On the High-Temperature Strength and Structural Change of Type 316 Austenite Stainless Steel Castings

Study on the austenite stainless steel castings-I

Abstract

High-temperature tensile test, creep-rupture test up to 10, 000hr at 650°C, aging treatment under various levels of stress at 650°C and stress-free aging treatment up to 6, 400hr at 650°C were carried out on the Type 316 and Type 316L stainless steel castings, and the structural changes daring these tests were observed in order to study the relations between high-temperature properties and structural changes of both alloys.
Carbide and σ-phase were identified by the following means.
(1) The electrolytic etching with concentrated strong hydrooxide solutions (10N. KOH, to color the a-phase; concentrated NH₄OH, to color the carbide).
(2) X-ray diffraction analysis of the residues obtained by the electrolytic extraction with 10% HC1 alcohol solution and HOC₆H₂ (NO₂) ₃, 5% HC1 alcohol solution.
(3) Examination of Fe, Ni, Cr distributions in carbide and a-phase decomposed from 6-fer rite by X -ray microanalyzer.
The short and long time high-temperature strength of Type 316 is a little 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 Type 316L. But the high-temperature long time load-carrying ability of both alloys seems to be fairly good.
There is a little difference between Type 316 and 316L in their structural changes during the tests. They take place generally in the following sequence:
(1) 8-ferrite decomposes into r-phase, carbide and a-phase.
(2) Carbide and σ-phase precipitate at austenite grain boundary.
(3) Carbide and a-phase precipitate in austenite matrix.
(1) Decomposition of 6-ferrite and (2) precipitation at austenite grain boundary are accomplished within 10, 100hr, but (3) precipitation in austenite matrix occurs gradually, taking long time.
Therefore it is probable that the precipitation of fine carbide and a-phase in austenite-matrix influences mainly the long time high -temperature strength of these alloys, and that Type 316L, is a little superior to Type 316L, since the precipitates of fine carbide and c-phase are dispersed uniformly in the austenite matrix of Type 316, while the precipitation in the austenite matrix of Type 316L occurs only round the region which has been formerly occupied by 6-ferrite.