JP2009270758A - Box type anti-oxidation heat treat furnace - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat treatment facility for performing intended heat treatment with the surface of a treated object in the anti-oxidation state, and its method. <P>SOLUTION: In a heat treatment device, a heating furnace 10 and a salt bath chamber 20 are partitioned by a door 100 and brought into contact with each other. Atomospheric gas is intensively introduced to the heating furnace, and a gas passage 110 is provided in the door between the heating furnace and the salt bath chamber so as to make the atomospheric gas introduced to the heating furnace flow into the salt bath chamber. The salt bath chamber includes a sealed structure and is shut off from the outside air. The atomospheric gas made to flow in from the heating furnace makes the atmosphere in the salt bath chamber become anti-oxidation atmosphere, and surplus gas is burned and exhausted through an exhaust port 300. The internal pressure of the heating furnace at this time is maintained at a higher level than that of the internal pressure of the salt bath chamber, to prevent the atomospheric gas from flowing back to the heating furnace by the salt bath chamber. A pressure relief valve 400 is provided in the upper part of the salt bath chamber and is opened/closed when the internal pressure of the salt bath chamber becomes higher than that of the heating furnace. The heat treatment device is further provided with a safety circuit for performing super purge during rapid decline in the furnace internal pressure generated when the door is opened/closed or when a treated object is immersed in the salt bath chamber. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a box-type non-oxidation heat treatment furnace, and more particularly to a box-type non-oxidation heat treatment furnace in which a heating furnace main body and a sealed salt bath are integrated and a processed product can be heated and salt-bath cooled in a non-oxidation atmosphere.

  Steel parts are rarely used in a machined state, and are used by extracting the mechanical properties of the material by heat treatment. Therefore, heat treatment is performed in accordance with the purpose of use. However, even in a heat treatment process including heating and cooling, the mechanical characteristic value varies greatly depending on the cooling method.

  The steel parts are once heated to the temperature of the austenite region for heat treatment such as carburizing, carbonitriding or austempering. At that time, in order to avoid oxidation of the steel parts, the steel parts are heated and held at a predetermined temperature in a non-oxidizing atmosphere furnace, and then cooled by a cooling device to obtain predetermined characteristic values. For cooling, oil, water or a salt bath is used as a refrigerant.

  FIG. 1 shows an example of a conventional heat treatment apparatus. The heat treatment apparatus shown in FIG. 1 includes a heating furnace 10 that heats and holds a processed product 50 at a predetermined temperature, a salt bath 22 for performing predetermined cooling, a pusher 31 that charges the processed product into the furnace, and a processed product from the furnace. A conveyor 39 for moving the product, an elevator 24 for immersing the processed product in the refrigerant, and a drawer 40 for taking out the processed product after the cooling is completed. The processed product finishes the heat treatment in a set sequence.

An atmosphere gas for maintaining the steel parts in a predetermined surface state is introduced into the heating furnace so as to be a non-oxidizing or carburizing atmosphere. The atmospheric gas is a modified gas (RX gas) made by mixing a hydrocarbon gas such as LPG or LNG and air, and is introduced into the main body of the heating furnace through a pipe. The treated product heated in this atmosphere is not oxidized and the surface is in a bright state. Thereafter, the treated product is taken out of the heating furnace and immersed in a refrigerant for predetermined cooling, but is oxidized by being exposed to air for a short time within one minute until it is immersed in the refrigerant.
FIG. 4 shows the surface state of a steel part that was in an oxidizing atmosphere for about 20 seconds until cooled.

  At present, many atmospheric heat treatment facilities that use oil as a refrigerant use an atmospheric furnace that can be quenched in an atmosphere in which a heating furnace and a quenching chamber are integrated to prevent oxidation. However, in a cooling bath using a salt bath or water as a refrigerant, water vapor generated when cooling a high-temperature processed product or evaporated gas from the salt bath enters the heating furnace, disturbing the atmosphere, and the generated gas is heated. It was installed at a fixed interval from the heating furnace for the purpose of prematurely degrading the heat insulating material of the furnace, and the cooling tank was composed of heat treatment equipment in an open top atmosphere.

  Recent automobile parts and precision machine parts have become more complicated, lighter and stronger. Therefore, the steel parts are more easily deformed by heat treatment (quenching). Quenching is a heat treatment that causes a stronger martensitic transformation by quenching the steel from the austenite temperature, but the martensitic transformation causes rapid volume expansion. For this reason, a heat treatment method in which the vicinity of the martensite transformation temperature (referred to as the Ms point, which is determined by the material composition of the steel) is allowed to pass through as slowly as possible at a uniform temperature state is preferred.

  This Ms point is around 240 ° C. in a general carburizing heat treatment, and in the case of quenching oil used as a refrigerant, it is not economical to maintain normally at that temperature because the deterioration is severe. Therefore, a nitric acid salt bath (for example, KNO 3, NaNO 2, etc.) is often used as the optimum refrigerant to be used by heating the refrigerant to this temperature. Currently, many continuous gas carburizing furnaces using this nitrate salt bath as a refrigerant are used, but all salt baths are open-type salt baths installed away from the carburizing furnace. In this case, the steel part subjected to the non-oxidation heat treatment is exposed to an oxidizing atmosphere in a high temperature state (approximately 800 ° C. or higher in the austenite temperature region) for a very short time, and the surface is oxidized.

  Furthermore, since the salt bath is installed away from the heating furnace, there is a disadvantage that the temperature is lowered before the predetermined heat treatment is finished and the cooling process is started. In order to cause martensitic transformation from the temperature in the austenite region, the steel part needs to be cooled from 800 ° C. to 500 ° C. within 25 seconds as an example, although it varies depending on the alloy composition. However, when the salt bath is away from the heating furnace, there is a risk of causing a pearlite transformation existing at approximately 600 ° C. to 700 ° C. due to the time required for transportation. Since general carburized steel parts have a carbon content of 0.7% or more, pearlite transformation does not occur during this time, but some of this pearlite transformation occurs inside the uncarburized part (core part). Waking up and producing proeutectoid ferrite. Since the steel part in which the pro-eutectoid ferrite is generated cannot obtain an initial mechanical characteristic value, it may be determined to be defective in the inspection result.

  In order to avoid such problems, the salt bath cooling is given up, and the upper limit temperature close to the use limit temperature of the quenching oil using the quenching oil that is a conventional refrigerant (150 to 200 ° C. depending on the properties of the quenching oil) This is done with the experience and intuition of heat treatment engineers, such as a method in which the temperature of the processed product is kept at that temperature for a certain period of time and then taken out from the quenching oil, and gradually cooled in the atmosphere. It is the actual situation.

  The above is a description of carburizing and quenching. Next, the present state of the austempering treatment and the background of the invention, which are most effective when treated in the non-oxidizing heat treatment furnace of the present invention, will be described.

  Austempering is one of the heat treatment methods for steel parts, and is a method for obtaining mechanical properties equivalent to or higher than normal quenching and tempering, and is widely used in spring parts and the like. In the same way as quenching, the steel part is once heated to the austenite temperature, held for a predetermined time and then immersed in a salt bath heated to 250 to 400 ° C. to transform the steel structure from austenite to bainite. Let Since a bainite structure can be obtained without causing martensitic transformation by quenching, rapid expansion of volume and transformation distortion during transformation can be avoided. Therefore, the effect which prevents the deformation | transformation and quenching crack of the processed goods by quenching often generated by quenching is large.

  In addition, the mechanical property values obtained by this austemper heat treatment are comparable to those obtained by ordinary quenching and tempering, and have a steel part that has excellent extensibility, drawability, and mechanical resistance. It is already known that it has excellent impact properties. Taking advantage of this feature, it is used for the patenting process of spring parts and wires. This austempering treatment needs to be cooled to 400 ° C. or less in a very short time in order to transform the steel part from the austenite temperature range to the bainite structure. There is a region that causes pearlite transformation in the middle temperature range of about 550 to 700 ° C. If cooling takes time, the pearlite transformation curve is crossed and pearlite is caused, and the initial mechanical strength cannot be obtained.

  In addition, carbon steel is generally used for processed products subjected to austemper treatment for the purpose of cost reduction. This carbon steel is poor in hardenability and is a material that easily causes pearlite transformation. Therefore, processed products made of carbon steel are limited to parts having a thickness of about 7-8 mm or less in order to obtain a uniform bainite structure.

  Atmosphere heat treatment furnaces currently used for this austempering treatment also have the above-mentioned restrictions, and atmosphere heat treatment furnaces in which a salt bath for cooling is provided below the final end of the heating furnace are used. Typical examples of the furnace include a mesh belt type austemper furnace and a shaker hearth type austemper furnace. In the facility shown in FIG. 1, it takes 30 seconds or more to be immersed in the salt bath from the heating furnace, and a uniform bainite structure cannot be obtained with carbon steel. FIG. 2 shows the most typical mesh belt type atmospheric furnace as an austemper furnace.

  In the case of FIG. 2, the heating furnace 500, the mesh belt 502 for charging the processed product, the preparation table 511 for putting the processed product into the furnace, the chute 503 for dropping the processed product into the salt bath, the salt bath 504, the dropped processing It is comprised with the conveyor 505 which pulls up goods. The heating furnace 500 is equipped with a pipe 510 for introducing atmospheric gas (such as RX gas), and the inside of the furnace can be made into a non-oxidizing atmosphere. First, the processed products are set side by side on the mesh belt 502 circulating in the furnace from the preparation table 511. The mesh belt circulates in the furnace with a predetermined heat treatment time. The heating furnace 500 is composed of a temperature raising zone and a soaking zone from the inlet side to the outlet side, and the processed product is placed on the mesh belt and passes through the soaking zone from the heating zone, and is suitable for quenching. Is held in the salt bath 504 through the chute 503 from the final end of the heating furnace 500. The processed product that has been dropped and cooled and held for an appropriate time is placed on the lifting conveyor 505 and carried out of the furnace, completing the austempering process.

  As described above, in the mesh belt type austemper furnace, the treated product is heated in the atmosphere and directly immersed in the salt bath, so that the surface is not oxidized. Furthermore, since the treated product is immersed in the salt bath from the austenite temperature in a short time, it is suitable for small steel parts.

  However, in this type of equipment, the shape of the processed product may be limited. For example, when thin and long parts are arranged on a mesh belt, the deformation is large. Furthermore, when it falls to a salt tub, it is often caught by a raising conveyor. In addition, troubles such as deformation and entanglement with the mesh belt may occur in a complicated shape.

As described above, the mesh belt type austemper furnace has many excellent aspects, but there are also drawbacks, and there has been a demand for the development of a generally used box-type and integrated non-oxidation heat treatment furnace.
Japanese Patent Application Laid-Open No. 08-246036

  An object of the present invention is to provide a box-type non-oxidation heat treatment furnace which does not cause erosion of the furnace material by the gas generated from the salt bath and disturbance of the atmosphere in the heating furnace.

  The present invention is a box-type heat treatment apparatus provided with a heating furnace body and a salt tub, and is configured as an integral type so that the heating furnace body and the salt tub are in contact with each other. Compartment with an openable and closable door, the atmosphere gas introduced into the main body of the heating furnace flows into the salt bath to make it a non-oxidizing atmosphere, and the entire heat treatment process of the processed products from heating to cooling is performed in a non-oxidizing atmosphere It is characterized by.

  Further, the present invention has an inlet for a processed product at one end and an outlet for the processed product at the other end, and is partitioned by an openable / closable door, and includes means for transporting the processed product from the inlet side toward the outlet. It includes a box-type atmosphere heat treatment furnace that integrally forms a heating furnace body that heats up, holds, and cools from the side, and a salt bath for cooling, and introduces atmospheric gas in the heating furnace body to introduce the internal pressure of the heating furnace body Is maintained higher than the salt bath internal pressure.

  The salt bath is a heat treatment furnace that cools at a predetermined temperature after heating, and is characterized by performing treatments such as austempering, gas carburizing and quenching, and gas carbonitriding and quenching that require surface non-oxidation. To do.

  The salt bath is provided with an exhaust port for discharging excess gas of the atmospheric gas continuously flowing from the heating furnace as exhaust gas, and an internal pressure adjusting valve for discharging gas when the salt bath internal pressure exceeds the heating furnace internal pressure. It is provided.

  In the heat treatment method of the present invention, after a predetermined heat treatment is performed in a heating furnace, a processed product heated to a high temperature (austenite state) is immersed in a salt bath of a salt bath, and at that time, the surface of the hot steel part and the refrigerant The heat treatment atmosphere in which the salt bath is in contact and the generated gas is exhausted from the salt bath is discharged as a carrier gas together with the carrier through the exhaust port, thereby preventing backflow into the heating furnace.

In addition, the problem of gas generated from the salt bath entering the heating furnace and degrading the refractory in the furnace was solved.
Due to the effect of preventing gas intrusion, it was confirmed that the lifetime was equivalent to the refractory in the furnace used in the conventional gas carburizing furnace, and the running cost could be reduced.

  Moreover, the time from the heating furnace to the salt bath immersion could be made within 10 seconds, which enabled the treatment of small parts of carbon steel, which could not be treated with non-oxidized austempering conventionally. . Furthermore, complex steel parts with a shape that often caused troubles in the conventional mesh bell type and long parts that are easily bent could be solved by using the present invention product.

  Furthermore, in the box-type non-oxidizing heat treatment furnace of the present invention, to cope with the instantaneous negative pressure in the furnace that occurs when the processed product is immersed in a salt bath or when the intermediate door that partitions the heating furnace and the salt bath is closed, A super purge circuit was provided, and the internal pressure of the heating furnace could be recovered quickly by flowing a large amount of atmospheric gas. This makes it possible to perform stable atmospheric heat treatment.

  Hereinafter, the present invention will be described in more detail with reference to the drawings.

  FIG. 3 is a view showing a box-type non-oxidizing heat treatment furnace in which a sealed salt bath is integrated with a heating body. The heating furnace 10 and the sealed salt bath 20 are integrally configured, an entrance door 11 is provided at the entrance of the heating furnace 10, and the intermediate door 100 divides between the heating furnace 10 and the salt bath 20, and the sealed salt bath 20 An exit door was provided at the rear.

Here, the heating furnace 10 is a sealed space defined by an entrance door 11 that opens and closes when the processed product 50 is loaded and an intermediate door 100 that opens and closes when the processed product is extracted after the heating is completed. Electric heating or other energy is burned to heat the atmosphere and furnace material inside the heating furnace, and the treated product charged therein is quickly heated to a predetermined temperature. In the present embodiment, an electric heating type radiant tube heater 15 is installed on the side of the furnace body, an agitation fan 16 for stirring the gas at the upper part of the furnace body, and a non-oxidizing atmosphere around the agitation fan 16 in the furnace. Or an introduction pipe 17 for introducing an atmospheric gas (such as RX gas) to be carburized.
Although FIG. 3 shows a single box furnace for processing products to be charged, a continuous box-type non-oxidizing heat treatment furnace in which a large number of processing products can be continuously inserted constitutes an integrated salt bath. I can do it. Therefore, it is not limited to the illustrated heating furnace.

  The salt bath 20 is a salt bath stirrer 21 for swirling the salt bath, although not shown here, when an electric or combustion heater for heating and holding the salt bath at a predetermined temperature and a processed product are immersed in the salt bath. In addition, a salt bath 22 provided with a cooling device that suppresses the rise in the salt bath temperature is provided, and an elevator 24 is also provided for throwing the processed product that has undergone predetermined heat treatment into the salt bath. Further, an exhaust port 300 for continuously discharging atmospheric gas is provided at the upper part of the salt bath, and an adjusting device 310 for regulating the exhaust amount is provided at this exhaust port. In addition, a pressure relief valve that opens and closes when the internal pressure of the salt bath rises above a specified level is provided. The intermediate door 100 that partitions the heating furnace 10 and the salt bath 20 is constructed of refractory bricks or ceramic fibers so as to block the heat of the heating furnace 10. Further, an intermediate door structure with a close contact structure is used to prevent the atmospheric gas from moving from the heating furnace 10 to the salt bath 20 or vice versa. Here, the atmospheric gas intensively introduced into the heating furnace flows into the salt bath through only the gas passage provided in the intermediate door 100.

  In FIG. 3, before and after the box-type non-oxidizing furnace configured integrally so that the heating furnace 10 and the salt bath 20 are in contact with each other, that is, on the inlet side, the processed product charging pusher 31 is in contact with the inlet door 11 of the heating furnace 10. Arrangement is made so that the entrance door 11 is opened and the processed product is charged into the heating furnace, and a gantry 40 with a drawer device used for taking out the processed product is provided behind the salt bath. Here, the method for loading and unloading this processed product is not defined.

Hereinafter, a heat treatment method using the heat treatment facility of the present invention will be described.
The inside of the heating furnace 10 is heated and held at a predetermined temperature in advance, and atmospheric gas is introduced to maintain the atmosphere in the heating furnace 10 in a predetermined atmosphere state. Furthermore, the atmospheric gas concentratedly introduced into the heating furnace 10 is introduced into the salt bath 20 through the atmosphere passage port of the intermediate door 100 to make the inside of the salt bath 20 non-oxidized, and the internal pressure in the heating furnace is set to 5 to 20 mmAq. adjust. The pressure inside the furnace may be set higher than this, but if it is excessively increased, the gas tends to leak from the seal part of each part of the furnace body that seals the atmospheric gas, and the furnace structure can be simplified and made inexpensive. Absent. Therefore, it is specified to make it an economic structure. Next, the internal pressure of the salt bath 20 is set to be lower than the internal pressure of the heating furnace 10 by 1 mmAq or more. The amount of exhaust gas is adjusted by the exhaust gas amount adjusting device 310 of the exhaust port 300 provided in the upper part of the salt bathroom 20 to adjust the internal pressure of the salt bathroom 20.

In a conventional atmospheric heat treatment facility provided with an oil quenching tank, the internal pressure of the heating furnace and the internal pressure of the cooling chamber are not adjusted, so that it is often seen that the gas in the cooling chamber flows back to the heating furnace. When the refrigerant used for quenching is oil, the evaporation gas does not particularly disturb the atmosphere, and it is not necessary to consider this point. However, when a salt bath is used as the refrigerant, the evaporating gas is an oxidizing gas and may further erode the furnace material, so that it must be avoided to flow back to the heating furnace 10. Accordingly, a pressure relief valve is provided, which operates when the internal pressure of the cooling chamber 20 exceeds approximately 100 mmAq when the intermediate door 100 is opened / closed, the processed product 50 is moved, or the furnace pressure is rapidly changed when immersed in the salt bath 22. Set as follows.
Furthermore, it is preferable to provide a super purge mechanism for instantaneously recovering the furnace pressure in the event of a sudden negative pressure in the furnace caused by opening / closing of the doors 11, 100, 200 or movement of the processed product 50, but this is not an absolute condition. .

  The heat treatment product opens the entrance door 11 of the heating furnace 10, and the treatment product 50 to be heat treated is charged into the heating furnace 10 using the charging pusher 31. The processed product 50 charged in the heating furnace 10 is heated and held in a pattern set to a necessary temperature and a necessary atmosphere. In the case of carburizing treatment, the necessary carburizing depth and the necessary carbon concentration are given to the treated product. In the case of austempering, the temperature and time are maintained in a non-oxidizing atmosphere. During this time, atmospheric gas such as RX gas is continuously introduced into the heating furnace 10 via the introduction pipe 17. The amount of gas introduced is determined according to the internal volume of the heating furnace 10. This gas flows into the salt bath through the gas passage 110 of the intermediate door 100.

  When a predetermined heat treatment process is completed in the heating furnace 10, the intermediate door 100 is opened, and the processed product 50 is moved by an arbitrary method on the elevator 24 of the salt bath 20 in a short time. For this movement, a pusher 31 system that operates quickly is suitable for preventing the temperature of the processed product 50 from being cooled. As soon as the processed product 50 pushed out by the pusher 31 moves onto the elevator 24, the processed product 50 descends to the salt bath and the processed product 50 is immersed in the salt bath 22. At the same time when the elevator is lowered, the intermediate door 100 is closed. Therefore, the gas generated from the salt tub after the elevator descends is discharged out of the furnace through the exhaust port 300 provided in the salt bathroom 20 using the atmospheric gas flowing from the heating furnace 10 as the carrier gas because the intermediate door 100 is already closed. .

  The treated product immersed in the salt bath 22 is held in a salt bath for a time required for quenching or austemper (5 to 60 minutes). During this time, the salt bath stirrer 21 is operated at a predetermined temperature in which heat is exchanged in the salt bath, and is uniformly cooled and held.

On the other hand, the processed product moves to the salt bath 20, and new processed products are continuously charged into the heating furnace 10 which has become an empty furnace through the entrance door 11. Therefore, the heating furnace 10 can also be continuously heat-treated with a new heat-treatment pattern, and is very efficient.
The treated product 50 that has been immersed in the salt bath 22 for a predetermined time rises with the elevator 24 over time, and the salt bath attached to the treated product 50 and the jig for a certain time is dropped in the salt bath 22 in that state, and the salt bath The outlet door 200 attached to the rear part 20 is opened, and the processed product withdrawing apparatus 40 including a gantry provided at the rear part is pulled out.

  At the same time that the exit door 200 is opened, the curtain frame 201 provided at the lower part of the exit door 200 burns so that air does not enter the salt bath from the outside of the furnace, and the combustion continues while the exit door 200 is open, As soon as the processed product 50 is pulled out, the exit door 200 is closed and the curtain frame 201 is extinguished. Air or combustion gas that partially enters when the outlet door 200 is opened is discharged from the exhaust port 300 to the outside of the furnace together with the atmospheric gas from the heating furnace 10. Thus, the first heat treatment is completed, and the heat treatment furnace returns to the first step. The removed processed product 50 is moved to the next step such as cleaning.

  As described above, the internal pressure of the heating furnace and the internal pressure of the salt bath are only set when the furnace is started up for the first time, and usually do not need to be managed. It is only necessary to measure and record the furnace pressure in a defined time cycle. If the furnace is continuously operated for a long time, the internal pressure of the heating furnace 10 and the internal pressure of the salt bath 20 may change. In this case, the state of the intermediate door 100 that partitions the heating furnace 10 and the salt bath 20 and the gas passage Check. This is the same as the periodic inspection performed in a general atmospheric heat treatment furnace. In this way, the facility of the present invention can be operated in a completely non-oxidized state that is stable for a long period of time.

  The present invention can be suitably used for heat treatment of steel parts using a salt bath as a refrigerant. Typical heat treatment includes gas quenching and gas carbonitriding quenching and martempering. It is also suitable for austempering and martempering of carbon steel and alloy steel.

It is an example of the heat processing apparatus using the conventional salt bath cooling device. This is a representative example of a mesh belt type austemper furnace. It is a box type batch type atmosphere furnace showing an example of an embodiment of the present invention. It is a photograph which shows the surface oxidation state at the time of heat-processing the conventional salt bathroom by an open air type.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Heating furnace 11 Inlet door 15 Heater 16 Atmosphere stirring fan 17 Atmosphere introduction pipe 20 Salt bath 21 Salt bath stirrer 22 Salt bath 24 Elevator 31 Charge pusher 39 Conveyor 40 Processed product drawing-out device 50 Processed product 100 Intermediate door 110 Gas passage 200 Exit door 201 Curtain frame 300 Exhaust port 310 Exhaust gas amount adjusting device 400 Pressure relief valve 500 Mesh belt type heating furnace 502 Mesh belt 503 Chute 504 Salt bath 505 Lifting conveyor 510 Atmospheric gas introduction pipe 511 Preparation table

Claims (4)

In a box-type heat treatment device with a heating furnace body and salt tub, the heating furnace body and the salt tub are integrated so as to be in contact with each other, and the salt bath is in a sealed atmosphere, and each can be opened and closed freely. The atmosphere gas introduced into the heating furnace body flows into the salt bath to be in a non-oxidizing atmosphere state, and the entire heat treatment process of the processed product from heating to cooling is performed in a non-oxidizing atmosphere. Box-type non-oxidizing heat treatment furnace.   Processed product inlet at one end, treated product outlet at the other end, partitioned by an openable / closable door, equipped with means to convey the processed product from the inlet side to the outlet, temperature rising and holding from the inlet side Including a box-type atmosphere heat treatment furnace that integrally constitutes a heating furnace body that cools the temperature and a salt bath for cooling, the atmosphere gas is concentrated in the heating furnace body to maintain the internal pressure of the heating furnace body higher than the salt bath internal pressure. A box-type non-oxidizing heat treatment furnace.   The salt bath is a heat treatment furnace that cools at a predetermined temperature after heating, and is characterized by performing treatments such as austempering, gas carburizing and quenching, and gas carbonitriding and quenching that require surface non-oxidation. The box-type non-oxidizing heat treatment furnace according to claim 1 or 2. The salt bath is provided with an exhaust port for discharging excess gas of the atmospheric gas continuously flowing from the heating furnace as exhaust gas, and an internal pressure adjusting valve for discharging gas when the salt bath internal pressure exceeds the heating furnace internal pressure. The box-type non-oxidizing heat treatment furnace according to claim 1, wherein the box-type non-oxidizing heat treatment furnace is provided.