The influence of steel aging period on standard parts

1. What is the limitation period 　　

There will be serious internal stress after steel smelting and rolling, mainly due to uneven carbon content on the surface, carbon segregation, pearlite present in large particles and uneven distribution, coarse grains, and large network of ferrite exist. In these cases, steel cannot be used. It must be stored in a natural environment for a certain period of time to eliminate internal stress. The process of pearlite precipitation of carbon to make the structure uniform and grain refinement is called the aging period.

2 The purpose of steel aging

1. Eliminate segregation and internal stress in the process of steel smelting and rolling. 2. Eliminate the dendrite segregation stress and the uneven distribution of pearlite ferrite in the process of steel smelting and rolling, so that carbon atoms can be better separated and resolved to achieve a balanced state of organization. 3. Eliminate internal stress to strengthen the matrix structure and improve the mechanical properties such as the strength and hardness of the steel. 4. Eliminate internal stress to stabilize the organization and ensure product dimensional tolerances.

Three The impact of the expiration date on the product 　　

If the aging period is not over, the parts will crack during the cold heading and plastic deformation processing. The reason is that the structure is uneven and the grains are coarse, and the coarse pearlite has high hardness and brittleness.　　

After the steel is annealed before the aging period, due to the uneven structure of the coarse grains after annealing, the coarse pearlite will crack during cold heading and cannot be processed. In order to refine the structure and grains, only through normalizing treatment, but the hardness of the steel after normalizing is very high (flaky pearlite) cold heading still cannot be processed. (Guide: The principles and methods of establishing a three-dimensional standard part library based on UG)

The influence of the aging period on the heat treatment and quenching. Due to the coarse structure and the large pearlite grains, the ferrite will inevitably lead to the coarse ferrite. It is known that the austenite will produce coarse grains on the pearlite grain boundaries during heating. Pearlite quickly transforms into austenite, but coarse ferrite is difficult to transform into austenite, because ferrite contains very little carbon. The maximum amount of carbon dissolved at 727°C is 0.0218%. Therefore, the ferrite remains in the form of a net after quenching, and it becomes martensite + net ferrite after quenching. After tempering, the ferrite will not change, and the hardness will be very different on the same sample. For example: GB5787 hexagon flange bolt M6x40, mechanical performance requirement is 8.8, according to GB3098.1-2000, some technical parameters: hardness HRC22-32, minimum guaranteed load is 16100N, observation under the same field of view under a metallographic microscope For the two types of organizations, the hardness values u200bu200bmeasured by the hardness tester (see Figure 1) are very different in heat treatment mechanical properties. The hardness is HRC27-29, and the local HRC18-20. When the minimum guaranteed load is greater than 12000N, it will be broken. For individual products, the hardness of one section is qualified, one section is unqualified, the thread section is qualified, the screw section is not qualified, and the hardness is severely uneven. In this way, it is not enough to reheat the product, only to store it for a period of time and then reheat it after natural aging.

Four aging period of steel 　　

There are different opinions on the issue of steel aging. Most steel manufacturers label it as 15 days, and some say 20 days. There is no standard requirement. In order to find out how long it takes to reach the expiration date, I did the following work: Cut off a section of the φ6.5 plate material 35# produced by Wuhan Iron and Steel Group Corporation 2003-11-29, with the furnace number 9701, for the expiration period inspection. On December 10, there is still four days of metallographic structure before the aging period (based on the 15-day aging period): coarse pearlite + coarse reticular ferrite. As shown in Figure 2, the grain size is two. On December 11, there is still three days of metallographic structure after the aging period (based on the 15-day aging period): coarse pearlite + fine reticular ferrite. As shown in Figure 3, the grain size is three grades. On December 12, there is still two days of metallographic structure after the aging period (15-day aging period): the pearlite transformation is finer, and the ferrite is obviously increased and thicker. As shown in Figure 4, the grain size is five grades. On December 13th, there is one day before the aging period (based on the 15-day aging period). There are more networked and more evenly distributed. As shown in Figure 5, the grain size is locally 5 grades, and the rest are 6 grades. The metallographic structure is close to the aging period on December 14 (based on the 15-day aging period): the pearlite is relatively small, and the ferrite changes from a network to a uniform distribution. . As shown in Figure 6, the metallographic structure of the first day after the expiry date on December 15 (take the 15-day expiry date as the standard): the distribution of pearlite is more uniform, and the distribution of ferrite is also more uniform. As shown in Figure 7, the grain size is 7 grades. The metallographic structure of the second day after the expiration date (subject to the 15-day expiration period) on December 16th. As shown in Figure 8, pearlite + ferrite are evenly distributed.

Five conclusions 　

From the analysis and testing, it is known that the annealing of the steel before the aging period is very harmful. The aging period is set as an ideal organization that does not meet the requirements for 15 days. The recommended aging period is 20 days. 1. After annealing, the pearlite and ferrite are unevenly distributed and the grains are coarse, which lays hidden dangers to the quality of the product. 2. The parts are cracked during the cold heading process. 3. Some parts of the products heated by heat treatment and quenching are not quenched, and there are soft spots. 4. The tensile test fails to meet the national technical parameter requirements.

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