EP1954848B1

EP1954848B1 - A process for raising the tempering resistance of a steel work piece

Info

Publication number: EP1954848B1
Application number: EP06793677.3A
Authority: EP
Inventors: Nils Lippmann; Rogerio Pitella
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2005-09-27
Filing date: 2006-09-20
Publication date: 2018-08-15
Anticipated expiration: 2026-09-20
Also published as: WO2007039468A3; BRPI0504417B1; BRPI0504417A; EP1954848A2; WO2007039468A2

Description

The present invention relates to a process for raising the tempering resistance of a steel work piece, especially injection-nozzle bodies of fuel-injection systems applied to internal combustion engines.

Description of the Prior Art

Steel is most widespread material for building injection-nozzle bodies of fuel injection systems of internal combustion engines. This is due to the different physical characteristics which steel has after a specific heat treatment.
Said treatment is carried out on injection-nozzle bodies, since application at high temperature and under high pressure is more and more common in feeding systems and, consequently, in said bodies. The temperature is in the range of 300 °C and the pressure of 2000 bar (200 MPa).
The most widespread initial treatment for said bodies is the cementation with subsequent quenching (rapid cooling). Optionally, one carries out austenitization after cementation and before quenching, in order to raise the toughness and fatigue strength of the steel.
The quenching may be carried out by a gaseous or liquid media. The gaseous medium has the advantage that there is no need to clean the bodies being treated afterwards. On the other hand liquid has the advantage of better raising the tempering resistance of the steel.
Tempering resistance means the resistance to the loss of hardness exhibited by the material in a tempering or annealing treatment after the quenching. The subsequent tempering or annealing aims at relieving the stresses generated in the material during the quenching process.
From document DE 103 18 135 , one knows processes for raising the tempering resistance in utilization in carburized steel, but its practical application is not so advantageous, since the alloys applied therein are silicon (Si) containing steels, which are little available on the market and consequently more expensive. The treatment thereof is less practical than that of other materials, namely, steel with alloy of 2 % Cr and 2 % Ni. The fatigue resistance is low and it still has nitrogen in its outer layer, which should be avoided, because it influences the dimensional stability during the operation. Further, in this process there is the formation of carbide (at least 15 %), that make the fatigue resistance even worse.
In the documents EP 1 432 841 and DE 102 54 846 processes for raising the tempering resistance by nitrating are disclosed, whereas high alloyed materials are used. Moreover, the costs of the nitrating treatment of these materials and of the material itself are higher than the use of a carburized steel. In addition, the documents WO 01/68933 A and GB 2 397 071 A disclose treatment procedures for a steel work piece including a quenching process in a gaseous medium.

Brief Description of the Invention

The present invention relates to a process for raising the tempering resistance of a steel work piece, which employs a gaseous medium to effect the quenching after cementation has been carried out, the steel work piece being preferably an injection-nozzle body of a fuel-feed system.

Brief Description of the Drawings

The present invention will now be described in greater detail with reference to an embodiment represented in the drawing. The figure shows:
Figure 1 - is a view of a graph with the steps of the process of the present invention.

Detailed Description of the Invention

The initial treatment step consists in carburizing, which is the introduction of carbon in the outer surface of the steel body of an injector nozzle. With the introduction of carbon the surface becomes harder after the quenching process.
This carburizing occurs in a furnace chamber, in which a vacuum is produced and then a carburizing gas is fed into the furnace chamber. This kind of carburizing is known as low pressure carburization.
Optionally, after the carburization, and before the quenching, one may employ an austenitization step. With this optional step the toughness and fatigue resistance of the steel is increased. However, the carrying out of the quenching after the carburization or after the austenitization is the step in which one achieves, in the structure of the steel, the desired martensite and the respective hardness.
The quenching may be carried out basically by two cooling media, as described in the prior art, namely a gaseous or a liquid medium.
It has been found that in gaseous media, the cooling of the work piece is relatively slow, due to the low reachable coefficient of heat transmission (800 W/m²K). It was believed that, even with this coefficient, due to the transition behavior of steel cemented with approximately 2 % Cr and 2 % Ni, one achieves in principle the desired martensite and the required hardness. On the other hand, one does not achieve a tempering resistance sufficient for the steel of injection-nozzle bodies not to fail mechanically. Therefore, said bodies might fail when used under high loads, the main failure being ruptures at the nozzle tip, or just wear of the seat.
In spite of the mentioned disadvantage with the utilization of gas in quenching, the process prevents that internal oxidation of the steel occurs, and a greater uniformity in the carburization is achieved.
On the other hand, in the quenching with liquid, it was proven that the achieved heat transfer coefficient is higher than in gas. Thus, this higher heat transfer coefficient provides higher tempering resistance of the steel. Therefore, the injection-nozzle bodies that utilize quenching in a liquid medium do not have the above-mentioned failures and resist the high temperature and pressure loads necessary in present-day fuel injection systems.
However, it is necessary to remove the quenching liquid from said quenched bodies after the quench treatment, thus making the mass production of the injection-nozzle bodies expensive. This additional step obviously does not occur in the high-pressure gas quenching.
Thus, the quenching of the present invention takes place in a gaseous medium with a heat transfer coefficient similar to that of the liquid medium, thus preventing the additional step of cleaning said bodies and the internal oxidation of the steel. Further, one achieves greater uniformity in the carburization and a higher tempering resistance.
As described before, in the prior art gas quenching is slower, because its heat transfer coefficient is of about 800 W/m²K. With this low coefficient, one has noted in laboratory studies that diffusion and precipitation of finer carbides occurred during quenching, so that the reduced carbon contents in the matrix lowers the tetragonability of martensite and consequently the tempering resistance of the steel.
In order to prevent this, the quenching intensity should be increased. When doing so, it has been found that the effect of precipitation of the carbides in the matrix was drastically reduced, or even totally eliminated. In order to raise the quenching intensity, a heat transmission coefficient higher than 1500 W/m²K was applied, with the use of high-pressure gas.
This high-pressure gas with a heat transfer coefficient higher than 1500 W/m²K can be achieved by using a mixture of gases such as, for example, helium gas and carbon dioxide, but the gas should be injected under a pressure of at least 2000 kPa. In addition to the pressure the gas used should be at a speed of at least 15 m/s for achieving the required coefficient of heat transfer. Such speed and pressure can be reached with equipment designed for this purpose, for example circulation turbines.
According to the invention, such a heat transfer coefficient is achieved by carrying out the quenching in small quantity of work pieces, or else with single work pieces, by means of a gas-jet field with injection of carbon dioxide with gas-flow speeds higher than 30 m/s are used.
Moreover, in spite of having the aim of not using liquids in the quenching, in order to avoid a subsequent cleaning of the work piece to be treated, it is possible to employ a fluid gas/liquid mixture, which uses liquid in evaporation, such as, for example, a mixture of polymer with nitrogen on a jet field with flow speeds higher than 10 m/s.
In figure 1, one can see a temperature graph T in function of the time t for the present process. The first step of the process is the carburization 1, which consists in raising the temperature of the work piece (injector-nozzle body) up to a cementation temperature in the range A from 880 to 960 °C.
Then, the work piece is kept at this temperature during a period of time t₁ to t₂ of about 30 to 150 minutes in a quasi vacuum in which a carburizing gas in introduced. In this atmosphere the carbon penetrates into the surface of the work piece in a uniform manner. Moreover, since this is a quasi vacuum, the final product will be free from internal oxidation, since oxygen is not applied.
The cementation 1 should not exceed the conditions of formation of a grain coarsening in the surface-near region of the steel. In this way, right after this carburization step 1, one may effect the quenching with a coefficient of heat transmission higher than 1500 W/m²K, as described above, and alter the structure of the martensite in the region of the carburized surface-near layer, thus obtaining a work piece that does not have the failures mentioned.
However, optionally, right after the carburization, one may include an austenitization step 2 for the work piece, in order to raise the toughness and fatigue resistance of the work piece. As shown in the graph, the austenitizing temperature 2 is lower than that of cementation 2 and is in the range B from 820 to 870 °C.
In other words, the temperatures of the range B are reached with the continuity of the present process from the mere reduction of temperature in the range A to B. When the temperature range B is reached, the work piece remains in the furnace chamber in the vacuum or a process-gas in order for the austenitization which occurs during a period of time t₄ to t₅ from 20 to 40 minutes. In this step, the oversaturation of austenite with carbon in the carburized layer is decreased in order to prevent retained austenite and thus soft points in said layer.
Once the austenitizing 2 has been completed, one carries out the quenching with high-pressure gas (or with a mixture of gases) commencing in the temperature range B, obtaining the work piece with the mentioned characteristics.
By using this quenching medium having a higher coefficient of heat transmission (> 1500 W/m²K), that is, high-pressure gas, one has prevented the loss of hardness steel in the subsequent high temperature load. In this way, the injector-nozzle now stands the mentioned high temperature and pressure loads. In comparison, with the prior art, the loss of hardness in the outer layer hardened by deep cementation of 0.1 mm in a temperature load of approximately 300 °C for 2 hours has been reduced by means of a quenching according to the invention (>1500 W/m²K) with respect to a temperature with 800 W/m²K in at least 40 HV1.
In order to ensure that a steel body which guarantees its functionality with said loads, it is recommendable, but not compulsory, after carrying out of the quenching at a subzero 3 treatment stage. This treatment is carried out at negative temperatures at a temperature range C from -60 to -196 °C (77 K to 213 K) for a time period from t₆ to t₇, which is 10 to 100 minutes. This guarantees a more complete transformation of the austenite and prevents the phenomenon of dimensional instability.
As a last step of the process of treating the steel piece, a tempering or annealing 4 process step is necessary to remove the stresses accumulated in the material and to transform more retained austenite. With the tempering the work piece will receive the definitive characteristic (hardness and resistance). The tempering gives a temperature range D from 160 °C to 220 °C for a time period from t₈ to t₉, which is 45 to 180 minutes. After tempering the steel is slowly cooled down to room temperature at a rate of approximately 100 to 300 K/h.
The metal work piece treated with this process has its main application in injection-nozzle bodies of injection systems, but it may also be employed in other applications than injection-nozzle bodies.
The material employed corresponds, in its physical characteristics in the non-carburized region, to the material 18CrNi8, this steel having good availability on the market, that is to say, its cost is lower than that of a steel that has more Si (as described in the prior art - DE 103 18 135 ). The lesser amount of Si is also advantageous for hot or cold extrusion, as well as for machining. Another advantage in using this process is that there is no step of adding nitrogen. Without nitrogen the stabilization of retained austenite is smaller, so that during the operation of the injection-nozzle bodies do not present the mentioned failures any longer. The surface carbon content corresponds to that of the usual carburization processes which is 0.6 to 0.8 %.
A preferred embodiment having been described, one should understand that the scope of the present invention embraces other possible variations, being limited only by the scope of the accompanying claims.

Claims

A process for raising the tempering resistance of a steel work piece, which comprises the following steps:
- vacuum carburization (1) of the work piece,

- carrying out a quenching process in a gaseous medium,
characterized in that
the quenching in the gaseous medium is carried out with the heat transfer coefficient between the steel work piece and the gaseous medium being higher than 1500 W/m²K, whereas the gaseous medium with said heat transfer coefficient is provided by means of high-pressure gas, and
that the high-pressure gas is a gas-jet field with injection of carbon dioxide at speeds higher than 30 m/s relative to the steel work piece and
that the vacuum cementation step (1) of the steel work piece takes place at a temperature range (A) from 880 °C to 960 °C for 30 to 150 minutes.
A process for raising the tempering resistance of a steel work piece according to claim 1, characterized in that the high-pressure gas is a fluid mixture of gas with liquid in evaporation at speeds higher than 10 m/s relative to the steel work piece.
A process for raising the tempering resistance of a steel work piece according to claim 1, characterized in that, after the cementation step (1), austenitization (2) of the work piece is carried out at a temperature range (B) from 820 °C to 870 °C for 20 to 40 minutes.
A process for raising the tempering resistance of a steel work piece according to claim 1, characterized in that, after the quenching, one carries out a subzero treatment (3) at negative temperatures (C) for 10 to 100 minutes.
A process for raising the tempering resistance of a steel work piece according to claim 4, characterized in that the negative temperatures (C) are from -60 °C to - 196 °C (77 K to 213 K).
A process for raising the tempering resistance of a steel work piece according to claims 4 or 5, characterized in that, after the subzero treatment (3), one carries out an annealing (4) for 45 to 180 minutes.
A process for raising the tempering resistance of a steel work piece according to claim 6, characterized in that the tempering temperatures are from 160 to 220 °C.
A process for raising the tempering resistance of a steel work piece according to claim 3, characterized in that the austenitization (2) is carried out in a furnace with process gases.
A process for raising the tempering resistance of a steel work piece according to claim 3, characterized in that the austenitization (2) is carried out in vacuum.
A process for raising the tempering resistance of a steel work piece according to claim 1, characterized in that the heat transmission coefficient is higher than 1500 W/m²K in the quenching chamber with a gas or a mixture of gases.
A process for raising the tempering resistance of a steel work piece according to claim 1, characterized in that the work piece is a fuel-injection-nozzle body.