Roller Coster

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Investigation of Roller Coaster

By: Farouk Merazka

Mate 25

Apr 28, 2014


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Cover Memo

To: Dr, sepka

From: Farouk Merazka

Failure investigation of wheel assembly and rail bent

The Goal of this investigation is to determine the reasons that causes the rail

bending in the roller coaster rides, and to find out if the ride is still risk for users.

The comparison of the hardness of the sample of steel that was tempered at a particular

temperature to the rail bend specimen indicates that that the hardness of 4140 steel

specimen is actually different from the hardness value of the heat treated 4140 steel. The

expected value of the hardness of rail is between 35 HRc and 40 HRC. The actual

hardness of the sample from roller coaster was 7.4 HRc. The chemical analysis indicts

that the specifications of the 4140 steel used for the roller coaster ride rail lies within the

expected value of 4140 steel according to AISI. Therefore the specimen is 4140 steel.

The microstructure of the steel shows that the specimen was not properly heated during

the heat process treatment.

The causes for the equipment failure are still unidentified. The Safe way to handle

situation like that is to shut down the rides until all the inspection finishes. The other part

of the roller coaster should be investigated by safety officers to insure that the steels

specifications confirm with the specifications expected. The part of the rails that was

bended should be removed from the site and replaced by new rails. The hardness should

be tested for the new rails as well as for old rails to make sure that the hardness is within

the expected value.

Introduction:

The Goal of this report is the determination of the reasons behind the equipment

failure of roller coaster ride causes a death of many people. In addition the report will


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determine whether it is safe to operate roller coaster or not. In order to determine the

causes, a bunch of tests should be conducted on bended part of the roller coaster such as

the measure of the hardness and the study of microstructure of the failure equipment.

The new design of the rails of the roller coaster rides is made by the 4140 steel that

contains between 0.38wt% and 0.43 wt% of carbon however the older designed is made

by 1018 steel, which contains 0.18 wt% of carbon. The carbon content inside the steel can

be measured by determination of the strength of steel. The harder the steel, the more

carbon content it has.

There are five phases of steel: austenite, pearlite, cementite, ferrite, and martensite

Austenite: is a steel iron and carbon at 912C that transforms the BCC structure of ferrite

into FCC crystalline iron. This process of heating is called austenitizing. The carbon gets

dissolve interstitially into alloy. The maximum level of solubility of the carbon is 2.0

wt%.

Ferrite: It is also known as the alpha iron, it has BCC crystalline structure, and its

maximum solubility of the carbon is 0.025 wt%

Pearlite: the steel is heated up to 727C and then let cool down slowly, two-phased will

appear lamellar structure is formed that is made up of alpha-ferrite and cementite. Its

content of carbon is 0.77wt%.

Cementite: Cementite is the iron carbide its chemical formula is Fe3C and its content of

carbon is 6.67 wt% and the rest is iron.

Martensite: This is a phase between crystalline arrangement of BCC and FCC crystalline

structure formed by quenching of austenite. During the cooling process the carbon atoms

do not get enough time to diffuse throughout the crystal. The austenite to martensite

transition occurs when there is not sufficient time for the carbon atoms to diffuse

properly. Due to fast cooling, an intermediate crystallization structure of BCC and FCC


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forms which is called martensite. The atoms of carbon trapped during fast cooling process

changes the shape the martensite lattice and causes internal stress. The martensite has a

low toughness, however it is brittle and it has high strength. In order to change the

material properties of steel, the still need to do tempered. The tempering of the steel will

restore the ductility and toughness of the steel. The tempering process is done by heating

martensite below 727C for considerable amount of time, which allows atoms of carbon

to diffuse in interstitial positions to form carbides.

Experimental Procedure:

First Part the specimen from the factory

1. Place six samples of steel specimen in furnace at 866C for an hour.2.

Take all the specimens out of the furnace after one hour.

3. Put 4 specimens immediately in water for rapid cooling, and Put the fifth specimen

in the room temperature to cool down.

4. Set the furnaces temperature at 205C, 370C, 482C, and 677C and put the four

specimens that were quench in water in the furnace for an hour.

5. Take the four specimens out of the furnace and let them immediately drop them in

water.

6.

Take the cooled specimen and determine the hardness of the six samples using

hardness test (Hardness Rockwell C Scale. (Figure 1) )

7. Perform Charpy test on all six samples. Calculate the impact energy and determine

the type of fracture for all specimens. (Figure 2 and 3)

8. Second Part the specimen form the bent rail:

9. Cut the specimen into a cubes shape specimen approximately .5 to .75 inch from

the bent wheel assembly rail using the Buehler Model 1000. (Figure 4)

10.Put the specimen on the mounting thermoplastic material of diameter .25 inch and

thickness .75 inch using Buehler SimpliMet 2 hot mounting press machine.

(Figure 5)


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11.Complete the coarse grinding of the specimen by the120 grit belt for a few minute;

rotate the specimen 90 and continue grinding. After finishing grinding on 120

grit, then move to fine grinding by using 240 grit followed by 320, 400 and600

grits. (Figure 6)

12.Polish the surface to remove scratches that were left during grinding. Make sure

that the specimen is clean and washed before starting.

13.Before polishing apply liquid slurry (contains alumina in de-ionized water) on

polishing wheel of 1 m alumina and hold the specimen on the wheel rotating it

30-40 angle, then move on to the next polishing wheel of 0.3 m alumina. Polish

on each wheel for 2 minutes. After polish, we will get smooth, shiny and bright

surface. (Figure 7)

14.Pour 1% nitric acid in ethanol on specimen for about 9 sec and clean the specimen

surface with water. (Figure 8 and 9)

15.Put the specimen under the microscope to see the microstructure of the specimen

and compare it to other samples that were tampered at different temperatures.

(Figure 10)

Discussion:

Data:


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The table shows the hardness, impact energy and type of fracture for the steel samples.

Treatment Avg

HRc

ImpactEnergy

(J)

Type of

Fracture

Air Cooled

No Tamper 19.6 17 Brittle

As Quenched

No Tamper56.3 3 Brittle

Quenched205 C

Temper

54.4 5 Brittle

Quenched

370 C

Temper

48.2 7 Brittle

Quenched

482 C

Temper

39.1 60 ductile

Quenched

677 C

Temper

28.38 136 Ductile


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Based on the collected data, we see that the hardness of quenched steel tempered

at 205C has the maximum hardness strength. The impact energy for quenched 677C steel

is 122 that make it ductile in fracture.

Graph 1: graph shows the hardness VS the heat treatment of the samples.

Data for the quenched steel tempered at different temperature is shown in the following

data table:

Temper Avg HRc

32C 17

205C 3

370C 5

482C 60

677C 136


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Graph 2represents the hardness of steel vs. the temperedtemperature that shows that the

hardness increases up to 205C temper and then start declining.

Graph 3: The graph shows the hardness vs. impact energy of steel

The expected hardness of rail bent specimen should be between RC35 and 40

measured by Rockwell hardness tester, however the actual average hardness of 4140 steel

is found between RC7.4. This gives an idea that there is something wrong with the steel.

To determine the cause of failure, the microstructure of the failure specimen need to be

determine.

19.6

56.354.4

48.2

39.1

0

10

20

30

40

50

60

32 205 370 482 677

HRc

Temperature Degree C

temperature vs HRc

0

10

20

30

40

50

60

70

19.6 56.3 54.4 48.2 39.1

Impactofengergy

HRc

HRc vs Impact of energy


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Chemical Analysis of believed 4140 steel rail bent sample is below.

Element SAE/AISI 4140

steel

(wt %)

Bent Rail sample

By analysis (wt

%)

Cr 0.801.10 0.93 + 0.01

Mo 0.150.25 0.21 + 0.01

Mn 0.751.00 0.93 + 0.02

Si 0.150.30 0.21 + 0.02

P 0.035 max. < 0.013

C 0.38 -0.43 0.41 + 0.01

Based on the compositions and the specification of the specimen, it is apparent that

the sample is 4140 steel. A microscopic picture of the specimen can help determine the

reason of failure.

Some of the photos of samples that were tempered at different temperatures and

rail bend specimen. These samples were magnified by the microscope at 400X..

Photo 1: The photo was taken by microscope at 400X that shows the microstructure of

1040 steel furnace cooled


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4140 steel furnace cooled.


4140 steel water quench, tempered at 205C


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4140 steel water quench tempered 370C


4140 steel water quench tempered 677C


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bent rail specimen etch for 10 sec.


bent rail specimen etch for 8 sec.

A comparison of the photos of the samples to the bent rail specimen. The

microstructure of the bent rail doesnt resemble the microstructure of the specimen that

was tempered at 480C. The microstructure of the bent specimen resembles to the photos

of sample of 4140 steel furnace cooled and 1014 steel furnace cooled.


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Conclusion:

The specifications and compositions of the specimen is similar to the 4140 steel,

however the hardness of the specimen is not the same as the 4110 steel. From the

microstructure of the bent steel specimen, it appear that probably the reason of failure is

the improper heat treatment of rails which could be a possible cause of making the 4110

steel less brittle. Quenching of steel after tempering can also be a possible reason for the

failure. The reason for the failure is still undetermined. I suggest to Cal-OSHA Safety

Director to keep the rides shut down until all the rails and wheel assembly are inspected

properly because the reason of failure is still unknown. The hardness of the all roller

coaster rails should be within the range of 3540 HR if one part of the rail is found to be

out of the rage, the roller coaster should be close for until the parts are replaced. Another

possible reason in the assembly. The assembly of the rail can affect the Hardness of the

steel especially if the assembly include heating


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Appendix:

Figure 1: Performing the

using Hardness Rockwell Ctype of fracture occurred on all

tester. steel samples.Figure 2: Using Charpytest, we hardness test ofsteel samples determinethe impact energy and


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Figure 3

Figure 4: Using the BuehlerModle1000, we cut a cube shape

specimen


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Figure 6: The grinding on 120, 240,320, 400 and 600 grits to remove

scratches from our specimen

Figure 5: mounting thethermoplastic material on ourspecimen using Buehler mpliMet2 hot mounting press machine.

Figure 7: This picture showsthe polishing of specimen on

0.1 and 0.3 m aluminarotating wheel.


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.

Figure 9: Cleaning the 4140 steelspecimen in water after

performing steel etching to makeit ready for microstructure study

Figure 8: Performing steeletching by pouring the

specimen in 1% nitric acid inethanol for 8 sec.


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Figure 10: Performing microstructure study of 4140 steel to determine the cause

of roller coaster equipment failure.

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