Rst effect of the hottest large free forging

Rst effect of large free forgings

for a long time, the quality problem of some large free forgings, which usually have high requirements on the tensile properties of materials, such as heat shrinkable film and tensile film, is that large-area dense defects appear in ultrasonic flaw detection, and the bottom wave decreases or even disappears completely in serious cases

in view of this kind of internal layered crack type defect, someone has made a theoretical analysis and put forward three reasons that are most likely to produce defects: ① unfixed porosity; ② Inclusion crack; ③ Hydrogen embrittlement or white spots. Then, identify and confirm one by one according to microscopic observation. However, the observations exclude the above three possibilities. After long-term observation and analysis, a new mechanical effect model, RST effect, is proposed

I. definition and production conditions of RST effect

the internal layered crack defect of large round cake and plate free forgings after undergoing a large amount of deformation in the forging process is caused by a special mechanical effect, which is defined as RST effect (rigidslidetearingeffect)

Figure 1 shows the rst effect in the forging process. Its feature is that when the size of the forging tool (anvil, etc.) on the contact surface with the forging in two directions (such as the length and width of the anvil, let's see, upsetting is the diameter) greatly exceeds the height of the blank, the upper and lower rigid areas in the blank meet. Then, under the continuous action of the press force, the layered rigid sliding deformation inside the rigid zone is generated and leads to tearing

(a) the upper and lower rigid areas "meet" during exercise

(b) rst cracks inside the exercise

Figure 1 Schematic diagram of RST effect generation conditions

the rigid area inside the blank is caused by the friction between the tool and the blank surface, also known as "dead zone" or "friction cone". In fact, in general, the metal in the rigid zone is not completely rigid, but a small strain rate distribution with a certain gradient from the symmetrical center of the surface to the interior of the billet. There are no strict regulations on the boundary of the rigid zone. Technicians often use the strain rate

(a) the internal rigid bodies meet during upsetting between plates

(b) the internal rigid bodies meet during flat anvil drawing

(c) the internal rigid bodies meet during tube sheet forming on the upper flat anvil and the lower platform

Figure 2 the rigid bodies inside the blank under different forging methods

, The preconditions leading to RST effect are:

(1) special boundary conditions and tool size conditions, such as friction coefficient μ， Length of anvil L, width of anvil W and height of blank H

(2) after the rigid bodies inside the blank formed under the above deformation conditions meet, continue to apply a certain amount of reduction deformation

(3) the metal inside the rigid body completely loses its elastic and plastic deformation capacity, continues to be forced to deform under pressure, and is forced to undergo layered rigid sliding (or layered brittle sliding) until it exceeds the shear strain strength of the material and tears

Second, the action mechanism of RST effect

RST effect does not act suddenly in an instant. Its mechanism is divided into three development stages according to mechanical properties: ① elastic compression deformation, ② rigid shear deformation, ③ rigid sliding tear

elastic compression deformation stage refers to the process of elastic deformation in the rigid body when the upper and lower rigid bodies inside the blank contact and continue to move opposite under the action of external force. This stage is generally short. After the elastic potential within the contact width of the two rigid bodies is fully released, this part of the material is "compacted", as shown in Figure 3

Figure 3 Schematic diagram of RST effect mechanism in the first stage

(a) "dead zone" is not contacted; (b) Just meet, elastic compression deformation begins; (c) Elastic compression deformation ends

rigid shear deformation stage refers to that when the "compaction zone" in the rigid body in the blank has no elastic compression capacity, it continues to be forced to compress, reducing its height, forcing the metal to have a layered transverse movement, as shown in Figure 4

Figure 4 characteristics of rigid shear deformation stage

Figure 4 also shows the distribution of transverse motion speed on the force center line. Because the blank is affected by the surface friction and related to the shape of the rigid body, the distribution of this velocity presents a certain gradient from the surface to the center. Due to the difference of lateral movement speed between the metal layers (rigid layers) in the "compaction area", it leads to the shear movement between adjacent layers, so it is called the rigid deformation stage

the rigid sliding tearing stage refers to the tearing failure process that begins when the shear deformation between rigid layers reaches a certain limit value (i.e. the shear strength of the material at this time). First, cracks are generated between some rigid layers, and then the sliding tearing is continued to expand until the deformation of the blank under external force is completed. In this process, the metal in the "compaction zone" has actually the characteristics of brittle material like rock. Figure 5 shows the internal characteristics of the blank at this stage

Figure 5 characteristics of rigid sliding tearing stage

in fact, it is not very appropriate to use "rigid" or "brittle" to describe the properties of the metal in the "compaction area" in this process. This is because rigidity means no deformation, but it is not like brittle materials, which fail at an angle of approximately 45 ° during compression

III. process criteria for avoiding RST effect

according to the action mechanism of RST effect, as long as the process parameters during forging are reasonably controlled so that the rigid bodies in the blank do not meet, the defect of "layered cracks" in the forging caused by RST effect can be completely avoided. Figure 6 shows the process parameters (anvil width W0, blank height h) and rigid body dimension parameters (friction angle) during flat anvil forging α、β， The relationship between the height of rigid body h, l)

Figure 6 Relationship between process parameters and rigid body size

generally, when forming tube sheet and forging plate, the size of contact with the blank in the length direction of anvil is always much larger than the blank height h, but as long as the anvil width W is reasonably controlled, the rigid body can be avoided. Considering the situation of deformation symmetry, the geometric parameters have the following relations:

α＝β， H = l (1)

friction coefficient under general high temperature (such as t ≥ 1100 ℃) deformation state μ About 0.37 ~ 0.42. When w/h = 1,

α＝ 33．3°～37.0° h＝l＝0．37～0．42H。

in the actual forming of tube sheet and slab, W is always less than h due to the large height and size of the blank in the initial stage, and w>h only occurs in the later stage of forming. Since the billet surface temperature has decreased (≤ 900 ℃), the friction coefficient will μ According to the calculation of 0.37, when w/h = 1.35, the removal method: turn on the oil pump, and the upper and lower rigid bodies in the billet will meet. In addition, considering the temperature at this time, the reduction deformation should not be too large（ ε H ≤ 15%), and during the pressing process, the W value increases (about 10%) due to the elongation and broadening of the blank. In addition, in order to ensure good internal deformation effect and make the initial w/h ≥ 0.5, the forging forming process criteria to avoid RST effect are specified as:

0.5 ≤ w/h ≤ 1.0, ε H ≤ 15% (3)

in the case of upsetting forming, the diameter height ratio of forgings should be limited to:

d/h ≤ 1.35 or h/d ≥ 0.74 (4)

, where formula (3) is also applicable to the case of asymmetric upper and lower deformation

IV. conclusion

rst effect is mainly related to the height of the rigid body inside the blank due to the influence of surface friction in the forging process. Its development process includes three stages: elastic compression deformation, rigid shear deformation and rigid sliding tear. The harmful effect of RST effect can be effectively avoided by reasonably controlling the anvil width ratio and reduction in the forging process