The authors studied the relative influences of C, Al and Cr contents in the Al-Cr steels up n the nitrided case as well as the mechanical properties before and after nitriding, and found that the good nitrided case having high hardness and moderate toughness is acquired in the steels of the following compositions; "Al 0.7-1.2%, Cr 0.8-1.5% and Al%<Cr%."
But even in the steels as the above compositions, their nitrided cases tend to show somewhat brittleness, as the contents of Al, Cr and C become higher. From the results of mechanical testing, a slight temperbrittleness was observed in Al-Cr steels by the prolonged heating of nitriding.
Therefore, the further investigation was carried out on the effect of the special elements upon the Al-Cr steel such as Ni, Mo, V and Cu, seperately or in any combination.
Researches were made on Al-Mo, Al-V, Cr-Mo, or Cr-V steels. And heat and corrosion resisting properties of nitrided case were also investigated on the above all steels.

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It has been appreciated for some time past that valuable prope. ties can be conferred on steels by the use of large proportions of special alloy additions. These properties, which include especially corrosion and heat-resistance, are largely due to the fact that the steels are in the austenitic condition Until quite recently, it has not been felt worth while investigating the use of similar additions in cast irons, in view of the relatively poor quality of the basis material. As a result of recent investigations, however, it has now been realised that the production of the austenitic structure in cast iron by the use of larger propertions of such elements as nickel, copper and manganese, provides materials which have many valuable industrial applications
The outstanding characteristics of the austenitic cast irons, as compare with ordinay cast iron, are as follows:
(a) A marked degree of corrosion resistance.
(b) A marked degree of errosion resistance.
(c) A superior resisatnce to the effects of heat.
(d) Non-magnetic, with suitable compositions.
(e) A high electrical resistance coupled with a low temperature coefficient of resistance.
(f) A high coefficient of thermal expansion.
(g) A good machinability
(h) A good weldability
(i) Moderate toughness and malliability in shock test.
(j) It is not so difficult in foundry practice.
(k) The cost is lower than brass and bronz.

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