The Fabrication Technology and Project Design of Casting Steel Wheel for Railway Vehicles
Wang Xiaobin, Beijing Central Engineering and Research Incorporation of Iron and Steel Industry (CERIS)

Abstract: The paper briefly introduces the fabrication technology of casting wheel for railway vehicles, and describes the project design.
Keywords: manufacturing technology of railway wheel, project design

1.    Forewords
New technology of casting wheel manufacturing was initiated in the United States for the fabrication of railway locomotives and wheels. The casting wheel fabrication technology consists of two processes: graphite mold pressure pouring and graphite mold pencil gate pouring. The casting wheels made through these two processes now find wide applications in railway trains in service in many countries such as the US, India, South Africa and Canada, etc. The graphite casting wheel technology was introduced to China through cooperation with the US in 1996 as the first casting wheel production line was launched at Datong Locomotive Works. The production technology and engineering design are described below.

    Graphite pressure pouring process
The casting wheels made by GRIFFIN from the US are produced through the process of arc furnace steel-making and graphite mold pressure pouring, with high dimensional precision, good surface quality, few casting defects and high safety, at low costs.
The production process is as follows:
Waste steel is smelted in an arc furnace with a tapping temperature of 1650?C. The molten steel is poured into a special ladle which is hoisted into an airtight buggy ladle and covered. There is a ceramic molten steel ascending tube on the cover which will be inserted into the molten steel as the cover falls. The connection between the ascending tube and the airtight chamber is surrounded by a vacuum insulated device.
Compress air into the airtight chamber to 0.2MPa to push the molten steel into the graphite mold through the ascending tube. The top and bottom parts of the graphite mold which have been heated to 200?C in a tunnel furnace are closed by a crane after leaving the furnace and carried to above the ladle cover, with its bottom hole right onto the mouth of the ascending tube. See Fig. 1 for the flask.






Fig. 1 Airtight graphite mold flask

Every graphite mold is filled with molten steel. The whole pressure casting operation takes about 20 seconds. After the mold is filled with molten steel, reduce the compressed air pressure, let the graphite check plunger core rod at the center of the mold descend to prevent the reflow of molten steel. The flask then immediately tilts and is moved by the crane to the conveyor line to cool for 7-12 minutes dependent upon the wheel size. When the flask cools to approx. 1100?C, remove the cape, and take the wheel blank out of the drag with a jig. The wheel blank is put into the tunnel furnace to cool and leaves the furnace at 540?C. Then remove the graphite check plunger and clear the wheel hub holes upon controlled cooling. Remove the 6 fluid infusion risers with the electric arc and cut through the wheel hub holes with flame. The flame cutting is so precise that the difference between the hole and the finished wheel is only 3mm in diameter. The cleared wheel blank is carried to a circular furnace for heat treatment, either normalizing or quenching as required by the customer, which is followed by tempering. Upon the treatment, remove the oxidized iron skins on the surface of the wheel blank with a shot blasting machine, test the web surface with a magnetic particle flaw detector and the rim with an ultrasonic flaw detector, and measure and check the warping of the inside rim surface. There is only a 0.5mm deviation from the requirement for finished products in each position of the wheels made by the company.
Qualified wheel blanks upon flaw detection will be machined by a boring unit for rim holes. Wheels for cargo trains will not be machined any more after boring, while those for passenger trains and locomotives need full machining.

    Graphite pencil gate pouring
Casting wheels made by ABEX from the US are manufactured through the process of arc furnace smelting and graphite mold pencil gate pouring. They are highly precise in dimensions, easily operated in casting and cost low. The mold flask is shown in Fig. 2.
Fig. 2 Pencil gate graphite mold flask

The production process is as follows:
Waste steel is smelted in an electric furnace with a tapping temperature of 1650?C. Pour the molten steel into a teapot ladle and add in ferro-silicon, ferromanganese and carburant, etc. Then pour the molten steel into a 4t plunger pouring box and add in alloy inoculant before pouring. The 4t plunger pouring box in a bottom pouring type is moved to the working position of pouring by a crane. The graphite mold on the pouring line is moved by a walking beam. The cores inside the graphite mold are: pencil gate core, floating core and central core. The molten steel flows out of the bottom of the plunger pouring box into the graphite mold, and the floating core rises up along with the rising molten steel to allow continuous inflow of the steel so as to prevent shrinkage cavities from being produced when the steel solidifies. In the mean time, the controlled flow of the molten steel also mitigates corrosion to the mold. After one flask is filled up, the graphite mold automatically moves on the chain conveyor line, slowly cooling. Put the wheel blank out of the opened flask into an annealing pit, eliminate the stress and control its cooling. For heat treatment, sprinkle water to quench the tread as required by the customer and then temper the wheel.
Unlike GRIFFIN, the company ABEX has the treads, rims and hubs of the wheel blanks upon tempering and shot blasting to be machined by a copying machine and bored by a boring machine. The machined wheels are on the automatic conveyor and check line for web surface magnetic particle flaw detection, rim ultrasonic flaw detection, warping measurement and check, and hardness check of inside rim surface, and finally for weighing, appearance check, measuring and code printing. They are selectively checked by batches by an X-ray flaw detector.

2.    Brief discussions on the process design of the new casting wheel project
China is a country where railway is a primary way of transportation and largely depended upon for the carriage of cargoes and passengers. For a long time, shortage, poor quality and out-of-date specification of wheels in China were far from the requirements for wheels for railway transportation, in particular high-speed, heavy-axle wheels.
Datong Locomotive Works was one of the 156 large enterprises built up under the supports of the Soviet Union and the largest steam locomotive manufacturer in China at that time. It set up a joint venture with ABC from the US, i.e. Datong ABC Casting Co., Ltd., renovating and constructing the first production line of casting steel wheel for railway vehicles in China on the site of former casting steel workshop in the plant. The line which was efficient in energy use, simple in equipment and low in cost was designed to produce 57,000t casting wheels (about 160,000 pieces) per annum for domestic sales or export.
For this project, a full set of designs was developed based on the special context in China while introducing the pouring technology from the US, that is, the application of the process of smelting waste steel in 5 10t electric furnaces and shaping wheels by direct pouring. Compared to the existing technology for making rolled steel wheels in China, there were no such processes as steel smelting, ingot casting, ingot cutting, forging pressing, etc. The new technology has such advantages as easy operation, energy efficiency, low metal consumption, high precision in dimensions of finished products, high safety and low production cost, etc. As for facilities, fully automated flow production equipment from mold preparation to pouring, heat treatment, machining and finished products inspection was developed and operated under the control of PLC.

2.1.    Process flow chart (see Fig. 3).


Fig. 3 Process flow chart 

2.2.    Wheel casting line 
In the process of wheel casting, to keep the pouring uninterrupted, the ladle needs to be frequently interleaving. Therefore, in the initial design stage, it was planned to import from the US exclusive equipment for the gating system, while in the shop drawing stage, the designer set the processes based on their experiences like this: a semi-gantry crane made in China lifted the bottom plunger ladle filled with molten steel to the casting position as the graphite mold was moving on the walking installation along the automatic conveyor line underneath. After the steel was poured into the graphite mold, the latter automatically moved forwards. The improved process design helped the company save investments. The means of graphite mold pencil gate pouring was adopted. These manufacturing processes and equipment were the pioneers in China at that time.
According to the pencil gate pouring technology, the riser and the gate were located in the same position. The molten steel for feeding was continuously fed in along the feeding path from the riser to achieve the best effect. To fill up a molding flask took approx. 20s, and the pouring operation last approx. 45s. Each plunger ladle full of molten steel can accommodate 8 molds. Upon the casting, the walking bean moved for one working position to allow the next mold right below the plunger ladle. The graphite mold with cast-in steel was moved by an automatic chain conveyor line for cooling.
The graphite mold moved slowly some 100m along the roller bed conveyor line for 35-42mins before the flask was opened. An emergency opener was equipped on the convey line and could open the cope of the mold to disconnect the riser in a timely fashion in case something went wrong in the previous working position and the conveyor line stopped moving. The timing for opening the flask was crucial as the molten steel would neither be solidified too much nor remain in a liquid state; if the steel were too hard, the riser would not be released.
As the graphite mold arrived in the position for flask opening, a special pneumatic flask opener lift up the cope cover, in the mean time, the riser was set loose. The opener put the empty cope back to the conveyor line which then carried it to the clearing preparatory line for cleaning. The wheel blank and the drag kept moving to the position where a special pneumatic wheel taker clamped its edge to take out the wheel blank, at the same time, the drag continued moving back to the cleaning preparatory line for use again together with the cope upon cleaning.
The wheel blank out of the mold was tilted by 180? by a tilter with the inner side upturned. Remove the core sands in the center of the axle hole. Lift the wheel blank with a crane moved by the control room to a slow cooling bucket for stacking and cooling under control. The slow cooling bucket was equipped with a gas heater. The slow cooling temperature fell from approx. 1100?C to 540?C to eliminate the thermal stress created out of uneven temperature drop in different parts of the blank during the cooling. Controlled cooling also helped the air inside the steel diffuse to prevent inside defects from forming due to hydrogen embitterment.
The wheel blank upon slow cooling was taken out of the buckle and stood upright by a wheel erector, and put into a heated wheel shot blasting machine through the roller bed for the sand and oxidized steel skin remaining on the surface to be removed. Upon the cleaning, it was laid horizontally again and sent to the position of the flame cutter by the roller bed for the remaining part of the riser to be cut off.

2.3.    Heat treatment line
The wheel blank upon slow cooling and cleaning was carried by the roller bed to the front of the circular furnace, and beneath a fixed feeder with the walking beam. Then the feeder put it into the circular heating furnace for heating. After the temperature of two rows of one-layer cloth on the inner side of the circular heating furnace rose to 870?C-900?C, a fixed discharger took out the blank. The wheel blank out of the furnace was sent to the position of quenching via the roller bed where it was picked up by a special hoisting device and the circular rotating sprinkler of a cover-type water injection system automatically sprinkled water to quench the tread and the rim. The blank upon quenching was loaded to a chain-type hook conveyor and fed into a furnace for immediate tempering. The tempering furnace was in a tunnel type. The blank upon tempering needed to be stacked and cooled to below 200?C, and then carried to the machining shop.

2.4.    Mold preparation
After the graphite mold returned to the cleaning preparatory line (2 flow lines for each cope and drag), the mold cavity was cleaned firstly. A mold tilter tilted the mold and emptied the sands in the mold, leaving the mold cavity upturned and spraying coating inside the cavity; then turned the mold cavity over and moved it to the position for sand shooting (water glass sand). Upon the sand shooting, carbon dioxide was blown into the cavity to solidify the sand lining. New sands were fed by wind and the remaining old sands upon cleaning were transported by an underground belt conveyor. Upon the solidification, the cavity was upturned for cores to be dropped in, firstly the pencil gate core, then the central core and the floating core.
The coated cope was carried to the position of pouring, where the mold tilter turned it by 180? to face down and a mold closer closed the cope and the drag, ready for casting.
The sand core was made of resin by a core shooter, and then dried in a far-infrared heated core oven for further use.

2.5.    Auxiliary facilities
This project design was difficult due to high requirements for the casting workshop, particularly in the aspect of environmental protection. In designing the previous works, environmental protection measures were sometimes neglected. But for this project, which is a Sino-US joint venture, the design requirements were very strict because of the environment concern of the foreign partner.
2.5.1    Ventilation and dedusting
Fume exhaust and dust removal from the electric furnaces had always been old and major issues. Even the existing dedusting system could not have a satisfying effect. We did a lot work in designing the dust removal for the electric furnaces.
There would be 5 electric furnaces when the production line was completely put into operation; hence the dedusting system for the electric furnaces was a focus of the project design. The old plant building was used as a smelting span, so the initial design was: hole 4 + roof top cover, by which exhaust and dust collection pipes were laid from the smelting operating platform to the ground, then through a 24m material span went out of the building. During the shop drawing design, it was found that the collection pipes were unable to go across underground. After repeated considerations, the design was changed into: lateral suction + clearstory hood, that is, the primary fume from 5 electric furnaces was exhausted from the lateral suction hood, the exhaust and dust collection pipes went from the smelting platform to the roof tops of the plant building, crossing the roof tops and the 24m material span through the truss supports. Each pipe was 1000mm-1600mm in diameter. The secondary fume was exhausted from the renovated clearstory of which the old transom windows were removed and sealed with windshields to allow connection between the exhaust and collection pipes and the clearstory. All pipes merged into a Ф3000mm main pipe at a height of 20m which entered into a 9000m2 duster.
The calculated exhaust and collection air volume was initially 630,000m3/h which was ideal for fume exhaust, but the installed capacity would be very high to increase the running cost during production. Therefore, it was recalculated based on 3 furnaces working simultaneously to result in 480,000m3/h. This figure was proved to be a right choice by experience.
The dedusting design led to such good effects after the system was put into service that almost all fume in the building was exhausted, and not a sign of a smelting workshop could be seen from the outside. The operators working in the building felt no fume. Unlike other steel-making works full of fume and dust, the production environment here was good.
Apart from the dust removal for the electric furnaces, bag houses were also equipped in a number of working positions like graphite machining, tilter, coating spray, etc. and played a satisfying role in cleaning the production environment in the plant buildings.
2.5.2    Gas decoking system
High quality gas was needed for facilities like heat treatment furnaces for this project, especially the circular furnaces and the tempering furnaces, with the required tar content < 40mg/m3. The plant was fuelled by gas-producers, from which the gas contained 360mg/m3 tar. If it were used directly, the automatic control system was unable to be used. In the design, the 70kV high-voltage electrostatic ionized decoking and steam cleaning system was adopted, by which oily waste water discharged from the decoking device flew into a cistern and newly developed cleaning chemicals were added into the purification tank to separate the tar and the water, leaving the clear water for reuse. The decoking system was successful in the first commissioning test run, reducing the tar content in the gas to below 40mg/m3 and meeting the requirement for industrial furnaces.

3.    Conclusions
This project was a highly mechanized and automatic leading production line of casting steel wheels in China. The implementation of the project enabled great changes in the manufacturing of railway wheels and filled the gap in this field in our country.