High-alloy Stainless Steels Research Articles

Korrosionsverhalten von hochlegiertem Stahlguß in gechlortem Meerwasser

The effect of chlorine on the corrosion behaviour of different high-alloy stainless cast steels in seawater was studied. The investigations show that chlorine acting as a strong oxidizing agent is shifting the redox potential to very positive values. Because of the corrosion potential of the materials being influenced in the same way, the pitting potential of steels having a low pitting resistance equivalent may be exceeded. But first of all only the local inhomogeneities of the cast materials are attacked by pitting corrosion, which may stop after a while if the chlorine concentration is very low. Increasing chlorine contents make a repassivation of these spots more difficult or even prevent it, so these pits may be the starting points for stabile corrosion

Accelerative Effect of Wear on Corrosion of High-Alloy Stainless Steel

The corrosive wear behavior of high-alloy stainless steels (SS) under sliding conditions was investigated in 20 wt% sulfuric acid (H2SO4) solution. Experiments showed a negative shift of the corrosion potential, positive shift of critical passivation potential, and steady passivation potential could be regarded as accelerative effects of wear on corrosion. For 18-8 austenitic SS (UNS S30302), it was concluded that the accelerative effect of wear on corrosion increased with increases of the corrosion rate.

Die Oberflächenbehandlung von Schweißverbindungen hochkorrosionsbeständiger 6% Mo‐Stähle und Nickel‐Basis‐Legierungen

AbstractEs wurde der Frage nachgegangen, inwieweit eine Oberflächennachbehandlung von Schweißnähten hochlegierter Stähle und Nickellegierungen (hier: ein 6% Mo‐Stahl und die Legierung Alloy 625) einen Einfluß auf die Beständigkeit gegenüber Lokalkorrosion hat. Dazu wurde an geschweißten Proben mit unterschiedlicher Oberflächenbehandlung die kritische Lochfraßtemperatur in sauren Chloridmedien (Standardtests) ermittelt. Im Ergebnis zeigt sich, daß Nachschleifen mit grobem Korn sowie Oberflächenstrahlen die Lokalkorrosionsbeständigkeit im Schweißnahtbereich vermindern; ein Nachbeizen verbessert sie wieder, insbesondere nach dem Strahlen. Das Beizen kann mit einer Flüssigkeit auf Basis Salpeter‐ + Flußsäure sowie mit kommerziellen Beizpasten erfolgen. Ein Nachbeizen von Schweißverbindungen ist insbesondere bei stark korrosiver Beanspruchung (Lochfraß‐, Spaltkorrosion) empfehlenswert.

Application of an austenitic nitrogen-alloyed 6MO stainless steel for seawater desalination (RO) and waste water treatment

In reverse osmosis plants, water from different sources and a variety of compositions has to be converted into fresh water. Water which contains chlorides and certain contaminants can be highly aggressive, particularly at elevated temperatures. Consequently, the piping material to be used has to show good corrosion resistance. In addition, when the R.O. process is run under increased pressure, a material with suitably improved mechanical properties is called for. The above requirements under specific R.O. conditions are met by a 6% Mo-containing austenitic stainless steel called Cronifer 1925 hMo - UNS N08926. Because of its high pitting resistance equivalent PRE N (% Cr + 3.3 × % Mo + 30 × % N) ⪖ 45 Cronifer 1925 hMo exhibits excellent resistance, notably in neutral and acidic chloride solutions. In addition, this special stainless steel possesses high resistance to general corrosion in a variety of process media. Specific investigations were carried out for the different types of components used in reverse osmosis installations, including seamless pipe, forged parts, plate and induction pipe bends. Furthermore, both longitudinal (PAW, SAW) and girth welds (GTAW, MMAW) were carried out and tested. The paper discussed the corrosion behaviour, the mechanical and metallurgical characteristics, and the high and low nickel versions of this alloy and compares them with other high-alloy stainless steels. In addition to Cronifer 1925 hMo*, a number of further materials suitable for use under various conditions in waste water treatment plants are briefly presented. These are Nicrofer 3127 hMo* - alloy 31 and Nicrofer 5923 hMo* - alloy 59.

The effect of σ-phase precipitation at 800°C on the mechanical properties of a high alloyed duplex stainless steel

High alloyed duplex stainless steels, referred to as super duplex, are being used increasingly for applications in corrosive environments. The higher Cr and Mo contents of these materials can lead to susceptibility to σ-phase precipitation, even during short time exposures to high temperatures. The 800°C aging of a high alloyed duplex stainless steel was investigated. Optical microscopy with image analysis, X-ray diffraction, and SEM were used to determine the σ-phase and to estimate the amount. Mechanical properties such as tensile strength, impact strength and hardness were investigated. It was found that the impact strength is most sensitive to aging at 800°C. The σ-phase transformation is discussed and the microstructural aspects are described.

Crevice Corrosion Resistance of Commercial and High-Purity Experimental Stainless Steels in Marine Environments—The Influence of N, Mn, and S

Abstract A major use of stainless steel is in chloride-containing environments; the limiting factor that most frequently determines an alloy's suitability to such environments is its crevice corrosion resistance. The objective of this work was to examine in detail the effect of minor elements on the crevice corrosion resistance of stainless steels ranging from the type 316 (UNS S31600) alloy to the 6% molybdenum austenitic stainless steels (SS) in order to define an alloy composition that would be suitable for seawater service. To this end, eight commercial stainless steels together with 31 high-purity experimental alloys have been assessed for resistance to crevice corrosion initiation in seawater environments using an accelerated test technique. Overall, the results indicate the feasibility of significantly improving the corrosion resistance of the high-alloy austenitic stainless steels to the point where they would be suitable for seawater service.

Performance of materials used in seawater reverse osmosis plants

Doha Reverse Osmosis Plant (DROP) has been in operation for more than 4 years. The plant consists of three different RO membrane configuration systems, each of which has its own materials requirements. Materials used for piping, pumps, valves and control systems were critically surveyed. A testing programme was initiated to generate data which will provide a sounder basis which these materials may be selected for use in reverse osmosis plants. A number of alloys were selected for the study to evaluate their behaviour in the various environments encountered in the reverse osmosis desalination plant. Corrosion performances and material behaviour were evaluated in terms of weight loss in the plant. Pitting and crevice susceptibility were assessed in the plant as well as by the electrochemical studies in the laboratory. A once-through pilot plant which could simulate most of the water conditions encountered in the reverse osmosis plant was constructed. Coupon weight losses of selected alloys are being determined under variables of PH, temperature and flow velocity. Preliminary work is still in progress. Results obtained from the programme, so far, appear from favour the use of high alloyed stainless steels in many area of reverse osmosis. plant. However, the economic incentive for extensive use of steel will depend on the results of long term studies current on the way.

Microstructure changes in high-speed hot rolling of steel rods

Transmission electron microscopy (TEM), quantitative texture analysis by means of neutron diffraction (QTA) and X-ray diffraction profile analysis (XDPA) were used to study the microstructure changes occurring in high-speed hot rolling (\(\dot \varphi \)≦1500 s−1;T=1073 K) of high-alloy stainless steel X8CrTi17 (ferrite) and X8CrNiTi18.10 (austenite). The investigations indicate that at higher deformation rates recrystallization due to adiabatic overheating becomes an important process of microstructure formation.

Development of a rapid solidification process with a double-roller method

A thin strip caster has been developed which consists of twin cooling rollers, a secondary cooling zone, tension control systems and a continuous-coiling machine. With this caster, fine crystalline strips were continuously produced to a near-net shape whose dimensions were 0.2–1 mm in thickness, 250–500 mm in width and 500 kg in unit coil mass. The characteristics of the thin strip caster are as follows. 1. (1) Many types of metal alloy whose melting points are less than 1800 K are rapidly solidified. 2. (2) Materials with a difficult workability, such as high silicon steel or high alloy stainless steel, are coiled in a net shape or near-net shape. 3. (3) Because of the cooling rate which is of the order of 10 2 K s −1 in a secondary cooling zone, the coils have such advantages of the rapid solidification process as a fine crystalline structure or decreased precipitation.

Duplex and high alloy stainless steels – corrosion resistance and weldability

AbstractConventional 18Cr–8Ni austenitic stainless steels have excellent resistance to corrosion in mildly aggressive environments, good mechanical properties, and are readily weldable. They are therefore used in a wide range of areas, including the food, petrochemical, and pharmaceutical industries. A major disadvantage to the use of austenitic stainless steels however is the drastically reduced resistance to pitting or crevice attack in aqueous environments containing chlorides. Furthermore, at temperatures above 65°C they are susceptible to failure from chloride stress corrosion cracking (SCC). Improvements in pitting or SCC resistance may be achieved by alloying additions, producing the high alloy austenitic stainless steels. Alternative solutions are to employ high Cr and Mo ferritic stainless steels, or the two phase, or duplex, ferritic–austenitic stainless steels. The choice of alloy depends on several factors, such as mechanical properties, economics, and weldability. The last property may be cri...

Corrosion engineering of high pressure piping in RO-plants

Through the utilization of high-alloy stainless steel for the high-pressure piping failures due to corrosion will be prevented. Furthermore, piping with reduced wall thickness compared to stainless steel 316L can be used which facilitates handling, welding and pipe-work assembly. In addition, the smaller wall thickness will allow that outlets for branch connections can be engineered for optimum corrosion resistance and cost effectiveness.

The sulfide stress corrosion cracking of high alloy stainless steels for oil and natural gas wells

The stress corrosion cracking resistance of some martensitic, precipitation hardening, duplex and high alloy austenitic stainless steels for oil or gas wells has been investigated. Different test methods have been utilized: constant load tests on smooth specimens with and without crevices, constant deformation tests on precracked specimens, and constant elongation rate tests on smooth specimens. The tests have been carried out in a NACE environment and in a modified NACE environment. The tests confirmed both the susceptibility to stress corrosion cracking of the martensitic and the precipitation hardening stainless steels and the resistance of the high austenitic stainless steel. Some tests showed the stress corrosion cracking susceptibility of the duplex stainless steel to be worse than expected from traditional tests.

Evaluation of biological monitoring among stainless steel welders.

Ten manual metal arc (MMA) high alloy stainless steel (SS) welders were studied during one week and the concentrations of chromium (Cr) and nickel (Ni) were determined in their urine and blood. Stationary and personal air samples were collected from the immediate work environment; they covered the entire work period. Spot urine samples were collected during the follow-up period. Whole blood and plasma samples were taken from the workers before and after one shift, and the retention rate of magnetic dust in the lungs was estimated with magnetopneumography. On the basis of the results, indices of short-term exposure to Cr and Ni were evaluated. Urinary Cr and Ni concentrations (corrected to creatinine) reflect both the body burden caused by long-term and short-term exposure to easily soluble fractions of these metals. The results indicated that the use of Cr and Ni urinary analyses as indices of short-term exposure is not as dependable as previously assumed. The Cr and Ni concentrations in whole blood and plasma did not correlate with the measured exposure, but the daily mean increase in the Cr concentration reflected exposure to total Cr and Cr(VI) very well. The large variation in the Cr concentration of the morning urine (0.01-2.7 mumol/1) and blood (0.05-1.43 mumol/1) indicated large personal variations of body burden among the exposed welders. The retention rate of magnetic dust in the lungs correlated well (P less than 0.01) with the daily mean increase of Cr in blood. Very good correlations (P less than 0.001) were found between the retention rate of magnetic dust and the personal air samples of Cr and Cr(VI).

Dynamic recrystallization behavior of austenite in Nb-bearing high strength low alloy steels and stainless steel.

Dynamic recrystallization behavior of austenite in Nb-bearing HSLA steels as well as 18-8 stainless steel was investigated by hot compression testing. The hot deformation equipment, incorporating a rapid cooling system and an on-line data analysis system, made it possible to investigate the microstructural changes associated with the restoration process by fully suppressing the static recovery and recrystallization. The stress-strain behavior to a strain of 0.70 was investigated in the range of strain rate from 5×10-4 to 10s-1 and of temperature from 900 to 1200°C. Dynamic recrystallization was observed in all the steels deformed under the condition of the Zener-Hollomon parameter being less than 1015s-1, but it was not observed with the highest strain rate of 10s-1. An increase of Nb content in the HSLA steels increased the flow stress for any deformation condition and also increased the strain at onset of dynamic recrystallization. Dynamically recrystallized grain size was primarily related to the value of the Zener-Hollomon parameter, but was not influenced by the initial grain size. The microstructural changes accompanying dynamic recovery and dynamic recrystallization were investigated by thin foil TEM observation and related to the results of hardness measurements.

Polarization Resistance Measurements of High Alloyed Austenitic Steels with Low Carbon Content in Boiling Solutions of Concentrated Nitric Acid

Abstract The relation between corrosion rate and polarization resistance (Rp) has been determined for high alloyed austenitic stainless steels with low carbon content as well as stabilized austenitic steels in boiling concentrated HNO3 solutions. It has been shown that the Rp value corresponding to corrosion reaction can be measured in spite of the presence of HNO3 as a rapid redox system. The relation for calculation of corrosion rate from the Rp value was found to be sufficiently constant with different types of austenitic steels and with different concentrations of the acid.

Physicochemical characteristics of powder boronizing

1. According to thermodynamic calculations, in the initial stage boronizing may occur through a dual mechanism: simultaneous active diffusion of atomic boron and direct reaction resulting in the formation of higher iron borides. 2. It is demonstrated that in powder boronizing the nature of the activator has a pronounced effect on the concentration and character of distribution of pores in the boride layer and hence on the quality of the resultant coating. 3. On the basis of data yielded by thermodynamic calculations and experimental results it is shown that iron oxides can be successfully employed as activators of the powder boronizing process. 4. Experiments have shown that a powder mixture consisting of 80% boron carbide and 20% magnetite is comparable in activity to mixtures with halide activators, but surpasses them in other respects: It possesses good processing characteristics, is nontoxic, shows no sinterability, and is capable of being used repeatedly without regeneration. 5. Mixtures with oxygen transport of boron offer greatest promise for the boronizing of high-alloy stainless steels.

Rate of development of fatigue cracks in some high-alloy martensitic stainless steels

Rate of development of fatigue cracks in some high-alloy martensitic stainless steels