Laser Hardening

Question 1

What are the two types of laser hardening? And what lasers are used for these?

Answer 1

Transformation hardening-CW lasers and shock hardening- Pulsed lasers

Question 2

Why do lasers compete successfully?

Answer 2

Due primarily to reduced distortion and high productivity. Reflectivity problems are overcome with surface coatings.

Question 3

What are the advantages of laser heat treatment?

Answer 3

Minimum part distortion
Selective hardening
No quenching required
Thin case capability
Case depth controllable
Eliminates post processing
Improved fatigue life

Question 4

What are the disadvantages of laser heat treatment?

Answer 4

High equipment cost
Coverage area is restricted
Absorbant coating helpful
Multiple passes lead to
inconsistent
hardness/tempering

Question 5

What are the advantages of heat treatment electron beam processes?

Answer 5

Minimum part distortion
Selective hardening
No quenching required
Thin case capability
Case depth controllable
Eliminates post processing
Improved fatigue life

Question 6

What are the disadvantages of heat treatment electron beam processes?

Answer 6

High equipment cost
Requires a vacuum
Low production rate
Very high processing costs

Question 7

What are the advantages of heat treatment induction processes?

Answer 7

Fast process rates
Deep case obtainable
Lower capital cost than laser
Coverage area

Question 8

What are the disadvantages of heat treatment induction processes?

Answer 8

Down time for coil change
Requires quenchant
Part distortion
Coil placement critical
Large thermal penetration
Surface degradation
Complex coils for 3-D
geometry

Question 9

What are the advantages of heat treatment flame processes?

Answer 9

Cheap, flexible and mobile

Question 10

What are the disadvantages of heat treatment flame processes?

Answer 10

Poor reproducibility
Lacks rapid quench
Component distortion likely
Environmental problems

Question 11

What are the advantages of heat treatment Arc(TIG) processes?

Answer 11

Relatively cheap, flexible
and mobile

Question 12

What are the disadvantages of heat treatment Arc(TIG) processes?

Answer 12

Section thickness limited
Large thermal penetration
Stirring takes place
Poor control to avoid
melting

Question 13

What are the 3 main thermochemical diffusion processes?

Answer 13

Carburising, nitriding and carbon nitriding

Question 14

What is transformation hardening?

Answer 14

Heat flow = power distribution.
The ideal power distribution is one which gives a uniform temperature over the area to be treated.
Requires complex power distribution since the heating effect is dependent on the surface heating and cooling at the edges.
Simply focused laser beam(high/low order) cannot obtain the ideal power distribution.

Question 15

What is the simplest hardening method to model?

Answer 15

Transformation model because there is no surface melting and therefore no unknown convection of latent heat aspects since there is no meltpool and surface heat losses follow the
conventional rules of convection and radiation.

Question 16

What is case depth?

Answer 16

The depth where metallurgical transformation has occurred.

Question 17

What is the benefit of the ashby and shercliffe model than the typical hardness relationship?

Answer 17

Dimensionless so it can be used for any material and laser. Theory is based on simplified heat conduction problem along the surface of the substrate. Graphs are quick and easy way to predict case hardening.

Question 18

What is the shape of hardening from gaussian laser intensity distribution?

Answer 18

Parabolic shaped hardening profile. Raster scanning of beam is used to harden larger areas.

Question 19

What are the 5 methods available for generating an ideal beam profile for laser transformation hardening?

Answer 19

1. Defocused high order mode beams;
2. Scanning patterns;
3. Kaleidoscopes;
4. Beam integrators; and
5. Diffractive optical elements.

Question 20

What is the main problem with varying the beam profile for hardening?

Answer 20

Variable hardness

Question 21

What is diffractive manipulation? and what are the advantages?

Answer 21

Reconstruct the beam into any custom shape and intensity.
Advantages:
 Reconstruction efficiency of >95%
 No moving parts
 Shape, Intensity and focus from a
single element
 Large depth of field
 Rapid switching of irradiance
distributions - a machine tool
 Highly customized intensity profiles
 In this case providing uniform
temperatures
 Even cooling rates

Question 22

What is a specific distribution?

Answer 22

Specific intensities for a uniform track. A diffractive pattern can be developed to produce the necessary
energy distribution
 This can then be made into a laser optic

Question 23

What patterns can be taken into account to make a real application of hardening application?

Answer 23

Because a kinoform diffraction
pattern can take into account raw
beam mode, angle of illumination
and process material
characteristics, hardness track like
those shown can be produced
 With constant iterations a diffractive
profile can be developed that can
harden the whole of the desired
region as desired without moving
the optic or beam

Question 24

What are the 8 steps for transformation mechanism of iron carbon?

Answer 24

1.On high heating, pearlite colonies first transform to austenite - such rapid heating is possible with lasers.
2. Carbon then diffuses outwards from these transformed zones into the surrounding ferrite(BCC).
3. The volume of high-carbon austenite(FCC) is thus increased.
4.On rapid cooling, there is not enough time for the carbon to diffuse out of these carbon-rich regions of austenite when it changes back to ferrite and pearlite.
5. The fast quenching results in a BCC structure with carbon left behind.
6.This stresses the lattice and causes it to shear - forming a non-equilibrium (metastable) structure called martensite.
7.Other structures also form, that are closer to the equilibrium ferrite/pearlite mix; e.g., bainite.
8.The non-equilibrium phases of a material can be charted using continuous cooling curves (CCC).

Question 25

What does a CCC indicate for phases?

Answer 25

1. A CCC(continuous cooling curves_ indicates the phases present if you plot the sample
   temperature against time.
2. Each of the regions represents the start and finish of
   formation of a specific phase.
   3.If the cooling line crosses that region, some of that phase
   will be present in the final microstructure.

Question 26

Why are some steels not suitable for hardening by traditional methods?

Answer 26

As the cooling rates required to form
microstructures containing large amounts of martensite are
too high for ordinary quenching.

Question 27

What is shock hardening?

Answer 27

Shot Peening (stream of metal shots). When impinging with the metal surface the shot produces craters which create compressive stresses in this surface. This in turn increases the fatigue strength of the article.
The depth of the treatment is generally of the order of 1 mm maximum and 0.25 mm average.
The magnitude of the compressive strength can be as high as 60% UTS.

Question 28

What are advantages of laser shock hardening ?

Answer 28

Less surface roughening;
 No embedded particles;
 Hardening into corners is possible;
 No recycling of material (shot);
 No wear on equipment; and
 Radioactive materials can be treated (shot not
contaminated).