Title: 530.352 Materials Selection
1530.352 Materials Selection
Lecture 21 Steels - part IIWednesday
November 2nd, 2005
2Transformations
- Equilibrium transformations (slow cooling)
? -gt ? ferrite ? -gt Fe3C iron
carbide ?? -gt ? Fe3C pearlite - Nonequlibrium (fast cooling / quenching) ?
-gt ? martensite
3Pearlite
4Normalized (slow cooled) steelsmechanical
properties
2,500
50
Strength (MPa)
Ductility ()
?f
UTS
500
YS
0
0 1
w/o Carbon
5Martensite
C
C
Fe
6Martensite mechanical prop.
- very hard and
- very brittle (too brittle !!!)
7Quench and Temper
- Temper heat treat at intermediate T (300-600
C) - Mechanical properties
- regains toughness (critical) with only a moderate
drop in hardness - Microstructurally
- C comes out of lattice a precipitates as Fe3C,
and distortion decreases with decrease amounts
of dissolved C. - Loss of distortion leads to bcc structure and
ductility. - Fe3C precipitates - precipitation strengthen the
?.
8Quenched and tempered
2,500
50
UTS
?f
YS
Strength (MPa)
Ductility ()
?f
UTS
normalized tempered
500
YS
0
0 1
w/o Carbon
9Heat treatments and cooling
- Normalizing (heating to form ?)
- T 1,000 C
- much easier to roll / forge / form at this
temperature - Quench - or - Slow cooling
- martensite pearlite
- Tempering (heating to soften martensite)
- carbides form, distortions relax
10Quench rates
- To form martensite in pure Fe
- Critical Cooling Rate (CCR) 100,000 C/sec
- To form martensite in mild steel Fe - 0.8 C
- CCR 200 C/sec
- To form martensite in alloy steels Fe -
0.2-.6C 2-7 (Mo, Mn, Cr, Ni) - CCR lt 1 C/sec
11Alloying elements are added to
- improve hardenability of the steel
- aides nucleation of martensite
- solution strengthen and precipitation hardening
- MxCy carbides form
- give corrosion resistance
- especially Cr which forms Cr203
- stabilize FCC austenite at RT
- especially Ni
- tougher, more ductile and easier to form
- non-magnetic and creep resistant (diffusion is
slower in FCC)
12Alloying of steels
Type of steel Fe ... Typical
uses Low-alloy .2C pressure vessels,
aircraft .8Mn,1Cr,2Ni parts, high ?
applications. High-alloy .1 C High T and
anti-corrosion, Stainless-steels .5Mn,18Cr,8Ni s
ilverware, medical, etc.
13Steel terminology (SAE-AISI)
- Plain carbon
- 10xx
- Manganese steels
- 13xx
- Nickel steels
- 23xx
- Ni-Cr-Mo steels
- 43xx
- HSLA
- 9xx
Note xx indicates carbon content in hundredths
of a percent
14Other steel standards
- SAE-AISI
- Society of Automotive Engineers American Iron
and Steel Institute - ASTM
- American Standards for Testing and Materials
- AMS
- Aerospace Materials Specifications
- DIN
- Deutsches Institut fur Normung
- JIS
- Japanese Industrial Standards Committee
- UNS
- Unified Numbering System