Stainless steel is a term used to refer to steel that is resistant to corrosion from weak acids such as air, steam and water, or that has stainless property.
Stainless steel has a history of more than 100 years since its creation.
The invention of stainless steel is a significant milestone in the world of metallurgy.
The advancement of stainless steel has played a crucial role in the development of modern industries and technological advancements.
Stainless steel has unique physical properties compared to other materials, including heat conduction, thermal expansion, strength, magnetism and density.
1. Heat conduction
It is commonly recognized that the heat transfer of stainless steel is slower compared to other materials, as demonstrated in Table 1. For example, the thermal conductivity of stainless steel is 1/8 and 1/13 for SUS304, compared to aluminum. Compared with carbon steel, it is 1/2 and 1/4 respectively, indicating a low thermal conductivity of stainless steel.
This low thermal conductivity poses challenges during the stainless steel annealing process. Stainless steel is an alloy material composed of iron with the addition of Cr and Ni.
So why is heat transfer in stainless steel worse than in iron? Simply put, the addition of Cr and Ni hinders the activity of free electrons in the metallic crystal, which conduct heat (electronic heat conduction). The activity of these free electrons is influenced by temperature and is therefore also related to heat conduction in the lattice, where the atoms vibrate in an irregular, elastic and wavy manner, gradually conducting heat in the lattice.
It is important to note that the thermal conductivity of stainless steel changes with temperature. The higher the temperature, the higher the thermal conductivity, especially for high-alloy steels such as stainless steel.
2. Thermal expansion
Thermal expansion is the phenomenon in which the length of a material increases by dL when the temperature increases by dT, given an initial temperature T and length L. The coefficient of linear expansion (a) can be expressed as:
a = (1/L) * (dL/dT)
For an isotropic solid steel, the coefficient of volumetric expansion (b) is equal to 3 times the coefficient of linear expansion, or b = 3a.
Table 1 shows the linear expansion coefficients of various materials. Compared to carbon steel, SUS304 has a larger coefficient of linear expansion, while SUS430 has a smaller coefficient of linear expansion. Additionally, aluminum and copper have higher coefficients of expansion than stainless steel.
Table 1 Thermal conductivity and coefficient of linear expansion of various materials at room temperature
| Material | Thermal conductivity（W/m℃）×10 2 | Linear expansion coefficient（×10 -6 ） |
| Silver Copper Aluminum Nickel Chrome Iron Carbon steel SUS430 SUS304 |
4.12 3.71 1.95 0.96 0.84 0.79 0.58 0.26 0.16 |
19 16.7 23 17 12.8 11.7 11 10.4 16.4 |
3. Endurance
The difficulty of electricity flow is called resistance or specific resistance and is generally expressed by the following formula:
Resistance = specific resistance' (conductor length / cross-sectional area)
Table 2 Specific electrical resistance of different materials
| Materials science | Specific resistance (at room temperature) | Temperature series | ||
|---|---|---|---|---|
| Conductor | pure metal | Silver Copper Aluminum No Cr Iron |
Ωcm 1.62×10 -6 1.72×10 -6 2.75×10 -6 7.2×10 -6 17×10 -6 9.8×10 -6 |
/℃ 4.1×10 -3 4.3×10 -3 4.2×10 -3 6.7×10 -3 2.1×10 -3 6.6×10 -3 |
| turns on | SUS430 (Fe-18% Cr) SUS304 (Fe-18%Cr) – 8%Ni SUS310S (Fe-25% Cr) – 20% Ni Fe-Cr-Al Alloy NiCr (nNiCr) Bronze (copper tin) |
60×10 -6 72×10 -6 78×10 -6 140×10 -6 108×10 -6 15×10 -6 |
0.8×10 -3 0.6×10 -3 0.5×10 -3 0.1×10 -3 0.1×10 -3 0.5×10 -3 |
|
| Semiconductor | Germanium Silicon |
5×10 3×10 5 |
–– | |
| Insulator | Paper Epoxy resin Quartz Glass |
10 10 ~10 12 10 3 ~10 15 >10 17 |
– | |
Stainless steel is a metal that can easily conduct electricity between various metals.
However, compared with pure metals, the specific strength of an alloy, including stainless steel, is generally higher. This is because stainless steel has a higher specific resistance than its constituent elements Fe, Cr and Ni.
It is important to note that SUS304 has a higher specific resistance than SUS430. And as the number of alloying elements increases, as in the case of SUS310S, the strength also increases.
The reason for the increase in specific electrical resistance due to alloying is that the movement of free charged electrons is stopped by the presence of alloying elements.
It is important to note that free electrons also play a role in heat conduction. Therefore, if the thermal conductivity of a metal is high, its electrical conductivity (reciprocal of specific resistance) will also be high.
This relationship between electrical and thermal conductivity is known as the Viedermann-Franz rule and is shown below:
L/s = TLo (where Lo is the Lorenz number and T is the temperature)
It is worth mentioning that the specific resistance also varies with temperature, as shown in Table 2.
4. Magnetism
Table 3 Magnetic Properties of Various Materials
| Materials science | magnetic properties | Magnetic permeability: μ （H=50e） |
| SUS430 | Strong magnetism | – |
| Iron | Strong magnetism | – |
| No | Strong magnetism | – |
| SUS304 | Non-magnetic (magnetic during cold working) | 1.5 (65% processing) |
| SUS301 | Non-magnetic (magnetic during cold working) | 14.8 (55% processing) |
| SUS305 | Non-magnetic | – |
5. Density
Table 4 Density of Various Materials (at room temperature)
| Materials science | Density (g/ cm3 ) |
| SUS430 | 7.75 |
| SUS304 | 7.93 |
| Aluminum | 2.70 |
| Iron | 7.87 |
| Cr | 7.19 |
| No | 8.9 |
| Silver | 10:49 am |
| Copper | 8.93 |
| Carbon steel | 7.87 |
| Wood (burnt) | 0.70 |
| Glass | 2.8-6.3 |
| Reinforced concrete | 2.4 |
| Celluloid | 1.35-1.60 |
