Abstract: In order to reduce the forging allowance as much as possible, the method of selecting the forging allowance is studied, the purpose is to save the metal material and reduce the machining time, so as to reduce the cost of forging. According to statistics, metal materials account for about 40% of the price of forgings, and some alloy steels even reach more than 50%. Therefore, saving metal materials plays an important role in reducing the cost of forgings. In line with this principle, the forging branch of a metallurgical plant in Tangshan has made a technical innovation attempt on forging production, and achieved good results.
How to choose the forging allowance, in the forging allowance and tolerance standards have instructions, generally according to its instructions to choose, but the specific choice of forging allowance, is very worthy of study and discussion. There are generally two types of forging allowance selection, one is for forgings without rough machining requirements, which is to simplify the part diagram and add the standard forging allowance; The other is for forgings requiring rough machining and quenching, which is to simplify the rough machining diagram and add the standard forging allowance. The choice of the latter margin is mainly studied and discussed here. The purpose of the margin selection is to reduce the forging margin as much as possible, save metal materials and reduce machining hours, thereby reducing the cost of forgings. According to statistics, the cost of metal materials accounts for about 40% of the cost of forgings, and alloy steel even reaches more than 50%. Therefore, saving metal materials plays an important role in reducing the cost of forgings.
In order to save metal materials, reduce the cost of forgings and improve economic efficiency, the forging industry has studied many forging methods to save metal materials in mass production, such as precision forging after less oxidation-free heating, no flash die forging, warm forging or cold forging and other process methods. However, for a single piece of small batch production, the method of saving metal materials is very little research, so the current single piece of small batch production of forgings still has the phenomenon of "fat head big ears", resulting in a great waste of metal materials and machining hours.
For many years, when the forging industry is preparing the forging process, for some forgings requiring rough machining and quenched and tempered, the standard forging allowance is added to the rough machining allowance in accordance with the provisions of the forging allowance and tolerance standard, which forms a double allowance. In addition, the ideology of "big insurance" of the operators forms a big increase. In this way, not only the waste of metal materials, but also greatly increased the machining hours, resulting in multiple waste between processes.
With the development of industrial technology, the requirements for product quality are also increasing. Therefore, more and more forgings are required to be tempered after rough machining, and the proportion of product forgings is relatively increasing. Therefore, it is of great significance to study the reduction of machining allowance for such forgings.
In order to solve this contradiction, the margin composition of all kinds of forgings and the relationship between the structure and deformation of all kinds of forgings are analyzed and studied. It is believed that it is possible and necessary to reduce a certain amount of machining allowance from the process for quenched and tempered forgings with finishing requirements. In order to reduce the margin without affecting the quality of forgings, try to make different provisions for all kinds of forgings, after more than a year of practice is proved to be effective, this method is called "small margin forging method".
2Small allowance forging method
The general principle of the "small margin forging" method is: when the small margin forging is selected, after the size of the forging reaches the original deviation, the actual size of the forging should be 1mm ~ 2mm larger than the size required for rough machining to ensure the rough machining requirements, and local black skin is allowed after rough machining (the margin at the black skin should ensure the finishing amount). The selection method of forging allowance for various forgings is as follows.
2.1 round shaft, square shaft, hexagonal shaft and flat square shaft forgings
The ratio of length to diameter (or side length) of this kind of forging is closely related to the deformation after heat treatment. Therefore, the specific method of selecting forging allowance is specified according to the ratio of length to diameter (side length):(1) When the ratio of length to diameter (or side length) of the part is 5, the standard forging allowance can be directly selected on the basis of the size of the part drawing, but it should conform to the general principle;(2) When the ratio of part length to diameter (or side length) is 5~10, the forging allowance shall be 80% ~ 90% of the standard forging allowance based on the rough machining pattern size;(3) When the ratio of part length to diameter (or side length) is greater than 10, the forging allowance shall be 90% ~ 95% of the standard forging allowance based on the rough machining pattern size.
If the steps of the shaft parts are very small, when forging is simplified to a round (square) shaft, the forging allowance can be selected according to the length of the remaining block. If the remaining pieces of forgings are very short, the forging allowance shall be determined according to the ratio of length to diameter (or side length). If the remaining piece of the forging is very long, the standard forging allowance can be selected according to the simplified size, instead of roughing the allowance.
In the length direction of the shaft forgings, in the preparation of the forging process, has increased the forging allowance, such as the hammer on the free forging of the shaft forgings, the length of the margin is 2 times the diameter direction, the hydraulic press free forging is 3~4 times. In addition, the end of the shaft forging will not be deformed due to heat treatment resulting in insufficient machining allowance, so there is no need to roughen the machining allowance in the length direction.
The allowance of such forgings shall be selected according to the specific situation after simplification. When there is no remaining piece in the simplified forging, the forging allowance can be selected according to its length-to-diameter ratio, and the calculation method of its length-to-diameter ratio should be based on the ratio of the minimum diameter of rough machining to the length. When the simplified forging has more than one block, the forging allowance should be determined according to the length of the remaining block. When the remaining block is longer, the standard can be selected on the basis of the simplified part drawing size.
Forging allowance; If the remaining block is very short, the forging allowance should be selected according to the size of the length-diameter ratio.
The allowance in the length direction of such forgings is the same as that of circular shaft forgings, and there is no need to roughen the allowance, but the standard forging allowance can be directly added to the length of the part drawing.
2.3 Hammer, Module, Short Cylinder, Shoulder Flange Forgings
This kind of forgings are chunky forgings, heat treatment deformation is very small, therefore, the provisions of this kind of forgings can be added on the basis of the size of the parts of the standard forging allowance, without having to add another rough machining allowance.
2.4 cake, perforated cake and plate forgings
The size of the deformation of such forgings during heat treatment is directly related to the ratio of its diameter (or side length) to its thickness. Therefore, the size of the forging allowance should be determined according to the ratio: when the ratio is less than 5, the standard forging allowance should be selected on the basis of the size of the part drawing; when the ratio is 5~10, 80% ~ 90% of the standard forging allowance shall be selected on the basis of the rough machining drawing size; when the ratio is> 10, 90% ~ 95% of the standard forging allowance shall be selected on the basis of the rough machining drawing size.
This kind of forgings in the diameter direction or length direction allows not to consider the rough machining allowance, only plus forging allowance can be.
The deformation of ring forgings during heat treatment is related to their wall thickness, and with the increase of the outer diameter of the forgings, the difficulty of forging production is also large, based on these conditions, the selection of its forging allowance is also made specific.
Assumption and regulations: when the outer diameter of the forging is ≤ 400mm and the inner diameter is ≤ 200mm, the standard forging allowance is allowed to be selected on the basis of the part drawing size; When the outer diameter of the forging is ≥ 400mm and the wall thickness is ≥ 200mm, 80% ~ 90% of the standard forging allowance shall be selected on the basis of the rough machining drawing size; When the outer diameter of the forging is ≥ 1000mm, the selection of the forging allowance can be determined by the process personnel according to the size of the wall thickness as appropriate.
The forging allowance for the outer diameter of cylindrical forgings can be selected according to the simplified shape with reference to the method of shaft or step forgings. The forging allowance of the inner hole is not specified for the time being. The margin at both ends can be added in coarse
On the basis of the size of the work drawing, 80% ~ 90% of the standard forging allowance is selected.
Refer to the forging allowance of shaft forgings, which is specified at the crank and journal (or other cylindrical part). Under normal circumstances, the forging allowance of the crank is relatively large, and the journal can be set by increasing the allowance of the multi-step shaft by 50%. Because the journal is a small part of the crankshaft, it is easy to deform here. Therefore, Different regulations have been made for the crank and the journal. For the forging allowance at the crank, the standard forging allowance can be selected on the basis of the simplified size of the part, while for the journal (or other cylindrical part), the standard forging allowance should be selected on the basis of the rough machining drawing size. The length direction of this type of forging shall be based on the size of the part drawing and the standard forging allowance shall be selected.
In line with the above principles and regulations, the forging production of a metallurgical plant in Tangshan forging branch of the technical innovation attempt, and achieved good results. After more than a year of production practice, it has been proved that this process not only ensures the quality of forgings, but also saves a lot of metal materials and machining man-hours, and achieves good economic benefits. According to statistics, as far as metal materials are concerned, in 500t forgings requiring rough machining and quenched and tempered, 36t of metal materials (including carbon steel and alloy steel) are saved, with a saving rate of 13.7 and a material saving value of nearly 200,000 yuan. It can be seen that the principles and provisions described in this paper are effective and need to be further explored, practiced and promoted.
