长时运行高温再热器12Cr1MoV/R102异种钢接头组织分析

分析了电站锅炉高温再热器中运行近8万小时的12Cr1MoV/R102异种钢接头组织变化及蠕变损伤状况,结果表明:母材与不完全重结晶区组织处于中度老化阶段,粗晶区组织老化程度最低;R102钢侧粗晶区硬度明显降低,不完全重结晶区出现软化现象;由于工作应力不高,接头各部位没有出现蠕变损伤.虽然R102钢蠕变断裂强度明显高于12Cr1MoV钢,接头存在力学性能不均匀性,但在界面附近并未发现蠕变孔洞.     

12Cr1MoVG钢再热器管焊接接头断裂原因分析

某12Cr1MoVG钢低温再热器管出口焊接接头运行7000 h后发生断裂,对其形貌、组织及硬度进行了分析。结果表明,焊接接头开裂出现在熔合区,沿晶界有微裂纹,断口呈冰糖状脆性沿晶断裂特征;同时,焊缝硬度偏高,易形成局部应力集中。结合裂纹产生的原因及特征,确定该焊接接头裂纹为再热裂纹。     

超超临界锅炉水冷壁T23/12Cr1MoV异种钢焊接接头焊后热处理裂纹分析

分析了T23/12Cr1MoV异种钢焊接接头焊后热处理裂纹的宏观和微观特征、断口形貌及接头的显微组织,测试了接头的硬度分布,在此基础上讨论了裂纹的性质及形成原因,并提出了防止裂纹的措施。结果表明,裂纹启裂于T23钢侧焊趾部位,沿粗晶粒热影响区(CGHAZ)晶界扩展,为典型的再热裂纹。焊后热处理明显降低了异种钢焊接接头两侧热影响区的硬度,T23钢侧CGHAZ产生再热裂纹与其在焊后热处理过程中晶界析出碳化物有关,其析出促进了孔洞的形成。在焊后热处理前,对异种钢焊接接头进行一次550 ℃×1 h的中间热处理有利于抑制再热裂纹的产生。     

低温再热器12Cr2MoWVTiB钢接头断裂失效机理

电厂低温再热器12Cr2MoWVTiB (R102)钢小径管接头焊接时取消预热和焊后热处理,工作温度在500～530 ℃,运行过程中发生脆性断裂失效.研究导致失效的裂纹启裂部位及扩展方向、裂纹附近的显微组织、硬度分布及沉淀相特征,判断裂纹性质,揭示裂纹的形成机理.结果表明,裂纹产生于粗晶区,终止于细晶区,性质为再热裂纹.粗晶区仍然为板条马氏体组织,板条内有高密度的位错,硬度值高达450 HV;晶内的析出相很少,但原始奥氏体晶界上析出富Ti的复合沉淀相,尺寸达到5 μm,它们的析出使晶界附近形成C和合金元素的贫化区,降低晶界强度,同时削弱晶间结合力,这是R102钢接头产生再热裂纹的主要原因.粗晶区的高淬硬状态也促进再热裂纹的形成.     

14MnNbq钢埋弧焊焊缝金属热裂纹的成因

对１４ＭｎＮｂｑ钢埋弧焊对接接头焊缝金属中含有裂纹的实物进了金相，扫描电镜及断口微区能谱分析，并采用可调拘束热裂纹敏感性试验方法对１４ＭｎＮｂｑ及１６Ｍｎｑ两种钢材进行了对比，分析了产生热裂纹的原因及其影响因素，提出了防止１４ＭｎＮｂｑ钢埋弧焊焊缝金属热裂纹的措施。     

高强度桥梁钢焊接接头疲劳裂纹扩展速率的研究

利用高频疲劳试验机对高强度桥梁钢的母材、热影响区和焊缝的疲劳裂纹扩展速率进行了测定,并结合疲劳扩展断口分析各部位疲劳裂纹扩展快慢的原因。结果表明,高强度桥梁钢焊接接头中焊缝裂纹扩展速率最低,抗疲劳裂纹扩展能力最高;母材疲劳裂纹扩展速率最高,抗疲劳裂纹扩展能力最差;热影响区介于二者之间。焊缝的疲劳断裂模式是以穿晶断裂为主,母材是以沿Z字形路径扩展的沿晶断裂为主,热影响区则介于两者之间。     

在役15X1M1Φ钢焊接接头早期失效分析及其预防

通过理化检验、裂纹形貌观察,研究了在役15X1M1Φ钢焊接接头裂纹性质及其原因。结果表明,裂纹起源于靠近熔合线的焊缝金属或热影响区粗晶区,可观察到宏观裂纹、微裂纹和空洞3种不同缺陷形貌。根据裂纹产生的位置、过程和形貌特征,可以判断该裂纹为焊接再热裂纹。焊后热处理温度场不合理产生的残余应力,焊接结构导致的强应力集中,支吊架不正常引起的附加应力,服役过程中接头塑性的降低,是导致该裂纹产生的主要原因。优化焊接和焊后热处理工艺以保证接头足够的临界塑性变形能力εc,服役过程中避免和降低应力集中和附加应力以降低接头塑性变形量εp,始终保证εc>εp,以避免具有再热裂纹敏感性的焊接接头服役过程中产生再热裂纹。     

12Cr1MoV钢水冷壁焊接裂纹分析及对策

通过理化检验、裂纹形貌观察、接头结构和焊接工艺分析,研究了小径厚壁管12Cr1MoVG钢接头裂纹性质及产生原因。结果表明,裂纹产生于接头焊接热处理后。沿接头周向分布,位于接头的热影响区粗晶区;在裂纹区域可见宏观裂纹、微裂纹和孔洞。根据裂纹产生的时间、位置和形貌,可以确定该裂纹为焊接再热裂纹。裂纹产生的原因是,该接头结构不合理导致较大拘束应力、制造尺寸偏差导致较大附加应力、焊后热处理时的温度导致接头位于再热裂纹形成敏感温度区间。通过改进接头结构、增加应力释放槽、建立合理的焊后热处理温度场,成功解决了该类接头的再热裂纹问题。     

新型低锰微铌钢与普通Q355钢焊接性能比较

通过焊接热影响区最高硬度试验、斜Y形坡口焊接裂纹试验、典型对接接头性能评价试验及典型对接接头疲劳性能试验，结合金相显微镜（optical microscope,OM）和扫描电子显微镜（scanning electron microscope,SEM）两种表征手段，深入研究了新型低锰微铌钢（MnNb钢）与普通Q355钢（Q355钢）焊接性能差异及机理。结果表明，在不预热条件下，Q355钢表现出强烈的焊接淬硬倾向，而MnNb钢无淬硬倾向；MnNb钢和Q355钢均具备优良的防止焊接冷裂纹性能。相比于Q355钢接头，Mn Nb钢对接接头抗拉强度稍低，但其热影响区冲击吸收能量明显更优，这主要是由于Q355钢粗晶区中存在细小的晶内组织和侧板条铁素体组织，而MnNb钢粗晶区具有大量针状铁素体与较小的原始奥氏体晶粒。MnNb钢接头的条件疲劳极限强度比Q355钢接头高57 MPa，其扩展区疲劳辉纹间距更小，裂纹扩展速度更慢，MnNb钢对接接头的疲劳性能显著优于Q355钢对接接头。     

经长时运行的异种钢焊接接头安全寿命分析

通过对运行5万多h的TP347H不锈钢与12Cr2MoWVTiB(G102)耐热钢异种钢焊接接头进行试验研究，结果表明，G102侧热影响区，特别是近缝区，为整个焊接接头的薄弱环节；该异种钢焊接接头的剩余寿命为132万h。鉴于该类焊接接头高温短时力学性能、高温持久强度及G102侧热影响区冲击韧度低于母材的标准要求，建议尽早更换。     

Proposed specimen for reheating cracking susceptibility and mechanical behaviour assessment in 2.25Cr–1Mo steel pressure vessel weld joint

Abstract

Post-welding heat treatment (PWHT) is carried out on 2.25Cr–1Mo steel used in pressure vessels, to relieve welding residual stress and diffused hydrogen and to soften the weld joint. However, it is known that reheating cracks often occur during PWHT. A standard specimen is available to pre-estimate the existence of cracks in pressure vessels according to rules such as those in Japanese Industrial Standard WES 3005. However, these specimens do not reflect the shape of the pressure vessels and do not reproduce the actual cracks correctly because of non-cylindrical welding. In the present paper, a numerical analysis has been conducted for various models, which can reproduce the occurrence of reheating cracks in the welded joint of a pressure vessel nozzle. The present paper also investigates mechanical behaviour such as the relief history of welding residual stress as well as the accumulated history of creep strains. Through the above processes, the mechanical behaviour of the welded pressure vessel nozzle joints has been elucidated and a model of reheating crack initiation has been selected. In addition, experiments on reheating crack initiation have been carried out for the WES 3005 specimen, and a new susceptibility assessment specimen has been proposed and developed to pre-estimate the existence of actual reheating cracks correctly.     

热轧方钢横裂纹缺陷的分析

从连铸和轧制两方面,对热轧方钢角部横裂纹缺陷的产生原因进行了排查分析,得出了处于包晶反应的钢应避免在700~900 ℃脆性区进行连铸矫直,并通过一次冷却和二次冷却采用弱冷却、减缓加热炉中钢坯的加热速度等措施,可以避免热轧方钢角部横裂纹的缺陷。     

二次加热条件对泡沫镁结构的影响

研究了两步法制备泡沫镁工艺中的二次加热过程,考察了二次加热温度和时间对泡沫镁结构的影响.结果表明:通过控制加热温度和时间,采用两步法可以制备出低密度、高孔隙率、气孔分布均匀的泡沫镁;随着二次加热时间或温度的增加,泡沫镁的孔洞尺寸逐渐变大,分布逐渐变得不均匀,并出现孔洞合并的趋势;当加热时间超过20 min或加热温度超过610℃后,泡沫镁的孔洞分布变得不均匀,出现孔洞氧化与合并的现象;随着加热时间或温度的增加,泡沫镁的密度逐渐减小,平均孔隙率逐渐增加,但当加热时间超过20 min时,密度略有升高,平均孔隙率略有下降.     

Analysis of thermoelastic characteristics for vertical-cracked thermal barrier coatings through mathematical approaches

Thermoelastic characteristics of thermal barrier coatings (TBCs) with vertical cracks were analyzed through mathematical approaches to investigate the thermoelastic behaviors of TBCs in a service temperature. TriplexPro?-200 system was applied to prepare the relatively dense TBC using METECO 204NS powder. The microstructure of top coat in the TBC was just controlled to create vertical type cracks by reheating without powder feeding in same equipment and rapid cooling process. A couple of governing partial differential equations were derived based on the thermoelastic theory, and a finite volume model was developed to the governing equations to evaluate the thermoelastic characteristics, such as temperature distribution profile, displacement, and stress, inducing a thermal fatigue. For the specimen with two or more vertical type cracks, smaller displacement appears to longitudinal direction and larger displacement to radial direction as the number of crack increases. In the longitudinal stress distribution profiles to z-direction, the tensile stress at the interface between the bond coat and the substrate converts into the compressive stress when the specimen has vertical cracks more than two, while larger magnitude undulation develops for the specimen with smaller number of crack in the radial stress distribution profiles. The results obtained demonstrate that multiple vertical cracks enhance the thermal durability and extend the lifetime of TBCs.     

换热器高压外壳的焊接

介绍了换热器高压壳体环缝的预热焊丝窄间隙埋弧焊、冷裂纹及再热裂纹敏感性较大的厚大锻件２０Ｍｎ封头下２０ＭｎＭｏＮｂ底部四通的焊接及密封阀座斯太利硬质合金堆焊的工艺。     

Microstructure evolution of semi-solid 2024 alloy during two-step reheating process

A two-step reheating process was proposed and applied to perform reheating experiments on the semi-solid 2024 alloy billet. In this process, the semi-solid billet was firstly heated over liquidus temperature and then isothermally held at solid-liquid zone temperature. Microstructure evolution of the semi-solid billet during two-step reheating was studied by optical microscope and compared with that during isothermal reheating. The results show that the remelting rate of the semi-solid billet during two-step reheating is faster than that during isothermal reheating. Under the same reheating time, the grains of the semi-solid billet reheated by two-step reheating process are finer and rounder than those by isothermal reheating process. The present experimental results indicate that accelerating the formation of liquid phase during the two-step reheating process can restrain the coalescence of grains to a certain extent, and thus refine the grain size and promote the grain spheroidization.     

加热工艺对AZ91D镁合金部分重熔组织的影响

提出半固态金属坯料先在液相线以上温度适当加热再降低温度至两相区温度继续等温保温的二次加热工艺。采用该工艺对晶粒细化AZ91D镁合金坯料进行部分重熔,研究了其组织演变规律,并与等温二次加热工艺进行比较。结果表明,与等温二次加热工艺相比,坯料先在液相线以上温度适当加热再降低温度至两相区温度继续保温,坯料重熔速度明显加快,相同加热时间时,晶粒更加细小和圆整。组织演变机理分析表明,加快液相形成速度可适当抑制晶粒的合并,降低晶粒长大速度,并促进晶粒球化。     

半固态坯料感应二次加热的模拟分析(1)

对半固态坯料感应二次加热的特点及规律进行了模拟分析,认为单段式感应二次加热难以获得较均匀的温度分 布,分段加热应先以高功率快速加热,再立即切换为小功率加热,然后辅以短时保温,可使坯料内温度分布更均匀。     

Influence of Liquid Film Characteristics on Hot Cracking Initiation in Al-Cu Alloys at the End of Solidification

In this study, Al-4Cu alloy specimens with spherical grains and liquid films were obtained by isothermal reheating treatment. The hot cracking of the solidification process was determined using a modified constrained rod casting experimental apparatus, and the effect of liquid film characteristics at the end of solidification on hot cracking initiation of Al-4Cu alloys was systematically investigated by combining molecular dynamics simulations and other methods. With the extension of soaking time, the liquid fraction (liquid film fraction at the end of solidification) and grain shape factor increased with higher isothermal reheating temperatures. Additionally, the widened filling channel decreased the hot cracking initiation temperature and the critical hot cracking shrinkage stress was found to increase, thus reducing the hot cracking severity in Al-4Cu alloys. Molecular dynamics simulations revealed that with the extension of soaking time, the composition of the liquid film changed at different isothermal reheating temperatures, but the short-range structure and atomic ordering of the liquid film remained the same. The activity of the liquid film increased in equilibrium, leading to a decrease in viscosity and an increase in fluidity, which contributed to the filling behaviour. After isothermal reheating at 640 °C for 60 min, the liquid fraction reached the maximum, and the viscosity of the liquid film was the minimum. In addition, almost no hot cracks were found.     

Design and application of dynamic secondary cooling control based on real time heat transfer model for continuous casting

The control of secondary cooling is extremely important for the quality of billet in the continuous casting. In order to minimise the variation in surface temperature and excessive reheating of billet, which have a considerable influence on the formation of cracks and other defects, a real time heat transfer model that can be used for simulation of varying casting operations was developed. In order to verify the dynamic performance of this model, a charge coupled device temperature measurement system that can eliminate the impact of the scales was presented. The actual shell thickness profile and billet temperature were calculated in real time by this model online and were taken into account to feedback control the casting process based on the desired target cooling pattern to improve the existing cooling system. The dynamic control system based on this model was developed and implemented on some caster, and the billet quality was obviously improved.