This module is intended highlight and summarize some of the major factors associated with heat treatment and simple alloys.  The Bruce text and Russ CD provide excellent illustrations that will not be repeated here in detail.  Rather, simplified illustrations will be used to emphasize the major phases of transformations and sub-alloys associated with steel.  The focus in this module will be on plain carbon steels.  However, a brief section will be dedicated to other selected alloys.

OBJECTIVES

 After completing this module, you should be able to do the following:

     Identify common constituents used in iron and carbon alloys (steel);
     Define properties affected by certain alloying agents in steel;
     Interpret a simple phase diagram;
     Interpret a simple equilibrium diagram;
     Explain the meaning of eutectic;
     Interpret the iron carbon diagram;
     Explain the properties affected by iron - carbon compounds in plain carbon steel;
     Explain selected heat treatment and conditioning terms.
 

TERMS:

     Compound
     Alloy
     Constituent
     Solvent
     Solute
     Liquidus
     Solidus
     Eutectic
     Phase Diagram
     Equilibrium Diagram
     Iron Carbon Diagram
     Austenite
     Ferrite
     Cementite
     Martensite
     Pearlite
     Tempering
     Normalizing
     Spheroidizing
     Annealing
     Surface Hardening
     Flame Hardening

INTRODUCTION

     In Module 2, we looked at the "building blocks" for metals and the characteristics associated with atomic bonds of materials. Properties of metals are ultimately related to the atomic structure, and the state of the material depends on the type, arrangement, and energy level of atoms.  In Module 3 some simple alloys will be considered as well as what changes take place when heat is used for treatment.  The Iron - Carbon alloy ( Steel) will be the primary focus.

METALS AND ALLOYS

     Metals in pure form are useful for some applications such as aluminum foil, copper wire, nickel plating, silver and gold contacts.  However pure metals have limited properties in their pure form, but ALLOYS provide a way of combining properties of and obtaining some desired characteristics of both elements.   Below is a phase diagram for two elements (alpha and beta).  Note that as the percentage of the alloying element changes the freezing point also changes.  When a transition from liquid to solid occurs without going through a "slushy" stage it is referred to as the eutectic point.
 
 

ALLOYS. By definition, alloys are two are more elements, one of which is metal.  There are three major types of alloys:

     TYPE 1:      Solid Solution - Two elements, complete solubility in solid and liquid;

     TYPE II :    Compounds - Complete solubility in liquid;

     TYPE III:   Mixtures -  Partial solubility in solid  ( interweaved crystals).

In general, metals do not like to mix.  Therefore, the most common type of alloy in metals is TYPE I (solid solution).  In this type of alloy, as crystals grow, the allow is formed by one of two ways:

              1.  Substitutional
              2.  Interstitial

The more abundant element is referred to as the solvent and the less abundant element is the solute.  Lets look at some of the requirements of substitutional type alloys with respect to atomic characteristics.

Substitutuional (TYPE I) Alloys.  Those alloys that are formed through substitution (two or more) elements)  must have the following characteristics:

               A.  Similar crystal structures

               B.  Difference in Atomic radii should be no more than 15 %.

Interstitial.  With interstitial type alloying, size of the atom becomes the major factor.   The solute atoms must be small in size to fit into the spaces between the larger solvent atoms.  Therefore the diameter of the solute atoms must be less than .59 of the solvent atoms.

The focus of this module will he on plain carbon steel alloys.  However some of the other alloying agents for steel should be mentioned.   Below is a brief list of some very important alloying agents that are often added to steel to change desired characteristics:

          Nickel -- Increases toughness and impact strength and corrosion resistance;

          Chromium -- Joins with carbon and forms chromium carbide.  Improves depth of hardness;

          Molybdenum -- Joins with carbon and increases the hardenability at elevated temperatures;
                hampers grain growth, therefore, finer grains.  Important for cutting tools, gears, crankshafts, etc.
                that require harder, tougher steel.

          Vanadium -- Promotes fine grain growth.

          Tungsten -- Increases heat resistance; prevents annealing at elevated temperatures for steel used as
               cutting tools.

THE MAJOR POINT OF MATERIALS BEING ALLOYED TOGETHER IS TO IMPROVE CHARACTERISTICS AND PROPERTIES.  Alloying is a very complex process, particularly when more that two elements are combined. However, even with a simple alloy (only 2 elements)  the percentage of solvent to solute will dramatically affect the characteristics of the material.  Additionally the rate of cooling also determines how secondary "compounds" are formed.    Heat treatment must be incorporated to condition steel alloys, generally for one of the following reasons:

           1.  To relieve internal stresses (stress relieving);
           2.  To refine grain size or produce uniform grains throughout;
           3.  To alter the micro structure;
           4.  To change the surface chemistry by adding or deleting elements.
 

STEEL ALLOYS:  Plain carbon steel is made up of Iron (solvent) and carbon (solute).  Wrought iron (meaning pure iron) is a very ductile material and has the the ability to undergo allotropic change from BCC to FCC to BCC as it cools from liquid to room temperature or vice versa.

  As the percent of carbon increases, the properties of the steel will change dramatically within the plain carbon steel group.  Plain carbon steels are divided into three classifications:

                 Low Carbon Steel  --  .05% to .30% Carbon;
                 Medium Carbon Steel -- .30% to .60% Carbon;
                 High Carbon Steel --  .60% to 1.2 % Carbon.

      NOTE:  If the carbon content exceeds 1.2%, the alloy is considered to be cast iron (very brittle).
                   Below .05 % is classified as "wrought" state and the properties cannot significantly be changed.

     Plane carbon steels are considered to be binary alloys (e.g. only two elements are combined).  When two elements are combined a eutectic composition usually occurs.  In the case of steel, the eutectic point occurs at .8 % carbon composition and 733 degrees C.   If the rate of cooling is controlled,  a eutectic microstructure of fine alternating layers of the two constituents occurs.  In steel this material has a special name--pearlite.

                           A                                                                                       B
 

     There are some critical compounds having significant characteristics that occur at particular temperatures with varying percent carbon content.  Those of critical importance are austenite, cementite, peralite, ferrite, and martensite.

        Austentite -  Occurs above the A3 line; have the ability to desolve carbon in solid solution.
                             Austenite (gamma iron) is a FCC structure that can dissolve and hold up to
                              2.11 % carbon in solid solution.

        Cementite - A very hard, brittle compound (Iron Carbide)  having over 6.67 % carbon.
                           Cementite is 100% IRON CARBIDE.

        Peralite  -   Alternating layers of alpha iron and cementitie.  Occurs only at eutectic composition.
                            The austenizing temperature must be reached and then the steel must be allowed to
                            cool and maintain equilibrium.

        Ferrite  -     Alpha iron has a BCC structure and holds only about .008% carbon at room temperature.

       Martensite - Extremely hard and brittle..occurs when austenite  is cooled rapidly through quenching.

          Note:   The severity of the quench determines the degree of formation of martensite.  Quenching
            media used for heat treatment of steel include (from most to least severe):

                                     1. Water
                                     2.  Brine
                                     3.  Oil
                                     4.  Air

The diagram shown below shows the regions of the iron - carbon equilibrium where these compounds occur.

HEAT CONDITIONING

     Heat treatment and secondary conditioning  is necessary to change the properties of steel into desirable and workable form.  Some of the of these conditioning treatments include annealing, recrystallization, tempering, spherodizing, surface hardening  treatment, and flame hardening.

Tempering - When steel is heated about the critical temperature (where allotropic change occurs) and quenched, it will become hard and brittle (provided sufficient carbon is present).  Tempering is a secondary heat conditioning process to reduce the brittleness and increase ductility and toughness.  Residual stresses are also reduced.  Tempering occurs below the critical temperature.

Normalizing -  This process is used to avoid softening steel to much.  Temperatures above the critical are reached and then air cooling is allowed.  The result is a higher strength and but lower ductility than full annealing.

Annealing - This is a very general term used for restoring the crystalline structure.  There are different types of annealing, from full to process.  The primary reason annealing is done is to relieve residual stresses and to make the part more machinable.  Parts are allowed to "soak" at elevated temperatures and cool slowly to create coarse grain structure and make the material soft and ductile with uniform grains.

Spheroidizing - This process is used to improve processing and re-orient the geometry of the grain boundaries.  For certain carbon steels, if the spheroidizing temperature is held for extended periods, the cementite will transform into small spheres and limit the stresses.  The result is a more ductile material, and processability is improved.

Surface Hardening - Low carbon steels that do not have sufficient carbon content can still be hardened by this process.  Carbon in introduced to the surface of the material at elevated temperatures and then quenched.  The result is a hardened shell or case around the softer core.  A type of surface hardening is case hardening.
 

Flame Hardening - This process is used for hardening only certain areas of a part, while allowing other regions to remain ductile.  An example would be hardening the teeth on a band saw blade.  The teeth or cutting surface need to be hard, while the remainder of the blade must be pliable in order to bend around the rollers that drive the blade.
Oil is typically used as a quenching medium after the part passes through an adjustable flame (for controlling temperature).
 
 

Properties of metals are ultimately related to atomic structure.   The state of the material depends on the type arrangement and energy levels of the atoms.  However, elements can be combined as alloys to achieve desired properties of each constituent.   Temperature and carbon content determine the properties of plain carbon steel.     Alloying occurs in metals primarily by substitution or interstitial atomic arrangement.    Certain elements such as chromium, tungsten, molybdenum, and vanadium are alloyed to achieve enhanced properties.

Because iron can undergo allotropic changes, the crystalline structure varies at different temperature.  Heat, and carbon content determine the formation of certain compounds such as austenite, cementite, ferrite, and martensite.  These compounds affect the properties of steel.

Heat is used to treat and condition steel, for increasing or reducing hardness, ductility, toughness, and machineability.