This paper presents results of a study on martensitic coarse grain heat affected zone that can appear in welded joints. Mechanical properties of martensitic coarse grain heat affected zone and its microstructure were investigated. Special attention was given to its behaviour during the cycle loading under stress concentration. Stress concentration was similar to the one in real welds. The S-N curve and the fatigue limit were determined. The Paris curve and the threshold for crack propagating were also determined.U radu se istražuje martenzitna grubozrnata zona utjecaja topline koja se može pojaviti pri zavarivanju. Istražena su mehanička svojstva i mikrostruktura ove zone. Posebna je pažnja posvećena njenom ponašanju tijekom cikličkog opterećenja kada je prisutna koncentracija naprezanja. Koncentracija naprezanja je slična onoj kod realnog zavarivanja. Određena je S-N krivulja i granica umaranja. Također je određena i Parisova krivulja i prag propagacije pukotine

Maruščak, P. O.

Samardžić, Ivan

Vuherer, Tomaž

Digital library of University of Maribor

301METALURGIJA 51 (2012) 3, 301-304T. VUHERER, P. MARUSCHAK, I. SAMARDŽIĆBEHAVIOUR OF COARSE GRAIN HEAT AFFECTED ZONE (HAZ)DURING CYCLE LOADINGReceived – Prispjelo: 2011-09-27Accepted – Prihvaćeno: 2011-12-28Original Scientifi c Paper – Izvorni znanstveni radISSN 0543-5846METABK 51(3) 301-304 (2012)UDC – UDK 621.791.032:620.17:620.18=111T. Vuherer, University of Maribor, Faculty of Mechanical Engineering, Slovenia, P. Maruschak, Ternopil State Ivan Plu’uj University, Depart-ment of Material Science, Ukraine, I. Samardžić, University of Osijek, Mechanical Engineering Faculty, Slavonski Brod, CroatiaThis paper presents results of a study on martensitic coarse grain heat aff ected zone that can appear in welded joints. Mechanical properties of martensitic coarse grain heat aff ected zone and its microstructure were investigat-ed. Special attention was given to its behaviour during the cycle loading under stress concentration. Stress concen-tration was similar to the one in real welds. The S-N curve and the fatigue limit were determined. The Paris curve and the threshold for crack propagating were also determined.Key words: Weld, coarse grain HAZ, microstructure, mechanical properties, fatigue limit, Paris curvePonašanje grubozrne zone utjecaja topline (ZUT) zavara tijekom cikličkog opterećenja. U radu se istražuje martenzitna grubozrnata zona utjecaja topline koja se može pojaviti pri zavarivanju. Istražena su mehanička svo-jstva i mikrostruktura ove zone. Posebna je pažnja posvećena njenom ponašanju tijekom cikličkog opterećenja kada je prisutna koncentracija naprezanja. Koncentracija naprezanja je slična onoj kod realnog zavarivanja. Određena je S-N krivulja i granica umaranja. Također je određena i Parisova krivulja i prag propagacije pukotine. Ključne riječi: Zavar, grubozrnata ZUT, struktura, mehanička svojstva, granica umaranja, Parisova krivuljaINTRODUCTIONA material is heated locally during an arc welding. Consumable material and a part of a base material are melted and solidiﬁ ed into weld metal during the cool-ing. The heat input needed for welding increases mate-rial temperature in the vicinity of the weld metal. Some microstructure changes appear in high heated area which is not melted. This region is called the heat af-fected zone (HAZ).Part of the HAZ that is heated the most is found nearby the weld metal, therefore crystal grains are coarsening during the heating. The coarse grain micro-structure, called the coarse grain heat affected zone (CG HAZ), results from this process; see Figure 1.In some cases, martensitic microstructure can ap-pear in the HAZ, if material contains enough carbon or other elements which increase hardness. The marten-sitic microstructure is usually hard and brittle with low impact toughness, and is therefore not always conven-ient for welded joints.In order to achieve better microstructure, preheating is used before welding and higher heat input is used dur-ing welding to reduce cooling speed of heated material. Result of such process is less hard and less brittle micro-structure. Heat input does not need to be too high because of grain coarsening and reduce of the HAZ impact tough-ness. Post weld heat treatment is also used to improve the microstructure, but some types of material do not allow post weld heat treatment like micro-alloyed steels. This is the reason why in some cases martensitic microstructure stays in the HAZ. Taking this into account, stress concen-tration can appear in welds due to weld toe’s shape where inconvenient coarse grain microstructure exists. This re-gion could cause problems especially when welded joint is loaded by cycle loading.Figure 1  Welded joint; BM (base material), HAZ (heat aff ected zone), WM (weld material)The aim of this study was to assess the 17CrNiMo7 CG HAZ microstructure and properties, and to deter-mine its behaviour during the cycle loading under stress concentration.MATERIALThe steel 17CrNiMo7 was used to prepare samples of CG HAZ microstructure. Chemical composition and 302  METALURGIJA 51 (2012) 3, 301-304T. VUHERER et al.: BEHAVIOUR OF COARSE GRAIN HEAT AFFECTED ZONE DURING CYCLE LOADINGmechanical properties of the steel are shown in Tables 1 and 2.EXPERIMENTAL PROCEDUREGMAW welding process with low heat input was simulated. The CG HAZ microstructure was investi-gated. It was prepared by using a weld thermal cycle simulator. The applied weld thermal cycle is shown in Figure 2.The peek temperature was 1300 °C, the holding time was 3 s on the 1300 °C and cooling time between 800 °C to 500 °C was 5 s. The crystal grains coarsened till 200 µm during simulation. Short cooling time ensured the martensitic transformation.Fifteen cylindrical specimens with a groove were prepared for rotation bending fatigue tests. The groove in the middle of the specimen causes the stress concen-tration during loading like a weld toe at real weld [1]. Other authors reported that the stress concentration due to the weld toe was approximately 1,74 [1,2,3] for gas metal arc welding. In order to obtain the same stress concentration in the specimens, the speciﬁ c geometry of the groove was used.  (1)The factors K1, K2, K3 and K4 depend on the type of loading (tension, bending, torsion, etc.), h is depth of the groove (D – d)/2, D is specimen diameter, d is in-side diameter of the groove. In the case of bending load-ing K1, K2, K3 and K4 are calculated by equation 2 [5].(2)Geometry of the grooved specimens used for this in-vestigation is shown in Figure 3. The groove causes the stress concentration of 1,74 during bending loading.Table 1 Chemical composition of the steel (weight / %)C Si Mn P S0,18 0,22 0,43 0,012 0,028Cr Ni Cu Mo Al1,56 1,48 0,15 0,28 0,023Table 2 Mechanical properties of the steelRp0.2/ MPaRm/ MPaA5/ %Z/ %Hardness/ HV 10489 633 26 72 217The groove was designed according to equations suggested by Peterson and Young [4,5]; in that way the theoretical stress concentration factor was calculated by using the equations 1 and 2, when the specimen was loaded by bending.Figure 2 Infl uence of weld thermal cycle on CG HAZ materialFigure 3 Specimen for rotation bending fatigue testingFigure 4 Preparation of the samples for TEM microscopeFatigue tests were done on the rotation bending ma-chine (Amsler UBM 200) in order to obtain a fatigue limit and behaviour of CG HAZ material during cycle loading. Fatigue crack growth tests were done on the Amsler cracktronic machine in order to obtain threshold for crack propagation and Paris curve, and ﬁ nally, anal-ysis of the microstructure was performed on light and TEM microscopes. Preparation of the samples for TEM microscope is shown schematically in Figure 4. The 303METALURGIJA 51 (2012) 3, 301-304T. VUHERER et al.: BEHAVIOUR OF COARSE GRAIN HEAT AFFECTED ZONE DURING CYCLE LOADINGCG HAZ material is only in grey region of the speci-men (see Figure 4). The samples for the TEM micro-scope were prepared from this region of the specimen.RESULTS AND DISCUSSIONMicrostructure was analysed by the light microscope at magniﬁ cation of 100×. The microstructure consists of laths martensite, as shown in Figure 5. The size of crys-tal grains is approximately 200 µm. approximately from the hardness by applying equations from the BS 7448-2 [6] and equations proposed by Pav-lina and Tyne [7], but in such cases error occurs up to 10%. Equations 3 and 4 refer to the BSI standard and equations 5 and 6 were proposed by Pavlina and Tyne. (3) (4) (5) (6)(HV is the Vickers hardness number)The yield stress and the tensile stress are presented in Table 3. Figure 5  Microstructure of CG HAZ material; light microscope 100 ×Figure 6  Martensitic laths microstructure; martensitic laths (a) and grain boundary (b)Figure 7 Curve force versus depth during Vickers indentationFigure 8 S-N curve of the CG HAZDetails of the CG HAZ microstructure were ana-lysed by the TEM microscope. Builds-up are of long martensitic laths. Their thickness is from 0,25 to 2,5 µm. Laths being 0,6 to 1,1 µm thick predominate, but some small regions of thicker or thinner laths also exist. The martensitic laths are shown in Figure 6a, and the boundary between them is shown in Figure 6b. Disloca-tion density was measured by a secant method and by using diffraction images from the TEM microscope. Dislocation density is high especially in the vicinity of grain boundary, where 8,5 1014 ± 2,5 1014 m/m3.Mechanical propertiesVickers’ hardness was measured by using load of 98,1 N on instrumented Zwick machine. Hardness was 455 HV10. Results are shown in Figure 7, presenting the curve force versus depth.It was not possible to measure the tensile strength and a yield stress by mechanical testing, because simu-lated HAZ region was too small. They were calculated Table 3  Yield stress and tensile strength CG HAZ and base materialMaterial Rp0.2 / MPa Rm / MPaCG HAZ 1271 (1) 1526 (1)1218 (2) 1599 (2)Steel 17CrNiMo7 474 (1) 761 (1)519 (2) 692 (2)(1) BSI 7448-2 standard (equations 3 and 4)(2) proposed by Pavlina and Tyne (equations 5 and 6)Results of fatigue testingResults of fatigue testing on the rotation bending machine are shown in Figure 8. 304  METALURGIJA 51 (2012) 3, 301-304T. VUHERER et al.: BEHAVIOUR OF COARSE GRAIN HEAT AFFECTED ZONE DURING CYCLE LOADINGLoad ratio R was -1 due to rotation bending. The continuing line represented the S-N curve of the CG HAZ microstructure. The fatigue limit was relative-ly low, only 330 MPa. This was the lowest stress limit beyond which specimen did not break after 10 million cycles. Results of fatigue crack growth testSpecimens of 10 × 10 × 55 mm were used for the fatigue crack growth test. Before starting the experi-ment, special thin foil crack gauge was attached on one side of the specimen. It was used for measuring the crack propagation during the loading. The 2 mm-long precrack was prepared with the low ∆K. The crack threshold was obtained step by step by reducing ∆K from ∆K, which was used for precrack creation. Crack threshold was low, only 10,7 MPam0.5. Test was contin-ued by increase of ∆K until the breakage of specimens. Results are shown in Figure 9. propagation. Paris law is valid in the middle liner part and is explained by equation 7.  (7)The constant C is 1,7 10-13 and the constant m is 3,6 for CG HAZ material.CONCLUSIONSThe CG HAZ microstructure consists of laths mat-rensite. Its hardness is 455 HV 10. The dislocation den-sity at the boundary of the martensitic laths is 8,5 1014 ± 2,5 1014 m/m3. The fatigue limit of the CG HAZ mate-rial is relatively low, according to its yield stress. It is only on one fourth of the yield stress. If compared to the unaffected base material, it is on two third of its yield stress, which is acceptable for welded joints, because base material fatigue limit will be lower. The threshold for crack propagation is low, being only 10,7 MPam0.5.REFERENCES[1] T. Vuherer, A. Godina, Z. Burzic, V. Gliha: Fatigue crack initiation from microstructurally small vickers indenta-tions, Metalurgija 46(2007)4, 237-243.[2] T. Vuherer, Lj. Milović, V. Gliha. Behaviour of small cracks during their propagation from Vickers indentations in coarse-grain steel: an experimental investigation. Int. j. fatigue, 33(2011)12, 1505-1513[3] T. Vuherer: Analiza vpliva mikro napak na trdnost pri utrujanju grobozrnatega TVP na zvarih (Analysis of in-luence of micro defects on fatigue strength of coarse grain HAZ in welds), Maribor, University of Maribor, 2008.[4] R.E. Peterson: Stress Concetration Factors, John Wiley & soons, New York, 1974.[5] W.C. Young: Roark’s Fourmulas for Stress Concentration Factors, New York, McGraw-Hill Book Company, 1989.[6] 7448-2 B. Fracture mechanics toughness tests, Method for determination of KIc, critical CTOD and critical J values of welds in metallic materials, British Standards Institu-tion, London, 1997.[7] E.J. Pavlina, C.J. Van Tyne: Correlation of Yield Strength and Tensile Strength with Hardness for Steels, J Mater Eng Perform, 17 (2008), 888-893.Note:  Proofreading; Martina Šuto, M.A., J.J. Strossmayer University of Osijek, CroatiaFigure 9 Paris curve of the CG HAZ materialObtained Paris curve represents CG HAZ material behaviour in case of crack and speed of eventual crack 

Ponašanje grubozrne zone utjecaja topline (ZUT) zavara tijekom cikličkog opterećenja

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