Printing Tool Steel: Maraging, H13 and What Happens After the Build

A printed steel insert is not finished when the build ends. It comes out of the machine welded to a plate, rough, stressed and, in most cases, not yet at working hardness. The steel grade decides how easily it prints, and the steps after the build decide whether it fits, polishes and lasts. This article compares the printable tool steels and sets out the post-processing route from build plate to finished insert.

Why carbon is the problem

In laser powder bed fusion each track of metal melts and solidifies in a fraction of a second. A carbon tool steel such as H13 transforms directly to hard, untempered martensite while the layers below are still shrinking. High hardness, low ductility and high residual stress together produce cracks, sometimes during the build and sometimes days later.

Maraging steel avoids this. It contains almost no carbon (below 0.03 %), so its martensite is soft and tough when it forms. The steel is printed in this soft state and then hardened by ageing at a moderate temperature, without quenching. That is why maraging steel 1.2709 became the standard material for printed tooling.

The printable grades

Three practical differences follow from the table.

• Maraging steels are machined soft and hardened last. At 33–37 HRC the as-built part mills and drills easily. Ageing then brings it to working hardness with very little distortion.
• H13 needs heat during the build. A build plate heated to 200 °C or more, and in some machines far higher, keeps the part above the temperature at which cracks form. Not every machine or bureau offers this.
• Thermal conductivity is lower than expected. Printed 1.2709 conducts about 15 W/m·K as built and about 20 W/m·K after ageing, against roughly 25 for H13 and 30 or more for pre-hardened mold steel. Conformal channels compensate by being closer to the surface. For inserts that need the highest heat removal, copper alloys such as CuCrZr can also be printed.

From build plate to finished insert

1. Remove powder. Clear the build chamber and blow out every channel while the part is still on the plate. Powder left in a channel sinters during heat treatment and cannot be removed afterwards.
2. Stress relieve on the plate. Residual stress is released by heating before the part is cut off. If the part is cut off first, it distorts. For H13, stress relieving at about 600–650 °C is essential. For maraging steel, the solution anneal or the ageing cycle does the same job.
3. Cut from the plate. Use wire EDM or a band saw and leave stock on the bottom face.
4. Solution anneal (optional). About 940 °C for two hours makes the structure uniform in all directions. It is used when toughness and consistent polishing matter.
5. Rough machine in the soft state. Establish datums and remove most of the stock while the hardness is low.
6. Age harden. 490 °C for six hours gives 1.2709 its peak hardness of about 54 HRC. There is no quench. The part contracts slightly and uniformly, by less than 0.1 %.
7. Finish machine. Hard mill, grind or EDM the fits, shut-offs and cavity to final size. The choice between milling and EDM follows the same logic as for wrought steel, described in Hard Milling vs. EDM for Mold Cavities.
8. Polish, texture or coat.
9. Flush and test. Flush the channels, then flow-test and pressure-test each circuit.

For H13 the route is different: stress relieve, cut off, then either double temper at 550–600 °C to reach the target hardness or fully harden and temper as for a wrought insert. Full hardening gives the most uniform structure and also the most distortion, so more stock is needed.

Surface treatment and repair

Any treatment above 490 °C overages maraging steel and lowers its hardness. This rules out high-temperature CVD coatings. The treatments themselves are compared in Surface Treatments for Tooling: Nitriding, PVD and When Each Fits.

Which grade for which tool

Common mistakes