Abstract: A method of carburizing treatment is proposed in which if carburizing is carried out at a low temperature, carbon will not turn amorphose and deposit on the surface of a titanium metal but reliably penetrate into between metallic atoms. It is a method of surface treatment of a titanium metal comprising the steps of heating the titanium metal to a temperature of 400-690° C. in a cleaning gas atmosphere containing hydrogen gas, subjecting the surface of the titanium metal to cleaning by applying a DC voltage of 200-1500 V, and plasma carburizing in an atmosphere comprising a carburizing gas having an atomic weight ratio of hydrogen atoms (H) to carbon atoms (C) adjusted to 1?H/C?9 at a pressure of 13-400 Pa and a temperature of 400-690° C. Ionization reaction in the gas is suppressed suitably. Because there exists no excessive carbon which is not used for carburization but turns soot or glass-like carbon, in the atmosphere during carburization, carburizing reaction progresses smoothly.

Abstract: Threaded parts for aircraft made of heat-resistant steel which will never seize even after subjected to large temperature fluctuations and vibrations for a long time. Thus they can be tightened sufficiently and also loosened for maintenance and inspection. The threaded parts are made of heat-resistant steel and having its surface subjected to carburizing so as to satisfy the following equations: 0≦X≦100, Y≧−10X+600, Y≦−6X+1000 where X is the depth (&mgr;m) from the surface of the threaded part and and Y is the Vickers hardness (HV).