Heat treatment changes the matrix of a ductile iron casting — to ferrite, pearlite, tempered martensite or ausferrite — and with it the trade between strength, hardness, ductility and machinability; the graphite nodules are fixed in the ladle and do not change. Most castings never need it. ASTM A536 assigns 65-45-12 and 80-55-06 to the as-cast condition, and those two grades cover most of the ductile iron castings we pour.
Heat treatment enters when the grade or the service asks for something the as-cast matrix cannot give: a ferritizing anneal for 60-40-18, normalizing or quench-and-temper for 100-70-03 and 120-90-02, austempering for the ADI grades of A897, and stress relief for any part that must hold size through finish machining.
Common Ductile Iron Heat Treatment Processes
Seven treatments cover ductile iron in practice, and what separates them is less the peak temperature — most of them austenitize in the same 850–950 °C band — than the way the casting comes back down, because the cooling path is what sets the matrix.
| Heat treatment | Typical temperature | Main purpose | Resulting structure |
|---|---|---|---|
| Stress relieving | 500–650°C | Reduce residual casting/machining stresses | Existing matrix largely unchanged |
| Ferritizing anneal | 850–950°C, followed by controlled cooling | Increase ductility and machinability | Mostly ferrite |
| Full annealing | ~850–950°C | Soften casting and reduce hardness | Ferrite + graphite |
| Normalizing | 870–950°C, air cool | Increase strength and hardness | Fine pearlite |
| Quench & temper | Austenitize ~850–950°C, quench, then temper | High strength and wear resistance | Tempered martensite |
| Austempering | Austenitize ~850–950°C, then hold ~230–400°C | Very high strength with good toughness | Ausferrite |
| Surface hardening | Local rapid heating + quenching | Improve surface wear resistance | Hard martensitic surface |
Surface hardening is a local induction or flame cycle on a finished journal or bore, and full annealing is the ferritizing anneal carried through to an all-ferrite matrix. The five that follow are the ones that get specified on drawings.
1. Ferritizing Annealing
A ferritizing anneal converts the as-cast pearlite to ferrite and produces the softest, most ductile and most machinable ductile iron — it is the treatment A536 assigns to 60-40-18, and to no other grade. Specify it when elongation or machinability matters more than strength: a valve body with a lot of drilling and tapping, or a part that takes shock loads at low temperature.
Chemistry decides how well the anneal works. Manganese, phosphorus, chromium and molybdenum all slow the pearlite-to-ferrite change, so a 60-40-18 order is best poured from a lean, low-manganese iron rather than annealed out of a pearlitic heat.
2. Normalizing
Normalizing — austenitize at 870–940 °C, then cool in still air — turns the matrix into fine pearlite and is the cheapest way to move a casting up in strength and hardness; with a temper after it, A536 accepts it as a route to 100-70-03 and 120-90-02, and 100-70-03 is where it is usually used. There is no quench, so distortion stays low and thin-walled parts survive it.
Whether air cooling delivers full pearlite depends on section and chemistry. The iron needs at least moderate manganese, around 0.3–0.5 % or higher, and silicon not too high. A heavy section cools too slowly to go fully pearlitic unless it carries nickel, molybdenum or extra manganese for hardenability, while a thin section of the same iron can overshoot into martensite or an acicular structure. Tell the foundry the section range when you specify a normalized grade, because the alloy is chosen to it.
Tempering usually follows, at 425–650 °C, to bring hardness to the target band. The price of normalizing is machinability and elongation — 100-70-03 gives 3 % against the 12 % of as-cast 65-45-12.
3. Stress Relieving
Stress relieving holds the casting at 510–565 °C for unalloyed ductile iron — higher for alloyed grades, up to 650 °C — long enough for residual stress from solidification and rough machining to relax, without changing the matrix, so the part holds its dimensions through finish machining and service.
It earns its place on castings with bore alignments or mating faces that are finish-machined after roughing — a gearbox housing, a pump casing with a bolted split line — and on shapes with large section changes, where the stress locked in at solidification is greatest. The hold is one hour plus one hour per inch of section, followed by a furnace cool, so it costs a full furnace cycle in the schedule, not just the hold time.
Stress relief is not free of side effects. The higher the temperature, the more completely stress is removed, but the upper end also takes something off hardness and tensile strength. Where a hardness minimum is on the drawing, keep the stress relief at the low end of its band.
4. Quenching and Tempering
Quench and temper reaches the highest hardness and strength conventional ductile iron offers — it is the usual route to 120-90-02 and an accepted one for 100-70-03 — by austenitizing at 845–925 °C, quenching to martensite, and tempering back to the hardness and toughness the part needs. As-quenched, the matrix is martensite with some retained austenite, and it peaks at 55–57 HRC when austenitized at the lower end, 845–870 °C.
Tempering at 425–600 °C trades hardness for toughness with each step up in temperature; 120-90-02 sits at the tough end of that trade, with 2 % elongation. The lower the temper, the harder the surface and the more wear it resists, which is why quench and temper rather than normalizing is the answer when the requirement is very high hardness.
Two cautions. Quenching distorts, and can crack, a casting with abrupt section changes or sharp internal corners, so quench-and-temper suits compact, uniform sections better than a thin-walled housing, and finish machining belongs after the temper. And for a part that needs 90 ksi yield with more toughness than 2 % elongation, ADI is the better answer.
5. Austempered Ductile Iron — ADI
Austempering — austenitize at 815–925 °C, then hold isothermally at 230–400 °C instead of quenching to room temperature — transforms the matrix to ausferrite, acicular ferrite in carbon-stabilized austenite, and gives the strongest ductile iron there is: the ASTM A897/A897M grades run from 110 ksi tensile with 11 % elongation to 230 ksi with 1 %. It is the treatment to consider when a ductile iron casting is being weighed against a forged or heat-treated steel part.
The hold temperature picks the grade. Transforming low, around 260 °C, gives a fine, high-strength, wear-resistant ausferrite; transforming high, around 370 °C, gives a coarser structure with high fatigue strength and good ductility.
| Grade | Tensile min, ksi | Yield min, ksi | Elongation min, % | Typical hardness, HBW |
|---|---|---|---|---|
| 110/70/11 | 110 | 70 | 11 | 241–302 |
| 130/90/09 | 130 | 90 | 9 | 269–341 |
| 150/110/07 | 150 | 110 | 7 | 302–375 |
| 175/125/04 | 175 | 125 | 4 | 341–444 |
| 200/155/02 | 200 | 155 | 2 | 388–477 |
| 230/185/01 | 230 | 185 | 1 | 402–512 |
Read the table against quench-and-tempered 120-90-02: 130/90/09 gives the same 90 ksi yield with 9 % elongation against 2 %. Hardness is shown for information only and is not an acceptance requirement, so put the grade on the drawing, not an HBW range.
Three conditions come with specifying ADI. The casting is normally machined before austempering, by agreement, because ausferrite machines hard. Weld repair is not acceptable on austempered castings under A897, so the casting has to be sound before it goes in. And the standard allows the casting, machining and austempering to be done by different manufacturers, so settle who certifies the properties.
How Heat Treatment Changes Ductile Iron
Heat treatment changes only the matrix between the graphite nodules; the nodules themselves — their count, size and roundness — are fixed when the iron is treated with magnesium and inoculated in the ladle, and no furnace cycle repairs poor nodularity. Every property that heat treatment moves is a matrix property: strength, hardness, elongation, machinability.
The matrix is set by the path down from austenite. Furnace cooling gives ferrite; air cooling gives pearlite; quenching gives martensite with some retained austenite; holding isothermally in the 230–400 °C band gives ausferrite. Tempering softens the normalized or quenched matrix into ferrite carrying small particles of iron carbide. Stress relieving never reaches austenite at all, and so leaves the existing matrix in place and only relaxes it.
Choosing the Right Treatment
Start from the property the part must have, name the A536 or A897 grade that carries it, and let the grade set the treatment — the standard already does: 65-45-12 and 80-55-06 as-cast, 60-40-18 by ferritizing anneal, 100-70-03 and 120-90-02 by quench and temper, normalize and temper, or isothermal treatment, and any A897 grade by austempering. For most pump, valve and housing castings the right answer is one of the two as-cast grades and no heat treatment at all.
| Required property | Typical treatment |
|---|---|
| Maximum machinability | Ferritizing anneal |
| High elongation | Ferritizing anneal |
| Dimensional stability | Stress relieving |
| Moderate-high strength | Normalizing |
| Higher wear resistance | Normalizing or quench & temper |
| Very high hardness | Quench & temper |
| Very high strength + toughness | Austempering / ADI |
| Hard surface with ductile core | Surface hardening |
Put the grade on the drawing, not the cycle; a bare “normalize” with no grade behind it leaves the foundry guessing at the property you wanted. The exception is stress relieving, which changes no grade and has to be called out as a process note, with the machining stage it comes after.
Two costs travel with the choice. Furnace time scales with section — an hour per inch is the working rule for annealing, normalizing and stress relief — so a heavy casting spends far longer in the cycle than a light one. And any treatment that goes through austenite (normalizing, quench and temper, austempering) adds distortion that the machining allowance has to absorb, which is one more reason to stay as-cast when the grade allows it.
Conclusion
Ductile iron is heat treated to change its matrix, and the grade you specify already decides whether it needs to be: 65-45-12 and 80-55-06 ship as-cast, 60-40-18 is annealed, 100-70-03 and 120-90-02 are normalized or quenched and tempered, and the A897 grades are austempered. Stress relief is the one treatment chosen for the part rather than the grade. Pick the property, name the grade, tell the foundry the section range, and the cycle follows.