| Corus has developed a new range of extremely clean steels that will help manufacturers of diesel fuel injection systems meet the demand for cars with improved performance, better fuel efficiency and lower costs. As well as being cleaner, the steels, which have been developed over a period of about five years in partnership with leading diesel manufacturers, also have consistent hardenability and increased fatigue resistance. These are all important properties for today’s diesel injectors, which are pushed to work at higher pressures with longer life expectancy, as well as needing to be easily machined. Fatigue life in steels is governed by their tensile strength and cleanness - a measure of the amount of small, nonmetallic inclusions in steel. Alumina inclusions are formed when aluminium is used in the steelmaking process and combines with oxygen to form these hard microscopic particles. These inclusions can cause problems in automotive parts, such as fuel injectors, because of their high cyclic loading. One of the controlling factors governing the manufacture of cleaner steels is the amount of oxygen in the melt. These new steels all have a greatly reduced amount of oxygen present during manufacture, and because of improvements in process control, variables such as oxygen can now be controlled more accurately to produce consistent batches.
It is difficult to test for these improvements in steel quantitatively Microscopy analysis is carried out on a number of samples through a cast, whose surfaces are polished. ‘In terms of comparison, a conventional steel matrix may have a handful of inclusions in a 1mm sample square, whereas in the new cleaner steels, a wider area may have to be taken to view any inclusions,’ said Martin Read, Senior Metallurgist, Corus Engineering Steels. All of the steels developed for this application are alloy steels, but the process could be used to control the production of any of the engineering steels to produce cleaner grades. The process is a balancing act, however, as cleaner steels are less suitable for mass construction as they are less easy to manufacture. Materials must chip easily in order to be drilled, and cleaner steels lose their ability to produce good swarf. The steels are also honed and hardened using heat treatment processes to improve their wear and fatigue life. Apart from the process technology, one of the biggest challenges that Corus faced in developing the new steels was actually knowing that the steel they were manufacturing was better than ones they had produced previously ‘Tests in the laboratory give an idea of the expected material performance, but it is hard to relate the material properties to actual performance in field. We know that cleaner steels give a better fatigue life, but it’s harder to predict what this will translate to as the true final assessment can only be made when an engine has done about 100,000 miles,’ said Read. ‘The life of a part becomes its final assessment, so it’s hard to persuade diesel manufacturers that your achievements are a benefit as it takes long a time get feedback - which is where the need for a partnership comes in.’ |