Experimental Investigation on Spall Fracture of 30CrMnSiNi2A Steel

Spall fracture is a damage that occurs in a body when two rarefaction waves interact and produce enough dynamic tension breakage,and then the spall zone is founded in the interior of the target.Behavior of materials in the spall phenomenon is strongly affected by the stress level,time of loading and so on.The spall properties of 30CrMnSiNi2A steel loaded by a gas gun were investigated using the planar impact experiment and the two-wave structural profiles of free surface velocities of specimens were recorded by velocity interferometer system for any reflector （VISAR） techniques.The Hugoniot elastic limit （HEL）,spall strength,spall thickness and strain rate were obtained in the plate impact experiments of 30CrMnSiNi2A steel specimens with different thickness.The result indicates that the values of HEL and spall strength increase with the increasing strain rate.The dynamic spall fracture and corresponding microscopic mechanism of 30CrMnSiNi2A at different strain rates were evaluated through the fracture analysis and fractography using a scanning electron microscope （SEM）.

Research on the properties and microstructures of 27Cr-7Ni-4Mo super duplex stainless steel

In this study, metallographic analysis and nanoindentation characterization were used to analyze the properties and microstructures of as-cast nitrogen alloyed 27Cr-7Ni-4Mo super duplex stainless steel （super-DSS）. The as-cast microstructure of the super-DSS was characterized by its ferrite and island-like austenite phases. During the solution annealing process, the austenite volume percentage of the steel decreased gradually with increased annealing temperature. As a main element, the chromium content in the ferritic and austenitic phases elevated slightly at first then decreased with increased annealing temperature. The chromium partition coefficient in the steel varied by around 1.0. The contents of nickel, another main alloy element, also increased in the ferritic and austenitic phases with increased annealing temperature, as did the nickel partition coefficient in the steel, which tended to be close to 1.0. The nanoindentation characterization results indicate that the hardness of the austenite phase is slightly greater than that of the ferrite phase. They were similar to each other within a certain temperature range from 1 050 ℃ to 1 100℃. This temperature range was consistent with the temperature range in which the content ratio of the two phases was close to 1： 1. We found the Young＇ s modulus of the ferrite phase to be greater than that of the austenite phase. With increased annealing temperature, the Young＇ s modulus of the ferrite phase decreased while that of the austenite phase remained almost unchanged.

A New Ductile Fracture Criterion for Various Deformation Conditions Based on Microvoid Model

To accurately predict the occurrence of ductile fracture in metal forming processes, the Gurson-Tvergaard （GT） porous material model with optimized adjustment parameters is adopted to analyze the macroscopic stressstrain response, and a practical void nucleation law is proposed with a few material constants for engineering applications. Mechanical and metallographic analyses of uniaxial tension, torsion and upsetting experiments are performed. According to the character of the metal forming processes, the basic mechanisms of ductile fracture are divided into two modes： tension-type mode and shear-type mode. A unified fracture criterion is proposed for wide applicable range, and the comparison of experimental results with numerical analysis results confirms the validity of the newly proposed ductile fracture criterion based on the GT porous material model.

A multi-parameter model of heat treatment process for soft magnetic materials on performance of HSERs

Abstract:Material properties play an important role in the performance of electromagnetic mechanism. For an aeronautic Hermetically-Sealed Electromagnetic Relay(HSER), more than 50%parts are made of soft magnetic materials. Therefore, the performance of soft magnetic materials directly determines the static and dynamic characteristics of the HSER. Based on the theory of crystal recrystallization, this paper analyzes cold extrusion and heat treatment in the processing of soft magnetic materials, simulates the grain change process of an armature at different heat treatment temperatures, establishes a correlation model of temperature, grain size, and magnetic energy,and verifies results by scanning electron microscopy. Results of heat treatment temperatures from 800 ℃to 920 ℃are obtained and compared. A sample soft magnetic material after heat treatment at different temperatures has the largest difference in the initial magnetization range, up to 22%. In order to verify the fluctuation of the overall output characteristics of an HSER caused by the difference between soft magnetic materials, a static and dynamic analysis model of a typical HSER is established, and the accuracy of the model is verified by a set of actual test system. The difference of dynamic characteristics under different heat treatment temperatures is nearly 3%.