- What…is a maraging steel?
- Maraging steel properties
- Grades of maraging steel
- Maraging steel uses
- Additive manufacturing and maraging steel
- Design considerations for 3D printing maraging steel
ABOUT MARAGING STEEL
If you are like most people, you’ve probably never heard of them. Stainless steel? Sure. Aluminum? Absolutely. Low carbon steel, very likely.
The term ‘maraging’ is descriptive; a combination of ‘martensitic’ and ‘aging’.
Martensitic is a reference to the hardened crystaline structure that is created during the heating and cooling process, and ‘aging’ refers to the heating process itself.
Originally developed in the 1950’s by Inco, maraging steels are a special class of low-carbon ultra-high-strength steels that derive their strength not from carbon, but from the precipitation of intermetallic (ordered structure between two or more elements) compounds. The principal alloying element for maraging steels is nickel, generally taking up 15 to 25 wt%. Secondary alloying elements can include cobalt, molybdenum, and titanium which are added to produce the intermetallic precipitates.
Why? See the list of characteristics below…
CHARACTERISTICS OF MARAGING STEEL
The absence of carbon and the use of intermetallic precipitation allows maraging steel to achieve combinations of high strength and toughness while maintaining relatively high ductility. Ductility refers to a material’s capacity to be deformed (like bending) without fracture.
Maraging steel offers the following advantageous properties…
- High yield strength and ultimate tensile strength
- High ductility
- Good machinability
- High impact strength
- High fatigue strength
- High resistance to crack propagation
- Heat treatment features
- Low coefficient of thermal expansion
GRADES OF MARAGING STEEL
There are grades of maraging steels, usually described by a number that indicates the approximate nominal tensile strength in thousands of pounds per square inch, while the compositions and required properties are defined by US military standards. The grades and composition below are listed in the MIL-S-46850D standard.
Its notable that the higher the grade, the greater the proportion of cobalt and titanium.
|Maraging steel compositions, by grade|
|Element||Grade 200||Grade 250||Grade 300||Grade 350|
|Tensile strength (MPa)||1379||1724||2068||2413|
Variable compositional grades have been developed to include stainless and cast steels, or to enhance strength or magnetic characteristics.
COOL, BUT WHAT’S IT FOR…?
Given the range of properties listed above, it should not come as a surprise that maraging steels are used in a wide range of applications.
Here’s a few…
- Rocket motor cases
- Arresting hooks (tailhooks) for aircraft and the Lunar Expedition Module
- Shock absorbers for the lunar rover
- Lightweight portable military bridges
- Connecting rods
- Drive shafts
- Casting and forging dies
- Gears for machine tools
- Pump impellers and casings
- Cable sockets
- Tensile test equipment
- Rotors for ultracentrifuges
- Fencing swords
- Guitar strings
- …and the list goes on.
As you can see from the list provided, maraging steels have found their way into a wide range of industries from aerospace, to music.
ADDITIVE MANUFACTURING AND MARAGING STEEL
Maraging steel lends itself well to additive processes and 3D printing and affords the user all the benefits of additive processes such as
- greater design complexity
- greater customization
- prototyping and faster product development
- reduced waste
- reduced inventory reliance
Interestingly, a study conducted by the Mechanical and Aerospace Engineering Department at the Naval Post Graduate School in California and printed by the MDPI in January 2020, tests were conducted on maraging steel to compare the effects of aging (heat treated) and print orientating against forged or casted steel.
The study found that ‘as-printed’ (no heat treatment) maraging steel either met or exceeded the ultimate tensile strength (UTS) of traditionally fabricated CNC parts.
Does this mean that all 3D printed parts will meet or exceed traditionally fabricated parts? No. But it does mean that with proper consideration and attention, 3D printed maraging steel have the capacity to reach that mark.
A Note about Conformal Molding….
Despite 3D printing’s vast benefits, injection molding is still the fastest and least expensive method to mass produce components. However even here, additive manufacturing can offer benefits. Injection molds make use of strategically positioned cooling channels used to cool the injected material to reduce the cycle time.
Traditionally produced molds make use of cooling channels positioned by the limitations of the tooling being used. Thus, it is very difficult to introduce channels that conform to more complex geometries…or even curves. 3D printing is an excellent way to circumvent that challenge.
A Note about Post Processing….
The ‘good machinability’ rating recieved by maraging steel has the benefit of making required post processing work easier than some other metals…like Inconel, which is notoriously difficult to machine. Maraging steels also offer ‘good weldability’, in those events when printing unitary components is not possible.
Additionally, the optimal aging temperature required by maraging steel is relatively low. In the study mentioned above, components aged at 490 C demonstrated a UTS increase of 2,000 MPa, with increasing temperatures having an adverse effect. In other words, a reduced MPa increase.
Selective Laser Melting
For a quick review of Selective Laser Melting, please click here.
SLM is arguably the most trusted means of 3D printing maraging steels because properly printed, it produces parts with near net dimensional accuracy and at densities approaching 100% which can result in properties meeting or exceeding traditionally cast or fabricated parts.
The print to the left was printed using the EPlus3D EP-M250 intelligent SLM printer and includes a metallographic microstructure.
Direct Energy Deposition
For a quick review of Directed Energy Deposition, please click here.
3D printing maraging steel can also be done with DED technology. One of the key advantages of DED over SLM technology is that DED systems are larger and ‘open’. (Meaning that: you can add to, print on or repair existing parts, or cladding.) Please refer to the picture to the left
The DED microstructure for printed components is similar to that of SLM. However, because of the enhanced speed and larger particle size, if finish and dimensional accuracy are considerations, milling may be required to attain required tolerances and finish.
For a quick review of binder jetting, please click here.
Binder Jetting can quickly print Inconel parts, but a word of caution is warranted with this method. Binder Jetting maraging steel requires post processing steps to burn out the binding agent, and possible tertiary steps to ensure optimal density.
However, it is essential that the user account for the geometric changes and dimensional variances that result in both the de-binding and subsequent sintering steps.
DESIGN CONSIDERATIONS FOR MARAGING STEEL
You will want to account for each of these in your product or part design. While many of the considerations apply to metals generally, work by Eplus3d application engineers have found that maraging steel is a generally accommodating material to work with, and have shared a few thoughts.
As with most metals a minimum wall thickness of any aluminum part should be 1mm. However, where post-processing is needed, wall thickness may need to be increased. (E.g. A part that will be milled or blasted before use.)
Maraging steel lends well to printing very fine detail.
While not the only application, this is a great benefit if the printed components need to be identified with a serial number. Fine detailing like engraved or embossed text will print very nicely if you outline the letters with a line thickness of 40µm and use a minimum height of 40µm (and/or a minimum depth of 15µm).
Some SLM printers like the Eplus3D’s EPM line can actually print in detail as small as 25µm.
Always an important consideration when 3D printing. Apart from aesthetics, part geometry can govern part strength, surface adhesion, support use and the amount of post-processing. Generally speaking, it is better to use steep angles (greater than 35°) as this makes it easier to achieve a quality smooth surface.
A note about supports…
The use of supports is a subset of part geometry but is important enough to warrant further consideration. As useful as supports can be, ideally the designer wants to minimize if not completely remove the need for supports. Supports are useful to keep the model rigid while building and prevent internal stress and deformations, but….
They need to be removed. Support structure removal can require a lot of work…much of it requiring considerable care to not damage the print…which is compounded by the number of supports. Here, maraging steel relative ease of machinability works in your favor making removal of such structures comparatively easy.
For more information on post-processing, please review our article: Mind of Metal: Post-Processing.
- ESCAPE HOLES
It is usually advisable to plan for lower overhang angles and bigger holes than when 3D printing steel.
For more information about Metal 3D Printing, feel free to check out our Mind of Metal blog articles:
- Mind of Metal: Inconel
- Mind of Metal: Lightweighting
- Mind of Metal: Post Processing
- Mind of Metal: Aluminum
We will be adding incrementally to the Mind of Metal series, so email us to Join our Newsletter and type “Join” in the subject bar to stay apprised.
If that wasn’t more than you wanted to know about Inconel and Inconel 3D printing, we’d love to the chance to answer your questions!