Heat-treating,

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Modifying Properties by Heat Treatment

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Heat-treating is a collective term used to indicate a group of thermal processes used to modify metal properties.

This page deals only with thermal cycles applied to steels.

Other metals are treated with different processes explained elsewhere in this website.

It has been remarked in our page on Welding Steel, that
the single reason that makes steel so important is its versatility.

By this one means the capability of presenting economically a very wide range of mechanical properties.

This ability is based on the fact that tailored chemical compositions make the material responsive in subtle ways to the application of precise heat treatments.

This page will present an overview of some of the important processes used for Heat-treating steels.

There is a strict correlation between the microstructure, describing the inner make up of the basic building blocks of metallic materials, and the mechanical properties displayed.

Heat-treating provides required Mechanical Properties

Modifying the composition of steels by alloying them with determinate elements, sometime in tiny amounts, can change their responsiveness to specific Heat-treating thermal cycles.

That is used to develop the mechanical properties needed to each application.

Metallic materials are characterized by being built of crystals, ordered tri-dimensional arrangements of atoms according to a repetitive pattern, specific to the material and to the temperature.

In the case of pure Iron (Fe), the main constituent of steel, at room temperature the basic crystal lattice or pattern, called unit cell, is described as a cube with one (Fe) atom at each vertex and one more in the cube center.

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Such a structure is called body centered cubic (BCC) alpha iron and is stable up to the temperature of 910 °C.

At that temperature a sudden transformation occurs, and the Iron atoms rearrange themselves at each vertex of a cube with one more atom in the center of each face.

This new structure is called face centered cubic (FCC) gamma iron and is stable within the range of temperatures between 910 and 1390 °C.

Upon further increasing of the temperature beyond 1390 °C, the structure reverts back to a body centered cubic delta iron stable up to 1539 °C, but this transformation has less importance from a practical Heat-treating point of view.

Unit cells as well as atoms cannot be seen under any microscope.

Their internal symmetry is inferred by x-ray diffraction methods.

Aggregates of crystals with the same orientation, called grains, are visible under the optical microscope after proper metallographic preparation (polishing and etching).

Grain size, described by established conventions can be measured with standard metallographic methods.

Steels are alloys of carbon and iron. Many of the interesting steel properties depend on the behavior of carbon.

It happens that carbon dissolves easily in gamma iron, usually called austenite, producing what is called an interstitial solid solution.

These are solutions in which atoms of the alloying element (in this case carbon) that are very small when compared to the size of the main atoms, occupy spaces between atoms of the solvent element (in this case iron), which retain their original lattice position.

The solubility of carbon in alpha iron is however much less than in gamma iron.

To study the behavior of a given steel as a function of changing Heat-treating temperature, one has first to know which is the carbon content.

The study is eased by observing the so called Iron-Carbon equilibrium diagram.

You may wish to inspect such an Iron-carbon phase diagram by visiting the page
http://www.substech.com/dokuwiki/doku.php?id=iron-carbon_phase_diagram

See another example of The Iron-Carbon Equilibrium Diagram at
http://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&NM=153

See also Principles of Heat Treating of Steels
http://www.keytometals.com/Articles/Art159.htm

On the horizontal axis the carbon content is depicted, starting with zero carbon on the left side (meaning 100% pure iron) and increasing toward the right until about 0.8 weight percent carbon, where something interesting is depicted, and then further to the right.

On the vertical axis temperature is marked, starting from zero up to 1539 °C at which point pure iron melts, becoming liquid.

In the diagram several drawn lines delimit areas where definite structures exist either singly, or coexist in mixed arrangements.

Heat-treating permits to manipulate the mechanical properties of a metal by controlling the rate of diffusion, and the rate of cooling within the microstructure.

Heat-treating cycles are generally divided into three parts: heating, holding and cooling.

Heating should be uniform to avoid large temperature differences between thick and thin sections in any given part which may cause strains because of differential thermal expansion.

For economic reasons this part of the cycle should be as short as possible, also to avoid grain growth which is generally detrimental to properties.

Holding at the prescribed temperature for the specific Heat-treating permits equalization and must be sufficient for the accomplishment of the required transformation, including diffusion of elements in solution.

At sufficiently high temperature, grains after severe plastic deformation recover and recrystallize losing hardness and gaining ductility.

Heat-treating intended to remove internal stresses is called Stress Relieving.

Annealing is the process used to remove completely internal stresses from a part, to reach minimum hardness and maximum ductility.

Removing an annealed part from the furnace and letting it cool down in air is called normalizing.

The cooling phase of the Heat-treating cycle may cause different outcomes depending on the actual cooling rate.

The following Heat-treating is called Hardening and Tempering.

The austenitic phase of a steel of suitable composition, quickly cooled by a proper method, transforms to a hard phase called martensite.

Cooling quickly is called quenching.

To retain hardness but to remove excessive brittleness, a further tempering treatment is performed, by heating again the hard martensite at a low temperature.

For applications requiring substantial mechanical properties, obtainable by such Heat-treating, it is important to consider a quality called Hardenability. (Click on the link to see the page.)

This characteristic provides (calculates or tests) the maximum size of a body of any given steel capable to develop in the center an agreed upon volume fraction of martensite of the required hardness upon quenching.

The larger the size of the part, the slower the achievable cooling rate, as heat removal is possible only from the external surface.

Therefore, if elevated hardness is required in a thick body, the steel composition must be such that it allows sluggish martensite transformations even at a rate as slow as air cooling.

Slower than quenching cooling rates (as in large bodies of non optimized composition) produce different intermediate microstructures with lower properties, known as troostite, bainite and sorbite, less important practically except for very special situations.

A slow cooling rate produces a structure called coarse lamellar perlite, characterized by low strength and hardness and high ductility.

Pearlite consists in alternate layers of ferrite (alpha iron) and iron carbide (called also cementite, a compound whose formula is Fe3C).

Other steel Heat-treating processes dealt with in this website are
Weld Preheating,
Stress Relieving
Case Hardening.

For a Book on this subject see

Heat Treating: ASM Handbook, Vol. 4
ASM International / 01-Jun-1991 / 1012 pages

An Article on Advances in Industrial Heat Treating was published (11) in Issue 128 of Practical Welding Letter for April 2014.
Click on PWL#128 to see it.

An Article on The Heat Treating Professional was published (11) in Issue 131 of Practical Welding Letter for July 2014.
Click on PWL#131 to see it.

An Article introducing the new ASM Handbook Volume 4C on
Induction Heating and Heat Treatment

was published in Issue 136 of Practical Welding Letter for December 2014.
Click on PWL#136 to see it.

An Article on Measuring Depth of Decarburization was published (3) in Issue 139 of Practical Welding Letter for March 2015.
Click on PWL#139 to see it.

An Article on Measuring Heating Rates in Vacuum Furnaces was published (3) in Issue 140 of Practical Welding Letter for April 2015.
Click on PWL#140 to see it.

An Article on HTPro - June 2015 was published (11) in Issue 143 of Practical Welding Letter for July 2015.
Click on PWL#143 to see it.

An Article on HTPro - October 2015 was published (11) in Issue 147 of Practical Welding Letter for November 2015.
Click on PWL#147.

An Article on Induction Coupled Thermomagnetic Processing was published (2) in Issue 155 of Practical Welding Letter for July 2016.
Click on PWL#155

An Article on Computer Modeling for Induction Hardening was published (3) in Issue 161 of Practical Welding Letter for January 2017.
Click on PWL#161.

An Article on Nanostructured Metal was published (7) in Issue 162 of Practical Welding Letter for February 2017.
Click on PWL#162.

An Article on New issue of HTPro was published (7) in Issue 165 of Practical Welding Letter for May 2017.
Click on PWL#165.

An Article on Improvements in Vacuum Furnace Design was published (7) in Issue 167 of Practical Welding Letter for July 2017.
Click on PWL#167.

To search for the most important titles of Articles published in all issues of Practical Welding Letter, click on Welding Topics.

For all the PAST ISSUES of Practical Welding Letter you may wish to explore, click on the Index of Past Issues of PWL.

Watch the following Video on

Elements of Tempering, Normalizing, and Annealing

http://www.youtube.com/watch?v=COasmrnxbqg

* * *

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To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on
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