Page 234 - Mechanical Engineer's Data Handbook

222 MECHANICAL ENGINEER’S DATA HANDBOOK

Alloy steels are classified according to increasing Chromium
proportion of alloying elements and also phase change
during heating and cooling as follows: A range of OM%, improves wear, oxidation, scaling
resistance, strength and hardenability. It also increases
low alloy steels
medium alloy steels high-temperature strength, but with some loss of
ductility. Chromium combines with carbon to form a
high alloy steels
wear-resistant microstructure. Above 12% the steel is
and according to the number of alloying elements as stainless, up to 30% it is used in martensitic and ferritic
follows: stainless steel with nickel.
ternary - one element Cobalt
quarternary - two elements
complex - more than two elements
Cobalt provides air hardening and resistance to scal-
ing. It improves the cutting properties of tool steel with
6.3.2 General description 8-10%. With chromium, cobalt gives certain high
alloy steels high-temperature scaling resistance.
Low alloy steels
Copper
These generally have less than 1.8% nickel, less than
6% chromium, and less than 0.65% molybdenum. The The typical range is 0.24.5%. It has limited applica-
tensile strength range is from 450-620 N mm-’ up to tion for improving corrosion resistance and yield
85O-lOoO N mm-2. strength of low alloy steels and promotes a tenacious
oxide film.
Medium alloy steels
Lead
These have alloying elements ranging from 5-12%.
They do not lend themselves to classification. They Up to 0.25% is used. It increases machineability in
include: nickel steels used for structural work, axles, plain carbon steels rather than in alloy steels.
shafts, etc.; nickel-molybdenum steels capable of
being case-hardened, which are used for cams, cam- Manganese
shafts, rolling bearing races, etc.; and nickel-
chromemolybdenum steels of high strength which The range used is 0.3-2%. It reduces sulphur brittle-
have good fatigue resistance. ness, is pearlitic up to 2%, and a hardening agent up to
1 Yo. From 1-2% it improves strength and toughness
High alloy steels and is non-magnetic above 5%.

These have more than 12% alloying elements. A Molybdenum
chromium content of 13-18% (stainless steel) gives
good corrosion resistance; high wear resistance is The range used is 0.3-5%. It is a carbide forming
obtained with austenitic steel containing over 11 YO element which promotes grain refinement and in-
manganese. Some types have good heat resistance and creases high-temperature strength, creep resistance,
high strength. and hardenability. Molybdenum reduces temper brit-
tleness in nickel-chromium steels.
6.3.3 Effect of alloying elements Nickel

Aluminium The range used is 0.3-5%. It improves strength,
toughness and hardenability, without affecting duc-
This acts as a deoxidizer to increase resistance to tility. A high proportion of it improves corrosion
oxidation and scaling. It aids nitriding, restricts grain resistance. For parts subject to fatigue 5% is used, and
growth, and may reduce strength unless in small above 27% the steel is non-magnetic. Nickel promotes
quantities. The range used is 0-2%. an austenitic structure.