High nitrogen stainless steel with nitrogen content of 0.75% was welded by gas metal arc welding with Ar–N2-O2 ternary shielding gas. The effect of the ternary shielding gas on the retention and ...improvement of nitrogen content in the weld was identified. Surfacing test was conducted first to compare the ability of O2 and CO2 in prompting nitrogen dissolution. The nitrogen content of the surfacing metal with O2 is slightly higher than CO2. And then Ar–N2-O2 shielding gas was applied to weld high nitrogen stainless steel. After using N2-containing shielding gas, the nitrogen content of the weld was improved by 0.1 wt%. As N2 continued to increase, the increment of nitrogen content was not obvious, but the ferrite decreased from the top to the bottom. When the proportion of N2 reached 20%, a full austenitic weld was obtained and the tensile strength was improved by 8.7%. Combined with the results of surfacing test and welding test, it is concluded that the main effect of N2 is to inhibit the escape of nitrogen and suppress the nitrogen diffusion from bottom to the top in the molten pool.
•Exploring the inhibition of moisture intrusion and the protection of perovskite solar cells in a CO2 and N2 sealed environment.•Established the model of moisture intrusion under the environment of ...N2, CO2 and SF6, and simulated the degree of moisture intrusion under the three gas environments.•In the practical application of perovskite solar cells, on the premise of considering sealing and cost, it provides guidance for the selection of shielding gas.
The stability and reliability of perovskite solar cells (PSCs) are severely affected by moisture. Therefore, it is very meaningful to study the barrier effect of gases with different compositions on moisture in a sealed environment. In this work, the stability of PSCs in a sealed environment filled with N2, CO2 with an external environment of 45 °C and 85% RH was investigated. After 240 h treatment, the PCE of the PSCs in the CO2 sealed environment decayed to 71.1% of the initial value. The PCE of the PSCs in the N2 sealed environment decayed to 51.8% of the initial value, while in the air environment as a control, the PCE decayed to 36.3% of the initial value. Through SEM, XRD film characterization and finite element simulation, it is found that both CO2 and N2 can be used as shielding gases to inhibit moisture intrusion, thereby slowing down the decay rate of perovskite solar cells. And CO2 is more effective than N2 in inhibiting moisture intrusion. On the one hand, CO2 molecules are larger than N2 molecules. When tiny gaps appear in the sealed environment, N2 molecules can escape, but CO2 cannot escape, which will lead to less moisture intrusion in the CO2 sealed environment. On the other hand, when there are large gaps in the sealed environment, both N2 and CO2 can escape, because the diffusion coefficient of moisture in the N2 sealed environment is larger, resulting in more moisture intrusion in the N2 sealed environment.
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%.
The influences of N2 content in shielding gas on microstructure and impact toughness of different zones in cold metal transfer and pulse (CMT-P) hybrid welded joint of duplex stainless steel (DSS) ...were systematically studied. The results showed that the N2-supplemented in shielding gas significantly facilitated austenite formation (weld root: 39.9%→41.2%, weld filler: 40.5%→43.7%, and heat affected zone (HAZ): 36.4%→39.6%). However, when the N2 content in the shielding gas exceeded 4%, there was no significant change in austenite content because of reaching the solubility limit of N atoms. In addition, γ2 precipitated both in the weld root and HAZ but not in the weld filler, which cannot be inhibited by N2 addition in the shielding gas. Besides, a great number of Cr2N and dislocations in addition to γ2 formed in the HAZ, and the content of Cr2N and dislocation significantly increased with the increase of N2 content in the shielding gas. Furthermore, the HAZ exhibited the lowest toughness in comparison with other zones. In addition, with the increase of N2 content in the shielding gas from 0% to 6%, the toughness increased first and then decreased, and reached the maximum when 4% N2 added to the shielding gas as (weld root: 122.0 J/cm2, weld filler: 135.0 J/cm2, and HAZ: 91.8 J/cm2). According to detailed microstructure analysis and toughness level, Ar+4% N2 was recommended as the shielding gas to join DSS by using CMT-P welding technique.
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•There was a solubility limit of N in weld pool with N2 addition in shielding gas.•Reheating from subsequent weld-pass was a prerequisite for γ2 formation.•N2 addition in shielding gas cannot inhibit γ2 precipitation in weld root and HAZ.•Cr2N and dislocation in HAZ increased with N2 addition in shielding gas.•N2 content in shielding gas had obvious influence on toughness of welded joint.
Super austenitic stainless steel (SASS) is widely used in extreme environments due to its excellent corrosion resistance and mechanical properties, and a feature of this grade is its relatively high ...nitrogen content. As valuable tools to study the solidification process and phase transformation, high temperature confocal microscopy (HTCM) and thermal analysis methods, such as differential thermal analysis (DTA) and differential scanning calorimetry (DSC), have been widely reported in the investigation of SASS. However, the inherent limitations of these two techniques make it difficult to explain the complex solidification behaviors of SASS, which involves multiple phases. Here, a unique design of HTCM-DTA was employed to investigate the nucleation and subsequent phase transformation of SASS. This in-house build apparatus enables contemporaneous in-situ observation of the surface and thermal analysis of the bulk thermal events. Using the novel HTCM-DTA device, we systematically revealed the four solidification modes of SASS and analyzed them in detail through thermodynamic arguments and multiple sets of experimental data under different conditions: (a) bimodal solidification with solid-state phase transformation, (b) bimodal solidification with liquid-state phase transformation, (c) pseudo-bimodal solidification, and (d) unimodal solidification. By comparing with the findings of earlier studies, the limitation of spatiotemporal separation employing HTCM and thermal analysis methods was pointed out. The results suggest that both the shielding gas and the time of retaining in the liquid state affect the N content in the melt, leading to a change of the primary nucleation phase. The cooling rate not only affects the undercooling and microstructure but also the transformation stage of δ-ferrite to γ-austenite, and when the phase transformation occurs in the liquid state it is usually accompanied by recalescence. These insights contribute understanding required for adjusting the continuous casting process of high nitrogen alloys.
•In-situ observations on surface and thermal analysis of the bulk in parallel.•Solidification mode and phase transformation highly depend on shielding gas.•Four different solidification modes of S31254 were found and analyzed in detail.•The recalescence phenomenon was observed and analyzed frame by frame.•C.R. and tL influence the solidification behavior.
Wire arc additive manufacturing (WAAM) has been considered as a promising technology for the production of large metallic structures with high deposition rates and low cost. Stainless steels are ...widely applied due to good mechanical properties and excellent corrosion resistance. This paper reviews the current status of stainless steel WAAM, covering the microstructure, mechanical properties, and defects related to different stainless steels and process parameters. Residual stress and distortion of the WAAM manufactured components are discussed. Specific WAAM techniques, material compositions, process parameters, shielding gas composition, post heat treatments, microstructure, and defects can significantly influence the mechanical properties of WAAM stainless steels. To achieve high quality WAAM stainless steel parts, there is still a strong need to further study the underlying physical metallurgy mechanisms of the WAAM process and post heat treatments to optimize the WAAM and heat treatment parameters and thus control the microstructure. WAAM samples often show considerable anisotropy both in microstructure and mechanical properties. The new in-situ rolling + WAAM process is very effective in reducing the anisotropy, which also can reduce the residual stress and distortion. For future industrial applications, fatigue properties, and corrosion behaviors of WAAMed stainless steels need to be deeply studied in the future. Additionally, further efforts should be made to improve the WAAM process to achieve faster deposition rates and better-quality control.
The composition of shielding gas exerts a crucial influence on high nitrogen austenitic stainless steel (HNASS) deposited by gas metal arc additive manufacturing. The proportion of N2 in shielding ...gas greatly affected the formation quality and microstructure of deposited parts. As N2 proportion improved, the formation quality became worse gradually and the nitrogen concentration of deposition metal increased. The maximum nitrogen concentration of deposition metal was more than 1%. However, N2 porosity in the deposition metal became serious when the N2 content was over 10 %. The microstructure of the deposition metal changed from two phases of austenite and ferrite in shielding gas of 0∼10 %N2 to complete austenite with 15 %∼25 %N2 content in shielding gas. With optimal shielding gas of 5%N2+2%O2+10%He+83%Ar, the deposited parts of HNASS demonstrated good formation and excellent properties. The average ultimate tensile strength of 920 MPa was achieved and the elongation was 27.7%.
CO2 shielding gas was innovatively employed for TC4/304SS dissimilar metal arc welding, overcoming the limitation of active gas application for Ti welding. The effect of CO2 addition on welding ...process was investigated. Our results show that the CO2+Ar hybrid shielding gas eliminated the problem of insufficient bonding at the backside of the steel plate, and was helpful to obtain a full weld joint. Moreover, the microstructure of the weld joint was modified due to the intense mass transfer inside the molten pool. The typical Ti/Cu interface was divided into two layers. No Ti–Fe compounds were found within the Ti/Cu interface for pure Ar shielding gas. However, the interface morphology was found to change due to induced CO2. Some TiFe2+Ti5Si3 dendrites were formed in layer Ⅱ. And the TEM results showed that the addition of CO2 did not contribute to the generation of a new phase, but the island-shaped Ti2Cu3 was transformed to a lath shape. In the weld seam center, Ti–Fe–Si ternary compounds was formed instead of Ti–Fe or Ti–Cu compounds. The hardness of the entire joint increased with increasing CO2 content. An average strength of 480 MPa was obtained for 3% CO2 content, which is an increase of 57.9% compared to that for pure Ar shielding condition. The fracture surface morphology showed small cleavage flatforms with some shallow dimples. Better weld formation and the optimization of the interface microstructure both contributed significantly to the improvement of the tensile strength of the Ti/steel joint.
Objective This paper seeks to address shortcomings in the conventional structural design of welding torch nozzles, such as prolonged development cycles, high costs, and uncertainty regarding the ...efficacy of shielding gas fow field protection. Methods The fuid dynamics computational software Fluent was utilized for numerical simulations, concentrating on critical parameters like shielding gas fow rates, welding torch nozzle structures, and nozzle-to-bevel spacings. The simulation results were subsequently verified and analyzed through gas dyeing experiments and practical welding operations. Results In the gas metal arc welding(GMAW) process context involving narrowgap bevels, a shielding gas fow rate of 20 L/min yielded effective protection to pipes. The conical nozzle and the fat conical nozzle presented ideal results in which the fat conical nozzle structure helped the shielding gas fow field obtain minimal conicity and straighter fow, fostering stable welding conditions by efficiently resisting crossw
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