Deformation Of 316L Stainless Steel Research Articles

The effect of thin surface layer of nitrogen-expanded austenite on bulk γ-α’ phase transformation in low-temperature deformation of 316L stainless steel

Deformation-induced γ-α’ phase transformation was studied in austenitic stainless 316L steel in low-temperature deformation regime (tension at 77 K). Before deformation, the surface of the steel specimens was saturated with nitrogen by ion-plasma nitriding at 673 K for 12 h. Thin 4-μm surface layer of nitrogen-expanded austenite (S-phase) does not influence mechanical properties but affects phase transformation in the bulk 1.5 mm-thick specimens. Surface nitriding effectively suppresses the inhomogeneity in deformation-induced martensite nucleation and formation of the α’-phase prior the macroscopic neck formation.

Influence of Compositionally Graded Interface on Microstructure and Compressive Deformation of 316L Stainless Steel to Al12Si Aluminum Alloy Bimetallic Structures.

Bimetallic structures offer properties that can be customized based on applications, manufactured in one operation. Such manufacturing options are fascinating, as joining two metallic materials, especially for two dissimilar metals without significant defects, is challenging. In this study, 316L stainless steel (SS) to Al12Si aluminum alloy structures were processed, tailoring the compositionally graded interface on a SS 316 substrate using a directed energy deposition (DED)-based additive manufacturing (AM) process. Applying such a compositionally graded transition for joining two dissimilar metals could mitigate the mismatch of mechanical and thermal properties. This study's objective was to understand the processing parameters that influence the properties of AM processed SS 316L to Al12Si bimetallic structures. Two different approaches were used to fabricate these bimetallic structures. The results showed no visible defects on the as-fabricated samples using four layers of Al-rich mixed composition as the transition section. The microstructural characterization showed a unique morphology in each section. The microstructural variation was caused due to various processing parameters such as laser power, powder feed rate, and laser scan speed. FeAl, Fe2Al5, and FeAl3 intermetallic phases were formed at the compositionally graded transition section. After stress relief heat treatment of the bimetallic samples, diffused intermetallic phases were seen at the compositionally graded transition. At the interface, as processed bimetallic structures had a microhardness value of 834.2 ± 107.1 HV0.1, which was a result of the FeAl3 phase at the compositionally graded transition area. After heat treatment, the microhardness value reduced to 578.7 ± 154.1 HV0.1 because of a more Fe-dominated FexAly phase formation. Compression test results showed that the non-HT and HT SS 316L/Al12Si bimetallic structures had a similar maximum compressive strength of 299.4 ± 22.1 MPa and 270.1 ± 27.1 MPa, respectively. These results demonstrated that the applied heat treatment conditions only had a minor impact on samples' compression strength.

A Comparison Study of Constitutive Equation, Neural Networks, and Support Vector Regression for Modeling Hot Deformation of 316L Stainless Steel.

In this research, hot deformation experiments of 316L stainless steel were carried out at a temperature range of 800–1000 °C and strain rate of 2 × 10−3–2 × 10−1. The flow stress behavior of 316L stainless steel was found to be highly dependent on the strain rate and temperature. After the experimental study, the flow stress was modeled using the Arrhenius-type constitutive equation, a neural network approach, and the support vector regression algorithm. The present research mainly focused on a comparative study of three algorithms for modeling the characteristics of hot deformation. The results indicated that the neural network approach and the support vector regression algorithm could be used to model the flow stress better than the approach of the Arrhenius-type equation. The modeling efficiency of the support vector regression algorithm was also found to be more efficient than the algorithm for neural networks.

Cyclic deformation of 316L stainless steel and constitutive modeling under non-proportional variable loading path

In order to investigate the effect of non-proportional loading history on cyclic deformation of 316L stainless steel, a series of multiaxial cyclic experiments with variable loading path are conducted at room temperature. It is found that the materials exhibit cyclic hardening at initial stage, followed by a long period of cyclic softening. Significant cyclic softening is observed when non-proportional loading path changes into proportional loading path and the rate of cyclic softening is related to non-proportional loading history. A visco-plasticity constitutive model based on Ohno-Wang kinematic hardening rule and Marquis isotropic hardening rule is used to characterize non-proportional cyclic behavior. Simulation results agree with experiments very well in terms of cyclic hardening effects. However, this model is found to be less effective in simulating cyclic softening effect, especially considering the influence of non-proportional loading history. A modified isotropic hardening model is thus proposed to simulate the cyclic softening behavior of 316L stainless steel by introducing a memory non-proportionality and qualifying a partial recoverable softening term of isotropic hardening rule. Simulation results of the proposed model agree much better with experimental values.

Thermo-mechanically coupled cyclic elasto-viscoplastic constitutive model of metals: Theory and application

Thermo-mechanically coupled cyclic deformations often occur in metallic components subjected to cyclic loading. A framework of thermo-mechanically coupled elasto-plasticity (including rate-independent or rate-dependent plasticity) is first presented in this work based on the thermodynamic laws and logarithmic stress rate. Then, a specific thermo-mechanically coupled elasto-viscoplastic constitutive model is constructed from the framework to describe the thermo-mechanically coupled cyclic deformation of 316L stainless steel by using combined nonlinear isotropic and kinematic hardening rules and considering the internal thermal production. The nonlinear kinematic hardening rule is extended from that originally proposed by Abdel-Karim and Ohno (2000) and Ohno and Abdel-Karim (2000) for small deformation; and a cyclic hardening-softening-hardening feature observed in the cyclic test of the steel is reflected by using a nonlinear isotropic hardening rule consisting of several component equations. The strain amplitude dependence of cyclic hardening is considered by introducing a memory surface of plastic strain proposed by Chaboche et al. (1979), and the additional hardening effect caused by the non-proportional multiaxial cyclic loading path is involved by using the non-proportionality parameter defined by Tanaka (1994). Furthermore, the proposed constitutive model is implemented into a finite element code (e.g., ABAQUS) by combining the user subroutines UMAT and UMATHT. Finally, the proposed model is verified by comparing the predictions with the experimental results of 316L stainless steel. It is shown that the predictions agree well with the corresponding experimental ones.

Creep Test of Type Austenitic 316LStainless Steel at High Temperature

The aim of this study is to investigate the creep test on static loading of stainless steel materials. Theestablished creep analytical modeling was used to predict the creep life of stainless steel at high temperature. In order to conduct creep testing, the materials were subjected to constant load and high temperature at 565°Cin comparisons with different stresses at the similar temperatures.Time dependent deformation of 316L stainless steel at high temperature at different loading was investigated in this study.The variationof creep strain rates is clearly shown that the secondary creep obeyed and well agreement with a power law relationship as the constant increase creep rate till the tertiary stages of specimen test.

Assessment of Tensile and Fatigue Damage in 316L Stainless Steel by Acoustic Emission Technique

Acoustic emission (AE) technique was applied to evaluate both tensile deformation of 316L stainless steel and bending-fatigue damage of socket-welded pipe. AE activity was monitored during tensile deformation of plate-type specimens subjected to various heat treatments. Variation in RMS voltage of AE signal was correlated with the amount of strain-induced martensite phase. Secondly, actual size of socket-welded pipe specimen was bending-fatigued under various stresses. Crack initiation was determined by observing an abrupt increase in AE count, and confirmed by radiographic examination before and after the crack initiation cycle. Potentials of AE technique for monitoring fatigue crack initiation were discussed.

EBSD Study of the Role and Activation of Mechanic Twinning in the Plastic Deformation of 316L Stainless Steel

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Microstructural refinement and deformation twinning during severe plastic deformation of 316L stainless steel at high temperatures

The present work focuses on the severe plastic deformation and deformation twinning of 316L austenitic stainless steel deformed at high temperatures (700 and 800 °C) using equal channel angular extrusion (ECAE). Very high tensile and compressive strength levels were obtained after ECAE without sacrificing toughness with relation to microstructural refinement and deformation twinning. The occurrence of deformation twinning at such high temperatures was attributed to the effect of high stress levels on the partial dislocation separation, i.e., effective stacking fault energy. High stress levels were ascribed to the combined effect of dynamic strain aging, high strain levels (∈ ∼ 1.16) and relatively high strain rate (2 s−1). At 800 °C, dynamic recovery and recrystallization took place locally leading to grains with fewer dislocation density and recrystallized grains, which in turn led to lower room temperature flow strengths than those from the samples processed at 700 °C but higher strain hardening rates. Apparent tension-compression asymmetry in the 700 °C sample was found to be the consequence of the directional internal stresses.

High temperature deformation of 316L stainless steel

A constitutive description for AISI 316L stainless steel deformed at elevated temperatures in a wide range of strain rates is proposed. The strain dependence is introduced by means of the exponential­saturation evolution law proposed by Sah et al., and the temperature and strain rate dependence of the saturation stress are introduced by means of the kinetic model advanced by Kocks. The derivation of this formulation is conducted in a sequential manner: first, by fitting of the Sah et al. evolution law to the stress–strain data under conditions of constant temperature and strain rate; second, by correlating the extrapolated values of the saturation stress with temperature and strain rate by means of the kinetic model advanced by Kocks; third, by imposing the condition that the initial work hardening rate of the material at the beginning of plastic flow should be a constant independent of temperature and strain rate. The overall constitutive description proposed in this manner requires only four material parameters besides an expression for the temperature dependent shear modulus of the alloy. The results indicate that the equation derived in this way is able to describe satisfactorily both the flow stress and work hardening rate of the material under most of the conditions of strain, strain rate, and temperature that have been analysed. At deformation temperatures in the range 850–900°C and strain rates in the interval 10–100 s-1, it has been observed that the extrapolated values of the saturation stress are overestimated, which is thought to be due, in part, to the proximity to the upper temperature limit where dynamic strain aging effects are present in this material. The kinetic analysis that has been conducted, emphasises the need to employ appropriate scaling parameters in order to compute the correct value of the activation energy for deformation.