Quite a number of models for hydrogen distribution in steels and welds have been developed in the past 20 years. They reach from simple analytical models to more complex two and three dimensional finite element simulations. So far, these models have been used to simulate hydrogen distribution in homogeneous microstructure. This paper contributes to numerical simulation of hydrogen distribution in heterogeneous microstructure, e. g. in a duplex stainless steel microstructure consisting of two phase fractions. Under appropriate conditions, such as cathodic protection, it is possible that hydrogen is absorbed leading to material embrittlement and possibly initiating hydrogen assisted cracking. In order to avoid hydrogen assisted cracking in duplex stainless steels, it is of great interest to know more about the diffusion behavior of the ferrite and austenite phase. A numerical model has been developed that operates on the mesoscale and enables simulation of hydrogen transport in the various phases of a metallic material. As a first application of this model, hydrogen distribution in a duplex stainless steel 1.4462, consisting of approximately equal portions of ferrite and austenite, was simulated using the finite element program package ANSYS. The results reflect the dependency of hydrogen distribution on the microstructural alignment of the ferrite and austenite phase fractions. Crack-critical areas can thus be identified, provided the critical strain-hydrogen combination is known for the respective microstructural phase.
Modeling of Hydrogen-Assisted Cracking (HAC) in Duplex Stainless Steels (DSS):
- Hydrogen causes significant degradation in DSS and consequently HAC
- Mechanisms of HAC have not been fully understood in two-phase microstructures
- Mesoscale numerical modeling for HAC in DSS was created, coupled to the macroscale, enabling simulation of HAC in both phases independent from predefined crack paths
This paper describes austenitic-ferritic duplex stainless steels, SAF 2205, in the presence of hydrogen. The duplex stainless steels (DSS) properties include excellent resistance to stress corrosion cracking, high strength and good weldability. Those steels are preferably used in industries combining hydrogen and loads. Hydrogen location in addition to hydrogen binding energy with the steel's defects are of great importance for the analysis of hydrogen embrittlement model in that steel. It is known from previously published works that the susceptibility to hydrogen embrittlement will depend on the competition between reversible and irreversible traps; meaning a direct relation to the hydrogen's state and position in the steel. In this work, we examine the local hydrogen concentration, trapping and distribution by two modern and advanced techniques: thermal desorption spectrometry (TDS) and we support it by time of flight-secondary ion mass spectrometer (ToF-SIMS). In this paper, we support and give for the first time new insights and better understanding to the hydrogen embrittlement mechanism in SAF 2205. The trapping energies levels were calculated using TDS and Lee and Lee's model. This model revealed reversible in addition to irreversible trapping sites. Also the trapping controlling mechanism was found to be a combination of detrapping controlled mechanism and diffusion controlled mechanism. The use of ToF-SIMS for local imaging the distribution of hydrogen species supports the discussion of the different hydrogen traps in this type of steel. The hydrogen embrittlemet phenomenon in SAF 2205 will be discussed in details in that paper.
Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and high-resolution scanning electron microscopy are well-acknowledged tools in materials characterization. The ability to map chemical species on the surface of an investigated sample with often low mass detection limits makes ToF-SIMS an essential tool in fields where many question marks concerning Degradation processes and damage mechanisms exist. The aim of this paper is to describe the power of data fusion of ToF-SIMS and high-resolution scanning electron microscopy results employing computational methods for multivariate data Analysis such as principal component analysis. As a case study the investigation of hydrogen distribution in an artificially charged Duplex stainless steel microstructure is presented aiming on a better understanding of hydrogen embrittlement.
Time-of-flight secondary ion mass spectrometry (TOF-SIMS) mappings provide a visualization of the distribution of chemical elements and phases on polished duplex steel surfaces as element and fragment secondary ion signals. The advantage of TOF-SIMS, compared to optical microscopy, is its potential to analyze the chemical composition with a lateral resolution at a submicrometer scale. TOF-SIMS mappings allow distinguishing between ferritic and austenitic phases because of the phase selective detection method without any requirement of etching or other processing.