Properties

The properties of metals include chemical, physical, and mechanical properties.  The chemical definition for metals was provided earlier in this book. The basic definition is repeated here. Let’s move forward with brief descriptions for properties of so we can differentiate the separate properties of metals.

Chemical properties define metals as possessing the following traits:

• High electrical conductivity
• Luster
• Reactivity to oxygen to form oxides
• Malleability
• Readily loses electrons to form positive ions (cations)
• Forms bases rather than acids in water
• Can be an element (e.g., iron), an alloy (e.g., stainless steel), or a molecular compound (e.g., polymeric sulfur nitride)
• Solid at room temperature (except mercury, which is a liquid at room temperature and gallium flows into liquid at 86˚ F.)

Physical properties refer to observable traits of a material, such as:

• Color,
• Melting point,
• and Density.

Metals may be silver or gold in color, yet all of them will have a shine or luster to them. If their luster is hidden by a layer of oxide, that is due to the metal’s chemical property for reacting to oxygen and form a layer of oxide, which will not be shiny.

We can think about the difference in soldering with lead or welding with steel to consider the variety of melting points across different metals.

The ability to measure mass and volume to determine a material’s density may rely on measuring gadgets found in a chemical laboratory; yet density is a physical attribute.

Mechanical properties describe how a material reacts to external forces like pushing, pulling, or twisting.

For metal workers such as machinists and welders, understanding the properties of metals is important and helpful. Even though, this section offers definitions that differentiate chemical, physical, and mechanical properties, the terms are not that clear cut.  You will hear different speakers and read different authors that state a specific property is chemical, physical, or mechanical.  In the end, they are all simply properties of metals. In this section, the focus on the properties most important to machinists, welders, and other metal workers.

In metallurgy, one may encounter many terms that have very specific meanings within the field, but may seem rather vague when viewed from the outside. Terms such as “hardness,” “impact resistance,” “toughness,” and “strength” can carry many different connotations, making it sometimes difficult to discern the specific meaning. Some of the terms encountered, and their specific definitions are:

• Strength – Resistance to permanent deformation and tearing. Strength, in metallurgy, is still a rather vague term, so is usually divided into yield strength (strength beyond which deformation becomes permanent), tensile strength (the ultimate tearing strength), shear strength (resistance to transverse, or cutting forces), and compressive strength (resistance to elastic shortening under a load).
• Toughness – Resistance to fracture, as measured by the Charpy test. Toughness often increases as strength decreases, because a material that bends is less likely to break.
• Hardness – A surface’s resistance to scratching, abrasion, or indentation. In conventional metal alloys, there is a linear relation between indentation hardness and tensile strength, which eases the measurement of the latter.
• Brittleness – Brittleness describes a material’s tendency to break before bending or deforming either elastically or plastically. Brittleness increases with decreased toughness but is greatly affected by internal stresses as well.
• Plasticity – The ability to mold, bend or deform in a manner that does not spontaneously return to its original shape. This is proportional to the ductility or malleability of the substance.
• Elasticity – Also called flexibility, this is the ability to deform, bend, compress, or stretch and return to the original shape once the external stress is removed. Elasticity is inversely related to the Young’s modulus of the material.
• Impact resistance – Usually synonymous with high-strength toughness, it is the ability to resist shock-loading with minimal deformation.
• Wear resistance – Usually synonymous with hardness, this is resistance to erosion, ablation, spalling, or galling.
• Structural integrity – The ability to withstand a maximum-rated load while resisting fracture, resisting fatigue, and producing a minimal amount of flexing or deflection, to provide a maximum service life.

We will focus on just a few of these and other properties in the upcoming chapters.

Derived from Tempering (metallurgy) – Wikipedia accessed and available online 16 January 2024.