Microstructural modification of austenitic stainless steels by rapid solidification

The microstructural modifications in three austenitic stainless steels (types 308, 310, and 312) were evaluated after rapid solidification. These three steels are commonly used weld filler metals. Two methods of rapid solidification were investigated, autogenous laser welding and arc-hammer splat quenching. The structure of 310 stainless steel was found to be 100 pct austenite, and did not vary over the range of conditions studied. On the contrary, the structures of types 308 and 312 steels were very sensitive to the cooling rates and solidification conditions. With the highest cooling rates, the type 308 structure was fully austenitic while the type 312 structure was fully ferritic. At lower cooling rates, the structures were duplex ferrite plus austenite. The results were interpreted in terms of faster kinetics of solidification of austenite compared to ferrite under the conditions examined. A comparison of the structures produced by the two rapid solidification techniques indicated the cooling rates are comparable.     

Grain structures in gas tungsten-arc welds of austenitic stainless steels with ferrite primary phase

The grain structures were investigated in full penetration gas tungsten-arc (GTA) welds in sheets of 304 and 321 austenitic stainless steels for a range of welding conditions. In type 321 steel welds, fine equiaxed ferrite dendrites were observed in the ferrite phase. The equiaxed structure was ascribed to heterogeneous nucleation of ferrite on Ti-rich cuboidal inclusions present in this steel, since these inclusions were observed at the origin of equiaxed dendrites. In type 304 welds, the ferrite grains were columnar, except in less complete penetration specimens, where a few coarse equiaxed dendrites appeared to originate from the weld surface. The secondary austenitic grain structure was columnar in both steels. In type 304 steel, the columnar austenitic grain structure did not necessarily correspond to the primary ferrite grains. In type 321 steel, the secondary austenite was columnar despite the equiaxed structure of the primary ferrite. Factors which affect the columnar-to-equiaxed transition (CET) are discussed. The failure to form equiaxed austenitic grains in type 321 steel is ascribed to austenite growing across the space between ferrite grains instead of renucleating on the primary equiaxed ferrite.     

Heat transfer during Nd: Yag pulsed laser welding and its effect on solidification structure of austenitic stainless steels

Theoretical and experimental investigations were carried out to determine the effect of process parameters on weld metal microstructures of austenitic stainless steels during pulsed laser welding. Laser welds made on four austenitic stainless steels at different power levels and scanning speeds were considered. A transient heat transfer model that takes into account fluid flow in the weld pool was employed to simulate thermal cycles and cooling rates experienced by the material under various welding conditions. The weld metal thermal cycles and cooling rates are related to features of the solidification structure. For the conditions investigated, the observed fusion zone structure ranged from duplex austenite (γ)+ferrite (δ) to fully austenitic or fully ferritic. Unlike welding with a continuous wave laser, pulsed laser welding results in thermal cycling from multiple melting and solidification cycles in the fusion zone, causing significant post-solidification solid-state transformation to occur. There was microstructural evidence of significant recrystallization in the fusion zone structure that can be explained on the basis of the thermal cycles. The present investigation clearly demonstrated the potential of the computational model to provide detailed information regarding the heat transfer conditions experienced during welding.     

Solidification Behavior and Weldability of Dissimilar Welds Between a Cr-Free,Ni-Cu Welding Consumable and Type 304L Austenitic Stainless Steel

The solidification behavior of a Cr-free welding consumable based on the Ni-Cu system was evaluated in conjunction with Type 304L stainless steel. The weld metal microstructure evolution was evaluated with optical and secondary electron microscopy, energy dispersive spectroscopy, X-ray diffraction, button melting, and thermodynamic (CALPHAD-based) modeling. Solidification partitioning patterns showed that higher dilutions of the filler metal by Type 304L increased segregation of Ti, Cu, and Si to interdendritic regions. Button melting experiments showed a widening of the solidification temperature range with increasing dilution because of the expansion of the austenite solidification range and formation of Ti(C,N) via a eutectic reaction. The model predictions showed good correlation with button melting experiments and were used to evaluate the nature of the Ti(C,N) precipitation reaction. Solidification cracking susceptibility of the weld metal was shown to increase with dilution of 304L stainless steel based on testing conducted with the cast pin tear test. The increase in cracking susceptibility is associated with expansion of the solidification temperature range and the presence of eutectic liquid at the end of solidification that wets solidification grain boundaries.     

Effect of welding consumables on hydrogen induced cracking of armour grade quenched and tempered steel welds

Abstract

Armour grade quenched and tempered (Q&T) steels are susceptible to hydrogen induced cracking (HIC) in the heat affected zone after welding. Austenitic stainless steel (ASS) consumables are selected for welding Q&T steels as they have higher solubility for hydrogen in the austenitic phase and it is the most beneficial method for controlling HIC in Q&T steel welds. Recent studies reveal that high nickel steel and low hydrogen ferritic steel consumables can be used to weld Q&T steels, which can give very low hydrogen levels in the weld deposits. In this investigation, an attempt has been made to study the effect of welding consumables on hydrogen induced cracking of Q&T steel welds by implant testing. Shielded metal arc (SMAW) welding process has been used for making welds using three different consumables, namely austenitic stainless steel, low hydrogen ferritic steel (LHF) and high nickel steel (HNS) to assess HIC by implant testing. The high nickel steel consumables exhibited a higher value of lower critical stress (LCS) and thus they offered a greater resistance to hydrogen induced cracking of armour grade Q&T steel welds than other consumables. The diffusible hydrogen content and the value of the LCS meets the specified limit for armour grade Q&T steel welds and hence, the LHF consumables can be accepted as an alternative to the to the traditionally used ASS consumables and the proposed HNS consumables.     

Effect of welding consumables and processes on dynamic fracture toughness (J 1d) of armour grade Q&T steel joints

Abstract

Austenitic stainless steel (ASS) welding consumables are being used for welding armour grade Q&T steels, as they have higher solubility for hydrogen in the austenitic phase, to avoid hydrogen induced cracking (HIC). Even with austenitic stainless steel consumables under high dilution, the risk of HIC prevailed. In recent years, the developments of low hydrogen ferritic steel (LHF) consumables that contain no hygroscopic compounds are utilised for welding Q&T steels. The use of ASS fillers for welding armour grade Q&T steels creates a duplex microstructure (austenite and δ ferrite) in the welds, which drastically reduces the joint efficiency (ratio of ultimate tensile strength of the joint and the base metal). On the other hand, the weld made using LHF fillers exhibited superior joint efficiency due to the preferential ferrite microstructure in the welds. The use of ASS and LHF consumables for armour grade Q&T steels will lead to formation of distinct microstructures in their respective welds. This microstructural heterogeneity will have a drastic influence on the dynamic fracture toughness of the armour grade Q&T steel welds. Hence, in this investigation an attempt has been made to study the influence on the welding consumables and processes on the dynamic fracture toughness properties of armour grade Q&T steel joints. Shielded metal arc welding (SMAW) and flux cored arc welding (FCAW) processes were used for fabrication of the joints using ASS and LHF welding consumables. The joints fabricated by SMAW process using ASS consumables exhibited superior dynamic fracture toughness values compared to all other joints.     

Solidification modeling and solid-state transformations in high-energy density stainless steel welds

The solidification and solid-state transformations which occur during the high-energy density (HED) welding of austenitic stainless steel were studied. Comparisons were made between structures observed in gas tungsten arc (GTA) welds and those of electron beam (EB) and laser welds using Fe-Ni-Cr ternary alloys with Cr/Ni ratios ranging from 1.5 to 1.85. Weld solidification and microsegregation was modeled using a finite difference analysis and compared with experimental results. These calculations were also used to help interpret the origin of the observed microstructures. Calculations showed that little solid-state diffusion occurs during the solidification and cooling of primary austenite solidified welds, whereas structures which solidify as ferrite may become almost completely homogenized as a result of diffusion. A change in solidification mode from primary austenite to primary ferrite was found to occur at higher Cr/Ni ratios with the HED welds than with GTA welds and is attributed to dendrite tip undercooling. A nearly segregation-free, single-phase austenite structure which appears to be unique to the rapid solidification velocities and cooling rates of HED welds was also observed. It is suggested that this structure is a product of ferrite solidification which transforms to austenitevia a massive transformation.     

奥氏体不锈钢中氮溶解度的热力学计算和实验研究

采用热力学分析方法,对固态不锈钢304、304L、301S和301L(γ-相)以及奥氏体不锈钢熔体中氮溶解度进行了计算,得出了氮溶解度的计算模型;同时通过1 kg MoSi电阻炉对4种奥氏体不锈钢在1520～1580℃和33～100 kPa压力下的渗氮行为进行了实验研究。结果表明,氮在固态奥氏体不锈钢的γ-相中的溶解度最高;在常压冷却、凝固过程中存在的液相、δ-相至γ-相的转变;当不锈钢熔体中相对于δ-相过饱和的氮在钢中以气泡形式析出,则降低了奥氏体钢的氮含量,所以采用常压快速冷却或加压浇注有利于冶炼高氮奥氏体不锈钢。     

不锈钢连铸凝固过程中夹杂物的捕捉规律研究

通过对不锈钢板坯连铸凝固组织枝晶间距的研究,探讨了连铸工艺下铸坯凝固特征参数与夹杂物捕捉之间的关系。首先分析了SUS304、SUS430、SUS409L三个不锈钢钢种正常工况下板坯连铸铸坯的凝固宏观结构及树枝晶微观结构,结合相关数值模拟,估算了这三个不锈钢钢种板坯的凝固系数以及坯壳凝固前沿的凝固速度、温度梯度和冷却速度,并据此推断出夹杂物粒子在结晶器内临界上浮尺寸。     

经济型双相不锈钢的研发进展

 经济型双相不锈钢是一种高性能低成本的氮合金化不锈钢新材料,具有典型的铁素体-奥氏体双相组织。利用氮取代镍元素的奥氏体化作用,降低成本的同时获得优良的力学性能和耐腐蚀性能。介绍了经济型双相不锈钢的发展历史,重点讨论了合金元素和热处理对相变、力学性能和耐腐蚀性能的影响规律,并与304和316进行对比;同时,分析了经济型双相不锈钢焊接性能和焊接工艺的研究进展。经济型双相不锈钢S32101、S32003、S32202等,已用于核电、桥梁、建筑、热交换器等行业,取代传统奥氏体不锈钢AISI 304和316。由于经济型双相不锈钢具有高强度和优良耐蚀性,同时镍、钼等贵金属的含量都较低,已成为未来不锈钢发展的方向之一。     

Welding L415/316L Bimetal Composite Pipe Using Post-Internal-Welding Process

The butt welding of bimetal composite pipes generally adopts single-side welding, which easily gives rise to the problems such as high cost or crack initiation. In this paper, the butt welding of L415/316L bimetal mechanical lined pipes was conducted using post-internal-welding process, which is double-side welding process, proposed by the authors. Firstly, the effect of groove shape on the weld process was discussed. Then, microstructures and mechanical and corrosion-resistance properties of welded joints welded using two different welding materials, 309MoL and 309L, were investigated. The results show that the most suitable groove is that L415 is V shape with angle of 60° and blunt edge of 1 mm and 316L is stripped 6–8 mm. The weld of both 309MoL and 309L is composed of austenite and a small amount of ferrite, but the presence of Mo can refine the grains and increase the content of ferrite phase. The width of transition layer is about 0.6–0.8 mm located at the weld junction of stainless steel weld and carbon steel weld, and the transition layer mainly contains martensite. The tensile and bending performances of the welded joints using both 309MoL and 309L do meet the standard requirements. The welding wire 309MoL can improve the corrosion resistance to Cl− compared to 309L. It is advisable to use the post-internal-welding process and 309MoL for the welding of bimetal composite pipes under environments containing Cl−.     

The influence of thermofluids phenomena in gas tungsten arc welds in high and low thermal conductivity metals

A comprehensive thermofluids numerical model of stationary gas tungsten arc (GTA) welding has been developed and used to examine the effects of thermofluids phenomena on the temperatures and flow velocities in the weld pool and their impact on the resultant weld dimensions for welds produced in high vs low thermal conductivity metals. A dynamic grid re-mapping technique was used to map a block of finite elements into the liquid weld pool to permit use of a k–ε turbulence model in the liquid. Simulations for low conductivity AISI 304 stainless steel show that good correlation with experimental data was only possible if the effects of fluid flow and turbulent mixing in the weld pool were modeled accurately. Conversely, simulations in higher conductivity metals, 6061 and 1100 aluminum, showed that while the average flow velocities and level of turbulence were higher, their effect on the final temperatures and weld pool dimensions were less significant.     

Effect of Oxide Fluxes on Depth of Penetration in Flux Bounded Tungsten Inert Gas Welding of AISI 304L Stainless Steel

Flux Bounded Tungsten Inert Gas (FBTIG) welding is a modified TIG welding process in which increased depth of penetration (DoP) can be achieved by laying thin flux coatings on either side of the weld centerline. The effect of three single component fluxes viz., SiO₂, TiO₂ and Cr₂O₃ on bead geometry of autogenous melt runs in AISI 304L stainless steel for the gap between the flux layers varying from 2 to 7 mm, is studied. Results show that DoP can be improved significantly in FBTIG process using single component fluxes. Nature of the flux and the gap between the flux layers influence the weld bead geometry. Among the three fluxes used, SiO₂ is more efficient in improving the DoP. Arc constriction is the predominant mechanism operative in improving the DoP in FBTIG welding. Possibility of change in solidification mode in FBTIG weld metals of stainless steels is highlighted.     

Improvement of weldability of TRIP steels by use of in‐situ pre‐ and post‐heat treatments

Low alloy TRIP‐aided steels are very interesting for the automotive industry as they combine both a high strength and an excellent formability. Though the actually developed TRIP steels can be considered as low alloyed when compared to the first generations of steels exhibiting TRIP effect, due to their chemical composition, they still exhibit a quite high carbon equivalent. This is particularly detrimental for the weldability of those materials. After solidification, welds are very hard and can show a brittle behaviour. The hardness of the heat affected zone of the welds can even exceed 500HV and cold cracking phenomena is prone to occur. In the automotive industry, spot welding is the main joining process. During spot welding of TRIP steels, the interface between the plates can act like a notch and promote fracture of the weld. This is particularly dangerous when brittle welds are submitted to peel stresses. The aim of the paper is to demonstrate that a careful choice of the process parameters can significantly improve the resistance of the welds. The selection of the welding cycle parameters is far from being an easy task as many different parameters are involved. Therefore, a design of experiment methodology (DOE) was chosen to optimise the welding cycle for a cold‐rolled TRIP steel with a tensile strength above 700 MPa. Mechanical properties of the welds were significantly improved by use of pre‐ and post‐heat treatments. Those improved welding cycles were realised without excessive extension of the total weld cycle on a conventional spot welding machine. This means that the optimised welds can be obtained in the existing production lines without any additional investment or significant decrease in productivity.     

The effects of process variables on pulsed Nd:YAG laser spot welds: Part I. AISI 409 stainless steel

The weldabilities of AA 1100 aluminum and AISI 409 stainless steel by the pulsed Nd:YAG laser welding process have been examined experimentally and compared. The effects of Nd:YAG laser welding parameters, including laser pulse time and power intensity, and material-dependent variables, such as absorptivity and thermophysical properties, on laser spot-weld characteristics, such as weld diameter, penetration, melt area, melting ratio, porosity, and sur-face cratering, have been studied experimentally. The results of this work are reported in two parts. In Part I, the weldability of AISI 409 stainless steel by the pulse laser welding process is reported. In Part II, the weldability of A A 1100 aluminum under the same operating con-ditions is reported and compared to those of the stainless steel. When welding AISI 409 stainless steel, weld pool shapes were found to be influenced most by the power intensity of the laser beam and to a lesser extent by the pulse duration. Conduction mode welding, keyhole mode welding, and drilling were observed. Conduction mode welds were produced when power in-tensities between 0.7 and 4 GW/m² were used. The initial transient in weld pool development occurred in the first 4 ms of the laser pulse. Following this, steady-state conditions existed and conduction mode welds with aspect ratios (depth/width) of about 0.4 were produced. Keyhole mode welds were observed at power intensities greater than 4 GW/m². Penetration of these keyhole mode welds increased with increases in both power intensity and pulse time. The major weld defects observed in the stainless steel spot welds were cratering and large-occluded gas pores. Significant metal loss due to spatter was measured during the initial 2 ms of keyhole mode welds. With increasing power intensity, there was an increased propensity for occluded gas pores near the bottom of the keyhole mode welds. Formerly Graduate Student.     

Effect of Treatment Time on the Microstructure of Austenitic Stainless Steel During Low-Temperature Liquid Nitrocarburizing

The effect of treatment time on the microstructure of AISI 304 austenitic stainless steel during liquid nitrocarburizing (LNC) at 703 K (430 °C) was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Experimental results revealed that the modified layer was covered with the alloy surface and the modified layer depth increased extensively from 2 to 33.4 μm with increasing treatment time. SEM and XRD showed that when the 304 stainless steel sample was subjected to LNC at 703 K (430 °C) for less than 4 hours, the main phase of the modified layer was expanded austenite. When the treatment time was prolonged to 8 hours, the abundant expanded austenite was formed and it partially transformed into CrN and ferrite subsequently. With the increased treatment time, more and more CrN precipitate transformed in the overwhelming majority zone in the form of a typical dendritic structure in the nearby outer part treated for 40 hours. Still there was a single-phase layer of the expanded austenite between the CrN part and the inner substrate. TEM showed the expanded austenite decomposition into the CrN and ferrite after longtime treatment even at low temperature.     

A new Bcc-Fcc orientation relationship observed between ferrite and austenite in solidification structures of steels

A new crystallographic orientation relationship (OR) between delta-ferrite and austenite has been observed in solidification microstructures of 304L and 309S austenitic stainless steels and a ternary Fe-Cr-Ni alloy. Evidence for the new OR was obtained from electron diffraction patterns in transmission electron microscopy (TEM). This relationship, (111)_fcc//(110)_bcc and , has not been previously reported for bcc-fcc systems. The 〈110〉_fcc//#x2329;110〉_bcc alignment is distinctive among known bcc-fcc ORs. The new OR is related to the Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) ORs by relative rotations of 35.26 and 30 deg, respectively, about the normal to the parallel close-packed planes. In 304L fabricated by laser-engineered net shaping (LENS), delta-ferrite with the new OR was found to coexist in the microstructure with both K-S and N-W oriented ferrite, but in separate austenite grains and with less frequent occurrence. In gas-tungsten arc (GTA) welds of 309S and the Fe-Cr-Ni alloy, the new OR was the only one observed within a few grains, whereas ferrite within other grains did not establish an apparent OR with the austenite matrix.     

Effect of Heat Input on Macro,Micro and Tensile Properties of Flux Cored Arc Welded Ferritic Stainless Steel Joints

This paper discusses the influence of Flux Cored Arc Welding Process parameters such as welding current, travel speed, voltage on bead profile, metallurgical and mechanical properties of welds of 2 mm thick 409M ferritic stainless steel sheets. The study reveals that, grain coarsening, volume fraction of martensite, hardness of heat affected zone and % of delta ferrite in ER 309 weld metal increases with increase in heat input. However, the results show that variation of heat input does not make any significant effect on tensile strength of the joint. Hence, welding parameters that provide uniform bead profile for the weld are recommended for fabrication.