Ca处理对Ti大线能量焊接非调质低合金高强钢组织与性能的影响

研究了Ca处理对含Ti大线能量焊接非调质低合金高强钢的组织与性能的影响.Ca处理钢组织结构得到细化,大线能量焊接热模拟热影响区(HAZ)晶界铁素体生长被抑制,其钢板及HAZ的强韧性均得到有效提高.Ca处理使含Ti钢热影响区氧硫化物复合夹杂物由片条状转变成球状,并从TiMnOS占主导变为TiCaMnOS-Al2O3占主导,提高了晶内针状铁素体形核能力.     

钒-钛微合金化钢焊接热影响区晶内铁素体与夹杂物的界面关系

采用聚焦离子束方法制备试样,利用最高加速电压为1 300 kV的高分辨电镜观察钒-钛微合金化钢100 kJ/cm大线能量热模拟焊接热影响区,对作为晶内铁素体形核核心的夹杂物形貌与结构及夹杂物与晶内铁素体界面关系进行了研究.结果表明,晶内铁素体在MnS V(C,N)复合夹杂物相上形核析出,夹杂物和晶内铁素体存在确定的晶体学位向关系.     

正三棱锥形夹杂物诱导晶内铁素体形核模型研究

通过建立晶内铁素体形核模型,结合数学推导发现,基底相为不规则形状的夹杂物比基底相为球形夹杂物形核功小,易诱导铁素体形核。根据数学模型分析球形夹杂物诱导晶内铁素体形核的影响因素。结果表明:夹杂物尺寸过小时不利于诱导晶内铁素体形核,其尺寸增加到0.15μm后,对铁素体的形核影响反而不大;夹杂物与钢液的润湿角越小,越易诱导铁素体形核;夹杂物与钢液的润湿角为70°~90°时,球形夹杂物易诱导正三棱锥形铁素体析出,而润湿角小于70°时,易诱导球形铁素体析出;析出相为正三棱锥形铁素体形核模型的形核率比析出相为球形铁素体模型的形核率高。通过实验验证了不规则夹杂物可以诱导铁素体形核,且夹杂物尺寸大于0.15μm后,对铁素体的形核影响不大。     

焊缝中夹杂物对针状铁素体形成的影响研究现状

综述了近年来国内外在夹杂物对针状铁素体形成的影响方面的研究进展,重点探讨了夹杂物诱发针状铁素体形核的4种机理,以及焊缝中常见的一些夹杂物对针状铁素体形成的影响。分析指出了目前研究工作中的不足和发展的方向。     

低合金钢焊缝中针状铁素体的形成及影响因素

综述了低合金钢焊缝中针状铁素体的形成及影响因素,包括铁素体形核位置、奥氏体晶粒尺寸、合金元素、夹杂物等,重点探讨了夹杂物的成分、尺寸及分布对针状铁素体形核的影响。     

热输入对1200MPa级HSLA钢焊缝组织性能的影响

为了研究焊接工艺条件对焊缝金属组织性能的影响,采用不同热输入对1200 MPa级低合金高强钢进行熔化极气体保护焊,利用OM、SEM、TEM观察并分析不同焊接热输入对焊缝组织及力学性能的影响规律。结果显示,当热输入为16、20、25 k J/cm时,焊缝组织主要以针状铁素体为主,并含有少量M-A组元以及粒状贝氏体。随焊接热输入增大,针状铁素体组织有所增多且板条宽度逐渐增大,而粒状贝氏体组织减少。焊缝内非金属夹杂物类型多为促进针状铁素体形核的Ti-Mn-Al-O-S系复合氧化物夹杂。焊缝金属硬度、冲击韧性及焊接接头强度随热输入增大基本呈下降趋势,并且各焊接热输入条件下焊缝金属具有良好的强韧性匹配。随热输入增大,焊缝金属断裂特征由韧性、脆性混合型断裂向脆性断裂转变。热输入为20 k J/cm时,焊接接头综合性能最佳。     

氧含量及夹杂物对高强钢金属芯焊丝E120C-K4熔敷金属冲击韧性的影响

本工作通过改变保护气体配比和Zr-Ti、Ti-B微合金化研究了金属芯焊丝E120C-K4熔敷金属中的氧含量及夹杂物对其微观组织和冲击韧性的影响。研究表明:在金属芯焊丝E120C-K焊接熔敷金属中,随着氧含量的增加,熔敷金属组织晶粒尺寸增大,粒状贝氏体含量增加,夹杂物尺寸增大、含量增加,熔敷金属的冲击韧性明显降低;添加微量Zr-Ti微合金的熔敷金属中夹杂物面积在单位面积所占比例降低,夹杂物的平均粒径减小,尺寸集中于0.4～0.7μm之间,为针状铁素体(AF)形核提供了潜在质点,从而提高了熔敷金属的冲击吸收功。     

X100级管线钢的动态塑性变形行为研究

采用原位拉伸扫描电镜试验对X100级管线钢的动态塑性形变行为进行观察,并运用EBSD微观取向分析技术对形变前后管体组织的取向变化进行分析。结果表明,X100级管线钢的微观组织由针状铁素体、粒状贝氏体和M/A岛组成。在拉伸应力的作用下针状铁素体首先发生形变,随着应变的增加,针状铁素体形变累积到一定程度后,导致粒状贝氏体发生形变。针状铁素体边界和贝氏体基体上的M/A组织钉扎位错使变形不易发生。由EBSD取向可知,晶体在发生形变后轧面的{110}晶面方向沿拉伸形变方向转动。由扫描电镜照片观察到,在外加应力作用下,夹杂物成为微裂纹形核核心并随着外加应力的增加而扩展,最后连接,导致裂纹贯穿基体直至失效。     

Mg处理石油储罐用钢大线能量焊接性能研究

采用Gleeble对不同焊接线能量下微量Mg处理石油储罐用钢粗晶热影响区(CGHAZ)的低温韧性进行了研究,并结合SEM,TEM等技术对CGHAZ进行了微观分析.结果表明:经微镁处理690MPa级石油储罐用钢,改变了钢中夹杂物的形态和分布状态,使钢中原有的氧化铝和硫化物夹杂物变成细小的弥散分布的含镁夹杂物,提高母材与焊接热影响区性能;随着线能量的增加,微镁处理690MPa级石油储罐钢CGHAZ的M-A组元均随着线能量的升高而粗化,M-A组元面积百分数随线能量的升高而降低,导致其低温冲击韧性降低.     

Ti对Fe-Ni膨胀合金凝固组织的影响

采用变质处理法和扫描电镜(SEM)研究了Ti对Fe-Ni膨胀合金凝固组织的影响.结果表明:经Ti变质处理后,Fe-Ni膨胀合金中形成了大量的Ti2O3-MnS高熔点复合夹杂物,尺寸约为2μm;Ti2O3复合夹杂物在凝固过程中作为非自发形核核心,使铸坯凝固组织细化.Ti在凝固组织的等轴晶和柱状晶晶界上以TiN,TiC,Ti(NC),Ti2O3形式存在,在一定程度上起到阻止晶粒长大的作用.     

Formation of acicular ferrite and influence of vanadium alloying

Abstract

The effect of vanadium microalloying in promoting a tough, acicular ferrite microstructure in C-Mn steels has been investigated. The microstructure obtained consisted of fine interlocking ferrite plates and was indistinguishable from acicular ferrite developed in steel weld metals, apparently from the intragranular nucleation of ferrite at inclusions. A number of variables were examined in high purity experimental steels including composition and heat treatment conditions, and related to a metallographic examination of the microstructure by high resolution micro-analytical transmission electron microscopy and surface analysis. A comprehensive study of the inclusions in the steels, containing different ratios of oxygen and nitrogen concentration, did not find any significant evidence that inclusion assisted nucleation was the sole determining factor in producing acicular ferrite. Moreover, no evidence could be found to relate vanadium alloying to significant vanadium nitride precipitation, either separately, or associated with the inclusions. Thus, in the present steels, any possible alternative influence of vanadium on intragranular ferrite nucleation is not obscured by effects associated with the inclusion population. The vanadium concentration appeared to be the most important influence in developing an acicular ferrite microstructure in these experimental steels, and this is not inconsistent with previous reports in the literature of a beneficial 'vanadium effect'. Evidence for vanadium segregation in the microstructure was found, which may be related to the effect of vanadium in encouraging the formation of acicular ferrite. Even when there is good evidence that inclusions are responsible for intragranular ferrite nucleation (as, for example, in steel weld metals), a 1 :1 inclusion-ferrite relationship has been difficult to establish. Thus, even an inclusion activated nucleation theory is likely to require additional intragranular ferrite formation without inclusion assistance, such as sympathetic or autocatalytic nucleation, and this could be reflected in the present study by vanadium atom clustering facilitating an alternative intragranular ferrite nucleation mechanism.     

Overview Inclusion assisted microstructure control in C–Mn and low alloy steel welds

Abstract

Inclusion assisted microstructure control has been a key technology to improve the toughness of C–Mn and low alloy steel welds over the last two to three decades. The microstructure of weld metals and heat affected zones (HAZs) is known to be refined by different inclusions, which may act as nucleation sites for intragranular acicular ferrite and/or to pin austenite grains thereby preventing grain growth. In the present paper, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the HAZ of steels are rationalised along with nucleation mechanisms. Acicular ferrite development is considered in terms of competitive nucleation and growth reactions at austenite grain boundary and intragranular inclusion nucleation sites. It is shown that compared to weld metals, it is difficult to shift the balance of ferrite nucleation from the austenite grain boundaries to the intragranular regions in the HAZ of particle dispersed steels because inclusion densities are lower and the surface area available for ferrite nucleation at the austenite grain boundaries tends to be greater than that of intragranular inclusions. The most consistent explanation of high nucleation potency in weld metals is provided by lattice matching between ferrite and the inclusion surface to reduce the interfacial energy opposing nucleation. In contrast, an increase in the thermodynamic driving force for nucleation through manganese depletion of the austenite matrix local to the inclusion tends to be the dominant nucleation mechanism in HAZs. It is demonstrated that these means of nucleation are not mutually exclusive but depend on the nature of the nucleating phase and the prevailing transformation conditions. Issues for further improvement of weldment toughness are discussed. It is argued that greater numbers of fine particles of a type that preferentially nucleate acicular ferrite are required in particle dispersed steels to oppose the austenite grain boundary ferrite transformation and promote high volume fractions of acicular ferrite and thereby toughness.     

大线能量焊接条件下低合金高强度钢针状铁素体形核影响因素及形核机理研究

综述了大线能量焊接条件下低合金高强度钢中针状铁素体形成的研究进展,概述了针状铁素体显微组织结构特点及其在低合金高强度钢中的主要性能,具体讨论了针状铁素体具有良好强韧性和抗断裂性等力学性能的原因,重点分析了大线能量焊接条件下影响焊接热影响区中针状铁素体形核长大的因素,包括合金元素及夹杂物的种类和尺寸,得出针状铁素体的形核机理是多种机理联合作用的结果。     

Effect of Cu addition on microstructure and impact toughness in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The effects of Cu content on microstructure and impact toughness in the simulated coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels were investigated. It has been observed that the microstructure in the simulated CGHAZ of Cu-free steel is dominated by a small proportion of acicular ferrite and predominantly bainite with martensite–austenite constituent. Whereas, in the 0.45 and 1.01% Cu-containing steels, the acicular ferrite increased significantly due to the effective nucleation on intragranular inclusions with outer layer of MnS and CuS. The formation of acicular ferrite is attributed to superior high heat-affected zone impact toughness in the 0.45% Cu-containing steel. Furthermore, the increasing martensite–austenite constituent and ε-Cu precipitates in the simulated CGHAZ of 1.01% Cu-containing steel caused degradation in impact toughness.     

Influences of titanium and manganese on high strength low alloy SAW weld metal properties

The objective of this work was to study the influence of titanium on API 5L-X70 steel weld metal properties at manganese levels of 1.4 and 2%. The best mechanical properties in the weld series were obtained in two compositions, i.e. 1.92%Mn–0.02%Ti and 1.40%Mn–0.08%Ti. In both groups of welds, acicular ferrite in the microstructure was increased with addition of titanium in the range of 0.02–0.08%. Manganese helped to refine and homogenize weld microstructures. Increased hardenability of the weld due to further addition of titanium or manganese encouraged grain boundary nucleation of bainite with greater frequency than intragranular nucleation of acicular ferrite. Also, the amount of manganese in inclusions was decreased with the addition of titanium to the weld. The impact toughness of the weld metal was improved by addition of titanium, but beyond the optimal titanium percentage, the quasi-cleavage fracture mode appeared in the specimens again.     

Formation of acicular ferrite at oxide particles in steels

Abstract

Experimental steels similar in composition to structural grades were prepared from weld metal deposits to study the formation of acicular ferrite under conditions experienced in the heat affected zone for a range of welding processes. The formation of acicular ferrite under these conditions is found to be dependent on the presence of a suitable distribution of oxide inclusions > 0·4 μm in size. The characteristics and proportion of acicular ferrite in the microstructure also depend on the prior austenite grain size and cooling rate. The relationship between these factors is presented in a simplified quantitative model, which is supported by data from limited welding trials. Metallographic observations suggest that acicular ferrite forms in two stages. The first involves the formation of relatively large primary acicular ferrite plates by multiple nucleation at intragranular inclusion sites, and the second involves the formation of many smaller acicular ferrite grains that grow sympathetically from the primary plates.

MST/1027     

Acicular ferrite nucleation and growth in API5L-X65 steel submerged arc welded joints

In the present study, acicular ferrite nucleation and growth were investigated from a crystallographic point of view in API5L-X65 steel weld metals. It was observed that acicular ferrite was nucleated on inclusions in weld metal. The titanium-enriched layer around inclusions was characterised as the TiO phase by selected area diffraction patterns. A scanning electron microscope equipped with an electron backscatter diffraction technique was applied to investigate the crystallographic orientation relationship. The results showed that acicular ferrite lathes had a Baker–Nutting orientation relationship with the TiO layer on the inclusion surface in the nucleation of acicular ferrite. However, it is confirmed that the growth of acicular ferrite lathes depends on the deviation from the Kurdjumov–Sachs relationship between lathes and primary austenite.     

The nature of acicular ferrite in HSLA steel weld metals

In this paper, the nature of the fine interlocking acicular ferrite microstructure in HSLA steel weld metals is investigated. The results strongly suggest the acicular ferrite is comprised of intragranularly nucleated Widmanstätten ferrite. Further, it is shown that the active nucleation sites for this ferritic product are weld metal inclusions. Sympathetic nucleation then takes place which leads eventually to the fine, interlocking microstructure which is a characteristic of acicular ferrite.     

Microstructures in a resistance spot welded high strength dual phase steel

The microstructure of a high strength dual phase steel resistance spot welded with tempering-pulse technology is characterized in this paper. In the fusion zone, there is a needle-like microstructure identified as acicular or side plate ferrite that has a cube-on-cube orientation relationship with respect to the surrounding martensite. In contrast to the microstructures produced by the lower cooling rate arc or laser welding techniques, the nucleation of this fine intragranular ferrite takes place independent of inclusions. Further, a leaf-like microstructure within the martensitic matrix is found to contain primitive orthorhombic Cr₃C₂ and face-centered cubic CrC chromium carbides, rather than Cr₂₃C₆ or Cr₇C₃ as is commonly observed in steel alloys. The formation histories of both the ferrite phase and the chromium carbides are analyzed.