Investigation of the weldability of dissimilar joint between high nitrogen steel and low alloy steel by comparing filler metals

Wu X., Lin H., Luo W., Jiang H.

The microstructure evolution and microhardness of Q&P980 steel heat-affected zone (HAZ) was investigated systematically using weld thermal simulation technique. The phase transformation temperatures were determined by the dilatometry. A c1 and A c3 were observed to be quite constant, 750 and 935 °C, at the heating rates higher than 150 °C/s. The different regions of HAZ were obtained varying the peak heating temperature in the simulation experiments. With the peak temperature increased from 300 to 1350 °C, the microstructure evolved from martensite/ferrite/retained austenite to tempered martensite/carbides/ferrite, then finally turned to single martensite phase with fine lath or coarse lath morphology. The volume percentage of retained austenite reduced dramatically from 13% to 2% due to retained austenite transformation at high temperature. The lowest microhardness of 268 HV was obtained in sub-critical HAZ and the highest microhardness of 485 HV was obtained in fine grained HAZ. The welding continuous cooling transformation (CCT) diagram of Q&P980 steel corresponding coarse grained HAZ was constructed by dilatometric methods. There was ferrite, bainite and martensite transformation regions when the cooling rates ranged from 0.1 to 100 °C/s. The microhardness kept at a high certain level of 450–460 HV at high cooling rates (≥20 °C/s) because of the formation of martensite. By the CCT diagram determined, the effect of cooling rate on microstructure and microhardness can be deduced and utilized for optimizing the welding parameters.

Li B., Lei Z., Wu S., Chen Y., Chen Y., Xiong Y.

The effect of powder feeding mode on the stability and nitrogen distribution of the high nitrogen steel (HNS) welding process was evaluated. The N loss of the joint was serious while the tensile strength was reduced to about 63.4 % in the absence of any effective measure. Four different configurations of powder feeding were designed to study the feasibility of powder feeding in the laser CMT hybrid welding process. And the powder feeding process was analyzed by arc shape and current-voltage curves which were collected by the Acuteye V4.0 welding observation system at 4000 frames/s. The most stable configuration was to fill the powder behind the arc. But non-uniformity was observed in the microstructure and properties in these configurations. The difference of ferrite content and tensile strength between the arc zone and laser zone reached 4.9 % and 150 MPa, respectively. A multi-stream powder feeding assisted laser-CMT welding method was proposed to improve the powder distribution and arc stability. The powder distribution was similar to the theoretical range and narrowed the ferrite content gap between the arc and laser zone to 1.3 %, and also the arc became stable even when a proportion of N2 was used. The tensile strength of the HNS welded nitride by CrN powder could reach 1198 MPa, which was 24.5 % higher than that of the non-nitride joint. However, the combination of N2 and powder nitriding needs to be explored in the future work.

Dongqing Y., Hanying X., Yong H., Dejun Y., Dong L., Yong P., Kehong W.

High nitrogen austenitic stainless steel (HNASS) wire with 0.78 wt.% nitrogen content was used for gas metal arc welding (GMAW) test. The characteristics of droplet transfer and weld formation in different welding parameters were investigated. There were three typical metal transfer modes in GMAW of HNASS, namely in terms of stable short circuit transfer, unstable short circuit transfer and spray transfer. With wire feed speed (WFS) of 3 m/min∼8 m/min and moderate arc voltage, the droplet transfer was stable short-circuiting transition mode. The transfer period and waveform of electrical parameters were regular and the weld formation was satisfactory with few spatters. With the increasing of arc voltage or WFS, the droplet grew up to abnormal large size due to the expansion of supersaturated nitrogen. The electric parameters were more disordered and the metal transfer became unstable with spatter. When wire feed speed was over 10 m/min with high arc voltage, the droplet transfer became spray transfer accompanied by numerous welding fumes. The weld formation of HNASS for unstable short circuit and spray transfer mode was inferior with serious spatters.

Liu Z., Fan C., Chen C., Ming Z., Yang C., Lin S., Wang L.

Three nitrogen-rich welding wires with nitrogen content of 0.15 %, 0.6 % and 0.9 % for high nitrogen stainless steel welding were developed by a modified Schaeffler diagram and equilibrium phase diagram calculation. The effect of the chemical compositions especially the nitrogen on the weld defects, element distribution, microstructure, mechanical properties and droplet transfer were studied to evaluate the performance of the welding wires. Nitrogen element in the droplets of nitrogen-rich welding wires was partially lost due to the escape of nitrogen during the welding process, so nitrogen in the welding materials was not completely transported to the weld as designed, resulting in a deviation of actual weld from the design. The joint of the welding wire with 0.6 % nitrogen content had the highest mechanical properties, the tensile strength reached 912.5 MPa, and the impact energy of the weld reached 138.17 J. Full-austenite weld was only obtained in the weld of the wire with 0.15 % nitrogen content, but the tensile strength of the joint was lowest because of the low nitrogen content in the weld. Welding wire with 0.9 % nitrogen content had the most serious nitrogen loss, a few of pores were found in the weld. Moreover, the content of skeleton ferrite in the weld with the welding wire containing 0.9 % nitrogen content was high, resulting in the decrease of the weld impact energy.

Khan W.N., Chhibber R.

In the present experimental work, an attempt has been made to investigate the effect of filler metal on solidification, microstructure, and mechanical properties of the dissimilar weld between super duplex stainless 2507 and high strength low alloy API X70 pipeline steel. This joint is widely used in offshore hydrocarbon drilling risers and oil-gas transportation pipelines. Welds have been fabricated by the gas tungsten arc welding process using super duplex 2594 and austenitic 309L grade filler. A comparative assessment of both the fillers has been done by investigating multiple aspects of weld's structural integrity. 309L fillers weld has skeletal ferrite morphology, whereas 2594 filler solidifies with precipitation of multiple reformed austenites in the ferrite matrix. Lower width unmixed zone is formed on the API X-70 side in 2594 filler as compared to 309L. The tensile and impact strength of 309L filler weld is superior to 2594 filler. Precipitation of reformed austenite in the weld zone leads to lower hardness in 2594 as compared to skeletal ferrite dominated 309L weld zone. High chromium concentration in the weld zone results in superior pitting corrosion resistance of 2594 filler. • Filler 309L solidifies with skeletal ferrite morphology in weld zone. • Filler 2594 weld zone has multiple reformed austenite formation. • Tensile, impact strength and hardness of 309L filler weld is superior to 2594 weld. • 2594 filler weld has better pitting corrosion resistance than 309L filler weld. • Unmixed zone on X70 side is narrower in 2594 filler weld.

Dak G., Pandey C.

In this review article, microstructure and mechanical behavior of the dissimilar welded joint (DWJ) between ferritic-martensitic steel and austenitic grade steel along with its application have been summarized in Ultra Super Critical (USC) power plant. Creep-strength enhanced ferritic-martensitic (CSEF/M) P91 steel was developed to sustain at extreme operating conditions of ultra-supercritical (USC) power plants, and later, P92 was developed to achieve better mechanical properties, higher creep-rupture strength and high operating temperature with the reduction in wall thickness as compared to P91 steel. The most common application of P91/P92 material in power plants includes high pressure and high-temperature steam piping, headers, super-heater tubing, and water-wall tubing. The other most commonly used material in the power plants is austenitic stainless steel, i.e., SS 304 L. The austenitic grade stainless steel offers high resistance to corrosion due to the high wt. % chromium and nickel content (18–20 and 8–12, respectively). Due to the low carbon content, the SS 304 L is less sensitive to the sensitization problem and offers excellent weldability. The joining of these dissimilar materials is frequently required in the power generation industry. The current review focuses on the main difficulty associated with dissimilar welding of martensitic P91/P92 and austenitic grade stainless steel. The different chemical composition, mechanical, physical and metallurgical properties of the martensitic P91/P92 and austenitic grade stainless steel leads to the problems such as hot cracking and carbon migration. The other weldability issues are the formation of a brittle intermetallic compound, the formation of soft transaction heat affected zone along with martensitic steel, δ ferrite formation in fusion zone, diffusion related problem, and residual stresses, which necessitates thorough study and qualification of welds. The effect of coarsening of various precipitates such as M23C6 carbides, MX carbonitrides, and effect of laves phase, z-phase, and sigma phase on mechanical property, and creep-rupture strength of DWJ are also discussed in detail. Based on the literature reviewed, it has been found that some of the above-stated problems can be solved by using nickel-based filler wire due to its intermediate physical and mechanical properties. The selection of the proper filler metal is another vital issue in dissimilar welds joint that is also covered in this review article. The reason behind the formation of the unmixed zone, filler deficient region, peninsula, island, beach, migrated grain boundaries, solidified grain boundaries, and solidified subgrain boundaries during DWJ of martensitic P91/P92 and austenitic grade stainless steel is also discussed. The heat treatment is required to eliminate the heterogeneous microstructure during the dissimilar welding. The effect of post-weld heat treatment (PWHT) on the microstructure and mechanical behavior of the DWJ also reviewed. The residual stress developed during the DWJ may cause the premature failure of the components under service, has also been discussed in detail. The effect associated with the residual stress deformation has been reviewed in the different conditions of the DWJ.

Liu Z., Fan C., Ming Z., Chen C., Yang C., Lin S., Wang L.

High nitrogen stainless steel has extensive applied foreground in industries. But the weldability limits the use of the steel. The weld without nitrogen can become the weakness of the joint in corrosion resistance and strength. In this study, response surface methodology was applied to optimize the Ar-N2-CO2 ternary shielding gas for a nitrogen-containing filler metal in high nitrogen stainless welding. The influence of the proportion of N2 and CO2 on the nitrogen content, the impact energy and the tensile strength were investigated by the statistical regression models. The results show that the tensile strength, nitrogen content and impact energy increase and then decrease with the increasing of CO2, which indicates that CO2 content should not be too high. N2 addition can increase the nitrogen content of the weld obviously. But the impact energy decreases when N2 content exceeds about 7%. Integrating the mathematic models of the three performances, the optimal shielding gas compositions were determined to be 87 %Ar-6.5 %N2-6.5 %CO2. With this optimal shielding gas, the tensile strength and impact energy reached 956.7 MPa and 166.8 J, respectively. The deviation between the experimental value and the predicted value was below 2%.

Liu Z., Fan C., Chen C., Ming Z., Liu A., Yang C., Lin S., Wang L.

The nitrides were added to the molten pool to optimize the microstructure and mechanical properties of high nitrogen stainless steel weld. The nitrides of the alloy element that promote the nitrogen dissolution were selected. The alloy element and nitrogen element were thereby simultaneously added to the weld. MnN was selected from TiN, VN, CrN and MnN because the nitrogen content of the autogenous tungsten inert gas weld that brushed the MnN powder was the highest and manganese is an austenite forming element. And then MnN was filled into the gas metal arc welding molten pool as the way of bypass MnN-cored wire. The proportion of ferrite of the weld reduced by 51.4 %, the tensile strength of the joint increased by 7.4 % and the impact toughness of the weld increased by 28.1 %. The action mechanism of MnN during the welding was discussed. MnN was decomposed under the high temperature of the arc and the nitrogen and manganese entered the molten pool respectively. Nitrogen escape occurred along with the mass transfer process.

Kumar N., Arora N., Goel S.K., Goel D.B.

The present study leads to the selection of matching filler materials to weld nitrogen alloyed austenitic stainless steel 21-4-N with the desired combination of properties. 7 mm thick 21-4-N hot rolled steel plates were butt welded by Gas Tungsten Arc Welding (GTAW) with ER2209 (nitrogen alloyed) and E309LMo stainless steel filler wire. Microstructure and mechanical properties of weld joint were studied. The various zones of weld joint were carried out using optical microscopy and field emission electron microscopy (FESEM). It was observed that the weld zone exhibited higher nitrogen content with ER2209 filler resulting in higher tensile strength and hardness.

Dev S., Ramkumar K.D., Arivazhagan N., Rajendran R.

Turbine blisk and shaft assembly in aero-engines make use of dissimilar joints of Inconel 718 and martensitic stainless steel, AISI 416. The joining of these 5 mm thick aerospace alloys carried out by adopting current pulsing in Gas Tungsten Arc Welding (GTAW) has been addressed. Two Nb free consumables namely ERNiCrMo-4, ERNiCu-7 and a duplex filler ER2553 were used to join these dissimilar combinations. The weldments were systematically characterized using optical microscopy (OM) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). It is endorsed from the studies that the welds obtained from these fillers were free from deleterious Laves phase. Irrespective of fillers, the tensile failures were experienced at the base metal side of AISI 416. Room temperature Charpy V-notch impact test indicated that the weld joints employing ERNiCu-7 witnessed better impact toughness than other weldments. This study is highly in demand in aerospace industries and successfully addressed the choice of fillers in supressing the Laves phase.

Velu M., Dixit S., Choure S.

This paper presents the results of metallurgical and mechanical examinations of Gas Tungsten Arc Welding of dissimilar steels AISI4140 and AISI410. Two different filler materials viz., ERNiCr3 and SS410 were used. The various properties of the weldments made using the fillers were compared to select the most appropriate one to get the sound joint. The ultimate tensile and yield strengths of the weldments of SS410 were greater than those of ERNiCr3. The fracture occurred at the weld in weldments made with ERNiCr3, whereas, in the base metal of AISI410 for weldments made with SS410. Microstructure of fusion zone of ERNiCr3 was fully austenitic. Microhardness values in the weld of SS410 were higher and fluctuating compared to those in the weld of ERNiCr3. From this research work, it shall be concluded that SS410 is the best filler material to weld these base materials.

Wang Q., Zhang M., Liu W., Wei X., Xu J., Chen J., Lu H., Yu C.

Failure behavior of SA508-3/EQ309L overlay weld interface was analyzed. Residual stress measurement showed weld interface was in low stress state at transverse and longitude directions. SA508-3/EQ309L interface was a typical sandwich structure containing a white bright band ended with multi micro-voids type-II boundary. Element changes little around type-II boundary, but hardness varied a lot. The property difference and feature of multi voids made type-II boundary unstable, affecting weld property significantly. Induced shear and tensile tests were applied to study type-II boundary behavior. Under shear stress, type-II boundary suffered work hardening while under tensile stress, type-II boundary cracked first before EQ309L necking. Energy Dispersive Spectrometer (EDS) showed little element variation around type-II boundary. Electron Back-Scattered Diffraction (EBSD) indicated that most part of type-II boundary was ∑3 CSL grain boundary. Under tensile stress, dislocations crossed the boundary, leading to further stress concentration, so as to make the boundary crack under low positive stress.

Vashishtha H., Taiwade R.V., Sharma S., Patil A.P.

High nitrogen austenitic stainless steels are in great demand due to their outstanding combination of strength and ductility, excellent work-hardening capability and corrosion resistance at relatively cost-effective than conventional stainless steels. The replacement compatibility of most widely used conventional austenitic stainless steel (type 304) was investigated by employing dissimilar weldments with high nitrogen austenitic stainless steel (type 201) using scanning electron microscope coupled with EDS and X-ray diffraction techniques. Dissimilar weldments between conventional and high nitrogen stainless steel was prepared using gas tungsten arc welding and shielded metal arc welding processes. The weld defects and their integrity were investigated by radiographic analysis. The effect of welding speeds on microstructural characteristics and grain boundary precipitation was analyzed. The subsequent effect on mechanical properties was studied using tensile test and microhardness evaluation. The faster cooling rate associated with higher welding speed resulted in 5% increment in ultimate tensile strength for GTAW process and 9% for SMAW process respectively.

Hosseini V.A., Wessman S., Hurtig K., Karlsson L.

Nitrogen loss is an important phenomenon in welding of super duplex stainless steels. In this study, a super duplex stainless steel was autogenously TIG-welded with one to four bead-on-plate passes with low or high heat inputs using pure argon shielding gas. The goal was to monitor nitrogen content and microstructure for each weld pass. Nitrogen content, measured by wavelength dispersive X-ray spectrometry, was after four passes reduced from 0.28 wt% in the base metal to 0.17 wt% and 0.10 wt% in low and high heat input samples, respectively. Nitrogen loss resulted in a more ferritic structure with larger grains and nitride precipitates. The ferrite grain width markedly increased with increasing number of passes and heat input. Ferrite content increased from 55% in base metal to 75% at low and 79% at high heat inputs after four passes. An increasing amount of nitrides were seen with increasing number of weld passes. An equation was suggested for calculation of the final nitrogen content of the weld metal as functions of initial nitrogen content and arc energy. Acceptable ferrite contents were seen for one or two passes. The recommendation is to use nitrogen in shielding gas and proper filler metals.

Soysal T., Kou S., Tat D., Pasang T.

Solute segregation on a macroscopic scale in a weld between two dissimilar metals or alloys has long been recognized, but fundamental understanding of macrosegregation in dissimilar-metal welding is still lacking. Two mechanisms for macrosegregation were proposed based on the liquidus temperature of the bulk weld metal, T LW , relative to the liquidus temperature of metal 1, T L1 , and the liquidus temperature of metal 2, T L2 . According to the mechanisms, two distinctly different macrosegregation features can form. A “peninsula” of an unmixed metal 1 can form if T LW T L1 . On the other hand, a “beach” of unmixed metal 2 irregular in shape can form if T LW  >  T L2 . To verify the mechanisms, a pure Cu sheet was butt welded to a low carbon steel sheet by gas-tungsten arc welding without a filler metal. Composition measurements were conducted inside and across the weld metal. A peninsula of unmixed steel and an irregular-shaped beach of unmixed Cu were observed, which verified the mechanisms. In addition, the bulk weld metal exhibited a layered structure caused by undercooling of the bulk weld pool into a metastable miscibility gap in the Cu-Fe phase diagram. Macrosegregation in previous studies on laser- and electron-beam welding of Cu to steel or stainless steel was discussed in light of the findings in the present study.

Yelamasetti B., Devi B.T., Omprakash B., Aslesha C.L., Shelare S., Sharma S., Rao V.N., Massoud E.E.

Tao Z., Liu Y., Yu S., Gao C., Yao C., Wu H., Gao X., Du L.

Yu C., Zhang D., Liu Z., Wu D., Zhong Y., Wu J.

Yuan J., Liu G., Yang L., Zhang Y.

Kostina V.S., Kostina M.V., Zinoveev D.V., Kudryashov A.E.

The processability of a material is directly related to the possibility of its production, operation and maintainability. One of the most important indicators of the processability of any metal is weldability. Austenitic steels with a high nitrogen content proved themselves as high-strength, corrosion- and cold-resistant materials, but the issue of their weldability is still not fully understood. The lack of welding filler materials on the market specifically designed for welding high-nitrogen steels is the primary obstacle to solving this problem. Thus, the goal of the work was to develop and obtain a laboratory sample of high-nitrogen welding wire. Based on calculations of nitrogen solubility and the phase composition of the weld metal, the chemical composition of Cr – Mn – Ni – Mo – V, N steel was selected for this wire. A defect-free ingot with 0.57 % N was obtained, and wire with a nitrogen content of 0.57 wt. % was produced using hot plastic deformation and drawing methods. Testing of this wire to obtain a welded joint of austenitic cast steel, close to it in chemical composition, with the welding process carried out according to the developed technological recommendations, made it possible to obtain a defect-free welded joint without loss of nitrogen in the weld metal. With a microhardness of the base metal of 252 HV50 , due to the alloying of the welding wire steel with nitrogen and vanadium, the metal of the weld and fusion line had a high microhardness (278 and 273 HV50 , respectively), significantly exceeding the microhardness of Cr – Ni cast austenite. The metal of the welded joint has high strength (0.9 of the base metal strength) and high impact toughness. The fracture of impact samples is characterized by a dimple structure characteristic of viscous materials. According to the obtained results, the new welding wire showed itself to be a promising material for welding austenitic high-nitrogen steels.

Cui B., Chen K., Yang Y., Lv Y., Zhang F., Liu S.

High nitrogen steel welded joints are prone to problems such as porosity defects and coarse grain size. To address these issues, ultrasonic-assisted laser-arc hybrid welding experiments were conducted on 8 mm-thick high nitrogen steel plates. The effect of ultrasonic vibration on the porosity defects, microstructure, and properties of welded joints was studied. The results indicate that with the increase of ultrasonic power, the pore ratio of the weld first decreases and then increases. The effects generated due to ultrasound such as cavitation and sound flow caused grain refinement while weakening the directionality of grain growth. The refined microstructure of the weld increased the microhardness. However, when the ultrasonic power was 240 W, the microhardness of the welded joint decreased due to the low nitrogen content. As the ultrasonic power was increased, the tensile strength and impact toughness initially exhibited an enhancement followed by a declining trend. The best mechanical properties were obtained when the ultrasonic power was 180 W. In contrast, the worst performance was obtained when the ultrasonic power was 240 W, which was due to higher porosity and severe nitrogen loss. Ultrasonic impact can reduce welding residual stress and improve the corrosion resistance of welds, and the effect becomes more pronounced with the increase of ultrasound power.