Jian Sheng Liu

Effect of Nitrogen Content on the Hot Ductility during Hot Deformation of Stainless Steels

Effects of nitrogen content on hot ductility of duplex stainless steels have been investigated. With the increase of nitrogen content in the duplex stainless steels, mechanical strength increased, while hot ductility and elongation decreased. With the same strain rates and deformation degree, the high nitrogen content led to the high optimum hot ductility temperature for the high nitrogen DSS alloy. These results indicated the importance of control over the shape and volume fraction of phases in duplex stainless steels to achieve the optimum hot ductility.

Simulation and Analysis on Microstructure Evolution of Large Generator Retaining Ring during Multi-Fire Forging

In order to investigate microstructure evolution of Mn18Cr18N retaining ring during the multi-fire forging, a series of constitutive equations for dynamic recrystallization, static recrystallization, meta-dynamic recrystallization and grain growth were developed and implemented into a Deform FE simulator. The single-axial hot upsetting test has been performed to investigate the process of microstructure evolution and to show validity and effectiveness of the developed program. Then based on the modified boundary condition, hot forging process for 300MW retaining ring was put into effect. The results have displayed that the microstructure prediction tool was validated by comparing the simulated grain structure with that of the experiment and it could provide a reference to optimize forging processes in the production of retaining ring.

Analysis and Study of Damage of Forging Cracks

This paper dealt with the outstanding problems which were forging cracks existing in the process of forging.The forming reasons of forging cracks were analyzed. From the perspective of damage mechanics, the conventional mesoscopic models and fracture criteria of ductile metal material damage were introduced. The problems in recent research of forging cracks were pointed out. Some corresponding suggestions for the future research of forging carcks were put forward based on the existing problems. The existed ductile fracture criteria have only predicted the damage of specific process yet ,while the diversities of metal forming processes require ductile fracture criterion to have wide scope of application,so,for reason given above,the emphasis is placed on the necessity of establishing new ductile fracture criteria and fracture parameter library with the method of combination of theory and experiment and numerical simulation.

Application of Ductile Fracture Criterion in Hot Forging Damage of Mn18Cr18N Steel

In this paper, the hot forging behavior of Mn18Cr18N steel is studied based on the viewpoint of meso-damage. According to high temperature tensile stress - strain curve of Mn18Cr18N steel under different temperatures, the relationships of void initiation strain and fracture strain with deformation temperature are obtained. The temperature dependence of damage threshold is obtained at 950°C~1200°C for Mn18Cr18N steel by Cockcroft & Latham criterion, That is, Ci,f value increases monotonously with temperature increasing. The corresponding deformation amounts of void initiation and fracture appearing are predicted accurately by combining Cockcroft & Latham criterion with finite element analysis during hot forging of Mn18Cr18N steel, which is in good agreement with the actual production. Under these experimental conditions, once upsetting deformation amount should be controlled in 50% based on hot upsetting test and finite element simulation. These provide theoretical instruction for preventing forging cracking of Mn18Cr18N steel.

Deformation Pattern of the Large Generator’s Retaining Rings during Hydraulic Bulging Strengthening

In accordance with hydraulic bulging strengthening technology for large generator retaining rings (Mn18Cr18N), the bulging deformation process of retaining rings is simulated by MARC software. The key factors such as the punch taper angle and contact pressure are analyzed which influence on the deformation. It has been proofed for carbon steels with small ratio mould .It is observed that adjusting the punch taper angle and controlling the contact pressure theoretically can make the rings deforming uniformly during the hydraulic bulging strengthening processes, and these results provided the credible theoretical bases of technology application.

Research on New Thermal Hole Flanging Process of Connection Tube Forming in Heavy Nuclear Power Thick-Wall Head

The hot hole flanging process of a connection tube in a thick-wall head was researched by finite element analysis and experiment in this paper. The deforming mechanism, main influencing factor and defects in hole flanging were analyzed. According to the stress state and the flow mode of deforming metal, the hole flanging process can be divided into two stages, that is, blending and hole flanging. Processing parameters, such as diameter of pre-holes, space ratio of die cavity, punch shape and size and their effect on forming were researched. The 1:3 scaled experiment was done in 30MN hydraulic press using A508-Ⅲ steel. The formed connection tube had a good quality and its end face was smooth without any crack. The measuring result showed that the error between simulation data and test data was less than 5%, which verified the feasibility of hot hole flanging process and provided an important reference for virtual production. Keywords: Connection Tube Thermal Hole Flanging Thick-wall Head Pro-hole Diameter Space Ratio Scaled Experiment

Study on the Static Recrystallization Behavior of Thermal Deformation of Austenitic Stainless Steel for Nuclear Power

Dual-interval compression test was carried out with Gleeble-1500D thermal simulation testing machine on the 304 austenitic stainless steel used for nuclear power. The first-interval strain was 0.3, and the second 0.05, and the dual-interval strain rates is 0.1s-1, dual-interval temperatures of thermal deformation were 950°C, 1000°C, and 1050°C respectively. The softening behavior with thermal deformation at different temperatures and time intervals was studied. It was shown in the study that the occurrence of static softening was very easy for the material; the activation energy for static recrystallization Qrex was 383.45 kJ / mol in case of true strain of 0.3 and strain rate of 0.1s-1.

Hot Deformation Behavior of 316LN Stainless Steel

Hot-compression experiments of 316LN stainless steel have been conducted on a Gleeble-1500D thermal-mechanical simulator. We have analyzed the flow stress-strain curve and acquired the constitutive equation of 316LN steel by calculating stress exponent, activation energy and Zemer-Hollomon parameter. Then, based on the material model theories and Prasad instability criterion, the iso-efficiency map at strain 0.6 of 316LN steel has been developed. The larger power dissipation rate is emerging at 1050~1200°C and lower strain rate. In addition, we have also analyzed the hot deformation microstructure mechanism of 316LN steel is discontinuous dynamic recrystallization by the observation of deformation microstructure. These results have been of great significance to understand microstructure evolution and to determine the optimum hot-working conditions in the production.

Research on the Intramode Canning Hot Forging Process for Mn18Cr18N Retaining Ring

In order to solve the problems such as cracking, uneven property and poor forming controllability in the traditional hot forging of Mn18Cr18N retaining ring, a new intramode canning hot forging process was developed. In this paper, the hot deformation behavior of the material was invstigated by thermo-mechanical molding tests. Based on the results of the tests, the new process was studied in detail and the main parameters were optimized by means of numerical simulation method. The experiment shows that the process was reasonable for forging Mn18Cr18N retaining ring.

Study of Static Recrystallization Microstructure Evolution of 304 Stainless Steel

The effect of various deformation degree and temperature on static recrystallization of 304 stainless steel during two-passes hot compression deformation with the strain rate of 0.1s-1 was investigated by use of Gleeble-1500D thermo-mechanical simulation. It is indicated that deformation degree is the most obvious factor to static recrystallization. Besides, the grain sizes after deformation were also measured by metallographic method. The results show that the grain sizes decreased dramatically with the conditions of deformation temperature (1050°C), holding time (30s) and total deformation degree (0.35). The result has provides the corresponding scientific basis for the quality forecast of 304 stainless steel during hot thermoplastic deformation.