Iron, Steel and their Variations and Alloys by Brian Read - History of Woodworking Tools in UK

Under different conditions one can get "martensite", a molecular supersaturated dispersion of iron carbide in alpha-Fe. Its formation is promulgated by rapid cooling of red hot steel and under the microscope shows a characteristic needle like pattern. This is the hardest of all the decomposition products of austenite and is responsible for the hardening of high carbon steel by quenching. It is inherently brittle and this is exacerbated by the stresses built up in the quenching process. To avoid this causing problems in the finished product it must be tempered by raising its temperature in a controlled manner and holding it there for a predetermined period. Too low a temperature or too short a time and the stresses are not removed. Too high a temperature or too long and further structural modification occurs and the martensite is replaced by cementite and pearlite. The grain structure of the steel is also liable to be changed.

Depending on how much the structure was changed it may be possible to re-harden, but it will then again need tempering. All hardening and tempering must be correct for the intended use. It is always a trade off between hardness, wear resistance, flexibility and brittleness. Repeated "blueing" of plane blades, drill bits and chisels by grinding on a high speed wheel can have a permanent effect not eradicable by either grinding back or even re-tempering.

The quoted transition temperatures apply to pure iron and carbon mixtures.

Complications arise because other elements are usually present, either intentionally or as residuals from smelting the ore and these have an effect, desirable or otherwise, on the overall properties either by changing the transition temperatures or by altering the crystal structure. Manganese, Chromium, Copper, Nickel, Silicon, Molybdenum, Vanadium and tungsten are all added in various amounts and combinations to confer specific properties on the finished alloy steel while Phosphorus, Titanium, Tin and Aluminum need to be carefully controlled because of possible adverse effects. They may however be deliberately added to reduce certain properties considered undesirable for a given use.

Beware of the use of scientific sounding trade names. Electro-boracic steel appears on various tools, I have seen it on plane blades, adzes and chisels.

It is a totally meaningless phrase, just a Victorian advertising gimmick.

More serious was a set of spanners I analyzed while working for British Rail. They were marked Chrome-Vanadium Spanners but contained neither Chromium nor Vanadium – it was just a Brand Name!

What is Cryogenic Steel?

During conventional heat treatment the structure of steel changes from having a soft coarse irregular grain to a finer grain, which is more resistance to wear. However with conventional heat treatment not all the steel is converted, the best processors will achieve 90% but it can be as little as 50%, however with cryogenic treatment the conversion can be nearly 100%.

There are a number of methods to cryogenically treat steel but generally the tool is taken down in temperature using nitrogen gas, the tool is soaked in liquid nitrogen for 20 hours, during this time micro-carbides are created which give additional wear resistance. The tool is then slowly returned to room temperature before being re-tempered to remove the brittleness this process creates. The tool needs treating only once as the changes to the steel structure that other, A2 being particularly suitable.

When a tool is cryogenically treated a very thin outer layer remains unaffected; this is sometimes referred to as the "thin film phenomenon" although it is easier to think of it as a skin effect. This layer is in the order of one tenth of a thousandth of an inch and needs to be removed by sharpening or honing before the benefits of the treatment become apparent.

The Rockwell scale or hardness

The Rockwell scale is a measurement of a material’s hardness. There are many different Rockwell scales for different materials. The most commonly used arte the "B" and "C" scaled. The "B" scaled is used to aluminum, brass and softer steels and the "C" scale is used to harder steels such as those used to plane blades. This is a linear scale the way the measurement is made, for the Rockwell C Scale at least, is for the material to be placed in a press, then a diamond cone, known as a Brale indenter, is pressed into the steel under a load of 10kg. This seats the point and ensures that any surface irregularities will not affect the results. A 150kg load is then placed on the indenter and the distance it now travels gives a value expressed as "HRC" or "Rc". Obviously the softer the steel the deeper the indent, but giving a lower Rockwell number. For instance most chisels are made from steel with a hardness of HRC 40 to HRC 45 whereas a plane blade is harder and commonly would be made from steel with a hardness of HRC 55 to HRC 62.

Just because a particular type of steel is softer does not mean that is inferior, it is all about balancing the characteristics of hardness, strength and brittleness.

This article is a re-print from The Tools and Trades Historical Society - TATHS Newsletter. We very much appreciate an agreement from Brian Read, Newsletter editor, to present this article to American readers. Brian is also the author of the article presented here.

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