Process for the heat treatment of iron-based metal parts in an active atmosphere with a high potential of carbon

Description

FIELD OF THE INVENTION

[0001] This invention refers to a process for the heat treatment of iron-based metal parts in an active atmosphere with a high potential of carbon, which makes it possible to increase the carbon content of the metal parts. Treatments to this type of problem includes cementation and carbonitriding.

BACKGROUND OF THE INVENTION

[0002] One already knows how to produce atmospheres with a high carbon potential by means of thermal dissociation of alcohols, particularly methanol. This dissociation determines the formation of a reducing species, such as hydrogen, and of a species which is carburizing as well as reducing, such as carbon monoxide. If it is necessary to attain certain values of concentration, these species can eventually be diluted with nitrogen from a cryogenic storage tank.

[0003] This known process for the production of a heat treatment atmosphere with a high content of carbon, however, involves certain disadvantages.

[0004] On the one hand, in fact, the quantities of hydrogen and carbon monoxide obtained are in a fixed ratio which is determined by the atomic composition of the alcohol utilized, and therefore this may be unsuitable for certain types of treatment.

[0005] On the other hand the utilization of raw material as an alcohol, particularly methanol, involves intrinsic problems of high cost and safety considerations.

SUMMARY OF THE INVENTION

[0006] In order to prevent these above-mentioned drawbacks, one object of this invention is a process for the heat treatment of iron-based metal parts in an active atmosphere having a high carbon potential, characterized by the fact that this atmosphere is obtained by catalytic reaction between a hydrocarbon combustible and an oxidant having an oxygen content of more than 21% by volume and said atmosphere containing at least 60% by volume of a carburizing species and/or reducing material.

DETAILED DESCRIPTION OF THE INVENTION

[0007] The raw materials used to generate a treatment atmosphere according to the invention are easily available at reasonable cost and present no particular safety hazard. In fact and in particular, it is possible to utilize natural gas or methane as combustible.

[0008] Furthermore, by suitably adjusting the ratio between fuel and oxidant, and the oxidant's oxygen content, it is possible to obtain an ample range of treatment atmosphere compositions usable for the most diverse requirements.

[0009] In particular, by increasing the oxygen content of the oxidant to 100% and correspondingly reducing the content of inert materials, especially nitrogen, it is possible to obtain 100% of carburizing and/or reducing agents, such as hydrogen and carbon monoxide in the treatment atmosphere.

[0010] Furthermore, reducing the content of inert materials makes it possible to reach a higher temperature during the combustion reaction. This provides the additional advantage of promoting the formation of desired materials, such as hydrogen and carbon monoxide, with respect to the completely oxidized undesired materials such as water and carbon dioxide.

[0011] The catalytic combustion reaction can be conducted without distinction either in a reactor separate from the chamber in which the heat treatment is carried out or in a reactor contained within it.

[0012] Preferably, the oxygen content of the comburant is included between 21% and 35% by volume. This oxidant may, for example, be obtained from the reflux of onsite plants serving for air separation by means of permeation and adsorption, or by mixing natural air with oxygen coming from the pressurized or cryogenic storage tanks, or yet by mixing nitrogen and oxygen coming from the pressure or cryogenic storage.

[0013] The reaction is carried out at a temperature preferably included between 400°C and 1100°C, and especially between 750°C and 1050°C.

[0014] As catalyst for the combustion reaction, use can be made of all types of catalysts known in the art for this type of reaction. For example one may use a catalyst supported on a refractory support based on nickel oxide or precious metals such as platinum, osmium, palladium and ruthenium or their oxides.

[0015] This refractory support can be of the ceramic type and preferably based on alumina, silica, zirconium oxide, titanium oxide or magnesium oxide.

[0016] This invention is now described in greater detail on the basis of the following examples given without limitation.

EXAMPLE 1 - Carburization

[0017] A combustion reactor is positioned on the outside of a pit oven in which the carburizing heat treatment is carried out. The reactor uses a catalytic system consisting of 1% platinum as catalyst over a support of alumina. The reactor is fed with a mixture of natural gas and oxidant coming from a pressurized bottle and with an oxygen content equivalent to 33% by volume of nitrogen. The ratio of oxidant/natural gas is equal to 1.5. The reaction carried out at a temperature of 930°C produces an atmosphere containing 73% of reducing agents (hydrogen and carbon monoxide), 24.5% of carburizing species (carbon monoxide) and 0.16% of carbon dioxide and a dew point of -3°C.

[0018] This atmosphere is injected in the pit oven in which a set temperature of 920°C is maintained in order to effect the carburization of a load of steel. In this manner a carbon potential equivalent to 1% is obtained. If necessary, this potential can be increased or decreased during carburizing treatment by injecting natural gas or air respectively directly into the oven.

[0019] The time required to carry out carburization of a load of steel [16MnCr5], which calls for an effective depth [hardness 525 HV] of 0.5 - 0.6 mm, is approximately 1 hr and 30 min.

[0020] The time required to carry out carburization of a load of steel [18NiCrMo5], which calls for an effective depth of 1 mm is approximately 3 hours.

[0021] The time required to carry out carburization of a load of steel [20MnCr5], which calls for an effective depth of 1.4 mm is approximately 4 hrs and 15 min.

EXAMPLE 2 - Carbonitriding

[0022] The combustion reactor is positioned outside of a chamber oven in which the heat treatment by carbonitriding is carried out. A catalytic system similar to that of Example 1 is utilized in the reactor. The reactor is fed with a mixture of natural gas and oxidant with an oxygen content equivalent to 31% by volume in nitrogen. The oxidant is obtained by enriching the plant's compressed air with oxygen from a cryogenic storage tank. The ratio of oxidant/natural gas is equivalent to 1.6. The reaction, conducted at a temperature of 930°C, produces an atmosphere containing 70% of reducing materials (hydrogen and carbon monoxide), 23.5% of carburizing species (carbon monoxide) and 0.15% of carbon dioxide, with a dew point of -4°C.

[0023] This atmosphere is injected into the chamber oven which is set for an operating temperature of 860°C for the purpose of carburizing a load of steel in a first phase. A carbon potential equivalent to 0.9% is thus obtained. If necessary, this potential can be increased or decreased during the carburizing treatment by means of injecting directly in the oven either natural gas or air.

[0024] Subsequently, a phase of nitriding is carried out by injecting into the oven, by a method known in the art, ammonia in various degrees of dissociation so as to control the nitriding potential.

[0025] The time necessary to effect carbonitriding of a load of C10 steel, for which an effective depth [hardness 525 Hv] is 0.2 mm, is approximately 1 hour.

[0026] The time necessary for carbonitriding of a load of C10 steel, for which an effective depth is 0.5 mm, is approximately 2 hours.

[0027] It goes without saying that, considering the principle of the invention, the details of execution and the embodiment can vary widely from what has been described above without departing therefor from the scope of this invention.

Claims

1. A process for the heat treatment of iron-based metal parts in an active atmosphere possessing a high carbon potential, said process being characterized in that said atmosphere is obtained by catalytic reaction between a hydrocarbon combustible and an oxidant having an oxygen content greater than 21% by volume, and contains at least 60% by volume of carburizing and/or reducing agents.

2. The process of claim 1, wherein said oxidant has an oxygen content between 21% and 35% by volume.

3. The process according to any one of previous claims, wherein said oxidant is obtained from the reflux of onsite air separation systems through permeation and absorption.

4. The process according to any one of previous claims 1 and 2, wherein said oxidant is obtained by mixing natural air with oxygen coming from pressurized or cryogenic storage tanks.

5. The process according to any one of previous claims 1 and 2, wherein said oxidant is obtained by mixing nitrogen and oxygen coming from pressurized cryogenic storage tanks.

6. The process according to any one of previous claims, wherein said hydrocarbon combustible is natural gas or methane.

7. The process according to any one of previous claims, wherein said reaction is carried out at a temperature included between 400°C and 1100°C, and preferably between 750°C and 1050°C.

8. The process according to any one of previous claims, wherein said reaction is carried out in the presence of a catalyst supported over a refractory support.

9. The process of claim 8 wherein said catalyst is a nickel oxide or a precious metal selected from the group consisting of platinum, osmium, palladium and ruthenium or their oxides.

10. The process of claim 8 wherein said refractory support is of the ceramic type selected from the group consisting of alumina, silica, zirconium oxide, titanium oxide and magnesium oxide.