Effect Of Prior Austenite Grain Size Research Articles

Influence of prior austenite grain size on martensite-austenite islands morphology and decomposition characteristics during two-step tempering in Mn-Ni-Mo steel

Martensite-austenite (M/A) islands are typical in the quenched microstructure of Mn-Ni-Mo steel, frequently utilized for fabricating reactor pressure vessels (RPV). These islands can decompose into harmful long rod-shaped Fe3C carbides during tempering above 600°C, quantifying the negative effect of M/A island decomposition on mechanical properties and indicating a potential issue in RPV fabrication. To address this issue, the present study investigates a two-step tempering process on three different prior austenite grain size (PAGS) specimens (5 µm, 47 µm, and 119 µm). The effect of PAGS on M/A island decomposition characteristics during sub-600°C tempering and their impact on precipitated carbide morphology is investigated. A preliminary pre-tempering treatment at 200°C, 350°C, and 500°C for 2 hours was followed by a 3-hour final tempering step at 650°C. The findings revealed unique temperature-dependent decomposition characteristics of M/A islands. For example, at 200°C, partial M/A island decomposition took place, resulting in long rod-shaped carbides during final tempering. Meanwhile, the 350°C pre-tempering specimen revealed aggregated carbide precipitates of varying sizes in former M/A island locations. In contrast, the 500°C pre-tempering specimen showed coarse globular carbides and the absence of carbide precipitate clustering. This result can be attributed to the decrease in surface energy associated with the carbide-matrix interface as size increases. After tempering, all specimens produced three types of precipitates: rod-shaped Fe3C carbides, rhombus-shaped Mn7C3, and globular-shaped Mn23C6 carbides. The formation of long rod-shaped carbides during M/A island decomposition reduces the critical cleavage stress for micro-crack initiation, jeopardizing steel mechanical properties. Aggregated carbides cause the simultaneous formation of numerous micro-voids in close proximity, which eventually merge to form a large crack, resulting in material failure. When compared to other pre-tempered and directly tempered specimens, pre-tempering at 500°C produced the most favorable microstructure in terms of impact properties.

Effect of prior-austenite grain size on the microstructure and mechanical properties of ultrafine-grained dual-phase layer-structured steel produced by hot rolling of intercritical annealed bainite

In this study, ultrafine-grained (UFG) ferrite/martensite (F/M) layer-structured (LS) steels were fabricated by hot rolling of intercritical annealed bainite steels with a rolling reduction of 75% to ease the strength-toughness trade-off dilemma, and the effect of prior-austenite grain (PAG) size of the bainitic steels on the microstructure and mechanical properties were systematically studied. The results showed that the layer thickness of martensite and ferrite was not affected by the PAG size, but the spacing of parallel lamellae bands (PLS), which evolved from the packet of bainite, decreased with the decrease of PAG size. The PLS had negligible influence on strength and ductility, but the decrease of the PLS led to the increase of delamination and both room temperature (RT) and cryogenic toughness enhancement. By reducing the PLS to introduce more delamination in this steel, the RT and cryogenic impact energy were increased 5.7 times (301 J) and 21 times (102 J at −196 °C), but without the sacrifice of strength (over 1.5 GPa) and ductility (total elongation of about 14%), compared with the initial bainite steel with the same composition respectively. The significantly improved toughness, especially the cryogenic toughness, was considered to result from the special UFG dual-phase layered structure as strain localization could be restrained and ductile fracture could be promoted by this special microstructure even at −196 °C. The cryogenic toughness improvement with the decrease of the spacing of PLS was confirmed to originate from the ductile crack path increase induced by delamination.

Effect of Prior Austenite Grain-Size on the Annealing Twin Density and Hardness in the Austenitic Stainless Steel

The present study examined the effect of prior austenite grain size on twin density and hardness of austenitic stainless steels (ASS). The 253 MA and 316L ASS were subjected to multi-pass cold rolling to reduce thickness up to 2.3 mm. Subsequently, the rolled steels were heat treated at 1100°C at 0, 900, 1800, 2700, and 3600 seconds in a tubular furnace in a hydrogen atmosphere. At the end of the annealing time, the rolled steel was quenched in the cooled zone of the tubular furnace until it reached room temperature in a hydrogen atmosphere. Then, microstructure observation of ASS was done to identify the austenitic grain size and annealing twin, and a hardness test was performed using the micro-Vickers hardness scale. The line intercept method was used to measure the changes in 253 MA and 316L austenitic grain sizes. ImageJ software was used to measure grain size and twin length. The results showed that austenite grains of both steels grew normally; 253 MA ASS had a lower SFE and K value than 316L ASS, which indicated that 253 MA ASS had sluggish grain growth, smaller grains, more easily formed annealing twins, and higher twin density. The Hall–Petch coefficient, K’, of 253 MA ASS was higher than 316L ASS, which resulted in a higher hardness value. The Sellars, Pande and Hall-Petch models were shown to predict austenite grain sizes, twin density, and hardness in 253 MA and 316L ASS.

Effect of cooling-rate during solidification on the structure-property relationship of hot deformed low-carbon steel

The present study investigates the combined effect of cooling-rate during solidification and hot deformation on the microstructure and mechanical properties of low-carbon steel, especially considering the beneficial effect of acicular ferrite. Low-carbon steel was cast in two different moulds to vary the cooling-rate during solidification. The cast ingots were soaked and hot forged at different temperatures. Relatively slower cooling-rate during solidification and deformation at relatively lower temperature were found to be beneficial for the acicular ferrite formation, which increased the hardness and strength without hampering the ductility. The difference in solute contents at the dendrite center and the interdendritic regions of the cast-ingots, as a result of microsegregation, and the compositional homogenization during soaking were predicted. Based on the predicted compositions, the microstructures of the cast and hot forged ingots were explained considering the effect of prior-austenite grain size, austenite conditioning during deformation, fraction of non-metallic inclusions and the segregation of Nb along the prior-austenite grain boundaries on the phase transformation.

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.

ショットピーニング後に人工腐食ピット加工を施した懸架ばね用鋼の疲労特性に及ぼす水素の影響

In order to reduce the weight of the suspension springs by increasing the steel strength, it is necessary to improve the corrosion fatigue property. However, the dominant factor governing the corrosion fatigue property of the suspension spring has not been satisfactorily revealed yet. Therefore, the investigation into elucidating the mechanism has been carried out to independently evaluate the influences of corrosion pits, shot peening, hydrogen, prior-austenite grain size, and so forth on the corrosion fatigue property. In this study, the influence of hydrogen on rotary bending fatigue test was investigated using specimens with artificial corrosion pits after shot peening. The dominant factors governing the corrosion fatigue property of the suspension spring were discussed based on the result of the present study in conjunction with the results of the previous study on the influences of shot peening and corrosion pit depth. The conclusions obtained were as follows: (1) In the case of the specimen without corrosion pit or with only shallow corrosion pits having still large compressive residual stress at the bottom of the pits, the fatigue property was fairly good but declined with the increment of charged hydrogen due to the diffusible hydrogen. On the other hand, in the case of the specimen with deep corrosion pits having only small compressive residual stress, the fatigue property was poor regardless of the charged hydrogen content. We inferred from this result that the deep corrosion pit releases the compressive residual stress at the bottom of pits. (2) The effect of prior-austenite grain size on fatigue properties of hydrogen charged specimens turned out to be almost negligible.

Effect of prior-austenite grain size and transformation temperature on nodule size of microalloyed hypereutectoid steels

The effect of prior-austenite grain size and transformation temperature on nodule size and colony size of hypereutectoid steels containing 1 pct carbon with different levels of vanadium and silicon was investigated. Specimens of the various steels were thermally processed at various temperatures ranging from 900 °C to 1200 °C and transferred to salt bath conditions at 550 °C, 580 °C, and 620 °C to examine the structural evolution of pearlite. The heat-treatment work showed that for only the hypereutectoid steel without vanadium there was a continuous grain boundary cementite network, the thickness of which increased with increasing reheat temperature. Analysis of the thermally processed hypereutectoid steels also indicated that the prior-austenite grain size and transformation temperature controlled the nodule size, while the colony size was dependent on the latter only.

A Discussion of the Paper “The Role of Inclusions on Mechanical Properties in High-Strength Steels”

The authors show that at high tensile strength fracture toughness is insensitive to steel cleanliness, and reduction of area is very sensitive to steel cleanliness. These conclusions are generally substantiated by work conducted at U. S. Steel. The U. S. Steel work amplifies that reported in the subject paper by examining the effect of residual elements (cleanliness) on the reduction of area and fracture toughness of 18Ni maraging steels and the effect of prior-austenite grain size and residual-element content on the reduction of area and fracture toughness of high-strength quenched and tempered alloy steels. The U. S. Steel work shows that for the maraging and alloy steels studied, reduction-of-area values are very sensitive to residual-element level but not affected much by tensile-strength levels in the range 230–300 ksi. The fracture toughness of the maraging steels shows a sensitivity to residual-element level that decreases as the strength is raised. The steels appear to reach a point of complete insensitivity to residual-element level at a tensile strength of 310 ksi. At this strength, the fracture toughness approaches a KIc value of 50 ksi( in.)12. The fracture toughness of the alloy steels is insensitive to residual-element level throughout the tensile-strength range 230–320 ksi, but is sensitive to prior-austenite grain size. The grain-size sensitivity decreases as the strength level increases and appears to reach a point of complete insensitivity at a tensile strength of about 310 ksi. Again, the fracture toughness of the steels at a tensile strength of 310 ksi approaches a KIc value of approximately 50 ksi in.12. It is concluded that lowering residual-element levels by vacuum melting should improve the fracture toughness of maraging steels at tensile strengths below 310 ksi but will probably not improve the fracture toughness of quenched and tempered alloy steel at tensile strengths in the range 230 ksi and higher. Lowering residual elements by vacuum melting improves the ductility, as measured by reduction of area, of both maraging and alloy steels over the entire range of strengths studied. Reduction of area is not very sensitive to tensile strength in the range studied.