Effects Of Austenite Grain Size Research Articles

Effect of microstructure and grain size on DWTT properties of 22 mm thick X80M hot rolled steel strip

Aiming at improving the performance of 22 mm thick X80M hot rolled steel strip DWTT performance, simulated tests and industrial trials were carried out on a 550 mm test mill and a 2250 mm hot tandem mill. The effects of different starting temperature and the end temperature of finish rolling, cooling rate, coiling temperature on the microstructure and DWTT properties of the test steel were studied. The effects of austenite grain size on DWTT properties were studied by comparing the rough rolling reduction rate and the accumulative deformation rate in the non-recrystallized zone. The results show that uniform and fine original austenite grains can obtain uniform, and fine acicular ferrite lath bundles. The microstructure of acicular ferrite, granular bainite and a large number of fine M/A islands dispersed on ferrite matrix with high density dislocations has better DWTT performance. Meanwhile, the average effective grain size of X80M pipeline steel can be refined to obtain more grain boundary area per unit volume. Large angle grain boundary is beneficial to prevent crack propagation. Improving grain size of X80M pipeline steel strip is an effective measure to improve fracture toughness. Under this chemical composition, the final pass reduction ratio in roughing mill of X80M pipeline steel is ≥26%, the thickness of intermediate slab is 68 mm,the starting temperature of finish rolling is 900 °C, the end temperature of finish rolling is 810 °C, the cooling rate is 25 °C/s, the coiling temperature is below 400 °C, and DWTT toughness fracture ratio can reach more than 85%, which can meet the application requirements of pipeline steel for the extreme conditions of −15 °C in the west-east gas pipeline.

Effect of austenite grain size on acicular ferrite transformation in a HSLA steel

Austenite grain size is well known to have a significant influence on various phase transformations in steels. Although the effects of many thermomechanical processing parameters on acicular ferrite (AF) transformation in HSLA steels have been investigated, little attention has been paid to the influence of austenite grain size. Therefore, in this research, different parameters of solid-solution heat treatment and rough deformation were adopted to generate austenite with different grain sizes and the effects of austenite grain size before deformation on the AF transformation and grain refinement were investigated. It was found that the reduction of prior-austenite grain size (PAGS) from 62.8 μm to 37.0 μm before austenite deformation promotes the AF transformation and simultaneously refines and homogenises the transformed microstructures. A reciprocally increased density of deformation induced dislocations with the reduction of PAGS was proposed to account for these results, which not only increase the nucleation sites of AF but also suppress the lengthening of BF laths. Further reducing PAGS from 37.0 μm to 22.3 μm, the fraction of AF is not increased and the transformed microstructure is not refined. Possible differences in the type, distribution, and density of austenite deformation substructures between austenite with PAGSs of 37.0 μm and 22.3 μm were argued to be responsible for the cease of grain refinement. The relationship between effective grain sizes and the Sv parameter was investigated and an equation relating effective grain sizes with processing parameters was also proposed.

Austenite to polygonal-ferrite transformation and carbide precipitation in high strength low alloy steel

AbstractBy differential scanning calorimetry measurement and microstructural examination, the effects of cooling rates on the kinetics and microstructure of polygonal ferrite transformation have been investigated in a high strength low alloy steel. The peak temperature of ferrite transformation exhibits an evident dependency on cooling rate. With the cooling rate decreasing, the ferrite fraction corresponding to the maximum transformation rate is decreased, as well as the size of polygonal ferrite grains. More ferrite is nucleated at lower cooling rate. On the basis of the effects of austenite grain size and carbide precipitation on ferrite formation, it is suggested that carbide precipitation plays a more dominant role in the kinetics of ferrite transformation. During austenite–ferrite transformation, the ferrite nucleation still proceeds, and the nucleation would not be saturated by pre-existing nuclei.

Effect of Austenite Grain Size on Hardenability and Impact Toughness of SCM435H

The effects of austenite grain size on hardenability and impact toughness were investigated. The results show that: Since the beginning of the two-phase region with quenching temperature, the austenite grain size from the initial 4+6 mixed crystal at 740°C, and gradually increased to 10 at 860°C; Austenite grain size and hardenability was directly proportional to the austenite grain size increased from 8μm to 36μm, the biggest change is the hardness 10HRC; Austenite grain size and impact toughness is linear, with the decrease of grain size, the impact energy increases linearly, and the austenite grain size and impact toughness curve fitting. Comprehensive analysis for ensuring the hardenability of cold heading steels should be considered optimal matching of material strength and plasticity.

極短パス間時間多パス圧延プロセスにおけるフェライト結晶粒微細化への内部組織変化の影響

The optimum conditions for manufacturing ultrafine-grained hot strips in super short interval multi-pass rolling (SSMR) process were discussed. As the first means of evaluation, the changes in austenite grain size and shape during multi-pass rolling were observed using experimental equipment. Deformed austenite grains could be observed with immediate quenching after each rolling pass. As the second means, a new numerical simulation method was used, which could predict the microstructure change of austenite, ferrite nucleation inside austenite grains, and ferrite grain growth. The effects of austenite grain size before finish rolling and also of temperature control between rolling passes on ferrite grain refinement were investigated. It was found that the temperature control between rolling passes is effective for grain refinement, whereas austenite grain refinement before finish rolling is not so effective. Therefore, deformation strain accumulation in austenite is important for ferrite grain refinement in SSMR process.

In situ observation of the formation of intragranular acicular ferrite at non-metallic inclusions in C–Mn steel

Intragranular acicular ferrite is regarded as a most desirable microstructure feature, in view of its strength and toughness, both in weld metals and in the heat-affected zone. This paper systematically investigated the effect of Ti addition on the evolution of intragranular acicular ferrite in the heat-affected zone of C–Mn steel. We also systematically studied the effects of austenite grain size, alloy content and the characteristic of inclusions on the formation of intragranular acicular ferrite. The nucleation and growth of intragranular acicular ferrite was directly observed by laser scanning confocal microscopy. Subsequently, microscopy analysis was used to quantitatively determine and distinguish the potent and inactive inclusions with respect to the nucleation of intragranular acicular ferrite. Finally, some possible reasons are given to explain the formation of intragranular acicular ferrite in the C–Mn steel.

Ultrafine grained structure formation in steels using dynamic strain induced transformation processing

The refinement of ferrite grain size is the most generally accepted approach to simultaneously improve the strength and toughness in steels. Historically, the level of ferrite refinement is limited to 5–10 μm using conventional industrial approaches. Nowadays, though, several thermomechanical processes have been developed to produce ferrite grain sizes of 1–3 μm or less, ranging from extreme thermal and deformation cycles to more typical thermomechanical processes. The present paper reviews the status of the production of ultrafine grained steels through relatively simple thermomechanical processing. This requires deformation within the Ae3 to Ar3 temperature range for a given alloy. Here, the formation of ultrafine ferrite (UFF) involves the dynamic transformation of a significant volume fraction of the austenite to ferrite. This dynamic strain induced transformation (DSIT) arises from the introduction of extensive intragranular nucleation sites that are not present in conventional controlled rolling. The DSIT route has the potential to be adjusted to suit current industrial infrastructure. However, there are a number of significant issues that have been raised, both as gaps in our understanding and as obstacles to industrial implementation. One of the critical issues is that it appears that very large strains are required. Combined with this concern is the issue of whether a combination of dynamic and static transformation can be used to achieve an adequate level of refinement. Another issue that has also become apparent is that grain sizes of 1 μm can lead to low levels of ductility and hence many workers are attempting to obtain 2–3 μm grains, or to introduce a second phase to provide the required ductility. There are also a number of areas of disagreement between authors including the role of dynamic recrystallisation of ferrite in the production of UFF by DSIT, the reasons for the low coarsening rate of UFF grains, the role of microalloying elements and the effects of austenite grain size and strain rate. The present review discusses these areas of controversy and highlights cases where experimental results do not agree.

Tempered Martensite Embrittlement in a 32NiCrMoV125 Steel

This study focused on tempered martensite embrittlement in a 32NiCrMoV125 steel through examination of the effects of austenite grain size and tempering temperature on the mechanical properties and fracture morphology of this material. Two different austenite grain sizes were obtained by austenitizing at 870 and 950 °C. After quenching, the specimens were tempered in the temperature range of 200–650 °C. The results obtained in this research indicate that by increasing the tempering temperature, the strength and hardness decrease, but ductility increases. However, impact testing indicated that tempered martensite embrittlement occurred when samples were tempered in the range of 250–400 °C. Fractography revealed intergranular and quasi-cleavage fracture. In summary, increasing the austenite grain size decreased strength, but increased impact toughness, except for samples tempered between 200 and 350 °C.

Effect of austenite grain size and bainite morphology on overall kinetics of bainite transformation in steels

An attempt is made to rationalise some contradictory observations on the effect of austenite grain size on the overall kinetics of the bainite transformation in steels. Experiments have been carried out on two steels which show opposite effects of austenite grain size on the reaction rate. General equations describing the reaction rate are derived by taking into account the morphology of the bainite in each steel. The equations derived can explain the contradictory effects of the austenite grain size on the overall reaction rate. The results are also found to be in good agreement with the published data.

The Formation of Deformation Induced Ferrite during Mechanical Testing.

The effects of austenite grain size, level of undercooling, and strain and strain rate in compression on the austenite-to-ferrite transformation were investigated in a 0.1 % C, 1.4% Mn steel. The influence of this transformation on the hot ductility was then examined. Straining was found to raise the effective Ar 3 temperature almost to the Ae 3 , and to accelerate the transformation significantly in both fine (∼ 25 μm) and coarse (∼ 200 μm) grained samples. At relatively low levels of undercooling ( ∼40°C), deformation induced ferrite only formed in regions that were significantly affected by the applied strain. In tension, it was observed that, if thin bands of grain boundary ferrite were present in the microstructure, the ductility decreased significantly and failure always occurred in this second phase. Increasing the strain rate significantly reduced the depth of the ductility trough across its entire width. It is concluded that, for the steel in question, unbending after continuous casting should be performed at strain rates in excess of 3 x 10 -2 s -1 to ensure good ductility.

The influence of austenite grain size and its distribution on chip deformation and tool life during machining of AISI 304L

In this article, the influence of austenite grain size and its distribution on chip deformation and tool life during machining of AISI 304L austenitic stainless steel bar is examined. Hot-forged bar and the quenched bars (at different quenching temperatures, 1050 °C, 1100 °C, 1150 °C, and 1200 °C) are machined at a high cutting speed. It was noted that the inhomogeneous distribution of grain size in the surface area, within a depth of 15 mm of the workpiece, resulted in tool edge breakage and lower tool life when machining the hot-forged bar compared with all of the quenched bars. In addition, a slight decrease in tool life was observed as the grain size increased in the quenched bars. The chip studies revealed that a higher segment height ratio of chip was gained when machining the hot-forged bar, compared to machining the quenched bars, due to the inhomogeneous distribution of grain size. Moreover, the thickness of the secondary shear zone was reduced as the grain size increased. Interestingly, it was noticed that the chip work hardened during the machining process due to strain-induced twinning and ɛ martensite transformation. The studies of tool wear and failure revealed that a crack was initiated on the flank face at the interface between the deposited workpiece and the tool substrate when machining the hot-forged bar. This crack was formed due to either the thermal and mechanical fatigue or plastic deformation of the tool substrate. The fatigue crack propagated into the tool substrate through the decohesion of interface between carbides. The criterion of tool life when machining all of the quenched bars was normal flank wear. Based on the studies of chip deformation and the mechanisms for tool wear and failure, the effects of austenite grain size and its distribution on tool life were explained.

Effects of the Austenite Grain Size and Deformation in the Unrecrystallized Austenite Region on Bainite Transformation Behavior and Microstructure.

The effects of austenite grain size and deformation in the unrecrystallized austenite region on bainite transformation behavior and microstructural characteristics were investigated under continuous cooling condition and compared with those in ferrite-pearlite transformation and martensite transformation. Austenite grain size does not affect the transformation temperature and resulting microstructure of bainitic ferrite except that nucleation site of bainitic ferrite is increased with the decrease in equiaxed austenite grain size. But the volume fraction of the transformed products depends on the temperature during continous cooling and does not change with the prior austenite grain size. These characteristics are quite different from those in ferrite-pearlite transformation. The deformation in the unrecrystallized austenite region raises the transformation temperature and decreases the hardness of bainitic ferrite. Kurdjumov-Sachs relationship holds between austenite and bainitic ferrite irrespective of prior austenite microstructure. No contribution of austenite grain boundary for the growth of bainitic ferrite seems to indicate the intensive involvement of the displacive mechanism in this transformation.

Effects of austenite grain size and cooling rate on Widmanstätten ferrite formation in low-alloy steels

Deformation dilatometry is used to simulate the hot rolling of 0.20 pct C-1.10 pct Mn steels over a product thickness range of 6 to 170 mm. In addition to a base steel, steels with additions of 0.02 pct Ti, 0.06 pct V, or 0.02 pct Nb are included in the study. The transformation behavior of each steel is explored for three different austenite grain sizes, nominally 30, 55, and 100 µm. In general, the volume fraction of WidmanstAtten ferrite increases in all four steels with increasing austenite grain size and cooling rate, with austenite grain size having the more significant effect. The Nb steel has the lowest transformation temperature range and the greatest propensity for WidmanstAtten ferrite formation, while the amount of WidmanstAtten ferrite is minimized in the Ti steel (as a result of intragranular nucleation of polygonal ferrite on coarse TiN particles). The data emphasize the importance of a refined austenite grain size in minimizing the formation of a coarse WidmanstAtten structure. With a sufficiently fine prior austenite grain size (e.g., ≤30 µm), significant amounts of WidmanstAtten structure can be avoided, even in a Nb-alloyed steel.

Effects of Austenite Grain Size on ε Martensitic Transformation in Fe-15mass%Mn Alloy

The effect of austenite (γ) grain size on the morphology of e martensite (e) and the transformation from γ to e has been investigated by means of optical microscopy, transmission electron microscopy and X-ray analysis in Fe-15 mass% Mn alloy, whose γ grain size was controlled between 1 and 130 μm by the reversion treatment of deformation induced bcc martensite to γ. With refining γ grain size, the formation of e tends to be suppressed and the starting temperature of γ-e transformation is also lowered. In the grain size range below 30 μm, the transformation is markedly suppressed. In small γ grains below 30 μm, one variant of e plates go through a γ grain from one grain boundary to the other of the opposite side

Effects of austenite grain size and grain boundary segregation of impurity atoms on high temperature ductility

The high temperature deformation characteristics of low alloy, high strength steel martensites have been studied with particular reference to the role of prior austenite grain size and the grain boundary segregation of impurity elements. It was confirmed that, in the region where the mean linear intercept of austenite grains is larger than 100 μm, slow tensile deformation at elevated temperatures nucleates microvoids on the boundaries and microvoid coalescence leads to final fracture. Elongation in this region is determined specifically by prior austenite grain size and is linearly proportional to the inverse of prior austenite grain size, if the mode of matrix strengthening is not varied. The addition of Cu or P impurities reduces austenite grain growth at temperatures above 1400 K and increases elongation. For higher P contents, the fracture mode transition from intergranular ductile to intergranular decohesion can be observed occasionally during the course of fracture. However, the effect of this transition on elongation cannot be detected by the unnotched tensile test used in these experiments because final fracture occurs following microvoid nucleation on prior austenite grain boundaries.

On the Temper Embrittlement of Manganese-Molybdenum-Nickel Steels

The manganese-molybdenum-nickel steels ASTM A533B and A508 are extensively used in the fabrication of reactor pressure vessels and steam generators in light water reactors. These components receive heat treatments during fabrication and in operational service that could lead to a possible degradation of toughness as a result of grain boundary segregation of alloying and impurity elements promoting temper embrittlement.The susceptibilities to temper embrittlement of commercially produced thick section A533B Class 1 and A508 Class 3 steels have been investigated by Charpy impact testing following isothermal heat treatments in the 300 to 600°C temperature range for periods up to 5000 h. In addition, the combined effects of austenite grain size and impurity content have been studied using experimental melts of the 533B/508 Class 3 type alloy composition doped with specific impurities. The lower and upper shelf fracture modes were examined as a function of aging treatment, and samples exhibiting a low temperature intergranular fracture mode were examined using Auger Electron Spectroscopy to determine the amount and types of elements segregated at the grain boundaries.While the commercial materials have been found to exhibit only small increases in the ductile-brittle transition temperature after isothermal aging at 450 to 500°C, large increases are observed for the experimental material with a high phosphorus content. The degree of embrittlement is strongly dependent on austenite grain size, increasing with increasing grain size. These results indicate the need for close control of chemical composition of the steel in inhibiting embrittlement and cracking in the weld coarse-grained, heat-affected zone regions.

18%Niマルエージ鋼の引張性質におよぼすオーステナイト粒度および時効前における冷間圧延の影響

18%Niマルエージ鋼の引張性質におよぼすオーステナイト粒度および時効前における冷間圧延の影響

The influence of grain size and stress on the morphology of a 300-grade maraging steel

The effects of austenite grain size and externally applied stress on the morphology of martensite formation for a 300-grade maraging steel were investigated. Austenite grain sizes ranging from ASTM 14 to 5 were examined. The growth pattern of the martensite was revealed by a selective aging treatment that involved heating to 815° C (austenitizing), cooling to 184° C (about 50 pct transformation to martensite), reheating to 400° C (partial aging of the martensite), and finally, cooling to room temperature (balance of austenite transforms). Blocky martensite formed in the fine-grain austenite, whereas for the coarse-grain size, a stringer-like structure developed. Electron transmission studies showed that the individual martensite units (laths or platelets) were similar in size for both types of morphologies. In both cases, the length of these units corresponded closely to the spacing between the twin boundaries or grain boundaries of the fine-grain specimens. Differences in morphology for different grain sizes are explained in terms of the relative ratios between the size of the martensite unit and the distance between boundaries intersecting the path of growing platelets. The application of an external stress to a coarse-grain specimen results in the delineation of the austenite annealing twins that normally cannot be readily detected. It is proposed that the application of a stress causes a preferential acceleration of the transformation in one of the two differently oriented, twin-related regions of an austenite grain. This argument is based on the differences in the maximum resolved shear stress for the most favorable orientation of the (112) {111} shear variants in each of the various possible twin-related austenite crystal orientations. Hardness and tensile data were also obtained. The absence of any significant variation in these properties for the different grain sizes is attributed to the similarity in size of the martensite units.

ステンレス鋼の溶接熱サイクル途上における高温延性に関する研究(第2報)

In this report, the continuation of authors' first report, various types of austenitic, martensitic and ferritic stainless steels were studied by RPI hot ductility test, the effects of working method upon the hot ductility were investigated about wrought round bars and rolled plates of austenitic stainless steels, and the effects of austenite grain size on the hot ductility were also studied. The following results were obtained.(1) It was found that the hot ductility during the cooling portion of thermal cycle is, in general, higher in rolled plates than in wrought round bars.In the 347 type stainless steels, the effect of working method on the hot ductility is remarkable, and it appears that this is one of the characteristic of these 347 type stainless steels. It was also found that the variation of hot ductility depending on the change is large in this steel.(2) The hot ductility measured at 1200°C to 1300°C during the cooling portion was the best in a 16-8-2 Cr-Ni-Mo alloy among various austenite stainless steels, followed by 316 type, 304L type, 304 type and 347 type in that order, the lowest value being given by 310 type. It was found by metallographic observation that the decrease of hot ductility of these austenitic stainless steels was due to the embrittlement caused by the partial liquation of grain boundary din heating up to 1340°C.(3) The behaviours of ferritic stainless steel 430 type and martensite stainless steel 410 type were quite different from that of austenitic ones.These steels did not show the embrittlement caused by liquation of grain boundary which occurred in austenitic stainless steel, since their melting points were much higher. Therefore, the decrease of hot ductility during cooling portion may be attributable to the growth of grains. In general, these steels showed good hot ductility, and it appears that the weld cracking of these steels may not be hot cracking but low temperature cracking due to martensitic transformation.(4) It was found by the RPI hot ductility test that the weldability of some austenitic stainless steels could be improved by the refinement of grain size and the steels with large grains inclined to suffer from more weld cracking.