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Liquid Nitriding (Salt Bath Nitriding):
Frequently Asked Questions

How Does Liquid Nitriding Work?

9: What is a compound layer, and a diffusion layer? How is CL different from case depth?
A compound layer is the outermost surface layer, or zone, formed during the nitrocarburizing process, and consists mostly of an iron nitride compound. This hard compound layer provides most of the desired wear and corrosion properties. The diffusion zone contains much less nitrogen, and forms immediately below the compound layer. The diffusion zone is the source of improved strength and fatigue properties. The "case" formed by Liquid Nitriding is composed of the compound layer, plus all or most of the diffusion zone.
     
10:   What is the temperature range in which LN is performed?
Liquid nitriding may be performed at temperatures as low as 500°C (932°F), and as high as 630°C (1166°F). Typically, however, the temperature range is between 540°C - 590°C (1000°F -1090°F). TS USA uses specially formulated nitriding chemistries whereby nitriding can be performed at 510°C (950°F) for certain types of steels, without compromising compound layer depth or quality and without any reduction in process productivity.
     
11:   What are typical compound and diffusion layer thickness and hardness ranges for different materials? Plain carbon steels? Low alloy steels? Medium alloy steels? Cast Irons? Martensitic and Austenitic stainless steels?
Both compound layer depth and hardness is dependent upon the composition of the material being treated. Hardness will increase with the amount of nitride-forming alloys (e.g. chromium, vanadium etc.), and depth will decrease with overall alloy content, including carbon. With ARCOR® Liquid Nitriding, compound layer depth can range from 0.0001 inches for stainless steels to 0.001 inches for plain carbon steels (0.003 mm - 0.025 mm OR 3 to 25 microns); and hardness from 600 HV for low carbon steels up to 1200+ HV for stainless steels (55 – 70+ HRC). Similarly, diffusion zone depths will also vary with alloy content, including carbon. These can range from 0.001 inches (0.025 mm) for austenitic stainless steels to 0.040 inches (1 mm) for plain carbon steels. It should be noted that austenitic materials, and certain martensitic PH grades, do not produce a typical compound layer. Rather, it is a relatively thin total "case" – usually 0.001 - 0.0025 inches - that consists of multiple layers of varying hardness and composition.
     
12:   What is the CL thickness uniformity on a single part and from part to part within a batch?
The compound layer thickness uniformity produced by the liquid medium is very consistent, normally varying ± 3 µm, assuming metallurgical uniformity of the surfaces to be nitrided.
     
13:   Do the dimensions of the part change after Liquid Nitriding?
All surface treatments will change part dimensions to some degree, but Liquid Nitriding produces comparatively little growth; typically 0.0002-0.0003 inches (5 - 8µm) on the diameter.
     
14:   What are typical processing times - both for Liquid Nitriding itself and typical pre and post nitriding steps?
Typical time cycles in the actual nitrocarburizing step are 60-90 minutes, at temperature. However, this can be as low as 10 minutes or as high as 4 hours, depending on the material and specific customer requirements. Pre- and post- nitriding steps may add 3-4 hours, for a total time of 5-6 hours, approximately. Typical gas nitriding times are between 24-30 hours, excluding pre and post nitriding times.