Understanding and Measuring Decarburization

Understanding and Measuring Decarburization Understanding the forces behind decarburization is the first step toward minimizing its detrimental effects. Decarburization is detrimental to the wear life and fatigue life of steel heat-treated components. This article explores some factors that cause decarburization while concentrating on its measurement. In most production tests, light microscopes are used to scan … Read more

Preparation and Characterization of Surface Treated and Coated Steels

ABSTRACT

A wide variety of surface treatments and coatings are applied to metals to enhance their performance, for example, to improve fatigue resistance, increase wear resistance, corrosion or oxidation resistance. Some of these treatments involve diffusion of one or more elements into the metal or alloy followed by post heat treatments. These processes included the familiar processes of carburizing, nitriding, and carbonitriding but also included less familiar processes such as ion nitriding and boriding. There are also a wide variety of coatings that are deposited by hot-dipping, electroless or electrolytic means, by physical or chemical vapor deposition, or by thermal or plasma spray. The technological significance of these processes is enormous.

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Microstructure of Surface Treatments, Platingsd and Coatings

Suggestions on Methods

  • Use the gentlest possible cutting method to minimize cutting damage and yield a smooth surface
  • SiC paper may be used for first grinding step
  • Start with the finest possible SiC grit size, 180-320 grit (P180 to P400), to minimize grinding damage
  • Diamond in slurry or suspension form in small sizes (< 6 μm) may become embedded in the surface of soft metals
  • Use diamond in paste form for sizes < 6 μm for soft metals
  • To minimize relief, use ChemoMet for the final step, but with higher than normal loads

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Microstructure of Stainless Steels

AISI 416 – Microstructure of 416 free free-machining martensitic stainless steel in the quenched and tempered condition. Etched with Vilella’s reagent. Original at 1 100X. Note the gray 00X. elongated sulfide inclusions and the elongated “stringers” of de delta ferrite (white, see lta arrows). The matrix is tempered martensite.

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Microstructure of Ferrous Alloys

Constituents of Steel

Products of diffusion-controlled transformations.
Products of diffusionless athermal transformations.
Products of precipitation before or during solidification due to limited solid solubility, or pick-up from external sources.

Grain Boundaries

The interface between two grains where the crystal lattice changes from that of one grain to that of the other grain.
There are three types of grain boundaries in Fe-based alloys: ferrite, austenite and prior-austenite. The prioraustenite grain boundaries are those of the parent (austenite) phase before transformation. Ferrite and austenite grain boundaries are those of the product phase, although there are compositions where allotropic transformations do not occur.

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Metallography of Stainless Steel and High Temperature Alloy Fasteners

Metallographic examination of longitudibally-oriented fastener specimens produces a great deal of useful information about fastener quality. Indeed, metallography is an indispensable tool for evaluating fasteners. Examination can reveal the presence of cracks or other surface flaws which may or may not, be harmful depending upon their location and nature. Metallography also can detect features associated with the manufacturing process and characterize the strenght of the fastener. All this depends upon proper selection and application of metallographic procedures.

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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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