WELDING RESEARCH was used for this coupled analysis instead of a full 3D model. Analysis Results and Discussions Effect of Coolto Temperature on CreepFatigue Strain and Stress The temperature history of the failed roll showed that the roll experienced an operating temperature between 982 and 1066C, with furnace temperature variations more than 500C. Occasionally, the furnace temperature dropped to 204 and 427C between temperature cycles. To identify the effect of the cool-to temperature between cycles on the creepfatigue strain and stress, sequentially coupled heat-transfer and creepfatigue analyses were conducted for two cases. Case 1 cooled the furnace to a temperature of 204C, and Case 2 cooled the furnace to a temperature of 427C between thermal cycles. Figure 9 shows the predicted distributions of effective creep strain for Case 1 and Case 2. The distributions of effective creep strain are identical between Case 1 and Case 2 since the only difference between these two cases is the temperature drops between loading cycles. Weld toes and weld root have high effective creep strain, as highlighted at locations 1–3 in Fig. 9. Location 1 has the highest creep strains among the three locations. The predicted creep strain is low (0.29%). Ten cycles were modeled, which are equivalent to a two-month time period. With the time increasing, the cumulated creep strain will be higher. Although the predicted creep strain is low, an elastic-plastic-creep analysis has to be performed to model the plastic and creep process during roll service. A sensitivity study showed that a pure elastic-plastic analysis cannot predict the same stresses as the elastic plastic-creep analysis. Figure 10 shows the predicted distributions 438-s WELDING JOURNAL / NOVEMBER 2016, VOL. 95 of the maximum principal stress for Case 1 and Case 2. Case 1 has a much higher maximum principal stress than Case 2. Compressive stresses were shown in the bottom of the weld with tensile stresses around it. These kinds of stress distributions resulted from the difference of the CTE expansion between the filler metal (N117) and the base metal (MORE ®1). The CTE of the base metal MORE ®1 is higher than the filler metal N117. At high temperatures, the base metal may expand more than the filler metal, while the filler metal restrains the base metal expansion to produce compressive plastic strains so the dimensions of the base metal become small. After cooling to a lower temperature, the filler metal will pull the base metal to the original dimensions. Thus, high tensile stresses show in the base metal and compressive stresses show in the bottom of the weld. Figure 11 shows that a crack initiated from the weld root and then propagated through the weld. The weld macrograph was prepared by cutting one of the failed rolls. The crack is most likely induced by the high tensile maximum principal stress shown at the weld root in Fig. 10A. Therefore, the experimental results indirectly verified the model predictions. Although compressive stresses are shown in the weld before cracking, the stress distributions could be changed after cracking propagates through the weld. The tension induced from the thermal and mechanical loads would overcome the compressive stress and induce tensile stresses in the crack tip during the crack propagation. Figure 12 shows the evolution of temperature, effective creep strain, and maximum principal stress at the weld root for Case 1. Effective creep strain and maximum principal stress increases as the time increases. The maximum principal stress increase results from the plastic strain accumulation. This result implies that the weld root is most likely the failure location since the stress range increases as the time increases. The weld-root crack shown in Fig. 11 confirms this analysis result. Figure 13A shows a hoop stress distribution in Case 2 where the weld is in compression, and the base metal near the weld is in tension. Hoop stress was plotted to understand the Fig 17. — Predicted maximum principal stress near the weld root after ten loading cycles with inside roll cooling. Fig. 18 — Predicted temperature, effective creep strain, and maximum principal stress near the weld root with inside roll cooling.
Welding Journal | November 2016
To see the actual publication please follow the link above