Age Hardening of Maraging 250: The Complete Heat Treatment Guide

Most alloys arrive at the heat-treat shop already close to their final strength, needing only a temper. Maraging 250 (UNS K92890, also known as Maraging C-250 and 18Ni-250; the trade name VASCOMAX™ 250 refers to the equivalent Carpenter Technology grade) is the opposite. It leaves the forge soft and stays soft through every drilling, turning and milling operation — then a single low-temperature soak transforms it into one of the strongest commercially forged steels available. That transformation is the whole point of the alloy, and it lives or dies on two temperature windows.

This guide walks through both stages of the cycle in the order a real part experiences them: solution annealing, then age hardening. Along the way it explains what is happening inside the metal, why the numbers are what they are, and where parts are most often lost. If you are specifying, buying, or inspecting heat-treated maraging forgings, this is the sequence your supplier's certificate is really describing. For the full material and product context, see our reference page on Maraging 250 (UNS K92890) forged parts.

01 / MECHANISMWhy Maraging Steel Hardens the Way It Does

Conventional high-strength steels — 4340, 300M, H13 — buy their strength with carbon. Quenching traps carbon in a distorted martensite lattice; tempering then claws back a little toughness. The carbon is both the source of hardness and the source of brittleness, and it drags weldability and corrosion resistance down with it.

Maraging 250 keeps carbon below 0.03% and hardens by a completely different route: precipitation. The name itself is a contraction of "martensite" and "aging." Strength does not come from the martensite being hard — the martensite here is soft and ductile. It comes from millions of nanometre-scale intermetallic particles that form inside that soft matrix during a low-temperature soak, each one pinning the dislocations that would otherwise let the metal deform. The two principal particles are Ni₃Ti and Ni₃Mo, with secondary Fe₂Mo contributions. Because they are coherent with the surrounding crystal lattice, they set up intense local strain fields that block slip in every direction at once.

The practical consequence is the property combination the alloy is famous for: tensile strength climbs from about 1,000 MPa in the soft state to over 1,860 MPa aged, while fracture toughness falls only modestly, landing at 80–100 MPa·m^0.5. No carbon-hardened steel matches that pairing at this strength level. But it only happens if both thermal stages are executed inside narrow limits.

02 / STAGE ONESolution Annealing — Building the Blank Canvas

Solution annealing is the reset button. Its job is to dissolve every previously formed precipitate back into solution and hand the machinist a clean, uniform, workable structure.

The cycle is straightforward on paper: heat to 820–845°C, hold for one hour per 25 mm of section thickness (with a one-hour minimum), then air cool to room temperature. On cooling, the alloy transforms fully to a low-carbon martensite. Its transformation temperatures are comfortably above ambient — martensite start near 200°C and finish near 150°C — which is why plain air cooling gives complete transformation without a quench tank. There is no drama, no cracking, no violent quench distortion.

The result sits around 28–32 HRC. That number matters: it is soft enough to machine efficiently with standard carbide tooling, yet already fully martensitic and dimensionally settled, so nothing shifts underfoot while you cut. This is the condition in which essentially all precision machining should be done.

03 / STAGE TWOAge Hardening — Where the Strength Is Made

With machining complete, the part goes back into a furnace at a far lower temperature: 480–500°C for 3–6 hours, then air cool. Nothing about the part looks different afterward — no colour change, no scale to speak of, dimensions essentially unchanged — but its tensile strength has nearly doubled.

Inside, the low soak gives titanium, molybdenum and nickel just enough atomic mobility to cluster and nucleate the Ni₃Ti and Ni₃Mo precipitates. The temperature is deliberately low: high enough for diffusion, low enough that the particles stay fine and densely dispersed rather than coarsening into a few large, useless ones. Time and temperature trade off against each other, but the window is narrow, and 480°C ±3°C for 3.5–4.5 hours tends to deliver the most repeatable peak strength for common section sizes of 50–200 mm.

The elegance of the low aging temperature is dimensional: because the transformation is precipitation within an existing structure rather than a phase change on quenching, the part barely moves. Typical linear dimensional change is under 0.05%. That single fact reshapes the entire manufacturing route, which we come back to below.

04 / THE CLIFF510°C and Austenite Reversion — The One Mistake You Cannot Fix

Everything about aging is forgiving until one line. Above roughly 510°C, the process reverses on itself. Instead of strengthening precipitates, small islands of soft reverted austenite begin to form and grow within the martensite matrix. Strength collapses — unpredictably, by anywhere from 150 to 400 MPa — and the part becomes non-conforming.

This is why serious maraging heat treatment is a furnace-control problem before it is a metallurgy problem. A shop running a single-zone furnace with one control thermocouple can easily have local hot spots 15–20°C above setpoint. Set that furnace to 500°C "to be safe on strength" and a corner of the load may quietly cross 510°C. The defensible practice is multi-zone furnaces with independent thermocouple logging on every charge, held to within a few degrees, so the aging plateau never approaches the cliff. Every part we ship is aged this way and its temperature record is archived with the certificate — see the process controls behind our custom Maraging 250 forgings.

05 / SECTION SIZESoak Time, Through-Thickness Heating and Heavy Sections

The "one hour per 25 mm" rule for annealing and "three hours per 25 mm" guidance for aging are not arbitrary padding — they exist because heat has to reach the centre of the part before the clock on transformation truly starts. A 300 mm-thick valve body does not become 490°C at its core the moment its surface does.

For heavy sections above 200 mm, add roughly half an hour of aging hold per additional 25 mm beyond the 200 mm baseline, so the core sees full time at temperature. Skip this, and the surface ages correctly while the centre under-ages, producing a part that passes a surface hardness check but fails a tensile taken from mid-wall. The failure is invisible to the quickest inspection and only shows up where it matters.

06 / WORKFLOWMachine-Then-Age: The Route That Saves 40–60% of Cutting Time

Here is where the under-0.05% dimensional change becomes money. Because the part barely moves during aging, you can do all your heavy metal removal while the steel is soft, then harden last:

• Forge near-net shape with controlled grain flow.
• Solution anneal → soft, uniform ~30 HRC.
• Rough and finish machine to IT6–IT7 on all critical dimensions in the soft state — fast tool life, clean surfaces.
• Age harden at 480–500°C → part rises to 50–53 HRC while holding size.
• Verify dimensions (expect under 0.05% change), inspect, ship.

Compared with hard-machining a fully aged 52 HRC forging, this route typically removes 40–60% of the machining time and, because there is no aggressive grinding of the hardened surface, it preserves the compressive residual stress that finish machining leaves behind — which is good for fatigue life. Only genuinely sub-5-micron tolerances justify a final light CBN grinding pass after aging.

07 / TROUBLESHOOTINGReading a Bad Result

When aged properties come back wrong, the cause is almost always one of a short list. The table below maps symptom to likely root cause — useful whether you are running the furnace or auditing a supplier's certificate.

The last row is worth underlining: not every property shortfall is a heat treatment fault. If strength is on target but toughness or fatigue life is poor, the problem usually traces back to melt cleanliness — titanium lost to TiN/TiO₂ inclusions — which no aging cycle can repair. That is the metallurgical case for triple-melt (VIM+ESR+VAR) feedstock — a melt-route cleanliness issue rather than anything the aging cycle can control.

08 / VERIFYHow Correct Heat Treatment Is Proven, Not Assumed

A heat treatment claim is only as good as its evidence. On a properly documented Maraging 250 part you should expect, at minimum: a per-charge temperature record from a multi-point thermocouple map covering the aging soak; a hardness survey confirming 50–53 HRC after aging; a tensile test to ASTM E8/A370 demonstrating the guaranteed 1,860 MPa minimum; and, where required, Charpy and fracture-toughness data. All of it is issued under an EN 10204 3.1 mill test certificate, which we issue as manufacturer; an EN 10204 3.2 certificate is available only when arranged with an independent third-party inspector. If a supplier cannot produce the aging temperature chart for your specific charge, they cannot actually prove the part never touched 510°C.

FAQQuestions Engineers Ask About Maraging 250 Aging

What temperature is Maraging 250 aged at?

Maraging 250 is aged at 480–500°C for 3–6 hours, air cooled. For consistent peak strength on medium sections, 480°C ±3°C for about 3.5–4.5 hours is the sweet spot. The one hard rule: never let any part of the load exceed 510°C.

Can over-aged Maraging 250 be recovered by aging again?

No. Once austenite reversion has occurred above ~510°C, re-aging will not restore strength. The only route back is a full re-solution-anneal at 820–845°C followed by a fresh aging cycle. Under-aging, by contrast, is fully recoverable with a second aging soak.

Should I machine Maraging 250 before or after aging?

Before. Machine to final IT6–IT7 tolerances in the solution-annealed condition (~30 HRC), then age. Because dimensional change during aging is under 0.05% linear, finished parts hold tolerance without hard machining in the great majority of cases.

Does Maraging 250 need quenching?

No. It air cools from both the solution anneal and the aging soak. Martensite forms on air cooling because the transformation temperatures sit well above room temperature, so there is no quench and none of the associated cracking or heavy distortion.

How much does the hardness change with aging?

From roughly 28–32 HRC in the solution-annealed state to 50–53 HRC after aging, with tensile strength rising from about 1,000 MPa to 1,860 MPa or higher — while fracture toughness stays high at 80–100 MPa·m^0.5.