Effect of constraint on fracture behavior of welded 17mn4 and aisi304 steels

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1 Journal of Mechancal Scence and Technology 5 (9) (011) 171~177 DOI /s Effect of constrant on fracture behavor of welded 17mn4 and as304 steels Bahadr Uyulgan 1, Hakan Cetnel,* and Tevfk Aksoy 1 1 Department of Metallurgcal and Materals Engneerng, Dokuz Eylul Unversty, Buca Izmr, Turkey Department of Mechancal Engneerng, Celal Bayar Unversty, Muradye Mansa, Turkey (Manuscrpt Receved December 30, 010; Revsed May 10, 011; Accepted May 11, 011) Abstract In ths study, 17Mn4 (P95GH) pressure vessels steel and AISI304 stanless steel were welded wth ER309L austentc consumable. In expermental part of the study, tensle tests were conducted on welded plates and varaton of hardness values along specmen was measured. J-ntegral fracture toughness values were nvestgated for dfferent crack locatons. In order to determne the regons where plastc deformaton dd not take place due to constrant, un-axal tensle test was performed on welded tensle specmen after attachng stran gauges. In numercal part of the study, fnte element (FE) analyses were conducted by fxng -D models precracked on dfferent locatons by usng ANSYS software. In these models, stress traxalty and plastc deformaton characterstcs around crack tp were determned for each crack locatons after stress stran analyses. The lmtaton on the extenson of plastc deformaton at dffuson lne causes extra ncrease n stress traxalty at crack tp. Keywords: Weld; J-ntegral; Constrant; AISI304; Msmatch; P95GH; Fracture toughness Introducton Ths paper was recommended for publcaton n revsed form by Assocate Edtor Mohammad Abdul Azz Irfan * Correspondng author. Tel.: , Fax.: E-mal address: hakan.cetnel@bayar.edu.tr KSME & Sprnger Mn4 (P95GH) s the carbon-manganese steel wdely used for producton of pressure vessels, steam bolers, ppe lnes etc., whch work under hgh temperature condtons, accordng to BS EN AWS ER309L s the austentc stanless steel rod used for tungsten nert gas (TIG) weldng of stanless steel and low alloy steels. These rods can be used n weldments exposed up to 300 C operatng temperatures [1]. Weldng of AISI 304 (X5CrN18-10) austentc stanless steel materals wth low alloy steels or plan steels s a very frequent stuaton. Welded jonts are more senstve parts of a structure wth regard to crack growth and falure, because of the nherent characterstcs of weldments. The fracture property analyss of a welded jont s a man part n any structure ntegrty assessment []. Snce the mechancal heterogenety n the welded jont affects the flow and fracture, as the materal researchers have pad great attenton to ths problem [3]. Spurrer et al. [3] researched the effect of fracture behavor of welded off shore structural steels usng crack tp openng dsplacement (CTOD) technque whch s a dfferent from technque from our method J-ntegral calculaton. They found that the mprovement n toughness on post weld heat treatng jonts has been quantfed and dstngushed from the practcal benefts from weld metal strength overmatchng. The man method to determne the fracture behavor of a materal s to analyze the stress-stran feld at the crack tp [4]. Xue & Sh [4] studed the effect of crack length on the sze of plastc regon for dfferent regons ncludng base metal, weld metal and heat affected zones (HAZs). Constrant due to hndrance of plastc deformaton spread could be nvestgated for overmatched and undermatched jonts. Because of the dffculty n drectly analyzng the stress stran feld, some fracture parameters have been used to represent the ntensty of the feld. There are two knds of constrants n weldments, namely the geometry constrant caused by loadng mode, crack sze and specmen dmensons, and the materal constrant caused by nhomogeneous materal propertes. These two constrants nvaldate the conventonal fracture mechancs parameters [5]. Thus, plastc deformaton spread and stress traxalty around crack tp should be nvestgated n order to determne constran effect. The J- ntegral s mostly used as the ntensty parameters n the post-yeld fracture mechancs. The three-pont bendng test s a method wdely used for determnng the fracture toughness of materal [6]. It s generally known that a welded jont can be regarded as a structure wth hybrd materal propertes due to the mcrostructural dfferences n the base metal (BM), weld metal (WM) and the heat affected zone (HAZ). HAZ propertes such as yeld strength, toughness and ultmate tensle strength are

2 17 B. Uyulgan et al. / Journal of Mechancal Scence and Technology 5 (9) (011) 171~177 dstnctly dfferent for these regons [7]. In another study, fracture behavor of overmatched weldments of nodular cast ron was also nvestgated [8]. Undermatched weldment gves better results for fracture behavor of welded ductle ron [9]. In our former studes, the base materal was ductle ron. In ths study, we have two dfferent steels as the base materals wth varous matchng levels. Effects of strength dfferences (msmatch) between weld metal and base metal, as well as local varatons of strength wthn weld HAZ zones, on the fracture toughness propertes of welded jonts were dscussed by another researchers as well [10]. Apart from our study, Rak & Treber [10] used CTOD method n order to determne fracture toughness of welded HT50 and HT80 steels. They found that The HAZ CTOD fracture toughness n weld jont produced on steels of dfferent strength level s excellent compared to WM CTOD one. We also showed to dfferences n fracture toughness values of varous regons ncludng weld metal, base metals and HAZ. In welded constructon, fracture s most lkely to occur ether n or near the weld as ths locaton contans the most severe resdual stress, potental weldng defects, and the lowest fracture toughness [11]. In our study, we chose certan regons ncludng weld metal, base metals and HAZ where can have crack potentally. Structural ntegrty assessments are therefore often based on toughness data derved by testng fracture specmens contanng welds. Msmatch between weld metal and base metal strength can alter the deformaton patterns n such specmens, whch complcates fnte sze effects on cleavage fracture toughness [1]. In our study, certan regons near dffuson lne were chosen n order to determne fracture toughness values affected by constrants due to obstructon of plastc deformaton. In ths study, the effect of msmatch due to yeld strength dfferences of HAZ, weld and base metals on fracture behavor of welded parts was nvestgated. Varaton of mcrohardness values on welded parts was measured. Constrants on dfferent regons ncludng HAZ, weld and base metals were determned. Fnally, J fracture toughness values were determned for dfferent regons on the welded parts. Table 1. Chemcal composton of the test materals. % AISI 304 (X5CrN 18 10) P 95 GH (17 Mn 4) ER309L (XCrN 4 1) C S P Mn S N Cr Mo N Co Cu Al Table. Mechancal propertes of the weld and the base materals. AISI 304 P 95 GH ER309L Yeld stress (MPa) Tensle strength (MPa) Elongaton (%) Table 3. Weldng parameters. Pass Voltage (V) Amperage (A) Weldng speed (cm/mn) Expermental study 17Mn4 (P95GH) pressure vessels steel and AISI304 stanless steel are welded wth ER309L austentc consumable. Accordng to nspecton certfcate, the chemcal composton and mechancal propertes of materals are gven n Tables 1 and, respectvely. The steel plates to be welded had 8 mm thckness, 150 mm wdth and 300 mm length. A standard V groove, wth mm root length, 50 o groove angle mm dstance between plates, was prepared. TIG (GTAW) weldng was conducted by usng ER309L. Fg. 1 shows three pass weldng steel plates schematcally. Weldng parameters for each pass are gven n Table 3 such as voltage, current and weldng speed. After weldng process, steel plates are cut by usng wre Fg. 1. The profle of the weld mouth. eroson process. Tensle test specmens were machned by lathe as cylndrcally standard test pece. Fg. shows welded steel plates and tensle test specmen locatons schematcally. Tensle tests were conducted at room temperature on a SIMADZU model AG50kNG unversal testng machne. The crosshead speed was 1 mm/mn. Durng testng, stress stran curves were recorded by a computer. The yeld stress, ultmate tensle stress and elongaton were determned from the tensle curves. In order to determne the regons where plastc deformaton s hndered, un-axal tensle test was performed on welded tensle specmen by attachng stran-gauge at fve dfferent

3 B. Uyulgan et al. / Journal of Mechancal Scence and Technology 5 (9) (011) 171~ Fg.. Two dfferent steels; 17Mn4 and AISI304 welded wth ER309L electrode and extracton regons of tensle test specmens from welded plates. Fg. 3. Dfferent cracks composed of notch and fatgue crack on the dffuson zones, weld and base metals. locatons of the sample. Nomnal stress stran dagrams are determned by usng fve stran-gauges for fve dfferent locatons. Durng the un-axal tensle test, stran values are recorded by constant tme perods. Nomnal stress was calculated as tensle load dvdng the cross secton area. The mcro-hardness values were measured along the longtudnal drecton of the specmens. The mcro-hardness values were measured on a SIMADZU model DUH-W01S ultra mcro-hardness testng machne wth Vckers ndenter under 1 Newton load. Reference pont of the welded sample for the mcro-hardness measurements was chosen as the fuson surface between weld metal and AISI304 base metal. Dstance measurements between mcro-hardness ndents were done wth respect to ths reference pont. The dffuson regon s nvestgated between the base metal 17Mn4 and the weld metal ER309L by scannng electron mcroscopy (SEM) and energy dspersve spectrometer (EDS). In order to examne the fracture behavor of the weld metal, base metal and HAZ n weld jont, the standard bend specmens for J fracture toughness test were prepared. The welded specmens were machned as rectangular bars (7.5 mm x 7.5 mm x 100 mm) for J-ntegral fracture toughness tests. The sngle edge-notches were machned at fve dfferent locatons n dfferent specmens n order to form precracks n fatgue tests usng three pont bend (3PB) loadng. The ntal crack length chosen was 4 mm. Sngle specmen technque was used, and calculatons of J-ntegral values were made from loadcrack mouth openng dsplacement (CMOD) curves usng the ASTM E standard. The sngle edge-notches were cut at fve dfferent locatons of selected specmens for fracture toughness tests (Fg. 3). The fve dfferent meshed locatons n FE models are shown n Fg. 4. The notch was cut as 1 mm wdth, mm depth and 30 o angle V type shape. The sngle specmen technque was used wth standard unloadng complance method n order to determne fracture mechancs parameter J. In the sngle specmen technque, by usng the standard J fracture toughness test fxture, tests were conducted at room temperature on SHIMADZU AG-50 kng unversal testng machne. The test technque nvolves unload/reload sequences steps Fg. 4. Meshed models for dfferent crack locatons. and n the each step, the specmen was loaded up to a crack mouth openng dsplacement level reaches 0.05 mm more than prevous level. In each step, after duraton of 0 seconds at maxmum load pont, the appled load was reduced untl CMOD level reached approxmately 0.03 mm less than actual level. Durng the tests, CMOD values were obtaned by usng a SHIMADZU type SG5-50 dsplacement gage. By repeatng these unload/reload sequences untl 1.5 mm CMOD value, the load crack mouth openng dsplacement (CMOD) curves were recorded by computer. The more accurate methodology for evaluatng J, as used n the advanced test procedure of ASTM E 1737 s: J = J + J (1) el pl where J el = elastc component of J and J pl = plastc component of J. For the sngle edge-notched bend specmen (SE (B)), the elastc component of J can be calculated at each pont V (CMOD), P (Load), from the LEFM stress ntensty as: J el ( ) ( ) K() 1 ν = () E where K () s stress ntensty factor of SE(B) specmen for

4 174 B. Uyulgan et al. / Journal of Mechancal Scence and Technology 5 (9) (011) 171~177 pont and gven by: K PS = BW () f 3 ( a W) (3) wth ( a W ) f a functon of specmen type, gven by: 1 a a a a a a W W W W W f = 3 W a a W W (4) Fg. 5. Representaton of unloadng complance technque for locaton 03 of 17Mn4. The subscrpt on a s present to emphasze that the crack length s changng here, and that the most recent value s used at each nstant. The plastc component of J can be calculated at each pont V, P ) from the ncremental equaton: ( J pl() η( 1) Apl( ) A pl( 1) a( ) a ( 1) = Jpl( 1) + 1 γ( 1) b( 1) B b( 1) (5) where for the SE(B) η ( 1 ) =. 0, γ ( 1 ) = 1. 0, a0 = the ntal crack length, W = the specmen wdth, B = the specmen thckness, b0 = ( W a0) s the uncracked lgament at the start of the test, and Apl = s the area under the load versus crack mouth openng dsplacement. Unloadng complance technques are used to obtan estmates of the crack length from accurate and precse measurements of the specmen complance made from perodc unloadngs durng the fracture test. Crack lengths are determned based on the measured specmen complance. Relatonshps between the measured complance and the specmen crack length are therefore very mportant and are provded n ASTM E For a sngle specmen method usng an elastc complance technque on three-pont bend specmens, wth crack openng dsplacements measured at the notched edge, the crack length s gven by (6): 1 U x = (7) 1 BWE C + 1 S 4 where E E = (for plan stran state) (8) (1 ν ) C = Specmen crack mouth openng elastc complance ( Δ V m ΔP) on an unloadng / reloadng sequence, ΔP = Increment of load, and ΔV m = Increment of crack mouth openng dsplacement measured at the specmen surface (Fg. 5). 3. Results and dscusson Tensle test results and producers data were compared n Table 4. Engneerng stress-stran curves determned by tensle tests are gven n Fg. 6. There s no remarkable devaton between expermental results and standards. The msmatch factor M s calculated from Eq. (9). WM YS BM M = σ σ (9) YS where WM σ YS BM σ YS : Yeld strength of the weld metal and : Yeld strength of the base metal. a W where = 3 [ U x +.981U x U x 4 5 x U x U ] (6) M values are 1.5 for materal 3 (AISI304) and materal (ER309L), and 1.14 for materal 1 (17Mn4) and materal (ER309L). Msmatch factors of both sdes of welded plates are over-match condtons but the msmatch factor s bgger on the AISI304 base metal sde than that of 17Mn4 base metal sde.

5 B. Uyulgan et al. / Journal of Mechancal Scence and Technology 5 (9) (011) 171~ Table 4. Strength values of weld and base metals. Base metal (X5CrN18-10) (AISI 304) Weld metal (ER309L) Base metal (P95GH) (17Mn4) ASTM A40/A40 M-04a Yeld strength (MPa) Tensle strength (MPa) Break elongaton (%) EN Tensle test results ESAB, ER309L electrode (OK Tgrod 16.53) Tensle test results EN EN Tensle test results Fg. 7. The Vckers mcro hardness values varaton on welded plates. Fg. 8. Vckers ndentaton traces on HAZ regon of 17Mn4 base metal. Fg. 6. Engneerng stress-stran curves of weld and base metals. The hardness values of the weld metal, base metal and HAZ were obtaned along a lne perpendcular to the weldng drecton of the specmen. The hardness values of the weld metal, base metal and HAZ are shown n Fg. 7. HAZ regon hardness values are rapdly ncreased from base metal hardness values to weld metal hardness values but the hardness values ncreasng regon was very narrow. The wdth of HAZ regon was detected accordng to results of the hardness values. Fg. 8 shows Vckers ndentaton traces on the heat affected zone between 17Mn4 and weld metal ER309L. Wdth of dffuson zone s narrow because of low heat affect durng weldng. Etchng HAZ regon could be seen by optcal mcroscope but hardness values n the HAZ couldn t change abruptly from hardness values of base metal 17Mn4 and weld metal ER309L. The SEM mcrograph also shows dffuson regon wdth between base metal 17Mn4 and weld metal ER309L (Fg. 9). Narrowness of the dffuson regon can be seen from the Fg. 9. SEM mcrograph of dffuson regon between 17Mn4 and ER309L. etched surfaces. The dffuson regon s nvestgated between the base metal 17Mn4 and the weld metal ER309L by scannng electron mcroscopy (SEM) and energy dspersve spectrometer (EDS). Iron, chromum, nckel and manganese dstrbutons are shown n Fg. 10. Ths graph also shows that the regon of dffuson s very narrow. After unload reload sequences each data pont on the load versus crack mouth openng dsplacement curves were obtaned, then J-ntegral and crack extenson values were calculated usng Eqs. (1) and (6). In the plastc part of J, A s the

6 176 B. Uyulgan et al. / Journal of Mechancal Scence and Technology 5 (9) (011) 171~177 Fg. 10. The varatons of chromum, nckel, manganese and ron amounts between ER309L weld and 17Mn4 base metals. Fg. 11. J resstance curves for all locatons ncludng heat affected zones, weld and base metals. stran, stran gages become unusable because of splttng from surface so that measurements taken by stran gages were ended. Fg. 1 shows dfferent plastc deformaton values on fve locatons because of dfferent plastc constrants. Plastc deformaton s started frstly n the base metal 17Mn4 and weld metal ER309L. After that plastc deformaton started n heat affected zone between ths two locatons. Preventng of plastc deformaton s observed also n two HAZ regon between base metal AISI304 weld metal ER309L and between base metal 17Mn4 weld metal ER309L. 4. Conclusons Mcro-hardness measurements on welded plate demonstrate that heat affected zones have very small area and dfference ts hardness wth other zones are nearly neglgble. Addtonally, the results of Iron, Chromum, Nckel and Manganese amounts determned by EDS analyss on scannng electron mcroscopy present dffuson between the weld and the base metal occurred n a very small zone. As result of the tensle test wth performed stran measurng, plastc deformaton s started frstly n the base metal 17Mn4 and the weld metal ER309L. After that, plastc deformaton started n heat affected zone between these two locatons. Preventng of plastc deformaton s observed also n two HAZ regons between the base metal AISI304 weld metal ER309L and between the base metal 17Mn4 weld metal ER309L. Accordng to J-ntegral fracture toughness test results, weld metal shows the mnmum resstance to crack propagaton. On the other hand, the resstance values of both base metals aganst the crack propagaton are the largest values. The other crack locatons between the base metals and weld metal have the average J-ntegral toughness values. Fg. 1. Nomnal stresses versus stran curves n dfferent locatons ncludng heat affected zones, weld and base metals. area under the load - CMOD curves, whch vas determned by usng mage analyzer. Fg. 11 depcts the J resstance versus crack propagaton curves of SE(B) specmens on fve dfferent crack locaton. Weld metal shows the mnmum resstance to crack propagaton. On the other hand, the resstance values of both base metals aganst the crack propagaton are the largest values. The other crack locatons between the base metals and weld metal have the average J-ntegral toughness values. In order to determne the regon where plastc deformaton s hndered, un-axal tensle test was performed on welded tensle specmen wth a number of stran gages attached (Fg. 1). Fve stran gages were attached on approxmately smlar locatons as on J fracture toughness tests. Durng the un-axal tensle tests stran and load values were recorded at the same tme. The load values are dvded to specmen cross secton area to fnd nomnal stress. After some amount of plastc References [1] Oerlkon Weldng Electrodes and Industry Co. Copyrght, 001, product_detal_new?forpc=0&selected_menu=0&selected_ sumenu=0&lng=trk&subsubcategory_d=53&product_d=406. [] C. Erpret, P. Hornet, Fracture toughness testng procedures for strength ms-matched structures, Ms-Matchng of Interfaces and Weld, Edted by K. H. Schwalbe & M. Koçak, GKSS Research Center Publcatons, Geesthacht, FRG (1997) [3] J. Spurrer, P. Hancock, J. P. Chubb, An assessment of weld ms-match, Engneerng Fracture Mechancs, 53 (1996) [4] H. Xue, Y. Sh, Effects of mechancal heterogenety on plastc zones of welded three pont bend specmens, Pressure Vessels and Ppng, 75 (1998) [5] Z. L. Zhang, C. Thaulow, M. Hauge, Effects of crack sze and weld metal msmatch on the haz cleavage toughness of wde plates, Engneerng Fracture Mechancs, 57 (1997)

7 B. Uyulgan et al. / Journal of Mechancal Scence and Technology 5 (9) (011) 171~ [6] ASTM E , Standard test method for J ntegral characterzaton of fracture toughness, West Conshohocken; ASTM Commttee, ASTM Internatonal (1996). [7] H. Cetnel, B. Uyulgan, T. Aksoy, The effect of yeld strength msmatch on the fracture behavor of welded nodular cast ron, Mater. Sc. Eng. A, (004) [8] H. Cetnel, Fracture behavour of overmatched ductle ron weldment, Int. J. Mater. Res. (Z. Metallkunde) 98 (0) (007) [9] H. Cetnel, B. Uyulgan, T. Aksoy, The effect of undermatchng on crack tp constrant n a welded structure of nodular rons, J. of Materals Processng Technology, 198 (1-3) (008) [10] I. Rak, A. Treber, Fracture behavor of welded jonts fabrcated n HSLA steels of dfferent strength level, Engneerng Fracture Mechancs, 64 (1999) [11] B. Petrovsk, M. Koçak, Evaluaton of the fracture behavour of strength ms-matched steel weld jonts wth surface cracked tensle panels and SENB specmens, Ms-Matchng of Welds, ESIS 17 (Edted by K.-H. Schwalbe & M. Koçak) Mechancal Engneerng Publcatons, London (1994) [1] M. T. Krk, R. H. Jr. Dodds, The nfluence of weld strength msmatch on crack-tp constrant n sngle edge notch bend specmens, Internatonal Journal of Fracture, 63 (1993) congresses. Hakan Cetnel was born n 1969 n Izmr, Turkey. He obtaned B.Sc. and M.Sc. degrees n Mechancal Engneerng n Dokuz Eylul Unversty. He attended several projects n Unversty of Manchester n England. He has nternatonal full papers ndexed by SCI and has gven presentatons at nternatonal Bahadır Uyulgan was born n 1974 n Izmr, Turkey. He obtaned B.Sc. and M.Sc. degrees n Metallurgy & Materals Engneerng n Dokuz Eylul Unversty. He has nternatonal full papers ndexed by SCI and has gven presentatons at nternatonal congresses. Tevfk Aksoy was born n 1947 n Izmr, Turkey. In 1989, he obtaned Professor Ttle n Mechancal Engneerng n Dokuz Eylul Unversty. He attended several projects. He has nternatonal full papers ndexed by SCI and has gven presentatons at nternatonal congresses.