Kanayev A.T., Kanayev A.A

Eurasian National University named after L.N. Gumilev

 

Plasma Surface Hardening of Crests of Wheels of a Rolling Stock

 

Wheels are one of the basic and most loaded elements of a running part of the railway rolling stock, directly cooperating with rails and brake pads. Accordingly they form pairs friction a wheel-rail and wheel-brake a pad.

The reason of wear process of such interfaced products is work of forces of friction. Under action of these forces there is a repeated deformation of sites of a contact surface, their hardening and rehardening allocation of heat, change of thin structure, development of processes of weariness and other complex processes.

In a basis of resistance steels to wear process in conditions of dry friction with sliding durability of a superficial layer of metal lays. The defining factor thus is the local characteristic of durability -hardness of steel.

Increase of hardness is directed on complicating plastic deformation and development of processes of weariness, and also to exclude micro cutting of surfaces of friction, having provided whenever possible elastic deformation of sites of contact. High hardness of a surface is necessary also for difficulty of deleting of contact surfaces at them prosliding.

One of the important operational characteristics defining intensity of deterioration, the parity of hardness of wheel and rail steel is. Now it is volumetric the tempered rails hardness 330-400 HV, which became a basis of the top structure of a way. Work in contact to seamless-rolled wheels and the bandages having hardness on the average smaller on 25-30% (80-100 HV) at average hardness of a rim 250-290 HV. After stacking in a way of such rails due to plastic deformation of a superficial layer of a head of a rail from wheels of a rolling stock there is an increase in hardness of a rail up to 420-480 HV. Quenching of crests of wheels on hardness from above 600 HV becomes effective way of struggle against lateral deterioration not only crests of wheels, but also rails [1]. The raised lateral deterioration, as is known, is connected with development of plastic wear process and the tease. The leading mechanism of lateral deterioration is jamming.

Suppression of plastic deformation by increase of superficial hardness by hardening even one element of the contacting pair (crests of wheels) on high hardness 600 HV and above not only does not render negative influence on other element (rail), but opposite, leads to improvement of its condition.

In work with the purpose of definition of an optimum parity of hardness of the strengthened layer of the bandage providing increases of wear resistance, various variants of hardness of pair "wheel-rail" were investigated. Hardness of a bandage of wheel pairs locomotives on a surface and in depth makes 280-290 HV and hardness of new rails R65 (on GOST grade) on their length and depth makes 330-400 HV. As it was specified above, after stacking in a way of such rails due to plastic deformation of a superficial layer of metal of a head of a rail from wheels of a rolling stock there is an increase in hardness of a rail up to 420-480 HV.

Thus, in pair friction "wheel-rail" the certain disproportion is traced, that negatively affects both deterioration of a wheel, and on deterioration of a rail. In this connection following parities of hardness of pair "wheel-rail" which probably to receive at operation of a rolling stock has been chosen (table 1).

Tests for wear resistance were spent by the machine of type MI1MU under the scheme «a rotating wheel -motionless pad» in oil abrasive environment under conditions of dry friction «metal on metal» with an abrasive layer. As the controller samples of rail steel R65 were used. Depth of the strengthened layer for all samples has been accepted 1 mm. Results of tests are presented in the table 1.

 

Table 1. Influence of a parity of hardness of a bandage and rail on wear resistance of pair «metal of a bandage - metal a rail».

 

Variant number

Bandage hardness, HVB

Rail hardness, HVR

Hardness relationship, HVB/HVR

Wear of tests, mm

1

263

467

0.56

74 initial state

2

277

456

0.61

62

3

481

462

1.04

50

4

554

453

1.22

32

5

657

465

1.41

18

6

783

458

1.71

12

7

877

471

1.86

9

8

956

455

2.10

6

 

From data of the table 1 it is visible, that the optimum range of hardness of a bandage is in an interval HV 554-877, [2] that is the optimum parity HVB/HVR is within the limits of 1.22-1.86. Increases in hardness from above 900-950 HV in real conditions of operation can lead crack formation and to painting of the strengthened layer, and also to intensive deterioration of a rail. Hardening of crests of a bandage of wheels of pairs on low hardness 277-481 it is inefficient, as wear resistance improves slightly.

Simultaneously with increase of wear resistance of a bandage it is necessary to provide high stability origin and distribution of cracks. For this purpose special tests for a shock bend with recording process of destruction in coordinates "effort-time" have been resulted.

Results of tests show, that impact strength at hardening by variants #4-7 decreases in 1.65-3.5 time (KF=0.17-0.008 MJ/mm2)  against 0.28 MJ/mm2 without hardening. The maximal destroying effort decreases within the limits of 1.2-1.5, but those so considerably as impact strength (P max = 6.7-7.5 kN) against 9.5 kN

The analysis of oscillograms and fractal-graphic researches of breaks of samples show basic difference of character of destruction of the strengthened samples from not strengthened metal of a bandage.

From oscillograms it is visible, that metal of a bandage not having the strengthened layer collapses on all section and process of destruction passes in two stages - origin of a crack and its distribution before full destruction of the sample.

It is visible the ascending branch of the schedule (growth of effort of destruction up to 9.5 kN) and a branch descending up to zero are visible at distribution of a crack. Such mechanism of destruction is caused by homogeneous structure of metal on all section of destruction that guarantees a constant complex of mechanical properties of metal on all section. In case of plasma hardening when metal consists of two layers - tempered (fragile) and initial (plastic) process of destruction proceeds on the "plural" mechanism. The crack arises on a surface of the strengthened layer and origin grows in the strengthened layer in depth where on border with initial soft metal stops as for its further distribution the much greater effort is necessary, than initiation in hardened layer.

Fractal-graphic breaks confirm the received character of destruction. So for not strengthened metal of a bandage it is characteristic fragile internal cleavage with the strongly pronounced twisting pattern, representing system converging steps a break. For metal of a bandage with the strengthened layer the break is characteristic quazicleavage. The size of facets crack (smooth surface) is much less than smooth surfaces in comparison with edges of a break of fragile metal that testifies to a high degree of depressiveness of structure of the strengthened layer. With increase in a degree of depressiveness of structure of the strengthened layer the critical pressure of fragile destruction in inverse proportion to the size of grain raises it cracks stability, as according to the physical theory of destruction of metals.

scr = k * d1/2

 

At reduction of the size of grain concentration of pressure on border that leads to increase of a limit of endurance decreases. From these positions plasma hardening of metal of a bandage provides the most power-intensive mechanism of its destruction that should affect operational characteristics of metal positively.

Tests of the samples strengthened, with melting has shown surfaces, that values of impact strength have the certain correlation with change of micro hardness of the melted off zone [2]. At reduction of micro hardness of the melted off zone of metal of a bandage, value of impact strength raises, that testifies to some increase crack stability comparison with metal of the bandage strengthened with melting of a surface, proceeds as intercrystalline break.

With the strengthened superficial layer and without it with breaking cracks on the mechanism of a curvature of its trajectory it is possible to explain change of character of destruction of metal of a bandage two factors:

1.  High plasticity of metal on border of the strengthened layer with an initial material.

2.  Transition to border of the strengthened layer with initial structure of pressure of compression in stretching which aspire to change directions of distribution of a crack.

The braking mechanism of distribution of a crack and curvature of its trajectory will be coordinated with the data received for other ways of superficial hardening who are characterized by presence of sharp border between zones with various structure of metal (nitrating, borronizing) [3].

Researches of distribution of residual pressure on a surface and on depth of the strengthened layer of metal of a bandage have shown that character of distribution of residual pressure depends on set of factors: parameters of a mode of hardening, an initial condition of metal of a bandage, speed of hardening, etc.

In figures 2-4 distribution of residual pressure across the strengthened layer of a bandage is presented at plasma hardening.

At processing without fusion of a surface in an interval of capacities of a jet P=5-10 kW in the center of the strengthened layer stretching pressure are observed ss =100-140 MPa, on border with the basic metal they increase up to 250 MPa.

With increase in capacity of a plasma jet (arch) in an interval P=10-15 kW  in the center of the strengthened layer is formed pressure of compression ss =180-250 MPa.

The Further increase in capacity up to P=20 kW with an output on a mode of micro-fusion is marked by increase of value of residual pressure of compression up to ss =250-380 MPa.

On size and character of distribution of residual pressure significant influence render speed of plasma hardening. At small speed of hardening 5-10 mm/s in the center of the strengthened layer stretching pressure that is caused by prevalence of pressure from thermal change of volume above pressure from structural pressure are formed. To increase in speed of processing from above 10 mm/s there is a change of a sign on pressure in the center of the strengthened zone [4].

At plasma superficial hardening various designs of electromagnets and other devices for reception of wide paths of hardening are used from 10 mm up to 60 mm. Depending on a degree of expansion of a plasma jet (arch) (frequency, the form of a signal, etc.) character of thermal processes a superficial layer of metal and as consequence, size and character of distribution of residual pressure changes also. It is noted, that with increase in width of a path of hardening from above 30 mm distribution of residual pressure on depth and width of the strengthened layer is observed [5]. It can negatively affect on crack stability of metal of a bandage process of operation. Character of distribution of residual pressure on depth of the strengthened layer is shown on fig. 1.

 

Fig. 1 Distribution of residual pressure on depth of the strengthened layer
depending on speed of
hardening (Vstr.):

1 - Vstr.=3.5 mm/s; 2- Vstr.=10.5 mm/s; 3 - Vstr.=14.6 mm/s

 

It is visible, that the maximal value of residual pressure of compression is in under a surface layer of metal of a bandage, and on border to initial structure of metal there is a change of a sign on residual pressure. Formation of residual pressure of compression in the strengthened layer of metal promotes increase crack to stability and fatigue durability.

 

References:

1.      A.T. Kanayev, K.T. Kusainova, A.A. Toktanayeva Plasma Hardening of Crests of Wheels. Herald of Kazakh academy of transport and communication, 2004, #6 (31),
P. 25-28.

2.      A.T. Kanayev, A.A. Kanayev Influence of Plasma Hardening on Structure of Crests of Wheels. //Lokomotiv, Moscow, 2006, #6, P. 59-63.

3.      S.V. Petrov, A.G. Saakov. Plasma Hardening Opportunities of Wheels//Lokomotiv, Moscow, 2001, #8.

4.      D.P. Markov. Hardening of Crests of Wheels of Rolling Stock to Strong Hardening. Herald of VNIIJT, 1997, #1, P. 36-42.

5.      Plasma Hardening of Wheels (Moscow railways experience)//Lokomotiv, Moscow, 1999, #3, P. 32-33.