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					Surface Hardening of Steel
                                   Overview
                                   Heat treatment processes vary greatly depending on how
                                   the material will eventually be used. Surface hardening is
                                   a particular branch in the heat treatment/hardening
                                   hierarchy. One of the most common diffusion surface
                                   hardening procedures is the carburizing of steel.
                                   Carburizing gives the operator the ability to harden the
                                   surface of a piece of steel that would otherwise not harden.
                                   The carburizing process depends on carbon diffusing, or
                                   dissolving, into a piece of steel. When carbon is in contact
                                   with a piece of steel, the steel acts like a sponge and
                                   absorbs it, and heating the steel speeds up the process. The
                                   result is an area (or case) of hard carbon steel enveloping
                                   the softer steel. This steel has a higher carbon content at
                                   the surface of the part than in the interior.
                                   In some cases, the parts to be carburized are packed in
                                   carbon rich materials such as activated charcoal granules,
                                   and then heated, speeding up the diffusion of the carbon
                                   into the surface of the low carbon steel. While still at heat,
                                   the carburized steel is quenched, then tempered, resulting in
                                   a case-hardened finish. Low carbon steel wears easily, but
                                   has good machinability, so low carbon parts are carburized
                                   to make them more wear resistant by providing them with a
                                   hard surface.
                                   In this Authentic Learning Task (ALT), you produce a
                                   carbon case around a piece of steel. You discover how to
                                   determine a heat treatment cycle, how to influence the
                                   atmosphere around the steel, and how to measure the effect
                                   of carbon on a piece of steel.
                                   After completing this ALT, you should be able to
                                   demonstrate the following competencies:
                                   • Identify and compare surface hardening procedures
                                      on steel.
                                   • Perform the surface hardening process of carburizing to
                                      produce a hard case around the core of a ferrous part.

                                   Safety and Disposal
                                   The particular safety issues for working around hardening
                                   furnaces in your facility depend entirely on the furnace size
                                   and the temperatures reached. Be sure to adhere strictly to
                                   your facility’s safety rules for any and all equipment and

Reproduced from Heat Treatment/Thermal Processes, published by Sinclair Community College.

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                                   procedures. Some basic general safety considerations are
                                   listed below:
                                   • Wear cotton clothing. Do not wear synthetic materials
                                        when close to a furnace as they may melt and stick to
                                        the skin.
                                   • Wear leather gloves to keep the heat and any oil away
                                        from hands. Elbow- length gloves provide the best
                                        protection. Wear gloves when handling any metal
                                        sample, since metal at 900 F and metal at room
                                        temperature are very similar in appearance.
                                   • Wear heavy leather boots to protect your feet from any
                                        hot objects that fall. Do not wear open-toed shoes or
                                        nylon tennis shoes.
                                   • Wear safety glasses at all times.
                                   • Restrain or otherwise pull back long ha ir. Hair will
                                        catch fire when too close to a furnace set at 1550 F.
                                   • When quenching steel specimens in oil, plunge the steel
                                        quickly into the oil and agitate the part to avoid
                                        popping. The slower the steel goes into the oil, the
                                        more likely the oil is to pop.
                                   • Make sure the quenched steel is oil- free before
                                        tempering. Any oil on the specimen to be tempered
                                        will result in a smoke filled room. Remove oil by
                                        washing the specimen in soap and water.
                                   • If you wear contact lenses, you must avoid getting too
                                        close to the furnace, even if wearing a protective
                                        facemask. The furnace heat may cause damage to
                                        lenses, and therefore to eyes.

                                   Activity
                                   1. Review the safety and disposal regulations of your
                                      facility. Refer to the Safety and Disposal section of this
                                      ALT and review the safety precautions there.
                                   2. Complete the tasks on Instruction Sheet: Carburizing
                                      of Steel.


                                   Assignment
                                   Completing This ALT
                                   To demonstrate achievement of competencies from this
                                   ALT, turn in:
                                   • Data Sheet: Carburizing Observations
                                   • carburized steel samples



Reproduced from Heat Treatment/Thermal Processes, published by Sinclair Community College.

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                                   Assessment
                                   Your facilitator may use Assessment Sheet: Surface
                                   Hardening of Steel to evaluate your performance and
                                   record whether you have adequately demonstrated the
                                   competencies developed in the ALT.




Reproduced from Heat Treatment/Thermal Processes, published by Sinclair Community College.

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Information Sheet: Carburizing of Steel
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The Carburizing Process
Carburizing is a primary process in the heat treating industry used to alter the surface of a piece of
steel. The carburizing process depends on carbon diffusing—or dissolving—into a piece of steel,
similar to the way sugar dissolves into water. When carbon is in contact with a piece of steel, the
steel acts like a sponge and absorbs it, and heating the steel speeds up the process. The result is an
area (or case) of hard carbon steel enveloping the softer steel. The most common reason for
carburizing a part is to make it more wear resistant.
Design engineers working with metallurgical/heat treatment professionals determine the final
mechanical properties of parts to be carburized. The specifications should include at least the
following:
•   a drawing of the part
•   the material (AISI/SAE or other standard designation)
•   the case hardening depth (in thousandths of an inch)
•   the resultant surface hardness of the areas to be surface hardened
It is the job of the heat treater to analyze these requirements for the specific steel alloy used and
devise a process to achieve the case depth and hardness requirements ordered by the customer.
•   Case depth can be checked by sectioning a carburized sample, and then polishing, etching, and
    analyzing the depth of the hardened zone.
•   Hardness can be checked with the Rockwell hardness tester. If the case is only a few
    thousandths of an inch, a superficial scale such as HR15N must be used.
The following five factors control case depth and final case hardness:
•   Carburizing temperature: Carburizing temperature is the temperature steel should reach to
    promote the diffusion of the carburizing agent into the surface. This temperature varies
    according to the base metal alloy and the carburizing media. A good approximation of the
    carburizing temperature is the austenizing temperature and can be found on the iron carbon
    diagram. Typical carburizing cycles can have carburizing temperatures from 1550 to
    1800 F.
•   Carburizing media: The more carbon in the atmosphere, the deeper the resultant cases,
    assuming all of the other variables stay the same. The carburizing process depends on carbon
    diffusing, or dissolving, into the surface of low carbon steel. When carbon rich material is in
    contact with a piece of heated steel, the steel absorbs some of the carbon into the surface. In
    some cases, the parts to be carburized are packed in carbon rich materials such as activated
    charcoal granules, cyanide, or other case hardening compounds. In gas carburizing, parts are
    heated in a special container and natural gas is injected. The carbon from the natural gas then
    diffuses into the surface of the part.




         Reproduced from Heat Treatment/Thermal Processes, published by Sinclair Community College.

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Information Sheet: Carburizing of Steel, page 2
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•   Time at temperature: Also called soaking time or time at heat. The times at carburizing
    temperature can range from 15 minutes for a very shallow case depth to many hours for a thicker
    case depth. Immediately after loading the samples, the furnace temperature drops, and must be
    brought back up to the carburizing temperature. When the furnace is at the desired temperature
    it is said to be at heat.
•   Quench rate: After carbon diffuses to the desired depth—while the part is at heat—it must be
    cooled quickly. The higher carbon case hardens to its maximum hardness, but the low carbon
    core still does not harden appreciably. The result is a hard, wear-resistant case over a softer,
    tough, low carbon core. The terms quench and quenchant refer to drawing the heat out of the
    steel. A quenchant is anything that can cool down or “quench” the heat from the steel.
    Quenchants can be oil, salt, atmosphere, lead, air, brine, or polymers mixed in water. Oil, the
    most common quenchant for carburized steel, is usually held at a temperature of 150 F or less.
    In industry, quenchants are agitated by mixing or stirring. Agitation moves the hotter quenchant
    away from the steel, thus removing the heat more uniformly.
•   Tempering temperature: This temperature depends on the as quenched hardness (maximum
    hardness) of the piece, and on the final hardness desired after tempering. Temper temperature
    and draw temperature are interchangeable terms. The term draw is used to convey the idea that
    a tempering step needs to be completed to “draw” the hardness down to a required level.
    Tempering also relieves some of the internal stresses created in the part caused by quench
    hardening. Tempering is usually carried out in an air atmosphere furnace. Temper temperatures
    can vary from 300–1200 F, depending on the desired final case ha rdness. Refer to ALT #1 for
    more information on tempering.
Note: Records of past heat treatment procedures—such as successful temperatures, time at
      temperature, carburizing media, quenchant and tempering temperatures—give the heat
      treater a starting point in achieving the desired case depth and hardness. Getting an exact
      carburizing procedure that achieves the exact result can require trials, testing, and
      documentation. Controlling and predicting the outcomes of the above listed factors when
      coupled with the multitude of possible base metals, part masses, diffusion rates, and heat
      treating equipment—plus many other factors—make even the best recipe only a starting
      point for a new carburizing process.




        Reproduced from Heat Treatment/Thermal Processes, published by Sinclair Community College.

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Instruction Sheet: Carburizing of Steel
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1. Read and follow all safety precautions required by your facility and your facilitator.
2. Your facilitator will instruct the entire group in the heat treatment process. Follow her or his
   guidance to perform the following steps.
3. Designate two pieces as the control specimens, to be used as received. Do not heat treat these;
   use them as a baseline against which to judge the other specimens.
4. Designate two specimens as “quenched only.” These specimens will not be carburized, but will
   be heated and quenched along with the carburized specimens.
5. Designate two specimens as “pack carburized,” then carburize them using the following
   procedure:
Pack carburizing steps:
6. Your facilitator will provide a packing box. Place parts to be hardened in packing box. The floor
   of the box should be covered with one inch of the carburizing compound. Tamp compound
   around pieces with one inch of compound available to all surfaces, and two or three inches of
   compound above pieces to allow for settling. The box should be full.
7. Seal the lid of the box tightly or use refractory cement, leaving a small gas-pressure escape vent.
Note to facilitator: Furnace temperature should be 1650–1700 degrees F. After box contents are at
                     full heat, leave under diffusion up to eight hours, depending upon thickness of
                     parts and thickness of case wanted (.015 in. to .125 in. normally).
8. Place the parts to be quenched only on top of the pack carburizing box or in the furnace while the
   carburizing parts are soaking.
“Quenched only” specimen steps:
9. Remove the quenched only parts after about one hour of heating.
1. Quench these specimens in water.
2. Visually observe and ana lyze these parts for hardness.
3. At the time specified, and using all of your facility’s required safety equipment, remove the steel
   box containing the specimens from the furnace.
4. Allow the box to cool, then remove the lid.
5. Remove the parts, shaking the carburizing compound from each piece. Allow the pieces to air
   cool completely.
6. Place parts in furnace again and soak through at 1450 F to 1750 F, depending on the desired
   properties.
7. Remove, again using all your facility’s required safety equipment, and quickly quench in water
   or oil.
8. After the steel pieces are quenched and cooled, your facilitator will place you and the other
   participants into teams of five or six.


        Reproduced from Heat Treatment/Thermal Processes, published by Sinclair Community College.

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Instruction Sheet: Carburizing of Steel, page 2
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9. Individually record the material descriptions at the top of Data Sheet: Carburizing Observations.
10. With your team, try using a file to shave some steel off each samp le.
11. Individually check each sample with the Rockwell C scale and the 15N scale, and record your
    answers on Data Sheet: Carburizing Observations.
12. Following all safety procedures for the cutoff saw, cut all samples open to expose the center (or
    core) of each one. Examine the sectioned regions.
13. Individually complete the remainder of Data Sheet: Carburizing Observations.




        Reproduced from Heat Treatment/Thermal Processes, published by Sinclair Community College.

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Data Sheet: Carburizing Observations
----------------------------------------------------------------
Participant:________________________________________ Date:_________________________

Specimen steel designation:
Pack carburizing media:
Pack carburizing soaking temperature:
Carburizing time:
Hardening soaking temperature:
Hardening soaking time:
Quenching media:

Answer the following questions about your carburizing process.
1. Using a file, compare the hardnesses of the three different samples: as received, quenched only,
   and pack carburized. Which one is hardest? Which one is softest? Did the as quenched
   specimen harden to any degree? If not, why not?




2. Were the hardness levels of the as received and the quenched only specimens similar when you
   performed the file test? If so, why do you think this is true? If not, why not?




3. What is the hardness of each sample?
   _______ as received, Rockwell C
   _______ as received, Rockwell 15N
   _______ pack carburized, Rockwell C
   _______ pack carburized, Rockwell 15N
   _______ as quenched, Rockwell C
   _______ as quenched, Rockwell 15N




        Reproduced from Heat Treatment/Thermal Processes, published by Sinclair Community College.

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Data Sheet: Carburizing Observations, page 2
----------------------------------------------------------------
Participant:________________________________________ Date:_________________________

4. How do the Rockwell readings differ on each scale?


5. Why do you think there is a difference in the Rockwell readings between the pack carburized
   piece and the non-carburized piece?




6. Examine the sectioned pack carburized piece. Can you see the case depth? If so, how deep is
   the case depth?


7. How is case depth determined by industrial heat treaters?




8. Based on the previous ALT, your experience, and the references you have consulted, what
   percentage of carbon do you think is on the surface of the steel on the pack carburized piece?




9. What will happen if the temperature is increased to 1850 F? Will the depth of carbon into the
   steel increase or decrease? What will happen to the core hardness?




         Reproduced from Heat Treating/Thermal Processes, published by Sinclair Community College.

                                            9
Assessment Sheet: Surface Hardening of Steel
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Participant:________________________________________ Date:_________________________

Use the following rubric to evalua te the participant’s performance.


     Participant        Highly Competent                Competent              Needs Improvement
     Deliverable
  Data Sheet:         Answers to questions are   Answers to questions are     Answers to questions are
  Carburizing         complete, correct, and     complete, correct, and       incomplete, incorrect, or
  Observations        show a basic knowledge     show a basic knowledge of    do not show a basic
                      of the carburizing         the carburizing process,     knowledge of the
                      process, along with a      along with a basic           carburizing process. No
                      high level of knowledge    knowledge of the             basic knowledge of the
                      of the differences         differences between the      differences between the
                      between the control        control specimens, the       control specimens, the
                      specimens, the             quenched only specimens,     quenched only
                      quenched only              and the carburized           specimens, and the
                      specimens, and the         specimens. Hardness          carburized specimens is
                      carburized specimens.      readings are measured and    shown. Hardness
                      Hardness readings are      recorded correctly.          readings are measured or
                      measured and recorded                                   recorded incorrectly.
                      correctly.

  Carburized steel                               Samples are correctly        Samples are incorrectly
  samples                                        carburized, with             carburized, or appropriate
                                                 appropriate case depths      case depths are not
                                                 achieved, and all safety     achieved, or any safety or
                                                 and operational guidelines   operational guidelines
                                                 were followed in their       were improperly followed
                                                 production.                  in their production.



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         Reproduced from Heat Treatment/Thermal Processes, published by Sinclair Community College.

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