C—Mn钢焊接接头的解理断裂应力的研究

测定了Ｃ－Ｍｎ钢焊接接头不同焊缝百分含量ｗ％的光滑试样,缺口试样和裂纹试样的解理断裂应力,发现不同试样有两个解理断裂应力表征了解理断裂起裂于母材、焊颖两区。而且发现裂纹试验解理断裂应力比缺口试样高,缺口试样解理断裂应力比光滑试样高,其原因是因为裂纹试样解理断裂临界事件最小,光滑试样最大。     

钢解理断裂应力的测量和物理意义

用二维平面应变有限元计算了不同尺寸四点弯曲（4PB）缺口试样缺口前的应力分布。通过测量解理起裂源的位置，精确测定了一种钢的细观解理断裂应力和σ1和宏观理解断裂应力σF。结果表明，σF的测量比σf更简单和方便，σf的值可以用σF估计。随试样尺寸的增加，σF和σf的值基本不变，σF的分散性不于σf。稳定的σF和σf的下限值可以用比Griffit-Owen试样尺寸大的试样测得。σf可精确用于细观解理断裂机理的分析，并表征了钢的本质韧性。建议σF作为一个潜在的工程参数，用于钢的理解断裂韧性评价和结构安全设计。     

JFE-HITEN610U2L钢组织形貌及其对韧性的影响

对JFE-HITEN610U2L钢的显微组织和断口形貌进行了分析。该钢的显微组织以针状铁素体为主，具有良好的强韧性。当针状铁素体比较粗大时，冲击试样的断裂方式从韧窝断裂转变为解理断裂。从材料使用的安全性考虑，建议对该材料增加显微组织、晶粒度和冲击试样断口形貌等内容的检验。     

幂硬化材料解理断裂应力σf的测定

提出了在测定幂硬化材料解理断裂应力σ１的方法，并且测定了１６ＭｎＲ母材及焊缝金属低温下的σｆ。     

X70管线钢焊接接头热影响区的冲击性能

对X70管线钢焊接接头热影响区进行了冲击试验,对结果进行了分析。结果表明：X70管线钢焊接接头热影响区冲击试样的冲击功与断口剪切面积基本成线性比例关系,冲击功越高,剪切面积越大。焊接接头热影响区冲击试样的断口由韧性区、脆性区和韧脆混合区组成。脆性区为解理断裂,韧脆混合区为准解理断裂。     

低碳钢解理裂缝源的微观分析

本文通过对低碳钢光滑拉伸试样及缺口弯曲试样在低温下断裂的断口表面形貌及其纵剖面金相所进行的SEM观察,揭示了解理裂缝萌生及扩展的随机性,分析了解理断裂过程的控制因素。建立了“解理裂缝源”的概念以与“解理初裂缝”相区别,据此并结合显微组织特点,指出了低碳钢解理断裂的微观机制。     

不同热输入下F460钢焊接粗晶热影响区韧脆转变的内在机理

使用Gleeble 3800热模拟试验机模拟F460钢单道次焊接条件下焊接粗晶热影响区的热循环过程,通过光镜(OM)、扫描电镜(SEM)分析热影响区的显微组织、确定临界事件,通过ABAQUS软件计算临界解理断裂应力σf,进而系统分析不同焊接热输入E下韧脆转变温度变化的内在机理。结果表明:随着E的提高,焊接粗晶热影响区显微组织依次为少量板条马氏体和大量细密的板条贝氏体,板条贝氏体较多的板条/粒状贝氏体,粒状贝氏体较多的板条/粒状贝氏体,粗大的粒状贝氏体。原始奥氏体晶粒、贝氏体团的最大尺寸随着E的提高而变大。在完全解理断裂的冲击断口上,寻找停留在缺口尖端附近的残留裂纹,通过对比残留裂纹长度、原始奥氏体晶粒大小、贝氏体团尺寸,发现不同E下解理断裂的临界事件尺寸都是贝氏体团大小,而临界事件尺寸越小,韧脆转变温度越低。此外,通过有限元模拟缺口尖端的应力分布得到σf,σf越大冲击韧度越好,随着E的提高σf降低,故进一步说明随着E的提高韧脆转变温度Tk上升的内在机理。     

损伤对低合金钢缺口试样解理断裂韧性的影响

对一种低合金钢的缺口试样在常温下进行了不同预加载荷的四点正、反弯曲实验,以在缺口前引入不同的微孔洞损伤量,然后通过高温回火消除残余应力和加工硬化。在-196℃低温下进行弯曲断裂实验,通过力学参数的测量和微观观察就损伤对缺口试样解理断裂韧性的影响进行了实验研究。发现,当预载荷比F0/Fgy<1.1时,预载荷对缺口解理断裂韧性基本无影响,其原因是在缺口前未产生微孔洞损伤。当F0/Fgy>1.1后,缺口解理断裂韧性随F0/Fgy的增加而迅速下降,其原因是缺口前的微孔洞损伤引起的局部高应力应变集中促使解理起裂发生在低载荷下。     

初始损伤对不同钢组织缺口试样解理断裂韧性的影响

通过对两种钢热处理后得到的三种材料组织的缺口试样在常温下进行了不同预加载荷的四点弯曲正反弯实验，以在缺口前引入不同的微孔洞损伤量，而后通过高温回火处理消除残余应力和加工硬化。随后在-196℃低温下进行弯曲断裂实验，通过力学参数测量和微观观察就初始损伤对不同钢组织缺口试样解理断裂韧性的影响进行了实验研究。研究表明，对于材料A和C，随预加载荷比P0／Pgy的增加，材料中的初始损伤量增加，从而使材料的缺口解理断裂韧性降低。其原因是初始损伤的长条形孔洞附近产生了局部高应力应变集中，促使了解理断裂过程的进行。细晶粒A材料的初始损伤量大，韧性下降幅度大。对于材料B，初始损伤主要为小尺寸的球形孔洞，并且其损伤量远小于材料A和C，故B材料的缺口解理断裂韧性几乎不随预加载荷比变化。     

反击式破碎机双转子断裂失效分析

利用光学金相显微镜、扫描电镜和热处理实验手段对反击式破碎机双转子断裂进行了分析。分析结果表明,其断裂属于解理和沿晶复合断裂,断口解理面较大,组织中有大块自由铁素体和粗大魏氏组织。此双转子未经热处理就投入使用以及该转子存在的缩松和气孔是导致该双转子断裂失效的主要原因。     

Fracture behaviour and cleavage initiation in hypoeutectoid pearlitic steel

The paper reports on an investigation of the micromechanism of cleavage fracture in hypoeutectoid pearlitic R7T steel, commonly used for producing railway wheels. The steel possesses extensive Lüders deformation, which somewhat complicates finite element (FE) modelling and analyses of fracture behaviour. Standard Charpy V-notch specimens were used in order to analyse the fracture behaviour at quasistatic and impact loading. Finite element 3D calculations were performed and the elastic-plastic behaviour of notched bars up to the fracture was simulated. Detailed fractographic analysis was carried out on a number of Charpy V-notch specimens in order to investigate the origin site of cleavage fracture initiation and its distance from the notch root. The suitability of the three-criterion micromechanical model (Chen et al. Acta Materialia 51:1841–1855, 2003) for cleavage initiation was verified. The R7T steel under investigation exhibited a cleavage fracture stress of 1,837 MPa. Its independence on temperature evidenced the micromechanism of cleavage fracture to be microcrack propagation-controlled. For the investigated blunt-notched bend bars, an active volume exists ahead of the notch root in which pearlite colony-associated initiation sites are located. The cleavage fracture initiation of the steel is thus governed by the sites lying in the active volume. The active volume is determined by the values of three parameters. A plastic strain lying in interval from to (for the steel investigated from 0.033 to 0.108) is necessary to create a cleavage crack nucleus at any location within the active volume depending on the local pearlite properties. A stress triaxiality parameter ranging from h _min to h _max (from 0.93 to 1.39) is supposed to prevent the blunting process at the site of the cleavage nucleus. Once the main principal stress σ ₁ exceeds the local cleavage fracture stress σ _CFmin, an unstable global cleavage fracture occurs in a blunt-notched bar.     

Charpy impact property related to {100} cleavage fracture in 15CrODS steel

Abstract

Based on the former results that delamination along {100} cleavage fracture leads to the improved absorbed energy, {100} cleavage plane was formed in parallel to the rolled plate by recrystallisation and cold roll processing in 15CrODS steel plate. Charpy impact properties were investigated in terms of notch direction against {100} cleavage plane. Notch with normal direction against {100} cleavage plane induced delamination fracture, but provided similar absorbed energy to that of conventional extruded bar. The transverse notch direction against {100} cleavage plane served higher absorbed energy at low temperature. Higher upper shelf energy and lower DBTT were attained in conventional hot extruded bar, which contains any crystallographic planes rotated around <110> axis at the transverse direction. The delamination fracture leading to high absorbed energy is significantly affected by a crystallographic morphology of the {100} cleavage plane perpendicular to transverse plane.     

Local approach concepts and the microstructures of steels

The paper describes cleavage fracture models that relate local fracture stresses and fracture toughness values to the sizes of brittle initiating particles. In a “quasi-homogeneous” steel, i.e. one possessing a smooth distribution of small particle sizes, a “typically coarse” particle is present in every sample tested and the failure stress or fracture toughness is single valued. When random experimental errors are included, the single valued function becomes a Gaussian distribution. Spatially heterogeneous microstructures produce quite different distributions and these are discussed with respect to extrapolations to low failure probabilities.     

Influence of statistical and constraint loss size effects on cleavage fracture toughness in the transition - A model based analysis

A database derived from tests on specimens with a large range of ligament (b) and thickness (B) dimensions was systematically analyzed to evaluate constraint loss and statistical size effects on cleavage fracture toughness. The objectives were to: (1) decouple size effects related to constraint loss, mediated by b and B, from those arising from statistical effects, primarily associated with B; and, (2) develop procedures to transfer toughness data to different conditions of constraint and B. The toughness database for a Shoreham pressure vessel steel plate, tested at a common set of conditions, was described in a companion paper. Quantification of constraint loss was based on an independently calibrated 3D finite-element critical stress-area, σ^∗-[K_Jm/K_Jc], model. The measured toughness data, K_Jm, were first adjusted using computed [K_Jm/K_Jc] constraint loss factors to the corresponding values for small scale yielding conditions, K_Jc=K_Jm/[K_Jm/K_Jc]. The K_Jc were then statistically adjusted to a K_Jr for a reference B_r = 25.4 mm. The B adjustment was based on a critically stressed volume criterion, modified to account for a minimum toughness, K_min, consistent with modest modifications of the ASTM E 1921 Standard procedure. The combined σ^∗-[K_Jm/K_Jc]-K_min adjustment procedure was applied to the Shoreham b − B database, producing a homogeneous population of K_Jr data, generally within the expected scatter. The analysis suggests that: (1) there may be a maximum B beyond which statistical size effects diminish, and (2) constraint loss in the three-point bend specimens begins at a relatively low deformation level. A corresponding analysis, based on a Weibull stress, σ_w-[K_Jm/K_Jc]-K_min, adjustment procedure, yielded similar, but somewhat less satisfactory, results. The optimized adjustment procedure was also applied to other K_Jm data for the Shoreham plate from this study, as well as a large database taken from the literature. The population of 489K_Jr data points, covering an enormous range of specimen sizes, geometries and test temperatures, was found to be consistent with the same master curve T₀ = −84 °C derived from the b − B database. Thus, calibrated micromechanical models can be used to treat size and geometry effects on K_Jm, facilitating using small specimens and data transfer to predict the fracture limits of structures.     

3D analyses of the effect of weld orientation in Charpy specimens

Three dimensional transient analyses of Charpy impact specimens are carried out. The material response is characterized by an elastic-viscoplastic constitutive relation for a porous plastic solid, with adiabatic heating due to plastic dissipation and the resulting thermal softening accounted for. The onset of cleavage is taken to occur when the average of the maximum principal stress over a specified volume attains a critical value. The weld analyzed here is overmatched, so that the yield strength for the weld is larger than that of the base material. Analyses are carried out for specimens where the notch is cut parallel to the surface of the test piece, as well as more complex geometries where the notched surface of the specimen is rotated relative to the surface of the test piece. It is found that even for a homogeneous material the brittle-ductile transition can be much affected by three dimensional effects; for example, curvature of the deformed free surface gives rise to a stress increase that promotes cleavage. Furthermore, for the rotated specimens the weld geometry relative to the notched specimen surface is so complex that only a full 3D analysis is able to account for the interaction of failure in the weld material, base material and heat affected zone (HAZ). For these rotated specimens the location where the notch crosses the thin layer of HAZ, i.e. whether this location is near the center of the specimen or near the free specimen edge, makes a large difference in the response.     

New developments of the Wallin, Saario, Törrönen cleavage fracture model

The Wallin, Saario, Törrönen (WST) model, published originally in 1984, assumes the cleavage initiators to be carbides. The preliminary trials of the model appeared promising. However, at the time, it was not considered possible to fully calibrate the model to provide actual predictions of fracture toughness based on microstructural and mechanical property information, because of lack of computational power. Today, more than 20 years later the computational means have evolved sufficiently so that a new version of the WST model can be developed. The new version is connected to a general statistical model and includes the effect of strain. It is with some success compared to actual material data, predicting the cleavage initiation location.     

低碳钢低温解理断裂应力计算

本文利用弹塑性大应变等参有限元法,对Chacpy缺口和任意裂缝四点弯曲试样的应力应变场进行了计算。并基于S_(co)—解理特征应力物理模型,对低碳钢低温解理断裂应力进行了计算。其计算结果与实验结果相吻合。本文还根据计算和实验结果。对工程塑脆转折温度进行了定义。