Inclusion effects on submerged-arc weld microstructure

Bead-on-plate welds were produced using twenty four fused reagent grade submerged arc welding fluxes, selected from three flux systems, SiO₂-MnO-FeO, MnO-CaF₂ and SiO₂-CaO-CaF₂. The welds were processed using AISI 1010 steel coupons, and a Lincoln L-50 AWS type A5.17) welding wire with a constant heat input of 3.0 kJ mm^–1. The three flux systems were selected because of their different oxygen potentials and their ability to produce welds with a wide oxygen range (70 to 1400 p.p.m.). Qualitative and quantitative metallography and chemical analysis were performed on the welds. Inclusion morphology and volume fraction are observed to be affected by flux composition Inclusions of 1 m in size and greater are associated with grain boundary and blocky proeutectoid ferrites, while inclusions 0.64 m and smaller are linked with the presence of acicular ferrite.     

Microstructure of copper-bearing C---Mn weld metal: As-welded and stress-relieved states

The effect of copper concentration in the range from 0.02 wt.% to 1.4 wt.% on the microstructure and hardness of multipass C---Mn welds has been studied both in the as-welded state and after a stress relief treatment of 2 h at 590°C. The microstructure has been investigated in terms of the proportions of the various ferrite morphologies, grain size, second-phase (carbides, martensite, retained austenite, …) volume fraction, inclusion distribution, and the extent of copper precipitation.

The main effects due to copper are (1) increase in hardness, (2) refinement of the microstructure, (3) increase in the second-phase volume fraction and (4) local precipitation of ε-Cu in the as-welded state. These results are discussed with respect to copper as a γ-stabilizing element. The local precipitation of copper is discussed in terms of composition variations due to segregation effects in the welds.

The stress relief treatment leads to (i) precipitation and spheroidization of carbides, and (ii) precipitation of ε-Cu, including grain boundary precipitation, in the 0.66% Cu and 1.4% Cu welds.     

Post weld heat treatment of a niobium stabilized austenitic weld metal

A study has been made of the effects of post weld heat treatments in the range 700°C to 900 °C on the microstructural, impact and creep crack growth properties of a 16Cr-8NI-6Mn-Mo, V, Nb, B austenitic weld metal. These treatments result in the progressive decomposition of delta-ferrite to M₂₃C₆ and a phase as well as enhanced precipitation of NbC; the time- temperature- precipitation characteristics have been determined. Impact energies are severely reduced, whereas the creep crack growth resistance of material heat treated at 800°C or 850°C is significantly better than for the as-deposited weld metal.     

Effect of TiO2-containing fluxes on the mechanical properties and microstructure in submerged-arc weld steels

This work presents a study of the effect of TiO₂ additions in fluxes on the mechanical properties and microstructure of the weld metal formed during Submerged-Arc Welding (SAW) of ASTM A-36 steel plates. Four fluxes with about 9, 12, 15 and 18% Ti were used with a low-carbon electrode. The welding conditions were kept constant. The microstructure of the weld metal for each flux consisted mainly of equiaxed ferrite and acicular ferrite. The increase in the percentage of acicular ferrite and a decrease in its length were observed with an increase in titanium content. The increase in titanium content in fluxes also improved the toughness and ductility of the welds.     

Effects of heat treatment process and niobium addition on the microstructure and mechanical properties of low carbon steel weld metal

The mechanical properties and microstructure were evaluated and analyzed by optical microscopy (OM) and transmission electron microscopy (TEM) for micro-alloy carbon steel weld metal with and without Nb addition, respectively, under different heat treatment processes including stress relief annealing, normalizing, and no treatment after welding. The strength and elongation of the weld metal without treatment after welding were improved with the addition of Nb element, and the impact toughness was not affected obviously with the Nb addition. After stress relief annealing, the strength decreased for the Nb-free weld metal, while the elongation and impact toughness increased. However, for the Nb-bearing weld metal, stress relief annealing improved the strength of the weld metal significantly, and deteriorated the elongation and impact toughness. In the case of normalizing treatment to the weld metal, it was shown that with the increase of the holding time at the normalizing temperature of 920 °C, for both the weld metals with and without Nb addition, the microstructure of the columnar grain zone (CGZ) was transformed from one of columnar grain into one of equiaxed grain. The grain size of the equiaxed grain zone (EGZ) increased initially, then remained almost unchanged with the prolonging of the holding time. The mechanical properties of the weld metal with and without Nb addition showed no obvious change with the increasing holding time. With the increase of the normalizing temperature, the strength of the Nb-bearing weld metal increased, while the elongation and impact toughness decreased significantly. OM and TEM analysis found that the fine NbC particles were precipitated at the normalizing temperature of 920 °C, which refined the grains of the weld metal and increased the impact toughness. With the increase of the normalizing temperature, the content of widmanstatten ferrite (WF) in the Nb-bearing weld metal increased, whereas the quantity of the NbC particles decreased, which improved the strength and lowered the impact toughness.     

焊接热循环对1 000 MPa级焊缝金属组织性能的影响

为研究多层多道焊对1 000 MPa级焊缝金属组织性能的影响,采用热模拟试验对末道焊缝金属进行一次及二次焊接热循环,并通过冲击、硬度试验,利用金相显微镜、扫描电镜、透射电子显微镜和电子背散射衍射技术分析热循环后焊缝金属组织性能的变化规律.结果表明:单次热循环下,随着峰值温度(Tp)的升高,冲击韧性逐渐下降,显微硬度先上升后下降;TP为800℃时出现了软化现象,显微硬度326 HV,冲击韧性较高为64 J;TP为950℃时,其奥氏体晶粒相对细小,具有良好的综合力学性能;TP为1 350℃时,显微硬度最高为383 HV,冲击吸收功只有19 J,其脆化原因是冷却时获得粗大原奥氏体晶粒以及粗大的、且方向性较为一致的板条马氏体和贝氏体硬脆组织;经800℃的二次循环后,再次出现软化现象,显微硬度为313 HV,并且焊缝热影响区中残余奥氏体体积分数为1.39%,冲击韧性得到改善为59 J.     

Effect of silicon on microstructure of Ti3Al alloys containing niobium

Abstract

The effect of silicon content on the microstructure and phases present in Ti–(20–23)Al–11Nb alloys has been studied in the temperature range 800–1270°C.Four phases, β_o, α₂, O, and a silicide, are formed. The parent β_o is ordered at 1270°C. At 1050°C α₂ is formed which exhibits a higher silicon solubility than the parent β_o. A peritectoid transformation β₀+silicide→α₂ is proposed. Assuming that niobium substitutes for titanium and silicon for aluminium, energy dispersive X-ray spectroscopic data suggest that the α₂ phase, unlike that in binary Ti–Al alloys, is highly stoichiometric and of the form (Ti+Nb)₃(Al+Si). Similarly the silicide corresponds to binary Ti₅Si₃ with the same site substitution as in the α₂ phase. The O phase is orthorhombic and similar in composition to the α₂ which it replaces: its formation is promoted by silicon.

MST/3088     

Relation between tensile properties and microstructure in type 316 stainless steel SA weld metal

Tensile properties of a thick, multipass, submerged-arc (SA) weld-deposited type 316 are investigated by tests at room temperature and at 400 ° C and by microstructural and compositional analyses. The as-deposited metal, which shows a lower yield strength, a comparable ultimate tensile strength and a lower total elongation compared to the (solution-annealed) parent metal, is characterized by systematic variations in tensile properties across its thickness, with the highest strength and the lowest ductility in the weld centre. These variations are related to material variability (mainly changes in dislocation density) within the weld metal due to local dissimilarities in thermal and mechanical histories during welding.     

The role of oxygen rich inclusions in determining the microstructure of weld metal deposits

The influence of the deoxidation product size distribution in determining the prior austenite grain and the subsequent balance between inter- and intragranular nucleation has been studied in C-Mn-Nb weld metals. The growth of austenite appears to be controlled by pinning of the grain boundaries by the deoxidation inclusions, and follows the classical Zener precipitate-boundary interaction equations. Some evidence of the direct nucleation by inclusions of the acicular ferrite phase has been obtained.     

Effect of artificial strain aging on microstructure and mechanical properties of a single-bead ferritic weld metal

Abstract

The microstructure and mechanical properties of a single-bead ferritic weld metal were studied in the as-welded and artificially strain-aged condition. It was found that the phenomena by which artificial strain aging increases hardness and strength, but decreases toughness, are caused by an increased dislocation density and the formation of precipitates at dislocations within the ferrite grains. These differences on a submicrometre scale are responsible for the observed changes in mechanical properties upon strain aging the ferritic weld metal.

MST/123     

T91钢焊缝及热影响区显微组织图象分析

T91钢具有良好的高温抗氧化性和抗腐蚀性,该钢含8％－9．5％Cr,合金含量复杂,焊接难度大,运用金相显微镜和扫描电镜（SEM）分析了不同爆接工艺条件下T91耐热钢焊缝及热影响区或区域显微组织特征,利用XQF－2000型显微图象分析仪对显微组织中各相的相对含量和奥氏体晶粒度进行了测量,分析了焊接线能量对T91钢焊接接头区组织性能的影响,结果表明,采用多道焊焊接工艺,严格控制焊接线能量在16kJ/cm左右,可以防止T91钢焊缝区奥氏体晶粒粗大,避免在热影响区出现声状铁素体组织,从而保证焊接接头区具有良好的组织性能。     

热输入对Q890高强钢焊缝组织及性能的影响

针对国内某钢厂最新研制的Q890高强钢,采用三种不同的热输入对其进行气体保护焊接,研究了不同热输入对焊缝金属组织、硬度及冲击韧性的影响.结果表明,3种热输入下,焊缝组织主要以板条贝氏体为主,并含有粒状贝氏体、少量的板条马氏体和残余奥氏体.随着热输入的增大,焊缝组织中贝氏体铁素体板条粗化,板条马氏体逐渐减少,而粒状贝氏体逐渐增多,部分残余奥氏体由薄膜状向块状转变;焊缝金属硬度随着热输入的增大而下降;焊缝金属的冲击韧性亦呈逐渐下降的趋势.     

Evolution of sigma phase in 321 grade austenitic stainless steel parent and weld metal with duplex microstructure

Abstract

Samples of 321 stainless steel from both the parent and welded section of a thin section tube were subjected to accelerated ageing to simulate long term service conditions in an advanced gas cooled reactor (AGR) power plant. The initial condition of the parent metal showed a duplex microstructure with approximately 50% ferrite and 50% austenite. The weld metal showed three distinct matrix phases, austenite, delta ferrite and ferrite. This result was surprising as the initial condition of the parent metal was expected to be fully austenitic and austenite+delta ferrite in the weldment. The intermetallic sigma phase formed during the accelerated ageing was imaged using ion beam induced secondary electrons then measured using computer software which gave the particle size as a function of aging time. The measurements were used to plot particle size, area coverage against aging time and minimum particle spacing for the parent metal. During aging the amount of ferrite in the parent metal actually increased from ～50 to ～80% after aging for 15?000 h at 750°C. Sigma has been observed to form on the austenite/ferrite boundaries as they may provide new nucleation sites for sigma phase precipitation. This has resulted in small sigma phase particles forming on the austenite/ferrite boundaries in the parent metal as the ferrite transforms from the austenite.     

Effects of Mn and Ti on microstructure and inclusions in weld metal of high strength low alloy steel

Abstract

The influences of alloying elements on chemical composition of non-metallic inclusions, impact toughness and microstructure in weld metals of high strength low alloy steels have been studied. Results indicated that microstructure had changed from a mixture of acicular ferrite, proeutectoid ferrite, ferrite side plates and microphases to a mixture of acicular ferrite, bainite and microphases due to the addition of Mn and Ti. The impact toughness of weld metal was improved correspondingly. The volume fraction and composition of inclusions both influenced the proportion of acicular ferrite. Mn and Si based oxide globular inclusions located at the boundary of acicular ferrite plates in the weld metal produced using C–Mn–Si–Cu wire. When Mn and Ti were added to welding wires, the inclusions within acicular ferrite plates permitted fewer primary acicular ferrite plates to grow into relatively larger dimensions. Secondary acicular ferrites nucleating on pre-existing ferrite plates refined microstructure effectively.     

On the solidification microstructure of copper-rich niobium alloys

The microstructure of copper-rich niobium alloys has been studied as a function of cooling rate. The main alloy composition investigated was Cu-7 wt% Nb. The cooling rate was varied from 40° C sec^–1 to approximately 7×10^4° C sec^–1. The microstructure as observed in the scanning electron microscope changes from well-dispersed niobium spheres for fast-cooling to an heterogeneous distribution of niobium flowers for slow-cooling.     

Pitting corrosion resistance of as welded and thermally aged nitrogen containing type 316 stainless steel weld metal

Abstract

Type 316 stainless steel weld metal with 0·07%N, prepared using nitrogen bearing 316 stainless steel filler wire by the manual metal arc (MMA) welding process, was evaluated for the microstructural changes and pitting corrosion resistance in as welded and aged (at both 1023 and 1123 K for 0·5, 1, 10, and 100 h) conditions. The initial delta ferrite content was about 5·5 ferrite number, which transformed from 70 to 100% as secondary precipitates depending on the aging conditions. Electrochemical potentiokinetic reactivation studies did not show any reactivation peak indicating the absence of Cr depleted zones. Pitting corrosion studies in a medium of 0·5M NaCl + 0·5M H₂ SO₄ (acidic chloride) by the potentiodynamic anodic polarisation method showed a significant variation in the pitting resistance which depended on the aging conditions. The pitting corrosion resistance has been correlated to the precipitation kinetics of the secondary phases such as sigma, carbide, and Cr₂ N in the weld metal.     

Effect of chromium content on the microstructure and mechanical properties of multipass MMA,low alloy steel weld metal

Effect of chromium content in the range of 0.05–0.91 wt% on the microstructure and mechanical properties of Cr–Ni–Cu low alloy steel weld metal was investigated. All welds were prepared by manual metal arc welding technique in flat position. Microstructure of the welds was examined by optical and scanning electron microscope in both columnar and reheated regions of the weld metal. The results showed increase in acicular ferrite and microphases formed at the expense of primary ferrite and ferrite with second phase with steady refinement of microstructure. According to these microstructural changes, yield and ultimate tensile stresses, Hardness and Charpy V-Notch impact toughness increased, whereas elongation decreased. Increase in Charpy impact value is thought to be due to fine dispersed spheroidized dark microphases at high chromium contents.