Ferrous Metallurgy & Foundry Heat Treatment, ASTM A897 Austempered Ductile Iron (ADI)
Austempered Ductile Iron (ADI) Ms & Ausferrite Formula
Calculate Initial Matrix Martensite-Start Ms (°C), Salt-Bath Austempering Superheat Above Ms (°C), T0-Limited Enriched Austenite Carbon C_gamma (wt%), and Enriched Ms (°C).
Why 2.4% silicon in Austempered Ductile Iron blocks cementite carbides—creating carbon-stabilized ausferrite stronger than forged steel!
ASTM A897 **Austempered Ductile Iron (`ADI`)** austenitizes nodular cast iron at `870–910 °C` (dissolving Matrix Carbon `C_0 ≈ 0.70–0.85` wt% from graphite nodules alongside `Mn` and `Mo`) and quenches into an isothermal nitrite/nitrate **Salt Bath at `T_{bath} = 260–390 °C`**—strictly ABOVE **Andrews Initial Martensite-Start `M_{s0} = 539 - 423 * C_0 - 30.4 * Mn - 7.5 * Mo`** (`°C`, Superheat `Delta_T = T_{bath} - M_{s0} > 40 °C`)! Because Ductile Iron’s `~2.4–2.8` wt% Silicon suppresses brittle `Fe_3C` cementite precipitation during Stage-I transformation, growing acicular ferrite rejects carbon INTO surrounding austenite until reaching Bhadeshia’s **`T_0` Carbon-Enrichment Ceiling `C_{gamma,T0} = (720 - T_{bath}) / 195`** (`~1.80–2.25` wt% C)! That massive carbon enrichment depresses **Stabilized Austenite `M_{s,enr} = 539 - 423 * C_{gamma,T0} - 30.4 * Mn - 7.5 * Mo`** below `-200 °C`—preserving tough, TRIP-toughened **Ausferrite** (`alpha_{ferrite} + gamma_{HC}`) all the way to Arctic service temperatures!
Verify on an ASTM A897 Grade 900-650-09 Tough Upper-Ausferrite Suspension Arm—Dissolved_Matrix_Carbon `C_0` = 0.78 wt% (B2), Matrix_Manganese `Mn` = 0.36 wt% (B3), Molybdenum `Mo` = 0.24 wt% (B4), and Isothermal_Salt_Bath `T_{bath}` = 360.0 °C (B5): Initial `M_{s0}` equals `196.3 °C` (E2), Salt-Bath Superheat Above `M_{s0}` equals `163.7 °C` (E3), `T_0`-Enriched Austenite Carbon reaches `1.846` wt% (E4), depressing Stabilized `M_{s,enr}` to `-254.7 °C` (E5)!
| ASTM A897 ADI Ausferrite KPI (C0=0.78%, Mn=0.36%, Mo=0.24%, Tbath=360 vs 285 °C) | Formula | Verified Output |
|---|---|---|
| Initial Matrix Ms0 (°C) | 539 - 423*C0 - 30.4*Mn - 7.5*Mo | 196.3 °C |
| Salt-Bath Superheat Above Ms0 (°C) | Salt_Bath_Temp_C - Initial_Ms0_C | 163.7 °C (88.7 °C @ lower-ADI 285 °C) |
| T0 Enriched Austenite C_gamma (wt%) | (720 - Salt_Bath_Temp_C) / 195 | 1.846 wt% (2.231 wt% @ 285 °C Grade 1400!) |
| Stabilized Ausferrite Ms_enr (°C) | 539 - 423*C_gamma - 30.4*Mn - 7.5*Mo | -254.7 °C (-417.4 °C cryogenic stability!) |
Andrews Martensite-Start & Bhadeshia T0 Carbon-Enriched Ausferrite Formula
Validate `Matrix_Carbon C_0` in `[0.50, 1.10]` wt%, `Mn` in `[0.10, 1.20]` wt%, `Mo` in `[0.00, 1.00]` wt%, and isothermal `Salt_Bath T_{bath}` in `[240, 420] °C` strictly above `M_{s0} + 25 °C`!
=LET(cz_w, B2, mn_w, B3, mo_w, B4, tb_c, B5, ms_z, 539 - 423 * cz_w - 30.4 * mn_w - 7.5 * mo_w, cg_w, (720 - tb_c) / 195, ms_e, 539 - 423 * cg_w - 30.4 * mn_w - 7.5 * mo_w, IF(OR(cz_w < 0.5, cz_w > 1.1, mn_w < 0.1, mn_w > 1.2, mo_w < 0, mo_w > 1, tb_c < 240, tb_c > 420, tb_c <= ms_z + 25), "Invalid ADI Schedule", ROUND(ms_e, 1)))Wear-Resistant Grade 1400 Lower-Ausferrite 285 °C Salt Bath Check (Set Salt Bath B5 = 285 °C)
Lower the isothermal salt bath from `360 °C` to `285 °C` (`B5 = 285`) for ultra-hard mining gear teeth: Superheat E3 narrows to `88.7 °C` (nucleating nanoscale acicular ferrite laths!), `T_0` Carbon Enrichment E4 rises to `2.231` wt%, and Stabilized `M_{s,enr}` E5 plunges to `-417.4 °C`!
=IF(ISTEXT(E5), E5, ROUND((720 - B5) / 195, 3))Try the example
Use the original values or change them to test your understanding.