Hot cracking tendency of flux-cored arc welding with flux-cored wires of types Ni 6625

Due to their mechanical and corrosive properties, nickel-based alloys are very important in several industrial sectors like power stations, chemical apparatus, and the oil industry. While flux-cored arc welding (FCAW) of carbon steels often uses flux-cored wires (FCW), the use of Ni-based flux-cored wires is industrially less common. The reasons for this include the lower degree of recognition and the higher material costs compared to solid wires. In comparison to solid wires, flux-cored wires have some technological benefits such as the possibility of welding without pulsed arc technology using low-cost standard mixed gases, which has a much lower tendency to weld defects due to high penetration depth. Depending on the slag, the flux-cored wires have a good weldability and excellent mechanical properties. Based on the self-stressed and externally stressed hot crack tests, the basic FCW showed a higher hot cracking susceptibility, contrary to the original assumption. Even if the causes have not yet been finally clarified, a negative influence of the comparatively high sulfur and oxygen contents in the basic FCW is suspected. The weld metal of the solid wires showed the highest hot crack resistance.

Effect of boron and manganese on hot crack resistance and toughness for flux cored arc weldments of 550 MPa grade tensile strength

The hot crack resistance and mechanical properties of flux cored arc (FCA) welds were investigated with three kinds of welding consumables having different boron (B) and manganese (Mn) contents for high strength carbon steel. The hot crack resistance measured from self-restraint testing strongly depended on the amount of B in the welding consumable. Welding consumable with higher B contents resulted in longer total crack length and an increased number of cracks. Boron was intensely detected near the grain boundary of the weld centerline by secondary ion mass spectrometry (SIMS) analysis, and precipitated with boron carbide (Fe23(C,B)6), as analyzed by transmission electron microscopy (TEM). This promoted hot crack propagation in the high strength carbon steel welds. However, removing B from the welding consumable decreased the low temperature toughness for root and face weld metal due to the growth of Ferrite Side Plate (FSP) in comparison with welding consumables having more B or Mn contents. The addition of Mn in the weld metal suppressed the formation of FSP and increased the low temperature toughness. Therefore, the minimization of B and the supplement of Mn successfully achieved hot crack resistance and low temperature toughness for high strength carbon steel welds of 550 MPa tensile strength.

Numerical simulation of nickel-based alloys’ welding transient stress using various cooling techniques

Nickel-based alloys play an important role in the field of high-temperature alloys, which are widely used in nuclear reactors, aerospace and components of turbomachinery. However, the high susceptibility of welding hot crack is a main shortcoming to nickel-based alloys. One of the methods that reduce hot cracking susceptibility is by adjusting element constitution of weld metal and another method is by reducing transient stress. This article used finite element method to study the effect of cooling source on transient stress of the nickel-based alloy weld joint. The selection of appropriate cooling technique can decrease the peak of the transient von Mises stress and make the tensile stress turn into compressive stress, which is beneficial to reduce hot cracking susceptibility. The peak of the transient von Mises stress decreases as the cooling intensity increases from 0 to 15,000 W/m2 K, but increases if the cooling intensity is ineffective. When the distance between cooling source and heat source reaches 35 mm, the weld can get larger region of compressive stress. The peak of the transient von Mises stress decreases with increasing radius of cooling source and reaches minimum value at 12 mm. Combined cooling is more effective in reducing the peak of this stress than the conventional single trailing cooling source.

Strategies for Increasing the Productivity of Pulsed Laser Cladding of Hot-Crack Susceptible Nickel-Base Superalloy Inconel 738 LC

A novel repair strategy based on decoupled heat source for increasing the productivity of wire-assisted pulsed laser cladding of the γ’-precipitation strengthening nickel-base superalloys Inconel 738 low carbon (IN 738 LC, base material) and Haynes 282 (HS 282, filler material) is presented. The laser beam welding process is supported by the hot-wire technology. The additional energy is utilized to increase the deposition rate of the filler material by increasing feeding rates and well-defining the thermal management in the welding zone. The simultaneous application of laser pulse modulation allows the precise control of the temperature gradients to minimize the hot-crack formation. Accompanying investigations such as high-speed recordings and numerical simulations allow a generalized statement on the influence of the adapted heat management on the resulting weld seam geometry (dilution, aspect ratio and wetting angle) as well as the formation of hot-cracks and lack of fusion between base and filler material. Statistical analysis of the data—the input parameters like laser pulse energy, pulse shape, hot-wire power and wire-feeding rate in conjunction with the objectives like dilution, aspect ratio, wetting angle and hot-cracking behavior—revealed regression functions to predict certain weld seam properties and hence the required input parameters.

Solidification Cracking Assessment of LTT Filler Materials by Means of Varestraint Testing and µCT

Investigations of the weldability of metals often deal with hot cracking, as one of the most dreaded imperfections during weld fabrication. The hot cracking investigations presented in this paper were carried out as part of a study on the development of low transformation temperature (LTT) weld filler materials. These alloys allow to mitigate tensile residual stresses that usually arise during welding using conventional weld filler materials. By this means, higher fatigue strength and higher lifetimes of the weld can be achieved. However, LTT weld filler materials are for example, high-alloyed Cr/Ni steels that are susceptible to the formation of hot cracks. To assess hot cracking, we applied the standardized modified varestraint transvarestraint hot cracking test (MVT), which is well appropriate to evaluate different base or filler materials with regard to their hot cracking susceptibility. In order to consider the complete material volume for the assessment of hot cracking, we additionally applied microfocus X-ray computer tomography (µCT). It is shown that by a suitable selection of welding and MVT parameter the analysis of the complete 3D hot crack network can provide additional information with regard to the hot cracking model following Prokhorov. It is now possible to determine easy accessible substitute values (e.g., maximum crack depth) for the extent of the Brittleness Temperature Range (BTR) and the minimum critical strain P m i n .