Effect of low temperatures on charpy impact toughness of austempered ductile irons

Impact properties of standard American Society for Testing Materials (ASTM) grades of austempered ductile iron (ADI) were evaluated at subzero temperatures in unnotched and V-notched conditions and compared with ferritic and pearlitic grades of ductile irons (DIs). It was determined that there is a decrease in impact toughness for all ADI grades when there is a decrease in content of retained austenite and a decrease in test temperature, from room temperature (RT) to −60 °C. However, the difference in impact toughness values was not so noticeable for low retained austenite containing grade 5 ADI at both room and subzero temperatures as it was for ADI grade 1. Furthermore, the difference in impact toughness values of V-notched specimens of ADI grades 1 and 5 tested at −40 °C was minimal. The impact behaviors of ADI grade 5 and ferritic DI were found to be more stable than those of ADI grades 1, 2, 3, and 4 and pearlitic DI when the testing temperature was decreased. The impact toughness of ferritic DI was higher than that of ADI grades 1 and 2 at both −40 °C and −60 °C. The impact properties of ADI grades 4 and 5 were found to be higher than that of pearlitic DI at both −40 °C and −60 °C. The scanning electron microscopy (SEM) study of fracture surfaces revealed mixed ductile and quasicleavage rupture morphology types in all ADI samples tested at both −40 °C and −60 °C. With decreasing content of retained austenite and ductility, the number of quasicleavage facets increased from ADI grade 1–5. It was also found that fracture morphology of ADI did not experience significant changes when the testing temperature decreased. Evaluation of the bending angle was used to support impact-testing data. Designers and users of ADI castings may use the data developed in this research as a reference.     

冷轧TRIP钢的热处理与残留奥氏体形成的关系

低合金TRIP钢的显微组织中的残留奥氏体使其具有优良的强度和延性组合。概述了低合金TRIP钢的热处理工艺与残留奥氏体形成的关系。分析了冷轧TRIP钢退火前组织、临界区退火以及中温等温处理对残留奥氏体形成的影响。对低合金TRIP钢中残留奥氏体的形成等物理冶金学进行研究将促进其发展和推广应用。     

The influence of cast structure on the austempering of ductile iron. Part 3. The role of nodule count on the kinetics,microstructure and mechanical properties of austempered Mn alloyed ductile iron

Austempering kinetic measurements and mechanical property measurements are reported for irons with different Mn contents and different nodule counts after austenitising at 870 °C and austempering at 375 °C. It is shown that increasing nodule count, which reduces segregation and changes the size and distribution of intercellular boundaries, increases the interphase boundary area between graphite and matrix and decreases the continuity of the unreacted austenite in the intercellular boundary. This accelerates the stage I reaction which broadens the heat treatment window and moves it to earlier austempering times. A high nodule count can be used to counter the delay of the stage I reaction caused by Mn additions used to increase the hardenability of the iron. A high nodule count produces a finer, more uniform ausferrite structure that increases the strength, ductility and impact energy of the austempered iron.     

"两步法" 制备等温淬火球铁的研究

由于单一温度下的等温淬火工艺不能同时达到高硬度和高韧性，开发了两步法等温淬火热处理工艺。其原理是：工件经奥氏体化后，先在低温盐浴保温一段时间，使其基体在过冷度较大时大量形核，然后再经高温保温使基体组织转变成具有含碳量较高的残余奥氏体，细化组织，进而提高力学性能。通过在低温不同保温时间下的组织对比，确定较佳的低温盐浴保温时间；然后进行单一温度下等温淬火和两步法热处理工艺试验。将两种工艺的结果进行对比，发现两步法等温淬火工艺得到的组织和性能介于单一温度等温淬火工艺的低温和高温之间。     

等温淬火工艺对高铬铸铁组织与性能的影响

通过与常规淬火与回火热处理工艺比较,研究了不同等温淬火热处理工艺对高铬耐磨铸铁的组织与性能的影响,等温淬火热处理工艺可以获得下贝氏体和马氏体为基体的组织,既提高铸铁的冲击韧性,又显著提高冲击磨损性。在320℃等温淬火1.5 h可获得较理想的材料冲击韧性和耐磨性。     

奥－贝球铁中残余奥氏体含量的研究

用Ｘ射线衍射分析仪测定了Ｓｉ、Ｃｕ、Ｍｏ含量和等温淬火温度、保温时间对奥－贝球铁中残余奥氏体含量的影响规律，在ＳＥＭ下现在分析了残余奥氏体量对试样断裂方式的影响；结果表明，Ｃｕ使残余奥氏体含量提高，Ｍｏ使残余奥氏体含量降低。一定的淬火温度、保温时间和含Ｓｉ量，可使残余奥氏体含量具有最高值，奥氏体含量减少使断裂方式向脆性断裂转变。     

Crl2MoV钢渗碳等温淬火组织分析

Ｃｒ１２ＭｏＶ钢采用渗碳及等温淬火处理后，获得了良好的力学性能和使用效果。采用ＴＥＭ和ＳＥＭ对此工艺处理后的Ｃｒ１２ＭｏＶ钢的渗层及心部组织形态进行观察分析，确定了心部贝氏体的形态；而渗层内在等温后期形成的特殊黑色针状组织，在该钢种的其他热处理方式中均不存在，不具备典型下贝氏体特征。本文为该钢种采用渗碳加等温淬火这种新的强化途径提供理论基础。     

Effects of Austempering on the Mechanical Properties of the Hot Rolled Si-Mn TRIP Steels

The effect of austempering on the mechanical properties of the hot rolled Si- Mn TRIP steels was studied. The mechanism of transformation induced plasticity （TRIP） was discussed through the examination of the microstructure and the mechanical properties of the specimens. The results stow that the microstructures of the steels were comprised of polygonal ferrite, granular bainite and a significant amount of stable retained austenite. The specimen exhibits excellent mechanical properties for the TRIP effect. Isothermal holding time for austempering affects the stability of retained austenite. The mechanical properties such as tensile strength, total elongation and strength ductility balance reach their optimal values （ 776 MPa , 33% and 25608 MPa% , respectively） when the specimen is held at 400℃ for 25 min.     

350℃-Thermal Stability of Austempered Ductile Iron

The thermal stability of an ADI has been studied by isothermal aging at 350℃ for 150 hours, measuring the amount of retained austenite and its carbon content by X-ray diffractometry. The influence of different austempering treatments, carried out at temperatures between 300 and 450℃ (300, 330, 360, 410 and 450℃) and holding times between15 and 60 minutes (15, 30, 45, 60), was considered. Thermal stability depends on whether austempering temperature is higher or lower than the ageing one. Thermal stability increases by increasing austempering temperatures, from 300° to410℃. Samples treated at 410° and 450° show a lower austenite decomposition than samples at 300-330-360℃. A drop in stability is shown by increasing the austempering temperature from 410° to 450℃. The results have been interpreted on the basis of the austenite stability out of the processing window, which in turn depends on the austempering parameters.     

贝氏体等温淬火及其在轴承上的应用

论述了国内外高碳铬轴承钢下贝氏体淬火的研究成果 ,对淬火后的显微组织、力学性能进行了对比分析 ,总结并分析了生产中贝氏体淬火工艺优势及应用 ,介绍了国内贝氏体淬火装备