[aoneill]

This post is totally unrelated to my earlier post regarding locating a service.
Can someone please explain to me when a 'soft ntride' process would be preferrable.
I understand this to be a gas process (with ammonia, CO and N) as opposed to salt or ion process.
Thanks in advance!

[heattreater]

I don't think I've ever heard the term "soft nitride". From the process gases you list, it sounds like a standard nitride process to me. Maybe some people call a regular nitride process "soft nitride" vs an FNC process "hard nitride". I'm not sure. Note that the FNC process requires the addition of a hydrocarbon to the other gases you list.

[josephmarkgreene]

When I read "soft nitride," my first thought was of nitriding a surface that is not already hardened & tempered. While not the standard process for nitriding, it is employed on a relatively infrequent basis.

Typically, a steel is prepared for nitriding by quench hardening. Nitriding steels containing a small percentage of aluminum certainly are appropriate, but high-strength low-alloy steels containing no aluminum respond well to nitriding and develop a less brittle compound layer. After quenching, the steel is tempered at slightly above the nitriding temperature to stabilize the microstructure and relieve stress to promote uniform nitriding. (The steel would otherwise temper during the nitriding process, which is said to yield unpredictable results.) Nitriding produces surface compressive stress and a hardness elevation of typically 350-400 HV above the core. Therefore a core at 32-36 Rc = ~320-360 HV typically ends up in the 700s HV = 60+ Rc at the nitrided surface, thus its enhanced wear and fatigue properties.

If the core is not hardened, then its hardness is lower. When I nitrided automotive components of AISI 5120, the core was just 180 HV, and thus the diffusion layer peaked in the 500s HV. Though this sounds moderate, the components passed all requirements, including fatigue and surface wear. They also experienced no significant distortion, a huge improvement over the manufacturer's standard process of carburizing with salt quenching.

[Brian]

I also wondered if it was not a nitride process on a surface that hadn't been quenched and tempered; but for the purposes of corrosion resistance, not wear. If I remember correctly, nitriding and especially oxy-nitriding give corrosion resistance benefits.

Brian

[josephmarkgreene]

Excellent observation by Brian. I should have thought of that possibility.

Nitriding by itself is frequently reported to provide little or no corrosion protection. Steam oxidation (alone) is perhaps slightly more effective, but nitrocarburizing followed by post-oxidation is a significant improvement over either treatment by itself.

The most elegant of such processes that I have ever run across is Nitrotec, which was developed in the U.K. by A. M. Staines and others. Nitrocarburized & post-oxidized layers often suffer from a couple of serious flaws, first of all that porosity in the compound layer allows corrosive media to penetrate beneath the oxide, compromising the protective value of the layer. Additionally, because epsilon compound layer is more stable at higher temperature, slow cooling allows the epsilon to decompose partially into gamma prime, resulting in a brittle mixed compound layer. Nitrotec addressed both issues by withdrawing the nitrocarburized workpieces HOT through air and immediately quenching into aqueous polymer. A few seconds through air effected the post-oxidation process. Quenching preserved as much epsilon as possible in the compound layer, and the polymer permeated the porosity to impart some additional corrosion protection. And the uniform dark black color looked terrific.

I seem to recall various folks reporting some impressive salt fog results, but Nitrotec didn't miraculously turn the parts into stainless steel. In fact, it performed best in dry ambient conditions. It didn't take much to strip the polymer out of the porosity. I once ran Nitrotec'd parts through a parts washer with hot soapy water, and was astounded at the change in appearance. The uniform black color turned to an uneven gray that was a lot less attractive, and the parts rusted shortly thereafter. I feel that the performance would be significantly enhanced by sealing with epoxy or other polymer that cures hard and doesn't easily wash away.