# Patent

## Title
Method for forming a common-rail injection system for diesel engine

## Abstract

A common-rail injection system is provided for a diesel engine and has excellent internal pressure fatigue resisting characteristics, vibrational fatigue resisting characteristics and cavitation resisting property and sheet face flawing resisting property, and can be made thin and light in weight. A main pipe rail is manufactured by transformation induced plastic type strength steel. After the main pipe rail is processed, residual austenite is generated by heat treatment, and the reduction processing of stress concentration of a branch hole and a main pipe rail side flow passage crossing portion is performed. Further, it is preferable that an induced plastic transformation is generated on the inner surface of the main pipe rail by autofrettage processing, and compression residual stress is left.

## Background
This application is a divisional of U.S. patent application Ser. No. 10/616,571, filed Jul. 10, 2003, which is now U.S. Pat. No. 7,125,051.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a common-rail injection system such as a high pressure fuel manifold or a block rail, etc. in an accumulating pressure fuel ejecting system of a diesel internal combustion engine, and particularly relates to a common-rail injection system for a diesel engine raised in internal pressure fatigue strength.
2. Description of the Related Art
As the common-rail injection system of this kind, for example, a common-rail injection system shown in FIG. 1, a common-rail injection system shown in FIG. 2, common-rail injection systems shown in FIGS. 3 and 4, an unillustrated block rail type common-rail injection system, etc. are conventionally known. In the common-rail injection system shown in FIG. 1, a boss 3c integrated with a main pipe rail 1 of the common-rail injection system is formed in this main pipe rail 1. A pressing seating face 2-3 formed in a connecting head portion 2-2 of a branch pipe 2 is abutted on a pressure receiving seating face 1-3 on the main pipe rail 1 side and is engaged with this pressure receiving seating face 1-3, and is connected by fastening a box nut 6 screwed onto a screw portion 3-2 arranged on the outer circumferential face of the above boss 3c. In the common-rail injection system shown in FIG. 2, a portion of a branch hole 1-2 communicated with an internal flow passage 1-1 of a circular section arranged in a circumferential wall portion on the side of a main pipe rail 1 is formed as a pressure receiving seating face 1-3 opened outward. A pressing seating face 2-3 formed in a connecting head portion 2-2 on the side of a branch pipe 2 as a branch connecting body formed in e.g., a tapering-off conical shape and enlarged in diameter by buckling molding in an end portion is abutted on the pressure receiving seating face 1-3 and is engaged with this pressure receiving seating face 1-3 by using a joint fitting 3 of a ring shape surrounding the outer circumferential portion of the main pipe rail 1 near the pressure receiving seating face. A portion of a screw wall 3-1 is projected outward from the main pipe rail 1, and is arranged in the joint fitting so as to be projected in the diametrical direction of the above main pipe rail 1. A nut 4 is assembled into the side of the branch pipe 2 in advance through a sleeve washer 5. The pressing seating face 2-3 is fastened and connected to the pressure receiving seating face 1-3 by pressing below a neck portion of the above connecting head portion 2-2 by screwing the nut 4 into the screw wall 3-1 portion. In the common-rail injection systems shown in FIGS. 3 and 4, sleeve nipples 3a, 3b of a sleeve shape instead of the joint fitting 3 of a ring shape are directly attached to the outer circumferential wall of the main pipe rail 1 by an irregular fitting screwing system, welding, etc. so as to be projected outward in the diametrical direction of the main pipe rail 1. A pressing seating face 2-3 formed in a connecting head portion 2-2 on the side of a branch pipe 2 is abutted on a pressure receiving seating face 1-3 on the side of the main pipe rail 1, and is engaged with this pressure receiving seating face 1-3, and is connected by fastening a nut 4 screwed into each of the above sleeve nipples 3a, 3b. 
However, in each of the above conventional common-rail injection systems, large stress is generated in a lower end inner peripheral portion P of the branch hole 1-2 by axial force applied to the pressure receiving seating face 1-3 by the internal pressure of the main pipe rail 1 and the pressing of the connecting head portion 2-2 of the branch connecting body such as the branch pipe 2. Therefore, a crack is easily caused with the lower end inner peripheral portion P as a starting point, and there is a possibility of generation of leakage of a fuel. The crack is next easily caused on the inner surface of the main pipe rail. This is because the main pipe rail is constructed by a thick cylinder, but a large tensile stress in the circumferential direction is caused on the inner surface since the main pipe rail has a large inside diameter.
The present invention is made in consideration of the above problems caused in the prior art, and an object of the present invention is to provide a common-rail injection system for a diesel engine in which the internal pressure fatigue strengths of the main pipe rail and the branch hole are raised by using transformation induced plastic type strength steel, and can be further improved by reducing the concentrating degree of stress generated in a crossing portion of the branch hole including the lower end inner peripheral portion with respect to the main pipe rail and a main pipe rail side flow passage.
SUMMARY OF THE INVENTION
The present invention resides in a common-rail injection system for a diesel engine constructed such that a branch hole communicated with a flow passage is formed in an axial circumferential wall portion of a main pipe rail having the flow passage within its axial core direction, and a branch connecting body is connected to the branch hole integrally with the main pipe rail or through a separate joint member, and characterized in that the main pipe rail is manufactured by transformation induced plastic type strength steel, and the main pipe rail is processed and residual austenite is then generated by heat treatment, and the processing hardening of an inner surface and compression residual stress are left by performing the reduction processing of stress concentration of the branch hole and a main pipe rail side flow passage crossing portion. The present invention is also characterized in that residual austenite is generated by heat treatment in the main pipe rail manufactured by transformation induced plastic type strength steel, and the main pipe rail is then processed and the processing hardening of an inner surface and compression residual stress are left by performing the reduction processing of stress concentration of the branch hole and a main pipe rail side flow passage crossing portion. Further, the present invention is characterized in that an induced plastic transformation is generated on the inner surface by autofrettage processing, and the compression residual stress is left after the reduction processing of the stress concentration of the branch hole and the main pipe rail side flow passage crossing portion is performed.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal sectional front view showing one example of a common-rail injection system of a boss integral type as an object of the present invention.
FIG. 2 is a longitudinal sectional side view of a main portion showing one example of the common-rail injection system using a joint fitting of a ring shape.
FIG. 3 is a longitudinal sectional side view showing one example of the common-rail injection system constructed by attaching a sleeve nipple of a sleeve shape to a main pipe rail by an irregular fitting screwing system.
FIG. 4 is a longitudinal sectional side view showing one example of the common-rail injection system constructed by attaching the sleeve nipple of the sleeve shape to the main pipe rail by welding.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Transformation induced plastic type strength steel in the present invention is developed for the purpose of making a press molding part around a foot in a passenger car light in weight in recent years. This transformation induced plastic type strength steel is ferrite (αf)+bainite (αb)+γR composite texture steel [TRIP type Dual-Phase steel, TDP steel], and bainitic ferrite (αbf)+γR steel [TRIP type bainite steel, TB steel] in which press molding property is greatly improved by utilizing the strain induced transformation (TRIP) of residual austenite (γR).
Here, the transformation induced plasticity is the large extension of an austenite (γ) layer existing in a scientifically unstable state caused in transformation to martensite by adding mechanical energy.
Namely, the TRIP steel is steel in which the metallic texture of a mixture of the residual austenite and the bainite texture with the grain boundary of an α-layer as a center is obtained by taking a specific heat treatment in the steel of a certain limited composition. As features of the TRIP steel having such a metallic texture, plastic deformation ability is high and the TRIP steel is high in strength and becomes hard since the TRIP steel becomes a martensite texture by plastic processing.
Since the common-rail injection system for a diesel engine in the present invention is manufactured by the transformation induced plastic type strength steel having such characteristics, the common-rail injection system has good processability at a forging time, and is easily formed in a desirable shape. In contrast to this, when no specific heat treatment is taken (when the residual austenite and bainite are small), both extension and tensile strength are low and cutting processing can be easily performed. In the case of the common-rail injection system using a pipe, a reduction at a pipe extending time is set to be large so that the number of pipe extending times can be reduced. Further, if the reduction is the same, processing can be performed by a small pipe extending machine and a small die.
Further, the transformation induced plastic type strength steel has characteristics (TRIP phenomenon) in which the austenite of a locally deformed portion is transformed to hard martensite, and its portion is strengthened. Accordingly, in the case of the common-rail injection system manufactured by this transformation induced plastic type strength steel, even when internal pressure fatigue is advanced, its fatigue portion is strengthened by the above characteristics and resistance force for preventing breakdown of the common-rail injection system is generated so that life is extended.
Further, since a branch hole and a main pipe rail side flow passage crossing portion are pressed in stress concentration reducing processing, compression residual stress is left around the branch hole. Further, since both hardness and tensile strength are improved by the deposition of processing induced martensite in the deforming portion, fatigue resisting characteristics are excellent.
In the heat treatment in the present invention, the main pipe rail is heated to 950° C., and is held for a predetermined time so that the main pipe rail is changed to austenite. Thereafter, the main pipe rail is held for a predetermined time at 350° C. to 500° C., and austemper processing is performed. A metallic texture having a residual austenite (γ) layer and a bainite texture mixed with each other is formed with the grain boundary of an α layer as a center by performing this austemper processing.
A method for leaving the compression residual stress by a pressing system is known as the reduction processing method of stress concentration of the branch hole and the main pipe rail side flow passage crossing portion in the present invention. As this method, for example, there are four methods described in JP-A-10-318081, etc. proposed by the present applicant. (1) In a first method, the compression residual stress is generated around a main pipe rail flow passage opening end portion of the branch hole by applying pressing force by an external pressure system. (2) In a second method, the compression residual stress is generated around the main pipe rail flow passage opening end portion of the branch hole by applying pressing force to the inner circumferential face of the main pipe rail near the branch hole by an internal pressure system. (3) In a third method, the compression residual stress is generated around the main pipe rail flow passage opening end portion of the branch hole by applying the pressing force by a pipe enlarging system for applying the pressing force to the inner circumferential face of the main pipe rail near the branch hole in the diametrical direction of the pipe from the interior of the main pipe rail. (4) In a fourth method, the compression residual stress is generated around the main pipe rail flow passage opening end portion of the branch hole by applying the pressing force by a diameter enlarging system for applying the pressing force to the inner circumferential face of the branch hole in the diametrical direction from the interior of the branch hole.
When the main pipe rail is excessively hardened by the heat treatment to raise the fatigue strength as steel (large in strength and small in extension), there is a case in which a crack is caused when pressing processing using the above pressing system is too strong. Further, a problem exists in that a tool (press pin) for pressing pressure is easily damaged, etc. However, in the case of the transformation induced plastic type strength steel (TRIP steel), there is no such a problem since strength is high and extension is large.
Autofrettage processing in the present invention is a method for plastically deforming only the inner circumferential surface by applying internal pressure. The main pipe rail is processed and hardened (both hardness and tensile strength are improved by the deposition of processed induced martensite) by the plastic deformation in the entire inner surface portion by this autofrettage processing. Further, the compression stress is left in the entire inner surface portion, and durability of the main pipe flow passage as the next weak point is also improved.
In the present invention, as mentioned above, both hardness and tensile strength are improved by the deposition of the processed induced martensite by taking the heat treatment and the pressing processing, and preferably further performing the autofrettage processing after mechanical processing of the TRIP steel although it was the austenite (γ) texture. Further, internal pressure fatigue resisting characteristics are also improved in the entire inner surface portion in addition to the branch hole and the main pipe rail side flow passage crossing portion by leaving the compression stress so that the durability of the main pipe flow path becomes excellent.
A round bar for forging manufactured by TRIP type bainite steel (TB steel) having components shown in Table 1 is cut to a predetermined size, and is heated until a hot forging temperature, and the raw material of a common-rail injection system (34 mmø in the outside diameter of a tubular portion) of a boss integral type is forged by die forging. Next, the processings of an inside diameter 10 mmø, a boss portion branch hole diameter 3 mmø, a sheet face, a screw portion, etc. in predetermined desirable portions are performed by cutting, etc. These processed portions are changed to austenite for 20 minutes at 950° C., and austemper processing is then performed by holding these portions for three minutes at 400° C. Thus, the common-rail injection system of the boss integral type having a texture having a residual austenite (γ) layer and a bainite texture mixed with each other with the grain boundary of an (α) layer as a center is formed. Thereafter, pressing force is applied to a branch hole portion of each boss of this common-rail injection system by an external pressure system described in JP-A-10-318081, and compression residual stress is generated around a main pipe rail flow passage opening end portion of the branch hole. Since the residual austenite layer and the bainite texture are small at a cutting processing time, tensile strength is low and extension is small so that processing is easily performed.
This common-rail injection system is repeatedly tested by a pressure tester, and its fatigue limit is examined. As a result, in the case of the common-rail injection system of the same size manufactured by normal high strength steel (SCM435) (C 0.33 to 0.38 mass %, Si 0.15 to 0.35 mass %, Mn 0.60 to 0.85 mass %, P 0.030 mass % or less, S 0.030 mass % or less, Cr 0.90 to 1.20 mass %, and Mo 0.15 to 0.30 mass %) used as a comparison material, the common-rail injection system is damaged by 800 thousand repeating tests using an oil pressure of 180 to 1500 Bar. In contrast to this, the common-rail injection system in the present invention is not damaged even by 10 million repeating tests at 2200 Bar and shows excellent internal pressure fatigue resisting characteristics.
In another example, a round bar for forging manufactured by TRIP type bainite steel (TB steel) having components shown in Table 1 is cut to a predetermined size, and is changed to austenite for 20 minutes at 950° C. Thereafter, austemper processing is performed by holding the round bar for three minutes in a range of 350 to 475° C. so that a texture having the residual austenite (γ) layer and the bainite texture mixed with each other with the grain boundary of an α-layer as a center is formed. This round bar is then forged by die forging so that the common-rail injection system (34 mmø in the outside diameter of a tubular portion) of the boss integral type is forged. Next, the processings of an inside diameter 10.6 mmø, a boss portion branch hole diameter 3 mmø, a sheet face, a screw portion, etc. in desirable portions are performed by cutting, etc. so that the common-rail injection system of the boss integral type is formed. Thereafter, pressing force is applied to a branch hole portion of each boss of this common-rail injection system by the external pressure system described in JP-A-10-318081, and compression residual stress is generated around a main pipe rail flow passage opening end portion of the branch hole. The residual austenite layer and the bainite texture exist at a forging time, and tensile strength is high but extension is large so that forging processing can be performed. Further, autofrettage processing is performed by applying internal pressure able to yield about 50% of the thickness of the tubular portion.
This common-rail injection system is repeatedly tested by a pressure tester, and its fatigue limit is examined. As a result, the common-rail injection system is not damaged even by 10 million repeating rests at 2400 Bar, and shows more excellent internal pressure fatigue resisting characteristic durability.
In another example, desirable processing of a branch hole diameter 3 mmø, a sheet face, a screw portion, etc. is performed by cutting, etc. in a common-rail injection system raw material (outside diameter 36 mmø and inside diameter 10 mmø of the pipe) obtained by cutting a seamless steel pipe manufactured by TRIP type bainite steel (TB steel) having components shown in Table 1 to a predetermined size. This common-rail injection system raw material is changed to austenite for 20 minutes at 950° C. Thereafter, austemper processing is performed by holding the common-rail injection system raw material for three minutes in a range of 350° C. to 475° C. so that the common-rail injection system having a texture having the residual austenite (γ) layer and the bainite texture mixed with each other with the grain boundary of an α-layer as a center is formed. Thereafter, pressing force is applied to a branch hole portion of this common-rail injection system by the external pressure system described in JP-A10-318081, and compression residual stress is generated around a main pipe rail flow passage opening end portion of the branch hole. Since the residual austenite layer and the bainite texture are small at the cutting processing time, tensile strength is low and extension is small so that processing is very easily performed.
This common-rail injection system is repeatedly tested by a pressure tester, and its fatigue limit is examined. As a result, in this embodiment, the common-rail injection system is also not damaged even by 10 million repeating tests at 2200 Bar, and shows excellent internal pressure fatigue resisting characteristic durability.
Similar effects are also naturally obtained in the case of a block rail manufactured by the TRIP type bainite steel (TB steel).












TABLE 1







C
Si
Mn
Al









0.17
1.41
2.02
0.032






(mass %)










As explained above, the common-rail injection system for the diesel engine in the present invention has excellent internal pressure fatigue resisting characteristics by processed induced martensite deposited in a crossing portion of the branch hole and the main pipe rail side flow passage and an inner peripheral portion of the branch hole and improved in both hardness and tensile strength, and compression residual stress. Further, this common-rail injection system has excellent internal pressure fatigue resisting characteristics over the entire inner surface of the common-rail injection system as well as the crossing portion of the branch hole and the main pipe rail slide flow passage and the inner peripheral portion of the branch hole by performing the autofrettage processing. Accordingly, durability at very high pressure can be secured. Further, there are also effects in that the common-rail injection system is excellent in vibrational fatigue resisting characteristics and cavitation resisting property and the flawing resisting property of a sheet face in addition to the excellent internal pressure fatigue resisting characteristics, and can be made thin and light in weight, etc.

## Claims

1. A method for forming a common-rail injection system for a diesel engine, comprising the steps of:
providing a main pipe rail formed from a transformation induced plastic strength steel, the main pipe rail having an axially-extending circumferential wall with an inner circumferential surface defining a flow passage through the main pipe rail, a branch hole extending through the axially-extending circumferential wall and communicating with the flow passage;
subjecting the main pipe rail to heat treatment sufficiently for converting at least portions of the transformation induced plastic strength steel to austenite;
subjecting the inner circumferential surface of the main pipe rail to autofrettage processing for applying an internal pressure and plastically deforming the inner circumferential surface of the main pipe rail for leaving a compression stress on the inner circumferential surface; and
applying a pressing force at a location surrounding the branch hole, whereby the autofrettage processing and the application of the pressing force produce process induced martensite at locations on the axial circumferential wall defining the inner circumferential surface and surrounding the branch hole.

2. The method of claim 1, wherein the heat treatment is carried out at approximately 950° C.

3. The method of claim 2, wherein the heat treatment is carried out for approximately twenty minutes.

4. The method of claim 3, further comprising performing austemper processing after the heat treatment and before the autofrettage processing.

5. The method of claim 4, wherein the austemper processing is carried out at a lower temperature than the heat treatment and for a shorter duration.

6. The method of claim 1, wherein the step of providing a main pipe rail further comprises die forging the main pipe rail to form a boss portion, the branch hole extending through the boss portion and communicating with the flow passage.

7. The method of claim 1 further comprising die forging the main pipe rail after the heat treatment step.