Fatigue Strength Improvement Due to Alloying Steel Weld Toes With WC-Tool Constituent Elements Through Friction Stir Processing

Friction stir processing (FSP) enables surface modifications using a rotational tool and can likely be applied as a new post-weld treatment for improving fatigue strength. When applying FSP to high strength materials, tool wear occurring at the interface between the tool tip and the topmost steel layer has been regarded as an unavoidable issue and is related to the tool rotational speed. The present study investigated the relationship between the tool rotational speed and fatigue strength of arc-welded high-strength low-alloy (HSLA) steel joints with weld toes subjected to FSP using a spherical-tip WC tool. FSP was conducted on the weld toe of HSLA steel joints with various tool rotational speeds. Tool wear increased with increase in tool rotational speed, and consequently contents of constituent elements of the WC tool increased in the topmost steel layer of weld toes, leading to large increase of fatigue strength. One reason for the increase with tool rotational speed is significant increase of solid solution hardening due to supersaturated W and C in the topmost steel layer consisting of martensite laths. The hardened topmost steel layer prevented fatigue crack initiation, and the increased fatigue strength depended on the contents of supersaturated W and C.

Geometric Parameters Optimization Design for Aluminum Alloy Welding Joint Based on Increasing Fatigue Strength

The geometric parameters of the welding joint had an important effect on fatigue strength, so the stress concentration of the typical double sided asymmetric V-notch joint on high speed train was calculated, the relationship between geometric parameters and the stress distribution of the welding joint was analyzed by FEM, also the geometric parameters optimization design of welding joint was proposed. According to theoretical optimization design scheme, the geometric parameters optimization adopted shaping the weld joint by manual TIG welding and processing circle arc on weld toes by milling respectively,. and the fatigue property of the welding joint optimized by different methods was tested. By comparing the experimental results, it was found that manual TIG welding was difficult to obtain the favorable weld toe angle to decrease the stress concentration, and the welding joint was softened obviously. Accordingly, it revealed that the fatigue property was lower than that of initial joint in fatigue testing. On the contrary, the radius of the weld toes circle arc can be controlled accurately by mechanical processing. The stress concentration was reduced obviously by rounding off the weld toe, as a result the fatigue strength of the welding joint could be increased remarkably. Keywords: A7N01, optimization design, geometric parameters of welding joint, FEM

Low-Cycle Fatigue Behavior of Large-Size Dissimilar Steel Welded Tube-Plate Structure

Based on the FEM analysis of the distributions and varieties of the stresses and the strains of the large-size dissimilar steel welded tube-plate structure under cyclic loading, the fatigue crack initiation sites of this structure were predicted. The low-cycle fatigue test was performed to verify the prediction and to study the cracks propagating paths as well. The results indicated that the weld toes linked to the two surfaces of the plate were the positions with the peak values of the maximum stress and the stress amplitude. The plastic deformations in the first loading cycle introduced the tensile-compressive repeated stress cycle at the weld toes during the subsequent loading cycles. It induced the fatigue cracks initiating at the weld toes linked to the two surfaces of the plate respectively. After initiating from the surfaces the cracks propagated along the fusion lines with a short distance then turn into the base metal in the sections vertical to the surfaces. The depth of the crack initiating from the compressed surface was shorter than the one from the tensioned surface.

Effect of Ultrasonic Impact Treatment on Fatigue Life in Butt Welded Joints of Austenitic Stainless Steel

Ultrasonic impact treatment (UIT), which is a type of peening method, is usually used as a post-weld treatment for mild steel in order to improve the fatigue strength of its welded joints. As there is insufficient fatigue data available on welded joints of austenitic stainless steel treated by UIT, the authors decide to conduct fatigue tests on butt welded joints of austenitic stainless steel treated by UIT. The results were compared with the fatigue lives of as-weld joints to investigate the effect of UIT on the fatigue lives of welded joints of austenitic stainless steel. The fatigue lives of butt welded joints treated by UIT were more than 1.5 times longer than those of as-weld joints. The following were considered as possible reasons for this improvement in fatigue life: change in residual stress near the weld toes, relaxation of stress concentration at the weld toes, and refinement of grains under the weld toes. The residual stress measured near the weld toes by using the X-ray diffraction method was transformed from tension to compression by the application of UIT. The stress concentration factors at the welded toes were reduced about 10% by the application of UIT.