Heat has the power to affect and alter many of the materials in our natural world. As steel is heated to various temperatures, its color and properties change. These transformations allow this metal to be used in a wide variety of applications to make our world a better place and improve life throughout society.
According to ASM International, “Steel is such an important material because of its tremendous flexibility in metal working and heat treating to produce a wide variety of mechanical, physical, and chemical properties.”
Definitions and Classifications of Steel
Before embarking on a discussion of how temperature changes the color and chemical properties of steel, it’s important to define and understand the basics of this amazing material.
Within industry, steel is defined as “an iron-based alloy, malleable in some temperature range as initially cast, containing manganese, usually carbon, and often other alloying elements. In carbon steel and low-alloy steel, the maximum carbon is about 2.0%; in high-alloy steel, about 2.5%. The dividing line between low-alloy and high-alloy steels is generally regarded as being at about 5% metallic alloying elements.”
This definition is then expanded to the practical understanding within the steel industry of the thousands of compositions and applications of steel that are possible. Although the specific number is impossible to accurately calculate, the five classes of steel generally accepted are: carbon steels, alloy steels, stainless steels, tool steels and special-purpose steels, which are usually proprietary in nature. Industry designation systems and subsequent grading of steel is done by the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI).
Phases of Steel
“In metallurgy, the term phase is used to refer to a physically homogeneous state of matter, where the phase has a certain chemical composition, and a distinct type of atomic bonding and arrangement of elements,” explains a document published by the Industrial Metallurgists.
Three phases exist for steel: ferrite, cementite, and austenite. Steel producers arrive at each phase through temperature changes. Ferrite is essentially iron, and at room temperature, conventional steels are a mix of this material and cementite. However, when steel is heated to above 1340 degrees Fahrenheit, the cementite dissolves and forms the austenite phase. Eventually, the steel loses its magnetic charge at this temperature.
How Metals Are Transformed Through Heat
According to Metal Supermarkets, metals are transformed through heat in terms of electrical resistance, thermal expansion, structure, and magnetism.
Electrical currents pass through metals at different levels of resistance. Heat speeds up electrons as they absorb energy, which actually increases electrical resistance.
Like most things, steel expands at higher temperatures. Heat actually boosts the movement of atoms within the metal.
Heat changes the allotrope structure of metals by atom relocation, which affects the hardness, strength and other properties of steel. For example, when iron is heated past 1,674 degrees Fahrenheit, it absorbs carbon. This affect hardens the steel product, which can then be used in high carbon steel applications like tool steel.
The iron component of steel holds the magnetic properties found in the metal. As steel is heated to 1,418 degrees Fahrenheit, it will lose its magnetization.
Changes in Color and Appearance
The different temperatures of steel will also bring the material through a change in coloration. According to Sciencing, “working with steel and modifying its color involves setting up a sufficient heat source, heating the steel to the desired color, then quenching and tempering it.” Here are the typical color changes steel will go through at various temperatures:
- At 480 degrees Fahrenheit, steel turns brown.
- At 520 degrees Fahrenheit, steel turns purple.
- At 575 degrees Fahrenheit, steel turns blue.
- At 800 degrees Fahrenheit, steel turns grey.
- Above 800 degrees Fahrenheit, steel produces incandescent colors.
- Between 1000 degrees Fahrenheit and 1500 degrees Fahrenheit, steel turns an increasingly brighter shade of red.
- Between 1600 degrees Fahrenheit and 1900 degrees Fahrenheit, steel turns orange and then yellow.
- At 2000 degrees Fahrenheit, steel turns bright yellow.
Once you achieve your desired color, remove the steel from the heat, quench it in oil and temper it as soon as possible.
Heat Treatment Affects Steel Properties
According to American Machine Tools, five forms of heat treatment affect the properties of steel.
Steel is hardened by heating it to a specific temperature and then cooling it rapidly in brine, oil or water. Although this process boosts the strength of steel, it also increases the brittleness. Hardened steel is used for everything from shovels to surgical instruments.
Because of the effects of the hardening process, most producers will want to temper the heated metal to a particular temperature before allowing the steel to cool on its own. Tempering steel, which requires lower temperatures, reduces the brittleness that occurs with hardening. Tempered steel is popular for construction and mining applications.
The annealing process involves bringing steel to a particular temperature, holding it there for a certain period of time, and then cooling it to room temperature. This process relieves internal stresses of the metal as well as softens steel, makes it more ductile, and refines the grain structure. Changing the rate of cooling will change the softness of the steel. Cooling is usually done by burying the steel in sand or ash by allowing a heated furnace to cool down with the steel inside. Sheet metal that has gone through a stamping process are frequently annealed.
This process requires heating steel to a higher temperature than either the hardening or annealing requires, followed by soaking it for uniform heating and air cooling. Normalization relieves internal stresses from machining, forging, or welding steels, and they are harder and stronger than annealed steels.
During this process, a low-carbon steel is heated to a given temperature along with another material that decomposes and leaves carbon on the steel surface. When rapid cooling occurs, the outside layer is hard but the inside is soft. The process is excellent for applications that require resistant surfaces such as gears and cams.
Steel is perhaps the more versatile metal available today. It can be easily changed, strengthened and altered with heat treatments to provide a wide range of solutions for applications throughout industries and around the world.