Maraging Steel for Die Casting Cores
When the thermal load on a core or a small insert goes beyond what a 5 percent chromium hot work steel will carry, die casters reach for a maraging steel. The name comes from martensite and ageing. These steels harden at a low temperature after machining instead of by quenching, so the part stays close to the size it was cut to, and that is what makes them usable for a core that has to keep its accuracy. This page sets out the four grades in general service, their composition, the treatment sequence that takes them from annealed to aged, the dimensional change that has to be allowed for, and where these steels have and have not worked in service.
Why a maraging steel instead of a hot work steel
A die casting core sits in the metal stream and takes the same thermal cycling as the cavity, only with less mass to carry the heat away. Where a hot work steel softens, a core loses the fit that keeps the cored hole true. The maraging steels answer that with a combination of high strength and toughness that holds up under severe thermal fatigue. They are low carbon steels produced by vacuum melting from high quality materials, and they carry nickel, molybdenum and cobalt with smaller but important additions of titanium, aluminium and silicon. Four compositions are in general service, classified by proof stress and ranging from 1400 to 2400 N per square mm. The grades are sold by several suppliers under their own designations, so the proof stress class is the reliable way to compare them.
The four grades in general service
The composition of each grade is close, and the differences are deliberate. Titanium is the element that decides how much strength the age brings and how much the part moves while it is brought. Molybdenum and cobalt carry the rest of the response. The table below gives the chemistry of the four grades as published in the source.
| Grade | C | Mn | Si | Mo | Ti | Al | Ni | Co |
|---|---|---|---|---|---|---|---|---|
| 18 Ni 1400 | 0.03 | 0.1 | 0.1 | 3.0-3.5 | 0.15-0.25 | 0.05-0.15 | 17-18 | 8-9 |
| 18 Ni 1700 | 0.03 | 0.1 | 0.1 | 4.6-5.1 | 0.3-0.5 | 0.05-0.15 | 17-19 | 7-8.5 |
| 18 Ni 1900 | 0.03 | 0.1 | 0.1 | 4.6-5.2 | 0.5-0.8 | 0.05-0.15 | 18-19 | 8-9.5 |
| 18 Ni 2400 | 0.01 | 0.1 | 0.1 | 3.5-4.0 | 1.6-2.0 | 0.1-0.2 | 17-18 | 12-13 |
Nominal composition of the four maraging steel grades in general service for die casting cores, in weight percent. Manganese and silicon are in effect 0.12 percent maximum, but the total of manganese plus silicon must not exceed 0.2 percent. Iron balances the analysis in every grade.
From annealed bar to aged core
The treatment has three stages, and the order matters because the part is machined between the first two. The steel arrives annealed and machines easily at about 30 HRC. After the initial machining it is solution treated and cooled fast enough that no age hardening can take place, which leaves it soft enough for the final cuts. The age comes last, on the finished shape, so that the dimensional change happens after the last cut is taken.
| Step | Temperature | Hold and cool | Hardness after |
|---|---|---|---|
| As supplied, annealed | Not applicable | Not applicable | About 30 HRC, and easy to machine |
| Solution treatment | 800 to 900 C | Cool in air or oil, fast enough to prevent age hardening | Soft martensite, ready for final machining |
| Age hardening | 480 C | 6 to 9 h according to section thickness, then air cool | About 50 to 54 HRC |
| Higher ageing temperature | Above 480 C | A shorter hold | A lower hardness |
The treatment sequence for a maraging steel core. Machining is done before the age so that the small dimensional change of the age falls on the finished part and not on the stock to be cut.
The dimensional change that decides the machining allowance
The age hardening does not leave the part the size it went in. These steels shrink during the age, and the amount depends on the titanium content and on the ageing temperature. The published figures for ageing at 480 C are small enough that the shrinkage can be carried in the finishing allowance, but it cannot be ignored on a core that has to hold a tolerance. A higher ageing temperature will move the part further, and the source notes that the difference is a function of the titanium content.
| Grade or condition | Dimensional change after ageing at 480 C | What drives it |
|---|---|---|
| 18 Ni 1400 | Less than 0.0004 mm per mm | The lowest titanium content of the four grades |
| 18 Ni 1900 | Less than 0.0008 mm per mm | A higher titanium content |
| Higher ageing temperature | A greater shrinkage | Must be allowed for in the machining |
Dimensional change during the age hardening of maraging steel grades at 480 C, and the titanium content that drives it.
Repair welding a die with a maraging filler
One practical difference between a maraging steel and a hot work steel shows up at the welding bench. A maraging steel is used satisfactorily for the weld repair of die steels, and unlike welding with H13 it needs no preheat at all. A cold die is in fact wanted, so that the maraging weld cools quickly. The filler is then deposited in successive small amounts as a soft and ductile material, which lowers the chance of weld cracks forming. A low temperature age after machining also serves to temper the rehardened zone that forms in the hot work steel alongside the weld. The general properties of the parent die steel are not affected by the welding.
Where maraging has worked and where it has not
Maraging steels used for cores and small die inserts have given good performance in aluminium die casting. Large inserts are the exception. On large inserts used in aluminium die casting the source records abnormal wear, caused by the high die surface temperature acting together with the erosion of the aluminium. Read that alongside the intended use. These are core and small insert materials, chosen for toughness and accuracy, and they are not a general replacement for a 5 percent chromium hot work steel across a whole die.
Where to go next
The hot work steel these cores usually support is covered on the H13 tool steel page, and the H13 grades sold into dies on the H11 page and the H21 page. Which grade goes into which part of the die and at what hardness is set out on the die casting die component materials page, and the core and insert choice on the core and insert selection page. The way a die actually fails is on the die casting die and insert page and the thermal fatigue cracking page. Repair welding across the tool steel range is covered on the tool and die welding page, and the cast metals the die has to survive on the die casting die steel selection page. The heat treatment of the hot work grades is on the hot work heat treatment page, and the softening that a die suffers in service on the H13 die softening page.
Before you use this as an alloy specification
This page is a reference summary of a published source and it is not an Aobo Steel specification. The compositions and the treatment response are reproduced from the source, which gives nominal analyses for grades sold under several trade names. The response a particular heat of maraging steel gives depends on the melting practice, the analysis and the ageing treatment. Final specification is confirmed on the job.
Source, Die Casting Metallurgy, A. Kaye and A. Street, Butterworths Monographs in Materials, 1982, Chapter 25, Pressure die casting dies.