Effects of heat treatment process on stress corrosion cracking and microstructures of 7A05 Al alloy plate

In order to improve the stress corrosion cracking(SCC) resistance of 7A05Al alloy, four different heat treatment processes were performed. After the heat treatments, the stress corrosion cracking resistance, mechanical properties and microstructures of 7A05A1 alloys were studied. The results show that the retrogression and reaging(RRA) treated specimens possess the highest SCC resistance with slightly reduced strength compared with the other specimens. Thus RRA is the best process in terms of the optimal combination of SCC resistance and strength. Further TEM observation reveals that the RRA treated specimens are characterized by the fine equiaxed grains, dispersed second phase and wide PFZ. The pre-aging treated specimens show lower SCC resistance but higher strength as compared with the RRA treated ones. Equiaxed grains are also observed in the pre-aging treated samples by TEM, indicating that the anisotropy is eliminated. However, the dual aging and deformation plus aging treated specimens are demonstrated by large anisotropy.

Healing of hydrogen-attacked cracks in split specimens with recovering heat treatment in vacuum

The healing mechanism of hydrogen-attacked cracks in low carbon steel andCr-Mo steel and its influencing factors during the healing process were studied by recovering heattreatment of split specimens in vacuum. The result showed that crack pacing turns much smaller underthe condition of pure heating, especially for crack tips. The healing effect is well related to thelength of cracks with the shorter in priority. By the primary mechanism of thermal diffusion, ironand carbon atoms must diffuse at the high speed in steel to realize that plasticity deformationenergy exceeds and overcomes surface tensile force energy. In addition, phase transformation andstress-stain relationship also have positive effects on the process.

EFFECTS OF HEAT TREATMENT AND ENVIRONMENT ON CRACK PROPAGATION RATE IN STEEL GC-4 UNDER CORROSION FATIGUE AND STRESS CORROSION

Comparative investigations were carried out of the effect of heat treatment regimes Jor steel GC-4(40CrMnSiMoVA)on its crack propagating rates,from corrosion fatigue, (da/dN)_(CF),or stress corrosion cracking,(da/dt)_(SCC),in media with various constituents and pH values.Both(da/dN)_(CF) and(da/dt)_(SCC) accelerate with the increase of yield stress of the steel,yet the former is far less than the later.In comparison with media,the (da/dt)_(SCC) in distilled water is slightly greater than that in 3.5% NaCl solution,and the (da/dN)_(CF) in distilled water is far less than that in 3.5% NaCl solution.With the pH value increasing in 3.5% NaCl solution,the(da/dN)_(CF) lowers down and the(da/dt)_(SCC) speeds up.An explanation was also proposed with concept of the cyclic hardening and softening at crack tip,as well as the crack closure and occluded cell effect.

Influence of heat treatment regimes on microstructures and fracture characteristics of 7055Al alloy containing Ag

Tensile properties, fracture characteristics and microstructures of 7055 aluminum-based alloy containing Ag after T6, T73 and RRA treatment were investigated. The results show that RRA treatment retains strength of 7055-T6 with higher electrical conductivity close to that of 7055-T73 alloy, but its elongation decreases greatly. SEM fractographs reveal that intergranular cracking and shear-type transgranular cracking are both presented on the fracture appearance of 7055-T6 specimen. After T73 treatment, the fractographs mainly consist of dimple-type transgranular cracking with minor intergranular cracking. For 7055-RRA specimen, intergranular cracking dominates with minor dimples on the fracture surface. The type and size of precipitates, width of grain boundary and the ability of precipitates to impede dislocation motion vary with heat treatment regimes. Three frature models were built on the basis of microstructural analyses.

INFLUENCE OF HEAT TREATMENT PROCESSES ON MECHANICAL PROPERTIES OF ACu－Ni－AI－Ti ALLOY

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S32750双相不锈钢仪表管接头早期疲劳开裂原因

某海上平台S32750双相不锈钢仪表管直通接头发生开裂现象,采用宏观观察、断口分析、化学成分分析、硬度测试、金相检验、扫描电镜和能谱分析等方法对仪表管接头的开裂原因进行分析。结果表明:由于接头表面存在损伤缺陷,且材料脆性较大,在交变载荷的作用下,裂纹在表面萌生并扩展,导致接头发生早期疲劳开裂现象。

连接底开裂原因分析及改进措施

对失效连接底金相组织、化学成分、裂纹形态等进行理化检测,分析连接底裂纹产生的原因。结果表明:连接底裂纹为淬火裂纹;连接底中心圆柱部与底部过渡弧R角半径较小是主要原因,致使淬火过程中应力集中导致开裂;原材料中心部位组织疏松和冷却速度较快,进一步促进裂纹萌生;通过增大R角、调整热处理工艺来降低淬火应力,从而避免连接底淬火开裂的产生。

超超临界锅炉集箱角焊缝开裂分析及优化

针对某电厂超超临界锅炉延伸侧墙出口集箱角焊缝开裂的问题,从材料成分、显微组织、硬度测试及运行工况等方面综合分析该集箱角焊缝失效的原因,并且从焊接工艺、热处理工艺及焊缝形式出发综合提出优化方案。结果表明:集箱管材料为12Cr1MoVG,失效集箱角焊缝的化学成分、显微组织及硬度均符合标准,裂纹起裂于角焊缝外表面,并向内表面扩展。开裂原因为焊接残余应力与服役过程应力产生复杂叠加。通过采用降低角焊缝处应力集中的优化措施,延长材料使用寿命,可以为同类型电站锅炉管道的日常监督与质量控制提供重要的借鉴。

Effect of deteriorated microstructures on stress corrosion cracking of X70 pipeline steel in acidic soil environment

In order to investigate stress corrosion cracking （SCC） of X70 pipeline steel and its weld joint area in acidic soil environ- ment in China, two simulating methods were used： one was to obtain bad microstructures in heat affected zone by annealing at 1300 ℃ for 10 min and then, quenching in water; the other was to get different simulating solutions of acidic soil in Yingtan in south- east China. The SCC susceptibilities of X70 pipeline steel before and after quenching in the simulating solutions were analyzed using slow stain rate test （SSRT） and potentiodynamic polarization technique to investigate the SCC electrochemical mechanism of different microstructures further. The results show that SCC appears in the original microstructure and the quenched microstructure as the polarization potential decreases. Hydrogen revolution accelerates SCC of the two tested materials within the range of-850 mV to -1200 mV vs. SCE. Microstructural hardening and grain coarsening also increase SCC. The SCC mechanisms are different, anodic dissolution is the key of causing SCC as the polarization potential is higher than the null current potential, and hydrogen embrittlement will play a more important role to SCC as the polarization potential lower than the null current potential.

Preliminary Study to Identify Criterion for Quench Crack Prevention by Computer Simulation

A criterion for preventing quench cracks has not been established although much research has been conducted in this field. A few predictions of cracking have been recently reported using simulated results of heat treatment processes. However, these results were insufficient to predict crack generation clearly because the mechanism relates not only to stress distributions in the parts but also to material characteristics. Systematic studies by the combination of experiment and simulation are therefore required to reliably identify a criterion for crack prevention by computer simulation. As a preliminary work before starting the full-scale research, a review of quench cracking research was conducted and existing test methods were classified. Several test results for cylindrical specimens were simulated for considering a cause of the crack generation. Lastly a tentative plan was made for the future research.

Metallurgical modeling of microcrack repairment during welding nonferrous materials: non-equilibrium grain boundary segregation(Ⅱ)

Metallurgical modeling of microalloying boron behavior in nickel-based superalloys during pre-weld heat treatment and welding has been systematically established. Non-equilibrium grain boundary resegregation is physically coupled with non-equilibrium solidification of the weld pool for improved quantitative understanding of the imminent detriment of boron near the as-transformed grain boundary of the mushy zone and weldability. A strategic priority of the reduction in boron through low heat input and pre-weld heat treatment to suppress massive boride nucleation and grain boundary liquation are introduced.Both factors are capable of reducing the material response to boron-assisted intergranular liquation cracking at the high-energy sites of the grain-coarsened heat-affected zone( HAZ) beneath the surface and are of practical importance to provide robust integrity of joints. The synergistic self-repairment arterial crack network with the crystallographic substructure of the backfill enables amelioration of the HAZ crack resistance. The theoretical predictions are in satisfactory agreement with the phenomenological microanalysis, indirectly. This metallurgical modeling is also applicable to other high-temperature aerospace materials with similar metallurgical properties.

Weldability and liquation cracking behavior of ZhS6U superalloy during electron-beam welding

The weldability of the ZhS6U nickel-based superalloy, which is prone to solidification cracking during electron-beam welding(EBW) repair processes, was investigated. The effects of two different pre-weld heat-treatment cycles on the final microstructure before and after welding were examined. Welds were made on flat coupons using an EBW machine, and the two heat-treatment cycles were designed to reduce γ′ liquation before welding. Microstructural features were also examined by optical and scanning electron microscopy. The results showed that the change in the morphology and size of the γ′ precipitates in the pre-weld heat-treatment cycles changed the ability of the superalloy to release the tensile stresses caused by the matrix phase cooling after EBW. The high hardness in the welded coupons subjected to the first heat-treatment cycle resulted in greater resistance to stress release by the base alloy, and the concentration of stress in the base metal caused liquation cracks in the heat-affected zone and solidification cracks in the weld area.

Mechanical properties of flax fibres treated by alkaline,enzyme and steam-heat

In recent years there has been interest in using flax fibres to produce composites because of a number of attributes, including low density, biodegradability and high mechanical properties. It was found that treatment of flax fibres may be required to improve the bond quality with a resin. These treatments also have an impact on the properties of the fibres themselves. The objective of this project was to evaluate the impact of three treatment methods on the mechanical properties of flax fibres. The three treatment methods were alkaline, enzyme and steam-heat. After treatment, flax fibres were tested in tension using a universal test machine. Results showed that tensile strength and Young's modulus of flax fibre can be enhanced significantly by the three treatment methods, compared with untreated flax fibres. Enzyme treatment was shown to be the best approach to improve mechanical properties of flax fibre than alkaline and steam-heat treatment.

Corrosion Testing of a Heat Treated 316 L Functional Part Produced by Selective Laser Melting

Selective Laser Melting (SLM) shows a big potential among metal additive manufacturing (AM) technologies. However, the large thermal gradients and the local melting and solidification processes of SLM result in the presence of a significant amount of residual stresses in the as built parts. These internal stresses will not only affect mechanical properties, but also increase the risk of Stress Corrosion Cracking (SCC). A twister used in an air extraction pump of a condenser to create a swirl in the water, was chosen as a candidate component to be produced by SLM in 316 L stainless steel. Since the main expected damage mechanism of this component in service is corrosion, corrosion tests were carried out on an as-built twister as well as on heat treated components. It was shown that a low temperature heat treatment at 450℃ had only a limited effect on the residual stress reduction and concomitant corrosion properties, while the internal stresses were significantly reduced when a high temperature heat treatment at 950℃ was applied. Furthermore, a specific stress corrosion sensitivity test proved to be a useful tool to evaluate the internal stress distribution in a specific component.

金属材料裂纹自愈合的热处理试验方法及结果分析

该文探究不同热处理方式对金属材料内部裂纹自愈合的影响。选择20钢和45钢这2种材料,采用钻孔压缩法制作内部带有裂纹的试件。试验设有2个变量,分别是加热温度(800、850、900、950和1000℃)和保温时间(30、45、60 min),对比不同加热温度和不同保温时间下,试件内部裂纹的自愈合情况。结果表明,加热温度越高,保温时间越长,内部裂纹的自愈合效果越好。在加热至1000℃、保温时间为60 min的情况下,原裂纹已经完全消失,实现裂纹的自愈合。

激光选区熔化Ti-6Al-4V钛合金疲劳裂纹扩展性能研究

采用激光选区熔化技术制备了Ti-6Al-4V钛合金紧凑拉伸试验件,对部分试验件进行热处理或热等静压处理,并分别对沉积态、热处理态和热等静压态的部分试验件进行表面机械加工,通过试验获取了不同状态激光选区熔化Ti-6Al-4V的裂纹扩展性能数据,对比研究了不同后处理方式的影响。结合对试验件微观组织及断口形貌的观测,分析了裂纹扩展性能变化的可能原因。研究结果表明,沉积态Ti-6Al-4V中针状马氏体α组织导致其韧性相对低,且由于成形残余应力的存在,其裂纹扩展性能明显低于常规成形材料。热处理和热等静压处理都在一定程度上实现了α相的粗化及β相含量的提高,结合成形残余应力的降低甚至消除,显著提升了材料的裂纹扩展性能,使其裂纹扩展速率与常规成形材料相当。当裂纹尺寸和应力强度因子超过一定范围时,表面机械加工处理对裂纹扩展性能几乎无影响。

Influence of Microstructures and Residual Stress on Fatigue Crack Propagation Rate of High Carbon Steels

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蜗杆齿面裂纹原因分析及工艺改进

针对蜗杆等齿轮类零件加工中易出现齿面裂纹这一问题,建立了以蜗杆齿面裂纹为顶事件的故障树,对造成该顶事件的故障原因进行逐级分解,从设计硬度、锻件毛坯、热处理、磨削工艺等方面对故障原因进行逐一分解,通过查阅设计标准、锻件毛坯化学分析对设计硬度及锻件毛坯原因进行了排除;分析过程中开展了材料热处理检查和金相组织检测,分析结果显示蜗杆齿面裂纹呈现二次淬火磨削烧伤特征和回火烧伤特征,蜗杆加工过程中磨齿工艺参数不合理,对磨削烧伤开展了故障机理分析,给出了工艺改进措施,最后采用磨削前低温回火消除内应力,调整砂轮转速和进给量等磨削工艺参数解决了蜗杆齿面裂纹问题。对齿轮类零件产生齿面裂纹的原因分析方法与实际问题的解决具有指导意义。

热处理温度对长期服役T23钢CGHAZ微观组织的影响

以服役时间超过6×104 h的T23水冷壁管对接接头作为研究对象,对焊接接头进行不热处理(焊态)、520℃×1 h,580℃×1 h,650℃×1 h,720℃×1 h及760℃×1 h等6种不同参数的热处理试验。为研究不同热处理后粗晶热影响区(CGHAZ)微观组织变化,分析合金碳化物在晶界的分布规律,同时探索CGHAZ再热裂纹形成原因与析出物之间的内在联系,重点对粗晶区进行了显微硬度分析、金相组织分析、扫描电镜形貌分析及能谱分析。结果表明,焊态T23钢CGHAZ为粗大板条马氏体与贝氏体的混合组织,经测算最大晶粒尺寸约100μm,大部分晶粒尺寸30~50μm;在580℃热处理时,由于晶粒细化及析出物在晶内弥散强化作用,CGHAZ出现了二次硬化现象,根据再热裂纹形成理论,此时晶内强度与晶界强度差较大,应力松弛主要集中于晶界,CGHAZ产生再热裂纹的倾向较大;在720℃热处理时,在CGHAZ晶界位置出现了密集孔洞及合金碳化物,并且空洞与合金碳化物之间存在伴生关系,但短时间内孔洞在后续正常服役条件(560~600℃)下能否发展成为再热裂纹有待商榷。

45钢螺帽断裂原因

在加工某型45钢螺帽时,数控车过程中发现有2个螺帽发生断裂。采用宏观观察、化学成分分析、扫描电镜及能谱分析、金相检验、硬度测试和热处理工艺分析等方法,研究了螺帽断裂的原因。结果表明:螺帽断裂处存在淬火裂纹;螺帽毛坯孔壁的厚度处于淬火裂纹敏感厚度的区间内,且底孔与内壁的过渡圆弧较小,产生了应力集中,并形成了淬火裂纹,裂纹沿晶扩展,最终在数控车过程中螺帽发生断裂。