Surface hardening of steel | flame, induction, laser beam, case hardening and nitriding
Summary
TLDRThis video script delves into surface hardening techniques for steel, crucial for enhancing wear resistance in components like gear wheels. It explores various methods including flame, induction, laser hardening, and nitriding, each affecting the steel's microstructure differently. The script highlights the importance of achieving a hard surface layer while maintaining core toughness, and touches on the challenges of distortion and embrittlement. It also discusses the role of carbon content in hardenability and the unique benefits of case hardening and nitriding for specific applications.
Takeaways
- 🔧 Surface hardening of steel is crucial for increasing wear resistance in components that experience contact and sliding, like gear teeth.
- 🛠️ Hardening the entire component can reduce toughness and cause embrittlement, so surface hardening is preferred to maintain core toughness.
- 🔥 Flame hardening involves passing a torch flame over the material to induce a microstructural change, creating a hard martensitic surface layer.
- 💧 Water jets are used to quench the heated surface in flame hardening, leading to the formation of martensite and enhancing wear resistance.
- 🌀 Induction hardening uses electromagnetic induction to heat the surface rapidly, allowing for precise control of the hardening depth through frequency adjustment.
- 🌡️ Laser hardening provides extremely short heating times, reducing distortion and scaling, and can be done without additional quenching due to self-quenching.
- ⚙️ Case hardening is used for low carbon steels, where carbon is diffused into the surface to increase hardenability, followed by quenching to form a hard surface layer.
- 🔩 Nitriding is a surface hardening process that involves diffusing nitrogen into the steel surface to form hard nitrides, improving wear resistance without structural transformation.
- 🔩 The depth of the hardened layer in surface hardening processes can be controlled by factors such as the speed of the flame in flame hardening or the frequency in induction hardening.
- ⏱️ Heating times and the potential for distortion vary across hardening methods, with laser hardening offering the shortest heating times and reduced risk of distortion.
Q & A
Why is surface hardening of steel components important?
-Surface hardening of steel components is important to increase wear resistance, particularly for parts in contact with each other, like gear tooth flanks. It hardens only the surface layer while retaining the core's toughness to prevent unpredictable material failure.
What is the primary disadvantage of traditional heat treatment over surface hardening?
-The primary disadvantage of traditional heat treatment is the reduction in toughness and embrittlement of the steel, which can lead to unpredictable material failure. Surface hardening mitigates this by only hardening the surface, keeping the core tough.
How does flame hardening work?
-Flame hardening involves passing a torch flame over the material's surface to be hardened, causing a microstructural change. The high temperatures induce austenite formation, and water jets quench the surface rapidly, trapping carbon in the martensitic structure, forming hard martensite.
What is the role of the water jets in flame hardening?
-The water jets in flame hardening are used to quench the heated surface immediately after the flame passes, ensuring rapid cooling that traps carbon in the martensitic structure, leading to the formation of hard martensite.
How does the thickness of the hardened surface layer in flame hardening relate to the flame passing speed?
-The thickness of the hardened surface layer in flame hardening is controlled by the speed at which the flame is passed over the workpiece. A slower speed allows deeper heat penetration, affecting the microstructure more and resulting in a thicker hardened layer.
What is induction hardening and how does it differ from flame hardening?
-Induction hardening uses electromagnetic induction to generate high-frequency alternating currents that heat the workpiece's surface. It differs from flame hardening by providing more precise control over the hardening depth and is more suitable for complex geometries and smaller components.
How does the skin effect influence the hardening depth in induction hardening?
-The skin effect refers to the increase in current density towards the outer areas of a conductor with increasing frequency of alternating current. This allows for greater control over the hardening depth by adjusting the frequency; higher frequencies result in thinner hardness layers.
What are the advantages of laser hardening over other surface hardening processes?
-Laser hardening offers shorter heating times, reducing distortion and scaling. It also allows for precise control over the hardened area, is suitable for hard-to-reach areas, and avoids the need for water quenching due to self-quenching.
How does case hardening differ from other surface hardening processes?
-Case hardening involves diffusing carbon into the surface layer of a low carbon steel to increase its carbon content to a hardenable level, followed by quenching. This results in a hard, wear-resistant surface while the core remains tough and ductile.
What is nitriding, and how does it provide a solution for surface hardening without microstructural transformation?
-Nitriding is a process where nitrogen-containing gases or salts diffuse nitrogen atoms into the surface of specially alloyed steels, forming hard nitrides without a martensitic transformation. This results in increased surface hardness and wear resistance without the distortion or scaling associated with other hardening methods.
Why is tempering often required after case hardening?
-Tempering is often required after case hardening to reduce the high residual stresses caused by the hardness differences between the surface and the core of the component. This helps to improve the component's dimensional stability and fatigue resistance.
Outlines
🔥 Surface Hardening Techniques
This paragraph discusses the necessity of surface hardening in steel components to increase wear resistance, particularly in parts that experience continuous contact and sliding, such as gear wheels. The traditional heat treatment method of hardening can lead to a reduction in toughness and embrittlement, so it's crucial to harden only the surface while maintaining the core's toughness. Surface hardening processes like flame hardening, induction hardening, laser hardening, case hardening, and nitriding are introduced. These processes involve microstructural changes through heating and quenching to increase hardness, with the exception of nitriding. Flame hardening is highlighted as one of the oldest methods, where a torch flame heats the surface, causing a microstructural change from ferrite to austenite, and subsequent quenching with water jets forms martensite, which is hard and wear-resistant. The paragraph also touches on the limitations of flame hardening, such as the bulky equipment and the challenges in hardening small components with complex geometries.
🌀 Induction and Laser Hardening
The second paragraph delves into induction hardening, which uses electromagnetic induction to generate eddy currents in the workpiece, leading to surface heating. This method allows for precise control over the hardening depth through the frequency of the alternating current and the use of the skin effect. Induction hardening is noted for its efficiency, as it generates heat primarily on the surface, resulting in less scale formation and distortion. It also does not produce toxic fumes. Laser hardening is mentioned as an even more precise and faster method, with the laser beam's high power density allowing for rapid heating of the surface to just below the melting point. This process minimizes distortion and scaling, and the self-quenching nature of laser hardening eliminates the need for water jets. The paragraph also discusses the economic benefits of induction hardening in automated production lines and the suitability of laser hardening for hard-to-reach areas.
🛠 Case Hardening and Nitriding
The final paragraph focuses on case hardening, a process used for low carbon steels to increase their surface hardness by diffusing carbon into the surface layer. This is achieved by exposing the steel to a carbon-rich environment at high temperatures, leading to a hardenable carbon content in the surface layer while the core remains low in carbon. The hardening process is completed by quenching the component, which can be done directly after carburizing or after a slow air cool. The resulting high surface hardness and residual compressive stresses enhance the component's wear resistance and fatigue strength. Nitriding is introduced as an alternative surface hardening process that does not involve a microstructural transformation. In this process, nitrogen atoms diffuse into the steel surface and form hard nitrides with alloying elements, increasing surface hardness without affecting the core properties. Nitriding is highlighted for its ability to improve fatigue strength and corrosion resistance, though it is noted for being time-consuming and expensive due to the long processing times required.
Mindmap
Keywords
💡Surface Hardening
💡Wear Resistance
💡Toughness
💡Flame Hardening
💡Induction Hardening
💡Laser Hardening
💡Case Hardening
💡Nitriding
💡Martensite
💡Residual Stresses
💡Distortion
Highlights
Surface hardening is essential for increasing wear resistance in components that experience friction, such as gear tooth flanks.
Hardening can lead to a reduction in toughness and embrittlement, which may cause material failure.
Surface hardening techniques are used to harden only the surface layer of a component while maintaining core toughness.
Flame hardening involves passing a torch flame over the material surface, inducing a microstructural change from ferrite to martensite.
Water jets are used to quench the heated surface in flame hardening, leading to the formation of martensite and increased hardness.
Flame hardening's limitations include its bulky setup and challenges in hardening small components with complex geometries.
Induction hardening uses electromagnetic induction to heat the surface rapidly, minimizing the heat-affected zone.
The skin effect in induction hardening allows for precise control of hardening depth through frequency adjustment.
Laser hardening provides even shorter heating times than induction, reducing distortion and scaling.
Case hardening is used for low carbon steels, where carbon is diffused into the surface layer to increase hardenability.
Nitriding is a surface hardening process that involves the diffusion of nitrogen atoms into the steel surface to form hard nitrides.
Nitriding can improve a component's fatigue strength and corrosion resistance due to the formation of residual compressive stresses.
Surface hardening processes aim to achieve a balance between hardness and toughness, ensuring component durability and reliability.
The choice of surface hardening process depends on the component's material, geometry, and the required depth of hardness.
Post-processing after surface hardening may be necessary to manage distortion, scale formation, and residual stresses.
Surface hardening techniques are critical for components subjected to dynamic stress, such as gears and drive shafts.
Innovations in surface hardening aim to improve efficiency, reduce costs, and enhance the performance of industrial components.
Transcripts
surface hardening of
Steel a hard surface is usually required
to increase the wear resistance of
components in contact with each other
this is the case for example with tooth
flanks of Gear wheels that permanently
slide on each other during
operation in these cases hardening is
basically a possible heat
treatment however the disadvantage is
the reduction in toughness and
embrittlement of the steel which can
lead to unpredictable material
failure for this reason it is necessary
to harden only the surface of a
component so that the core retains its
toughness this is known as surface
hardening typical examples of surface
hardening are geers or guide
rails however crankshafts or camshaft
are also usually surface hardened after
quenching and
tempering so note surface hardening only
hardens the surface layer to increase
where resistance leaving the core of the
component
tough depending on the type of of
surface hardening there are different
processes such as flame hardening
induction hardening laser hardening case
hardening or
nitting with the exception of nitriding
the increase in hardness of All Surface
hardening processes is based on micr
structural changes during Heating and
subsequent quenching as in normal
quenching and tempering more information
can be found in the linked
video in this video we take a closer
look at the surface hardening processes
mentioned let's first take a closer look
at at flame hardening in flame hardening
a torch flame is passed over the surface
of the material to be hardened the high
temperatures tinze the surface and the
steel changes from the body centered
cubic ferite lattice to the face-
centered cubic tinite lattice the carbon
previously Bound in the cementite
becomes soluble in the tinite micro
structure water jets are positioned
directly behind the burner Flames to
quench the heated surface the rapid
cooling ensures that the carbon can no
no longer diffuse out of the
transforming lattice the forcibly
dissolved carbon forms a distorted
lattice structure known as
Marite this Marite micro structure is
very hard and is largely responsible for
the wear resistance after
quenching unlike traditional quenching
and tempering subsequent tempering is
not common in flame hardening this
generally also applies to other surface
hardening processes as the unhardened
core already has sufficient toughness so
that the component does not become
brittle during qu Ing and thus retains a
certain formability
anyway the thickness of the hardened
surface layer can be controlled by the
speed at which the flame is passed over
the surface of the workpiece the slower
the speed the deeper the heat can
penetrate and atiz the micro structure
and the thicker the surface layer after
quenching at the same time of course the
cooling rate required for the formation
of Marite in the deeper surface layers
must be provided as alloying elements
generally reduce the critical Cooling
rate deeper layers can be hardened in
high alloy
Steels flame hardening is one of the
oldest methods of surface
hardening however it has limitations due
to the relatively bulky arrangement of
torches and water nozzles especially for
small components with complex
geometries for example in the case of
very small gears not only the flanks are
hardened but also the are is further
inside due to the relatively large heat
affected
Zone with large surfaces to be hardened
the risk of unwanted hardening is lower
but the design effort is higher because
the shape of the burner and the water
nozzle must always be adapted to the
shape of the workpiece flame hardening
is also generally inferior to induction
and Laser hardening in terms of the
accuracy of the hardening
depth in all surface hardening processes
heating should be carried out as quickly
as possible to minimize the heat
affected Zone at undesirable areas
otherwise there is a risk of thermal
stress or Distortion of the component
geometry
in addition long heating times lead to
increased scale formation which usually
requires
postprocessing although rapid heating is
always desirable it should be noted that
the thermodynamic equilibrium in the
micr structure is no longer present this
leads to a shift of the austenization
temperature to higher
values in principle the burner flame and
the long heat up times during flame
hardening result in a relatively large
heat affected Zone in the case of small
geometries such as a thin shaft this can
lead to undesirable hardening over the
entire
cross-section in such cases induction
hardening can be used to harden only the
surface of such thin Walt components to
within a few tenents of a
millimeter induction hardening is based
on the phenomenon of electromagnetic
induction which is also used in
induction cookers or
Transformers a highfrequency alternating
current is generated in a copper
electrode which is formed to the shape
of the workpiece to be hardened
this electrode is also known as an
inductor and forms the primary coil of a
transformer in a figurative sense the
alternating current creates a constantly
changing magnetic field around the
electrode this alternating field
penetrates the adjacent workpiece and
generates Edie currence there in a
figurative sense the workpiece
represents the so-called secondary coil
of a
transformer the very high Edy currents
of sometimes several thousand amp per
square mm lead to a strong heating of
the
workpiece during induction hardening the
tinti surface is usually quenched using
water nozzles which are moved along with
the inductor over the
workpiece in cases where only very low
hardening depths are achieved quenching
can also be carried out without water
through the relatively cool core of the
workpiece this is then referred to as
self-quenching as a very high surface
hardness can be achieved during
induction hardening High residual
stresses can occur this may require
subsequent tempering at low
temperatures the fact that heat is
mainly generated on the surface and less
on the inside of the material is due to
the so-called skin effect while the
current density in a conductor
cross-section is constant with direct
current with alternating current the
current density increases with
increasing frequency in the outer areas
and decreases in the
interior the frequency of the Edy
currents in the workpiece depends on the
frequency of the AC current in the
electrode the this makes it relatively
easy to control the hardening depth by
the
frequency the higher the frequency the
greater the skin effect and the thinner
the hardness layers that can be
achieved the AC frequency to be set will
therefore depend on the hardening depth
to be achieved at a frequency of 50 HZ
hardening depths in the range of 10 to
20 mm can be achieved in the frequency
range from 1 to about 10 khz hardening
depths of about 1 to 5 mm can be
achieved if in the high frequency range
up to several megaherz hardening layers
as thin as a few tenents of a millimeter
are
possible heating times for induction
hardening are generally much shorter
than for flame hardening as the specific
heat output can be around 10 times
higher at several kilowatt per square
cm this has the advantage that scale
formation is relatively low and the
amount of post-processing required is
reduced the risk of hardening Distortion
is also greatly
minimized in addition induction
hardening does not produce any toxic
fumes as is the case with flame
hardening another advantage of induction
hardening is the more even heating of
the surface provided the inductor is
optimally adapted to the
workpiece however this requires a high
level of tool design in advance meaning
that induction hardening is particularly
economical in automated production lines
with large batch sizes due to the high
electricity costs cost Effectiveness
increases if only small areas of a
workpiece need to be hard
hardened laser hardening offers even
shorter heating times for the surface to
be hardened than induction hardening
this significantly reduces the already
low levels of distortion and scaling
oxidation of the surface can even be
avoided completely when using inert
gases in laser hardening a laser beam
with around 10 times the specific power
compared to induction hardening is moved
over the surface to be hardened the
enormous heat of the diode laser of
several kilowatts leads to a heating of
the surface layer to just below the
melting temperature in a very short
time as the heat input is limited to the
focal spot of the laser unnecessary
heating of unwanted areas is avoided as
a result the locally heated area is
quickly quenched by the cooler
surrounding areas this self-quenching
makes quenching with water jets
obsolete depending on the focusing and
process control the laser spot covers a
track width of 1 to approximate 50 mm
large surface layers must therefore be
scanned line by line with the laser
typical hardening depths for laser
hardening are in the range of 0.1 to 2
mm as with induction hardening the
smaller the area is to be hardened and
the thinner the hardening depths the
greater the cost- effectiveness of laser
hardening laser hardening is
particularly suitable for areas that are
very difficult to access such as
recesses or blind
holes in principle the toughness of
Steels increases as the carbon content
decreases as there are then fewer
brittle cementite components in the micr
structure so if components are to be
very tough they must inevitably be
relatively low in
carbon at the same time however the low
carbon content reduces the hardenability
of the material since it is precisely
the carbon that is forcibly dissolved in
the lattice that leads to the necessary
Marite
formation in order to harden such low
carbon Steels a process called case
hardening is used
in case hardening a low carbon steel
with a maximum of 0.2% carbon is first
exposed to a carbon containing
environment in the past the steel was
placed in boxes filled with carbon
granules and then sealed airtight hence
the name case hardening the steel is
then placed in a furnace for several
hours at temperatures between 900 and
1,000°
C nowadays the components are usually
analed in carbon containing atmospheres
in gas furnaces or in molten
salts however the physical processes
explained in the following are basically
the
same as a result of the high
temperatures the carbon in the
environment diffuses into the surface
layer of the component and causes the
carbon content there to increase to a
hardenable level of around 0.8% carbon
while the core remains low in carbon
this accumulation of carbon in the
surface layer is known as
carbonization the depth of carbonization
is typically between 0 .1 and 5
mm after carburising the surface layer
the actual hardening takes place by
quenching the component in water or in
another quenching medium depending on
the process control the steel is
quenched directly after carburising
which is referred to as direct hardening
or the steel is first cooled slowly in
air after carburising and the hardening
process is carried out
subsequently after case hardening
tempering is generally required to
reduce the high residual stresses caused
by the large hardness differences
between the Surface and the core of the
component after case hardening the
surface of the component is not only
very hard and we resistant but the
residual compressive stresses generated
also increase the fatigue strength to a
particularly High degree this makes case
hardening particularly suitable for
dynamically stressed components such as
gears or drive
shafts a common feature of the surface
hardening processes presented so far is
that the hard surface is achieved by a
martensz iic transformation of the micro
structure however such a transformation
of the micro structure is a problem for
ensuring precise production with as
little post-processing as possible as
the transformation of the micr structure
generally leads to
Distortion the scale layers that form
can also make postprocessing
necessary in such cases nitriding can
provide a solution in which no micr
structural transformation takes
place during nit rting special steals
known as nitri in Steels which contain
alloying elements such as aluminum
chromium malanum vadium or titanium are
exposed to a nitrogen containing
atmosphere at temperatures of around
500°
C the nitrogen atoms diffuse into the
steel surface and combine with the
alloying elements to form hard and wear
resistant
nitrides while the surface hardness
increases significantly due to the
nitrides formed the properties of the
core of the component remain almost
unchanged as the nitrides only form on
the surface surface layer thicknesses
typically range from 0.1 to 1 mm thicker
nitride layers are only possible at
Great expense the long and Ealing times
sometimes several days can make
nitriding very timeconsuming and
therefore
expensive the nitrides formed on the
surface also create stresses in the
material however far from weakening the
material the residual compressive
stresses increase the fatigue strength
of the component the nitride layer ALS
Al improves corrosion
resistance I hope you enjoyed the video
and found it helpful thanks for
watching
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