JPS6238430B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for hardening the surface of a high-pressure fuel injection pipe made of carbon steel pipe for high-pressure piping, and a nitriding device applied to the method.

In general, injection pipes used in the fuel supply path of diesel internal combustion engines are produced by cold working carbon steel pipes for high-pressure piping.

Semi-annealing, gas nitrocarburizing, and carburizing are methods used to improve strength, but the strength of semi-annealing is approximately 1.5 times that of cold-worked products, and the strength of gas nitrocarburizing and carburizing products is approximately 1.5 times that of cold-worked products. is approximately 1.5 for cold-processed products.
~1.8 times.

Gas nitrocarburizing treatment aims to improve strength by forming a compound layer on the inner surface through nitriding.
The compound layer is susceptible to bending deformation, cracks occur when bent three times or more, and the hardening depth is shallow, about 10 μm. In addition, many fuel injection pipes are long and have complex shapes, and their inner diameters are small (2 to 2.4 mm), so in a typical gas soft nitriding furnace or gas carburizing furnace, the gas circulation is poor and the inner diameter of the injection pipe is hardened to the center. At present, the hardness of the inner surface is _Hv 100 to 120, which is about the same as that after cold working, and the effect of the hardening treatment is not recognized.

As the performance of engines and vehicles improves, high-pressure steel pipes that can withstand high flow pressure are desired. Due to manufacturing constraints, it is impossible to improve the strength of small diameter pipes such as fuel injection pipes by changing the material, so it is necessary to improve the strength of the inner diameter part by some type of surface hardening method.

As a result of intensive studies to meet this need, the present inventors found that steel has the property of improving hardenability and lowering the transformation point by diffusing nitrogen. Focusing on the fact that quenching is not effective even if quenched as it is, the carbon content is low (less than 0.15%), and nitrogen gas is first uniformly diffused on the inner surface of small-diameter pipes such as fuel injection pipes. It was concluded that the strength could be improved by hardening the steel and then quenching it.

However, in the case of small-diameter tubes such as fuel injection tubes, it is not possible to obtain a uniform nitrided layer up to the center of the inner tube using conventional in-furnace stirring.

Therefore, the present inventors heated the raw gas (nitrogen gas) by, for example, high-frequency heating immediately before it enters the furnace to raise it to an appropriate nitriding temperature, and used a pipe, particularly preferably a tapered pipe, to heat the raw gas (nitrogen gas) into the furnace. We developed a device that forcibly sends nitrogen gas to the inner surface of the injection tube from each joint while adjusting the flow rate of nitrogen gas. Nitrogen forcibly diffused by the device uniformly enters the center of the inner surface of the tube, improving hardenability. After that, if a quenching process is performed in which the carbon steel for high-pressure piping is held at the quenching temperature for a certain period of time and then rapidly cooled, the nitrogen compound layer diffused on the inner surface of the pipe is maintained at the quenching temperature and the core of the base material is heated. At the same time, it is possible to prevent embrittlement of the ferrite grain boundaries on the inner surface.
Further, by rapid cooling, the brittle nitride compound layer becomes a martensitic structure, which increases the toughness and improves the bending workability. Furthermore, if it is desired to improve the toughness, low-temperature tempering treatment may be performed.

That is, the present invention, (1) Nitrogen gas heated to the nitriding temperature of the inner surface of a carbon steel high-pressure fuel injection pipe for high-pressure piping is forcibly introduced to nitridize the inner surface, and then quenched. A method for hardening the surface of a high-pressure fuel injection pipe, characterized by:

Among the above methods, equipment suitable for nitriding treatment includes: (2) a pipe installed in the center of the equipment body through which nitrogen gas flows; a case surrounding the pipe; and a case surrounding the lower part of the pipe. a heating device for heating nitrogen gas in the pipe; a high-pressure fuel injection pipe connection joint attached to the pipe; a high-pressure fuel injection pipe connection joint attached to the surrounding case; A nitriding device for hardening the surface of a high-pressure fuel injection pipe, comprising a communication pipe for communicating the inside.

It is related to.

Hereinafter, the method and apparatus of the present invention will be described in more detail with reference to the accompanying drawings.

FIG. 1 is a longitudinal sectional view showing an embodiment of the apparatus of the present invention, and FIG. 2 is a top half sectional view showing an example of a method for attaching a fuel injection pipe to be treated.

In Figures 1 and 2, 1 is the main body of the apparatus (nitriding furnace), 2 is the assembled wall of the nitriding furnace main body, 3 is a gas feed pipe, 4 is a case surrounding the pipe 3, 5 is a high-frequency heating device, and 6 is a lid. , 7 is Juan.

The pipe 3 is installed in the center of the nitriding furnace body 2 with a tapered shape as shown in the figure or a parallel pipe with a closed end.

Reference numeral 8 denotes a joint, which is fixed to the pipe 3 in an inclined manner from the inner lower side to the outer upper side. 9 is a communicating pipe, which is inclined from the outer lower part to the inner upper part to connect the pipe 3.
is fixed to. These joints 8 and communication pipes 9 are formed hollow and communicate between the inside and outside of the pipe 3.

The case 4 includes a plate 11 arranged horizontally, a cylinder 12 fixed to the outer peripheral surface of the plate 11, a cylinder 13 fixed to the upper surface of the plate 11, a cover 14 attached to the cylinder 12, and a cylinder 13 attached to the cylinder 13. cover 1
It consists of 5. Plate 11 is fixed to pipe 3.

A plurality of joints 16 are fixed to the cylindrical body 13. The joint 16 is formed hollow, and the cylindrical body 1
It connects the inside and outside of 3.

17 is a communication hole provided in the cylinder 12 below the plate 11, and 18 is a communication hole provided in the cover 14.

21 is a fuel injection pipe to be processed, and after it is molded according to the shape of use, joints 8 and 16 are placed at appropriate positions.
connected to. FIG. 2 shows an example of the connection method, in which a joint 22, a sleeve 23, and a nut 24 are fixed to the tip of the pipe 21 by welding or the like. When connecting the fuel injection pipe 21, a large number of joints 8, 16 are provided at arbitrary positions, so that the connection can be made without difficulty by appropriately selecting the joints 8, 16. Further, the unused joints 8 and 16 are shut off with plugs 25 and 26.

FIG. 3 is a longitudinal sectional view showing another embodiment of the device of the present invention.

In the case of FIG. 1, the plate 11 and the cylindrical body 12 constituting the case 4 were fixed together, but in FIG.

The mode of operation of the device of the present invention configured as described above is as follows.

The raw gas (nitrogen gas) sent from below the pipe 3 is heated to the nitriding temperature by the high-frequency heating device 5, and then forced through the joint 8 to the fuel injection pipe 21.
sent inside.

The gas that has passed through the fuel injection pipe 21 is transferred to the joint 16
is discharged to the outside from the communication hole 1 by the fan 7.
8 to the outside of the case 4, and the communication hole 1
7, is returned to the communication pipe 9, and is again supplied to the pipe 3.

In the above process, if a tapered pipe is used as the pipe 3, the amount of gas supplied between the joints 8 at the upper and lower positions of the pipe 3 will be approximately the same,
This is preferable in order to make the nitriding treatment conditions more uniform.

After nitrogen is diffused into the inner surface of the fuel injection pipe 21 in this manner, it is removed and subjected to normal hardening treatment. That is, after holding the quenching temperature at 800 to 900°C for 30 to 60 minutes, rapid cooling is performed.

The inner surface hardness after treatment becomes H _Mv 500 or more.

If a bending angle of 45 degrees or more is required, add an additional 180 degrees.
Low-temperature tempering may be performed at ~230°C.

Depending on the material of the injection tube, fine cracks may occur on the outer circumferential surface along the grain boundaries during quenching, so in such cases, decarburization prevention treatment and a slight nitrogen diffusion treatment on the outer circumferential surface may be necessary. do it.

FIG. 4 is a diagram showing the effect of the present invention, in which A shows the hardness in the case of no treatment, B shows the hardness in the case of conventional gas nitrocarburizing, and C shows the hardness in the case of the hardening heat treatment of the present invention.
It can be seen from the figure that the surface hardness, hardening depth and core hardness are significantly improved in the case of the present invention.

In addition, A, B, and C in Fig. 4 are fuel injection pipes (inner diameter 2.2 mm) made of STS35 (carbon steel pipes for high pressure piping).
φ, outer diameter 6.2 mmφ, length 600 mm), and in the case of C, the hardening heat treatment of the present invention, an apparatus of the embodiment shown in Fig. 3 (the dimensions of each part of the apparatus are as shown in Fig. 5 (unit: (mm), and the joints 8 and 16 were each installed radially at an angle pitch of 45°, and the wall thickness of the pipe 3, plate 11, cylinder 13, cover 15, and cylinder 31 was 10 mm) at 530°C. After nitriding for 20 hours by supplying nitrogen gas at a supply rate of 1.5 m ³ /Hr during soaking, the specimens were taken out from the above equipment and subjected to water-cooled quenching at 830°C for 30 minutes in an electric furnace (not shown). In the case of B, conventional gas nitrocarburizing,
It was treated at 570°C for 6 hours.

In addition, the tensile strength is 35 to 38 Kgf/mm ² for the untreated product and 50 to 60 Kgf/mm ² for the conventional gas nitrocarburizing treatment and carburizing treatment.
On the other hand, it has been confirmed that the strength is 45 to 55 Kgf/mm ² as is after gas nitriding by the hardening treatment of the present invention, and that the strength is significantly improved to 120 to 120 Kgf/mm ² by applying the hardening treatment.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of an embodiment of the device of the present invention;
FIG. 2 is a detailed view of a part of FIG. 1, FIG. 3 is a diagram showing another embodiment of the device of the present invention, FIG. 4 is a diagram showing the effects of the present invention, and FIG. FIG. 4 is a diagram showing the dimensions of the apparatus (of the embodiment shown in FIG. 3) used in the example carried out to obtain the hardness C of the present invention shown in the figure.

Claims (1)

[Claims] 1. Nitrogen gas heated to a nitriding temperature is forcibly fed into the inner surface of a carbon steel high-pressure fuel injection pipe for high-pressure piping to nitridize the inner surface, and then to quench the inner surface. A method for hardening the surface of a high-pressure fuel injection pipe, characterized by: 2. A pipe installed in the center of the device body through which nitrogen gas flows, a case surrounding the pipe, a heating device installed around the bottom of the pipe to heat the nitrogen gas in the pipe, and the pipe. A high-pressure fuel injection pipe comprising a high-pressure fuel injection pipe connection joint attached to the surrounding case, a high-pressure injection pipe connection joint attached to the surrounding case, and a communication pipe for communicating the inside of the pipe and the inside of the case. Nitriding equipment for surface hardening.