Prior Austenite Grain Size Research Articles

The Influence of Lath, Block and Prior Austenite Grain (PAG) Size on the Tensile, Creep and Fatigue Properties of Novel Maraging Steel.

The influence of martensitic microstructure and prior austenite grain (PAG) size on the mechanical properties of novel maraging steel was studied. This was achieved by looking at two different martensitic structures with PAG sizes of approximately 40 µm and 80 µm, produced by hot rolling to different reductions. Two ageing heat-treatments were considered: both heat-treatments consisted of austenisation at 960 °C, then aging at 560 °C for 5 h, but while one was rapidly cooled the other was slow cooled and then extended aged at 480 °C for 64 h. It is shown that for the shorter ageing treatment the smaller PAG size resulted in significant improvements in strength (increase of more than 150 MPa), ductility (four times increase), creep life (almost four times increase in creep life) and fatigue life (almost doubled). Whereas, the extended aged sample showed similar changes in the fatigue life, elongation and hardness it displayed yet showed no difference in tensile strength and creep. These results display the complexity of microstructural contributions to mechanical properties in maraging steels.

The microstructure analysis of white etching layer on treads of rails

Abstract Through the utilization of the scanning electron microscope (SEM) and transmission electron microscope (TEM), the microstructure of Martensite White Etching Layers (WELs) on the surface of treads of U71Mn rail(used for a long time and taken from curves of normal railroad tracks) was analyzed and also its formation mechanism was discussed. As the result showed, the WELs observed from the cross-section was crescent-shaped and discontinuously distributed, with the thickness between 30 and 100 μm. Within the microstructure of WELs, the grain size of prior austenite at high temperature was ultra-fine, with the maximum diameter of approximate 1 μm at the outermost layer. As depths increased in the WELs, the grain size of prior austenite inside the WELs gradually became smaller; when the temperature on rail surface reached the temperature of austenitic transformation, austenite grains were less than 200 nm. Under the condition of room temperature, the microstructure inside the WELs was mainly composed of martensite. However, nanocrystalline α-phase, the structure of which was body-centered tetragonal, existed within 300 nm in the outermost layer of the WELs, and its misorientation generally featured random distribution. The organizations in the subsurface of the WELs were plate martensite, and undissolved and ultra-fine cementite, the amount of which rose as depths increase. The formation of the above-mentioned WELs was caused by rapid temperature rises and the subsequent cooling down of rail surface when a train suddenly braked or crept on a track.

Effect of ultrasonic vibration on microstructural evolution and mechanical properties of underwater wet welding joint

The propagation of high-intensity ultrasonic vibration into weld pool can influence on microstructure and mechanical properties of welded joints. This research introduced a hybrid method for improving weld quality by using superimposed ultrasonic vibration on workpiece directly in underwater flux cored arc welding (FCAW). With the purpose of revealing how macrostructure, microstructure and mechanical properties of welded joints are affected by ultrasonic vibration within the weld metal, the present study has been carried out. The results indicated that proeutectoid ferrite, ferrite side plate and acicular ferrite constitute the microstructures of weld metal. Ultrasonic vibration can affect the size and morphology of prior austenite grain during weld pool solidification and eventually accelerate the formation of fine ferrite microstructure of weld metal in high proportion in the subsequently solid-state phase transformation. Excellent balance of high tensile strength and impact toughness is realized as more fine columnar microstructures in weld metal with using ultrasonic vibration. These findings suggest that the addition of ultrasonic vibration is an effective way to improve the tensile strength and impact toughness of weld metal during underwater wet welding.

Improvement of strength-toughness combination in austempered low carbon bainitic steel: The key role of refining prior austenite grain size

Abstract A low carbon bainitic steel with ultrahigh strength of ∼1650 MPa and elongation and toughness of ∼16% and ∼72 J/cm2 respectively, was obtained through austempering. The resulting microstructure and mechanical properties for varying prior austenite grain size (PAGS) are described. The refinement of the PAGS resulted in finer packet/block size, higher volume fraction and higher stability of retained austenite. The coalescence of bainitic laths was effectively hindered when the prior austenite grain size (PAGS) was refined to ∼10 μm. On the other hand, the coalescence of bainitic laths with crystallographically homogeneous characteristic was accompanied by disappearance of film-like retained austenite and crack can propagate without inhibition, which is detrimental to toughness. Eliminating the coalescence of bainite and increasing the volume fraction of retained austenite and its stability by means of refining PAGS contributed to ultrahigh strength – high ductility and toughness combination.

Effect of austenite grain size on the bainitic ferrite morphology and grain refinement of a pipeline steel after continuous cooling

The effect of prior-austenite grain size (PAGS) on bainitic ferrite (BF) morphology and effective grain size in a pipeline steel under an industrial cooling condition was investigated. With PAGSs of 22.3μm and 37.0μm, BF laths with a parallel morphology could be seen, while as the PAGS is increased to 52.4μm or 62.8μm, more interlocking BF microstructures were observed. Detailed crystallographic analysis identified that intense BF variant selection occurs for the small-grained austenite, but due to the large austenite grain quantity, and thus the large total austenite grain boundary area per unit volume of the small-grained austenite, with the reduction of PAGS from 62.8μm to 22.3μm, the effective grain size of the transformed microstructure gradually decreases from 9.5μm to 5.6μm. The small-grained austenite transformation was dominated by one close-packed plane (CP) group, but for the larger austenite grain, different CP groups were observed to be adjacent to each other, producing the interlocking structure. A mechanism based on the carbon distribution near the BF/austenite interface is proposed to account for this difference.

Effects of Microalloy Additions and Thermomechanical Processing on Austenite Grain Size Control in Induction-Hardenable Medium Carbon Steel Bar Rolling

Three AISI 1045 steels: a base steel, one modified with vanadium (V), and one modified with V and niobium (Nb) were studied to evaluate microstructural conditioning prior to induction hardening. Simulated bar rolling histories were evaluated using fixed-end hot torsion tests with a Gleeble® 3500. The effects of chemical composition and thermomechanical treatment on final microstructures were examined through analysis of laboratory simulations of steel bar rolling and induction hardening processes in order to provide additional insights into the morphological evolution of austenite of microalloyed steels. Analysis of prior austenite grain size (PAGS) is complemented with analysis of austenite recrystallization and pancaking during rolling. The potential for utilizing TMP, in conjunction with microalloy additions, to enhance bar steel microstructures and subsequent performance is assessed by evaluating the induction hardening response of each steel systematically processed with different preconditioning treatments.

Effect of quenching temperature on martensite multi-level microstructures and properties of strength and toughness in 20CrNi2Mo steel

The martensite multi-level microstructures of 20CrNi2Mo steel, which were quenched at the different temperatures of 900–1200°C and tempered at 200°C, were investigated by optical microscope (OM), scanning electron microscopy (SEM), electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM), and the relationship between the microstructures and properties of strength and toughness was discussed by the classic formula of Hall–Petch. The results show that the size of prior austenite grain (dr), martensite packet (dp) and block (db) increase with increasing of the quenching temperature, while the martensite lath (dl) size is opposite. On another hand, the confusion degree of the martensite packets changes from disorder to order. The boundaries of prior austenite grain, packet, block and the martensite lath are high angle boundaries (HBs) and low angle boundaries (LBs), respectively, and the ratio of the low angle boundaries increase with the quenching temperature by calculating to the multi-level microstructure size with the mathematical model established by myself. In addition, the relationship between the packet/block and strength follows the classical formula of Hall–Petch, and the size of db is far lower than the size of dp, db is the effective control unit of the strength. Meanwhile, dl is the effective control unit of toughness because it strongly impacts the crack initiation and propagation and follows also the Hall-Petch with toughness in 20CrNi2Mo steel.

Object kinetic Monte Carlo study of the effect of grain boundaries in martensitic Fe–Cr–C alloys

Fe–Cr–C alloys with chromium concentrations in the range from ∼2 to 12 wt.% form ferritic‐martensitic structures by rapid cooling from the austenite state already in the presence of relatively low carbon concentrations. In this process it is possible to obtain different ratios of ferrite and martensite, as well as formation of carbides, by varying the thermal treatment. The presence of ferrite or martensite might have an influence on the nanostructural evolution under irradiation of these alloys. Here, considering a tempered martensite reference alloy with 9% Cr, we make use of an already validated object kinetic Monte Carlo (OKMC) model in order to study the possible effect of the formation of martensite laths on the material nanostructural evolution under neutron irradiation, assuming that the relevant boundaries act as sinks for radiation defects. The results show that the reduction of the grain size (including in this definition the average size of prior austenite grains, packets, blocks, and laths) does not play any relevant role until sizes of the order of ∼0.5 µm are reached: for smaller grains, the number of defects being absorbed by the boundaries becomes dominant. However, this threshold is lower than the experimentally observed martensite lath dimensions, thereby suggesting that what makes the difference in martensitic Fe–Cr–C alloys with respect to ferrite, concerning events and mechanisms taking place during irradiation, are not the lath boundaries as sinks. Differences between the nanostructural evolution under neutron irradiation in ferrite and martensite should therefore be ascribed to other factors.

Characterization of as-welded microstructure of heat-affected zone in modified 9Cr–1Mo–V–Nb steel weldment

Non-equilibrium microstructure of the heat-affected zone (HAZ) in the as-welded modified 9Cr–1Mo–V–Nb pipe steel (P91) weldment deposited by gas tungsten arc welding (GTAW) and flux core arc welding (FCAW) has been characterized by field-emission scanning electron microscope (FESEM) and electron backscatter diffraction (EBSD). The heterogeneous structures in the sub-layers of the as-welded HAZ are attributable to phase transformations caused by the welding thermal cycles and the local structure variations in the as-received base metal. Coarse-grained heat-affected zone (CGHAZ) has a prior austenite grain (PAG) size of 20μm. Fine uniformly-distributed precipitates and a higher fraction of MX carbonitrides are observed in the CGHAZ. Fine-grained heat-affected zone (FGHAZ) consists of the finest grains (1.22μm measured by EBSD, 5μm PAG size), coarse undissolved M23C6 carbides within the PAG boundaries and fine nucleated M23C6 particles within the martensite laths. Inter-critical heat-affected zone (ICHAZ) consists of partially austenitized grains and over-tempered martensite laths. EBSD kernel average misorientation (KAM) map in the FGHAZ close to the ICHAZ illustrates the greatest local strain variations with a moderate normalized KAM value of 0.92°. The majority (88.1%) of the matrix grains in the CGHAZ are classified as deformed grains by EBSD grain average misorientation (GAM) evaluation. The FGHAZ close to the ICHAZ has the most recrystallized grains with an area fraction of 14.4%. The highest density variation of precipitates within grains in the FGHAZ originates from the inhomogeneous chemistry in the base metal.

Effect of large-size M23C6-type carbides on the low-temperature toughness of martensitic heat-resistant steels

The relationship between the low-temperature toughness of and the microstructures found in a martensitic heat-resistant steel 10Cr12Ni3Mo2VN was studied using heat treatment and microscopic microstructure analysis. This study discusses the effect of large-size M23C6 type carbides on the low-temperature toughness. Recent results show that the large-size M23C6-type carbides that remain undissolved during austenitizing have a notably adverse effect on the low-temperature toughness after quenching and tempering. The large-size M23C6-type carbides form during normalization and can be eliminated by austenitizing at temperatures of 1050 °C and above. Refinement of prior austenite grains can markedly improve the low-temperature toughness. To meet the requirement of FATT50≤−35 °C in 10Cr12Ni3Mo2VN steel, the average size of prior austenite grains should be less than 65 μm. No large-size M23C6 type carbides should be present after quenching, and the tempering temperature should be 690 °C.

Effect of Partitioning Treatment on the Mechanical Property of Fe-0.19C-1.47Mn-1.50Si Steel with Refined Martensitic Microstructure

In order to understand the effect of microstructural features on the mechanical property, quenching and partitioning (Q&P) and quenching and tempering (Q&T) treatments were carried out on a cold-rolled low-carbon Fe-0.19C-1.47Mn-1.50Si steel sheet. It has been shown that because of the rolling in advance, the grain size of prior austenite was dramatically reduced, which resulted in a great decrease in martensite packet/block size and an increase in dislocation density in martensite in the as-quenched state. However, there was no obvious change in average lath size. Different from Q&T treatment, Q&P not only stabilized a large amount of retained austenite, but also led to a serious carbon depletion in martensite as revealed by X-ray diffraction and three-dimensional-atom-probe analyses. In Q&T and Q&P samples, refining martensitic microstructure improves both the strength and impact toughness markedly but does not affect the elongation very much. Compared with Q&T sample, Q&P one is softer due to the existence of considerable amount of retained austenite and less carbon content in martensite, i.e., it has higher elongation and impact toughness but lower strength. Analyses indicated that the strength loss caused by carbon depletion in martensite is critical which has even completely covered up the strengthening effect of microstructural refinement. On the other hand, the carbon depletion in martensite is more essential in improving impact toughness, comparing the role of microstructural refinement and the existence of more retained austenite. Through a combination of rolling and Q&P processes, the refined Q&P microstructure was achieved for a greatly improved product of strength and elongation and a much lower ductile-to-brittle transition temperature.

Microstructures and mechanical properties of dual phase steel produced by laboratory simulated strip casting

Conventional dual phase (DP) steel (0.08C–0.81Si–1.47Mn–0.03Al wt.%) was manufactured using simulated strip casting schedule in laboratory. The average grain size of prior austenite was 117±44μm. The continuous cooling transformation diagram was obtained. The microstructures having polygonal ferrite in the range of 40–90%, martensite with small amount of bainite and Widmanstätten ferrite were observed, leading to an ultimate tensile strength in the range of 461–623MPa and a corresponding total elongation in the range of 0.31–0.10. All samples exhibited three strain hardening stages. The predominant fracture mode of the studied steel was ductile, with the presence of some isolated cleavage facets, the number of which increased with an increase in martensite fraction. Compared to those of hot rolled DP steels, yield strength and ultimate tensile strength are lower due to large ferrite grain size, coarse martensite area and Widmanstätten ferrite.

Influence of Prior-Austenite Grain Structure on the Mechanical Properties of Ultrafine Elongated Grain Structure Steel Processed by Warm Tempforming

0.4%C-2%Si-1%Cr-1%Mo steels with different austenite grain structures were quenched and tempered at 773 K and deformed by multi-pass caliber rolling with a rolling reduction of 78% (i.e. warm tempforming, (WTF)). The microstructures, in addition to the tensile and Charpy impact properties of the warm tempformed (TF) steels, were investigated in relation to the prior-austenite grain (PAG) structures. The TF samples demonstrated hierarchical microstructures consisting of “packet bands” and “ultrafine elongated grains (UFEGs)”. The PAG structure had little influence on the transverse grain size, grain shape, <110>//rolling direction (RD) fiber texture, and carbide particle distribution in the UFEG structure, while the size of the packet bands decreased as the PAG size decreased. The PAG size had little influence on the strength of the TF sample. However, the effect of the WTF on the tensile ductility and toughness was enhanced as the PAG size decreased. It was demonstrated that as the PAG size decreased, the inverse temperature dependence of impact toughness (resulting from delamination) became significant at lower temperatures. The delamination toughening mechanism was discussed in relation to the UFEG and packet band structures.

A procedure for indirect and automatic measurement of prior austenite grain size in bainite/martensite microstructures

An alternative procedure for indirect and automatic measurement of the prior austenite grain size (PAGS) in bainite/martensite is proposed in this work. It consists in the determination of an effective grain size by means of statistical post-processing of electron backscatter diffraction (EBSD) data. The algorithm developed for that purpose, which is available on-line, has been applied to simulated EBSD maps as well as to both nanocrystalline bainitic steel and commercial hot-rolled air-cooled steel with a granular bainitic microstructure. The new proposed method has been proven to be robust, and results are in good agreement with conventional PAGS measurements. The added value of the procedure comes from its simplicity, as no parent reconstruction is involved during the process, and its suitability for low-magnification EBSD maps, thus allowing a large step size and coverage of a substantially broader area of the sample than the previous methods reported.

Effect of Titanium and Niobium on Modifying the Microstructure of Cast K100 Tool Steel

The effects of Ti and Nb on the microstructure of cast K100 tool steel were studied by optical and scanning electron microscopes. The amounts of Ti as 0.3, 0.7, and 1 wt pct and Nb as 0.2 and 1 wt pct were added to the studied steel. The addition of 0.3 wt pct Ti did not result in a considerable change in the size of carbides and prior austenite grain size. However, microstructure of K100 with 0.7 and 1 wt pct Ti was considerably modified (about 55 pct) and a uniform grain size was obtained at different positions (bottom, middle, and top) of the ingot. With addition of 0.2 and 1 wt pct Nb, microstructure was modified and a more uniform grain size was obtained all over the ingot. The average modification of microstructure in the bottom, middle, and top of the ingot was about 22 pct. Both Ti and Nb could effectively decrease the segregation of Cr and C from the bottom (high cooling rate positions) to the top of the ingots (low cooling rate positions). The homogeneity of chemical composition increased with increasing Nb or Ti. In alloy with 0.7 to 1 wt pct Ti, the average size of prior austenite grains was finer than alloys with 0.2 to 1 wt pct Nb. Therefore, Ti was found more capable than Nb in the modification of microstructure and decreasing the segregation of Cr and C in cast K100 tool steel.

Impact Toughness of 0.3 Mass% Carbon Tempered Martensitic Steels Evaluated by Instrumented Charpy Test

The effect of tempering temperature on the impact toughness of 0.3 mass% carbon martensitic steels with prior austenite grain (PAG) size of about 6 μm and 30 μm were investigated. Instrumented Charpy impact test (ICIT) method was used to evaluate the impact toughness. The tempering temperature of 723K gives the largest difference in the Charpy impact energy at room temperature (RT) between the specimens with two different PAG sizes. Investigation of the test temperature dependence of Charpy impact energy in the 723K tempered steels shows a steep transition at around 200 K for the 6 μm PAG specimen, while it shows a continuous slow transition in a wide range of temperature for the 60 μm PAG specimen. ICIT waveform analysis shows that the fracture propagation energy in stead of the fracture initiation energy mainly controls the temperature dependence of the impact energy. The carbide size distribution in these two specimens was investigated by SEM and TEM. The 60 μm PAG specimen shows the distribution of coarser carbides than does the 6 μm PAG specimen, which seems to be the main reason for the observed difference between them in the Charpy impact energy and the other properties of impact fracture.

Strengthening and Toughening Mechanisms in Martensitic Steel

Low carbon martensitic steels are often produced by reaustenitizing and quenching (RA/Q). Direct quenching (DQ) has gained interest in the past few decades and requires quenching immediately after working above or below the austenite recrystallization temperature to form martensitic microstructures. In the current study, microalloyed ASTM A514 steel is used to produce martensite from either equiaxed or pancaked prior austenite grain (PAG) microstructures. The equiaxed PAG conditions simulate microstructures produced by RA/Q and the pancaked PAG conditions simulate microstructures produced by controlled rolling (CR) before DQ. Controlled rolling followed by DQ was simulated with double hit compression in a Gleeble® 3500. The prior austenite grain size (PAGS) was varied between 9 and 75 μm prior to controlled rolling. The strengthening and toughening mechanisms are being investigated in the as-quenched (AsQ), low temperature tempered (LTT: 200 °C), and high temperature tempered (HTT: 600 °C) conditions. The equiaxed PAG condition has a Hall-Petch (H-P) relationship between yield strength (or microhardness) and PAGS in the AsQ condition. There is not a H-P relationship between PAGS and microhardness in the CR-DQ conditions. The CR-DQ conditions generally exhibit higher microhardness than the RA/Q conditions with similar PAGS, with the most significant differences in the larger PAGS conditions. Toughness was only measured in the equiaxed PAG conditions. The smallest PAGS has the lowest ductile-to-brittle transition temperature (DBTT) with the highest strength in the AsQ and LTT conditions. The smallest PAGS has the lowest DBTT and the lowest strength in the HTT condition.

Microstructural effects on discrepancy between toughness under sharp cracks and blunt notches in Ti-added 8Ni-13Co secondary hardening steel

In this study, the toughness under sharp cracks and blunt notches is investigated in terms of the prior austenite grain size (PAGS) and inclusion particles as a function of the austenitizing treatment in a Ti-added 13Co-8Ni secondary hardening steel. For the quantitative analyses of the inclusion and precipitate particles, small-angle neutron scattering analyses are conducted under austenitizing conditions of 1050 °C, 1200/1050 °C, and 1200/1200 °C; the impact toughness values are 32, 30, and 24 J for each austenitizing condition, respectively. In contrast, the fracture toughness values under the same conditions are 66, 78, and 103 MPa·m1/2. Thus, the fracture toughness significantly improves under 1200/1200 °C austenitizing conditions with coarse PAGS; however, the impact toughness deteriorates. The adverse effect of the grain size on the toughness under sharp cracks and blunt notches is elucidated in terms of the effective microstructural factors that control the fracture process inside the plastic zone, the size of which varies with the notch sharpness. In particular, through considering the density of the slip bands as a function of the grain size in the small confined plastic zone before the sharp crack, the complicated problem regarding an increase in the fracture toughness with an increasing grain size is described from micromechanical and microstructural perspectives.

Effect of Heat Treatment on Ductile-Brittle Transition Behaviour of 9Cr-1Mo Steel

The 9Cr-1Mo steel used in fast breeder reactor is exposed to irradiation during service which severely affects the dynamic fracture resistance by increasing the ductile to brittle transition temperature (DBTT). Thus, even at room temperature, the steel can become brittle and prone to cracking. The microstructural features especially prior austenite grain size (PAGS), martensitic lath and packet size originating from prior processing routes has an effect on the toughness properties of 9Cr-1Mo steel, which is yet to be understood fully. In the present investigation, three different austenitizing temperatures (950°C, 1025°C and 1100°C for 1h) have been selected to vary the PAGS. Impact energy, lateral expansion, dynamic yield stress as the function of temperature has been estimated from Charpy impact tests carried out on normalized and tempered specimens. The results show lower DBTT for the samples reheated at 1025°C followed by tempering.

Metallographic etching and microstructure characterization of NiCrMoV rotor steels for nuclear power

The grain size of prior austenite has a distinct influence on the microstructure and final mechanical properties of steels. Thus, it is significant to clearly reveal the grain boundaries and therefore to precisely characterize the grain size of prior austenite. For NiCrMoV rotor steels quenched and tempered at high temperature, it is really difficult to display the grain boundaries of prior austenite clearly, which limits a further study on the correlation between the properties and the corresponding microstructure. In this paper, an effective etchant was put forward and further optimized. Experimental results indicated that this agent was effective to show the details of grain boundaries, which help analyze fatigue crack details along the propagation path. The optimized corrosion agent is successful to observe the microstructure characteristics and expected to help analyze the effect of microstructure for a further study on the mechanical properties of NiCrMoV rotor steels used in the field of nuclear power.