微合金钢焊缝金属中针状铁素体相变动力学分析

采用热膨胀法成功地测出不同热输入下微合金钢焊缝金属组织中针状铁素体（AF）相变开始温度及终止温度，并绘制出AF转变开始温度和终止温度曲线，得出焊缝金属组织中AF相变温度范围为668—541℃。在热模拟和定量金相试验基础上，通过对试验数据的分析，建立了微合金钢焊缝金属中AF相变的各种T-tc-f动力学曲线。通过对AF各种相变动力学曲线的分析和讨论，得出微合金钢焊缝金属组织中AF相变的一般规律。     

X80管线钢焊缝金属中的针状铁素体

探讨了X80管线钢焊缝中针状铁素体的形成条件、对焊缝韧性的影响及针状铁素体控制机理。结果表明,X80管线钢焊缝组织是大量针状铁素体+少量先共析铁素体的混合组织。在针状铁素体影响因素中,起决定作用的是焊缝的化学成分和冷却速度。焊缝中针状铁素体形态和数量与焊缝韧性之间存在对应关系,焊接热输入对焊缝韧性的影响较复杂,存在一个热输入最佳值。优化的焊缝合金系统和化学成分是控制焊缝针状铁素体形成的必要条件,而合理的工艺方法和焊接参数(含热输入)则是控制针状铁素体形成的充分条件。     

低合金高强度钢焊缝金属中针状铁素体的微观组织

利用光学显微镜,扫描电子显微镜及EBSD(electron backscattering diffraction)分析技术对800MPa级低碳微合金高强度钢的焊缝金属进行了分析.结果表明,焊缝金属组织主要为针状铁素体和贝氏体.针状铁素体以夹杂物为核心形核,该夹杂物主要是以Al2O3为核心形成的钛氧化物.针状铁素体以夹杂物为核心多维形核呈放射状生长,仅某些方向的针状铁素体晶核迅速长大,生长方向存在取向择优.EBSD分析同样表明针状铁素体晶粒存在取向择优.观察到由同一夹杂物生长,沿同一直线方向背向生长的针状铁素体取向相同,沿不同方向生长的针状铁素体取向不同,说明其沿原奥氏体惯习面生长,并非与夹杂物共格.     

低合金钢焊缝的针状铁素体微观组织

针对X70管线钢多道焊焊缝金属,利用Gleeble1500热模拟机进行了热模拟试验,对焊缝金属中初生针状铁素体组织及其与非金属夹杂物之间的关系以及经历二次热循环后感生形核的针状铁素体组织进行了SEM及TEM分析.结果表明,针状铁素体边界存在着一层富碳的薄膜,其长大既存在切变转变的特征,同时也伴随着C原子的扩散过程.共感生成的二次针状铁索体是在初生针状铁索体基体的高密度位错处形核并以一定的速度迅速长大到有限的尺寸.初生针状铁素体和共感形核针状铁素体都是在奥氏体晶内形核、长大,都具有细化奥氏体晶粒、提高多道焊焊缝金属和焊接热影响区韧性的作用.     

焊缝中针状铁素体形成影响因素

综述了低合金高强钢焊接时针状铁素体的形成机理,重点探讨了影响针状铁素体形核的因素:夹杂物、焊接热输入等。分析指出了目前研究工作中的不足和发展方向。     

Zr含量对大热输入焊缝金属组织及冲击韧性的影响

采用气电立焊大热输入焊接方法,制备了3种不同Zr含量的焊缝金属,研究分析了Zr含量对大热输入焊缝金属组织与冲击韧性的影响。结果表明:在185 k J/cm大热输入焊接条件下,Zr的质量分数为0. 12%的1号焊缝金属中夹杂物数量有限,形成了大量粗大的等轴铁素体晶粒,不利于冲击韧性的提高; Zr的质量分数为0. 35%的2号焊缝金属中夹杂物数量明显增加,形成了致密的针状铁素体(AF)组织,大幅度提升了焊缝金属的冲击韧性; Zr的质量分数为0. 66%的3号焊缝金属中Zr的固溶量增加,提高了焊缝金属的淬透性,导致相变温度降低,促进了贝氏体的生成,恶化了焊缝金属的冲击韧性。     

微合金钢焊缝组织中针状铁素体形核与长大驱动力

利用KRC模型对焊缝金属中针状铁素体转变的热力学驱动力进行了理论计算。计算结果表明，焊缝金属中AF(针状铁素体)相变开始温度的理论值为1100K(827℃)。试验所测得的焊缝金属中针状铁索体转变开始温度实际值为670℃，针状铁素体的相变热力学驱动力数值为△G^→AF γ1≤-570J／mol。随着转变温度的降低和焊缝金属中碳含量的降低，针状铁素体的形核和长大驱动力增加针状铁索体的总相变驱动力小于其形棱和长大的驱动力．这种差距随转变温度的降低而增大。     

低合金高强钢焊缝针状铁素体转变动力学及其仿真

合理控制低合金高强钢焊缝中针状铁素体的转变行为，对获得理想的焊缝性能具有重要意义。建立了基于夹杂物惰性界面直接生长的焊缝针状铁素体的连续转变动力学模型，仿真结果与实际试验结果能很好地吻合。     

Mechanical properties and grain refinement by acicular ferrite in high strength high toughness weld metal

The volume fraction and morphology of acicular ferrite evolution in a high strength high toughness weld metal were studied and the mechanical properties of weld metal under heat input of 21 kJ/cm with and without fast cooling were tested.The results show the weld metal can obtain a large proportion of acicular ferrite during a wide range of cooling rate and the sizes of acicular ferrite in length and thickness decrease with cooling rate increasing.The weld metal exhibited high tensile strength(895 MPa and 870 MPa) and good low temperature toughness(average A_(KV-30℃) 104 J and 79.2 J).The higher tensile strength and better low temperature toughness of the weld metal under fast cooling are due to the more refined grain of acicular ferrite.     

In situ observation of morphological development for acicular ferrite in weld metal

Abstract

Acicular ferrite is regarded as the most desirable microstructural feature, in view of strength and toughness, in mild and low alloy steel weld metals. Recent evolution and diversity of mechanical property for base metal demand the same property as the weld. Therefore, understanding of the formation mechanism for acicular ferrite microstructure in weld is one of the essential problems for low alloy steel weld. In the present work, the morphological development of acicular ferrite in situ, was observed during weld cooling. The sample designed to form acicular ferrite microstructure was schematically heated and cooled by infrared imaging furnace and the morphological developments were directly observed using laser scanning conforcal microscopy. The nucleation and growing at inclusion, sympathetic nucleation and impingement event of acicular ferrite were directly shown in high time resolution.     

B元素对药芯焊丝焊缝金属针状铁素体形成的影响

利用金相组织观察、冲击试验和热膨胀试验,研究了B元素含量变化对高强钢药芯焊丝焊缝金属中针状铁素体形成的影响,得到了不同试验温度下焊缝金属冲击吸收功. 结合透射电镜分析和分级淬火试验从热力学和动力学的角度对B元素影响机理进行了分析. 结果表明,焊缝金属组织晶界中含有自由状态的B元素具有抑制晶界铁素体形核利于针状铁素体生成的作用;N元素含量增加会降低晶界B元素含量,并提高奥氏体向铁素体转变的温度,减少针状铁素体含量;针状铁素体是在以Ti元素和Mn元素的氧化物为核心,以Cu元素和Mn元素的硫化物为外层,以BN为过渡层的复杂结构上形核并长大的;针状铁素体含量的增加有利于提高焊缝金属冲击吸收功,–60 ℃冲击吸收功最大为70 J.     

针状铁素体对超细晶铁素体钢力学性能的影响

针对利用形变强化铁素体相变工艺制备出的超细晶铁素体钢,其抗拉强度很高但伸长率偏低的问题,对其进行了组织精细分析。精细分析结果表明,针状铁素体的形成主要与轧后冷速有关,冷速越大,针状铁素体越多,针状铁素体对钢的力学性能具有双重影响,一方面钢的抗拉强度随着针状铁素体体积比的增加而增加;另一方面又因针状铁素体其内部的精细结构特征、形态特征、针尖效应及其尺寸等影响因素,使钢的伸长率大为降低。当将试验钢中针状铁素体体积比控制在7.28%左右时,可使钢的抗拉强度在高达740 MPa时,仍具有26%的理想伸长率。     

The influence of PTFE used as basic covered electrode binder on weld metal acicular ferrite formation

Weld metals obtained from covered electrodes agglomerated with polytetrafluorethylene presented ultra-low diffusible hydrogen content and appreciable quantities of acicular ferrite. This article investigates the polymer influence on its constituent formation. The microstructure produced by consumables containing this component presented acicular ferrite quantities higher than expected in the literature for weld metals with the same carbon, silicon and manganese content. This difference could not be related to welding parameters employed. The residual chemical element evaluation indicated nitrogen as being mainly responsible for the microstructural variances observed. The operational characteristics of classic E7018 and polymer-agglomerated electrodes showed the lesser weld metal nitrogen absorption must be associated with two main factors. The dissolved nitrogen of the metallic drops collected during the polymer-agglomerated electrode welding showed most effective shielding atmosphere. The carbon content of the polymer-agglomerated electrode indicated an atmosphere rich in this element. This is in accordance with a best shielding process hypothesis. The metallic drops’ average size, dissolved nitrogen content as a function of size and voltage oscillograms indicated metallic transfer mode changes.     

Effect of grain boundary ferrite on susceptibility to cold cracking in high-strength weld metal

Due to the practical limitations of lowering the diffusible hydrogen content of flux-containing welding consumables, it is now felt that modification of the weld microstructure would alleviate the risk of weld metal cracking in multi-pass weld deposits. Thus, this study aimed to identify and evaluate the effect of the weld microstructure on the cold cracking susceptibility of FCAW weld metals and then to provide a basic guideline for designing new welding consumables from a microstructural point of view. In order to identify the parameter(s) that can quantify the microstructural susceptibility of multi-pass weld deposit, two sets of FCAW wires with tensile strength of about 600 MPa were prepared by controlling the Ni content to allow sufficient variation in the weld microstructure, but with little change in weld metal strength. The cold crack susceptibility of those two chemistries was evaluated by a multi-pass weld metal cracking test at various levels of diffusible hydrogen content. All of the cold cracks developed were Chevron-type, and the occurrence of such cracks depended upon the proportion of grain boundary ferrite (%GF) as well as the diffusible hydrogen (HD) content. In fact, at the same level of HD, higher Ni (1.5%Ni) wire showed better resistance to cold cracking than lower Ni (0%Ni) wire even though the latter was stronger and also higher in carbon equivalent. This result could be explained solely by the difference in grain boundary ferrite content between those two welds since Chevron cracking preferentially initiates at and propagates along grain boundary ferrite. Therefore, we propose the use of a value of %GF as a parameter to quantify the microstructural susceptibility of ferritic multipass weld deposit with a strength level of about 600 MPa. It was further suggested that, in addition to the hydrogen control approach, microstructural modification in the form of reducing the %GF can be pursued to develop welding consumables with improved resistance to cold cracking.     

Crystallographic analysis for acicular ferrite formation in low carbon steel weld metals

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view in low carbon low alloy steelweld metals. The samples from electro slag welding (ESW) and submerged arc welding (SAW) deposits with various cooling rates were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from inclusions. The inclusions contributing to acicular ferrite formation were of multi-phase type consisting of amorphous phase, spinel type and MnS. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction patterns and energy-dispersive X-ray spectrometer analyses that the Ti-enriched layer was TiO. The acicular ferrite had a Baker–Nutting orientation relationship with the TiO layer on the inclusion surface. The misfit was 3.0% at the interface between the acicular ferrite and TiO. Therefore, it is considered that TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit. However, themorphologies of ferrite growth which nucleated from inclusions were different in both samples. Whereas the growth of ferrites nucleated from TiO was enough in ESW, the size of nucleated ferrite in SAW was a few hundred millimetres in size. In the early stage of nucleation from TiO, ferrite had small deviation from Kurdjumov–Sachs orientation relationship (K–S relationship) in both ESW and SAW. However, there was a difference in the growth stage of ferrite. The ferrite orientations were gradually changed to fit to the K–S relationship in ESW. On the other hand, the nucleated ferrite in SAW stopped growing and the newly nucleated ferrite which had K–S relationship prior to austenite was formed adjacently because of large super cooling due to small heat input.     

夹杂物对微合金钢熔敷金属针状铁素体形核的影响

研究了微合金钢熔敷金属中夹杂物大小、尺寸分布和化学成分对针状铁素体形核的影响。结果表明,作为针状铁素体形核核心的夹杂物,其尺寸大多数位于0．2．0．6μm之间,并且是含有多种元素的复合夹杂物,具有化学成分不均匀的性质。夹杂物作为一种高能的惰性表面,降低了形核能垒,促进了针状铁素体的形核。当一个针状铁素体以夹杂物为初始核心先期形核后,又可诱发大量的相互交错的针状铁素体感应形核。     

微合金管线钢针状铁素体等温转变行为

通过Gleeble模拟热变形后的等温转变过程,研究了一种微合金管线钢的中温转变行为,对等温过程膨胀量变化情况及相变组织观察,研究了试验钢针状铁素体相变过程及其组织特征。结果表明,试验管线钢针状铁素体转变主要发生在550℃左右,针状铁素体组织中具有较高密度的位错及亚结构,其转变具有形核与生长两阶段特征。