ABSTRACT Processing conditions were designed to produce alloys 718 (UNS N07718), 725 / 725HS (UNS N07725), and 925 (UNS N09925) with essentially clean grain boundaries, partial coverage of grain boundaries by the precipitates, and full coverage of grain boundaries by the precipitates. Grain boundary precipitates were found to degrade room temperature impact strength. Partial coverage of grain boundaries by second phase particles did not adversely affect the properties in slow strain rate (SSR) tests conducted in the oil patch environments. However, the materials having grain boundaries fully covered with second phase particles had inferior properties in SSR testing.
INTRODUCTION Corrosion environments encountered in oil and natural gas production are rather aggressive (1). They may contain significant levels of hydrogen sulfide, carbon dioxide, chlorides, and free sulfur. Further, some of these environments are at high pressure and temperature, up to 450°F (232°C). Processing of oil and natural gas under these environmental conditions requires special materials.
Nickel-base alloys 718, 725, and 925 are commonly used in oil and natural gas production These alloys contain high Cr/Mo contents for aqueous corrosion, and also considerable levels of hardeners like Ti/Nb/A1 to form gamma prime and gamma double prime precipitates for strength. Being heavily alloyed multi-component systems; these materials require special consideration for processing and heat treatments. Time-Temperature-Transformation (TTT) diagrams can be used as road maps to determine precipitation of various phases under different processing conditions. This paper shows the effect of intergranular precipitates on mechanical properties and SSR test results in sour oil patch environments.
EXPERIMENTAL PROCEDURE Nominal chemical compositions of alloys 718, 725 / 725HS, and 925 are listed in Table 1. Solution annealing and age hardening treatments of these alloys are listed in Table 2.
Alloy 925 was produced in the mill with two different microstructures. Standard material containing isolated grain boundary carbides and non-standard material containing heavy continuous grain boundary eta and carbide precipitates, Figures 1 a and 1 b. The former will be referred to as 925-S and the latter will be referred to as 925-T. Processing of material 925-T was specially designed to generate the above mentioned microstructure.
Commercially produced alloys 725 and 725HS were used for the testing. Alloys 725 and 725HS have the same UNS chemical composition but differ in the way they are processed and heat treated. Alloy 725 had essentially clean grain boundaries whereas 725HS had isolated eta precipitates on the grain boundaries, Figures 2a and 2b. The purpose of grain boundary eta in alloy 725HS is to refine the grain size for higher strength. In addition, a lower temperature solution anneal of alloy 725HS compared to the regular alloy 725 (Table 2) results in a higher residual work which enhances age hardening on subsequent heat treatment. Fine grain size and a specially designed age hardening treatment of alloy 725HS is responsible for its higher yield strength compared to regular alloy 725.
Commercially produced hot worked alloy 718 was solution annealed and age hardened in the lab to produce two different microstructures, Figures 3a and 3b. Heat treatment conditions are listed in Table 2. The material for Figure 3a was heat treated using the standard oil patch specification conditions. This material will be referred to as 718-S. The material for Figure 3b was subjected to a specially designed heat treatment to precipitate continuous fine grain boundary delta precipitates. This material will be refe
ABSTRACT The U.S. Bureau of Mines has studied the effects of cold working and subsequent heat treatments on the susceptibility of a high-nitrogen stainless steel to localized corrosion. The ,degree of sensitization (DOS) of a cold worked high-nitrogen stainless steel was measured using procedures specified in ASTM G 108 and compared to DOS values measured for the non-worked steel. Cold working was done at the 20% level and heat treatments ranged from 600 to 800°C for up to 100 hours. Sensitization of high-nitrogen stainless steels occurs in a manner similar to that of carbon-containing stainless steels with the exception that chromium nitrides are predominantly responsible for the sensitization. Micrographs comparing the effect of cold working showed that the formation of both grain boundary and intragranular Cr2N precipitates occurred more rapidly for the cold worked samples.
The effect of heat treatment on the non-worked high-nitrogen stainless steels is to increase the DOS with increasing aging time at both 600 and 700°C. At 800°C, however, the DOS decreases or heals for aging times greater than 30 hours. The healing process has been related to the chromium depletion of the matrix material. Cold working prior to heat treatment increases the DOS values but also decreases the aging time necessary to heal the high-nitrogen stainless steel. Thus at 700°C, aging of cold worked material causes healing for times greater than 30 hours while the non-worked samples showed no healing in similar time periods. Also, at 800°C healing of the old worked samples occurs in 10 hours compared to 30 hours for the non-worked samples.
INTRODUCTION High-nitrogen stainless steels (HNSS) have the potential to become an increasingly important class of materials. The presence of interstitial nitrogen not only increases yield and tensile strengths by a factor of 2-3.5 over A1S1300 series steels but also improves Focalized corrosion resistance. Nitrogen is known to be more soluble than carbon and this accounts for the greater increases in physical and chemical properties compared to commercially available C-containing stainless steels. In spite of these advantages, HNSS are affected by some of the same problems as C-containing stainless steels. One of these problems is due to the thermal instability of the metal matrix and results in a state similar to the sensitization normally caused by the formation of metal carbides. In the case of HNSS, nitrides are formed and these cause chromium depletion of the surrounding matrix and subsequent sensitization.
The effect of cold work on the corrosion and sensitization behavior of AISI 300 Series stainless steels has been wall documented. The observed effect of cold work on the 300 series stainless steels indicates that both corrosion and sensitization are adversely affected up to a threshold level of cold work. ?~hat threshold level ranges from 15 to 20% depending on the research report. Above that level, both corrosion and sensitization decrease to levels that are even lower than nonworked materials. One of the reasons postulated for this behavior was that the higher levels of deformation induce rapid precipitation of carbides throughout the entire matrix with no significant precipitation at the grain boundaries. Another study showed that the average diameter, the spacing, and the aspect ratio of grain boundary precipitates decreased with deformation up to a value of? 1So/O. Analogies will be made later in this paper between the behavior of the carbon-containing stainless steels and the nitrogen-containing stainless steels. The sensitization of high-nitrogen stainless steel was reported by Simmons and others, with some preliminary indication of the effect of cold-
ABSTRACT It is well-known that high strength Al-Zn-Mg alloys are susceptible to stress corrosion cracking (SCC). However, Al-Zn-Mg alloys are widely used in aircraft, trains, bridges, some military equipment, etc., not only because of their high strength but also because of their superior weldability relative to other aluminum alloys. Therefore, an understanding of the SCC resistance of Al-Zn-Mg alloys after welding, especially within the heat-affected zone (HAZ), becomes very important. The wrought aluminum alloys stressed in the short transverse (ST) direction are more susceptible to SCC than those stressed in longitudinal (L) and long transverse (LT) directions due to the elongated planar grain structures and precipitate stringers in the grain boundaries. When alloys are stressed in ST direction, the SCC propagation path along grain boundaries is the shortest one among those three directions, and the tensile stress is directly normal to the grain boundaries.1 Therefore, understanding the SCC resistance of Al-Zn-Mg alloys loaded in ST direction becomes more significant. The SCC resistance of HAZ of Al-Zn-Mg alloys have been studied by many researchers, but the SCC behavior of the HAZ in the ST direction has not been widely investigated. Therefore, the present study focuses on this special combination of HAZ and ST direction loading. To investigate this combination, two types of specially modified double-cantilever beam (DCB) specimens were adopted, and a normal DCB specimen was also used to compare the SCC behavior of base metal. In addition, three other methods were also used to study the base metal and weldment loaded in L and LT directions, respectively.
EXPERIMENTAL PROCEDURES A hot-rolled and then overaged 1-in. or 10-mm thick Al-3.7wt%Zn-2.5wt%Mg alloy was used in this work. Its chemical composition is listed in Table 1.
ABSTRACT This paper presents a study of the microstructures of A1-5Mg alloys with additions of the alloying elements So, Zr, and Ag and the effects of these elements on the polarization and stress corrosion cracking behavior of this material in a 3.5%NaCI solution (pH=8) at 50°C. The results show that Sc+Zr additions have a stronger effect on inhibiting recrystallization than does Zr alone, and that in a sample aged for 24 hours the Zr+Sc alloy had less precipitation of AI3Mg2 along the grain boundaries relative to the alloy that contained only Zr additions. In the alloy that contained Ag and Zr additions, there was significantly more precipitation of A13Mg2 along the grain boundaries than in the aUoy that contained only Zr. Additions of Sc and Ag improve the protective nature of the passive film in A1-5Mg-0.15Zr alloys in 3.5%NaCI solution. Addition of Sc to an AI-5Mg-0.15Zr alloy improves stress corrosion cracking resistance, while addition of Ag causes the alloy to become more susceptible to stress corrosion cracking in the quenched (510°C) and aged (175°C/24h) conditions.
INTRODUCTION AI-5Mg alloys are of interest in the automotive industry due to their light weight and high strength. However, these alloys are susceptible to stress corrosion cracking (SCC) if the Mg content is higher than 3.5% (wt) t~,2]. During the aging of these alloys at temperatures ranging from 100 to 200°C, AI3Mg2 (IB-phase) will precipitate if the Mg content is greater than 3.5% (wt) [31. It is generally accepted that AIaMg2 precipitates that have formed along grain boundaries provide an active path for SCC 0-41, although other mechanisms for failure have been proposed, such as hydrogen induced cracking  and degradation of the passive film at the crack tip trl. Doig and Edington t71 indicated that the corrosion potential of AI3Mg2 is several hundred millivolts more anodic than that of the matrix; consequently there will be a localized corrosion reaction at the precipitate/matrix interface. Cui and Wu tsl found that the SCC resistance is improved in AI-Mg alloys when the 13-phase along the grain boundaries changes from a continuous film of precipitates to discrete precipitates.
Various studies have shown that the addition of alloying elements such as Zr, Sc and Ag can change the microstructure and the mechanical properties of AI-Mg alloys [9-121. The addition of small amounts of Sc causes the formation of fine, coherent, spherical precipitates of AI3Sc which can pin grain boundaries and inhibit recrystallizationt91; this can result in a great strengthening effect 001. Zr plays a similar role, but it is not as effective in inhibiting recrystallization [131. The addition of Ag promotes age- hardening in the AI-Mg systems 01, ~21 However, less is known about the effects of these alloying additions in small amounts on the polarization and the stress corrosion cracking response of AI-5Mg alloys. In this paper we will present a study of the electrochemical polarization and stress corrosion cracking of these alloys in 3.5% NaCI solution (pH=8) at 50°C and also describe their microstructures after specific heat treatments.
EXPERIMENTAL PROCEDURE The materials we used in this study were AI-5Mg-0.15Zr-0.05Sc, A1-SMg-0.15Zr, A1-5Mg-0.2Ag and AI-5Mg-0.15Zr-0.2Ag alloys. Table 1 shows their chemical compositions. The ingots were extruded at 450°C to bars with rectangular cross-sections (65 mm x 9 mm). The heat treatment procedure involved solutionizing at 510°C for 30 minutes, followed by water quenching. The samples were then aged at 1750C for 24 hours. Microstructures were examined in detail before and atter the heat treatment by optical microscopy (OM) and transmission electron microscopy (TEM). Samples f
ABSTRACT Commercial magnesium anodes were evaluated using ASTM G97-89 as standard test and attached to the ASTM test arrangement a technique of electrochemical impedance was also used . Cylindrical samples were cut from the as-cast Mg anodes. Several treatments and different cooling rates were carried out. The anodic efficiency determined as a function of the cooling rate, showing an increment around 10 to heat was 1270 above the efficiency showed by the non-treated commercial anodes (as-cast condition). The efficiency increments were related to the microstructural characteristics i.e. particle size and/or grain size. Also some morphological aspects were considered .The appearance of second phase particles influences the different forms that the corrosion process exhibits.
INTRODUCTION The increasing demand for cathodic protection in the past few years for commercial and domestic uses has given rise to widespread interest in new developments and applications of galavanic anodes. Considerable amount of work has been done in the past in order to improve the most important properties of the anodes. Considering mainly the metal purity, the alloying elements , and backfill material(4). These studies concentrate their efforts towards the improvements in current efficiency, polarization characteristics and the distribution of the corrosion attack in the anodes.
Magnesium anodes are particularly recommended for high-resistivity environments where the anodes inherent negative potential and high current output per unit weight is desirable i.e. their capacity to drain the current. Magnesium anodes generally have a current efficiency below that of other galvanic anodes. In practice, current efficiency rarely exceeds 50%. This figure is unfavorable compared with that of other anodes, such as Zn and Al which have current efficiencies better than 90%.. Several factors have been attributed to the low efficiency of the Mg anodes such as the thermal history related to the structure effects and the alloying elements, changes in anion and cation concentrations, which occur close to the dissolving Mg surface and the anodic electrochemistry.
Some significant local consumers of Mg-based sacrificial anodes, and specifically the Mexican national pipeline system are concerned with respect to the casting procedures and the efficiency of the sacrificial anode material which now ranges between 30% and 35%. Cathodic protection is now well established and required by law for pipeline protection. There is, therefore, current interest in improving efficiency of Mg anodes. Salinas et al. pointed out that the metallurgical features of sacrificial anodes (i,e.castings conditions, heat treatments, etc.) are related to the anode operation potential and also to their own efficiency. In the Mg anodes, contrary to the requirements for anode materials, the corrosion occurs by pitting rather by uniform corrosion, shifting the potential to more electronegative values. However a very recent work reported that pitting was found under a local standard that use artificial sea water for the anodes evaluation. Furthermore, the same authors 0) report that when using the ASTM standard for GALVOMAG (trademark of the Dow Chemical Co.) type anodes evaluation, they 201/2 found uniform corrosion, being this difference due to the solution used.
On the other hand, local consumers of Mg anodes pointed out that at present they have several tons of low-efficiency Mg-anodes already cast in stock and they are interested that these anodes could be rehabilitated instead of rejecting them. The low efficiency is related to the hetreogeneity on their chemical composition obtained from several batch of anodes (i.e.higher ratios Fe/Mn esp