Metallurgy

Material processing is part of the 3-legged stool, along with material selection, and material structure. Processing includes: cold working, hot working, forging, melting, extrusion, heat treating, surface treatment, casting, and roll forming which affect the structure and performance. Heating steel to 1750oF and cooling in air, versus quenching in water will provide different properties depending on the composition; high carbon steels respond significantly to water quenching as demonstrated by high hardness, brittleness, and increased propensity for cracking.

Reitz, W., “Assessment of Weld Embrittlement in A516 Steel due to Multiple Weld Repairs,” J. of Failure Analysis & Prevention, 11, (6), 618-623, 2011.

Steel represents approximately 90% of metallic component compositions. Aluminum represents approximately 5% and copper plus all others make up the remainder. Some steels are strong and brittle, while others are weak and ductile, some have great corrosion resistance, some are not expensive, and some are biocompatible. Understanding the operating environment and expected performance enables the proper selection. Proper material selection includes consideration of: strength, corrosion, density, melting temperature, recrystallization temperature, crystal structure, ease of processing, ease of joining, availability, cost, quality, etc.

Reitz, W., “Metallurgy of Stainless Steel Alloys – Welding & Corrosion,” Soc. of Manufacturing Engineers, TP06PUB19, http://www.sme.org/ProductDetail.aspx?id=48250&terms=tp06pub19, 26 pg., 3/8/06.

Structure of a metal is revealed by its 1) crystal structure (FCC, BCC, HCP, etc.), 2) grain structure (microstructural), and 3) processing defects (voids, porosity, dendrites, etc.).

Wear is the undesired removal of material from contacting surfaces by mechanical action. There are numerous types of wear that include: abrasive, erosive, corrosive, 3-body, fretting, adhesive, cavitation, etc. Corrosion is the deterioration of a metal by chemical or electrochemical reaction with the environment. There are numerous types of corrosion that include: galvanic, uniform, crevice, SCC, concentration cells, etc.

Reitz, W., “Materials Basics for Corrosionists,” ASM Metals Handbook, 10th ed., Vol. 13A, Materials Park, OH, 980-991, 2003.

General is the uniform and low level attack against the entire metal surface with little or no localized penetration. It is the least damaging of all forms of corrosion. Generalized, or uniform, corrosion usually occurs in environments in which the corrosion rate is inherently low or well controlled.

Corrosion - Galvanic
Galvanic is an aggressive and localized form of corrosion between two or more dissimilar metals in an electrically conductive environment. It occurs because the anodic material is attacked by the cathodic material.

Stress Corrosion Cracking
Stress Corrosion Cracking (SCC) is the cracking develops from the combination of tensile stress and a corrosive environment on a susceptible material. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses.

Corrosion - Crevice
Crevice is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices (shielded areas) such as those formed under gaskets, washers, fastener heads, surface deposits, and clamps.

Corrosion - Pitting
Pitting can be present over the entire metal surface, creating an irregular or very rough surface profile. In other instances, pits are concentrated in specific areas, leaving the majority of the metal surface in like new condition. Pitting is accelerated, localized corrosion that can result in leaks without measurable metal loss of the entire component.

Corrosion - Microbiological
Microbiological, a.k.a., MIC produces large and deep pits due to the microorganism's production of strongly corrosive metabolic by-products such as sulfuric acid that is capable of corroding metal.

Dealloying
Dealloying a.k.a., selective leaching, refers to the selective removal of one element from an alloy by corrosion processes. A common example is the dezincification of unstabilized brass, which results in a weakened, porous copper structure. An understanding of component configuration and environment and prior processing is important. Corrosion by-products are analyzed via SEM / EDS to characterize the chemical reactions that are necessary to produce the corrosion film.

Erosion
Erosion is the gradual and selective deterioration of a metal surface due to mechanical wear and abrasion. It is usually caused by entrained air bubbles, and / or high velocity particulates.

The structure of polished and etched metals as revealed by a microscope at magnifications greater than 50X.

Reitz, W., “Metallurgical Investigation into Automotive Fire,” J. of Failure Analysis & Prevention,” https://talbottassociates.com/downloads/Apr06CaseHistoryReitz2s.pdf, 6, (2), 18-21, 2006

This is analytical chemistry to determine alloy composition.

This is an examination technique that does not alter the physical characteristics of the material, e.g., X-ray, ultrasonic, liquid dye penetrant.

This is a simple, straight-forward test to measure the ultimate tensile strength of the material. Hardness measures the resistance to penetration.

Optical examination of a component at low magnification (1X – 50X) searching for fracture features.

Fatigue is the phenomenon leading to fracture under repeated fluctuating stresses having a maximum value less than the tensile strength of the material. Fatigue characterizes the life of the component under alternating stresses; how many times can you bend a paperclip before it breaks? There are three stages: 1) crack initiation, 2) crack propagation, and 3) failure.

Forces acting on a body that stretch or elongate the component until fracture.

Shear is a type of force that causes or tends to cause two regions of the same part or assembly to slide relative to each other in a direction parallel to their plane of contact; similar to sliding the top half of a deck of cards.

Torsion is a twisting action applied to a shaft-like member. The twisting may be either reversed (back and forth) or unidirectional.

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Example Topics

• Basic Metallurgy - crystal structure, phase diagrams, heat treatment, strengthening mechanisms.
• Ferrous metals - iron, stainless steel, cast iron.
• Nonferrous metals - aluminum, nickel, titanium.
• Nonmetallic materials - polymers, ceramics.
• Characterization techniques - scanning electron microscopy, auger spectroscopy.
• Mechanical testing - tensile, impact, fatigue.
• Surface engineering - laser, CVD, diffusion, thermal spray.
• Failure analysis - failure modes, surface features, conducting an investigation.
• Surface degradation - wear, corrosion, oxidation.
• Surface cleaning - mechanisms, environmental issues, systems available.
• Diamonds - thin films, properties, thermodynamics.