The present disclosure relates to a method of carbonitriding a work piece. The present disclosure particularly relates to a method of carbonitriding a work piece on a continuous furnace.
DEFINITIONS
As used in the present disclosure, the following terms are generally intended to have the meaning as set forth below, except to the extent that the context in which they are used to indicate otherwise.
Carbon Potential: A measure of the capability of a furnace atmosphere to impart carbon into a steel during heat treatment. The carbon potential of an atmosphere is defined as the carbon content of a thin sheet of pure iron in equilibrium with the atmosphere.
Carbonitriding Process: A metallurgical surface modification technique that is used to increase the surface hardness of a metal, thereby reducing wear. During the process, atoms of carbon and nitrogen diffuse interstitially into the metal, creating barriers to slip, increasing the hardness and modulus near the surface.
Endothermic gas atmosphere: A gas atmosphere that inhibits or reverses oxidation on the surfaces it is in contact with. Endothermic gas is the product of incomplete combustion in a controlled environment. An example is hydrogen gas (H2), nitrogen gas (N2), and carbon monoxide (CO).
HRC: An abbreviation for Rockwell Hardness measured on the C scale. The Rockwell scale is a hardness scale based on indentation hardness of a material. The Rockwell test measures the depth of penetration of an indenter under a large load compared to the penetration made by a preload. C scale load corresponds to 150 kgf

Lambda probe: An electronic device that monitors and control a nitrogen-methanol atmosphere in the furnace.
BACKGROUND
The background information herein below relates to the present disclosure but is not necessarily prior art.
Carbonitriding is a process which is employed to improve the surface hardness and wear resistance of a work piece. In the carbonitriding process, carbon and nitrogen are allowed to diffuse into the work piece so as to impart hardness to the work piece. A work piece is fed into a furnace under an oxygen-free endothermic gas atmosphere. As carbon and nitrogen diffuse into the work piece, the surface of the work piece becomes more and more enriched in the amount of carbon and nitrogen. Both carbon and nitrogen first get dissolved in the solid solution of the work piece and then on reaching super saturation, carbon forms carbides and nitrogen forms carbonitrides. These carbides and carbonitrides are mainly responsible for the increased hardness and wear resistance of the work piece.
The amount of carbon and nitrogen present in the work piece play a very important role in determining the hardness and therefore the wear resistance of the work piece. The conventional carbonitriding methods are carried out in a batch type sealed quench furnace, wherein the furnace atmosphere is controlled by oxygen probe and adjusted as per oxygen probe reading.
However, it is difficult to directly measure the amount of the nitrogen and carbon permeating into the surface of the work piece during the continuous carbonitriding process. If, untrammeled amounts of the source of nitrogen and carbon are used then the work piece so obtained after carbonitriding may have skewed amounts of nitrogen and carbon, which will offset the desired level of hardness of the work piece.

There is, therefore, felt a need for a continuous carbonitriding method, where the amount of carbon and nitrogen permeating into the work piece can be effectively controlled so as to produce work pieces having the desired amount of carbon and nitrogen.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows.
It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
An object of the present disclosure is to provide a method for carbonitriding a work piece.
Another object of the present disclosure is to provide a method of controlling the amount of carbon permeating into the work piece during the carbonitriding of the work piece.
Other objects and advantages of the present disclosure will be more apparent from the following description, which is not intended to limit the scope of the present disclosure.
SUMMARY
The present disclosure relates to a method for carbonitriding a work piece.
The method of the present disclosure comprises introducing methanol and nitrogen gas into a roller hearth furnace maintained at a temperature in the range of 800 to 900 °C to generate an oxygen-free endothermic gas atmosphere. The molar ratio of methanol to nitrogen is 60: 40. A work piece is then introduced into the furnace.

In the next step, ammonia is added in a controlled manner into the furnace at a flow rate in the range of 1.2 to 1.6 m /hr, followed by addition of the gaseous carbon source. The furnace atmosphere is analyzed continuously and the flow rate of the gaseous carbon source is adjusted such that the concentration of carbon lies in the range of 0.5 to 0.7% of the furnace atmosphere.
The work piece is allowed to reside in the roller hearth furnace to form the carbides and nitrides on the surface of the work piece, until the carbon content of the work piece lies in the range of 1 to 1.3%, the carbonitrated work piece is then ejected from the roller hearth furnace.
The present disclosure further provides an apparatus for the carbonitriding of the work piece. The apparatus comprises a roller hearth furnace equipped with base plate and rollers adapted to carry the work piece inside the furnace; a gas inlet adapted to feed methanol, nitrogen gas, the gaseous carbon source and ammonia respectively into the furnace; valve means coupled to the gas inlets adapted to control the feeding of methanol, nitrogen gas, the gaseous carbon source and ammonia respectively into the furnace; a heater adapted to heat the furnace; gas analyzers adapted to continuously monitor the furnace atmosphere; and lambda probe adapted to monitor the carbon concentration of the furnace atmosphere.
The carbonitrated work piece obtained by the method of the present disclosure has hardness in the range of 60 to 65 HRC.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
The present disclosure will now be described with the help of the accompanying drawing, in which:
Figure 1 illustrates a schematic diagram of the method of carbonitriding a work piece using a carbon and ammonia.

DETAILED DESCRIPTION
Embodiments, of the present disclosure, will now be described with reference to the accompanying drawing.
Embodiments are provided so as to thoroughly and fully convey the scope of the present disclosure to the person skilled in the art. Numerous details are set forth, relating to specific components, and methods, to provide a complete understanding of embodiments of the present disclosure. It will be apparent to the person skilled in the art that the details provided in the embodiments should not be construed to limit the scope of the present disclosure. In some embodiments, well-known processes, well-known apparatus structures, and well-known techniques are not described in detail.
The terminology used, in the present disclosure, is only for the purpose of explaining a particular embodiment and such terminology shall not be considered to limit the scope of the present disclosure. As used in the present disclosure, the forms "a," "an," and "the" may be intended to include the plural forms as well, unless the context clearly suggests otherwise. The terms "comprises," "comprising," "including," and "having," are open ended transitional phrases and therefore specify the presence of stated features, integers, steps, operations, elements, modules, units and/or components, but do not forbid the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The particular order of steps disclosed in the method and process of the present disclosure is not to be construed as necessarily requiring their performance as described or illustrated. It is also to be understood that additional or alternative steps may be employed.
Carbonitriding is a process for improving the surface hardness and wear resistance of a work piece. The process involves diffusion of the carbon and nitrogen into the work piece, wherein carbon forms carbides with the metal and nitrogen forms

carbonitrides. These carbides and carbonitrides are mainly responsible for the increased hardness and wear resistance of the work piece. However, the amount of carbon and nitrogen permeating into the work piece are difficult to control, resulting in a work piece devoid of the desired level of hardness.
The present disclosure, therefore, envisages a continuous method for carbonitriding a work piece where the amount of carbon and nitrogen permeating into the work piece can be effectively controlled so as to impart the desired level of hardness to the work piece. The method involves following steps:
Initially, methanol and nitrogen gas are introduced into a roller hearth furnace maintained at a temperature in the range of 800 to 900 °C to generate an oxygen-free endothermic gas atmosphere. The molar ratio of methanol to nitrogen is 60: 40. A work piece is then introduced into the furnace.
In the next step, ammonia is introduced in a controlled manner in the roller hearth furnace. The flow rate of the ammonia is maintained in the range of 1.2 to 1.6m7hr. The gaseous carbon source is then introduced in the furnace. The furnace atmosphere is analyzed continuously and the flow rate of the carbon source is adjusted such that the concentration of carbon lies in the range of 0.5 to 0.7% of the furnace atmosphere.
The work piece is allowed to reside in the roller hearth to form carbides and nitrides on the surface of the work piece, until the carbon content of the work piece lies in the range of 1 to 1.3%, the carbonitrated work piece is then ejected from the roller hearth furnace.
In accordance with the present disclosure, the mixture of methanol and nitrogen gas is supplied into a furnace to provide the requisite oxygen-free endothermic gas atmosphere in the furnace. Endothermic gas is chemically inert to the surface of the work piece to be treated. Further, even a little amount of oxygen in the furnace atmosphere will cause oxidation of the work piece and hence not desirable.

The methanol cracks on entering the furnace to form carbon monoxide and hydrogen in accordance with the following reaction:
CH3OH + N2 ► CO + 2H2 + N2
This chemical reaction typically takes place inside the furnace. Carbon monoxide is a strong carburizing agent. It has a high carbon potential and becomes increasingly more stable at elevated temperatures. However, it is only at lower temperatures that carbon monoxide will supply carbon in the form of soot in the so-called carbon reversal reaction. The reversible reaction of CO to form carbon (C) and C02 is of particular interest in a furnace atmosphere. Soot causes most of the maintenance-related issues with gas generators and heat-treating furnaces.
The mixture of methanol and nitrogen also acts as carrier for the carbon source and ammonia.
In accordance with an exemplary embodiments of the present disclosure, heating of work piece in a furnace is carried out at a temperature in the range of 850 to 880 °C. In accordance with the present disclosure, at temperatures lower than 800 °C, diffusion of carbon and nitrogen would not take place. Further, at temperatures higher than 900 °C, grain coarsening will take place which will lower strength of the work piece.
In accordance with an exemplary embodiments of the present disclosure, heating of the work piece in a furnace is carried out for a time period of 7 hours.
In accordance with an exemplary embodiment of the present disclosure, the carbon source is liquid petroleum gas (LPG).
The flow rate of the source of nitrogen is maintained constant throughout the carbonitriding process, thereby maintaining the concentration of nitrogen in the furnace.

The present disclosure provides a method for carbonitriding a work piece to provide an optimum amount of carbon and nitrogen enrichment. It is found that the amount of carbon source present in the furnace atmosphere has a reverberating effect on the amount of carbon enrichment in the work piece. During carbonitriding of the work piece, carbon present in the work piece primarily depends upon the amount of carbon source present in the furnace atmosphere. When the amount of carbon source present in the furnace atmosphere is more than 0.7 w/w, the amount of carbon diffused into the work piece is more than the required i.e. 1.1 to 1.3 wt% which taints the hardness of the work piece. Therefore, the present disclosure provides a method of carbonitriding a work piece while controlling the amount of carbon present in the furnace so as to produce work pieces having the desired level of hardness.
The reading of the carbon potential in the furnace is taken by lambda probe technique to control the flow rate of the source of carbon. Based on the lambda probe reading of the carbon potential, the flow rate of the source of carbon is reduced if the carbon potential is less and is increased, if the carbon potential is high. Also, the furnace atmosphere is continuously monitored using multiple gas analyzers which show the furnace atmosphere based on percentage amounts of CO, CO2, CH4 and H2 gases present in the furnace atmosphere. Under these conditions, carbon and nitrogen start diffusing into the surface of the work piece, typically into a depth of 0.1 to 0.3 mm therefrom, creating barriers to slip and increasing the hardness of the work piece.
A carbonitrited work piece obtained by the method of the present disclosure has hardness in the range of 60 to 65 HRC.
In an embodiment of the present disclosure, the method for carbonitriding a work piece is performed using the apparatus as shown in Figure 1. The method for the carbonitriding of a work piece using the apparatus of Figure 1 is described herein below.

The apparatus comprises a roller hearth furnace '1' operating at a temperature in the range of 800 °C to 900 °C. Methanol and nitrogen gas are introduced via inlets '5' and '6' respectively into the furnace '1' to provide the requisite oxygen-free endothermic gas atmosphere, which is maintained throughout the process. Work pieces '2', placed on a base plates '3', are introduced into the furnace '1' via rollers '4' through one end of the furnace.
Thereafter, ammonia and a gaseous carbon source are added into the furnace '1' via inlets '7' and '8' in a controlled manner. Gas analyzers '9' are used to continuously monitor the furnace atmosphere. Lambda probe '10' is used to monitor the carbon concentration of the furnace atmosphere.
Under these conditions, carbon and nitrogen start diffusing into the work pieces. The supply of ammonia into the furnace is maintained constant a rate of 1.4 m /hr; whereas, the flow rate of the carbon source is varied based on the carbon potential value and the amount of methane present in the furnace atmosphere so as to produce work pieces of the desired hardness or amount of carbon enrichment. After the carbonitriding process is completed, the work pieces are taken out of the furnace '1' for further processing.
In an embodiment of the present disclosure, iron work pieces are used for carbonitriding. When the iron work pieces are subjected to carbonitriding, carbon and nitrogen, both diffuse into the iron, and the surface of the work pieces becomes enriched in carbon and nitrogen. Both carbon and nitrogen first get dissolved in the solid solution of austenite then after reaching super saturation, carbon forms iron carbides and nitrogen forms carbonitrides which are responsible for the increased hardness and wear resistance of the work pieces.
The foregoing description of the embodiments has been provided for purposes of illustration and not intended to limit the scope of the present disclosure. Individual

components of a particular embodiment are generally not limited to that particular embodiment, but, are interchangeable. Such variations are not to be regarded as a departure from the present disclosure, and all such modifications are considered to be within the scope of the present disclosure.
The present disclosure is further described in light of the following experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. The following experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial scale.
EXPERIMENTAL DETAILS
Experiment 1: Carbonitriding of the iron piece
Methanol and nitrogen gas were introduced into a roller hearth furnace '1' maintained at 850 °C via inlets '5' and '6' to provide an oxygen-free endothermic gas atmosphere. The molar ratio of methanol to nitrogen gas is 60: 40. Iron work pieces '2' were then introduced through one end of the furnace '1'.
Thereafter, LPG as a carbon source and ammonia gas as a nitrogen source were fed to the roller hearth furnace '1' via inlets '7' and '8' in a controlled manner. The flow rate of the ammonia is maintained in the range of 1.4 m3/hr. The furnace atmosphere was continuously analyzed with the help of gas analyzers '9' and Lambda probe '10' is used to monitor the carbon concentration of the furnace atmosphere. The flow rate of the carbon source is adjusted such that the concentration of carbon lies in the range of 0.5 to 0.7% of the furnace atmosphere. All the data was compiled and correlated using Minitab statistical software.
The work piece was allowed to reside in a heated oxygen-free endothermic gas atmosphere inside the roller hearth furnace for resident time of 7 hours. After

ascertaining that the carbon content of the work piece till depth of 0.2 mm from surface is 1.2%, the carbonitrated work piece was ejected from the other end of the roller hearth furnace.
The carbonitrated work piece obtained had hardness of the work piece was 64 HRC.
TECHNICAL ADVANCEMENTS
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of a method for carbonitriding a work piece, which:
- is capable of controlling the amount of carbon diffusing into a work piece; and
- provides a carbonitrated work piece having the desired/controlled hardness.
The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
The foregoing description of the specific embodiments so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and

range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
The use of the expression "at least" or "at least one" suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.
While considerable emphasis has been placed herein on the components and component parts of the preferred embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other changes in the preferred embodiment as well as other embodiments of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to

be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

WE CLAIM:

1.A continuous method for carbonitriding a work piece, the method comprising:
a. introducing methanol and nitrogen gas in a roller hearth furnace maintained at a
temperature in the range of 800 to 900 °C to generate an oxygen-free
endothermic gas atmosphere within the furnace;
b. introducing a work piece into the furnace;
c. introducing ammonia into the furnace at a flow rate in the range of 1.2 to 1.6
m3/hr;
d. introducing a gaseous carbon source into the furnace;
e. analyzing the furnace atmosphere continuously and adjusting a flow rate of the
carbon source such that the concentration of carbon lies in the range of 0.5 to
0.7% of the furnace atmosphere;
f allowing the work piece to reside inside the furnace to form carbides and nitrides on the surface of the work piece, until the carbon content of the work piece lies in the range of 1 to 1.3%.
2. The method as claimed in claim 1, wherein in step (a), the molar ratio of methanol to nitrogen is 60: 40.
3. The method as claimed in claim 1, wherein the gaseous carbon source is liquid petroleum gas (LPG).
4. A carbonitrited work piece obtained by the method as claimed in claim 1 has hardness in the range of 60 to 65 HRC has.
5. An apparatus for carbonitriding a work piece comprising:
i. a roller hearth furnace equipped with at least one base plate and rollers adapted to carry a work piece inside the furnace;

gas inlets adapted to feed methanol, nitrogen gas, the carbon source and ammonia respectively into the furnace;
valve means coupled to the gas inlets adapted to control the feeding of methanol, nitrogen gas, the carbon source and ammonia respectively into the furnace;
a heater adapted to heat the furnace;
gas analyzer adapted to continuously monitor the furnace atmosphere; and lambda probe adapted to monitor the carbon concentration of the furnace atmosphere.