Materials and Metallurgy - Talbott Associates, Inc

A “3-legged stool” is a metaphor sometimes used to describe the 3 components of materials design and their critical inter-relationship: Processing, Material Selection, and Material Structure. Processing topics include cold working, hot working, forging, melting, extrusion, heat treating, surface treatment, casting, and roll forming which affect the structure and performance.

Steel represents approximately 90% by weight of all metallic components. Correspondingly, steel and steel technology are core principles in Metallurgical and Materials Engineering. Recognizing that this dominance is shrinking, and being displaced by other metals and materials, effort is made to understand each, and the potential values each brings to a design. Proper material selection includes consideration of strength, corrosion, density, serviceable temperatures, changes induced by temperature and other environmental factors, structure, ease of processing, ease of joining, availability, cost, quality, etc.

Structure of a materials is complex, but for each type of material the structure can be modified to influence properties, and thus service performance. For metal structure is revealed by its 1) crystal structure (FCC, BCC, HCP, etc.), 2) grain structure (microstructural), and 3) processing defects (voids, porosity, dendrites, etc.). For metals and non-metals, such features are inherent to the manufacturing process – some benefit the materials’ performance, while some are flaws. Flaws which are found to be outside of acceptance criteria and/or lead to service failure are “defects.”

External forces almost constantly act upon materials. Some are direct, yet other appear subtle, indirect, or induced. Analysis of and for these effects sheds light on failures and characterizes serviceability. The various types of corrosion are described here.

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

Corrosion - Galvanic
Galvanic is typically aggressive and localized attack of a surface having two or more dissimilar metals in an electrically conductive environment. It occurs because the anodic material is attacked by the cathodic material.

Environmental Stress Corrosion Cracking or SCC
Stress Corrosion Cracking (SCC) is a surface cracking which develops from the combination of tensile stress and a corrosive environment on a susceptible material. The required tensile stresses may be directly applied or residual. These effects can be seen in most types of materials, each having its own unique mechanism for weakening a surface layer and permitting progressive weakening of underlying material.

Corrosion - Crevice
Crevice corrosion is 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. See also Corrosion-Microbiological.

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 most of the surface unaffected. Localized pitting can lead to leaks without measurable metal loss of the bulk of the 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, typically acids, capable of corroding metal, leading to pits or pinholes, or the appearance of crevice corrosion.

Dealloying / dezincification / graphitization
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 un-stabilized brass, which results in a weakened, porous copper structure. Another is the progressive leaching of iron from certain cast irons, leaving behind only a matrix of graphite. An understanding of component configuration, service environment and prior processing is important.

Erosion
Erosion is the gradual and selective deterioration of a metal surface due to mechanical wear and abrasion. In fluid transfer, erosion of piping interiors can be influenced by entrained air bubbles, high velocity particulates, or other fluid mechanics.

Embrittlement
Materials are sometime inherently tough or brittle, but each can be affected by outside forces. An elastomeric engine gasket is inherently tough and flexible, alternatively a chef’s knife is tough enough to chop into meat, and rigid enough to be sharpened to a razor’s edge. The toughness of the gasket can be negatively affected by the engine overheating – it becomes no longer tough and flexible, but brittle, and soon cracks, the seal leaks, and you’ve got problems. During processing of the chef’s knife if the heating atmosphere is contaminated with hydrogen, the steel can become brittle. It will hold an edge, but can easily crack with little effort, like chopping.

The internal structure of a material as examined by an optical microscope or Scanning-Electron Microscope (SEM) typically at magnifications greater than 50X. This compares to the macrostructure, which can be viewed by the naked eye, or with the assistance of minimal magnification.

For metals this is analytical chemistry to determine alloy composition. Other methods are utilized to determine compositional makeup of ceramics, polymers, elastomers, etc., such as FTIR, DSC-OIT, and XRF.

These are evaluation techniques that do not alter the physical characteristics of the material, e.g., Visual inspection, Radiography (X-ray, neutron), ultrasonic, dye-penetrant, magnetic particle, eddy current, leak testing.

For many materials this test measures the standardized resistance of a material to penetration by an external object. For some other materials it is instead the rebounding of an object off of its surface. For some materials, the measured hardness is equated to other material properties such as wear or strength, and thus hardness can be a useful and inexpensive predictor of performance.

Materials in the real world frequently are impacted by inertial forces, not just pushed, or pulled. The bullet-stopping ability of armor is one example of a material designed around its impact properties. These can be measured at various temperatures to characterize the impact properties in various environments. Testing apparatus include Charpy and Izod impact testers, among others.

Fatigue is repeated fluctuating stresses having a maximum value less than the tensile strength of the material. As a failure mode, this appears contradictory since the stress is not greater than the material’s ultimate strength. A material’s fatigue properties can characterize the life expectancy of the component under alternating stresses; how many times can you bend a paperclip before it breaks?

Forces acting on a body that stretch or elongate the component in excess of its strength until permanent deformation occurs - exceeding the yield strength - or fracture -exceeding the ultimate tensile strength.

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 unidirectional or alternating.

Non-metallurgists can make effective and safe process control decisions if they have a basic understanding of what they are doing and why they are doing it. Does your workforce need to increase its understanding of a particular operation? Or perhaps just improve its overall understanding of Materials Science and Metallurgy? The topics listed here, and others, are typically inquired. A custom training program can be created to address specific problems or introduce your entire operation to the general topic “Materials for the Non-Metallurgist.” A training certificate can be issued showing the number of training hours.