Processing: the furnace writes the datasheet
Introduit| English | Sens |
|---|---|
| heat treatment | the furnace and quench schedule that sets a metal final properties |
| quenching | cooling from the austenite field fast enough to form martensite |
| tempering | reheating quenched martensite to restore toughness at some strength cost |
| casting/ˈkæstɪŋ/ | shaping by freezing liquid metal in a mould |
| solidification/səˌlɪdɪfɪˈkeɪʃn/ | the freezing clock t = B(V/A)^2, volume per cooling surface squared |
Four routes, one grade
- Same 0.40% steel, four routes: as-rolled $\mathbf{280}$ MPa at 24%, normalised $\mathbf{260}$ at 26%, quench + temper 600°C $\mathbf{620}$ at 16%, at 200°C $\mathbf{950}$ at 8%.
- Heat treatment 热处理 - quenching 淬火 to martensite, then tempering 回火 to buy toughness back - dials the trade-off.
*Same steel, four datasheets - and the riser sizes the casting
Quenched and tempered at 600 C: yield strength in MPa (nearest integer)?
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The menu line.
Quenched and tempered at 200 C: yield in MPa (nearest integer)?
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Strongest, least ductile.
For the impact-loaded mount bracket, the sensible route is:
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Impact energy is absorbed by plastic area.
The freezing clock
- Casting 铸造 shapes by freezing; solidification 凝固 time is Chvorinov's $t = B(V/A)^2$ - volume over cooling surface, squared.
- With $B = 2.0 imes 10^6$ s/m²: the 20 mm cube $\mathbf{22.2}$ s, the 100×100×10 plate $\mathbf{50.0}$ s - $2.25 imes$ longer despite being thinner, the $(V/A)^2$ ratio exactly.
Which process, which change?
Route each process change to its microstructural cause.
Chvorinov with B = 2.0e6 s/m2: freezing time of a 20 mm cube (one decimal)?
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B (V/A)^2 with V/A = 3.33 mm.
Same B: the 100x100x10 mm plate, faces only (one decimal)?
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V/A = 5.00 mm.
Plate-to-cube freezing time ratio (two decimals)?
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(5.00/3.33)^2.
A thinner section always freezes sooner than a thicker one.
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V/A times the clock, not thickness - the plate outlasted the cube.
The riser must be greediest
- A riser (the shrinkage reservoir) must have a larger $V/A$ than any section it feeds, or it freezes first and the casting sucks in air.
- $V/A = 6.0$ mm gives 72.0 s - greediest, last to freeze, sound casting.
A riser with V/A = 6.0 mm: its freezing time (one decimal)?
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B (6e-3)^2.
Three processing slips
- Buying to grade, not state. The menu is the material.
- Tempering too cold for impact service. The mount needs the 600°C temper's 16%.
- Comparing freezing times by thickness. $V/A$, not thickness, times the clock.
Recap
- Routes: 280/24, 260/26, 620/16, 950/8 - MPa against % elongation.
- $t = B(V/A)^2$: 22.2 s cube, 50.0 s plate, ratio 2.25 = the squared $V/A$ ratio.
- Design the riser to freeze last: 72.0 s at $V/A = 6$ mm.