Fracture Behavior of High-Nitrogen Austenitic Stainless Steel Under Continuous Cooling: Physical Simulation of Free-Surface Cracking of Heavy Forgings

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Superheavy forgings are increasingly used in the nuclear industry. The strain rate is extremely low during hot forging due to the huge size of the superheavy forging; in fact, the surface temperature of the forging decreases obviously during each deformation step. Hot-deformation behavior differs from that of isothermal deformation. In this study, 18Mn18Cr0.6N steel was selected as a model material. Hot-compression tests were conducted using a Gleeble 3800 simulator at a strain rate of 10−4 s−1 and continuous cooling rates of 0.0125 Ks−1 and 0.025 Ks−1. The microstructure was observed using electron backscatter diffraction analysis and transmission electron microscopy. The flow stress increased with increasing strain: the higher the cooling rate, the higher was the hardening rate. Continuous cooling inhibited dynamic recrystallization by delaying its nucleation. The subgrain/cell size increased linearly with increasing final temperature of deformation in the temperature range 1273 to 1448 K. An intense <001> texture formed in 0.8-strained specimens and the matrix exhibited a low Taylor factor orientation. Most dislocations were separately distributed in subgrains and did not entangle with each other or with the subgrain boundary. Dislocation arrays transferred easily through boundaries and dislocation accumulation at boundaries was weak. This study contributes to understanding the hot-forging process of superheavy forgings.

Section: Resultssupporting

confidence: 94%

Section: Introductionmentioning

confidence: 99%

Hot-Deformation Behavior of High-Nitrogen Austenitic Stainless Steel under Continuous Cooling: Physical Simulation of Surface Microstructure Evolution of Superheavy Forgings during Hot Forging

Wang

2019

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“…High-N austenitic stainless steels (HNASS) have attracted great interest due to their excellent mechanical properties, high N content and absence of the Ni element, compared with common austenitic stainless steels [ 8 , 9 , 10 ]. High-N austenitic stainless steel has been used for implant devices by Yang et al [ 11 ] and Li et al [ 12 ], for example as coronary stent material, showing superior strength and better hem-compatibility.…”

Section: Introductionmentioning

confidence: 99%

Microstructure and Properties of Porous High-N Ni-Free Austenitic Stainless Steel Fabricated by Powder Metallurgical Route

Ngai

Peng

et al. 2018

Porous high-N Ni-free austenitic stainless steel was fabricated by a powder metallurgical route. The microstructure and properties of the prepared porous austenitic stainless steel were studied. Results reveal that the duplex stainless steel transforms into austenitic stainless steel after nitridation sintering for 2 h. The prepared high-N stainless steel consists of γ-Fe matrix and FCC structured CrN. Worm-shaped and granular-shaped CrN precipitates were observed in the prepared materials. The orientation relationship between CrN and austenite matrix is [011]CrN//[011]γ and (-1-11)CrN//(1-11)γ. Results show that the as-fabricated porous high-nitrogen austenitic stainless steel features a higher mechanical property than common stainless steel foam. Both compressive strength and Young’s modulus decrease with an increase in porosity. The 3D morphology of the prepared porous materials presents good pore connectivity. The prepared porous high-N Ni-free austenitic stainless steel has superior pore connectivity, a good combination of compressive strength and ductility, and low elastic modulus, which makes this porous high-N Ni-free austenitic stainless steel very attractive for metal foam applications.

“…In previous studies, the influence of preheating temperature [ 6 ], strain rate [ 7 ], continuous cooling [ 8 ], grain size [ 9 , 10 ], and delta ferrite [ 11 , 12 ] on the hot ductility of HNASs was clarified in detail. However, these conditions differ from those encountered during the actual production of heavy-section HNAS components.…”

Section: Introductionmentioning

confidence: 99%

“…In addition to the above factors, precipitates that form easily from 600 to 1050 °C [ 13 , 14 ] also affect the hot ductility of HNASs significantly [ 15 ]. During the hot forming of heavy-section HNAS components, where the surface may be cooled to a low temperature during forming [ 8 , 16 ], cracks form almost simultaneously with precipitation. However, the combined effect of precipitates and other factors remains unclear.…”

Section: Introductionmentioning

confidence: 99%

Grain Size Effect on the Hot Ductility of High-Nitrogen Austenitic Stainless Steel in the Presence of Precipitates

Wang

2018

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Precipitation occurs easily during the hot forming of high-nitrogen austenitic stainless steels, which reduces their hot ductility significantly. The effect of grain size on the hot ductility of high-nitrogen austenitic stainless steel in the presence of precipitates was investigated. Different grain sizes of 18Mn18Cr0.5N steel specimens, with and without precipitates, were hot-tension tested. The precipitate morphology, fracture surface, and cracks were studied by scanning electron microscopy, transmission electron microscopy, and electron backscatter diffraction analysis. For the 18Mn18Cr0.5N steel, damage-formation strains of all grain-size specimens were reduced by the precipitates during the hot-tension test. Crack-formation sites were located at grain boundaries and were independent of the Taylor factor. A larger grain size resulted in an increased sensitivity of the fracture strain to precipitates. When the grain size was smaller than 51 μm, the fracture strain became insensitive to the precipitates. A method was suggested to mitigate surface cracking for metal materials with a high precipitation tendency.