Liesbeth Barbé

Characterization of the metastable austenite in low-alloy FeCMnSi TRIP-aided steel by neutron diffraction

Abstract The detailed analysis of X-ray diffraction data obtained from intercritically annealed and isothermally transformed low-alloy FeCMnSi TRIP-aided steels reveals that the microstructure contains athermal plate martensite and Fe2C η carbide in addition to ferrite, bainite and residual austenite. Neutron diffraction shows that athermal plate martensite can be formed at room temperature in the isolated austenite phase. Whereas the formation of athermal martensite leads to compressive strains in the austenite, the formation of strain-induced martensite results in tensile straining of the austenite. The strain-induced transformation leads to the formation of a martensite of low tetragonality. Low-temperature annealing leads to the formation of g carbide in both the athermal and strain-induced martensite.

Texture and Orientation Relationships in Phosphorus Added TRIP Steels

Phosphorus added TRIP steels were subjected to Electron BackScattering Diffraction (EBSD) measurements in order to study the texture of the three different phases and in order to evaluate the orientation relations that are active during the transformation of austenite to bainite. The retained austenite had a maximum intensity on the Brass component and the intensity gradually decreased along the alpha and the beta fibre. The intensity of both BCC phases was comparable, but the bainite texture displayed clear transformation products that could be related with the dominant Brass component of the intercritical austenite. A detailed evaluation of the crystallographic orientation relationships between about 360 retained austenite grains and their BCC neighbours was performed. Three relationships were considered, namely Kurdjumov-Sachs, Nishiyama-Wassermann and Pitsch. It was found that the majority of the austenite grains had at least one neighbour that could be related with one of the three orientation relationships. The Kurdjumov-Sachs relationship appeared to be dominant and no strong indication for variant selection could be retrieved from the studied data.

Neutron diffraction analysis of martensite ageing in high-carbon FeCMnSi steel

Abstract A high-carbon metastable austenite phase was quenched in liquid nitrogen to obtain fresh athermal martensite. Different ageing treatments were performed in order to study the transformation of the martensite phase and the formation of carbides. Neutron diffraction experiments give detailed information on the transformations during the annealing treatment itself. The tetragonal athermal martensite transformed immediately, i.e. during the heating to the ageing temperature, to cubic martensite. Ageing at 400°C resulted in the decomposition of the austenite in ferrite and carbides and the formation of bainite. Those carbides could be determined as -carbides transforming in -carbides after extended annealing times. As was expected, ageing at 170°C resulted in the formation of a small amount of carbides while the austenite phase remained stable.

Influence of composition on crack sensitivity of ferritic stainless steel.

Abstract The high temperature cracking of 17 wt-%Cr ferritic stainless steel during continuous casting was investigated in detail. No relationship between the high temperature internal cracking and the ductile to brittle transition temperature (DBTT) was found. The experimental results imply that the cracking is owing to a combination of factors: a high ferrite potential, a hot ductility gap, and the weaker grain boundary cohesion. The 17 wt-%Cr Ti stabilised grades were found to be the most sensitive ferritic grades for crack formation at high temperatures. The ductility was evaluated by means of hot tension tests. At 1200°C, the fracture surfaces of the Ti free ferritic steel had an intergranular character. At temperatures between 700 and 1100°C, the fracture surfaces were dimpled. Below 600°C cleavage fracture occurs. The 17 wt-%Cr Ti stabilised steel contained solidification related cracks. These cracks widen during the further cooling and could clearly be seen on the surface of a cracked specimen. The Ti stabilised grades had a ductility gap at 1100°C. The DBTT was measured by means of Charpy tests. Most grades had a DBTT higher than 200°C. Type AISI 430Ti and AISI 409 ferritic steel both had a lower DBTT of ~ 90°C.