Heat Treat Chemistry

Introduction
Unit Cells
Transformation Temperature
Grain Size
Iron-Carbon Phase
Finding Balance
QuenchinBalanceg Mediums
Heat Treat Process Details
Protective Atmospheres
Glossary of Heat Treat Terms

 

Introduction

Iron and steel can be heated to a temperature where they melt into a liquid.  When this liquid cools, crystals form.  Tiny crystals form first and they keep growing until they form regular precise formations.  These regular patterns of precise rows are called the space lattice.  A Space Lattice is the arrangement of the atoms in a crystal.

The smallest fundamental arrangement in the lattice is called the Unit Cell.  Therefore a lattice is simply a group of unit cells in which each unit cell is identical.  There are several types of unit cells.

 

Unit Cells

  • Body-Centered (bcc) consists of eight atoms in the corners of a square cube.  In the center of these eight atoms is a ninth atom.  Ferrite Iron takes the form of bcc.
  • Face-Centered Cubic (fcc) has eight atoms in the corners of a square cube.  However, rather than an atom in the center of the cube like bcc, fcc has an atom in the middle of each of the six faces of the cube.  So a fcc unit cell consists of 14 atoms.  Austenitic Iron takes the form of fcc.
  • Body-Centered Tetragonal (bct) is almost identical to bcc.  The difference is that bct has rectangular faces.  Martensitic Iron takes the form of bct.

As iron goes through temperature changes, its atoms realign themselves into new geometric patterns.

  • Ferrite Iron is bcc at room temperature that has not been heat treated
  • Austenitic Iron is fcc at elevated temperatures.  When iron is heated above a critical temperature is “transforms”, or its atoms rearrange to form fcc unit cells.  In other words,  Austenite is formed above the transformation temperature.  If Austentite is slowly cooled it returns to bcc.
  • Martensitic Iron is bct and is Ferrite that has been heated until transforming into Austenite and then quenched (fast cooled).  This process tends to harden the iron.  The iron goes from bcc to fcc to bct.

 

Transformation Temperature

As Ferrite is heated and changes to Austenite, there are two important temperatures to recognize and understand.

The Lower Transformation Temperature is the temperature where the bcc structure STARTS to change to the fcc.  (Ferrite starts to change to Austenite.)

The Upper Transformation Temperature is the temperature where all the bcc structure has COMPLETELY changed to fcc. (All the Ferrite has changed to Austenite.)

Depending on how it is cooled, Austenite will change to Martensite or back to Ferrite.  Martensite is formed with a rapid quench and Ferrite is formed with slow cooling at room temperature.

The Lower Transformation Temperature for all irons and steels is around 1,330F.  The Upper Transformation Temperature varies with each metal, but is between 1,330 and 2,000F.

 

Ferrite

Austenite

Martensite

Body Centered

Cubic Lattice Formation

Face Centered

Cubic Lattice Formation

Body Centered

Tetragonal Lattice Formation

Exists at Low Temperature Exists at High Temperature Exists at Low Temperature
Magnetic Non-Magnetic Magnetic
Less Hardness No Hardness Most Hardness
Less Strength No Strength Most Strength
Ductile N/A Brittle

 

For minimum distortion a Heat Treater should always use the minimum temperature required to ensure complete transformation.  To do this, the Heat Treater must know the carbon content of the parts/steel.

Grain Size

As molten metal cools, it aligns itself in to a precise regular structure that is called Unit Cells (bcc, fcc, bct).  As the Cells form, they combine to form “Nucleation Sites”.  This process continues in all three dimensions, forming “Colonies” of Unit Cells.  When the boundaries of the Colonies touch each other, a “Grain” or “Grain Boundary” is formed.  In general, the slower the cooling rate, the fewer the Nucleation Sites and therefore the larger the Colonies or the larger the Grain Size.  Conversely, the faster the cooling rate the more Nucleations Sites and the smaller the Grain Size.  The Grain Size effects the mechanical properties of the steel.

The smaller the Grain Size (cooled fast) the greater the strength.  The larger the Grain Size (cooled slowly) the more ductile it is.  Therefore, a part is cooled according to the desired property.

 

Iron-Carbon Phase

Steel is Iron with 0.0 to 2.0% carbon content.  The location of the carbon atom within the Unit Cell is called the Interstitial Spots of the Unit Cell.

Steel with very little carbon is called Ferrite.   Steel that has approximately 0.8% carbon is called Pearlite. Steel with a carbon content above 0.8% to 2% is called Cementite.

At more than 2% carbon, it is called “Cast Iron”.

Ferrite is almost pure iron.  It has little ‘desire’ to dissolve carbon and so there is little carbon in it.  Since carbon gives steel the ability to become strong and hard, Ferrite is very weak steel.  Ferrite exists at low temperatures only and is magnetic.

Pearlite is a mixture of Ferrite and Cementite.  Pearlite exists at room temperature and is magnetic.

Cementite is a compound of iron and carbon known as “iron carbide”.  Its chemical formation is Fe3C.  Cementite contains 6.67% carbon by weight.  However, Cementite is present in the alloy between 0.8% and 6.67% carbon.  As the percentage of carbon increases, more and more Cementite is present, until at 6.67% the entire mixture is Cementite.  Below 2% it is considered steel and above 2% it is Cast Iron (carbon content by volume).

 

Finding Balance

The hardness, brittleness, ductility, and grain size of a steel is a result of the heating and cooling method that is used in the Heat Treat Cycle.

When steel is heated above the Transformation Temperature to form Austenite, and then quenched very rapidly it will most often transform into Martensite.  It will be hard, strong and have a small grain size, but will also be brittle.

When steel is heated above the Transformation Temperature to form Austenite, and then cooled very slowly, it will transform into Ferrite, Pearlite and Cementite.  This structure is comparatively softer, less strong, more ductile, and has a larger grain size.

The ideal properties for heat treated steel is hardness, strength, ductility and small grain size.

The selection process comes down to:

  • If a steel must hard and strong, quench rapidly.  However, it will be brittle.
  • If a steel must have great ductility for machining, cool slowly.  However, will not be very strong.
  • If the steel must have both strength and ductility, alloys can be added, but costs will increase.

 

Quenching Mediums

Water Quenching

Water is the most common quenching medium.  It is inexpensive, convenient to use, and provides very rapid cooling.  It is used primarily for low carbon steels where the heat must be extracted from the steel rapidly in order to obtain good hardness and strength.  Although water provides a sudden, drastic quench, it can cause internal stresses, distortion, or cracking.  For best results, water should be used at room temperature.

Oil Quenching

Oil is more gentle than water and is used for more critical parts which have thin sections or sharp edges.  Since oil is more gentle it develops less internal stresses, distortions, or cracking.  However, oil does generally does not produce as hard or strong of a steel as water.  Therefore, the decision must be made by the designer of a part which is more important, hardness and strength or minimizing cracking and distortion.

Air Quenching

Air quenching is much less drastic than either oil or water.  High speed fans blow room  temperature air over the steel parts.  The slower rate decreases the distortion, internal stress and cracking.  However, it will not be as strong unless special alloys have been added into the metals.  Therefore, air quenching is usually used on high alloy metals such as chromium and molybenum.

Brine Quenching

Brine is water with 5 – 10% salt added.  The speed is slightly faster than water and therefore more drastic in regards to cracking and distortion.  It is also effective at removing surface scale from the parts, since the salt causes mini ‘explosions’ on the part’s surface that blows off the scale.

 

Heat Treat Process Details

Normalizing

A process for ferrous metals.

The purpose of normalizing varies considerably.  Normalization may increase or decrease the strength and hardness of a given steel in a given produce form depending on the thermal and mechanical history of the product.  The functions of normalizing may overlap with or be confused with those of annealing, hardening and stress relieving.  Improved machinability, grain-structure refinement, homogenization, and modification of residual stresses are among the reasons for normalizing.

Homogenization of casting by normalizing may be done in order to break up or refine the dendritic structure and facilitate a more even response to subsequent hardening.  Similarly, for wrought products, normalization can obliterate banded grain structure due to hot rolling,as well as large grain size or mixed large and small grain size due to forging.

Normalizing is an austenitizing heating cycle followed by cooling in still or slightly agitated air.  Typically, the work is heated to a temperature about 100F above the upper transformation temperature.  The heating portion must produce an austenitic phase prior to cooling.  This requires holding the temperature for about 1 hour per inch of part thickness.  The cooling must take place very slowly, to the point of “black heat”, or they are no longer red-hot as when removed from the furnace.  After this, they may be quenched or cooled more rapidly to reduce total cooling time.

 

Annealing

A process for non-ferrous and certain ferrous metals at higher temperatures.

In its simplest explanation, Annealing is the same as Normalizing (see above) except it is ALSO done to non-ferrous metals, such as aluminum, brass, copper, etc., and Cast Iron (iron with more than 2% carbon).

Its purpose is to soften and homogenize metals that have been partially processed.  It can also be used as a solution process for elimination of entrapped gases, age hardening and corrosion resistance.

Surface or Case Hardening

A process in a vacuum or controlled atmosphere furnace that changes mainly the surface (the wear) portions of the part.

 

Protective Atmospheres

Oxidizing

 

Neutral

 

Reducing

 

 

Glossary of Heat Treat Terms

Open a 31 Page Heat Treat Glossary PDF

 

 

Sources:  Heat Treat Network, Handbook of Industrial Gas Utilization 1977, Metals Handbook, American Society of Metals Source Book. TechPro DTE Energy 2001.

© Energy Solutions Center Inc. All Rights Reserved