Tensile properties and fatigue crack propagation of solution-annealed and aged superaustenitic stainless steel

The effect of aging in air at 650°C for 100~1000 h on the tensile properties of superaustenitic stainless steel in the range RT-750°C and the fatigue crack growth behaviour at RT and 650°C was studied. Yield strength and ultimate tensile strength were almost the same between the as-received and the aged specimen. The fracture strain, however, decreased significantly from aging, and the fracture surface of the aged specimen at RT test was intergranular. The fatigue crack growth rate at RT is enhanced by aging at the high stress intensity factor range. This is due to the occurrence of an intergranular fracture in the aged specimen. At 650°C the fatigue crack growth behaviour of both the as-received and the aged specimen was almost same with no intergranular fracture.     

渗氮及表面增碳对3Cr2W8V钢冷热疲劳行为的影响

用板状缺口试样研究了在自约束条件下，表面增碳和渗氮对３Ｃｒ２Ｗ８Ｖ钢冷—热疲劳裂纹萌生，扩展及裂纹形态特征的影响，结果表明，表面增碳及渗氮均使钢的热疲劳裂纹萌生及扩展抗力降低，渗氮尤为显著，经渗氮处理的试样在冷—热疲劳试验中除缺口尖端出现主裂纹外，试样表面较早出现大量裂纹，最后形成龟裂；表面增碳的试样在试验中也出现少量表面裂纹，而表面化学成分无变化的试样则未出现表面裂纹。     

Zum Einfluß der mechanischen Belastung auf die interkristalline Spannungsrißkorrosion von unlegiertem Stahl in Carbonat/Bicarbonat-Lösung

Effect of mechanical stress on intergranular stress corrosion cracking of unalloyed steel in carbonate/bicarbonate solution Intergranular stress corrosion cracking of a carbon steel in 0.5 M NaHCO₃ + 0.5 M Na₂CO₃ solution at 70°C was examined by normal, interrupted and cyclic CERT testing. Within the critical potential range, the lengths of intergranular cracks increase with increasing strain rate, and decrease when the material is cold worked. The fracture time indicates that the crack velocity increases with increasing strain rate. This is also observed with tensile specimens subjected to interrupted and cyclic strain rates. Detailed information on crack propagation and threshold stress levels can be obtained by interrupted and cyclic CERT tests. Intergranular cracks arise if surface region of the material undergoes plastic deformation in excess of a definite amount. In the case of a 60°-notch specimen the plastic zone must be greater than 0.3 mm. For cyclically strained specimens the crack velocity can markedly decrease with time or can become even zero as a consequence of strain hardening. From this one can conclude that stress corrosion cracking can only occur if stress levels are very high.     

Untersuchung des SwRK-Verhaltens des hochreinen martensitaushärtenden Stahles X5CrNiCuNb 17 4 PH im Vergleich zum weichmartensitischen Stahl X4CrNiMo 16 5 1 ESU in chloridhaltigen wäßrigen Medien. Teil 2: SwRK-Untersuchungen und Rißeinleitungsmechanismen

Investigations of the corrosion fatigue behaviour at a super pure martensitic stainless steel X5CrNiCuNb 17 4 PH in comparison to the soft martensitic stainless steel X4CrNiMo 16 5 1 ESR in chloride containing aqueous media — Part 2: Corrosion fatigue tests and crack initiation mechanisms The following report concerns the study of the corrosion fatigue behaviour of the soft martensitic steel X4CrNiMo 16 5 1 ESR and the precipitation hardened X5CrNiCuNb 17 4 PH in sodium solution in the temperature range between 20° and 150 °C and the determination of their general corrosion properties and the mechanism of crack propagation. Their corrosion fatigue limits were compared with each other. A comparision was also made between an electro-slag-remelted soft martensitic steel and a charge without an ESR aftertreatment. Microfractographical fracture and crack path investigation were carried out for interpretation of the experimental results. It was observed that in both super pure steels (soft martensitic and precipitation hardened) the oxidic inclusions are not responsible for the crack intiation, as it was found in the non ESR treated steels. In the 17-4 PH steel copper containing inclusions in the crack initiation areas were observed. In concentrated sodium solution pitting corrosion was found at both steels.     

海洋柔性立管用钢的疲劳断裂行为

采用GPS100高频疲劳试验机研究了海洋柔性立管用钢在温轧工艺下的疲劳断裂行为。结果表明,海洋柔性立管用钢疲劳寿命随加载应力幅的升高而不断下降;疲劳裂纹产生于试样表面,裂纹源区可见变形带及河流花样,裂纹扩展区存在大量二次裂纹,瞬断区疲劳裂纹呈韧性断裂特征。在疲劳裂纹扩展过程中,裂纹的扩展能力因大角度晶界的铁素体和高强度贝氏体的相互作用而受到限制,同时在晶界处形成的二次裂纹同样可减缓扩展速率,达到提高疲劳寿命的效果。     

Determination of corrosion‐assisted stress crack growth rate in 304L stainless steel welds

The corrosion assisted stress crack growth rate of 304L stainless steel welds, was determined by the rising displacement test method. The experiments were carried out in a Boiling Water Reactor (BWR) environment, using a refreshed autoclave loop system at 288 °C and 8 MPa of pressure. Pre‐fatigued CT specimens were used and the crack length was monitored by the potential drop technique. After testing, fractographical analysis and measurement of the initial and final crack length were made. The displacement rates of 3.6 and 2.37 μm/h allowed the initiation and propagation of cracks by stress corrosion cracking, and the test results agree with the crack growth rate values determined with another method. The fracture surfaces are evidence of stress corrosion crack propagation with a transgranular appearance in some cases and transgranular with localized areas of intergranular propagation in others.     

Fatigue crack growth behavior of laser-processed 304 stainless steel in air and gaseous hydrogen

Fatigue crack growth test was performed to evaluate fatigue behavior of 304 stainless steel specimens with or without laser processing (welding and surface treatment) in air and gaseous hydrogen. As the crack propagation normal to the laser welding or scan direction, the laser-processed specimens exhibited a higher resistance to crack growth in the low stress intensity factor range (ΔK) than the as-received steel plates regardless of testing environments. However, the marked retardation of crack growth behavior vanished for welded specimens subjected to a 850 °C/h stress relief treatment or with a shorter distance from notch tip to the weld centerline in the test.Fatigue-fractured appearance of the steel plate tested in air was composed of mainly transgranular fatigue fracture and some flat facets, along with a small amount of intergranular fracture. While quasi-cleavage fracture and few twin boundary separations were observed for the same specimen in hydrogen. On the other hand, the lower crack growth rate of laser-processed specimens in both air and hydrogen was accompanied with rubbed areas on the fracture surfaces. It was found that the extent of quasi-cleavage fracture was related to the formation of strain-induced martensite, which would contribute to an increased fatigue crack growth rate of all specimens in gaseous hydrogen.     

Acoustic detection of stress-corrosion cracking of nitrogen austenitic steels

Structural changes and resistance to the stress-corrosion cracking of the nitrogen-bearing austenitic steels 04Kh20N6G11M2AFB and 09Kh20N6G11M2AFB (with 0.04 and 0.09 wt % C, respectively) after different treatments, including thermomechanical action, quenching from 1200°C, and aging at 700°C for 2 and 10 h, have been studied. It has been shown that aging at 700°C of the air-melted austenitic steel 09Kh20N6G11M2AFB leads to a decrease in the strength of samples with an induced crack upon the cantilever bending in air and in a 3.5% aqueous solution of NaCl as compared to the strength of the steel 04Kh20N6G11M2AFB-EShP with a smaller carbon content after high-temperature mechanical treatment or quenching from 1200°C. The smallest resistance to stress-corrosion cracking is observed in the samples of 09Kh20N6G11M2AFB steel after 10 h of aging, which is accompanied by the most intense acoustic emission and by brittle intergranular fracture. This is explained by the high rate of the anodic dissolution of the metal near chromium-depleted grain boundaries due to the formation of continuous chains of grain-boundary chromium-containing precipitates of carbides and nitrides.     

In situ observation of short fatigue crack propagation in oxygenated water at elevated temperature and pressure

A new method is demonstrated for in situ measurement of short crack propagation rates during a corrosion fatigue test of an austenitic stainless steel within a windowed autoclave at 250 °C, 50 atmospheres, oxygenated water. Digital image correlation is used to monitor crack growth through measurement of the opening displacements of defects; either focused ion beam (FIB) milled notches from which fatigue cracks initiated, or stress corrosion cracks that subsequently propagated by fatigue.     

STUDIES OF FATIGUE CRACK PROPAGATION AND MICROFRACTUREGRAPHY IN HIGH STRENGTH STEELS

本文用宏观和微观方法测量了几种高强度钢的疲劳裂纹扩展速率,用金相和断口复型法观察了疲劳断口的微观形态以及与显微组织的关系。结果表明,宏观与微观测量的结果基本一致;疲劳裂纹扩展速率随平均应力的增加而增加;18CrMn2MoBA钢显微组织中的颗粒相(M+A)可以降低疲劳裂纹扩展速率。     

Elektrochemische Untersuchung der Rißbildung bei Schwingungsrißkorrosion im stabil-passiven Werkstoffzustand

Electrochemical investigation of crack initiation during corrosion fatigue of stainless steels in the passive state The corrosion fatigue behaviour of three stainless steels - ferritic (12% Cr), austenitic (type 316 Ti) and austenitic-ferritic (type 31803; Duplex stainless steel) - was studied under rotating bending moments in aqueous sulphuric acid of 30°C. An instrumental set-up for recording the transient currents of specimens during potentiostatically controlled corrosion fatigue is described. Based on this transient current signal technique, three stages on the corrosion fatigue process can be discerned. In the incubation period, small stochastic current transients are caused by the response of the passive layer to alternating stresses and environmental conditions. The appearance of sinusoidal current signals indicates crack initiation whereas the phase angle between a fixed marker - i.e. a light barrier signal -, and the anodic amplitude represents the site of initiating cracks. Finally, the crack growth period is characterized by an increasing cell current and steadily growing sinusoidal current signals caused by the interplay of microplastic and repassivation processes at the crack tip.     

X80管线钢焊接热模拟热影响区疲劳裂纹扩展行为的分析及预测

通过焊接热模拟方法对X80双相管线钢不同焊接热影响区疲劳裂纹扩展行为进行了研究,并分析实际焊接试样与焊接热模拟试样的热影响区疲劳裂纹扩展行为的差异,讨论了焊接热模拟技术应用于疲劳裂纹扩展寿命预测的可行性。结果表明,当裂纹在粗晶热影响区(CGHAZ)和临界热影响区(ICHAZ)上扩展时,由实际焊接试样与焊接热模拟试样热影响区所测得的疲劳裂纹扩展速率曲线(da/dN-DK曲线)存在明显差异,其原因与显微组织变化引起的裂纹扩展阻力大小不同有关。因此,建议采用实际焊接试样完整的热影响区所测得的da/dN-DK曲线来评估疲劳裂纹扩展寿命。     

高速列车车轴用EA4T钢腐蚀疲劳性能研究

目的 研究腐蚀环境中EA4T车轴钢疲劳性能,为车轴的腐蚀检测和使用寿命评估提供依据.方法 采用旋转弯曲疲劳试验机,在人造雨水模拟的腐蚀环境和空气环境中,对EA4T车轴钢试样进行疲劳试验,以获得不同环境下试样的疲劳S-N曲线、表面损伤以及裂纹扩展规律.然后对扩展裂纹进行概率统计,通过扫描电镜对疲劳失效的断口进行观察,并分析对比不同环境中裂纹扩展门槛值的变化.结果 空气环境中,试样的疲劳极限为355 MPa,而在腐蚀环境中,试样不存在疲劳极限,107循环周次对应的疲劳强度降低到245 MPa,相比空气环境中降低了31％.Gumbel分布统计与Weibull双参数分布统计相比,更适合描述EA4T车轴钢试样表面腐蚀裂纹长度随加载次数的变化.腐蚀环境中,疲劳裂纹萌生于表面腐蚀坑,并存在多个裂纹源.腐蚀环境显著降低了试样裂纹扩展门槛值,空气环境下,该值为6.29 MPa·m1/2,腐蚀环境下降低到4.1 MPa·m1/2.结论 腐蚀环境降低EA4T钢疲劳寿命的主要原因是,腐蚀环境降低了裂纹扩展门槛值,加快了裂纹萌生以及短裂纹扩展.而当裂纹达到一定长度时,腐蚀环境对裂纹扩展几乎没有影响.     

低温卷取热轧双相钢的显微组织及疲劳性能

采用扫描电镜、拉-拉疲劳试验机等研究了低温卷取热轧双相钢的显微组织及疲劳性能.结果表明:热轧双相试验钢的疲劳极限约为530 MPa;低温卷取工艺生产的热轧双相试验钢夹杂物平均尺寸多在5 μm以下,晶粒比较细小,马氏体组织较细小且弥散均匀分布,具有良好的综合力学性能.热轧双相试验钢疲劳裂纹源位于样品表面的棱角处,疲劳裂纹...     

Structure and fracture of granulated alloy ÉP741NP after fatigue tests

The structure and properties of high-alloy refractory nickel alloy ÉP741NP after quenching and aging in a standard mode with additional heating to 650°C (τ = 4 h), 870°C (τ = 10 h), and 1000–1050°C (τ = 2.5 h) that corresponds to operating conditions and production processes of specific parts are studied. The rate of growth of fatigue cracks is determined as a function of the amplitude of the stress intensity factor at 20 and 650°C. Data on the types of fracture surface textures for fatigue crack propagation and various values of this factor are obtained.     

Intergranular fracture on fatigue fracture surface of 2.25Cr-1Mo steel at room temperature in air

Low-alloy steels serving for a long time at high temperature (∼500 °C) are very sensitive to temper embrittlement due to segregation of various trace elements at prior austenite grain boundaries and/or carbide/matrix interfaces. This type of segregation in combination with various environmental effects can adversely affect the fracture resistance and fatigue crack propagation rate with subsequent change in the fracture morphology of low-alloy steels. The present work describes the effects of heat treatments on impurity element segregation and its subsequent effects on fatigue fracture behavior of 2.25Cr-1Mo steel under different environmental conditions and temperatures. It has been found that either prior impurity element segregation caused during the heat treatment or hydrogen-induced embrittlement due to the presence of water vapor in laboratory air alone cannot produce intergranular fracture on the fatigue surfaces of 2.25Cr-1Mo steel at room temperature in air. The occurrence of intergranular fracture on the fatigue surfaces results from the combined effect of impurity element segregation-induced grain boundary embrittlement and hydrogen-induced embrittlement, and that the proportion of intergranular fracture is a function of prior impurity element segregation provided that the grain boundary segregation level exceeds a certain critical value.     

Effect of Boron on the Kinetics of Low-Temperature Decomposition of Martensite in Quenched Medium-Carbon Steel

The effect of boron on the microstructure, microhardness, and kinetics of low-temperature decomposition of martensite in the 40Kh and 30KhRA steels quenched at different cooling rates has been studied. It has been shown that the low-temperature decomposition of martensite in the boron-containing steel after quenching from 1050°C at a high cooling rate is strongly decelerated at the initial stage of decomposition. At low quenching cooling rates, the martensite decomposition in the steels under investigation is characterized by a similar kinetics.     

Untersuchungen zur transkristallinen und interkristallinen Spannungsrißkorrosion austenitischer Chrom-Nickel-Stähle in heißen Magnesiumchlorid-Lösungen

Investigations Into transgranular and intergranular stress corrosion cracking of austenitic stainless steels In hot magnesium chloride solutions The stress corrosion cracking (SCC) of austenitic stainless steels in hot magnesium chloride solutions is known to be transgranular. Therefore the slip-step-dissolution model is most favourable when explaining the failure mechanism. Constant load and constant extension rate tests (CERT) show that both methodes are almost equivalent. Moreover constant extension rate tests in more concentrated magnesium chloride solutions at 135°C reveal a small potential range of intergranular stress corrosion cracking more negative than the range of transgranular SCC. Observations of crack nucleation and crack propagation make plain that crack nucleation is a localized corrosion process. Pitting produces crack nucleis in the elastic range whereas cracks start along slip lines after plastic deformation. Fractography of specimens which failed by intergranular and transgranular SCC show macroscopically brittle fracture surfaces. Therefore a model is proposed which explains crack propagation by hydrogen-induced intermitted cracking at high-stressed sites at the crack tip.     

The effect of grain orientation on fracture morphology during high-cycle fatigue of Ti–6Al–4V

Three different product forms of Ti–6Al–4V, unidirectionally rolled and cross-rolled plate and forged bar, have been cyclically loaded within the high-cycle fatigue regime to investigate fatigue crack initiation. The fracture surfaces of fatigued specimens contained large regions of neighbouring facets. The majority of facets examined had a near-basal fracture plane. It was shown that grains favouring crack initiation were primarily those with misorientations between 15° and 40° from the loading direction, whereas other orientations served as crack growth paths. This implies that the formation of a facet requires a combination of a moderately high Schmid factor for basal slip coupled with a tensile component perpendicular to the basal plane. The large regions of neighbouring facets on the fracture surface were found to be a consequence of cracking within a macrozone unfavourably oriented for slip found within the microstructure, i.e. one with its c-axis near to the loading direction.     

EFFECT OF CREEP FRACTURE TOUGHNESS ON CRACK INITIATION AND PROPAGATION UNDER CREEP FATIGUE INTERACTION

本文研究了蠕变断裂韧性对三种低合金Cr-Mo-V钢在蠕变一疲劳交互作用下裂纹开裂和扩展的影响.试验表明,材料韧性对裂纹开裂和扩展起重要作用,脆性状态时,裂纹开裂时间比蠕变时短,二者间的裂纹扩展速度无明显差别;韧性状态时,裂纹开裂时间不仅比蠕变时短,而且其裂纹扩展速度比蠕变时大得多.此外,低合金Cr-Mo-V钢经蠕变一疲劳交互作用后有脆化倾向,其脆性程度取决于钢的原始韧性值.韧性状态时,蠕变-疲劳交互作用显著促使三种Cr-Mo-V钢由韧性向脆性转变;而在脆性状态时,这种脆性转变不明显。