J. Soc. Occup. Med. (1976) 26, 141-142

Manufacturing Processes Hard Metal L. R. PAYNE Employment Medical Adviser, Coventry Hard metal, a man-made product from powders, was developed in Germany after World War I. The product resembles metals in many of its properties—the chief characteristic is extreme hardness (90-95 per cent of diamond) and abrasion resistance. Hard metal is a cemented metal carbide consisting of small carbide particles of the highmelting-point heavy metals (usually tungsten) cemented together by binders from the iron group (often cobalt). Mixtures of titanium, tantalum, vanadium, molybdenum and chromic carbides may be added for specific characteristics. Manufacture (Fig. 1) Typically a hard metal would be 80-90 per cent tungsten carbide, 8-18 per cent titanium carbide and 6-10 per cent cobalt. Tungsten Carbide Imported as wolfram (Fe/Mn/WO 4 ), the ore is crushed and ground to liberate the tungsten minerals. These are concentrated by gravity and/or flotation, the concentrate being separated by leaching, roasting and electromagnetic methods. Contaminating sulphide minerals are usually removed by flotation using a reagent Ce.g. xanthate). The resultant tailings are treated with gangue (quartz and calcite) depressants and collectors for the tungsten minerals. Electromagnetic separators remove magnetic impurities. Acid leaching removes phosphorus and calcite. After decomposition of the ore and concentrate, the preparation of pure tungsten oxide follows and it is reduced to pure tungsten powder. A mixture of this powder and imported lamp black is now heated to 1400-1500°C in a stream of hydrogen to produce tungsten carbide powder.

extracted by the usual mining methods, then concentrated by settlement, flotation or magnetic techniques. Alternatively the slag from smelted ore produces a titanium concentrate. Refining to produce good quality titanium dioxide is carried out by precipitation following hydrolysis using sulphuric acid. The titanium oxide is mixed with lamp black and the mixture heated in a reducing atmosphere to produce titanium carbide powder. Cobalt Oxide Most cobalt is obtained as a by-product during the processing of other metals, especially copper. The Wolfram

Titanium oxide

1 Pure tungsten powder Cobalt oxide

1

Mixed

1

1 Reduced

Slaved

1

Heated in hydrogen at 600-1000 C

1

Heated in hydrogen at 1400-1500 C

Heated in hydrogen at I40O-150OX

I

Titanium carbide

Cobalt metal powder

i

1

Pure tungsten carbide

i

Sieved

1 Sieved

Sieved I

»

Mixed

1

•*

1

Ball milled (wet ground to very small particle size)

i Centnfuged

1 Vacuum dried

Fig. 1. Diagrammatic representation of manufacturing process.

particular process will depend on the characteristics of the ore used, but precipitation of different Titanium Oxide metals results and cobaltous oxide and cobaltic Titanium ores (ilmenite, rutile, titanite etc.) are oxide are disassociated. The oxides after sieving are 141

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OCCUPATIONAL MEDICINE

heated in a hydrogen atmosphere to be reduced to cobalt powder. The tungsten carbide, titanium carbide and cobalt metal powders are sieved and mixed. Wet grinding in a ball mill (to produce very small particles) follows, then the powder is centrifuged and vacuum dried to produce the final hard-metal powder. The hard-metal powder is moulded into shape by hot or cold pressing using a little wax as a binder. Machinable blanks are obtained by presintering the moulded shape at about 1000°C. After grinding to the correct dimensions, sintering at 1500°C in a hydrogen atmosphere completes the manufacturing process. The finished product is now ground to final dimensions. Grinding wheels of aluminium oxide as used for cutting steel tools are ineffective where hard metal is concerned—the wheel wearing away before the hard metal. Silicon carbide or cubic boron nitride wheels may be used, but other problems arise, so in practice synthetic or natural diamond wheels are the grinding wheels of choice. The cost is about ten times that of aluminium oxide. Uses The TLV of hard metal is 5 mg/m 3 and the maximum allowable concentration in Russia is 6 mg/m3. Hard metal increases the cutting-edge life of a tool 8-100 fold, but because of the very high cost

it is only brazed on to the edge or tip of the parent tool. Typical products making use of hard metal are rock drills, masonry drills, the teeth of coal cutters, cutting tools in engineering and working dies. Hard metal is also used militarily as protective armour plate, and for the nose cones of armourpiercing shells, bullets and torpedoes. As the cost of sharpening a machine tool tipped with hard metal is high, 'throw-away' disposable hard-metal edges have been developed. These 3- or 4-sided tips are clamped on to the tool body (instead of being brazed on as in the usual hardmetal-tipped machine tools) thus providing 3 or 4 edges if a positive rake is used, or 6 or 8 edges using a negative rake. Location of each edge is accurate when the tip is rotated to a new position, so the cost of machine inactivity during a resharpening process and the expense of tool resetting is avoided, Advantages of Hard-metal Machine Tools 1. Tool life is longer. 2. Higher machine speeds are possible resulting in a higher output. 3. Component dimensions are more consistent as tool wear .is so much slower. 4. The metallurgical quality of the product is enhanced as there is less 'bruising' of the component by slightly blunted steel tools, i.e. the component is cut rather than bruised.

Requests for reprints should be addressed to: Dr L. R. Payne, EM AS, c/o HM Factory Inspectorate, 4 Copthall House, Station Square, Coventry, CVI 2PP.

SUBSCRIPTIONS The subscription rate for Volume 27 of the Journal of the Society of Occupational Medicine, commencing January, 1977, will be £900. This subscription is now due and should be forwarded without delay.