Electrical resistivity of steel and metals, also called specific electrical resistance, describes the ability of a material to resist the flow of current. It is designated by a Greek letter ρ (rho), and the basic unit is Ω·mIn materials technology, units are particularly convenient μΩ m and Ω mm²/mResistivity is important in the design of heating elements, electrical devices, measuring systems, welded joints, induction heating, and in the analysis of steel behavior under variable temperature conditions.
The specific resistance is material feature under given conditions, while the electrical resistance R of a specific product depends additionally on its length and cross-sectional area. Two rods of the same steel 1.4301 have similar resistivity in the same state and temperature, but different resistance if they differ in length or diameter. Therefore, when comparing grades, the terms "resistivity" and "resistance" should not be interchanged.
The most important relationship: for a uniform conductor with a constant cross-section R = ρ · L/AThe greater the resistivity or length of a conductor, the greater its resistance. Increasing the cross-sectional area lowers the resistance. The conductivity, in turn, is σ = 1/ρ.
What is Electrical Resistivity? Definition and Mechanism of Conduction
In metals, electric current is carried primarily by conduction electrons. Their motion is disrupted by, among other things, vibrations of the crystal lattice, alloying atoms, defects, dislocations, and grain boundaries. These interactions influence the resistivity. In simple terms, lower resistivity means easier current flow under comparable conditions.
In stainless steel alloys, numerous alloying elements, primarily chromium, nickel, molybdenum, and other elements, alter the conductivity compared to pure iron. Consequently 316L steel or duplex 2205 have a resistivity significantly higher than the copper used for electrical wiring. However, this does not mean that stainless steel is an insulator: it is still an electrical conductor.
Electrical conductivity also depends on temperature, alloy composition, magnetic alignment, and processing history. In severely worked steels or precipitation-hardened alloys, values may differ from the catalog data for the annealed material.
| Size | symbol | Unit | Meaning |
|---|---|---|---|
| Electrical resistivity | ρ | Ω m, μΩ m | Specific resistance of the material |
| Element resistance | R | Ω | Resistance of a finished conductor or part |
| Specific electrical conductivity | σ | S/m, MS/m | Reciprocal of resistivity |
| Length of the guide | L | m | The length of the current flow path |
| Cross-sectional area | A | m², mm² | Cross-section perpendicular to the current flow |
| Current | I | A | Charge flowing per unit of time |
| Voltage | U | V | Potential difference |
| Temperature coefficient of resistance | αR | 1 / K | Approximate relative resistance change per 1 K |
Resistivity units – Ω m, Ω mm²/m, μΩ m and μΩ cm
In the metallurgical documentation and manufacturer's cards you can find the following records: Ω mm²/m, μΩ m i μΩ cmThese are three ways of expressing the same quantity. Confusing the units can lead to errors of 10, 100, or more when calculating resistance.
| Record | Substitute | Example |
|---|---|---|
| 1 Ω mm²/m | 1 μΩ m = 100 μΩ cm | Numerically equivalent units after conversion |
| 1 μΩ m | 10−6 Ω·m | 304: 0,73 μΩ m = 7,3 10−7 Ω·m |
| 1 μΩ cm | 0,01 μΩ m | 129 μΩ cm = 1,29 μΩ m |
| 1 nΩ m | 0,001 μΩ m | 172 nΩ m = 0,172 μΩ m |
| 1 MS/m | 106 S / m | A unit of conductivity, not resistivity. |
Conversion: 1 Ω mm²/m = 1 μΩ m = 100 μΩ cm.
Example: in the card Alloy 625 value 129 μΩ cm at approximately 21°C it is equivalent to 1,29 μΩ m or 1,29 Ω mm²/mAll three notations specify the same resistivity. These values do not represent the 1,29 Ω resistance of each Alloy 625 element.
Basic formulas for resistivity, resistance, and conductivity
| Dependence | Pattern | Application |
|---|---|---|
| Conductor resistance | R = ρ · L/A | For a homogeneous element with a constant cross-section |
| Resistivity from measurement | ρ = R · A/L | Determination of specific resistivity based on a sample |
| Specific conductivity | σ = 1/ρ | Conversion between resistivity and conductivity |
| Ohm's Law | U = I · R | The relationship between voltage, current and resistance |
| Power dissipated at the resistance | P = I² · R = U · I | Joule Losses and Resistance Heating |
| Energy at constant power | E = P · t | Energy released during time t |
| Temperature change R | R(T) ≈ R(T0) · [1 + αR · (T − T0)] | Approximation over a limited temperature range |
In calculations with technical units it is most convenient to assume ρ in Ω mm²/m, L in meters and A in mm². Then R is obtained directly in ohms. When using Ω m, the cross-section must be consistently converted to m².
Calculation example: 304 steel bar or strip
Consider a solid section of 304 steel with a length of 10 m and constant cross-section 2 mm², at a temperature of about 20°C. At a resistivity value of ρ = 0,73 Ω mm²/m, the resistance is R = 0,73 × 10/2 = 3,65 ΩFor a copper wire of the same dimensions and a resistivity of approximately 0,0172 Ω mm²/m we would get R ≈ 0,086 ΩThe steel element therefore has approximately 42 times greater resistance than the copper element, given the same length, cross-section and temperature.
If a current flows through a steel section 5 A, the power dissipated in it will be P = 5² × 3,65 = 91,25 WFor the same copper section it will be approximately 2,15 WThis comparison assumes a constant temperature for the materials; during actual heating, the resistivity and resistance may vary and the temperature distribution may not be uniform.
Important when designing: High resistivity alone does not imply suitability for use in a heater. The material must also maintain adequate oxidation resistance, structural stability, strength at operating temperature, and durability during thermal cycling.
Table of electrical resistivity of stainless steels and nickel alloys at 20°C
Below is presented typical values at room temperature, taken from manufacturer's material data sheets. Specific grades are given, and where relevant, commercial material or condition. These data should not be considered as permissible delivery tolerances or universal properties for all product varieties.
| Material / species | ρ [Ω mm²/m] | temp | Data |
|---|---|---|---|
| 1.4301 / AISI 304 | 0,73 | 20°C | Outokumpu Core 304 |
| 1.4404 / AISI 316L | 0,75 | 20°C | thyssenkrupp Materials |
| 1.4016 / AISI 430 | 0,60 | 20°C | thyssenkrupp Materials |
| 1.4462 / Duplex 2205 | 0,74 | 20°C | Alleima Sanmac 2205 |
| 1.4410 / Superduplex 2507 | 0,83 | 20°C | Alleima SAF 2507 |
| Alloy 625 / 2.4856 | 1,29 | 21°C | Special Metals; 129 μΩ cm |
| Alloy 718 / 2.4668 | 1,18 | 20°C | VDM Alloy 718; 118 μΩ cm |
| Nickel 200 / 201 | 0,090 | 20°C | VDM Nickel 200/201 |
| MONEL 400 / 2.4360 | 0,511 | 21°C | Special Metals; card reference condition |
| Annealed copper, standard | 0,01724 | 20°C | 100% IACS; reference value |
1 Ω mm²/m = 1 μΩ m. The values in one column are individually comparable, but come from different cards and do not replace testing a given batch of material.
For comparison, the manufacturers' lists provide: Outokumpu Core 304 — 0,73 Ω mm²/m; thyssenkrupp 430 — 0,60 Ω mm²/m; Alleima 2205 — 0,74 μΩ m. The materials belong to different microstructural groups and have different operational requirements.
How does electrical resistivity change with temperature?
In most electronically conductive metals, an increase in temperature leads to more intense electron scattering and increase in resistivityIn alloys with a high addition content, this region may have a more complex course, and with magnetic and structural transformations, the relationship may be nonlinear. Therefore, a simple expression with αR is an approximation, and for large temperature ranges the ρ(T) tables of the specific alloy must be used.
| Temperature | Duplex 2205 | Superduplex 2507 | Monel 400 |
|---|---|---|---|
| 20-21 ° C | 0,74 | 0,83 | 0,511 |
| 100°C | 0,85 | 0,89 | 0,537 |
| 200°C | 0,96 | 0,96 | 0,559 |
| 300°C | 1,00 | 1,03 | 0,574 |
| 400°C | 1,10 | 1,08 | 0,587 |
ρ in μΩ m. Alleima data for 2205 and 2507 and Special Metals for MONEL 400. The 20–21°C line indicates different starting temperatures in the source cards.
In the duplex 2205 configuration, the resistivity increases from 0,74 to approximately 1,10 μΩ·m between 20°C and 400°C. This is an increase of almost half. For superduplex 2507, the value at 400°C is 1,08 μΩ·m. These values refer to material data and do not automatically authorize the use of either grade at 400°C—specifically, for duplex and superduplex, limitations resulting from structural stability, corrosion, and mechanical properties must be independently assessed.
Temperature coefficient of resistance αR
Coefficient αR informs about the approximate relative change in resistance when the temperature changes by 1 K. It should not be confused with linear expansion coefficient αL: the first relates to changes in electrical resistance, and the second to changes in material dimensions. In practice, resistance changes due to both resistivity and geometry changes during expansion. Catalog data αR therefore, they require a specific measurement method and temperature range. More information about changing dimensions can be found in linear expansion coefficient of steel and metals.
Resistivity of Nickel Alloys – Alloy 625, Alloy 718, Nickel 200 and Monel 400
Nickel alloys differ in composition and structure, so their electrical properties can vary greatly. For example: technically pure nickel 200/201 has a resistivity at 20°C of approximately 0,090 μΩ m, while multi-component Alloy 625 about 1,29 μΩ mThe term "nickel alloy" alone does not allow for the prediction of electrical resistance without specifying the grade.
| temp | Alloy 625 | Alloy 718 | Nickel 200/201 |
|---|---|---|---|
| 20-21 ° C | 1,29 | 1,18 | 0,090 |
| 100°C | ≈1,32 | 1,20 | 0,130 |
| 200-204 ° C | 1,34 | 1,23 | |
| 300-316 ° C | 1,35 | 1,25 | |
| 400-427 ° C | 1,36 | 1,27 |
ρ in μΩ m. Data: Special Metals for Alloy 625 (21, 93, 204, 316, and 427°C — the value of approximately 1,32 at 100°C is an approximation based on data for 93°C and 204°C); VDM for Alloy 718 (20, 100, 200, 300, 400°C); VDM Nickel 200/201 (20, 100°C). The sign "—" denotes no value given here.
In the bulletin Special Metals – INCONEL alloy 625 resistivity increases from 129 μΩ·cm at 21°C to about 136 μΩ·cm at 427°C. In turn, VDM Alloy 718 shows a change from 118 μΩ·cm at 20°C to 127 μΩ·cm at 400°C. Both alloys have higher resistivity than austenitic 304 steel, but their strength properties and stability are also decisive in high-temperature design.
What influences the resistivity of steel? Composition, phases, and processing.
| Factor | Typical influence | Technical meaning |
|---|---|---|
| Temperature | Often, the increase in ρ during heating | Change in losses and heating parameters |
| Alloying additives | Change in electron scattering | Differences between carbon steel, Cr-Ni and Ni alloys |
| Microstructure and phases | Possible change of ρ during transformations | Important for hardening and aging |
| Crumple and defects | Additional electron scattering | The influence of rolling and drawing history |
| Divisions | Influence on matrix composition and defects | Important in strengthened alloys |
| Stresses, joints and contacts | Influence on element and contact resistance | Joint measurements and welding |
| Magnetic field | In some materials, the change in resistance (magnetoresistance) | Sensors and electromagnetic systems |
In ferritic and martensitic steels, magnetic behavior must also be considered. Curie temperature The magnetic ordering changes, which can affect electron scattering. However, the Curie temperature is not the same as the melting point or the ferrite-to-austenite transformation.
In industrial settings, it's crucial to distinguish between the properties of the material and the entire measuring system. Oxide layers, uneven contact pressure, roughness, welds, and local constrictions can introduce significant additional resistance. contact, even though the nominal steel grade and its resistivity have not changed.
Resistivity and thermal conductivity – is there a relationship?
In metals, electron transport can contribute to both electrical and thermal conduction. This is one of the physical foundations of the relationship known as Wiedemann–Franz law, which approximately connects the electronic part of the thermal conductivity λe with electrical conductivity σ and absolute temperature T. For the idealized case, one writes λe/(σ · T) ≈ L0, where L0 is the Lorenz number.
This does not mean, however, that the total resistivity can be precisely calculated from the resistivity alone. thermal conductivity any steel. In industrial alloys, heat transport through the crystal lattice, electron scattering on impurities, and the temperature dependence of properties are also important. Therefore, in technical calculations, separately verified values of λ(T) and ρ(T) for a given alloy should be used.
Resistivity in resistance and induction heating
In resistance heating, current flowing through the element causes Joule heat to be released. In a simple circuit with constant current, power increases proportionally to resistance: P = I²R. However, heating materials are selected with respect to combined electrical, thermal and chemical characteristicsIn addition to the ρ value, the permissible component temperature, resistance to the operating atmosphere, creep and oxidation resistance, and resistance stability during operation are important.
W induction hardening of steel Heating is associated with eddy currents and—in ferromagnetic materials—magnetic effects. The current distribution depends simultaneously on resistivity, frequency, and magnetic permeability. The characteristic current penetration depth for a simplified model of a homogeneous material is δ = √[ρ/(π f μ)], where f is the frequency and μ is the absolute magnetic permeability. In practice, these parameters may vary with temperature and field strength.
At high temperatures, ferromagnetic steel can lose some of its magnetic properties, which changes the electromagnetic coupling conditions. Therefore, designing induction heating requires knowledge not only of ρ but also of the magnetic characteristics, temperature, and dimensions of the workpiece.
Material resistivity and contact, weld and surface resistance
In systems with screwed, soldered, and welded connections, additional resistance occurs due to the geometry and contact area. Oxides, contamination, insufficient pressure, or a small effective contact area can increase resistance and cause local overheating. Simply providing the chemical composition or steel type is not sufficient to assess the safety of an electrical connector.
For example, elements from 316L stainless steel They may be beneficial in terms of corrosion resistance, but they will not automatically replace low-resistance copper conductors. In installations designed to carry high currents, the first thing to check is the cross-section, voltage drop, heating, insulation, and connection conditions.
How is electrical resistivity measured?
In laboratory measurements, resistivity can be determined from the sample's resistance and its dimensions. For very small resistances, the method is particularly useful. four-wire (Kelvin), in which separate paths are used to apply current and measure voltage drop. This limits the influence of wire resistance and measurement contacts.
The tested material should have a known geometry, a uniform cross-section, and a controlled temperature. It is advisable to describe the result, including the sample dimensions, delivery condition, temperature, method, uncertainty, and measurement direction if material anisotropy is possible.
| Document/method | Range | An important distinction |
|---|---|---|
| ASTM B193-25 | Resistivity measurement of metallic conductive materials | Test method standard, not steel grade standard |
| ASTM E1004-23 standard | Measurement of electrical conductivity using the eddy current method | Method intended for non-magnetic materials |
| Four-wire (Kelvin) method | Measuring low resistances and voltage drops | Requires geometry and temperature correction |
| ρ(T) measurements | Determining the dependence of resistivity on temperature | Checking the sample condition and temperature stabilization |
| Product standard, e.g. EN 10088 | Requirements for specific steel products | It does not replace the ρ designation if the design requires measurement |
ASTM B193-25 covers the determination of resistivity of metallic conductive materials based on resistance measurement and sample geometry. ASTM E1004-23 covers the determination of conductivity by electromagnetic methods for non-magnetic materials, so it cannot be applied without reservation to ferromagnetic steels. Sales documentation should also verify whether the resistivity parameter is even covered by the product requirements.
How to use resistivity when selecting steels and alloys?
| Application | What to evaluate | Why |
|---|---|---|
| Resistance heating | ρ(T), heat resistance, durability | Determining the resistance and temperature of the element |
| Induction heating | ρ(T), μ(T), frequency | Penetration depth and power distribution |
| Electrically conductive element | ρ, cross-section, voltage drop | Reducing Joule losses |
| Welded or butt joint | Contact resistance, geometry, surface condition | Avoiding local overheating |
| Resistive sensor | Repeatability ρ(T), αR | Temperature measurement stability |
| Corrosive elements in the installation | Corrosion resistance, electrical properties | Meeting two different sets of requirements |
When comparing materials, it is worth comparing their resistivity with thermal conductivity, specific heat, linear expansion i densityThis reduces the risk of selecting an alloy that is advantageous in one parameter but unsuitable in terms of heat transfer, mass, durability or expansion.
Frequently Asked Questions – FAQ
What is electrical resistivity?
Resistivity ρ is a property of a material that describes its resistance to the flow of current. In the SI system it is expressed in Ω m, and in metallurgy also in μΩ m and Ω mm²/m.
Do resistivity and resistance mean the same thing?
No. Resistivity is a material parameter, while resistance R applies to a specific object and also depends on its length and cross-section.
What is the formula for the resistance of a rod?
For a uniform rod with constant cross-section R = ρ · L/A. In material practice, ρ can be given in Ω · mm²/m, length in m and cross-sectional area in mm².
What is the resistivity of 304 stainless steel?
The Outokumpu data sheet for 1.4301 / 304 steel gives a typical value of 0,73 Ω mm²/m at 20°C.
What is the resistivity of 316L steel?
According to thyssenkrupp, the typical resistivity of 1.4404 / 316L is 0,75 Ω mm²/m at 20°C.
What is the resistivity of 430 ferritic steel?
For steel 1.4016 / AISI 430, thyssenkrupp data indicates 0,60 Ω mm²/m at 20°C.
What is the resistivity of 2205 duplex steel?
Alleima gives a value of 0,74 μΩ m at 20°C for Sanmac 2205. This property may vary slightly from batch to batch.
What is the resistivity of the superduplex 2507?
According to Alleima SAF 2507 the value at 20°C is approximately 0,83 μΩ m.
Is Alloy 625 conductive?
Yes. It is a conductor, but its resistivity at room temperature, approximately 1,29 μΩ·m, is much higher than copper and many structural steels.
Does resistivity increase with temperature?
In many metals, yes, but the relationship is different for each alloy and may be non-linear, especially in the areas of phase and magnetic transformations.
What is the difference between resistivity and electrical conductivity?
Conductivity σ is the reciprocal of resistivity: σ = 1/ρ. The higher the resistivity, the lower the conductivity.
Does the specific resistivity depend on the thickness of the sheet metal?
The resistivity of a material under steady-state conditions does not directly depend on the sheet thickness. However, thickness influences the cross-sectional area of current flow and, therefore, the resistance of the entire element.
Is resistivity important in induction hardening?
Yes. Together with magnetic permeability and frequency, it influences the eddy current distribution and the depth of heating.
How to measure very low metal resistance?
Four-wire measurement is often used to limit the influence of wires and contacts. Sample dimensions and temperature control are also necessary to determine resistivity.
What standards describe the measurement of resistivity and conductivity?
For example, ASTM B193-25 describes the resistivity test of metallic conductive materials, and ASTM E1004-23 describes the conductivity test of non-magnetic materials using the eddy current method.
Summary – resistivity and material selection
Electrical resistivity ρ is one of the fundamental physical properties of steels and alloys. It allows for determining the resistance of elements with specific geometry, assessing Joule losses, and understanding some of the phenomena occurring during resistance and induction heating. For example, 304 steel typically has ρ ≈ 0,73 μΩ m, duplex 2205 ≈ 0,74 μΩ m, Alloy 625 ≈ 1,29 μΩ m, and pure nickel 200 ≈ 0,090 μΩ m at room temperature.
This parameter depends on the alloy type and temperature; values should not be transferred between grades or equated with the resistance of a specific component. For demanding projects, use the manufacturer's ρ(T) data sheets, verify the delivery status, and determine whether measurement is necessary for the delivered batch.
Need stainless steel, structural steel or a nickel alloy for a specific application? Please provide the grade, required form, dimensions, and operating conditions. Alfa-Tech will help you select the appropriate material specification and prepare a quote. Contact: kontakt@alfa-tech.com.pl.