General
Properties
The Alloy 303 is a free-machining stainless steel
specifically designed to exhibit improved machinability.
Sulfur is added to produce the free-machining
characteristics while maintaining good mechanical
and corrosion-resistant properties.
Alloy 303 is a non-magnetic, austenitic
stainless steel which is not hardenable by heat
treatment. It is the free-machining modification
of the basic 18% chromium – 8% nickel stainless
steel.
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Typical
Anlayses
| Element |
Percent |
| Carbon |
0.15 max |
| Manganese |
2.00 max |
| Silicon |
1.00 max |
| Chromium |
17.00 - 19.00 |
| Nickel |
8.00 - 10.00 |
| Phosphorous |
0.20 max |
| Sulfur |
0.15 min |
| Molybdenum |
0.60 max* |
| Iron |
Balance |
*Optional
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Resistance
to Corrosion
The addition of certain elements to stainless
steels to impart better machining characteristics
also slightly lowers corrosion resistance. For
dry conditions, and in most mildly corrosive environments,
the free-machining grade is practically comparable
with its corresponding unmodified type. Where
moist atmospheres are involved, some free-machining
grades may tend to form a rust film, and in certain
severe environments, they may show somewhat increased
corrosion as a result of the free-machining additions.
In a majority of cases, they will perform nearly
the same as the basic parent composition.
Because the free-machining grades
have a slightly reduced corrosion resistance compared
with unmodified basic stainless steel, bright
machined parts may be susceptible to surface dulling
or etching by final treatment with nitric acid
solutions. Caution in the use of such treatments
is suggested. For mild action, mixtures containing
as little as one or two percent by volume nitric
acid, with additions of an inhibitor, may be allowable
for short periods of time at 120 to 140°F
(49-60°C). A solution of 12 percent nitric
acid and 4 percent copper sulfate is also satisfactory
where mild action is required. In some instances,
the complete elimination of treatment with nitric
acid solutions may be desirable.
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Resistance
to Oxidation
Alloy 303 has good resistance to oxidation at
temperatures up to 1700°F (927°C). In
extreme oxidizing atmospheres, irregular scaling
may be encountered, particularly above 1400°F
(760°C).
The rate of oxidation of all stainless
steels is greatly affected by the service atmosphere,
by heating and cooling cycles, and by design considerations.
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Physical
Properties
| Melting
Point |
2552°F
(1400°C) |
| Density |
0.290 lb/in3
or 8.03 g/cm3 |
Coefficient of Thermal Expansion
| Temperature
Range |
|
| °F |
°C |
Per °F x 10-6 |
Per °C x 10-6 |
| 68 - 212 |
20 - 100 |
9.2 |
16.6 |
| 68 - 932 |
20 - 500 |
10.4 |
18.8 |
| 68 - 1450 |
20 - 787 |
10.9 |
19.6 |
Thermal Conductivity at 212°F
(100°C)
| W/m•K |
Btu•in/hr•ft2•°F |
| 16.4 |
113.2 |
Electrical Resistivity
| Temperature |
|
| °F |
°C |
Microhm-in |
Microhm-cm |
| 68 |
20 |
28.3 |
72.0 |
| 212 |
100 |
30.7 |
78.0 |
| 392 |
200 |
33.8 |
86.0 |
| 752 |
400 |
39.4 |
100.0 |
| 1112 |
600 |
43.7 |
111.0 |
| 1472 |
800 |
47.6 |
121.0 |
Magnetic Permeability
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Mechanical
Properties
The mechanical properties of these steels are
functions of their carbon, nickel and chromium,
or chromium plus manganese contents. Within the
range of composition for these steels, as furnished
in the annealed condition, the properties will
vary within the following limits.
Room Temperature Properties -
Annealed Condition
Yield Strength
0.2% offset |
30 - 40 ksi
207 - 276 MPa |
| Tensile Strength |
75 - 90 ksi
517 - 621 MPa |
| Elongation in 2
in., % |
35 - 50 |
| Reduction in Area, % |
50 - 60 |
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Fabricating
Properties
Welding
Although the free-machining grades are not recommended
for welding, they may be welded with some difficulty.
Alloy 303 may be welded with Alloy 310 electrodes
and should be annealed after welding to redissolve
precipitated carbides, thereby increasing the
resistance of the material to intergranular corrosion.
Machining
The same machining methods commonly used for mild
steel are applicable to Alloy 303. High machining
rates can be obtained for these materials in the
annealed condition with hardnesses in the range
of 200 to 240 Brinell. However, modifications
in machining techniques are necessary to adjust
to the special characteristics of each type of
stainless steel.
Since Alloy 303 will work harden, it should be
machined at reduced surface feet per minute and
heavier feeds to prevent glazing at the tool interface.
Annealing Temperatures
For maximum ductility, Alloy 303 should be annealed
near the upper limit of the 1800 - 2000°F
(982 - 1043°C) range. The material should
be water quenched from the annealing temperature
to prevent harmful carbide precipitation. For
the same reason, heating within the 800 - 1500°F
(427 - 816°C) temperature range should be
avoided unless the material can be subsequently
annealed.
Hardening Temperature
Alloy 303 is austenitic and cannot be hardened
by heat treatment.
Structure
Alloy 303 is austenitic at all temperatures. When
heated within the range 800 - 1500°F (427
- 816°C), carbides will precipitate at the
grain boundaries. Upon reannealing, the carbides
will be redissolved.
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