Wheel Defects


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I tried to cover as much information as possible from various sources about manufacturing , properties and various wheel defects in wheels of trains in railway indutsry

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Wheel Defects

  1. 1. WHEELS INTRODUCTION Wheels are most critical part of rolling stock. Utmost care is taken right from the manufacture to its last use on the trains. Wheels are most stressed components of railway vehicles. They carry axle load of up to 25 tonnes and more. They guide the train on the tracks through curves and switches and are subjected to constant wear process. Different functional parts of the wheel (as shown in Fig 1 below) such as flange (4), rim (3), centre (2) or hub (1) fulfill different task and have therefore different material properties. Railway wheel is high safety part and special attention is given during production process of wheel to the reliability, availability, and safety of the product. Railway wheels are usually made of unalloyed or low alloyed steels with a high degree of purity. Tight tolerances for the single alloying elements are desired in order to assure a low variation of material properties from heat to heat. European standard EN 13262 defines four different steel grades which mainly consist of up to 0.6wt% of carbon, up to 0.8 wt% of manganese and up to 0.4wt% silicon. National and international standards such as UIC 812-3V, GOST 10791, AAR M107-84, EN 13262 define the wheel steel grades and partly their manufacturing. Page 1
  2. 2. MANUFACTURING PROCESS  STEEL MAKING PROCESS Steel for the production of railway wheels is melted in the electric furnace at 3200deg F with the charge of 250 tonnes and tapping in two ladles. High quality characteristic of steel is provided by the subsequent processing at the out of furnace complex of steel treatment. The ladles with metal are delivered in turn to the unit furnace-ladle, where the metal finishing and refining takes place. Steel blowing in the ladle by argon along with the refining process provide for the sculpture content in the finished metal equal to 0.010% phosphorus – 0.015% and less, and uniform distribution of another elements. Degassing process is affected by means of removing the hydrogen, nitrogen, oxygen dissolved in metal at the vacuum degassing set simultaneously with argon blowing. Hydrogen content in steel is within the limits up to 2 ppm.  WHEEL MANUFACTURING From electric furnace molten metal is transferred to ladle which pours metal in graphite moulds to make ingots. Each ingot is approximately 20 feet long. Ingots are then cut into small pieces called billets. Each weight around about 1000 pounds. Billets are then transferred to rotary furnace where they are heated to 2400 deg F. From other end of furnace machine transfer these billets to scaling unit where their outer most layers are removed. After that these billets are subjected to pressure of 9000tonnes in a press which forge these billets to rough shape of a wheel. The train wheel increases 20% in diameter when it rolled on rolling mill. Then it goes to final shaping press where centre hole is punched in it. After heat treatment, machine sprays cold water to harden the steel. Wheels are machined to correct shape of rim, axle hole and imbalance checking. Various Non destructive tests like magnetic particle and ultrasonic tests are carried out to check any abnormalities in the wheel. The chemical composition of steel is checked by spectrometer. Automated brinell is used for measuring hardness. Page 2
  3. 3. The Technology of the Production of Railway Wheels Open-hearth furnace Ladle Ladle-furnace Vacuum degassing set Notching and breaking Billets inspection Billets heating in Hydroscaling of Open forging on the of ingot into initial billets and repairing the rotary furnace cinder press 20MN Closed forging on the Punching on the press Wheel rolling on the Wheel calibration on the press 50MN 100MN rolling mill press 35/8 MN and piercing of central hole Intermediate cooling Cooling of the wheel Machining Wheels heating and anti-flocking in open air isometric holding Rim quenching Tempering in pit Adjustable cooling CNC machining, in the furnaces imbalance checking Automated non- Wheels inspection and Testing of mechanical Storage destructive measuring, hardness and other properties control (ultrasonic test, determination on the samples magnetic particle inspection etc.) Page 3
  4. 4. MAJOR WHEEL DEFECTS IN RAILWAY INDUSTRY New designs and modern equipment have culminated in a shift in the mechanisms of wheel damage over the last 15 years. In 1992 when the last metallurgical survey was carried out by BR Research, about 80% of wheels were reprofiled due to thermal damage, largely as a result of inefficient wheel slide protection (WSP) systems. The other causes of premature wheel turning were identified as wear, including flange wear and thermal damage caused by wheel slide and tread braking. Today it is estimated that up to 80% of wheels are reprofiled due to rolling contact fatigue (RCF), and although an increase in total wheel life has been achieved, improvements are still sought. The British Standard and European specifications for railway wheels describe classes of heat treated wheels. The choice of the class of wheel to be used for any particular type of rolling stock and service is based on the conditions to be met. WEAR Wheel life depends largely on the resistance of the wheel to wear and its immunity to tread failures caused by thermal cracking and shelling as a result of RCF. Wear of wheels occurs on the wheel tread and flange. This can be minimized by Correct alignment of the wheels, flange lubrication, material of wheel and rail being similar and equipment in proper mechanical condition. Every effort should be made to avoid the abnormal loss of tread metal caused by thermal cracking and shelling. The most effective form of flange wear reduction is by flange lubrication, which can reduce wear by at least six times. Where this is cost prohibitive or not practical, wear can be improved by increasing the carbon content of the steel, and by promoting the morphology of the pearlite microstructure by altering the quench rate. A typical wear profile is shown in Figure given below EXAMPLE OF FLANGE WEAR Page 4
  5. 5. HOLLOW WHEEL OR DEEP FLANGE While the wheel moves there is constant wear on the tread and thus the diameter of wheels at tread start reducing. Due to which wheel flange height increases. Deep flange is dangerous because it starts damaging fish plate, fish plate bolts, distance blocks, points and crossings etc. Moreover inclination of 1 in 2.5 and 1 in 20 (1in 50 in Dubai Metro trains ) practically vanishes which results in higher friction and there is every possibility of wheels to derail on curves for two wheels on same side cannot be suitably converted with different diameters to suit longer outer and shorter inner rails automatically. HOLLOW WHEEL OR DEEP FLANGE RADIUS TOO SMALL AT THE ROOT OF FLANGE Root radius in Dubai Metro trains is R15. In service radius at the root of flange is subjected to maximum wear on curves and by snaking effect of the wheels. When it is reduced to R13 gauge will fit properly as shown in fig below. Defect results in to increased friction between rail and flanges because of reduction in taper of 1 in 2.5 given on wheel flange which affects hauling capacity of the train besides wearing effect on the rails also. FIGURE SHOWING RADIUS TOO SMALL AT ROOT OF THE FLANGE. Page 5
  6. 6. THIN FLANGE Flanges are subjected too severe side thrusts during the run and as such the strength of the flanges ought to be adequate to sustain all the severest side thrusts and maintain smooth running. When they wear thin they become weaker and there are cases when flanges could not sustain side thrusts and broke causing mid section accidents. Besides thin flanges cut through the partly opened facing points due to any signal or permanent way or other defect causing two roads under same train and serious accidents follow thereafter. THIN WHEEL Wheels become thinner because of continuous wear and metal removal in wheel reprofiling. Thin wheels are considered unsafe because it can further be thinned due to hammering impact at rail joints resulting into cracks and breakages etc. SHARP FLANGE Tip of the flange is not square but given the radius R10 and R18. Flange wears sharp when continuously wheel negotiates curves and during snaking effect of the wheels. When top radius at the corner towards tread reduces beyond tolerable limit (5mm in IR) is called sharp flange also known as knife edged flange. It is highly dangerous as it mounts the rails at points and nose and heel of the switch rails and crossing. It also mounts rail on the curves and easily cause accidents if happens to negotiate outer rail. Page 6
  7. 7. WHEEL FLAT Jamming of brakes or seizure of wheels cause skidding of wheels continuous for some distance thus damaging tread which wears excessively at the point of contact with rail and becomes flat to certain length and depth. In such cases wheel starts rolling but with unpleasant noise one like we hear on rail joints. This defect also irks passengers and add to their discomfort. This cause hot axles, journal breakage, derailments and skidding if allowed for long. It also cause false flanges on tread which is highly detrimental to safe running of train Tread Damage occurs from a number of mechanisms including severe tread braking at high speeds or high speed slip, caused for example by faulty WSP systems or WSP activity combined with low adhesion conditions, resulting in a heat input into the wheel tread. This locally heated metal then quenches rapidly due to the ‘colder’ bulk material of the wheel, which acts a heat sink. This phase is typically 20-30mm wide and 1mm deep. A typical example is shown in Figure below. This damage on the wheel tread may develop into larger cracks through rolling contact and thermal input and must be turned out. Resistance to thermal damage can be improved by lowering the carbon content of the steel. There are also other tread damage mechanisms such as thermal fatigue, which is associated with tread braking, but would generally be worn away due to the action of scraping whilst braking on the tread. Low speed slide can induce local heating below the transformation temperature and at an increased depth. The temperature is high enough, however, to overload the wheel due to loss of strength as the temperature increases which leads to mechanical damage of the wheel tread in the form of a ‘flat. WHEEL SLIP /SLIDE DAMAGE ON TREAD ROLLING CONTACT FATIGUE Rolling Contact Fatigue is the failure of the wheel tread due to cyclic fatigue. In Britain, there are two notions of rolling contact fatigue; a) Generally fatigue of the tread contact area due to high loads leading to shelling of the surface, an example is shown in Figure1 below. This surface breakdown can be greatly accelerated if abnormal conditions exist and may occur under relatively light static loads. Page 7
  8. 8. b) Curving forces experience by the wheel will also cause rolling contact fatigue of the wheel tread; it is generally seen off centre of the tread towards the field side. This type of rolling contact fatigue is generally associated with the low of an axle in a curve and leads to chevron type indications on the field side of the tread, as shown in Figure2 below. It can occasionally be seen towards the flange root on some wheels and is attributed to the action of the high wheel in curves. FIGURE 1 FIGURE 2 Fatigue failures of wheels can be surface induced, where initiation is due to gross plastic deformation of the wheel close to the running surface. This is normally due to high loading and/or low material strength, and leads to cracks that grow some millimeters into the wheel before deviating back to the surface and leading to small sections falling away from the tread. This is a progression from the initial RCF crack initiation shown in Figure 1 and 2 above. Sub surface fatigue failures occur below the running surface and initiate on a macroscopic defect, although they can occur in a virtually defect free material if the stresses are too high. These defects can typically grow to 30mm below the tread before deviating back to the surface, so larger sections of the tread can break loose. This type of failure is therefore potentially very serious as can be seen in Figure 3 below. Figure 3 Page 8
  9. 9. The following factors are found to be detrimental to wheel fatigue life: High wheel loads High impurity levels Small wheel diameters Small rail radius Tensile residual stresses Other damage mechanisms related to premature wheel tread turning are local tread collapse; indentation damage, rim face bulging, and tread roll over. High strength and higher carbon content are required for maximum resistance to shelling. On the other hand, thermal cracking is minimized by lowering the carbon content. These two causes of failure, thermal cracking and shelling, call for remedies which are the opposites of each other. For this reason, it is not possible to precisely specify the appropriate class of wheel for the severity of service which develops under various conditions. The following five factors have an important influence on the wheel life: • Static stress in the wheel treads • Maximum train speed • Braking requirements • Track conditions • Design and condition of equipment Bibliography  Wheel Steel Handbook By Rail Safety and Standards Board  The Journal of Rail wheel interaction  KLW Wheelco, Ukraine  The Book of Railway Engineering By PC Gupta , Carriage and Wagon inspector, HQ N. Railway, New Delhi  European Railway Review Prepared By Nishan Singh Rolling stock Technician Date 30/06/2010 Page 9
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