Tension and Jominy Testing Number

Tensionand Jominy Testing

Number:

Tensionand Jominy Testing

Determiningproperties of materials is an important aspect in many differentfields of applications such as engineering and material mechanics.Through the tensile test, there is the tensile load that is obtained,which is in turn calculated to an Ultimate Tensile Strength (UTS).UTS is the maximum tensile stress that any given material can sustainwithout fracture. After a material goes through tensile testing, onecan determine its yield stress, stress obtained before plasticdeformation occurs in the material, and a material tensile strength.The purpose of this lab was to determine the properties of stress ona sample of nylon. It was proven that stress can affect a material.Also, the temperature can play a big factor in the strength of thematerial. It changes how the material behaves. The purpose of thislab was to test the hardenability of a metal by quenching in waterand how annealing changes the hardness of a material. The flatness ofa surface can affect the change in hardness. Also, the time a metalcools down changes the hardness as well as the grains. Larger grainsmean greater deformation. This paper is a report on a performedexperiment in tensile testing

Introduction20/25

Intensile testing (tension testing) a material is pulled in tension toaccurately obtain a stress vs. strain graph that can be used to learninformation about the material. Various points on a stress-straingraph are significant, which include the elastic modulus, a yieldpoint, and a fracture point all holding important information on thematerial. Elastic moduli are the part of the graph that measures thestiffness in a material before plastic deformation occurs. This isrepresented by the linear part of the graph. As long as tension puton the material is within the elastic modules the nylon will retractto its original state, but after the linear part ends the plasticwill still retract but not as far as before. When the test materialis changed from linear to a curve, it starts to form plasticdeformation, this point is known as the ‘yield point’. The nextmajor point on the graph is the ultimate tensile strength which givesthe max stress put on the material. Lastly the fracture point of thematerial is also achieved through tension testing this helps todetermine the toughness of the material I question, area under thegraph while showing us where the specimen failed.Many importantfeatures of a material are obtained by looking at a tress straincurve.

Inthe real world this experiment can and is used often to gaininformation and test tension strength of different materials.Manufacturers have to test their material to make sure it can hold upunder certain conditions. This test can be done cold, hot, or at roomtemperature to give different graphs and results. With this they cantell how temperature will affect the material being used, to makesure it will withstand a certain load at given temperatures that areneeded. Tensile tests are fairly inexpensive and can really tellmanufacturers how the given material will react under tension, theelasticity, and elongation of a material that is tested at a certaintemperature. &nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp

Metal‘Hardenability’ is the extent to which a given material ishardened after being subjected to heat treatment process. Itindicates how deep into a material, a specific hardness can beachieved, and this can be achieved by the use of the Jominy bar test.The test consists of quenching a hot rod of metal with water, oil, orcoolant and testing its hardness along the length of the specimen.

Hardenabilityis dependent on how fast a metal is able to cool and the amount ofmartensite that is formed during the quenching process, themicrostructure of the metal before quenched, and the speed it iscooled. The metal has to be heated to a temperature that allows it tocrystallize into a face-centered-cubic structure called martensite.When one end of the metal is cooled rapidly with water, the austenitewill form martensite (very strong material) or non-martensite. Theend hit with water cools rapidly forming mostly martnsite on theoutside and depending on the thickness could form it all the waythrough while the end not hit directly with water cools at a slowerrate forming less martnsite and a softer outer shell.

Thegreater the difference in hardness between the two ends of thequenched bar results in a lower hardenability. This comes from thefact that when the outside is cooled rapidly it creates a strongermaterial that can be hardened as easy where the slowly cooled sidecan still increase in hardness giving it a greater hardenability. Inmost cases the higher the carbon content of the steel will result ina greater hardenability then low carbon steels.

Oneapplication in using hardenability is welding. The lower the abilityto be hardened, the lower the hardenability level the easier it willbe to weld the material. Welders can do this test between two metalsto determine if the metal can be welded together and what methods canbest be used to weld those materials together.

PROCEDURE20/25

Asstated above, the tensile test experiment was determining thestress-strain relation of nylon at three different temperatures roomtemperature, hot, and cold. Load versus extension was measured usingthe equipment in Table 2, below.

Table 2. Tensile Testing Tools

Equipment

Model

Serial Number

Appendix A

Instron Tensile Tester

5882

41157

Figure 1

IBM-compatible PC with a recording software

Data Acquisition System

(USB-6008 or SCXI-1000)

Othermaterials used were three identical nylon samples in the dogboneshape, see Figure 2 below.

Figure2, Dogbone tensile testing shape.

(Missingfigure)

Acomputer program was set up in the beginning of the lab to measureand record the load versus extension data taken from the tensiletester. Load resulted from the force exerted by the tester, and anextension was the resulting change in the length of the nylon sample.To start with, a room temperature dogbone of 20°C was placed intothe bottom of the tensile tester and the top clamp was lowered untilthe top of the dogbone fit inside it. For the room temperaturesample, the feed rate was set at 1.0 inches per minute. Both top andbottom clamps were tightened and the testing started. The machine ranand the computer automatically gathered data until the sample broke,at which point testing stopped. The broken sample was removed, itsdata saved for further analysis, and the next sample loaded.

Thetesting repeated for the hot sample which had been heated to 115°CMuffle furnace and then lastly, the cold sample which had been cooledin liquid nitrogen and had a temperature of -196°C. Insulation waswrapped around the neck of the sample in the tester so that it wouldshatter and cause a safety hazard. Insulation also had the benefit ofhelping to maintain the sample’s temperature so the experiment wasmore repeatable.

Theload versus extension data for all three samples was converted intostress versus strain and further analyzed below.

Asstated above, the Jominy bar experiment tested the hardenability of ametal specimen. A metal bar, a Jominy bar, was heated uniformly andthen quenched at one end in water while the other end was cooled byroom temperature air. Hardness was measured at regular intervalsalong the length of the bar, on the HRB scale. The equipment in Table1, below, was used.

Table 1. Jominy Testing Tools

Equipment

Model

Serial Number

Appendix A

Rockwell Hardness Tester

8534-T

97226304

Figure 2

Jominy Bar Testing Mount

n/a

n/a

Figure 3

Jominy Quenching Apparatus

n/a

n/a

Figure 4

Theonly material used was the Jominy bar, a steel cylinder-type shapewith many straight sides suitable for hardness testing.

(Insertpicture)

Onlyone of these sides was measured during this experiment, but the wholebar was heated and quenched equally. The bar was heated in a Mufflefurnace at 900°C for 45 minutes. It was then removed and immediatelyplaced in the Jominy Quenching Apparatus see Figure 4 in Appendix A.This was a metal bucket with a plate across the top with a holeslightly larger than the diameter of the Jominy bar such that the barcould be inserted vertically and held in place with a clamp whilewater from a hose quenched one end of the bar, see Figure 1 below.

Figure1, Jominy Quenching Apparatus

Afterthe entire bar had achieved a comfortable handling temperature, about15 minutes later, the side to be tested which had been tested beforewas polished with sandpapers to smooth it and remove scales formedthrough oxidation from being heated. The bar was then placed into thetesting mount, Figure 3 in Appendix A, side of interest up. Startingnear but not on quenched end, hardness was measured every 1/16” for1 ¼”, then every 1/8” for the rest of the bar. This data wasanalyzed further as can be seen below.

Resultsand Discussion23/25

TheStress vs. strain plot is useful for gleaning information aboutvarious materials. The plots can be used to determine the maximumtensile strength of materials, elastic modulus, and even yieldstrength. In this experiment, nylon was the subject of study intothose material properties. In the Figure 1 below, sample of nylon wasat 20⁰C and was subjected to tensile loading until failure. Onexamination of Graph 1 and Table 1, one can observe that thedifference in Ultimate tensile strength and yield strength is small.Meaning that after yielding, the sample did not require much moreforce to plastically deform to the point of failure. But that alsomeans that the sample will undergo considerable stress while stillremaining in the elastic portion of its stress- strain curve. Theelastic portion being the straight part below approximately 9000 psi.Towards the end of Graph 1 at approximately 0.375 inches of strain,it can be seen that the load required continuing to deform the sampledecreases and then stops. This decrease is likely due to the samplenecking and the cross sectional area decreasing, but as the samplestretches during its last moments, it weakens as less material isthere to support it, thereby reducing the force required to deformit. The reason behind the abrupt stops in Graph 1 is due to the nylonsample failing the test as when it was heated, it crabbed together,eventually, breaking altogether.

Graph1,Stress, Strainn curve of Nylon at room temperature

Young`s Modulus €

Yield strength (psi)

Ultimate Tensile Strength (psi)

145901.175

8585.990

10755.00

Table1, Key points from Graph 1, Of Nylon at 20oC

Thesecond sample of nylon underwent the same tensile loading test withone notable exception the sample was heated to 115oCduring testing. At such temperatures, the sample behaved differentlythan the control sample at room temperature. Looking at Graph 2 andTable 2, one can observe that the sample plastically deforms foralmost the entirety of the load cycle. This is proven by thedifference in yield strength and maximum tensile strength being verylarge. Comparing this data with the control data, increasing thetemperature of a nylon sample drastically reduces the strength of thematerial as the yield strength of the control sample is even greaterthan the ultimate tensile strength of the heated sample. This is mostlikely due to fact that as a material is heated, the number ofdislocations in it decreases and as dislocations inhibit slip withina material, the slip in the heated material occurs more often andmore easily than the same material at room temperature.

Graph2, Stress, Strain plot of nylon at 115oC

Young`s Modulus €

Yield Strength (psi)

Ultimate Tensile Strength (psi)

113526.4128

966.3300

8213.3300

Table2, Key points from Graph 2 depicting the Stress Strain curve of nylonat 115oC

Thethird and final experiment with tensile loading of a nylon sample wasperformed with a nylon sample identical to the others but with atemperature of -196 degrees Celsius. As observed before with a heatednylon sample, changing the temperature can drastically alter how thematerial behaves. Looking at Graph 3 and Table 3, one can see that ina very cold environment, the nylon sample does not deform nearly asmuch. In fact, once the sample has begun to deform, the sample failsafter only.04 inches of additional strain. In comparison with theother two samples, the cold nylon undergoes only a 4thof the total strain before failure. Meaning the sample barelystretched at all before fracture. However, the yield strength of theold sample is the highest out of all the samples tested.

Indicatingthat the cold sample is the hardest and can resist much higher stressbefore plastically deforming

Graph3, Stress, Strain curve of nylon at -196oC

Young`s Modulus €

Yield Strength (psi)

Ultimate Tensile Strength (psi)

191638.343

16653.343

19098.68

Table3, Key points from Graph 3

Belowis an image of all the samples after tensile testing and failure aswell as a control sample that has not been tested to demonstrate howthe shape changes during testing. Each are labeled by whattemperature they were tested, the unlabeled sample is the untestedcontrol. As shown, both the room temperature sample and the hotsample necked as a result of the tension. However, the hot samplestretched slightly more than the control. The cold sample did notneck hardly at all, harder samples of any material are more brittleand therefore undergo less strain before fracture and break morecleanly.

Picture1, all tested Nylon samples labeled by relative temperature. Theuntested sample included to show relative changes in the sample dueto testing

Aswith any experiment there is a certain level of error that couldoccur during testing. Error in precise measurement is due to theinexactness of measurement devices, temperature is not exactlyconstant throughout the entirely of testing as heat radiates out andthe sample cools or the sample heats up as it is exposed to a warmerenvironment. To combat this error, each sample was wrapped in aninsulating blanket to prevent major temperature fluctuation. One wayto improve the accuracy of temperature control would be to performeach experiment in a carefully controlled environment with either arefrigeration system to keep the sample cool, or an oven to keep thesample hot during the duration of the tensile loading cycle.

Asteel Jominy bar was used to test the hardenability of a metal byquenching in water. The bar was quenched from one end in order tocreate a gradient of phases along its length. Steel quenched from itsaustenite temperature will transform into several different phasesdepending on the time it took to cool. From this, it was possible totest the length of the Jominy bar to gain hardness values using aRockwell tester. Once knowledge of the hardness of the material atdifferent points was ascertained it was possible to observe thedifferent phases of the steel. Looking at Plot 1, it can be observedthat as the distance from the quenched end of the sample increases,the hardness of the steel decreases. This is due to the specificquenching process used. For example, the harder the steel is, thefaster the temperature was reduced, which means that it was closer tothe water used to quench it. The hardest sections of the Jominy barare likely the martensite. This is due to the fact that, it is formedby rapidly cooling steel from its austenite temperature of 900⁰Cdown to 200 ⁰C. Bainite on the other hand is likely form at themiddle sections of the bar as they take slightly longer to form butare much more ductile than martensite. The portions of the barfurthest from the quenched end are most likely pearlite. Pearlite isformed by cooling from austenite to lower temperatures over thecourse of several minutes. Pearlite is not nearly as strong asbainite, and is much more ductile.

However,there are some outliers in the data. These were most likely caused bya thick layer of oxidized steel on the surface of the sample whichwas not entirely removed prior to testing. This oxidation may nothave the same composition or phase as the main sample and would alterthe results of the test by not giving the hardness value of theJominy bar. One interesting product of quenching could be that thecenter of a bar of steel would take longer to cool and will mostlikely be a more ductile phase of the steel sample, leading to alayered structure that is hard on the outside and softer on theinside. So when designing a structure that had to be equally hard allthe way through, it may have to be thinner. Which is likely thereasoning behind an I-beam? It still has a fairly large crosssectional area to support more loading, but is thin enough to bemostly of a harder steel phase.

Plot1, Hardness vs. Distance from quenched end of Steel Jominy Bar

Conclusionand Recommendations 10/15

Inconclusion, the results from the completion of this experiment, leadsto the conclusion that, hot temperatures make materials more ductilewhile lower temperatures makes them less ductile. Temperatureaffects how sample behaves under stress. The nylon sample with atemperature of -196oChad the highest tensile strength. This happened because in lowertemperatures, particles move slowly and cannot move freely. It wasclear that the sample broke without significant strain and absorbedlittle energy prior to fracture. The nylon sample with a temperatureof 20C had a strong yield strength and tensile strength, but not asstrong as the case of a cold sample. The data obtained from theJominy test are essential in determining whether any particularmaterial such as steel can be adequately hardened, and in diversequenching media. The nylon sample at a temperature of 115oChad the lowest yield strength. This is due to the fact that, at hightemperatures, particles move fast and freely. The hot temperaturealso made the nylon sample more ductile. The sample plasticallydeformed for almost the entirety of the load cycle. The heat treatingprocess is used to change the composition and structure of amaterial. Heat is used to alter any given material, metallurgicallyin order to examine and test its characteristic.

Afterthe experiment, it is clear that temperate is a key determinant ofhardenability aspects of any given material. For example thehardness of steel metal was decreasing with an increase intemperature. The end that was quenched in the water had a higherhardness than the end that was air cooled. The decrease in hardnessover the distance was almost at a constant rate.

Futureexperiments can be improved if the participants could use more metalsamples during the Rockwell hardness measurement so they could learnmore about the hardness of more materials. The experiment can be usedto test the hardness of many other materials. The experiment would agreat gain for sectors such as engineering. The heat treatmentexperiment and the hardenability are very helpful and significant indetermining property of steel. It helps in determining the rate atwhich, any given steel should be quenched.

Futureexperiments should be conducted by controlling the temperature duringthe tensile loading cycle. This is due to the fact that, maintainingthe same temperature, will help to get accurate and reliable date.When one needs a softer or harder material, it can be achievedthrough heat treating the material and in turn cooling it to achievethe desired hardness.