Materials Strengthening
Metallurgy Background 3
Lesson Objectives
When you finish this lesson you will
understand:
• Strengthening Mechanisms in materials
• Processes for restoring ductility in
strengthened materials
Learning Activities
1. View Slides;
2. Read Notes,
3. Listen to lecture
4. Do on-line
workbook
5. Do homework
Keywords:
Solid Solution Strengthening, Transformation Strengthening, Grain
Refinement, Deformation Strengthening, Precipitation
Strengthening, Quench, Temper, Recovery, recrystalization,
Solution heat treatment, aging, overaging
Materials Strengthening
• Solid Solution Strengthening
• Transformation Strengthening
– Grain Refinement
– Non-equilibrium Transformation Strengthening
• Deformation Strengthening
• Precipitation Strengthening
Materials Strengthening
• Solid Solution Strengthening
• Transformation Strengthening
– Grain Refinement
– Non-equilibrium Transformation Strengthening
• Deformation Strengthening
• Precipitation Strengthening
AWS Welding Handbook,
8th Ed Vol 1
Turn to the person sitting
next to you and discuss (1
min.):
• The BCC iron crystal
structure is represented here.
The “x” are the interstitial
positions where the carbon
atom might occupy. What
percent expansion is
necessary between the iron
atoms to fit this carbon?
•Phosphorous is about 40% larger than carbon. What do you
think would happen if phosphorous was alloyed with the BCC
iron?
Materials Strengthening
• Solid Solution Strengthening
• Transformation Strengthening
– Grain Refinement
– Non-equilibrium Transformation Strengthening
• Deformation Strengthening
• Precipitation Strengthening
Materials Strengthening
• Solid Solution Strengthening
• Transformation Strengthening
– Grain Refinement
– Non-equilibrium Transformation Strengthening
• Deformation Strengthening
• Precipitation Strengthening
Steel
Equilibrium Arrangement of Iron
Atoms in Steel
Ferrite
•
•
•
•
Austenite
Delta ferrite
Ferrite - room temperature to 1670°F
Austenite - 1333°F up to 2715°F
Delta ferrite - 2353°F up to 2795°F
Exact temperatures depend on % carbon in the iron
Austenite Transformation to
Ferrite-Pearlite
Steel
Time for iron atoms
to rearrange and
carbon atoms to
diffuse
Austenite
Ferrite + Pearlite
• Low carbon and alloy contents (AISI 1020,
ASTM A36)
• High heat input processes give slower cooling
rates
0.1.1.5.1.T7.95.12
Austenite Transformation to
Martensite
Weld cools too
fast to allow atom
diffusion and
rearrangement
Austenite
Martensite
• Increased carbon and alloy contents (AISI
4340)
• Low heat inputs, thick sections give fast
cooling rates
Steel
Martensite: Good or Bad?
GOOD
• High strength and hardness
compared to ferrite +
pearlite
– Many steels are designed to
be quenched to form
martensite then tempered to
improve their ductility and
toughness
• 4130
• 4340
Steel
BAD
• A ferrite + pearlite
base material might
have hard, brittle
martensite in the weld
joint
• Hard, brittle
martensite is a factor
involved in hydrogen
cracking
Heat
Treatment
Of 1040
Linnert, Welding Metallurgy
AWS, 1994
Maximum As Quenched Hardness
Tempering of Martensite
Turn to the person sitting next to you
and discuss (1 min.):
• Often a pre-heat is required by
codes to slow down the cooling rate
from welding. What might happen
to the as welded structure if a
preheat of 400F were specified?
600F?
Materials Strengthening
• Solid Solution Strengthening
• Transformation Strengthening
– Grain Refinement
– Non-equilibrium Transformation Strengthening
• Deformation Strengthening
• Precipitation Strengthening
Turn to the person sitting next to you and discuss (1 min.):
• When we weld a cold worked structure, the temperature in
the heat affected zone near the weld metal is very high but
as we move further and further from the weld, the peak
temperature gets less until it reaches room temperature at
some large distance. What might this temperature gradient
across the HAZ do to the cold worked structure?
Materials Strengthening
• Solid Solution Strengthening
• Transformation Strengthening
– Grain Refinement
– Non-equilibrium Transformation Strengthening
• Deformation Strengthening
• Precipitation Strengthening
Turn to the person sitting next to you and discuss (1 min.):
• Can precipitation hardening take place without first going
through a solutionizing heat cycle?