Nitriding vs. FNC
How well do you know hardness processing? Can you draw the line where nitriding and ferritic nitrocarburizing (FNC) differ? In this Technical Tuesday feature, skim this straight forward data that has been assembled from information provided by four heat treat experts: Jason Orosz and Mark Hemsath at Nitrex, Thomas Wingens at WINGENS LLC – International Industry Consultancy, and Dan Herring, The Heat Treat Doctor at The HERRING GROUP, Inc.
Let us know what you think! What is the next comparison you'd like to see? What facts were you surprised by? Email Heat Treat Daily editor Bethany Leone at bethany@heattreattoday.com.
| Nitriding | Descriptor | Ferritic Nitrocarburizing |
|---|---|---|
| 480º-590C (896º-1094ºF) typical | Temperature Range | 565º-590ºC (1049ºF-1094ºF) typical |
| Wrought and powder metallurgy materials including alloy steels (e.g., 4140), stainless steel (e.g., 304L, 420), tool steels (e.g., H11, H13) and special nitriding steels (e.g,, Nitralloy 135M, Nitralloy EZ) are typical examples. Many other steel grades are possible. | Materials Commonly Processed | Plain and medium carbon steels (e.g., 1015, 1018, 1045), alloy steels (e.g., 4140, 4340) and tool steels (e.g., H11, H13) are typical examples. Many other steels grades are possible. |
| Wear (as in abrasion resistance), bending, torsional and rolling contact, fatigue resistance, lubricity, and adhesive strength improvements. | Materials Commonly Processed: Why to Process Them with These Methods | Wear resistance, lubricity, fatigue, and corrosion resistance are primary benefits with improved fatigue strength and adhesive strength possible. |
| 3-48 hours at temperature. May be up to 72 hours. | Relative Cycle Times | 2-6 hours at temperature. |
| Pit retort furnaces and front load retort furnaces for gas nitriding, although bell retort furnaces have also been used. | Equipment Types Used for the Process | Pit retort furnaces and front load retort furnaces for gaseous ferritic nitrocarburizing. Bell retort furnaces have also been used. |
| Ammonia and nitrogen or ammonia and dissociated ammonia. | Atmospheres Used/Required | Ammonia and nitrogen and carbon-bearing gas such as CO2, CO, or endothermic gas. |
| Dies, gears, pump bodies, springs, gun barrels, shafts and pinions, pins, brake rotors and may other types of component parts produced from bar, plate, rod, forgings and castings formed by stampings, machining, rolling, forging, casting, etc. | Typical Parts Processed | Wear plates, washers, clutch plates, gas pistons, brake pistons, brake rotors, barrels, slides, differential cases and other types of component parts produced from bar, plate, rod, etc., and formed by stampings, rolling, machining, casting, etc. |
| Automotive, aerospace, oil & gas, industrial machinery (e.g., pumps), and tool & die. | Typical Industries Served | Automotive and industrial machinery hydraulics. |
| Cost is often higher for gas nitriding as opposed to other case hardening processes (including FNC) based on the type of component parts run. In many cases, cost is a function of the longer cycle time and/or more labor involved. | Relative Cost Per Unit | Cost is often lower than many other case hardening processes (including gas nitriding) based on the type(s) of component parts run. In many cases, cost is a function of a shorter cycle time and/or less labor involved. |
| Basic specifications are easily achieved with good equipment and/or controls; difficulty increases when attempting to produce specialized layer compositions/phases. | Ease of Use/Control | Basic specifications are easily achieved with good equipment and/or controls; difficulty increases when attempting to produce specialized layer compositions/phases. Hardware/control requirements are more complicated than for nitriding when controlling for carbon potential. |
| It can range from very simple to medium-high depending on application. | Relative Expertise Necessary to Perform | Medium-high depending on the application. The user will want to look for clean parts, a good loading system, and PLC controlled cycle. |
| Aqueous (clean chemistry) including rinse/dry, vapor degreasing (clean chemistry). | Cleaning Requirements | Aqueous (clean chemistry) including rinse/dry, vapor degreasing (clean chemistry). |
| White glove | Handling Requirements | White glove |
| Pre- and post-oxidation | Process Options | Pre- and post-oxidation |
| AMS 2759, AMS 2759/10, (latest revisions) | Applicable Specifications | AMS 2757, AMS 2759/12, AMS 2759/13 (latest revisions) |
| Time, temperature, gas flow, nitriding potential (Kn) and/or percent dissociation, hydrogen sensors. | Controls | Time, temperature, gas flow, nitriding potential (Kn), carbon potential (Kc) and oxygen potential (Ko). Hydrogen sensor and oxygen (carburizing) sensor may be used. |
| electric and gas-fired equipment | Fuel Source | electric and gas-fired equipment |
| Hardness (surface, core), case depth determination (via microhardness – typically core hardness + 50 HV), microstructure (compound and diffusion zone depths), composition, core structure, presence of absence of nitride networking (aka nitride needles), and the presence or absence of cracking or spalling of the case. | Testing Required | Hardness (surface, core), case depth determination (via microhardness – typically core hardness + 50 HV), microstructure (compound and diffusion zone depths), composition, core structure, porosity (type and depth), and the presence or absence of cracking or spalling of the case. |
| Warm wall plasma nitriding, as well as advances in controls, sensors, temperature uniformity, and reduced gas volumes. | Latest Advances | Black oxide, hydrogen sensors, and fast cooling techniques as well as advances in controls, sensors, and temperature uniformity. |
| (1) simple equipment, (2) can offer beneficial tribological changes part/metal, (3) performed after part machining, (4) little-to-no distortion. | Pros (Strengths) | (1) fast, cheap, repeatable results, (2) excellent corrosion resistance, especially with (black) oxide, (3) performed after part machining, (4) minimal distortion/almost distortion free |
| (1) long cycle time, sometimes a multi-day process if deep case is required, (2) effective pre-cleaning required, (3) weldability becomes reduced, (4) ammonia is used, (5) embrittlement with too much white layer. | Cons (Weaknesses) | (1) Focused on part surface, mainly with inexpensive materials, (2) effective pre-cleaning required, (3) weldability becomes reduced, (4) ammonia is sometimes a concern. |
A leading automotive supplier in the U.S. recently received a large oven for its operations. This furnace was customized with a heavy duty cast work tray which sits on the floor of the chamber inside the liner area to support the workload and protect the floor brick.
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