1965 Volume 14 Issue 137 Pages 101-110
Stainless steel-castings Type 316 and Type 316L were subjected to high-temperature tensile test, creep-rupture test up to 10000hr at 650°C, and aging treatment under various levels of stress at 650°C and stress-free aging treatment up to 6400hr at 650°C, and the structural changes during these test were observed in order to study the relations between the high-temperature properties and the structural changes of both alloys.
Carbide and σ-phase were indentified by the following means.
(1) The techniques of electrolytic etching by using concentrated strong Hydrooxide solutions (10N, KOH, to color the σ-phase, concentrated NH4OH, to color the carbide)
(2) The X-ray diffraction analysis of the residues obtained by the electrolytic extraction with 10% HCl alcohol solution and HOC6H2 (NO2)3 5% HCl alcohol solution.
(3) Distribution of Fe, Ni and Cr in carbide and σ-phase decomposed from δ-ferrite, examined by X-ray microanalyzer.
The short and long time high-temperature strength of Type 316 is slightly superior to that of Type 316L. The superiority of Type 316 is attributed to the solid solution hardening effect of carbon and the more uniform distributions of fine carbide and σ-phase in the austenite matrix than in the case of Type 316L. But the high-temperature long time load-carrying ability of both alloys seems to be fairly good.
There has been a little difference observed between Type 316 and 316L in their structural changes during the tests, but they take place generally in the following sequences;
(1) The δ-ferrite decomposes into γ-phase, carbide and σ-phase.
(2) The carbide and σ-phase precipitate at austenite grain boundary.
(3) Carbide and σ-phase precipitate in austenite matrix.
(1) Decomposition of δ-ferrite and (2) precipitation at austenite grain boundary are accomplished within 1000hr but (3) the precipitation in austenite matrix gradually proceeds for a long time.
Therefore it is probable that the precipitation of fine carbide and σ-phase in austenite matrix influences mainly the long time high-temperature strength of these alloys, and that Type 316 is a little superior in the high temperature strength to Type 316L, since the precipitates of fine carbide and σ-phase are dispersed uniformly in the austenite matrix of Type 316, while the precipitation in the austenite matrix of Type 316L is limited only round the regions which have been formerly occupied by δ-ferrite.
