matthewwh":2wrk2s2w said:
From the 1970's onwards, after yours was made, the demand for ever cheaper tools that the sheds (spit) could sell for tuppence a bucketful, led to widespread adoption of impulse hardening. The chisel is held by a robot inside a coil for a moment and heated insanely fast, then it drops into a bath of oil to quench it. The problem with this is that the result after tempering can be anywhere from RC55 (mush) to RC65 (chippy) and in some cases both issues within the same blade. If a chisel chipped, people were more likely to return them to the sheds, so they asked for softer blades to reduce their return rate. Result - good ones and bad ones with an excellent chance of mush, even though the steel itself was capable of being outstandingly good.
That's interesting - because the usual advice when hardening steels (of any grade) is to ensure they are heated through before quenching, by soaking at hardening temperature for long enough to ensure an even heat right through - one hour per inch of thickness is one 'rule of thumb'. The speed of impulse hardening would presumably mean the case was hardened, but the core remained soft. Fine for some duties, but not necessarily ideal for a high quality edge tool.
Further to the notes on 'cast steel' - the straight carbon steel of about 1% carbon content with no alloying element additions - this is a material that is quite hard to harden; it needs a fast, aggresive quench in water or brine, and even then, it only hardens to a relatively shallow depth. It has another couple of problems, too. Firstly, it can change dimensionally when hardened; not a lot, but measurably. This isn't a problem to edge-tool makers, but it is to engineering gauge-makers. The second problem is that because of the very fast quench, it is prone to the locking-in of quite severe internal stresses which can cause items to distort, or in extreme cases, crack.
During the later part of the 19th century, a lot of experimentation took place to find solutions to these problems. It was found that the addition of quite small quantities of some other elements had significant benficial effects. One result was the fore-runner of our favourite 01 oil-hardening steel, which because it responds to a slower, more gentle oil quench, is much less prone to distortion (and not prone to dimensional change, which is why it is often called 'gauge plate', since it was much more suitable for the making of accurate measuring instruments than cast steel). Another advantage is that it hardens to a much greater depth. Some feel that it doesn't have quite the supreme edge-taking capabilities of plain carbon steels, but most feel that it gives a perfectly adequate edge. It thus tended to supplant plain carbon steels for edge-tool making from the end of the 19th century onwards. As some of it was still made by the crucible steel process, it might still be called 'cast steel', though. From WW1 onwards, more tool steels tended to be made by electric arc furnaces, producing steel every bit as good as 'cast steel'.
Many other alloy steels came about during this period of experimentation, not all of them of direct use to edge-tool makers. In fact, it's probably fair to say that no steel has ever been developed specifically for edge-tool use (except possibly cementation steel, and that mainly for swords and miitary knives); their main applications have always been for some other application - Huntsman steel for clock springs, 01 for engineering gauges, and so on. However, the edge-tool makers have always used what's available, often to very good effect.
