CASE DEPTH MEASUREMENT

1. Effective case depth (ECD) and total case depth (TCD)

• Effective Case Depth (ECD)

Effective case depth or ECD is the perpendicular distance from the surface of a hardened case to the deepest point at which a specified level of hardness is maintained. The hardness criterion except when otherwise stated is 150 HV higher than the core hardness. ECD will be the distance from the surface when the reading is 150 HV higher than the core hardness. ECD can be up to 0.6mm for some applications.

• Total Case Depth (TCD)

Total case depth is defined as the perpendicular distance from the surface of a hardened case to the point at which differences in chemical and physical properties between the case and the core can be distinguished.

• Surface hardness

Surface hardness of a plasma nitrided part is between 350-1200 HV.

2. Case depth measurement

• Hardness transverse method
  • From the sample, cut a test piece perpendicular to the surface and mount it.
  • Polish the test piece
  • The case depth is measured utilizing a Vickers’ hardness tester. An indentation is made using a load of 100 -300 gf at the interval of 0.1 mm from the surface until there are no changes in the size of the indentation, as shown in Fig.1.
  •  The effective case depth will be the distance from the surface where the hardness reads 150 HV higher than the core or otherwise as stated.

Fig.1 Impression of hardness indenter

• Microscopic method
• From the sample, cut a test piece perpendicular to the surface and mount it.
• Polish and etch the test piece.
• Measure the darkened zone for the total case depth.

A micrograph indicating plasma nitrided layer is shown in Fig.2.

Fig.2 Plasma nitriding layer after etching (200x)

3. An example

Material: CA6N cast stainless steel
Process: Plasma nitrocarburised at 550 deg C for 30 hrs.
Hardness: As shown in Table 1 and Fig.3

Table 1 Hardness values for a sample

Fig.3 Hardness profile