The Effect of Welding on the Fatigue Crack Growth Rate in a Structural Steel

Transcription

1 The Effect of Weldng on the Fatgue Crack Growth Rate n a Structural Steel Poor steelmakng practces and non-low-hydrogen electrodes were found to accelerate fatgue crackng BYG. O. RADING ABSTRACT. Fracture mechancs prncples have been used to study the effects of weldng on the rate of fatgue crack growth (FCG) n a low-carbon structural steel. The steel concerned s used wdely n the fabrcaton of the structural framework of passenger buses. Tests were carred out on the base metal (BM), heat affected zone (HAZ) and weld metal (WM). Both the near threshold and mdrange regmes of crack growth were studed. In the mdrange regme, the FCG rate was hghest n the HAZ and lowest n the WM. Near the threshold, the FCG rate was hghest n the BM and lowest n the WM. The results are explaned n terms of mcrostructural changes due to weldng, weldng resdual stresses, and fracture mechansms. Recommendatons to reduce the ncdence of fatgue crackng have been made. Introducton Most of the structural steel that s currently used n Kenya s cast and rolled locally. The bulk of the raw materal conssts of an assortment of steel and cast ron scrap that s arc melted and then G. O. RADING s a Graduate Research Assstant n the Department of Metallurgcal and Materals Engneerng, Unversty of Alabama, Tuscaloosa, Ala., presently on leave from Mo Unversty, Kenya, Afrca. rolled. Manufacturers who buld passenger buses use ths materal to fabrcate the structural framework. The operators of the sad buses reported a hgh ncdence of fatgue falures. On examnaton, t was revealed that most of the fatgue cracks occurred n the heat-affected zone (HAZ). Fgure shows the extent of crackng n one such bus as t was brought n for repars. Investgaton of the meltng practces and mll procedures ndcated several weaknesses wth regard to qualty control. There was lttle control of the qualty of scrap, no attempts to degas the melt, no close control of the chemstry, lttle control of temperature durng KEY WORDS Effects of Weldng Low-C Structural Steel Fatgue Crack Growth Base Metal Heat-Affected Zone Weld Metal Mcrostructure Changes Resdual Stresses Fracture Mechancs Mechancal Propertes rollng and contact of hot metal wth water durng rollng. The purpose of ths nvestgaton was to study the effects that weldng has on fatgue crack growth n ths materal. In partcular, to study the rate of fatgue crack growth n the base metal (BM), heat-affected zone (HAZ) and weld metal (WM). The nvestgaton covered both mdrange (where the Pars law s applcable) and near threshold (below a crack growth rate of approxmately 3 X 0" 6 mmcycle) regmes of FCG. Materals and Test Procedures The materal studed s a low-carbon structural steel. The chemcal composton and mechancal propertes are gven n Table, whch also ncludes the requrements of ISO 630 (Ref. ) for ths grade of steel. The fatgue crack propagaton tests were performed on center cracked tenson specmens. The dmensons of the specmens are shown n Fg. 2. The thckness of the specmen s the same as that used n the fabrcaton of the bus body frameworks (about 2 mm.). To assess the rate of FCG n the weld metal, the materal was cut nto two and rewelded. The center notch was then machned such that the crack would grow along the WM. To preserve the thckness, the WELDING RESEARCH SUPPLEMENT I 307-s

2 I DOOR WAY Fg. The extent of crackng n a vehcle framework brought n for repars The arrows ndcate postons of the cracks. I IJ. ****^**- :? rc II ( \ \ ll,. ' I I Fg. 2 Dmensons of the specmen. weld bead was ground flush. Specmens for testng FCG n the HAZ were smlarly prepared, but the notch was machned adjacent to the weld to make the crack grow along the HAZ (parallel to the weld). The poston of the notch relatve to the weld was determned by pror macro-etchng of a smlar weld. The tests were performed on a rg desgned for ths purpose. The load was measured by a stran gauge based load cell attached to an osclloscope. The stress ntensty factor was calculated from (Refs. 2, 3) AK AP na na 4 sec B \2W 2 where, a = 2aW, and AP s the alternatng load. The crack length was measured usng a travelng mcroscope accurate to 0. mm. The crack growth rate, dadn, was then calculated (determned) usng the followng two methods: ) In the mdrange regme (regme II), a graphcal method was used,.e., a graph of a vs. N was plotted and the gradent determned at varous ponts. 2) Near the threshold, the secant method was used,.e., dadn was determned from da_ dn (2) where a = 'A (a + + a-,), and the subscrpt,, denotes the 'th data pont. All tests were performed at an R-rato of 0.2 and a frequency of 24 Hz n laboratory ar. All the mdrange tests were performed at a constant AP n accordance wth the ASTM standard (Ref. 3), whle near the threshold, the decreasng K technque, (Ref. 3), was used. In all cases, precrackng was carred out under the same condtons (frequency, R-rato, envronment, maxmum load, etc.) as the actual test, followng the procedures n ASTM 647 (Ref. 3). Results The mdrange FCG curves for the base metal, weld metal, and heat-affected zone are shown n Fgs. 3, 4, and 5, respectvely. The graphs show the range of stress ntensty calculated from Equaton plotted aganst dadn determned by the graphcal method as explaned earler. The results are seen to follow the Pars law dn CAK" (3) For AK n MPa m 2 and dadn n mmcycle, the values of C and m determned were as shown n Table 2. Fgure 6 shows the results of the near threshold FCG tests for the BM, HAZ and WM. In ths regme, the Pars equaton s no longer applcable. In Fg. 7, all the results are plotted together on the same dagram. Fgures 3 to 7 show the followng trend n FCG for ths materal: ) In regme II of FCG, the rate s hghest n the HAZ and lowest n the WM. The dfference between the rates n the HAZ and BM dmnshes at low values of stress ntensty range (exemplfed by the dfference n the exponent, m). 2) Stll n the mdrange, the dfference between the FCG rates n the BM and WM s hghest at low values of stress ntensty range, the dfference dmnshes at hgher values of stress ntensty range. 3) Near the threshold, the FCG rate s hghest n the BM and lowest n the WM. There s thus a cross-over n the FCG curves pertanng to the BM and that pertanng to the HAZ. Dscusson Weld Metal The results gven above show that the rate of FCG s lowest n the weld metal. Ths s so both n the md-range and near threshold regmes. There are several factors whch contrbute to ths. Resdual Stresses The weldng process leaves behnd stresses dstrbuted as shown n Fg. 8 Table - The Chemcal and Mechancal Propertes of the Materal Tested. Yeld Stress UTS Elong. C (MPa) (MPa) (%) (%) Mn S (%) (%) P (%) \ (%) Table 2 - Results Summary m of Md range FCG C Materal ISO mn. 530 mn. max max. max max max. BM HAZ WM * lo" 9 2.9* 0" 0.6* 0" 308-s I JULY 993

3 4; u 0" TJ o- o-» SfEC.I SPEC. II 0" 5 - <Q-«l....I.I.I......,... I S 7» '0 SO AK Mjjfnvg Fg. 3 Mdrange FCG rate data for the base metal. 0- ' *.... I I I I I I I I ul I I * I I t I I I I I t r o JO A K MN m-» Fg. 4 Mdrange FCG rate data for the HAZ. (Ref. 4). These stresses may be as hgh as the yeld stress (Ref. 5). For a thn plate, such as the one tested here, the stresses can be taken as unform through the thckness. No apprecable tunnelng was evdent. As can be seen from Fg. 0, the resdual stresses are compressve n the part of the specmen where the crack s growng, and wll result n a reducton of stress ntensty such that AK.. = AK-K r (4) where K res s the stress ntensty due to the compressve stress. Fgure 8 shows a reducton n the magntude of the compressve stress as the dstance from the center of the plate ncreases. Thus, the contrbuton of K,. es should decrease at hgher values of stress ntensty. Ths s ndeed what s observed n Fg. 5. A smlar trend has been observed by several workers (Refs. 6-2), and some of the workers have shown that when the resdual stresses are releved, the rates of FCG though the BM and WM become essentally the same (Refs. 6, 7, 8, 2, 3). Weld Mcro-Imperfectons and Crack Tp Morphology Small, rounded mcro-mperfectons lke porosty, slag ncluson, etc., (but excludng weld mcrocracks) wll result n crack tp bluntng. Ths reduces the sharpness of the crack and, hence, the effectve value of stress ntensty, resultng n a lower FCG rate. Furthermore, t may be necessary for the crack to re-ntate or the nature of the mperfecton may lead to crack tp knkng and branchng. All these factors lead to a reduced effectve stress ntensty and, hence, a lower FCG rate. A smlar reasonng was advanced by Maddox (Ref. 4). However, the benefcal effects of mcro-mperfectons are lkely only f ther sze and number are below some crtcal value. Ths crtcal value depends on the number of stress cycles requred to re-ntate the crack, compared to the number of cycles requred to grow the crack through sound materal by an amount equal to the dameter of the defect. Ths number of cycles can be estmated by ntegratng Equaton 3 and usng the values of C and m from Table 2. It s expected that a sutable nondestructve test would detect those mperfectons that exceed the crtcal sze. Changes n Mcrostructure and Mechancal Propertes The mechancal propertes and mcrostructure of the weld metal are df-. The equaton does not nvoke crack closure arguments. K res s calculated from resdual stress at each pont,.e., K res = f(a res ) = f(a), and ths value s then substracted from the mechancally appled stress ntensty. Crack closure can occur wthout resdual stress (e.g., oxde nduced closure or mode l nduced dsplacement). Conversely, resdual stresses can occur wthout crack closure. For example, f the resdual stresses are tensle, Equaton 4 s stll applcable, except that K res s added rather than subtracted. But tensle resdual stresses do not cause crack closure. Ths also clarfes the pont concernng the equatons' range of applcablty. Where there s no resdual stress, K res s set to zero. Where the stress s tensle, K, s added rather than subtracted. WELDING RESEARCH SUPPLEMENT I 309-s

4 0" 0- I u >> E 0 o T3J CT 6 t THWESKX.D» WEL p HAZ» PARENT M, 0" 5 o- 7 - WEIO HETAl tret PARENT Mt f)-t..... V t I 0 Fg. 5 Mdrange FCG rate data for the WM. I..J lo *o so AK MNm-»J 0' I. I r AK[MNm'h] Fg. 6 Near threshold FCG data (all zones). ferent from those of the BM. Ths becomes mportant n near threshold growth where the FCG rate s very senstve to the mcrostructure (Refs. 5, 6). Furthermore, the WM has a much coarser gran structure. A coarse gran structure s more resstant to FCG (Ref. 5), whch contrbutes to the lower rate of FCG near the threshold n ths case. Heat-Affected Zone The results presented prevously ndcate a faster rate of FCG n the HAZ than n the BM n the mdrange. The graphs cross over as the threshold s approached. Ths s a clear departure from most reported results, whch ndcate a lower rate of FCG through the HAZ (Refs. 9-2). Ths behavor can be attrbuted to the reasons stated below. Mcromechansms of FCC In the mdrange of FCG, the growth of fatgue cracks occurs by straton formaton (Refs. 7, 8). Ths materal was seen to have an unacceptably hgh concentraton of ntrogen. The heatng that occurs n the HAZ due to weldng s lkely to cause the formaton of brttle ntrdes n the HAZ. Such an effect was observed by Gardner (Ref. 9). In fatgue, these brttle partcles wll fal by a "statc" mechansm most lkely debondng from the rest of the materal. These "statc" falure mechansms, supermposed on the straton formaton mechansm, wll result n a hgher rate of FCG through the HAZ. Near the threshold, the stresses are too low for the "statc" modes of falure to take place and, as s evdent from Fg. 4, the gap between FCG rates n the HAZ and BM closes. Hydrogen Embrttlement As was explaned n the ntroducton, lack of adequate qualty control procedures leads to the hot metal comng nto contact wth water durng rollng. Thus, some water vaporhydrogen s lkely to be trapped n the metal. Ths effect may be further aggravated durng weldng when hydrogen, both from resdual water vapor and from the electrode cover (all the weldng was done usng the shelded metal arc method) dffuse to the HAZ. The result s hydrogen embrttlement n the HAZ, leadng to the hgher rate of FCG. Hgher rates of FCG caused by hydrogen embrttlement n structural steels has been reported by Marrow, etal. (Ref. 20), Cotterll and Kng (Ref. 2), and Hppsley and Lane (Ref. 22), among others. Coarser Grans Smlar to what was sad for the weld metal, coarser grans n the HAZ lead to a lower rate of FCG n the near threshold regme (Ref. 5). The most serous anomalous behavor of the materal subject to study was the hgher rate of FCG through the HAZ. Ths s attrbutable to the lack of qualty control both durng castngrollng of the steel and durng fabrcaton of the bus body frameworks. It s recommended that both the steel rollng mlls and the bus body fabrcators nsttute qualty control assurance procedures whch should nclude: ) proper control of the chemstry of the melt through chemcalspectrophotometrc analyss. 30-s I JULY 993

5 0 E E 0 D 0- -VE VE -LQNGTUDONAL 0 VE 0"' a porenl mol» w*ld metal HAZ * brdgng ponts T -VE TRANSVERSE Q 8 'I. I. I, I. I _ L_J I. I AK Fg. 7 Combned regme I and II FCG data (all zones). MNrrr* Fg. 8 Schematc dstbuton WELD St weldng resdual stresses. 2) approprate degassng of the melt. 3) preventon of excessve contact between the hot metal and water durng rollng. (The water s used to cool the rollers. The soluton could be to ether use an alternatve method of coolng or to ensure better sealng of the water ppes.) 4) dry the weldng electrodes pror to use to exclude water vapor. 5) usng electrodes of hgher qualty to exclude the possblty of hydrogen absorpton. It may also be of nterest to quantfy the effects of the factors mentoned above,.e., the resdual stresses, crack branchng and knkng, as well as the ntrde formaton tendency. Except for resdual stresses, for whch several researchers (Refs. 7, 23) have suggested equatons, quantfyng the other factors s no easy task. A statstcal approach may be the only path. Conclusons The followng conclusons may be drawn from ths study of the effects of weldng on the rate of FCG n a low-carbon structural steel: ) In the mdrange of FCG, the FCG rate s hghest n the HAZ and lowest n the WM. 2) The hgh FCG rate through the HAZ may be attrbuted to the formaton of ntrdes, and hydrogen embrttlement n the HAZ, though other factors may also be contrbutng. 3) The slower FCG rate through the weld metal was due to resdual stresses and weld mcro-mperfectons, whch lead to changes n crack tp morphology. 4) The threshold stress ntensty range ncreased n the order BM < HAZ < WM. 5) The greater resstance of the HAZ and the WM to near threshold FCG was due to the coarser gran structures n these zones, and, n the case of WM, partally to compressve resdual stresses. Acknowledgments Fnancal support for ths study was provded by the German Academc Exchange Servce (DAAD). References. ISO 630, Structural Steels. Internatonal Organzaton for Standardzaton, (980). 2. Cartwrght, D. )., and Rooke, D. P. Compendum of Stress Intensty Factors. HMSO, London, (976). 3. ASTM E647, Test Method for Constant Ampltude Fatgue Crack Growth Rates Above 0" 8 mcycle. Annual Book of ASTM Standards, part 0. (980). 4. Gurney, T. R. Fatgue of Welded Structures, Cambrdge Unversty Press, (979). 5. Nakamura, H., etal.). Testng and Evaluaton, Vol. 6. No. 3, p. 280, (988). 6. Yuen, J. L., and Coperland, J. F. ;). Eng. Mat. and Tech., Vol. 0, p.24, (979). 7. Itoh, Y. Z. ef al. Eng. Fracture Mech.; Vol. 33, No. 3, p. 397, (989). 8. Greasley, A. and Naylor, S. G. W. Eng. Fracture Mech., Vol. 24, No. 5, p.7l7, (986). 9. Clark, Jr., W. G. ASME paper No. 70- PVP-24, (970). 0. Bucc, R. J. Greene, B. N. and Pars, P.C. ASTM STP 536, (973).. Shahnan, P., et al. Weldng Journal., Vol. 5, No., (972). 2. James, L. A. HEDL-TME 76-93,Westnghouse Hardford Co. Rctuand,WA, (977). 3. James, L. A., and Mlls, W. ). J. Eng. Mat. and Tech. Vol. 07, p. 34, (985). 4. Maddox, S. J. Weldng Journal, Vol. 53, No. 9, (974). 5. Rtche, R. O. Metal Scence, (979). 6. Shh, T. T., and Donald, J. K. J. Eng. Mat. and Tech., Vol. 03, (98). 7. Pelloux, R. M. N. Eng. Frac. Mech. Vol.. (970). 8. Lard, C. ASTM STP 45, (967). 9. Gardner, J. Weldng Engneerng Scence and Metallurgy. Norman Prce Ltd., (972). 20. Marrow, T. J. Cotterll, P. J., and Kng, J. E.; Acfa Metall. Mater., Vol. 40, No. 8, pp , (992). 2. Cotterll, P. J., and Kng, J. E.; Int. J. Fatgue, Vol. 3, No 6, pp , (99). 22. Hppsley, C.A., and Lane, C. E. Mat; Sc. Tech., Vol. 6, pp , (990). 23. Beghn, M., and Bertn, J. Eng. Fracture Mech., Vol. 36, No. 3, p. 397, (990). Appendx Nomenclature a C dadn K K max half crack length constant n the Pars equaton fatgue crack growth rate stress ntensty factor maxmum value of K durng cyclc loadng WELDING RESEARCH SUPPLEMENT I 3-s

6 ^mn AK K r AK,h m N P R mnmum value of K durng cyclc loadng range of stress ntensty plane stress fracture toughness threshold stress ntensty range exponent n the Pars equaton number of stress cycles load stress rato K mn K max Abbrevatons ASTM CCS FCG(P) HAZ Amercan Socety for Testng and Materals Center Cracked Tenson Specmen Fatgue Crack Growth (Propagaton) Heat-Affected Zone ISO NTFCG BM WM Internatonal Organzaton for Standardzaton Near Threshold Fatgue Crack Growth Base Metal Weld Metal WRC Bulletn 369 December 99 Ntrogen n Arc Weldng A Revew By IIW Commsson II In 983, Commsson II of the Internatonal Insttute of Weldng (IIW) ntated an effort to revew and examne the role of ntrogen n steel weld metals. The objectve was to comple n one source, for future reference, the avalable nformaton on how ntrogen enters weld metals produced by varous arc weldng processes, what forms t takes n these welds, and how t affects weld metal propertes. Ths bulletn contans 3 reports and several hundred references related to Ntrogen n Weld Metals that has been prepared as a revew to show the mportance ntrogen has n determnng weld metal propertes. Publcaton of ths report was sponsored by the Weldng Research Councl, Inc. The prce of WRC Bulletn 369 s $85.00 per copy, plus $5.00 for U.S. and $0.00 for overseas, postage and handlng. Orders should be sent wth payment to the Weldng Research Councl, Room 30, 345 E. 47th St., New York, NY 007. WRC Bulletn 370 February 992 Recommendatons Proposed by the PVRC Commttee on Revew of ASME Nuclear Codes and Standards Approved by the PVRC Steerng Commttee The ASME Board on Nuclear Codes and Standards (BNCS) determned n 986 that an overall techncal revew of exstng ASME nuclear codes and standards was needed. The decson to ntate ths study was renforced by many factors, but most mportantly by the need to capture a pool of knowledge and "lessons learned" from the exstng generaton of techncal experts wth codes and standards background. Project responsblty was placed wth the Pressure Vessel Research Councl and actvty ntated n January 988. The drecton was vested n a Steerng Commttee whch had overvew of sx subcommttees. The recommendatons provded by nuclear utltes and ndustry were combned wth the ndependent consderatons and recommendatons of the PVRC Subcommttees and Steerng Commttees. Publcaton of ths document was sponsored by the Steerng Commttee on the Revew of ASME Nuclear Codes and Standards of the Pressure Vessel Research Councl. The prce of WRC Bulletn 370 s $30.00 per copy, plus $5.00 for U.S. and $0.00 for overseas, postage and handlng. Orders should be sent wth payment to the Weldng Research Councl, Room 30, 345 E. 47th St., New York, NY s I JULY 993