Finite Element Analysis of Plastic Collapse and Crack Behavior of Steel Pressure Vessels and Piping Using XFEM

This article aims to study the plastic collapse and crack behavior of steel pressure vessels and piping using the extended finite element method (XFEM). First, the plastic collapse loads of steel cylinders under the internal pressure are predicted, and the numerical results are compared with experimental data. In addition, the computational efficiency and accuracy using different methods including the XFEM, nonlinear stabilization algorithm, and arc-length algorithm are compared. Particularly, effects of different initial crack configurations, element sizes, damage initiation, and evolution criteria on the crack behaviors are investigated. Second, the crack initiation and propagation properties of buried pipelines due to deflection in the landslide area are explored, and the numerical results are compared between testing data and current study. Besides, the effects of internal pressure, wall thickness, soil property, and width of landslide area on the critical deflection displacement of buried pipeline are studied. This research provides a fundamental support for safety evaluation and life prediction of pressurized structures.     

Graphitization in CK 45 Steel

The purpose of this study was to determine the influence of heat treatment cycle on graphite phase formation on CK 45 steel. The presence of well distributed graphite in the matrix is responsible for the good mechanical and thermal properties of this kind of alloy. Such properties include excellent wear resistance, higher resistance to thermal shock, and higher resistance to oxidation at high temperature. A number of specimens were made up of appropriate design to provide the experimental materials. The transformation phase to a free carbon microstructure during graphitization under different conditions was then examined for the most successful experimental steels. Austenitising temperature of 920℃ ahd the following isothermal heat treatment of 750℃ at different holding times were used. Microstructures were examined by OM (optical microscopy) and SEM (scanning electron microscopy). Furthermore, it was found that isothermal transformation at 750℃for different soaking times produced a typical microstructure. Also, the amount of graphite increased with increasing isothermal heat treatment time. Heat treatment leading to supersaturation of iron with carbon was described and some of the consequences of the supersaturation were presented. Finally, the formation of the thermodynamically stable state of the graphite taken from the supersaturated solid solution was discussed.     

TC4外压容器的设计计算

在没有TC4材料外压计算曲线的条件下,提出了TC4制容器的外压计算(包括加强圈的设计)的公式及计算方法,据此确定厚度和设置加强圈是安全的.     

低合金钢的大气腐蚀规律研究

大气腐蚀受自然环境影响因素很多.通过暴露试验,对09CuPTiRE等8种低合金钢和普通碳钢(A3钢)在宜昌、三斗坪、秭归3个试验点进行了大气腐蚀试验,获得了1,2,5 a的腐蚀数据.采用扫描电镜、X射线衍射仪和电子探针分别进行了锈层形貌观察、结构分析及锈层内合金元素分布的测试,采用电化学腐蚀测试仪对锈层进行了阳极极化曲线测定.结果表明:试样在宜昌站的平均腐蚀速率(16.9～23.9 μm/a)是三斗坪站(9.4～14.6 μm/a)的1.5倍左右;耐蚀性最好是09CuPTiRE钢,暴露5 a后其相对耐蚀性在三斗坪约为A3钢的1.54倍,在宜昌约为A3钢的1.41倍.锈层的物理分析及阳极极化曲线的测定验证了暴露试验结果.     

低周疲劳下16MnR微孔的裂纹萌生与扩展

采用宏观与微观结合的方法,在疲劳试验机上进行疲劳试验,在光学显微下观察裂纹的起裂与扩展.研究了压力容器用钢16MnR在低周疲劳下微孔(40～200μm)的裂纹萌生与扩展规律.研究表明裂纹的萌生机制:滑移带启裂和疏松带启裂,前者由剪应力起主要作用,后者由正应力起主要作用.而滑移带的局部性和裂纹开叉是低周疲劳下微裂纹的两大典型现象.微观缺陷尺寸、应力水平对疲劳寿命有显著影响,当应力水平较低时,微孔尺寸对寿命的影响明显.而当应力水平较高时(超过屈服极限),孔径对寿命的影响不敏感.在同一应力水平下,微缺陷尺寸存在临界值dt,当d＞dt时,疲劳寿命下降很多.     

Corrosion Evolution of Low Alloy Steel in Deaerated Bicarbonate Solutions

Corrosion evolution during immersion tests(up to 43 days) of Ni Cu steel in deaerated 0.1 mol/L bicarbonate solutions was investigated by electrochemical measurements, scanning electron microscopy(SEM) and X-ray diffraction(XRD). Results show that Ni Cu steel transformed from the anodic dissolution in the early stage of immersion to a metastable passive state in the final stage as the open-circuit potential value shifted positively, which was aroused by the precipitation of corrosion products. This process was mainly promoted by the trace amount of oxygen. Simultaneously, dominant cathodic reaction transformed from the hydrogen evolution in early stage to reduction processes of corrosion products in later stages. Possible corrosion processes were discussed with the assistance of a corresponding Pourbaix diagram.     

390MPa级低合金高强钢的低温韧性

采用系列冲击试验研究了控轧控冷技术生产的390MPa级低合金高强钢的低温韧性,并分析了其低温韧性与组织特征的关系。结果表明：该钢具有良好的低温韧性,在-40℃时的冲击功为127J,远大于相关标准的技术要求,按照能量法确定的韧脆转变温度为-56℃;由于该钢晶粒十分细小,裂纹在扩展过程中频繁改变断裂路径,提高了其抵抗解理断裂的能力,从而使其具有良好的低温韧性。     

低碳低合金钢热处理退火工艺参数优化研究

合理的热处理退火工艺参数匹配是获得均匀、细化微观组织材料的关键。以低碳低合金钢20CrMnTi为研究对象,在临界-亚临界温度Ac附近加热和保温试样,获得细小的片状珠光体和细化的铁素体晶粒。以加热温度和保温时间为设计变量,通过正交试验设计退火工艺方案,获取材料的塑性数据(硬度、屈服强度、断面收缩率和伸长率)。通过多层次分析法计算多目标综合塑性的权重,然后运用灰色系统理论计算退火工艺参数的优化组合,进一步用优化的工艺参数组合退火处理试样。应用扫描电镜测试并优化工艺处理材料的微观组织,获得分布均匀的铁素体和细小片状渗碳体晶粒组织;通过拉伸应力-应变试验曲线分析材料力学性能,均证明该材料的综合塑性明显提高,表明该优化退火工艺参数的方法是正确、可行的。     

热连轧Q345R压力容器板性能研究

研究了传统轧制和热连轧工艺生产的Q345R压力容器板的力学性能及焊接接头性能,分析了热连轧Nb-Ti微合金化对性能的影响。试验结果表明,低碳、Nb-Ti微合金化处理的热连轧钢板冲击韧性较传统轧制钢板的提高2～3倍,热连轧钢板焊接接头冲击韧性明显好于传统轧制钢板的,粗晶区淬硬程度低;热连轧钢板良好的性能来源于TMCP工艺下碳当量的降低及Nb-Ti微合金化处理改善的基体及接头微观组织。     

低碳中合金钢破碎机锤头的研制

研究了一种新型的破碎机锤头,对其采用合金化处理并经过热处理,实验表明:该低碳中合金耐磨钢的硬度大于51HRC,冲击韧性大于141J/cm2。经热处理后的显微组织为回火马氏体+残余奥氏体+弥散的碳化物,比高锰钢破碎机锤头性能有很大提升。     

S275低合金钢焊接接头的组织与力学性能

通过拉伸、冲击、硬度测试和金相检验等方法对S275低合金钢熔化极活性气体保护焊(MAG)焊接接头的力学性能与显微组织进行了研究。结果表明:采用JM一56焊丝对S275低合金钢进行焊接时,接头具有较好的抗剪强度;开45。坡口,拉伸后均断在离焊缝位置较远的母材上,焊接接头抗拉强度满足技术要求。焊缝组织晶内为细小密集的针状铁素体,有极少量珠光体,焊缝局部有魏氏组织;熔合区、过热区为沿晶界析出的块状先共析铁素体和向晶内生长的条状铁素体以及少量的珠光体;正火区组织为细小的铁素体和珠光体;母材组织为铁素体和珠光体,晶粒也比较细小。最后得出S275低合金钢用于转向架焊接构架是可行的。     

Mechanism of Residual Stress Reduction in Low Alloy Steel by a Low Frequency Alternating Magnetic Treatment

Residual stress reduction in low alloy steel by a low frequency alternating magnetic treatment and its mechanism were investigated. Experimental results revealed that average stress reductions of 20%-24% were obtained in the welded samples. Moreover, compared with the zones with lower initial stress levels, more remarkable stress reductions were obtained in the stress concentration zones. The microstructures and magnetic domains were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Based on the analysis of the microstructure and magnetic domain changes, the mechanism of stress reduction by the magnetic treatment has been concluded: (1) the magneto-plastic deformations mainly due to the more uniform redistribution of dislocations are the fundamental cause of stress relaxation; and (2) surface topography is also proved to affect the magnetic treatment results to some degree by influencing magnetic domains.     

低合金高强钢的延迟裂纹和应力腐蚀裂纹

本文介绍了低合金高强钢制的球罐在定期检验时发现的焊接接头裂纹。作者从应力腐蚀裂纹和氢致裂纹的机理、影响因素、形貌、发生区域、发生时间来论证球罐的应力腐蚀裂纹和氢致裂纹的内在机理是一样的,属于氢致开裂机理。理论分析和检验检测实践证明在球罐的焊接加工阶段采取措施防止氢致裂纹就能大大降低应力腐蚀裂纹敏感性。     

低碳低合金钢的连续冷却相变组织特征及其形成机制

利用Formastor-Digital全自动相变仪测定低碳Mn,Ni,Mo,Cr,V,Ti等低合金化钢的静态CCT曲线,结合光学显微镜、透射电镜、显微硬度法分析冷却速率对相变组织演变规律的影响。结果表明:当冷速为0.03℃/s时,相变组织为多边形铁素体(PF)+珠光体(P);冷速为0.06℃/s时,出现退化珠光体(PD);冷速为0.29℃/s时,出现针状铁素体(AF);冷速为1.7℃/s时,组织为粒状贝氏体(GB)+板条贝氏体(LB);冷速为42℃/s时,出现马氏体(M)。提出极低冷速下低碳钢中出现退化珠光体是由于多元低合金元素的耦合交互作用,引起碳活度的变化所致。针状铁素体在晶内形核并长大,对后续相变组织起到分割作用,利于细化组织。低碳钢中局部孪生马氏体的形成归因于淬透性元素聚集对钢局域相变切应力过大所致。     

反应堆压力容器用SA508Gr.4N钢的热变形行为

利用Gleeble-1500D热模拟试验机,在温度为1050~1250℃、应变速率为0.001~0.1s-1、真应变量0.16的条件下,研究和分析SA508Gr.4N钢高温塑性变形及动态再结晶行为。结果表明:SA508Gr.4N钢的高温真应力-应变曲线主要以动态再结晶为特征,峰值应力随变形温度的降低或应变速率的升高而增加,属于温度和应变速率敏感材料;在真应力-应变曲线的基础上,建立材料热变形本构方程,较好地表征了材料高温流变特征,其热激活能为383.862kJ/mol;其硬化率-应力(θ-σ)曲线均呈现拐点且-dθ/dσ-σ曲线出现极小值;临界应变随应变速率的增大与变形温度的降低而增加,且临界应变(εc)与峰值应变(εp)之间具有一定相关性,即εc/εp=0.517;临界应变与Z参数之间的函数关系为εc=8.57×10-4 Z0.148。     

碳纳米纤维低温下的石墨化转变

“实心”碳纳米纤维在高温下的微结构转变分别利用电子显微镜和激光Raman光谱进行了表征。研究发现当热处理温度高于1000℃时，原来为无序结构的碳纳米纤维开始发生石墨化转变，有序化程度提高；而且随着热处理温度越高，石墨化程度越明显。分析认为碳纳米纤维的石墨化转变温度低是由于纳米结构小尺寸效应引起的。     

低合金高强度钢的微观组织和疲劳裂纹扩展行为

用透射电镜观察了３０ＣｒＭｎＳｉＮｉ２Ａ钢等温的微观组织，疲劳裂纹扩展行为、裂纹尖端塑性区和位错结构，结果表明，等温状态组织由马氏体和贝氏体组成。在一个奥氏体晶粒内一般存在四个板条领域、裂纹尖端的塑性区内存在主位错带，疲劳断裂的基本组织单元为板条晶或板条束。裂纹遇到板条束界时方向发生较大偏斜。     

基于拉伸试验的低合金钢韧性表征参量分析

对700组低合金钢材料的拉伸试验和冲击试验数据进行了统计分析,结果表明,拉伸试验确定的强度和塑性参量与冲击韧性间存在一定的相关性,在该统计分析结果基础上提出了基于拉伸试验的韧性表征参量——强度比SR=R_m/R_(p0.2_·(1-Z);低合金钢的冲击韧性和强度比满足统计关系:KV_2=-83.413 67+61.471 08·Rm/R_(p0.2)·(1-Z)或KV_2=-221.538 54+286.857 87·ln/RmRp0.2·(1-Z)。     

低合金汽车结构钢动态响应模型研究

目的 研究低合金汽车结构钢不同应变速率下的动态响应行为,并提供一种简单准确的动态增长因子的预测方法,为内高压成形工艺提供参考.方法 进行高应变速率拉伸(动态拉伸)实验,并用高速摄像机和扫描电子显微镜对材料的拉伸行为和断口形貌进行表征,利用实验数据建立一系列结构钢的动态响应模型,研究不同应变速率下的响应特性.结果 随着应...