Total case depth 1

[gblanchard]

I am seeking the best functional definition of TOTAL case depth. A carburiuzed and hardened part specifies a TOTAL case depth of.015"-.022". There appears to be multiple technical definitions. In my history, total case depth was determined by graphing traverse microhardness measurements, selecting the point where the curve levels to a gradual core slope. Is this a correct and practicable determination of total case depth?

 

[dbooker630]

For routine checks I typically use the visual method, polish-etch-measure with a Brinell glass (SAE J423 section 6.2).

I understand what you are trying to do, I would think quench temperature would be a factor in getting a clear change of slope.

In my company we refer to SAE J423 for the technical definition. I took a glance but I do not see a graphical method listed in the SAE document for total case measurement.

 

[CoryPad]

Same here, SAE J423.

 

[gblanchard]

Thank you folks. Perhaps a bit more information. The material is 8620 and it is usually metallographically evaluated after quench and temper. I find it difficult to see much visually discernable difference in microstructure below the 50 HRC depth, probably contributed to by the part's section thickness, yet I know the total case extends some 1/3 to 1/2 deeper than the effective case. Any thoughts? The part is bushing-shaped with a wall thickness about 3/8" in the area evaluated.

 

[israelkk]

Here is the methods for case depth measuring as per MIL-sTD-1876, NITRIDING, GASEOUS ATMOSPHERE.


5.3.1.1 Micro-hardness survey method. Test coupons or part shall be sectioned in accordance with figure 5 and prepared for evaluation in accordance with ASTM E3. Hardness survey ihail be conducted in accordance with ASTM E384 from surface to core using appropriate load and selection of
incre=nts to avoid stress deformations between indentations. First Impression shall be taken at 0.002 inch from the surface and progressing in increments of 0.004 inch until the case has been traversed. The case depth is
the distance from the nitrided surface to the point where the maximum of the core hardness specified on the engineering drawing is obtained. Recommended load is 500 grams.

5.3.1.2 Chemical etch.method. The surface of the applicable test coupon (prepared in accordance with 5.3.1.1) shall be etched in 2 percent nital etch (ASTM E407) solution (ref 4.2.5) for a time sufficient to reveal a microstructure. The case depth shall be determined by measuring the distance
from the surface to the edge of the dark band (case material) in the location specified in figure 4 using a Brinell glass or equivalent.

Here is the methods for case depth measuring as per MIL-STD-1878 CARMJRIZING, GASEOUS ATMOSPHERE

5.5.1.1.3.1 Microhardness traverse survey. This method shall be used as the referee method in case of any dispute. Load washer processed through the entire carburizing and hardening cycle with the parts, shall be prepared in
accordance with SAE J423 or ASTM E3. Hardness survey shall be conducted in accordance with ASTM E384 from surface to core using an appropriate load and selection of increments to avoid stress deformation between indentations. First impression shall be taken at a point 0.002 inch from the carburized surface. The effective case depth is the perpendicular distance from the finished surface of the case to point of Rockwell HRC50 equivalent.

5.5.1.1.3.2 Microscopic etch. Surface of the test sample (see 5.5.1.1.3.1) shall be chemically etched in a 2 to 5 percent nital solution in accordance with ASTM E407 for a time sufficient to reveal the microstructure. Effective case depth is the distance from the surface to metallographic structures, usually 50 percent martensite which have been shown to be equivalent to Rockwell HRC50.

Note the Rockwell HRC50 lower limit to establish case depth.

 

[TVP]

blanchard,

There are two separate points that should be clarified. One is the definition of total case depth and the other is the best method for determination of total case depth. SAE J423 uses the following definition of total case depth:

Total Case Depth — The distance (measured perpendicularly) from the surface of the hardened or unhardened
case to a point where differences in chemical or physical properties of the case and core no longer can be
distinguished.

Now, regarding the best method for determination of total case depth, I agree with you that a hardness traverse method best identifies the point at which "case and core no longer can be distinguished". SAE J423 does not make a distinction on which method is best for determination of total case depth, only for effective case depth. The standard states how each method is conducted [Chemical, Mechanical (hardness testing), Visual (includes microscopical methods)] but does not explicitly state a referee method for total case depth. I suggest that you modify your drawing or procedure to indicate the following

TOTAL case depth 0.015 - 0.022 inches according to SAE J423 (alternate cross-section method, Figure 3)