Metal treatment – Process of modifying or maintaining internal physical... – Carburizing or nitriding using externally supplied carbon or...
Reexamination Certificate
2002-02-22
2004-08-17
Sheehan, John P. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Carburizing or nitriding using externally supplied carbon or...
C148S222000, C148S225000, C266S044000, C266S249000, C266S258000, C266S275000
Reexamination Certificate
active
06776854
ABSTRACT:
The present invention relates to a process for the partial thermochemical vacuum treatment of metallic workpieces.
The thermochemical treatment of workpieces of metals in a gaseous atmosphere of decomposable carbon compounds and/or nitrogen compounds, optionally mixed with other gases, for example inert gases and/or hydrogen is known. For example, DE 41 15 135 C1 describes a process for the treatment of, inter alia, hollow bodies such as injection nozzles or structural parts with bores that are similarly difficult to access. In this process the workpieces are loaded as loose items without any particular arrangement or alignment in the batch receiver. As a result the bore treatment depth is difficult to control, the external surfaces of the workpieces being preferentially treated. If the external surfaces are to be subsequently machined, this becomes difficult or impossible, since after the machining a hardening is out of the question on account of the hardening distortion.
EP 0 818 555 A1 too is concerned with the carburization of hollow bodies with blind holes, though here too the carburization preferentially takes place on the external surface of the hollow bodies.
EP 0 695 813 A2 discloses the use of a plasma with a pulsed voltage of between 200 and 2000 volts for the carburization. Here too however the total external surface of the workpieces is always carburized.
The Applicants' in-house publication “Vakuumgestützte Kohlungsverfahren mit Hochdruck-Gasabschreckung”, [“Vacuum-Assisted Carburization Processes with High Pressure Gas Quenching”] W2004d/9.97/2000/St., discloses complete process sequences both in a single-chamber vacuum furnace as well as in a multi-chamber through flow unit. The treatment of the external surfaces of drive/transmission parts such as gearwheels and shafts is described in particular. EP 0 313 888 B2 specifically relates to high pressure gas quenching for the hardening of steel workpieces.
It is also known to carburize workpieces partially in conventional gas carburization by “sealing” with a covering paste those external surface regions that are not to be hardened. Such covering pastes are however not suitable either for vacuum processes or for plasma processes since the covering pastes are unable to withstand the ion bombardment of the plasma. Attempts have also already been made mechanically to cover screw threads by encapsulation or plugging, but in this case also “subcreepage” of the coverings readily occurs due to the different expansions involved, which often can be eliminated only with difficulty and resulting in damage. Also, the threads that are thermally co-treated are no longer dimensionally accurate after the treatment.
It is known from DE 29 20 719 A1 to carburize in a zone-like manner individual annular workpieces such as gearwheels, coupling parts, running rings for roller bearings and the like so that the zones that are not to be carburized are screened by means of reusable sheathings against the carburization gas. This is achieved for example by covering the front faces of the workpieces with disc-shaped moulded parts of metal or briquetted metal powder that engage via annular flanges partially in the bores of the workpieces, and that are stepped or have an annular groove in order to protect the ends of the workpieces. In each case the largest proportions of the internal and also of the external surface regions are exposed to the carburization gas. Due to the carburization and hardening of the external surfaces a subsequent mechanical treatment, e.g. thread cutting, is made difficult. Although a continuous fabrication involving placing the workpieces on a porous conveyor belt and transportation through a throughput furnace are in fact disclosed, nevertheless this always involves the treatment of individual parts. Injection parts for engines and the non-carburization of the external surfaces of these parts are not disclosed.
It is known from WO 00/58531 A1 when coating workpieces with aluminium and/or chromium and compounds thereof to protect partial regions of the workpieces, for example the seat or roots of turbine blades, against the influence of the coating material by providing these partial regions with reusable masks or caps comprising ceramic components that do not react with the workpieces. However, the “masking” of individual workpieces and the coating of external surfaces of the said workpieces are always involved. Injection parts for engines are not disclosed, and in particular the non-carburization of all external surfaces of these parts is not mentioned.
Also, it is known from WO 99/13125 to protect a partial length, i.e. the end of tubular workpieces, for example drill elements, against a thermochemical surface treatment by providing the end of the workpiece with a cap that screens the aforesaid partial length against the influence of the thermochemical surface treatment. The largest part of the external surfaces is however subjected to the thermochemical treatment. Here too it is the masking of the ends of individual workpieces that is described however. Injection parts for engines are not disclosed.
From DE 35 02 144 A1 it is known to protect the internal surfaces of annular workpieces comprising plane front surfaces such as slitted piston rings against a nitriding treatment by protecting these internal surfaces with for example a coating of copper, nickel, chromium or tin. By axially arranging in rows and congruently tensioning the front faces of several workpieces against one another on a carrier it can also be achieved that only the cylindrical external surfaces are subjected to the nitriding treatment. This is the exact opposite of the invention, in which all external surfaces are to be protected against a thermochemical treatment. The process is neither intended nor suitable for workpieces other than annular workpieces that can be mounted on one another in a plane-parallel manner.
From DE 28 51 983 B2 it is known in the carburization of hollow bodies with different wall thicknesses, such as for example in the case of nozzles for diesel engines, to encase the surface regions of the thin-walled sections in jackets, in which a carburization process takes place at a lesser intensity than on the remaining surface regions, in order to avoid a so-called “through-carburization” and an embrittlement. This also applies to the embodiment in which several thin-walled sections of the nozzles are introduced through bores into a common, box-shaped cavity. For all embodiments it is true however that all surface regions, i.e. also the external surfaces of the workpieces, are to be carburized, and that the surroundings of both the thick-walled and also the thin-walled sections of the hollow bodies participate in a mutually throttled manner, for example via the nozzle bores themselves, in the periodic gaseous exchange in a vacuum furnace. The carburization and subsequent hardening of the external surfaces is extremely disadvantageous for a subsequent metal-cutting machining of the workpieces.
None of the aforementioned publications deals with the following problem:
1. It is difficult if not impossible to cover irregular and/or rough surfaces that have been formed for example by casting or forging processes against the penetration of for example carburization gases.
2. On heating to the conventional temperatures for gaseous treatments, which are carried out at 900° C. and above, the covering effect may be reduced or even destroyed by heat distortion, different expansions, etc.
3. Thin-walled extensions of otherwise thick-walled workpieces tend to undergo considerably more severe embrittlement.
4. With the partial covering of workpieces, the boundary between treated and untreated surface regions may be displaced during the treatment as a result of different thermal expansions.
5. Workpieces of relatively large batches, in particular in a mass production run, are exposed to identical process parameters in all predetermined surface regions.
The object of the invention is accordingly to provide a process of the generi
Bardelmeier Udo
Minarski Peter
Fulbright & Jaworski L.L.P.
Sheehan John P.
Vacuheat GmbH
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