核电主管道用316LN不锈钢焊缝热老化脆化的趋势

开展了核电站主管道用316LN不锈钢焊缝在325、365和400℃下1.5万h的加速热老化试验,测量了不同热老化时间下焊缝的冲击性能和焊缝铁素体的纳米硬度。以纳米硬度作为热老化脆化参量,利用Arrhenius方程得出该焊缝热老化激活能约为93.1kJ/mol;并以焊缝室温冲击功为预测参数,通过拟合的方法获得了焊缝热老化脆化预测方程;利用热老化激活能和热老化脆化预测方程预测了主管道用316LN焊缝在服役温度下60年寿命期内的热老化脆化趋势。试验结果表明,随着热老化时间的增加,焊缝的冲击韧性显著下降,焊缝中铁素体纳米硬度快速增加。预测结果表明,在运行15年内主管道焊缝韧性迅速下降,在随后的运行过程中下降趋缓。     

Z3CN20—09M铸造奥氏体不锈钢的低周疲劳行为

采用径向应变控制研究了Z3CN20-09M奥氏体不锈钢在室温和350℃高温下的低周疲劳行为.Z3CN20-09M不锈钢表现为先硬化后软化的循环特性,但硬化的程度取决于温度和应变幅.随着应变幅的增加,Z3CN20-09M钢的低周疲劳循环寿命逐渐减短,而相同循环次数下应力幅也随之提高.温度对Z3CN20-09M钢的低周疲劳行为影响较大,与室温相比高温下的循环硬化程度更高,相同应变幅下高温的低周疲劳寿命也高于常温下的寿命.通过疲劳实验的原位观察发现,奥氏体内的滑移面、夹杂物及奥氏体和铁素体两相的界面是疲劳裂纹可能的形核位置,奥氏体和铁素体两相的不协调变形使相界处产生应力集中,导致疲劳裂纹容易沿两相界面扩展.      

控轧控冷对X80M钢强韧性的作用

采用热模拟压缩试验和热膨胀试验研究了X80M钢奥氏体高温变形行为和连续冷却相变规律,制定了控轧控冷参数,进行了工业生产.研究结果表明:控轧控冷工艺中末道次变形量大于25％可发生完全再结晶,晶粒尺寸显著细化,冷却速率在20～25℃/s时,获得比例合适的针状铁素体+贝氏体.钢带屈强比为0.86左右,-20℃冲击功不低于370 J,-15℃落锤剪切面积不小于95％,夏比V型冲击韧脆转变温度为-45℃,保证焊管后具有低包辛格强度变化、低韧脆转变温度和优良的抗屈曲失稳特性.     

Cu/Sb/Sn/Mo/W对新国标09CrCuSb钢组织及力学性能的影响

采用金相显微镜、扫描电镜、电子探针、拉伸力学性能测试、夏比冲击测试,研究了新国标NB/T47019-2021的09CrCuSb钢合金元素Cu、Sb、Sn、Mo和W对其微观组织和力学性能的影响。研究表明,新标准中增加的合金元素将促进钢中δ-铁素体的析出,同时也会增加材料强度但会降低其塑性;δ-铁素体将显著降低材料冲击力学性能[其中,R_p0.2=365.64 MPa,夏比冲击功KV₂为243 J(室温)和192 J(-40℃)],使得冲击断口由塑性断口向脆性断口特征转变,特别是对-40℃的冲击功降低作用尤为显著。在新国标成分框架下,须通过合理的热加工工艺调整,降低钢中δ-铁素体的析出,从而提高材料力学性能。     

细晶组织耐候钢热影响区粗晶区的组织和性能

 采用焊接热模拟技术研究了焊接热循环对细晶组织09CuPCrNi钢热影响区粗晶区 (CGHAZ)显微组织和性能的影响。结果表明,在800~500 ℃冷却时间t8/5≤8 s时,该钢CGHAZ组织为贝氏体和少量马氏体。随着t8/5延长,在原奥氏体晶界上逐渐析出先共析铁素体,当t8/5＞18 s时,显微组织由大量铁素体和珠光体组成,且原奥氏体晶粒明显粗化,先共析铁素体含量增加。在-20 ℃和0 ℃下,t8/5对冲击吸收功影响较小,在-40 ℃时,随着t8/5延长,冲击吸收功下降显著,而且随着t8/5延长,CGHAZ硬度逐渐下降,但硬度值均高于母材,焊接热影响区相对于母材未出现软化倾向。     

核岛用P280GH碳锰钢热老化的组织和性能

 P280GH碳锰钢用于制造压水堆核电站核岛蒸汽系统和辅助系统管道,其在280～350 ℃的长时服役过程中易于出现老化现象。为了研究其热老化机理,在400 ℃温度下进行了P280GH碳锰钢的10 000 h的加速热老化试验。测定了不同老化时间下铁素体和珠光体的显微硬度、试样拉伸性能和冲击性能。结果表明,铁素体和珠光体的显微硬度随老化时间的增长而逐渐减小;老化时间少于300 h时,拉伸强度和屈服强度随老化时间增长而增加,老化时间大于300 h后,抗拉强度逐步降低,屈服强度缓慢降低;冲击吸收功随老化时间的增加而减小。上述力学性能变化与热老化过程中的珠光体形貌变化及铁素体中的位错移动有关。     

临界区热处理工艺对X80大变形管线钢力学性能的影响

采用拉伸、夏比冲击、落锤撕裂(DWTT)及SEM等试验方法,研究了临界区热处理工艺生产的X80大变形管线钢微观组织和力学性能.研究结果表明:采用800～860℃保温15 min淬火后,强度可达到X80大变形管线钢要求,并具有优异的变形性能和极佳的低温冲击韧性,-20℃夏比冲击功均值大于240J,夏比冲击剪切面积均值大于90％,-15℃DWTT剪切面积均值大于85％,同时拉伸曲线呈拱顶型.上述工艺参数条件下淬火后,得到铁素体加马氏体双相组织,马氏体体积分数为16.1 ％～18.2％.     

大热输入条件下氮含量对低碳Mo-V-Ti-B微合金钢粗晶热影响区组织与冲击韧性的影响

采用Gleeble研究了大热输入条件下,氮含量对低碳Mo-V-Ti-B微合金钢焊接粗晶热影响区组织和冲击韧性的影响。结果表明,在100 kJ/cm热输入条件下,85 N、110 N和144 N试验钢粗晶热影响区-20℃冲击功分别为186 J、97 J和57 J。随着试验钢中氮含量的增加,试验钢粗晶热影响区冲击功逐渐降低。当氮含量从85 ppm增加至110 ppm时,低碳Mo-V-Ti-B微合金钢焊接粗晶热影响区组织由块状铁素体+大量针状铁素体转变为块状铁素体+少量的针状铁素体,组织发生粗化,故粗晶热影响区冲击功降低。当氮含量从110 ppm进一步增加至144 ppm时,粗晶热影响区组织由块状铁素体+少量的针状铁素体转变为块状铁素体+珠光体,虽然块状铁素体晶粒发生细化,但是粗晶热影响区组织中的“硬相”珠光体的含量增加,并且珠光体尺寸较大,在冲击试样变形过程中“硬相”珠光体与基体形变不协调,从而导致裂纹的萌生,造成冲击功进一步下降。     

新型工程机械用Q550钢气保焊性能研究

采用Φ1.2mm的WH70-G的实芯气保焊丝对16mm厚的新型Q550D钢板进行富氩气体保护对接焊,针对焊接接头分析了显微组织,进行了拉伸、弯曲、冲击、硬度等力学性能检测。接头断面上没有发现气孔、夹杂、裂纹等明显缺陷,焊缝区组织为针状铁素体+先共析铁素体,热影响区粗晶区组织为粒状贝氏体;接头抗拉强度达到760 MPa,断裂位置均位于母材,D=3a,180°冷弯试验合格,接头塑性良好,焊缝、熔合线、热影响区-20℃冲击功均大于47J。表明所采用不预热的焊接工艺能够满足Q550D钢的焊接技术要求。     

长期热老化后核电站主管道材料的力学行为

大型压水堆核电站主管道多采用综合性能优良的铸造奥氏体不锈钢,但该材料在中温下长期服役会发生热老化脆化现象。总结了国内外的最新研究成果,主要综述了长期热老化过程中主管道材料的相变规律及其影响因素,以及长期热老化后材料的纳米硬度、拉伸、冲击和疲劳行为。     

Study of Aging-Induced Degradation of Fracture Resistance of Alloy 617 Toward High-Temperature Applications

For the Alloy 617, the effect of aging on the fracture energy degradation has been investigated after aging for different time periods at 1023 K (750 °C). A sharp reduction in impact energy (by ~55 pct vis-à-vis the as-received material) after 1000 hours of aging, as evaluated from room-temperature Charpy impact tests, has been observed. Further aging up to 10,000 hours has led to a degradation of fracture energy up to ~78 pct. Fractographic examinations using scanning electron microscopy (SEM) have revealed a change in fracture mode from fibrous-ductile for the un-aged material to intergranular mode for the aged one. The extent of intergranular fracture increases with the increasing aging time, indicating a tendency of the material to undergo grain boundary embrittlement over long-term aging. Analysis of the transmission electron microscopy (TEM) micrographs along with selected area diffraction (SAD) patterns for the samples aged at 10,000 hours revealed finely dispersed γ′ precipitates of size 30 to 40 nm, rich in Al and Ti, along with extensive precipitation of M₂₃C₆ at the grain boundaries. In addition, the presence of Ni₃Si of size in the range of 110 to 120 nm also has been noticed. The extensive precipitation of M₂₃C₆ at the grain boundaries have been considered as a major reason for aging-induced embrittlement of this material.     

Atom probe examination of thermally ages CF8M cast stainless steel

The precipitation reactions which occur in the ferrite phase of cast type CF8M stainless steels after low-temperature (300 °C to 400 °C) aging have been investigated using atom probe field ion microscopy (APFIM). Both spinodal decomposition and G-phase precipitation were found in two heats of material aged in this temperature range for times varying from 700 to 30,000 hours. The extent of spinodal decomposition and G-phase precipitation increased with both aging temperature and aging time. The activation energy for spinodal decomposition was estimated to be 260 ± 50 kJ/mole, consistent with the activation energy for chromium diffusion. The kinetics of G-phase precipitation showed less dependence on aging temperature than did the kinetics of spinodal decomposition. Also, the composition of the G-phase precipitates was observed to change with aging temperature but not with aging time. The faster embrittlement kinetics of these two heats in comparison to most other heats of CF8M can be attributed to the large extent of G-phase precipitation. formerly with the Department of Materials Science and Engineering formerly with the Department of Materials Science and Engineering     

The Evaluation of Tempered Martensite Embrittlement in 4130 Steel by Instrumented Charpy V-Notch Testing

Tempered martensite embrittlement (TME) was studied in vacuum-melted 4130 steel with either 0.002 or 0.02 wt pct P. TME was observed as a severe decrease in Charpy V-notch impact energy, from 46 ft-lb. at 200 °C to 35 ft-lb. at 300 °C in the low P alloy. The impact energy of the high P alloy was consistently lower than that of the low P alloy in all tempered conditions. Fracture was transgranular for all specimens; therefore, segregation of P to the prior austenitic grain boundaries was not a factor in the occurrence of TME. Analysis of load-time curves obtained by instrumented Charpy testing revealed that the embrittlement is associated with a drop in the pre-maximum-load and post-unstable-fracture energies. In specimens tempered at 400 °C the deleterious effect of phosphorus on impact energy became pronounced, a result more consistent with classical temper embrittlement rather than TME. A constant decrease in pre-maximum-load energy due to phosphorus content was observed. The pre-maximum-load energy decreases with increasing tempering temperature in the range of 200 °C to 400 °C, a result explained by the change in work hardening rate. Carbon extraction replicas of polished and etched as-quenched specimens revealed the presence of Fe₂MoC and/or Fe₃C carbides retained after austenitizing. Ductile crack extension close to the notch root was related to the formation of fine microvoids at the retained carbides.     

Low temperature aging behavior of type 308 stainless steel weld metal

The aging behavior of welded type 308 stainless steel was evaluated by mechanical property testing and microstructural examination. Aging was carried out at 475°C for up to 20,000 h. The initial material consisted of austenite with approximately 10% ferrite. Upon aging, the ferrite hardness increased up to 100%. This hardening was accompanied by a noticeable increase in the ductile—brittle transition temperature and a drop in the upper shelf energy, as measured by Charpy impact tests, and a degradation in fracture toughness, as determined by J-integral test. Tensile properties did not change significantly with aging. Microstructural analysis indicated that the ferrite decomposed spinodally into iron-rich α and chromium-enriched α′. In addition, abundant precipitation of nickel- and silicon-rich G-phase was found within the ferrite and M₂₃C₆ carbide formed along the austenite-ferrite interface. These effects are similar to the aging behavior of cast stainless steels. Occasionally, large G-phase or α precipitates were also found along the austenite-ferrite interface after aging more than 1000 h. After comparison of the mechanical property changes with the microstructural features, it was concluded that both spinodal decomposition as well as G-phase formation contribute to ferrite hardening. Spinodal decomposition results in embrittlement of the weld insofar as the ductile-brittle transition temperature is raised. G-phase formation and carbide precipitation are associated with a degradation in the ductile fracture properties, as shown by a drop in the upper shelf energy and a decrease in the fracture toughness.     

Embrittlement of titanium alloys by long time,high temperature exposure

α stabilized titanium alloys are known to exhibit embrittlement after long-time exposures above ∼800°F. The time-temperature dependency of this embrittlement phenomenon in the Ti-6Al-2Sn-4Zr-2Mo and Ti-5Al-6Sn-2Zr-lMo-0.25Si alloys was observed using a substandard fracture mechanics test. Room temperature slow-bend tests of fatigue precracked Charpy specimens were used to monitor toughness degradation after unstressed thermal exposures in the temperature range of 800° to 1100°F for times to 5000 hr. The activation energy for the embrittlement process was found to be ∼25 to 28 kcal per g mole, which approximates that for diffusion of aluminum or tin in α-Ti. The embrittlement is attributed to the Ti₃X (X = Al, Sn) phase with the rate controlling step that of diffusion controlled growth of the Ti₃X phase domains. The embrittlement process is reversible by heat treatment at temperatures above the α + Ti₃X two phase region.     

The evaluation of tempered martensite embrittlement in 4130 steel by instrumented charpy V-notch testing

Tempered martensite embrittlement (TME) was studied in vacuum-melted 4130 steel with either 0.002 or 0.02 wt pct P. TME was observed as a severe decrease in Charpy V-notch impact energy, from 46 ft-lb. at 200 °C to 35 ft-lb. at 300 °C in the low P alloy. The impact energy of the high P alloy was consistently lower than that of the low P alloy in all tempered conditions. Fracture was transgranular for all specimens; therefore, segregation of P to the prior austenitic grain boundaries was not a factor in the o°Currence of TME. Analysis of load-time curves obtained by instrumented Charpy testing revealed that the embrittlement is associated with a drop in the pre-maximum-load and post-unstable-fracture energies. In specimens tempered at 400 °C the deleterious effect of phosphorus on impact energy became pronounced, a result more consistent with classical temper embrittlement rather than TME. A constant decrease in pre-maximum-load energy due to phosphorus content was observed. The pre-maximum-load energy decreases with increasing tempering temperature in the range of 200 °C to 400 °C, a result explained by the change in work hardening rate. Carbon extraction replicas of polished and etched as-quenched specimens revealed the presence of Fe₂MoC and/or Fe₃C carbides retained after austenitizing. Ductile crack extension close to the notch root was related to the formation of fine micro voids at the retained carbides. This paper is based on a presentation made at the “pcter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

Ductility exhaustion mechanisms in thermally exposed thin sheets of a near-<Emphasis Type="Italic">β</Emphasis> titanium alloy

This study examines the effects of thermal exposure in air environment on the plastic elongation of thin sheets of Ti-15Mo-2.7Nb-3Al-0.2Si (Timetal-21S) alloy. Specimens with thicknesses of 0.12, 0.39, and 1.0 mm were exposed in air environment to temperatures ranging from 482 °C to 693 °C. Tensile tests conducted on these specimens at room temperature show a reduction of plastic elongation proportional to the thermal exposure parameters, time and temperature. Furthermore, a change in the failure mode into a quasi-brittle fracture was observed in the near-surface region. The depth of this region depends on both exposure time and temperature. The kinetics of embrittlement is studied through theoretical considerations of gas diffusion into metal. This approach shows that two distinct embrittlement mechanisms operate in this alloy. The characteristics of each of these mechanisms depend on the corresponding temperature range. At temperatures higher than 545 °C, the embrittlement activation energy is 41.2 kcal·mol⁻¹, indicating that the embrittlement process is governed by an enhanced diffusion of oxygen into Timetal-21S. Below this transitional temperature, the embrittlement activation energy approaches zero, a characteristic of slow kinetics transformation. The effects of solid-solution hardening, precipitation-hardening mechanisms, and alloying-element partitioning on ductility exhaustion processes are analyzed and discussed.     

Influence of Isothermal Aging on the Embrittlement of 00Cr25Ni7Mo4N Super Duplex Stainless Steel

The effect of an isothermal aging treatment on the embrittlement of 00Cr25Ni7Mo4N super duplex stainless steel was studied by combining metallographic, SEM and TEM observation. The results showed that thermal aging treatment below 500°C and above 1050°C did not have any effect on the impact toughness. The observed embrittlement behaviour was mainly attributed to the generation of intermetallic phases in the steel matrix, which included the R phase formation below 600°C, the R phase and especially the σ phase formation at 700°C, and the formation of the σ phase in the range of 800‐1000°C. The impact toughness decreased to a minimum after a thermal aging for 30 minutes in the temperature range of 800°C to 900°C. A trend of decreasing impact ductility with increasing amount of precipitated σ phase was observed. The impact energy started to decrease as the amount of the σ phase reached about 10‐15 volume‐% and reached a minimum when the amount of the σ phase increased to about 30 volume‐%.     

Mechanistic similarities between hydrogen and temperature effects on the ductile-to-brittle transition of a stainless steel

Charpy V-notched impact test studies on an Fe-Mn-Al austenitic stainless steel over a range of temperatures (293 K to 77 K) demonstrated that hydrogen charging promoted multiple crack initiation, reduced the energy absorbed by the material, and shifted the ductile-to-brittle transition curve to higher temperatures. These observations suggest that some stainless steels can exhibit changes in their ductile-to-brittle transition behavior in the presence of hydrogen. Further, the observation that embrittlement exists even at liquid nitrogen temperatures indicates that little or no localized rearrangement of hydrogen during the test is required or that relatively high strain rate effects on hydrogen embrittlement need not be necessarily attributed to enhanced transport of hydrogen atmospheres by mobile dislocations. The data presented in this paper are consistent with a model that predicts the lowering of interfacial strengths in the presence of an environment.