Method for hardening steel pipes

Abstract

This invention relates to a method wherein the setting angles of cooling water jetting orifices an outer quenching head and an inner quenching head are made 30.degree..+-.10.degree. in the pipe advancing direction with the pipe axis and the number of jetting orifices of the inner quenching head are greater for purposes of higher density for the front one-third of the length of the head and lower for purposes of lower density for the rear two-thirds of the length of the head. Further, the first contact point of inner quenching water within the inner surface of the pipe is so set as to be delayed by 20 to 150mm. from the first contact point of quenching water with the outer surface of the pipe, the outer quenching being carried out when the steel pipe heating temperature is above the A.sub.3 transformation point and the inner quenching is carried out when the temperature of the inner surface of the pipe is just below the A.sub.1 transformation point. As both heads have a fixed angle, water or steam will not enter the unquenched part and the quenching velocity and the hardness of the cross-section of the steel pipe will be improved.

Dependencies are inferred from the claim text.