"case depth" standards

[stevewils]

Please could anyone help me with a surface engineering problem i have. I am doing some research into the nitriding/carburising of metals.
I am struggling to find any detailed information on the standards associated with the case depth that is created during nitriding etc.
I have very little background knowledge in this field so any help on this subject would be grateful.
Cheers
Steve Wilson

 

[shooter45]

General information is in the ASM Handbook on Heat Treating.
Specific requirements depend on the applications - for example gear teeth case depth recommendations are included in AGMA 2001.

 

[metman]

Steve,
Here are some references from the ASM website:

4. Section: The Final Analysis
From: Volume 11, Failure Analysis and Prevention, Article: Failures of Gears
...According to the heat-treating specification, the gears were required to satisfy the following requirements: (1) a carburized case depth of 0.4 to 0.6 mm (0.015 to 0.025 in.), (2) a case hardness of 77 to 80 HR30N, and (3) a core hardness of 36 to 44 HRC....

1. Section: Medium-Alloy Air-Hardening Steels
From: Volume 4, Heat Treating, Article: Heat Treating of Ultrahigh-Strength Steels
...about 525 °C (980 °F). The nitrided case depth depends on time at temperature. For example, gas nitriding in 20 to 30% dissociated ammonia for 8 to 48 h normally produces a case depth of about 0.2 to 0.35 mm (0.008 to 0.014 in.)...

2. Section: Wear and Failure Modes in Die Casting
From: Volume 18, Friction, Lubrication, and Wear Technology, Article: Friction and Wear of Dies and Die Materials*
...of H13 steel. However, the nitrided case depth should not exceed 0.13 mm (0.005 in.); deeper cases can result in flaking or spalling damage at parting lines and sharp edges. Spalling of a nitrided surface (which causes undercuts in the spalled area) leads to tearing of the casting...

Another source is a book entitled "Steel Selection" by Suess (pronounced Seece) and co-authored by another whose name I cannot recall. They give in depth (no pun intended) methods for calculating Hertzian stresses etc for determining required case depth. I have the book at work and will try to remember to get the publisher name etc.

As you can see from the above excerpts, the processing time get be quite long as shown for nitriding. Carburizing time as I recall is inversly proportional to the square root of case depth. Therefore it behooves one to correctly analyze the minimum depth required from the standpoint of cost.

Hope this helps

 

[metman]

Table 39 Typical characteristics of diffusion treatments
Process Nature of case Process
temperature,
°C ( °F) Case hardness,
Typical case depth HRC Typical base metals Process characteristics
Carburizing
Pack Diffused carbon 815-1090 (1500-2000) 125micro m-1.5 mm (5-60 mils) 50-63(a) Low-carbon steels, low-carbon alloy steels Low equipment costs, difficult to control case depth accurately
Gas Diffused carbon 815-980 (1500-1800) 75micro m-1.5 mm (3-60 mils) 50-63(a) Low-carbon steels, low-carbon alloy steels Good control of case depth, suitable for continuous operation, good gas controls required, can be dangerous
Liquid Diffused carbon and possibly nitrogen 815-980 (1500-1800) 50 m-1.5 mm (2-60 mils) 50-65(a) Low-carbon steels, low-carbon alloy steels Faster than pack and gas processes, can pose salt disposal problem, salt baths require frequent maintenance
Vacuum Diffused carbon 815-1090 (1500-2000) 75micro m-1.5 mm (3-60 mils) 50-63(a) Low-carbon steels, low-carbon alloy steels Excellent process control, bright parts, faster than gas carburizing, high equipment costs
Nitriding
Gas Diffused nitrogen, nitrogen compounds 480-590 (900-1100) 125micro m-0.75 mm (5-30 mils) 50-70 Alloy steels, nitriding steels, stainless steels Hardest cases from nitriding steels, quenching not required, low distortion, process is slow, is usually a batch process
Salt Diffused nitrogen, nitrogen compounds 510-565 (950-1050) 2.5micro m-0.75 mm (0.1-30 mils) 50-70 Most ferrous metals including cast irons Usually used for thin hard cases <25micro m (1 mil), no white layer, most are proprietary processes
Ion Diffused nitrogen, nitrogen compounds 340-565 (650-1050) 75 m-0.75 mm (3-30 mils) 50-70 Alloy steels, nitriding steels, stainless steels Faster than gas nitriding, no white layer, high equipment costs, close case control
Carbonitriding
Gas Diffused carbon and nitrogen 760-870 (1400-1600) 75micro m-0.75 mm (3-30 mils) 50-65(a) Low-carbon steels, low-carbon alloy steels, stainless steel Lower temperature than carburizing (less distortion), slightly harder case than carburizing, gas control critical
Liquid (cyaniding) Diffused carbon and nitrogen 760-870 (1400-1600) 2.5-125micro m (0.1-5 mils) 50-65(a) Low-carbon steels Good for thin cases on noncritical parts, batch process, salt disposal problems
Ferritic nitrocarburizing Diffused carbon and nitrogen 480-590 (900-1090) 2.5-25micro m (0.1-1 mil) 40-60(a) Low-carbon steels Low-distortion process for thin case on low-carbon steel, most processes are proprietary

Source: Ref 64