In order to ensure the heating quality, it is necessary to strictly control the initial forging temperature and terminal forging temperature, and accurately measure the metal billet temperature, especially important metal materials and forgings with high internal performance.
The measurement of metal heating temperature is mainly to measure the temperature of the metal billet during the heating process in the furnace, so as to control the heating process of the billet. However, measuring the temperature of the metal billet in the furnace is more difficult than measuring the furnace temperature. The furnace temperature is always higher than the metal billet temperature. After finding out the law of the two, the temperature measurement of the controllable furnace is often used to replace the metal temperature measurement.
There are many methods to measure the heating temperature of metal billets, such as visual measurement of metal, heating temperature, thermocouple temperature measurement, infrared thermometer temperature measurement, etc.
When the steel is heated to above 530 ℃, it will emit light waves (fire) of different colors due to different temperatures. With the gradual increase of temperature, the color also changes from dark to light or from dark to bright, so the color of billets at different temperatures corresponds to the corresponding temperature of billets.
Visual measurement of metal heating temperature is a common and simple method for measuring metal heating temperature in forging workshops. However, due to the influence of day, night, sunny day, workshop brightness, operator proficiency and other aspects, the visual temperature error is large. Generally speaking, fire is black when measured in a bright place, and relatively bright when measured in a dark place. The visual temperature error of experienced heating workers and forging engineering can reach ± 15 ℃~40 ℃.
The standard color card can also be used to judge the heating temperature of the blank. For medium frequency induction heating furnace, the standard color card is generally hung beside the heating furnace and controlled by the operator.
The advantage of using this method is that for some steel or high tolerance forgings with less strict forging temperature range requirements, visual temperature measurement is very convenient and fast, which can determine whether there is a problem with the thermometer, and can also be used as a supplement to the instrument temperature measurement. The disadvantage is that the error is too large, which does not meet the requirements for measuring the heating temperature of steel billets and precision die forgings; At the same time, it can stimulate the eyes to a certain extent.
The common die forging method can not meet the requirements of the final part shape, dimensional accuracy and surface roughness. The main reasons are: the blank surface oxidation, decarburization and other pollution phenomena under high temperature; Unqualified surface mechanical properties or other defects; The blank volume change and the final forging temperature fluctuation make it difficult to control the forging size; Due to the need of forgings, the side wall of the die is inclined, and the forgings are correspondingly added with dressing; Die wear and dislocation of upper and lower dies lead to dimensional deviation of forgings.
Therefore, all or part of the surface of the forging has or part of the surface to be machined. On these surfaces, metal layers shall be left for machining, which is called machining allowance. All parts requiring machining shall be provided with machining allowance. The size of machining allowance mainly depends on the shape and size of parts, machining accuracy and surface roughness, as well as the heating quality in the forging process, the accuracy of equipment and mold and the level of operation technology. Excessive machining allowance will increase processing time and metal loss; If the machining allowance is insufficient, the scrap rate of forgings will increase.
Segregation refers to the uneven distribution of chemical composition and impurities in steel. Generally speaking, those above the average component are called positive segregation, and those below the average component are called negative segregation. There are still macro segregation, such as regional segregation and micro segregation, such as dendrite segregation and intergranular segregation. Forging segregation is closely related to ingot segregation, and the degree of ingot segregation is related to steel grade, ingot type, smelting quality and pouring conditions. Alloying elements, impurity content and gas in steel all intensify the development of segregation. The larger the steel ingot is, the higher the pouring temperature is, the faster the pouring speed is, and the more serious the segregation is.