Intercritical Region Research Articles

Correlation of Creep Properties of Simulated and Real Weld Joints in Modified 9 %Cr Steels

This paper examines the creep properties of modified 9–12% Cr steels, especially the P91 grade. This steel is widely-used in many power plant constructions all over the world. Welding P91 steel components is one of the most important technological procedures. The aim of the investigation performed is to why creep rupture on welded joints at high temperatures is often observed in the heat-affected zone (HAZ), especially in the so-called inter-critical zone (IC-HAZ). The creep rupture strength (CRS) of cross-weld joints is usually about 20 to 30% lower than that of the base material, in the case of creep-resistant modified chromium steels. Precipitation strengthening (PS) depends on the inter-particle spacing (IPS) of secondary phases — MX nanoparticles and/or M23C6 particles. With increasing IPS of secondary phases, the proof stress and creep rupture strength decreases, while creep rate increases. With respect to the small size of each in the HAZ region and to determine the IPS in the HAZ, the simulation technique was used for estimation of the microstructure and properties. The Smitweld machine was used to reproduce the thermal cycles in the HAZ. The microstructure of the HAZ-simulated regions was analysed by transmission electron microscopy. Real creep tests were carried out on specimens with HAZ-simulated regions and also on the P91 real welded joints. It was found that inter-particle spacing of secondary phases in the inter-critical region of the HAZ is much higher than that of the base material. Therefore, the proof stress and hardness, as well as creep rupture strength of the HAZ are lower than that of the base material. Creep test results of the real welded joints and simulated HAZ regions are in good correlation with all theoretical presumptions.

Influence of Processing Conditions on Obtaining an Ultrafine Grain Structure

For low carbon steels, thermomechanical processing in the intercritical (austenite–ferrite) temperature region has potential for generating an ultrafine grained structure. By using compressive deformation, the influence of temperature, strain rate and strain on the transformation kinetics was considered for developing a room temperature microstructure of fine ferrite grains. The flow behaviour was also examined to determine the critical conditions for dynamic transformation and/or strain induced transformation using the stress–strain and strain hardening rate curves. Characterization of the microstructure after deformation in the intercritical region confirmed the occurrence of grain refinement with the presence of homogeneous and ultrafine ferrite grains.

Formability Evaluation of TRIP-aided Steel Sheets with Different Microstructures: Polygonal Ferrite and Bainitic Ferrite Matrix

Recently, application of TRIP-aided steel sheets has been developed due to their good balance of strength and formability. In this work, a comparative study was performed on the formability of two different microstructures of TRIP-aided steel with different retained austenite characteristics. Microstructures along with their specific retained austenite characteristics such as volume fraction, stability and morphology were achieved by annealing of cold rolled sheet at fully austenite and intercritical region during subsequent isothermal treatment in the range of 350–410°C. SEM and XRD measurements were used to evaluate retained austenite characteristics. Formability and mechanical properties of sheets were determined by Erichsen test and tensile test, respectively. Results indicate that steel sheets with polygonal ferrite (PF) matrix present better formability. However, at identical volume fraction of retained austenite, sheets with bainitic ferrite (BF) matrix show better stretch formability due to high retained austenite stability.

Thermomechanical processing in the intercritical region and tensile properties of dual-phase steel

Thermomechanical process of 50% rolling reductions were applied at different intercritical temperatures on a low alloy steel containing 0.09% C, followed by quenching in the iced brine solution. Fibrous (ferrite + martensite) microstructures with variable contents of martensite were obtained. Rolling caused an increase in tensile strength and ductility in the longitudinal than the transverse directions at all levels of martensite, expect 35% of martensite where ductility is little affected. Microvoids were found to nucleate and grow in the necks of tensile specimens by fracture of martensite and by decohesion of the martensite/ferrite interface. The changes in density of the microvoids with distance from the fracture surface defined the straining behavior of the steel with different volume fractions of martensite. The growth of microvoids appeared to be interrupted when a critical stress was reached at which the ferrite cleaved. The martensite fibers in longitudinal directions appeared to be positioned for receiving stresses from ferrite. The increased straining with tensile deformation promoted ductile dimpling on fracture surfaces which were determined by the extent of void growth and coalescence prior to interruption by cleavage.

Type IV Cracking Susceptibility in Weld Joints of Different Grades of Cr-Mo Ferritic Steel

Relative type-IV cracking susceptibility in 2.25Cr-1Mo, 9Cr-1Mo, and 9Cr-1MoVNb ferritic steel weld joint has been assessed. The type-IV cracking was manifested as preferential accumulation of creep deformation and cavitation in the relatively soft intercritical region of heat affected zone of the weld joint. The type-IV cracking susceptibility has been defined as the reduction in creep-rupture strength of weld joint compared to its base metal. The 2.25Cr-1Mo steel exhibited more susceptibility to type-IV cracking at relatively lower temperatures; whereas, at higher temperatures, 9Cr-1MoVNb steel was more susceptible. The relative susceptibility to type-IV cracking in the weld joint of the Cr-Mo steels has been rationalized on the basis of creep-strengthening mechanisms operating in the steels and their venerability to change on intercritical heating during weld thermal cycle, subsequent postweld heat treatment, and creep exposure.

AVALIAÇÃO DO AMACIAMENTO ESTÁTICO A QUENTE E A MORNO DE UM AÇO SILICIOSO

Double hit compression tests were performed on an ultra low carbon Si-Al alloyed steel. The true strain in both deformation steps was kept constant (0.2), while varying the strain rate (1 s -1 , 10 s -1 and 30 s -1 ), the temperature (1,050°C-austenite phase, 950°C-intercritical region and 850°C-ferritic phase) and the interpass time (0.5; 1; 5 and 50 s). The temperatures used are within the industrial range applied in the finishing rolling for this steel. The analysis of the flow curves obtained allowed the evaluation of the static softening characteristics of the steel. It was shown that the steel has a similar softening condition between passes at the three temperatures studied, with the softening fraction increasing with the increasing time and strain rate. The characteristics of the softening curves and microstructural features indicate that at the two higher temperatures (austenite phase and upper part of the intercritical region) the softening process is governed by recrystallization, with the nucleation and growth of subgrains. At the lower temperature, however, the softening is mainly due to strain induced boundary movement (SIBM) and recovery.

Effect of Niobium Addition on the Texture Formation of High Strength Cold-Rolled Low Carbon Steel Sheets

The effect of niobium (Nb) addition on recrystallization texture formation in cold-rolled low carbon steel sheets containing 2% manganese (Mn) was investigated. The microstructures of hot-bands were significantly refined by Nb addition, which led to the development of the cold-rolling texture in both the γ-fiber (<111>//ND-fiber) and the α-fiber (<110>//RD-fiber). Recrystallization was retarded by Nb addition, in particular, the growth of <110>//ND grains was retarded. However, the γ-fiber and {112}<110> grains developed during annealing even in the intercritical (α-γ) region. Consequently, the r-value increased as the content of Nb was increased due to the development of the intensity ratio of the <111>//ND texture to the <100>//ND texture, which is desirable for deep-drawable high strength steel sheets.

Microstructures and Properties in Simulated Heat-Affected Zones of 685 MPa Grade Copper-Bearing Steel

Abstract The experiments were carried out upon the determination of SH-CCT diagrams and the characteristic of microstructures and properties in simulated coarse grain heat-affected zone of hot continuously rolled copper-bearing steel under single and double welding thermal cycle using physical simulation. The results show that with the help of SH-CCT diagram of copper-bearing steel, it is proposed to choose t8/5 ranging from 7s to 35s during welding. The copper-bearing steel has a narrow range of heat-input and brittlement is easy to happen in the region of CGHAZ with higher heat-input. Granular bainite transformed from austenite leads to brittlement. Softening starts when t8/5 is more than 7s. The dissolution of ɛ-Cu and coarse lath bainite and more ferrite cause softening. Moreover, the impact toughness decreases dramatically and obvious brittlement happens in the intercritical region of CGHAZ. The reason is pearlite formed on the interface of original austenite and coarse granular bainite, which can reduce the impact toughness.

Formation of Microstructure and Texture in Intercritically Rolled C-Mn Steel

Series of trials were conducted on a laboratory rolling mill to evaluate the influence of intercritical rolling on the microstructure and texture of steel with 0.082%C, 1.54% Mn, 0.35% Si, 0.055%Nb and 0.078%V. Two parallel rolling schedules A and B were designed on the base of the experimentally deduced CCT diagram of the steel. In rolling schedule A the material was subjected to accelerated cooling and coiling simulation after final rolling in the intercritical region, whereas in rolling schedule B the last rolling pass in the intercritical region was replaced by a water quench at the same temperature of the intercritical rolling pass in schedule A. Microstructure and texture were characterized by means of light optical microscopy, scanning electron microscopy, EBSD and XRD. It was found that the average grain diameter and the texture depend significantly on the final rolling temperature in the intercritical region. The decrease of the intercritical rolling temperature leads to an increase of the {111}〈uvw〉 /{001}〈uvw〉 ratio, but at the same time the increase of the average ferrite grain size was also observed. A phenomenological model based on the K–S orientation relationships was used to predict the texture formation in the intercritical region.

NbC Precipitation Kinetics during and after γ/α Transformation of a Nb Microalloyed Steel

The precipitation kinetics of NbC carbide during and after y/a transformation of a Nb microalloyed steel was investigated experimentally. The mole fractions of NbC precipitates formed at different isothermal temperatures were measured as a function of holding time by using the electrical resistivity method. The NbC precipitation happened during γ/α transformation and was closely related to ferrite transformation kinetics in the intercritical region while it occured in the bainite matrix after the bainitic transformation was almost done below bainite start (Bs) temperature.

Effect of Thermomechanical Processing on Microstructures of TRIP Steel

Abstract In order to control retained austenite, the effect of hot deformation in the intercritical region on the micro-structure of hot-rolled transformation-induced plasticity (TRIP) steel was studied on a Gleeble 1500 hot simulator. Compressive strains varying in amounts from 0 to 60% were imposed in the intercritical region, and effects on the formation of polygonal ferrite, carbide-free bainite and retained austenite were determined. With increasing the hot deformation amount and the ferrite content and decreasing the carbide-free bainite content, the volume fraction of retained austenite decreases. Increased dislocation density, grain refinement of ferrite and carbon enrichment are the main factors which control retained austenite stability.

Characterization of Microstructures across the Heat-Affected Zone of the Modified 9Cr-1Mo Weld Joint to Understand Its Role in Promoting Type IV Cracking

In the postweld heat-treated (PWHT) fusion welded modified 9Cr-1Mo steel joint, a soft zone was identified at the outer edge of the heat-affected zone (HAZ) of the base metal adjacent to the deposited weld metal. Hardness and tensile tests were performed on the base metal subjected to soaking for 5 minutes at temperatures below Ac1 to above Ac3 and tempering at the PWHT condition. These tests indicated that the soft zone in the weld joint corresponds to the intercritical region of HAZ. Creep tests were conducted on the base metal and cross weld joint. At relatively lower stresses and higher test temperatures, the weld joint possessed lower creep rupture life than the base metal, and the difference in creep rupture life increased with the decrease in stress and increase in temperature. Preferential accumulation of creep deformation coupled with extensive creep cavitation in the intercritical region of HAZ led to the premature failure of the weld joint in the intercritical region of the HAZ, commonly known as type IV cracking. The microstructures across the HAZ of the weld joint have been characterized to understand the role of microstructure in promoting type IV cracking. Strength reduction in the intercritical HAZ of the joint resulted from the combined effects of coarsening of dislocation substructures and precipitates. Constrained deformation of the soft intercritical HAZ sandwich between relatively stronger constitutes of the joint induced creep cavitation in the soft zone resulting in premature failure.

Procesos metalúrgicos de aceros microaleados de alta resistencia 450 EMZ tipo II en la zona afectada térmicamente debido al efecto multipasada en soldaduras por arco sumergido

A considerable number of crack tip opening displacement tests in the heat affected zone (HAZ) of multipass welds, performed in accordance with standards BS 5762 and EEMUA, are rejected since the crack is not inside the coarse grain region at 0.5 mm from the fusion border, as well as the quantity of the crack length in percentage, that the crack goes through the intercritical region instead of in the grain coarse region as it would correspond. This circumstance make advisable to carry out a metallographic study of the intercritical zone in the HAZ as well as the corresponding tests, in order analyze the intercritical region brittleness reasons. The study is performed on a HSLA 75 mm thick panel 450 EMZ type II, welded under a SAW process with heat input and welded parameters controlled, without any post weld heat treatment

Influence of Severe Accumulative Rolling in a Low Carbon Microalloyed Steel

Effect of the severe deformation by multi-pass rolling on microstructure and tensile properties was analyzed in terms of rolling temperature, plate thickness, and cooling rate for a modified API X65 steel containing B. The plates, 80 and 50 mm thickness, were rolled six times by 20%/pass (total 75%) to 20 and 12 mm, at 1023 K of unrecrystallized γ region or 973 K of intercritical (α+γ) region, and then quenched in water or oil. All specimens except one oil-quenched condition showed relatively high UTS 700-830 MPa and the continuous yielding(YR～0.6), typical mode of the (ferrite + martensite (bainite)) dual phase microstructure. In contrast, one oil-quenched specimen with the 973 K-20 mm condition, exhibited the discontinuous yielding (YR～0.8), indicating that the microstructure basically consists of ferrite plus pearlite, as well as a relatively low UTS 660 MPa. The degree of deformation really occurring within materials, i.e., strain hardening seems to be enhanced with a decrease in deformation temperature. As the degree of deformation increases, the remaining austenite, not dynamically transformed to fine ferrite, becomes increasingly unstable. A lower hardenability of this remaining austenite thus would lead to a higher possibility to transform into the (ferrite + pearlite) structure of lower strength rather than the (ferrite + martensite (bainite)) of higher strength.

Grain Growth after Intercritical Rolling

The distribution of the characteristic texture components between the ferrite grains of different size classes has been studied in a steel with 0.082%C, 1.54% Mn, 0.35% Si, 0.055%Nb and 0.078%V after different rolling schedules with a final rolling temperature above or below Ar3. Microstructures and textures were characterized by means of optical microscopy and orientation microscopy. A strong grain refining effect together with a bimodal grain size distribution was observed in the steel both after final rolling in the intercritical region or in the austenite region, close to the Ar3 d temperature. The differences in grain size were interpreted on the basis of three potentially acting mechanisms: (i) transformation- induced recrystallization, (ii) increased mobility of specific grain boundaries and (iii) fast nucleation of ferrite grains on specific sites of the parent austenite microstructure. The experimental data clearly favoured the third of these assumptions as the responsible mechanism for the observed bimodal grain size distributions.

Characteristics of the Static Recrystallization Kinetics of an Intercritically Deformed C-Mn Steel

The austenite recrystallization kinetics in the intercritical region of a C-Mn steel is investigated by means of stress relaxation tests. It is found that the Avrami exponent, n, decreases significantly with decreasing temperature, i.e. with increasing ferrite fraction. This behaviour deviates from that of austenite recrystallization in the purely austenitic state, in which case the Avrami exponent is constant and independent of temperature and deformation. To interpret this, the influence of spatial variation of the plastic strain in the intercritical austenite grains on recrystallization kinetics is modelled quantitatively. The modelling results seem to indicate that the strain heterogeneity is responsible for the decreasing Avrami exponent with decreasing intercritical temperature.

A metallurgical interpretation of the static recrystallization kinetics of an intercritically deformed C-Mn steel

The austenite recrystallization kinetics in the intercritical region of a C-Mn steel is investigated by means of stress relaxation tests. It is found that the Avrami exponent, n, decreases significantly with decreasing temperature, i.e., with increasing ferrite fraction. This behavior deviates from that of austenite recrystallization in the purely austenitic state, in which case the Avrami exponent is constant and independent of temperature. To resolve the origin of the changing Avrami exponent, the influence of the austenite/ferrite interface boundary area and that of the spatial variation of the plastic strain in the austenite grains is modeled quantitatively. The modeling results seem to indicate that the strain heterogeneity rather than the reduced interface boundary length is responsible for the decreasing Avrami exponent with decreasing temperature.

Structural characteristics of multipass flux cored arc steel welds

Abstract The effect of heat input, shielding gas, and welding technique on the structure of the weld metal has been investigated in all weld multipass test plates produced by flux cored arc welding using 1.2 mm diameter boron microalloyed rutile, metal cored, and rutile flux cored wires. Linear and areal measurement techniques have been used on cross-sections of seventeen test plates welded using three different consumables to determine the volume fraction of reheated (at least partially reaustenitised) region and as solidified weld metal. Using the simplifying assumption that the structural gradient in the heat affected zone of reheated weld metal can be characterised in terms of the grain coarsened, grain refined, and intercritical regions, the volume fractions of these regions were also determined. Some of the reheated regions evolve under the thermal influence of a second overlapping weld bead, resulting in doubly reheated weld metal structures that can be defined in terms of nine subzones. The presen...

Processing of ultrafine ferrite steels

Recently, the grain refinement of metallic materials has been investigated intensively. There are two basic methods, i.e., severe plastic deformation and thermomechanical processing, to refine grain sizes of bulk steels for the ultrafine or submicron range. By severe plastic deformation submicron-grained microstructure (less than 1 μm) in steels can be obtained. For example, the microstructure with ultrafine grains of 0.3 μm in a commercial 0.15 wt.%C–1.1 wt.%Mn–0.25 wt.%Si steel was obtained after ECAP at an accumulative strain of about 4, and the microstructure with ultrafine grains of 0.4 μm in a Ti-added interstitial free (IF) steel was obtained after ARB at an accumulative strain of about 5.6. As a result of thermomechanical processing for the grain refinement, the ultrafine ferrite (UFF) grains (about 1 μm) of steels could be obtained through hot rolling in the intercritical region, warm rolling in the ferrite region, or strip rolling under the condition of strain-induced transformation from austenite to ferrite. A composite microstructure, consisting of UFF grains in the surface layers penetrated to about one-quarter to one-third of thickness and coarser ferrite (about 5–10 μm) and pearlite in the core of the strip, is introduced by strain-induced transformation. Finally, the relationship of strength and ductility is discussed. The strength of ultrafine-grained steels increases with decreasing ferrite grain sizes, following the Hall–Petch relationship. On the other hand, ductility decreases with decreasing ferrite grain sizes. The composite microstructure of UFF surface and coarser core in steels has an advantage to keep both high strength and high ductility.

D310 CHARACTERISTICS OF THE WELDED JOINTS OF FERRITIC HEAT RESISTANT PIPE AND TUBE STEELS FOR ADVANCED STEAM POWER PLANTS

Effects of welding processes and thermal histories on mechanical properties and failure behaviors have been investigated in P/T91,P92 and P122 steels for utility boiler components. In T/P91 steel, the low hardness value below about 170Hv was remarkably influenced by the abnormal thermal exposure at the inter-critical region between the Ac1 and the Ac3 and subsequent heating at the temperature just below the Ac1. The T/P91 steel with low hardness of 155Hv showed low creep rupture strength below the allowable stresses of T/P91. The continuous fatigue life was similar for both P92 and P122 at the temperature range. The fatigue life of the CFI test with 10 min. hold time also indicated nearly a similar value in the base metal, while lower values by a factor of 2&acd;4 in the weld metal and the HAZ compared to that of the continuous fatigue test. The fatigue crack growth rates at 600℃ and 700℃ were faster in the HAZ than in the base metal at lower values of ΔK. However, the trends was gone into reverse at higher values of ΔK. At a given temperature, the creep crack propagation life of P122 is a little longer than that of P92. The correlation between a^^・ and C_t could be described as a^^.&acd;[C_t]^<1.149> in P92 and a^^.&acd;[C_t]^<0.9567> in P122 at the temperature range of 600℃ to 650℃.