Seamless stainless steel tubes for gasoline engines in cars
16.01.2018 | Initiative Automotive
The requirements placed on high-pressure fuel lines in gasoline engines have increased dramatically in recent years. The seamless stainless steel tubes used there must meet these demands.
More aggressive types of gasoline are placing increased requirements in terms of corrosion and are consequently ruling out the material groups used in the past. At the same time, more efficient engine concepts mean that the lines that carry fuel are subjected to steadily increasing mechanical loads, resulting in new, previously unknown challenges. In view of their potential characteristics, austenitic stainless steels are a good foundation to overcome these challenges. In addition to the materials' original parameters, the available cost-relevant manufacturing procedures also play a crucial role in the successful use in automotive mass production.
Electrochemical corrosion from biofuels
Some years ago German automakers agreed to lay the foundation for the increased use of biofuels in order to meet environmental demands for reduced CO2 emissions from gasoline engines. When these biofuels come into contact with metallic materials, however, the result can be electrochemical corrosion that damages a material's passive layers in the form of local corrosion. Austenitic stainless steels spontaneously form a thick chromium oxide layer on the surface that prevents further "rusting" of the base material. This is important here since it is absolutely necessary to avoid corrosive damage to the component, because otherwise extremely hot fuel can eventually be released and possibly cause a fire in the engine area.
Higher injection pressures
Recent years have seen a gradual increase in the injection pressures in gasoline direct injection engines. Today the latest models of these engines have values of 350 bar, which is significantly below that of today's diesel versions. Due to the corrosion requirements addressed above, however, the high strength, low-alloy steel grades used there cannot readily be used, so that here there is a stronger focus on austenitic materials. In principle, however, this class of high-alloy steels has lower mechanical strength than other common iron materials.
High demands on test engineering
The first step in the zero-error manufacture of seamless tubes as components for lines that carry gas is a critical analysis of the overall production chain. The hot (extrusion) and cold (pilgering) forming steps during the tube manufacture must consequently be coordinated in a way that avoids macroscopic defects caused by overloading the material. At the same time, the microstructural homogeneity of the alloy must be guaranteed across the entire tube length and geometry. Quality control using non-destructive tests, such as ultrasonic inspection of the end tube, ensures this. Due to the compact tube geometries, the test engineering must satisfy a stringent metrological requirement profile in order to identify any small flaws clearly and sort out the corresponding tube sections.
Safety aspects have priority
Particularly microfractures and the smallest blow holes (hollow spaces or dents) can lead to critical crack growth and consequently to swift failure of a pressure line as the result of high frequency pulse stresses during constant operation. This is why before series production begins, a comprehensive series of fatigue property tests is conducted on tube samples in special internal pressure pulse test machines in order to determine the compressive stress that results in 1 ppm failure probability. This ensures that the designed limits for operating pressures during engine operation are always below the gas tube’s mechanical load limits. Although the cost aspects must also be kept in mind along with the technical aspects, the safety aspect clearly has priority because of the possible risks to life and limb.