Metalurgia Fisica

Lecture 6: Strengthening of Plain Carbon Steels
MMat 380

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 tode-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

31xxx - %C in steel

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Strengthening low C steels (0-0.3%C)
2 major ways: • increase carbon content
– sacrifice % elongation; toughness because of Fe3C

•decrease grain size

– increase strength but doesn’t affect ductility −1 / 2 – Hall-Petch Equation σ y = σ o + kd d = grain size (mm) ∴ better strength without sacrificingductility and toughness trend: use fine grained steels and lower C content
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Examples
ASTM No 5 8 12-13 Grain Coarse Fine Very fine

σy (MPa)
250 300 500

6Grain size and strength

335 MPa

540 MPa
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ASTM grain size
Grain boundaries act as barriers to dislocation motion

n=2N-1

n = # grains/in2 @ 100x mag N =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 # 1
16 grains/mm2 x (25.4)2 = 10 323 gr/in2 = 1.03 gr/in2 @ 100xASTM # 8
2048 grains/mm2x(25.4) 2 =1 321 287 gr/in2 = 132 gr/in2 @ 100x not 128

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ASTM grain size

1

3

6

8

10

Same magnification
11Effect of grain size on strength

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Applications
Grade 1006 1010-1025 1030 %Mn 0.25-0.4 0.3-0.6 0.6-0.9 Product Sheet Structural Heat treated
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Low carbon steels (