Jean-Luc Fihey

A COMPARISON OF RESIDUAL STRESS IN HAMMER-PEENED, MULTI-PASS STEEL WELDS - A514 (S690Q) AND S41500

As part of a program to assess the in-situ weldability and mechanical performance of the candidate high strength low alloy A514 (S690Q) steel as an alternative to the S41500 martensitic stainless steel for hydro-turbines, three aspects of the welds were studied: residual stress, Charpy toughness and cavitation erosion resistance. The experimental set-up involved robotized gas metal arc welding (GMAW), performed on U-groove and double-V weld preparation cut into 50 and 75 mm-thick steel plates. Half of the welds were robotically hammer-peened after each weld layer, except for the root pass. Strain gauges measured longitudinal and transverse strains during welding and hammer-peening. Once the weld cooled down to room temperature, the strain gauges provided the surface residual stress level at their location. Two-dimensional, sub-surface, longitudinal, residual stress distributions were measured on cut sections with the contour method, using an optical profilometer. The results showed that hammer-peening completely eliminates the near-surface tensile welding residual stress on the A514 steel, whereas on the S41500 steel, the process is less useful due to the already beneficial effect of the low temperature martensitic transformation during weld cooling. Furthermore, hammer-peening the last weld layer confines tensile residual stress inside the weld, while inducing compressive stress at the weld surface. Charpy test results showed that the A514 weld presented better toughness than the S41500 weld and comparable cavitation erosion resistance. Finally, hammer-peening showed a beneficial effect on cavitation resistance of the weld surface.

Residual Stresses Induced by Robotized Hammer-Peening

The strains induced by hammer-peening were measured by strain gauges on a mild steel plate during the hammer-peening operation. This process has recently been robotized by the research institute of Hydro-Québec (IREQ), so the hammer-peening performed for this study was done with the help of a Scompi robot. The resulting stresses calculated from the strain measurements were compared with residual stress measurements made with the hole drilling technique. The comparison shows a very good correlation of the two sets of measurements. Residual stress measurements were also made in the hammer-peened zone: as expected we found a highly biaxial compressive state of stress in this zone. An unexpected region of transverse tension was found at the end of the hammerpeened zone. This region can be very critical if hammer-peening is made with the objective of improving fatigue behaviour.

ON LINE WELD PENETRATION MEASUREMENT USING AN INFRARED SENSOR

A new Approach to weld penetration monotoring in GTAW or PAW is proposed. Analytical as well as numerical calculations show that a strong correlation exists between the weld penetration and the solidification time of the weld pool. The physical origin of such a correlation is that the dissipation time of the heat of fusion is larger for a deeper molten pool. The experimental verification of the basic principle is carried out by means of a rugged and inexpensive fiber-optic sensor which probes the temperature distribution of the weld pool. A discussion is presented of the relevant parameters such as the spatial and temporal variation of the thermal gradients around the weld pool as well as the infrared emissivity of the surface.

Technical challenges in narrow-gap root pass welding during tandem and hybrid laser-arc welding of a thick martensitic stainless steel

As part of a collaborative program to develop advanced manufacturing processes for next-generation hydraulic turbines, this study investigated the technological challenges for joining 25-mm thick martensitic stainless steel (MSS) plates using tandem and hybrid laser-arc welding. Although candidate materials for the intended application typically include wrought AISI 415 and cast CA6NM, a martensitic 410 stainless steel (SS) was especially selected in this study due to its greater crack sensitivity. A narrow-gap groove was designed to minimize the amount of 410NiMo filler metal required to fill the groove using a multi-pass single-sided welding technique. All the welding trials were performed using a 5.2 kW fiber laser. The root-pass quality was characterized in terms of weld bead geometry, defects and microstructure. The main technical challenges observed for the root pass were lack of penetration, lack of fusion and cracking, as detailed in this work.

Weld pool geometry variation in GTA welding of austenitic stainless steel

A number of Weldability problems have been reported with automatic GTA welding of austenitic stainless steels. Variation in penetration from heat to heat and weld pool shift have raised concern in the welding community about the reliability of the GTAW process for these applications. The effect of slight variation in the composition of the base metal is now recognized. Our experience is that sulfur and oxygen content of the parent metal as well as oxygen content of the shielding gas may affect the penetration pattern. In this work the weldability characteristics of a low sulfur 317 stainless steel plate is investigated. A correlation is made between the measured thermal profile of the weld pool, the visual observation of the weld pool surface and the anode movement. A new experimental set-up was designed to monitor the anode position. The experiments show that any shift of the weld pool is accompanied by a smaller shift of the anode. It is suggested that the formation of a blue vapor on the weld pool surface plays a major role in the heat redistribution process.