Surface hardening of steel | flame, induction, laser beam, case hardening and nitriding

Summary

This 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

00:00

🔥 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.

05:00

🌀 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.

10:02

🛠 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.

Keywords

💡Surface Hardening

Surface hardening is a heat treatment process that increases the surface hardness of a material while maintaining the toughness of the core. This is crucial for components like gear wheels that experience wear during operation. The video discusses various methods of surface hardening, emphasizing the importance of this process in improving wear resistance without compromising the overall durability of the component.

💡Wear Resistance

Wear resistance refers to the ability of a material to resist surface damage due to friction or contact with other surfaces. In the context of the video, surface hardening is used to increase wear resistance, which is particularly important for components that experience continuous sliding or contact, such as tooth flanks of gear wheels.

💡Toughness

Toughness is the material's ability to absorb energy and plastically deform without fracturing. The video highlights the need to maintain the core's toughness in components after surface hardening, as this property is essential for preventing material failure under stress.

💡Flame Hardening

Flame hardening is a surface hardening technique where a torch flame is used to heat the surface of the material, followed by rapid cooling to create a hard surface layer. The video explains how this method works by transforming the steel's microstructure and creating a martensitic structure that is hard and wear-resistant.

💡Induction Hardening

Induction hardening uses electromagnetic induction to heat and harden the surface of a workpiece. The video describes how this method is more precise and efficient than flame hardening, as it allows for better control over the hardening depth and reduces the risk of distortion.

💡Laser Hardening

Laser hardening is a process that uses a high-power laser beam to heat the surface of a material to a temperature just below its melting point, followed by rapid cooling. The video mentions that this method offers even shorter heating times than induction hardening, which minimizes distortion and scaling.

💡Case Hardening

Case hardening is a process used for low carbon steels where the surface is enriched with carbon to a level that allows for hardening. The video explains that this is achieved by exposing the steel to a carbon-containing environment, leading to a hard and wear-resistant surface layer while the core remains tough.

💡Nitriding

Nitriding is a surface hardening process where nitrogen is diffused into the surface of a material to form hard and wear-resistant nitrides. The video points out that unlike other hardening processes, nitriding does not involve a microstructural transformation, which can be beneficial for maintaining dimensional stability.

💡Martensite

Martensite is a hard and brittle microstructure formed in steel when it is rapidly cooled from austenite. The video discusses how the formation of martensite is a key factor in the increased hardness and wear resistance achieved through various surface hardening processes.

💡Residual Stresses

Residual stresses are the internal stresses that remain in a material after the external forces have been removed. The video explains how certain hardening processes, like case hardening, can result in residual compressive stresses at the surface, which can improve the component's fatigue strength.

💡Distortion

Distortion refers to the change in shape or dimension of a material due to heat treatment processes. The video emphasizes the importance of minimizing distortion during surface hardening to maintain the precision and quality of the components.

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