DEVELOPMENT OF ANTI-DARK SPOT RAILWAY RAILS

随着铁路运输速度的提高，铁路系统中产生了各种各样的缺陷。其中之一是钢轨暗点缺陷或钢轨剥壳。暗点缺陷的特征在于主裂纹，其在接触表面处开始并且在交通方向上以与轨道表面成锐角的方式扩展，以及第二裂纹，其在与交通方向相反的方向上从表面上的几乎相同的点以稍微不那么浅的角度缓慢地增长。主裂纹具有许多子裂纹，并且在某些阶段，这些子裂纹中的一个可能横向延伸穿过轨头，导致轨道的灾难性拉伸破坏。因此，这种缺陷是非常危险的钢轨故障。作者找到了黑斑开裂的机理。此外，他们还制造了一种贝氏体钢，这种钢似乎对滚动接触疲劳失效的发生有效。本研究的目的是为开发者提供设计指导， 关于我们 根据黑斑裂纹产生的机理，研制了抗黑斑缺陷钢轨。主要研究结果如下：1.由于局部直接接触而产生的高牵引力会产生裂纹。在湿运行过程中，裂纹在水的渗透作用下扩展.在平均牵引力较低的条件下，贝氏体钢的抗暗斑能力远高于普通珠光体钢.在平均牵引力相当高的条件下，由于塑性变形而产生表面损伤。钢的硬度的增加对这种损伤的增强是有效的.贝氏体钢的耐磨性比珠光体钢差得多。而贝氏体钢的耐磨性可以通过提高硬度和碳含量来改善.虽然普遍认为较高的磨损率会阻止裂纹的萌生，但事实并非如此.贝氏体钢轨焊接部位的接头性能与普通珠光体钢轨几乎相同.目前，贝氏体钢钢轨（HB=250-350）的滚动接触疲劳抗力试验正在实际铁路上进行。研究发现，贝氏体钢轨的抗暗点缺陷能力比普通珠光体钢轨高得多，因此，为了使贝氏体钢轨的应用能满足JIS标准的珠光体钢轨钢强度和日本铁路公司制定的电阻标准，目前正在研究贝氏体钢轨的化学成分。少

As traffic speed on railroads increases, various kinds of defects have been produced in rail systems. One of them is the rail dark spot defect or rail shelling. The dark spot defect is characterised by a principal crack, which initiates at the contact surface and propagates at an acute angle to the rail surface in the direction of traffic, and a second crack, which grows more slowly from almost the same point on the surface, in the opposite direction to traffic and at a slightly less shallow angle. The principal crack has many sub-cracks and at some stage one of these may extend transversely across the railhead leading to a catastrophic tensile failure of the rail. Thus, this defect is a very dangerous rail failure. The authors have found the mechanism for the dark spot cracking. Furthermore, they have made a bainitic steel, which seems to be effective for the occurrence of rolling contact fatigue failures. The purpose of this study is to establish the design instruction for the develo … More pment of anti-dark spot defect railway rails on the basis of the mechanism for dark spot cracking. The main results obtained are summarised as follows :1. Cracking is produced by a high traction force due to a local direct contact. The crack initiated propagates by a hydraulic pressure effect due to water penetrated into the crack in a period of wet running.2. Under conditions where the mean traction force is rather low, the dark spot resistance of bainitic steel is much higher than that of conventional pearlitic steel.3. Under conditions where the mean traction force is rather high, surface damages due to the plastic deformation are produced. The increase in hardness of steels is effective for the enhancement of such damages.4. The bainitic steel is much worse than the pearlitic steel in the wear resistance. However, the wear resistance of bainitic steel can be improved by increasing the hardness and the carbon content.5. Although a common opinion that the higher wear rate prevents the crack initiation is believed, the fact is somewhat different.6. The welded parts of bainitic steel rails are almost the same joint performance as those of conventional pearlitic steel rails.7. Presently, the rolling contact fatigue resistance test of bainitic steel railroads (HB=250-350) is in progress using actual railways. It has been found that the dark spot defect resistance of bainitic steel rails is much higher than that of conventional pearlitic steel rails.Considering above results, the chemical composition of bainitic steel is under investigation now for application of bainitic steel rails so as to satisfy JIS standard on pearlitic rail steel strength and the electrical resistance standard established by the Japan Rail companies. Less