Maraging Steels 3

[butelja]

I've heard of "maraging steel" as having phenomenal strength and toughness. What are some of the ASTM grades of these steels, and what are the typical chemical compositions and heat treats?

 

[bilge]

"Maraging" is metallurgical argot for "martensitic aging." The alloys have high nickel content (18%) and as low a carbon content as is possible (0.03% or less). They also contain relatively high concentrations of cobalt, titanium, molybdenum, and a pinch of aluminum.

The secret is in the high nickel content (which allows the steel to transform to martensite at fairly high temperatures) and in the low carbon content (which stops other transformation products, common in normal steels, from forming). On cooling from the austenitizing temperature you get no pearlite or bainite, only a very low-carbon martensite; and essentially all of the austenite transforms. This lath martensite, because of its low carbon content, is weak but ductile. Strengthening comes from a post-cooling heat treatment at around 900F. During this treatment Fe2Mo and Ni3Ti precipitate.

P.S. These steels have good weldability.

 

[TVP]

First, most of the specifications for maraging steels are SAE AMS (Aerospace Material Specification) and not ASTM. ASTM A 579 - 01 covers forgings using maraging steels, and there may be several ASTM F specs that reference them for surgical screws, plates, etc. Try searching for maraging steel at

http://www.astm.org

for more info on ASTM specs. Click on the following link for the relevant SAE AMS specs:

http://www.sae.org/servlets/search?...ARCH&using=AND&ofType=ALL&COMMON_SUCCESS=TRUE

Now, the term maraging comes from Martensite Age Hardening, which describes the process of age hardening a low carbon, iron-nickel lath martensite matrix. One grade of maraging steel (specifically Carpenter's NiMark 250) has the following composition:

C 0.030
Mn 0.10
P 0.010
S 0.010
Si 0.10
Ni 18.0 to 19.0
Mo 4.7 to 5.0
Co 7.0 to 8.0
Ti 0.30 to 0.50
Al 0.050 to 0.15
Ca 0.050
B 0.0030
Zr 0.030
Iron Balance

As you can see, carbon is basically an impurity in this alloy. The age hardening process is similar to that of aluminum alloys: solution heat treating is typically 1 hr at 820 C, then artificial aging for 3-9 hrs at 455-510 C. Using these alloys and heat treatments, an outstanding combination of mechanical properties (strength, elongation, and fracture toughness) can be obtained. For example, a an 18Ni(250) grade, like NiMark 250, can have the following properties:
yield strength = 1700 MPa
tensile strength = 1800 MPa
elongation = 8 %
K1c plane-strain fracture toughness = 120 MPa m^1/2

As a comparison, spring steels that have similar properties would have K1c approximately one order of magnitude less than this. The cost of the alloys are quite high, due to the cobalt content. Cobalt-free alloys have also been developed. You can find more information at

http://www.cartech.com/.

 

[metman]

buteja-
Both bilge and TVP answered your quetions very clearly and specifically which is of great help to metallurgical types such as myself.
To add some practical flavor to your question. We had 3.5 inch diameter splined shaft made of 4340 alloy that was loaded in bending and torsion which was failing in fatigue. We had used every metallurgical and mechanical contrivance known to man to prolong the fatigue life of this shaft including shot peen, micropolish of radii, flame harden, carburizing, aircraft quality material spec and we considered going to a special Gogan quench.

After some sophisticated rotary (dynamic) strain gage testing we learned that we had a reverse torsional loading durng delayed startup of dual connected drives which greatly reduced our fatigue life and therefore it was a geometry problem because our shaft size was limited by a sun gear in a planetary which the shaft had to pass thru. To redesign and tool up for a larger shaft would be a major project which eventually had to happen over a perod of several years.

Meanwhile I suggested 18% Ni Maraging steel as an interim fix. This increased the worst case scenarios of 3 month failures to 6 months. So in simple terms at least in this instance the fatigue life was increased by a factor of two or to put it another way, in our case the one order of magnitude difference in Kic (measure of fracture toughness mentioned by TVP) equated to double the life since fracture toughness is a very key factor in fatigue.

The down side is that 18%Ni Maraging steel is about ten times the cost of the next best material alternative. It was worth the cost in the example listed above and we were able to pass along most of this cost to the customer because of the horendous downtime and replacement costs. by comprison, we would like to be using 18%Ni Maraging for an airplane landing gear application but we cannot justify the cost. Jesus is the WAY