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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
In the Name of Allah
Metallurgy in Production
1
Chapter7-Lecture 12
First semester 95-96
Mohammad Ali Mirzai
University of Hormozgan
Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Chapter 7 - Part 3: Heat Treatment of Steels -
Surface Hardeninig
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Chapter7-Lecture 12
Metallurgy in Production University of Hormozgan, Mirzai, 95-96
HEAT TREATMENT
BULK SURFACE
ANNEALING
Full Annealing
Recrystallization Annealing
Stress Relief Annealing
Spheroidization Annealing
AUSTEMPERING
THERMAL THERMO-
CHEMICAL
Flame
Induction
LASER
Electron Beam
Carburizing
Nitriding
Carbo-nitriding
NORMALIZING HARDENING
&
TEMPERING
MARTEMPERING
An overview of important heat treatments
A broad classification of heat treatments possible are given below. Many more
specialized treatments or combinations of these are possible.
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Surface Hardeninig
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Surface Hardeninig
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Selective Hardeninig - Flame Hardeninig
Heat is applied to the part being hardened, using an oxy-
acetylene (or similar gas) flame on the surface of the
steel being hardened and heating the surface above the upper critical
temperature before
quenching the steel
in a spray of water.
The result is a hard
surface layer
ranging from 0.050"
to 0.250" deep.
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Selective Hardeninig - Flame Hardeninig
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Selective Hardeninig - Induction Hardeninig
Induction hardening is a process used for the
surface hardening of steel and other alloy components.
The parts to be heat treated are
placed inside a water cooled
copper coil and then heated
above their transformation
temperature by applying an
alternating current to the coil.
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Selective Hardeninig - Laser Hardeninig
Power lasers offer rapid, high-quality and reproducible heat
treatment methods. Laser surface hardening consists of the
rapid heating of a material's surface bylaser beam, a short
hold at the target temperature, and intensive cooling due to
the high thermal conductivity of the material.
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Selective Hardeninig-Electron beam Hardeninig
Electron beam hardening (EBH)
is one of the most modern surface-
hardening technologies, with some
special characteristics in
comparison to other heat treatment
technologies. Electron beam
hardening is similar to other surface
heat treatment processes in that
the material to be hardened is
heated to the austenitization
temperature, held at that
temperature for a short period, and
then quenched.
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig - Carburization
Carburization is a diffusion-controlled process, so the longer
the steel is held in the carbon-rich environment the greater the
carbon penetration will be and the higher the carbon content.
The carburized section will have a carbon content high
enough that it can be hardened again through flame or
induction hardening.
The carbon can come from a solid, liquid or gaseous
source; if it comes from a solid source the process is
called pack carburizing. A heating period of a few hours
might form a high-carbon layer about one millimeter thick.
Liquid carburizing involves placing parts in a bath of a molten
carbon-containing material, often a metal cyanide; gas
carburizing involves placing the parts in a furnace maintained
with a methane-rich interior.
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Pack Carburising
Packing work in the heat resisting steel boxes with 50
mm gap with carburising material.
Heated slowly to 850 – 9250 C, maintained for 8 hrs
according to depth needed.
Temperature
Ca
rbu
ris
ing
Tim
e
Depth of case
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Pack Carburising
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
• Charcoal
With Barium carbonate (10 to 15%).
Process depends on presence of CO
2C + O 2 2 CO
At surface, releases C atoms
2CO CO2 + C
C dissolved interstitially at surface of
steel.
Ba CO3 Ba O + CO2
CO2 + C 2CO
Pack Carburising
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig – Liquid Carburization
Mixture of salts of Sodium Cyanide, Sodium carbonate,
Sodium/barium Chloride
Melted in pots to 870- 950 C, work immersed for 5 min to 1
hour
Then basket quenched- hard and clean surface
For shallow- 0.1 to 0.25 ,mm;
Usually for small parts
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig – Gas Carburization
In batch type or continuous
furnaces.
Far widely used
Clean compact plant
Heated to 900 C for 3 to 4
hours
Hydrocarbons methane and propane partly burnt in furnace,
diluted with carrier gas to get required carbon POTENTIAL ( ie
carbon content maintained in equilibrium in the surface film- of
0.8% desirable)
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig – Plasma Carburization
Plasma carburization is increasingly used to improve the
surface characteristics (such as wear, corrosion resistance,
hardness and so on).
It also provides an even
treatment of components
with complex geometry
(the plasma can
penetrate into holes and
tight gaps), making it
very flexible in terms of
component treatment.
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig - Nitriding
Nitriding is a heat treating process that diffuses nitrogen
into the surface of a metal to create a case-hardened surface.
These processes are most commonly used on low-carbon,
low-alloy steels. They are also used on medium and high-
carbon steels, titanium, aluminium and molybdenum.
Nitriding heats the steel part to 482–621 °C in in the nitriding
atmosphere such as ammonia gas.
Typical applications include gears, crankshafts, camshafts,
cam followers, valve parts, extruder screws, die-casting tools,
forging dies, extrusion dies, firearm components, injectors and
plastic-mold tools.
The four main methods used are: gas nitriding, salt bath
nitriding, solid nitriding and plasma nitriding.
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig - Nitriding
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig - Nitriding
The time the part spends in this environment dictates the
depth of the case. The hardness is achieved by the formation
of nitrides. Nitride forming elements must be present for this
method to work; these elements include Cr, Mo, and Al.
The advantage of this process is that it causes little
distortion, so the part can be case-hardened after being
quenched, tempered and machined. No quenching is done
after nitriding.
•Obtain High Surface Hardness
•Increase Wear Resistance
•Increase Tensile Strength and Yield Point
•Improve Fatigue Life
•Improve Corrosion Resistance (Except for Stainless Steels)
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig – Gas Nitriding
In gas nitriding the donor is a nitrogen rich gas, usually
ammonia (NH3). When ammonia comes into contact with the
heated work piece it dissociates into nitrogen and hydrogen.
The nitrogen then diffuses onto the surface of the material
creating a nitride layer.
The advantages :
- Precise control of chemical potential of nitrogen.
- Large batch sizes possible
- With modern computer control of the atmosphere
- Relatively low equipment cost
The disadvantages:
- Reaction kinetics heavily influenced by surface condition
- Surface activation is sometimes required to treat steels with
a high chromium content
- Ammonia as nitriding medium Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig – Bath Nitriding
In salt bath nitriding the nitrogen donating medium is a
nitrogen-containing salt such as cyanide salt.
The advantages of salt nitriding is that it achieves higher
diffusion in the same period time compared to any other
method.
The advantages :
- Quick processing time - usually in the order of 4 hours
- Simple operation - heat the salt and submerge workpieces
disadvantages:
- The salts used are highly toxic
- Only one process possible with a particular salt type - since
the nitrogen potential is set by the salt, only one type of
process is possible
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig – Plasma Nitriding
Also known as ion nitriding, plasma ion nitriding or glow-
discharge nitriding.
In plasma nitriding, the reactivity of the nitriding media is not
due to the temperature but to the gas ionized state (electric
fields). Such highly active gas with ionized molecules is called
plasma. The gas used for plasma nitriding is usually pure
nitrogen. There are hot plasmas typified by plasma jets used
for metal cutting, welding, cladding or spraying. There are also
cold plasmas, usually generated inside vacuum chambers, at
low pressure regimes.
Advantages:
- the close control of the nitrided microstructure
- Increase the fatigue strength
- resistance to wear
- the surface hardness of tool steels can be doubled. Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig – Plasma Nitriding
A plasma nitrided part is usually ready for use. It calls for no
machining, or polishing or any other post-nitriding operations.
Thus the process is user-friendly, saves energy since it works
fastest, and causes little or no distortion.
Plasma nitriding is often
coupled with physical
vapor deposition (PVD)
process and labeled
Duplex Treatment, with
enhanced benefits.
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig – Solid Nitriding
The solid nitriding consists in provide the diffusion of nitrogen
into the steel using a solid materials such as cyanide salt or
solid granulated (Fe4KCN). The samples are surrounded by
this compound and confined inside a metalic (Exp. Aluminum)
bowl. This pack is introduced in a muffle oven and the
temperature is set to 560 °C.
solid nitriding is a technique not well diffused of treatment, in
comparison with the processes of gas and plasma nitriding.
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig –Carbonitriding
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig –Carbonitriding
Carbonitriding is similar to gas carburization with the addition
of ammonia to the carburizing atmosphere, which provides a
source of nitrogen. Nitrogen is absorbed at the surface and
diffuses into the workpiece along with carbon. Carbonitriding
(around 850 °C) is carried out at temperatures substantially
higher than plain nitriding (around 530 °C) but slightly lower
than those used for carburizing (around 950 °C) and for
shorter times. Carbonitriding tends to be more economical
than carburizing, and also reduces distortion during
quenching. The lower temperature allows oil quenching, or
even gas quenching with a protective atmosphere.
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig –Nitrocarburizing
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig –Nitrocarburizing
Nitrocarburizing is a shallow case variation of the nitriding
process. Advantages of the process include the ability to
harden materials which are not prehardened, the relatively
low temperature of the process which minimizes distortion,
and relative low cost in comparison to carburizing or other
case hardening processes. This process is done mainly to
provide an anti-wear resistance on surface layer and to
improve fatigue resistance.
An additional advantage is that nitrocarburizing can be applied
to the same materials as is nitriding, as well as unalloyed
materials, where good wear resistance and some improved
fatigue resistance are needed at a low cost. It is widely used
for stampings, as an alternative to hard plating or even
carbonitriding.
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig –comparison
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
Diffusion Hardeninig –comparison
Chapter7-Lecture 12
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Metallurgy in Production University of Hormozgan, Mirzai, 95-96
End
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Chapter7-Lecture 12
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