Examination of steel at the microscale
Luke Severson, Dr. John Kirk, University of Wisconsin-Stout
Introduction
Manufacturing Process
Steel is used commonly throughout society. Steel contains Iron and Carbon, plus a
mixture of various elements. It is the various elements that give steel the unique
prosperities. Such prosperities as corrosion resistance, mechanical strength and
elasticity, and electrical. In recent events steel structures have failed due to stress
such as the I-35W bridge in Minnesota.
At the point of failure the steel elongates and ruptures. In Figure 1 at number three
is the point of rupture. The point of rupture was examined using the SEM (Scanning
Electron Microscopy).Three different types of steel were chosen because of
common use; Type 304- Stainless steel, 1018 Cold Rolled, and 1045 Medium Tensile
steel.
Steel is classified by numbers. There are several classification systems in existence
today. SAE (Society of Automotive Engineers), AISI (American Iron and Steel
Institute), UNS (Unified Numbering System), are the most often used in the United
States. SAE is the most used because it is a four numbers only. The first two
numbers of the set describe what kind of alloy it is. For an example
1018 C.R. and 1045 are used for structural purposes due to the ease of
manufacturing the desired carbon amount , cost , and tensile strength. Type 304
Stainless steel is used for the corrosive resistance to the environment.
SAE Grade
Composition
Type 304 "Stainless
Steel"
Fe balance, <0.08% C, 17.5-20%
Cr, 8-11% Ni, <2% Mn, <1% Si,
<0.045% P, <0.03% S
1018 Cold Rolled
98.81-99.26% Fe, 0.14 -0.20 % C,
0.60 -0.90 % Mn, < 0.04 % P,
0.05 % S
1045 Medium
Tensile Steel
Fe balance, 0.43 - 0.5 % C, 0.1 0.35 % Si, 0.60 - 0.9 Mn
*Type 304 Stainless Steel - Annealed
* 1045 Medium Tensile steel - Annealed
Tensile Strength
(Mpa)
~515
~480
~585
•Tensile strength is different than Young’s Modulus,
and Yield Strength
Objectives are to:
• To determine what types of structures at the microscale contribute to overall
strength
Type 304 Stainless steel
Type 304 is a basic stainless steel . It cannot
be heated treated after being poured, but
instead must be cold worked to increase
tensile strength.
Type 1018 Cold Rolled
1018 cold rolled steel is a low carbon steel.
After it is heated at 1850 F for 8 hours, then
cooled till 1450 F. It is then reheated and
quench in water or oil. When cooled rapidly
it reinforces the allow to withstand higher
pressure.
Type 304 Stainless steel
1045 Medium Tensile steel
Type 1045
10145 Medium Tensile steel is heated to
2280 F until it is uniformly melted. It is then
cooled in the furnace to prevent cracking
and other stress fatigues.
Figure 1.
1018 Cold Rolled
Results and Conclusions
Procedure
The metals examined during this process
were Type 304 Stainless steel, 1018 Cold
Rolled, and 1045 Medium Tensile steel. The
tensile strength was taken from the
averages.
•Once the pieces of steel were obtained, the
tips had to be cut off. A horizontal band saw
was used to cut the desired shape.
•After, only the tips remained. The tips were
then placed inside the SEM.
•By using the SEM, the examination at the
microscale was achieved.
The material composition of 1018 Cold Rolled and 1045 Medium Tensile
steel are similar, except for the different amounts of certain elements.
Carbon is a vital part in steel, since it increases the hardness of the steel
alloy. A mixture of sulfur and phosphorus also contribute to the overall
strength of the steel.
Besides the elements that contribute to the hardness and strength of the
steel, the structure of the atoms contributes also. 1018 is Body Centered
Cube, but when heated to its melting point it becomes Face Center Cube.
1045 Medium Tensile Steel is a Body Centered Cube structure which
traps carbon atoms inside of the BCC when being annealed which
increases the strength of an alloy.
An analysis of 1018 and 1045 structures in the images above, provides
information of the fracture points. 1018 reveals a considerable amount of
fracture points than that of 1045 . The topography of 1018 is also far more
rigid than 1045. 1045 however can withstand more force than that of 1018
because of the atomic cube structure.
Type 304 Stainless steel is a different than the other steels as it is used in
harsh environments and it is aesthetically pleasing to the eye. The
structure as shown above creates a smooth looking finish since the layers
are stacked on top of each other, The addition of other elements such as
nickel and chromium. The chromium which is added to increases the
effective resistance to oxidation and weldability.