Ferrite Grain Boundaries Research Articles

SCC initiation for X65 pipeline steel in the “high” pH carbonate/bicarbonate solution

The initiation of stress corrosion cracking (SCC) was studied using scanning electron microscope observations of linearly increasing stress test specimens. SCC initiation from the following surfaces was studied: (i) initiation from the commercial pipe surface covered by the Zn coating, (ii) initiation from a mechanically polished surface with a deformed layer, and (iii) initiation from an electro-polished surface. SCC initiation involved different features for these surfaces as follows. (i) For the Zn coated commercial pipe surface, a crack in the Zn coating led to the dissolution of the deformed layer and when the deformed layer was penetrated, intergranular SCC initiation became possible. (ii) For a mechanically polished surface with a deformed layer, cracks in the surface oxide concentrated the anodic dissolution to such an extent that there was transgranular SCC in the deformed layer. SCC was intergranular when the deformed layer had been penetrated. Transgranular stress corrosion cracks were stopped at ferrite grain boundaries (GBs) oriented perpendicular to the SCC propagation direction. (iii) For an electro-polished surface, the surface oxide film was cracked at many locations, but intergranular SCC only propagated into the steel when the oxide crack corresponded to a GB. An oxide crack away from a GB is expected to be healed. The observed SCC initiation mechanism was not associated with simple preferential chemical attack of the ferrite GBs.

Microstructure and grain boundary microanalysis of X70 pipeline steel

Grain boundaries (GBs), particularly ferrite: ferrite GBs, of X70 pipeline steel were characterized using analytical electron microscopy (AEM) in order to understand its intergranular stress corrosion cracking (IGSCC) mechanism(s). The microstructure consisted of ferrite (alpha), carbides at ferrite GBs, some pearlite and some small precipitates inside the ferrite grains. The precipitates containing Ti, Nb, V and N were identified as complex carbo-nitrides and designated as (Ti, Nb, WC, N). The GB carbides occurred (1) as carbides along ferrite GBs, (2) at triple points, and (3) at triple points and extending along the three ferrite GBs. The GB carbides were Mn rich, were sometimes also Si rich, contained no micro-alloying elements (Ti, Nb, V) and also contained no N. It was not possible to measure the GB carbon concentration due to surface hydrocarbon contamination despite plasma cleaning and glove bag transfer from the plasma cleaner to the electron microscope. Furthermore, there may not be enough X-ray signal from the small amount of carbon at the GBs to enable measurement using AEM. However, the microstructure does indicate that carbon does segregate to alpha : alpha GBs during microstructure development. This is particularly significant in relation to the strong evidence in the literature linking the segregation of carbon at GBs to IGSCC. It was possible to measure all other elements of interest. There was no segregation at alpha : alpha GBs, in particular no S, P and N, and also no segregation of the micro-alloying elements, Ti, Nb and V. (C) 2003 Kluwer Academic Publishers.

Cyclic Crack Resistance of Grey and High-Strength Cast Irons with Increased Phosphorus Content

We investigated the influence of phosphorus (0.02–0.76%) on the microstructure, short-term strength, cyclic crack resistance characteristics, and micromechanism of fatigue fracture of grey and high-strength cast irons. It was established that the low cyclic crack resistance of grey and high-strength cast irons with increased phosphorus contents (0.7–0.8%) is caused by the propagation of a fatigue crack via intergranular cleavage, initiated by a discontinuous or continuous network of precipitates of the ternary fine-grained phosphide eutectic along boundaries of ferrite grains. We showed that, from the viewpoint of cyclic crack resistance, it is admissible to alloy cast irons of the ferritic and ferritic–pearlitic class with phosphorus up to 0.3% when the phosphide eutectic forms in amounts of 3–5% for grey cast irons and 4–7% for high-strength cast irons without significant decrease in their resistance to brittle fracture.

Effect of H 2S on the electrochemical behavior of steel weld in acidic chloride solutions

Three different welding heat inputs, 15, 30 and 45 kJ cm −1, were applied to A516 carbon steel plates via submerged arc welding process. Electrochemical behavior of weld metal, with grain boundary ferrite (GF)/acicular ferrite (AF) structure, of A516 carbon steel weldment in saturated H 2S containing acidic chloride solution at 25°C was investigated by potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS) measurements. Potentiodynamic polarization curves reveal that the presence of H 2S in acidic chloride solution leads to a decrease in the cathodic current density and corrosion potential of weld metal as compared with that without H 2S, regardless of heat input. EIS data show that the corrosion process for weld metal in the H 2S containing solution is governed by the formation of sulfide film and the presence of pores on the metal surface. The polarization resistance increases by increasing the heat input, or by increasing the volume fraction ratio of GF/AF in the weld metal.

Boundary characterisation of X65 pipeline steel using analytical electron microscopy

Analytical electron microscopy was used to characterise grain boundaries (GBs) and interphase boundaries (IBs) of X65 pipeline steel. There was no segregation of P or S at the proeutectoid ferrite GBs. This indicates that contrary to literature expectations, P and S are unlikely to be involved in the mechanism of SCC of pipeline steels. There was Mn segregation at IBs between pro-eutectoid ferrite and pearlitic cementite, and desegregated from the IBs between pro-eutectoid ferrite and pearlitic ferrite. This pattern of Mn segregation is attributed to diffusion in the process zone ahead of the pearlite during the austenite to pearlite transformation and diffusion in the IBs between the proeutectoid ferrite and pearlite. A new mechanism was proposed for pearlite formation. A GB carbide first forms at an α : α GB, and then grows along the α/γ interface. Subsequently pearlite initiates from this interface carbide.

Measurement of grain boundary composition for X52 pipeline steel

Analytical electron microscopy was used to measure the composition of grain boundaries (GBs) and interconstituent boundaries (IBs) of X52 pipeline steel using specimens about 40–60nm in thickness. All elements of interest were examined with the exception of carbon. With this caveat, there was no segregation at proeutectoid ferrite GBs. This indicated that the commonly expected species S and P are not responsible for preferential corrosion of GBs during intergranular stress corrosion cracking of pipeline steels. Manganese was the only species measured to segregate at the IBs. Manganese segregated to the IBs between proeutectoid ferrite and pearlitic cementite, and desegregated from IBs between proeutectoid ferrite and pearlitic ferrite. The pearlitic cementite was Mn rich. There was no Mn segregation at the IBs between pearlitic ferrite and pearlitic cementite. The pattern of Mn segregation could be explained in terms of diffusion in the process zone ahead of the pearlite during the austenite to pearlite transformation and diffusion in the IBs between the proeutectoid ferrite and pearlite.

Highly resistive grain boundaries in doped MnZn ferrites for high frequency power supplies

The power loss of MnZn ferrites and its relation to average grain size and grain boundary properties was investigated. The power loss was found to be dependent on the average grain size and on the highly electrically resistive grain boundaries which are formed by introduction of aliovalent ions in the outer grain region of a MnZn ferrite. Analysis of ac impedance data, assisted by Auger electron spectroscopy, elucidated the chemical and electrically diversity of doped and undoped samples and its influence on eddy current loss.

Differences in impact values of core and rim in electroslag weld metal: Improvement of toughness properties in ESW weld metal in constructional steels (2nd Report)

Summary The purpose of this paper is to investigate the impact properties of weld metal produced under different welding conditions with special reference to electroslag welding (ESW) as a high heat input welding process generally applied in the fabrication of four‐sided thick‐plate box columns for general multi‐storey buildings. The paper focuses in particular on the impact properties of ESW weld metal at its centre (core (C)) and periphery (rim (R)). The results obtained may be summarised as follows: The vE value of the weld metal core is lower than that of the weld metal rim. The absorbed energy transition temperature is also higher. The vE values of the weld metal core and rim altered with a change in the welding heat input Q (varied at six levels in the 10.1 ∼ 126.7 kJ/mm range), generally decreasing with an increasing heat input. The vE value of the core is lower than that of the rim. In the weld metal produced at the maximum heat input (126.7 kJ/mm), however, the core and rim have much the same vE values. The vE value of the weld metal core shows little change with a change in the steel type (one type of TMCP and two types of SM490A), and remains at a low value. When there is a change in the surrounding gas (oxygen (O2), air, and argon gas), the vE value decreases in an Ar, air, O2 sequence. The weld metal core vE value is little dependent on any change in the flux type (three types of fused flux) and weight of flux used (0.5 or 0.7 N). The microstructural observations suggest that grain‐boundary ferrite (GBF) occurs at the coarse prior austenite grain boundaries of the rim, with fine acicular ferrite (AF) being mainly found trans‐granularly. In the core, grain‐boundary ferrite is formed at high density at the fine prior austenite grain boundaries, with massive polygonal ferrite (PF) being formed at high density transgranularly. There are thus distinct differences in the coarse ferrite morphology of the rim and core. The SEM observations of the fracture surfaces suggest that the core weld metal has a large grain size, a feature corresponding to its low vE value. The observations made at the fracture surface periphery indicate that numerous secondary cracks are initiated in the grain‐boundary ferrite of the core, suggesting that the ferrite has a low toughness. The results of the SEM simultaneous fractographic‐microstructural observations suggest that fracture selectively propagates along the grain‐boundary ferrite. This indicates that the low vE value of the core is due to the presence of high‐density grain‐boundary ferrite and massive transgranular polygonal ferrite.

Designing Shielded Metal Arc Consumables for Underwater Wet Welding in Offshore Applications

The use of underwater wet welding for offshore repairs has been limited mainly because of porosity and low toughness in the resulting welds. With appropriate consumable design, however, it is possible to reduce porosity and to enhance weld metal toughness through microstructural refinement. New titanium and boron-based consumables have been developed with which high toughness acicular ferrite (AF) can be produced in underwater wet welds. Titanium, by means of oxide formation, promoted an increase in the amount of acicular ferrite in the weld metal, while boron additions decreased the amount of grain boundary ferrite (GBF), further improving the microstructure. Porosity reduction was possible through the addition of calcium carbonate at approximately 13 wt percent in the electrode coating. However, weld metal decarburization also resulted with the addition of carbonate.

The influences of microstructure and composition on the electrochemical behavior of a516 steel weldment

ASTM-A516 (Grade 70) low carbon steel weldments have been characterized in terms of the effect of metallurgical stimuli on electrochemical behavior. Potentiodynamic polarization measurement and electrochemical impedance spectroscopy (EIS) were made for the specimens, prepared with three different welding heat input conditions (15, 30 and 45 kJ cm −1) either by submerged arc welding or by weld thermal simulation with a Gleeble machine, in NACE solution (5 wt% NaCl + 0.5 wt% CH 3 COOH) of pH 2.8 at 25°C. The polarization resistances of the base metal, thermal simulated heat-affected zone and weld metal obtained using linear polarization method and EIS were compared. The results show that weld metal is distinguished in having the greatest polarization resistance. Furthermore, the polarization resistances of the weld metal and simulated grain-coarsened HAZ increased with increasing welding heat input. The electrochemical behavior of the weld metal is dependent upon the volume fraction ratio of grain boundary ferrite (GF) to acicular ferrite (AF), while that of the thermal simulated grain-coarsened HAZ is associated with the coarsening of Widmanstätten ferrite (WF).

Effect of ferritic grain size on the structure and growth of austenitic colonies during cementation of chromium steels

The fixed growth of the austenite-carbide colonies during carbonization of chromium alloys is prevented close to the grain boundaries of the initial ferrite. Restructuring of the colonies under various conditions is associated with diffusion of carbon and chromium in the ferrite and the creation of a thermodynamically unstable martensitic phase ahead of the recrystallization front, which is supersaturated with chromium and carbon. Thus, in the initial stages of growth, as the degree of supersaturation in the ferrite increases, lamellar rod-shaped colonies grow whose main growth direction is along the carbon concentration gradient. A decrease in the supersaturation of carbon in the ferrite as one moves away from the advancing front toward the center of the grain makes the isolated growth of the coarsely agglomerated phase along the boundaries of the initial ferritic grains possible. The less the amount of defects in the grain boundaries, the more the body of the ferritic grains becomes supersaturated. This leads to the simultaneous growth of the new phase at some distance from the general growth front of the colonies.

Structural mechanism of the development of thermal embrittlement in steels with a temper-bainite structure

1. Thermal embrittlement of steels 10G2N2MFA and 15Kh2NMFA-A is caused by variation in the size and distribution of disperse-phase particles. During holding to 3000 h at 350°C, finely disperse carbides are segregated in the body of ferrite grains, block the dislocations, strengthen the steel in turn, and, correspondingly, increase its semibrittleness temperature. A further increase in holding (right up to 10,000 h) leads to coagulation of the carbides, and to a reduction in the resistance to plastic deformation and in the semi-brittleness temperature of the steel. In steel 10G2N2MFA, a monotonic increase in T50 during holding at 350°C is caused by preferential segregation of carbide particles along the ferrite-grain boundaries. 2. A different pattern of variation in the semibrittleness temperature during holding at 350°C for steels 15Kh2NMFA-A and 10G2N2MFA is governed by differences in the structure of the metal in the initial state. Steel 15Kh2NMFA-A has a preferentially subgrain-cellular structure, whereas steel 10G2N2MFA, in addition to a subgrain structure, has a significant amount of recrystallized ferrite grains; in this case, the dislocation density is three times lower in steel 10G2N2MFA than in steel 15Kh2NMFA-A. 3. The relation between the degree of thermal embrittlement and the percentages of intercrystalline fracture in the brittle fracture zones of impact specimens of the steels under investigation is ascertained.

Effect of carbon, phosphorus and nitrogen contents in steel on machined surface and cutting force.

The effect of carbon, phosphorus and nitrogen content on the surface roughness and cutting force was studied with regard to the microstructure of specimen and the behavior of void formation and built-up edge during cutting. Specimens to plunge-cutting were prepared by adding each element into pure iron.Two-phase microstructure such as ferrite-pearlite and ferrite-grain boundary where phosphides precipitate, increase the surface roughness and reduce the cutting force. On the contrary, the single-phase microstructure, that is to say, ferrite containing soluble nitrogen, slightly improves the surface roughness but raises the cutting force.Since the ductility of each phase in the two-phase microstructure is different from each other, the formation of voids takes place along the interface due to cutting force. The void formation behavior in the primary shear zone is closely related to the projection length of the built-up edge and the surface roughness.

Microstructure of low-alloy cast iron with spheroidal graphite and high crack resistance

1. The results of investigations of the resistance of cast irons with different structures and shapes of graphite inclusions to brittle failure suggest the absence of correlation between KIc and δ, and the presence of a weak correlation between σu and σo.2. The critical embrittlement temperature can be estimated qualitatively from the results of tests on the resilience of specimens with an acute notch (Charpy-type). 2. Two-stage annealing at 920°C and 720°C with subsequent heating to Ac2, holding for 15...30 min, and cooling in air to 600...500°C, and thereafter in water or oil makes it possible to fix ductile austenite, which is stable to the normal temperature, along the ferrite-grain boundaries; in this case, redistribution of Mn, Ni, Cr, Mo, Cu, Al, and Si occurs between the α- and γ-solid solutions, and the fracture toughness KIc is increased as compared with the KIc of cast iron with an initial ferritic structure. 3. An increase of up to a total of 2.0...3.5% in the extent to which ferritic cast iron with spheroidal graphite is alloyed with Ni, Cu, Cr, Mn, Mo, and Al contributes to an increase in the amount and stability of austenite, even during slow cooling in air. 4. The investigations that we conducted to determine the feasibility of increasing the resistance to brittle failure, which were conducted on compact specimens for eccentric tensioning and which were computed by methods used for steel components, make it possible to expand the range of application of low-alloy cast iron with spheroidal graphite.

2相ステンレス鋼におけるα粒界から析出するμ目の形態と結晶学的特徴

2相ステンレス鋼におけるα粒界から析出するμ目の形態と結晶学的特徴

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.

Relationship between microstructures and magnetic properties of ferrites containing closed pores

A model for the effect of demagnetizing field caused by closed pores and grain boundaries in ferrites was developed. This paper presents the expressions to transform the magnetic properties free from the demagnetizing field into the apparent magnetic properties. The expression for the apparent permeability is \mu_{app} = \frac{(1 - P)\mu_{1}}{(1 + \frac{P}{2})(1 + 0.75 \frac{t}{D}\frac{\mu_{1}}{\mu_{b}}} , where p is porosity, D is average grain size, t is effective thickness of the grain boundary region, μb is the permeability of the grain boundary region and μl corresponds to the permeability free from the demagnetizing field, it was found that the demagnetizing field caused by the pores is proportional to the magnetic field, whereas that caused by the grain boundaries is proportional to the flux density, and that the grain boundary region is important for the control of the magnetic properties of ferrites.

Redistribution of the carbide phase during heating of high-strength steels

1. Holding quenched carbon steel at high temperatures leads to intense redistribution of carbides in the direction of the intercrystalline boundaries. The carbide phase concentrates in the boundaries of grains and subgrains formed as the result of the polymorphous γ→α transformation. The transfer of carbides from the subboundaries to the boundaries of ferrite grains occurs at temperatures above 500°C, where, as the result of recrystallization, the intragranular boundaries disappear and the formation and growth of new ferrite grains begins. 2. The development of the graphitizing process in carbon steel on prolonged holding at high temperature is preceded by the redistribution of cementite in the direction of the grain boundaries, and thus carbide precipitates in the grain boundaries of the α-solid solution undergo graphitization.

Peculiarities of the diffusion of carbon in iron

1. Diffusion of carbon inα andγ-irons is non-uniform: at 550 to 950°C, carbon enriches preferentially the grain boundaries; the diffusivity of carbon along grain boundaries of ferrite is three to four higher than inside the grain. An enrichment with carbon of theα-solid solution is also observed in regions adjoining the grain boundaries. 2. The diffusion rate and the distribution of carbon within a single grain are changed by alloying. Addition of substantial amounts of certain elements curtailing theγ loop considerably increases the penetration depth of carbon on account of boundary diffusion developing inγ-iron. The concentration gradient of carbon between boundary and interior of a grain is less in alloyed than in unalloyed iron.

低炭素鋼の焼鈍脆性に關する研究

In case of annealing low carbon steel, sometimes an "anneal-brittleness", (named by the author) occurs accompanied by its softening and a remarkable decrease in impact value. This report treats of the results of inspection on the cause of developing "anneal-brittleness" and its prevention. Some of chief conclusions are as follows:1. Anneal-brittleness occurs remarkably in low carbon rimmed steel, while it seldom occurs in killed steel.2. Anneal-brittleness occurs, because ferrite grains become coarse and cementite separates out around the boundaries of ferrite grains when low carbon steel is cooled slowly from high temperature.3. Anneal brittleness increases remarkably with decrease of the C content and with increase of the N, O or P content., 4. From the viewpoint of quality, killed steel should be used instead of rimmed steel if anneal brittleness is undesirable. In case of rimmed steel anneal-brittleness can be avoided by such normalizing in which aircooling rate is high.