Stainless Steel Forging

Stainless steel forgings refer to objects in which stainless steel materials are subjected to pressure to shape the required shape or suitable compressive force through plastic deformation. For important parts in machinery with high load and severe working conditions, in addition to rolling plates, profiles or welded parts with simple shapes, forgings are often used to eliminate defects such as loose as-cast metal produced in the smelting process, and optimize the use of forgings. Microstructure, mechanical properties of forgings are generally better than castings of the same material.

Stainless steel forgings refer to objects in which stainless steel materials are subjected to pressure and plastically deformed to shape the required shape or suitable compressive force. This force is typically achieved through the use of a hammer or pressure. Through forging, defects such as as-cast looseness produced in the smelting process can be eliminated, and the microstructure can be optimized. At the same time, due to the preservation of metal streamlines, the mechanical properties of forgings are generally better than those of castings of the same material. The preparation before forging stainless steel forgings includes raw material selection, material calculation, blanking, heating, calculation of deformation force, selection of equipment, and design of molds. Before forging stainless steel forgings, it is necessary to choose a good lubrication method and lubricant. Forging materials involve a wide range. It is well known that the quality of products is often closely related to the quality of raw materials. Therefore, for forging workers, it is necessary to have the necessary material knowledge and be good at selecting the most suitable materials according to the process requirements. Material calculation and blanking is one of the important links to improve the utilization rate of materials and realize the refinement of blanks. Too much material is not only wasteful, but also increases die bore wear and energy consumption. If the blanking does not leave a little margin, it will increase the difficulty of process adjustment and increase the scrap rate. In addition, the quality of the blanking end face also affects the process and the quality of stainless steel forgings. The purpose of heating is to reduce the forging deformation force and improve the metal plasticity. But heating also brings a series of problems, such as oxidation, decarburization, overheating and overburning. Accurate control of the initial forging and final forging temperature has a great influence on the product structure and performance. Flame furnace heating has the advantages of low cost and strong applicability, but the heating time is long, oxidation and decarbonization are easy to occur, and the working conditions need to be continuously improved. Electric induction heating has the advantages of rapid heating and less oxidation, but it has poor adaptability to changes in product shape, size and material. Forging is produced under the action of external force. Therefore, correct calculation of deformation force is the basis for selecting equipment and checking molds. Stress and strain analysis inside the deformed body is also indispensable for optimizing the process and controlling the microstructure and properties of stainless steel forgings.

1. According to the moving method of the billet, forging can be divided into free forging, closed die forging and closed upsetting.

Free forging. Using impact force or pressure to deform the metal between the upper and lower irons to obtain the required forgings, there are two types of manual forging and mechanical forging.

Die forging is divided into open die forging and closed die forging, and can be divided into cold heading, roll forging, radial forging and extrusion, etc.

2. According to the deformation temperature, forging can be divided into hot forging. Between 300 and 800 ℃, it is called warm forging or semi-hot forging.

Forging method After hot working deformation, due to the deformation and recrystallization of stainless steel, the coarse dendrites and columnar grains become equiaxed recrystallized structures with finer grains and uniform size, so that the original segregation, looseness, slag inclusion, etc. Compaction and welding improve mechanical properties. It can ensure the mechanical properties and long service life of the parts. Die forging is used, which is unmatched by castings.

Reasonable use of different forging processing technologies during forging can greatly improve the quality and production efficiency of stainless steel forgings. Precision forging refers to the forming technology that only needs a small amount of processing or no processing to meet the requirements of the dimensional accuracy of the parts after the parts are forged and formed.

There are two main ways to achieve precision forging: refining the blank, that is, directly forging a blank that meets the requirements of precision machining. For stainless steel forgings, the whole or some parts of the parts are directly processed by the precision forging process, thereby reducing the amount of machining.

There are many precision forging processes currently used in production. According to different forming temperatures, it can be divided into warm forging, hot precision forging, cold precision forging and conformal precision forging.

Warm forging is a precision forging technique in which the metal is heated to a suitable temperature below the recrystallization temperature. It has the advantages of hot forging and cold forging at the same time and avoids their defects, effectively reduces the load of the equipment and the die, improves the plasticity and fluidity of the metal, and does not require forging and annealing.

Hot precision forging refers to the precision forging process in which forging is performed above the recrystallization temperature. Because the deformation temperature is high, the deformation resistance of the material during forging is low, and the plasticity is good, so it is easy to form parts with complex geometric shapes.

The cold precision forging process is a precision forging forming technology that is forged at room temperature. Due to room temperature forming and avoiding dimensional errors caused by thermal expansion and contraction, the shape and size of cold precision forged workpieces are easier to control. The forging accuracy of precision forging and warm precision forging is lower than that of cold precision forging.

The compound precision forging process combines the cold, warm and hot forging processes to complete the forging of a workpiece. It can take advantage of the advantages of cold, warm and hot forging and avoid the shortcomings of cold, warm and hot forging. At the same time, the mechanical properties, dimensional accuracy and surface quality of parts produced by composite precision forging technology are improved compared with those produced by single forging technology. At present, the commonly used composite precision forging processes mainly include warm forging-cold finishing, hot forging-cold forging, warm extrusion-cold rolling, warm precision forging-cold extrusion, hot precision forging-cold rolling, etc.

The problem of defects in stainless steel forgings is a complex issue involving multiple disciplines, especially for specific problems. Due to the numerous influencing factors, it is not easy to analyze accurately. The purpose of defect analysis of stainless steel forgings is to clarify the problem, find out the reasons, take corresponding countermeasures, manufacture forgings that meet the requirements of technical standards, meet the requirements of product design and use, and formulate practical measures to prevent similar defects from happening again, constantly Improve the quality of stainless steel forgings.

If the forging process is improper, a series of forging defects may occur. The general forging process consists of the following steps, namely blanking, heating, forming, post-forging cooling, pickling and post-forging heat treatment. The heating process includes furnace loading temperature, heating temperature, heating speed, holding time, furnace gas composition, etc. If heating is improper, such as heating temperature is too high, heating time is too long, it will cause defects such as decarburization, overheating, overburning, etc. For billets with large section size, poor thermal conductivity and low plasticity, if the heating rate is too fast and the holding time is too short, the temperature distribution will be uneven, which will cause thermal stress and cracking of the stainless steel forging billet.

The forging forming process of stainless steel forgings includes deformation method, deformation degree, deformation temperature, deformation speed, stress state, mold and lubrication conditions, etc. If the forming process is not appropriate, it may cause coarse grains, uneven grains, various cracks, etc. . During the cooling process after forging, if the process is not appropriate, cooling cracks, white spots, network carbides, etc. may occur.

Since stainless steel forgings must undergo heat treatment, surface treatment and machining after forging, they can be put into use after they are made into parts, so the quality analysis of stainless steel forgings includes quality analysis of forgings after forging, as well as quality analysis. Problems found during heat treatment, surface treatment, cold working engineering and use of stainless steel forgings. At this time, on the stainless steel forgings or finished parts, there may also be quality problems caused by improper follow-up processes after forging, improper use and maintenance, or improper design and material selection, as well as other influencing factors other than improper forging processes.