The Microstructure Evolution and Dynamic Softening Mechanism of a 00Cr27Ni7Mo5N Hyper Duplex Stainless Steel

Abstract

Duplex stainless steels combine the advantages of ferritic and austenitic properties with excellent strength, plastic toughness, and corrosion resistance, [1][2][3][4][5] resulting in a wide range of applications in the marine industry, chemical industries, and heat exchangers. To meet the needs of deep-sea oil and gas exploration, desalination, and other harsh corrosion conditions of high chloride, pitting resistance equivalent value greater than 48, such as 00Cr27Ni7Mo5N hyper duplex stainless steel has received more and more attention. It is well known that the differences between ferrite and austenite in duplex stainless steels, such as the stacking fault energy (SFE), crystal structure, and grain boundary migration ability, affect the dislocation motion, resulting in a complex softening mechanism of both phases during hot deformation. [6][7][8][9][10][11][12][13][14][15][16] Liu et al. [17] reported that continuous dynamic recrystallization (CDRX) tended to occur in both phases of LDX2101 duplex stainless steel during hot deformation. Deng et al. [18] indicated that both phases of 23Cr-2.2Ni-6.3Mn-0.26N duplex stainless steel would tend to soften via dynamic recovery (DRV) under low temperature and high strain rate conditions, while its softening was dominated through the DRX of austenite under high temperature and low strain rate. Chen et al. [19] investigated the dynamic softening mechanism of 2205 duplex stainless steel. The results showed that the softening mechanism of ferrite changed from a (sub)crystalline agglomeration mechanism at low strain rates to the CDRX mechanism at medium strain rates, and then the discontinuous dynamic recrystallization (DDRX) mechanism occurred at high strain rates. Meanwhile, austenite softened through the CDRX mechanism at low strain rates, while twins formed at high strain rates softened via the DDRX mechanism.Owing to its relatively high SFE, ferrite is prone to undergo DRV during hot deformation. As the strain increases, dislocations and (sub)grains are absorbed, and the transition from low-angle boundaries (LABs) to high-angle boundaries (HABs) occurs, which is called "extended" DRV. It can also be referred to as CDRX. [19][20][21][22][23][24] However, some recent studies [11,[25][26][27][28][29] reported ferrite can soften through the DDRX mechanism under certain deformation conditions, among which, Haghdadi et al. [28,29] proposed that the softening mechanism of ferrite shifted from the CDRX to DDRX mechanism as the strain rate increased during hot deformation. The softening of ferrite was predominated through the CDRX mechanism, conversely, tended to the DDRX mechanism, when the Z parameter values [30] were above some critical limit (i.e., at high temperature and low strain rate). However, Castan et al. [31] reported that the softening of ferrite in Fe-8% Al low-density steels preferentially through the DDRX

Effect of Cooling Rate on Crystallization Behavior during Solidification of Hyper Duplex Stainless Steel S33207: An In Situ Confocal Microscopy Study

“…The composition of each phase was almost the same as the one in the as-received sample. Point F seems to be a mixture of both phases since the morphology of the newly formed austenite after solidification seems to resemble a tiny island morphology [25]. The obtained finding regarding the unbalanced microstructure is the normal case when using Ar as the protection gas during solidification.…”

“…Alternatively, points A and D hold the higher Ni and lower Cr, which can be recognized as the γ austenite phase. Of note, in the solidified microstructure, the fraction of γ austenite was not really high, which has been confirmed by a separate work using electron backscatter diffraction (EBSD) [25]. This fact was deemed to be due to the loss in nitrogen during solidification, as well as lacking a post-heat treatment for austenite growth.…”

“…Shin et al [24] assessed the effects of the cooling rate after heat treatment on the pitting corrosion of DSS S32750, and they found that the ferrite volume fraction increased accordingly with the increase in the cooling rate. In a former study by the current authors, the solidification behavior of the different grades of DSSs have been investigated in situ [25] using a combinational approach of HT-CLSM and differential scanning calorimetry (DSC) [26]. Till now, the δ→γ transformation has been reported in previous works, but the evolution mechanism of δ-ferrite during the solidification process is still not clear regarding the DSSs.…”