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Lecture 6:Strengthening of Plain Carbon Steels
MMat 380
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Contributions to strength in steels
Base material
Solid solution strengthening (i.e., %Mn)
Grain size (ferrite)
Precipitates (distance between ppts)
Cold work (dislocation density)
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Steels - Fe-C-Mn alloys
Mn added as ferromanganese Helps to de-sulpherize steel MnS
(Fe-S brittleness)
Powerful solid solution strengthener Powerful effect on heat treating med. %C
(0.3%C ) and high %C steels which are
usually Q&T
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1xxx - %Cin steel
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Strengthening low C steels(0-0.3%C)
2 major ways: increase carbon content
sacrifice % elongation; toughness because of Fe 3C
decrease grain size increase strength but doesnt affect ductility Hall-Petch Equation d = grain size (mm)
better strength without sacrificing ductility and
toughnesstrend: use fine grained steels and lower C content
2/1+= kd o y
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Examples
500Very fine12-13
300Fine8
250Coarse5
y (MPa)GrainASTMNo
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Grain size and strength
335 MPa 540 MPa
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ASTM grain size
Grain boundaries act as barriers todislocation motion
n=2 N-1 n = # grains/in 2 @ 100x magN = ASTM grain size No
Note: Table 2.1 pg. 74 Smith
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ASTM grain size
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Unit conversions
ASTM # 116 grains/mm 2 x (25.4) 2 = 10 323 gr/in 2
= 1.03 gr/in 2 @ 100x
ASTM # 82048 grains/mm 2x(25.4) 2 =1 321 287 gr/in 2
= 132 gr/in 2 @ 100x not 128
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ASTM grain size
1 3 6 8 10
Same magnification
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Effect of grain size on strength
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Applications
Heattreated0.6-0.91030
Structural0.3-0.61010-1025
Sheet0.25-0.41006
Product%MnGrade
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Low carbon steels (
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Low carbon steels (
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HSLA steels
Solid solution hardening (Mn) Decrease ferrite grain size by
Controlled rolling Controlled cooling
Precipitation hardening Nb (C,N) VC
Typical x70 pipeline steel%C = 0.06; %Mn = 1.50; %Nb and/or V ~0.04Controlled rolling to produce very fine grain size
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Strengthening HSLA steels
Obtain fine grain size(ASTM 10-13) by: Controlled rolling Controlled cooling
Can increase yieldstrength by 100-134MPa (i.e. 300-440 MPatotal)
670P
600Be
100Si
94Mn
80Ni
67Ti
40Al27Mo
20W
13.4V
13.4Co
6.7Cr
y increase of
per 1% additionElement
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Controlled rolling
Normal finish T in hot rolling: 900-1000 C Finish at temperatures: 750-800 C
Lower temperature R x & grain growth after hot rolling takes
longer end up with smaller g grain size\ smaller
grain size
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Controlled rolling
transformation of deformed gives finest grainsize
want to T of R x and make R x more difficult Nb in small amounts does this (~0.04%Nb added)
Need massive roll force to give required deformation Controlled rolled plate typically < 1 in thickness Thick plate usually has larger grain size
because it is finished at a higher T and has a larger grain
size
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Controlled rolling schematic
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Controlled cooling and grain size
Slow coolingHigher transformation
temperature
Fast cooling
Lower transformationtemperature
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Thick sections
Why do thick sections have lower yield stress?
When finish rolling at higher T (larger grain size) thick sections cool slower
raises T transf fewer nuclei grow to larger grain
size lower y
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Precipitation hardening
Microalloying Nb, V, Ti Nb (C,N) precipitate during hot rolling in
restricts grain growth
refines grain size retards R x & raises T transf Pancake grains
nuclei closer together therefore finer R x grain size
V VC precipitate on cooling after rolling
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Strength in HSLA steels
+ Standard C-Mn Steel 200-300 MPa+ Decrease grain size 100-134 MPa+ Increase Mn 67 MPa+ Increase Nb, V, Ti ppt hardening 67-100 MPa
Total: 434-600 MPa Can now afford to lower the C content and still
have 470-500 MPa steel
Can have any strength level wanted by varyingthe degree of strengthening components
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Effect of austenite grain size
Deformed
Small
Large
Ferrite grainsAustenite grains TRANSFORM TO
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