Process for obtaining a pipe for supplying fuel to an internal combustion engine

The present invention relates to a process for producing a pipe, particularly for supplying fuel to an engine, comprising the steps of:

FIELD OF INVENTION

The present invention relates to a process for obtaining a fuel supply pipe made of stainless steel, in particular for fuel injection to internal combustion engines, as well as a process for obtaining the same.

BACKGROUND OF INVENTION

The pipes for injecting gasoline into engines are normally made of stainless steel, preferably austenitic steel, which in addition to having good mechanical properties also has optimal resistance to corrosion caused by the gasoline. Stainless steel pipes or tubes have already been proposed; at whose ends a respective enlarged or pointed oval head is welded which has maximum diameter greater than the outer diameter of the pipe. The head is intended to abut against a receiving seat, in use, e.g. formed in a mouth or relief in an injector body or in a pump body or “fuel rail”.

As is known, when a liquid or fluid flows through a pipe, it is affected by electric charges due to the so-called “triboelectric” effect. In such a manner, galvanic currents are generated in the pipe that in the long run can compromise effectiveness of the welding, with consequent fuel leaks, injection pressure lowering and engine performance drop.

Moreover, during manufacture it is necessary to carry out two welds, operations which require the use of sophisticated and costly equipment as well as personnel with a high level of training.

SUMMARY OF THE INVENTION

The main object of the present invention is that of providing a process for producing a stainless steel pipe for supplying fuel to an injection engine which is safe and relatively easy and quick to obtain.

Another object of the present invention is that of providing a stainless steel pipe for supplying fuel to an injection engine that is fully reliable over time and particularly resistant to galvanic currents.

Another object of the present invention is that of providing a stainless steel pipe for supplying fuel to an injection engine which has a competitive production cost.

These and still other objects, which will be clearer below, are achieved by a process for producing a pipe for supplying fuel to an engine comprising the steps of prearranging a stainless steel pipe, carrying out a hot-pressing operation at at least one end of the pipe, and subjecting said at least one end of the pipe to heat treatment followed by cooling.

Advantageously, a pipe obtained according to the abovementioned process is characterized in that the at least one end thereof subjected to hot-pressing sequentially has a frustoconical end section, a substantially cylindrical intermediate section with outer diameter greater than the frustoconical end section, and a frustoconical section for connection to the pipe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to the drawing, a pipe1is illustrated that is designed to supply fluid or gaseous fuel to an injection engine, in particular a gasoline injection engine. The pipe1is preferably made of austenitic stainless steel and has one end3thereof configured as an attachment head with a substantially frustoconical (preferably pointed) end section3a, followed by a substantially cylindrical intermediate section3bwith outer diameter greater than the adjacent frustoconical section3a, and an internal frustoconical section3cfor connection to the remaining part of the pipe and having decreasing cross section towards the remaining portion of the pipe1. Due to the slight increase, between the frustoconical terminal section3aand the cylindrical portion3b, an annular shoulder3dis preferably delimited.

The end configured as an attachment head3is obtained via hot-pressing, an operation which causes a hardening of the head itself and in particular of its pointed section3a. For the purpose of softening and thus reducing the risk of breakage, the molded attachment head is subjected to heat treatment, i.e. it is heated via induction, after which it is allowed to cool or fast cooling thereof is caused in air or inert atmosphere.

The heat treatment provides for heating so as to bring the carbides contained in the stainless steel, in particular in the austenitic stainless steel, in solution; it also allows a partial recrystallization of the microstructure or “grain” of the steel. In such a manner, a reduction of the hardness is caused, at least of the end section3a, from about 350° to 200-190° Vickers. Preferably, heating is carried out at a temperature in the range of about 800° C. to about 1300° C.

If desired, both ends (the end3and the other end not shown in the drawings) of the pipe1are heat treated by induction after the hot-pressing step, as indicated above.

A pipe1according to the present invention can be e.g. installed on, and fixed to a frustoconical, hollow appendage or relief4, which delimits a frustoconical reception seat4ain fluid communication with an injector body5or a pump body or “fuel rail” for an injection engine, in particular a gasoline injection engine. The hollow appendage or relief4, known as the “connection interface”, is externally threaded at4bfor the removable fixing of a tightening connector or nut2thereon; by acting on the internal frustoconical section3cof the attachment head3, such connector or nut2being suitable for tightening the frustoconical end section3aagainst the frustoconical receiving seat4ain the hollow relief4.

It will be understood that a pipe according to the present invention is easily obtainable, with uniform composition and free of welding spots or lines, and thus resistant to galvanic currents. It is therefore not liable to suffer corrosion attack nor give rise to fluid or gaseous fuel leakage.

After hot-pressing, the steel of the pipe1becomes at least partly work-hardened and thus is very hard, so that it would not be suitable for ensuring a secure fluid seal between the frustoconical end section3aand the receiving seat4a, also made of stainless steel. Moreover, the rubbing of these components could in the long run compromise the integrity of the relatively softer seat4aof the section3a. Due to the induction heating, a softening is caused that is suitable for bringing its hardness level back to the initial level (before the hot-pressing operation)—at least at the frustoconical end section3a, for its optimal arrangement in the receiving seat4a. The latter occurs without the risk of scraping the seat, which would compromise its seal at work pressures, usually on the order of about 0-3000 bar.

The pipe described above is susceptible to numerous modifications and variations within the scope of protection defined by the claims.