1. HEAT TREATMENT OF METALS
SME Video – Additional Processes
Heat Treating (vts_04)
Annealing
Martensite Formation in Steel
Surface Hardening
Heat Treatment Methods and Facilities
In-class Assignment
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

2. Heat Treatment Various heating and cooling processes
Structural changes in a material
Mechanical properties
Metals
Glass‑ceramics
Tempered glass
Powder metals and ceramics
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

3. Annealing
Heating and soaking metal at suitable temperature for a certain time, and slowly cooling
Reasons for annealing:
Reduce hardness and brittleness
Soften metals
Recrystallize cold worked metals
Relieve residual stresses
Full annealing
Normalizing
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

4. Martensite Formation in Steel
Iron‑carbon phase diagram
Assumes cooling from high temperature is slow enough to permit austenite to transform into ferrite and cementite (Fe3C) mixture
Equilibrium is prevented under rapid cooling
Austenite transforms into a nonequilibrium phase called martensite
Martensite is hard and brittle
Austenite - face‑centered cubic (FCC)
Martensite - body‑centered tetragonal (BCT)
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

5. Iron-Carbon TTT curve Cooling trajectory (0.80% C)
Iron‑carbon phase diagram
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

6. Hardness of Plain Carbon Steel
Tempering to relieve stress in Martensite structure
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

7. Quenching Media and Cooling Rate
Brine -salt water
Agitated brine (fastest cooling rate)
Still fresh water
Still oil
Air (slowest cooling rate)
Fast cooling rate
Internal stresses
Distortion
Cracks
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

8. Tempering of Martensite
Reduce brittleness, increase toughness, and relieve stresses
Treatment involves heating and soaking at a temperature below the eutectoid for one hour
Slow cooling
Precipitation of very fine carbide particles
Gradually transforming BCT to BCC
BCT body centered tetragonal
Tempered martensite
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

9. Hardenability Capacity of a steel to be hardened
Capacity of transformation to martensite
Depth of hardening below quenched surface
Steels with good hardenability
Hardened more deeply below the surface
Do not require high cooling rates
Hardenability does not refer to the maximum hardness that can be attained
Hardenability of steel increased by alloying
TTC curve is moved to the right
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

10. Jominy End-Quench Test for Hardenability
(a) setup, showing end quench of the test specimen
(b) hardness readings as a function of distance from quenched end
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

11. Surface Hardening Thermochemical treatments applied to steels
Case hardening
Carburizing, nitriding, and carbonitriding
Applied to low carbon steel parts
Hard, wear‑resistant outer shell
Tough inner core
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

12. Chromizing Higher temperatures Longer treatment times
Applied to low carbon steels
Casing is hard and wear resistant
Heat and corrosion resistant !!!
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

13. Furnaces for Heat Treatment
Fuel‑fired furnaces
Normally direct‑fired - the work is exposed directly to combustion products
Fuels: natural gas or propane and fuel oils that can be atomized
Electric furnaces
Electric resistance for heating
Cleaner, quieter, and more uniform heating
More expensive to purchase and operate
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

14. Batch vs. Continuous Furnaces
Batch furnaces
Heating system in an insulated chamber, with a door for loading and unloading
Production in batches
Continuous furnaces
Generally for higher production rates
Mechanisms for transporting work through furnace include rotating hearths and straight‑through conveyors
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

15. Selective Surface Hardening
Typical induction heating setup
High frequency alternating current
Quenching follows
Quenched by surrounding metal
HF resistance heating
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

16. Selective Surface Hardening Methods
These methods heat only the work surface, or local areas of the work surface
They differ from surface hardening methods in that no chemical changes occur
Methods include:
Flame hardening
Induction hardening
High‑frequency resistance heating
Electron beam heating
Laser beam heating
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

17. SME Video
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

18. In-class Assignment
In a surface grinding operation performed on hardened plain carbon steel, the grinding wheel has a diameter = 200 mm and width = 25 mm. The wheel rotates at 2400 rev/min, with a depth of cut (infeed) = 0.05 mm/pass and a cross‑feed = 3.50 mm. The reciprocating speed of the work is 6 m/min, and the operation is performed dry.
Determine (a) the length of contact between the wheel and the work, (b) the volume rate of metal removed. (c) If there are 64 active grits/cm2 of wheel surface, estimate the number of chips formed per unit time. (d) What is the average volume per chip? (e) If the tangential cutting force on the work = 30 N, compute the specific energy in this operation?
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

19. Additive Manufacturing
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

20. Time Permitting Content
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

21. Precipitation Hardening
(a) phase diagram of an alloy system consisting of metals A and B
(b) heat treatment:
(1) solution treatment (2) quenching (3) precipitation treatment
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

22. Precipitation Hardening
Temperature and time during precipitation treatment (aging)
(a) High precipitation temperature
(b) Lower precipitation temperature
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

23. Other Furnace Types Atmospheric control furnaces
Desirable in conventional heat treatment to avoid excessive oxidation or decarburization
Include C and/or N rich environments for diffusion into work surface
Vacuum furnaces
Radiant energy is used to heat the parts
Disadvantage: time needed each cycle to draw vacuum
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e