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Effect of cooling rate on microstructure and properties of microalloyed HSLA steel weld metals
(2015)
Two high strength Nb/Ti microalloyed S690QL steels were welded with identical filler material, varying welding parameters to obtain three cooling rates: slow, medium and fast cooling. As cooling rate increased, the predominantly acicular ferrite in Nb weld metal (WM) is substituted by bainite, with a consequence of obvious hardness increase, but in Ti WM, no great variation of acicular ferrite at all cooling rates contributed to little increment of hardness. The transition between bainite and acicular ferrite has been analysed from the point view of inclusions characteristics, chemical composition and cooling rate. Excellent Charpy toughness at 233 K was obtained with acicular ferrite as predominantly microstructure. Even with bainite weld of high hardness, the toughness was nearly enough to fulfill the minimal requirements. WM for Ti steel showed to be markedly less sensitive to the variations of cooling rate than that for Nb steel.
Welded components of P91 9% Cr steel demand for careful welding fabrication with necessary post weld heat treatment (PWHT). Before the PWHT, a hydrogen removal heat treatment is necessary for avoidance of hydrogen assisted cracking (HAC). In this context, the microstructure and temperature-dependent hydrogen diffusion is important, and reliable diffusion coefficients of P91 weld metal are rare. For that reason, the diffusion behavior of P91 multi-layer weld metal was investigated for as-welded (AW) and PWHT condition by electrochemical permeation experiments at room temperature and carrier gas hot extraction (CGHE) from 100 to 400 °C. Hydrogen diffusion coefficients were calculated, and the corresponding hydrogen
concentration was measured. It was ascertained that both heat treatment conditions show significant differences. At room
temperature the AW condition showed significant hydrogen trapping expressed by to seven times lower diffusion coefficients. A preferred diffusion direction was found in perpendicular direction expressed by high permeability. The CGHE experiments
revealed lower diffusion coefficients for the AW condition up to 400 °C. In this context, a hydrogen concentration of approximately 21 ml/100 g was still trapped at 100 °C. For that reason, a certain HAC susceptibility of as-welded P91 weld metal cannot
be excluded, and hydrogen removal should be done before PWHT.
9 %-Cr steel P91 is widely used in power plants due to the excellent creep-resistance. Components of this steel are typically welded and demand for careful welding fabrication, whereas a so-called post weld heat treatment (PWHT), must be conducted to increase the toughness and decrease the hardness of the martensitic as-welded (AW) microstructure. Before the PWHT, a hydrogen removal (or dehydrogenation) heat treatment is necessary as hardened AW martensitic microstructure is generally prone to delayed hydrogen assisted cracking (HAC). The microstructure and temperature dependent hydrogen diffusion is an important issue as it determines how long a potential crack-critical hydrogen concentration could remain in the microstructure. In this context, reliable hydrogen diffusion coefficients of P91 weld metal are rare. Hence, the diffusion behavior of P91 multi-layer weld metal was investigated in two different microstructure conditions: AW and further PWHT (760 °C for 4 h). Two different experimental techniques were used to cover a wide range of hydrogen diffusion temperatures: the electrochemical permeation technique (PT) at room temperature and the carrier gas hot extraction (CGHE) for a temperature range from 100 to 400 °C. From both techniques typical hydrogen diffusion coefficients were calculated and the corresponding hydrogen concentration was measured. It was ascertained that both heat treatment conditions show significant differences in hydrogen diffusivity. The biggest deviations were identified for room temperature. In this case, the AW condition shows significant hydrogen trapping and up to seven times lower diffusion coefficients. Additionally, PT investigations showed a preferred diffusion direction of hydrogen in the weld metal expressed by the diffusion coefficients and the permeability for both heat treatment conditions. The CGHE generally revealed lower diffusion coefficients for the AW microstructure up to 200 °C. In addition, the AW condition showed hydrogen concentrations up to 50 ml/100 g (considering electrochemical charging). Nonetheless, this hydrogen was not permanently (reversibly) trapped. Nonetheless, this temperature is approximately 100 °C below recommended dehydrogenation heat treatment (DHT). This has two main consequences: (I) in case of welding is interrupted or no DHT is conducted, a HAC susceptibility of hardened martensitic P91 weld metal cannot be excluded and (II) DHT can be conducted at temperatures around 200 °C below the recommended temperatures.
9 %-Cr steels like P91 and P92 are widely used in power plants due to the excellent creep-resistance. Components of this steel are typically welded and demand for careful welding fabrication, whereas a so-called post weld heat treatment (PWHT), must be conducted to increase the toughness and decrease the hardness of the martensitic as-welded (AW) microstructure. Before the PWHT, a hydrogen removal (or dehydrogenation) heat treatment is necessary as hardened AW martensitic microstructure is generally prone to delayed hydrogen assisted cracking (HAC). The microstructure and temperature dependent hydrogen diffusion is an important issue as it determines how long a potential crack-critical hydrogen concentration could remain in the microstructure. In this context, reliable hydrogen diffusion coefficients of P91 and P92 weld metal are rare.
A common approach to describe hydrogen traps is by their activation energy that is necessary to release hydrogen from the trap. In the present study, Cr-Mo-V steel T24 (7CrMoVTiB10-10) base material and TIG weld metal were investigated. Electrochemically hydrogen charged specimens were analyzed by thermal desorption analysis (TDA) with different linear heating rates. The results show two different effects. At first, the microstructure effect on trapping is evident in terms of higher hydrogen concentrations in the weld metal and increased activation energy for hydrogen release. Secondly, it is necessary to monitor the real specimen temperature. A comparison between the adjusted heating rate and the real specimen temperature shows that the calculated activation energy varies by factor two. Thus, the trap character in case of the base material changes to irreversible at decreased temperature. Hence, the effect of the experimental procedure must be considered as well if evaluating TDA results.
Quite a number of models for hydrogen distribution in steels and welds have been developed in the past 20 years. They reach from simple analytical models to more complex two and three dimensional finite element simulations. So far, these models have been used to simulate hydrogen distribution in homogeneous microstructure. This paper contributes to numerical simulation of hydrogen distribution in heterogeneous microstructure, e. g. in a duplex stainless steel microstructure consisting of two phase fractions. Under appropriate conditions, such as cathodic protection, it is possible that hydrogen is absorbed leading to material embrittlement and possibly initiating hydrogen assisted cracking. In order to avoid hydrogen assisted cracking in duplex stainless steels, it is of great interest to know more about the diffusion behavior of the ferrite and austenite phase. A numerical model has been developed that operates on the mesoscale and enables simulation of hydrogen transport in the various phases of a metallic material. As a first application of this model, hydrogen distribution in a duplex stainless steel 1.4462, consisting of approximately equal portions of ferrite and austenite, was simulated using the finite element program package ANSYS. The results reflect the dependency of hydrogen distribution on the microstructural alignment of the ferrite and austenite phase fractions. Crack-critical areas can thus be identified, provided the critical strain-hydrogen combination is known for the respective microstructural phase.
Laser beam weldability of austenitic and austenitic-ferritic high-manganese stainless steel sheets
(2010)
Thick-walled components made of duplex stainless steels are used in the semi-finished products as well as in machinery, apparatus and plant construction. Electron beam welding (EBW) of these components may be recommended for economic and quality reasons. To guarantee the necessary mechanical and technological properties and the corrosion resistance, the duplex stainless steels are welded with filler material and afterwards undergo a post-weld heat treatment. The present work shows interim results of investigations concerning the development of an electron beam multi-process technology for welding these steels without filler material and post-weld heat treatment. The studies were performed on standard duplex stainless steel of type 1.4462 (X2CrNiMoN22-5-3). When welding duplex stainless steels, the cooling rate and the chemical composition have a crucial influence on the final result. Based on fundamental investigations relating to the influence of the process parameters on the effusion of nitrogen and the cooling rates, the resulting austenite formation, mechanical properties and the corrosion resistance were taken into account to develop appropriate electron beam multi-process techniques. The ferrite content was measured metallographically and by magnetic induction, the impact toughness was measured at –40 °C and the determination of critical pitting temperatures was performed using electrochemical noise measurements.
Single-pass solid-state laser welding of plates in the thickness range of 10 to 20 mm became possible with the invention of the fibre laser. This new technique provides excellent beam quality at powers as high as 20 kW or more, and has proved applicable in several industrial applications. By replacing conventional methods with the fibre laser, it is possible to avoid multiple-pass welding that requires time-consuming bevelling. The high energy density of the fibre laser beam also reduces the heat input and consequently the distortion. However, the rapid solidification and cooling associated with laser welding can cause imbalance of the microstructure of duplex stainless steel weldments, where excessively high ferrite contents may reduce the corrosion resistance and the ductility of the material. The solution is normally to add nickel-based filler wire and to increase the heat input. By using a hybrid welding process where the laser beam and the gas metal arc (GMA) process act in a common process zone, filler metal can be added to the molten pool at higher heat input and at the same time, higher welding speed and deeper penetration can be achieved. In this work, 13.5 mm thick 2205 (EN 1.4462, UNS S31803) was fibre laser-GMA hybrid welded in a single-pass using 14 kW of laser power and ISO 22 9 3 N L as filler wire for the GMA process. The resulting welds were free from defects, with smooth surfaces and full penetration. The investigation examines the weld metal microstructure and the effect on corrosion resistance and mechanical properties. The option to add nickel foil, when hybrid welding, was also investigated, as comparison, and the effect on austenite formation was evaluated.
Single-pass solid-state laser welding of plates in the thickness range of 10 to 20 mm became
possible with the invention of the fibre laser. This new technique provides excellent beam
quality at powers as high as 20 kW or more, and has proved applicable in several industrial
applications. By replacing conventional methods with the fibre laser, it is possible to avoid
multiple-pass welding that requires time-consuming bevelling. The high energy density of the
fibre laser beam also reduces the heat input and consequently the distortion. However, the
rapid solidification and cooling associated with laser welding can cause imbalance of the
microstructure of duplex stainless steel weldments, where excessively high ferrite contents
may reduce the corrosion resistance and the ductility of the material. The solution is normally
to add nickel-based filler wire and to increase the heat input. By using a hybrid welding
process where the laser beam and the gas metal arc (GMA) process act in a common
process zone, filler metal can be added to the molten pool at higher heat input and at the
same time, higher welding speed and deeper penetration can be achieved. In this work,
13.5 mm thick 2205 (EN 1.4462, UNS S31803) was fibre laser-GMA hybrid welded in a
single-pass using 14 kW of laser power and 2209 (ISO 22 9 3 L N) as filler wire for the GMA
process. The resulting welds were free from defects, with smooth surfaces and full
penetration. The investigation examines the weld metal microstructure and the effect on
corrosion resistance and mechanical properties. The option to add nickel foil when hybrid
welding was also investigated as comparison and the effect on austenite formation was
evaluated.