We continue our discussion on how to interpret carburized case depths by focusing on measurement techniques. These methods are also applicable to cases produced by nitriding, nitrocarburizing, boronizing, and induction or flame hardening. Let’s learn more.

    Methods used to determine the depth of case can be categorized as visual, chemical or mechanical in nature. Sample parts or representative test specimens are most often of the same grade of steel as that being case hardened and ideally from the same heat of steel. At the very least, one should know the chemistry and/or properties. Some companies, however, prefer to use a single steel (e.g., SAE 8620 for carburizing) and perform tests on it for comparative purposes with the added benefit of gaining insights into furnace performance over time.

Visual Methods

Visual interpretation falls in two broad categories: macroscopic and microscopic, and both are valuable.

Macroscopic Techniques

Figure 1.  Example of demarcation line in the Ms method

Macroscopic methods are often used on the shop floor for routine process control, primarily because of their simplicity and the short time required for determination. They are typically done using the unaided eye, a loop or with a stereomicroscope up to magnifications of 40X. Accuracy of results can be improved by correlation with other methods used to measure the case depth of the parts being processed.

    Visual methods are normally applied to hardened specimens. Induction- and flame-hardened samples are prime examples since they usually have excellent contrast between case and core when macroetched (e.g., 10% Nital). Other methods include the use of fracture bars, which is an efficient and quick way to test every case-hardened load. The depth of hardening is well defined and easily interpreted by fracture methods. The outside has a flat but slightly grainy appearance associated with brittleness, while the inside has an irregular, rather fibrous appearance associated with toughness.

    The important point is that the fracture changes from one to the other quite abruptly. Case depth can be measured by a Brinell scope or with a scale on a stereomicroscope. If the core is soft, the fractured surface will exhibit good contrast between case and core. If the core is hard, bluing the fracture on a hot plate^[3] can enhance the contrast.

    The M_s method^[4] is another, but more involved, visual technique. It is based on the fact that the martensite-start temperature (M_s) varies with carbon content. Quenching (typically in a salt bath) and then holding the steel for a short time at the M_s temperature corresponding to a given carbon content tempers the martensite formed at all lower-carbon levels. Subsequent water quenching transforms austenite at all higher-carbon levels to untempered martensite. Polishing and etching reveals a sharp line of demarca-tion between tempered and untempered martensite (Fig. 1).

Microscopic Techniques

Microscopic methods are commonly used for determining case depth and have been described in detail by others.^[3,5] What is often overlooked is that their accuracy depends on the nature of the case and core microstructures. For example, carburized depth is easier to evaluate in an unhardened sample while nitrided cases are, in general, difficult to estimate. It is important that the sectioning of the sample be perpendicular. Otherwise, the taper angle must be known. Perhaps the single-most overlooked step is to ensure good edge retention by use of proper mounting methods and procedures.

    Microscopic techniques require that specimens first be given a full polish and etch before the evaluation, usually at 100X. Effective case-depth determination of hardened specimens relies on comparison to metallographic structures found to be equivalent to 50 HRC by other methods. A structure that is approximately 85% tempered martensite and 15% mixed transformation products often corresponds to 50 HRC. Total case depth is the demarcation line between the case and core (between dark and light regions after etching). This line is far from distinct for alloy steels.

Chemical Methods

These methods usually rely on analysis of chips from turned bars. Test specimens must be carburized with the parts or in a manner representative of the process. If the parts and test specimens are quenched after carburizing, the specimens should be tempered at approximately 595-650?C (1100-1200?F) and straightened to 0.04 mm (0.0015 inch) maximum TIR (total indicator runout) before machining is performed. The time at temperature should be kept to a minimum to avoid excessive carbon diffusion even at these low temperatures.

    Machining intervals between 0.05 and 0.25 mm (0.002 and 0.010 inch) are typically chosen depending on the accuracy desired and expected depth of case. Chips from each increment must be kept separate and analyzed individually for carbon content in a carbon analyzer or other suitable device. In some cases, especially for deep-case carburizing, taper bars can be used. They are machined, and spectrographic analysis is performed along the length of the bars at a spacing of at least one turn diameter apart.

Mechanical Methods

These methods are preferred for an accurate determination of effective case depth and for determining total case depth in parts that have been shallow case hardened. The use of this method is based on obtaining and recording hardness values at specific intervals through the case. The sample is considered through-hardened if the hardness level does not drop below the effective case-depth value.

    Considerable care should be exercised during preparation of specimens for case-depth determination by any of the mechanical methods. Serious errors can be introduced if the specimen has not been properly prepared. In the case of microhardness measurements, it is important to avoid cutting or grinding burns. It is always a good idea to use an etchant for burn detection as a general precaution, although this is almost never done in practice.

    If the specimen is to be tested directly on a Rockwell scale, the cutoff technique done on it is critical. The hardness indentations must be made perpendicular to the surface, and in no case can the angle from parallel of the top and bottom surfaces be greater than 2 degrees. Otherwise, the readings will be erroneous.

    When using microhardness methods, surface finish of the specimen is important and is a function of the indenter load. For accurate readings, the hardness impressions must not be affected by the surface condition. For example, a Knoop (500-gram) hardness profile can be performed on a specimen that was final polished on 600-grit (15-micron) paper. (Remember, the larger grit numbers correspond to smaller particle size and smoother surface finish with finer scratches.) The lighter the indenter load, however, the finer the polish necessary. Also, the hardness traverse should be started far enough below the surface of the case to ensure proper support from the metal between the center of the impression and the surface. A common error is to use too heavy an indenter load too close to the edge of the specimen, which results in deflection at the edge and a false (low) hardness value.

    Another common error is to bunch the readings too close together. Making an indentation cold works the surface in the vicinity of the impression. If a subsequent reading is taken too close to a previous one, the resultant hardness value will be distorted (too high). For light and medium cases, up to 0.75 mm (0.030 inch), the indentations should be spaced along a 45-degree diagonal, a minimum of one indenter width apart. For deeper cases readings under one another are acceptable.

    A typical Vickers or Knoop (500-gram) microhardness traverse would have an initial reading at 0.06 mm (0.0025 inch) and subsequent readings at 0.13-mm (0.005-inch) intervals to 0.75 mm (0.030 inch) and then at 0.25-mm (0.010-inch) intervals until readings above and below the 513 HV value are observed. Interpolation or additional indentations can be done to determine the exact value.

Summary

Regardless of the technique used to determine total and effective case depth in carburized components, it is important that the method be consistent, accurate and correlate to actual physical and mechanical properties as they relate to the performance and characteristics of the part in service. IH

References

1. Bernard III, William J., “Methods of Measuring Case Depth in Steel,” ASM Handbook, Volume 4A: Steel Heat Treating, Fundamentals and Processes, ASM International, 2013, pp. 305-413

2. Herring, Daniel H., “Carburized Case Depth,” Heat Treating Progress, January/February 2002

3. Vander Voort, George, Metallography, Principles and Practice, ASM International, 1999

4. Herring, Daniel H., “A Quench-and-Temper Technique for Evaluating Carburized Case Depths,” Industrial Heating, September 2009

5. Vander Voort, George, “Microstructural Characterization of Heat Treated Irons and Steels,” ASM Handbook, Volume 4C (in preparation), ASM International