爆轰处理对20CrMo 钢氢脆敏感性的影响

本文研究了爆轰处理对不同组织和强度水平的20CrMo 钢氢脆敏感性的影响。结果发现爆轰处理加高温时效可显著降低钢的氢脆敏感性。对屈服强度分别为830MPa 和620MPa 的均相珠光体组织、爆轰处理对其氢脆敏感性没有或略有影响。爆轰处理显著增加了屈服强度为680MPa,组织为铁素体加珠光体材料的氢脆敏感性系数,同时显著降低它对不可逆氢脆的敏感性系数。     

珠光体组织的等径弯曲通道变形

具有全珠光体组织的65Mn钢在650℃以C方式等径弯曲通道变形(ECAP)后,珠光体组织中的渗碳体片层以周期性的弯曲变形、周期性的剪切变形、剪切断裂等形式协调ECAP的强烈塑性变形.渗碳体表现出很强的塑性变形能力,在其内部导入了大量的晶体缺陷,为渗碳体的球化打下了能量基础.变形五道次后,片层状的珠光体组织演变成了超细的渗碳体颗粒均匀分布于铁素体基体的组织.铁素体基体为均匀的等轴晶,平均晶粒大小为～0.3 μm.渗碳体的球化可能以两种机制进行:破碎渗碳体片的非均匀长大(Ostwald熟化)和细小球状渗碳体颗粒的形核长大.     

钢中渗碳体特性的理论计算与实验研究

采用第一性原理方法计算了钢中渗碳体的电子结构、力学与物理性能.确定了不同结构Fe3C、Fe2C及Fe5C2等铁碳化物之间的稳定性,从原子和电子层次揭示了添加合金元素与附加应力对渗碳体相稳定性的影响规律与机制.研究了珠光体渗碳体在大冷变形及退火过程中的演变规律,建立了共析珠光体钢在大冷变形过程中铁素体的碳过饱和度与冷轧压下率问的定量关系,阐明了添加元素Cr、Mn、Si对冷变形过程中渗碳体溶解行为的影响规律,给出了重度冷轧后珠光体钢的退火组织分为细晶区与粗晶区的有力判据,分析了粗晶区与细晶区中渗碳体颗粒随退火时间的变化规律,发现2个区域内珠光体渗碳体颗粒的粗化动力学均符合关系式d=ktn,但渗碳体颗粒的粗化机制不同,而且进一步研究了Cr、Mn、Si等元素对2个区域内渗碳体颗粒粗化机制的影响规律.此外还研究了中高碳钢在温变形与热变形条件下渗碳体的球化规律,提出了高碳珠光体钢在不变形、中碳钢在珠光体区大变形条件下渗碳体的快速球化工艺,为进一步深化与丰富钢铁材料的超细化理论及其工业化应用提供了理论与实验支持.     

GB/T 13299-1991《钢的显微组织评定方法》的修订建议

GB/T 13299-1991《钢的显微组织评定方法》规定了低碳、中碳钢的钢板、钢带和型材的游离渗碳体、低碳变形钢的珠光体、带状组织和魏氏体组织的级别评定方法。在过去20多年的实际应用过程中,该标准在使用范围、取样和检验面、碳含量界限、低碳变形钢的珠光体和带状组织评级等方面都暴露出了不少问题。通过对这些问题进行分析讨论,对该标准的修订完善提出了一些建议。     

高碳钢超塑性形变组织的研究

本文研究了高碳钢在893k 温度下超塑形变过程中组织结构的变化。尽管含碳1.2wt％的钢形变前的原始组织中存在一些片层状珠光体,但以0.31mm/min 的速度拉伸后仍获得了442％的延伸率。由于这些片层状珠光体在形变过程中迅速球化,因而对钢的超塑性能无明显妨害。在形变中,球状渗碳体发生彼此相连,含碳1.6wt％的钢中的渗碳体形成沿晶界的条状和网状,考虑到钢的室温力学性能,作者建议用于超塑成形的钢的含碳量选择在0.8～1.2wt％范围。     

过时效工艺对低碳冷轧搪瓷钢组织性能的影响

使用连续退火模拟实验机研究连续退火线上的过时效工艺对搪瓷钢力学性能和氢渗透行为的影响,并探讨了性能与组织的关系。结果表明:与传统等温过时效相比,降温过时效退火板晶粒内的渗碳体颗粒不但体积分数更大,且更加细小。降温过时效退火板的强度比等温过时效退火板的稍低,断后伸长率明显更高。含量更高且更加细小弥散的晶粒内渗碳体具有更好的氢陷阱效果,明显迟滞了氢在钢板中的扩散。与传统等温过时效相比,降温过时效的实验钢退火板的氢扩散系数DL降低了约18%。改进的过时效工艺,使低碳冷轧搪瓷钢的力学性能和抗鳞爆性能明显提高。     

焊后热处理对1.25Cr0.5Mo钢在循环水中腐蚀行为的影响

目前有关焊后热处理对1.25Cr0.5Mo钢的腐蚀行为的影响规律尚无文献报道.为此,对正火态1.25Cr0.5Mo钢分别进行淬火、高温回火及调质,模拟不同的焊后热处理,对其金相组织进行分析,并对其腐蚀行为进行电化学测试,研究了不同热处理对1.25Cr0.5Mo钢组织和耐蚀性能的影响.结果表明:正火态1.25Cr0.5Mo钢的金相组织为铁素体和珠光体,淬火组织为板条马氏体,高温回火组织为粗大铁素体和珠光体,调质态组织为均匀的回火马氏体和细小碳化物;淬火处理加速了 1.25Cr0.5Mo钢在工业循环水中的腐蚀速度;高温回火处理对腐蚀行为影响不大;调质处理显著提高了 1.25Cr0.5Mo钢的耐蚀性,是因为调质热处理细化了材料的组织,细化后的组织较晶粒粗大的淬火及淬火后高温回火组织更容易形成氧化物膜,阻碍电极的放电过程,从而获得了较好的耐蚀性能.     

超快冷对碳素钢中渗碳体析出强化行为的影响

用超快速冷却技术并控制轧后冷却温度,研究了3种碳含量不同的碳素钢热轧后组织中渗碳体的析出行为和强化机制。结果表明,在超快速冷却条件下0.04%C和0.5%C(质量分数,下同)实验钢的主要强化方式分别是细化晶粒和细化珠光体片层间距,没有纳米级渗碳体颗粒析出,而在0.17%C实验钢的组织中则有大量弥散的纳米级渗碳体析出,颗粒直径范围为10-100 nm,通过超快速冷却技术实现了在不添加微合金元素的条件下纳米级渗碳体的析出。随着超快速冷却终冷温度的降低纳米渗碳体的析出强化作用使0.17%C钢的屈服强度提高110 MPa,强化效果明显。在超快速冷却的工艺基础上若继续采用形变热处理工艺,可进一步提高0.17%C实验钢的位错密度,促进渗碳体均匀形核,实现纳米级渗碳体颗粒在整个组织中更加均匀弥散的分布,达到更好的均匀强化效果。在超快速冷却和形变热处理工艺条件下0.17%C钢的屈服强度可达到650 MPa以上,强化效果提高300 MPa以上。     

淬火对含ZrC的20Mn2钢的组织及力学性能的影响

研究了淬火冷却对含0.2%(体积分数)ZrC颗粒(粒径0.2～1.2μm)的20Mn2钢在大形变量轧制后的组织与力学性能的影响.结果表明,ZrC粒子作为大形变量热轧变形中的奥氏体形变核心及再结晶核心,对奥氏体晶粒的超细化有显著的作用,而显著细化的晶粒提高了完全获得马氏体组织的临界冷却速度,使水冷淬火态马氏体的数量、长度和宽度减小,使油淬态珠光体的数量减少、渗碳体片变短、变薄.在冷却过程中,细小的奥氏体晶粒转变为更细小(1～2μm)的铁素体晶粒,保留了热轧变形时的形变位错组织,使水淬态20Mn2钢的延伸率不下降的同时强度成倍提高,油淬态20Mn2钢在强度大幅度提高的同时延伸率成倍提高.     

高碳钢中珠光体在温楔横轧过程中的组织演变

采用温楔横轧方法制备出表层具有超微细复相组织的高碳珠光体钢棒件,研究了珠光体组织在温变形过程中的演变.结果表明,珠光体组织中的渗碳体片层主要以弯曲扭折的形式协调塑性变形,表现出较强的塑性变形能力;剧烈塑性变形促进了渗碳体片层的球化,表层球化完全的渗碳体颗粒粒径均小于0.2μm;温楔横轧后铁素体基体发生了动态连续再结晶,等轴铁素体平均晶粒尺寸为0.3～0.4 μm,0.5R处和心部的渗碳体球化不完全,铁素体再结晶也不完全;铁素体晶粒的超细化和渗碳体片层的球化明显改善高碳珠光体钢棒件的塑性,温变形过程中应变、应变速率及温度分布的不均匀是引起组织性能差异的根本原因.     

Characterisation of long term aging behaviour of 9Cr–1Mo ferritic steel using ultrasonic velocity

Abstract

The measurement of ultrasonic velocity of 9Cr-1Mo ferritic steel thermally aged at 793 and 873 K exhibited four distinct regimes in the variation of ultrasonic velocity with aging time. These different regimes have been correlated with the progressive evolution and coarsening of precipitate microstructure studied using TEM and microhardness measurements. The study revealed that ultrasonic velocity can be used to examine the secondary precipitation in the steel and the use of this technique as such can be extended to the health assessment of a component during service.     

Utilization of Ultrasonic Measurements for Determining the Variations in Microstructure of Thermally Degraded 2205 Duplex Stainless Steel

The aim of this study is to investigate the changes in attenuation of longitudinal waves and velocity of shear wave modes for assessing variations in the microstructure of thermally degraded 2205 duplex stainless steel samples that were aged isothermally at 700°C and 900°C for different time intervals. The evaluation of material microstructural changes such as phase transformation and second phase precipitation which are related to material properties is of primary importance to ensure quality of components. This paper presents evidence that indicate that the attenuation coefficient of the longitudinal mode is sensitive to gradual microstructural changes produced by the aging treatments. The gradual changes in microstructure are associated with reduction of impact properties. On the other hand, for samples aged at 700°C, the velocity of the fast mode of shear wave show changes at high aging times and does not show changes at early aging times were the materials properties are already degraded.     

长期时效对20Cr1Mo1VTiB螺栓钢的组织和力学性能的影响

对20Cr1Mo1VTiB螺栓钢进行拉伸和冲击实验、TEM和SEM观测并测量其XRD谱，研究了其在500℃进行0~2000 h时效处理后的组织和力学性能。结果表明：随着时效时间的增加原板条状贝氏体分成若干个包体，其平均尺寸由时效前的14 μm减小到时效2000 h后的9 μm。在0~2000 h时效过程中碳化物尺寸没有明显增大，主要析出相VC和TiC弥散分布在晶内、板条界和板条内。VC相呈椭球状，随时效时间的延长TiC相由长条状变为方形。随着时效时间的延长贝氏体板条的边界逐渐模糊，时效2000 h后板条的宽度明显增加。在长时间时效过程中20Cr1Mo1VTiB螺栓钢的力学性能没有大幅度的变化而保持着较高的值，表明其可在高温下长期使用。     

On-line ultrasonic velocity measurements for characterisation of microstructural evaluation during thermal aging of β-quenched zircaloy-2

Ultrasonic non-destructive evaluation (NDE) technique has been used for characterisation of evolution of microstructure in β-quenched and thermally aged zircaloy-2 specimens. On-line ultrasonic velocity measurements have been made in β-quenched state of zircaloy-2 (A specimen) during heating at different heating rates up to 573 K (B specimen), 603 K (C specimen) and 623 K (D specimen) with holding time periods of 5 h for specimens B and C, and 2 h for specimen D, at the corresponding maximum temperature, by employing a specially designed experimental set-up. The observed change in velocity at room temperature (298 K) before and after ageing for specimens B and D is 0.52% and 0.48%, respectively, and this reveals that intermetallic precipitates are formed during the aging treatment. Ultrasonic measurements are correlated with the hardness, density and microstructural changes.     

ECAP变形对20MnSi钢组织与性能的影响

采用C方式等径弯曲通道变形(Equal Channel Angular Pressing,简称ECAP)对20MnSi钢进行了4道次室温变形,研究了变形道次对显微组织和力学性能的影响.结果表明,铁素体组织随变形道次的增加逐渐演变为等轴状大角度晶界的亚微晶组织;试验用钢的硬度和强度随变形道次的增加而增加,4道次后强度有所降低;在本试验条件下,珠光体组织中的片状渗碳体表现出很强的塑性变形能力.     

Evaluation of Railroad Wheel Steel with Lamellar Duplex Microstructures Using Diffuse Ultrasonic Backscatter

The effects of lamellar duplex microstructure within grains that contain alternating phases of cementite and ferrite on ultrasonic scattering in railroad wheel steel are evaluated using a diffuse ultrasonic backscatter technique. A new singly scattered response (SSR) model that considers the lamellar duplex microstructure within grains is developed based on a previous SSR model. The results show that the amplitude of ultrasonic scattering decreases with decreasing lamellar space. Corresponding experiments are performed with 10 MHz and 15 MHz focused transducers by scanning both unquenched and quenched wheels. The experimental results show that the ultrasonic scattering amplitudes drop dramatically near the quenched tread surface, a result which is attributed to the creation of duplex microstructure (pearlite phase) within grains due to the quenching process. The lamellar spacing within grains increases progressively from the tread surface to the deeper locations due to the non-uniform cooling rate. The distribution of lamellar spacing within grains as a function of depth is quantified with the modified SSR model. Good agreement with optical microscopy is observed. The diffuse ultrasonic backscatter technique exhibits strong sensitivity to microstructure changes, an outcome that may be applicable for quality control during manufacturing.     

Microstructure and microhardness of pearlitic steel after laser shock processing and annealing

The microstructure evolution of the high carbon pearlitic steel after laser shock processing (LSP) with different laser pulse energy and high temperature annealing was investigated. After LSP, the cementite lamella were bent, fractured and broken into granules. Fragmentation and dissolution of the cementite lamella were enhanced by increasing the laser pulse energy. Results show that the ferrite lattice parameter increased due to carbon atom dissolution in the ferrite matrix, and the corresponding ferrite X-ray diffraction peaks shifted significantly towards the smaller diffraction angles. After annealing at 650°C for 30?min, an ultrafine duplex microstructure (ferrite+cementite) was formed on the surface. After LSP with a high energy, equiaxed ferrite grains were refined to 400?nm and the cementite lamella were fully spheroidised with the particle diameter of ～150?nm. The corresponding grain size of ferrite and cementite under low pulse energy was 500 and 300?nm respectively. After annealing, the ferrite peaks significantly shifted towards the higher diffraction angles, and the ferrite lattice parameter decreased. The microhardness initially increases after LSP and then slightly decreases after subsequent annealing but remained higher than without LSP.     

The microstructure and mechanical properties of prolonged and lower temperature aged Fe–Ni–Mn–Mo–Ti–Cr maraging steel

In this study, the microstructure and mechanical properties of Fe–Ni–Mn–Mo–Ti–Cr maraging steel at low temperature and prolonged aging condition were investigated. Optical and scanning electron microscopy examinations, tensile and hardness tests were conducted to study the microstructure, aging behavior and mechanical properties of the cold‐rolled steel. The results showed that aging of cold rolled Fe–Ni–Mn–Mo–Ti–Cr maraging steel resulted in the formation of Mo rich and Ti rich Lave phase precipitates. Existence of many dislocation cores due to cold rolling and subsequently, low temperature aging caused to formation of uniform distribution of very fine precipitates. The presence of these precipitates increased the yield and ultimate tensile strengths but couldn't improve the uniform tensile ductility. This alloy showed ultra‐high fracture stress of about 1950 MPa with a negligible tensile elongation (about 2 %) at the peak aged condition. The fractographic studies indicated this alloy shows semi‐brittle fracture in the subsequent aging treatment.     

Influence of initial microstructure on microstructural stability of 20% cold worked Ti modified austenitic stainless steel

Abstract

The influence of initial microstructure at different solution annealing conditions on the stability of 20% cold worked microstructure of a Ti modified austenitic stainless steel has been studied. Solution annealed and 20% cold worked samples were isothermally annealed at different temperatures in the range 923-1123 K for 1-1000 h. Mechanical property measurements, ultrasonic velocity measurements, and optical metallography were carried out to assess the stability of the cold worked microstructure during annealing. The extent of recrystallisation was measured using quantitative metallography. The variation of hardness and velocity with annealing time exhibited a three stage behaviour for the conditions leading to complete recrystallisation. Based on the microstructural observations, the three stages identified are recovery, progress of recrystallisation and completion of recrystallisation. There is one to one correspondence between the hardness and velocity measurements in assessing the microstructural changes. However, variation in the velocity measurements with annealing time correlated well with the extent of recrystallisation compared to that of the variation of hardness with annealing time. With an increase in solution annealing temperature from 1173 K to 1473 K, more primary TiC precipitates dissolve into the matrix and there is an increase in the secondary precipitation of TiC during annealing of the 20% cold worked alloy. The increase in secondary precipitation of TiC retards recovery and recrystallisation of the cold worked microstructure effectively and hence enhances the stability of cold worked microstructure. Thus, an increase in the solution annealing temperature from 1373 K to 1473 K enhances the stability of cold worked microstructure in Ti modified austenitic stainless steel.     

Martensite structure in TP304H austenitic stainless steel after long-term service and aging

The microstructure change of TP304H steel after long-term service and aging at high temperatures is investigated through XRD, OM, SEM, TEM, SAED and EDS. The results indicate that after long-term service, α’-martensite and ε-martensite are found in TP304H steel. α’-martensite distributes in stripes along grain boundaries; the substructure of α’-martensite is high-density tangling dislocations. ε-martensite distributes in parallel, a little further from grain boundaries than α’-martensite; the substructure is stacking fault. After long-term high-temperature aging, α’-martensite is found in TP304H steel. With the extension of aging time, the amount of α’-martensite increases and the lath morphology becomes clearer. The increase of martensitic transformation start temperature besides grain boundaries due to the decrease of Cr content in this area is the main reason for the formation of martensites in TP304H steel after long-term service and being aged at high temperature.