JPH08246044A - Heat treatment method for spheroidal graphite cast iron and differential device obtained by executing heat treatment to spheroidal graphite cast iron-made member by this method - Google Patents

Abstract

PURPOSE: To improve the wear resistance of sliding part and the machinability of the other part of a differential case. CONSTITUTION: Side gears 17, 19 at an output part side are constituted so as to be mutually connected through pinion gears 39, 41 freely rotatably housed in housing spaces 35, 37 of a spheroidal graphite cast iron-made differential case 3 which is rotationally driven by an engine. After austenizing the differential case 3 coated with a decarburize-preventive on its sliding parts 35, 37, 59, 61 with each of the gears by holding in a decarburizing atmosphere at 850-950 deg.C for 0.5-2hr, the case is rapidly cooled, and further, tempering is executed thereto at 300-450 deg.C for 0.5-1.5hr.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for spheroidal graphite cast iron and a differential apparatus for heat treating a spheroidal graphite cast iron member by this method.

[0002]

2. Description of the Related Art Japanese Unexamined Patent Publication (Kokai) No. 4-325624 discloses "austempering method for spheroidal graphite cast iron". This heat treatment method is for improving the machinability of a strength member such as a machine part.

The above-mentioned publication describes the following steps.

First, a member made of spheroidal graphite cast iron is heated to 900 ° C to 9 ° C.
The decarburized layer is formed on the surface by keeping the decarburized atmosphere at 50 ° C. for 3 hours to 5 hours.

Next, this member whose surface has been decarburized is retained at, for example, 900 ° C. to 950 ° C. for 0.5 hours to 1.5 hours to be austenitized.

Further, it is cooled to 300 to 400 ° C.
After holding for 5 hours to 1.5 hours, the austenite is subjected to isothermal transformation (austempering treatment) and then air-cooled.

By such heat treatment, the surface layer portion decarburized into the ferrite material becomes a pearlite structure by austempering, and the deep layer portion not decarburized becomes a bainite structure by austempering. Thus, the surface layer portion has high machinability due to the property of pearlite, and the deep layer portion has high hardness and toughness due to the property of bainite, which improves the strength of the entire component.

On the other hand, induction hardening, which is often used for quenching steel materials and cast members, has an advantage that it can be partially quenched. However, since quenching is performed by generating an overcurrent in the member with a high frequency induction current due to electromagnetic induction, if the quenching part has a complicated shape and a closed circuit is not formed, uniform overcurrent will not occur and the desired quenching will occur. In addition, it is impossible to improve the mechanical properties of the entire member because the deep layer is not quenched due to the skin effect of electric current.

Further, FIG. 6 is disclosed in JP-A-5-280596.
Such a differential device 201 is described.

This is a differential case 203, a helical pinion gear 205 and other helical pinion gears meshing with the differential case 203, and output side helical side gears 207, 209 connected through these helical pinion gears.
It has and. The helical pinion gear 205 is slidably and rotatably housed in the housing hole 211 of the differential case 203,
The other helical pinion gear is slidably and rotatably housed in another housing hole. These storage holes have a cylindrical shape in which a part is cut for meshing with each helical pinion gear and the helical side gears 207 and 209.

The driving force of the engine causes the differential case 203 to rotate so that each helical pinion gear moves from the side gear 20.
It is transmitted to the wheel side via 7, 209. At this time, due to the meshing reaction force and meshing thrust force of each gear, a frictional resistance is generated between the gears and between each gear and the differential case 203, and the torque sensitive differential limiting force is obtained by these frictional resistances. There is.

As described above, the differential case 203 is required to have a high abrasion resistance in the storage hole in which the helical pinion gears are pressed by the meshing reaction force and the sliding portion in which the gears are pressed by the meshing thrust force.
However, we want to lower the hardness at the cutting location and improve the cutting performance.

[0013]

[Problems to be Solved by the Invention] In induction hardening,
Since partial hardening is possible as described above, the wear resistance of the hardened part is improved, and the part that is not hardened has good machinability. For example, like the storage hole of the differential case 203, Hardening is not possible at locations with complicated shapes, and hardening is not possible at deep layers.
It is not possible to improve the mechanical properties of the whole 03.

Further, in the heat treatment method disclosed in Japanese Patent Laid-Open No. 4-325624, since partial quenching is not possible, wear resistance of the sliding portion and machinability of other portions like the differential case 203 are not maintained. It cannot be applied to a member that needs to improve both (if it is applied, the abrasion resistance of the sliding part is reduced).

In addition to this, the time for decarburization treatment is 3 hours to
Since it is as long as 5 hours, the low carbon decarburization layer becomes thicker during that time, and the strength of the entire heat-treated member such as the differential case 203 decreases accordingly, and the member is obtained by holding the casting at a high temperature of 900 ° C to 950 ° C for a long time. Deformation occurs.
Moreover, as the decarburization time is long, the productivity is poor, and the heat energy loss is large.

Therefore, the present invention provides a heat treatment method for spheroidal graphite cast iron capable of improving the wear resistance of a required portion and improving the machinability of other portions, and a differential device in which a member is heat treated by this method. With the goal.

[0017]

According to a first aspect of the present invention, there is provided a heat treatment method for spheroidal graphite cast iron, wherein a spheroidal graphite cast iron member having a decarburization inhibitor applied to a portion where wear resistance is desired to be improved is 850 ° C to 950 ° C.
It is characterized in that it is held in the decarburizing atmosphere for 0.5 to 2 hours to be austenitized and decarburized, followed by rapid cooling and further tempering.

The heat treatment method for spheroidal graphite cast iron according to claim 2 is:
Tempering at 300 ° C to 450 ° C for 0.5 hours to 1.5
The heat treatment method for spheroidal graphite cast iron according to claim 1, which is carried out under the condition of time.

The heat treatment method for spheroidal graphite cast iron according to claim 3 is
An austenitizing and decarburizing treatment is performed by holding a spheroidal graphite cast iron member coated with a decarburization inhibitor on a portion where wear resistance is desired to be enhanced, in a decarburizing atmosphere at 850 ° C to 950 ° C for 0.5 to 2 hours. After that, the heat treatment method for spheroidal graphite cast iron is characterized by performing an austempering treatment of maintaining the isothermal transformation temperature and air cooling.

The heat treatment method for spheroidal graphite cast iron according to claim 4 is as follows:
The heat treatment method for spheroidal graphite cast iron according to claim 3, wherein the austempering treatment is carried out by salt bath quenching or the like under the conditions where the temperature is kept at 240 ° C to 270 ° C for 0.5 hour to 1.5 hours.

The heat treatment method for spheroidal graphite cast iron according to claim 5 is as follows:
The heat treatment method for spheroidal graphite cast iron according to claim 1, 2, 3 or 4, wherein the decarburizing atmosphere is oxygen, and nitrogen is mixed into the oxygen to control the oxygen concentration.

According to a sixth aspect of the present invention, a differential device includes a differential case made of spheroidal graphite cast iron which is rotationally driven by a driving force of an engine, a pair of output side gears rotatably supported inside the differential case, and a rotary case inside the differential case. A pinion gear that is freely supported and is meshed and connected to allow differential rotation of a pair of output side gears,
A differential case having a decarburization inhibitor applied to a sliding portion with each gear is heat-treated by the method of claim 1, 2, 3, 4 or 5.

According to another aspect of the differential device of the present invention, the pinion gear is slidably rotatably housed in a housing hole formed in the differential case radially outside the pair of output side gears, and the first gear meshes with the pair of side gears. And at least a pair of pinion gears having a second gear portion meshing with each other.

[0024]

[Function] Generally, in the case of carburized steel, etc., a method of applying a carburizing agent to a portion which is not desired to be hardened and heat-treating it in an atmosphere having a high carbon content has been conventionally practiced. This method is not applicable due to the high carbon content of. Therefore, the heat treatment method of the present invention is based on a completely opposite idea.As shown below, a decarburization inhibitor is applied to a portion to be hardened with spheroidal graphite cast iron, and austenite is formed in a decarburizing atmosphere. While performing decarburization treatment.

In the heat treatment method for spheroidal graphite cast iron according to claims 1 to 4, a decarburization inhibitor is applied to a portion of the spheroidal graphite cast iron member where decarburization is desired to be prevented (a portion where abrasion resistance is desired to be enhanced), and the temperature is 850 ° C.
Decarburization treatment is carried out by holding in a decarburization atmosphere at 950 ° C for 0.5 hours to 2 hours while austenitizing (primary treatment step).

By this primary treatment step, decarburization is prevented in the portion coated with the decarburization inhibitor, the carbon content is kept high and quenching is possible, and the other portion is decarburized and the surface layer has low carbon content. A decarburized layer is formed.

After that, in the first aspect, the decarburized member is rapidly cooled and then tempered. In the second aspect, the tempering is performed at 300 ° C. to 450 ° C. for 0.5 hours to 1.5 hours. Further, in claim 3, the decarburized member is subjected to an austempering treatment in which the member is kept at a constant temperature transformation temperature and air-cooled. In claim 4, the austempering treatment is performed at 240 ° C.
It is carried out under a condition of keeping a constant temperature in a salt bath at ˜270 ° C. for 0.5 to 1.5 hours (secondary treatment step).

In the first and second aspects, in the high carbon content portion where decarburization is prevented by the secondary treatment step, as shown in FIG. 4, the whole layer 101 from the surface to the deep portion has high hardness and toughness. As shown in FIG. 5, in the decarburized portion that becomes the tempered structure of the site, a mixed structure of ferrite and pearlite with good machinability is formed in the surface layer 103, and the deep layer portion 10
5 has a tempered structure of martensite.

In the third and fourth aspects, in the high carbon content portion where decarburization is prevented by the secondary treatment step, the entire layer 101 has a bainite structure having high hardness and toughness as shown in FIG. , The decarburized part, as shown in FIG.
A mixed microstructure of ferrite and pearlite having good machinability is formed in the deep layer 105, and the deep layer 105 has a bainite microstructure.

In this manner, the sliding portion and the like can be increased in hardness and toughness to provide a large amount of wear resistance, and the other portions can be improved in machinability, shortening the processing time and extending the life of the cutting tool or the like. it can.

Further, since the parts other than the decarburized surface part are baked, the mechanical strength of the whole member is greatly improved.

As described above, since it is possible to quench the necessary portion only by applying the decarburization preventing agent, it is possible to easily apply it to a member or a place where the induction hardening cannot be applied due to its complicated shape.

Further, the decarburization time is shortened to 0.5 hours to 2 hours, which is much shorter than the conventional example, and the decarburization temperature is 850 ° C. to 9 hours.
Since the temperature is lowered to 50 ° C., the strength of the member is not lowered due to excessive decarburization, the casting is prevented from being deformed by heating for a long time, the productivity is greatly improved, and the heat energy loss is reduced.

The heat treatment method for spheroidal graphite cast iron according to claim 5 is as follows:
The heat treatment method for spheroidal graphite cast iron according to claim 1, 2, 3 or 4, wherein the decarburizing atmosphere is oxygen and nitrogen is mixed into oxygen to control the oxygen concentration, and the carbon in the member to be heat treated is Decarburized by reacting with oxygen. Further, the reaction rate between the decarburizing atmosphere and carbon can be controlled by changing the amount of nitrogen mixed, and the thickness of the decarburizing layer can be adjusted to a desired value.

In the differential device of the sixth aspect, in addition to the function of the first, second, third, fourth or fifth aspect of the invention, a decarburization preventive agent is applied to the sliding portion of the differential case made of spheroidal graphite cast iron with the side gear and the pinion gear. Sufficient abrasion resistance is obtained because it is applied and cured by heat treatment.

In the differential device of the seventh aspect, in addition to the operation of the sixth aspect of the invention, the pinion gear is slidably rotatably accommodated in the accommodating hole formed in the diff case made of spheroidal graphite cast iron. Since the decarburization-preventing agent is applied to this sliding portion as well as the other sliding portions and is cured by heat treatment, sufficient abrasion resistance can be obtained.

[0037]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIGS. This embodiment is the differential apparatus according to claims 6 and 7, and is heat-treated by the method according to claims 1, 2, and 5. FIG. 1 shows a differential device 1 of this embodiment. The left and right directions are the left and right directions in FIG. 1, and members and the like without reference numerals are not shown.

As shown in FIG. 1, in the differential case 3 of the differential device 1, the cover 7 is attached to the casing body 5 by the bolts 9.
And the retaining plate 11 is fixed with bolts. The differential device 1 is disposed inside the differential carrier, and the left and right boss portions 13 and 15 of the differential case 3 are supported by the differential carrier via bearings. The differential carrier is provided with an oil sump, and the lower part of the differential device 1 is immersed in this oil sump in a stationary state, and when rotated, the oil splashes up from the oil sump.

Inside the differential case 3, left and right side gears 17 and 19 (output side gears) each formed of a helical gear are arranged.

The respective side gears 17 and 19 have hollow boss portions 21 and 23, respectively, which are pivotal support portions 25 and 27 of the differential case 3.
Is supported by. The left and right wheel side output shafts pass through the boss portions 13 and 15 of the differential case 3, respectively, and
7 and 19 are splined to the boss portions 21 and 23. Washers 29 and 31 are arranged between the side gears 17 and 19 and the differential case 3, respectively.
A washer 33 is arranged between 7 and 19.

The differential case 3 has long and short storage holes 35, 37.
Are formed in plural sets in the circumferential direction. These storage holes 3
Long and short pinion gears 39 and 41, which are helical gears, are housed in reference numerals 5 and 37 so as to be slidably rotatable.

The long pinion gear 39 is composed of first and second gear portions 43, 45 and a small diameter shaft portion 47 connecting them, and the first gear portion 43 meshes with the left side gear 17. The short pinion gear 41 is composed of first and second gear portions 49 and 51, the first gear portion 49 meshes with the right side gear 19, and the second gear portion 51 meshes with the second gear portion 45 of the pinion gear 39. ing.

The driving force of the engine for rotating the differential case 3 is from the pinion gears 39, 41 to the side gears 17, 1.
It is distributed to the left and right output shaft sides via 9. Further, for example, when a driving resistance difference occurs between the output shafts while traveling on a rough road, the driving force of the engine is differentially distributed to the left and right sides by the rotation of the pinion gears 39 and 41.

During transmission of torque, each pinion gear 39, 4
The tooth tip of No. 1 is pressed against the wall surfaces of the storage holes 35, 37 by the reaction force of meshing with the side gears 17, 19, and frictional resistance is generated. In addition, the meshing thrust force of the helical gear causes the left end of the pinion gear 39 to move to the thrust washer 29.
It is pressed against the cover 7 via and the right end is pressed against the retaining plate 11. Further, the left end of the pinion gear 41 is pressed against the casing body 5, and the right end thereof is pressed against the retaining plate 11. Furthermore, the side gear 17
Is pressed against the cover 7 via the thrust washer 29, and the side gear 19 is pressed against the casing body 5 via the thrust washer 31. Also, each side gear 1
7 and 19 press each other through the thrust washer 33. In this way, frictional resistance is generated in each sliding portion, and a torque-sensitive differential limiting force is obtained by these frictional resistances.

The differential case 3 is provided with openings 53 and 55, and spiral oil grooves 57 and 57 are formed on the inner circumferences of the boss portions 13 and 15. When the differential device 1 rotates, the oil splashed from the oil sump is
When stationary, the oil in the oil sump flows into and out of the differential case 3 through the openings 53, 55 and the oil grooves 57, 57 and is supplied to the storage holes 35, 37 and the meshing parts of the gears.
Lubricate these.

Thus, the differential device 1 is constructed.

In the vehicle equipped with the differential device 1, the torque sensitive differential limiting force of the differential device 1 stabilizes the behavior of the vehicle body when a large torque is applied such as at the time of starting or accelerating, and is excellent in maneuvering. And stability are obtained.

As described above, a high abrasion resistance is required for the sliding portion between the differential case 3 for generating the differential limiting force and each gear. In addition to this, the cutting portion of the differential case 3 is required to have good cutting performance. Therefore, the differential case 3 is subjected to the following heat treatment in order to satisfy these contradictory requirements.

The heat treatment step and the cutting steps performed before and after the heat treatment step will be described with reference to FIGS. 2 shows a casing body having the same shape as the casing body 5 of the differential device shown in FIG. 1, and the reference numerals are the same as those in FIG.

The differential case 3 is made of spheroidal graphite cast iron.

First, the housing holes 35, 3 of the casing body 5
7, the sliding portion 59 between the casing body 5 and the side gear 19 (thrust washer 31) and the sliding portion 61 between the casing body 5 and the pinion gear 41 are cut before the next heat treatment. .

Next, as shown in FIGS.
Decarburization preventive agent (mainly Cu
System).

Casing body 5 coated with a decarburizing inhibitor
For 1 to 2 hours in a decarburizing atmosphere at 900 ° C.,
The structure is austenitized and decarburized. If the wall thickness of the casing body 5 is thick, this holding time is long within this range, and if thin, it is shortened. The decarburizing atmosphere is one in which oxygen is the main component and nitrogen is mixed to adjust the oxygen concentration.

During this heat treatment, the decarburization-preventing storage holes 35 and 37 and the sliding parts 59 and 61 are prevented from decarburization, and all layers including the surface layer remain as the casting base material having a high carbon content. Become.
Further, in the portion that does not prevent decarburization, the carbon in the surface layer reacts with oxygen in the decarburizing atmosphere to decarburize, and a decarburized layer is formed on the surface, and the casting base material with a high carbon content is left inside. Here, by changing the amount of nitrogen mixed in the decarburizing atmosphere, the reaction rate with carbon in the decarburizing atmosphere and the thickness of the decarburizing layer can be controlled. The thickness of the decarburized layer is set to 0.1 mm to 0.2 mm in consideration of the machining allowance in the cutting process in the subsequent process.

Then, the casing body 5 is furnace-cooled to 850 ° C. in a decarburizing atmosphere and kept for 15 minutes to sufficiently austenize the inside of the thick portion of the casing body 5 (primary treatment step).

Next, the casing body 5 after the primary treatment is
Quench with oil to 20 ° C., then air cool and quench.

By this quenching, the storage holes 35, 37
Part and all the layers of the sliding parts 59 and 61 and the deep layer of the decarburized part have a martensitic structure, and the decarburized layer has a mixed structure of ferrite and pearlite.

Next, the casing body 5 after quenching
Is held at 400 ° C. for 1 hour, then cooled in the atmosphere and tempered (secondary treatment step).

As a result of this tempering, as shown in FIG. 4, the storage holes 35, 37 and the sliding portions 59, 61 are covered with all layers 101.
Shows the tempered structure of martensite, and in the decarburized portion, as shown in FIG. 5, the surface layer portion (decarburized layer) 103 has a mixed structure of ferrite and pearlite, and the deep layer portion 105 has a tempered structure of martensite. .

The tempered structure of martensite has high hardness and toughness, and the storage holes 35, 37 and the sliding portions 59, 61 are provided with excellent wear resistance. Further, the mixed structure of ferrite and pearlite has good machinability. This cutting portion is cut with a thickness of the decarburized layer of 0.1 mm to 0.2 mm or less.

Thus, the diff case 3 is provided with high hardness and toughness in the housing holes 35 and 37 and the sliding portions 59 and 61, which require great wear resistance, and the durability is greatly improved.
Further, in other portions, the machinability of the surface layer portion is improved, the cutting time is shortened, and the life of the cutting tool is extended.

Further, since the parts other than the decarburized layer 103 are baked, the mechanical strength of the differential case 3 as a whole is significantly improved.

Further, as shown in FIG. 3, the storage holes 35 and 37 have a complicated cylindrical shape, a part of which is cut away for meshing of the pinion gears 39 and 41 and the side gears 17 and 19. Although quenching is difficult, as described above, according to the heat treatment methods of claims 1, 2 and 5, quenching can be easily performed by only applying a decarburization inhibitor to a necessary portion.

Further, the decarburization time was shortened to 0.5 hours to 2 hours, which is much shorter than the conventional example, and the decarburization temperature was 850 ° C to 9 hours.
Since the temperature is lowered to 50 ° C., strength reduction due to excessive decarburization of the differential case 3 and deformation due to long-time heating are prevented, productivity is greatly improved, and heat energy loss is reduced.

Further, according to the heat treatment method of claim 5, wherein nitrogen is mixed into oxygen in the decarburizing atmosphere to control the oxygen concentration.
By controlling the decarburization rate by changing the amount of nitrogen mixed,
The thickness of the decarburized layer can be adjusted to a desired value.

Next, the second embodiment will be described. This embodiment has the features of claims 3, 4 and 5, and is applied to the heat treatment method for the differential case 3 of the differential device 1 as described in claims 6 and 7 as in the first embodiment.

Following the first embodiment, the heat treatment step and the cutting steps performed before and after the heat treatment step will be described by taking the casing body 5 as an example.

First, the housing holes 35, 3 of the casing body 5
After cutting 7 and sliding parts 59 and 61 and applying a decarburization preventive agent to these, casing body 5 is retained in a decarburizing atmosphere at 900 ° C. for 1 to 2 hours to be austenitized and decarburized. Further, the casing body 5 is furnace-cooled to 850 ° C. in a decarburizing atmosphere and held for 15 minutes to sufficiently austenize the inside of the casing body 5. The primary processing steps up to this point are the same as in the first embodiment.

Next, the casing body 5 is held in a salt bath at 260 ° C. for 1 hour to be austempered (secondary treatment step).

As a result of this austempering treatment, as shown in FIG. 4, all layers 101 of the storage holes 35 and 37 and the sliding portions 59 and 61 have a bainite structure, and the decarburized portion of FIG. Thus, the surface layer portion (decarburized layer) 103 has a mixed structure of ferrite and pearlite, and the deep layer portion 105 has a bainite structure.

The bainite structure has high hardness and toughness, and the storage holes 35, 37 and the sliding portions 59, 61 are provided with excellent wear resistance. Further, the mixed structure of ferrite and pearlite has good machinability. This cutting part has a decarburized layer thickness of 0.1.
It is cut to a thickness of mm to 0.2 mm or less.

In the differential case 3, thus, the storage holes 35, 37 and the sliding portions 59, 61, which require great wear resistance, have a bainite structure with high hardness and toughness, and the durability is greatly improved. In addition, the surface layer portion of the other portion has a mixed structure of ferrite and pearlite with good machinability, which shortens the machining time and prolongs the life of the blade and the like. Further, since the parts other than the decarburized layer 103 are baked, the mechanical strength of the differential case 3 as a whole is significantly improved.

According to the heat treatment method of claims 3 and 4,
Quenching is easy even in a complicated shape such as the storage holes 35, 37. According to the heat treatment method of claim 5, the decarburization rate is controlled by changing the amount of nitrogen mixed, and the decarburization layer is formed. The thickness can be adjusted to the desired value.

Further, since the decarburizing time is greatly shortened and the decarburizing temperature is lowered as compared with the conventional example, the strength reduction due to the excessive decarburization of the differential case 3 and the deformation due to the heating for a long time are prevented, and the production Properties are greatly improved and the loss of heat energy is also reduced.

The tempering according to claim 1 and the austempering treatment according to claim 3 may be performed under different conditions from the tempering conditions according to claim 2 and the austempering treatment according to claim 4, respectively.

Further, in the differential device of the seventh aspect, the pinion gear and the side gear may be constituted by a spur gear instead of a helical gear.

This differential device is either a rear diff (rear wheel side axle diff), a front diff (front wheel side axle diff), or a center diff (differential device for distributing the driving force of the engine to the front and rear wheels). Also used for.

[0078]

The heat treatment method for spheroidal graphite cast iron according to claims 1 to 4 comprises applying a decarburization inhibitor to a portion of a spheroidal graphite cast iron member such as a sliding portion where wear resistance is desired to be enhanced, and 850 ℃ ~ 95
Decarburization treatment is carried out while austenitizing by holding in a decarburizing atmosphere at 0 ° C for 0.5 to 2 hours, and while maintaining a high carbon content in the portion coated with the decarburizing inhibitor, the surface layer of other portions is Forming a low carbon layer (primary treatment step), then
In claim 1, tempering is performed after quenching the decarburized member, and in claim 2, the tempering is performed at 300 ° C to 45 ° C.
It is carried out at 0 ° C. for 0.5 hours to 1.5 hours. Further, in claim 3, an austempering treatment is carried out in which the decarburized member is held at a constant temperature transformation temperature and air-cooled, and in the claim 4, the austempering treatment is carried out in a salt bath of 240 ° C to 270 ° C.
It is performed under the condition that the temperature is kept constant for 5 hours to 1.5 hours (secondary treatment step).

In the first and second aspects, the decarburization-prevented portion is decarburized by a martensite tempered structure having high hardness and toughness in the entire layer from the surface to the deep portion by these secondary treatment steps. In the portion where the deep layer is formed, the surface layer has a mixed structure of ferrite and pearlite, which has good machinability, and the deep layer has a tempered structure of martensite.

Further, in claims 3 and 4, the part where decarburization is prevented by these secondary treatment steps has a bainite structure with high hardness and toughness in the entire layer, and the decarburized part has the surface layer. A mixed structure of ferrite and pearlite with good machinability is used, and a bainite structure is formed in the deep layer.

Since the decarburized layer is not burnt and the machinability is improved, the other parts than the decarburized layer are baked, so that the abrasion resistance of the surface and the mechanical strength of the entire member are significantly improved.

Thus, decarburizing only necessary portions to improve machinability, shorten machining time and prolong the life of blades and the like, and increase hardness and toughness except for the decarburized layer to improve wear resistance and machine resistance. Strength can be given.

Further, since it is possible to quench the necessary portion only by applying the decarburization preventing agent, it is possible to easily apply it to a member or a portion having a complicated shape and to which induction hardening cannot be applied.

Further, the decarburization time was shortened to 0.5 hours to 2 hours, which was much shorter than that of the conventional example, and the decarburization temperature was 850 ° C to 9 hours.
Since the temperature is lowered to 50 ° C., the strength of the member is not lowered due to excessive decarburization, the casting is prevented from being deformed by heating for a long time, the productivity is greatly improved, and the heat energy loss is reduced.

The heat treatment method for spheroidal graphite cast iron according to claim 5 is as follows:
The heat treatment method for spheroidal graphite cast iron according to claim 1, 2, 3 or 4, wherein the decarburizing atmosphere is oxygen and nitrogen is mixed into oxygen to control the oxygen concentration, and the amount of nitrogen mixed is changed. Thus, the reaction rate between the decarburizing atmosphere and carbon can be controlled, and the thickness of the decarburized layer can be adjusted to a desired value.

According to the differential device of claim 6, in addition to the effect of the invention of claim 1, 2, 3, 4 or 5, the sliding parts of the differential case made of spheroidal graphite cast iron with the side gear and the pinion gear prevent decarburization. Since the agent is applied and cured by heat treatment, sufficient abrasion resistance can be obtained.

According to the differential device of claim 7, in addition to the effect of the invention of claim 6, the pinion gear is slidably rotatably housed in a housing hole formed in the differential case made of spheroidal graphite cast iron. This sliding part of the differential case is coated with decarburization prevention agent along with other sliding parts,
Since it is hardened by heat treatment, sufficient abrasion resistance can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view showing a first embodiment of the present invention.

FIG. 2 is a sectional view of a casing body having the same shape as the casing body used in the first embodiment.

3 is a view on arrow A in FIG. 2. FIG.

FIG. 4 is a cross-sectional view showing the structure of a portion where decarburization of the surface is prevented by the heat treatment method of each example.

FIG. 5 is a cross-sectional view showing the structure of a portion whose surface is decarburized by the heat treatment method of each example.

FIG. 6 is a cross-sectional view of a conventional example.

[Explanation of symbols]

 1 Differential Device 3 Differential Case 17, 19 Output Side Gear 35, 37 Storage Hole (Sliding Part) 39, 41 Pinion Gear 43, 49 First Gear Part 45, 51 Second Gear Part 59, 61 Sliding Part

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display area F16H 48/20 F16H 1/45

Claims (7)

[Claims] 1. A spheroidal graphite cast iron member having a decarburization inhibitor applied to a portion where wear resistance is desired to be improved is kept in a decarburizing atmosphere at 850 ° C. to 950 ° C. for 0.5 to 2 hours. A heat treatment method for spheroidal graphite cast iron, which comprises quenching after quenching after denitrifying and stenitizing, and further tempering. 2. Tempering at 300 ° C. to 450 ° C.
The heat treatment method for spheroidal graphite cast iron according to claim 1, which is carried out under the condition of 5 hours to 1.5 hours. 3. Austenite by holding a spheroidal graphite cast iron member coated with a decarburization inhibitor at a portion where wear resistance is desired to be improved in a decarburizing atmosphere at 850 ° C. to 950 ° C. for 0.5 to 2 hours. A heat treatment method for spheroidal graphite cast iron, which comprises subjecting to a carbonization and decarburization treatment, and then subjecting to an austempering treatment of keeping at a constant temperature transformation temperature and air cooling. 4. Austempering treatment is performed at 240 ° C. to 27 ° C.
The heat treatment method for spheroidal graphite cast iron according to claim 3, wherein the heat treatment is carried out by salt bath quenching or the like under the condition that the temperature is kept constant at 0 ° C for 0.5 hour to 1.5 hours. 5. The decarburizing atmosphere is oxygen, and nitrogen is mixed into the oxygen to control the oxygen concentration.
Or the heat treatment method for spheroidal graphite cast iron according to item 4. 6. A differential case made of spheroidal graphite cast iron that is rotationally driven by a driving force of an engine, a pair of output side gears rotatably supported inside the differential case, and a pair rotatably supported inside the differential case. 7. A differential case, comprising: a pinion gear meshingly connected to allow differential rotation of an output side gear of FIG. 3, wherein a decarburization preventive agent is applied to a sliding portion with each gear. A differential device characterized by being heat treated by a method. 7. The pinion gear is slidably housed in a housing hole formed in a differential case on the radially outer side of the pair of output side gears, and the first gear portion meshes with the pair of side gears and the second gear meshes with each other. 7. The differential device according to claim 6, wherein the differential device is at least a pair of pinion gears having a gear portion.