Ring Die Processing
Ring die is one of critical parts in pellet mill, which is wearing part and expensive,.Whether ring die’s quality is good and stable will directly have an effect on its service life, output of pellet mill, and quality of granules. Furthermore, it will eventually affect the production costs. The main failure form of ring die is abrasive wear of die hole and inner ring surface, with cracks of ring die and blocking of die hole. The service life is mainly connected with the die material and processing, for the same die material and processing, which also relates to raw materials, technical parameters and operation. The discharging in initial pressure test also relates to surface smoothness of die hole.
At present, common die materials in domestic pellet die are high quality carbon
structure and alloy constructional steel, such as 20#, 35#, 45# steel, 20Cr, 40Cr, 20CrMnTi, 40CrMnMo etc low carbons. Because the cost of stainless steel is high and process is difficult, it is adopted in the specific pelletizing production or custom-made according to customs’ requirements. For the same die material, different process will directly influence the quality and stability of ring die. Therefore, reasonable die process is very important, which is the key to prolonging service life and improving good quality. Die forging blank is supplied by forging plants. A reasonable die process includes forging, rough turning, finish turning, drilling hole, grinding inner bore and heat treatment arranged in the machine processing
Heat treatment process in ring die processing:
There are common heat treatment methods in die material processing, such as
normalizing, tempering, quenching, carburization and nitriding. According to different die materials, the features of heat treatment methods should be taken into account synthetically so that the methods are adopted in the machine process reasonably
Normalizing: With the temperature 30℃～50℃, heat the ring die at Ac1 or above Acm, keep warm for a while(45~90 minutes), then chilling in the air. Normalizing is used for eliminating defects happened in former procedure and internal stress, preparing for next procedure. It is set before rough machining and finish turning or after forging. Ring die through normalizing treatment has improved the cutting performance and surface smoothness.
Tempering: With the temperature 30℃～50℃, heat the ring die at Ac3 or above Ac1, keep warm for a while(45~90 minutes), then quick chilling (quenching or oil quenching), high temperature tempering. It aims to gain general mechanical performance with high strength and toughness, especially to maintain the mechanical performance in the centre of ring die. It is set before finish turning, chambering or after rough machining, preparing for heat treatment. For high quality carbon structure and structural alloy steel, the time interval between quenching and high temperature tempering should not be too long. Otherwise, complicated structure of ring die may lead to heat treatment cracks.
Quenching: With the temperature 30℃～50℃, heat the ring die at Ac3 or above Ac1, keep warm for a while (45~90 minutes), then quick chilling. Common cooling medium is water and oil. If 0.15％～0.30％ of Polyvinyl alcohol is added, the cooling will be between water and oil and then ring die will get better heat-treated structure. In usual, the quenching process is arranged before abrasive machining or after bearizing, it can be the finalized heat-treated process.
Carburization: Carburize the carbon atom in surface of die hole and inner ring to make carbon iron compound organization with higher hardness and tightness, improve the hardness of die hole, surface of inner ring and abrasive resistance, finally, improve the working efficiency. Carburization is mainly aimed at high quality structural steel whose carbon content is 0.15％～0.25％ and low alloy steel, such as 20# steel, 20Cr, 20CrMnti and so on. Time of carburizing is related with the thickness of Carburizing layer, which is measured by experience.
The usual way is solid carburization. The method is embedding ring die in carburizing case which is filled with charcoal and infiltration agent (Na2CO3 or BaCO3). Heat to 920℃～950 ( 930 ℃ ℃ in general)，keep warm for a while（time depends on thickness of carburizing layer, take it out in air cooling, oil cooling or water cooling. There is also gas carburizing. The usual method is stacking the ring die on end in carburizing furnace which is well type and sealed. Pass into carburizing gas or dripping into liquid which is carbonaceous. Keep warm in 930℃ then use air cooling, oil cooling or water cooling, it can be the finalized heat-treated process.
Nitriding: It is finalized heat-treated process and ensures that the heart section’s performance and surface will have nitrogen iron compound which is wearable and tight. Nitriding is commonly used in high quality structural steel and medium carbon alloy steel, such as 45# steel, 40Cr, 40CrMnMo and so on. The hardness of nitriding surface can reach HV1000~1200(equal to HRC65~72) after nitriding. With higher corrosion resistance, ring die can resist the corroding of water and water vapor. But the time of nitriding is too long and the cost is too high, which leads to the price of ring die is a little higher
The usual nitriding has well type furnace gas nitriding and glow ion nitriding. Note: For ring die with small hole (below 4), it is not suitable to use glow ion nitriding. Because the hole is small and the bore diameter is long, it will make glow overlap on die hole surface, the temperature on die hole will be too high and cause local annealing, which will decrease the hardness and abrasion resistance then shorten the lifetime. When using well type furnace gas nitriding, the temperature should not be too high and be kept 500℃～520℃. Otherwise, nitride organization will be coarse and hardness will be de decreased, shortening the service life of ring die.
Process of die hole：
The surface smoothness of die hole will directly influence discharging smoothly when producing the pellets. Generally, using artificial drilling process is difficult to meet the requirements of the smoothness. But the price of imported multi-station drilling machine tool is so expensive and the drill needs to rely on import, the manufacturing costs of ring die will increase. Some factories adopt special purpose machine tool, though meet the requirement, the manufacturing costs will be very high.
Here introduce another way which uses common drilling machine, after improving and with the necessary tools, drill hole semi-automation can be achieved. Then get satisfied finishment on surface and higher production efficiency, decrease manufacturing cost.
The method is to use SBS to control two stepping motors, one of which controls the method of drill entrance, another one controls the corner of ring die to achieve rotating automatically. After processing the die surface smoothness will be 0.8, the quality of products is stable and manufacturing costs will decrease, the degree of automation will be improved greatly. Different programming can change drills of bore and feed movement of milling cutter so as to process the same hole by one step or more. Changing different diameters of drills or milling cutter can process ring die with different diameter’s die hole. Therefore, it can process semi-automated coarse
drill and fine reaming of die hole.
Process of contact end face and ring die inner ring surface：
Because both ends of ring die connect with moving side plate and stock pan
respectively, so contact end face must be rough machined. We can’t use rough turning directly, because the later heat treatment process will result in transformation, which makes installation inconvenience, so it is set before finalized heat treatment or after intermediate heat treatment. As the ring die inner face transforms in heat treatment process, inner face of ring die should be grinding process in order to work steadily.
Grinding process should be set in last machining process. In order to improve
production efficiency of ring die and make sure the surface quality of inner ring, we should design specialized frock head tool.
Other process：
It’s better to use vacuum furnace processing with non-oxidation heating in heat
treatment of ring die. Cooling medium is nitrogen air cooling or oil quenching, so that ring hole surface of ring die will get less oxide layer. It is helpful to discharge smoothly of pellet production. But as a result of vacuum furnace cost is higher and more complicated, small and medium-sized enterprise may not have these equipments and often use ordinary chamber furnace and pit type furnace in general. So in the heat treatment process, the die hole’s surface is easy to form a layer of oxide. The presence of oxide layer will have a direct impact on whether it can discharge smoothly of new ring die initial production, sometimes lead to die hole plugging. Therefore, oxide layer should be cleared, then paint rust-preventative oil.
Common removal oxide method is pressure oil furfural (adding a small amount of emery or grinding sand in chaff). Now introduce a method before pressure oil furfural: Rapid rust removing with hydrochloric acid and water. Hydrochloric acid and water are fully mixed with baume measurement for 13 ~ 15°Bé, then according to the gross and scale (1 ton adds 40~50kg fast rust-removing additive), with ring die being soaked 5 ~ 30 minutes, until the surface oxide layer peeling off, then put it in alkaline (pH > 8) water after washing, then use water cleaning and drying. Before painting anti-rust oil, phosphorus passivation to ring die is benefit to anti-rust of the ring die and can appropriately improve the surface finish and good for pressure test of ring die.
Phosphorus passivation method: put ring die into phosphorus passivation solution, place 20~30 minutes, then wash and dry it!
Process of typical ring die materials：
1. 20# steel, 20Cr, 20CrMnTi
Scheme 1: Board cutting→ Forging→ Normalizing→ Rough turning→ Tempering→Finish turning→ Drill hole→ Carburization→ Grinding→ (Removing oxide layer)→Pressure test→ (Phosphorus passivation)→ Painting anti-rust oil→ Checking and putting in storage
Scheme 2: Board cutting→ Forging→ Rough turning→ Normalizing→ Finish
turning→ Drill hole→ Carburization→ Grinding→ Pressure test→ Painting anti-rust oil→ Checking and putting in storage
2. 45# steel
Scheme 1: Board cutting→ Forging→ Normalizing→ Rough turning→ Tempering→Finish turning→ Drill hole→ Quenching→ Carburization→ Grinding→ (Removing oxide layer)→ Pressure test→ (Phosphorus passivation)→ Pressure test→ Painting anti-rust oil→ Checking and putting in storage
Scheme 2: Board cutting→ Forging→ Rough turning→ Tempering→ Finish
turning→ Drill hole→ Carburization→ Grinding→ Pressure test→ Painting anti-rust oil→ Checking and putting in storage
3. 40Cr, 40CrMnMo
Scheme 1: Board cutting→ Forging→ Rough turning→ Normalizing→ Finish
turning→ Tempering→ Finish turning(end face)→ Drill hole→ Carburization→
Grinding→ Pressure test→ Painting anti-rust oil→ Checking and putting in storage
Scheme 2: Board cutting→ Forging→ Normalizing→ Rough turning→ Tempering→ Finish turning→ Drill hole→ Quenching→ Grinding→ Removing oxide layer→
Pressure test→ (Phosphorus passivation)→ Pressure test→ Painting anti-rust oil→ Checking and putting in storage |
