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In the nuclear fuel reprocessing systems, the demands for high borated stainless steels are increasing due to their excellent thermal neutron shielding properties. However, the eutectic borides precipitating around the matrix grains could easily result in the severe edge cracking for the ingot-casting steels during processing. In this paper, a 3-layered composite plate consisting of 2.1 wt% B stainless steel and boron-free austenite steel was fabricated by welding and hot-roll-bonding. Besides, the microstructure evolution and the tensile properties of the composite and non-composite plates were comparatively investigated. It was found that the edge cracking of the composite plate was significantly suppressed comparing with the non-composite plate prepared by conventional hot rolling. Moreover, the ductility of the composite plate at room-temperature was greatly enhanced. The tensile elongation reached to 15.5% for the specimen after 1100 °C × 30 min solution treatment, and that was approximate 2.5 times as high as the elongation of the non-composite specimen. In addition, the inhibition mechanism of edge cracking during hot-roll-bonding, and the enhancement mechanism of tensile plasticity for the composite plates were both explained. The present work not only develops a new high borated stainless steel with the excellent mechanical properties, but also provides a promising method to suppress the edge cracking for some other metallic materials with poor hot workability. [ABSTRACT FROM AUTHOR]

High borated steels have been applied in the nuclear power industry as excellent neutron-shielding materials, but their ductility was relatively low due to the large and brittle borides, restricting their wider utilization. In this work, the high-performance borated steels containing 2.40 wt% B were fabricated through a designing combination of titanium-alloying along with twin-roll strip casting technology. It was found that titanium addition facilitated the increasing boron content of eutectic point for steels. The TiB 2 particles solidifying firstly from molten steel depleted massive boron atoms in advance, leading to an as-cast microstructure consisting of more γ-Fe and less M 2 B-type borides comparing with titanium-free steels. Besides, TiB 2 particles were 200–240 nm, and they could act as heterogeneous nucleation cores for M 2 B borides. Thus dispersive and fine M 2 B borides smaller than 5 μm were produced in as-cast microstructure. After subsequent hot-rolling and solution-treating, ultra-fine borides were obtained with more than 80% of which in a size range of 0.1–2 μm. In this case, the steels exhibited an excellent total elongation of 16.2% that was twice as high as that of titanium-free steels. Additionally, the relations between microstructure and mechanical properties of hot-rolled and solution-treated steels were clarified. The ductility of steels was determined by the volume fraction of γ-Fe matrix that could accommodate large deformation, but the strength of steels was dominated by the volume fraction of borides that provided the strengthening effects including grain refinement, load transfer term and hindering the dislocation slip. This work provided a new promising method to enhance the ductility of such high borated steels. [ABSTRACT FROM AUTHOR]

The increase in casting slab thickness causes sharp increase in both the size and uneven distribution of brittle eutectic borides in high borated austenitic stainless steel, thus remarkably deteriorating the hot workability and the final room-temperature mechanical properties. Hence, it is undesirable to produce thick plates by rolling an extra-thick slab. Here it is reported that a specially designed hot-roll-bonding method for producing Fe–19Cr–15Ni-1.2B austenitic stainless steel thick plates using the conventional thick slab as starting materials. The main aim is to obtain refined and uniformly distributed borides so as to improve the room-temperature ductility and toughness by increasing total rolling compression ratio via hot-roll-bonding. It was validated that double-layered thick slabs could be successfully hot-roll-bonded though massive brittle ceramic (Cr,Fe) 2 B particles exist at the contacting interlayer interfaces. The hot-roll-bonding mechanism was clarified in terms of severe plastic flow, recrystallization and interface-diffusion. The effects of total rolling compression ratio on the morphological change and size distribution of (Cr,Fe) 2 B particles in both the mono-layered plates and the double-layered composite plates were investigated in details with the consequent effects on room-temperature comprehensive mechanical properties. It was found that the initial large and brittle (Cr,Fe) 2 B broke into more spheroidal and refined particles and significantly enhanced the ductility and toughness with increasing the total rolling compression ratio up to 10. The present work provides not only a practicable method to fabricate high borated austenitic stainless steel thick plates but also a promising way to improve the ductility and toughness of other metals containing a large number of hard and brittle particles. [ABSTRACT FROM AUTHOR]

To further understand the effects of LSP on large components of high strength steel and provide improved solutions, this paper conducts research on effects of surface integrity on fatigue life in the LSP process with absorbent coating (LSPC), without absorbent coating (LSPN), without absorbent coating but with axial polishing (LSPP) respectively. The research also put a baseline specimens (BSL) before LSP for a comparison. Results show that average fatigue life in the LSPN process is 47.8% shorter than that of baseline specimens, and crack site area presents an obvious splitting-like tearing step. In the LSPC process, fatigue life is 31.7% longer than that of baseline specimens, and crack initiation site develops from surface to subsurface. In the LSPP process, the specimens show the longest fatigue life of 2.53 × 105cycles, with multiple fine microcracks developing simultaneously. Axial polishing removes ablative layer and tensile residual stress in the LSPN process and reduces dislocation pile-ups and refines grains in the LSPC process, resulting in a surface roughness of R a 0.61 μm, surface residual stress of −531MPa and microhardness of 405HV 0.025.Thus, the fatigue performances were found to be significantly affected by multiple surface integrity indicators induced by different LSP processes. In this paper, the effect of LSP on surface integrity on the subface and its influence on crack initiation and propagation have also been discussed, the crack initiation mechanisms in different processes were explained by using Murakami's K max equation. [ABSTRACT FROM AUTHOR]

High borated steel sheets containing 0.25 wt% to 4.0 wt% boron including hypoeutectic (0.28 wt% B and 2.11 wt% B), eutectic (2.43 wt% B) and hypereutectic (4.01 wt% B) compositions were tried to be fabricated by a novel strip casting technology, and the effects of boron content on sub-rapid solidification behavior and subsequent microstructural evolution together with mechanical properties was studied. It was found that the morphology of borides depended greatly on the boron content. When increasing boron, the morphological change of borides was network-like → grainy → cluster-like → plate-like. Various orientation relationships between different morphological borides and γ-Fe in as-cast steels were also identified. After subsequent hot-rolling and solution-treating, ultra-fine (<10 μm) borides were obtained in hypoeutectic and eutectic steels. Benefiting from that, excellent mechanical properties was achieved, which was much better than that of steels prepared by traditional ingot casting. Particularly, when the boron content approached eutectic composition, of which the effect on the microstructure and mechanical properties was more sensitive. The steels containing 2.11 wt% and 2.43 wt% boron exhibited different morphological borides with a total elongation of 14.1% and 8.0%, respectively. Moreover, a thin hypereutectic borated steel sheet was also prepared, but the borides were very large and the total elongation was very low. The correlation between microstructure and strength was clarified based on the Hall-Petch behavior and intermetallic strengthening effect. Smaller γ-grains and higher volume fraction of borides were responsible for higher strength. The plastic behavior of steels was influenced by strain-hardening rate that was determined by the volume fraction of γ-Fe matrix. A better sustainable strain-hardening capability was beneficial to extend the stage of uniform deformation and enhance the ductility. In addition, the mechanical properties of steels were also decided by the fracture mechanism that was dominated by the size of borides. [ABSTRACT FROM AUTHOR]

Abstract High borated stainless steel sheets have been widely applied in the nuclear power industry because of good thermal neutron absorption property. However, the large and network-like borides existing around the austenite grains in the conventional ingot casting steel seriously deteriorate the hot workability and the mechanical properties at ambient temperature. In this work, we tried to acquire ultra-fine microstructure by sub-rapid solidification using a novel twin-roll strip casting technology so as to enhance the mechanical properties of 2.1%B austenitic stainless steel. Surprisingly, dispersive and very fine borides mostly smaller than 5 µm were produced in the as-cast microstructure without network-like distribution. The morphologies and crystal structure of borides as well as the stacking faults in borides were investigated in detail. After subsequent hot-rolling and solution treatment, ultra-fine borides were observed with more than 50% of which in a size range of 0.3–1.5 µm, significantly smaller than those of the conventional ingot casted and hot rolled steel. Benefiting from the ultra-fine borides, excellent mechanical properties which had not been reported were obtained. In particular, a total elongation of 14.1% was exhibited, which was twice as high as that of the conventional ingot casted and hot rolled steel. Thus a new structure-function combining high borated stainless steel sheet was achieved. The strengthening and plasticity increasing mechanism was discussed based on Hall-Petch relationship, Orowan mechanism and strain-hardening rate analysis. The fracture behavior in tensile deformation was studied in detail. A characteristic fracture process accompanied with the initiation and coalescence of cavities in austenite matrix was found. This work not only developed a new processing way to produce high borated stainless steel with excellent mechanical properties, but also provided a potential solution for some other hard-worked metallic materials with brittle eutectic phase. [ABSTRACT FROM AUTHOR]

Nano/ultrafine-grained 18Cr–8Ni stainless steel was produced by severe cold deformation followed by annealing. The effect of annealing temperature on the fraction of high-angle grain boundaries (HAGBs), the kernel average misorientation (KAM), and Schmid factor of steels are revealed through electron backscatter diffraction analysis. The analysis on HAGB, KAM values and Schmid factor frequency provide a mechanistic understanding of the impact of structure on mechanical properties, including hardness, strength, toughness and ductility. In this study, a novel application of persistent homology (PH) was employed to reduce the dimensionality of the information describing the complex processing-structure-property relationship. PH emphasizes the relationship between processing (annealing temperature), structure (distribution of the Schmid factor) and property (strength). Specifically, the PH formalism translates multidimensional data sets into clusters, distinguished by common PH features. In this analysis, two clusters emerge. First, at low annealing temperature, incomplete reversion of austenite in austenitic stainless steels results in materials with a greater fraction of grain boundaries, resulting in high strength and low ductility. Second, at high annealing temperature, fully reversed austenite results in lower, though still acceptable strength and superior ductility. The PH approach is applicable for identifying salient features in complex processing-structure-property relationships and is amenable to analysis of large data sets based on machine learning. [ABSTRACT FROM AUTHOR]