Four-stroke engines splash oil on the bore wall for lubrication and cooling. To scrape off the excess oil, the contact pressure of the oil control ring is set
high. This decreases oil consumption. Hence, the bore wall should have higher wear resistance. To raise durability, a hard gray cast iron containing phosphorus (P) is often used (high-P cast iron in Table 2.2). Figure 2.16 shows the microstructure of high-P cast iron. The increased P crystallizes from the melt as hard steadite. It has a chemical composition of Fe3P. The curious shape of steadite stems from its low freezing point. The iron crystal solidifies first. Then, the residual liquid solidifies to form steadite in the space between the iron crystals. This alloy composition has good wear resistance because of high hardness, but low machinability. Hence, instead of using this composition for an entire block, it is typical to enclose it as a wet or dry liner in a normal cast iron block (2 in Table 2.1) or aluminum block (3 and 5 in Table 2.1, described later).
The mileage required for commercial diesel engines is very high, being as much as 1,000,000 km. These engines have high combustion temperatures. Engines requiring very long durability use additional heat treatment on the bore surface. A nitrided liner4 is often enclosed to increase hardness. A phosphate conversion coating on the liner also prevents corrosion. Instead of enclosing a hard liner, interrupted quench hardening by laser or induction heating can also be applied to the bore wall of the monolithic cast iron block.
The science and technology of materials in automotive engines
Hiroshi Yamagata
Woodhead Publishing and Maney Publishing
on behalf of
The Institute of Materials, Minerals & Mining
CRC Press
Boca Raton Boston New York Washington, DC
WOODHEAD PUBLISHING LIMITED
Cambridge England
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