High Speed Steel
Not all high-speed steels contain cobalt, but possibly the newest and the best ones do.
High-speed steels are also steel but with large additions of refractory metals – tungsten, chromium, molybdenum, vanadium and, in specialised cases, cobalt. The other element in steel, namely “carbon”, forms “carbides” in carbon steels with just iron and in high-speed steels, with all the alloying additions except cobalt which has other functions. So, in essence, a high speed steel is a steel containing large amounts of refractory carbides which proved hardness, high temperature strength, wear resistance to tempering, with cobalt enhancing high temperature strength.
Structure is of paramount importance in tools steels and the aim is to get a very fine distribution of carbides. To this end, complex heat treatment schedules have been devised, often with two or even three tempering stages.
Three current methods of manufacture have evolved: i) air melt cast and work, ii) vacuum melt cast and work, iii) atomise – cold isostatic press – sinter – hot isostatic press and work.
The newer ASP alloys made by method (iii) are superior to other grades and the best of these contain high levels of cobalt (8-10%).
The benefit of the powder route is in the structure. Casting produces segregation by its very nature and further work and heat do little to change it. Atomising a homogeneous molten metal gives such rapid cooling that each “mini-ingot” (powder particle) is homogeneous unlike its large cast brethren. The rest of the process is to stick these little ingots back together into a pore-free, homogeneous form.
Why is cobalt in high-speed steels?
A good question as it doesn’t form carbides.
The reasons that have brought cobalt to prominence in these latest alloys are the same as they always were.
Cobalt dissolves in iron (ferrite and austenite) and strengthens it whilst at the same time imparting high temperature strength (temperature on cutting surfaces can be 850°C) During solution heat treatment (to dissolve the carbides), cobalt helps to resist grain growth so that higher solution temperatures can be used which ensures a higher percentage of carbides being dissolved. Steels are quenched after solution annealing and the structure is then very hard martensite, plus the retained high temperature phase austenite plus carbides peppered throughout the structure.
Tempering will precipitate the ultrafine carbides still in solution and maximum hardness will be attained. Here, cobalt plays another important role, in that it delays their coalescence. This is important as it means that during cutting, the structure is stable up to higher temperatures. Thus, cobalt-containing tool steels are capable of retaining strength to higher temperatures – They cut faster for longer.
Tools, however, are not longer as simple as they were. The surface can be modified by coating – with TiN or TiC for example, put on by plasma or vapour deposition. These coatings increase cutting life by large factors (4 or 5 times) and do so even after regrinding.
Cobalt in Cemented Carbides
The ability to cut metal faster and faster is to a great extent at the heart of the economic growth in the 20th Century. Up until World War I, cutting tools were made from high carbon steels and cutting speeds of 25 ft/min were the norm. 1896 saw the start of tungsten carbide manufacture when Moissan in France melted/fused tungsten and carbon together to make diamonds. He didn’t but WC resulted. Although mixtures of WC and MoC did get used for cutting, the great leap forward came when Schroeter and Osram produced a carbide material consisting of crushed tungsten carbide in cobalt. Iron was the first choice but it was cobalt for reasons which only became clear subsequently, which was the most successful binding material. The need for a binder is paramount as carbide alone is brittle and has little impact strength. The actual driving force however was not for cutting tools but as wire drawing dies.
Osram was cut off by a blockade from its sources of diamonds for dies and the carbide route was the alternative they developed. The cutting properties however were quickly exploited and by the 1920’s, 150 ft/min cutting speeds were commonplace.
Although nickel has also been used as a binder, cobalt reigns supreme. Why should this be?
There are several criteria which govern the performance of a binder for carbides:
a) It must have a high melting point – Cobalt: 1493°C
Cobalt fulfils all the needs of a binder whilst others, like Ni, Fe, etc., only fulfil some. It is this fact that has kept it irreplaceable in carbides.
b) It must have high temperature strength – Cobalt does
c) It must form a liquid phase with WC at a suitable temperature – Cobalt does at 1275°C. This pulls the sintered part together by surface tension and eliminates voids.
d) It must dissolve WC – Cobalt forms a eutectic with WC at 1275°C/1350°C and at that temperature dissolves 10% WC.
e) On cooling, WC should reprecipitate in the bond – in cobalt it does, giving hardness combined with toughness.
f) The binding agent should be capable of being ground very finely to mix with the hard carbide particles – cobalt can be produced very finely and grinds down to << 1µ. On grinding, it reverts to the close packed form which is brittle although in the carbide product, it retains the more ductile cubic form at room temperature.