The thesis is dedicated to investigation of peculiarities of formation of welded joints of elements of railway frogs from steel 110G13L with rail steel M76, made through an intermediate insert and without it using flash butt welding (FBW).
The work studies peculiarities of thermal cycles in FBW in M76, 08Kh18N10T and 110G13L steels using mathematical modeling of the process. Based on received thermal cycles, effect of residual stresses in welded joints of austenite and pearlite steels produced by FBW was investigated.
It is shown that increase of mechanical properties of welded joints requires application of intermediate chrome-nickel austenite insert, which has a yield limit of 200-220 MPa at high indices of ductility (percent elongation over 40%).
The most favorable order of joint welding, from point of view of mechanical properties of the part, was selected for welding of frog core with rail end through the insert. The insert (first joint) is welded to the rail end and then cut to necessary length. High manganese core (second joint) is welded to the insert. At insert width of 10-30 mm, re-heating of heat affected zone (HAZ) of steel M76 does not result in detrimental transformation of sorbite structure in contrast to HAZ of steel 110G13L, which in re-heating will lead to additional carbide formation and, as a result, decrease of plastic properties of the welded joints.
Investigations of stresses state of the welded joints showed that rolling surface (over rail head) and flange include compressive stresses that has favorable effect on structure fatigue life for one type of the joint as well as another. Longitudinal residual stresses for all combinations of austenite and pearlite steels being welded are virtually the same, so they are determined by non-uniformity of cooling of rail profile.
The optimum modes were developed for FBW of steels M76+08Kh18N10T and 08Kh18N10T+110G13L, which provide stable results that correspond to the requirements of Ukrainian and European standards as for welded joint quality.
The optimum size of the intermediate insert and method of its production were determined. In process of operation of the welded frogs there is a need to optimize the size of insert taking into account accumulated experience of their operation at Ukrainian railways. The following reasons effect the selection of the optimum width of the insert, namely increased wear-out of the insert at product operation as the softest element of the frog; effect of insert ductility on the results of mechanical tests of the samples. In order to determine the effect of insert width on value of frogs’ wear-out in process of operation, the examination was carried out for frogs of various type laid at different time under diverse amount of passed cargo from pilot batch produced at PJSC “Driptopetrovsk railway switch plant” that are under author’s control of the plant.
It is determined that in process of operation of the welded frog there is equalization of hardness on the rolling surface due to hardening of high-manganese core and chromium-nickel insert. Deviation from linearity of the frog rolling surface in the insert zone appears at initial stage of operation. It is proved by measurements carried for the same frog after passing cargo of different tonnage.
Carried metallographic examinations showed that insert width within 10-30 mm does not have significant effect on structural transformations in re-heating of M76+08Kh18N10T joint in welding of the second joint through the insert.
It is determined that width of the insert within 10-30 mm does not have significant effect on service properties of the welded frogs. In process of operation of the welded frog there is hardness equalizing on the rolling surface due to hardening of steel 110G13L and insert. Local wear-out of the frogs (0.5-0.6 mm) is formed at initial stage of operation and does not grow further on.
It is proposed to use a cast intermediate insert with reduced content of titanium instead of rolling one at production of the welded iron frogs.
It is determined that in application the cast intermediate insert the fracture force and deflection are 1.15, and 1.25 times more than in use of rolling one and 1.4 and 2.4 tomes more than without application of the intermediate insert.
Effect of the casting defects (pores, cavities), appearing as a result of their coming into HAZ, on strength properties of welded joint and their determination by radiography testing was determined. Entering of the casting defect (cavity or pore with inclusions) in the welded joint zone can provoke two variants of formation of welding defect, i.e. the first one, when defect size allow squeezing out slag inclusions in form of melt to the pore periphery, than melting zone is formed from the melt, in which no slag inclusion are present.
It is determined that radiography testing of the ends of castings of the frog cores, which are subjected to flash butt welding, allows detection of the cast detects
