Low Heat Input Conditions Research Articles

모델 가스터빈 연소기에서 합성가스 연소성능시험 - Part 2 : NOx/CO 배출특성, 온도특성, 화염구조

본 논문은 부게넘 및 태안 IGCC플랜트의 연료를 대상으로 석탄으로부터 생성된 합성가스의 NOx/CO배출 특성, 온도특성, 화염의 구조에 대해 기술하였다. GE7EA 모사 가스터빈 연소기를 대상으로 상압 고온 연소시험을 수행하여 입열량 및 질소희석에 따른 연소특성을 관찰하였다. 입열량이 감소하고, 희석제량이 증가할 때 단열화염온도의 감소로 NOx가 감소하였고, 저부하에서는 희석량이 증가할수록 불완전 연소로 인해 CO가 증가하였다. 이러한 NOx 감소 및 CO발생은 각각 <TEX>$1500^{\circ}C$</TEX> 와 <TEX>$1250^{\circ}C$</TEX>의 특정 화염온도에서 관찰되었다. 또한 질소 희석이 증가할수록 단열화염온도 및 연소기 라이너의 온도가 감소했고, 화염 부상과 같은 화염구조의 변화로 인해 특이점이 관찰되었다. 본 연구결과를 통해 질소희석이 NOx/CO배출 및 온도, 화염구조에 미치는 영향이 검토되었으며, 시험 데이터는 향후 태안 IGCC 플랜트의 운전시 최적 조건 선정을 위한 기초자료로 활용될 예정이다. This paper describes on the NOx/CO emission characteristics, temperature characteristics and flame structures when firing coal derived synthetic gas especially for gases of Buggenum and Taean IGCC. These combustion characteristics were observed by conducting ambient-pressure elevated-temperature combustion tests in GE7EA model combustor when varying heat input and nitrogen dilution ratio. Nitrogen addition caused decrement in adiabatic flame temperature, thus resulting in the NOx reduction. At low heat input condition, nitrogen dilution raised the CO emission dramatically due to incomplete combustion. These NOx reduction and CO arising phenomena were observed at certain flame temperature of <TEX>$1500^{\circ}C$</TEX> and <TEX>$1250^{\circ}C$</TEX>, respectively. As increasing nitrogen dilution, adiabatic flame temperature and combustor liner temperature were decreased and singular points were detected due to change in flame structure such as flame lifting. From the results, the effect of nitrogen dilution on the NOx/CO and flame structure was examined, and the test data will be utilized as a reference to achieve optimal operating condition of the Taean IGCC demonstration plant.

Influence of Zr addition on TIG welding–brazing of Ti–6Al–4V toAl5A06

TIG welding–brazing of Ti–6Al–4V to Al5A06 alloys with pure Al and Al–Cu–Zr fillers was investigated. The Zr and Cu additions enhanced the spreadability of Al-based fillers on Ti–6Al–4V substrate under TIG heating condition, and a sound Ti/Al dissimilar butt joint was obtained. Influenced by Zr addition, a discontinuous TiAl3 layer with a thickness of 2–4μm was observed at the Ti/seam interface under a low heat input condition, which was replaced by TiAl3+L–(Ti,Zr)Al3+H–(Ti,Zr)Al3 multi-sublayers when a high heat input was adopted. The joint with Al–Cu–Zr filler has higher tensile property and wider processing parameters than that with pure Al filler. The largest tensile strength of the joint produced by Al–Cu–Zr filler reaches 284MPa up to 85% of base Al5A06 alloys, and the fracture occurred by quasi-cleavage mode at the TiAl3 sublayer. Zr addition was favorable to enhance the spreadability on Ti substrate, and the welding processing parameters were broadened by the presence of Zr addition.

Effect of single pass welding heat input on microstructure and hardness of submerged arc welded high strength low carbon bainitic steel

The bead on plate test method was employed to investigate the effect of single pass welding heat input on microstructure and hardness of submerged arc welded high strength bainitic steel. The weld metal microstructure contains very high volume fraction of acicular ferrite regardless of heat input. By contrast, with the increase in heat input, the main microstructure in coarse grained heat affected zone (HAZ) changes from a mixture of lath martensite and ferrite sideplate to full ferrite sideplate. Microhardness measurement showed that maximum hardness values always present at the coarse grained HAZ under lower heat input condition, which indicates that the minimum heat input of experimental steel should be higher than 0·92 kJ mm−1 to avoid the formation of welding cold crack. The size distributions of complex inclusions in the weld metal are very similar for all heat inputs mainly because the short residence time does not make the inclusion grow rapidly.

Effect of welding heat input and post-welding natural aging on hardness of stir zone for friction stir-welded 2024-T3 aluminum alloy thin-sheet

The hardness of stir zone (SZ) at as-weld state and post-welding natural aging (PWNA) state under different heat input conditions were investigated for friction stir-welded joints of 2024 aluminum alloy thin-sheets. Results show that welding heat input has obvious effect on the hardness of the SZ. Under the high welding heat input condition, the hardness of the SZ is lower than that of the base metal (BM). However, under low welding heat input condition, the hardness of the SZ exhibits strong dependency on the rotational speed of the stirring tool. With increasing the rotational speed of the stirring tool, the hardness of the SZ increases. The average hardness in the SZ of the joint welded at 1500rpm and 1000mm/min achieves the BM level. In comparison, the effect of the PWNA on the improvement of the hardness in the SZ is limited. The variation in the strengthening-phase particles plays a more important role than does grain size in the SZ for the improvement of hardness.

Formation and distribution of brittle structures in frictionstir welding of aluminium and copper: influence of process parameters

Morphological, metallographic and structural analyses of aluminium–copperdissimilar welds produced under different friction stir welding conditionswere conducted in order to analyse the mechanisms of intermetallic phasesformation, its relation with welding conditions and its consequences in theweld structure and morphology. Under lower heat input conditions, only a thinintermetallic layer distributed along the aluminium/copper interface was depictedinside the nugget. Increasing the heat input promoted material mixing andformation of increasing amounts of intermetallic rich structures. The intermetallicphase content and the homogeneity of the mixed area increased with increasingheat input, evolving from structures containing Al, Cu, CuAl2 andCu9Al4 to structures predominantly composed of Cu9Al4and Cu(Al). In order to explain these results, the mechanisms ofintermetallic phases formation are discussed, taking into account the processparameters and material flow mechanisms in friction stir welding. Importantrelations between intermetallic formation and weld surface morphology werealso found.

Analysis of WC/Ni-Based Coatings Deposited by Controlled Short-Circuit MIG Welding

This study investigates the recently developed controlled short-circuit metal inert gas (CSC-MIG) welding system for depositing WC/Ni-based claddings on carbon steel substrates. WC/Ni-based coatings deposited by CSC-MIG were analyzed by optical light microscopy and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) capabilities. X-ray diffraction (XRD) and hardness measurements of depositions are also reported. The CSC-MIG welding system provides a significant amount of user control over the current waveform during welding and has lower heat input when compared with traditional MIG welding. Heat input for the analyzed coatings ranged from 10.1 to 108.7 J/mm. Metallurgically bonded coatings free from spatter and with 0.75% average porosity were produced. It was found that the detrimental decarburization of the WC particles seen in thermal spray systems does not occur when welding with the CSC-MIG. Precipitation of a reaction layer around the reinforcing phase was identified as WC; the average thickness of which increases from 3.8 to 7.2 μm for the low and high heat input condition, respectively. Precipitation of newly formed WC particles was observed; their size distribution increased from D50 of 2.4 μm in the low heat input weldment to 6.75 μm in the high heat input weldment. The level of dilution of the reinforcing phase increases significantly with heat input. The hardness of the deposited coatings decreases from 587 HV10 to 410 HV10 when the energy input was increased from 10.1 to 108.7 J/mm.

이종 알루미늄 합금 A6K31/A5J32 겹치기 마찰교반 접합부의 인장성질에 미치는 접합조건의 영향

The scope of this investigation is to evaluate the effect of joining parameters on the microstructural features and mechanical properties of dissimilar aluminum alloys, 1mm-thickness fixing AA6K31 at the top position and fixing AA5J32 at the bottom position. The friction stir lap welds were studied under various welding conditions, rotation speed of 1000, 1250, 1500rpm and welding speed of 100, 300, 500, 700mm/min, respectively. Mechanical test has been investigated in terms of tensile shear test and hardness test. The results showed that three type nugget shapes such as onion ring, zigzag type, hooking with the void, have been observed with revolutionary pitch. All welding conditions fractured at the HAZ of top plate, A6K31 and also the strength compare with base metal of lap joints were low efficiency, 52~63%. The thickness of fractured position was decreased with the lower heat input conditions. The relationships were excellent due to linear between the effective thickness of fractured position and peak load. The fractured position was the interface between joint area and not joint area. Also the strength efficiency compared with base metal was lower than decreasing rate of thickness because the hardness was decreased at fractured position due to softened material.

The influence of stabilization with TITANIUM on the HEAT-AFFECTED ZONE SENSITIZATION of 11 to 12% chromium ferritic stainless steels under low heat input welding conditions

The occurrence of sensitization during continuous cooling after welding was investigated for two low chromium ferritic stainless steels: a non-stabilized steel conforming in composition to EN 1.4003, and a corresponding grade stabilized with titanium to increase its sensitization resistance. These steels transform partially to austenite in the high temperature heat-affected zone (HTHAZ) adjacent to the fusion line during cooling, with the austenite subsequently transforming to martensite below the Ms temperature. In the non-stabilized condition these alloys are known to sensitize during continuous cooling after welding at low heat inputs levels (below approximately 0.5 kJ/mm). The rapid cooling rates associated with low heat input welds suppress austenite nucleation in the HTHAZ, resulting in almost fully ferritic microstructures. Chromium-rich carbides precipitate at the continuous ferrite-ferrite grain boundaries, resulting in chromium depletion. Slower cooling rates promote the formation of more austenite in the HTHAZ during cooling. The austenite absorbs excess carbon, preventing supersaturation of the ferrite and inhibiting carbide precipitation. Titanium stabilization does not prevent sensitization during low heat input welding, and may even be detrimental. The high temperatures experienced by the HTHAZ promote the dissolution of titanium carbides, and the titanium is retained in solid solution due to fast cooling. At lower temperatures chromium-rich M23C6 precipitates nucleate at the ferrite-ferrite grain boundaries, resulting in sensitization. The titanium in solid solution acts as a strong ferrite-former, increasing the ferrite content in the HTHAZ. The non-stabilized 1.4003 steel contained considerably more grain boundary martensite in the HTHAZ after low heat input welding than the titanium-stabilized grade, despite having very similar Kaltenhauser ferrite factors. The presence of grain boundary martensite in the non-stabilized grade increased the resistance to sensitization under low heat input conditions.

Development of high strength, high conductivity copper by friction stir processing

The objective of this study is to obtain a high strength, high conductivity copper by friction stir processing. Three milli meter thick pure copper plate was friction stir processed to a depth of 2.8 mm at low-heat input conditions by varying the travel speed from 50 to 250 mm/min at a constant rotation speed (300 rpm) to obtain fine grains. Grain size of the nugget decreased from 9 to 3 μm and the hardness increased from 102 to 114 H V by increasing the traverse speed from 50 to 250 mm/min. Yield strength, microhardness and ultimate tensile strength increased with decrease in grain size in the nugget region and the yield strength obeyed σ s = 223.8 + 0.07 d −1/2 Hall–Petch relationship, where d is the grain size in m. Electrical resistivity measurement at room temperature showed that there was no change in the resistivity of the processed samples compared to the base metal.

Effect of Heat Input Conditions on Microstructure and Mechanical Properties of Friction-Stir-Welded Pure Copper

Defect-free friction stir welds of 5-mm-thick pure copper plates were produced in relatively low heat input conditions. The characteristics of the microstructure and mechanical properties of the welded joints were investigated. The stir zone (SZ) exhibited equiaxed recrystallized grains, whose size decreased as the heat input was decreased. The percentage of high-angle grain boundaries (grain boundary misorientation angle >15 deg) in the SZ was quite high (90.2 to 94.5 pct) and increased as the heat input was increased. When the heat input was decreased, the percentage of the twin boundaries (TBs) dropped, and the number of the twin lamellas was reduced. Under a very low heat input condition, the typical characteristics of thermomechanically affected zone (TMAZ) were discernible; however, the TMAZ was characterized by a recrystallized grain structure at higher heat input conditions. The grains in the heat-affected zone (HAZ) were slightly coarsened compared to those in the parent material (PM), but the grain size varied a little under different parameters. The hardness of the SZ increased as the heat input was increased, and the lowest hardness appeared at the HAZs where the welds failed. The ultimate tensile strength (UTS) was similar to that of the PM under various heat input conditions, but the yield strength (YS) and elongation were lower. The YS increased as the lowest hardness value increased, and the elongation decreased due to the enhanced strain localization.

Friction stir weld nugget temperature asymmetry

Two different hardness distributions were observed to result from the friction stir welding of an Al–Mg–Si alloy using identical welding speeds but different heat inputs. In one case, the distribution was relatively symmetric about the weld centreline and in the other, the hardness declined continuously from the advancing to the retreating sides of the weld nugget. Based on in-probe temperature measurements, full-field grain size mapping, and response to post-weld heat treatment, it has been ascertained that under low heat input conditions, the weld nugget attained a higher temperature on the advancing side than on the retreating side. Such a temperature asymmetry has often been proposed and has been predicted by simulation but, has not been convincingly demonstrated before this time.

Characterisation of microstructure and corrosion behaviour of thin section AISI 430 stainless steel gas tungsten arc weldment

Thin section ferritic stainless steels (AISI 430) welded by autogenous gas tungsten arc welding with low heat input conditions were characterised for the microstructural features, hardness and corrosion behaviour. Development of very coarse grained heat affected zones (HAZ) was observed even with the low heat input level applied. The weld metal was found to be over matched in terms of mechanical properties (hardness and tensile strength). Both the parent material and the weld metal/HAZ region have shown poor corrosion resistance in neutral and acidic chloride solutions. The corrosion resistance of these regions was found to be adversely influenced with increasing chloride level in the test electrolyte. The weld metal/HAZ region of this weldment has shown a higher susceptibility to localised corrosion damage compared to the parent alloy in both the neutral and acidic chloride solutions.

Crystal Growth in Laser Surface Melting and Cladding of Ni-Base Single Crystal Superalloy

Crystal growth behaviours in the laser surface-melted region and overlay weld metal of/on Ni-base single crystal superalloy CMSX-4 has been investigated. The Ni-base superalloy Rene 142 was used as the filler metal for overlay welding. Surfaces of the specimens were prepared as to be coincident with the (001) plane. The surface melting and overlay welding were conducted using a 2 kW diode laser along the [100] direction. Electron back scattering analysis for the crystal orientation revealed that, under low heat input conditions, the surface-melted region solidified as a single crystal with unidirectional dendrites which grew from the base metal epitaxially along the [001] direction. For intermediate heat input conditions, the surface-melted region was also found to be single-crystalline; however, it consisted of orthogonal dendrites along the [001], [010] and [100] directions. In contrast, under high heat input conditions, stray crystals were observed in some parts of the surface-melted region resulting in the melted region becoming polycrystalline. A numerical analysis of the dendritic growth direction and the constitutional supercooling at solidification front indicated that the overlay weld metal grew epitaxially on the substrate and stray crystals tended to form under conditions of higher laser power and (V f / V w) ratio (V f: wire feeding speed, V w: laser scanning speed), which agreed with the metallographic observation. A numerical calculation combining CET (columnar to equiaxed transition) theory with an inverse heat conduction analysis allowed us prediction of the conditions needed to suppress the formation of stray crystals and to promote single-crystal production in the overlay welded region with epitaxial dendritic growth of [001] orientation and/or dendritic growth of [100] and [010] orientations. Using these predictions, a single crystalline layer of 3 mm thickness on a single crystal substrate could be achieved by laser overlay welding with ten passes. Furthermore, creep rupture properties of single-crystalline surface-melted region were comparable or slightly better than that of the CMSX-4 base metal.

Duplex Stainless Steel Welds and their Susceptibility to Intergranular Corrosion

Duplex stainless steels (DSS) as alternatives to conventional austenitic stainless steels for the construction of pipelines is becoming more wide-spread, particularly for sour service applications where corrosion resistance / stress corrosion cracking resistance is required in aggressive chloride /sulphide environments. While these steels show many superior characteristics, limitations are associated with the welding of these steels, particularly controlling the weld structure and properties and understanding how the weld metallurgy may influence the susceptibility to intergranular corrosion (IGC). The focus of this paper is to report on the findings from a detailed study of the various weld sections within a DSS pipeline, as a function of heat input and type of weld, in terms of the metallurgical structure, composition and mechanical properties and to assess the susceptibility to IGC. Welding was performed using the manual Gas Tungsten Arc Welding (GTAW) technique at both high and low heat input conditions. Two different join configurations (double bevel single V bevel and double bevel single U joint configuration) were adopted. Structural analysis consisted of (i) optical microscopy of welded specimens; (ii) ferrite content determination; (iii) Vickers hardness measurements; (iv) Charpy impact studies and (v) transverse tensile testing. Two test methods namely a modified ASTM A262 and a modified Double Loop Electrochemical Potentiokinetic Reactivation (DL-ERP) test was used to determine susceptibility to IGC. The electrolyte solution used was 0.5M H2SO4 + 0.001 M TA (thioacetamide). The test was conducted at 60 °C. The potential was scanned from -500 mV (SCE) to +200 mV (SCE) and back to 500 mV (SCE) at a rate of 1.67 mV/s. The ratio of the reactivation charge to the passivation charge was calculated. From the results obtained it can be shown that the fill region was most susceptible to IGC for all weld conditions.

Ni基単結晶超合金の表面溶融部の組織学的検討

Crystal growth behavior in the surface-melted region of Ni-base single crystal superalloy CMSX-4 was investigated. The surface melting was conducted using 2kW diode laser or gas tungsten arc. Laser power and scanning speed in laser surface melting were varied while defocusing distance and shielding gas (Ar) flow rate were fixed. For arc surface melting, arc current and welding speed were varied. Specimen surfaces were arranged parallel to the (001) of the base metal. The microstructure and crystal orientation in the melted region were analyzed by using optical microscopy, scanning electron microscopy(SEM) and electron backscattered pattern analysis(EBSP). The microstructure and crystal orientation in laser surface-melted region were markedly influenced by heat input. The laser surface-melted region under low heat input conditions was found to be solidified as the single-crystalline state with unidirectional dendrites which grew along the [001] directions. The laser surface-melted region under intermediate heat input conditions was also single-crystalline with dendrites which grew along the [001], [010] and [100] directions. In contrast, in the case of high heat input conditions, the laser surface-melted region was consisted of polycrystal with stray crystals. Such tendencies were also observed in the melted region with gas tungsten arc. These results demonstrate that heat input significantly affects single-crystallinity regardless of the type of heat source, such as laser and arc. Furthermore, stray crystal formation in surface-melted region was investigated in detail by using SEM and EBSP. From the results of these experiments, it is revealed that stray crystal formation was affected by constitutional supercooling and dendrite growth direction.

Development of a fine-grained microstructure and the properties of a nugget zone in friction stir welded pure copper

Defect-free pure copper welds were achieved under low heat input conditions of 400–800 rpm for a traverse speed of 50 mm min−1. The grain size of the nugget zones decreased from 9 to 3.5 μm with decreasing rotation rate from 800 to 400 rpm. “Onion rings”, consisting of different-sized grain bands, were observed at 400 rpm, but disappeared at 600–800 rpm. Variations of both microhardness and yield strength of the nugget zones with grain size followed the Hall–Petch relationship.

Grain Growth Behavior and Mechanical Properties of the Friction Stir Welded Zone of 7055 Al Alloy Followed by Post Weld Heat Treatment

The grain growth behavior and mechanical properties in the friction stir weld zone after post weld heat treatment (PWHT) have been investigated. As PWHT temperature increased, a normal grain growth of as-welded equaxied grains ceased and abnormally grown grains with elongated shape coarsened. Huge elongated grains changed into smaller equaxied grains at 500°C. In case of lower heat input condition, abnormal grain growth initiated faster due to smaller initial grain size. The weld zone with bigger initial grains had advantages to maintain the thermal stability at high temperature. The hardness near the weld zone was almost recovered to the 95% of the unaffected base metal at 500 °C and the weld zone under lower heat input condition resulted in the homogeneous recovery through the whole weld zone.

Study of Microstructure in Surface-Melted Region of Ni-Base Single Crystal Superalloy CMSX-4

This work investigated microstructure in the surface-melted region of Ni-base single crystal superalloy CMSX-4 by using a diode laser beam as a heating source. Such processing parameters as laser power and scanning speed in laser surface melting were varied while defocusing distance and shielding gas (Ar) flow rates were fixed. Specimen surfaces were arranged parallel to the (001) of base alloy. The microstructure in the melted region was analyzed by optical microscopy and SEM. Crystal orientation of the melted region was analyzed using electron backscattered pattern analysis. The microstructure was remarkably changed when the heat input of surface melting was varied. The surface-melted region was found to solidify into a single crystal with directional dendrites that grew along the [001] directions under low heat input conditions. The surface-melted region was also a single crystal with disoriented dendrites that grew along the [100] or [010] directions under medium heat input conditions. In contrast, the melted region consisted of poly crystals with stray crystals under high heat input conditions. Such tendencies were also observed in the melted region with gas tungsten arc. These results demonstrate that the surface-melted region can solidify into a single crystal under conditions in which larger temperature gradient and higher solidification rates can be achieved.

An Understanding of HSLA-65 Plate Steels

HSLA-65 plate steels can be produced using one of five plate manufacturing techniques: normalizing, controlled rolling (CR), controlled rolling followed by accelerated cooling (CR-AC), direct quenching and tempering (DQT), or conventional quenching and tempering (QT (b) eliminate or substantially reduce the need for preheating during welding; (c) resist susceptibility to hydrogen-assisted cracking (HAC) in the weld heat affected zone (HAZ) when fusion (arc) welded using low heat-input conditions; and (d) depending on section thickness, facilitate high heat-input welding (about 2 kJ/mm) without significant loss of strength or toughness in the HAZ. However, application of this plate manufacturing process and of these controls produces significant differences in the metallurgical structure and range of mechanical properties of the HSLA-65 plate steels both among themselves and versus conventional higher strength steel (HSS) plates. For example, among the HSLA-65 plate steels, those produced by Q&T exhibit minimal variability in mechanical properties, especially in thicker plates. Besides variability in mechanical properties depending on plate thickness, the CR and CR-AC plate steels exhibit a relatively higher yield strength to ultimate tensile strength (YS/UTS) ratio than do DQT and Q&T steels. Such differences in processing and properties of HSLA-65 plate steels could potentially affect the selection and control of various secondary fabrication practices, including arc welding. Consequently, fabricators must exercise extreme caution when transferring allowable limits of certified secondary fabrication practices from one type of HSLA-65 plate steel to another, even for the same plate thickness.

Effect of microstructure on hydrogen embrittlement of weld-simulated HSLA-80 and HSLA-100 steels

HSLA-80 and HSLA-100 steels have been subjected to weld-simulated grain-coarsened heat-affected zone (GCHAZ) and grain-refined heat-affected zone (GRHAZ) treatments at peak temperatures of 1350 °C and 950 °C, respectively, followed by varying cooling rates to approximate the weld heat inputs of 10 to 50 kJ/cm. Subsequent slow strain rate testing in synthetic seawater has been employed to assess the hydrogen embrittlement (HE) propensity of the materials. It is indicated that in spite of an increase in strength after weld simulation, further ductility deterioration, compared to the base material under similar testing conditions, did not occur in GCHAZ HSLA-100 steel and for low heat input condition of GRHAZ HSLA-80. This has been attributed to their HE resistant microstructures. Predominant acicular ferrite or lath martensite or a combination of both imparts resistance to HE, as observed in the case of grain-coarsened HSLA-100 and for the low heat input grain-refined HSLA-80 steels. The deleterious effect of bainitic-martensitic microstructure has been reflected in the ductility values of grain-coarsened HSLA-80, which is in agreement with the observation of higher susceptibility of the as-received HSLA-100 steel having a similar structure. However, contrary to its beneficial effect in the as-received HSLA-80, an acicular ferrite structure has shown vulnerability toward HE for high heat input grain-refined HSLA-80. This has been attributed to the presence of polygonal ferrite and to the development of an HE susceptible substructure on GRHAZ weld simulation.