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Microstructures of Hot and Cold Worked Metals and Alloys

HotWorking Temperatures Hot working occurs at a temperature that is relatively close to the melting point of the metal or alloy. This temperature is normally well above the normal recrystallization temperature. A homogenization cycle may be used prior to hot working to permit alloy diffusion and enhance chemical homogeneity. Too high a temperature must be avoided so that “burning” or grain-boundary liquation (incipient melting) does not occur. The temperature during the last hot working pass is also important as it controls the grain size in the as-rolled microstructure and may influence problems such as “banding” in steels. If the finishing temperature is low, recrystallization will not occur and the grain structure will be coarse and elongated and will contain residual deformation (dislocations). “Warm” working occurs below the recrystallization temperature.

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Microstructures of Alloys Used in Medical Devices

Alpha Alloys

  • Microstructure of CP Ti, ASTM F 67, Grade 4 (UNS R50700) (longitudinal plane, specimen was annealed at 704 °C) prepared using the three-step method and etched with Kroll’s reagent to reveal the grain structure.
  • Microstructure of CP Ti, ASTM F 67, Grade 2 (UNS R50400; specimen was in the as-rolled condition) prepared using the three-step method and viewed with polarized light to reveal the grain structure. Note the mechanical twins in the grains (arrows).
  • Microstructure of CP Ti, ASTM F 67, Grade 2 (UNS R50400; specimen in the as-rolled condition) prepared using the three-step method, followed by a vibratory polish, and viewed with crossed polarized light to reveal the grain structure. Note the deformation twins (arrows).

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Microstructure, Its Evolution

Microstructure – As-Cast, Cold Worked and Annealed, Hot Worked and Annealed and Heat Treated DOWNLOAD THE PRESENTATION HERE

Microstructure of Nonferrous Alloys

Super Pure Aluminum

Equiaxed alpha grains in the interior of a super-pure aluminum specimen anodized with Barker’s reagent, 30 V dc, 2 min. Viewed with crossed polarized light plus sensitive tint. Original at 50X. The dark spots are intermetallic phases.

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Microindentation Hardness Testing – Presentation

Vicker’s Test

In 1925, Smith and Sandland of the UK developed a hardness test using a square-based pyramidal shape made from diamond to overcome the limitations of the Brinell test (developed in 1900) that used a spherical hardened steel ball (metals harder than about 48 HRC can not be tested). The indenter shape chosen produced hardness numbers similar to those obtained by the Brinell test.
The first low-load Vickers tester was built by Lips and Sack in 1936.

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Microindentation Hardness Testing – Paper

IN MICROINDENTATION HARDNESS TESTING (MHT), a diamond indenter of specific geometry is impressed into the surface of the test specimen using a known applied force (commonly called a “load” or “test load”) of 1 to 1000 gf. Historically, the term “microhardness” has been used to describe such tests. This term, taken at face value, suggests that measurements of very low hardness values are being made, rather than measurements of very small indents. Although the term “microhardness” is well established and is generally interpreted properly by test users, it is best to use the more correct term, microindentation hardness testing.

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Metallography of Welds

Welding is an important joining technology, and is highly dependent on the process choice, consumables used, operating parameters, and operator proficiency.

Thus, inspection procedures, both nondestructive and destructive, are required to control the process and guarantee quality. Metallographic examination is a key tool in the destructive examination of weldments, both as a process control tool and as a post-mortem examination of failed components. Macrostructure must also be examined, which can be done on sections after grinding or polishing. Macrostructural examination is used to learn about the weld geometry, the depth of weld metal penetration, the magnitude of the heataffected zone, and to detect cracks and voids. Microstructural examination is used to determine the mode of cracking and the cracking mechanism and to identify phases or constituents in the weld metal, heat-affected zone, and base metal including nonmetallic inclusions, as related to governing specifications, fitness for service, or cause of failure.

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Metallography of Precious Metals

Preparation of Precious Metals

For 18-karat gold alloys, and similar highly noble precious metals: it is necessary to use an attack-polish agent in the MasterPrep alumina abrasive step. Mix 10 mL of the attack-polish solution with 50 mL of MasterPrep alumina slurry. The attack-polish agent can be: H2O2 (30% conc.), or aqueous 5-20% CrO3, or aqueous 10% oxalic acid, etc.

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Metallography and Microstructures of Stainless Steels and Maraging Steels

STAINLESS STEELS are complex alloys containing a minimum of 11% Cr plus other elements to produce ferritic, martensitic, austenitic, duplex, or precipitation-hardenable grades. Procedures used to prepare stainless steels for macroscopic and microscopic examination are similar to those used for carbon, alloy, and tool steels. However, certain types require careful attention to prevent artifacts. Because the austenitic grades work harden readily, cutting and grinding must be carefully executed to minimize deformation. The high-hardness martensitic grades that contain substantial undissolved chromium carbide are difficult to polish while fully retaining the carbides. The most difficult to such grades to prepare is AISI 440C. For the most part, preparation of stainless steels is reasonably simple if the basic rules for metallographic preparation arefollowed. However, unlike carbon, alloy, and tool steels, etching techniques are more difficult due to the high corrosion resistance of stainless steels and the various second phases that may be encountered.

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