Mechanical Properties, Corrosion Resistance and Microstructure of Both Regular and Titanium Hardened 625 Alloys

Abstract

The addition of titanium to standard 625 grade enhances the age-hardening response, enabling yield strengths greater than 800 MPa to be obtained by simple, industrially compatible, heat treatments. The effect of such treatments on microstructure have been studied, together with their influence on the strength/toughness trade-off and the resistance to intergranular corrosion, compared to regular 625 alloy.

The Influence of Processing Variables on the Microstructure and Properties of PM 625 Alloy

“…When the HIP or annealing temperature does not exceed 95O"C, abundant precipitation of an "MeC" -type phase is observed, containing Ni, MO, Cr, Nb and Si, together with significant amounts of nitrogen and oxygen. The composition of this phase, at least as regards the metallic elements, is quite similar to that found in conventional 625 alloy [4]. At 1050°C and above, the "Mac" is replaced by Nb, MO carbides or carbonitrides M(C,N), together with a nitrogen-rich Nb, Cr, MO-containing phase.…”

“…235 The "M C " -type precipitates are apparently associated mainly with prior particle surfaces2in% represent the major constituent decorating these sites. In spite of their small size, they remain stable over the temperature range studied, contrary to their behaviour in cast and wrought 625 alloy [4]. This stability may be due to the presence of oxygen and nitrogen.…”

Influence of Heat Treatments on Corrosion Behaviour of Alloy 625 Forged Rod

“…Intergranular corrosion takes place after heat treatments between 700 and 95O"C, at the same time as precipitation of chromium-and molybdenum-rich M23C6 and M6C carbides in grain and twin boundaries. Intergranular corrosion is due to chromium and probably also molybdenum depleted zones along grain and twin boundaries, as shown in (14) and (22). The two maxima observed at 700-750 and 950°C for the ASTM G28 results correspond to the depletion due respectively to M23C6 and M6C precipitation.…”

The Role of Niobium in Wrought Precipitation-Hardened Nickel-Base Alloys

“…The 53% nickel-19% iron matrix is strengthened mainly by 5.3% niobium that forms γ′′ (∼18 to 20%) giving alloy 718 a higher yield strength than other superalloys strengthened by an equivalent amount of γ′. However, γ′′, being metastable, can transform to δ after long periods at temperatures at and above ∼650°C resulting in some loss of strength (36)(37)(38)(39)(40)(41). Like alloy 706, alloy 718 precipitates γ′′ as fine coherent platelets in the γ matrix.…”