Metal treatment – Process of modifying or maintaining internal physical... – Carburizing or nitriding using externally supplied carbon or...
Reexamination Certificate
1998-09-10
2001-05-29
Rachuba, M. (Department: 3724)
Metal treatment
Process of modifying or maintaining internal physical...
Carburizing or nitriding using externally supplied carbon or...
C148S213000
Reexamination Certificate
active
06238489
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and apparatus for masking a drill member, prior to a thermo chemical surface treatment thereof.
BACKGROUND OF THE INVENTION
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
1
a
. 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.
SUMMARY OF THE INVENTION
The objects and advantage of the invention are achieved by a method of masking a metallic percussive drill member prior to a thermo chemical surface treatment thereof. The drill member is of elongated shape and has a central channel formed therein. The method comprises the steps of:
A. positioning on an end portion of the drill member a lid such that an end wall of the lid abuts a free end of the drill member, and a side wall of the lid extends along a surface of the drill member with a gap formed therebetween;
B. heating the lid and the drill member to a heat-treating temperature in a furnace whereby a portion of the surface of the drill member not covered by the lid is heat treated to a depth, and whereby furnace atmosphere enters the gap and causes the heat treatment depth to be gradually reduced in a direction toward the end wall;
C. cooling the lid and the drill member; and
D. removing the lid from the drill member.
The present invention also relates to a drill member which has been hardened to a depth along a first portion thereof and non-hardened along a second portion thereof. The depth of hardening becomes gradually smaller at a junction between the first and second portions and defines a transition. The transition has a longitudinal length. A ratio of: (a) the length of the transition to (b) a depth of the hardening along the first portion, is greater than one.
The invention also pertains to a heat treatment lid for protecting a metal surface of a member from the effects of a thermo chemical treatment. The heat treatment lid comprises an end wall and at least one side wall extending from the end wall to form a recess opening in a direction opposite the end wall and adapted to receive the member.
REFERENCES:
patent: 3527624 (1970-09-01), Biretta et al.
patent: 3873376 (1975-03-01), Ritzka
patent: 4062702 (1977-12-01), Kunst et al.
patent: 4165243 (1979-08-01), Sarnes et al.
patent: 4664722 (1987-05-01), Clinkscales et al.
patent: 4867015 (1989-09-01), Kane et al.
patent: 27549 (1898-04-01), None
Burns Doane Swecker & Mathis L.L.P.
Rachuba M.
Sandvik AB
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