By analyzing the solid solution and precipitation mechanism of nitrogen in F55 duplex stainless steel within the forging temperature range, the forging process is improved to solve the problems of surface cracks and delayed surface cracking in large-sized F55 duplex stainless steel after forging. The nitrogen precipitated along the grain boundary is difficult to re-dissolve in the structure, resulting in the formation of significant internal stress. During the forging deformation process, tensile stress promotes nitrogen precipitation in the microstructure, while compressive stress prevents and promotes nitrogen diffusion into the microstructure. Therefore, the stress state inside the billet should be controlled during the forging process to keep it in a compressive stress state, avoiding excessive non-deformation and micro-deformation zones inside the billet. Using a “V” shaped anvil during forging is an effective measure. At the same time, the method of first small deformation and then large deformation is used to change the internal deformation stress state, effectively avoiding the problem of large tensile stress formed inside when the circular section is forged into a square. When the included angle of the “V” shaped anvil is 135 °, the compressive stress state inside the billet is the best.
What is F55 duplex stainless steel?
F55 duplex stainless steel, as a high nitrogen super duplex stainless steel, is prone to cracking after forging. Much research has been conducted on the cracking mechanism of high-nitrogen stainless steel domestically and internationally. Different viewpoints have been proposed, mainly focusing on the study of various microstructure precipitation and crack formation and expansion mechanisms under stress, such as σ phase, nitride precipitation, or brittle alumina precipitation, under the action of internal stress, form cracks along the precipitated phase. Adequate research has been conducted on the solid nitrogen solution in different temperatures and tissues. Still, there is limited research on the diffusion and enrichment of nitrogen in tissues and the resulting stress concentration and cracking.
The conventional form of domestic F55 duplex stainless steel is a round shaft, and special tools are used to make the forging billet in a state of compressive stress. This process is relatively mature at present, and the quality is stable. However, the forging research of large square F55 duplex stainless steel is not involved, and there is no standardized hot working process. There needs to be more discussion on the nitrogen diffusion mechanism and the causes of cracks in large billets. This article analyzes the reasons for external and internal cracks in the forging of F55 duplex stainless steel square valve bodies through the possible diffusion forms of nitrogen in the structure. The forging process of the blank has been improved and optimized, providing a reference basis for the forging and production of other large nitrogen-containing stainless steels.
1. Existing F55 stainless steel square valve body forging process
1.1 Dimensions and Materials of F55 Stainless Steel Valve Body
Nitrogen in α, the solubility of the phase is relatively low, γ solubility of the phase high, and a certain proportion of nitrogen is added to austenite and duplex stainless steel. Through solid solution treatment, the strength of the stainless steel is strengthened, and the material’s corrosion resistance is improved. The improvement of comprehensive performance has made stainless steel more widely used. During the cooling process, nitrogen precipitates and forms nitrides. When the nitrogen content is high, such as in F55 duplex stainless steel, the nitrogen content is as high as 0.23%. Excessive precipitation of nitrogen can form a free gas state, which is difficult to re-dissolve at high temperatures. Large internal stress occurs inside the stainless steel, causing cracks on the forged surface and delayed cracking after machining, especially in the forging of larger products. Elaborate on optimizing the forging process for large-sized F55 duplex stainless steel.
The size of F55 duplex stainless steel valve body forgings is 820mm × the chemical composition of the 2250mm F55 duplex stainless steel valve body material is shown in Table 1, and its mechanical performance requirements are shown in Table 2.
Table.1 Chemical composition (%, mass fraction) of F55 duplex stainless steel valve body
| C | Si | S | P | Mn | Cr | Ni | Mo | W | Cu | N |
| 0.02 | 0.35 | 0.01 | 0.03 | 0.72 | 24.2 | 6.83 | 3.25 | 0.62 | 0.55 | 0.23 |
Table.2 Mechanical Property Requirements for F55 Duplex Stainless Steel Valve Body
| Parameter | Yield strength ReL/MPa | Tensile strength Rm/MPa | Elongation A/% |
| Numerical Value | ≥550 | ≥750 | ≥45 |
1.2 Existing F55 Duplex Stainless Steel Valve Body Process
The raw material is steel ingot, and the steelmaking process is electric furnace LF + VODC + electroslag, with specifications of Ф 1000mm x 2100mm. The forging process is as follows: the initial forging temperature is 1150 ℃, the final forging temperature is 950 ℃, and the forging heat is 3 times. The forging is carried out on a 60000 kN press. The deformation process is raw material upsetting, stretching, upsetting, stretching, and forming products.
1.3 Quality issues with F55 duplex stainless steel valve body
After forging the F55 duplex stainless steel valve body, cracks appeared on the surface of the blank (Figure 1) but could be removed after mild polishing. The crack depth was ≤ 2mm, but cracks appeared again after processing, which could be removed after mild polishing. This phenomenon occurred repeatedly in different positions. When the product is sawn down to a depth of 50mm, the overall transverse cracking of the blank results in a tearing-like fracture (Figure 2). According to UT inspection, the interior is coarse-grained with a grain size of level 2.
Figure.1 Surface Cracks
Figure.2 Surface transverse cracking fracture surface
2. Forging Quality Analysis of F55 Duplex Stainless Steel Valve Body
2.1 Stress cracking mechanism of F55 duplex stainless steel valve body
F55 duplex stainless steel within the forging temperature range inside the material α、γ variation curve of nitrogen content in the two phases with temperature is shown in Figure 3. Nitrogen in γ content in the phase is greater than its α content of nitrogen in the phase is between 950 and 1150 ℃ γ content in the phase is 1.4% -1.5%, indicating its presence in α 10 times the content in the phase.
Figure.3 Nitrogen in α Xianghe γ Content curve in phase
The nitrogen content in F55 duplex stainless steel is 0.23%, and the proportion of nitrogen in the structural changes within the forging temperature range; But as the forging temperature decreases, the nitrogen α As the content of the phase decreases, nitrogen will α precipitated nitrogen at the grain boundary is difficult to dissolve γ Phase again solidly. When the forging temperature is between 650 and 950 ℃, some nitrogen forms in the form of Cr2N α/γ Precipitation at the phase boundary; When the temperature rises, γ Phase reduction, γ Phase transition to α When in phase, nitrogen increases from high solubility γ Precipitation in the phase also increases α/γ enrichment of nitrogen content in the phase boundary increases the internal stress. In multi heat forging, the temperature repeatedly increases and decreases. When the temperature rises, γ Phase transitions to low nitrogen content α Phase, nitrogen from γ Precipitation in the phase; When cooling, nitrogen is present in the α solubility of the phase decreases from α Precipitation in the phase. Therefore, multiple rounds of forging can cause nitrogen to shift from γ Phase and α Precipitation in the phase. However, accumulating a large amount of free nitrogen precipitated at grain boundaries will ultimately form significant internal stress, making it difficult to effectively dissolve and diffuse nitrogen even through large deformation through the compressive stress state generated by forging. Due to the precipitation and enrichment of nitrogen during the heating and cooling processes of forging, significant internal stress is formed, and shallow surface cracks are directly formed on the surface of the forged billet. The internal stress causes delayed surface cracking after the billet is sawn or processed
2.2 Forging Process Analysis of F55 Duplex Stainless Steel Valve Body
According to the analysis of the stress cracking mechanism of the F55 duplex stainless steel valve body, multiple rounds of forging lead to repeated temperature increases decrease, and nitrogen along the α/γ. The main reason for cracking is the precipitation and enrichment of phase boundaries and the difficulty of effective solid solutions within the forging temperature range after nitrogen precipitation. During the forging deformation process, tensile stress promotes the precipitation of nitrogen in the microstructure, especially in areas with high nitrogen content γ Precipitation in the phase, and compressive stress promotes the precipitation of nitrogen towards α Phase and γ Intraphase diffusion; therefore, during the forging process, the stress state inside the billet should be controlled to keep it in a compressive stress state to avoid excessive nondeformation and micro deformation zones inside the billet. At the same time, rapid cooling should be carried out after forging to avoid Cr2N, Cr23C6, and σ Phase precipitation.
3. Process optimization
3.1 Control of stress state of billets
3.1.1 Forging control
When a circular billet is forged into a square product, there is inevitably a large tensile stress in the center of the circular shaft billet, which promotes nitrogen orientation α/γ. The precipitation and enrichment of phase boundaries form significant internal stresses. A “V” shaped anvil is effective when forming square products from circular billets to avoid forming tensile stress inside the billet. At the same time, the deformation method of first small deformation and then large deformation is used to change the stress state of internal deformation, effectively avoiding the large tensile stress formed inside the billet when forming square products. When the included angle of the “V” shaped anvil is 135 °, the compressive stress state inside the billet is the best.
The original single large anvil surface pressing caused many non-deformation and micro-deformation areas inside the billet. After improving the process, a combination of large anvil surface pressing and small square anvil pressing was adopted, which not only increased the near-surface deformation of the billet but also made the overall deformation of the billet uniform, ensuring that the interior of the billet is always in a compressive stress state during the deformation process.
3.1.2 Temperature control
F55 duplex stainless steel has poor thermal conductivity and a large blank size. To prevent a significant increase in internal temperature due to excessive deformation speed, it is necessary to lower the initial forging temperature from 1150 ℃ to 1120 ℃, while controlling the deformation speed from 3 heats of forging to 9 heats of forging and adjusting the deformation amount from 30% -50% to 20% -25%. The forging temperature increases, and in F55 duplex stainless steel γ Phase reduction, γ Phase transition to α When in phase, nitrogen increases from high solubility γ Precipitation in the phase increases the content of nitrogen-enriched at grain boundaries, increases internal stress, and controlling temperature can effectively reduce the degree of nitrogen enrichment at grain boundaries – rapid cooling after forging to prevent nitrogen, Cr2N, Cr23C6, and σ Phase precipitation.
3.2 Process validation products
3.2.1 Surface and internal quality
After process improvement and the forging of the blank, the product’s surface no longer shows cracks, and there are no delayed cracks after processing. There are no defects detected through PT testing. After UT testing, the internal quality of the product is qualified.
3.2.2 Mechanical performance test
Table 3 shows the mechanical performance test results of the product, which meet the product design requirements.
Table.3 Mechanical Property Test Results of Products
| Parameter | Yield strength ReL/MPa | Tensile strength Rm/MPa | Elongation A/% |
| Numerical Value | 650 | 770 | 67 |
4. Conclusion
- (1) During the forging process of the F55 duplex stainless steel valve body, when the forging temperature increases, the high nitrogen content γ Phase reduces, transitioning to the lower nitrogen content α Phase, nitrogen from γ Precipitation in the phase; When the forging temperature decreases, nitrogen α content of phase decreases from α Precipitation in the phase. Nitrogen continuously precipitates and accumulates along grain boundaries, making it difficult for the precipitated nitrogen to solidly dissolve again α Xianghe γ phase structure forms significant internal stress, leading to surface cracks in the forged billet and delayed surface cracking after processing.
- (2) During the forging deformation process, tensile stress promotes the precipitation of nitrogen in the microstructure, while compressive stress prevents and promotes the precipitation of nitrogen towards the α Phase and γ Phase diffusion; therefore, during the forging process, the interior of the billet should be kept in a compressive stress state to avoid excessive nondeformation and micro deformation zones inside the billet. The forging adopts a “V” shaped anvil and a method of small deformation followed by large deformation, which changes the internal deformation stress state and effectively avoids the large tensile stress formed inside when forging a circular section into a square section. When the included angle of the “V” shaped anvil is 135 °, the internal compressive stress state of the billet is the best.
Author: Sun Changfen