PURPOSE: To prevent an attack due to cavitation on the internal wall surface of a small diameter, thick walled pipe, either by quenching the pipe from outside after it is carburized at the internal wall and is heated as a whole, or by allowing it to cool after it is carburized and its inner wall alone is heated by high frequency induction.

CONSTITUTION: A small diameter, thick walled steel pipe 1 for charging fuel to a diesel engine is carburized at the inner wall side 4. The pie is heated as a whole, it is then cooled as fast as possible from outside. Or after the carburizing as described above, the inner wall alone of the pipe is heated rapidly by high frequency induction, then it is allowed to cool. By the treatment, the hardness of the thick walled part 2 of the pipe 1 is increased to give a hardness distribution of about HV 150 to HV 230 from the outside to the inside. With increase in hardness, the attack due to cavitation is prevented efficiently because a residual compressive stress layer 5 of high carbon content is formed in the peripheral direction including the longitudinal axis direction, and a dense crystal structure is formed.