Precipitation of disperse vanadium carbides at the interphase boundary upon the pearlitic transformation of a steel

It has been shown that upon pearlitic transformation of a high-carbon vanadium steel, vanadium carbides 5–10 nm in size precipitate in the ferrite interspaces of the pearlite in the form of periodic rows (with a period of ∼20 nm). A correspondence between the distribution of vanadium carbides and the shape of the γ/α interphase boundary in the pearlite has been found. It has been supposed that in commercial steels the pearlitic transformation is accompanied by the formation of segregates of different atoms at the discretely moving γ/α interphase boundary.     

Influence of copper on the structure and mechanical properties of pearlitic steels

The structure and mechanical properties of pearlitic steels, which contain ~0.6% carbon and copper in the amount of 1.25 and 1.4%, have been studied in the states immediately after the pearlitic transformation (with different rates of cooling) and after tempering at 500°C. It has been established that tempered pearlitic steel with copper is 10–15% stronger than the steel of similar composition without copper. The strengthening of copper-containing pearlitic steel after tempering is caused by the precipitation of copper particles 5–20 nm in size in the ferritic regions of pearlite and in grains of free ferrite.     

Vermeidung der Wasserstoffversprödung vergüteter Stähle bei galvanotechnischen Prozessen durch thermische Legierungsbildung

Avoidance of hydrogen embrittlement of high strength steels during electroplating processes by thermal alloying Low alloyed high strength steels are often electroplated by metal layers protecting against corrosion. For ultra high strength, quenched and tempered steels with yield strengths > 1000 Nmm^?2 embrittlement by hydrogen being envolved during the electrochemical pretreatment as well as metal deposition has to be avoided. More over the corrosion protecting layers should form a diffusion barrier for hydrogen which can be formed during corrosion processes under special circumstances. In this paper two problem solutions including thermal alloying processes will be discussed. Plating the steel substrate with a nickel layer subsequently annealed at a temperature above 800°C in an inert gas atmosphere an austenitic iron-nickel-alloy at the boundary is formed, being a high efficient diffusion barrier for hydrogen. Further zinc plating is improving the corrosion resistance avoiding at the same time pitting corrosion problems. Plating the steel substrate with a copper and a following nickel layer on top and annealing it at the temperature of 800°C a highly corrosion resistant copper-nickel-alloy is formed showing excellent barrier behaviour for hydrogen diffusion. In both cases hydrogen being formed during the plating process itself and penetrating into the base metal does not lead to embrittlement as it is effusing during the annealing procedure.     

Effect of carbide volume fraction on the oxidation of austenitic Fe‐Cr‐C alloys

A series of Fe‐15Cr‐(2‐3)Mo alloys (compositions in weight percent) was produced with different carbon concentrations, to control the distribution of chromium between matrix metal and M₂₃C₆ precipitates. The alloys were oxidized in the austenitic state at 850°C in pure oxygen, with and without a pre‐oxidation treatment at low oxygen potential, where no iron oxide could form. Protective, chromia‐rich scaling took place if the chromium concentration at the metal‐scale interface was high enough. This concentration was controlled by the original alloy matrix chromium concentration, and whether or not a high diffusivity ferrite zone developed at the surface by decarburization. Ferrite zone formation was assisted by pre‐oxidation at low oxygen potentials. The value of the carbides as suppliers of additional chromium was demonstrated by comparison with the oxidation performance of carbide‐free alloys of corresponding matrix chromium levels. However, because dissolution of the coarse carbides could be slow, alloys with high volume fractions of large carbides were unsuccessful.     

Metal dusting in CCR platforming unit

The mechanism of metal dusting of 9Cr-1Mo steel in CCR platforming unit, based on examinations of a charge heater tube, is presented. The tube operated for 10 years, and the metal skin temperature was about 600 °C. The feed was composed of hydrotreated naphtha and hydrogen gas. The mechanism of the corrosion was elucidated using scanning electron microscopy, electron probe microanalysis and X-ray diffraction technique. It has been found that the carbon deposition on the steel surface and its inward diffusion into the steel is accompanied by the outward diffusion of carbide forming elements, i.e. chromium and molybdenum. At an advanced stage of the metal dusting process a thin layer of fine chromium-rich carbides beneath the steel surface exists. The layer is followed by a porous zone composed of big degraded primary carbides and fine carbides instead of alloy ferrite, with chromium and molybdenum content higher than the ferrite inside the tube wall. On the steel surface, a layer of coke composed of graphite, iron and M₇C₃ carbides is formed and the uniform wastage of the steel takes place. Possible influence of some sulphur additions to the CCR platformer feed during the future service on degradation of the subsurface material has been considered.     

Effect of different thermochemical treatments on the electrochemical behaviour of a hot working steel

The X 37CrMoV5 1 KU hot working steel has been quenched and tempered, overcarburised, nitrided and nitrocarburised. The surface layers constituted after the thermochemical treatments were characterised by means of X‐ray diffraction (XRD) analyses, optical and scanning electron microscope (SEM) observations, micro‐ and macrohardness indentations. The electrochemical behaviour was evaluated in two different aggressive environments: 0.1 M Na₂SO₄ and 0.1 M NaCl solutions. While the overcarburised and the nitrocarburised samples exhibit a poor electrochemical behaviour, the presence of a nitrided surface layer enhances the corrosion resistance. This is attributable to the protective action of the nitrided layer that hinders the anodic dissolution reaction of the matrix, whereas the presence of complex carbides in the overcarburised or nitrocarburised samples causes the preferential dissolution of the ferritic matrix due to galvanic coupling phenomena. Finally, the poor corrosion resistance exhibited by the nitrocarburised sample can be ascribed to the presence of a large amount of ferrite together with little ε solid solution in the surface layer.     

Growth of lamellar pearlite in the weld zone between dissimilar steels

Transmission electron microscopy is used to study the welds between high-carbon pearlitic and chromium–nickel austenitic steel workpieces performed by flash butt welding. It has been established that lamellar pearlite colonies alloyed with chromium and nickel are formed in the weld zones between dissimilar steels. Thin austenite interlayers have been detected in the center of ferrite plates. The structure formed presents the C–F–A–F–C–F–A–F (and so on) sequence of three plate-shaped phases. The ferrite–cementite structure in alloyed-pearlite colonies is finer than that in unalloyed pearlite.     

Corrosion behaviour of duplex stainless steels sintered in nitrogen

Duplex stainless steels obtained through powder metallurgy (PM) technology from austenitic AISI 316L and ferritic AISI 430L powders were mixed on different amounts to obtain biphasic structures with austenite/ferrite ratio of 50/50, 65/35 and 85/15. Prepared mixes of powders have been compacted at 750 MPa and sintered in N₂-H₂ (95% and 5%) at 1250 °C for 1 h. Corrosion behaviour, using electrochemical techniques such as anodic polarization measurement, cyclic anodic polarization scan and electrochemical potentio-kinetic reactivation test and double loop electrochemical potentio-kinetic reactivation double loop test were evaluated. For duplex stainless steels, when austenite/ferrite ratio increases the corrosion potential shifts to more noble potential and passive current density decreases. The beneficial effect of annealing solution heat treatment on corrosion behaviour was established and was compared with corrosion behaviour of vacuum sintered duplex stainless steels. The results were correlated with the microstructural features.     

Electrochemical corrosion response of a precipitation hardening moulds steel in a NaCl solution

Investigations concerning the electrochemical corrosion behavior of mould steels are scarce and limited. In the present study, the corrosion resistance of a precipitation hardening moulds steel submitted to aging treatments has been assessed in a 0.1 M NaCl aqueous solution by means of potentiodynamic polarization and electrochemical impedance spectroscopy measurements. The electrochemical characterization of the steel not submitted to hardening treatments was also examined and a comparative study was made between the different specimens. Scanning electron microscopy and electron probe microanalysis were used to obtain information about the morphology and chemical composition of the steels surface after the electrochemical tests. Increasing the aging temperature an increase of secondary hardness is observed, attributable to an increase of secondary phases (re‐precipitated carbides and metastable GP zones) amount, positively reflecting on the mechanical properties. However, the electrochemical results show a progressive worsening of the corrosion behavior, the as‐received steel sample exhibiting the best corrosion resistance. The observed decrease of corrosion resistance with increasing the aging temperature is ascribed to enhanced galvanic coupling phenomena between the secondary phases, acting as cathodes, and the surrounding matrix which, consequently, experiences preferential dissolution.     

Microstructures and Mechanical Properties of Bearing Steels Modified for Preparing Nanostructured Bainite

Mo containing high-C-Cr bearing steel was modified with Si (0.8–1.5 wt.%) and 0.8Si–1.0Al to prepare nanostructured bainite by low-temperature isothermal heat treatment. The modified steels were isothermal held at 220 to 240 °C after partial austenitization in an intercritical gamma+carbide region, and the resultant microstructure and mechanical properties were studied. Carbide-free nanostructured bainite with plate thickness below 100 nm and film retained austenite, as well as a small amount of undissolved carbide particles, was obtained in the modified steels except in 0.8Si steel, in which carbides precipitated in bainitic ferrite. As Si content increased, the mean thickness of bainitic ferrite plates modestly decreased, whereas the fraction of retained austenite markedly increased. The thickness of bainitic ferrite plate and the fraction of retained austenite in Si-Al-modified steel were smaller than those in Si-modified steels. The hardness and elongation of the Si-Al-modified steel were lower than those of Si-modified steels. The yield strength of Si-Al-modified steel was superior to that of Si-modified steels. Mid-level ultimate tensile strength and impact toughness were achieved in Si-Al-modified steel. For bearing applications, Si-modified steels could provide higher hardness and toughness but lower dimensional stability. Meanwhile, Si-Al-modified steel could offer higher dimensional stability but lower hardness and toughness.     

Identification of the selective corrosion existing at the seam weld of electric resistance-welded pipes

The selective corrosion existing at the seam weld of high frequency electric resistance welded pipes of carbon steel with low sulfur content in electrolyte solutions is revealed by localized electrochemical measurements. The seam weld, mainly consisted of ferrite, has more negative open circuit potential and higher anodic dissolution current density than the base metal consisting ferrite and pearlite. Between the seam weld and the base metal, there is a galvanic coupling effect accelerating the dissolution kinetics of the seam weld such that V-shaped corrosion groove preferentially occurs at the seam weld.     

The precipitation of copper in abnormal ferrite and pearlite in hyper-eutectoid steels

The precipitation of copper within abnormal ferrite and pearlitic phases in hyper-eutectoid Fe-C-Cu and Fe-C-Mn-Cu experimental steels has been examined, principally by transmission electron microscopy, in terms of morphology, mechanism and crystallography. Interphase precipitation of copper during austenite decomposition was found to be a primary precipitation mechanism. However, precipitation due to ageing was also observed in both abnormal and pearlitic ferrite. The latter case appeared either as uniformly distributed precipitates within the ferrite or in association with dislocations. Copper precipitates adopted either the Kurdjumov–Sachs or the Nishiyama–Wasserman orientation relationship with respect to the ferrite. Disc-shaped precipitates thought to form by ageing within abnormal and pearlitic ferrite develop on {001}_ferrite habit planes. Twinning observed within copper precipitates formed by interphase precipitation was attributed to a requirement to accommodate the growth of precipitates at the interphase boundary. Multi-twinned precipitates were found in pearlitic ferrite and may be evidence for an intermediate transitional state between b.c.c. and f.c.c. structure.     

Interpretation of high-temperature tensile properties by thermodynamically calculated equilibrium phase diagrams of heat-resistant austenitic cast steels

High-temperature tensile properties of three heat-resistant austenitic cast steels fabricated by varying W, Mo, and Al contents were interpreted by thermodynamically calculated equilibrium phase diagrams of austenite, ferrite, and carbides as well as microstructural analyses. A two-step calculation method was adopted to cast steel microstructures below the liquid dissolution temperature because the casting route was not an equilibrium state. Thermodynamically calculated fractions of equilibrium phases were well matched with experimentally measured fractions. Ferrites existed at room and high temperatures in both equilibrium phase diagrams and actual microstructures, which has not been reported in previous researches on austenitic cast steels. In the W2Mo1Al1 steel, 38% and 12% of ferrite existed in the equilibrium phase diagram and actual microstructure, respectively, and led to the void initiation and coalescence at ferrites and consequently to the serious deterioration of high-temperature strengths. The present equilibrium phase diagrams, besides detailed microstructural analyses, effectively evaluated the high-temperature performance by estimating high-temperature equilibrium phases, and provided an important idea on whether ferrite were formed or not in the heat-resistant austenitic cast steels.     

Influence of Ti,C and N concentration on the intergranular corrosion behaviour of AISI 316Ti and 321 stainless steels

Intergranular corrosion behaviour of 316Ti and 321 austenitic stainless steels has been evaluated in relation to the influence exerted by modification of Ti, C and N concentrations. For this evaluation, electrochemical measurements – double loop electrochemical potentiokinetic reactivation (DL-EPR) – were performed to produce time–temperature–sensitization (TTS) diagrams for tested materials. Transmission (TEM) and scanning electron microscopy (SEM) were used to determine the composition and nature of precipitates. The addition of Ti promotes better intergranular corrosion resistance in stainless steels. The precipitation of titanium carbides reduces the formation of chromium-rich carbides, which occurs at lower concentrations. Also, the reduction of carbon content to below 0.03 wt.% improves sensitization resistance more than does Ti content. The presence of Mo in AISI 316Ti stainless steel reduces chromium-rich carbide precipitation; the reason is that Mo increases the stability of titanium carbides and tends to replace chromium in the formation of carbides and intermetallic compounds, thus reducing the risks of chromium-depletion.     

Graphitization in high carbon commercial steels

Graphitization kinetics in two commercial high carbon steels, AISI types 1075 and 1095, are studied by conducting a series of isothermal annealing treatments in the temperature range of 560 to 680‡ C for periods of time ranging from 20 to 500 h. The samples selected were collected along the processing route in a commercial production line dedicated to the fabrication of thin strip. The structures studied were those of hot rolling (consisting of fine pearlite), cold rolling (spheroidized carbides within a deformed ferritic matrix), and subcritical annealing (spheroidized carbides in undeformed ferrite). The samples obtained from hot rolled coils do not graphitize, whereas those cold rolled graphitize at a rate that depends on the type of steel and degree of deformation. No graphite was found in samples from the lower carbon steel, which were subcritically annealed, although they were observed in specimens from the other steel, which were cold rolled to a reduction of 50% prior to the subcritical annealing.     

Characterization of the Trace Phosphorus Segregation and Mechanical Properties of Dual-Phase Steels

The segregation behavior of trace amount of phosphorus (P) and the mechanical properties of dual-phase (DP) steels have been systematically studied. The microstructure of DP steels is mainly composed of martensite, ferrite and nanoscale carbides. For the DP steels with different trace amounts of P (≤ 0.015 wt%), P has almost no effect on the mechanical properties. Atom probe technology (APT) analyses confirm that P segregation was only found at the precipitate/matrix interface. Moreover, the precipitates of (Ti, Mo) C are widely distributed in the ferrite, martensite and ferrite/martensite interface regions. The special segregation feature of P would not concentrate at specific regions such as ferrite/martensite interface and/or martensite lath interface, which reveals that trace amounts of P (≤ 0.015wt%) have almost no effect on the mechanical properties of DP steels. It is proved for the first time that the MC-type carbides of (Ti, Mo) C can reduce or eliminate the damage effect of P on the mechanical properties of steels, which provides a new way for the design of alloys to reduce P damage. This work will promote to increase the P content control standard in DP steels from 0.01 to 0.015 wt%, which will not change the mechanical properties, but greatly reduce the scrap rate and increase the energy efficiency of the manufacturing process.     

Electrochemical noise characterisation of base metal and weld metal of an ASTM 516 steel in H2S containing environment

The inspection of steel structures exposed to sour environments has revealed a higher incidence of cracks at welded joints. This tendency is associated to several factors, including residual stresses, different microstructures and dissimilar electrochemical behaviour between base metal and weld metal. The crack incidence in general is attributed to hydrogen absorption due to the cathodic reduction of this species. In this work, the electrochemical noise behaviour of weld and base metals of an ASTM 516, C‐Mn steel exposed to a H₂S containing environment is studied. Current and voltage fluctuations are measured and the spectral noise impedance (R_sn) is determined. The current fluctuations are measured by means of a Zero Resistance Ammeter and voltage fluctuations with respect to a calomel reference electrode. In this case, residual stresses and microstructures of both materials can be assumed as secondary factors to hydrogen cracking. However, electrochemical noise measurements indicated the existence of different electrochemical processes for base metal and weld metal in the environment considered.     

Effects of aluminum on the oxidation of 25Cr-35Ni cast steels

The oxidation kinetics and morphological development during reaction of two cast austenitic steels at 1000°C in pure dry oxygen at 20 kPa are reported. Both steels contained approximately 25 wt.% Cr and 35 wt.% Ni and, in addition, one steel contained 3.3 wt. %. Both steels oxidized to form external scales consisting mainly of Cr₂O₃ with a thin outer layer of manganese rich spinel. Scale growth kinetics were parabolic, and somewhat faster rates were observed for the aluminum bearing steel. In both steels, deep internal oxidation occurred at the site of primary (interdendritic) carbides. The kinetics of this process were parabolic, and rate control was attributed to oxygen diffusion along the interface between internal oxide and matrix metal. In the aluminum-free steel, interdendritic carbides were converted to chromium rich oxide, but when aluminum was present, a sheath of aluminum rich oxide formed around the carbides. In this latter case, the rate of interdendritic penetration was somewhat slower. The aluminum bearing steel also formed large numbers of rod-shaped Al₂O₃ precipitates within the austenitic dendrites. Deepening of the Al₂O₃ precipitate zone also proceeded according to parabolic kinetics at a rate consistent with rate control by diffusion of oxygen along the oxide-alloy interfaces.     

Oxide films formed during chemical polishing of steels

Abstract

Some mild steels when chemically polished in oxalic acid/hydrogen peroxide solutions developed oxide films, containing chromium, which were unusually resistant to removal. The same steels developed similar films during electrochemical passivation in sulphuric acid. Susceptibility to chromium enrichment seems to be not solely influenced by chromium content but is affected by heat-treatment and possibly depends on some chromium remaining in solution in the ferrite. For mild steels, chromium enrichment does not prevent brightening, but for steels deliberately alloyed with chromium it may.

Chromium enrichment of the surface of a stainless steel was also found after electrochemical polishing. The use of anodic oxidation at constant current for the examination of stainless steel surfaces is discussed.