1. Heat Treatment of metals
Damian Keenan
Summerhill College
March 2013

2. Fundamental Metallurgy Terms
Cementite - Iron carbide Fe3C chemical compound of iron and carbon

4. Fundamental Metallurgy Terms
Ferrite - Pure iron
Cementite - Iron carbide Fe3C chemical compound of iron and carbon
Pearlite - Grain structure resulting from a mechanical combination of ferrite and cementite in layer formation.

5. Terms cont. Austenite will dissolve carbon and alloying elements.
Austenite - grains of ferrite and pearlite change when steel is heated to transformation temperature.
Austenite will dissolve carbon and alloying elements.
Martensite - Formed when carbon steel is rapidly cooled by quenching. Untempered martensite is the hardest and most brittle of the microstructures.

7. Heat Treatment
An operation, or series of operations, involving the heating and cooling of steel in the solid state to develop the required properties.
Related to the crystalline structure of carbon and iron.

8. Heat Treatment
Low carbon steels are generally used as rolled and in most cases do not respond well to heat treating
High carbon steels and alloys use heat treatment as the means of achieving the ultimate property capabilities on the metals

9. Four Types Stress Relieving Normalizing Annealing
Hardening and Tempering

10. Stress Relieving
Reduces internal stresses that may have been caused by machining, cold working or welding.
Heat the metal to a temperature below the critical range (1100ºF)
Hold until temperature is reached throughout the piece.
Allow to cool slowly

11. Normalizing
Promotes uniformity of the structure and alters mechanical properties.
The steel is heated to a determined temperature above the critical range ( º F)
Cooled to below that range in still air.
Molecular structure changes
Results in higher strength, hardness, and less ductility
Cools faster than stress relieving or annealing

12. Annealing May be used for the following: To soften steel
To develop a structure like lamellar pearlite or spheroidized carbide.
To improve machinability or facilitate cold shaping
To prepare the steel for additional heat treatment

13. Annealing cont. to reduce stress to improve or restore ductility
to modify other properties
Steel is heated to a point at or near the critical range ( ºF)
Cooled slowly at a predetermined rate.

14. Hardening and Tempering
Hardens the metal and tempering reheats to relieve internal stress.
Uses 3 operations:
Heating the steel above the critical range, so it approaches a uniform solid solution
Hardening the steel by quenching in oil, water, brine or fused salt bath
Tempering by reheating to a point below the critical range to get the proper combination of strength and ductility

15. Hardening and Tempering
Molecular structure changes to small grain Austenite
Quenching locks in hard structure
Reheating and tempering to relieve brittleness and make the steel tough

16. Fundamental Metallurgy
Metal structures determined by molecular shapes
Body centered cube
9 atoms: 8 at cube corners and 1 in the center
Can be worked cold

17. Fundamental Metallurgy
Metal structures determined by molecular shapes
Face centered cube
14 atoms: 8 at cube corners and 1 each on the six faces
Not plastic and cannot be worked cold

18. Basic Guide to Fundamental Metallurgy
Grain size is unchanged as temperature increases from ambient (room temperature) up to transformation range
At extremely low temperature, impact resistance is low
In the transformation range, grain size becomes small as temperature increases
Transformation (critical temperature) is the lowest at the 0.83% Carbon (Eutectoid steel) level

19. Basic Guide to Fundamental Metallurgy
Lower carbon levels have a higher critical temperature
Carbon steels are body centered cubic structures at room temperature and are a face centered cubic structure at the transformation temperature

21. Hardenability and Weldability are influenced by four factors
Carbon content – Weldable  .35% C Hardenable
Heating Cycle – maximum temperature
Cooling Cycle – minimum temperature
Speed of cooling

22. Methods of Hardening Steel
Quenching
Brine – severe, fast
Cold water – medium rate
Warm Oil – slow rate
Tempering – reheating and re-quenching at temperature desired
Cold chisel – allow heat from upper end to reheat lower portion

23. Surface Hardening Benefits Resists wear and deformation
Two zones result, avoiding brittleness
Surface hardness is increased without sacrificing desirable mechanical properties

24. Surface Hardening Flame Hardening
Heating steel to above the critical temperature by use of a flame
Followed by quenching
Can harden small areas (i.e., push rod ends)

25. Surface Hardening Induction Hardening
Heat is generated by electrical induction
Frequency between 1000 to 3,000,000 cycles per second used
Maximum hardness by these two processes is a function of the carbon content
Used on: gears (teeth), shafts, cams, crankshafts, cylinders, and levers

26. Surface Hardening Carburizing
Hardening the surface in the presence of a carbonaceous material as a gas, solid, or liquid
Surface is hardened and the core remains as original material
The depth of the case acquired is governed by temperature, time, activity of carburizing medium, and the analysis of the ferrous alloy used.
Since hardness increases with carbon content, increasing the carbon content of the surface of a low carbon steel (by diffusion) results in high hardness at the surface and toughness at the core

27. Surface Hardening Pack or Box Carburizing
Work is placed in a pack or box filled with a solid carburizing agent
Heated to º F
CO reacts with steel and dissolves into austenite
Quench harden after heating, or let cool slowly and reheat and quench after working

28. Surface Hardening Liquid carburizing
Molten salts containing cyanides and chlorides
Heat to º F and place work in cyanide salt solution
Length of time determines thickness of surface hardened
Quench in oil or brine after removing from salt solution
No moisture can be on metal when placed in salt solution or an explosion could result