Forging production of tube sheets
Introduce the process characteristics of the new forging method used in the forging of tube sheets. Forge ∅ 1000 mm with 4.5t steel ingot × after mechanical performance testing and two rounds of flaw detection, grain size and white spot crack detection, the new forging method can effectively remove hydrogen without producing white spots, and qualified forging products can be obtained.
The quality of forgings required for the manufacturing of large and high-performance machinery and equipment in modern heavy industry is becoming increasingly high, such as thermal and hydroelectric power equipment, pipe plate end caps, impellers, and head forgings for pressure vessels in petrochemical industry, which require high technical requirements. In the past, the main problem during the production of tube sheets with a size of ∅ 1000 mm and a height of about 300 mm was that the core defects exceeded the standard. Through anatomical analysis, it was identified as a white spot crack defect. The main reasons for this are: firstly, the high hydrogen content of steel ingots produced by alkaline electric energy smelting and atmospheric pouring is prone to white spot cracks; Secondly, the heat treatment has a short hydrogen removal time and a fast cooling rate; The third is that the segregation in the forging changes from strip like to sheet like defects. Due to poor cooling conditions in the central part, there is more residual austenite that is difficult to remove hydrogen, resulting in cracks and scrapping of the forging. This requires forging workers to study and develop reasonable forging deformation processes, in order to eliminate the porosity and micropores inside the forged steel ingot, refine the coarse grain structure in the cast state, and carbonize the coarse network carbides, resulting in the diffusion and segregation of non-metallic impurities, in order to facilitate hydrogen removal, reduce and avoid waste products with white spot defects, and ensure the internal quality of the forging, in order to meet strict technical requirements and achieve good economic benefits.
1. Determination of forging process plan
After analysis and research, it is determined to use the process of “drawing into a round bar material” to remove hydrogen in the middle and then forged the tube plate.
1.1 Process parameters and process preparation
The previous forging process was as follows: first heat: pressing the jaws, chamfering, removing the bottom, drawing and cutting ∅ 600 mm × 1280 mm, second heat: upsetting to the finished product size before producing the finished product. These two tube sheets are pressure vessel products, and the forgings are inspected and accepted according to JB755-85IV level. The chemical composition is controlled for residual elements Cr and Ni, ultrasonic testing is conducted, and mechanical properties, grain size, non-metallic inclusions, finished product analysis and other inspection items are carried out. The technical requirements are strict. Due to the limited quantity, only electric furnace smelting and atmospheric pouring of steel ingots can be used. In order to ensure quality, we have adopted some measures in the forging deformation process and heat treatment process based on past experience and lessons learned in the production of tube sheets.
Forging process: The tube plate material is a typical pressure vessel material of 15CrMo, using 4.5t steel ingots with a total forging ratio of 6.45. The detailed process is as follows: the first heat: pressing the jaw, chamfering, and removing the bottom; the second heat: upsetting, H=700- ∅ 900 mm, elongation and cutting ∅ 600 mm × 1280 mm, hot delivery heat treatment is used for intermediate dehydrogenation treatment, and the dehydrogenation curve is shown in Figure 1. The forging ratio is 2.25, and the third heat: the billet is reheated and roughened to produce the finished product, as shown in the size diagram of the finished product (Figure 2). The forging ratio is 4.2, and after forging, the heat treatment curve is shown in Figure 3.
The selected forging deformation equipment is a 1250t hydraulic press, and the deformation temperature of the forging billet is 1200-800/750 ℃. In order to ensure the quality of the forging, we have determined the characteristics of the forging deformation process:
Figure.1 Dimensions of Finished Forgings
Figure.2 Intermediate Dehydrogenation Curve
(1) The forging ratio per heat is greater than 2.
(2) By using the form of upsetting, drawing, and forming round rods for cutting, the as-cast structure is fully broken to become the forged structure.
(3) Convert the round bar forging blank to heat treatment for one intermediate dehydrogenation treatment.
(4) Reheat and upset to the finished product size.
Even if there is segregation white spot crack defect during the first dehydrogenation, the defect can still be welded during the final forging, because the last forging ratio was 4.2. We have conducted experimental research on the forging of cake shaped parts and impeller white spots, and as long as the forging ratio is greater than 2, the white spot defect can be forged.
(5) Continue to undergo positive tempering treatment after forging.
Figure.3 Heat Treatment Curve
1.2 Actual Process Execution
The alkaline electric furnace is used to smelt atmospheric cast steel ingots, which are sent to the hot water press plant. The chemical composition is qualified, and the residual elements do not exceed the standard. The forgings are strictly implemented in accordance with the process regulations. The two tube plate products forged through this deformation process have been inspected to meet the appearance dimensions and quality of the forgings, and both have been rated as first-class qualified forgings.
1.3 Final heat treatment and quality inspection of machining
After inspection, the forgings were qualified and sent to the machining workshop for rough machining. The first ultrasonic inspection was conducted, which met the standards. Then, the final quenching and tempering treatment was carried out. After treatment, tangential specimens were cut for mechanical performance inspection, grain size, non-metallic inclusion inspection, and hardness inspection all fully met the technical requirements. Then, a second ultrasonic inspection was conducted before leaving the factory, and no exceeding standard defects were found.
2. Analysis and Discussion
When our factory produces medium-sized tube sheets, in order to ensure the quality of the tube sheets and reduce the occurrence of white spot defects, the following methods are adopted:
- (1) Alkaline electric furnace smelting and atmospheric pouring, using methods such as long heat treatment and hydrogen removal time for forgings, can affect the production cycle and waste energy.
- (2) Alkaline electric furnace smelting, vacuum casting, and forging, although the heat treatment and dehydrogenation time are relatively short, the quality is also unstable, and this method is only used when large batches are needed.
- (3) Steel ingots produced by refining furnace smelting and atmospheric pouring have a high hydrogen content, short heat treatment and tempering time for hydrogen removal, and still exhibit white spot cracks. This type of piece can only be used in large quantities and is expensive.
- (4) Open hearth smelting LB4 pouring adopts two methods: intermediate heat treatment cooling or non cooling during forging. Although the yield in forging is higher than the above two methods, it can only be used in large batches.
We adopt a process plan of alkaline electric furnace smelting, atmospheric pouring, and adding a hot material dehydrogenation treatment in the middle of forging deformation for small batch of medium tube sheets.
This process method has been successfully applied for the first time in our factory. We believe that the cooling conditions for drawing billets into round bars are good, the transformation of austenite into pearlite is easy, and the residual austenite is less prone to diffusion. Even if white spots are generated, the final forging ratio is greater than 2, and it can still be forged. However, directly forging into a cake shaped forging results in more residual austenite in the central part due to poor cooling conditions. Not easy to diffuse, it is easy to produce white spot defects during cooling, resulting in scrapping of forgings.
It is feasible to adopt some process schemes for forging the middle tube sheet, which can ensure quality without generating waste, reduce heat treatment cycle, save energy, and be economically reasonable.
Author: Wang Shouhua