stuff was used to try to quell the blaze. Finally, the fire was brought under control. The drug-store didn’t ignite, and the excitement ended with a toolroom full of smoke and several red faces.

Having replaced certain instrument parts which varied in thickness, and choosing to go via the case-hardened mild steel route, the question of final brittleness was always a consideration.