The Internet is a wonderful place, full of invaluable information available, literally, at our fingertips. Did you ever wonder, however, how much of the information you’re viewing is truly accurate? An intriguing question raised by a reader about hardness testing of carburized case depths is one such example. Let’s learn more.
The Question
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?”
The Answer
There is indeed a minimum case depth that will allow its accurate determination by indentation surface hardness measurements using standard and superficial hardness tests (Table 2). Note that the values are listed for effective case depths in this table, which are different than the total case depth information in Table 1.
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.
Understanding Case Depth[3]
Here are the most commonly accepted definitions for case depth in terms of carbon content.
• Total case depth:The depth at which the carbon content of the steel is 0.04% above the core carbon content of the steel. In other words, total case depth is the point at which differences in the chemical or physical properties of the case and core no longer can be distinguished (Fig. 1). Many metallurgists determine the total case depth by (nital) etching a steel sample and measuring the zone affected by the etchant.
• Effective case depth: The depth at which the carbon content of low-alloy steel is 0.40%, or approximately 0.30% for medium-alloy and high-alloy steels.
Since it is often time consuming to measure the carbon concentration as a function of depth in a carburized component to determine effective case depth, other techniques (such as microhardness) are commonly used. In certain situations involving very deep case depths, hardness is measured directly on the A or C scales. In these instances, the following definitions are used.
• Effective case depth (U.S.): The distance from the surface (in inches) to a point within the case where the hardness is 50 HRC (542 Knoop or 513 HV). This measurement is done on a cross section by microhardness techniques with a Knoop or Vickers indenter (Fig. 2). These values are then converted to Rockwell C hardness if a 500 gram-force load is used per ASTM E384 (latest revision). Interpolation is required to arrive at the proper case-depth value. The effective case depth depends on the carbon gradient and the case hardenability, but 50 HRC is typically equated to a carbon content of 0.40% in low-alloy steels and 0.30% in medium- and high-alloy steels.
• Effective case depth (international): The distance from the surface (in millimeters) to a point within the case where the hardness is 550 HV (approximately 52.5 HRC). This measurement is done by a Vickers microhardness method with a pre-defined load of 1 kg and is not converted to Rockwell C. The effective case depth still depends on the carbon gradient and the case hardenability. For highly alloyed case-hardening steels, the carbon content for 550 HV is typically in the range of 0.25-0.30%. For medium-alloy grades, it is approximately 0.30-0.35% C, and for low-alloy steels, it is approximately 0.35-0.40% C.
Case-Depth Callouts on Prints
Case depth on engineering drawings should be stated either as total case depth, effective case depth or finished effective case depth after a defined amount of material is removed from the surface. It is important to note that if neither total nor effective case depth is specified, one must notassume that total case depth is intended (although at one time many years ago this was a commonly held belief).
A “rule of thumb” to determine total case depth from the effective case depth is to multiply by 3/2, or to determine effective case given the total case depth is to multiply the total case depth 2/3. The steel grade, chemistry and hardenability of material (as well as case depth) can affect this percentage.
The following are several recommendations for writing clear specifications that unambiguously define the desired case.[3]
• Use wording and sketches, as needed, to define test locations and methods. However, do not use verbiage to define a hardened case.
• Specify the test procedure at each location and for each characteristic. State the maximum and minimum specification tolerance.
• Specify the condition of the part at the time of testing (as-hardened, after temper, after final grind).
• Show the draft print and specification to your test lab and part producer and check their interpretations of the specification against your requirements.
Next Time
We continue this discussion and focus on other methods of measuring case depth in August. IH
References
1. SAE Handbook, Part 1, 1982. Society of Automotive Engineers, 1982.
2. Fundamentals of Rockwell Hardness Testing, Bulletin WB1226, Instron, 2004.
3. Herring, Daniel H., “Carburized Case Depth,” Heat Treating Progress, January/February 2002.
4. Herring, Daniel H., “A Quench-and-Temper Technique for Evaluating Carburized Case Depths,” Industrial Heating, September 2009.
5. Satisfactory Case Hardened Part Performance Requires Proper Specifications, Case Studies from Element Experts, Element Materials Technology.
6. Low, Samuel R., NIST Recommended Practice Guide, Special Publication 960-5.
Podcast Copy:
In this month’s IH Monthly Prescription Podcast we talk about exothermic atmospheres. Every month, Dan Herring sits down with IH’s editor, Reed Miller, to talk technical. If you have a topic you would like them to discuss, drop us an e-mail at reed@industrialheating.com. Find the podcast on our website. IH Monthly Prescription is sponsored by ALD-Holcroft.
