CARBURISING TREATMENT OF A STEEL WITH REDUCTION OF THE HYDROGEN CONTENT IN THE CARBURIZED LAYER

Abstract: Steel is carburized at a temperature within the austenitic range and in an atmosphere which comprises nitrogen, hydrogen and carbon monoxide, by lowering the hydrogen concentration in the treatment atmosphere prior to cooling the steel in oil, salt or water, so as to reduce the hydrogen concentration in the carburization layer of the steel, by outward diffusion of hydrogen from the carburization layer.

Description: Carburizing Treatment of a Steel with Reduction o f t h e Hydrogen Content in the Carburized Layer. The present invention relates to a method of carburiz¬ ing steel at a temperature within the auεtenitic range and in an atmosphere which comprises at least nitrogen, hydrogen and carbon monoxide. Parts which are subjected to heavy stresses and strains, such as gear wheels, are often manufactured from steel which has been surface-hardened in a carburizing process. In one known method for carburizing steel parts, the parts are cooled slowly in propane gas down to room temperature, so that the hydrogen taken-up in a car¬ burization layer during the carburization process, through dissociation of propane gas, is able to diffuse outwards from the carburization layer, at least par¬ tially (US-PS 3 737 204). In those cases where the carburization cycle is followed by direct quenching of the parts in oil, salt or water, some of the atomic and molecular hydrogen will remain in the carburization layer. In the case of nickel-alloy steel, this hydrogen results in ■ embritt ent of the hardened layer of the steel, which can result in dangerous crack formation. Accordingly, one object of the present invention is to provide a method of carburizing steel parts, particu¬ larly parts comprised of low-alloyed nickel-alloyed steel, in which embrittle ent of the hardened layer ' of said steel parts due to hydrogen dissociated from the carburizing gas is significantly avoided. Another ob¬ ject is to provide a method which can be defended economically. SUBSTITUTE SHEST These objects are achieved with a method having the characterizing features set forth in Claim 1. The object of the invention is achieved with the inven- tive method, by treating the steel parts in an inert gas, such as nitrogen, during the last phase of the carburizing process. The inert gas thus functions as a sheilding gas so that neither oxidation, decarburiza- tion or carburization will take place on the surfaces of the steel parts. By flushing the furnace chamber with inert gas over a given period of time, the hydro¬ gen concentration in the furnace atmosphere will de¬ crease and part of the hydrogen taken up by the car¬ burizing medium during carburization of the carburiza- tion layer is able to diffuse out of said layer. Consequently ' , only an extremely short heat-treatment time is required for removal of hydrogen from the car¬ burization layer, thereby rendering the inventive me- thod highly economical, due to the short treatment time required and the low energy input. Other advantageous features of the inventive method will be apparent from the dependent Claims. The steel parts are normally carburized at a tempera¬ ture of between 900 and 980°C. Carburization is either effected in a batch-type furnace having integrated or separate cooling baths for cooling medium, such as oil, or in a continuous furnace with integrated cooling baths. The carburization atmosphere used is a carbon monoxide/hydrogen mixture, normally obtained through the dissociation of methanol. The carbon potential of the furnace atmosphere is normally regulated by adding hydrocarbons, such as propane or methane. The carburization time varies from between 1 and 100 hours. The undesirable absorption of hydrogen in the steel takes place during the carburizing process. The amount of hydrogen taken-up in the steel increases with in- creasing carburization times and with increasing pro¬ portions of hydrogen gas in the furnace atmosphere. According to the present invention, steel parts are treated in the final stage of the carburization cycle and in the carburizing furnace with flowing nitrogen, which is introduced into the furnace at the same time as the introduction of the carburization atmosphere ceases. The steel parts are treated in the flowing nitrogen atmosphere for a period of time such that substantially all dissociated hydrogen will diffuse out from the carburization layer. Instead of using dissociated methanol as the carburi¬ zing agent, another gas, e.g. methane, can be used which breaks down into carbon and hydrogen at the car¬ burizing temperature used.

Claims: CLAIMS 1. A method of carburizing steel at a temperature within the. austenitic range and in an atmosphere com- prising nitrogen, hydrogen and carbon monoxide, c h a r a c t e r i z e d by lowering the hydrogen concentration of the treatment atmosphere prior to cooling (quenching) the steel in oil, salt or water, such as to reduce the hydrogen concentration in the carburization layer of the steel by outward diffusion of hydrogen from said carburizated layer. 2. A method according to Claim 1, c h a r a c ¬ t e r i z e d by introducing hydrogen and carbon mono- xide into the treatment atmosphere, through dissocia¬ tion of supplied methanol. 3. A method according to Claim 2, c h a r a c ¬ t e r i z e d by reducing the methanol content in the treatment atmosphere towards zero, such as to lower the hydrogen concentration of said atmosphere. 4. A method according to Claim 3, c h a r a c ¬ t e r i z e d by carrying out the method in a batch furnace and terminating the carburizing process by totally cutting-off the supply of methanol thereto, and thereafter flushing the furnace with nitrogen gas. 5. A method according to Claim 3, c h a r a c t e r i z e d by carrying out the method in a contin¬ uous furnace, and by maintaining a nitrogen atmosphere in the last zone of the furnace before the cooling zone.

Inventors: HOLM TORSTEN, HOLM, TORSTEN
Applicants: AGA AB
Application year: 1990
Main classification: C21D3
IPC codes: C21D3, C23C8