Hard Shell, Tough Heart
Highly stressed vehicle components need excellent materials. Material researchers and production technicians at Bosch have thus turned their attention to techniques that will turn “normal” steels into high performance material. The methods of choice are a combination of low-pressure carburization and high-pressure gas quenching.
Steel is the very symbol of hardness and strength. But that is not the way that materials researchers see the picture: Hard materials are also brittle, and things that are brittle can hardly be considered strong.
Within brittle materials, cracks can form, and the component threatens to “fail,” as researchers say. Hardly any other industry puts such high demands on materials as does the automotive industry – in terms of stress on the materials and the required length of service life.
In modern diesel injection systems, like those based on the common rail principle (CR), pressures up to 1,800 bars occur within the components. The injector and the CR high-pressure pump are subjected to extreme stresses: The surfaces of the moving components have to be as free of wear as possible and thus hard. The entire component must be strong and tough. Which is easy to understand, considering that each injection triggers pulses of stress throughout the entire system.
During the entire service life, about 10 million such reversals of load have to be endured.
Materials researchers working at Bosch are examining a number of techniques that could be used for best realizing this apparent contradiction of “hard surface, tough heart” in components. The key elements are found in the area of production: Hard, high-alloy steel cannot be used because it is so difficult to cut. For this reason, researchers are turning to the option of low-alloy steel, which can be shaped easily through soft cutting. Finally, a heat treatment is applied in order to achieve the desired characteristics (see graphic).
The Bosch engineers produce a hard and wear-resistant surface in steel components by using low-pressure carburization. In the process, carbon atoms diffuse into the surface layer of the steel. This occurs at a temperature of about 1,000 degrees Celsius and a low pressure of several millibars.
At these high temperatures, carbon is relatively soluble in steel. The atoms diffuse down to depths of one to several millimeters and occupy interstitial sites. This carburization of the surface to about 0.7 percent carbon content produces a considerable increase in hardness.
To control the carburization depths and surface carbon content, the Bosch researchers are looking for the optimal parameters among several factors – pressure, type of gas and processing time. The challenge that the research team is facing is to treat the geometrically complex components with deep bores in the shortest possible times.
To further finish the surface layer, the entire component is hardened in a final step, quenching. In the process, the crystal lattice of the austenite phase that is stabile at 1,000 degrees Celsius changes into the hard martensite phase. But this can occur only when the steel is brought down to room temperature within a matter of seconds. Otherwise, it returns to its soft initial phase (ferrite/perlite).
In order to lower the temperature, the high-pressure gas quenching process employs a stream of nitrogen at a pressure of up to 20 bars. The inert gas offers several advantages: It does not oxidize, for example. The components do not need to be cleaned as is the case with those used in traditional forms of quenching that employ salts or oils. And the gas does not represent a danger to humans or the environment.
To take full advantage of these economic and environmental benefits, Bosch researchers are working to integrate high-pressure gas quenching into the process chain. By employing such additional process steps as deep-freezing and tempering, it is possible to individually tailor the hardness and ductility of the component.
It is also very important to investigate the distortion that occurs during quenching – deformities that occur during structural changes are unavoidable. Simulations should help to provide a clearer understanding of what causes the distortion and how it can be kept to a minimum. That will reduce the time and expense needed for the final finishing work.
Within brittle materials, cracks can form, and the component threatens to “fail,” as researchers say. Hardly any other industry puts such high demands on materials as does the automotive industry – in terms of stress on the materials and the required length of service life.
In modern diesel injection systems, like those based on the common rail principle (CR), pressures up to 1,800 bars occur within the components. The injector and the CR high-pressure pump are subjected to extreme stresses: The surfaces of the moving components have to be as free of wear as possible and thus hard. The entire component must be strong and tough. Which is easy to understand, considering that each injection triggers pulses of stress throughout the entire system.
During the entire service life, about 10 million such reversals of load have to be endured.
Materials researchers working at Bosch are examining a number of techniques that could be used for best realizing this apparent contradiction of “hard surface, tough heart” in components. The key elements are found in the area of production: Hard, high-alloy steel cannot be used because it is so difficult to cut. For this reason, researchers are turning to the option of low-alloy steel, which can be shaped easily through soft cutting. Finally, a heat treatment is applied in order to achieve the desired characteristics (see graphic).
The Bosch engineers produce a hard and wear-resistant surface in steel components by using low-pressure carburization. In the process, carbon atoms diffuse into the surface layer of the steel. This occurs at a temperature of about 1,000 degrees Celsius and a low pressure of several millibars.
At these high temperatures, carbon is relatively soluble in steel. The atoms diffuse down to depths of one to several millimeters and occupy interstitial sites. This carburization of the surface to about 0.7 percent carbon content produces a considerable increase in hardness.
To control the carburization depths and surface carbon content, the Bosch researchers are looking for the optimal parameters among several factors – pressure, type of gas and processing time. The challenge that the research team is facing is to treat the geometrically complex components with deep bores in the shortest possible times.
To further finish the surface layer, the entire component is hardened in a final step, quenching. In the process, the crystal lattice of the austenite phase that is stabile at 1,000 degrees Celsius changes into the hard martensite phase. But this can occur only when the steel is brought down to room temperature within a matter of seconds. Otherwise, it returns to its soft initial phase (ferrite/perlite).
In order to lower the temperature, the high-pressure gas quenching process employs a stream of nitrogen at a pressure of up to 20 bars. The inert gas offers several advantages: It does not oxidize, for example. The components do not need to be cleaned as is the case with those used in traditional forms of quenching that employ salts or oils. And the gas does not represent a danger to humans or the environment.
To take full advantage of these economic and environmental benefits, Bosch researchers are working to integrate high-pressure gas quenching into the process chain. By employing such additional process steps as deep-freezing and tempering, it is possible to individually tailor the hardness and ductility of the component.
It is also very important to investigate the distortion that occurs during quenching – deformities that occur during structural changes are unavoidable. Simulations should help to provide a clearer understanding of what causes the distortion and how it can be kept to a minimum. That will reduce the time and expense needed for the final finishing work.
