Examination of steel at the microscale Luke Severson, Dr. John Kirk, University of Wisconsin-Stout

Introduction

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.

Manufacturing Process 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 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.

Type 304 Stainless steel Figure 1.

1045 Medium Tensile steel 1018 Cold Rolled

SAE Grade

Type 304 "Stainless Steel" 1018 Cold Rolled 1045 Medium Tensile Steel

Composition

Fe balance, <0.08% C, 17.5-20% Cr, 8-11% Ni, <2% Mn, <1% Si, <0.045% P, <0.03% S 98.81-99.26% Fe, 0.14 -0.20 % C, 0.60 -0.90 % Mn, < 0.04 % P, 0.05 % S Fe balance, 0.43 - 0.5 % C, 0.1 0.35 % Si, 0.60 - 0.9 Mn

Tensile Strength (Mpa)

~515 ~480 ~585

*Type 304 Stainless Steel - Annealed * 1045 Medium Tensile steel - Annealed

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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

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.

Results and Conclusions

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.