Indirect heating furnace for the surface treatment of a metal or the like employing a salt bath

Claim 1

An indirect heating furnace for surface treatment of a metal or the like, employing a salt bath, comprising: a furnace body (1) made of a non-conductive refractory material and having a concave portion (11) with an open top end, for containing a thermal medium (2) mainly composed of a chloride; a heat-resistant vessel (3) having an upper opening, positioned in said concave portion and supported at the upper portion of said furnace body, in a manner to be replaceable, said vessel containing a treatment material mainly composed of a borate for surface treatment of a metal or the like; and at least one pair of electrodes (12) provided in said furnace body (1) and arranged at a specified location in the inner wall of said furnace body, which defines said concave portion (11), said at least one or more pairs of electrodes (12) being so located that a line (1) connecting the upper ends of the electrodes (12) of each a pair is located below said heat-resistant vessel (3), and that the smallest distance (M) between said line connecting the upper ends of the electrodes of that pair which is located nearest to said heat resistant vessel and said heat-resistant vessel (3) is not less than 1/4 of the distance (L) between said pair of electrodes.