Mechanism governing nitrogen absorption by steel weld metal during laser welding

The existence of monatomic nitrogen in the plasma just over the keyhole during CO₂ laser welding was confirmed by the monochromatic image of a specific spectrum line emitted by monatomic nitrogen. The smaller reaction area of the molten pool with monatomic nitrogen is considered to lead to less nitrogen absorption during CO₂ laser welding than that during arc welding. The effect of the penetration mode shows that the nitrogen absorption during CO₂ laser welding mainly occurs on the upper surface of the molten pool. The nitrogen content in a reduced-pressure nitrogen atmosphere during CO₂ laser welding is in good agreement with that obtained during yttrium aluminum garnet (YAG) laser welding within the range of low nitrogen (partial) pressures. This result supports the supposition that the different behaviors of nitrogen absorption between CO₂ laser welding and YAG laser welding can be reasonably attributed to the lesser amount of monatomic nitrogen during YAG laser welding.     

Nitrogen desorption by high-nitrogen steel weld metal during CO<Subscript>2</Subscript> laser welding

Nitrogen desorption by high-nitrogen steels (HNSs) containing 0.32 and 0.53 pct nitrogen during CO₂ laser welding in an Ar-N₂ gas mixture was investigated and the obtained data were compared with those for arc welding and at the equilibrium state predicted by Sieverts’ Law. Although the nitrogen content in the weld metal during CO₂ laser welding was lower than that in the as-received base material in all conditions, the nitrogen desorption was larger in the top part of the weld metal than in the keyhole region. The nitrogen desorption in the Ar atmosphere was less during CO₂ laser welding than during arc welding. With the increase in nitrogen partial pressure, the nitrogen content in the weld metal sharply increased during arc welding, but only slightly increased during CO₂ laser welding. The nitrogen absorption and desorption of the HNS weld metal were much smaller during CO₂ laser welding than during arc welding.     

Effect of Compositional Variation in TiO<Subscript>2</Subscript>-Based Flux-Cored Arc Welding Fluxes on the Thermo-physical Properties and Mechanical Behavior of a Weld Zone

The effect of compositional variation in TiO₂-based flux-cored arc welding fluxes on viscosity, wettability, and electronegativity was studied. The thermo-physical properties of the retrieved fluxes and their relationship with the mechanical properties of the weld zone, including tensile strength and micro-Vickers hardness, after welding were identified. Microstructural observation under similar welding conditions revealed significant grain coarsening at a corrected optical basicity (Λ^corr) of 0.62, resulting in reduced strength and hardness due to greater heat transfer. Welding fluxes containing TiO₂-based simple structural units should result in greater heat transfer due to the deficiency in complex [AlO₄]^5?- and [SiO₄]^4?-based structural units, as identified through spectroscopic analyses using fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The electronegativity of the retrieved fluxes was also evaluated since higher electronegativity results in greater absorption of electrons in the arc, resulting in arc condensation towards the center direction. Consequently, deeper penetration could be obtained, where the highest electronegativity was identified to be approximately 0.62 of the corrected optical basicity. Thus, both the thermal conductivity and electronegativity of the welding fluxes were identified to determine the heat transfer phenomenon during flux-cored arc welding.     

Marangoni convection in weld pool in CO<Subscript>2</Subscript>-Ar-shielded gas thermal arc welding

Small CO₂ additions of 0.092 to 10 vol pct to the Ar shielding gas dramatically change the weld shape and penetration from a shallow flat-bottomed shape, to a deep cylindrical shape, to a shallow concave-bottomed shape, and back to the shallow flat-bottomed shape again with increasing CO₂ additions in gas thermal arc (GTA) welding of a SUS304 plate. Oxygen from the decomposition of CO₂ transfers and becomes an active solute element in the weld pool and reverses the Marangoni convection mode. An inward Marangoni convection in the weld pool occurs when the oxygen content in the weld pool is over 80 ppm. Lower than 80 ppm, flow will change to the outward direction. An oxide layer forms on the weld pool in the welding process. The heavy oxide layer on the liquid-pool surface will inhibit the inward fluid flow under it and also affects the oxygen transfer to the liquid pool. A model is proposed to illustrate the interaction between the CO₂ gas and the molten pool in the welding process.     

Investigation on Tensile Behaviour of Laser Welded Al–Mg–Sc–Zr In Situ TiB<Subscript>2</Subscript> Reinforced Metal Matrix Composite

The objective of the present work is to investigate the tensile behavior on laser welded Al–Mg–Sc–Zr in situ nano TiB₂ composite. Al–3.5Mg–0.15Sc–0.075Zr–1TiB₂ composite was melted in a resistance heating furnace. TiB₂ was formed during in situ reaction of K₂TiF₆ and KBF₄ salt mixture at 750 °C for 60 min. Welding was done using Nd:YAG pulsed laser source JK 600 (GSI make) using a robotic (IRB1410 of ABB) laser set up. The autogenous welding experiments were carried out using some of the significant parameters such as frequency—75 Hz, laser beam energy—8.10 J, pulse width—2.5 ms and welding speed—5 mm/s. It was observed that laser beam power played a major role and lower value of energy with higher repetition rate resulted better and uniform weld bead with full penetration. Five different processing methods were utilized to investigate the mechanical and metallurgical properties namely: (a) as cast (AC), (b) as cast followed by welding (AC + W), (c) as cast followed by aging (AC + A), (d) as cast followed by aging and then welding (AC + A + W) and finally (e) as cast followed by welding and then aging (AC + W + A). The ageing treatment followed was heating the samples at 300 °C for 5 h followed by air cooling. The obtained results infered that apart from the obvious superior properties shown by as cast followed by aging treatment (AC + A: 248 MPa); the AC + W + A specimens showed better properties (235 MPa) along with AC + A + W specimens (226 MPa). The fracture surface analyses revealed the following: (a) the weld region in the laser welded as cast material did not show any TiB₂ in the structure probably due to the fact that temperature experienced during laser welding process might have melted the particles and was dissolved in the solid solution, (b) the interface of the weld-base region showed the presence of few TiB₂ particles which lost their hexagonal shape due to preferential melting along the edges. (c) The fracture morphology of both AC + A + W and AC + W + A specimen’s showed typical mixed mode fracture with fine precipitates along the interface. The strength increased in AC + W + A at the expense of ductility due to formation of Al₃Sc precipitates.     

Study of the Reduction of Industrial Grade MoO<Subscript>3</Subscript> Powders with CO or CO-CO<Subscript>2</Subscript> Gases to Prepare MoO<Subscript>2</Subscript>

Industrial grade MoO₂ powders have a plenty of advantages relative to MoO₃ in the direct alloying steelmaking processes. In this work, the reduction of industrial grade MoO₃ powder with CO gas or the mixed gases of CO and CO₂ has been investigated in detail in order to prepare industrial grade MoO₂ powder. It is found that reaction temperature has a significant effect on the product composition. Using pure CO as the reducing gas, for temperatures below 868 K (595 °C), the main product is MoO₂ with some whisker carbon; for temperatures above 868 K (595 °C) the main reaction products are MoC and amorphous carbon; as the reaction temperature further increased, the final reaction product is Mo₂C. In addition, Mo₄O₁₁ is always formed as an intermediate product during the reaction processes both at lower and higher temperatures, which is similar to that observed on reduction of MoO₃ by H₂. It is found that adding CO₂ to the reducing gases eliminated carbon formation but still allows the formation of MoO₂ during the reaction process. This method may be applied to produce industrial grade MoO₂.     

高氮钢制备及焊接过程中氮的溶解与释放

综述了高氮钢制备及焊接过程中氮的溶解与释放规律;论讨了不同制备及焊接工艺下钢中氮溶解度的计算公式、适用条件及影响因素等;指出大气压力下的GTA焊接过程是一个非平衡过程,焊缝处的氮含量与保护气体中的氮分压之间不满足Sievert定律,焊缝处的氮含量主要取决于钢中平衡氮含量和Cr的含量;选择GTA焊接时,在较低的氮分压下,便可对焊缝氮含量进行控制,但由于氮的吸收和释放较快,采用GTA焊接时焊缝氮含量不能精确控制;采用（CO2,YAG）激光焊接时需要考虑焊缝氮的释放;氮质量分数大于1．0％的高氮钢焊接方法亟待开发．     

Study of welding ultra‐fine grained ferrite steel by CO2 laser

Ultra‐fine grained ferrite steels have higher strength and better toughness than the normal ferrite steels because of their micrometer or sub‐micrometer sized grains. In this paper the ultra‐fine grained steel SS400 is welded by CO₂ laser. The shape of weld, cooling rate of HAZ, width of HAZ, microstructures and mechanical properties of the joint are discussed. Experimental results indicate that laser beam welding can produce weld with a large ratio of depth to width. The cooling rate of HAZ of laser beam welding is fast, the growth of prior austenite grains of HAZ is limited, and the width of weld and HAZ is narrow. The microstructures of weld metal and coarse‐grained HAZ of laser beam welding mainly consist of B_L + M (small amount). With proper laser power and welding speed, good comprehensive mechanical properties can be acquired. The toughness of weld metal and coarse‐grained HAZ are higher than that of base metal. There is no softened zone after laser beam welding. The tensile strength of a welded joint is higher than that of base metal. The welded joint has good bending ductility.     

Thermodynamic Assessment of the Reduction of WO<Subscript>3</Subscript> by Carbon and Silicon

An interesting process in terms of resource conservation is the arc surfacing of worn components by means of powder wire in which the filler contains tungsten oxide WO₃ and a reducing agent (carbon and silicon). Thermodynamic assessment of the probability of 21 reactions in standard conditions is based on tabular data for the reagents in the range 1500–3500 K. This range includes the temperatures at the periphery of the arc and in the upper layers of the surfacing bath. The reactions assessed include direct reduction of WO₃ by carbon and silicon, indirect reduction of WO₃ by carbon, and reaction of tungsten compounds with carbon and silicon to form tungsten carbides and silicides. The possible reaction products considered are W, WC, W₂C, WSi₂, W₅Si₃, CO, CO₂, SiO, and SiO₂. The reduction of the oxide is written for 1 mole of O₂, while the reactions of tungsten compounds with carbon and silicon compounds are written for 2/3 mole of tungsten W. The probability of the reactions is estimated in terms of the standard Gibbs energy. In the range 1500–3500 K, the standard states of the reagents are assumed to be as follows: W(so); WO₃(so, li), with phase transition at 1745 K; WC(so); W₂C(so); C(so); CO(g); CO₂(g); WSi₂(so, li), with phase transition at 2433 K; W₅Si₃(so, li), with phase transition at 2623 K; Si(so,li), with phase transition at 1690 K; SiO(g) and SiO₂(so, li), with phase transition at 1996 K. To assess the influence of the possible evaporation of tungsten oxide WO₃ in the arc (T_b = 1943 K) on the thermodynamic properties, the thermodynamic characteristics of two reactions are considered; the standard state in this temperature range is assumed to be WO₃(g). Thermodynamic analysis of the reduction of tungsten oxide WO₃ shows that the temperature of the melt and the composition of the powder wire may affect the composition and properties of the layer applied. At high melt temperatures (>2500 K), the formation of tungsten and also tungsten carbides and silicides is likely. These reactions significantly change the composition of the gas phase, but not that of the slag phase in the surfacing bath. Below 1500 K, the most likely processes are the formation of tungsten silicides and tungsten on account of the reduction of WO₃ by silicon. In that case, the slag phase becomes more acidic on account of the silicon dioxide SiO₂ formed. However, this temperature range is below the melting point of WO₃ (1745 K). In the range 1500–2500, numerous competing reduction processes result in the formation of tungsten and also tungsten carbides and silicides in the melt. The reaction of tungsten compounds with carbon and silicon to form carbides and silicides is less likely than reduction processes. Evaporation of tungsten oxide WO₃ in the arc increases the thermodynamic probability of reduction; this effect is greatest at low temperatures.     

Decomposition of ferrite in commercial superduplex stainless steel weld metals; microstructural transformations above 700 °C

The microstructural stability at temperatures above 700 °C of weld metal of type 29Cr-8Ni-2Mo-0.39N and weld metal of type 25Cr-10Ni-4Mo-0.28N has been compared. Multipass welding was employed using the gas tungsten arc welding technique with a shielding gas of Ar+2 pct N₂. The quantitative assessment of the intermetallic phase was performed using automatic image analysis in the light optical microscope (LOM). Detailed microanalysis was also performed using scanning and transmission electron microscopy. A computer program developed by the authors was used to calculate a continuous cooling-temperature (CCT) diagram on the basis of the experimentally determined time-temperature-transformation (TTT) diagram. Thermodynamic calculations for estimating phase stabilities and for interpreting experimental observations were performed. It was found that weld metal of type 29Cr-8Ni-2Mo-0.39N was microstructurally more stable than weld metal of type 25Cr-10Ni-4Mo-0.28N. A lower molybdenum concentration and a higher nitrogen concentration in the former alloy could explain the higher stability with respect to the intermetallic phase. The higher nitrogen concentration also provides a rationale for the higher stability against the formation of secondary austenite in weld metal of type 29Cr-8Ni-2Mo-0.39N. This effect, which is associated with a lower thermodynamic driving force for precipitation of secondary austenite during multipass welding, can be explained by nitrogen-enhanced primary austenite formation.     

Effect of Shielding Gas on the Microstructural Evolution in Laser Beam Welded AISI 410 Martensitic Stainless Steel

Laser welding of AISI 410 martensitic stainless steel was attempted in a diffusion cooled RF excited CO₂ slab laser under Gaussian mode with argon and nitrogen as shielding gas. The effect of shielding gas and energy density on the resultant weld bead geometry, microstructure and hardness were assessed and discussed. It has been observed that welds obtained under nitrogen shielding conditions had higher and uniform hardness across the weld metal on account of reduced ferrite content.     

Formation of CaS on Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO inclusions during solidification of steels

In this article, the effect of CaS formation on the evolution of Al₂O₃-CaO inclusions in low-carbon Al-killed and Ca-treated steel during the solidification process is investigated through high-temperature confocal scanning laser microscopy (CSLM). The inclusions started as mostly liquid-globular inclusions that did not agglomerate with each other on the melt surface but during solidification were seen to change shape into an irregular morphology. The shape change was found to be due to the reaction between the Al₂O₃-CaO inclusions with the dissolved S and Al in the melt, resulting in the formation of dense CaS shells around the inclusions. The melt composition during solidification, estimated from the observed solid δ-front advance rate, was compared to the thermodynamic limit for CaS precipitation. The observed growth rate of the CaS shell was found to initially increase with decreasing temperature because of the higher, solid δ-front advance rates at lower temperatures, which results in higher rates of S and Al partitioning. Once CaS had precipitated, the inclusions were found to form agglomerates on the melt surface because of fluid flow, initially, and later, the capillary depression.     

Characterization of a continuous CO<Subscript>2</Subscript> laser-welded Fe-Cu dissimilar couple

Continuous CO₂ laser welding of an Fe-Cu dissimilar couple in a butt-weld geometry at different process conditions is studied. The process conditions are varied to identify and characterize the microstructural features that are independent of the welding mode. The study presents a characterization of the microstructure and mechanical properties of the welds. Detailed microstructural analysis of the weld/base-metal interface shows features that are different on the two sides of the weld. The iron side can grow into the weld with a local change in length scale, whereas the interface on the copper side indicates a barrier to growth. The interface is jagged, and a banded microstructure consisting of iron-rich layers could be observed next to the weld/Cu interface. The observations suggest that solidification initiates inside the melt, where iron and copper are mixed due to convective flow. The transmission electron microscopy (TEM) of the weld region also indicates the occasional presence of droplets of iron and copper. The microstructural observations are rationalized using arguments drawn from a thermodynamic analysis of the Fe-Cu system.     

Surface Treatment of S355JR Carbon Steel Surfaces with ZrB<Subscript>2</Subscript> Nanocrystals by CO<Subscript>2</Subscript> Laser

To improve mechanical properties of S2355JR carbon steel, pre-synthesized ZrB₂ nanocrystals were used to coat the metal surface by laser cladding using 2000 W CO₂ laser. ZrB₂ nanocrystals were synthesized by mechanochemical process. The effect of laser power on the coating layers was examined for optimizing the most effective coating conditions. Microstructural studies were carried out using optical microscope, scanning electron microscope and X-ray diffraction to analyze phase structures of the coated layers. Mechanical characteristics of the laser coated layers were evaluated by studying microhardness, wear and scratch resistance properties. Maximum hardness of the coated layers was observed while cladding with 75 and 125 W laser powers, when other processing parameters and conditions were kept at optimum levels. EDS analysis of these laser cladded layers indicated the formation of complex boro-nitrides, nitrides and carbides of Zr and Fe that contributed to vast increase in hardness of the laser-clad coating on S2355JR steel. Depending upon the laser powers used, the thickness of the coated layers was found to be in the range of 15–37 µm. The wear and micro-scratch tests results revealed significant improvement in wear properties.     

Experimental evidence of crystal fragmentation from highly undercooled Ni<Subscript>99</Subscript>B<Subscript>1</Subscript> melts processed on an electrostatic levitator

Employing an electrostatic levitator (ESL) equipped with a CO₂ laser heating setup, we solidified Ni₉₉B₁ bulk crystals through containerless processing at high undercoolings and observed grain-refined microstructures. The electron backscatter diffraction pattern (EBSP) and analysis of the twin directions were accomplished, from which the primary growth traces with a cellular-like structure were revealed on a macro-millimeter scale. In comparison with the strong mechanical electromagnetic stirring in a sample processed on an electromagnetic levitator, the ESL provides a quite quiescent state for the melt, which enables identification of the primary growth traces after solidification. The present observation supplied experimental evidence that the refined microstructure in the Ni₉₉B₁ alloys at the high undercooling regime was due to fragmentation of the primary growth crystal, rather than dynamic nucleation.     

Effect of Na<Subscript>2</Subscript>CO<Subscript>3</Subscript> Addition on Inclusions in High-Carbon Steel for Saw Wire

Inclusions cannot be sufficiently stretched to adapt extremely strict requirement of saw wire with only conventional inclusion softening art. In order to explore a potential new method to further enhance the deformability of inclusions, Na₂CO₃ addition should be comprehensively investigated due to the extremely low melting temperature of inclusions containing Na₂O. In the present study, an effective method of Na₂CO₃ addition was put forward and a presumable reaction mechanism between Na₂CO₃/steel/inclusion/slag was deduced by studying the effect of Fe/Na₂CO₃ (weight ratio), Na₂CO₃ addition amount and reaction time on inclusions using a graphite tube resistance furnace. The relations between Na₂O content, melting temperature, deformability and crystallization of inclusions were also briefly discussed. Through these studies, the deformability of inclusions was significantly improved on the whole.     

Synthesis and Characterization of Microwave Absorbing SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Nanocomposite

Zinc ferrite and strontium hexaferrite; SrFe₁₂O₁₉/ZnFe₂O₄ (SrFe_11.6Zn_0.4O₁₉) nanoparticles having super paramagnetic nature were synthesized by simultaneous co-precipitation of iron, zinc and strontium chloride salts using 5 M sodium hydroxide solution. The resulting precursors were heat treated (HT) at 850, 950 and 1150°C for 4 h in nitrogen atmosphere. The hysteresis loops showed an increase in saturation magnetization from 1.040 to 58.938 emu/g with increasing HT temperatures. The ‘as-synthesized’ particles have size in the range of 20–25 nm with spherical and needle shapes. Further, these spherical and needle shaped nanoparticles tend to change their morphology to hexagonal plate shape with increase in HT temperatures. The effect of such a systematic morphological transformation of nanoparticles on dielectric (complex permittivity and permeability) and microwave absorption properties were estimated in X band (8.2–12.2 GHz). The maximum reflection loss of the composite reaches −26.51 dB (more than 99% power attenuation) at 10.636 GHz which suits its application in RADAR absorbing materials.