Breaking Resistance of Forging and Heating Method

Heat Treatment Overview

Heat treatment is a technology that changes the physical and chemical properties of metal materials through heating and cooling processes. Its purpose is to improve the hardness, strength, toughness and other mechanical properties of materials to meet specific engineering requirements. Heat treatment usually consists of three basic steps of heating, holding and cooling, and the specific process and conditions depend on the type of material being treated and the final performance requirements.

In the heat treatment process, the metal material will go through different stages, including heating to a specific temperature range (such as annealing, normalizing, quenching, etc.), during which the crystal structure of the material will change. For example, in the annealing process, the metal is heated to a high temperature and held there for a period of time, and then cooled slowly, thereby reducing internal stress and increasing toughness; In the quenching process, the metal is cooled rapidly at high temperatures, usually using water or oil, to obtain higher hardness and strength, but this can also bring stress and brittleness problems.

Heat Treatment of Forgings and Roll Performance

The matrix structure, hardness and mechanical properties of forgings can be adjusted by heat treatment. However, due to its ultra-high carbon material, forgings are prone to quenching cracking during heat treatment, especially compared with forged steel rolls, which have a greater tendency to quenching, so it is difficult to withstand too intense quenching process. We believe that from the point of view of use conditions, such rolls do not rely on the hardened matrix, but mainly rely on a large number of carbides to ensure its wear resistance at high temperatures. If martensite or bainite matrix is formed after quenching, its thermal cracking resistance is usually poor. Therefore, it is suggested that the matrix of this kind of roll should be selected after high temperature tempering pearlescent structure.

Test Method for Sulfur Prints

Sulfur print test is a macroscopic method to check the distribution of sulfur in steel. With the crystallization of molten steel and the process of gas precipitation, a variety of different substances produce segregation as a result of separation crystallization. In general, the larger the ingot, the more serious the segregation phenomenon, which will have varying degrees of impact on the performance and quality of the steel. The basic principle of sulfur print test is that sulfur exists in the form of sulfide in steel, reacts with dilute sulfuric acid to generate hydrogen sulfide gas, and then reacts with silver bromide on the photo paper to form silver sulfide precipitation. The sediment is a dark brown color, which will show up on the photographic paper.

When the sulfur content of steel is high, the dark brown silver sulfide traces on the paper are larger and darker. On the contrary, when the content is small, the traces are smaller and the color is lighter. By analyzing the size, color depth and distribution of the brown marks on the forging paper, the size and content of sulfide in the steel can be reflected, so as to make a preliminary evaluation of the quality of the forging, and qualitatively judge the degree of segregation of sulfur in the steel according to the inspection standard.

Final Heat Treatment Process

Final heat treatment is performed after rough machining and rough opening. Practical final heat treatment processes include normal tempering and spray quenching + tempering. The heating temperature of normalizing is 850-930℃, and when cooling in the still air, it needs to be spread out on the pad iron, and it can also be accelerated by spraying air. After normalizing, tempering at a high temperature of 500-650℃. At this time, the structure of the forging is lamellar pearlite, bulk and strip primary eutectic carbide and granular secondary carbide.

The normal tempering process is mainly used for forging semi-steel rolls. After this process, the surface hardness of the roll is 38-42, and the strength can reach the level of 850 MPa. The normal tempering process is easy to operate, the quenching stress is small, and can meet the hardness requirements of general rolls. However, this process may lead to the formation of partial network carbides, especially at higher normalizing temperatures, which is more serious. Although these network carbides are relatively small and not completely continuous, they will still damage the strength and toughness of the roll, and reduce the ability of the roll to break.

Sum up

In summary, the heat treatment of forgings needs to be carefully adjusted to avoid quenching cracking and optimize performance. The sulfur print test is an important means to evaluate the quality and sulfur distribution of the steel, and the final heat treatment process is a key step to ensure the performance of the roll. Through these measures, the quality and performance of forging and rolling can be effectively improved to meet the needs of different conditions of use.