Percentage of Coeflicient of Expansion Nickel. ' per degree C. 26 000001312 28 000001131 28-7 000001041 30 -4 0 000004 58 3 I -4 0 00000340 34-6 0-00000137 35-7 0 '00000087 37-3 0-00000356 39-4. 000000537 44 -4. 0 00000856 475 . . 0-00000870 Ordinary Mild Steel 0-00001078 Ordinary Hard Steel 0-00001240

There is little doubt that the progress in the development of the engine was at times held up almost entirely until a more suitable material could be found to meet the case.

In the very high speed engines of large output, metals of higher conductivity were inevitably called for, and those made use of are special aluminium alloys for pistons, cylinder covers, etc. In the forged state in particular, such materials give excellent results.

Some progress has taken place also in the use of magnesium alloys because of their low specific gravity, coupled with very good heat conductivity. Here again the forged conditions produce a much more lasting material than with castings.

For crankshafts, connecting rods, etc., special steels were developed with exceptionally high stress and impact values. An example is shewn in Fig. 21.

For exhaust valves to resist the great heat, high nickel chrome alloys have been successfully introduced, a strength of 34-5 tons being retained at 700°C.

The great need for the surface hardening of parts for quick running engines to resist wear has at last resulted in the invention of a system whereby, after treating special steel with ammonia gas, an extraordinary hardness is obtained. Brinell hardness figures range from 900 to 1,100, equivalent to Shore scleriscope of 102 to I 13 (Fig. 22). This looks to be of real benefit for the wearing