STANDARD
AISI and SAE STEELS
Studies have been made in the steel industry
for the purpose of establishing certain standard steels and eliminating
as much as possible the manufacture of other steels which vary only slightly
in composition from the standard steels, These standard steels are selected
on the basis of serving the significant metallurgical and engineering
needs of fabricators and users of steel products.
STANDARD
CARBON STEELS
Definition. By common custom. steel is considered
to be carbon steel when no minimum content is specified or required for
aluminum, boron. chromium, cobalt, columbium, molybdenum. nickel, titanium,
tungsten, vanadium or zirconium, or for any other element added to obtain
a desired alloying effect; when the specified minimum for copper does
not exceed 0.40 per cent; or when the maximum content specified for any
of the following elements does not exceed the percentages noted: manganese
1.65, silicon 0.60, copper 0.60.
Numbering System. In the AISI system of
identification. the prefix B is used to designate acid bessemer steel.
The letter L within the grade number is used to identify leaded steels.
A four-numeral series is used to designate
graduations of chemical composition of carbon steel. The last two numbers
of which are intended to indicate the approximate middle of the carbon
range. For example, in the grade designation 1035, 35 represents a carbon
range of 0.32 to 0.38 per cent.
It is necessary, however. to deviate from
this rule and to Interpolate numbers in the case of some carbon ranges
and for variations in manganese, phosphorus or sulphur with the same carbon
range.
The first two digits of the four-numeral
series of the various grades of carbon steel and their meanings are as
follows:
10xx Nonresulphurized carbon steel grades
11xx Resulphurized carbon steel grades
12xx Rephosphorized and resulphurized carbon steel grades
I5xx Nonresulphurized high manganese carbon steels.
STANDARD
ALLOY STEELS
Definition. Steel is considered to be alloy
steel when the maximum of the range given for the content of alloying
elements exceeds one or more of the following limits: manganese, 1.65
per cent; silicon, 0.60 per cent; copper, 0.60 per cent; or in which a
definite range or a definite minimum quantity of any of the following
elements is specified or required within the limits of the recognized
field of constructional alloy steels: aluminum, boron, chromium up to
3.99 per cent, cobalt, columbium, molybdenum, nickel, titanium, tungsten,
vanadium, zirconium or any other alloying element added to obtain a desired
alloying effect.
Numbering System. In the AISI numbering
system, the prefix letter E is used to designate steels normally made
only by the basic electric furnace process. Steels without a prefix letter
are normally manufactured by the basic open hearth or basic oxygen processes,
but may be manufactured by the basic electric furnace process with adjustments
in phosphorus and sulphur limits.
The last two digits of the four-numeral
series are intended to indicate the approximate middle of the carbon range.
For example, in the grade designation 4142, 42 represents a carbon range
of 0.40 to 0.45 per cent. (Where a five-numeral series occurs, the last
three digits indicate the carbon content.) It is necessary, however, to
deviate from this rule and to interpolate numbers in the case of some
carbon ranges, and for variations in manganese, sulphur, chromium, or
other elements.
The first two digits indicate the type of
alloy according to alloying elements as follows:
13xx Manganese 1.75 per cent
40xx Molybdenum 0.20 or 0.25 per cent
41xx Chromium 0.50, 0.80 or 0.95 per cent Molybdenum 0.12, 0.20 or 0.30
per cent
43xx Nickel 1.83 per cent Chromium 0.50 or 0.80 per cent Molybdenum
0.25 per cent
44xx Molybdenum 0.53 per cent
46xx Nickel 0.85 or 1.83 per cent Molybdenum 0.20 or 0.25 per cent
47xx Nickel 1.05 per cent Chromium 0.45 per cent
48xx Nickel 3.50 per cent Molybdenum 0.25 per cent
50xx Chromium 0.40 per cent
51xx Chromium 0.80, 0.88, 0.93, 0.95 or 1.00 per cent
5xxxx Carbon 1.04 per cent -- chromium 1.03 or 1.45 per cent
61xx Chromium 0.60 or 0.95 per cent -- Vanadium 0.13 per cent or 0.15
per cent min.
86xx Nickel 0.55 per cent --Chromium 0.50 per cent-- Molybdenum 0.25 per
cent
87xx Nickel 0.55 per cent -- Chromium 0.50 per cent -- Molybdenum 0.35
88xx Nickel 0.55 per cent --Chromium 0.50 per cent -- Molybdenum 0.35
92xx Silicon 2.00 per cent
EFFECTS
OF COMMON
ALLOYING ELEMENTS IN STEEL
By definition, steel is a combination of
iron and carbon. Steel is alloyed with various elements to improve physical
properties and to produce special properties, such as resistance to corrosion
or heat. Specific effects of the addition of such elements are outlined
below:
Carbon
(C), although not usually considered as an alloying element, is the most
important constituent of steel. It raises tensile strength, hardness and
resistance to wear and abrasion. It lowers ductility, toughness and machinability.
Manganese
(Mn) is a deoxidizer and degasifier and reacts with sulphur to improve
forgeability. It increases tensile strength, hardness, hardenability and
resistance to wear. It decreases tendency toward scaling and distortion.
It increases the rate of carbon-penetration in carburizing.
Phosphorus
(P) increases strength and hardness and improves machinability. However,
it adds marked brittleness or cold-shortness to steel.
Sulphur
(S) Improves machinability in free-cutting steels, but without sufficient
manganese it produces brittleness at red heat. It decreases weldability,
impact toughness and ductility.
Silicon
(Si) is a deoxidizer and degasifier. It increases tensile and yield strength,
hardness, forgeability and magnetic permeability.
Chromium
(Cr) increases tensile strength, hardness, hardenability. toughness, resistance
to wear and abrasion. resistance to corrosion and scaling at elevated
temperatures.
Nickel
(Ni) increases strength and hardness without sacrificing ductility and
toughness. It also increases resistance to corrosion and scaling at elevated
temperatures when introduced in suitable quantities in high chromium (stainless)
steels.
Molybdenum
(Mo) increases strength, hardness, hardenability and toughness, as well
as creep resistance and strength at elevated temperatures. It improves
machinability and resistance to corrosion and it intensifies the effects
of other alloying elements. In hot-work steels, it increases red-hardness
properties.
Tungsten
(W) increases strength, hardness and toughness. Tungsten steels have superior
hot-working and greater cutting efficiency at elevated temperatures.
Vanadium
(V) increases strength, hardness and resistance to shock impact. It retards
grain growth, permitting higher quenching temperatures. It also enhances
the red hardness properties of high speed metal cutting tools and intensifies
the individual effects of other major elements.
Cobalt (Co)
Increases strength and hardness and permits higher quenching temperatures.
It also intensifies the individual effects of other major elements in
more complex steels.
Aluminum
(Al) is a deoxidizer and degasifier. It retards grain growth and is used
to control austenitic grain size. In nitriding steels it aids in producing
a uniformly hard and strong nitrided case when used in amounts 1.00% -
1.25%.
Lead
(Pb), while not strictly an alloying element, is added to improve machining
characteristics. It is almost completely insoluble in steel, and minute
lead particles, well dispersed, reduce friction where the cutting edge
contacts the work. Addition of lead also improves chip-breaking formations.
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