The ring die is usually made of carbon structural steel, alloy structural steel and stainless steel through forging, cutting, drilling, heat treatment and other processes. The materials used in the ring die and each processing procedure have a direct impact on its service life, granulation quality and output. Carbon structural steel is mainly 45 steel, its heat treatment hardness is generally HRC45~50, its wear resistance and corrosion resistance are poor, and it is basically eliminated; alloy steel mainly has 20CrMnTi material, which is through surface carburization and other surfaces Heat treatment, the processing hardness is above HRC50, and has good comprehensive mechanical properties. The ring die made of such materials has high strength and wear resistance better than 45 steel, but the disadvantage is that the corrosion resistance is not good. Although the cost of a single ring die is relatively low, the production cost per ton of material is higher than that of a stainless steel ring die, which is now being phased out; stainless steel materials are mainly 4Cr13, and these materials have good rigidity and toughness. The heat treatment is overall quenching, and the hardness is greater than HRC50, with good wear resistance and corrosion resistance, long service life, the lowest cost per ton material ring die. The following describes the effect of each processing procedure of 4Cr13 material ring die on its quality: a. Ring die material composition
For the ring mold of 4Cr13 material, the quality source must start from the steel ingot: the chemical composition (mass fraction %) of the 4Cr13 steel ring mold is: C content ≤0.36～0.45, Cr content 12～14, Si content ≤0.60, Mn content ≤0.80 , S content ≤0.03, P content ≤0.035; ring mold with Cr content of about 12% in actual use
The service life of the ring die with 14% Cr content is reduced by more than 1/3 under the same conditions as other treatments; so the source of the ring die quality is from the steel deposit, not only to ensure that the Cr content is more than 13%, It also requires that the size, shape, etc. conform to forging. b. Quality of ring die forging
For 4Cr13 material, the ring die forging process requires suitable heating temperature and time. The common ring die forging problems are:
1). The heating temperature is too high and the time is too long, the forging billet is yellow-white, the estimated temperature exceeds 1200 ℃, at this time the forging billet is prone to surface overburning, the overall overheating, and the coarse grain unevenness phenomenon;
2) Poor furnace temperature uniformity. During the heating process, due to uneven temperature, the forgings are overheated or partially overfired, resulting in coarse grains and poor homogeneity of the forging billet. The larger the forging size, the more serious this phenomenon. It is difficult to eliminate this defect in the subsequent heat treatment process, which is also a common cause of ring die cracking.
3) Due to defects such as segregation, non-metallic inclusions, and looseness in the ingot, it is necessary to improve and eliminate by forging deformation, which is the basis for the forging to achieve good comprehensive mechanical properties. For the forgings with higher longitudinal and transverse performance requirements, a good combined forging process should be used, and it is difficult to ensure quality with a single forging process. 4). The cooling temperature and speed after forging are not well controlled. If the cooling rate is very slow, austenite recrystallization is likely to occur. The grains that have been refined during forging will grow abnormally and cause mixed crystals.
5) Annealing treatment: The hardness of the ring die blank should be controlled. The hardness is preferably between HB170 and 220. If the hardness is too high, the drill bit is likely to break during drilling, and it will cause dead holes. If the hardness is too low, it will affect the die hole The finish. c. The effect of heat treatment on the quality of the ring die
The 4Cr13 material adopts the overall quenching process. Quenching is to heat the metal material or parts above the phase transition temperature. After heat preservation, the Cr compound is dissolved in the structure and is rapidly cooled at a rate greater than the critical cooling rate to obtain the heat treatment of the martensite structure. Craftsmanship. Quenching is to obtain the martensite structure, and after tempering, the workpiece can be used well, so as to give full play to the potential of the material. After making holes, the ring mold should be heated in a vacuum furnace. The ring die after vacuum heat treatment can effectively ensure the smoothness of the die hole (to avoid the oxidation of the die hole). For each part at three equal parts in the circumferential direction, take no less than 3 points to measure the average value of hardness. The difference in the hardness values ??of the parts should not be greater than HRC4.