Metals and alloys: the state ships
Introduit| English | Sens |
|---|---|
| phase diagram | the map from composition and temperature to the phases present |
| eutectoid | the 0.76 wt% carbon reaction at 727 C where austenite decomposes to pearlite |
| pearlite | the lamellar mixture of ferrite and cementite from slow cooling |
| lever rule | the tie-line balance that weighs phase fractions from composition |
| strain hardening | strengthening by accumulated plastic strain, sigma = K eps^n |
| martensite | the supersaturated hard phase a quench traps, brittle until tempered |
| precipitate/prɪˈsɪpɪteɪt/ | a fine second phase that impedes dislocations and raises strength |
The map of phases
- The phase diagram 相图 maps composition and temperature into phases: read it before you buy.
- The eutectoid 共析 point - $\mathbf{0.76}$ wt% C at $\mathbf{727°}$C - is where austenite turns to pearlite 珠光体 on slow cooling.
*Read the phases off the map; weigh them with the lever rule
Eutectoid composition of the iron-carbon system in wt% C (two decimals)?
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The classic constant.
Eutectoid temperature in degrees C (nearest integer)?
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The horizontal of the eutectoid.
Weighing the tie-line
- The lever rule 杠杆定律 for the 0.40% steel just below 727°C: pearlite $= (0.40 - 0.022)/(0.76 - 0.022) = \mathbf{0.512}$, ferrite $\mathbf{0.488}$ - they sum to one.
- Strain hardening 应变硬化: $\sigma = K\varepsilon^n$ with $K = 530$ MPa, $n = 0.20$ → $\mathbf{401.7}$ MPa true at 25% cold work.
Which microstructure, which road?
Route each composition and cooling path to its product.
Pearlite fraction for 0.40 wt% C just below 727 C (three decimals)?
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(0.40-0.022)/(0.76-0.022).
True stress at 25% cold work with K = 530 MPa and n = 0.20 (one decimal)?
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530 x 0.25^0.20.
Strength mechanisms add linearly, so the certificate can be computed by summing them.
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They interact; the certificate is measured.
The quench detour
- Fast cooling misses pearlite entirely: martensite 马氏体 forms - hard, brittle, and never shipped untempered.
- Precipitation is the aluminium road: precipitates 析出相 (Mg₂Si in 6061) pin dislocations the way boundaries do.
Martensite is best described as:
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Hard and brittle as quenched.
Pearlite is:
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Mixture, not phase.
Two parts of the same steel grade always ship with the same properties.
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The state - processing history - decides, not the grade.
Three alloying slips
- Phases read above their field. Ferrite below 727°C stops at 0.022% C.
- Strength mechanisms added. They interact - the certificate is measured, not summed.
- Cementite confused with pearlite. One is a phase, the other a mixture.
Recap
- Eutectoid 0.76% at 727°C; 0.40% steel below it: 51.2% pearlite + 48.8% ferrite.
- $\sigma = K\varepsilon^n$: 401.7 MPa true at 25% work, ductility 28% → 12%.
- The state ships, not the grade - buy annealed, form, then process.