They say that you can’t teach an old dog a new trick, but what about teaching a new dog an old trick? Did you know there is a simple and highly accurate shop-floor technique to measure case depth that does not involve microhardness testing or carbon-bar analysis? Let’s learn more.

Fig. 1. Case/core demarcation by Ms point method (0.50% C sample) (a) Ms test specimen (2-3X) that has been heat treated, mounted, polished and etched. The lighting makes the unetched (fresh martensite) zone on the outer edge look black, but it is actually a shiny polished surface. (b) Close-up of the demarcation zone between the etched (tempered-martensite) zone and the unetched (fresh-martensite) zone. The line of demarcation is not exact. Generally, the depth is “read” using a Brinell scope at approximately 12-15X. When you read the depth, you read to the depth where the tempered martensite layer is consistent (not to the outer edges of the tempered martensite wisps). At 12-15X the line is not as fuzzy and the depth can be pretty consistently measured to +/-0.001 inch (0.025 mm) or so.

The Method

The method was developed by Rowland and Lyle[1] and rests on the principle that you can create a clear and precisely measureable line of demarcation (Fig. 1) between tempered martensite formed in a quench bath held for a short time at the martensite-start (Ms) point of the steel and the freshly formed martensite produced by subsequent water quenching.

The technique involves taking a small specimen from a carburized load and re-austenitizing it followed by quenching into a bath maintained at the martensite-start point of the steel corresponding to the carbon content at which the case-depth measurement is desired. The method is quick for a large number of samples and has good repeatability.

The Theory

The method relies on the principle that a properly austenitized sample can be quenched into a bath maintained at the Ms temperature corresponding to the carbon content at which the case measurement is desired, held at this temperature for a very short time (typically seconds) and water quenched to achieve a line of demarcation between the case and core. Case depths measured at various carbon levels typically range from 0.40–0.70% C.

The effect of carbon on the Ms point is independent of either the type or amount of alloying elements associated with it. The effect of elements other than carbon on the Ms point is sufficiently small that normal variation of analysis – from heat to heat within a given type – can be generally ignored and a single set of conditions used. Chemical segregation or extreme variations of both manganese and chromium in the same direction away from the middle of the range can affect local Ms temperature and have to be taken care of by slight adjustments.

A great deal of work has already been done to accurately determine the Ms temperature for most steels.[3] It is well known that the Ms temperature is strongly dependent on the composition of austenite, the parent phase. The effect of individual alloying elements upon the Ms temperature for iron-based (binary) alloys has also been extensive studied. The results show that Al, Ti, V and Co effectively increase the Ms temperature, whereas Nb, Cu, Cr, Mo, Ni, C and N cause it to decrease.

The Process

Typically, a small sample (e.g., 2.25-inch OD x 1.25-inch ID x 1.25-inch long) is austenitized in either a standard laboratory box furnace or a salt bath. If a box furnace is used, the samples are typically coated with either a boric-acid slurry or a stop-off paint to prevent decarburization. The samples are heated and held long enough to produce complete carbon solution at the desired carbon level. The sample is then rapidly quenched into a salt or oil bath and held for the appropriate temperature/time (Table 1) prior to water quenching. For higher Ms temperatures, salt quenching is common. For lower temperatures (up to around 300–350°F), oil quenching is often used (provided the oil doesn’t smoke too much).

This technique works for any steel for which you can calculate or experimentally determine the Ms temperature for the desired carbon level. The technique is easy to perform and does not restrict control of conditions except on very low-carbon or low-alloy steels. It is important to hold at the isothermal quench temperature longer at lower temperatures so that the martensitic portion formed at that temperature will isothermally temper and etch dark (2% nital) while the outer portion, which remains austenitic until the final water quench transforms it to fresh martensite, does not etch. At higher isothermal hold temperatures, the auto isothermal tempering occurs more rapidly, reducing the need for long hold times.

Pros and Cons

The biggest advantage of this test is that it is quick and easy. It is also highly accurate for carbon levels above 0.40%.

One of the great advantages of this method is that the line of demarcation between the tempered and freshly formed martensite (due to quenching) is sufficiently sharp to permit the average heat treater to determine case depth within 0.002 inch (0.050 mm) using a simple calibrated loop or low-power stereomicroscope. This level of accuracy is true whether comparing to microhardness or carbon-bar (gradient) methods.

Further, none of the test conditions require close control (except for carbon and very low-alloy steels); a change of 6°F (3.5°C) represents only one point (0.01% C) of carbon. And almost any sized specimen convenient for (hand) polishing can be used. Finally, the effect of elements other than carbon on the Ms point is sufficiently small that normal variation of analysis from heat to heat can generally be ignored allowing a single set of parameters to be used.

The principal limitation of this method lies in its application to low-carbon (e.g., SAE 1018/1020) and very low-alloy (e.g., 4028) steels because of bainite formation in the constant-temperature bath. In these cases, a maximum specimen thickness of 1/16 inch (1.60 mm) is required, and this presents both difficulties in handling and control of quench-bath temperature. For example, a 1020 steel quenched to the 0.40% C level at 635°F (335°C) held for three seconds is too short a holding time for good delineation, five seconds is satisfactory while 10 seconds is too long (completely obscuring the line of demarcation because of bainite formation). Precautions should also be taken to avoid pearlite formation on quenching as well as to ensure complete carbon solution at the desired level of measurement before quenching.

Finally, the dividing line between the higher-carbon fresh martensite and the lower-carbon tempered martensite is often not a perfectly straight line, and different operators will read slightly different values depending on where they judge the line of demarcation to be. Readings can vary from operator to operator, especially if the depth is determined by a Brinell scope.

Tips

To make the job easier:

1. Samples must be cut before applying this technique and not afterward to avoid tempering the freshly quenched martensite.
2. Good etching practices are essential as the sample should be etched just enough to develop maximum contrast.
3. Material that is heavily banded can still be evaluated using this method, but tight control of the process parameters is mandatory.

Summing Up

This method is simple, straightforward and can be easily performed by shop personnel, making it a valuable addition to the heat treater’s arsenal of testing methods. IH

Acknowledgment
The author would like to thank Mr. Craig Darragh, senior product technologist-steel, The Timken Company, for providing both the idea and inspiration for this article.