WO2018131993A1

WO2018131993A1 - Low-pressure carburisation method

Info

Publication number: WO2018131993A1
Application number: PCT/MX2017/000002
Authority: WO
Inventors: Niko MATTHIAS BOUSKA
Original assignee: Thyssenkrupp Presta De México S.A. De C.V.
Priority date: 2017-01-13
Filing date: 2017-01-13
Publication date: 2018-07-19
Also published as: MX2017000565A

Abstract

The present invention describes a low-pressure carburisation method which, by means of a suitable mixture of gases, prevents the carbon residue from each process from impregnating the entire surface of the chambers in which the carburisation is being carried out, the chambers, including the thermal insulation and heating elements (graphite resistors), being constructed from graphite, thereby preventing the contamination of the chambers and keeping the chambers in the same conditions, which thus prevents greater variation in the results obtained and reduces energy consumption.

Description

LOW PRESSURE CARBURIZED PROCESS

TECHNICAL FIELD OF THE. INVENTION The present invention relates to the technical field of metalworking, materials science, fluid dynamics, the calculation of atmospheric pressures, thermochemistry and mechanical strength, since it provides a low pressure carburizing process.

BACKGROUND OF THE INVENTION

Cementation or carburization is one of the oldest methods to produce a hardening on the surface of low carbon steel parts and low carbon alloy steels, generally from 0.08 to 0.25% C. This process consists of supplying carbon to the surface of the steel so that it diffuses inside the piece. This thermochemical treatment is carried out in an endothermic atmosphere plus an enrichment gas, to obtain a sufficient carbon potential, capable of enriching the carbon percentage layer and the time necessary to obtain the desired layer depth, tempering in oil to harden the layer and core of the piece. A subsequent tempering to eliminate the tensions caused during tempering, and obtain that the surface has the resistance, hardness to the required wear and the ductile core. Its main advantage is presented by using low carbon steels that, having a soft core and hard surface, can become a stubborn tool, this is due to the high carbon content, but is achieved by hardening. The main reason to subject the pieces to this treatment is to harden them and make them resistant to use. This process allows obtaining parts with a soft heart and hardened periphery and is performed when it is required to increase the carbon content at the periphery of the piece. The purpose of cementing is to harden the surface of a piece without modifying its core, resulting in a piece formed by two materials: that of the steel core (with low carbon index) tenacious and resistant to fatigue, and the part of the surface ( Steel with higher carbon concentration) 0.2% carbon.

As a consequence, the reactive (endothermic) atmosphere used in this process must protect the steel from oxidation and provide enough carbon to increase its surface content. This increase is based on the adsorption and subsequent diffusion of carbon in the steel at high temperatures, once the carbon penetration is desired, the part is removed by tempering it or normalizing it.

There is low pressure carburation (LPC) which is a boxed process that is carried out in a vacuum furnace, using hydrocarbon gases at very low pressure and elevated temperatures, to obtain a hardened surface layer of martensite turned and a core Lasted. The treatment is used to increase the wear resistance and fatigue of the components.

The main benefits of low pressure carburization are:

- The ratio between side and bottom of the carburized layer (box depth) in the gears is practically 1: 1 (uniform).

- High hardness below the surface, compared to conventionally carburized parts. - Faster cycles.

- Parts can be carburized between 930 and 1,000 ° C (1,700 and 1,830 ° F).

- Carbon penetration in deep blind holes, resulting in a uniform hardness throughout the profile.

- Carburetion of small holes and blind holes.

- Unnecessary cleaning of parts after heat treatment due to rapid cooling by high pressure gas (dry fast cooling).

- Reduction of dimensional alterations by heat transfer independent of temperature during rapid cooling by high pressure gas.

- Improved mechanical properties: elimination of the intergranular oxidation layer, increased fatigue resistance properties.

- Dimensional control: low distortion, predictable and repeatable.

- Environmentally friendly.

- Reduction of manufacturing stages, such as rectification, cleaning and subsequent inspection.

- Improved cleaning of the products.

- Precise control of the box depth, microstructure and hardness.

- Greater uniformity of the cement box for complex shapes. The uniformity of the depth of the box can be maintained, in most cases, within ± 0.002 ".

The carburization process can be summarized in the following steps:

a) Heat the piece to the austenization temperature (815-1090 ° C).

b) Subject the piece to a fuel medium (solid, liquid or gas) for a certain time (from a few hours to days, depending on the fuel medium). c) Temper the piece: The carbureted surface, with greater hardenability, martensite shape. The core of the piece, whose chemical composition is not altered, has lower hardenability and does not form martensite.

d) Coming from the superficial martensitic microstructure.

As a consequence, a hardness gradient is generated, which is larger on the surface and decreases inwards. This creates a hard surface (50-65 HRC) and resistant to wear, preserving the toughness inside the piece.

The carburization can be liquid, solid or gas.

Carbonaceous gas

In this process, keeping the pieces for 1 to 8 hours in a fuel atmosphere, at temperatures of 850 ° C to 950 ° C, cemented layers of 0.2 to 1.5 mm depth are obtained.

Only in special cases, such as the manufacture of shields, are longer processes used and cemented layers of greater thickness are obtained. Although the installations are quite complicated and expensive, the cementation of the pieces is very simple and fast. Very regular results are obtained and large quantities of pieces can be cemented in a very short time. At present, gas cementation is the method of surface hardening most used in large workshops, car factories, motorcycles, etc., where significant quantities of cemented parts are prepared and is not highly recommended for small workshops that work only with small series . This procedure is replacing in many cases the solid cementation (in boxes), and the liquid cementation (in a salt bath).

Parts of low carbon steels exposed to fuel atmospheres (eg, methane or propane) will be carburized at temperatures from 850 ° C. In the most primitive form of In this process, the source of carbon in the fuel atmosphere was so rich that the limit of carbon solubility in austenite for that temperature was reached. As a consequence, Fe3C could be formed on the surface of the steel at high temperature, since the austenite is saturated with carbon.

This is not a very favorable condition for subsequent tempering. In addition, if only methane or propane is used as a fuel atmosphere, a large amount of soot is generated on any surface inside the oven, including the part. Although this mode of carburization is still used in some places with limited resources, the main objective in the modern practice of carbonated gas is to be able to control the carbon content in the furnace atmosphere in such a way that:

- The final carbon concentration on the surface of the pieces is below the solubility limit of austenite.

- The deposition of soot inside the oven is minimized.

This is the reason why controlled fuel atmospheres are used. They are produced by mixing a carrier gas with a rich gas (fuel gas). The latter is usually natural gas (methane), propane or butane. One type of carrier gas is the endothermic gas, which is produced in an independent generator by mixing, in a certain proportion, a hydrocarbon gas (natural gas, propane or butane) with air. Subsequently, it decomposes at high temperature (1000 ° C) and in the presence of a catalyst in an oven. This is the principle of the endothermic atmosphere generator. In this way, the carrier gas is a mixture of CO, H2, N2 and to a lesser extent CO2, CH4 and water vapor. It has the function of diluent, decreases the generation of soot and accelerates the reaction of carburization on the surface of the piece. The types of furnaces for gaseous carburization can be divided into two categories: continuous ovens and batch type ovens. In batch ovens, the pieces are loaded and unloaded from the oven in each batch. The most common are pit, horizontal and fluidized bed furnaces. In all of them, the pieces are placed in baskets or hung with wires, depending on their shape and size. Then, they are introduced into the oven through which the fuel gas circulates, generally agitated by a fan. Once the carburization is finished, the oven lid is moved and the basket where the pieces are located is taken out to be taken quickly to another oven (which is at a lower temperature) and then the pieces are passed to the tempering bath. In particular, some horizontal furnaces may have an integrated quenching system, in which it is not necessary to remove the part from the fuel atmosphere. In these cases, the pieces are taken from the oven to a hall that ends in an oil quenching pool. In this case, since the furnace atmosphere also flows into the lobby, oxidation of the pieces before tempering is avoided.

In continuous ovens, the pieces are transported through the oven from one end to the other by means of trays or mobile chains. There are several designs (rotary, rotary retort, with conveyor belt, etc.) and can also be designed with an integrated tempering system. These types of furnaces are characterized by high productivity in the carburation of similar parts.

The successful operation in the gas carburation process depends on the control of three main variables:

i. Temperature;

ii. Weather? Y,

iii. Composition of the atmosphere As secondary variables can be named the content of alloys of the piece to be carbureted, and the degree of circulation of the oven atmosphere. TEMPERATURE: The maximum speed at which carbon can enter steel is limited by the diffusion rate of carbon in austenite. This diffusion rate increases sharply with increasing temperature. For example, carbon is incorporated into steel 40% faster by going from 870 ° C to 925 ° C.

A widely used carburization temperature is 925 ° C, for allowing a reasonably fast carburization rate without excessive deterioration of the interior of the oven. This temperature can be raised to 955 ° C and 980 ° C, shortening the carburization time for parts that require greater layer depth.

Conversely, if lower layer depths are required, lower carburization temperatures are used, obtaining a much more precise control of the carbureted layer. To obtain a consistent result in carburization, the temperature must be uniform throughout the piece. Since the parts located outside the load are the first to reach the carburization temperature, they begin to carbure much earlier than a piece inside the load. The consequence is a variability in the depth of layer from one piece to another. Therefore, to improve this situation, the load is first preheated to the carburization temperature in an endothermic atmosphere

(both in batch and continuous furnaces). Once the desired temperature is reached, the carburization begins when the fuel gas is added.

TIME: The carburization time decreases if the temperature increases. The graph was calculated assuming saturated austenite on the surface of the piece. In case of controlling the content of surface carbon, and that this is less than saturation, the layer depth will be less.

COMPOSITION OF ATMOSPHERE XA: The atmosphere consists of an endothermic gas (produced from methane) that is enriched by the addition of methane, which is ultimately the source of carbon.

The main constituents of the resulting fuel atmosphere are: CO, N2, H2, CO2, H2O and CH4.

Of all these constituents, N2 is inert, acting only as a diluent.

Finally, the amounts of CO2 and H2O remain constant and the change in the composition of the carburation atmosphere results in a reduction of methane content and an increase in the amount of H2. In this way, the parameters that are monitored in order to control the fuel atmosphere are: water vapor content (measuring the dew point of the gas), CO2 content (by infrared gas analysis), amount of O2 (by zirconia sensors

Solid carburization

In solid carburization, or carburization in boxes, the part to be treated is heated in a closed container (box) and in contact with a solid carburization compound.

This is the oldest method of carburization processes, and was the most used for many years. However, its use has declined significantly in recent years due to the inherent limitations of the process and the improvement of the other carburization techniques, mainly, gas carbonylation. Despite the limitations of this process, solid carburization still offers some attractive advantages, which makes it still used in some workshops that do not require large-scale production. The main advantages of solid carburization are:

- A great variety of ovens can be used, since the process generates its own atmosphere contained in the carburization box.

- Ideal for parts that must be machined after carburization and before final heat treatment, because low cooling speeds can be achieved from the carburization temperature.

- Low distortion of the parts during the carburization, because the carburizing compound is used to support the parts.

- Compared to carbonated gas, this technique allows more options in terms of selective carburization techniques.

The main disadvantages of solid carburization are:

- It is not suitable for producing shallow layers, where strict tolerances are required in terms of layer depth. This is mainly due to the temperature variation inside the box. For the same reason, not all parts inside the box will have the same layer depth.

- Compared to gaseous carburization, solid carburization does not provide flexibility or accuracy regarding the control of process variables and therefore, the final results.

- It has difficulty to directly temper the pieces from the boxes. It requires an additional operation to pass the newly carbureted piece to the tempering bath, being impossible the automation of the tempering process when working with large serles.

- It requires more processing time due to the heating and cooling of the box and the cementing compound.

- Higher fuel / electricity consumption, as a consequence of the previous item.

- Requires considerable labor. High cost of preparation and placement of the pieces in the boxes.

- It has environmental problems associated with the disposal of carburization compounds containing barium.

The most commonly used commercial carbonization compounds or hardener powders are made up of charcoal, petroleum coke or tar, mixed with 10 to 20% barium carbonates, calcium and sodium.

If only coal is used as a hardener powder, carbonated layers with a carbon content of less than 0.65% would be obtained. Mixing it instead with alkaline or alkaline earth carbonates, up to 1.20% of carbon is reached on the surface. Therefore, these carbonates are called activators. On the other hand, in ancient times it was thought that the transfer of carbon to steel occurred directly from the solid carbon from the carburization compound. However, if a piece of steel is placed surrounded by fuels and heated to 925 ° C in a vacuum, there would be no gases around the part and the steel would not absorb carbon. In this way, it can be verified that the true fuel agents are the gases that are released from the high temperature hardening powder.

The success of solid cementation depends largely on the ability of the fuel medium to supply and surround the carbon steel surface, in a state that can be called active. In this way, the absorption of carbon by steel takes place quickly. Experience has shown that the nascent carbon that forms inside the cement boxes when the carbon oxide decomposes, is in a very favorable state for the cementation to occur.

Furnaces heated with coal, gas, fuel oil or electricity are used. In them, the cement boxes are placed, with the pieces and the cementitious mixtures inside. The joints of the boxes must be tightly closed, using mud or refractory clays.

The main characteristics that an oven must have to be used in solid carburization are: to provide good thermal capacity, temperature uniformity and to have adequate supports for the boxes.

As for the material of the boxes, carbon steel containers (can be coated with aluminum), cast iron, welded steel sheets, stainless steel or Fe-Cr-Ni alloys can be used for high temperatures. The function of aluminum as a coating of carbon steel is to extend the life of carbon steel. This significantly reduces the cost of the boxes. The choice of material will depend on the amortization given. That is, the cheapest material could become a Fe-Cr-Ni alloy, with the largest initial investment, but amortized based on the number of years of use and continuity of the

production.

It is convenient that the boxes have legs or are placed copper supports so that the heat penetrates below and the underside can be heated easily. The carburization in boxes has the same variables to control as those already mentioned in the carbonated gas: temperature, time and composition of the atmosphere. Ά1 as in carbonated gas, the velocity of formation of the carbureted layer increases rapidly with temperature. The normal temperature range is 815 ° C to 955 ° C. However, in recent years the temperature of carburization has been raised up to 1095 ° C, due to improvements in the manufacturing processes of steel (fine grain practices). Some steels can keep the fine grain up to 1040 ° C. Above this temperature, grain growth would occur after a long enough time. Finally, the increasing improvement of the material of the boxes and the fine grain steels allow the use of a great variety of temperatures.

The layer depth at a given carburization temperature is proportional to the square root of time. In this way, the carburization rate is higher at the beginning of the process and gradually decreases over time.

As for the composition of the atmosphere, the amount of nascent carbon in the generated atmosphere is directly increased by increasing the ratio of carbon monoxide to carbon dioxide. Therefore, the amount of carbon on the surface of the part is greater if energizers and carburization compounds are used that promote the formation of carbon monoxide.

Even with good control of process variables, it is difficult to obtain parts with a variation in layer depth less than 0.25mm. Therefore, commercial tolerances for layer depths in solid carburization begin at +/- 0.25imti. For higher layer depths, this tolerance can reach up to +/- 0.8mm. Lower carburization temperatures decrease the variation in layer depth.

Due to the variation of the layer depth, inherent in the process, and the cost associated with the assembly of the boxes, it is not common to use this process for parts that require layer depths less than 0.8mm.

Liquid carburization

In liquid carburization, the piece is immersed in a bath of carburizing salts at a temperature greater than Acl. Like solid carburization, liquid carburization no longer has the commercial importance it had in the past. This is mainly due to environmental issues, due to the difficulty and cost associated with the disposal of salts, especially those that contain cyanide. In addition, the removal of salts can be very difficult in some parts.

Due to the characteristics of the thermal transfer of the salt bath to the piece, the piece heats up much faster and the liquid carburization takes place in less time than the soda. In general, the pieces are tempered after the bath, followed by a temper.

Most carburization salts contain cyanide, in which cases both carbon and nitrogen are introduced into the layer. However, there are also salts that do not contain cyanide and only introduce carbon to the piece.

In this process carbon and nitrogen are introduced to the steel. The surface hardness achieved can be considered almost exclusively to the action of carbon. The influence of the small percentage of nitrogen in the layer is minor and can be almost negligible. The salts used can be classified into two groups, according to the layer depth:

a) Salts with low tanperature or average penetration: Cemented layers of 0.2 to 1.5mm are achieved. In general they are used in the range of 850 to 900 ° C. In some specific cases, this range extends from 790 to 925 ° C.

b) Rooms with high tanperature or great penetration: Cemented layers of 1 to 3mm are achieved. In general they are used in the range of 875 to 950 ° C. In some cases depths of up to 6mm can be achieved. The most important use of these salts is the rapid formation of 1 to 2mm layers.

The salts are arranged in crucibles that can be cast iron, sheet steel (embedded or welded) or refractory stainless steel. The heating of the crucibles can be with coal, gas, gasoline or electricity.

In electric muffle furnaces the contact of cyanide vapors with the resistors should be avoided, since it attacks them, shortening the oven's life.

A type of electric oven widely used consists of a group of electrodes that are submerged in the bathroom. As the molten salts are conductive, the electric current passes through them and the heat generated by the passage of this current is sufficient for heating the bath. Since solid salts are not conductive of electricity, commissioning is somewhat complicated. It begins by placing between two electrodes a piece of coke of appropriate size, which is pressed between them with an auxiliary iron bar to facilitate contact. The passage of the current heats the coal, which becomes red. Then, the salts around it begin to melt. Subsequently, the rest of the dough is heated and melted. The temperature is regulated with great accuracy by modifying the electrode voltage. Is It is necessary to have bells on the crucibles, to absorb the harmful vapors that are released.

There is also low-pressure carburation with rapid cooling by high-pressure gas, which is mainly used for transmission gears and diesel fuel injectors, which require a uniform carburized layer on all surfaces of the part and minimal dimensional movement. The process produces a very clean surface, thus eliminating the need to clean the part after heat treatment.

A search of the state of the art of LOW PRESSURE CARBÜRIZED PROCESSES was carried out and the patent document number ES2276161 (T3) was found, with the title "METHOD FOR PRESSURE CARBÜNIZING STEEL PARTS" published on June 12, 2007 , said document describes a method for pressurizing carbon steel parts by introducing an active nitrogen carrier in the vacuum oven chamber, using ammonia at a pressure of 1 to 500 mbar, characterized in that the ammonia begins to be introduced into the vacuum chamber of the oven at the moment in which the load reaches the temperature of 400 ° C, and is continuously introduced into the chamber until the moment in which the load reaches the temperature necessary to begin the cementation process, that is, the moment when the coal carrier begins to be introduced.

Another document found is the patent application US5205873 (A), published on April 27, 1993, entitled "PROCEDURE AND INSTALLATION OF LOW PRESSURE METAL ALLOY PARTS", describes a fuel mixture consisting of Hydrogen and Ethylene by reason 2 to 60% ethylene by volume and the oven is heated between 820 oven (50), called double vacuum, consisting of a tank (55) with its internal device for distribution of cementation / carburization gases, by an annular space (56) surrounding the tank, by a lid crossed by pumping and arrival ducts of Hydrogen (51) and Ethylene (52) that flow into the different levels of the tank in several regularly distributed places, thermocouples (CT), a microcomputer (61). Its application is in auto parts.

One more document is US 6187111 (Bl), published on February 13, 2001, with the title "VACUUM CARBURATION METHOD" and describes an improved vacuum carburization method that is easy to handle or the costs of maintenance and low without using hydrogen gas equipment or acetylenic gas. The improved vacuum cementation method comprises a cementation treatment of work pieces made of steel material is heated to approximately 900 to 1100 ° C. Performed by introducing an ethylenee gas as a carburation gas at a vacuum of 1 to 10 kPa in a cement kiln chamber.

Finally, there is patent document number US 4104129, entitled "CARBURIZATION OF LOW TEMPERATURE AND DESULFURATION OF CARBON UNDER ELEVATED PRESSURES" and describes a process of carburization and desulfurization in which the high pressures are used so that the product gases contain enough hydrogen for its use as a recycling sequence in carburization and desulfurization. The high pressure allows product recovery systems for gas products that use the high pressures of carburization and desulfurization. One of the main drawbacks of low temperature carburization processes is that in the furnaces used in vacuum carburization processes, the chambers where the process is carried out are made of graphite, both the thermal insulation and the elements heating (Graphite Resistors); during production continuous, the carbon remnants of each process will be impregnated on the entire surface of the chamber, this remnant will contaminate the chamber and change its conditions, therefore, there will be greater variation in the results obtained, in addition to the consumption of Energy could be adversely affected due to this change.

OBJECT PE IMVMrClÓK It is therefore the object of the present invention to provide a low pressure carburizing process, which solves the problems described above.

Another object of the present invention is to provide a suitable mixture of gases, the main advantage is the reduction of carbon remnants and therefore less pollution, preserving the original conditions of the carburizing chamber and achieving repeatability between batches.

DETAILED DESCRIPTION OF THE INVENTION

The present invention describes a low pressure carburizing process, it is composed of the following steps:

i. Preheat the carbon-containing steel from 0.15 to 0.30%, in a preheating chamber, using Nitrogen as a convective medium at a temperature greater than 700 ° C, the steel charge remains at this temperature for a period of time between 30 minutes at 90 minutes;

ii. Transport the steel load to a heating chamber;

iii. Heat the carbon-containing steel in vacuo to a carburization temperature greater than 900 ° C, heat transfer occurs by radiation and the carbon-containing steel load remains at this temperature for a time range of 60 minutes to 150 minutes;

Iv. Inject by means of nozzles, a mixture of gases to generate an atmosphere enriched with carbon, in a time range of 1 to 7 minutes, with a flow range of 75 to 300 liters per minute;

v. Adjust by means of pumps the pressure of the heating chamber between 100 and 1000 passages, to generate the movement of the gas mixture; saw. Generate a vacuum through the pumps, between 50 and

100 pascaies, allowing the temperature of the heating chamber to remain stable; vii. Transport the carbon-containing steel load to a cooling chamber or preheating chamber;

viii Cool the carbon steel load using nitrogen as a convective medium; ix. Maintain a temperature between 600 ° C and 700 ° C for a time range of 20 to 90 minutes, to refine the austenitic grain;

x. Transport the carbon steel load to the heating chamber;

xi. Vacuum heating the carbon-containing steel with a temperature range of 820 to 880 ° C, allowing a heat transfer by radiation;

xii. Transport the carbon steel load to the cooling chamber; and, xiii. Dip the carbon steel load in oil for at least 30 minutes.

Steps iv to vi are repeated as many times as necessary, to meet the specific specifications. The gas mixture described in step iv is to generate an enriched enough to carburizing areas larger than 20m ² carbon steels low carbon atmosphere.

Said mixture is composed of:

- 60% Acetylene;

- 20% Ethylene; and,

- 20% Hydrogen

Where Acetylene and Ethylene are in a 3: 1 ratio in a temperature range between 900 ° C and 1000 ° C; and Hydrogen is in a 1: 3 ratio to Acetylene to maintain a clean reaction. Acetylene is used as the main carbon carrier, and Ethylene will increase the efficiency of Acetylene.

The low carbon steel parts that are carburized are mainly intended for automotive applications.

An advantage of the gas mixture described above, is that it allows the reduction of carbon remnants and therefore less pollution, preserving the original conditions of the carburizing chamber and achieving repeatability between lots, in less time, as well as, achieving volumes greater than 200 x 200 x 400 mm, modifying process flows and consumables for a carburizing area greater than 0.4 m ² .

Claims

CLAIMS Having sufficiently described the invention, the content of the following claims clauses is claimed as property.

1. A low pressure carburizing process, characterized in that it comprises the following steps:

i. Preheat the carbon-containing steel, in a preheating chamber, using Nitrogen as a convective medium at a temperature greater than 700 ^C C, the steel load remains at this temperature for a period of time between 30 minutes to 90 minutes;

ii. Transport the steel load to a heating chamber;

iii. Heat the carbon-containing steel in vacuo to a carburization temperature greater than 900 ° C for a time range of 60 minutes to 150 minutes;

iv. Inject by means of nozzles, a mixture of gases containing Acetylene, Ethylene and Hydrogen, to generate a carbon-enriched atmosphere, for a time range of 1 to 7 minutes, with a flow range of 75 to 300 liters per minute ; v. Adjust by means of pumps the pressure of the heating chamber between 100 and 1000 pascals, to generate the movement of the gas mixture; saw. Generate a vacuum through the pumps, between 50 and 100 pascals, allowing the temperature of the heating chamber to remain stable; vii. Transport the carbon-containing steel load to a cooling chamber or preheating chamber;

x. Transport the carbon steel load to the heating chamber;

xi. Heat the carbon-containing steel in a vacuum with a temperature range of 820 to 880 ° C;

xii. Transport the carbon steel load to the cooling chamber; Y,

xiii. Dip the carbon steel load in oil for at least 30 minutes.

2. The process of previous claim, wherein the carbon content in the steel is from 0.15 to 0.30%.

3. The process of claim 1, wherein the heat transfer of steps iii and xi is by radiation.

4. The process of claim 1, wherein the gas mixture of step iv has the following concentration:

i. 60% Acetylene;

ii. 20% Ethylene; and,

iii. 20% Hydrogen

5. The process of claim 4, wherein the Acetylene and Ethylene are in a 3: 1 ratio in a temperature range between 900 ° C and 1000 ° C.

6. The process of claim 4, wherein Hydrogen is in a 1: 3 ratio to Acetylene.

7. The process of the preceding claims, wherein steps iv to vi are repeated as many times as necessary, so that the carbon-containing steel meets the desired austenitic grain refinement.