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。     

GH864合金蠕变/疲劳裂纹扩展速率及a-N曲线分析

研究了GH864合金不同保载时间下650℃蠕变/疲劳裂纹扩展行为,分析了裂纹扩展过程中蠕变和氧化的作用,以及a-N曲线的转折点含义。结果表明:保载5s时GH864合金以穿晶断裂为主,疲劳作用占主导;保载90s时GH864合金以沿晶断裂为主,蠕变作用占主导。利用Saxena模型可较好地表征本实验条件下650℃蠕变/疲劳交互作用的裂纹扩展速率曲线,可估算较高应力强度因子和较低应力强度因子的裂纹扩展速率。另外,用Saxena模型可求出蠕变和疲劳的表达式,对比分析高温蠕变/疲劳交互作用的裂纹扩展过程中蠕变和疲劳的作用及所占的比例。最后针对a-Ni/Nf、da/dN-a曲线及da/dN-N曲线变换中出现的拐点,结合断口形貌分析了转折点对应的含义。高温合金及其它材料的裂纹扩展速率曲线也适用于以上曲线分析方法。     

Fracture toughness and fatigue crack behaviour of A3003/SUS304 lap friction stir welded joints

Friction stir welding (FSW) can weld dissimilar metal joints without a thick and brittle intermetallic compound layer at the weld interface. In this study, the dissimilar lap joint of A3003 aluminium alloy and SUS304 stainless steel was successfully welded by FSW, and the joint obtained was tested to examine the properties of fracture toughness and fatigue crack growth rate. Its fracture toughness was different by the directions of crack propagation. The fracture toughness of advancing side (AS) to retreating side (RS) was stronger than that of RS to AS, and that of cryogenic temperature was stronger than that of room temperature. Its fatigue crack growth rate also showed the same tendency as its fracture toughness. These data were compared with the past data and discussed.     

夹杂物对超高强度钢低周疲劳裂纹萌生及扩展影响的原位观测

采用扫描电镜原位观测了数微米大小夹杂物对超高强度钢疲劳裂纹萌生及扩展的影响,并用有限元法解释了夹杂物尺寸和形状对疲劳裂纹萌生及初期扩展的影响程度,得到了超高强度钢低周疲劳裂纹萌生与扩展特性和夹杂物对疲劳破坏的关键尺寸的估计值．同时,讨论了现行实验方法对确定铁基材料裂纹扩展速率的作用．     

A NOVEL PARAMETER FOR EVALUATING THE FATIGUE CRACK GROWTH RATE IN CARBON STEELS

1. Introduction Estimation of the fatigue life of a component under cyclic loading is of great interest in the modern engineering design against plastic deformation or conventional fracture. The fatigue process of materials may be approximately divided into two stages, i.e. the fatigue crack initiation and its propagation. The latter is more significant in service, because most of the fatigue life of many engineering components is spent on the crack growth, especially the small crack growth. …     

钢在中性水溶液中腐蚀疲劳的研究

本文对16Mn、08PVXt、440C和4340钢在蒸馏水和3.5％NaCl水溶液中的疲劳裂纹扩展速率(FCGR)进行了实验研究,并与一些钢在同类介质中的腐蚀疲劳(CF)实验结果进行了对比。结果表明:一般钢在前种介质中的疲劳寿命(FL)较后者长;08PVXt、440C及4340钢在蒸馏水中的FCGR较NaCl水溶液中高。而16Mn钢则有相反的结果。作者认为,这种情况是由于Cl~-在裂纹萌生和宏观裂纹扩展过程中起的不同作用所致。     

A model of heating a flux-cored wire in arc metallizing and analysis of the structure of the coating

Summary

This paper describes HAZ‐notched CTOD tests of multipass welds in SMYS = 420–460 MPa class high‐strength steels for offshore structural applications. The weld metal strength overmatch causes different fracture behaviour depending on the actual CGHAZ toughness. When the CGHAZ is completely embrittled, the weld metal strength overmatch leads to the lower bound critical CTOD value. This is due to elevation of the local stress in the CGHAZ caused by the restraint effect of the overmatched weld metal. The fracture surface is generally flat, and brittle fracture originates from the CGHAZ sampled by the fatigue crack front. A larger fraction of the CGHAZ along the crack front gives a smaller critical CTOD value. When the CGHAZ has moderate toughness, however, the weld metal strength overmatch may produce a higher critical CTOD value at brittle fracture initiation. This is due to crack growth path deviation towards the base metal. Plastic deformation preferentially accumulates to a greater extent on the softer base metal side before the critical stress conditions for brittle fracture initiation occur in the CGHAZ. This asymmetrical plastic deformation promotes deviation of ductile crack growth from the crack tip CGHAZ. In this case, the critical CTOD value does not always reflect the CGHAZ toughness itself.

A notch location nearer the weld metal sometimes causes fracture initiation in the weld metal if the fatigue crack tip samples the CGHAZ. Such experimental data do not reflect the real CGHAZ toughness.

The significance of the critical CTOD value obtained in the tests must be determined in the fracture toughness evaluation of the weld CGHAZ. This paper presents a procedure for evaluation of CTOD test results obtained for HAZ‐notched welds that considers the strength mismatch effect.     

CORROSION FATIGUE OF HSLA STEELS IN SALT SPRAY

The corrosion fatigue propagation behaviour of high strength low alloy Cr-Ni and Si-Mnsteels has been investigated in 3.5 %NaCl solution spryly at the frequencies of 0.1 and 5.5 Hz.It was shown that the fatigue crack initiation resistances of the two steels are significantly re-duced in salt spray;the fatigue crack growth rates of steels increase with the decrease of fre-quency and are much higher in salt spray than in air within low ΔK range.A critical stress in-tensity factor was observed for each steel and the crack growth stoppage will occur if the ΔKvalues are lower than it.It was found that the active dissolution makes great contribution tothe fatigue crack growth within low ΔK range.     

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.     

IMPROVEMENT OF CARBIDE MORPHOLOGY AND MECHANICAL PROPERTIES OF Cr12MoV STEELS

1. IntroductionCr12MoV steel is used widely as an important high wear resistallt cold die steel aroundthe world[1]. It is well known that Cr12MoV steel has networked carbide in the solidification structure, which leads to the intergranular brittleness. Moreover. The networkcarbide is too stable to be changed during heat treatment even at high temperaturely]. Inthe conventional process, forging is needed to break etwork carbide into a granular form.However, partly owing to Cr12MoV steel hav…     

CREEP CRACK PROPAGATION IN CONVENTIONAL CAST AND DIRECTIONALLY SOLIDIFIED SUPERALLOY RENE 80

The creep crack propagation in superalloy René80 of two different microstructures,i.e.,equiaxed grain structure by conventional casting and columnar grain by directionalsolidification,was investigated under static load at 1123 K.The creep crack growth rate,da/dt,seems to be correlated with the stress intensity factor,K.The creep crack growth ratein the directionally solidified alloy is lower than that in the conventional cast alloy,owing tothe elimination of transverse grain boundaries.The effect of microstructure on creep crackpropagation has also been discussed.     

HIGH TEMPERATURE CRACK PROPAGATION AND FRACTUREOF SUPERALLOYS

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Failure mechanisms of high strength steels in bicarbonate solutions under anodic polarization

High strength steels used in prestressing concrete structures are not exempt from the effects induced by corrosion on the normal concrete reinforcement. Carbonation of surrounding concrete or mortar is not unlikely for prestressing tendons and strands. Moreover, these steels undergo to brittle fracture as a consequence of stress corrosion cracking phenomena. To evaluate if concrete carbonation can promote this kind of failure, constant load tests in bicarbonate aqueous solutions under anodic polarization were carried out on high strength steel wires. Microscopic examination pointed out that the wires exhibited a brittle fracture mode, while its natural feature is ductile, as indicated by air testing. Failure mechanism was evaluated by a fracture mechanic approach. Cracks initiation was attributed to an anodic dissolution mechanism, while its propagation, interpreted by means of the surface mobility theory, was related to interaction between hydrogen atoms and magnetite at a crack tip.     

EFFECT OF CRACK CLOSURE ON SLOW PROPAGATION IN DUAL-PHASE STEEL

The effect of ferrite content in ferrite-martensite dual-phase steel on the initiation and prop-agation of fatigue crack and the plastic deformation at crack tip has been studied.In the rangeof ferrite content from 24.2 to 41.5％,the optimum seems to be 33.8％,of which the crack ini-tiation will be prolonged,the threshold value increased,the propagation rate decreased and theclosure stress intensity factor increased.As the propagation force is described by effectivestress intensity factor,three steels with various ferrite contents will show the same propagationbehaviour on da/dN vs △ K_(eff)curve.It is shown that the closure effect increases with thedecrease in △K at the fatigue crack tip.When △K equals to △K_(th),the closure effect reachesa maximum value of0.7 in a dual-phase steel with 33.8％ferrite.     

几种低合金钢在NaCl水溶液中腐蚀疲劳的研究

本文对几种低合金钢在3.5%NaCl 水溶液中的 CF 进行了实验研究。首先在室温下对其抗 CF性能作了比较,进而研究了温度对18Nb 钢 CF 裂纹扩展速率的影响.求出该钢在3.5%NaCl 水溶液中 CF 裂纹扩展激活能为7.4～7.9kcal/mol。认为氧的去极化反应可能是控制18Nb 钢 CF 裂纹扩展速率的主要因素。     

（F＋M＋A）三相钢低周疲劳的扫描电镜动态研究SCIEI

用扫描电子显微镜疲劳加载台进行动态原位观察，研究了奥氏体（Ａ）在（Ｆ＋Ｍ＋Ａ）三相钢低周疲劳裂纹萌生和扩展过程中的作用．结果表明，在低周疲劳裂纹的萌生和早期扩展阶段，三相钢中的奥氏体会降低疲劳缺口敏感性，延长裂纹萌生期，使裂纹路径弯折并降低扩展速率．在高应变疲劳阶段，因奥氏体应变诱发相变生成马氏体，其边界是裂纹扩展的低能通道，从而加速裂纹的扩展．并提出了一个能量判据：当裂尖扩展功小于Ａ→Ｍ转变的能量时，奥氏体会提高材料的疲劳抗力，反之奥氏体会加速疲劳裂纹的萌生和扩展．     

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。     

THE CREEP AND FRACTURE BEHAVIOR OF TWO NICKEL-BASED SUPERALLOYS

1.IntroductionTheevolutionofsuperalloyshasbeengreatlymotiVatedbythedevelopmentofmodernjetengines.Somepartsoftheengineareoperatedathightemperaturesandsubjectedtobothstaticanddynamicforcesaswellasthermalgradients.Therefore,creepcanoccuratsignificantratesandthentheestimationofcreepstrainandtheassociateddamageaccumulationbecomethecriticalconsiderationinthedesignoftheenginebladesoperatingathightemperatures.ThepurposeofthisinvestigationwastoexaminethecreeppropertyandfracturebehaviorofthecastK417and…     

喷丸对H13钢单边带缺口试样疲劳裂纹扩展行为的影响

喷丸工艺通过改变试件的表面形貌、微观组织及残余应力等表面完整性可以影响裂纹的萌生和扩展;对于存在缺陷的试件,喷丸的作用机制和影响结果有所不同。采用白光干涉仪、SEM、XRD及显微硬度计等对单边带缺口的H13钢薄板试样喷丸前后的表面完整性进行了测定。并借助原位SEM开展了系列裂纹扩展试验,分析了喷丸对试样疲劳寿命、裂纹扩展速率以及疲劳断口特征的影响。研究结果表明,虽然残余压应力诱发裂纹闭合,但由于喷丸后表面粗糙度的大幅提高增强了缺口效应,表面加工硬化使得韧性有所降低,以及残余应力在加载过程中发生松弛等因素,喷丸后单边带缺口试样的裂纹萌生过程缩短,疲劳寿命降低,且裂纹扩展速率的变化较小。喷丸前后疲劳断口形貌均为准解理特征,喷丸后断口近表面处的撕裂棱特征消失。     

Specific Features of the Nucleation and Growth of Fatigue Cracks in Steel under Cyclic Dynamic Compression

The processes of the fracture of 40Kh and U8 steels under cyclic dynamic compression are studied. It has been found that the main cause for the fracture of the cyclically compressed specimens is the propagation of cracks due to the effect of residual tensile stresses, which arise near the tips of the cracks at the stage of the unloading of the specimens. The growth rate of a crack has the maximum value at the initial stage of its propagation in the vicinity of the stress concentrator. As the crack propagates deep into the specimen, its growth rate decreases and depends only slightly on the real cross section of the specimen. The model of the process of the fatigue fracture of the steels under dynamic loading by a cyclically varied compressive force is proposed. It has been found that the high fatigue endurance is provided by tempering at 200°C for the 40Kh steel and at 300°C for the U8 steel.