Terminology

STRETCHING A%
It measures the stretching of the material when taken to rupture point using tensile testing.
it is a data that allows engineers to know how much steel can be stretched before reaching the rupture point.
NON MAGNETIC
It’s a steel showing ferromagnetic properties and that can be demagnetized with adequate heat treatment, which consists of exceeding the magnetism critical point (769 °C), remaining at that point for a period of time, and cooling normally in air or furnace.
ANISOTROPHY
The crystal shape is different in all directions as well as the physical properties (refraction index, thermal conductivity, mechanical and magnetic properties, etc.) which vary depending on the direction taken.
ANISOTROPIC
The specific physical properties inside a material have different values in different directions.
SPECIFIC HEAT CAPACITY
Heat quantity necessary to raise temperature of a mass unit by 1 °C.
The specific heat capacity at 20 °C of ferritic and martensitic steels is slightly less than that of austenitic steels, but grows quicker with raising temperature.
MAGNETIC FIELD
Magnetic field forming around a magnet or an electrical circuit. It’s marked by H and measured by Ampere/meter.
ULTIMATE STRENGTH Rm-R
The ultimate strength determined by stress proportional samples, measured in N/mm2.
This occurs within a solid subject to stress causing the molecular bonds to break.
This testing is used by engineers for dimensioning supporting structures.
YIELD STRENGTH Rp0.2
Load corresponding to a non-proportional extension, widely used as yield strength at 0.2%.
The value is obtained from tensile testing, measured in N/mm2 and is useful to engineers to determine sections and safety margin to adopt for structures of a determined construction.
CONDUCTIVITY
It’s the property of a material’s ability to conduct heat or sound energy or electric current.
Symbol: Siemens • m/mm2.
THERMAL CONDUCTIVITY
Thermal conductivity is the W / (m • K) measurement of a body’s ability to conduct heat. It depends from the nature of the material and not from its shape.
This property is a lot higher in carbon steels ~ 50 W / (m • K) and bonded steels ~ 40 W / (m • K), it lowers to ~ 30 W / (m • K) in stainless steels at chromium 15% and lowers ~ 15 W / (m • K) in nickel and molybdenum steels.
Thermal conductivity proportionally increases to the part heating and it is a parameter used in heat treatments to define the heat rate.
HEAT TRANSFER COEFFICIENT
Defines the variation of magnetic properties at temperature variation. Normally expressed in variation % of the part for each degree of temperature.
COERCIMETER
Automatic instrument used to measure coercive force of steel samples of any shape (regular or irregular) and of specific manufacture products alone or assembled with other materials.
Normative widely used for checks: ASTM 341 or IEC 404-7.
COERCIVITY
Defined as the magnetic field, expressed by Hc, needed to reduce induction B or magnetization M to value zero. Normally measured in Oersted or Ampere/meter. Needed to measure resistance to demagnetization of a magnetized material.
CONDUCTIVITY
It’s the property of a material’s ability to conduct heat or sound energy or electric current.
Physical size equal to the inverse of resistivity.
HYSTERESIS CURVE
Graphic representation of the curve obtained by measuring induction B (air + material) or magnetization M in presence of a magnetic field H. It describes a complete cycle between defined limits for induction or magnetization saturation from the first to third quadrant. 

B = Magnetic flow density Br = Residual magnetic induction
H = Magnetic field Hc = Coercive force
DEMAGNETIZATION CURVE
Graphically it is the curve sector of the hysteresis cycle of the second quadrant, which defines the main magnetic characteristics of a magnetized material. The demagnetization curve describes the induction change or emanation of the residual value at zero applying a negative magnetic field.

DENSITY (MASS PER UNIT VOLUME)
This ratio m/v between the mass (m) of a body and its volume (v) is also called specific weight and is measured in Kg/dm3.
The iron specific weight is 7.86 Kg/dm3 and its atomic mass is 55.847 while that of the main elements of stainless steel is: 51.996 for chromium, 58.69 for nickel and 95.94 for molybdenum.
Thereby alloys rich in chromium are lighter than iron, while nickel and molybdenum alloys are heavier. The following example compares the estimated weight of three bars sized 180 mm square and 1500 mm long.
Density values are reported in the data sheets specifications.
42CrMo4 steel (1,80 • 1,80 • 7,85 • 15,00) = Kg 381,51
1.4006 steel (1,80 • 1,80 • 7,70 • 15,00) = Kg 374,22
1.4567 steel (1,80 • 1,80 • 8,027 • 15,00) = Kg 390,11
FLOW DENSITY
Defines the induction field as magnetic lines of force per unit area.
DIELECTRIC
Is an electrical insulator that can be polarized by an applied electric field with accumulated energy. It has the function of separating parts with different potentials and forcing the current into one single direction.
HARDNESS
Characteristic depending on molecular cohesive forces. Measured through various sizes (HB, HRC, HV, etc.) within empirical tables, among which only some correlation exists, all in ratio with traction rupture load.
THERMAL EXPANSION
The thermal expansion or growth is the physical phenomenon which appears when material has a volume increase, due to temperature increase during heat treatment, function or welding.
The value can be determined by dilatometer testing or according to data sheet specifications contained in the volume Designing with steel by Lucefin Group.
This data lies within the formula for the theoretical linear e volumetric growth in mm, that steel will endure when heated at temperatures between: 20 and 100 °C, 20 and 200 °C, 20 and 300 °C, etc. 

Lo = Initial length of the bar or the part in mm
ro = Initial radius in mm
E = Constant value from data sheet specification (i.e. as 10-6 • K-1 equal to 10.4 insert 0.0000104)
Δt = Temperature difference between the part to be heated and the environment
L = Length in mm after heating at °C.
V= 3 mm volume after heat treatment
Linear expansion L = Lo • (1 + (E • Δt))
Volumetric expansion V = 3,14 • r2 • Lo • (1 + (2 • E • Δt)) • (1 + (E • Δt)

FLOW
Number of magnetic lines of force measured in Gauss o Tesla. The lines can seen using dry iron powders or in humidity.
FERRITICS
Consist of non metallic materials made of iron oxides and a bivalent metal (Mg, Mn, Zn, Cu, etc.) and can be assimilated to ceramic materials for its harness and brittleness. They have a very low conductivity, thus are suitable to form ferromagnetic nuclei for high frequency applications (5 – 500 kHz). The most common ferritics are Mn-Zn, Ni.Zn, Mg-Mn.
MAGNETIC FIELD FORCE
It is the magnetizing or demagnetizing force, generally measured in Oersteds, which determines the ability of an electric current, or a magnetic body, to induce a magnetic field at a given point.
FLOWMETER
Devised used to measure change of magnetic induction flow.
COERCIVE FORCE
Demagnetizing force necessary to reduce residual induction to zero, as measured on a saturated magnet. Calculated in Oersted or A/m and KA/m. Symbol: Hc.
INTRINSIC COERCIVE FORCE
Measures the resistance of a magnetic material to demagnetization and shows its stability degree at high temperatures. Symbol: Hci.
Gauss
Unit of measure of magnetic induction in the CGS electromagnetic system. It shows flow lines for cm2.
GAUSSMETER
Device used to measure the instantaneous value of magnetic induction and residual magnetism.
MAGNETIC INDUCTION (B)
Is the magnetic field induced by an applied field resulting from the laid down field and from the matter. Also defined as magnetizing or demagnetizing force measured in Oersted, which determines the current ability or of a magnetic material to induce a magnetic field in a given point. B = µo • H
In any given material: B = µr • µo • H
ISOTROPIC
A magnet is isotropic when its properties are identical in all directions. In the metal material field, magnetic orientation of particles does not have a preferred direction thus allowing an all round magnetization.
HYSTERESIS
It is a ferromagnetic substance characteristic, in which magnetization intensity does not uniquely depend from the magnetic field applied, but also from the previous evolution in the magnetic field. It is defined as the tendency of a magnetic material to retain its magnetization in a demagnetizing energy presence.
MAGNET
Is a ferromagnetic body artificially or naturally magnetised. Only certain types of substance are able to acquire a satisfying permanent magnetization, after adequate treatments.
RESIDUAL MAGNETISM
It is the remaining magnetism in a steel material after having been in contact with an applied magnetic field (usually lift magnets, induction processes, etc.). Its intensity depends on several factors, some of the most important are: chemical structure, magnetic field intensity at source, temperature of material.
MAGNETIZATION
Magnetism for each volume unit, measured in Ampere/meter.
MAGNETOSTRICTION
It is a property that can cause changes of shape or dimension during the process of magnetization.
MAXIMUM ENERGY
In the hysteresis curve it is represented by the point of maximum out come between magnetizing force H and induction B. Also defined as the energy that a magnetic material can transfer to an external magnetic circuit in a given point of the demagnetization curve. Symbol: BH max.
ANTIFERROMAGNETIC MATERIALS
The magnetic structures a and b inside anti-ferromagnetic materials, are precisely equal but opposite, resulting in null magnetization.
Hematite is the best anti-ferromagnetic material.
DIAMAGNETIC MATERIALS
Are those materials whose magnetization is inversely inducted to that of the inductive field. They are composed of non magnetic atoms placed in complete orbitals without free electrons. This causes an opposition when in a magnetic field. In other words, a negative magnetization is created, exactly the opposite of what happens in ferromagnetic materials. Among these diamagnetic materials are quartz, calcite, water and organic substances.
FERROMAGNETIC MATERIALS
One of the main characteristics of these materials is the spontaneous magnetization without a magnetic field and can be increased until it reaches magnetic saturation. Saturation is at high temperatures and moderate magnetic fields.
In particular, each ferromagnetic material at a given temperature called Curie temperature, which differs from material to material, loses the electrons configuration and becomes paramagnetic. Furthermore, ferromagnetic materials can retain a magnetic memory from previously. The best ferromagnetic elements are: iron, nickel and cobalt. Ferritic and martensitic stainless and duplex steels are among this category.
PARAMAGNETIC MATERIALS
These are made of atoms and ions with unpaired electrons and incomplete orbitals. They have a net magnetism and can magnetize when exposed to a magnetic field. However it is a weak magnetization, which disappears when the magnetic field is taken away. Liquid oxygen, aluminium, biotite, pyrite, siderite are among the paramagnetic materials. Austenitic stainless steels belong to this category (stable austenitic structure).
Maxwell
The measurement unit of the flux produces by a intensity magnetic field in the CGS system in an area of 1 cm2. One Maxwell is 10-8 Weber and equals one line of magnetic flux. Symbol: Mx.
ELASTIC MODULUS
Defined as the strain stress and deformation ratio, in mono-axial loads and elastic behaviour of material. Used by engineers for flexural strain verification, under exercised stress, to establish the maximum load factor of a construction. The longitudinal elastic modulus for each steel and the different temperatures to which a stainless steel product can be worked at, all are reported in the data sheet specification.
m = 1/Poisson coefficient (the Poisson coefficient is reported within some data sheet specification contained in the volume Stainless Steels by Lucefin Group © 2011).
E = Longitudinal elastic modulus G = Tangent elastic modulus
The result is measured in GPa (GigaPascal).
E = G / (m / 2 • (m + 1)) G = (m / (2 • (m + 1)) • E
MAGNETISM
It is measured in Am2 (Ampere for each m2).
POISSON’S RATIO
When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular to the direction of compression. This phenomenon is called Poisson effect. The Poisson ratio is the ratio of the percent of expansion divided by the percent of compression. It is often used when calculating elasticity and structural projects.
Oersted
Unit of measure for the intensity of the magnetic field and measuring the magnetic force.
1 Oe = 1 Gauss = 0,79 A/cm. Symbol: Oe.
PERMEABILITY (µo)
Is the propagation ability of the magnetic flux in a classical vacuum:
µo = 1,26 • 10-6 • H/m
µr < 1 Diamagnetic materials (if the magnetic field is weakened inside the material)
µr > 1 Paramagnetic materials (if the field is strengthened inside the material)
µr >> 1 Ferromagnetic and ferromagnetic materials (if the field is strengthened a lot inside the material)
INITIAL PERMEABILITY
Is the ratio between field B and field H measured when H has a zero tendency. Relative permeability or ratio from material and free space (air) permeability is more useful. Used to indicate weak ferromagnetic steels used for transformers.
ABSOLUTE MAGNETIC PERMEABILITY
Is the parameter of all materials given by the ratio between magnetic induction B produced inside the material by the magnetic field and intensity H of the applied field.
Symbol: m. The 1/m opposite of permeability is called specific reluctance.
RELATIVE MAGNETIC PERMEABILITY
In physics terms it defines the ability of a substance to be magnetized by a magnetic field. Symbol: µr and it is the ratio between absolute permeability µ of a generic material and the permeability µo of free space (vacuum). Ferritic and martensitic stainless steels are defines magnetic (a magnet attracts them) when at room temperature and they lose this characteristic when heated over 769 °C. Austenitic steels are classified as non-magnetic and their permeability is around 1.02 µr. They can be slightly magnetized during cold drawing treatment, but a later re-crystallization return them to the initial non-magnetic state.
PERMEAMETER
A device able to do hysteresis cycles and measure magnetization of mild magnetic steels (i.e. stainless for electro-valves and nuclei). It performs in automatic and determines the following parameters: Br, Hc, Bsat, Jsat, µmax. Standards used for checking: ASTM 341 or IEC 404-4 for straight samples or bars.
MAGNETIC POLARIZATION
A substance exposed to a magnetic field directs its atoms magnetism to magnetize for induction. Generally, term used to describe an alteration of physical state, where some phenomena from isotropic become vector.
RESILIENCE
Indicates material tenacity when exposed to violent shocks. Resistance measured in J (work – energy), it is determined through previously cut samples and usually of Kv types. This value also indicate steel predisposition for certain uses.
ELECTRICAL RESISTIVITY (also known as SPECIFIC ELECTRICAL RESISTANCE)
It is the resistance of a conductor of unit length and area section unit, measured in Ω • mm2/m. The resistivity of a conductor depends on its nature, temperature, and in some cases on the intensity of magnetic field in which it is. Under heat treatment the resistivity of the material is null (absolute zero) and increases by 6% each 100 °C. The increase of resistivity in a material can be obtained by altering its composition (i.e. 4-4.5% silicon increase).
RESIDUAL MAGNETISM
It represents residual magnetism when the magnetic field applied is zero. In the graph it is the intersection of the curve with the y-axis. Symbol: Mr.
SATURATION
Phenomenon occurring when, in a ferromagnetic substance subject to a sufficiently intense magnetic field, magnetization remains practically constant to every increase of magnetic field.
CREEP (DEFORMATION)
The tendency of a solid material to deform under the influence of constant stresses at high temperatures. The sample is subject to a determined constant load and temperature for long periods. The obtained results simulate steel behaviour in time.
DEMAGNETIZATION
This unwanted magnetic force (found in products for grinding, lapping and polishing, galvanic treatments, etc.) attracts filings or iron powder and causes unacceptable surface finishes. It can be diminished or removed by bringing steel at temperatures above 769 °C or let it through demagnetizing tunnels. Another rather efficient method is the stress-relieving heat treatment for long periods.
SOLENOID
It is a coil wound into a tightly packed helix, mounted on a cylindrical support bigger than the helix diameter.
CURIE TEMPERATURE
It is the temperature above which ferromagnetic materials become paramagnetic. It normally depends on the chemical composition of the magnetic material, once the material reaches it, loses all of its permanent magnetic properties and can no longer keep magnetism. Symbol: Tc = 769 °C.
MAXIMUM OPERATING TEMPERATURE
The maximum temperature of exposure that a material can resists without mechanical or structural changes.
Tesla
Density unit of magnetic flux: 1 T = 10000 Gauss.
MAGNETIC VISCOSITY
It describes the variation of magnetization delay in a ferromagnetic material when the external magnetic field abruptly changes intensity.
Weber
It is the unit for magnetic flux which when linked at a uniform rate with a single turn electric circuit during an interval of 1 second, will induce an electromotive force of 1 volt. 1 Weber = 10-8 Maxwell. Simbol: Wb.