Possibility of Fatigue Crack Extension Under Cyclic Large Compressive Stress by Medium Frequency Induction Hardening

Abstract

Induction hardened axles have been used since the start of the Shinkansen (Japanese Bullet train) service in 1964. Axles are subjected to cyclic loading, and induction hardened axles are used under cyclic compressive stress conditions because of the large compressive stress state caused by induction hardening along the axle surface. Japanese Railways regularly inspect its axles, and if any crack larger than 0.15 mm in depth is found, the axle is taken out of service. The compressive stress is around −500 MPa, and magnetic particle inspection is conducted at intervals of 2.2×108 and 4.4×108 rotation. The issue of whether or not a crack more than 0.15 mm in depth will propagate under cyclic large compressive stress conditions is of great importance from the viewpoint of railroad safety. From the above mentioned background, a literature survey and experiment of fatigue crack extension under a cyclic large compressive stress condition were conducted. The results show that the crack did extend in the range of around 3.6–5.0×108 cycles under the condition of −500∼0 MPa cycle. Elastic-perfectly plastic stress analysis was also conducted to understand the possibility of tensile residual stress origination, which is considered to be a cause of extension. The analysis of −500∼0 MPa cycle showed that a large tensile stress of about 89.5 MPa originated at the crack tip when unloading. Whereas, the analysis of −560∼−440 MPa cycle for actual maximum stress in the field showed compressive when unloading.

Fatigue limit of induction hardened railway axles

“…Figure 2 also shows the dependence of HV30 hardness on the depth below the surface of an induction hardened axle made of S38C steel (equivalent to AISI 10308). 15 The higher surface hardness of the S38C steel after induction hardening compared with a surface hardened EA4T steel axle 23 is caused by the difference in chemical composition, primarily the higher carbon content in S38C steel.…”

“…This produces martensitic structure in the surface layer to a depth of up to 6 mm, increasing hardness by up to three times near the surface compared with the axle core; it also produces a compressive stress state of around À800 MPa in the surface layers. 5,7,15,18,19,23 From a technological perspective, the values of residual compressive stress in the surface layers as well as the hardness gradient are strongly dependent on the hardening frequency used in the inductor.…”

“…The use of lower inductor frequencies produces higher values of residual compressive stress. 5,7,15 These findings for induction hardened axles made of S38C medium carbon steel 24 (equivalent to AISI 10308) have led to the reduction of the hardening frequency used in the inductor from 10 to 3 kHz. 5 When accompanied by a reduction in tempering temperature from 230 to 200°C, the residual compressive stress in the surface layers of the axle increased.…”

“…These results, labelled EA4T-IH, are given in the third line of Table 3. The table also lists the same mechanical properties for S38C steel used in the production of Shinkansen train axles; 15 these values are given in the bottom line of the table.…”

“…In addition to these basic design concepts, studies also focus on the effects of structural materials, surface treatments, methods of heat treatment and subsequent machining, and geometric parametersparticularly the effect of axle body diameter and wheel seat diameter on fatigue strength with a focus on high-speed train axles. [13][14][15][16][17][18][19][20][21][22] One technology applied to increase axle fatigue strength is surface induction hardening, which involves the relatively rapid heating of surface layers of pre-machined axles up to the hardening temperature using an inductor, followed by the rapid cooling of the axle by a water jet positioned behind the inductor and also by the transfer of heat by the axle body. As a consequence, the fatigue limit is significantly increased, and short fatigue cracks in surface layers are hardly able to propagate.…”