ALEXANDRIA
PRODUCTION ENGINEERING
UNIVERSITY
DEPARTMENT
Lecture 11:
HEAT TREATMENT
Dr. Mohamed Abd elmonem Daha
Assistant Professor, Prod. Eng. Department
Faculty of Engineering
Alexandria University 1
Heat Treatment
Principles f Heat Treatment
Heat Treatment may be defined as a sequence of heating and cooling designed
to get the desired combination of properties in the steel.
The changes in the properties of steel after heat treatment are due to the phase
transformations and structural changes that occur during the heat treatment.
The factors which determine and control theses structural changes are called
the principles of heat treatment.
The important principle of heat treatment are as follows:
Phase transformations during heating.
Effect of cooling rate on structural changes during cooling.
Effect of carbon content and alloying elements
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Heat Treatment
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Heat Treatment
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Heat Treatment
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Heat Treatment
ANNEALING
It is a softening process in which iron base alloys are heated above the transformation range
held there for proper time and then cool slowly below the transformation range in the furnace
itself.
Objectives of Annealing
The purpose of annealing is to achieve the following
1. Soften the steel.
2. Relieve internal stresses
3. Reduce structural in-homogeneity.
4. Refine grain size.
5. Improve machinability.
6. Increase or restore ductility and toughness.
Annealing is of two types
(a) Process annealing
(b) Full annealing.
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Heat Treatment
Process annealing
Used to negate the effects of cold
work to soften and increase the
ductility with somewhat decrease
in internal stresses of a previously
strain-hardened steel.
In this, metal is heated to
temperature some below or close
to the lower critical temperature
generally it is heated 550°C to
650°C holding at this temperature
and it is slowly cooled. This causes
completely recrystallisation in steel.
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Heat Treatment
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Heat Treatment
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Heat Treatment
Objectives
1. To reduce tensile strength
2. To increase ductility
3. To ease machining
4. To impart structure for subsequent hardening process 10
Heat Treatment
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Heat Treatment
TEMPERING
If high carbon steel is quenched for hardening in a bath, it becomes extra hard, extra brittle
and has unequal distribution internal stresses and strain and hence unequal harness and
toughness in structure. These extra hardness, brittleness and unwanted induced stress and
strain in hardened metal reduce the usability the metal.
Therefore, these undesired needs must be reduced for by reheating and cooling at constant
bath temperature.
In tempering, steel after hardening, is reheated to a temperature below the lower critical
temperature and then followed by a desired rate of cooling. Reheating the of hardened steel
is done above critical temperature when the structure is purely of austenite and then
quenching it in a molten salt path having temperature in the range of 150-500°C. This is done
to avoid transformation to ferrite and pearlite and is held quenching temperature for a time
sufficient to give complete formation to an intermediate structure referred to as bainite then
cooled to room temperature. The temperature should not be held less than 4 to 5 minutes for
each millimeters of the section.
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Surface Hardening of Steels
Surface Hardening (Case Hardening)
Production of parts that have hard, wear resistance surfaces, but with softer and or tougher cores.
Why Surface Hardening?
- To improve wear resistance
- To improve resistance to high contact stresses
- To improve fracture toughness
- To improve fatigue resistance, and, sometimes,
- To improve corrosion resistance
Components usually surface-hardened
- gears - bearings - valves - shafts - bearing races
- cams - hand tools - rolls - machine tools - sprockets
Methods
Diffusional: carburizing, nitriding, carbonitriding, nitrocarburizing, boronizing, chromizing, ...
Selective Hardening: Flame hardening, induction hardening, laser and electron beam hardening
Case Depth
CHD (case hardened depth) and defined as the depth from the surface to the point where the
hardness is 550HV, as shown in the Figure. Sometimes a hardness other than 550HV is used to
define the case depth.
Carburizing
A process of adding carbon to the surface of steels. This is done by exposing the part to
a carbon-rich atmosphere at an elevated temperature and allowing diffusion to
transfer the carbon atoms into steel.
Note: Diffusion will work only if the steel has low carbon content, because diffusion depends on
concentration gradient at a given temperature. If, for example, high-carbon steel had is heated in a
carbon-free furnace, such as air, the carbon will tend to diffuse out of the steel resulting in
decarburization.
Types of Carburizing:
- Pack carburising - Vacuum carburizing
-Gas carburizing - Plasma carburizing
Carbon content achieved: 0.7 to 1.2 wt.%
Suitable for: Low-carbon steels and alloy steels
containing 0.08 to 0.2 wt.%C.
Carburizing temperature: 850-950 °C (1550-1750°F)
Carburizing Time: 4 to 72 h.
Carburizing
Surface hardness achieved: 55 to 65 HRC
Case Depth: No technical limit. In practice, 0.5 to 1.5 mm
Applications: Gears, cams, shafts, bearings, piston rings, clutch plates, sprockets
Quenching:
After carburizing, the part is either slow cooled for later quench hardening, or quenched directly
into various quenchants. The part is then tempered to the desired hardness.
Nitriding
A process of diffusing nitrogen into the surface of steel. The nitrogen forms
nitrides with elements such as aluminum, chromium, molybdenum, and
vanadium. The parts are heattreated and tempered before nitriding.
Suitable for: Low carbon alloy steels containing Al, Cr, Mo, V, Ni
Nitriding temperature: 500 to 600 °C (subcritical, below A1).
Surface hardness achieved: up to 1000 VHN Case Depth: 0.1 to 0. 6 mm
Applications: Gears, valves, cutters, sprockets, pump boring tools, fuel-injection pump
parts.
Methods:
•Gas
•Liquid
•Plasma
•Bright
•Pack
Selective Hardening Methods
W hen is selective hardening necessary?
Selective hardening is applied because of one or more of the following reasons:
(1) Parts to be heat-treated are so large that conventional furnace heating and quenching become
impractical and uneconomical - examples are large gears, large rolls and dies;
(2) Only a small segment, section, or area of the part needs to be heat-treated-typical examples
are ends of valve stems and push rods, and the wearing surfaces of cams and levers;
(3) Better dimensional accuracy of a heat-treated part; and
(4) Overall cost savings by giving inexpensive steels the wear properties of alloyed steels.
Flame Hardening
Flame hardening is the process of selective hardening with a combustible gas flame
as the source of heat for austenitizing. Water quenching is applied as soon as the
transformation temperature is reached. The heating media can be oxygen acetylene,
propane, or any other combination of fuel gases that will allow reasonable heating
rates. For best results, the hardness depth is 3/16 inch.
Methods:
(1) Spot Flame Hardening: Flame is directed to the spot that needs to be heated and hardened.
(2) Spin Flame Hardening: The workpiece is rotated while in contact with the flame
(3) Progressive Flame Hardening: The torch and the quenching medium move across the surface
of the workpiece.
Suitable for: At least medium-carbon steels containing ≥ 0.40 wt.%C, cast irons
Surface Hardness Achieved: 50 to 60 HRC
Case Depth: 0.7 to 6 mm
Typical Applications: Lathe beds and centers, crankshafts, piston rods, gear and sprocket
teeth, axles, cams, shear blades
Flame Hardening
Induction Hardening
Here, the steel part is placed inside copper induction
coils and heated by high-frequency alternating
current and then quenched. Depending on the
frequency and amperage, the rate of heating as well
as the depth of heating can be controlled.
Suitable for: Medium carbon steels (wt.% C ≥
0.4), cast irons
Hardening tem perature:
The induced current i within the steel then produces
heat according to the relationship: Heat = I2R,
where R is the electrical resistance of the steel.
Surface hardness achieved: 50 to 60 HRC Case
Depth: 0.7 to 6 mm
Typical Applications: see flame hardening
Induction Hardening
Induction Hardening
Case Study: Complex Surface Hardening
Material: Carbon concave
Steel, AISI 1070
convex
Automotive parts from
Delphi Inc., Sandusky,Ohio
•Concave and convex on surface
of workpiece make the heating Real spindle to be hardened
process not easy to control. Geometry Model
•FEM is employed for the analysis.
•Mesh should be much finer at
locations of convex and concave
in both coil and workpiece.
FEA model and B.C. Mesh generated by ANSYS
