Ferrium® C61™


Ferrium C61 (AMS 6517) is a new high strength and high fracture toughness carburizable steel that also has high temperature resistance and hardenability. C61 steel is a higher performance upgrade from 9310, X53 (AMS 6308), EN36A, EN36B, EN36C, XTrac, and 8620. It can achieve a surface hardness of ~61 HRC via vacuum carburization.

C61 steel is double vacuum melted (i.e., vacuum induction melted and then vacuum arc remelted or "VIM/VAR") so that it is very clean, which helps it have a much greater fatigue strength.

An air-melt version of C61 steel is currently under development.

Improved C61 Steel Properties versus Incumbent Materials

The following table shows that for example C61 has a Yield Strength that is 70 ksi greater than 9310 and a much greater toughness:

Additional information can be found in the C61 Datasheet or on Carpenter's Ferrium C61 page.


One leading application for C61 steel is the Block 2 Upgrade on Boeing's CH-47 Chinook, where C61 steel is being considered as a drop-in replacement for the incumbent 9310 steel. By replacing the incumbent steel 9310, C61 steel provides ~20% weight savings or 20% increase in power at the same geometry. Three prototype forward rotorshafts were successfully produced and delivered in 2013, and successful component rig testing was completed in 2014. Qualification for full-scale production is expected in 2018.

Other applications include gears, integrally-geared driveshafts, actuators and power transmission components where durability, compactness, weight savings, or high temperature resistance is valued.

Its performance is race-proven, with more than 100 ring-and-pinion sets made of C61 steel used in highly demanding SCORE International® off-road races such as the BAJA 1000. Other applications such as armor plate may also be considered.

If you need even greater surface hardness, see Ferrium C64.


Benefits of using C61 steel vs. alloys such as 9310/EN36 or 8620 for power transmission applications can include:

  • Smaller, lighter-weight gears, or greater throughput or durability, including improving existing gears by replacing current materials with C61 steel. Gears and gearboxes using C61 steel can handle higher impact loads and internal stress than comparable designs using traditional materials, or in some cases be reduced in size and weight, due in part to C61 steel's very high fracture toughness and bending fatigue resistance. As one example, C61 ring-and-pinion sets used in SCORE International off-road racing have demonstrated a ~3-4x improvement in lifetime vs. identical parts made of 9310 (see below).
  • Smaller, lighter-weight driveshafts, or greater throughput or durability, including improving existing driveshafts by replacing current materials with C61 steel. Integrally-geared driveshafts using C61 steel can often handle approximately 20-25% higher loads than comparable driveshafts using traditional materials, or be reduced in size and weight by comparable amounts. C61 steel’s core UTS of 240 ksi is a ~39% increase vs. 9310, for example. As one illustration, the weight of the main rotor shaft of a CH-47 helicopter can potentially be reduced by 15-25% without making significant changes to the production process (see below).
  • Reduced manufacturing times and costs, with increased flexibility and control. C61 steel was designed to resist grain growth even at high temperatures, have high hardenability, and use vacuum carburizing with a direct low pressure gas quench, to thus: maintain good properties in large, thick-sectioned components (even with vacuum carburizing); reduce final machining/finishing costs by eliminating intergranular oxide formation and reducing quench distortion; eliminate the time, expense, equipment and non-uniformity of the traditional after-carburizing oil quench “hardening” step; and permit “dial-in” control of carburized case hardness profiles. AGMA technical paper 11FTM27 provides a detailed comparison of costs and time (see below).
  • Superior high temperature operability and survivability such as in oil-out emergency conditions or high-temperature environments. The 900°F tempering temperature of C61 steel is 400-600°F higher than most incumbent alloys, yielding superior thermal stability and allowing gearboxes to run hotter (thus reducing cooling system weight, size, drag, etc.).
  • Greater corrosion resistance. Under a Navy-funded project, QuesTek has shown C61 steel's corrosion resistance to be greater than that of both 9310 and Pyrowear 53.

Forging, Machining, Heat Treatment and Finishing Processes

C61 steel was designed for high-temperature carburizing. This allows solution heat treatment to be combined with the carburizing treatment and C61 steel is therefore quenched directly from the carburizing temperature. After quenching to room temperature C61 steel is subjected to liquid nitrogen immersion to assure a complete martensitic transformation. C61 steel is typically tempered at 900°F (482°C) and has excellent thermal resistance approaching this temperature. If desired, nitriding can be used in lieu of carburizing in order to achieve higher surface hardness. Using both nitriding and case carburizing may result in a brittle surface, resulting in sub-surface spalling initiation and significantly lower fatigue life; users should complete internal trials before considering this combination.

For detailed information about forging, machining, heat treating and finishing processes of C61 steel, please refer to our Manufacturing and Thermal Processing of C61 Steel document.

C61 Steel Availability and Pricing

Carpenter Technology produces and typically keeps an inventory of C61 steel in round bar form of the following sizes, usually with immediate shipment and no minimum order: 1.25", 2", 3", 4", 6.5" and 10.5" OD.

Other sizes will be added in time, and they can produce custom diameters, round corner square and octagon, although longer lead times and minimum orders in the range of 800 lbs are likely.

For pricing, inventory availability and lead time please contact Carpenter Latrobe facility by phone (800-241-8527) or e-mail at service@cartech.com. You may also contact the Reading facility by phone (800-572-2800 or 800-245-5030) or e-mail at sales@cartech.com

Technical Assistance

If you have a specific application engineering or technical question that is not answered by the processing document above, then feel free to Contact Us.

Design and Development Background

We computationally designed C61 steel in response to stated needs for improved materials that could yield higher-density and more high-temperature-resistant power transmission.

In 2011 we co-authored an AGMA technical paper that analyzed total gear manufacturing costs and compared Ferrium C61 and C64 to baseline alloys (see below).

In 2011 SAE International issued AMS 6517 specification for C61 steel.

In 2010 we began a U.S. Army-funded SBIR Phase II project to evaluate the use of C61 steel to replace 9310 as the material of construction for the forward rotor shaft of the Boeing-designed CH-47 Chinook helicopter (see below). It is expected that the weight of this large shaft can be reduced by 15-25%, and that total thermal processing and manufacturing operations can be simplified.

Relevant files


Material Data Sheets


Press Releases