Effect of notch location on fatigue crack growth behavior of strength-mismatched high-strength low-alloy steel weldments

Welding of high-strength low-alloy (HSLA) steels involves the use of low-strength, equal-strength, and high-strength filler materials (electrodes) compared with the parent material, depending on the application of the welded structures and the availability of filler material. In the present investigation, the fatigue crack growth behavior of weld metal (WM) and the heat-affected zone (HAZ) of undermatched (UM), equally matched (EM), and overmatched (OM) joints has been studied. The base material used in this investigation is HSLA-80 steel of weldable grade. Shielded metal arc welding (SMAW) has been used to fabricate the butt joints. A center-cracked tension (CCT) specimen has been used to evaluate the fatigue crack growth behavior of welded joints, utilizing a servo-hydraulic-controlled fatigue-testing machine at constant amplitude loading (R=0). The effect of notch location on the fatigue crack growth behavior of strength mismatched HSLA steel weldments also has been analyzed.     

火焰矫正对屈服强度900 MPa级低合金高强钢焊接接头组织与性能的影响

对屈服强度900 MPa级的高强度钢焊接接头进行加热温度为700℃的模拟火焰矫正,发现其焊接热影响过热区出现了大量的M-A组元,焊接接头强度基本不变,但焊接热影响区低温冲击韧性下降45.3%。结果表明:屈服强度900 MPa级高强钢高强钢宜采用机械矫正,若采用火焰矫正,需控制好加热温度。     

一种锻造斗齿用低合金耐磨钢热变形行为

为研究一种锻造斗齿用新型低合金耐磨钢高温热变形行为,采用Gleeble-1500D热模拟试验机对其试样进行等温压缩试验,获取在变形温度为1173~1473 K、应变速率为0.01~10 s-1下的真应力-真应变曲线,对高温流动应力特征进行研究,分析了高温热变形行为的物理本质,拟合计算材料本构方程中的参数值,为斗齿锻造成形工艺的数值模拟提供材料性能数据。研究结果表明:热变形温度区间内,当应变速率一定时,该材料的流动应力随着变形温度的升高而减小;当变形温度一定时,该材料的流动应力随着应变速率的增大而增大;本构方程能够比较精确地描述材料在热变形过程中的流动应力,预测值与实验值的平均相对误差为2.29%,最大仅为5.37%。     

35CrMoV钢高温塑性变形行为及其本构方程建立

为了研究35CrMoV钢的高温变形行为,借助Gleelble 3800型热模拟试验机,在应变速率为0.01~10 s^-1、变形温度为950~1150℃的条件下进行轴向单道次高温压缩试验,并根据试验结果绘制35CrMoV钢的流动应力-应变曲线。分析研究了变形温度、应变速率对流动应力的影响,计算了变形激活能Q及参数n、A、α的取值。试验结果表明:35CrMoV钢在950~1150℃进行压缩试验时,存在动态再结晶和动态回复两种流动应力-应变关系,当应变速率为0.01和0.1 s^-1时,其流动应力-应变曲线主要表现为动态再结晶型;当应变速率为1和10 s^-1时,其流动应力-应变曲线主要表现为动态回复型。在试验条件下获得35CrMoV钢的平均变形激活能Q为310.433 kJ·mol^-1,建立了用于描述35CrMoV钢流动应力、应变速率和变形温度三者之间关系的本构方程。     

T92/HR3C异种钢焊接接头高温拉伸变形及断裂行为

采用500℃和625℃拉伸试验,研究T92/HR3C异种钢管接头的高温变形及其断裂行为。结果表明,在高温拉伸过程中,焊缝、T92侧热影响区(HAZ)及母材(不包含颈缩段)均未发生明显的塑性变形及组织结构的变化,而HR3C侧母材晶粒明显被拉长,HR3C侧HAZ的拉伸变形不明显。HR3C母材塑性变形量随温度升高而明显降低,孪晶回复越少。高温拉伸断口位于T92侧HAZ的细晶区(FGHAZ),呈正断加剪切断的混合断裂方式,均与室温状态下该焊接接头的拉伸变形及断裂行为不同。应力三轴度理论可很好地解释该接头高温短时拉伸变形及断裂特征。     

高温塑性变形中孔隙性缺陷自修复机理

通过物理模拟研究20MnMo材料内部孔隙性缺陷的修复过程,建立了孔隙性缺陷高温修复的修复再结晶机理。将孔隙性缺陷修复分为3个阶段:修复再结晶晶粒形核准备、修复再结晶晶粒形核、修复再结晶晶粒长大和改建,其中修复再结晶形核主要存在3种方式。孔隙性缺陷修复再结晶机理的提出对深入研究塑性变形中缺陷修复以及塑性变形对性能的影响作用具有重要理论意义,对缺陷修复规律在生产实际中应用具有重要的实用价值。     

Prediction of the cutting conditions giving plastic deformation of the tool in oblique machining

A method is described for predicting cutting conditions at which the cutting edge starts to deform plastically when machining with oblique nose radius tools. It is shown how tool stresses and temperatures determined from machining theory can be used together with experimental high temperature compressive strength data for the tool material to make these predictions. A comparison made between predicted and experimental results for two plain carbon steel work materials and a range of cutting conditions shows good agreement.     

热轧双相钢塑性变形和断裂行为的研究

文章利用SEM、TEM研究了热轧双相钢单向拉伸过程中不同变形量下的形变位错结构和断口组织特征。结果表明,双相钢中铁素体的塑性变形是由位错增殖、位错运动和交互作用、滑移带形成、位错缠结产生及其密度的增加、位错胞出现及细化等一系列演变构成;变形10%时,板条马氏体已产生塑性变形,其板条中出现形变带,并且随马氏体变形的继续,形变带不断形成和发展。变形颈缩时,双相钢中的微孔主要通过相界面结合力的丧失和马氏体断裂产生,而马氏体断裂主要出现在马氏体的尖角处。     

剧烈塑性变形珠光体钢丝退火组织与性能

研究了经剧烈塑性变形制备的珠光体扁钢丝在不同温度退火后的力学性能和组织.结果表明,在低于200℃退火时,由于应变时效的作用,强度和硬度有所增加;退火温度＞200℃时,强度和硬度不断降低.同时,在变形时引入的高应力和退火温度的作用下,片层状渗碳体在200℃以上退火时开始溶解,并随温度的升高,渗碳体溶解的速度加快,最终形成球化组织.     

Phase-stability dependence of plastic deformation behavior in Ti-Nb-Ta-Zr-O alloys

The authors investigated the effects of alloy content on mechanical properties to make clear a correlation between plastic deformation behavior and β-phase stability in Ti-Nb-Ta-Zr-O alloys. It was realized that there was specific compositional area in which the alloy exhibited little work hardening and minimum Young’s modulus value. The specific area was expressed by the bond order (Bo), based on theDV-Xα method, of 2.87 and the averaged electron/atom ratio (e/a) of 4.24, which corresponded to those of a multifunctional β titanium alloy, “Gum Metal.” These electronic conditions also minimized ideal strength required for plastic deformation without any dislocation activity. The deformation behavior of alloys in the specific compositional area revealed that the unique behavior could be characterized by a “giant fault.” It was also confirmed that such a compositional area corresponded to the phase boundary between the α″ martensite and β phases at room temperature. This paper was presented at the Beta Titanium Alloys of the 00’s Symposium sponsored by the Titanium Committee of TMS, held during the 2005 TMS Annual Meeting & Exhibition, February 13–16, 2005 in San Francisco, CA.     

Special features of thermal and deformation processes in welding high-strength steels with different edge preparation

Investigations were carried out into the special features of thermal and deformation processes in single- and twin-arc welding of joints with different edge reparation. On the basis of the experimental results it was established that deformation of all types when using the constricted shapes of the welding gap is lower than when using the standard welding gaps. The level of strain in the component can be additionally reduced by the application of twin-arc welding. The investigation results also show that in the final stage of welding it is necessary to correct the welding conditions in the immediate vicinity of the edge of the welded component in order to eliminate the superheating effect caused by the reflection of the heat flow from the welding edge.     

退火工艺对强塑性变形珠光体钢丝组织和性能的影响

用TEM、XRD研究了强塑性变形（ε=2．60）SWRH72A钢丝经不同温度（100—400℃）退火处理后的微观结构变化。XRD分析显示强塑性变形引起SWRH72A钢丝中渗碳体回溶,部分碳原子溶入铁素体形成过饱和铁素体,过饱和铁素体中含碳量约O．34at％。由TEM分析可知强塑性变形钢丝经200—300℃退火,由于渗碳体颗粒的弥散析出,钢丝强度升高到2400MPa。退火温度进一步升高到400℃,渗碳体片开始球化,钢丝强度大幅降低,但塑性显著增加。     

基于热拉伸试验的DP590高强钢变形本构关系及热加工图

在变形温度为1123~1423 K和应变速率为0.01~10 s-1条件下,对DP590高强钢进行高温热拉伸试验,得到其真应力-真应变曲线,分析了温度和应变速率对DP590高强钢热变形时流动应力的影响。结果表明,当应变量一定时,流动应力随应变速率的升高和温度的降低而增大。基于热拉伸试验数据,通过线性回归分别确定了在峰值应力下DP590高强钢的高温材料常数:应变硬化指数n=3.194,热变形激活能Q=508.29 k J·mol-1,α=0.0153和A=2.126×1017s-1。构建了DP590高强钢的Arrhenius双曲正弦本构关系,与试验值相比,模型的最大误差为7.8%,最小误差为0.18%。根据DMM动态材料模型建立了DP590高强钢在应变为0.3条件下的热加工图,确定了DP590高强钢的适宜热成形区为:应变速率为0.01~0.1 s-1,变形温度为1250~1375 K。     

Effect of electron beam surface hardening on fatigue crack growth rate in AISI 4340 steel

This paper studies the effect of electron beam (EB) surface hardening on the fatigue crack growth rate in AISI 4340 steel. The heat treatment conditions were varied to consider the influence of microstructure and residual stress. The results show that increasing the EB heat input increases the compressive residual stress in the hardened layer. Thus EB surface-hardening treatment improves the fatigue crack growth resistance. This effect increases with increasing EB heat input but disappears as the ΔK value increases. The fracture mechanism of the hardened layer is intergranular fracture, while that of the base material is transgranular quasi-cleavage.     

大型"H"型钢压力矫直塑性变形失稳区域及侧压力的估算

针对国内首次研制的大型"H"型钢矫直机,提出了塑性变形区域及侧压力估算方法,并进行了分析与讨论,以供本行业科技人员参考与借鉴.     

A study on the surface severe plastic deformation behavior of a Zr-based bulk metallic glass (BMG)

A surface treatment process, which can generate the severe plastic deformation in the near-surface layer of crystalline materials, is successfully applied on the Zr₅₀Cu₄₀Al₁₀ bulk metallic glass (BMG). The experiment is implemented using 20 WC/Co balls with a velocity of about 5 m/s to bombard the surface of the samples in a purified argon atmosphere. The plastic-flow deformation in the unconstrained sample edge was observed, which exhibits the good intrinsic ductility of this material under the experimental condition. In the sub-surface layer, the bombardment-induced shear-band operations generate the extrusion and intrusion marks on the side face. Differential scanning calorimetry (DSC) shows that the free volumes of the deformed BMG have increased, and possible crystallization may occur during the process. X-ray diffraction (XRD) and synchrotron high-energy X-ray diffraction techniques were used to inspect the possible crystal phase. A nanoindentation test shows that on the side surface, the hardness increases and, then, decreases with the distance from the processed surface. Four-point-bending-fatigue behavior has been studied and related to the modified surface structure and the compressive residual stress introduced by the surface treatment.     

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.     

Fracture behaviour and numerical study of resistance spot welded joints in high-strength steel sheet

Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out using 980 MPa high strength steel sheet of 1.6 mm thickness. In proportion, as nugget diameter increased from 3√t to 5√t (where t is thickness), cross tension strength (CTS) increased while fracture morphology simultaneously transferred from interface fracture to full plug fracture. In cases of interface fracture, circumferential crack initiation due to separation of the corona bond arose at an early stage of loading. The crack opening process without propagation was recognized until just before fracture and then the crack propagated to the nugget immediately in a brittle manner around CTS. In full plug fracture, main ductile crack initiation from the notch-like part at the end of sheet separation occurred with the sub-crack initiated at an early stage. The ductile crack propagated toward the HAZ and base material to form full plug fracture. The mode I stress intensity factor was considered as a suitable fracture parameter because the circumferential crack behaved pre-crack for brittle fracture in the nugget region at the final stage. Based on the FE analysis, the mode I stress intensity factor was calculated as 116 MPa √m at CTS as fracture toughness for the nugget. With respect to full plug fracture, ductile crack initiation behaviour from the notch-like part was expressed by concentration of equivalent plastic strain. On the assumption that the ductile crack arose in critical value of equivalent plastic strain, the value was calculated as 0.34 by FE analysis. Reasonable interpretation for interface fracture and full plug fracture in the resistance spot welded joint was proposed due to first crack initiation by stress concentration, brittle fracture by using mode I stress intensity factor, and ductile crack initiation by using equivalent plastic strain.     

Nanocrystallization process and mechanism in a nickel alloy subjected to surface severe plastic deformation

Bulk components made of a Ni-base C-2000 alloy with a face-centered cubic crystal structure and a very low stacking fault energy have been severely plastically deformed at the surface region to attain a grain size gradient ranging from nanocrystalline at the surface to coarse grained in the bulk. The evolution of microstructural characteristics has been studied as a function of the processing time employing a variety of analytical techniques, including extensive conventional and high-resolution transmission electron microscopy analyses. The thickness of the nanocrystalline surface layer is found to increase with the processing time. Deformation twinning is ubiquitous and occurs at the earliest stage of deformation and the deepest region of the material where plastic deformation has taken place in the surface severe plastic deformation process. A grain-refinement mechanism led by deformation twins and complemented by dislocation activity has been put forth to explain the nanocrystallization of the coarse-grained material employed in this investigation.     

Detection of plastic deformation and estimation of maximum value of residual stress in low carbon steel by X-ray stress analysis using statistical techniques

Residual stress distribution on plate-type tensile specimens of low carbon steel with applied plastic strains up to 6.45% has been measured by X-ray stress analysis. Standard deviation for the frequency distribution profile of the residual stress was calculated by statistical processing to estimate the amount of macroscopic plastic strain applied on the specimen. Since adequate understanding of the maximum value of the residual stress (σ_max) has been important for the maintenance of the nuclear power reactor components, the σ_max was also estimated from the limited number of the experimental data by statistics of extreme. It was found from above estimation that the X-ray stress analysis supported by the statistical techniques was a promising non-destructive evaluation (NDE) method to detect the plastic deformation and to estimate the σ_max in S25C low carbon steel.