An ever-larger number of high alloy tool steels are being made to fit customer-specific applications. As a result, quality control teams need to perform metallographic preparation and analysis of high alloy tool steel in increasingly high volumes. What are the key considerations in the metallographic preparation and analysis of high alloy tool steel?
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Fig. 1: Plastic mold steel etched with 5% picral, revealing some singular needles and plates at high magnification in an otherwise amorphous martensite (Mag: 1000x, DIC)
Avoiding thermal damage
As heat treatability of high alloy tool steels is a quality criterion, thermal influence during cutting has to be avoided in order to ensure a true representation of the actual microstructure. When cutting larger sections, this preparation step has to be carried out with great care.
Fig. 2: Thermal damage due to faulty cutting conditions
Preserving carbides and inclusions
The main difficulty during grinding and polishing of high alloy tool steels is ensuring that carbides and non-metallic inclusions are retained. In cold working tool steels, the primary carbides are very large and fracture easily during grinding. In fully annealed conditions, secondary carbides are very fine and can easily be pulled out from the softer matrix.
Fig. 3: Fractured primary carbides (Mag: 200x)
Large volume processing of high alloy tool steels
For quality control teams working within high alloy tool steel production, processing large sample volumes requires a very efficient organization of the workflow, automatic equipment and standard procedures.
Table 1: Preparation method for high alloy tool steel on large automatic equipment.
DiaPro diamond suspensions can be substituted with DP-Diamond suspension P as follows: For FG with 9 μm, DP 2 with 1 μm used with DP-Blue/Green lubricant.
Table 2: Preparation method for high alloy tool steel on table-top semi-automatic equipment.
DiaPro diamond suspensions can be substituted with DP-Diamond suspension P as follows: For FG with 9 μm, DP 1 with 3 μm, DP 2 with 1 μm used with DP-Blue/Green lubricant.
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High alloy tool steel samples are usually initially examined unetched to identify inclusions and carbide size and formation. To reveal the microstructure, various concentrations of nital or picral are used.
For example, to show the carbide distribution in cold work steel, a 10% nital ensures the matrix is dark and the white primary carbides stand out. For fine globular pearlite, a brief submersion into picric acid followed by 2% nital gives a good contrast and avoids staining.
Nital etching solution:
100 ml ethanol
2-10 ml nitric acid (Caution: Do not exceed 10% of the solution as it becomes explosive!)
Picral etching solution:
100 ml ethanol
1-5 ml hydrochloric acid
1-4 g picric acid
Fig 5: Cold work tool steel etched with 10% nital, primary carbides stand out white
Fig. 6: Hot work tool steel etched with picral and nital, globular pearlite (Mag: 500x)
All images by Kelsey Torboli, Applications Engineer, USA
For specific information about the metallographic preparation of tool steel, contact our application specialists.
