Interpreting Carburized Case Depths

jixieman

As part of suggested guidelines for designers, it was recommended that hardness on drawings be specified as Rockwell “C’’ with a minimum spread of 5 points or as a minimum hardness value, again in Rockwell C. The statement was then made that if total case depth is less than 0.76 mm (0.030 inch), an accurate reading cannot be obtained on “C” scale because a 150-kg load will drive the penetrator through the case and into core material, giving a false reading. Various Rockwell scales were recommended (Table 1) for use with minimum total case depths. The question was, “How accurate is this data?”

jixieman

UNDERSTANDING HARDNESS TESTING OF CASE-HARDENED PARTS

In general, there are four controlling factors in the selection of the proper scale for hardness testing, namely type of material (chemistry and hardenability), thickness of the specimen, width of the area to be tested and scale limitations. It is this last factor that is often open to interpretation.

    In the case of Rockwell C, typical applications (per ASTM E18) include “deep case-hardened steel.” In the case of Rockwell A, its use includes “shallow case-hardened steel.” These statements, while interesting, do not in and of themselves tell us if the data in Table 1 is correct.

    The regular or superficial Rockwell scales are established such that an infinitely hard material will read 100 on the diamond penetrator scales (or 130 on the ball penetrator scales). Thus, one regular Rockwell number represents a penetration of 0.002 mm (0.000080 inch). Therefore, a reading of 60 HRC indicates penetration (from minor to major load) of 100 – 60 = 40 x 0.002 = 0.08 mm (0.0032 inch). By contrast, in Rockwell superficial testing, one superficial Rockwell number represents a penetration of 0.001 mm (0.000040 inch). Therefore, a reading of 90 HR15N indicates penetration from minor to major load of (100 – 90) x 0.001 = 0.01 mm (0.0004 inch).[2] However, Rockwell superficial testing, due to the lighter applied load, has a greater margin for error, which is why Rockwell C or Rockwell A testing is preferred.

oliver

Discussion:

    I don't think the Rockwell scale recomended on Table 1 is correct to test surface hardness on case hardened parts.
    For example, when case depth is more than 0.76 mm and "C" scale is used, even penitrator may not drive through the case, the case might collapse since the core is lower carbon and soft. That will cause inaccurate reading.
    HV scale should be more proper to recomend for testing surface hardness on case hardened parts.