Ni在奥贝球铁上贝氏体第一阶段转变中的作用及其机制研究

采用光学显微镜、扫描电镜、透射电镜、电子探针、X射线衍射仪、图象处理仪、磁性秤、差热分析仪、磨粒磨损机、拉伸和冲击试验机等,研究分析了Ni、Mn在奥贝球铁中的行为、分布和对奥贝球铁热处理工艺参数的影响。 深入探讨了 Ni及 Ni与 Mn复合作用、奥贝球铁处理工艺参数对奥贝球铁贝氏体转变动力学、热稳定性、组织形貌、性能的影响与机制。 系统提出并论述了金相法、硬度法、磁性法、残余奥氏体量法四种确定贝氏体第一阶段转变表面结束时间和实际结束时间的有效方法和原理。 发现并解释了奥贝球铁中存在的网球状石墨及其形成机制和作用。 提出了Ni、Mn对奥贝球铁组织均匀性、强韧化和贝氏体转变动力学的复合作用机制。指出了保证奥贝球铁生产工艺稳定性和最佳综合性能的关键因素。     

等温淬火球铁等温转变过程与奥贝球铁生产的若干问题

本文介绍了等温淬火球铁（ＡＤＩ）和它的一个品种－－奥贝球铁。通过对文献资料的综述和分析，简述了ＡＤＩ等温转变的过程和由此引发而来的生产高质量奥贝球铁时应注意的若干问题。     

等温淬火球铁等温转变过程与奥贝球铁生产的若干问题

本文介绍了等温淬火球铁（ＡＤＩ）和它的一个品种──奥贝球铁。通过对文献资料的综述和分析，简述了ＡＤＩ等温转变的过程和由此引发而来的生产高质量奥贝球铁时应注意的若干问题。     

基于奥贝球墨可锻铸铁性能的影响因素

介绍了奥贝球墨可锻铸铁性能影响因素。奥贝球墨可锻铸铁是在球墨可锻铸铁的基础上通过等温淬火处理获得奥贝基体组织,具有类似于奥贝球铁的组织和性能,可放宽原材料对Mn、P元素的含量限制,是一种有前途的新型结构材料。     

60Si2Mn钢中的上贝氏体

本文用光学显微镜、电子显微镜和 X 射线衍射仪,对60Si2Mn 钢经870℃×20分钟奥氏体化后在320～500℃等温不同时间所获得的组识形态和结构进行了研究。认为这种钢的上贝氏体在等温形成的前期阶段为无碳化物贝氏体或称 BI 型上贝氏体,后期阶段为典型粒状贝氏体。粒状贝氏体的形成是 BI 型上贝氏体中贝氏体铁素体长大与富碳奥氏体为减少界面能而进行球化过程的结果。BI 型上贝氏体中奥氏体量达22%,其含碳量为1.52%,经-78℃处理和300℃以下回火显示了高度的稳定性。此外,初步探讨了 BI 型上贝氏体的机械性能。     

Ni在球墨铸铁中的行为及作用

采用电子探针、图像处理和差热分析仪等测定观察了Ｎｉ在球铁中的偏析分布特性，研究分析了Ｎｉ对铸态态球铁组成比例，Ｆｅ－Ｃ－Ｓｉ平衡相图Ｍｓ点和贝氏体转变动力学的影响及其机制，结果表明：Ｎｉ在球铁中呈连续负偏析，其加入可以改变铸态球铁中各组成相的比例，使得Ｍｓ点降低，奥氏体稳定性增加，等温转变过程中，贝氏体转变速度减慢。     

250℃～350℃等温淬火过程中超高碳钢微观组织转变及其机制

本文研究了超高碳钢(1.40wt%C,1.50wt%Cr,1.50wt%Al)等温淬火贝氏体形貌及形成机制。在250℃~350℃温度范围内等温淬火研究结果表明,随着等温温度升高,贝氏体的孕育期缩短,贝氏体片的长度变短,厚度增加,呈现侧向长大特征,贝氏体转变完全程度降低。贝氏体转变结束后,继续延长等温时间,残余奥氏体发生了渗碳体和铁素体分解。在贝氏体转变较少的试样中,在随后进行的空冷过程中贝氏体将继续形成。经分析认为,贝氏体转变由应力与原子扩散两种因素控制;等温温度较低时,应力占主导地位,碳原子的扩散起辅助作用,贝氏体切变形成,呈薄片状;随等温温度的升高,扩散作用增大,贝氏体长大,出现侧向增厚,长度变短的组织变化特征。     

9Cr2Mo钢中贝氏体的组织形貌

采用光学显微镜和扫描电子显微镜等技术手段观察了经1100℃奥氏体化的9Cr2Mo钢在不同温度的盐浴中等温淬火后贝氏体的组织形貌。结果表明：9Cr2Mo钢1100℃奥氏体化后,在410℃的硝盐浴中等温得到羽毛状的经典上贝氏体组织,在350℃的硝盐浴中等温得到针状（或片状）的下贝氏体组织;上贝氏体铁素体铁素体是在奥氏体晶界处形核并向晶内生长,碳化物在铁素体条间分布;下贝氏体是在奥氏体晶内形核,碳化物分布在铁素体片中间,碳化物大多数与片条的主轴方向交角排列,但角度不等。     

影响GCr15钢M／B复相组织强韧性的主次因素初探

通过正交设计试验和方差分析方法讨论了奥氏体化温度，形成贝氏体的等温温度及贝氏体的相对量对ＧＣｒ１５钢Ｍ／Ｂ复相组织的冲击韧性与硬度的影响。结果表明：贝氏体的相对量显著影响复相组织的冲击韧性及硬度，而奥氏体化温度和开成贝氏体的等温温度则对其影响不大。     

感应淬火热处理工艺对10CrNi3MoV对称球扁钢力学性能的影响

研究了感应淬火热处理工艺对10CrNi3MoV对称球扁钢力学性能的影响。结果表明,10CrNi3MoV对称球扁钢的淬火温度随着感应淬火轨道频率的提高而显著降低,控制轨道频率7～11 Hz,即可控制球扁钢加热温度840～890℃。感应淬火温度和淬火水量对球扁钢的力学性能存在显著影响,随着感应淬火温度的提高以及淬火水量的降低,腹板部位的屈服强度显著降低,低温韧性变化不明显。二次调质热处理时钢的屈服强度显著降低,应提高钢的奥氏体化温度及奥氏体化均匀程度。感应加热淬火时,高温停留时间短,奥氏体化程度低,淬火后获得了细小的马氏体和贝氏体混合组织,钢的屈服强度较加热炉加热显著提高。     

Effect of modified austemper on tensile properties of 0·52%C steel

Abstract

The effect of a modified austemper on the tensile properties of 0·52%C steel has been studiedfor the purpose of developing the mechanical properties of upper bainitic steel. The modified austempering treatment involved intercritical annealing at 1018 K in the two phase region offerrite (α) and austenite (γ) followed by austempering at 673 K and subsequent water cooling. The results have been compared with those obtained from conventionally austempered steel, and quenched and tempered steel with a similar ultimate tensile stress. The modified austempered steel consisted of a mixed structure of upper bainite and 10 vol.-% ferrite in which ferrite appeared as layers along the rolling direction. The modified austempering treatment wasfound to significantly increase the product of ultimate tensile stress and total elongation, and also the notch tensile stress at 193 K. Conventional austenitising at 1173 K followed by subcritical annealing at 998 K in the two phase region of ex and y, and then austempering at 673 K and subsequent water cooling produced the same mixed structure of upper bainite and 10 vol.-% ferrite. However, this treatment yielded inferior mechanical properties to those obtained with the modified austempering treatment, independent of the test temperature. The results are described and discussed.

MST/3102     

High temperature decomposition of austempered microstructures in spheroidal graphite cast iron

Abstract

Spheroidal graphite (SG) cast iron is often plasma nitrided for corrosion resistance, and plasma nitriding has been proposed as a surface engineering treatment to improve wear resistance. However, the microstructure of austempered SG iron comprises constituents that may be unstable at nitriding temperatures. Therefore, the thermal stability of austempered SG cast iron has been studied at high temperature. Differential scanning calorimetry shows that microstructures obtained by austempering at low (300°C) and intermediate (380°C) temperatures, and which contained retained austenite, underwent a large exothermic transition during heating to typical nitriding temperatures. The transition began at approximately 470°C and peaked at 510–520°C, and was due to the decomposition of retained austenite to ferrite and cementite. A microstructure obtained by austempering at a higher temperature (440°C), and which consisted entirely offirst and second stage bainite, was stable up to nitriding temperatures. After tempering for 2 h at 570°C all austempered microstructures consisted offerrite and cementite, but cementite was most finely distributed in the material that had been austempered at 300°C, and coarsest in that austempered at 440°C. It is concluded that if SG cast iron is to be nitrided conventionally at temperatures >500°C, then prior austempering to obtain controlled microstructures is of limited value.

MST/3106     

Multiphase microstructure formation and its effect on fracture behavior of medium carbon high silicon high strength steel

This work investigated the evolution of multiphase microstructure and impact fracture behavior of medium carbon high silicon high strength steel subjected to the austempering treatment at 240,360,and 400 ℃.The results show that martensite,bainite,and retained austenite (RA) are the main microstructural phases.The austempering treatments at 360 and 400 ℃ caused the formation of carbon-poor ferrite in the matrix,and the transformation of ultrafine bainite into coarse lath bainite and granular bainite,respectively.Thick filmy RA was distributed between bainite laths.The polygonal martensiteaustenite islands and blocky RA formed along the grain boundaries.The average carbon concentration in the matrix decreased with the temperature increase,while the impact toughness initially increased and then dropped with temperature.The quasi-cleavage brittle fracture dominated the impact fracture mechanism of the sample austempered at 240 ℃ by forming tearing surfaces and tearing steps.The microcracks disappeared in the RA on the prior austenite grain boundaries.On the other side,the fracture surface of the sample austempered at 360 ℃ exhibited ductile fracture with deep dimples and brittle fracture with cleavage river patterns.The polygonal martensite-austenite islands or blocky RA constrained the microcracks.After austempered at 400 ℃,the brittle fracture was dominant,showing river patterns,and the microcracks propagated through the granular bainite without any resistance.     

Microstructure and mechanical properties of austempered AISI 9255 high-silicon steel

The present investigation is focused on evaluating the microstructure and mechanical properties of American Iron and Steel Institute 9255 high-silicon steel austempered at different temperatures and durations. Material characterisation was done using a scanning electron microscope and an X-ray diffractometer. Results show the bainite microstructure over a temperature range of 280–400°C. Bainite structure gains coarseness at higher temperatures at 360 and 400°C. A significant improvement in the tensile properties was observed for all austempered specimens; with a maximum tensile strength of 1852?MPa and elongation up to 35%. An excellent strain hardening response was observed from the samples austempered at temperatures of 360 and 400°C. Tensile properties were found to be superior at 15?min of austempering duration for all austempering temperatures.     

Characterization of transformed and deformed microstructures in transformation induced plasticity steels using electron backscattering diffraction

The microstructure of transformation induced plasticity (TRIP) steels was characterized by means of electron backscattering diffraction (EBSD) technique to identify and quantify their different microstructures such as ferrite, bainite, and retained austenite. Further, the strain distribution in ferrite and retained austenite was analyzed during deformation. The TRIP steels were annealed by austempering for different durations to investigate the effect of the austempering time on the volume fraction change of the microstructural constituents. The quantitative analysis by EBSD coupled with an image contrast analysis revealed that the amount of retained austenite decreased and the amount of bainite increased with increasing austempering time. The mechanical properties of the TRIP steels were also affected by the austempering time. The maximum elongation was obtained in the sample austempered for 5 min, probably because of the good stability of retained austenite. The strain distribution in bcc and fcc phases during tensile deformation was characterized by evaluating the changes in the average local misorientation of the phases.     

The Influence of Austempering Conditions on the Machinability of a Ductile Iron

Austempering conditions such as temperature and time and their influence on austempered ductile iron machinability were analyzed. Austenitization at 910°C for 90 min and austempering into molten salt bath at 300°C, 360°C, and 420°C for 30, 60, and 90 min each were performed. Microstructures were analyzed by optical microscopy and hardness measurements. Samples were further machined in a lathe for machinability tests. The lathe was instrumented considering power and cutting time and machinability evaluation performed referring to cutting force and material removal. Microstructures at 300°C for 30 min showed ausferrite with retained austenite and martensite. Retained austenite decreased and acicular ferrite sheaves appeared at 60-min austempering time. Mixed bainite was also present at 90-min austempering. Ausferrite and retained austenite were observed in all austempering periods at 360°C, whereas at 420°C only bainite and fine pearlite were present. Hardness increased with increasing temperature at 30-min austempering and decreased with increasing time. However, an exception was observed at 420°C. The highest machinability performance was achieved at 360°C at 60-min austempering, and the lowest performance at 420°C at 90-min austempering.     

Formation of strain-induced martensite in austempered ductile iron

The present work has been taken up to study the influence of microstructure on the formation of martensite in austempered ductile iron. Ductile iron containing 1.5 wt.% nickel and 0.3 wt.% molybdenum was subjected to two types of austempering treatments. In the first, called as conventional austempering, the samples were austempered for 2 h at 300, 350 or 400 °C. In the second treatment, called as stepped austempering, the samples were initially austempered at 300 °C for 10, 20, 30, 45 or 60 min. These were subsequently austempered for 2 h at 400 °C. Tensile tests revealed considerable variation in the strain-hardening behaviour of the samples with different heat treatments. In the case of samples subjected to conventional austempering, it was found that strain-hardening exponent increased with increasing austempering temperature. In the case of samples subjected to stepped austempering, increased strain hardening was observed in samples subjected to short periods of first step austempering. Study of the microstructures revealed that increased strain hardening was associated with the formation of strain-induced martensite. There was a greater propensity for the formation of strain-induced martensite in the samples containing more of blocky austenite. Retained austenite in the form of fine films between sheaths of ferrite was relatively more stable. Studies revealed that the morphology, size and carbon content of the retained austenite were important parameters controlling their tendency to transform to martensite.     

Effect of austenisation temperature on bainite transformation below martensite starting temperature

Effects of austenisation temperature on martensite and bainite transformation behaviour, microstructure, and mechanical properties of a bainitic steel austempered below martensite starting temperature were investigated in this study. Results show that the amount of athermal martensite gradually increased with the increase of austenisation temperature, whereas the amounts of bainite and retained austenite initially increased and then decreased, resulting in the trend of the first increase and then decrease in the product of tensile strength and elongation. In addition, the transformation rate of isothermal bainite after athermal martensite formation revealed a trend of deceleration and then acceleration with austenisation temperature at the beginning period. Moreover, the size of bainite plates decreased first and then increased with austenisation temperature.     

Transformation characteristics of ductile iron austempered from intercritical austenitizing temperature ranges

In the present work, the transformation characteristics of ductile iron austempered from intercritical austenitization temperature ranges were investigated. For this purpose, an unalloyed ductile cast iron containing 3.50 wt% C, 2.63 wt% Si and 0.318 wt% Mn were intercritically austenitized (partially austenitized) at various temperatures and then rapidly transformed to a salt bath held at the 365 °C for austempering for various times to produce dual matrix structure with different ausferrite volume fractions in ferrite matrix. A microstructure map was created to illustrate the transformation of products quantitatively as a function of austempering time for a particular intercritical and austempering heat treatment temperature and time. It was demonstrated that the total volume fraction of transformed phases was approximately constant for all austempering times after rapidly transforming samples from a particular intercritical temperature to austempering temperature. It was found out that the new ferrite (It is also called epitaxial ferrite) introduced into the intercritically austenitized structure during austempering and its content was dependent on the intercritical austenitizing temperature and austempering time.     

Dry sliding wear of low alloyed austempered ductile iron

Abstract

A multiple low alloyed ductile iron with 0.8 wt-%Ni and 0.25 wt-%Mo was austempered in single and two step processes at 300 and 400°C for 120 min. Specimens were used to study the effect of austempering conditions on the wear behaviour of this material. Sliding wear tests were carried out using a pin on disc apparatus, the tes tmaterials rubbing under dry atmospheric conditions against a surface of hardened steel (55 HRC) at speeds of 0.6, 0.7 and 1.0 m s^-1 and normal loads of 15.82 and 22.84 N. Test durations were 30, 60, 90 and 120 min. Scanning electron microscopy was used to examine the worn surfaces of test specimens. It was found that two step austempered specimens exhibited wear resistance that was higher than that of specimens austempered at 400°C, and almost as high as that of specimens austempered at 300°C. These two step austempered specimens, moreover, gave the highest impact energy and showed the best combination of mechanical properties. During two step austempering, the first stage reaction (formation of ausferrite) was completed in the intercellular area before the undesired second stage reaction (precipitation of carbides) had started in the eutectic cells. The two step treatment resulted in a duplex structure: upper and lower bainitic ferrite without formation of carbides. This structure was responsible for the improvement of mechanical properties and the good wear resistance. The results show that a well balanced choice of smaller additions of multiple alloying elements can reduce the negative effects of segregation and resulting structural inhomogeneity. MST/5472