Effect of strain paths and residual delta ferrite on the failure of cold rolled austenitic stainless steels, type 304L

Abstract

This study was carried out to assess the effect of strain paths and residual delta ferrite on failure characteristics of austenitic stainless steels cold rolled to 20% reduction. Optical metallography was carried out to determine grain size and quantify residual delta ferrite. Mechanical tensile tests to failure along three orthogonal directions were carried out on annealed and 20% cold rolled samples to study the effects of strain paths on mechanical properties of the material. Post-mortem scanning electron microscopy was used to study fracture surface and cross-sectional views of the failed specimens. The yield strength of material cold rolled to 20% reduction increased relative to annealed material along the rolling and transverse directions by twice as much along the normal direction. The increase in the yield strength occurred at the expense of ductility which decreased by about half in all directions. It also emerged that material loaded along rolling and transverse directions showed a gradual failure (from rupture strength), while that loaded along the normal direction exhibited a rapid failure. Correlation between ferrite morphologies on the three orthogonal planes and failure characteristics on fracture surfaces were established. The scanning electron microscopic micrographs suggest that materials loaded along transverse and rolling directions failed with characteristic features of pure ductile failure, while specimen loaded along normal direction showed mixed features of ductile and brittle failure.

Deformation path effects on the internal stress development in cold worked austenitic steel deformed in tension

“…Although 304 stainless steel is essentially a single phase material [3] (there is some residual ferrite but the volume fraction is very low [3]), the grains in a polycrystal are anisotropic and therefore will also develop incompatibilities between them [4,30,31]. In other words, some grains deform plastically less than others, simply because of their crystallographic orientation, and therefore act like "hard" grains.…”

“…Diffraction lattice strain measurements were carried out on the Engin-X Diffractometer at ISIS neutron spallation source, Rutherford Appleton Laboratory (RAL) in Oxford in line with established procedures detailed elsewhere [3,9,12,[19][20][21][22]37]. The sample configuration used for lattice strain measurement was the same as those used for mechanical testing (see Fig.…”

“…Single peak fitting of the diffraction pattern was carried out with Open Genie from which lattice spacings were obtained. The lattice strain was calculated from the peak shift (change in the lattice spacing) according to the following expression [3]:…”

“…On the other hand, when the re-straining direction is reversed, the yield strength of material appears much lower than the prior flow stress. The reverse flow stress also appears much lower than the forward flow stress, evident in the much higher permanent softening [3,4]. This is an example of the Bauschinger Effect (BE), which has origin on the development of internal stresses caused by the heterogeneous nature of deformation at the microstructural scale [5], often observed in dual phase materials.…”

Expansion of Additive‐Manufactured Tubes: Deformation and Metallurgical Analysis

“…The vertical lines that are seen in the microstructure of the CW tubes correspond to residual δ-ferrite phases that were elongated during the production of the tubes by rolling and profiling. [29] Because the as-supplied CW tubes were annealed to homogenize the microstructure resulting from rolling and profiling, their final microstructure is equiaxial with stable polygonal grain features, [30] as shown in the rightmost image of Figure 7b.…”