Very long life fatigue behaviors of 16Mn steel and welded joint

Yongjie Liu,Chao He,Chongxiang Huang,Muhammad K. Khan,Qingyuan Wang 국제구조공학회 2014 Structural Engineering and Mechanics, An Int'l Jou Vol.52 No.5

Very long life fatigue tests were carried out on 16Mn steel base metal and its welded joint byusing the ultrasonic fatigue testing technique. Specimen shapes (round and plate) were considered for both the base metal and welded joint. The results show that the specimens present different S-N curve characteristics in the region of 105-109 cycles. The round specimens showed continuously decreasingtendency while plate specimens showed a steep decreasing step and an asymptotic horizontal one. The fatigue strength of round specimen was found higher than plate specimen. The fatigue strength of as-welded joint was 45.0% of the base material for butt joint and 40% for cruciform as-welded joint. It was found that fracture can still occur in butt joint beyond 5×106 cycles. The cruciform joint has a fatigue limit in the very long life fatigue regime (107-109 cycles). Fatigue strength of butt as-welded joint was much higher as compared to cruciform as-welded joint. Improvement in fatigue strength of welded joint was found due to UPT. The observation of fracture surface showed crack mainly initiated from welded toe at fusion areas or geometric discontinuity sites at the surface in butt joint and from welded toe in cruciform joint.

Influence of heterogeneous mechanical properties on the performance of forward static bending test of rail welded joint

Pingpo Fu,Zhiming Zhu 대한기계학회 2018 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.32 No.9

The contact FE model for forward static bending test of the rail joint welded by narrow gap arc welding (NGAW) was established, in which the welded joint with heterogeneous mechanical properties was divided into multiple regions with gradual and step-changed mechanical properties determined according to the hardness distribution characteristics of the actual rail welded joint. Then the stress and strain distribution of the heterogeneous rail welded joint was calculated and analyzed under the forward static bending load, referring to the standard of static bending test for the rail joint welded by flash butt welding. The calculated results indicate that the magnitude of equivalent stress (Von Mises stress) presents positive correlation with the value of strength in the rail welded joint, while the failure tendency presents negative correlation with the value of strength, and the lowest strength region in HAZ is the weakest and most vulnerable part of the rail welded joint. Therefore, increasing the strength of soften zone in HAZ by appropriate post-weld heat treatment process is effective to improve the performance of forward static bending test. Further analysis was made by adjusting the value of strength in the weld zone within the heterogeneous rail welded joint, and it was found that the weld zone with high strength match could reduce the value of Von Mises stress of all other zones and improve the performance of forward static bending test of the rail welded joint. All these researches are expected to provide valuable reference and guidance for the adjustment and optimization of the process of NGAW and post-weld heat treatment, and the improvement of the performance of static bending test of rail welded joint.

Very long life fatigue behaviors of 16Mn steel and welded joint

Liu, Yongjie,He, Chao,Huang, Chongxiang,Khan, Muhammad K.,Wang, Qingyuan Techno-Press 2014 Structural Engineering and Mechanics, An Int'l Jou Vol.52 No.5

Very long life fatigue tests were carried out on 16Mn steel base metal and its welded joint by using the ultrasonic fatigue testing technique. Specimen shapes (round and plate) were considered for both the base metal and welded joint. The results show that the specimens present different S-N curve characteristics in the region of $10^5-10^9$ cycles. The round specimens showed continuously decreasing tendency while plate specimens showed a steep decreasing step and an asymptotic horizontal one. The fatigue strength of round specimen was found higher than plate specimen. The fatigue strength of as-welded joint was 45.0% of the base material for butt joint and 40% for cruciform as-welded joint. It was found that fracture can still occur in butt joint beyond $5{\times}10^6$ cycles. The cruciform joint has a fatigue limit in the very long life fatigue regime ($10^7-10^9$ cycles). Fatigue strength of butt as-welded joint was much higher as compared to cruciform as-welded joint. Improvement in fatigue strength of welded joint was found due to UPT. The observation of fracture surface showed crack mainly initiated from welded toe at fusion areas or geometric discontinuity sites at the surface in butt joint and from welded toe in cruciform joint.

Study of Fire Resistance Performance of Stiffened Welded Hollow Spherical Joint Under Axial Tension

Xiaobin Qiu,Tao Chen,Bingsheng Huang,Jingrui Zhu,Zhen Zhang,Haoyu Song 한국강구조학회 2023 International Journal of Steel Structures Vol.23 No.2

To study the fire resistance performance of stiffened welded hollow spherical joints under axial tension, two specimens with load ratios of 0.4 and 0.6 were subjected. The temperature distribution, failure mode, and fire resistance performance of stiffened welded hollow spherical joints under axial tension were obtained. The test results show that the failure mode of the joint was a pull-out failure. During the heating process, the closer to the spherical equator, the higher the spherical temperature, and the temperature of the sphere at the stiffener is lower than that of the sphere at the non-stiffener. The load ratio has a great influence on the refractory performance of the stiffened welded hollow sphere joint. When the load ratio of the specimen is reduced from 0.60 to 0.40, the fire resistance time of the specimen increases by 4.47 min, and the critical temperature increases by 21.3 °C. According to the European Code, the finite element model for the stiffened welded hollow spherical joints is established. The effect law of various influencing factors on the fire resistance performance of stiffened welded hollow spherical joints is studied. The research results show that reducing the load ratio and increasing the thickness of the sphere can significantly improve the fire resistance performance of the joint. The calculation formula of the critical temperature of steel members in the current standard not suitable for the calculation of the critical temperature of the welded hollow spherical joint. The critical temperature values with different load ratios recommended by the current standard were relatively more conservative. Taking the ambient temperature as the fire resistance temperature, the finite element analysis results were fitted, and the formula for calculating the fire resistance temperature of the stiffened welded hollow spherical joint was proposed.

Fatigue Strength Improvement of Welded Joints by Blast Cleaning for Subsequent Painting

김인태,정영수 한국강구조학회 2013 International Journal of Steel Structures Vol.13 No.1

In the fabrication of steel bridges, prior to painting, blast cleaning is performed to clean the surfaces and to increase the adhesive properties of the surfaces for the subsequent painting. In addition to these objectives, blast cleaning can improve the fatigue strength of welded joints, as a result of the impact of forcibly propelled abrasive materials on the welded joints. A series of fatigue tests were carried out on the five different types of welded joints: one-side out-of-plane gusset fillet welded joints,both-side out-of-plane gusset fillet welded joints, non-load-carrying rib fillet welded cruciform joints, butt welded joints, and a large-scale model beam with out-of-plane gusset welded joints, and under the three different types of load conditions: uniaxial tension, out-of-plane bending, or in-plane bending stress cycles. Drawing on previous fatigue test results that include unpublished data, this paper presents the beneficial effects of blast cleaning for the subsequent painting applications on the fatigue strength improvement of welded joints and also presents a simple quality control method for the blast cleaning process that improves the fatigue strength.

Critical effect of heat input on the joint quality of resistance element welding

박영도,Md Abdul Karim,진우성,남대근 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11

This study evaluates the influence of heat input on the joint formation and mechanical performance of resistance element welding (REW) between press-hardened steel and aluminum alloy sheets. REW is the joining process of combining thermal (metallic bond between the element and bottom-sheet similar materials) and mechanical (interlock between the element-head and top-sheet dissimilar materials). REW has already been identified as a superior joining process with comparatively higher joint strength than conventional welding or riveting between steel and aluminum dissimilar materials. However, the intensity of heat input is critical for REW joint quality due to the possibility of aluminum sheet melting around the element shank. Excessive melting of aluminum sheets reduces the load-bearing area of the joint; in addition, it forms weak Fe-Al intermetallic compounds. The competition between the weld nugget and aluminum sheet determines the joint strength. When the heat input increases, the failure mode transits from weld interfacial to aluminum sheet failure. The nugget dominates joint strength if the heat input is comparatively low; however, the aluminum sheet plays a significant role with the increase of heat input. The maximum peak load and energy absorption of the REW joints enhance with heat input until a critical value. The heat input has a negative effect on joint strength after the critical value because of aluminum sheet melting and squeezing out and subsequent reduction of load-bearing area of the aluminum sheet. Therefore, increasing nugget size cannot improve joint strength in this stage. Conversely, the joint strength slightly decreases due to the reduction in the load-bearing area; furthermore, the strength of the base aluminum sheet degrades due to the formation of Fe-Al intermetallic compounds. Consequently, determining the appropriate heat input level is crucial to achieving an excellent performance of the REW joints. The finite element method (FEM) can effectively predict the nugget size and aluminum sheet melting characteristics to determine the level of heat input before mass production. In this present study, the simulation results satisfy the experimental observations of the aluminum sheet melting characteristics and achieve less than 11.5% of error in nugget size.

Numerical Analysis for Controlling Residual Stresses in Welding Design of Dissimilar Materials Girth Joints

Jie Xia,Hui Jin 한국정밀공학회 2018 International Journal of Precision Engineering and Vol.19 No.1

Dissimilar metal girth welded joints have been widely used due to their efficient implementation and cost-effectiveness. It is of critical significance to understand the knowledge of residual stresses by numerical technology to determine precision requirements for manufacturing practice. A multi-layer welding process of a dissimilar welded joint is simulated in this literature by coupled thermalmechanical analysis. The welding simulation model is verified by previous experimental work. The magnitudes and distributions of residual stresses in a dissimilar girth pipe joint are predicted. This study describes the influence of welding sequences and the welding start/stop positions on residual stress profiles by the simulation model. Constraint conditions is considered to analyze residual stresses in such a dissimilar welded joint. The results demonstrate that residual stresses are asymmetrical in dissimilar pipe joints due to the difference in material properties. Residual stress profiles are considerably different since the effect of welding sequence and welding start/stop positions. The variations of residual stresses on both sides of the weld centerline are more complex than those in similar welded joints. Optimized welding sequence and appropriate constraint conditions are necessary to control welding residual stress and distortion of such dissimilar welded structures.

Three-dimensional weld toe magnification factors for various welded joints

한정우,이권희,한승호 대한기계학회 2012 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.26 No.7

To effectively calculate the stress intensity factors in welded components, a weld toe magnification factor is introduced here that will allow for the influence of geometrical effects. This factor was derived from the data obtained in a parametric study performed by FEM. Several sets of weld toe magnification factor solutions have been presented, but these are applicable only to non-load-carrying cruciform or T-butt joints, due possibly to the requirement of very complicated calculations. In the majority of cases for various welded joints such as cruciform, cover plate and longitudinal stiffener joints, the currently used weld toe magnification factors do not adequately describe the behavior of weld toe cracks. In the present study, the weld toe magnification factor solutions with exponents and fractions that have polynomial functions in terms of a/c and a/t - i.e., crack depths normalized by corresponding crack lengths and specimen thickness - were developed through a parametric study using 3-dimensional finite elements for the above mentioned three types of welded joints. These weld toe magnification factor solutions showed a tendency to increase at a/t < 0.2. Meanwhile, for 0.2 < a/t < 0.7, the effect of the attachment on the weld toe magnification factor decreased asymptotically. When the a/t ratio exceeded this range, the weld toe magnification factor became almost one of unity. The fatigue crack propagation life was evaluated by using the proposed weld toe magnification factor and by considering the propagation mechanisms of multiple-surface cracks, and it showed good agreement-to within a deviation factor of 2-between the experimental and calculated results for the fatigue crack propagation life for all welded joint.

철도차량 차체 필렛 용접이음재의 피로설계에 관한 연구

김영신(Yeong-sin Kim),배동호(Dong-ho Bae),박근수(Keun-soo Park) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11

Stainless steel sheets are widely used as the structural material for the railroad cars and the commercial vehicles. These kinds structures used stainless steel sheets are commonly fabricated by using the gas welding. For fatigue design of gas welded joints such as fillet and plug type joint, it is necessary to obtain design information on stress distribution at weldment as well as fatigue strength of gas welded joints. And also, the influence of the geometrical parameters of gas welded joints on stress distribution and fatigue strength must be evaluated. Therefore, in order to establish a reasonable and systematic fatigue design criterion for the long life design of the gas welded body structure, in this study, stress distribution around the gas welded joints subjected to tensile load was numerically analyzed. Also, the ΔP-N<SUB>f</SUB> curves were obtained by fatigue tests. Using these results, ΔP-N<SUB>f</SUB> curves were rearranged in the Δσ-N<SUB>f</SUB> relation with the maximum stress at the edge of fillet welded joint.

TP310HCbN 용접 이음부의 미세조직과 고온 기계적 물성

황정호(Jeong Ho Hwang),송근동(Geun Dong Song),김대웅(Dae-Woong Kim),정인현(In Hyun Cheong),홍성구(Seong-Gu Hong),한준희(Junhee Hahn) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.4

TP310HCbN 은 고온에서의 내식성 및 기계적 특성의 탁월한 조합으로 인해 초초 임계 급 화력발전 보일러 튜브 열교환기의 핵심 후보 소재이다. 본 연구에서는 모재와 용접 접합부의 미세 구조 특성, 인장 및 저주기 피로 특성을 평가했다. 용접 금속과 모재는 완전 오스테나이트 조직으로 입증되었고, 용접 금속의 하위 입자인 dendrite core는 nanoindentation 시험을 통해 soft zone으로 입증되었다. 용접 금속에서 인장 파괴가 발생했으며, 이는 용접 금속에 soft zone 이 존재하기 때문이다. 용접 이음의 강도는 모재보다 훨씬 낮게 평가되었으나 최대 허용 응력 계산 결과는 국제 규격보다 높기 때문에 신뢰성이 확보되었다. 용접 이음의 피로 파괴는 용접 금속에서 발생하였으며, 용접 이음의 파괴 거동은 피로 균열이 dendrite core에서 시작되어 이 영역으로 전파되었음을 보여주었다. 용접 이음부와 모재 모두 피로 변형 시 주기적으로 경화되므로 소성 변형 에너지 밀도가 피로 수명 예측에 가장 적합한 피로 매개 변수로 확인되었다. 피로 수명의 온도 의존성과 용접 공정에 대한 피로 저항 감소를 동시에 고려할 수 있는 에너지 기반의 통합 수명 예측 모델이 제안되었으며, 적용가능성을 실험 데이터와 비교하여 확인하였다. TP310HCbN constitutes a key candidate material for thermal power boiler tube heat exchangers operating under ultra-supercritical conditions due to its excellent combination of corrosion resistance and mechanical properties at elevated temperature. In this study, microstructural characteristics, tensile and low-cycle fatigue properties were evaluated for the base metal and the welded joint. Our results showed that the weld metal and base were proved as fully austenite grain, and the dendrite core which was subgrain of the weld metal, was proved as softest zone at welded regions by nanoindentation test. As for the tensile behavior, the tensile-fracture occurred at the weld metal, it was attributed to the presence of the soft zone at weld metal. The strength of the welded joint was evaluated much lower than that of base metal, however, the maximum allowable stress calculation results were higher than those of international standard, therefore, the integrity of was secure. The fatigue fracture of the welded joint occurred at weld metal, the fracture behaviors of welded joint showed that fatigue crack initiated at the dendrite core and propagated to this region also. As both the welded joint and the base metal periodically hardened during fatigue deformation, the plastic strain energy density was found to be the most suitable fatigue parameter for fatigue life prediction. An energy-based unified life prediction model was proposed that can consider simultaneously the temperature dependence of fatigue life and the fatigue resistance reduction to welding process. Its effectiveness was verified by comparison with experimental data.