Metallographic Techniques for Tool Steels

TOOL STEELS can be prepared for macroscopic and microscopic examination using the same basic procedures used for carbon and alloys steels. However, because many tool steels are highly alloyed and are generally heat treated to much higher hardness than most carbon and alloy steels, specific aspects of their preparation differ slightly. The reasons for these differences and the required procedural modifications are discussed in the following sections. Also covered are the effects of hot working, composition, austenitizing, and tempering on microstructure.

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Metallographic Characterization of the Microstructure of Tool Steels

Abstract

Examination of selectively etched tool steel microstructures by light microscopy provides more information than standard etchants, such as nital, picral or Vilella’s reagent. Further, the images are more suitable for quantitative measurements, especially by image analysis. Specimens must be properly prepared, damage free, if selective etchants are to be applied successfully. A number of etchants have been claimed to selectively etch certain carbides in tool steels. The response of these etchants has been evaluated using a variety of well-characterized tool steel compositions. While many are selective, they are often selective to more than one type of carbide. Furthermore, their use in image analysis must be evaluated carefully as measurements showed that the amount and size of the carbides are often greater after selective etching as many of these reagents outline and color or attack the carbides. Selective etching of the matrix, leaving the carbides unaffected works well, but no one etchant will cover the broad spectrum of tool steel compositions. No etchant has been found that will color retained austenite in tool steels and image analysis of retained austenite in tool steels are always much lower than by x-ray diffraction unless retained austenite is the dominant phase present in grossly over-austenitized steels.

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Metallographic Characterization of the Microstructure of Tool Steel

Five Five-Step Method for Preparing Tool Steel

  1. 120-to 240-grit SiC, water cooled, 240-300 rpm, 6 Lbs/specimen, UP
  2. 9-μm diamond on UltraPolsilk cloth, 150 rpm, 6 Lbs/specimen, 5 min
  3. 3-μm diamond on TriDentcloth, 150 rpm, 6 Lbs/specimen, 4 min
  4. 1-μm diamond on TriDentcloth, 150 rpm, 6 Lbs/specimen, 3 min
  5. 0.05-μm MasterPrepAlumina suspension on MicroCloth, 120-150 rpm, 6 Lbs/specimen, 1-3 min

Use contra rotation (head rotates in opposite direction to platen if the head speed is <100 rpm.

Step 4 is optional; use with more difficult to prepare specimens.

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Martensite & Retained Austenite - (Article)

ih0409-mct-fig3-mdMartensite development is critical to many heat-treatment processes. This paper examines the conditions under which austenite is retained and the problems associated with its presence, with detecting it and with measuring it.

 

Depending upon the carbon content of the parent austenite phase, either lath (low-carbon) or plate (high-carbon) martensite may form, as well as mixtures of the two. In general, lath martensite is associated with high toughness and ductility but low strength, while plate martensite structures are much higher strength but may be rather brittle and non-ductile. By George F. Vander Voort

 

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Imaging Phases in Steels

Some techniques learned from experience can help metallographers identify certain phases in steel.

The microstructure of metals and alloys can be quite complex, particularly in certain alloy systems such as iron-based alloys. A good understanding of physical metallurgy, the role composition and processing on structure formation and modification, and an idea of the approximate expected hardness of different phases or constituents will help narrow the range of possible phases that might be present.

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Etching Isothermally Treated Steels

The microstructure of iron-based alloys is very complicated, being influenced by composition, homogeneity, processing, and section size. Microstructures of
coarse-grained steels are much easier to observe than those of fine-grained steels. Of course, steels are normally made with a fine grain size to optimize
their mechanical properties. In general, it is easiest to identify heat treated structures after transformation and before tempering. However, in most applications, hardened steels must be tempered and are usually examined in this condition. If a mixed microstructure of bainite and martensite is formed during quenching, these constituents will become more difficult to identify reliably as the tempering temperature increases toward the lower critical temperature (Ac1).

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resourcesThe articles and presentations that can be down-loaded from this web site are based upon work done by GFV while employed at Bethlehem Steel (1967-1983), Carpenter Technology (1983-1996), Buehler Ltd. (1996-2009) and Struers (2009-Present) and from the authors consulting work for companies such as, Latrobe Steel, Scot Forge, etc., and from his litigation work. GFV's bylined articles appearing in various issues of the ASM Handbook series have been listed here courtesy of ASM International, Materials Park, Ohio.

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