Ok smart guys - heat treating steel vs magnetics

[WolfeMacleod]

Jeebus, something I don't know about magnetics has come up and I need to find some info.

I'm trying to figure how how heat treating various steels affect it's Paramagnetic properties and magnetic susceptibility.

In other words, if two otherwise identical pieces of, say, 1018 steel are applied to a magnet, will the heat treated part offer better magnetic "conductivity?"

[M.D. Holloway]

From Wikipedia...

Electrical steel is an iron alloy which may have from zero to 6.5% silicon (Si:5Fe). Silicon significantly increases the electrical resistivity of the steel, which decreases the induced eddy currents and thus reduces the core loss. Manganese and aluminum can be added up to 0.5%.

Increasing the amount of silicon inhibits eddy currents and narrows the hysteresis loop of the material, thus lowering the core losses. However, the grain structure hardens and embrittles the metal, which adversely affects the workability of the material, especially when rolling it. When alloying, the concentration levels of carbon, sulfur, oxygen and nitrogen must be kept low, as these elements indicate the presence of carbides, sulfides, oxides and nitrides. These compounds, even in particles as small as one micrometer in diameter, increase hysteresis losses while also decreasing magnetic permeability. The presence of carbon has a more detrimental effect than sulfur or oxygen. Carbon also causes magnetic aging when it slowly leaves the solid solution and precipitates as carbides, thus resulting in an increase in power loss over time. For these reasons, the carbon level is kept to 0.005% or lower. The carbon level can be reduced by annealing the steel in a decarburizing atmosphere, such as hydrogen.

[edit] Physical properties examplesMelting point: ~1,500 °C (example for ~3.1% silicon content) [1]

Density: 7,650 kg/m3 (example for 3% silicon content)

Resistivity: 47.2×10−8 (Ω·m) (example for 3% silicon content)

[edit] Grain orientationThere are two main types of electrical steel: grain-oriented and non-oriented.

Grain-oriented electrical steel usually has a silicon level of 3% (Si:11Fe). It is processed in such a way that the optimum properties are developed in the rolling direction, due to a tight control (proposed by Norman P. Goss) of the crystal orientation relative to the sheet. Due to the special orientation, the magnetic flux density is increased by 30% in the coil rolling direction, although its magnetic saturation is decreased by 5%. It is used for the cores of high-efficiency transformers, electric motor and generators. Cold Rolled Grain-oriented steel is often abbreviated to CRGO.

Non-oriented electrical steel usually has a silicon level of 2 to 3.5% and has similar magnetic properties in all directions, which makes it isotropic. It is less expensive and is used in applications where the direction of magnetic flux is changing, such as electric motors and generators. It is also used when efficiency is less important or when there is insufficient space to correctly orient components to take advantage of the anisotropic properties of grain-oriented electrical steel. Cold Rolled Non Grain-oriented steel is often abbreviated to CRNGO.

[M.D. Holloway]

SpringerLink - Metal Science and Heat Treatment, Volume 5, Number 11

[M.D. Holloway]

More info...

HEAT TREATMENT



If a piece of austenitic stainless steel has been made to respond to a magnet by cold work this can be removed by a solution treatment – the standard treatment of heating to about 1050°C (depending on the grade) followed by water quenching or other rapid cooling. The high temperature allows the “strain-induced martensite” to re-form as austenite and the steel returns to being non-magnetic. It is also returned to being low strength.



DOES MAGNETIC RESPONSE MATTER?



Magnetic response has no effect on any other property. Cold drawn 304 (and to a lesser degree 316) is attracted to a magnet, but this has no effect on the corrosion resistance. Some of the most highly corrosion resistant stainless steels are strongly magnetic… examples are the duplex and super duplex grades and highly alloyed ferritic grades such as 29-4C.



Cold drawn 304 also has high tensile strength, but this is not due to the magnetic response – both the magnetic response and the high strength are due to the cold work. Applications where absence of magnetic response may be required include MRI equipment and in naval mine-hunter vessels. Specialist guaranteed low magnetic response stainless steels can be sourced for such applications.



MAGNETICALLY SOFT STAINLESS STEELS



Magnetically soft steels are used in electrical applications involving changing electromagnetic induction. Solenoids and relays are typical examples, and where these components also need to have corrosion resistance a ferritic stainless steel can be a good choice. For critical applications specialist ferritic bar grades are available (subject to mill enquiry) with guaranteed magnetic properties.

[RPKESQ]

Originally Posted by WolfeMacleod

Jeebus, something I don't know about magnetics has come up and I need to find some info.

I'm trying to figure how how heat treating various steels affect it's Paramagnetic properties and magnetic susceptibility.

In other words, if two otherwise identical pieces of, say, 1018 steel are applied to a magnet, will the heat treated part offer better magnetic "conductivity?"

Yes, depending on the type and heat treatment.

SpringerLink - Russian Journal of Nondestructive Testing, Volume 44, Number 10

[M.D. Holloway]

The major objectives of the different kinds of thermal treatments are:

Soften the material for improved workability.
Increase the strength or hardness of the material.
Increase the toughness or resistance to fracture of the material.
Stabilize mechanical or physical properties against changes that might occur during exposure to service environments.
Insure part dimensional stability.
Relieve undesirable residual stresses induced during part fabrication.
Different metals respond to treatment at different temperatures. Each metal has a specific chemical composition, so changes in physical and structural properties take place at different, critical temperatures. Even small percentages of elements in the metal composition, such as carbon, will greatly determine the temperature, time, method and rate of cooling that needs to be used in the heat treating process. Depending on the thermal treatment used, the atomic structure and/or microstructure of a material may change due to movement of dislocations, an increase or decrease in solubility of atoms, an increase in grain size, the formation of new grains of the same or different phase, a change in the crystal structure, and others mechanisms.

Since there are so many ways in which metals are heat treated, it is not practical to discuss them all. But, as an example, let’s look at how heat treatment is used to strengthen a copper aluminum alloy.

Precipitation Hardening
In designing alloys for strength, an approach often taken is to develop an alloy with a structure that consists of particles (which impede dislocation movement) dispersed in a ductile matrix. Such a dispersion can be obtained by choosing an alloy that is a single phase at elevated temperature but on cooling will precipitate another phase in the matrix. A thermal process is then developed to produce the desired distribution of precipitate in the matrix. When the alloy is strengthened by this thermal treatment, it is called precipitation strengthening or hardening.

Precipitation hardening consists of three main steps: solution treatment, quenching, and aging. Solution treatment involves heating the alloy to a temperature that allows the alloying atoms (called the solute) to dissolve into the solution. This results in a homogeneous solid solution of one phase. Quenching rapidly cools the solution and freezes the atoms in solution. In more technical terms, the quenching cools the material so fast that the atoms of the alloying elements do not have time to diffuse out of the solution. In the as-quenched condition, the solute is supersaturated meaning that the lattice is overly stressed by the alloying atoms. Aging is the process where the solute particles diffuse out of solution and into clusters that distort and strengthen the material.

The precipitation hardening process for a copper-aluminum alloy is shown graphically in the image below. On the right is phase diagram, which is a very useful tool for understanding and controlling polyphase structures. The phase diagram is simply a map showing the structure of phases present as the temperature and overall composition of the alloy are varied. The images on the right in the image show the resulting microstructure at each step in the process.




Common Heat Treating Processes
A few of the more common terms used in heat treating are introduced below. It should be noted that not all of the term are applicable to all alloys.

Age Hardening is a relatively low-temperature heat treatment process that strengthens a material by causing the precipitation of components or phases of alloy from a super-saturated solid solution condition.

Annealing is a softening process in which metals are heated and then allowed to cool slowly. The purpose of annealing is to soften the material for improve machinability, formability, and sometimes to control magnetic properties.

Normallizing is much like annealing, but the cooling process is much faster. This results in increased strength but less ductility in the metal. Its purpose is to refine grain structure, produce more uniform mechanical properties, and sometimes to relieve internal and surface stresses.

Precipitation Heat Treatment is the three step process of solution treating, quenching, and age hardening to increase the strength or hardness of an alloy.

Solution Heat Treatment involves heating the material to a temperature that puts all the elements in solid solution and then cooling very rapidly to freeze the atoms in place.

Stress Relieving is a low temperature heat treat process that is used to reduce the level of residual stresses in a material.

Tempering involves gently heating a hardened metal and allowing it to cool slowly will produce a metal that is still hard but also less brittle. This process is known as tempering.

Quenching is the rapid cooling of a hot material. The medium used to quench the material can vary from forced air, oil, water and others. Many steels are hardened by heating and quenching. Quenching results in a metal that is very hard but also brittle.

[M.D. Holloway]

[M.D. Holloway]

To answer your question, heat treating (annealing) does effect magnetic properties.

Here is the data...

Effect of annealing parameter on microstructure and
magnetic properties of cold rolled non-oriented electrical steel

LI Min(..)1, XIAO Yu-de(...)1, WANG Wei(..)1, ZHOU Juan(..)1,
WU Guang-liang(...)2, PENG Yue-ming(...)2


1. School of Materials Science and Engineering, Central South University, Changsha 410083, China;
2. Hunan Valin Lianyuan Iron and Steel Company, Loudi 417722, China
Received 15 July 2007; accepted 10 September 2007
Abstract: The microstructure and magnetic properties of cold rolled non-oriented electrical steel, annealed at 200-1 000 . for
0-240 min with different heating rates, were investigated by optical microscopy, scanning electron microscopy, Epstein frame, and
transmission electron microscopy. The results show that the magnetic properties of cold rolled non-oriented electrical steel can be
improved by controlling the annealing process to obtain uniform coarse grains with critical sizes after the recovery, recrystallization
and growth of grains. Additionally, the annealing temperature influences the magnetic properties more significantly than annealing
time, and with the increase of heating-up rate during the annealing process, the magnetic properties of the cold rolled non-oriented
electrical steel increase.

Key words: non-oriented electrical steel; annealing process; magnetic property

1 Introduction

Cold rolled non-oriented electrical steels, having
good magnetic and workability properties, play an
important part in electric machine, rectifier, and electric
transformer. It is a new kind of soft magnetic material.
Currently, Japan and Germany hold the key technology
to produce stable products[1]. With the combination of
domestic markets with aboard markets, the markets for
the energy saving electrical machines are huge. At
present, researchers have tried many ways to enhance the
magnetic properties by materials design, cast process
control, dispersed precipitate control, hot/cold roll
process, heat treatment and so on. Obviously, annealing
parameter of heat treatment affects microstructure and
magnetic properties of rolled non-oriented electrical steel
effectively[2].

In this work, influences of annealing temperature,
annealing time and heat-up rate on the microstructure
and magnetic properties of cold rolled non-oriented
electrical steels were discussed.

2 Experimental

The cold rolled non-oriented electrical steel samples
were provided by Hunan Valin Lianyuan Iron and Steel
Company. The compositions (mass fraction, %) of the
samples are listed in Table 1.

The steel plates were cut into the size of 300 mm×
30 mm, and the amounts of the sample perpendicular to
the rolling direction and parallel to the rolling direction
were the same. The annealing experiments were carried
out in the self-made high-temperature tube resistance
furnace. The annealing temperature was 0-1 000 .,
annealing time was 0-240 min, heating rate was 0-100
./min, and the temperature fluctuation was 1 .. The
mixture of 65% H2 and 35% N2 was applied as protective
atmosphere during the annealing process and the current
of air velocity was 3 L/min.

The microstructure of the samples after different
heat treatment processes were observed on the
Polyvar-MET optical microscope after corrupted by
sweet spirit of 3% aqua fortis and grain size was analysed
by Q550 quantitative metallographic analyser. The
magnetic properties were measured by Epstein frame
method. The amounts of the samples perpendicular to the
rolling direction and parallel to the rolling direction were
the same and the samples were jointed to a square. The
total mass was approximately 1 kg. The magnetic loss
(P15/50) was measured under the condition of 1.5 T and 50
Hz. The magnetic induction density was measured in
magnetic field intensity of 5 kA/m. The microstructures
of the samples were characterized by KYKY-Amray
2008B scanning electron microscopy and Philips
TecnaiG220 transmission electron microscopy.

3 Results and discussion

3.1 Microstructure and magnetic property
The optical microstructure of cold rolled non-
oriented electrical steels is shown in Fig.1. The
microstructure of the cold rolled samples consists mainly
of refined and uniform equiaxed grains, and no typical
cold rolled fibrous tissues are observed. It might be due
to the fact that the cold rolling process is carried out at
the higher temperature but not at room temperature, and
this phenomenon that temperature rises can be caused by
deforming heat during the rolling process[3].

There is adequate driving force for the recrystallization
process, which is beneficial to the recrystallization and
the grain growth[9]. Additionally, it is also helpful for
ameliorating the texture, resulting in the improvement of
the magnetic properties[10].

4 Conclusions

1) With the increase of annealing temperature and
extension of annealing time, recovery, recrystallization
and grain growth take place in the rolled plates. The
magnetic properties improve along with the growth of
the grains. A critical grain size exists, which can make
the iron loss least.

2) The annealing temperature influences the
magnetic properties more than the annealing time.

3) The magnetic properties are improved with the
increase of the heat-up rate.

[WolfeMacleod]

Thanks, Lube!

In fact, I was hoping that it either didn't do anything, or had an adverse effect....

Good info though, thanks