Characteristics of High Temperature Tensile Properties and Residual Stresses in Weldments of High Strength Steels

Transcription

1 Materials Transactions, Vol. 47, No. 2 (26) pp. 348 to 354 #26 The Japan Institute of Metals Characteristics of High Temperature Tensile Properties and Residual Stresses in Weldments of High Strength Steels Kyong-Ho Chang 1 and Chin-Hyung Lee 2 1 Dept. of Civil & Environmental Engineering, Chung-Ang University, Seoul , Korea 2 Institute of Technology and Science, Chung-Ang University, Seoul , Korea In this study, high temperature tensile properties of high strength steels, POSTEN6 and POSTEN8, whose tensile strengths were 6 and 8 MPa respectively, were investigated through the elevated temperature tensile test. Residual stress measurements were also carried out to estimate the residual stress relaxation due to phase transformation (martensite transformation) in the process of cooling after welding. A finite element (FE) model which was able to include the volumetric changes due to the austenite! martensite phase transformation was developed on the basis of the experimental results. The three-dimensional thermal elastic-plastic FE analyses using the FE model were conducted to determine residual stresses in weldments of the high strength steels. The results show that the extents of residual stress relaxation due to the austenite! martensite phase transformation in the process of cooling after welding are approximately.85 x = Y and.75 x = Y in the FZ and HAZ of POSTEN6 and POSTEN8, respectively. And residual stresses of weld line direction in the base metal (BM) which is adjacent to HAZ, therefore, do not undergo martensitic transformation increase (655 MPa < 87 MPa) with increasing tensile strength of the high strength steels (POSTEN6 < POSTEN8). (Received August 19, 25; Accepted December 2, 25; Published February 15, 26) Keywords: high temperature tensile properties, high strength steels, martensite phase transformation, residual stress relaxation 1. Introduction As the spans of bridges are getting longer and the stories of buildings are getting higher, there are strong demands for steels with high strength. The application of high strength steels makes it possible to design not only light weight structures, but also simple structures with simple weld details. But the fabrication of structural member using high strength steels always involves welding process. And due to the localized heating during welding, complex thermal stresses are inevitably generated. Residual stresses are stresses that remain in a material as a result of liquid-tosolid phase transformation associated with weld solidification and the subsequent non-uniform cooling of the weld altered by phase transformation (martensite transformation) in the solid state. And it is well known that, especially in high strength steel weldments, the solid-state transformation on cooling of austenite to martensite could have a major influence on the relaxation of residual stresses. 1 4) Residual stresses that develop in and around the welded joint are detrimental to the integrity and the service behavior of the welded part. High tensile residual stresses in the region near the weld might promote brittle fracture, reduce the fatigue life, and promote stress corrosion cracking during service. So, it is very important to clarify the characteristics and production mechanism of residual stress. The examination of residual stress is generally performed through numerical method due to the limitation of experimental investigation as structure becomes more complex. And the mechanical property changes depending on temperatures are required for the welding simulation by numerical method. The present research was undertaken to determine residual stresses in weldments of POSTEN6 and POSTEN8, newly developed high strength steels for the use in bridges, pressure vessels and pipelines and offshore construction, etc. High temperature tensile properties of the high strength steels were investigated through the elevated temperature tensile test. Residual stress measurements were also carried out to investigate the residual stress relaxation due to phase transformation (martensite transformation) in the process of cooling after welding. A finite element (FE) model which was able to include the volumetric changes due to the austenite! martensite phase transformation was developed on the basis of the experimental results. The model also included the mechanical property dependence on temperatures through the elevated temperature tensile test. Threedimensional thermal elastic-plastic FE analyses using the FE model were conducted to determine residual stresses in weldments of the high strength steels. 2. High Temperature Tensile Properties of the High Strength Steels The elevated temperature tensile tests were carried out to determine the mechanical properties of the high strength steels at elevated temperatures which were the required input to the FE code. 2.1 Test specimens The chemical compositions of POSTEN6 and POST- EN8 are shown in Table 1. The carbon equivalents (C eq ) of the high strength steels calculated using the formula: C eq ¼ C þ Si/24 þ Mn/6 þ Ni/4 þ Cr/5 þ Mo/4 þ V/14 are.4 and.46%, respectively. The dimensions of tensile test specimens were determined in accordance with KS D 26 5) as shown in Fig. 1. There were spirals at both ends of each specimen to enable fixing to the loading shafts located at the top and bottom ends of a furnace. These spirals were designed so that they did not affect the specimen fracture that occurred in the middle of the specimen. 2.2 Test procedure An universal testing machine equipped with a specially made electrical furnace heated by thermal rays was used for

2 Characteristics of High Temperature Tensile Properties and Residual Stresses in Weldments of High Strength Steels 349 Table 1 Chemical compositions of materials used (mass%). Materials C Si Mn P S SAL Cr Ni Cu V Mo B POSTEN POSTEN :9 þ unit:mm over over 3 8 UTS YS Fig. 1 Dimensions of tensile test specimens Temperature, T/ o C 1 8 UTS YS Fig. 2 Test rig Fig. 3 Yield stress and ultimate tensile strength at elevated temperatures: POSTEN6 steel, POSTEN8 steel. the elevated temperature tensile tests (see Fig. 2) and a hydraulic actuator was used to apply the tension load to specimens. A load cell was used to measure the applied tension load, whereas the elongation of the specimen was measured in the middle of the specimen using a specially modified extensometer. Tests were carried out in the elevated temperature range of 2 to 8 C at intervals of 1 C with a strain rate of 1 mm/ min. And the temperature was controlled to within 2 C. In this study, thermal expansion was allowed by maintaining zero tension load during the heating process. Each specimen was held for approximately 2 min at the testing temperature before testing began to make sure the temperatures evenly distributed throughout the specimens. After each test, data for load versus displacement were converted into engineering stress versus strain curves which were analysed to determine the yield stress (YS), ultimate tensile strength (UTS), percentage elongation to fracture and elastic modulus. 2.3 Deterioration of mechanical properties with increasing temperature The variations of YS and UTS of high strength steels with testing temperature are shown in Fig. 3. It is seen that both YS and UTS of POSTEN6 steel decrease at 1 C and then increase with increasing temperature up to 3 C due to blue Elastic Modulus, σ / GPa POSTEN6 POSTEN Fig. 4 Elastic modulus at elevated temperatures. brittleness, after which they decrease for all conditions. On the other hand, YS and UTS of POSTEN8 steel decrease with increasing temperature up to 2 C and then increase at 3 C due to blue brittleness, after which they decrease slowly until 6 C. Elevated temperature also leads to the deterioration of elastic modulus. Figure 4 shows the variations of elastic modulus at elevated temperatures. From this figure, it can be seen that elastic modulus of POSTEN6 steel

3 35 K.-H. Chang and C.-H. Lee o C 3 o C RT unit=mm End Tap o C 4 o C 5 o C 6 o C 7 o C L= o C Strain (%) 1 o C 3 o C 2 o C 4 o C Strain (%) RT 5 o C 6 o C 7 o C 8 o C 6 2/3 1/3 3 6 B=5 Fig. 6 Joint configuration. Table 2 Chemical compositions of weld metals (mass%). Weld metal C Si Mn P S SUPERCORED MGS Fig. 5 Stress strain curves at elevated temperatures: POSTEN6 steel, POSTEN8 steel. Table 3 Mechanical properties of weld metals. decrease slowly with increasing temperature up to 4 C and then decrease radically at higher temperatures. On the other hand, elastic modulus of POSTEN8 steel decrease radically with increasing temperature up to 4 C and then decrease slowly until 6 C, after which they decrease radically. The stress strain curves obtained at elevated temperatures are also shown in Fig. 5. As like room temperature, POSTEN6 specimens show higher strain hardening rate than that of POSTEN8 specimens and strain hardening rate decrease radically after 4 C for both specimens. 3. Experimental Investigation into Residual Stresses in Weldment of High Strength Steels Residual stress measurements together with metallographic observations were performed to investigate the microstructures and the extent of residual stress relaxation due to phase transformation in the FZ and HAZ of high strength steels. Rolled plates of 3 mm thickness have been used as the base materials. Double V butt joint configuration, as shown in Fig. 6, has been prepared for joining the plates. The joint for the specimen of POSTEN6 was welded in the flat position with six passes using flux cored arc welding (FCA) procedure with SUPERCORED81 electrode of 1.2 mm in diameter and the joint for the specimen of POSTEN8 was welded with five passes using gas metal arc welding (GMA) procedure with MGS-8 electrode of 1.2 mm in diameter. The chemical compositions and mechanical properties of the weld metals are presented in Tables 2 and 3, respectively. The welding conditions and process parameters used in the fabrication of the joints are given in Table 4. Weld metal Yield stress (MPa) Ultimate strength (MPa) Elongation (%) SUPERCORED MGS Table 4 POSTEN6 PASS Current (A) Welding conditions and process parameters. Voltage (V) Velocity (Time) Remarks Preheat temperature ( C) Turn Over Interpass (Gausing) temperature ( C) Under POSTEN8 PASS Current (A) Voltage (V) Velocity (Sec) Remarks Preheat temperature ( C) Turn Over (Gausing) Interpass temperature ( C) Under 2

4 Characteristics of High Temperature Tensile Properties and Residual Stresses in Weldments of High Strength Steels 351 L = 5 (unit = mm) x 5 µm B = y Fig. 8 Specimen with measurement locations. (c) 5 µm 5 µm Residual Stress, σ x /σ Y FE w/o phase transf. Experiment Distance from the weld center line, l /mm (d) 5 µm Fig. 7 Light optical microstructure of the FZ and HAZ specimens: POSTEN6 (FZ 5), POSTEN6 (HAZ 5), (c) POSTEN8 (FZ 5), (d) POSTEN8 (HAZ 5). Residual Stress, σ x /σ Y FE w/o phase transf. Experiment Distance from the weld center line, l /mm Fig. 9 Residual stress comparison with the case of no phase transformation effect: POSTEN6, POSTEN Microstructure Microstructures were analyzed using a OLYMPUS PME3 optical microscope after welding. Samples used for microstructural analysis were cut from the FZ and HAZ. They were finally polished with 1 mm diamond paste on a cloth polishing wheel, and etched with Nital s etchant for about 2 3 s. Results are shown in Fig. 7. Figures 7 and, representing FZ and HAZ of POSTEN6, exhibit a dual phase structure (ferrite-plus-martensite). On the other hand, welding of POSTEN8 produced a microstructure nearly wholly martensitic in the FZ and HAZ [refer to Figs. 7(c) and (d)]. Therefore, it can be inferred that the FZ and HAZ heated over the austenitic temperature during welding undergo martensitic transformation and the extent of martensitic transformation in POSTEN8 is larger than that of POSTEN Residual stress measurements Residual stress measurements were carried out on the twoaxis strain gauge with the saw cutting method. Figure 8 shows the measurement positions which are located on the specimen. Gauges were intensively attached at the FZ and HAZ in order to investigate the residual stress relaxation due to phase transformation. Residual stresses were measured by calculating the difference between initial strains and residual strains after saw cutting. Figure 9 shows the residual stresses

5 352 K.-H. Chang and C.-H. Lee ρ, c, λ, a Density ρ ( g / mm 3 ) x 1-3 Specific heat c (J / g C ) x 1-1 Heat transfer coeff. 2-5 a ( J / mm s C) x 1 Heat conductivity λ ( J / mm s C ) x 1-2 Fig. 1 Analysis model Temperature, T/ C of weld line direction perpendicular to the weld line. A solid line represents the results of three-dimensional thermal elastic-plastic FEM analysis for the experimental model without taking into account the phase transformation effects. The calculating conditions for the analysis are the same as those used in the fabrication of the experimental model. Considerable differences between the experimental values and the analysis results in the FZ and HAZ are shown. Lower stresses in the FZ and HAZ can be explained by the volume change of the materials that undergo the austenite! martensite phase transformation. And the extents of residual stress relaxation in POSTEN6 and POSTEN8 are approximately.85 x = Y and.75 x = Y, respectively. 4. Numerical Investigation into Residual Stresses in Weldments of High Strength Steels 4.1 Model for analysis Figure 1 shows the coordinate systems and sizes for the analysis. Two 1 mm 5 7 mm plates were assumed to be welded by one pass with a heat input of 12 (J/mm) and welding velocity of 6 mm/s. The residual stress distribution was computed using an uncoupled thermo-mechanical FE formation 6 11) to incorporate the phase transformation effects in the thermal and mechanical analysis. The computation employed threedimensional, eight-noded, hexahedral elements in a full model and used temperature-dependent thermo-physical and mechanical properties of the BM. Figures 11 and show the temperature dependency of physical 6 11) and mechanical properties. The thermal analysis was based on the threedimensional non-steady heat conduction formulation with the moving heat input. In thermal analysis, the heat conduction into the surrounding solid materials as well as the convective heat transfer in ambient temperature was considered. Thermal and mechanical analyses were uncoupled and conducted sequentially. First, the thermal analysis was carried out calculating the transient temperature distributions during welding. The mechanical part relied on the thermal analysis results and calculated the stress strain distribution on the basis of the temperature history. The same FE mesh as in the thermal analysis was used in the mechanical model, except for the applied boundary conditions. The movement of bottom plate was restrained to approximately model the actual welding conditions. 4.2 Numerical reproduction of phase transformation Phase transformation is occurred by the temperature σ u (w2) σ yu (w2) σ y (w2) σ u (w1) σ yu (w1) σ y (w1) σ y : Yield Stress σ yu : Upper Yield Stress σ u : Ultimate Tensile Strength σ(w1) : POSTEN6 σ(w2) : POSTEN Temperature, T / C Fig. 11 Temperature-dependent thermo-physical and mechanical properties of the materials: Physical constants, Mechanical properties. changes due to rapid cooling after welding and temperature history is changed by the increase of specific heat due to the volumetric expansion originated in the phase transformation. And the volumetric expansion eventually becomes the cause of residual stress relaxation. Thus, while it is very difficult to reproduce the effect of phase transformation on residual stress relaxation, it is necessary to develop a FE model which is able to include the volumetric changes due to the austenite! martensite phase transformation for the accurate prediction of the mechanical behavior in welds. The correlation between temperature and metallography is largely dependent on specific heat (SH) change and the correlation between stress strain field and metallography is largely dependent on coefficient of thermal expansion (CTE) change. Figure 12 shows the change of specific heat and coefficient of thermal expansion in the region of phase transformation with respect to temperatures. Residual stress relaxation due to the austenite! martensite phase transformation in the process of cooling after welding was simulated by obtaining the value of Cp and which yield a good agreement with the experimental results through iterative analyses assuming specific heat and coefficient of thermal expansion were varied linearly in the transformation temperature range. The Cp and used to numerically model the volumetric expansion in the process of cooling after welding were 5: 1 1 (cal/g C) and 1:5 1 5 respectively for the case of POSTEN6 and 8: 1 1 (cal/g C) and 2:5 1 5 respectively for the case of POSTEN8. The thermo-physical and mechanical properties of the materials were used for the entire model during heating.

6 Characteristics of High Temperature Tensile Properties and Residual Stresses in Weldments of High Strength Steels 353 Specific Heat, Cp P-T end Cp modified P-T start original unit = c( cal / g oc ) x Residual POSTEN6 POSTEN Thermal Expansion Coefficient, α 3.E-5 2.E-5 1.E-5.E+ -1.E-5-2.E-5 P-T end α P-T start modified original Residual POSTEN8 POSTEN8-3.E Fig. 12 Variation of material properties within a transformation region: Specific heat, Thermal expansion coefficient Fig. 13 FE results with phase transformation: POSTEN6, POST- EN8. Depending on the peak temperature that a particular point reached during the heating transient, the decision was made whether the point underwent the martensitic transformation or not. The temperature of the materials transformed into the austenitic phase was determined from the equilibrium phase diagram. The volume change was approximated by introduction of a modified specific heat and coefficient of thermal expansion. The analysis distinguished between the heating and cooling cycle of each point and the modified CTE and SH curves were used (Fig. 12). Note that the part of the CTE and SH curves that represent the austenite! matensite phase transformation were only used on the cooling cycle for the points that satisfied the transformation criteria explained above. The onset temperature of phase transformation is determined through the empirical formulae of Andrews: 12) Ms = C 3.4Mn 12.1Cr 17.7Ni 7.5Mo. 4.3 Results and discussion Figure 13 shows the residual stress distribution perpendicular to the weld line at the center of the weldments (x ¼ 5 mm). Figures 13 and show the results of POST- EN6 and POSTEN8 respectively. The residual stresses of weld line direction in the FZ and HAZ which undergo martensitic transformation are as high as 47 and 545 MPa respectively, which correspond to each degree of residual stress relaxation due to the volume expansion by phase transformation. Stresses in the BM adjacent to HAZ increase significantly (655 and 87 MPa, respectively) and then gradually decrease to compressive stress in the far BM. The temperature in the BM adjacent to HAZ do not reach the Residual Residual POSTEN6 POSTEN POSTEN8 POSTEN Fig. 14 FE results without phase transformation: POSTEN6, POSTEN8. transformation temperature, therefore, no phase transformation occurs there, which means no volume change to offset high stresses. The results presented in Fig. 13 take into

7 354 K.-H. Chang and C.-H. Lee account the phase transformation effects. Another analysis was carried out without considering these effects, and the results are shown in Fig. 14. The results show a significant effect of phase transformation on residual stresses. This can be explained by the microstructural changes (Fig. 7) that occur during the austenite! martensite phase transformation in the FZ and HAZ that affect the SH and CTE which have a significant effect on the residual stress. 5. Conclusions This study presented high temperature tensile properties and a FE model for residual stress analysis in weldments of high strength steels which accounts for the phase transformation effects. Three-dimensional thermal elastic-plastic FE analyses using the FE model were conducted to determine residual stresses in weldments of high strength steels. The following conclusions can be made. (1) Both YS and UTS of POSTEN6 decrease at 1 C and then increase with increasing temperature up to 3 C due to blue brittleness, after which they decrease for all conditions. On the other hand, YS and UTS of POSTEN8 decrease with increasing temperature up to 2 C and then increase at 3 C due to blue brittleness, after which they decrease slowly until 6 C. (2) Elastic modulus of POSTEN6 decrease slowly with increasing temperature up to 4 C and then decrease radically at higher temperatures. On the other hand, elastic modulus of POSTEN8 decrease radically with increasing temperature up to 4 C and then decrease slowly until 6 C, after which they decrease radically. (3) The extents of residual stress relaxation due to the austenite! martensite phase transformation in the process of cooling after welding are approximately.85 x = Y and.75 x = Y in the FZ and HAZ of POSTEN6 and POSTEN8, respectively. (4) Residual stresses of weld line direction in the base metal (BM) which is adjacent to HAZ, therefore, do not undergo martensitic transformation increase (655 MPa < 87 MPa) with increasing tensile strength of the high strength steels (POSTEN6 < POSTEN8). REFERENCES 1) B. Taljat, B. Radhakrishnan and T. Zacharia: Mater. Sci. Eng. A 246 (1998) ) B. A. B. Anderson: Trans. ASME 1 (1978) ) K. Satoh and T. Terasaki: J. JWS 45 (1976) p ) K. Satoh and T. Terasaki: J. JWS 45 (1976) p ) KS D 26: Korean Standards, (1997) pp ) Y. C. Kim, K. H. Chang and K. Horikawa: Trans. JWRI 27-2 (1998) ) Y. C. Kim, K. H. Chang and K. Horikawa: Trans. JWRI 27-1 (1998) ) Y. C. Kim, K. H. Chang and K. Horikawa: Proc. WTIA. 46th Int. Conf. (1998) ) K. H. Chang, C. H. Lee and W. C. Cho: Proc. 6th Japan-Korea Joint Seminar on Steel Bridges (JSSB-JK6) (21) ) K. H. Chang, C. H. Lee and Y. C. Kim: Proc. 7th Int. Welding Symposium 2 (21) ) J. U. Park, C. H. Lee and K. H. Chang: Proc. of, IWC-KOREA (22). 12) K. W. Andrews: J. Iron Steel Inst. 23 (1965)