Engine valves " intake and exhaust valves "

Valves are subject to both thermal and mechanical loads,the latter being applied by the springs and actuating gear. All these loads are so severe as to justify an assertion that the valves are the most heavily loaded components in an engine.

Modes of potential failure include Tensile elongation or fracture , either hot or cold corrosion , wear, burning, and flow of metal from the seating area,cold corrosion being caused by condensation containing acid products of combustion.

For protection against Corrosion , a wide variety
of coatings has been applied.

They include Alloys containing materials such
as nickel,manganese,cobalt,chromium,silicon
,and ,less commonly,molybdenum,tungsten,and
titanium.

These, however,are all costly .The outcome is
a smooth permanently adhering coating of
aluminium on an iron-aluminium alloy underlay.
finishing isn't necessary . which are known as
Coated valves .

Remedies are mainly in the choice of materials
having adequate corrosion resistance and hot
strength and hardness.

Exhaust valves , in particular , operate for long periods at high temperatures 700-800.

A significant property is coefficient of expansion . The martensitic steel have low and The austenite materials high coefficients of expansion.Consequently, the stem of an exhaust valve of austenitic material has to be tapered to compensate for the temperature gradient along its length so that,at operating temperatures
,its flanks remain as nearly parallel as possible.

In many instances, a martensitic material having good wear resistance and low coefficient of expansion is used for the stem only, a head of a more costly austenitic material resistant to high temperatures being friction-welded on to it .