Steel: The Framework of Our Civilization

Lisa Marie Dougherty
Materials Science and Technology
Los Alamos National Laboratory
 

More than any other metal, our civilization relies on steel. Everywhere we look, we see it. Steel covers the bodies of cars, trains, and buses. Steel bars reinforce concrete walls and bridges, and steel I-beams support the floors and roofs of buildings. Steel is used in so many things that we take it for granted as we use our steel tools, tables, chairs, staples, zippers, and utensils without a second thought. And yet, such common use of steel has only been possible since modern steelmaking practices were developed about 150 years ago.
 
Anthropological history is commonly divided into three ages: the Stone Age, the Bronze Age, and the Iron Age. Each age is associated with a change in weapon and tool material so significant that it revolutionized civilization. The earliest tools and weapons were made from stone about three million years ago during the early part of the Stone Age. This period concluded around 6000 BC when the art of extracting copper from its ore was developed. This process, called smelting, allowed for the first production of metal implements. Around 4000 BC, the discovery that arsenic or tin can be added to copper to make a harder material initiated the Bronze Age. This method of improving select material or mechanical properties by combining two or more metals into a homogenous mixture is called alloying. Armor and weapons fashioned from this new copper alloy were far superior to anything that had come before. Civilizations that could smelt bronze rose in power.
 
Around 1200 BC, the smelting of iron, which requires a higher temperature than that of copper or tin, was achieved. Even though iron is weaker than bronze, it became the metal of choice for tools and weapons because it was cheaper, more abundant, and easier to sharpen. Iron, like many metals and alloys, can form into more than one type of solid phase, where a "phase" is a unique organization of atoms into a structure that possesses uniform chemistry and physical properties. The addition of carbon to iron, producing an alloy called steel, dramatically increases the strength of iron by changing the solid phases that make up its microstructure. The earliest steel artifact has been dated to around 300 BC when wootz steel, a high-carbon alloy with a banded appearance due to the distribution of carbon-rich phases, was first developed in India. Wootz steel was used to make the famous Damascus swords possessing almost legendary strength and sharpness, the source of which remained a subject of debate among metallurgists until a few decades ago when metallography revealed the phase structure of the steel in the blades.
 
Although steel implements were produced during the Iron Age, steel was not widely available until advances in steelmaking in the late 1800s allowed for its cheap mass production. Once steel production was commercialized, all parts of the construction and transportation industries turned to the iron-carbon alloy as their metal of choice due to its excellent mechanical properties at a relatively low cost. Its primary components are plentiful and easily recycled, and its properties can be controlled through thermomechanical processing (i.e., cycles of severe plastic deformation alternating with baking the deformed metal at high temperatures), surface treatments, and alloying.
 
Steel has become such a commonplace material in our lives few contemplate its significance or amazing characteristics. Only a smidgeon of carbon turns a weak bar of iron into a beam that can withstand tons of stress without bending. Adding chromium and nickel or manganese transforms a metal that readily rusts into an alloy that can withstand corrosive environments as extreme as seasides and nuclear reactors. Its strength can be dramatically increased simply by rolling it into a flat sheet or cooling it very quickly. And if carbon is forced into its surface, steel blades and bits can stand up to the intense heat and friction of high-speed cutting or drilling.
 
What is it about steel that makes it so versatile? How can one material span the range of so many physical properties: from soft to hard, ductile to brittle, magnetic to non-magnetic, weak to strong, and corrodible to corrosion-resistant? The answer lies in the complex microstructure of iron and iron-carbon phases, and their chemical properties when combined with other elements. Unlike many modern-day alloys and engineered materials, steel began as an empirical material, produced for specific applications through trial and error. As such, the number of different types of elements that are used in steels is more extensive than in any other alloy system.
 
This variety complicates the selection of the right type of steel for a particular application. When the wrong steel is selected for a structural application, failure can result regardless of time in service. For example, some steels are subject to a transition from ductile to brittle behavior at temperatures that are not uncommon in arctic waters and cold climates. This has resulted in storage tanks suddenly bursting during the winter and ships literally breaking in half. Failure can also result from impurities introduced during steel production. Slag retained in the steel from the smelting process used to make rivets for the Titanic has been proposed to have caused its tragic sinking.
 
As with any material, steel structures need to be inspected regularly and replaced when their reliability has degraded. In structural applications, steel members typically support heavy loads for long periods of time. If the structure is in a corrosive environment, detecting even a minor crack early can prevent an unexpected failure due to stress corrosion cracking. Maintenance of steel structures becomes more critical as their use exceeds load and lifetime engineering designs, which is becoming more common as our population and traffic density increases. The failure of structural steels in the 1967 Silver Bridge collapse over the Ohio River that killed 46, the 1983 Mianus River Bridge collapse in Connecticut that killed 3, and the 2007 Mississippi River bridge collapse in Minnesota that killed 13 have been attributed in large part to high loads and inadequate inspections. But with careful inspections and maintenance, the steel framework of our civilization can perform satisfactorily for generations to come.
 
Steel is an amazing alloy and the most important metal in modern civilization. But the more we depend upon it, the more we take it for granted. Most popular science magazines and television programs focus on new materials with exotic properties and carefully engineered structures. Yet even though the media features nano-scaled materials and devices, our civilization relies most heavily on macro-scaled constructions and vehicles, most of which are made, at least in part, of steel. It may not be exotic, but steel will always be as fascinating as it is essential to our way of life.