EP0512254B2 - Process for uniform thermochemical treatment of steel pieces having difficult accessible areas - Google Patents

Abstract

Hollow bodies or holes in steel components are subjected to thermomechanical treatments above 450 deg.C with reactive gases, partic. ammonia with 5-95 vol.% N or a C-generating gas, pref. methane at 870-1000 deg.C, at gas pressures above 0.2 MPa. Pref. the treatment is carried out at constant pressure, between 0.2 MPa and 10 MPa, depending on furnace capability.

Description

The invention relates to a method for uniform thermochemical carburizing of hollow bodies and bores or areas that are difficult to access from the outside Steel components with carbon-emitting gases Temperatures from 870 to 1000 ° C.

The thermochemical treatment of metallic components for the production of wear and corrosion resistant Edge layers are wide in machine and gear construction spread. Coming as a thermochemical process for example carburizing for steel. Preferred process media are carbon donors Gas mixtures that make the diffusible or compound-forming substances on the workpiece surface Deliver at the selected treatment temperatures. Next Temperatures, gas compositions and types of steel are the Treatment results also from the shape of the components and on the nature of the batch within the Heat treatment system and the flow of the batch with depending on the carbon donating gas.

Despite many years of experience and numerous Process modifications it is technical and only economically possible in such gas mixtures complicated shaped components on the outside difficult carburize accessible inner surfaces. Among them are e.g. Steel parts with undercuts, internal holes or Blind holes, or nozzle body with to be hardened Understood interior surfaces. The lack of uniformity between inside and outside surfaces is lacking Availability of the carbon donating gas within the To justify hollow body. The gas fills the cavity, however, this process gas becomes poor in chargeable Material, since the Gauss exchange with the environment is not takes place because the inside of the hollow body Flow rate of the gas is close to zero. Usually with gas pressures of 0.1 MPa absolute worked, that is under normal pressure.

Also a periodic pressure change in the vacuum area, as described, for example, in US-A-4,160,680, improves the carburization layers in hard-to-reach areas Areas only marginally.

Furthermore, DE-A-28 51 983 describes a method for uniform carburizing of holes and hollow bodies with different wall thicknesses known in which parts of the hollow body covered or one be exposed to less carburizing activity. This However, procedure is very time consuming and unfriendly.

A number of known methods concern Gas nitriding of steel components under pressure.

JP-A-52-145 343 describes the nitriding of Cassette recorder housings with ammonia under pressure described. The nitriding process according to the DE-A-0 485 686 for steel workpieces works under pressure. US-A-2 779 697 describes a process in which steels in gaseous ammonia under pressure in a pressure vessel be nitrided. In GB-A-749 992 there is also a Nitriding process for steel surfaces disclosed, whereby Ammonia is used under pressure.

EP-A-0 226 729 describes a process for carburizing Rolling bearing components known, the carburization at Temperatures from 800 to 1000 ° with methane and / or propane as carbon donating gases takes place.

It was therefore an object of the present invention Process for the uniform thermochemical carburization of Hollow bodies and holes or difficult to access from the outside Steel components with surfaces carbon-emitting gases at temperatures from 870 to 1000 ° C to develop that is easy to do and even treatment layers on the outside supplies difficult to access areas.

This object is achieved in that the Treatment at pressures above 0.2 MPa takes place.

For carburizing, methane or is preferably used Methane mixtures. Besides, it is advantageous treatment with the carbon donor Gas under constant pressure.

It has surprisingly been found that in one Compression of the carbonization gas during the treatment phase the pressures mentioned above greater than 0.2 MPa do not occur. The higher pressure of the coal gas inside a hollow body causes a quick and complete formation of boundary layers. The in Carbonization gas is stored amount of the reactive substance sufficient, the typically required Hardening depths or connection layer thicknesses too produce. An impoverishment within the cavity reactive substances occur, but not up to the concentration in which, for example, growth is disturbed by connection layers. The concentration of the Carbonation gas is so high inside and outside that the Delivery of carbon to the goods to be treated in total available concentration less than at Treatment under normal pressure or negative pressure. The further the Treatment pressure is above 0.2 MPa, the lower the percentage change in the composition of the Process gas by diffusion into or reaction with the workpiece. This also means that so far known undesirable differences between indoor and External surfaces of hollow bodies to be treated eliminated.

This means that there are differences between inside and Outside areas become very small. Additional gas injection or gas circulation in the interior is therefore not required.

The pressures are generally between 0.2 and 10 MPa, the upper pressure limit depends on the Available oven.

The following example is intended to show the advantages of the invention Process clarify:

An injection nozzle made of a material 16MnCr5 is in a pressure-resistant furnace with methane at 0.8 MPa for 2 Carburized at 930 ° C for hours. The injectors are as bulk goods in the batch room without any special arrangement or alignment placed. After completing the treatment is the thickness of the carburizing layer outside on the nozzle and measured in the inner bore. The result is uniform Carburization depths inside and outside.

It is noteworthy that the outlet holes of the nozzle the nozzle tip with an initial diameter of 0.2 mm and a length of 1.5 mm in the inner surfaces are carburized. The layer thickness is equal to that Layer thickness in the entire inner bore of the nozzle.

The high concentration of reactive elements in the Treatment gas, enables a tight packing of the Batch items. So far, have to be uniform Treatment of the individual parts certain, empirically determined Clearances between the individual parts are observed. These distances can be achieved at pressures> 0.2 MPa decrease without reducing the uniformity of the layers suffer from.

All known carburizing gases can be carburized use.

Claims (3)

1. A process for the uniform thermochemical carburization of components composed of steel having inner surfaces poorly accessible from the exterior, such as components with undercuts, blind holes or nozzle bodies, with carbon-emitting gases at temperatures of 870 to 1000°C, characterised in that the treatment takes place at pressures exceeding 0.2 MPa.
2. A process according to claim 1, characterised in that methane is used.
3. A process according to claim 1 or 2, characterised in that the treatment takes place under constant pressure.