Heat Treatment

1
Heat Treatment

• Metals and Welding

2
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.

3
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

4
Four Types

• Stress Relieving
• Normalizing
• Annealing
• Hardening and Tempering

5
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

6
Normalizing

• Promotes uniformity of the structure and alters
  mechanical properties.
• The steel is heated to a determined temperature
  above the critical range (1600-1700º 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

7
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

8
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 (1600-1700ºF)
• Cooled slowly at a predetermined rate.

9
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

10
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

11
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

12
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

13
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

14
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

15
(No Transcript)
16
Fundamental Metallurgy Terms

• Cementite - Iron carbide Fe3C chemical compound
  of iron and carbon

17
Fundamental Metallurgy Terms

• Ferrite - Pure iron
• Pearlite - Grain structure resulting from a
  mechanical combination of ferrite and
  cementite in layer formation.

18
Terms cont.

• 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.
• Click Here for more crystal structure information

19
(No Transcript)
20
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

21
Methods of Hardening Steel

• Quenching
• Brine severe, fast
• Cold water medium rate
• Warm Oil slow rate
• Tempering reheating and requenching at
  temperature desired
• Cold chisel allow heat from upper end to reheat
  lower portion

22
Surface Hardening

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

23
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)

24
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

25
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

26
Surface Hardening

• Pack or Box Carburizing
• Work is placed in a pack or box filled with a
  solid carburizing agent
• Heated to 1550 - 1750º F
• CO reacts with steel and dissolves into austenite
• Quench harden after heating, or let cool slowly
  and reheat and quench after working

27
Surface Hardening

• Liquid carburizing
• Molten salts containing cyanides and chlorides
• Heat to 1600-1750º 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

28
Heat Treating Definitions

• See Pages 64-65

29
Reading Assignment

• Oxy-Acetylene Welding and Cutting
• Oxy Acetylene Equipment - Pages 115-128