Steel Processing Tempering

MatE 25 San Jose State University Lab Notes

Rev. 4.0 7 - 1 Steel Processing and Microstructure 1

Chapter 7: Steel Processing 1

Processing and Microstructure

1.0 Learning Objectives After successfully completing this laboratory workshop, including the assigned reading, the lab worksheets, the lab quizzes, and any required reports, the student will be able to:

List the phases in steel (austenite, ferrite, pearlite, cementite, and martensite), what temperatures they are stable at, and their mechanical properties.

Describe the influence of tempering, annealing, and quenching on the mechanical properties of steel.

Identify ductile and brittle fractures. Use standard heat treating equipment and methods. Operate the notch-impact testing machine. Demonstrate the relationship between heat treatment and mechanical properties.

2.0 Resources [1]. Callister, Materials Science and Engineering: An Introduction 8th edition, (John Wiley and Sons, New York, 2009), Ch 9.18-9.20, 10.1-10.9.

[2] Industrial Heating articles on Martensite and Retained Austenite http://www.buehler.com/application_support/MartensiteRetainedAusteniteIHApril2009.pdf

3.0 Materials Applications Steel is an alloy of iron, carbon, and small additions of a variety of other elements. Steel is one of the most versatile engineering materials, and is used in a wide variety of structural applications. There is an abundant supply of iron ore distributed in many locations in the world. Also iron and steels are readily recyclable, a very important property in material selections. The technology for reusing iron and steel is making great progress and is a current subject of research in many universities and industries worldwide.

Some of the key properties that make steel so versatile are high strength and toughness. There are many alloys of steel, and there are many processes for steel production that enhance the properties to suit design needs. Processes must be done very carefully to achieve the desired results, and we will explore one such process in this lab exercise.

One application where steel is used is amusement park rides. This brings us to our accident investigation.

4.0 The Big Twist Problem During a recent spring weekend, an accident involving an equipment failure at an amusement park in Florida set in motion a chain of events that involves you in an investigation to learn the root cause of the equipment failure and to prevent a reoccurrence of the accident at a similar park in the local area.



In Florida a new ride called The Big Twist went into service in early spring of 2010. After about eight weeks of operation, the warming weather and school spring break brought a line of eager riders resulting in full carloads of riders and full capacity operation of the ride. The Big Twist raises a car with eight riders to the top of a 60 meter tower from which the car runs down a twisting and looping track for one and a half minutes. This feature giving the ride its name, Big Twist, involves a fast descent in a tightly curled helical section of the track involving six complete circles of 360 degrees each in a path having a diameter of about 7 meters.

On the day of the equipment failure, at about 2:15 p.m. one of the cars separated from the track about midway down the twist. The car and eight occupants fell to the ground about 30 meters below. Two of the occupants were killed immediately, two died later at the hospital and the other four suffered a range of non-fatal injuries, some of which will result in permanent disabilities.

Florida officials immediate shut down the ride and began an investigation to determine the cause of the car separating from the track. Investigators determined that the sharp descent in the helical twist would place high side loads on the wheel mechanisms in addition to the normal vertical loads. Even with the impact damage from the 30+ meter fall, it was determined that the outside track wheel assembly was inclined at more than 30 degrees from its normal vertical orientation. This was due to bending of the support bracket rail that connected the wheels to the underside of the car.

When the Director of Safety for Cal-OSHA received report of the Florida amusement park accident, she quickly determined that three such rides of similar design had been installed in California during the winter and all three had been inspected and certified to start operation within the past three weeks. She immediately issued an order shutting down those three rides. She ordered another inspection, and ordered the rides closed until there was a determination of a cause for the equipment failure. Of the three California locations, two were in southern California and one in the San Jose area. Teams of inspectors were dispatched to each of the sites with instructions to inspect the cars and all associated equipment. Particular attention was to be given to the condition of the wheel assemblies and support rails. Within 12 hours the reports came back from southern California that the inspectors found no problems. However, the team in northern California reported wheel assemblies tilting 10-15 degrees from the normal orientation and bending of the support rails on the left side on 3 out of 24 cars at the site.

The park management at the two southern California parks requested lifting of the order shutting down their rides. They tell the Director of Safety that since the inspection team found no problems, they should be allowed to resume operations. The Director refuses to lift the stop order. She still wants to find the root cause of the Florida crash, and her concern escalates when she receives the report of the problem found on the northern California ride. However, she mentions in a staff meeting that the pressure to lift the order will intensify as the weekend approaches and the parks anticipate capacity crowds.

5.0 Failure Investigation

5.1 Your Job You have been selected by the Cal-OSHA Safety Director to be part of a team of experts to investigate the wheel assemblies and bent rails discovered both in Florida and California.



As a materials engineer, you are asked to determine why the Florida roller coaster failed, and if the other coasters are at risk. As part of your investigation, you study how the microstructure and hardness of steel changes with thermal processing. As a first step, you obtain reference samples of 4140 steel. You austenize, quench, and temper a series of these samples to gain an understanding of how the microstructure, hardness, and notch-impact toughness change as a function of quench and tempering temperature. You then need to apply that knowledge to a bent rail sample to determine if the hardness, toughness, and microstructure match what should be expected.

5.2 Investigation To Find the Facts To start in the investigation, you arrange a conference call with representatives of the company that designed and manufactured the ride. During the call you make notes about the material details mentioned by the designers. From your notes you plan the tests to be performed once the suspect rail is received.

1. The designer says that this ride is a new model based on a previous design in use for several years. The previous model is smaller. It makes loops and turns but less sustained twisting motion, such as the helical drop that is a new feature introduced for this ride.

2. The designer identified that the wheel assembly could disengage from the track if the rail bends enough to allow the wheel shaft to tilt more than 15 degrees from its normal horizontal direction.

3. Calculations made during design, indicate this new ride will generate higher side loading of the wheel assembly and rail. Because of the higher expected loads a stronger wheel assembly rail was needed.

4. To handle the higher loads, it was decided to make the rail out of a higher strength material, keeping the dimensions the same as for the older car design. This avoided a redesign of other portions of the car.

5. In the previous car design, the rails were made of low carbon steel with commercial designation of 1018 steel. The material specification called for the finished rail to have a Rockwell B-scale hardness of HRB 75-85 (which is roughly equal to HRC 2-3).

6. For the new ride design, the design team specified a change to a medium carbon steel (4140), which gives the material higher strength than the previously used 1018 steel if processed correctly.

7. This company has now built more than a hundred cars using this design. Manufacturing the new wheel assembly and rail was complex. After machining to dimensions, rails were sent out to another shop in batches to be heat-treated. The process involved heating to 844oC for 1 hour followed by water quench, then post-quench tempering for 1 hour at 480oC. Parts were then cleaned of surface scale by grit blasting. The material specification called for a final Rockwell C-scale hardness of HRc 35-40.

8. The designer and his supervisor say they have rechecked the loading calculations, and both say the wheels and rail should be safe under the maximum expected loads plus design margin, provided the rail material is at least 0.40 wt% carbon and the rail received the specified heat treatment.

9. Your investigation team finds that the first car manufactured with the new design has a complete record of all the testing done on the materials and components that go into



making the car, and everything passed. After that, all the subsequent cars do not have records of material tests during manufacturing.

10. You participate in an investigation team traveling to Florida, and obtain a small specimen of the bent rail from the crashed car, and bring it back to your lab in California. The Cal-OSHA machine shop makes that specimen into a notch-impact toughness Charpy test sample. You perform hardness tests, and a Charpy test on this sample. You then cut and polish a section for microscope examination, and take photographs of the microstructure to document your investigation. A small portion of the retrieved bent rail specimen was sent to a chemical analysis lab to determine the element content of the steel.

From the data you gather from the reference samples, and the data from the retrieved bent rail specimen, you are able to piece together the problem. Your data eliminates all other potential causes, and you are able to determine conclusively the root cause of the. Further, you are able to recommend additional testing to determine which of the roller coaster cars, if any, should be allowed to return to operation.

Most importantly, now you need to document your results and send the report to the Director for her review. She is under pressure to issue a ruling on operational status for the amusement parks, and needs these results immediately. Also, almost certainly other safety organizations, and litigation teams will question her ruling. The report must be of high quality and have technical accuracy.

6.0 Writing Assignment - Engineering Report with Cover Memo

6.1 The Audience The Cal-OSHA Safety Director, Ms. Irene Huang, has a bachelors degree in Industrial Engineering, and an MBA. She deals with all types of safety issues and engineering problems, but is not very familiar with specific materials tests and their interpretation. Assume Irene is familiar with general engineering concepts, but not necessarily an expert in material properties and failure modes.

6.2 Length and Time Guidelines The length should be sufficient to cover the details requested below, and to demonstrate your understanding of the concepts we discussed in class.

The assignment is to write both a Cover Memo and an Engineering Report.

6.3 Cover Memo (maximum of 1 Page) It is recommended you write the cover memo after completing the report. That way, you can select salient points out of the report and write 2-3 sentences on each item shown here. Each of these should be a separate paragraph.

Introduce your purpose for writing by reminding Irene what you did and why. Summarize your research and findings. Explain the significance of the results by stating what caused the failure. Give a recommendation as to whether the rides can safely be put back in operation. If not,

state what should be tested first to determine if the cars are safe.



6.4 Engineering Report Write at an appropriate level of detail for your audience. Write in your own words.

6.4.1 Introduction State why you did the research Briefly explain relevant materials engineering concepts:

o Amount of carbon in 4140 steel compared to 1018 steel and what effect that has on strength

o Phases of steel at various temperatures, as shown on the phase diagram, and how that effects the amount of carbon that can be held interstitially

o The austenite to martensite transition, including the importance of fast quench o The process of converting martensite to tempered martensite, and the effects on

strength, hardness, and toughness.

6.4.2 Experimental Procedure Describe what processing you did to prepare some reference samples. Describe the mechanical tests you performed on the reference samples. Describe how this compares with the design specifics for the manufactured car. Describe the tests you performed on the retrieved sample. Describe what you did to prepare the reference samples and the retrieved sample for

microscope examination.

6.4.3 Results and Discussion Present your results from the experiments you ran on both the reference and retrieved

samples. Remember to show results in properly formatted tables and/or graphs. Answer how the retrieved sample compare to the reference samples in hardness and

Charpy tests. Answer how did the retrieved sample compare to expected results under microscope

examination. What was the microstructure you observed? What did the chemical analysis indicate, and was that expected. Describe if that indicates

a possible causes of the failure.

6.4.4 Conclusions and Recommendations From the tests and observations you made, state the processing treatment the suspect rail

received in manufacturing. Explain how that would cause the rail to have the properties it did and how that caused the failure.

What is your recommendation to the Cal-OSHA Safety Director concerning reopening the California rides?

If you suspect some cars are safe and some at risk, how do you recommend finding the cars that are not safe so they can be taken out of service and repaired?

6.4.5 References List all references you consulted to write this report. Use the proper format.



6.5 Grading Rubric



Scoring

Attribute 1

Unacceptable 1.5

2

Marginal 2.5 3

Proficient 3.5 4

Excellent

Writing Mechanics Wt Assignment Instructions and Requirements

Assignment instructions not followed

Some assignment instructions followed

Most assignment instructions followed

Report fully complies with instructions & requirements

1

Grammar, mechanics and spelling

Consistently inadequate grammar, mechanics and/or spelling; Errors impair meaning

Many errors, which affect writing clarity

A few errors, which do not impair meaning

Consistently correct use of grammar, mechanics and spelling

1

Figure and Table Format and Quality

Figures/tables consistently not labeled or not referenced in text

Some figures/tables missing labels, missing descriptive captions, not referenced and/or not discussed in text

All figures/tables neatly labeled with title, figure no. and descriptive caption, discussed, explained in text

2

Units and Significant Figures

No units given for any table headings, plot labels or values; or wrong number of significant figures

Some table headings, plot labels or values missing units and/or wrong number of significant figures

Tables formatted correctly, with all table heading; Plot labels and values given with correct units and significant figures

2

Formulas All symbols and formulas written by hand

Some symbols and formulas handwritten or word-processed

All symbols and equations written with equation editor and mathematical notation; All variables defined, all equations numbered

1

Writing Quality Writing Quality Organization lacks coherency;

Language and sentence structure is poor; Report is difficult to read

Organization of some sections is coherent; Language and sentence structure is average; Report requires some effort to understand

Overall organization is coherent; Language and sentence structure is sophisticated; Report is easy to read and understand

1

Originality Writing is plagiarized from other sources*

Author restates or paraphrases ideas from other sources; Writing does not clearly demonstrate authors understanding

Writing is original, shows clarity, and demonstrates depth of understanding

1

Scoring Attribute

1 Unacceptable

1.5

2 Marginal

2.5 3 Proficient

3.5 4 Excellent



Technical Quality Wt Mastery of Theoretical and Technical Concepts

Author appears to lack comprehension of subject matter

Author demonstrates some comprehension of subject matter

Comprehension of subject matter is clearly demonstrated

3

Methods Experimental methods not described

Experimental methods are difficult to understand

Experimental methods described using specific terms and procedures, but general method is unclear

Experimental methods described and explained; Appropriate technical language and level of detail is used

3

Figure(s) of Experimental Set-up

No figure of experiment set-up

Generic figure of experimental setup, but relevant details omitted

Detailed figure of experimental setup; Figure not related to text

Detailed figure of setup, Figure complements text;

1

Data, Presentation, and Calculation of Results

Result of the work not stated or unclear; Calculations not shown; Results incorrect

Partial or incomplete results of the work

All required results present; Calculations presented

All required results clearly presented, distinguished from data, and correct; Calculations clearly demonstrate how results were determined

3

Discussion of Results Results presented but not explained

Results presented and compared to theoretical or expected values

Results presented and compared to theoretical or expected values; Appropriate sources of error are stated

Results interpreted in terms of theoretical or expected values; Sources of error discussed and explained

3

Conclusions No summary given Results summarized, but no outcome stated

Report summarized and outcome stated

Report summarized; opinion stated concerning outcome supported by work

2

References Sources not cited Some sources cited; Common format not used

Sources cited, using appropriate format (ie IEEE)

1

Total Rubric Score:__________out of 100 * Note that any plagiarism will result in a 0 on the entire report. If you are unclear on what constitutes plagiarism, consult our instructor.



7.0 Background on Steel Processing The range of properties produced by alloying and by applying various heat treatments is extensive. The initial step in the heat treatment of steel involves heating to a temperature at which all of the carbon in the alloy goes into solution by dissolving interstitially in the face-centered cubic (FCC) iron. Austenite is the name of the elevated temperature FCC crystalline iron. The maximum level of carbon solubility in austenite is about 2.0 wt%. The process of dissolving carbon in austenite is termed austenitizing. This is shown on the Fe-C phase diagram, in Callister Figure 9.24. High hardness is then achieved by rapidly cooling the steel to room temperature by quenching it in water (or oil).

The body centered cubic (BCC) iron is called ferrite, and the solubility limit of carbon in is only about 0.025 wt%. It is this large difference in carbon solubility between austenite and ferrite that is primarily responsible for the range of properties available in steels. When there is insufficient time for the redistribution of carbon in cooling austenite, a metastable phase called martensite forms. Martensite is a crystalline arrangement intermediate to the FCC and BCC structures; see Callister Figure 10.20. The carbon, locked in during rapid cooling of austenite, distorts the martensite lattice. This causes internal stress in the crystal structure. The grain size for martensite is very small. As a result, martensite formed upon rapid cooling is an extremely hard and brittle material. Quenched martensite is of limited use as an engineering material because it is so hard and brittle.

Toughness, and therefore the usefulness, of quenched steel is greatly improved through a process called tempering. Diffusion of carbon and iron atoms is increased in a controlled manner by tempering. Some of the distortion of the martensite crystal structure is reduced, and some of the carbon moves out of the trapped interstitial positions. Since tempering is adjusted by time and temperature controlling the diffusion, a large range of strength and toughness is possible. The properties of steel thus can range from extremely hard and brittle (un-tempered martensite) to very soft and ductile (annealed) which has a predominantly ferrite microstructure. Tempering never goes above the Eutectoid temperature (727C), where austenite will reform.

7.1 The Heat Treatment of Steel Alloys are heat-treated to improve or tailor mechanical properties. In steel there are many potential heat treatments for accomplishing this. In this experiment we examine a few of the general principles.

Understanding heat treatment processes begins with the concept of an equilibrium state, the state for which the total free energy of the system is at a minimum, and thus the net change with time in the system is nil. A phase diagram, such as Callister Figure 9.29, shows the equilibrium phases predicted at given temperatures and compositions. However, phases not predicted by the equilibrium diagram can appear during heat treatments. These are metastable phases, and they will eventually transform to the equilibrium phases, if given sufficient time and/or temperature. (Diamond at atmospheric pressure and room temperature is an example of a metastable phase that persists for an extremely long time.) The time and temperature dependent transformation of martensite to the equilibrium ferrite plus carbide phases provides the engineer a broad range of mechanical properties from extremely hard and brittle, to very soft and highly ductile.



7.2 Hardening of Steel First consider the equilibrium heating and cooling of a 0.77 wt% carbon steel. As shown in the iron-carbon phase diagram (Callister Figure 9.24), one can see that an alloy of iron and carbon containing 0.77 wt% carbon heated long enough above 727C will become a single phase solid solution of carbon in gamma (FCC) iron. Upon slow (equilibrium) cooling, a eutectoid reaction occurs at 727C. The gamma phase decomposes completely into two new solid phases, alpha Ferrite (BCC) and Fe3C (cementite). Both phases are stable below 727C. The resulting structure consists of a lamellar mixture (alternate plates) of the two phases, which is referred to as pearlite. This is a low strength, soft, ductile structure capable of being machined or shaped relatively easily. Very slow (many hours) furnace cooling of steel from the austenite region produces coarse pearlite. If steel is transferred from furnace to air, more rapid cooling occurs and finer pearlite is produced. This can be seen in Callister Figure 10.15. Aside from being easily machined and formed, pearlite steel is not used as a structural material because of low strength.

If carbon steel is heated to the fully austenite phase and is rapidly quenched, then there is insufficient time for the carbon to diffuse out of the FCC interstitial positions, and there is insufficient time for full conversion of FCC crystal to BCC. The resultant crystal is martensite with a Body Centered Tetragonal (BCT) structure. If the cooling was rapid enough the structure is fully martensite, which is a metastable phase, and very little (if any) alpha Ferrite or Fe3C is formed.

Martensite has very high strength, but is also very brittle with low toughness. This is because the crystal slip planes have little mobility. Slip plane motion is constrained because Martensite grain size is small, the carbon is still held in the interstitial sites causing distortions, and the crystal is BCT, which has fewer slip planes. The as-quenched martensitic structure steel is too hard and brittle for practical engineering applications.

7.3 Tempering of Steel A process known as tempering is necessary to restore some ductility and toughness to the rapidly quenched steel. Heating martensite to some temperature below 727C allows some of the carbon to diffuse out of interstitial positions, where it combines with iron to form carbides (Fe3C). A mixture of cementite and ferrite results, with the effect being reduced distortion caused by the carbon atoms being trapped. At the same time the grain sizes increase, and some of the BCT is transformed to BCC. All of this increases slip plane mobility in a gradual and controlled manner. This decreases the strength of the steel, but with a substantial increase in ductility and toughness. Notice the drop in hardness with tempering shown in Callister, Figure 10.32.

On a microscopic scale the grain size of tempered martensite increases only a small amount and the appearance is similar to that of un-tempered martensite, if the tempering is done correctly.

8.0 Austenitizing and Tempering Carbon Steel

8.1 Equipment and Materials Hardening and tempering furnaces Charpy impact testing machine Quenching tank Standard V-notch Charpy impact specimens



Abrasive paper

8.2 Safety Precautions Do not use Rockwell hardness tester until you have received instructions. Be sure specimen is firmly seated, and that test and base surfaces are flat and free of scale.

Stand to the side and away from the front of Charpy machine while testing. Eye protection must be worn, and you must have a second person watching and verifying safety procedures are followed.

8.3 Procedures 1. Check to see that furnaces are at proper temperatures. The austenitizing furnace

temperature should be at set 844C. Place 6 specimens in the furnace for one hour. 2. Remove 5 specimens from the austenitizing furnace and immediately quench in water.

Remove one specimen and allow to air cool. 3. Set tempering furnaces at 205C, 370C, 482C, and 677C. Place a quenched specimen

in each furnace for one hour. One quenched specimen does not get tempered, but is left as full martensite.

4. Remove tempered specimens from furnaces and allow cooling. Remove surface scale on two parallel longitudinal faces that do not have the notch.

5. Measure Hardness Rockwell C Scale (4 places each specimen) on each of the four temper treatment specimens, the one air-cooled specimen, and the one as-quenched full martensite specimen. Test hardness on one face only.

6. Perform Charpy Impact Tests for each of the six specimens. For operation of the Charpy Impact Tester follow these steps:

o Lift pendulum to safety catch. o Put specimen in place with notch away from pendulum. o Lift pendulum to uppermost position. o Manually move pointer to far left reference mark on the scale. o Release hammer and take reading on scale.

7. Note the appearance of the fracture surfaces, and if available take pictures.



Worksheet 1: Tempering of Steel Key Member (Encourages all team members to participate, ensures everyone understands the material, and organizes/divides the tasks amongst the team members):

Other Group Members:

This week you austenize, quench, and temper steel samples to determine hardness and other properties for reference samples. Next week you will test a retrieved sample of steel from the wheel assembly failure, and compare with the reference sample results.

Reference Sample Results: Type of Steel ______________

Treatment

HRC#1

HRC#2

HRC#3

HRC#4

Avg. RC

Impact Energy (J)

Type of Fracture

Air-Cooled, No Temper

As-Quenched, No Temper

Quenched, 205C Temper




1. What process will result in a Ferrite plus Pearlite microstructure? List the reasons why this

structure will have low strength.

2. What process will result in a Martensite microstructure? List the reasons why this structure will have high strength.

3. What is happening to the microstructure during the temper process?

4. For 4140 Steel, how many iron atoms per carbon atom are there? Show work neatly on back.

5. What are your sources of error?

6. Did you calibrate the HRC? Did you calibrate the Charpy?

Documents

Steel Processing Tempering