Constitution

Abstract

The constitution of high interstitial steels describes the state of atomic order in thermodynamic equilibrium. It depends on the three variables of state: concentration of alloying elements in iron, temperature and pressure. A region that is in the same state of order is known as a phase. Nitrogen is a volatile element and therefore the gas phase and especially the partial pressure of N 2 has to be taken into account. The liquid phase appears during melting, solidification and welding. Of the solid state phases austenite, d-ferrite, carbides, nitrides and sigma phase are to be expected. To cope with such a complex alloy system the commercial software program THERMO-CALC TM , version R with TCFE4 database [1] was used to calculate the constitution of multi-component HIS.It depends on experimental data and theoretical models covering a wide range of steel compositions by minimizing the Gibb's free energy [2]. The program provides phase diagrams as isothermal or isoplethal sections through a system. Also the mole, mass or volume fraction of phases in a given steel may be plotted over the temperature. In addition the chemical composition of each phase is available. In the high temperature range from solidification to solution annealing, which are of practical importance, less deviation from the calculated equilibrium is to be expected than at lower temperatures. However during quenching the kinetics of precipitation are of interest only. It is shown in the next chapter that the atoms in homogeneous austenite are not necessarily distributed evenly, but that microsegregation (mm range) or short range atomic decomposition (clustering, nm range) may cause a chemical inhomogeneity which is not covered by the calculations. Tramp elements are not considered to reduce the computing time except for manufactured grades.Limited experimental verification was in good agreement with the simulation. Therefore the calculated phase diagrams are taken as a reasonable guideline to reveal tendencies.

Barkhausen Noise Emission in AISI 321 Austenitic Steel Originating from the Strain-Induced Martensite Transformation

“…Diffraction angles 2θ hkl were taken as the centre of gravity of {311} diffraction doublet MnKα and {211} diffraction doublet CrKα. To determine residual stresses, the Winholtz and Cohen method and X-ray elastic constants 1 2 s 2 = 7.18 TPa −1 , s 1 = −1.2 TPa −1 and 1 2 s 2 = 5.75 TPa −1 , s 1 = −1.25 TPa −1 were used for the austenite and the martensite phase, respectively. Phase compositions, crystallite size (the size of coherently diffracting domains), and microdeformation were determined from the X-ray diffraction (XRD) patterns obtained using cobalt radiation and subsequent Rietveld refinement, performed in MStruct software (used for the fitting of the measured XRD patterns).…”

“…The presence of different phases and the corresponding phase partitioning is very important, since the alterations of the phases, along with the different degree of plastic straining, significantly differ from those observed in single-phase materials [1]. The main distinction can be found in the strain hardening, which is not exclusively associated with slip dislocations, as the superimposing aspect of the strain-induced phase transformation also plays a significant role.…”

“…Austenitic stainless steels are well-known iron-based alloys that are widely used for various applications because of their high strength, corrosion resistance and ductility. The stability of austenite at room temperature is mainly due to the presence of austenite stabilizers, such as Ni and Mn [1]. However, Cr and Mo elements are also important for their acceptable level of corrosion resistance.…”

“…Stainless steels are used in food, oil, gas, and chemical industries [1]. Components made of these steels are manufactured via a variety of technological operations which alter the microstructure and the corresponding functional properties of the steel.…”