The Good, The Bad and the Ugly of Heat Treating Metal

The Good (Strength and Ductility)
The Bad (Part Distortion; High Alloy Costs)
and The Ugly (High Energy Use; Environmental Issues;
Part Cracking and Failure)

I. Basic Definitions:

1. What is heat treating of metal?

Heat treating of metal is the controlled, application and removal of heat to a metal to change the internal, molecular structure of the metal for the purpose of obtaining the desired physical properties; e.g., hardness, tensile strength, ductility, formability, machinability, etc.

2. What is “heat treat hardening” or “quench and temper?”

The strengthening of steel by heat treatment usually means “hardening” with a “quench and temper” (Q+T) process. The Q+T process works by, first, heating the steel to its transformation temperature, the “austenitizing” temperature, or the “critical temperature.” Steel is ~ 90% to 98% iron, plus less than 2% carbon, and other small amounts of other alloys, such as, nickel, chrome, vanadium. (Higher carbon than about 2% carbon is called “cast iron.”) The austenitizing temperature is 1330F or above, depending primarily on the carbon content of the steel. One of the key characteristics of steel at the transformation temperature, is that the steel’s carbon atoms are put into “solution.” Once these “magic carbon atoms” are in “solution,” they are free to move around the still solid iron atoms, and rearrange themselves into different crystal structures or “phases” for that material. If the heated steel is rapidly cooled (“quenched”), the new structure is “frozen” into a hardened structure, called “martensite.” This new structure, or arrangement of carbon and iron atoms, gives the part its hardness and its strength.

The as-quenched, martensitic structure, while very strong, is usually too brittle to be of practical use, so we then temper the part to “draw down” some of the hardness and make the part more ductile. There is usually a trade-off, or a balancing act, that part designers need to consider when specifying a heat treating, hardening process; to balance the need for tensile strength (hard) with ductility (soft).

“Playing in God’s Playground:” Rearranging the atoms to get different properties in the steel parts.

Most parts are machined or stamped when the steel is in its annealed or soft state. In its annealed state, the steel atoms are arranged as shown in Figure 1, called Body Centered Cubic (BCC).

The atoms on the corners of the cube are iron, and the “body” in the center is a carbon atom. The first step of “heat treat hardening” is to heat the part to slightly above its austenitizing temperature (1330F, or above, depending on the type of alloy steel, but always below the melting point for the material). After the part is soaked thoroughly at its austenitizing temperature, some of the atoms in the part (primarily carbon atoms) are able to “move” or dislocate in the iron matrix and reconfigure themselves – the carbon atoms are said to be “in solution.” The part is still solid – it is in a state of “solid-solution” with the iron atoms remaining in a solid state, but the atoms of carbon (and other alloying elements) are free to move about inside the part and rearrange themselves. Metallurgists study the phenomena of combining different alloying agents (nickel, chrome vanadium, boron, etc.) in various amounts and at various temperatures, and what effect they have on the physical characteristics of the steel – better ductility for a given hardness, corrosion resistance, high temperture strength, etc. (We heat treaters just call this “Playing in God’s Playground.”)

The orientation of the atoms in the hot, austenitic phase is called Face Centered Cubic (FCC) and the carbon atoms move to the face of the iron cubes in the metal’s matrix. See Figure 2. At this point, if the part is cooled rapidly enough (“quenched”), the part converts into a hardened, “martensitic” phase; the atoms are arranged as Body Centered Tetragonal (BCT). Because the iron matrix is now longer on one side, tetragonal in shape and not cubic, the hardened structure is now approximately 4% larger in volume than the unhardened part. See Figure 3. (If you cool the part slowly from the austenitizing temperature, it will go back to the soft, “annealed” state, with a BCC structure.)

The martensitic structure is very hard and strong – with a high tensile strength. Unfortunately the as-quenched part is also usually very brittle. To make the part more ductile it is then “tempered” to “draw down” or “draw out” some of the hardness and make the part more ductile. To temper the part it is reheated to a temperature below the critical temperature and soaked for a period of time. Generally, the higher the tempering temperature, the lower the hardness (and lower tensile strength) the part will have, but the higher ductility of the part. Hardness and strength versus ductility and impact resistance – it’s a balancing act that heat treaters constantly struggle to find just the right compressive residual stresses on the part surface. We will discuss the issue of residual compressive stress management in a later blog.

NEXT TIME: We will discuss “Intensive Water Quenching Processes” (Intentiquench®) and how they differ from traditional quenching in oil, polymer or high pressure gas.

More definitions of heat treating processes are available at our Akron Steel Treating Company website: www.akronsteeltreating.com.

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