Studies of cryotreatment on the performance of integral diaphragm pressure transducers for space application

The integral diaphragm pressure transducers machined out of precipitation hardened martensite stainless steel (APX4) are widely used for propellant pressure measurements in space applications. These transducers are expected to exhibit dimensional stability and linearity for their entire useful life. These vital factors are very critical for the reliable performance and dependability of the pressure transducers. However, these transducers invariably develop internal stresses during various stages of machining. These stresses have an adverse effect on the performance of the transducers causing deviation from linearity. In order to eliminate these possibilities, it was planned to cryotreat the machined transducers to improve both the long-term linearity and dimensional stability. To study these effects, an experimental cryotreatment unit was designed and developed based on the concept of indirect cooling using the concept of cold nitrogen gas forced closed loop convection currents. The system has the capability of cryotreating large number of samples for varied rates of cooling, soaking and warm-up. After obtaining the initial levels of residual stress and retained austenite using X-ray diffraction techniques, the pressure transducers were cryotreated at 98 K for 36 h. Immediately after cryotreatment, the transducers were tempered at 510 °C for 3 h in vacuum furnace. Results after cryo treatment clearly indicated significant reduction in residual stress levels and conversion of retained austenite to martensite. These changes have brought in improvements in long term zero drift and dimensional stability. The cryotreated pressure transducers have been incorporated for actual space applications.     

Corrosion fatigue of a manganese–nitrogen stabilized austenitic stainless steel

Austenitic chromium–manganese–nitrogen stabilized stainless steels have been developed to replace chromium–nickel–nitrogen stainless steels in certain applications. In comparison, chromium–manganese–nitrogen steels have improved mechanical properties and acceptable corrosion resistance in hot, high chloride containing media. In this paper, corrosion fatigue investigations of a solution annealed and for practice more relevant 14% cold worked high alloyed chromium–manganese–nitrogen steel have been done. Inert glycerine was used as reference media and 62% calcium chloride solution as corrosive media, both aerated at a temperature of 120 °C. The stress ratio between upper and lower stress levels was 0.05 (tension–tension loading) to avoid the destruction of the fracture surfaces. As testing frequency for the dynamic experiments, 20 Hz was chosen considering possible application areas. Maximum stress versus number of cycles curves were recorded and representative specimens were investigated in a scanning electron microscope. In addition, electrochemical tests, exposure tests and constant load tests were done. This paper shows results on the corrosion fatigue of a manganese–nitrogen stabilized austenitic steel in a hot high chloride containing salt solution and helps to get a better understanding of occurring failure mechanisms.     

汽车拨叉热处理工艺研究

采用正交试验法，确定的汽车拨叉零件热处理工艺参数组合，满足使用要求；分析了该工艺形成的拨叉表层碳浓度情况和碳浓度梯度分布，获得浅层碳氮共渗碳浓度梯度分布对性能的影响，采用在升温阶段手动通入适量氨气的工艺调整方法，可以缩短均温时间，保证表面硬度达到技术要求，探索了合适氨气流量确保表层不出现过量残余奥氏体和黑色组织的机理，选用MT25656分级淬火油，可以满足零件变形要求。     

Effect of austenite microstructure and cooling rate on transformation characteristics in a low carbon Nb-V microalloyed steel

Deformation dilatometry has been used to simulate controlled hot rolling followed by cooling of a Nb-V low carbon steel, looking for conditions corresponding to wide austenite grain size distributions prior to transformation. Recrystallization and non-recrystallization deformation schedules were applied, followed by controlled cooling at rates from 0.1 °C/s to about 200 °C/s, and the corresponding continuous cooling transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling rates to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformations, displacing the CCT curve to higher temperatures and shorter times. However, it was found that the accelerating effect of strain on bainite transformation weakened as the cooling rate diminished and the polygonal ferrite formation was enhanced. Moreover, it was found that plastic deformation had different effects on the refinement of the microstructure, depending on the cooling rate. An analysis of the microstructural heterogeneities that can impair toughness behavior has been done.     

Mechanical stabilisation of retained austenite in δ-TRIP steel

Recent research suggests that extraordinary combinations of strength and ductility can be achieved in the so-called δ-TRIP steels, which contain ferrite, bainite and austenite. A part of the reason for the ductility of almost 40% elongation at a strength of some 900 MPa, is believed to be the optimal stability of the austenite to plastic deformation. We demonstrate here that mechanical stabilisation plays an important role in preserving the austenite to large plastic strains.     

Microstructure and Mechanical Properties of Gear Steels After High Temperature Carburization

High temperature carburization is becoming more and more attractive because it can remarkably reduce processing time and increase productivity. However, the commonly used gear steels which are microalloyed by Al are not suitable for high temperature carburization due to abnormal grain coarsening. The gear steel 20CrMnTiNb, which is microalloyed with 0. 048% Nb and 0. 038% Ti, has been compared with the gear steel 20CrMn in terms of microstructure in the case of hardened layer and in the core after carburizing at 1000 °C for 4 h and mechanical properties after carburizing and pseudo-carburizing. The results indicate that the fine austenite grains exist in the carburized case of 20CrMnTiNb steel, while there is abnormal coarsening and duplex grain structure in the case and core of steel 20CrMn. The average prior austenite grain sizes are 19.5 and 34.2 μm for the steels 20CrMnTiNb and 20CrMn, respectively. In addition, the mechanical properties of 20CrMnTiNb steel are superior to those of 20CrMn steel. In particular, the HV hardness of the former is higher than that of the latter by about 40–70 in the range of less than 0.7 mm in depth. Therefore, the steel 20CrMnTiNb is suitable for high temperature carburization.     

卷取温度对热轧TRIP钢残奥及力学性能的影响

为了研究卷取温度对热轧TRIP钢的残余奥氏体和力学性能的影响,使用金相显微镜、扫描电镜、x-射线衍射、拉伸实验等方法对三种卷取温度下制备的热轧TRIP钢进行分析.结果显示,随着卷取温度的降低,残余奥氏体晶粒尺寸变小,残奥体积分数和碳的质量分数也变小.450 ℃和400 ℃卷取温度下制备的热轧TRIP钢的残奥形貌的圆整性相差不大,而350 ℃卷取温度下制备的热轧TRIP钢的残奥形貌较圆整.热轧TRIP钢的力学性能随着卷取温度的降低表现为高的屈服强度和低伸长率,450 ℃卷取温度下制备的热轧TRIP钢的综合力学性能最优.     

残余奥氏体的诱发相变对Cr12钢耐磨性的影响

利用金相、Ｘ射线、扫描电子显微镜等手段研究了Ｃｒ１２钢中残余奥氏体的形变诱发马氏体相变对耐磨性能的影响，探讨了强化机制．结果表明，Ｃｒ１２钢经１１５０℃淬火得到大量处于变塑状态的残余奥氏体，并在磨损过程中发生形变诱发马氏体相变，使表面硬度和耐磨性显著提高，超过常规的９８０℃淬火．残余奥氏体对耐磨性的强化，为逐层诱发马氏体的循环强化机制．     

ZGOCr13Ni4Mo中的逆变奥氏体和氢脆

用热处理工艺控制钢中的逆变奥氏体含量,用阴极渗氢的时间控制钢中的氢含量。本文通过对具有不同逆变奥氏体含量和不同氢含量的试样的慢速拉伸图的分析,研究了逆变奥氏体、氢含量与塑性、断裂韧性间的相互影响。用扫描电镜观察各种试样的断口,研究了断口形貌和逆变奥氏体、氢之间的关系。结果表明,ZGOCr_(13)Ni_4Mo对氢脆敏感。逆变奥氏体能够降低该钢的氢脆系数。适当地提高逆变奥氏体含量还能提高钢的断裂韧性。     

不锈钢换热器板片泄漏原因分析

316不锈钢换热器板片在使用中发生泄漏。对失效件进行了金相检验、腐蚀产物的成分分析，以及应力状态分析，对腐蚀起主要作用的氯离子的来源，以及氯离子在应力腐蚀过程中循环作用的机理作了探讨。结果表明，该换热器板片泄漏为应力腐蚀所致。