Preparing Tool Steels for Microstructural Examination

Tool steels are essential materials and are available in various compositions, ranging from water-hardening carbon steels or simple plastic molding die steels, to high-alloyed high-speed steels. Powder metallurgy can be used to achieve more exotic compositions.

However many common features exist, the first being that all tool steels are iron-based materials. Metallographers use relatively soft annealed tool steels, as-forged or as-rolled steels with a broad range of hardness, and microstructural constituents, to heat-treated microstructures usually comprising of high-strength martensite and various carbide types. A broad range of microstructures, including some undesirable ones, are encountered when performing tool and die failure analysis.

Usually tool steels are not very difficult to prepare for microstructural analysis, however some problems have to be taken into account. The use of weakly bonded abrasive blades to avoid burning can make the sectioning process difficult. Edge retention is one of the requirements to prepare specimens, for example in the rating of decarburization or analysis of heat-treated specimens, especially those in failures. If rating has to be carried out for the inclusion content, inclusion retention is also important. The proper retention of graphite is essential for graphitic tool steels. High silicon grades face staining issues, and high alloy grades encounter voids associated with carbides in the center of the sections or cracking of carbides. The metallographer has to determine if the voids are real or have been produced during the preparation process. This article discusses the metallographic preparation of tool steels.

Preparation Procedures

Sectioning

Specimens that are relatively soft (less than 345 HRV or 35 HRC) can be sectioned using hacksaws or band saws. However these operations result in significant deformation zones below the cut, and rough surfaces. This damage has to be removed with a rough grinding process (80- to 120-grit silicon carbide).

Water-cooled abrasive cut-off wheels are used to section higher-hardness specimens. To avoid the burning effect and to allow effective cutting, the blade should have weak bonding. Submerged cutting processes restricts the generation of heat, which is most severe when sectioning quenched or as-quenched tool steels or lightly tempered tool steels. Improper techniques can lead to heat generation and can result in a highly tempered appearance in the martensites, and can re-austenize the surface if the heat is substantial. This damage cannot be removed by subsequent grinding operations.

When working with as-quenched high-alloyed steels it is useful if the specimen is fractured to form a flat surface free from damage, because of the steels’ brittleness. For analysis, the fractured surface is then ground and polished. The use of a cubic or diamond boron nitride wheel for the sectioning process will result in minimum damage with high-quality surfaces for high-alloy, high-hardness steels. The sectioned surfaces are smooth, although the rate of cutting is slow. Grinding can be proceeded with fine grits (240- to 320-grit silicon carbide).

Mounting

Many advanced automatic polishing machines can carry out polishing even when the specimens are not mounted. If edge retention is important, mounting is required. Though not necessary, optimum results can be obtained if the surface is plated before mounting. EpoMet® Resins and other compression-mounting epoxy polymers offer better edge retention even if the specimens are not plated, and better edge retention is obtained with automatic polishing instead of hand polishing. Grinding and polishing in central force mode provides a better flatness than when carried out in single force mode.

Mounting is preferred for odd-shaped or small specimens. Most mounting mediums are acceptable if edge is not very important. However many mounts can be degraded if heated etchants are used, and some do not have a high resistance to alcohol and other solvents. These issues can be resolved if compression-mounting epoxies are used. Transparent methyl methacrylate compression-mounting material is utilized if a transparent mount is needed to control the grinding process to a specific feature. Cast epoxies are also satisfactory, and can produce true adhesive bonding to the sample, less heat during curing, and are preferred when the sample cannot withstand the high heat utilized in compression mounting. Low-cost phenolic compression-molding materials are preferred if heat degradation is not expected, or when edge retention is not needed.

The Traditional Grinding and Polishing Approach

Manual (hand polishing) or automatic polishing is utilized in traditional methods. For grinding, water-cooled silicon-carbide paper (200 to 300mm or 8 to 12in. diameter) is used, with the initial grit size depending upon the approach used for the sectioning process. The common grit sequence is 120, 240, 320, 400, and 600-grit. When carbide pullout is an issue, finer grit sizes can be utilized for high-alloy tool steels. Grinding pressure can be moderate to heavy, and the grinding process can be proceed for 1 to 2min for removing scratches and deformation caused by the earlier step. It is advisable to use fresh paper; deformation can result if loaded or worn paper is used.

Polishing is usually carried out by utilizing one or more diamond abrasive stages, and then one or more final abrasive stages, usually with alumina abrasives. For routine operations, polishing by using 6µm and 1µm diamond is sufficient. Diamond abrasive in slurry or paste form is applied to the polishing cloth. Low-nap or napless cloths are used for coarser diamond abrasives, and a medium-nap cloth is preferred for finer diamond abrasives. An “extender” or lubricant matching with the abrasive can be added for wetting the cloth and minimizing the drag effect. Wheel speeds range from 100 to 150rpm, and with moderate pressure, polishing can be realized in approximately 2min.

Manual or automatic polishing is also undertaken for the final polishing step. Alumina (Al2O3) abrasives, 0.3µm α-Al2O3 and 0.05µm γ- Al2O3 are commonly used with medium-nap cloths. Effective polishing can also be realized by the use of colloidal silica (SiO2) with the particle size ranging from 0.04 to 0.06µm. The polishing time, wheel speed and the pressure applied are the same as adopted in rough polishing with diamond abrasives. As tool steels have high hardness, they are relatively easy to polish, and an artifact-free and scratch-free surface can be obtained.

Contemporary Approach

The modern technique uses automated equipment for the grinding and polishing processes. Specimens can be placed in a holder which can hold several samples of different sizes, either in unmounted or mounted condition. 200, 250 or 300 mm (8, 10 or 12 in.) diameter formats are used. Modern abrasives enables the achievement of surface qualities that exceed those required for research studies in as little as three steps.

Table 1. shows a 4-step process that offers good quality surfaces suitable for any requirement. Satisfactory results for routine analysis can be obtained even if step 4 is avoided for production work. The results are better and publication-quality photographs can be obtained if step 4 is included in the process.

Table 1. Four-step method for preparing tool steels.

Abrasive & Surface Lubricant RPM Head Platen/ Direction Load per Specimen Time (mins.)
120 to 240grit* (P120 to P280) SiC CarbiMet® 2 waterproof abrasive paper, or 125 to 45 μm Apex® DGDs or DGD Red or Purple water 240-300 Contra 6 lbs [27 N] Until Plane
9 μm diamond on an UltraPol® silk cloth (or UltraPad® polyester cloth or ApexHercules® H rigid grinding disc) MetaDi® Fluid 120-150 Contra 6 lbs [27 N] 5
3 μm diamond on TriDent® cloth (or TexMet® chemotextile pad) MetaDi® Fluid 120-150 Contra 6 lbs [27 N] 3
MasterPrep® Alumina Suspension on a MicroCloth® pad No other lubricant is needed 120-150 Contra 6 lbs [27 N] 1-3

* Use 120-grit for specimens > 60 HRC; use 180-grit for specimens at 35-60 HRC; use 240-grit for specimens < 35 HRC

Table 2. shows a 3-step process that also offers research/publication-quality micrographs and excellent surfaces. UltraPad® Cloth can be used for step 2, though best results are obtained when the UltraPad® Silk Cloth is employed. However, edge flatness is excellent when either of the surfaces is used.

Table 2. Three-step method for preparing tool steels.

Abrasive & Surface Lubricant RPM Head Platen/ Direction Load per Specimen Time (mins.)
120- to 240-grit* (P120 to P280) SiC CarbiMet® 2 waterproof abrasive paper, or 125 to 45 µmApex® DGDs or DGD Red or Purple water 240- 300 Contra 6 lbs (27 N) Until Plane
3 µm diamond on an UltraPol® silk cloth cloth (or UltraPad® polyester cloth or ApexHercules® H rigid grinding disc) MetaDi® Fluid 120- 150 Contra 6 lbs (27 N) 5
MasterPrep® Alumina Suspension on a MicroCloth® pad No other lubricant is needed 120- 150 Contra 6 lbs (27 N) 5

* Use 120-grit for specimens > 60 HRC; use 180-grit for specimens at 35-60 HRC; use 240-grit for specimens < 35 HRC.

The sample holder (head) rotates in the same direction during complementary rotation, in contra rotation the rotation directions are reversed. Contra rotations achieve more removals. When contra rotation is employed, the lubricant and the abrasive will be retained in the surface for a longer period when the holder rotation is <100 rpm. In complementary rotation, as a result of centrifugal forces, the liquids are thrown off the platen surface when it is added. If the speed is >100 rpm, contra rotation may result in the liquids being thrown across the room. Step 4 can be repeated using complementary rotation, if relief patterns are noticed around sulfides or oxides after the step, to ensure their removal. This phenomenon is specimen-specific and is rarely observed.

Cutting can begin at a faster rate if paste is used when the cloth is charged with diamond. Diamond needs to be applied generously, and is then spread with a clean fingertip. The lubricant is applied and polishing starts. In order to maintain a high cutting rate a diamond suspension (e.g. MetaDi® Supreme) can be squirted, where the particle size is same as that of the paste. There is no need to add MetaDi® Fluid lubricant as slurries contain the lubricant, although small amounts may be added occasionally even if diamond is employed in slurry form.

The particle size is 0.05 µm in MasterPrep® Alumina Suspension, which is made through sol-gel processes instead of the conventional calcinations process, the suspension is also free of agglomerates. If these steps are followed, from cutting to polishing, the final step will last for 5 minutes, avoiding relief or edge-rounding problems. Avoiding heavy cutting damage, avoiding shrinkage gaps by mounting with EpoMet® Resin, beginning grinding with fine silicon carbide abrasive (or a same sized-abrasive in another from, such as DGD discs), and employing contra rotation mode with low head speeds are the important steps to obtain perfect results of a true microstructure.

Etching

Nital is the most commonly used etchant in concentrations ranging from 2 to 10%. Nital in 2 to 3% concentration is sufficient for a majority of tool steels, while for high-alloyed tool steels including the D types, a 10% concentration is needed. Storing stock solutions with more than 3% HNO3 in pressure-tight bottles should be avoided. When higher concentrations are needed as a stock reagent, bottles with pressure-relief valves have to be used. Alternatively ethanol can be replaced by methanol, but being a cumulative poison, methanol’s use should be limited.

Irrespective of the predicted microstructural constituents, nital is usually utilized for tool steels. Even though nital is better than picral, which is 4% picric acid in ethanol, to etch martensitic structures, usage of picral yields better results for analyzing annealed samples.

When analyzing spheroidized annealed tool steels (a common annealed state), picral shows the interfaces between ferrite and carbide. Nital also shows the ferrite grain boundaries, which usually obscure the carbide shape. Spheroidization ratings become more difficult when carbides with ferrite grains are poorly delineated as a result of nital being orientation sensitive.

For martensitic steels a 2% nital solution is usually used. Higher concentrations result in increased etching speeds, which cannot be easily controlled. 5% concentration is needed to etch martensitic high-alloy steels, including high-speed tool steels, while 10% concentration is needed for the D types. Etching with picral or nital is carried out by immersion. Light pressure needs to be applied to avoid smearing issues when swabbing is employed.

Due to the broad range of tool steel compositions and since etch response can be altered by heat treatment, etching times are not easy to generalize. Trial and error methodology can help to discover the degree of dulling needed to realize the appropriate degree of etching.

Other etchants can be used. Table 3 provides the compositions of various reagents to realize selective etching or to improve contrast among microconstituents. Figure 1 shows the application of a three-step preparation process with annealed O6 graphite tool steel. Irrespective of the surface used, graphite has been fully retained. There are no scratches or residual deformation in the ferrite, and the cementite clearly shows up on the use of picral. Figure 2 displays the etched microstructure of spheroidize annealed W1 water hardening tool steel, after it has been etched with 4% picral along with alkaline sodium picrate and Klemm’s I reagent.

The cementite particles appear to be outlined, as picral evenly dissolves the ferrite. The Klemm’s I reagent leaves out the cementite matrix but colors the ferrite, allowing the cementite to be easily distinguished through image analysis. Alkaline sodium picrate evenly colors the cementite, and avoids attacking or enlarging the particles. The use of Klemm’s I reagent or alkaline sodium picrate allows the measurement of the cementitie to be statistically equivalent.

Table 3. Etchants for tool steels.

Composition Comments
1-10 mL HNO3 99-90 mL Ethanol Nital. Most commonly used reagent. Reveals ferrite grain boundaries and ferrite-carbide interfaces. Excellent for martensite. Do not store solutions with >3% HNO3 in a tightly stopped bottle.
4 g Picric acid 100 mL Ethanol Picral. Recommended for annealed structures or those containing pearlite or bainite. Does not reveal ferrite grain boundaries. Addition of a few drops of zephiran chloride increases etch rate. Add 1-5 mL HCl to improve etch response for annealed higher alloy tool steels.
1 g Picric acid 5 mL HCl 95 mL Ethanol Vilella’s reagent. Reveals structure of higher alloyed tool steels.
50 mL sat. Aqueous Sodium thiosulfate 1 g Potassium metabisulfite Klemm’s I tint etch. Immerse specimen until the surface is colored violet. Colors ferrite blue and red while martensite is brown. Carbides are unaffected. Works well only on low alloy and carbon tool steels.
0.6 mL HCl 0.5-1.0 g Potassium metabisulfite 100 mL water Beraha’s reagent. Immerse specimen until the surface is colored. Colors ferrite and martensite, carbides are not affected. Good for most tool steels.
3 g Potassium metabisulfite 2 g Sulfamic acid 0.5-1.0 g Ammonium bifluoride 100 mL water Beraha’s sulfamic acid reagent No. 4. For carbon and low-alloy tool steels, leave out the NH4F•HF. Immerse until the surface is colored. Ferrite and martensite are colored; carbides are not affected. Good for high chromium tool steels.
2 g Picric acid 25 g NaOH 100 mL water Alkaline sodium picrate. Colors cementite and M6C carbides. Immerse specimen in solution at 80-100 °C for 1 minute or more.
10 g K3Fe(CN)6 10 g NaOH or KOH 100 mL water Murakami’s reagent. Use at 20 °C to outline and darken M7C3 and M6C, and to outline M2C. M23C6 is faintly colored.
4 g KMnO4 4 g NaOH 100 mL water Groesbeck’s reagent. Use at 20 °C to outline M2C and to outline and darken M6C. M7C3 is faintly colored.
1 g CrO3 100 mL water Blickwede and Cohen’s etch. Use at 2-3 V dc, 20 °C, for 30 seconds with a stainless steel cathode. Outlines M23C6, outlines and colors M7C3, colors MC and attacks M2C.

Annealed microstructure of type O6 graphitic tool steel prepared (top) with the three-step method using the UltraPol® silk cloth for step 2; and (bottom) using the ApexHercules® H rigid grinding disc for step 2 (magnification bars are 10µm long; 4% picral etch).

Annealed microstructure of type O6 graphitic tool steel prepared (top) with the three-step method using the UltraPol® silk cloth for step 2; and (bottom) using the ApexHercules® H rigid grinding disc for step 2 (magnification bars are 10µm long; 4% picral etch).

Figure 1. Annealed microstructure of type O6 graphitic tool steel prepared (top) with the three-step method using the UltraPol® silk cloth for step 2; and (bottom) using the ApexHercules® H rigid grinding disc for step 2 (magnification bars are 10µm long; 4% picral etch).

Spheroidize annealed W1 water-hardened carbon tool steel etched with (top) 4% picral to reveal the general structure, with (middle) Klemm’s I to color the ferritic matrix, and with (bottom) alkaline sodium picrate at 90ºC for 60 seconds to color the cementite.

Spheroidize annealed W1 water-hardened carbon tool steel etched with (top) 4% picral to reveal the general structure, with (middle) Klemm’s I to color the ferritic matrix, and with (bottom) alkaline sodium picrate at 90ºC for 60 seconds to color the cementite.

Spheroidize annealed W1 water-hardened carbon tool steel etched with (top) 4% picral to reveal the general structure, with (middle) Klemm’s I to color the ferritic matrix, and with (bottom) alkaline sodium picrate at 90ºC for 60 seconds to color the cementite.

Figure 2. Spheroidize annealed W1 water-hardened carbon tool steel etched with (top) 4% picral to reveal the general structure, with (middle) Klemm’s I to color the ferritic matrix, and with (bottom) alkaline sodium picrate at 90ºC for 60 seconds to color the cementite.

Several experiments were performed with many tool steel grades in annealed or quenched and tempered states, or both, for assessing the selectivity of etchants provided in Table 3 that claim to color, outline, or outline and color, or attack certain carbide types. Electron-back scattered diffraction (EBSD) was used for characterizing the carbides. The specimens were re-prepared prior to the usage of each etchant. Table 4 shows the results of the etchings.

Table 4. Results of the etching experiments.

Etchant M3C M23C6 M7C3 M6C MC M2C
Alk. Na Pic. Colors NA NA Colors NA NA
Murakami NA Faint Outlined/ Colored Outlined/ Colored NA Outlined
Groesbeck NA NA Faint Outlined/ Colored NA Outlined
1% CrO3 NA Outlined Outlined/ Colored NA Colors Attacks

NA – no affect

Conclusions

To observe the true structure and correct interpretation, it is necessary to prepare the specimens in a proper manner. Tool specimens can be quickly prepared with perfect and consistent results if modern semi-automated equipment is employed. Simple and easy 3-step and 4-step procedures have been explained in this article. Important aspects concerning the preparation of samples were also defined. First specimen sectioning needs an abrasive blade suitable for metallographic studies, and for the specimen’s hardness to avoid heavy damage. Second for edge examination, the best possible resin has to be used for mounting. Grinding has to be performed with the finest abrasive. Sufficient abrasives have to be used during polishing to facilitate effective cutting. Napless woven or pressed cloths can be used, except for the final step. Finally in order to observe the structure in good contrast, the best etchant needs to be selected.

This information has been sourced, reviewed and adapted from materials provided by Buehler.

For more information on this source, please visit Buehler.

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