新型耐热钢Super304H高温时效后的组织与性能

采用光学显微镜、扫描电子显微镜及X射线衍射等手段并通过显微硬度和冲击实验,研究了Supre304H钢经750～1350℃时效后的微观组织和性能.结果表明:高温时效后Super304H钢的微观组织为γ相+析出相;随时效温度的不同,基体晶粒尺寸及析出相的种类、分布发生不同的变化.在750℃左右因微细沉淀强化及细晶组织使得显微硬度达到最大值,而后随温度升高以及析出相、晶粒尺寸与固溶元素的变化,显微硬度呈现先快速下降后缓慢下降的趋势;时效试样的冲击功值随温度升高在850℃左右,由于M23C6沿晶界大量析出导致晶界脆化而达到最低值,后又因析出相的再溶解致使晶界脆化效果趋弱而逐渐升高.     

固溶时效对1Cr21Ni5Ti双相不锈钢组织的影响

本工作以1Cr21Ni5Ti双相不锈钢为原材料,对1 000~1 350℃固溶30 min+650~1 000℃时效1~1 440 min后的显微组织及σ析出相进行观测,描述了不同处理条件下的组织特征,绘制出σ相析出TTP曲线图。结果表明:随着固溶温度的升高,铁素体含量增加,奥氏体含量减小,双相不锈钢组织发生再结晶和晶粒长大。铁素体与奥氏体中Cr、Ni元素发生均匀化,各相中的含量差异降低。σ相优先在铁素体与奥氏体相界处形核,随着时效温度的升高和时效时间的延长,σ相长大、粗化并向铁素体基体延伸;时效时间越长,析出相越多;当温度达到750℃,σ相析出速度最快,之后随着温度的升高而降低。σ相析出温度范围为650~850℃,析出鼻尖温度为750℃。     

火焰矫正温度对SA-213TP347H不锈钢管组织和性能的影响

通过改变火焰矫正温度,从火焰矫正后钢管的硬度、显微组织以及析出相三个方面研究了火焰矫正温度对SA-213TP347H不锈钢管组织和性能的影响。结果表明:火焰矫正温度愈高,矫正后钢管的硬度则愈低;不锈钢管在火焰矫正过程中没有发生相变,始终是单相奥氏体,但矫正温度过高容易产生晶粒长大、过热甚至过烧。     

超超临界锅炉用E911钢管的高温性能

对德国Vallourec＆Mannesmann公司生产的159mm×27mm的E911钢管进行了力学性能、显微组织、抗氧化性能和持久强度的试验,并对供货状态的E911钢管进行了时效试验和时效后的显微组织测定。试验结果表明,在650℃下E911钢管基本不氧化,其常温性能、高温性能和持久强度都优于T91钢,完全可代替TP304H,TP321H和TP347H不锈钢管,解决了异种钢焊接问题,并可降低锅炉的制造成本。     

稳定化处理对Super304H奥氏体耐热钢晶间腐蚀敏感性的影响

用光学显微镜(OM)、扫描电子显微镜(SEM)、电解萃取法、草酸电解实验、X射线衍射(XRD)和双环电化学动电位再活化法(DL-EPR)等方法,研究了固溶和稳定化处理工艺对Super304H钢的显微组织(包括晶粒度,析出相分布、数量和类型)及晶间腐蚀敏感性的影响。结果表明,经过1150℃×15 min固溶处理后Super304H钢的晶粒度维持在7-10级;经不同温度的稳定化处理后析出相的数量较固溶态明显增加,其中950℃仍为敏化温度,有大量M23C6(M=Fe,Cr)沿晶界析出;随着稳定化温度的升高Nb(C,N)的析出数量随之增加,抗晶间腐蚀性能不断提高;当温度达到1100℃时Nb(C,N)的析出量达到最多,其抗晶间腐蚀性能较固溶态有明显提高;1100℃处于Super304H钢的固溶温度范围,明显高于传统1Cr18Ni9Ti型不锈钢的稳定化温度900℃,说明Super304H钢供货态的固溶处理工艺实际上兼顾了固溶与稳定化的双重作用。     

23Cr-14Ni高氮奥氏体不锈钢σ相析出行为EI北大核心CSCD

采用经验公式、热力学计算方法、Gleeble热/力模拟实验技术,结合光学显微镜、扫描电镜及透射电镜分析,研究了23Cr-14Ni高氮奥氏体不锈钢中σ相的析出行为。结果表明,23Cr-14Ni高氮奥氏体不锈钢中σ相可在960~1030℃析出,高于1050℃溶解。σ相析出具有异常快速的动力学特征,在经过1030℃保温1 min固溶处理后,σ相可直接从奥氏体晶界快速析出,析出先于碳氮化物相。σ相析出动力学行为及相对碳氮化物的析出次序和传统奥氏体不锈钢显著不同。铬、锰、钼元素含量较高且钼元素在晶界处偏聚提高了σ相平衡析出温度,是加速σ相析出的主要原因。     

再热器TP347H钢受热面管开裂原因

某电厂后屏再热器TP347H奥氏体不锈钢受热面管发生开裂，采用宏观观察、扫描电镜和能谱分析、室温拉伸试验等方法分析其开裂原因。结果表明：TP347H奥氏体不锈钢受热面管发生脆性开裂，其显微组织老化级别为4.5级；受热面管长期过热运行后，晶界析出粗大的碳化物，组织发生劣化，材料的脆性增加，导致管子开裂。     

Q235钢、Cu-Cr合金铸铁和TP304H不锈钢在熔融Zn-5%Al-RE合金中的耐蚀性研究

为研究铁基材料的成分和显微组织对其耐熔蚀性能的影响,将Q235钢、Cu-Cr合金灰铸铁、TP304H不锈钢及喷丸的TP304H试样在熔融Zn-5%Al-RE合金中进行了熔蚀试验(试验温度为470℃),并进行了熔蚀速率测定和截面形貌观察(SEM)。结果表明:在试验条件下,Q235钢在熔融合金中不能形成连续致密的Al-Fe金属间化合物层;Cu-Cr合金铸铁中的石墨及珠光体对Al、Fe原子扩散起到了阻挡作用;TP304H不锈钢由于奥氏体结构的致密性,Al原子在其中的扩散速率低,同时在Al-Fe化合物层之外形成了Zn-Al-Cr-RE相,对化合物层起到一定的保护作用;TP304H不锈钢经喷丸处理后由于生成马氏体,使Al原子扩散速率提高,熔蚀速率增大。4种试验材料在熔融Zn-5%Al-RE中的耐蚀性能从高到低的排序:TP304H喷丸处理的TP304HCu-Cr合金铸铁Q235钢。     

固溶温度对TP347HFG耐热钢组织和性能的影响

采用金相显微镜、扫描电镜、布氏硬度测试、拉伸测试等手段研究了不同固溶温度下TP347H FG钢的组织和性能.结果表明,随着固溶温度升高,TP347HFG耐热钢晶粒度降低,固溶温度为1 180℃时晶粒尺寸、形状均匀;固溶温度为1 210℃时晶粒明显长大,且尺寸不均匀;TP347HFG耐热钢第二相由大颗粒相和小颗粒相组成,其主要成分均为NbC;固溶温度为1 120℃和1 180℃时小颗粒第二相在晶界析出,对晶界强化作用显著,1 210℃时第二相大部分在晶粒内部析出,并有Ostwald熟化现象发生,细小第二相消溶而较大颗粒第二相变大,从而影响基体性能;固溶温度为1 120℃和1 180℃时其抗拉强度和Rp0.2最高,随着固溶温度的升高,伸长率增加而硬度降低.     

Al对GH4169合金冲击性能的影响

研究了Al对GH4169合金晶界相的析出和冲击性能的影响.结果表明,提高Al含量可抑制晶界δ相的析出,促进晶界Laves相、M7C3相和σ相等有害相的析出;随着Al含量的提高,GH4169合金的室温冲击性能明显降低,冲击断口由穿晶型转变为沿晶型.提高Al含量所导致的Laves相等有害脆性相的析出降低了晶界强度,使晶界裂纹更容易萌生和扩展,降低了GH4169合金的冲击韧性.     

Martensite structure in TP304H austenitic stainless steel after long-term service and aging

The microstructure change of TP304H steel after long-term service and aging at high temperatures is investigated through XRD, OM, SEM, TEM, SAED and EDS. The results indicate that after long-term service, α’-martensite and ε-martensite are found in TP304H steel. α’-martensite distributes in stripes along grain boundaries; the substructure of α’-martensite is high-density tangling dislocations. ε-martensite distributes in parallel, a little further from grain boundaries than α’-martensite; the substructure is stacking fault. After long-term high-temperature aging, α’-martensite is found in TP304H steel. With the extension of aging time, the amount of α’-martensite increases and the lath morphology becomes clearer. The increase of martensitic transformation start temperature besides grain boundaries due to the decrease of Cr content in this area is the main reason for the formation of martensites in TP304H steel after long-term service and being aged at high temperature.     

Influence of Aging treatment on the microstructure and mechanical properties of T92/Super 304H dissimilar metal welds

Influence of aging treatment on the microstructure and mechanical properties of T92/Super 304H dissimilar steel joints was investigated. The microstructure of T92/Super 304H dissimilar steel joints was characterized using optical microscopy, scanning electron microscopy and energy dispersive spectrometer. The results show that the tensile strength of dissimilar metal welds (DMWs) after 10,000 h aging treatment met the ASME T92 and Super 304H standards. Rupture positions were located in the T92 base metal because of the precipitates formed along the sub grain and prior grain boundaries. The tensile strength of DMWs initially increased with time up to 4000 h, then decreased between 4000 to 6000 h, and finally came to almost a constant value from 8000 to 10,000 h exposure. The decrease in the tensile strength resulted from the nucleation and growth of Laves phases at the sub-grains and prior austenitic grain boundaries. The low absorption of impact energy in the weld metal was related to the coarse grains and its grain orientation.     

CONTINUOUS SEM OBSERVATIONS OF CREEP-FATIGUE DAMAGE PROCESSES

In order to examine the relation between damage evolution and changes in microstructure, e.g. from creep cavities, surface micro-cracks and dislocation structures at high temperature, strain controlled creep-fatigue tests were performed and interrupted at several damage levels on Types 304 and 316 stainless steels. The creep-fatigue tests on Type 304 stainless steel at a low strain level were conducted in a high-temperature fatigue testing machine combined with a scanning electron microscope, and the micro-crack initiation and growth behaviour were continuously observed to clarify the damage extension mechanism. It was found that even though many cavities were initiated and grew on the internal grain boundaries of the specimens during the strain-controlled tests, the failure life was governed by the propagation of surface cracks. On the other hand, micro-cracks of about the order of one grain size were initiated mainly along grain boundaries normal to the loading axis under low stress creep-fatigue, and the crack propagation rate of the micro-cracks was slow and random due to the nature of the microstructures. The micro-cracks gradually opened in the loading direction with increasing number of cycles and coalescence contributed to growth.     

Microstructure and property evolutions of a novel Super304H steel during high temperature creeping

High-temperature creep tests of a novel Super304H steel under 650 °C/195 MPa were conducted and the evolutions of microstructure and property with creep time of the material were investigated by using optical microscope, scanning electron microscope, micro Vickers hardness tester and electrochemical workstation. The results show that M₂₃C₆ carbides precipitated along grain boundaries of austenite matrix in a chain distribution and then got coarsened with the increase of creep time. Creep cavities started to form near the surface when the steel was crept for 2500 h. Afterward creep cavities increased, developed, interconnected and finally formed micro cracks along grain boundaries till fracture at the time of 4578 h. The hardness of the steel increased dramatically at the early stage of creeping and reached a high level at 500 h, and then kept a stable state at the succedent stage till fracture. Intergranular corrosion susceptibility of the steel increased first and then declined gradually, indicating the occurrence of sensitization – desensitization process of the steel during creeping.     

Variations of the microstructure and mechanical properties of HP40Nb hydrogen reformer tube with time at elevated temperature

Hydrogen reformer furnaces have been widely used in the petrochemical industry to produce the hydrogen-rich gas from a mixture of hydrocarbons and steam at high temperature. However, the degradation of material microstructure was frequently encountered in the tubes due to high temperature service, leading to their premature failure. The aim of this paper was to address the variations of the microstructure and mechanical properties of HP40Nb hydrogen reformer tubes after aging treatment and long-term service at temperature of 900 °C. The results showed that the grain boundaries became coarsening due to the precipitation of the chromium-rich carbides and the secondary carbides precipitated in the matrix after aging treatment and long-term service. The mechanical properties of the HP40Nb tube obviously degraded after short-term service and then almost kept unchanged. The interdendritic carbide content can be used as a key index for the life predication of the used tube since there was a linear relationship between the logarithm of carbide content and the logarithm of time.     

Failure analysis of styrene reactor tubes

The analysis of failure of the styrene reactor tubes made of stainless steel has been reported where the tubes contained catalyst as well as ethyl benzene in vapour form along with superheated steam at 570 °C. The tubes were exposed to flue gases causing heating of tubes to 790 °C in the upper part where cracking and failure were mostly noticed. The studies included microstructural examinations of cracked and uncracked tubes, fracture surface investigations and estimation of creep rupture strength, etc. The degradation in microstructure such as extensive grain coarsening, sigma phase formation, carbide formation along grain boundaries, etc., led to development of cracks/voids within the grains and along the grain boundaries and also excessive oxidation of tubes. Possible chemical attack by the feed on the inner surface of tubes further deteriorated the tube life. Suggestions for avoidance of failure have been listed.     

Quantitative observations of precipitation in 2.25Cr–1Mo steel exposed to different creep conditions in a power station

Samples of 2.25Cr–1Mo steel in the form of tubes have been examined in the new condition and after extended service in a power station. The differences in microstructure in terms of precipitates have been characterized using a TEM; the various precipitates present have been identified individually by using extraction replicas and energy dispersive spectroscopy analysis. The area fraction of each type of precipitate was calculated. The results have shown that the quantity of M₇C₃ precipitated within grains as well as on grain boundaries decreased markedly with the time in service while the quantity of M₆C increased in both regions. It was observed that precipitates of Mo₂C and (CrMo)₂CN, detected after aging respectively on the samples with the shorter and the longer aging times, were present only within grains. The procedure used was not accurate for identification and quantification of small VC and (VNb)C precipitates. The results of the relative quantity of M₇C₃ and M₆C, as a function of time in service, particularly on grain boundaries, showed a well defined interdependence. These results can be used as a complementary analysis for creep damage and remaining life estimation for this class of steel operating at high temperature under steady state conditions.     

Diffusion Behaviour of Fe-Cu Interface of Copper Brazed Double-wall Steel Tubes

By means of electron probe(EPMA),scanning electron microscope(SEM),and optical microscope(QM),the diffusion behaviour on the Fe-Cu interface of copper brazed double-wall steel tubes andthe microstructure of the diffusion layer have been investigated.There are three kinds of metallurgicalbonds between copper plating layer and steel substrate: (1)the Cu diffusing into steel substratealong grain boundary of ferrite;(2)the Cu diffusion into grain bulk of ferrite: (3)the Fe diffusing intoCu layer.The copper brazed double-wall steel tubes are formed by the combination of the diffusionsmentioned above and this is the reason for excellent mechanical and technological properties of thecopper brazed double-wall steel tubes.