During thermo chemical surface treatment such as carburizing, nitrocarburization, boriding, etc. it is sometimes required that certain surfaces shall maintain the same chemical composition as before the heat treatment. Normally, this happens by coating these surfaces with some preparation (i.e., masking the surfaces) which prevents the steel surface from being directly exposed to the furnace atmosphere. The coating forms a more or less diffusion sealed layer for elements in the furnace atmosphere which is not in equilibrium with steel. The coating can be done in a number of ways such as by brushing, dipping, spraying of fluid coating, sticking of pastes or by electrolytic copper coating.
During, for example, the carburizing of tubes, if one wishes that the ends and a portion about 20 mm along the tube length are not carburized, neither internally nor externally, a usual method is to dip the tube end in a coating material, referred to as paint, which then may dry under increased temperature to form a layer. For the layer to be dense, drops must not be formed, and the layer must not be too thin. Therefore often an adjustment must be done with a brush or with another tool after the dipping for smoothing the layer. The method is expensive since it requires large exactness both with regards to the paint (viscosity, composition etc.) and the application step.
Depicted in FIG. 7 is a drilling apparatus comprised of a drill member 1 and a cutting head 50 attached thereto. The drill member is surface treated except at an end portion where the cutting head is to be attached. The heat treatment is prevented there by a masking step. FIG. 1 shows an example of a hollow drill member or a tube 1, an end 2 of which has been masked with a protective coating in a conventional manner through dipping. The protective coating is applied at the end surface and along a part of the surface of the tube length and somewhat along the inside hole 1a. Arrows depict the motion of the furnace atmosphere on the surfaces. FIG. 2 shows characteristics of a carburized tube 1 whose end surface 2 had been painted with a protective coating before the carburizing.
The carburized layers 4 have a depth d usually of about 1 mm and a relatively abrupt transition 5 at the junction with the protected surface. The length a of that transition zone is often about 1 mm. The abruptness of the transition 5 can be calculated by dividing the transition zone length by the depth, i.e., a/d (the shorter the length a, the greater the abruptness). Thereby, the abruptness amounts to about 1 in the conventional method for heat treatment such as disclosed for example U.S. Pat. No. 4,165,243. The abruptness of the transition creates unfavorable states of strain in the surface for example if the tube or the rod is submitted to bending or impact stresses. A thermo chemical treatment of a down-the-hole drill bit is described in U.S. Pat. No. 4,867,015, wherein the insert holes in the drill bit are covered with print before the heat-treatment is started.
One object of the present invention is to provide a method for masking a member before a thermo chemical surface treatment in a simple and thereby cost-effective manner.
Another object of the present invention is to provide a method for masking a member prior to a thermo chemical surface treatment for improving the strength of the member.
Still another object of the present invention is to provide a drill member with successively varying heat treatment.
Still another object of the present invention is to provide a device which enables control of the heat treatment.