Around the turn of the century in the United States, fundamental metallurgical work was being undertaken by F.W. Taylor and his associate M. White and by 1901, these researchers had greatly improved the overall tool steel and slightly modifying its composition with a material that was to be known as High-speed steel (HSS), enabling cutting speeds to approach 19 m·min–1. High-speed steel was not a new material, but basically an innovative heat treatment procedure. The typical metallurgical composition of HSS was: 1.9% carbon, 0.3% manganese, 8% tungsten, and 3.8% chromium, with iron the remainder. Taylor and White’s tool steel mainly differed from that of Mushet’s by an increased amount of tungsten and a further replacement of manganese by chromium. By 1904, the content of carbon had been reduced, allowing for more ease in forging this HSS. Further rapid development of the HSS occurred over the next ten years, with tungsten content increased to improve its ‘hot-hardness’. Around this time, Dr J.A. Matthews found that vanadium additions had improved the material’s abrasion resistance. By 1910, the content of tungsten had increased to 18%, with 4% chromium and 1% vanadium, hence the wellknown 18:4:1 HSS had arrived, its metallurgical composition continued with only marginal modifications over the next 40 years. Of the modifications to HSS during this time, of note was that in 1923 the so-called ‘super’ HSS was developed, although this variant did not become commercially viable until 1939, when Gill reduced the tungsten content to enable the tool steelto be successfully hot-worked. Around 1950 in the United States, M2 HSS was introduced, having some of the tungsten content replaced by that of molybdenum. This gave the approximate M2 HSS metallurgical composition as: 0.8% C, 4% Cr, 2% V, 6% W and 5% Mo – Fe being the remainder. In this form, the M2 HSS could withstand machining temperatures of up to 650°C (ie the cutter glowing dull red) and still maintain a cutting edge.
In 1970, Powder Metallurgy (P/M) by metallurgical processing via hot isostatic pressing (HIP), was introduced for the production of HSS, with careful control of elemental particle size; afterward the sintered product is forged then hot-rolled. This HSS (HIP) processing gave a uniformly distributed elemental matrix, overcoming the potential segregation and resulting non-homogenous structure that would normally occur when ingot-style HSS forging. Such P/M processing techniques enable the steel-making company to ‘tailor’ and specify the exact metallurgical composition of alloying elements, this would allow the newly-developed sintered/forged HSS tooling to approach that of the performance of cemented carbides, in terms of inherent wear resistance, hardness and toughness. In Fig. 3, a comparison of just some of thetooling materials is highlighted, here, fracture toughness is plotted against hardness to indicate the range of influence of each tool material and the comparative relative merits of one material against another, with some of their physical and mechanical properties tabulated in Fig. 3b. A typical sintered micro-grained HSS of today, might contain: 13% W, 10% Co, 6% V, 4.75% Cr and 2.15% C – Fe the remainder. One reason for the ‘keen’ cutting edge that can be retained by sintered micrograined HSS, is that during P/M processing the rapid atomisation of the particles produces extremely fine carbides of between 1 to 3 μm in diameter – which fully support the cutting edge, whereas HSS produced from an ingot, has carbides up to 40 μm in diameter. By way of illustration of the benefits of the latest micrograined HSS – in the uncoated condition – when compared to its metallurgical competitor of cemented carbide, HSS has a bend, or universal tensile strength of between 2,500 to 6,000 MPa – this being dependent on metallurgical composition, whereas cemented carbide tooling has a bend strength of between 1,250 to 2,250 MPa. These metallurgical tool processing techniques have significantly improved sintered micrograined HSS enabling for example, high-performance drilling, reaming and tapping to be realised.
Coating by either single-, or multiple-coating has been shown to significantly enhance any tooling material, but this is a complex subject and more will be said on this subject shortly.
