Effect of alpha prime due to 475 °C aging on fracture behavior and corrosion resistance of DIN 1.4575 and MA 956 high performance ferritic stainless steels

The 475 °C embrittlement in stainless steels is a well-known phenomenon associated to alpha prime (α′) formed by precipitation or spinodal decomposition. Many doubts still remain on the mechanism of α′ formation and its consequence on deformation and fracture mechanisms and corrosion resistance. In this investigation, the fracture behavior and corrosion resistance of two high performance ferritic stainless steels were investigated: a superferritic DIN 1.4575 and MA 956 superalloy were evaluated. Samples of both stainless steels (SS) were aged at 475 °C for periods varying from 1 to 1,080 h. Their fracture surfaces were observed using scanning electron microscopy (SEM) and the cleavage planes were determined by electron backscattering diffraction (EBSD). Some samples were tested for corrosion resistance using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. Brittle and ductile fractures were observed in both ferritic stainless steels after aging at 475 °C. For aging periods longer than 500 h, the ductile fracture regions completely disappeared. The cleavage plane in the DIN 1.4575 samples aged at 475 °C for 1,080 h was mainly {110}, however the {102}, {314}, and {131} families of planes were also detected. The pitting corrosion resistance decreased with aging at 475 °C. The effect of alpha prime on the corrosion resistance was more significant in the DIN 1.4575 SS comparatively to the Incoloy MA 956.     

Corrosion properties of S-phase layers formed on medical grade austenitic stainless steel

The corrosion properties of S-phase surface layers formed in AISI 316LVM (ASTM F138) and High-N (ASTM F1586) medical grade austenitic stainless steels by plasma surface alloying with nitrogen (at 430°C), carbon (at 500°C) and both carbon and nitrogen (at 430°C) has been investigated. The corrosion behaviour of the S-phase layers in Ringer’s solutions was evaluated using potentiodynamic and immersion corrosion tests. The corrosion damage was evaluated using microscopy, hardness testing, inductive coupled plasma mass spectroscopy and X-ray diffraction. The experimental results have demonstrated that low-temperature nitriding, carburising and carbonitriding can improve the localised corrosion resistance of both industrial and medical grade austenitic stainless steels as long as the threshold sensitisation temperature is not reached. Carburising at 500°C has proved to be the best hardening treatment with the least effect on the corrosion resistance of the parent alloy.     

Einsatzhärten mit Stickstoff für nichtrostende Wälzlager und Werkzeuge

Case hardening of stainless bearings and tools using nitrogen Case hardening of low alloy steels is widely used for bearings, tools, gear wheels etc. Carburizing of high alloy stainless steels, however, leads to the precipitation of carbides and lowers the corrosion resistance. This may be avoided by case hardening with nitrogen instead of carbon. Dissolved nitrogen increases the hardness, the corrosion resistance and the compressive residual stresses of the martensitic case. Secondary hardening during tempering allows for service temperatures up to about 350°C with only little loss of corrosion resistance. The new heat treatment, which is quite different from nitriding, may be applied e. g. to advanced bearings for aircraft or tools for processing aggressive polymers.     

Influence of heated forming tools on corrosion behavior of high strength aluminum alloys

In this study forming tools temperated at 24 °C and 350 °C were used to systematically investigate the influence of different cooling rates on the mechanical and corrosion properties of a high strength aluminum alloy AA7075 within a novel thermo-mechanical process that combines forming and quenching simultaneously. The samples formed within heated tools reveal higher ductility and lower material strength compared to the parts processed in cold tools. In addition, the corrosion behavior changed between samples formed with 24 °C forming tools and 350 °C forming tools, respectively. Through cyclic polarization in chloride containing aqueous media a change in the hysteresis and shift of open circuit potential was observed. Metallographic investigation revealed that there was also a very different corrosion morphology for the samples formed within the heated tools. No change in average grain size could be detected but changes of the microstructure in subgrain scale that occur during the forming within the heated tools are responsible for this effect. In further research, the effect of various cooling rates on mechanical and corrosion behavior and the microstructure will be investigated by variation of the forming tool temperature.     

Influence of nickel alloying on hot corrosion of stainless steels in SO2-environment

Flue dust produced by copper flash smelting causes problems by forming corrosive melts on the walls of heat recovery boiler used to recover the process heat value and to separate flue dust from SO₂-rich gas, derived from smelting sulphidic copper ore, before manufacture of sulphuric acid. The corrosion phenomena of molybdenum containing low and high nickel stainless steels AISI 316 and Sanicro 28 were studied by simulating the conditions prevailing in the heat recovery boiler in laboratory in the temperature range of 250–350°C. ZnCl₂ in copper smelter flue dust resulted in partial melting of the deposit containing sulphates and oxides of copper, zinc, arsenic, iron and lead, which increased the rate of corrosion dramatically. Chlorination of the steel was the dominant corrosion mechanism. The high nickel steel corroded more than low nickel steel due to reactivity of nickel under molten sulphates.     

Elevated temperature creep of pearlitic steels: an experimental–numerical approach

Pearlitic steels are well known for their high strength and hardness. This makes them the natural choice for applications in which structural integrity and minimum irreversible deformation over time are required. Although their room-temperature mechanical response has been intensively studied in the past, little information can be found in the literature regarding the effect of temperature on the mechanical response of pearlitic steels. In this paper, an experimental–numerical approach is used to study the mechanical response of pearlitic steels in the temperature range 20–500 °C. A finite-strain thermo-viscoplastic model is presented together with a set of elevated temperature tests (tensile and creep tests). The aim of the tests is twofold: first, to provide insight into the elevated-temperature mechanical response of the material; and second, to provide the data required to identify the corresponding material parameters. Furthermore, the model and the experimental data are instrumental in showing that the influence of temperature on the mechanical behavior of pearlitic steels becomes significant for temperatures above 350–400 °C.     

Corrosion behaviour of steels and refractory metals in flowing Lead-Bismuth Eutectic at low oxygen activity

The corrosion behaviours of AISI 316L and T91 steels, and several refractory metals as W, Mo and Ta, were investigated in flowing lead-bismuth eutectic (LBE) at 400°C. The tests were performed in the LECOR (Lead Corrosion) loop, at low oxygen activity in LBE. The results obtained show that all materials exhibited a weight loss after exposure to the flowing LBE, except for the case of tantalum, which exhibits a weight gain. The resistance to corrosion offered by refractory metals has been found higher in comparison to the case of both steels. Moreover, the austenitic steel behaved more resistance to corrosion induced by LBE than the martensitic steel, under the test condition adopted.     

Irradiation behavior of nanostructured 316 austenitic stainless steel

In order to get information about radiation resistance of ultrafine grained austenitic stainless steels, a 316 steel was deformed by high pressure torsion. The mean diameter of the grain after deformation was 40 nm. This material was annealed at 350 °C for 24 h or irradiated with 160 keV iron ions at 350 °C. Changes in the microstructure during annealing or irradiation were characterised by transmission electron microscopy (grain size) and laser assisted tomographic atom probe (solute distribution). Results indicate that this annealing has no influence on the grain size whereas the grain diameter increases under irradiation. Concerning the solute distribution, atom probe investigations show evidence of radiation-induced segregation at grain boundaries. Indeed, after irradiation, grain boundaries are enriched in nickel and silicon and depleted in chromium. On the contrary, no intragranular extended defects or precipitation are observed after irradiation.     

12. Controlled deposition of protective aluminum films on aircraft steels

A laboratory system for controlled vacuum deposition of aluminum films was assembled in order to conduct anodic protection coating experiments on CrMo aircraft steels. High deposition rates were achieved employing a high-power electron-beam gun for source evaporation. A special substrate-heater was constructed for controlled variation of coating temperature. Process variables and their effect on the relevant properties of the films, mainly adhesion and corrosion resistances, were studied. It was found that these properties improved with increasing substrates temperature in the range of 200–450°C. Optimum conditions were obtained for the following set of process variables: temperature = 400–450°C. Films of about 5 microm thickness were obtained conditions and their structure was studied using scanning electron microscope and X-ray diffraction methods. It was found that the films had a (111) preferred orientation evidenced by 0.5–1.0 microm crystallites, with the larger sizes corresponding to the higher temperatures.     

Wear Protection in Plastics Processing by Using Ceramic Components

Still, wear and corrosion of plastics processing machines, e. g. plasticizing units, are a common problem in the plastics processing industry. Whenever glass fibre reinforced resins are processed the common steels and hard facings used for plasticizing units reach their limits concerning the wear properties. A solution of this problem is the substitution of steel based materials by high performance ceramics. When substituting steel by ceramics the specific conditions of a plastics processing device have to be taken into account. The most critical conditions are the temperature range from 25 to 350 °C and the cyclic pressure load up to 400 bar. Furthermore a possible corrosive attack, e. g. by halogenated resins as polyvinyl chloride, has to be taken into account. To screen different high performance ceramics for their suitability in plastics processing machines the so called platelet apparatus was used. It could be shown that compared with common steels like 34CrAlNi7 the wear could be reduced by one magnitude.     

Corrosion resistance evaluation of the UNS S31803 duplex stainless steels aged at low temperatures (350 to 550°C) using DLEPR tests

This work describes the investigation of the corrosion evaluation of UNS S31803 duplex stainless steels aged at low temperatures using double loop electrochemical potentiokinetic reactivation tests (DLEPR). A wrought duplex stainless steel (DSS) UNS S31803 was aged at four temperatures in the 350—550°C range for times up to 1000 h. The hardening and embrittlement effects due to aging were also determined. An important decrease in corrosion resistance was observed in the samples aged at 475 and 550°C, but the samples aged at 350 and 400°C were not affected. Healing was observed in the samples aged for long times (1000 h at 475°C and 500 h at 550°C).     

The influence of aqueous environment of coolant of power plants on cyclical crack resistance of steels

The results of investigations of cyclic corrosion crack resistance of carbon steel of grade 20 and low-alloyed pipe steels of grade 15GS and 12Kh1MF are presented. The variants of water standard conditions of power plants are used as test environments. A significant activating impact of the aquatic environment on the kinetics of growth of fatigue cracks of the steels under study is shown. The most significant corrosive effect is observed when tested in an aqueous environment with addition of organic acid. In the low-frequency range of cyclical loading (0.04–0.0008 Hz), the frequency does not influence the characteristics of the cyclic corrosion crack resistance of steels. To eliminate the influence of stress cycle asymmetry on the diagram of corrosion-fatigue crack resistance of steels, it was proposed to use as a parameter of crack growth rate (CGR) the effective range of stress intensity factor (SIF), functionally associated with the factor of stress cycle asymmetry. When the temperature of the medium is increased from 80 to 150°C and to 280°C, the form of kinetic diagram of cyclic corrosion crack resistance is changed, resulting in a decrease in crack growth rate at the middle and upper sections of the diagram.     

Intergranular corrosion resistance of nanostructured austenitic stainless steel

Nanostructured metals and alloys possess very high strength but exhibit limited plasticity. Enhancement of the strength/ductility balance is of prime importance to achieve wide industrial applications. However, post-deformation heat treatment, which is usually used to improve plasticity, can lead to a decrease in other properties. In the case of austenitic stainless steels, heat treatment in the range from 480 to 815 °C can increase their susceptibility to intergranular corrosion. The aim of the work reported in this paper was to determine if nanostructured austenitic stainless steel is susceptible to intergranular corrosion if heat treated for 1 h at 700 °C. Samples of 316LVM austenitic stainless steel were hydrostatically extruded, in a multi-step process with the total true strain of 1.84 to produce a uniform microstructure consisting of nanotwins. These nanotwins averaged 21 nm in width and 197 nm in length. Subsequent annealing at 700 °C produced a recrystallised structure of 68-nm-diameter nanograins. The heat treatment improved the ductility from 7.8 to 9.2 % while maintaining the ultimate tensile strength at the high level of 1485 MPa. Corrosion tests were performed in an aqueous solution consisting of 450 ml concentrated HNO₃ and 9 g NaF/dm³ (according to ASTM A262-77a). The evaluation of the corrosion resistance was based on transmission and scanning electron microscopic observation of the microstructure and chemical analyses. The results revealed that both the as-received and HE-processed samples are slightly susceptible to the intergranular corrosion after annealing at 700 °C for 1 h.     

Structural characteristics of low temperature plasma carburised austenitic stainless steel

Abstract

A low temperature plasma carburising process has recently been developed to engineer the surfaces of austenitic stainless steels to achieve combined improvements in wear and corrosion resistance. The present paper discusses the structural characteristics of the carburised layers produced on AISI type 316 steel at temperatures between 400 and 600°C. It was found that at low temperatures (<520°C), the carburised layers produced were precipitation free and comprised a single phase, which had a face centred cubic structure and was identified as expanded austenite owing to the supersaturation of carbon in austenite. The carburised layer was in a deformed and distorted state. High densities of twins, stacking faults, and dislocations were found in the expanded austenite. The degree of lattice expansion was estimated and was found to vary with processing temperature and depth in the layer. Precipitation of carbides (mainly Cr₇ C₃ ) occurred when the carburising temperature was relatively high (for example 550 and 600°C). In addition, stress induced martensite was found, particularly in the carburised layers produced at relatively high temperatures.     

Influence of heat treatments on toughness and sensitization of a Ti-alloyed supermartensitic stainless steel

Supermartensitic steels are a new class of martensitic stainless steels developed to obtain higher corrosion resistance and better toughness through the reduction of carbon content, and addition of Ni and Mo. They were developed to more critical applications or to improve the performance obtained with conventional grades AISI 410, 420, and 431. In this study, the influences of the tempering parameters on the microstructure, mechanical properties (hardness and toughness), and sensitization of a Ti-alloyed supermartensitc stainless steel were investigated. The material showed temper embrittlement in the 400–600 °C range, as detected by low temperature (−46 °C) impact tests. The degree of sensitization measured by double loop reactivation potentiodynamic tests increased continuously with the increase of tempering temperature above 400 °C. Healing due to Cr diffusion at high tempering temperatures was not observed. Double tempered specimens showed high amounts (>20%) of reverse austenite but their toughness were similar to specimens single tempered at 625 and 650 °C.     

Influence of the grain size on deleterious phase precipitation in superduplex stainless steel UNS S32750

In the present work, the effect of grain size on deleterious phase precipitation in a superduplex stainless steel was investigated. The materials studied were heat treated isothermally at 800 °C, 850 °C and 900 °C for times up to 120 min. Hardness tests, light optical microscopy, scanning electron microscopy and X-ray diffraction were carried out to detect sigma and other harmful precipitate phases. The ferritic and austenitic grain sizes in the solution treated condition of the two steels analyzed were measured by electron backscattered diffraction (EBSD). Cyclic polarization corrosion tests were performed to evaluate the effect of grain size on the corrosion resistance. The results presented show that the precipitation of deleterious phases such as χ, σ and γ₂, which can occur during welding and forming operations, is retarded by grain growth.     

Struktur und Eigenschaften nichtrostender Stähle nach einer Plasmaimmersionsionenimplantation und einer Plasmanitrierung

Structure and properties of stainless steels after plasma immersion ion implantation and plasma nitriding Stainless steels can be nitrided at temperatures ≤ 400 °C to increase their hardness and wear resistance without a decreasing of their excellent corrosion resistance. Structure and properties of the surface layers produced by plasma nitriding and plasma immersion ion implantation in this temperature range were tested. There are negligible differences in the structure of the produced surface layers in spite of different interaction principles of the used technologies. However there are clear differences between the case of different steels. The case of ferritic chromium steels mainly consists of ε-nitride. Whereas the cases of austenitic and ferritic austenitic steels are characterized by expanded austenite. The corrosion resistance of the steels is reduced by nitriding only, if evident CrN-formation occurs.     

Effects of the high temperature plasma immersion ion implantation (PIII) of nitrogen in AISI H13 steel

In this paper we report the effect of high temperature PIII of nitrogen on the chemical and physical properties of AISI H13 steel. The implantation of H13 steels was carried out at different temperatures ranging between 300 °C and 720 °C. After the treatment, the surface morphology was drastically changed as observed by SEM analysis. Nitrogen penetration depth reaching up to 12 μm was achieved at 620 °C and 720 °C. The maximum hardness of about 592 HV was obtained for the sample treated at 470 °C that is 17% higher than for untreated specimen. There was a decrease of the hardness values for temperatures above 470 °C. The same hardness behavior with the temperature was confirmed by nanoindentation testing. Although an enriched nitrogen layer was obtained, no evidence of nitride compounds was detected by XRD analyses. On the other hand, improvements of the H13 steel tribological properties and corrosion resistance were obtained. The wear tests were conducted by pin-on-disk tribometer (rotating mode). The wear volume decreased by factor of 4.5 compared to the standard tempered and annealed H13 steel and 2.6 times reduction of the coefficient of friction was achieved. The electrochemical measurements were performed in 3.5% NaCl solution, pH = 6. Open circuit potential curves showed that the potentials are nobler for the PIII treated samples than for untreated specimen. In addition, the corrosion current density of the samples treated at 620 °C and 720 °C diminished to 3 × 10⁻⁸ A/cm².     

Influence of nitriding on corrosion resistance of martensitic X17CrNi16‐2 stainless steel

Nitriding increases surface hardness and improves wear resistance of stainless steels. However, nitriding can sometimes reduce their corrosion resistance. In this paper, the influence of nitriding on the corrosion resistance of martensitic stainless steel was investigated. Plasma nitriding at 440 °C and 525 °C and salt bath nitrocarburizing were carried out on X17CrNi16‐2 stainless steel. Microhardness profiles of the obtained nitrided layers were examined. Phase composition analysis and quantitative depth profile analysis of the nitrided layers were preformed by X‐ray diffraction (XRD) and glow‐discharge optical emission spectrometry (GD‐OES), respectively. Corrosion behaviour was evaluated by immersion test in 1% HCl, salt spray test in 5% NaCl and electrochemical corrosion tests in 3.5% NaCl aqueous solution. Results show that salt bath nitrocarburizing, as well as plasma nitriding at low temperature, increased microhardness without significantly reducing corrosion resistance. Plasma nitriding at a higher temperature increased the corrosion tendency of the X17CrNi16‐2 steel.     

Spannungsrißkorrosionsuntersuchungen bei hohen Temperaturen. Verhalten höherlegierter Stähle unter praxisnahen Bedingungen

Stress Corrosion Cracking at Temperature in the range of 180 °C . Tests are described to find out the stress corrosion behaviour of some stainless steels of the austenitic and ferritic-austenitic type in low concentrated aqueous chloride media with temperatures up to 180°C and pressures up to 10 bar. A sensitivity of the austenitic steels could be determined during 6 to 55 hours, the ferritic-austenitic steel X 2 CrNiMoN 22 5 however didn't suffer any SCC during 235 h with the exception of the welded state. In this way – by the use of a pressure vessel – it is possible, to carry out laboratory tests under conditions of practice.