Measurement of Case Depth | Surface Hardening | Steel | Metallurgy

Case and surface-hardening methods such as carburising, nitriding, carbonitriding, cyaniding, induction and flame- hardening, result in a ‘depth of case’ which is substantially harder than the inside section of the component called the ‘core’. In every instant, the chemical composition or mechanical properties, or both, are changed by the process. An accurate method to measure the case depth is essential for the quality control of the process and also to confirm to the specifications.

Various methods for measuring the case depth have been adopted based on the difference in either chemical composition, or mechanical properties or visual colour. Each method of measurement of case should be properly specified. Normally, the measured case depth is properly specified with a notation about the method used for measurement.

Each procedural method has its area of application established through experience, and no single method is advocated for all applications. Different methods may lead to different values of case-depth. A proper choice of method to be used and yet be cheap is very essential.

On a broad basis, the methods used for measuring case depth can be classified as:

1. Chemical Methods.

2. Mechanical Methods.

3. Visual Methods.

and the specimens or parts may undergo certain tests in either the soft or, hardened condition. The measured case depth is reported as actual or equivalent effective case depth and as total case depth on hardened or unhardened specimens.

‘Total case depth’ may be defined as the perpendicular distance from the surface of a hardened or unhardened case to a point at which differences in chemical or physical properties of the case and the core can no longer be distinguished. For example, the point could be where the carbon content is 0.04% more than the carbon content in the core in case of carburised cases.

‘Effective case depth’ is defined as the perpendicular distance from the surface of a hardened case to the farthest point at which a specified hardness value is obtained. This hardness criterion is HRC 50, except when otherwise specified. Effective case depth is always determined on representative samples, or the part itself but in the heat treated condition.

1. Chemical Methods:

This method is generally used for carburised cases, but can also be used for cyanided or carbonitrided cases, and is considered most accurate for measuring the total case depth. The carbon content and nitrogen content when applicable) is determined at incremental depths below the surface. The combustion carbon analysis method or spectrographic method may be used to know the carbon content.

Procedure for Carburised Parts:

Test specimens are normally of the same grade of steel as the parts being carburised. Test specimens may be the actual components, rings, or bars and should be suitable for subsequent machining into chips to be analysed chemically. The test specimen is carburised along with components, or in a manner representative of the procedure to be used for components. Precise control should be exercised to avoid distortion and decarburisation while cooling test specimens after carburising.

When the components and test specimens are quenched after carburising, the test specimens should be tempered at approximately 600 to 650°C and straightened to 0.0015 max total indicator reading before machining is done. The time at the temperature should be minimised to avoid excessive carbon diffusion.

The cleaned test specimens are then machined dry in increments of predetermined depth. The chosen increments usually vary between 0.002 and 0.010 inch depending on the accuracy desired and expected case depth. The machined off chips of each increment is analysed individually for its carbon content. Total case depth is then the perpendicular distance from the surface to the depth of the last increment of machining whose chips show carbon content 0.04% higher than that of the known carbon content of the core.

2. Mechanical Methods:

In this method, a hardness traverse is made at known intervals through the case and core of the specimen, after preparing the cross section by one of the three procedures. Automatic micro-hardness testers could be used which digitally display the result as well as print out the hardness readings and depth at which they were taken and in just around 5 minutes, and give hardness numbers in Diamond pyramid or Knoop. Rockwell superficial A or C scales can be used in some instances on flame and induction hardened cases.

This method is most useful and accurate and is preferred for measuring effective case depth (i.e. depth up to 50 HRC or approved equivalent) and also total case depth of thin cases (0.25 mm or less) like the nitrided cases.

For induction and flame hardened cases, the criterion for measuring effective case depth up to different hardness values depending on the carbon content of the steel is given below:

Preparation of the specimens for case depth measurement by any of the three following methods should be done very carefully particularly taking care that no grinding or cutting burn occurs. As a precaution, the use of an etchant for burn detection is recommended as a practice as serious errors are resulted when these are present.

Cross-Section Procedure:

In this method, cut the specimens perpendicular to the hardened surface at a critical location. Be careful to avoid any cutting or grinding method which would effect the original hardness. Grind and polish the specimen. The surface of the area to be traversed shall be polished finely enough so that the hardness impressions are unaffected, i.e., the lighter the indenter load, the finer the necessary polish. After polishing, hardness traverses in made.

Fig. 8.80 illustrates the procedure normally recommended for the measurement of light and medium cases, whereas the procedure illustrated in Fig. 8.81 is normally recommended for the measurement of medium and heavier cases.

Further precautions needed are:

The hardness traverse should be started far enough below the surface to ensure proper support from the metal between the centre of the impression and the end of the surface. The subsequent impressions are spaced far enough apart so as not to distort hardness values. The distance from the surface of case to the centre of impression in measured on a calibrated optical instrument, micrometer stage, or other suitable means.

Taper-Grind Procedure:

This method is used for measuring light and medium cases. The procedure is illustrated in Fig. 8.83. A shallow taper is ground through the case. The angle of taper is so chosen that readings spaced equal distances apart will represent the hardness at the desired increments below the surface of the case. The hardness measurements are made along the surface thus prepared.

Unless special anvils are used, a parallel section (as illustrated in Fig. 8.83, its bottom line) should be prepared so that readings are taken at right angles to the tapered surface. All the precautions should be taken while grinding to prepare the shallow taper.

Step-Grind Procedure:

This method is normally used for measuring case-depths which are medium to heavy. The procedure is illustrated in Fig. 8.83. It is essentially similar to the taper- grind procedure except that steps are ground at known distances below the surface and hardness readings are made on these steps.

A slight variation in this procedure is to grind at two predetermined depths. If the hardness on first step, close to surface is greater than HRC 50, and is less than HRC 50 on the 2nd step, the effective case depth (based on 50 HRC) lies between the two depths.

3. Visual Methods:

In this category are methods which use any visual procedure, with or without the aid of magnification to determine the case depth. Samples may be prepared by the combination of fracturing, cutting, grinding and polishing methods. Etching with a suitable chemical reagent is normally required to produce a contrast between the case and the core. Nital (concentrated nitric acid in alcohol) of various strengths are frequently used as the reagents for producing this contrast.

Visual methods are classified into two categories:

i. Macroscopic Visual Procedure:

Here the surface may be ground up to 000 emery papers and magnifications used are up to 20 diameters.

ii. Microscopic Visual Procedure:

The specimens are polished well and etched and the case depth is read at a magnification of 100 X diameters.

i. Macroscopic Visual Procedures:

Macroscopic procedures are very commonly used for routine process control mainly because of quickness in determining the case depth and also of minimum specialised equipment and trained personnel required. These methods could be used to measure all types of cases. However, the accuracy can be improved by correlation with other methods more in keeping with engineering specifications for the parts being processed. These methods are applied to either hardened or unhardened cases.

Except that a variety of etchants may be employed with equal success, the following procedures are typically commonly used:

1. Fracture:

The specimen is fractured and is examined at a magnification not to exceed 20 diameters, with no further preparation.

2. Fracture and Etch:

Water quench the samples directly from the carburising temperature. Fracture, and then etch in 20% nitric acid in water for a time established to develop maximum contrast. Rinse in water and read while wet.

3. Fracture or Cut, and Rough Grind:

Prepare specimen by either fracturing or cutting, and then rough grinding. Etch in 10% nital for a period of time established to provide a sharp line of demarcation between the case and the core. Examine at magnification not to exceed 20 diameters and read all of the darkened area for approximate total case depth.

4. Fracture or Cut, and Polish or Grind:

Prepare sample by fracturing or cutting. Polish or grind through No. 000 or finer emery paper, or both. Etch in 5% nital for approximately 1 minute. Rinse in clean alcohol or water. Examine at magnification not to exceed 20 diameters and read all of the darkened zone. After correlation, effective case depth can be determined by reading from external surface of specimen to a selected line of the darkened zone.

ii. Microscopic Visual Procedure:

Microscopic methods are generally used in laboratory and require complete set up for polishing, etching and microscopic examination. The examination is made most commonly at 100 diameters.

Carburised Cases:

The microscopic method is used to determine the total case depth and the effective case depth in the hardened condition. In the annealed sample, the total case depth and the depth of the various zones-hypereutectoid, eutectoid, hypoeutectoid-also can be determined quite precisely.

Procedure for Hardened Condition:

1. Fracture, or cut specimen at right angles to the surface.

2. Prepare specimen for microscopic examination and etch in 2 to 5% nital.

3. For effective case depth, read from the surface to the metallographic structures that have been shown to be equivalent to HRC 50 (it is normally 85% martensite and 15% other transformed products).

4. For total case depth, read to the line of demarcation between the case and the core. In alloy steels, quenched from a high temperature, the line of demarcation is not sharp. Read all of the darkened zone that indicates a difference in carbon from the uniform core structure.

Procedure for Annealed Condition:

1. The specimen to be annealed may be protected by copper plate, or any other suitable means for preventing loss of carbon (decarburisation).

2. Pack in a small, thin-walled container with a suitable material such as charcoal.

3. Place the container in furnace at 25 to 50°C above the upper critical temperature (Ac₃) for the core, i.e., around 870 to 925°C.

4. Leave in the furnace long enough for specimen to reach furnace temperature (but not for an excessive time at this temperature, because carbon diffusion will increase the total case depth).

5. Cool the sample from the annealing temperature at the following rates:

i. Carbon Steels:

A satisfactory cooling rate is 150°C per hour for most steels to 425°C and cools as desired below 425°C. A simple method of cooling the container in mica, lime, or other insulating material is good enough.

ii. Alloy Steels:

The best results are obtained when the alloy steel has undergone isothermal transformation. Even slow cooling at 75°C/hour to 425°C from the annealing temperature results in reasonable result. If martensite is retained in the structure, tempering at 540° to 600°C results in better contrast after etching.

6. Section, prepare and etch specimen as described under the heading procedures for ‘hardened condition’. Etching time is a bit longer.

7. Total case-depth can be read up to the depth at which no further change in microstructure occurs.

Following zones can be detected:

(a) Hypereutectoid Zone:

Depth up to which, carbide is present either as a network at grain boundary, or in globular form.

(b) Eutectoid Zone:

It is the distance from the point where carbide is no longer visible and the point where ferrite first appears.

(c) Hypo Eutectoid Zone:

The point beginning where, ferrite first appears, and extending down to and including the core % Pearlite roughly gives an idea about the carbon content of the point

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For Specimens Cooled Slowly after Carburising:

If the method used in carburising cycle is such that cooling rate similar to that described in for annealed condition is obtained, then the specimens may be prepared and examined as it is, otherwise the specimen has to be reheated after carburising and cooled slowly to examine the steel.

Carbonitrided Cases:

Total case depth is measured in the hardened condition. However, the high quenching temperatures, large amount of alloying elements in steels and the high carbon content of the core result in inaccuracies of readings obtained by this method.

Procedure:

Section, polish, etch and read for carburised cases, procedure for Hardened condition.

Cyanided Cases:

As cyanided cases are very thin, only microscopic method is recommended for accurate case-depth determination. The cyanided case consists of, starting from the surface- a light etching layer; then totally martensitic structure; then martensite with increasing networks of other constituents depending on the type of steel cyanided. Cyanided cases are read in the hardened state and the result is reported as total case depth.

Procedure:

1. Section, prepare and etch the specimen for carburised cases, Procedure for hardened condition.

2. Read to the line of demarcation between the case and the core. If a sharp line of demarcation does not exist, then the mechanical method of hardness-traverse is used.

Nitrided Cases:

If the available sample cannot readily be prepared for more recommended Hardness-Traverse method, then the second best microscopic method is used.

Procedure:

1. Section, prepare, and proceed for carburised cases, Procedures for Hardened Condition.

2. Etch in 10% nital.

3. Read all the darkened zone for total case depth.

Flame or Induction Hardened Cases:

As no change in the chemical composition occurs in cases obtained in flame, or induction hardening, case depth in determined in the hardened, or hardened and tempered condition only. A procedure for reading effective case-depth may be established by correlating microstructures with a Hardness-Traverse method.

The minimum hardness of Rockwell 50 HRC or its equivalent is commonly used, but some other point may be selected or required, for example, in low carbon steels which don’t reach the hardness of HRC 50 when fully hardened (less than 0.3% C). The microstructure at the selected location will differ depending on the steel composition, prior treatment and the hardness level chosen.