Ferritic Weld Research Articles

Underwater wet welding of high-strength low-alloy steel using self-shielded flux-cored wire with highly exothermic Al/CuO mixture

Underwater wet welding processes with minimal energy consumption and high efficiency is the future goal for researchers in the field. Al/CuO exothermic mixtures in consumables for land welding process can improve welding process stability and decrease the electrical dependence of the welding process. To confirm the beneficial effects of Al/CuO thermite and gain a better understanding of metallurgical properties of the thermite welds in underwater environment, this study investigated underwater wet self-shielded flux-cored arc welding (FCAW-S) of high-strength low-alloy steel with highly exothermic Al/CuO mixture. The effects of Al/CuO thermite on arc stability, weld metallurgical behavior and melting characteristics of flux-cored wire were clarified. The ratio of chemical heat in the flux to the heat required to melt welding wire increased from 4.7% to 38.4% with the addition of Al/CuO, potentially accelerating the melting of flux-cored wires. The arc stability with thermite addition was superior to that without the addition of thermite. The presence of copper as a result of metallurgical reactions had a substantial impact on metallurgical behavior of underwater wet welds. The ferrite weld was a non-equilibrium Cu supersaturated solid solution, which increased the microhardness and tensile strength of wet welds. The measured cooling time proved that the incorporation of Al/CuO had the capacity to reduce cooling rate. The incorporation of 50 wt% thermite yielded a 7.8% increase in welding process efficiency compared to the base formulation. Current work provided an important direction for the development of a chemically self-heating flux-cored wire.

ANALYSIS OF STRESS CORROSION CRACKING OF HETEROGENEOUS WELDED JOINTS IN SIMULATED PRIMARY WATER ENVIRONMENT

The paper deals with the stress corrosion cracking of heterogeneous welded joints. The welded joints were made of austenitic base metal (AISI 321) and ferritic weld metal by arc welding. Two methods were used to analyze the stress corrosion cracking of samples of this type of welded joint. The first method was a slow strain rate test (SSRT) using a tabletop device at a temperature of 60°C with a graded strain rate for three specimens (10−5, 10−6, and 10−7 1/s). The second method was exposure in a corrosion autoclave at a temperature of 270°C and pressure of 12.26 N/mm2, while prestressing devices were used to achieve the prestressing at the level of yield strength and 3% plastic deformation. One specimen was bent to 60°. Boric acid solutions were used as the medium, which was supposed to simulate the environment of the primary circuit of nuclear power plants, a type of water-water energetic reactor. The surface and the presence of corrosion products, cracks, fractures, or pits on specimens were monitored using scanning electron microscopy, stereomicroscopy, confocal microscopy, and light microscopy. A gravimetric analysis was performed, as well, to determine the corrosion rate after exposure to the autoclave.

Effect of titanium content on solidification structure of ferritic stainless steel gas-tungsten and gas-metal arc welds

Ferritic stainless steel is utilized to fabricate automotive exhaust systems using a ferritic weld metal. Ductility of the weld metal is higher if its microstructure contains a significant proportion of equiaxed grains. The formation of equiaxed (rather than columnar) grains is favoured by a higher titanium weld metal content. In this study, the Ti content of ferritic stainless steel weld metal was changed by using Ti-free (Type 436) and Ti-containing (441) ferritic stainless steel as base metals. The metal-cored welding consumable contained 0.4% Ti. Gas-tungsten arc welding and gas-metal arc welding processes were compared. The weld metal Ti content ranged from zero to 0.5% Ti, as determined from scanning electron microscopy supplemented by inductively coupled plasma optical emission spectroscopy. Cross-sections of the weld beads were subjected to point counting (to estimate the fraction of equiaxed grains) and image analysis (to estimate the average grain size). Point counting proved to be more reliable. The fraction of equiaxed grains was sensitive to the Ti content, but not to the welding process. Below 0.4% Ti, the fraction of equiaxed grains gradually increased with an increase in the weld metal Ti content; above 0.4% Ti, the fraction of equiaxed grains rapidly increased with increasing Ti content. The transition in behaviour at 0.4% Ti corresponded to a Ti content at which Ti-rich precipitates became stable at the estimated liquidus temperature of the weld metal.

Prediction of hydrogen cracking in the wet welding of structural steels with ferritic stick electrodes down to 20 m

Hydrogen cracking (cold cracking), both in the heat-affected zone (HAZ) and in weld metal (WM) is one of the main factors responsible for losses in ductility and toughness in wet welds. These losses make it difficult to comply with structural quality requirements for underwater wet welding. This work presents and discusses experimental results focusing on such cracks in ferritic weldments. The welding tests performed in the laboratory used a mechanized system operating at pressures corresponding to water depths from 0.3 m to 20 m. The relationships between diffusible hydrogen, chemical composition, hardness, and cracking incidences were assessed for both the weld metal and heat-affected zone. Boundary conditions for the occurrence of cracks are suggested. The influence of welding depth on cracking occurrence was also considered. The main contribution of this work is the proposition of limits on diffusible hydrogen, hardness, carbon content, carbon equivalent, and critical metal parameter (Pcm) to avoid hydrogen cracks both in the weld metal and in the heat-affected zone. Emphasis is placed on the estimation of conditions to avoid hydrogen cracking in the welding of structural ship steels. Suggestions for future studies are presented.

Influence of different filler weld wire chemistries on metallurgical and mechanical behavior of ultrahigh strength steel welded joints

Three different filler combinations comprising of ferritic, austenitic, and (ferritic + austenitic) were used to fabricate butt welded joints on 15 mm thick ultrahigh strength steel using hybrid arc welding processes. Owing to different weld metal compositions, a significant variation in metallurgical properties of these welds was observed, which consequently affected their mechanical properties in terms of tensile and impact toughness. Acicular ferrite with relatively soft zones formed in the ferritic weld metal imparted better impact toughness and ductility, whereas the joints welded using austenitic filler wire due to formation of hard martensitic structure showed high hardness across all their zones which resulted into higher tensile strength but poor ductility and impact toughness. SEM fractographs facilitated studying of shear lip formation and percentage shear area, and could be correlated with the ductility and impact toughness of the welded joints to a reasonable extent. Among all the welds, ferritic filler showed relatively less joint efficiency as well as ultimate tensile strength, but could be considered as a better choice over the austenitic as well their combination (ferritic + austenitic), as it performed better in terms of tensile ductility as well as impact toughness.

Effect of shielding gas composition on intergranular corrosion of stabilized ferritic stainless steel GMA welds

The effect of shielding gas composition on intergranular corrosion of the fusion zone formed by bi-stabilized ferritic stainless steel plates (AISI 441) and ER430Ti and ER430LNb filler metals during gas metal arc welding was investigated. Double loop electrochemical potentiokinetic reactivation tests were conducted to examine the influence of the shielding gas content (Ar + 2%O2, Ar + 8%CO2 and Ar + 25%CO2) on intergranular corrosion of the ferritic stainless steel welds. It was possible to observe an increase in the tendency toward sensitization with the increase in the CO2 content in the shielding gas, especially with the ER430Ti filler metal. Analysis of the formed precipitates by means of optical microscope, scanning electron microscope an energy-dispersive X-ray spectroscopy allowed to notice that high levels of CO2 blended in the shielding gas increase the intergranular precipitates and lead to significant intergranular corrosion, which was induced by the Cr-depletion zone formation, especially for the welds produced with the ER430Ti filler metal.

Development of 25%Cr SMAW welding materials – development of high Cr ferritic overlay welding materials for recovery boiler

ABSTRACTIn place of the 18%Cr overlay welding material, which has been used for furnace water wall tubes of recovery boilers, a 25%Cr overlay welding material with good corrosion resistance has been newly developed. In consideration of dilution with a carbon steel tube, the Cr content of the new welding materials are designed to be more than 20%Cr. And in order to fine the grains of the weld metal, some elements, such as Nb, Al and Ti, are added to these welding materials. Mainly two types of welding materials, which have Austenite + Martensite + Ferrite of materials for the three-phase and fully ferritic materials, are examined. As results of weldability tests and bending properties, the fully ferritic welding materials are superior to the three-phase materials, respectively. On the point of the corrosion test, which are exposed into some smelt ashes obtained from the recovery boilers, the corrosion rate of the full ferritic material was confirmed to be the about half of that of 18%Cr overlay welding material. The 25%Cr overlay welding material was found to exhibit excellent corrosion resistance. This study introduces the development of 25%Cr ferritic welding material and methods for reliable overlay welding and examines the weldability, the bend property and the corrosion resistance of the 25%Cr overlay welded metal on tube.

Microstructural features, mechanical properties and high temperature failures of ferritic to ferritic dissimilar welds

ABSTRACTDissimilar metal welds (DMWs) between ferritic steel grades are found extensively in the construction of thermal power plants. The potential combinations and approaches for joining dissimilar ferritic steels are nearly limitless. For DMWs, the difference in alloy composition (specifically chromium and carbide-forming elements) provides the main driving force for carbon diffusion during welding, post-weld heat treatment and long-term service at elevated temperatures. Since the high temperature creep strength of local, carbon-denuded zones can be dramatically reduced from that of the parent or filler material, the service performance of ferritic DMWs can be severely reduced. This article reviews experimental observations on microstructural evolution in dissimilar ferritic welds, activities to describe the observed phenomena by modelling and simulation and discusses the performance of these welds at high temperature. Lastly, a well-engineered approach to the design of ferritic DMWs is discussed in the context of thermal power plants which are subject to damage by creep.Abbreviations: HAZ: heat affected zone; PWHT: post weld heat treatment; GMAW: gas-metal arc welding; SMAW: shielded-metal arc welding; GTAW: gas tungsten arc welding; SAW: submerged arc welding; DMW: ferritic dissimilar metal weld; CGHAZ: coarse-grained heat affected zone; FGHAZ: fine-grained heat affected zone; CDZ: carbon-denuded zone; CEZ: carbon-enriched zone; CSEF: creep strength enhanced ferritic

Susceptibility of Acicular Ferrite and Upper Bainite Microstructures to Hydrogen Assisted Cold Cracking Propagation

Acicular ferrite (AF) and upper bainite (UB) are microstructural constituents commonly found in ferritic weld metals. Both microstructures are formed within a similar temperature range and by the same type of transformation mechanisms. They have however, substantially different morphologies and microstructural features that govern both their mechanical properties and hydrogen embrittlement susceptibility. This work shows that despite substantial microstructural differences, the mechanical properties of both microstructural constituents were quite similar. However, the microstructural differences were found to significantly affect the hydrogen crack propagation resistance. Hydrogen assisted cold cracking (HACC) propagates along a path of least resistance through the surrounding microstructure. The unit crack path was significantly shorter for AF than for UB, which implied more frequent changes in direction and thus increased dissipation of energy from the crack driving force. These results suggest that AF, possessing fine interlocking grains and high angle grain boundaries (HAGB), increases the localised resistance to HACC propagation more than UB due to the impediment of brittle, cleavage-like crack propagation at HAGB’s.

Quenched and tempered steel welded with micro-alloyed based ferritic fillers

Quenched and tempered steel welds were made using one ferritic and two different micro-alloyed based ferritic fillers using shielded metal arc welding process. Variable metallurgical compositions in the weld zones of these joints resulted into wide variation in the microstructural phases and microhardness across different zones of these welds, which consequently influenced their tensile and impact toughness performance significantly. Joint welded using filler containing Cr and Mo with Nb, Ti, Al, V, Cu and N as micro-alloying additions resulted into weld metal where martensitic refinement occurred and thus showed highest microhardness of more than 400 HV_1. So this joint exhibited highest yield and ultimate tensile strength but gave least percentage elongation and showed the highest joint efficiency, whereas the other two joints welded using ferritic and less micro-alloyed (containing small additions of Ni, Mo and Cu) fillers fractured in their respective weld zones. High matching index (ratio of yield strength of weld joint to base metal) shifted the plastic strain from the weld metal to the HAZ during tensile loading. High micro-alloyed weld metal also showed the highest Charpy impact toughness value followed by low micro-alloyed and conventional ferritic weld metal. Weld metal with microalloying additions made to the weld metal via using suitable filler resulted into microstructural refinement of the weld metal which improved the tensile and impact toughness performance of quenched and tempered welded joints.

Systematic Expansion of the Microstructural Characterization of Ferritic Weld Metals

Abstract This paper deals with the analytic gap during the microstructural examination of ferritic weld metals and points out which methods can be used to perform a meaningful evaluation of the weld metal microstructure. The designers of filler materials and the respective metallographs are provided with a tool to specifically search for weaknesses in the microstructural composition and invalidate them by alloying on the basis of their metallurgical context.

Assessment of delta ferrite in multipass TIG welds of 40 mm thick SS 316L: A comparative study of ferrite number (FN) prediction and measurements

Austenitic stainless steels are widely used in the fabrication of fusion reactor major systems like vacuum vessel, divertor, cryostat and other structural components development. Multipass welding is used for the development of thick plates for the structural components fabrication. Due to the repeated weld thermal cycles, the microstructure adversely alters owing to the presence of complex phases like austenite, ferrite and delta ferrite and subsequently influences the mechanical properties like tensile and impact toughness of joints. The present paper reports the detail analysis of delta ferrite phase in welded region of 40 mm thick SS316L plates welded by special design multipass narrow groove TIG welding process under three different heat input conditions. The correlation of delta ferrite microstructure of different type structures acicular and vermicular is observed. The chemical composition of weld samples was used to predict the Ferrite Number (FN), which is representative form of delta ferrite in welds, with Schaeffler’s, WRC-1992 diagram and DeLong techniques by calculating the Creq and Nieq ratios and compared with experimental data of FN from Feritescope measurements. The low heat input conditions (1.67 kJ/mm) have produced higher FN (7.28), medium heat input (1.72 kJ/mm) shown FN (7.04) where as high heat input (1.87 kJ/mm) conditions has shown FN (6.68) decreasing trend and FN data is compared with the prediction methods.

Method for Assessing Grain Boundary Density in High-Strength, High-Toughness Ferritic Weld Metal

A method for measuring peak values on the maxlength-area fraction curve and the perimeter-area fraction curve with morphological photos using Image Pro Plus 6.0 Soft for assessing grain boundary density in high-strength, high-toughness ferritic weld metals is developed. Results show the sizes of the peak values have a tough relationship with grain boundary densities in that a larger peak value stands for a larger grain boundary density. As ferrite transforms into a certain orientation relationship, this semi-empirical method provides handy references for judging the sizes of effective grain boundary densities.

A study on the relationship between microstructure and mechanical properties of acicular ferrite and upper bainite

Acicular ferrite and bainite are microstructural constituents commonly found in ferritic weld metal and many other ferrous alloys. These often highly localised microstructural volumes are known to have a significant impact on the mechanical properties of the bulk material they constitute. It is well established that acicular ferrite and bainite have substantially different morphologies and features that can be directly linked to their intrinsic mechanical properties. However, in the bulk material they usually co-exist and interact with other microstructural constituents and features. Therefore, the individual contributions of acicular ferrite and bainite still remain unclear. This work utilises micro-testing techniques to evaluate and compare the mechanical properties of these two micro-constituents with an ultimate objective to understand their impact on bulk properties of ferrous alloys. Microscopic regions consisting of either acicular ferrite or upper bainite were first selected and then characterised using a high resolution Scanning Electron Microscope (SEM) and Electron Backscattered Diffraction (EBSD). Conventional nano-indentation and an advanced characterisation procedure were implemented to evaluate elastic modulus, yield strength, hardness and strain hardening exponent of both micro-constituents. The fracture resistance was estimated from micro-fracture tests that were conducted within the selected regions. The experimental studies have indicated that, at the micro-scale, despite their different microstructures, acicular ferrite and upper bainite have very similar mechanical properties. Nevertheless, the fracture resistance of upper bainite was more dominated by the contribution of plastic deformations.

Residual Stresses and Micro-Hardness Testing in Evaluating the Heat Affected Zone’s Width of Ferritic Ductile Iron Arc Welds

Shielded Metal Arc Welding (SMAW) in Ductile Irons (DI) is often required by foundries for practical manufacturing reasons. The mechanical properties of the welded structures are strongly dependent on their HAZ’s width. A model based on the behaviour of the ferritic matrix of high-Si DIs in order to make an approach in measuring their HAZ’s width is developed in this study. A series of thermal treatments on 3.35 and 3.75 wt% Si as-cast DIs and spot SMAWs is applied on these materials. The applied SMAWs are done on non-preheated and preheated samples (150℃ - 300℃). For welding we modify the amperage (100 - 140A). The micro-hardness Vickers changes in the ferrite of the as-cast samples and inside the HAZ of the welded ones can be attributed to the existence of residual stresses (RS) in the ferritic matrix and assist in estimating the HAZ’s width.

25%Cr被覆アーク溶接棒の開発

In place of the 18%Cr overlay welding material, which has been used for furnace water wall tubes of recovery boilers, a 25%Cr overlay welding material with good corrosion resistance has been newly developed. In consideration of dilution with a carbon steel tube, the Cr content of the new welding materials are designed to be more than 20%Cr. And in order to fine the grains of the weld metal, some elements, such as Nb, Al and Ti, are added to these welding materials.Mainly two types of welding materials, which have Austenite + Martensite + Ferrite of materials for the three-phase and fully ferritic materials, are examined. As results of weldability tests and bending properties, the fully ferritic welding materials are superior to the three-phase materials, respectively. On the point of the corrosion test, which are exposed into some smelt ashes obtained from the recovery boilers, the corrosion rate of the full ferritic material was confirmed to be the about 1/2 of that of 18%Cr overlay welding material. The 25%Cr overlay welding material was found to exhibit excellent corrosion resistance.This study introduces the development of 25%Cr ferritic welding material and methods for reliable overlay welding and examines the weldability, the bend property and the corrosion resistance of the 25%Cr overlay welded metal on tube.

Investigation of hydrogen assisted cracking in acicular ferrite using site-specific micro-fracture tests

Hydrogen assisted cracking (HAC) is a common type of failure mechanism that can affect a wide range of metals and alloys. Experimental studies of HAC are cumbersome due to various intrinsic and extrinsic parameters and factors (associated with stress, hydrogen and the materials microstructure) contributing to the hydrogen crack kinetics. The microstructure of many materials consists of diverse constituents with characteristic features and mechanical properties which only occur in very small material volumes. The only way to differentiate the effect of these individual constituents on the hydrogen crack kinetics is to miniaturise the testing procedures. In this paper we present a new experimental approach to investigate hydrogen assisted crack growth in a microstructural constituent, i.e. acicular ferrite. For this purpose, sharply notched micro-cantilevers were fabricated with a Focus Ion Beam within this selected microscopic region. Acicular ferrite can be found in many ferrous alloys including ferritic weld metal and has specific features that control its intrinsic susceptibility to HAC. These features were characterised via Electron Backscatter Diffraction and the specimens were subsequently loaded under uncharged and hydrogen charged conditions with a nano-indenter. The outcomes of the testing, demonstrated that the threshold stress intensity factor, Kth, to initiate crack propagation in acicular ferrite ranges between 1.56MPam1/2 and 4.36MPam1/2. This range is significantly below the values of Kth reported for various ferrous alloys in standard macro-tests. This finding indicates that the mechanisms and resistance to HAC at micro-scale could be very different than at the macro-scale as not all fracture toughening mechanisms may be activated at this scale level.

Comparative study of evolution of residual stress state by local mechanical tensioning and laser processing of ferritic and austenitic structural steel welds.

Complex thermal stresses generated in welded structures are undesirable but inevitable in fusion welding. The presence of residual stresses can be detrimental to the integrity of a welded joint. In this research, redistribution of residual stress magnitude and profile was studied and compared in two multi-pass welded structural alloys (API X100 and 304L stainless steel) after cold rolling and laser processing. The residual stress field was studied by neutron diffraction using the SALSA strain scanner at their reactor neutron source at ILL, Grenoble. In addition to a complex distribution of residual stress state, multi-pass welds also forms dendritic grain structure, which are repeatedly heated, resulting in segregation of alloying elements. Dendritic grain structure is weaker and segregation of alloying elements may result in formation of corrosion microcells as well as reduction in overall corrosion prevention due to depletion of alloying elements in certain areas. The modification of as-welded residual stress state was done by cold rolling which was followed by laser processing to create a recrystallized microstructure to minimise segregation. The main objective of this study is to understand the suitability of this novel manufacturing technique to create a stress free weldment with recrystallised grain structure. Hardness evolution in the welded structures was scanned following welding, post weld cold rolling and cold rolling followed by laser processing. Hardness distribution in both the structural alloys showed a significant evidence of plastic deformation near the cap pass of the weld metal. Residual stress redistribution was observed up to 4 mm from the capping pass for ferritic steel, while in austenitic steel weld, post weld cold rolling was effective in modifying the residual stress redistribution throughout the entire thickness. Laser processing in both cases reinstated the as-welded residual stress distribution and resulted in softening of the strained area.

Study of Weld Hardness of Pipes at Elevated Temperature Application in Power Plants

Weld hardness has close relationship with creep strength and ductility of the welded structures. Hence it is important for any weld to achieve certain level of weld hardness. This study aims at identifying the parameters that affect the quality of weld in achieving the required weld quality in terms of hardness and to establish the relationship between different factors that affect the weld hardness. The focus is to study and explore the welding input parameters for the gas tungsten arc welding (GTAW) of elevated temperature piping used in power plants and to predict the weld hardness on construction sites. The Design of Experiment (DoE) is used in analyzing the related parameters. The data generated through experiments has been validated for the hardness based on input process parameters (welding current, welding voltage, travel speed, welding rod diameter). The findings from the study revealed that the most important factor influencing the hardness of creep-strength enhanced ferritic (CSEF) material welds is the voltage while other factors have minimum or the least influence for the studied ranges and factors.

The Influence of Austenite Grain Size during Welding Simulations of Ferritic Steels

In recent years, considerable progress has been made in the simulation of ferritic steel welding processes. The successful validation of a single-pass autogenous TIG beam weld in SA508 Gr.3 Cl.1 steel has identified key simulation variables required for the accurate prediction of post-weld residual stress in ferritic weldments. The present work outlines a sensitivity study performed to examine the influence of austenite grain growth on predicted solid-state phase transformation kinetics and consequently, residual stress predictions.