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High and Low Thermal Expansion Steels
There are cases in engine construction where steel has to work in conjunction with light alloys, such as cylinder-head bolts, valve seating, or cylinder liners in aero engines. The comparatively high thermal expansivity of aluminium leads to looseness unless the steel has a similar coefficient of expansion. The austenitic steel of the following composition
C, 0,59; Ni, 12; Mn, 5,1; Cr, 3,4
has a thermal expansion of 0,000021 per degree C up to 400°C, which is only slightly lower than that of aluminium, and it combines good mechanical properties with resistance to abrasion.
Cold rolled austenitic stainless steel is another alternative. Where an abnormally low coefficient of expansion is required, Inver, containing 36% Ni, is used.
Ball-race steel. A typical composition is C, 1,0; Mn, 0,5; Cr, 1,36%. After quenching in oil from 810°C the steel is usually tempered at 100-200°C to
a) reduce hardening stresses,
b) reduce cracks in grinding.
Tempering at 100°C also increases the hardness slightly, e.g.:
Tempering temperature nil 100 200 250 VPN 800 876 750 736
so whats all that mean in a nutshell?
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