Cold soaking gives hot performance

The lives of industrial cutting tools can be enhanced by as much as ten times, and wear resistance up to eight times, by a careful cold treatment at liquid nitrogen temperatures The effect was first put down mainly to the improved elimination of retained austenite, the softer form of steel that tends to exist at high temperatures, but is now known to be largely due to the formation of particles of 'eta' or 'n' carbide. Sizes are typically 5 to 10nm across and 20 to 40nm long. Cold treatment of steels began 150 years ago when Swiss watchmakers developed the practice of burying their watch parts in the snow for a winter because they noticed that it would make them more durable so the watches kept better time. In the 1930s, tool and die makers noticed that freezing tool steel would help it hold an edge better than tool steel that had not been frozen. However, the real breakthrough came in the 1960s when NASA scientists noted that many of the materials used in spacecraft were improved once they had been subjected to the heat and extreme cold of space. The basic heat treatment of most steels is essentially the same as that discovered by the first smiths thousands of years ago. Steel is heated until the crystal structure of the iron changes from ferrite to austenite, in which form it can dissolve carbon. It is then quenched to transform the austenite to the hard, strained, form of ferrite called martensite. This is hard, but also brittle, so it has to be tempered to allow the precipitation of fine carbide particles, which provide strength combined with toughness. With plain carbon steels and even more with alloy steels containing other elements, benefit can be obtained by freezing the steel to complete the austenite to martensite transformation, with subsequent temper precipitation at a low enough temperature to ensure precipitated carbide particles are of small size. Long ago, it was realised by theoretical metallurgists that the finer the particle size of a hard second phase, the stronger the material would be. However apart from the obvious hazard, dropping hot steel parts into liquid nitrogen would only crack them. The proper treatment, which in the UK is available from Cryogenic Treatment Services in Nottingham, is to carefully cool the steel in gaseous nitrogen to liquid nitrogen temperatures in computer controlled cryogenic cabinets, and then slowly heat it up again. The charge for processing an average single carbide insert is £1.50, drills average £2.50 and the company accepts orders from £75 plus VAT. Cryogenic Treatment Services Pointers * Careful cooling to liquid nitrogen temperatures followed by equally careful re-heating can increase tool life by up to 1000%, in the case of a stamping die made of high carbon/chromium die steel * Other tools, even those made of plain carbon steel, show life enhancements from 250% to 600% and wear resistance improvements from 92% to 500%