55SiCr钢铸坯表面孔洞原因分析

55SiCr钢坯在轧制过程中表面出现孔洞现象,影响到了盘条成品的质量。本文利用低倍酸洗、金相观察、能谱分析等手段对钢坯产生孔洞的原因进行了分析。结果表明:铸坯内部存在严重的内部裂纹,铸坯内部缺陷与外部相连通,连铸坯内部存在的穿晶裂纹为铸坯内部较大内应力所致,并在加热和轧制过程中裂纹不断扩展,最终形成孔洞,通过加强冷却均匀性可以控制此类铸坯孔洞。     

Q345B钢铸坯断裂原因分析与预防措施

对Q345B钢连铸坯的断口形貌进行了宏观分析,结合光学显微组织的观察与分析,研究了连铸坯断裂的原因。结果表明：Q345B钢连铸坯内部存在中间裂纹,冷却过程中在热应力、相变应力及铸坯自重共同作用下进一步扩展,导致铸坯断裂。通过加强设备的维护与检修,优化扇形段二冷制度, 低过热度浇铸和稳定铸坯拉速等工艺措施预防铸坯断裂。     

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 某轧钢厂正常轧制的钢材表面存在裂纹缺陷。利用扫描电子显微镜和金相显微镜对裂纹的形貌进行观察,发现裂纹附近存有高温氧化产物,经腐蚀后裂纹附近脱碳严重。由此可知：为连铸坯的表面裂纹,在经加热的过程中产生高温氧化,而正常的热轧工艺无法使其焊合,最终在钢材表面表现为裂纹缺陷。     

铸坯裂纹造成钢材缺陷的特征及控制措施

采用金相显微镜(OM)及扫描电子显微镜(SEM)等仪器对铸坯裂纹造成钢材缺陷的特征及成因进行分析。结果表明:钢筋拉拔断裂、钢板表面结疤及麻坑缺陷均是由铸坯裂纹遗传至轧制过程所导致的。由铸坯裂纹引起的缺陷往往在缺陷处可观测到一薄层高温氧化铁,附近分布着大量细小高温氧化圆点,且在中高碳钢材中缺陷处往往伴随有脱碳迹象。为改善钢材品质,全面分析了铸坯裂纹的形成原因,并针对相关原因提出了相应的改进措施。     

SUS304J1含铜不锈钢热轧板表面裂纹缺陷分析

对304J1含铜不锈钢热轧板表面的裂纹缺陷进行了微观形貌及微区成分观察和检测,并调查了现场生产工艺参数。结合检测试验结果和现场工艺调查分析,认为裂纹主要是由于铸坯中成分偏析及加热过程中铜选择性富集氧化造成的。通过优化调整冶炼、连铸和热轧工艺参数,缺陷发生率大幅降低,缺陷得到有效控制。     

铸坯质量缺陷引起钢板边裂的特征及机理分析

采用SEM、EDS和金相显微镜研究了某钢厂钢板边部开裂缺陷的形成原因。结果表明:铸坯气泡、铸坯中间裂纹、铸坯纵裂纹是引起钢板边部开裂的主要原因。钢板开裂边缘附着一层高温氧化铁,附近分布着大量细小高温氧化圆点,且部分边裂缺陷存在脱碳现象。钢板边部开裂缺陷主要来源于铸坯质量缺陷,并在轧制过程中得到扩展。针对相关原因提出了相应的改进措施以降低边裂缺陷率,对实际生产具有一定的指导意义。     

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热轧板表面条纹缺陷为热轧板常见缺陷,且较难治理。为了弄清热轧板表面的条纹缺陷产生原因,利用金相显微镜和扫描电镜等设备进行了分析。结果表明:缺陷区存在再结晶不完全的大晶粒和脱碳现象,以及内氧化质点。说明铸坯原生缺陷有高温加热、轧制的痕迹。大晶粒的形成可能是由于缺陷处于临界应变状态或细化晶粒的元素被氧化掉,或两种因素共同作用的结果。裂纹与热轧板表面夹角很小,裂纹毗邻金属存在脱碳及内氧化斑点等,缺陷性质也可能是热轧过程中形成的折叠。     

低碳钢异物缺陷分析

在热轧生产中钢板边部附近出现了数量较多的异物缺陷,严重影响了热轧产品质量。本文对用作冷轧原料的热轧低碳钢钢板表面异物缺陷的产生原因进行了多组对照实验和微观组织分析,初步确认异物缺陷来源于铸坯。推断异物缺陷产生过程为:异物落在铸坯表面,被轧制压入铸坯基体;受除鳞水的影响,异物迅速降温,在热轧过程中形成了纤维状组织及碳化物;钢板母材与异物的接触处的组织受异物低温和铸坯高温的影响,提前转变为铁素体,并在铸坯基体温度的缓慢下降过程中不断长大,形成了异常粗大的铁素体。     

连铸坯内部裂纹形貌及其在热轧无缝管过程中的形踪

<正> 1.引言所谓连铸坯内部裂纹(简称内裂),就是在连铸坯内部而不露出铸坯表面的裂纹1(见图1)。一般对连铸坯内裂只在某一断面(多以横断面)进行研究,而对连铸坯内裂的空间形貌研究甚少,至于铸坯内裂在热轧无缝钢管过程中变化情况的研究,更不多见。因此,有必要对连铸坯内部裂纹形貌及其在热轧无缝钢管过程中的形踪进行研究,从而进一步认识连铸坯的内部裂纹,这对裂纹的研究具有一定的意义。     

金相组织和夹杂物对40Cr边部裂纹形成的影响

针对永洋特钢集团公司40Cr轧材边部裂纹的成因和影响因素进行系统调查研究,通过一系列取样、检验和分析发现,某纵裂纹内含有与钢渣反应产物、结晶器保护渣类似的残存物,结合160 mm×225 mm 铸坯热轧工艺过程,最终得出该钢种产生裂纹的主要原因是由于脆性夹杂物较多、铸坯表面粘渣或有细小裂纹,以及加热过程脱碳严重、轧制变形制度不合理。     

Oxidation Behaviors of Different Grades of Ferritic Heat Resistant Steels in High-Temperature Steam and Flue Gas Environments

For steam tubes used in thermal power plant, the inner and outer walls were operated in high-temperature steam and flue gas environments respectively. In this study, structure, microstructure and chemical composition of oxide films on inner and outer walls of ex-service low Cr ferritic steel G102 tube and ex-service high Cr ferritic steel T91 tube were analyzed. The oxide film was composed of outer oxide layer, inner oxide layer and internal oxidation zone. The outer oxide layer on the original surface of tube had a porous structure containing Fe oxides formed by diffusion and oxidation of Fe. More specially, the outer oxide layer formed in flue gas environment would mix with coal combustion products during the growth process. The inner oxide layer below the original surface of tube was made of Fe-Cr spinel. The internal oxidation zone was believed to be the precursor stage of inner oxide layer. The formation of internal oxidation zone was due to O diffusing along grain boundaries to form oxide. There were Fe-Cr-Si oxides discontinuously distributed along grain boundaries in the internal oxidation zone of G102, while there were Fe-Cr oxides continuously distributed along grain boundaries in that of T91.     

Identification of diffusing species and the dynamic nature of diffusion paths during oxidation of a dilute Ni-Cr alloy

The results of an investigation of oxidation of a Ni-1 at.% Cr alloy are presented. Photolithographic marker experiments revealed that the markers were found to reside at the interface between a predominantly columnar outer NiO layer and a very fine grain inner layer of NiO, indicating that substantial oxygen ingress had occurred through the columnar scale. New oxide growth at the metal-oxide interface requires the oxidant to be transported across the oxide layer. Since the measured diffusion rate of oxygen ions along grain boundaries and through the lattice is much too slow to account for the observed microstructural growth (1: 1 ratio of inner and outer layers), it is necessary to postulate that the oxidant traverses the scale along some type of short-circuit path other than grain boundaries. Extensive formation of elongated pores and pipelike channels was observed along columnar oxide grain boundaries. Thus, it appears that the transport of oxygen occurs via voids (pores) formed by vacancy coalescence at the columnar grain boundaries. These pores appear to open and close continuously. Formation of new fine-grained oxide in these pores was observed to have sometimes completely resealed the void, suggesting a dynamic nature of the voids.     

Microstructural evidence for short-circuit oxygen diffusion paths in the oxidation of a dilute Ni-Cr alloy

A comparative study of high-temperature oxidation of Ni containing 1 at.% Cr and pure Ni was carried out. Instead of the conventional kinetics study using thermogravimetry, a microlithographic marker experiment was designed. Observation of the markers using cross-sectional TEM and SEM has revealed striking differences in the scale morphology, microstructures, and oxidation mechanisms between pure Ni and the Cr-doped Ni substrates. In particular, the results suggest that a small addition of Cr promotes significant inward transport of oxygen. Marker experiments revealed that NiO grown on pure Ni is wholly attributable to outward-cation diffusion. In contrast, NiO grown on Ni–1 at.% Cr exhibited formation of a substantial inner layer having a submicron grain size, established by the markers to have formed from oxygen ingress. For pure Ni, voids were observed to be distributed only within oxide grains. In contrast, for Ni containing 1 at.% Cr, elongated pores formed extensively along oxide-grain boundaries. Formation of new fine-grain oxide in these pores was observed to have sometimes completely resealed the void. It is, therefore, proposed that the transport of oxygen in the case of oxide scale grown on Ni–1 at.% Cr occurs via voids (pores) formed by vacancy coalescence at the grain boundaries.     

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 对22SiMn2钢铸坯断裂原因进行了分析,发现断坯部位柱状晶较发达,沿晶分布着较多气孔、裂纹和硫化物、钛化物夹杂,使得铸坯晶界处成为薄弱环节。由于铸坯堆放环境恶劣,反复的升降温使得铸坯内应力得不到充分释放而集中在铸坯晶界处,裂纹沿塑性较差夹杂物扩展,最终导致了铸坯的断裂。研究表明,降低钢水浇铸时过热度,可解决22SiMn2钢铸坯断裂的问题。     

EFFECT OF CREEP FRACTURE TOUGHNESS ON CRACK INITIATION AND GROWTH

本文研究了蠕变断裂韧性对二种低合金耐热钢蠕变裂纹开裂和扩展的影响。试验表明:随着蠕变断裂韧性提高,抗蠕变裂纹开裂和扩展能力增加。材料呈韧性或脆性状态时,蠕变裂纹萌生和扩展过程不同。韧性状态时,裂纹为穿晶和晶界二种混合形式:穿晶裂纹可在晶内碳化物处发生,或在晶界上形核后向晶内扩展,晶界裂纹仍是由晶界上空洞形成和相互连接而成,裂纹可沿晶界和晶内扩展,但不连续。脆性状态时,裂纹沿晶界发生,它是由晶界形成空洞和相互连接而成,扩展仅沿晶界发生。     

高温再热器SA-213TP347H钢管裂纹原因分析

通过结构分析、断口检查、硬度测试、抗拉试验、显微组织观察及能谱分析,对电厂高温再热器SA-213TP347H钢管进行裂纹原因分析。结果表明:高温再热器在运行中容易晃动,裂纹处于管夹挤压的凹坑;裂纹处管材的拉伸性能、硬度均符合ASME A213/A213M对SA-213TP347H钢的要求;显微组织为奥氏体和沿晶界分布的含Cr碳化物,裂纹是沿晶裂纹,由管外壁向管内扩展;裂纹处腐蚀产物均含有O、S等非金属元素。分析表明:产生裂纹的原因是由于管夹挤压管道,同时管道结构易晃动,增加了挤压变形处的应力,促进了含Cr碳化物沿晶界析出,形成贫Cr区而导致晶界弱化,并在腐蚀性介质的作用下,导致晶间应力腐蚀开裂。     

IMPROVEMENT OF TYPE IV CRACKING RESISTANCE OF 9Cr HEAT RESISTING STEEL WELDMENT BY BORON ADDITION

Creep lives of high Cr ferritic heat resisting steel weldments decrease due to Type Ⅳ fracture, which occurs as a result of formation and growth of creep voids and cracks on grain boundaries in fine-grained heat affected zone (HAZ). Because boron is considered to suppress the coarsening of grain boundary precipitates and growth of creep voids, we have investigated the effect of boron addition on the creep properties of 9Cr steel weldments. Four kinds of 9Cr3WSCoVNb steels with boron content varying from 4.7×10-5 to 1.8×10-4 and with nitrogen as low as 2.0×10-5 were prepared. The steel plates were welded by gas tungsten arc welding and crept at 923K. It was found that the microstructures of HAZ were quite different from those of conventional high Cr steels such as P91 and P92, namely the fine-grained HAZ did not exist in the present steel weldments. Boron addition also has the effect to suppress coarsening of grain boundary carbides in HAZ during creep. As a result of these phenomena, the welded join     

Ni-Cr-W合金在1100 ℃的氧化膜演变

对新型Ni-Cr-W合金在1100 ℃下不同保温时间下的恒温氧化行为进行了研究。采用扫描电镜（SEM）以及能谱（EDS）对合金热暴露后的表面氧化膜形貌、元素含量以及合金基体的恶化情况进行了分析,表面氧化膜的相组成通过XRD进行确定。结果表明：在氧化初期（<3 h）,合金表面生成的单层氧化膜主要由Cr2O3组成,随着氧化时间的延长（>7 h）,在Cr2O3外逐渐形成了一层具有尖晶石结构的NiCr2O4。一旦外表面被均匀致密的尖晶石膜所覆盖,双层氧化膜NiCr2O4·Cr2O3便能有效的减慢合金基体被进一步氧化。合金亚表层的恶化形式包括晶界的内氧化、空洞以及无碳化物区的形成。合金中高的W含量并没有明显恶化合金的抗氧化性能。     

Intergranular oxidation and internal void formation in Ni-40% Cr alloys

Internal void formation and intergranular oxidation behaviour have been studied during the oxidation of two Ni-40Cr alloys in 1 atm oxygen at 1000° to 1200°C. The development of an external Cr₂O₂ scale causes vacancies to be generated in the alloy at the alloy-scale interface as chromium diffuses into the scale, and others to be generated in the alloy due to the different diffusion rates of chromium towards the interface and of nickel back into the bulk alloy. At 1200°C, internal void formation results from condensation of such vacancies at inclusions in the grains and at the grain boundaries. The intergranular oxidation observed at 1000°C, 1100°C and to a lesser extent. 1200°C results from preferential condensation of vacancies to form voids in the alloy grain boundaries. Significant depletion of chromium in the alloy adjacent to the scale facilitates the supply of oxygen from the scale and its penetration into the alloy grain boundaries to form intergranular oxide. Such intergranular oxide develops deep into the alloy following diffusion of this oxygen through a porous network in the oxide, which arises because of the vacancy condensation, and oxidation of chromium at the tip of the intergranular penetration.     

Microstructural investigation of the oxide formed on TP 347H FG during long‐term steam oxidation

The long‐term oxidation behaviour of TP347H FG in ultra supercritical steam conditions was assessed by exposing the steel in test superheater loops in a Danish coal‐fired power plant and characterising the oxide layer with reflective light and electron microscopy. Double layered oxide scales formed during steam oxidation. TEM investigations reveal that the inner oxide layer consists of particles of metallic Ni/Fe and Fe? Cr spinel in the interior of the former alloy grains and a compact layer of Fe? Cr spinel and Cr₂O₃ along the former alloy grain boundaries. The morphology suggests that the inner layer grows by internal oxidation of the interior of the alloy grains. The thickness of the inner oxide layer did not change significantly with oxidation time and temperature for exposure times up to 30 000 h. Faster Cr diffusion within the fine‐grained alloy at higher temperatures is held responsible for this observation. This hypothesis is supported by kinetic data. The oxide thickness at low and high temperatures after 58 000 h exposure was higher than expected.