The role of Cu and Al addition on the microstructure and fracture characteristics in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels with superior toughness

The effects of Cu and Al addition on the microstructure and fracture in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels with superior toughness were studied and compared with the X70 pipeline base steel counterpart. The microstructure in base steel was dominated by a small fraction of acicular ferrite and predominantly bainite. However, acicular ferrite microstructure was obtained in Cu-bearing steel, which nucleated on complex oxide with outer layer of MnS and CuS because of Cu addition. The microstructure in Al-bearing steel consisted of bainite with ultrafine martensite–austenite constituent, which was refined by Al addition. CGHAZ in Cu-bearing and Al-bearing steels had superior impact toughness and ductile fracture, which were attributed to acicular ferrite and ultrafine martensite–austenite constituent, respectively.     

Effect of dissolution and precipitation of Nb on the formation of acicular ferrite/bainite ferrite in low-carbon HSLA steels

Effect of dissolution and precipitation of Nb on the phase transformation during cooling was investigated. It is firstly recognized that either the formation of acicular ferrite or the separation of bainite ferrite could be adjusted by the preparation of the steel specimens with different amounts of solute Nb and Nb-precipitates in austenite (isothermally holding at 850 °C for different durations). An increase in isothermal duration at 850 °C would spawn more Nb(CN) precipitates, leading to a microstructural evolution from bainite ferrite to acicular ferrite/bainite ferrite dual phase, and eventually to acicular ferrite in the final microstructure. This could be explained by the solution of Nb in the austenite, due to the solute dragging effect of Nb, can decrease the A_r3 temperature and promote the formation of bainite ferrite, while the precipitation of NbC can increase the A_r3 temperature and promote the formation of acicular ferrite by increasing the nucleation sites of acicular ferrite. Thus, the properties of acicular ferrite/bainite ferrite dual phase steel can generally be improved by appropriately controlling the state of Nb (Nb(CN) as precipitates and Nb in solution) in the austenite before cooling, which provides a new approach to the modification of acicular ferrite/bainite ferrite ratio.     

Microstructure,phase equilibria,and transformations in corrosion resistant dual phase steel designated 3CR12

Abstract

To optimize the properties of the new corrosion resisting steel 3CR12 the microstructure has been studied as a function heat treatment. The kinetics of both the decomposition of austenite and the reaustenization reactions have been investigated using a series of isothermal anneals. The steel has a dual phase ferrite–austenite structure between 800 and 1350°C and the amount of austenite is maximum at about 1050°C. At this temperature a higher nickel version of the alloy is fully austenite. On cooling to ambient temperature, the austenite transforms to a lath-type martensite. Heat treatments at temperatures up to 800°C cause the slow tempering of the martensite, the recovery and recrystallization of original ferrite regions, and the nucleation and growth of newly formed ferrite. The growth of ferrite requires the concomitant precipitation of carbides and nitride particles from the austenite or martensite and these particles mark the stepwise movement of the interface. In contrast the reaustenization does not require any immediate redistribution of elements. Consequently, the hardness of the resulting martensite is a function of both the temperature and time of the austenization treatment. These findings can be used to advantage by the producers, fabricators, and end users of the steel since variations in thermomechanical treatments promote differences in formability, strength, and toughness.

MST/493     

Metallography of bainitic transformation in austempered ductile iron

Abstract

An unalloyed nodular cast iron has been used to investigate the development of microstructure on heat treating in the bainite temperature region. Specimens were austenitised at 900°C for 1·5 h, then austempered for 1, 2, or 3 h at 250,300, and 350°C, respectively, and examined by light, transmission electron, and scanning electron microscopy. Experimental results indicate a microstructure consisting of a stable, highly enriched, retained austenite with one of two lower bainitic ferrite morphologies. One of these morphologies is carbide free acicular ferrite for specimens austempered at 350°C for 1 h and the other is bainitic ferrite in which carbide is distributed within the ferrite produced by different heat treatment conditions. Austempering at 350°C for 2 h and at 300°C for 1 and 2 h resulted in the formation of transition carbides in bainitic ferrite platelets. The η carbide was formed at 350°C for 2 h by precipitation from a bainitic ferrite supersaturated with carbon. By contrast, ? carbide was associated with austempering at 300°C for 1 and 2 h and precipitates either on the austenite twin/bainitic ferrite boundaries or within the bainitic ferrite. The fracture mode of tensile and impact specimens in the austempered condition was fully ductile compared with as cast specimens, which had mixed fracture characteristics.

MST/1646     

Discussion

Abstract

An examination by transmission electron microscopy has been made of the austenite → ferrite + carbide reaction in an Fe–C–V low-alloy steel over the transformation temperature range 550–850°C. Thin films of interlath retained austenite in the untransformed martensite regions of partially transformed specimens have been used to investigate the crystallographic relationships between parent and product phases. Grain boundary ferrite allotriomorphs nucleate with a rational orientation relationship to at least one, but also occasionally to both, of the two adjacent austenite grains. Interphase precipitation of vanadium carbide can occur in the ferrite irrespective of the austenite–ferrite crystallographic relationship and, in particular, it is shown that the single habit plane adopted by the carbide is not a consequence of a three-phase crystallographic relationship between parent austenite and the product ferrite and carbide phases. It is thought that the precipitate adopts that habit plane which lies at the smallest angle to the plane of the transformation interface, and additional experimental evidence to support this is presented.

MST/664     

Effect of hydrogen charging on fracture behaviour of 304L stainless steel

Abstract

Slow tensile straining of a series of specimens of 304L stainless steel after thermally charging with hydrogen at 31·0 MPa and 350°C resulted in a ductility loss compared with uncharged specimens. The susceptibility to embrittlement was shown to be dependent on the formation of martensite during deformation and, hence, the stability (and composition) of the austenite, but the interface between the austenite and any ferrite stringers acted as a nucleation site for cracking and as a weak propagation path for fracture.

MST/1088     

Microstructure of as-quenched 3.5NiCrMoV rotor steel. Part I. General structure and retained austenite

The microstructural features have been examined for 3.5NiCrMoV steam turbine rotor steel, in the as-quenched state and tempered at 500 °C. Quenching produces lath martensite, with bands of retained austenite at the lath boundaries and, to a lesser extent, at prior austenite grain-boundaries. Autotempering occurs during the quench, resulting in loss of tetragonality of the martensite and extensive carbide precipitation in the matrix and to a lesser degree at prior austenite grain boundaries, but not at lath boundaries. Tempering at 500 °C leaves the lath structure largely intact, but causes retained austenite to transform to bands of ferrite and cementite. This transformation does not correlate with the reduction in stress corrosion crack velocity which occurs on tempering. The strength of 3.5NiCrMoV steel in the as-quenched and 500 °C tempered conditions is most probably due to the combination of carbide precipitation strengthening and substructure strengthening.     

Influence of hot working parameters on microstructure evolution,tensile behavior and strain aging potential of bainitic pipeline steel

The microstructure evolution, tensile properties and aging behavior were studied in a low-carbon pipeline steel by performing a number of physical simulations in a thermo-mechanical simulator. The austenite status, before entering the finishing stage, was varied by varying parameters in the austenitization and the roughing stages. The finishing parameters and the subsequent cooling strategy were kept fixed throughout all applied simulations. It was found that starting the finish rolling stage with pancaked austenite is more effective in motivating the formation of acicular ferrite and refining martensite/austenite phase than refining the prior austenite grains. The former effect resulted in improving both of ultimate tensile strength and proof stress without significant ductility decrease. Increasing the deformation during roughing stage resulted in stimulating the transformation kinetics during subsequent processing and consequently decreasing the untransformed austenite that forms the martensite/austenite microconstituent. Furthermore, increasing the austenitization temperature is found to enhance both of strength and ductility when starting the finishing stage with pancaked austenite. On the other hand, attaining aging effect in the studied steel was only possible after creating new dislocations by pre-straining. An increase up to 63 MPa in the yield strength is recorded due to aging after 2% pre-straining.     

High temperature mechanical properties of triple phase steels

A 0.15% C–1.2% Si–1.7% Mn steel was intercritically annealed at 780 °C for 5 min and then isothermally held at 400 °C for 4 min followed by oil quenching to room temperature and the annealed microstructure consist of 75% ferrite , 15% bainite and 10% retained austenite was produced. Samples of this steel with triple phase structure were tensile tested at temperature range of 25–450 °C. Stress–strain curves showed serration flow at temperature range of 120–400 °C and smooth flow at the other temperatures. All of the stress–strain curves showed discontinuous yielding at all testing temperatures. Both yield and ultimate tensile strength decreased with increasing temperature, but there exists a temperature region (120–400 °C) where a reduction of strength with increasing temperature is retarded or even slightly increased. The variation in the mechanical properties with temperature was related to the effects of dynamic strain aging, high temperature softening, bainite tempering and austenite to martensite transformation during deformation.     

Effect of Cu addition on microstructure and impact toughness in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The effects of Cu content on microstructure and impact toughness in the simulated coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels were investigated. It has been observed that the microstructure in the simulated CGHAZ of Cu-free steel is dominated by a small proportion of acicular ferrite and predominantly bainite with martensite–austenite constituent. Whereas, in the 0.45 and 1.01% Cu-containing steels, the acicular ferrite increased significantly due to the effective nucleation on intragranular inclusions with outer layer of MnS and CuS. The formation of acicular ferrite is attributed to superior high heat-affected zone impact toughness in the 0.45% Cu-containing steel. Furthermore, the increasing martensite–austenite constituent and ε-Cu precipitates in the simulated CGHAZ of 1.01% Cu-containing steel caused degradation in impact toughness.     

Fabricating Defect-Free API X65 Steel Welds under Underwater Wet Conditions using Friction Taper Plug Welding

The experiment on API X65 steel was carried out using friction taper plug welding (FTPW) under underwater wet conditions at room temperature. Sound weld formation can be achieved vastly at axial force ranging from 25 to 40 kN and a rotating speed of 7000 rpm. The microstructure of the weld zone consists of lath martensite, upper bainite, granular bainite, polygonal ferrite, and acicular ferrite. The microstructure of heat affected zone (HAZ) mainly consists of lath bainite as well as a bit of lath martensite and granular bainite. The tensile properties of defect-free welds are excellent. The maximum impact energy of the V-notch of bonding area can reach 110 J, which was conducted at 0°C with an error of ±12.5 J. The typical SEM impact fracture morphology presents areas of cleavage, quasi-cleavage, and dimples.     

Tensile behaviour of duplex stainless steel at low temperatures

Abstract

The tensile behaviour of the ferrite and austenite phases of Fe–22Cr–5Ni (wt-%) duplex stainless steel containing a maximum of 17·2% austenite was investigated in the temperature range 65–298 K. The results indicate that mechanical twinning occurred in the testing temperature range, and that austenite impeded the growth of twinning. Mechanical twinning in ferrite was well decorated with a ‘dislocation shell’, and the density of dislocations at the coherent twin boundary and within a twin was much higher than in the matrix above the ductile–brittle transition temperature (DBTT). This supported the occurrence of slip localisation next to coherent twin boundaries. Dislocations in the material with no austenite tested below the DBTT were characterised by coplanar slip dislocation on the { 110} plane, and both coplanar slip on { 110} and cross-slip dislocations were observed above the DBTT. Dislocation in ferrite was negligibly affected by the presence of austenitic particles. Strain induced martensite transformation occurred in austenitic particles at or below 220 K, and the characteristics of the transformation were essentially similar to those in type 304 stainless steel. The DBTT of the material was lowered from ～140 to 110 K in the presence of austenite, independent of the volume fraction of austenite. This suggests that the decrease in the DBTT of the material was mainly due to austenite scavenging carbon and other interstitial elements from the ferritic matrix. The fracture of the material at low temperatures was primarily controlled by the fracture of twin boundaries in ferrite.     

Effects of long term thermal aging on high temperature tensile deformation behaviours of duplex stainless steels

Abstract

Tensile deformation behaviours of unaged and thermal aged duplex stainless steels (DSS) were investigated at 350°C in order to understand the effects of long term thermal aging on the high temperature deformation behaviours of DSS. After aging for 20?000?h, the strength of DSS has a slight increase, the plasticity has a considerable decline, and the tensile fracture transfers from ductile to brittle. Nanoindentation tests indicate that ferrite has a considerable increase in hardness, and austenite has only a negligible increase with aging time. Thermal aging embrittlement is primarily concerned with ferrite. After long term thermal aging, spinodal decomposition and G-phase precipitation occur in ferrite and these reactions result in the dramatic decline of the ferrite phases' deformation ability. Cleavage cracks can easily initiate and propagate in ferrite of long term thermal aged DSS.     

Thermal aging of 16Cr – 5Ni – 1Mo stainless steel Part 1 – Microstructural analysis

Abstract

Specimens of 16Cr - 5Ni - 1Mo stainless steel were solution treated at 1050 ° C for 1 h followed by heating in the temperature range 400 - 750 ° C for different holding times (1 - 16 h). After heat treatment, optical microscopy, scanning (SEM) and transmission (TEM) electron microscopy, and X-ray diffraction examinations were conducted. The microstructure of all aged specimens was found to consist of martensite with variable fractions of δ ferrite and reversed austenite. Very fine precipitates of Mo carbides were revealed in the specimens aged at 475 ° C. The specimens aged at 625 ° C showed a decrease in the dislocation density and a high volume fraction of austenite and precipitation of Fe₂Mo Laves phase was detected by X-ray analysis. Above 625 ° C, Cr₂₃C₆ and TiC became the predominate carbides heterogeneously precipitated in the martensitic matrix. Partial transformation of reversed austenite to unaged martensite was observed at temperatures above 625 ° C.     

Effects of aging temperature on the microstructure and mechanical properties of 1.8Cu-7.3Ni-15.9Cr-1.2Mo-low C, N martensitic precipitation hardening stainless steel

The effects of aging temperature on the microstructure and mechanical properties of a newly designed martensitic precipitation hardening stainless steel, which is 1.8Cu-15.9Cr-7.3Ni-1.2Mo-low C, N steel, for improving the toughness, ductility and corrosion resistance of stainless steel of 1000 MPa grade tensile strength were experimentally investigated. The specimen aged at 753 K for 14.4 ks has a typical lath martensitic structure with about 12% interlath austenite, while the specimens aged at 813 K and 853 K for 14.4 ks have the lamellar duplex microstructure of the reverted austenite and the aging hardened martensite. The formation process of reverted austenite is controlled by diffusion of Ni in martensite. The mean size of precipitates which are enriched with Cu increases with rising aging temperature, however, it is about 30 nm even after aging at 853 K for 14.4 ks. The specimens aged at 813 K and 853 K for 14.4 ks, in which the reversion of martensite to austenite is observed, have the excellent combinations of strength, ductility and toughness.     

Effect of thermal aging on pitting corrosion resistance of 16Cr – 5Ni – 1Mo precipitation hardening stainless steel

Abstract

Specimens of precipitation hardening 16-5-1 stainless steel were solution treated at 1050°C for 1 h followed by aging at temperatures in the range 400 – 750°C for various holding times (1 – 16 h). After heat treatment, two types of corrosion test (accelerated and immersion testing) were conducted in 6% ferric chloride solution. The results showed that the pitting corrosion resistance was affected by austenite content, δ ferrite and precipitation of molybdenum and chromium carbides. Three critical temperature ranges were identified, which were related to the phases formed: (a) high corrosion rate at 475°C (δ ferrite and Mo₂ C); (b) low corrosion rate at 550 – 625°C (reversed austenite and Laves phase); (c) intermediate corrosion rate at 750°C (Cr₂₃ C₆ and TiC). The morphology of the pitting was dependent on the form of the δ ferrite and carbides.     

Study of the austenite quantification by X-ray diffraction in the 18Ni-Co-Mo-Ti maraging 300 steel

In this paper, quantifications of the austenitic phase in a maraging 300 steel heat treated at different temperatures and periods of time were carried out using the direct comparison method by X-ray diffraction. The influence of taking into account the chemical compositions of austenite and martensite phases in the results by the direct comparison method was evaluated. In order to analyze the instability of austenite under plastic deformation, the quantifications were carried out with and without previous grinding of the samples. The behavior of the austenite volume fraction against aging time at 560°C, 600°C and 650°C were determined. The variation of the martensite lattice parameter with aging time was also analyzed. The results show an increase of the austenite content with aging time at 560°C and 600°C. At 650°C, however, the austenite content present at room temperature decreases and the martensite parameter increases with the aging time above 1 h.     

Effect of aging-deformation-treatment on the formation of intragranular ferrite in V-microalloyed steel

The influence of the aging-deformation-treatment on the formation of intragranular ferrite (IGF) in a vanadium microalloyed medium carbon steel (Fe–0.34C–1.53Mn) was studied. Effects of aging time on the volume fraction of IGF and austenite grain size were also investigated. The effect of the aging-deformation recrystallisation process on the amount of ferrite and the mechanism of IGF formation was discussed. The results show that aging-deformation-treatment makes the precipitation position of the carbonitride transfer from austenite grain boundary to intragranular, and the precipitated intragranular carbonitrides become the nucleation cores of the IGF during the recrystallisation. The content of precipitated carbonitrides, and the volume fraction of ferrites and the grain size of austenites, increase with the increasing aging time.     

Influence of heat treatments on microstructural parameters and mechanical properties of P92 steel

Abstract

The microstructural parameters (dislocation density, martensite lath width, precipitate diameters, and volume fractions) have been measured for the 9%Cr steel P92 (NF616) after different heat treatments. The austenitising temperatures were 970, 1070, and 1145°C and the tempering temperatures 715, 775, and 835°C. Increasing the austenitising temperature led to an increase in the austenite grain size and in the martensite lath width, but no significant effect on the tensile properties at 20, 600, and 650°C was observed. The creep strength was, however, reduced by tempering at 835°C due to rapid recovery of the martensitic structure with a sharp decrease in dislocation density. The lowest creep strength was found for the P92 steel subjected to a heat treatment that produced a fully ferritic microstructure; the secondary creep rate was four orders of magnitude higher than that of the steel in the usual martensitic condition.     

Effect of thermal treatment on the evolution of delta ferrite in 11Cr–3Co–2.3W steel

Effect of thermal treatment on the evolution of delta ferrite was studied in 11Cr–3Co–2.3W steel. Ferrite-forming alloy elements promoted the formation of delta ferrite during thermo-mechanical processing. The results of JMatPro and chromium equivalent calculations indicated that 11Cr–3Co–2.3W steel partially contained delta ferrite. Cr and W were segregated in/around delta ferrite. Delta ferrite had a bamboo-type morphology and it contained MX-type carbonitrides inside and the Laves phase at the interface between delta ferrite and martensite. On annealing, delta ferrite dissolved especially during the early stage of annealing. After ageing at 600°C for 500?h, some Laves phases were observed in delta ferrite which is regarded as a favourable nucleation site. In the meantime, its precipitation resulted in the reduction of dissolved W and Mo in the matrix.