Abstract
Duplex stainless steels are widely used by the oil and gas and chemical and process industries because of their combination of high strength and corrosion resistance. The alloys are usually used in the solution annealed condition and must be fast cooled from the annealing temperature to prevent the precipitation of third phases, such as sigma, chi, nitrides and alpha prime. Alpha prime precipitates in the temperature range 550 to 300°C. It forms less readily than sigma phase and other intermetallic precipitates and so is not normally found in commercially produced duplex alloys. However, poor cooling of duplex steels through this temperature range or repeated excursions in to this temperature range can result in its formation. Alpha prime dramatically reduces impact toughness and increases susceptibility to hydrogen induced stress corrosion cracking due to cathodic protection. However, corrosion resistance in chloride environments appears to be little affected by this phase. Alpha prime cannot be seen under optical microscopes, because of its very small size. This means that the combination of corrosion testing in ferric chloride solution, microstructural examination and impact testing, can be ineffective in detecting alpha prime precipitation, principally because of the low toughness acceptance level of 45 Joules at -46°C that is commonly specified. This paper describes a simple electrochemical reactivation test to detect alpha prime that can be used with both 22%Cr and 25% Cr alloys. The paper also presents some case studies where low toughness occurred but no third phases were visible in optical microsections. The test was used to confirm the presence of alpha prime and determine what should be done to correct the problem.
Introduction
Duplex stainless steels are widely used by the oil and gas and chemical and process industries because of their combination of high strength and corrosion resistance, particularly to stress corrosion cracking (SCC). The most commonly used grades are 2205 (UNS S32205), 2507 (UNS S32750) and Z100 (UNS S32760). In order to get the best properties the alloys must be solution annealed at high temperature for sufficiently long to dissolve any third phases and to allow homogeneity in both the ferrite and austenite1. The alloys must then be transferred rapidly to a water quench tank so that the metal can be cooled sufficiently fast to prevent any third phases from forming. Figure 1 shows a typical CCT curve for superduplex stainless steel and it can be seen that sigma, chi and nitrides form at high temperatures, while alpha prime forms between 550 and 300°C. Although it requires a greater time at temperature for alpha prime to form, compared with sigma and chi, the cooling rate is slower at lower temperatures. This is a bigger problem for thick sections and Francis and Hebdon have discussed in detail the requirements in terms of quench tank volume to charge mass, as well as start and finish bath temperatures, to ensure a sufficiently rapid quench2.
