Author: Broytman O.A., Orlova A.M., Savushkin R.A., Bezobrazov Yu.A.
Source: Proceedings of the 18th Int. Wheelset Congress 7-10 November 2016, Chengdu, China
The current operating conditions of wagon wheels involve high loads on the axles and speeds, leading to stringent requirements regarding their mechanical property characteristics and resistance to cyclic stresses. The production method must ensure the absence of critical defects that could cause the wheel body to fail.
The present work investigates the feasibility of manufacturing wagon wheels in the most economically reasonable manner: by pouring metal into a temporary sand mold. This method compares favorably with known practices of producing cast wheels that require special maintenance for permanent and semi-permanent molds made of materials with significant differences in their heat dissipation capabilities.
The production of railcar wheels by steel casting is not as widespread in the world as wheel production by hot deformation. Widespread use of cast railcar wheels is found in the USA, Latin American countries, Canada, China, and India. In European countries, including Russia, national standards specify the use of rolled-steel wheels only.
The hot deformation method for wheel production ensures strain hardening due to achieving a highly dispersed metal structure, density increase, and compensating or removing initial cast structure discontinuities. The steel casting wheels production procedure does not include the above-mentioned operations of material structure and properties improvement. For that reason, high material strength characteristics must be provided with a special chemical composition of the steel, cast structure solidity, and following heat-treatment operations. Steels for rolled wheels are not fully applicable for casting, so there is a need for the development of special steels for the production method.
Available information resources do not contain unquestionable data on fundamental performance advantages of rolled wheels vs. cast wheels [1-4, etc.]. The benefits of cast wheel production consist of a significant decrease in the number of processing operations, which leads to shop cost reduction. The present work investigates the possibilities of cast wheels production from specially developed steels; moreover, the wheels must have such a configuration and mechanical properties combination that may make them acceptable for use in countries of the 1520 mm gauge railway system.
The typical operational cycle of a wheelset in the 1520 mm gauge space presents a significant challenge in developing an easily manufacturable cast wheel part design. The rim of the new wheel must be massive to ensure several numbers of periodical machining operations during the whole wheelset operational period, as the tread surface experiences step-by-step wear. The width of the non-machinable rim of a new USA-type railcar cast wheel part is 35-40 mm, while for the Russian new wheel, the value must initially be 78 mm. As new machining operations are performed for the wheelset, it is acceptable to decrease the rim width up to 22 mm. It seems that non-machinable wheels with initially narrow rim areas are not perspective for the present-day Russian railway market.
The designing process was supported with computational analysis. The stress level in the newly developed wheel was estimated with static strength calculations according to appropriate European norms [5], and the fatigue endurance limit was calculated with the simulation based on the applicable standard method [6]. As a result of the calculations, taking into account the data revealed in [7], the new cast wheel design was developed (Fig. 1) in such a way that it complies with the requirements of the interstate standard [8] which is in force on the territory of Russia.
Figure 1. The configuration and principal dimensions of railcar cast wheel compatible with operational cycle at 1520 mm railway gauge
The developed wheel features a half-moon-shaped disk and a stiffening element in the vital disk-to-rim area. Shape variations for other part areas are strictly limited by the requirements of [8]. The weight of the new cast wheel part must be approximately 410 kg.
A railcar wheel is a simple shape casting; however, the requirements for inner solidity, structural, and material properties uniformity are very tough. One of the main factors that adds complexity to the casting procedure is the need to operate with high-carbon and medium-carbon steels. These steels have a wide solidification range and lowered casting characteristics.
The absolute majority of cast wheels are produced at present days by two technologies [9], each of which involves graphite as a main component of the foundry mold. The so-called ABC technology is based on metal pouring in a graphite mold with a sand lining on the part that forms a disk surface. The sand lining works as a thermal insulator, therefore it is achievable to feed the thick rim through the thin disk. As a result, the solidification unidirectionality is realized for the whole volume of the casting: the rim solidifies first, the disk is next, and the hub is last. Griffin technology provides wheel production by low-pressure die casting, and the die is made of graphite. Hot spots in the disk-to-rim area are compensated with risers. Unidirectional solidification isn’t observed for the whole casting body: the disk solidifies first, then the rim and hub solidify independently.
Technology development for predeveloped wheel production was carried out through computational analysis in PoligonSoft and ProCAST casting simulation software. Calculation results showed that wheels shaped in a way compatible with the Russian market may be hardly produced by conventional technologies. In the case the ABC concept is used, there is a problem of guaranteed shrinkage defects in the disk-to-rim area (Fig. 2, a). A very thick disk only may feed the problem area, but the measure leads to an unacceptable part weight increase. Calculations proved the effectiveness of additional disk heat insulating with a low-conductivity insulator integrated into the sand lining – the measure [10] adds complexity to the technology, however, initially, it was described and applied for a relatively thin rim. Calculations imitating the Griffin technology concept showed that only installing 14 massive heat-insulated risers of 70 mm diameter might feed the problem area in the cast wheel of the desired design (Fig. 2, b).
Figure 2. Temperature-phase fields at the critical moment of wheel casting formation according to computational analysis of ABC (a) and Griffin (b) casting technologies.
Both methods are based on the production of graphite molds, which requires purchasing and appropriate servicing of special machinery equipment, as well as maintenance of the reusable graphite die. Under the conditions of access to a railway castings foundry utilizing vacuum-film (so-called V-process) and no-bake sand molding, it was decided to fix the reachable molding processes as a basis for the casting technology development. The optimal technology solutions discovered and proved by computer modeling were later put into practice and finally improved through appropriate trials.
Castings were placed horizontally in the mold, flange down, to ensure guaranteed good quality for the element. Calculations and practical trials showed that the presence of a small protrusion on the flange doesn't break the rim solidification unidirectionality. Both the thick rim and hub need feeding in case the wheel is produced in sand molds. The disk may tend to non-directional solidification. It was proved that a decrease in pouring temperature is an effective measure for removing the mentioned tendency and preventing disk axial porosity. If strict control of the pouring temperature can't be achieved, configuration features may directly improve solidification directionality. This measure is connected with adding appropriate tapering from left and right sides to the disk center to make the area thinnest (Fig. 3). However, this measure presents some problems. In case the geometry is changed with thinning of the central area, the wheel part becomes weaker, while thickening of right and left disk areas makes the part heavier and may amplify hot spots in the dangerous rim area.
Figure 3. Disk thickness influence on porosity occurring in the event of significant melt overheating and thermal reasons for defects formation.
As a result of calculations and trials, the final technology concept included rim feeding with regularly spaced 10-12 risers in exothermic sleeves. Zones between risers must be chilled from the bottom with steel chills to guarantee porosity prevention. The central sand core installed into the hub makes the element lighter, helps to feed it easier, and decreases the amount of machining work.
Two casting technologies were introduced – bottom pouring of two castings in the mold and top pouring through the ceramic filter installed in the central hub riser surrounded by an exothermic sleeve (Fig. 4). The sleeve allows heating up the riser and decreasing its size, while the filter, which is initially installed in the sleeve bottom, floats up to the liquid free surface after the end of pouring. So the filter ensures laminarization of the liquid metal stream and finally works as additional heat insulation for the riser. The rim feeding system is the same for both technologies.
Figure 4. Casting technologies for the production of wheel pilot batches: a – top pouring through the hub riser; b – bottom pouring of two castings.
The method of pouring through the hub riser showed low compatibility with V-process molds. At the end of pouring, the metal and appropriate mold area form a closed 'pocket' at the top hub region, which caused the failure of the mold at that place. It was proved experimentally that the failure could be prevented by installing vents inclined in such a way as to avoid intercrossing with the hub riser. However, this measure makes the molding procedure too complicated.
For this reason, it was decided to produce the mold for top pouring with ester-cured phenolic sand only, while the molds for bottom pouring were still produced with V-process technology. An additional benefit of the bonded no-bake mold is the improved stability of its surface covered with refractory coating while it interacts with liquid metal for a long period. This fact allows for the stabilization of the wheel castings' surface quality and prevents sand drops on it. A disadvantage of the mold is an increased tendency to gas evolving due to binder gasification, which may lead to appropriate casting defects. This tendency can be controlled through a decrease of binder content in the mix, avoiding over-compacted sand regions with poor gas permeability, and arranging the vent pin holes during the molding procedure.
In terms of automation, no-bake molding for the wheel is more favorable than the V-process because the casting technology requires installing a large number of riser sleeves. Planning of large-scale manufacturing of wheels in sand molds must take into account the prolonged solidification time in comparison with graphite molds: complete solidification of the casting, including risers, takes about 1.5 hrs., and cooling in the mold prior to shake-out needs 3.5 hrs. Top pouring allows a 17% metal economy per one casting in comparison with bottom pouring. The ratio of the mass of the casting without risers and gating system to the liquid metal gross is no less than 75% for top pouring. Considering the totality of advantages, the technology of top pouring through the wheel’s hub riser into a bonded sand mold is considered prioritized.
Figure 5. Experimental cast wheels at different production stages: a – after shot-blasting and removing risers and gating system; b – after rim quenching and final machining; c – solidity checking with liquid penetrant inspection.
As a result of calculative and experimental work, the casting technologies were tuned to a sufficient level which ensures defect-free wheel production (Fig. 5). The manufacturing operations sequence that allows the achievement of good wheel parts from sand mold castings is given in Fig. 6.
Figure 6. Manufacturing operations sequence for the production of pilot batches of cast wheel parts.
The main objectives of the study were the development of a chemical composition of steel for cast wheels and the search for optimal parameters of all the wheel processing steps to achieve the desired level of mechanical properties and performance characteristics. The basic requirements for steels are high strength, including a hardness of at least 320 HB at a distance of 30 mm from the tread surface, high impact strength (up to –60°C), and a low degree of structure imperfections [8].
A marked improvement in mechanical properties and performance characteristics of steels for wheel castings can be reached by the addition of micro-alloying elements. Vanadium helps to reduce the austenite grain size, increase yield strength, ultimate strength, and impact strength; chromium is essential for increasing hardenability and for uniform hardness distribution. The experiments have shown that for the considered steel types, an increase in chromium content by 0.1% leads to an increase in the material strength characteristics by 50 MPa.
Modern chemical composition design concepts for freight cars railway wheels [11-14] were taken into account while developing the steel compositions, which are the following: reduction of carbon content and additional alloying by chromium. This leads to the formation of a bainitic structure, which provides a more stable level of strength and performance characteristics.
Initial search of optimal steel composition was performed by means of computer thermodynamic simulation. The used physically based models allowed calculating the material strength characteristics, depending on the steel chemical composition and cooling conditions. As a result, several experimental steel compositions were developed:
A series of industrial heats of the named steels were produced using an electric arc furnace. For the refinement and improvement of the initial as-cast structure, the addition of alloys containing barium and rare earth metals and argon bubbling of the steel melt was carried out in the ladle. Comprehensive studies of the as-cast structure and the influence of heat treatment conditions on the steels' structure and properties were carried out on the cast wheels with different chemical compositions, alloying conditions, etc.
Figure 7. Panoramic photos of the as-cast microstructure of the rim area of the experimental steels (×50): a – Steel1; b – Steel2; c – Steel3.
Typical results of studies of the as-cast structure (grain size, microporosity, nonmetallic inclusions) obtained for the experimental steels are given in Table 1. Representative photographs of the as-cast structure of these steels are given in Fig. 7.
Table 1. As-cast Structure Characteristics of the Experimental Steels
After the conducted analysis of the experimental data, it can be concluded that the level of non-metallic inclusions of steel meets the requirements of the applicable standard [8]. The castings made from Steel1 and Steel3 show marked microporosity and rough as-cast structure characterized by extensive borders of primary crystallites (4-6 mm and more); at the same time, the castings made from Steel2 did not show such pronounced structure defects. These imperfections of the as-cast structure can be the cause of low mechanical properties, especially impact strength and plasticity [15, 16]. After all the necessary processing steps (Fig. 6), a set of mechanical tests, microstructure analysis of the material, and fracture toughness tests of the wheel rim were carried out on the wheel castings selected from the experimental heats of different steels. Table 2 provides the information on the best mechanical properties achieved on the discussed steels, first of all, by realizing the optimal regimes of rim quenching and subsequent tempering. Fracture toughness tests demonstrated that all the steels meet the applicable requirements [8]: more than 50 MPa∙m1/2.
Table 2. Mechanical Properties of Cast Wheels
Steel2 showed the best combination of strength, plasticity, and toughness (especially at a temperature of -60°C). This fact can be attributed to an optimal combination of the major alloying elements (C, Mn, and Cr), finer as-cast microstructure compared to other steels, and low microporosity level. Fig. 8 shows the typical photographs of the microstructure of the investigated steel in the rim area of cast wheels.
Fig. 8. Photographs of rim microstructure (30 mm from the tread surface) (×500): a – Steel1 (fine perlite, pearlite, and incomplete ferritic network); b – Steel2 (fine perlite and ferritic network); c – Steel3 (fine perlite and rare ferrite inclusions).
As a result of the experiments and taking into account the proximity of the achieved properties to the standard requirements, the chemical composition, melting conditions, alloying, casting, and heat treatment of castings made from the experimental Steel2 were found to be the most appropriate for the manufacture of wheels in sand molds [8].
The result of the present work shows the possibility of producing railcar cast wheels with an acceptable level of solidity utilizing sand molds. The prioritized casting technology for that is based on steel pouring into a no-bake mold through a ceramic filter placed at the hub riser cavity.
The highest and more stable level of strength, plasticity, and impact material properties was achieved with the pouring of medium carbon steel with decreased carbon content, alloyed with chromium and vanadium due to the optimal combination of main alloying elements, fine as-cast structure, and minimal level of structural imperfections. The improvement of the above-mentioned characteristics of mechanical properties of wheel steel may be associated with melt cleanliness control, the development of inoculation and microalloying technologies for obtaining a fine as-cast structure, and heat-treatment regimes, including thermocycling for the refinement of austenite grains.
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