How Does High-Performance Alloy Tool Steel Work?

Each individual element in a tool steel imparts certain and specific  properties to the steel according to the percentage.  The effects of a single alloying element can be modified by the presence of other elements.  Below is a chart showing the alloying elements in Cold Work Tool Steels.  Read on to learn the effects of each alloying element.

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Carbon - C - This is the most important alloying element in steel.  With increasing carbon content, the strength and hardenability of the steel increase, but its ductility, formability, weldability, and machinability can be decreased.

Manganese - Mn - Manganese is used as a deoxidizer.  It contributes to strength and hardness, but to a lesser extent than carbon.  Manganese has a strong effect on increasing the hardenability of steel by reducing the critical cooling rate.

Silicon - Si - Silicon is used as one of the main deoxidizers in steelmaking.  Silicon is also absorbed in the melt during steelmaking from furnace and ladle refractory brick.  Silicon is a strong promoter of hardenability, improved forgeability and increases scale resistance.

Sulfur - S - For most steels a maximum sulfur limit is specified.  Sulfur is intentionally added to certain steels to  improve machinability.  Sulfur decreases weldability and, in most steels, increases impact toughness and ductility.  In Particle Metallurgy steels, however, a controlled sulfur level does improve machinability with negligible  effects on other properties.

Nickel - Ni - Nickel increases strength and hardness without sacrificing ductility and toughness.

Aluminum - Al - This is the most effective and frequently used deoxidizer in steelmaking.  Small additions are used to insure small grain size.  Aluminum will form with nitrogen and form hard aluminum nitrides, which is why it is added to nitriding steels.

Carbon Forming Elements

Chromium - Cr - Chromium is generally added to steel to increase resistance to corrosion and oxidation, to increase hardenability, and to improve high temperature strength.  Chromium is a carbide former, which increases edge retention and wear resistance.

Molybdenum - Mo - Molybdenum is usually alloyed together with other elements and is a pronounced carbide former.  Molybdenum promotes fine grain structure and improves secondary hardening during tempering.  Molybdenum increases strength, hardness and overall toughness.

Tungsten - W - Tungsten is a very pronounced carbide former.  It improves toughness and prevents grain growth.  Tungsten increases high temperature strength and red hardness.  It is primarily used in high speed steels and hot work tool steels.

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Vanadium - V - Vanadium is a pronounced carbide former, which increases wear resistance.  Vanadium increases strength, hardness and retards grain growth.  Vanadium enhances red hardness properties for high speed steels and  intensifies the effects of other alloying elements.

Overcoming difficulties in the metallographic preparation of high alloy tool steels

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.

Recommendations for the grinding and polishing of high alloy tool steel

When preparing high alloy tool steels for metallographic analysis, the form, size and amount of carbides must be accurately represented. In addition, non-metallic inclusions must be retained in an undeformed matrix.

• Large volumes are best processed on fully automatic grinding and polishing machines, which guarantee a fast and efficient workflow and reproducible results.
• Tool steels are hard. Therefore, fine grinding with diamond is more efficient and economical than grinding with silicon carbide foil.
• Sometimes a final oxide polish can be useful for contrasting and identifying carbides.

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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Recommendations for the etching of high alloy tool steel

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)

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