EFFECT OF TEMPERING TEMPERATURE ON MICROHARDNESS AND RESIDUAL STRESSES OF QUENCHED C25 CARBON STEEL HOLLOW CYLINDER

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1 Adedayo, S. M., Adekunle, A. S., Ajiboye, A. Oiginal Scientific Pape EFFECT OF TEMPERING TEMPERATURE ON MICROHARDNESS AND RESIDUAL STRESSES OF QUENCHED C5 CARBON STEEL HOLLOW CYLINDER Received: 15 Septembe 14 / Accepted: 1 Octobe 14 Abstact: Effect of tempeing tempeatues on esidual stesses and micohadness acoss quenched hollow C5 cabon steel was investigated. Residual stesses wee induced by sepaately quenching in wate and vegetable oil (Aachis hypogoe). Quenched specimens wee stess elieved at, 4 and 6 o C. Two othogonally mounted 1 ohms esistance stain gauges wee used on each specimen to measue stain elaxation. Induced machining longitudinal and tansvese stess wee measued and factoed in final stess values. Modified Sach s boing equations was applied to obtain longitudinal and tansvese esidual stesses acoss the quenched cylinde. Hadness of C5 cabon steel and cooling ate of the quenchants was also detemined. Key wods: Residual stesses, quenching, Sach boing technique, esistance stain gauges, tempeing Uticaj tempeatue tempeovanja na mikotvdoću i zaostale napone kaljenog šupljeg cilinda od ugljeničnog čelika C5. U ovom adu je ispitivan uticaj tempeatue tempeovanja na zaostale napone i mikotvdoću kod kaljenog ugljeničnog čelika C5. Zaostali naponi su unošeni odvojenim kaljenjem u vodi i biljnom ulju (Aachis hipogoe). Kaljeni uzoci su otpuštani na, 4 i 6 o C. Dve otogonalno montiane mene take otpoa 1 oma su koišćene na svakom uzoku za meenje napona otpuštanja. Uzdužni i popečni naponi geneisani obadom su takođe meeni i uzeti u obzi kod ačunanja ukupnih vednosti napona. Modifikovana Sačova jednačina je koišćena za dobijanje vednosti uzdužnih i poečnih zaostalih napona u kaljenom cilindu. Tvdoća ugljeničnog čelika C5 i bzina hlađenja medijuma je takođe odeđivana. Ključne eči: Zaostali naponi, kaljenje, Sačova tehnika pošiivanja, otpone take, tempeovanje 1. INTRODUCTION Residual stesses can be descibed as stesses which emain locked in a body which is stationay and at equilibium with its suoundings o stesses locked in a system even though extenal foces ae not acting on it [1-3, 1]. Liscic et al. and Palaniadja et al. [4, 5] diffeently studied the effect of esidual stess on manufactued pat and found that esidual stesses aising in poducts being made and used goven thei lifetime significantly. Residual stess analysis in thick plates using neutons and Fe modeling showed that actual and opeating stesses may diffe consideably fom design values due to a possible summation of woking and esidual stesses in sepaate poduct potions [6]. Developed model to educe distotion of a cabuized and quenched steel gea showed that detemination of esidual stesses is a pincipal poblem in engineeing pats [7]. Palkowski et al. [8] used numeical simulation and neuton diffaction analysis of dawn tubes to show that the magnitude and distibution of esidual stesses in a component o stuctue is a significant souce of uncetainty in mechanical engineeing design and one that can affect subsequent machining as well as life pediction and assessment of stuctual integity. Chinnaaj et al. [9] used numeical and expeimental investigation of esidual stesses in cold fomed tuck fame ail sections and concluded that esidual stesses ae an unavoidable concomitant of almost all manufactuing and fabication pocesses and can also aise duing sevice. James [1] studied esidual stess influences in mechanical engineeing using Fe modeling while Bin et al. [11] studied esidual stesses in machined and shink-fitted assemblies and they found that esidual stess may occu unde any set of cicumstances that lead to diffeential expansion o contaction between adjacent pats of a body in which the local yield stength is exceeded; simila esult was obtained by Totten et al. and Guo et al. [1, 13] in thei wok esidual stess, nanohadness and micostuctue changes in whildwind milling of GC15 steel. Buch et al. [14] in his wok mechanical popeties and coosion esistance of duplex stainless steel diffeently showed that the ceation of esidual stesses duing heating, quenching and machining is consideed as one of the key paametes since it has diect effect on the fatigue life of a machined component. Although theoetical and numeical analyses ae being widely used to estimate the esidual stesses in components, it is necessay to undetake expeiments to validate the pedictions. Residual stess measuement techniques ae geneally categoized as non-destuctive and fully destuctive methods. Most of the nondestuctive methods such as conventional X-ay and magnetic techniques ae esticted to nea suface measuements [15]. In contast the neuton diffaction method is able to penetate up to depths of about 5mm [16]. Kingston et al. [17] in his wok novel application of the deep hole dilling technique fo measuing though thickness esidual stess distibutions showed that semi-invasive methods such as the cente hole dilling povide nea suface measuements up to depths of about 1mm. Moe ecently Smith et al. [18] in the wok measuement and pediction of esidual stesses in thick section steel weld showed that the 13

2 deep-hole dilling technique has povided thoughdepth measuements to depths in excess of 7mm. Heat teatment such as induction hadening, flame hadening and quenching altes the metallugical stuctue of the metal unde esidual stess; with such metal poducing high compessive esidual stesses, with fatique life impovement of up to 1% [19]. Similaly duing quenching, sufaces that cools fist end up in esidual bi-axial compession, whilst the inne coe is in a state of ti-axial tension [-1]. Sometimes compessive stesses ae intentionally intoduced on the sufaces in ode to incease the fatique stength of the mateial. Heat teatment esults that ae of common inteest include the volume factions of phases, hadness, esidual stesses and pat distotion. This pape descibes the outcomes of two independent studies of the measuement of esidual stesses and hadness in hollow C5 cabon steel afte hadening by quenching in wate and Jatopha oil with subsequent tempeing.. EXPERIMENTAL.1 Mateials Composition and Specimen Pepaation Photo-spectomety tests wee caied out on wok piece mateials. Obseved composition is as shown in Table.1 Elements Volume % C.354 Si.351 S.6 P.7 Mn.74 Ni. C.17 Mo.17 V.1 Cu.38 W.4 As.1 Sn.4 Co.13 Al.4 Pb.1 Zn.4 Fe Table 1. Chemical Composition of C5 quench metal [vol.%] The cylindical test specimens wee cut fom the paent metal and machined to an oveall height of 9 mm, inne and oute diametes of 1 and 48 mm espectively. Pepaed test specimens wee austenised at 9 o C 3 o C in an electic funace at the ate of 5 o C/min and soaked fo 3 minutes to achieve complete homogenization. Test specimens wee sepaately quenched lateally in 1 ml of wate and goundnut oil (Aachis hypogae). Themal histoy of quench pocess was taken using a K-type themocouple with immesion tempeatue of the specimen been 84 o C. The quench pocess was followed by tempeing at tempeatues, 4 and 6 o C. Each tempeing pocess condition was epeated thice with aveage values of esidual stesses esults pesented.. Instumentation and Calibation A wheatstone bidge with the dummy ams made of electic esistance stain gauges of type FLA 5 11 of esistance 1Ω and gauge facto.11 was constucted. Stain indications on the wheatstone bidge wee caied out in ode to establish a elationship between known stain values in the active am and coesponding eadings of the digital stain indicato. A vaiable esisto was connected in paallel with the active am, which allowed fo vey small changes of esistance to be effected on the active am as shown by Adedayo (1998). A vaiable esisto (Rs) setting of 18Ω acoss a 133Ω esisto gave an effective esistance of 1Ω on the active am. At this setting, all the fou ams of the wheatstone bidge wee balanced with ead-out instument indicating zeo. The mathematical elationship used to obtain active am stain coesponding to any esistance change is given in equation (1) below 1 dr (1) F R (b) Fig.1 (a & b) Wokpieces showing stain gauges and hadness test points (a) 14

3 .3 Longitudinal and Tansvese Residual Stess Evaluation.3.1 Stain elaxation and measuement Stain measuements wee caied out though use of stain gauges. Specimen sufaces was popely pepaed though emoval of scales, dits, buns and degeasing using diffeent gades of emey pape. Ethanol and methylated spiit was used fo degeasing the suface fo pope chemical affinity. Gauge location was scibed on the specimen. Positioning and oientation of the gauge wee caefully maintained duing application of a cynoacylate adhesive. Gauges wee mounted on the cylindical wok-pieces in longitudinal and tansvese oientations as depicted in Figue 1(a). Mild pessue was applied in mounting the gauges in ode to squeeze out excess adhesive and poduction of a stong thin bond line. A suitable wheatstone bidge was built. A digital stain mete ( Model No. E1MKII ) was used to monito stains. Bonded stain gauges wee effectively potected against abasion and moistue contacts. The zeo eadings of stain gauges wee obtained fo a peiod of seven days with aveage eadings used as zeo eadings fo the test esults. Stains wee ead fo both longitudinal and tangential diections using the successive boing technique. A Stoebo lathe, Model No. GS6 was used to make the successive intenal diamete cuts fo stain elaxation. Machine speed was 135 ev/min and coolant was applied duing the pocess to minimize wok piece tempeatue. A depth of cut of.5 mm was taken evey boing day. Relaxed longitudinal and tansvese stains indicated as ε L and ε T espectively wee measued 4 hous afte the metal emoval. The tue values of ε L and ε T wee obtained by subtacting the zeo eading fom the indicated eadings on the digital stain mete and actual stain values obtained fom the calibation cuve. Obtained stains wee fitted into modified Sach s equations to detemine the tansvese and longitudinal stesses..3. Longitudinal and Tansvese Residual Stesses The longitudinal (σ L ) and tansvese (σ T ) esidual stesses wee obtained fom modified Sach s equations [] as shown in equation (a & b). 1 d L (a) d T 1 d d 1 (3.) 1 (b) o = extenal adius of cylinde = cuent intenal adius of cylinde afte laye emoval. (4.) Λ= L T (5.) θ= T L L = change in longitudinal suface stain due to emoval of an inne laye T = coesponding change in tansvese suface stain E = Young s Modulus ( N/m ) ν = Poisson s atio (.3) Stain values wee obtained fom the digital stain mete eadings afte successive elaxations. The value of the functions and ae zeo at both inne and oute sufaces so that extapolation of functions is to zeo instead of an unknown value. In this fom, the stesses ae calculated by plotting the functions of and against, so that the longitudinal and tansvese stesses can be obtained espectively at any value of by multiplying the slope by a constant facto (see equation a and b). The adial stess values wee neglected in this wok due to the elatively small thickness of the cylinde walls. The following pocedues wee adopted in ode to evaluate the values of both longitudinal and tansvese esidual stesses. (i) The value of was obtained, and it was constant fo all the specimens (ii) The values of afte each successive laye emoval opeation wee noted. (iii) Longitudinal stain and the tansvese wee obtained fom stain gauge measuements. (iv) The values of Λ and θ wee also calculated fo all the specimens fom equations 4. and 5. espectively. (v) The values of and stain T wee obtained fo each specimen (vi) The gaphs of and vesus wee plotted fo each specimen. (vii) The gadients of the gaphs plotted in (iv) and d (vii) above gave the values of d and d d (viii) The constant L 1 was calculated. (ix) The values obtained in (vi) and (vii) wee substituted in equations (a) and (b) to give the longitudinal and tansvese stesses. (x) Longitudinal and tansvese machining stesses 15

4 wee similaly obtained by machining an annealed wokpiece. Obtained Machining longitudinal and tansvese stesses wee applied fo compensation by subtacting fom the quenching stesses. The final values ae pesented in the esults metallic cystal paticle bonds ae weakened at high tempeatues thus accounting fo the highe pecentage stess eliefs at high tempeatues..3.3 Machining Residual Stesses A fully annealed wok-piece of same dimension as the quenched wokpieces was fitted with stain gauges on oute diamete fo longitudinal and tansvese esidual stesses measuement. Successive machining was caied out and esults of the stain changes wee fitted into the modified sach boing expession to obtain the machining stesses..4 Metallogaphic Examination Test specimens wee sectioned and sufaces ginded with silicon cabide papes of 4, 3, 4 and 6 git sizes. They wee subsequently polished using a cloth impegnated with alumina until a mio suface was obtained. Ginded suface wee cleaned with wate and ethanol. Etching with % Nital was done and micostuctues obseved using a high poweed optical micoscope. Fig. Effect of tempeing tempeatue on longitudinal esidual stesses in wate quenched Specimen.5 Hadness Test Measuements The wate and oil quenched C5 steel mateial was subjected to micohadness test along the adial diection fo both tempeed and untempeed wokpiece. Hadness measuement of the specimen at diffeent adii locations was epetitively done thee times using micohadness teste LM7AT unde an applied load of 49.3 mn with a dwelling time of 1 seconds. Aveage values of depth of penetation of the indente was ecoded and pesented. The specific locations along which the longitudinal and tansvese hadness values wee taken ae shown in Fig.1(b). 3. RESULTS AND DISCUSSION 3.1 Effect of Tempeing Tempeatue on Longitudinal Residual Stesses Figs and 3 show the effect of tempeing tempeatue of, 4 and 6 o C on longitudinal esidual stesses fo wate and oil quenched specimens espectively. The maximum values of longitudinal esidual stesses of specimens quenched in wate and oil unde no themal elief conditions ae 65 MN/m and MN/m espectively as shown in Figs. and 3. Unde stess elief tempeatues of, 4 and 6 o C, maximum longitudinal esidual stesses ae 9, 188 and 153 MN/m espectively fo wate quenched and 179, 15 and 14 MN/m espectively fo oil quenched specimens. These indicate a esidual stess elief of 7.5, 34.4 and 45.3% fo wate and 1.9, 5.4 and 3.3% fo oil at, 4 and 6 o C themal elief tempeatues espectively. Fig. 4 shows the elative magnitudes of longitudinal esidual stesses of wate and oil quenched specimens in a puely quenched and 4 o C tempeed state. The quench seveity of wate is highe than oil thus esulting in highe values of esidual stesses with o without tempeing and the Fig.3 Effect of tempeing tempeatue on longitudinal esidual stesses of oil quenched specimen Fig 4 Distibution of longitudinal esidual stesses in wate and oil quenched specimen 16

5 3. Effect of Tempeing Tempeatue on Tansvese Residual Stesses Figues 5 and 6 shows the effect of tempeing tempeatues of, 4 and 6 o C on tansvese esidual stesses fo wate and vegetable oil quenched specimens espectively. The maximum values of tansvese esidual stesses of specimens quenched in wate and oil unde no themal elief conditions ae 46 MN/m and 411 MN/m espectively. Unde stess elief tempeatues of, 4 and 6 o C, maximum tansvese esidual stesses ae 436, 46 and 345 MN/m espectively fo wate quenched and 387, 363 and 311 MN/m espectively fo oil quenched specimens.. These indicate a esidual stess elief of.5, 7.4 and 5 % fo wate and 5.8, 11.7 and 4.6 % fo oil at, 4 and 6 o C themal elief tempeatues espectively. Fig.7 compaes magnitudes of wate and oil quenched specimens with and without tempeing at 4 o C. The quench seveity of wate is highe than oil thus esulting in highe values of tansvese esidual stesses. Fig.5 Effect of tempeing tempeatue on tansvese esidual stesses of wate quenched specimen Fig.6 Effect of tempeing tempeatue on tansvese esidual stesses of oil quenched specimen Fig.7 Distibution of tansvese esidual stesses in wate and oil quenched specimen 3.3 Machining Longitudinal and Tansvese Residual Stesses Table. shows the values of the machining longitudinal and tansvese esidual stesses at evey successive machining stage. Cut No Radius (mm) Longitudinal (MN/m ) Tansvese (MN/m ) Table. Machining Residual Stesses 17

6 Maximum tensile machining longitudinal and tansvese esidual stesses of 5.78 MN/m and 3.1 MN/m was attained at the 18 th and 17 th cut with intenal adius of 15. and 14.5 mm espectively. The coesponding maximum compessive stesses was MN/m and -5.5 MN/m occuing at adius 4 mm fo longitudinal and tansvese esidual stesses espectively. Magnitudes of the machining stesses ae small elative to the quenching stesses. This is due to the elatively low cutting foces due to small depths of cut and coesponding low themal gadients acoss mateial thickness. 3.4 Effect of tempeing tempeatue on metal hadness The micohadness fo both wate and oil quenched C5 mateial at tempeing tempeatue of o C, 4 o C and 6 o C is as shown in Fig. 8 and 9 espectively. Slight incease in micohadness values is obseved between adii 3 and 6 mm fo all tempeing tempeatue with highest value been 98 H v at tempeing tempeatue of 6 o C; while fo oil quenched mateial the hadness only inceases between adial distances 3 and 6 mm fo tempeing tempeatue of o C with highest attained hadness value of 17.5 VHN Fig. 9. Beyond adial distance of 6 mm fo the wate and oil quenched mateial the hadness values deceases fo all tempeing tempeatues. The hadness of wate quench C5 mateial was highe than that of oil quenched fo all tempeing tempeatues which shows highe quench seveity and cooling ate of wate than oil as shown in Fig 11. In geneal highest hadness was obtained fo the C5 mateial at tempeing tempeatue of 4 o C fo both wate and oil quenched. Fig. 1. Fig 9. Effect of tempeing tempeatue on micohadness of oil quenched C5 mateial Fig 1. Vaiation of hadness in wate and oil quenched at 4 C tempeing tempeatue Fig 8. Effect of tempeing tempeatue on micohadness in wate quenched C5 mateial Fig. 11. Cooling ate fo vaious quenchants 18

7 3.5 Micostuctues of Quenched Specimens Figs.1(f-h) shows mixtue of matensite and fine pealite indicative of quenching condition but the gain stuctues ae not as fine as the specimens quenched in wate, which indicates that oil has lowe seveity of quenching. 4. CONCLUSIONS Magnification: x Figs. 1 ( a e ): Micostuctues of wate quenched.3% C steel specimens (a) As eceived (annealed) (b) Wate quenched (c) Wate quenched and tempeed at C (d) Wate quenched and tempeed at 4 C (e) Wate quenched and tempeed at 6 C Fig.1(a) Compises of coase pealite in a feite matix indicative of fully annealed condition. Figs.1(b - e) shows a mixtue of matensite and fine pealite indicative of quenching condition but the gain stuctues ae fine than the specimens quenched in oil, which shows that wate has highe seveity of quenching. To study the effect of tempeing tempeatue on esidual stesses of quenched C5 cabon steel hollow cylindes, successive stess elaxation ae pefomed with stain gauges used to monito the stain changes. Modified sach boing technique is applied to obtain values of longitudinal and tansvese esidual stesses. Also effect of tempeing tempeatue on hadness was studied and the vaiation of hadness along the adial distances was detemined fo the quenched mateial using wate and oil. Fom the expeimental esults the following conclusions ae deduced: (1) Highe levels of esidual stesses of as much as 4% ae induced in wate quenched mateials in compaison with vegetable oil quenched mateials. () Residual stesses acoss the quenched hollow cylinde ae compessive at inne and oute sufaces and tensile in the inne pat thus maintaining equilibium acoss the coss section. (3) Tempeing educes magnitude of esidual stesses induced by quenching by as much as 4%. (4) The highe the tempeing tempeatue the moe the esidual stesses elieved in both longitudinal and tansvese diection. (5) Maximum tensile tansvese esidual stess of wate quenched and untempeed mateial is highe than that of longitudinal stess by as much as 73.5%. (6) Maximum longitudinal machining stesses ae as high as 5.78 MN/m and tansvese 3.1 MN/m in tension. They ae significant enough and theefoe deducted to obtain actual quench esidual stesses. (7) Tempeing at 4 o C gives highe hadness fo C5 mateial than othe tempeing tempeatues and the highest hadness was 81.5 VHN and.4 VHN fo wate and oil quenched espectively. Hadness deceases with incease in adial distance beyond 6 mm. 5. REFERENCES Magnification: x Fig.1 (a, f-i): Micostuctues of oil quenched.3% C steel specimens (f) Oil quenched (g) Oil quenched and tempeed at C (h) Oil quenched and tempeed at 4 C (i) Oil quenched and tempeed at 6 C [1] James, M. N., Hughes, D. J., Chen, Z., Lombad, H., Hattingh, D. G., Asquith, D., Yates, J. R., Webste, P. J. Residual Stesses and Fatigue Pefomance, Engineeing Failue Analysis. England, 1, p [] Gokunov, E. S., Zadvokin, S. M., Gouleva, L. S. Estimating Residual Stesses in heat-teated Cabon Steels by Magnetic Paametes, 18 th Wold Confeence fo Nondestuctive Testing, South Afica, Apil 16 1, [online] accessed 4 th Febuay 14 [3] Totten, G. E. Steel Heat Teatment, Metallugy and 19

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