Tadashi Chihaya1, Ryoichi Urao1 and Shinichiro Yoshioka1
1Faculty of Engineering, Ibaraki University, Hitachi
Metals are nitrided by nitrogenion-bombarding on their surfaces. In order to apply this method as a method of surface hardening, it would be desirable to permeate a large number of nitrogen atoms into metals for a short time, i.e. to bombard them with a high ion current density at high temperature. From this point of view, the ion-nitriding method in glow discharge is being empoyed industrially, but many experimental basic data are not available. We tried to ion-bombard some steels in N2-H2 mixing gases under some different bombarding conditions, and examined their hardness, nitride products formed in the surfaces layer, etc. Results obtained are as follows.
(1) The iron and steel surfaces are hardened by nitrogen-ion-bombarding. The hardnesses of the steels containing Cr, Mo and Al are much higher than that of iron and plain low carbon steel.
(2) The highest hardness is obtained by nitriding iron and S 15 C steel in the gas of 70 vol% N2 and SCM 21, SACM 1 and SUS 304 steels in the gase of 30 to 50 vol%N2, respectively. The maximum hardness and the hardness change, in accordance with the relation between hardness and gas concentration, decrease with the increase in gas pressure.
(3) The highest hardness is obtained by nitriding iron and S 15 C steel at from 550 to 600°C and SCM 21, SACM 1 and SUS 304 steels at 500 to 550°C, respectively.
(4) ε -Fe2 --3N, γ′-Fe4N, γ -Fe and α -Fe phases are detected from nitrided surfaces by X-ray diffraction. The ε -Fe2 --3N phase is formed in the gas of high N2 concentration and is hardly formed in the gas lower than 30 vol%N2.
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