Research On Fracture And Damage Condition For316LN Steel During Hot Deformation

Nuclear power equipment works under complex stress states at hightemperatures. Due to the particularity of nuclear power equipment, safetyperformance of the device is very important. Hot deformation is one of themain processing ways for key parts of the nuclear power equipment, and it isresponsible for large forgings’ quality and shape. How to design reasonableprocesses and parameters in hot deformation to the quality and safety ofnuclear power construction equipment has widespread concern in the last fewdecades, and this major topic continue. In the hot deformation, ductilefracture occurs at high temperatures in many cases. It needs adequateresearch on steel’s stress-strain relationship and other relevant characteristicsto minimize and avoid thermal cracking phenomenon.Used in nuclear power equipment as structural materials, the316LNaustenitic stainless steel is mainly processed for the FBR nuclear powerequipment and main pipes. Based on the Bonora model of continuousdamage mechanics, this work focuses on316LN’s thermal ductile fracturecharacteristics. A damage evolution equation at high temperatures based onBonora model is proposed. Hot tensile and upsetting experiments weredesigned; the comparison between experiments and numerical simulationsverified the reasonableness of the damage evolution equation. The main workis as following:Enough attention should be paid to the fact that large forgings areproduced at variable temperatures and different strain rates when researchingon the damage evolution law of large forgings’ hot deformation.316LNaustenitic stainless steel tensile tests were carried out at five differenttemperatures and four different stain rates in the Gleeble thermal simulationmachine to get stress-strain curves. The peak stress, deformation activationenergy, the threshold strain, fracture strain, critical damage values were also obtained from experiments based on theories and formulas of continuumdamage mechanics and hot deformation. The quantitative analysis for therelationships of material parameters and temperature, strain rate were alsocarried out.Taking into account the effect of the Zener-Hollomon parameter inplastic deformation under high temperatures, the modified form of Zparameter was used in the establishment of316LN damage evolutionequation. The equations describing the relationships among temperature,strain rate and threshold strain, fracture strain, critical damage were attainedand then damage evolution equation was established which included sixfactors. The combined effects of the threshold strain, fracture stain, strain rate,temperature, and the stress triaxiality were considered.In order to verify the validity and accuracy of316LN thermal crackingdamage evolution equation, the316LN experimental load-stroke curves andthe corresponding simulation results were compared. Due to the fact thatupsetting is the main deformation approach for billet of large size, theupsetting experiments of four sets of80specimens with different shapes anddimensions at uniform/non-uniform temperature distributions were conductedto get critical reductions at fracture. The equation was inputted to the MSC.Marc for the simulations. The results of the experiments and simulationswere compared and verified the accuracy and effectiveness of damageevolution equation in terms of predicting316LN thermal fracture. Thisequation showed a good geometric transferability in shapes. As a conclusion,this equation can provide insight value in designing reasonable processparameters for forging forming.