Showing papers on "Austenitic stainless steel published in 2011"

A Study on Microstructural Evolution and Dynamic Recrystallization During Isothermal Deformation of a Ti-Modified Austenitic Stainless Steel

Abstract: Dynamic recrystallization (DRX) behavior in hot deformed (by uniaxial compression in a thermomechanical simulator in the temperatures range 1173 K to 1373 K [900 °C to 1100 °C]) Ti-modified austenitic stainless steel was studied using electron back scatter diffraction. Grain orientation spread with a “cut off” of 1 deg was a suitable criterion to partition dynamically recrystallized grains from the deformed matrix. The extent of DRX increased with strain and temperature, and a completely DRX microstructure with a fine grain size ~4 μm (considering twins as grain boundaries) was obtained in the sample deformed to a strain of 0.8 at 1373 K (1100 °C). The nucleation of new DRX grains occurred by the bulging of the parent grain boundary. The DRX grains were twinned, and a linear relationship was observed between the area fraction of DRX grains and the number fraction of Σ3 boundaries. The deviation from the ideal misorientation of Σ3 boundaries decreased with an increase in the fraction of Σ3 boundaries (as well as the area fraction of DRX) signifying that most Σ3 boundaries are newly nucleated during DRX. The generation of these Σ3 boundaries could account for the formation of annealing twins during DRX. The role of Σ3 twin boundaries on DRX is discussed.

Interatomic potential to study plasticity in stainless steels: the FeNiCr model alloy

Abstract: Austenitic stainless steels are commonly used materials for in-core components of nuclear light water reactors. In service, such components are exposed to harsh conditions: intense neutron irradiation, mechanical and thermal stresses, and aggressive corrosion environment which all contribute to the components' degradation. For a better understanding of the prevailing mechanisms responsible for the materials degradation, large-scale atomistic simulations are desirable. In this framework we developed an embedded atom method type interatomic potential for the ternary FeNiCr system to model movement of dislocations and their interaction with radiation defects. Special attention has been drawn to the Fe–10Ni–20Cr alloy, whose properties were ensured to be close to those of 316L austenitic stainless steel. In particular, the stacking fault energy and elastic constants are well reproduced. The fcc phase for the Fe–10Ni–20Cr random alloy was proven to be stable in the temperature range 0–900 K and under shear strain up to 5%. For the same alloy the stable glide of screw dislocations and stability of Frank loops was confirmed.

Dislocation structure evolution and its effects on cyclic deformation response of AISI 316L stainless steel

Abstract: The cyclic deformation response of an austenitic stainless steel is characterised in terms of its cyclic peak tensile stress properties by three stages of behaviour: a hardening stage followed by a softening stage, and finally a stable stress response stage. A series of tests have been performed and interrupted at selected numbers of cycles in the different stages of mechanical response. At each interruption point, specimens have been examined by transmission electron microscopy (TEM) with different beam directions by means of the tilting function in order to investigate the formation and the development of dislocation structures from the as-received condition until the end of fatigue life. A new 3D representation of dislocation structure evolution during cyclic loading is proposed on the basis of the microstructural observations. The 3D representation provides a deeper insight into the development of dislocation structures in AISI 316L during low cycle fatigue loading at room temperature. By investigating the dislocation evolution, the study shows that the hardening response is mainly associated with an increase of total dislocation density, whereas the softening stage is a result of the formation of dislocation-free regions. Further development of the dislocation structure into a cellular structure is responsible for the stable stress response stage.

Development and microstructural characterization of microwave cladding on austenitic stainless steel

Abstract: In the present work microwave cladding was explored as a new processing method for enhancement of surface properties of austenitic stainless steel (SS-316). Cladding of nickel based powder (EWAC) was developed using microwave radiation as the heating source. This paper explains the possible mechanism of clad formation using microwave hybrid heating with the help of a schematic model. The developed clads were characterized using field emission scanning electron microscope (FE-SEM), energy dispersive X-ray spectroscope (EDS), X-ray diffraction (XRD) and measurement of Vicker's microhardness. Typical X-ray diffraction (XRD) pattern of the clad showed the presence of chromium carbide, nickel silicide and nickel iron phases that eventually contribute to enhancement in microhardness of the clads. Clads of approximately 1 mm thickness were developed without any visible interfacial cracking and had significantly less porosity (1.09%). Microstructure of clad transverse section revealed good metallurgical bond with SS-316 substrate by partial mutual diffusion of constituent elements. The microstructure of the clad was found dominantly cellular in nature. Chromium was observed segregated around the cell boundaries while iron and nickel were identified inside the cells. Chromium carbides (Cr 23 C 6 , Cr 3 C 2 ) were formed during the processing and appeared at the cell boundaries. Vicker's microhardness study revealed that the hardness profile varies within the clad zone and the average microhardness of the developed clad was observed to be 304 ± 48 H v .

Overview of Strategies for High-Temperature Creep and Oxidation Resistance of Alumina-Forming Austenitic Stainless Steels

Abstract: A family of creep-resistant, alumina-forming austenitic (AFA) stainless steel alloys is under development for structural use in fossil energy conversion and combustion system applications. The AFA alloys developed to date exhibit comparable creep-rupture lives to state-of-the-art advanced austenitic alloys, and superior oxidation resistance in the ~923 K to 1173 K (650 °C to 900 °C) temperature range due to the formation of a protective Al2O3 scale rather than the Cr2O3 scales that form on conventional stainless steel alloys. This article overviews the alloy design approaches used to obtain high-temperature creep strength in AFA alloys via considerations of phase equilibrium from thermodynamic calculations as well as microstructure characterization. Strengthening precipitates under evaluation include MC-type carbides or intermetallic phases such as NiAl-B2, Fe2(Mo,Nb)-Laves, Ni3Al-L12, etc. in the austenitic single-phase matrix. Creep, tensile, and oxidation properties of the AFA alloys are discussed relative to compositional and microstructural factors.

Effects of laser shock processing on mechanical properties and micro-structure of ANSI 304 austenitic stainless steel

Abstract: The aim of this article is to address the effects of a single laser shock processing (LSP) impact on the nano-hardness, elastic modulus, residual stress and phase transformation of ANSI 304 austenitic stainless steel. Residual stress distribution of the LSP-shocked region is determined by X-ray diffraction (XRD) with sin 2 ψ method, and the micro-structural features in the near-surface layer are characterized by using cross-sectional optical microscopy (OM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). By comparing with the untreated samples, LSP can clearly improve nano-hardness, elastic modulus, and residual stress in the LSP-shocked region. The underlying enhancement mechanisms of LSP on nano-hardness, elastic modulus and residual stress of stainless steel ANSI 304 are also revealed. These studies may provide some important insights into surface modification for metal materials.

The Effects of KCl, K2SO4 and K2CO3 on the High Temperature Corrosion of a 304-Type Austenitic Stainless Steel

Abstract: The oxidation of 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated at 500 and 600 °C in 5% O2 + 40% H2O. Prior to exposure the samples were sprayed with KCl, K2CO3 or K2SO4, the amount of salt corresponding to 1.35 μmol K+/cm2. For reference, salt-free samples were exposed in 5% O2 + 40% H2O and in 5% O2 (N2 was used as carrier gas). The oxidized samples were analyzed with SEM/EDX, XRD, IC and FIB. KCl and K2CO3 strongly accelerate the corrosion of 304L while K2SO4 has little influence on the corrosion rate and on the morphology of the corroded surface. KCl and K2CO3 react with the chromium-rich oxide on the sample surface, forming K2CrO4. The resulting chromium depletion of the protective oxide causes rapid oxidation and the formation of a thick duplex scale consisting of an outer hematite layer and a inner layer made up of FeCrNi spinel-type oxide. The differences in the corrosivity of the three salts are directly connected to their ability to form chromate on the surface and, hence, to the relative stability of the corresponding leaving groups (HCl, CO2 and SO3).

Comparison of creep rupture behaviour of type 316L(N) austenitic stainless steel joints welded by TIG and activated TIG welding processes

Abstract: Creep rupture behaviour of type 316L(N) austenitic stainless steel base metal and its weld joints fabricated both by single-pass activated TIG (A-TIG) and multi-pass conventional TIG (MP-TIG) welding processes were studied at 923 K over a stress range of 160–280 MPa. Both the weld joints possessed lower creep rupture lives than the base metal. The A-TIG weld joint displayed higher rupture lives than the MP-TIG weld joint. Failure in the weld joints occurred in the weld metal. Progressive localization of creep deformation in the weld metal of both the joints led to the premature failure. Accumulation of creep deformation at higher rate was observed in the weld metal of the MP-TIG joint than in the A-TIG joint. Finer microstructural features and higher amount of δ-ferrite was observed in the weld metal of MP-TIG joint than in the A-TIG weld joint. Orientation of the columnar grains and δ-ferrite was nearly transverse to the welding direction in the A-TIG joint, whereas it was towards short transverse to the welding direction in the MP-TIG weld joint. TEM investigation of creep exposed weld metal showed the extensive formation of M 23 C 6 carbides, σ-phase and Laves phase along the boundaries in MP-TIG joint, which were less prevalent in the A-TIG joint. With creep exposure, the δ-ferrite transformed to σ and Laves phases, and creep cavitation was found to be associated with the intermetallic phases. Creep cavitation was more pronounced in the MP-TIG weld joint than in the A-TIG weld joint.

Failure mode transition in AHSS resistance spot welds. Part II: Experimental investigation and model validation

Abstract: The objective of this paper is to investigate and analyze the transition criteria from interfacial to pullout failure mode in AHSS resistance spot welds during the tensile-shear test by the use of both experimental and analytical approaches. Spot welds were made on three dual phase steel grades including DP600, DP780 and DP980. A low strength drawing quality special killed (DQSK) steel and AISI 304 austenitic stainless steel were also tested as a baseline for comparison. The microstructure and mechanical strength of the welds were characterized using metallographic techniques and the tensile-shear testing. Correlations among critical fusion zone (FZ) size required to ensure the pullout failure mode, weld microstructure and weld hardness characteristics were developed. It was found that critical FZ size increases in the order of DQSK, DP600, DP980, DP780 and AISI304. No direct relationship was found between the tensile strength of the base metal and the critical FZ size. It was concluded that low hardness ratio of FZ to pullout failure location and high susceptibility to form shrinkage voids are two primary reasons for high tendency of AHSS to fail in interfacial mode. HAZ softening can improve RSW mechanical performance in terms of load bearing capacity and energy absorption capability. This phenomenon promotes PF mode at smaller FZ sizes. This fact can explain smaller critical FZ size measured for DP980 in comparison with DP780. The results obtained from the model were compared to the experimental results and the literature and a reasonable agreement was obtained.

Deformation mechanisms of a modified 316L austenitic steel subjected to high pressure torsion

Abstract: The present work is assigned to the microstructural evolution of a modified 316L stainless steel during high pressure torsion (HPT) in a temperature range between −196 °C and 720 °C. The aspect of microstructural evolution is similar to that of materials with low stacking fault energy: at high deformation temperatures (Tdef > 450 °C) the dominant deformation mechanism is dislocation glide whereas for medium temperatures (450 °C > Tdef > 20 °C) mechanical twinning is observed. At very low deformation temperatures (20 °C > Tdef > −196 °C) mechanical twinning is replaced by the deformation induced martensite transformation γ(fcc) → ɛ(hcp). Based on the present results, the formation mechanisms of nanocrystalline austenite are discussed.

Comparative study on mechanical behavior of low temperature application materials for ships and offshore structures: Part I—Experimental investigations

Abstract: Austenitic stainless steels (ASSs), aluminum alloys, and nickel alloys are potential candidate materials for cryogenic applications owing to their superior mechanical properties under low temperatures. In the liquefied natural gas (LNG) industry, these materials are widely used in the construction of thermal barriers for the insulation systems of storage tanks and LNG equipment. Among the typical material nonlinearities of ASSs, phase transformation induced plasticity (TRIP)-related nonlinear hardening characteristics have been experimentally and numerically reported in a number of detailed studies [1] , [2] ; however, to the best of our knowledge, quantitative studies on aluminum and nickel alloys are not available for reference. Moreover, although these materials are used under various temperatures and strain rates, their temperature- and strain rate-dependent properties have not been determined thus far. In this study, a series of tensile tests is carried out under various temperatures (110–293 K) and strain rate (0.00016–0.01 s−1) ranges as a preliminary step in the overall process for understanding the material characteristics of ASSs, aluminum alloys, and nickel alloys. On the basis of the experimental results, the essential mechanical properties are summarized in a quantitative manner in terms of the temperature and strain rate. The strain-hardening rate and strain sensitivity, which can be used to describe cryogenic temperature dependent material nonlinearities, are also proposed for the selected materials.

Analysis and mathematical modelling of elevated temperature flow behaviour of austenitic stainless steels

Abstract: High temperature flow behaviour of various grades of austenitic stainless steels viz. 304L, 304, 304 (as-cast), 316L and 15Cr–15Ni–Ti modified austenitic stainless steels (alloy D9) were analyzed by performing isothermal hot compression tests in a wide range of temperatures (1073 K to 1473 K for 304L, 304, 304 (as-cast), 316L and 1123 K to 1523 K for alloy D9) and strain rates (0.001–1 s−1). It has been observed that all these materials show strain hardening, strain rate hardening, thermal softening, coupled effect of temperature and strain, and temperature and strain rate on flow stress in the hot working domain. The modified Zerilli–Armstrong (MZA) model which considers the above significant effects on flow stress has been applied to predict the flow behaviour of these materials. The material constants of the MZA model for each material have been evaluated and subsequently applied to predict the flow stress. It has been demonstrated that the MZA model could adequately represent the elevated temperature flow behaviour of these materials over the entire ranges of strain, strain rate and temperature.

Microstructure and mechanical properties of annealed SUS 304H austenitic stainless steel with copper

Abstract: An experimental investigation into the effect of Cu on the mechanical properties of 0 and 3 wt.% Cu added SUS 304H austenitic stainless steel upon annealing at 700 °C for up to 100 h was conducted. Optical microscopy reveals grain coarsening in both the alloys upon annealing. Observations by transmission electron microscopy revealed the precipitation of nanometer-sized spherical Cu particles distributed within the austenitic grains and the presence of carbides at the dislocations. Both the yield and ultimate tensile strengths of the alloys were found to remain invariant with annealing. Tensile ductility and the threshold stress intensity factor range for fatigue crack growth for 3 wt.% Cu added alloy increase with annealing. These are attributed to the grain coarsening with annealing. In all, the addition of Cu to SUS 304H does not affect the mechanical performance adversely while improving creep resistance.

Hydrogen Solubility and Diffusion in Austenitic Stainless Steels Studied with Thermal Desorption Spectroscopy

Abstract: Hydrogen solubility and diffusion in austenitic stainless steels, namely AISI 310, AISI 301LN and AISI 201, are studied with thermal desorption spectroscopy (TDS) after electrochemical potentiostatic hydrogen pre-charging. Temperature dependencies of hydrogen desorption for all studied steels manifest a complex main peak caused by hydrogen releasing from the steel lattice by diffusion. Depending on the steel and heating rate the peak is situated from 350 to 500 K and its shape reflects a specific of hydrogen diffusion in stainless steels, which are multicomponent alloys. Analysis of the TDS curves is based on the hydrogen diffusion model taking into account trapping of hydrogen atoms in the energetically deep interstitial positions in the steel crystal lattice. Diffusion coefficient of hydrogen and its total content after the same charging procedure are obtained from the TDS curves and compared for the studied steels.

Effect of very high cutting speeds on shearing, cutting forces and roughness in dry turning of austenitic stainless steels

Abstract: Behavior of austenitic stainless steels has been studied at very high cutting speeds. Turning tests were carried out using the AISI 303 austenitic stainless steel. In particular, the influence of cutting speed on tool wear, surface quality, cutting forces and chip geom- etry has been investigated. These parameters have been compared when performing machining at traditional cutting speeds (lower than 350 m/min) versus high cutting speeds. The analysis of results shows that the material undergoes a significant change in its behavior when machining at cutting speeds above 450 m/min, that favors the machining operation. The main compo- nent of cutting forces reaches a minimum value at this cutting speed. The SEM micrographs of the machined surfaces show how at the traditional cutting speeds the machined surfaces contain cavities, metal debris and feed marks with smeared material particles. Surfaces machined at high cutting speeds show evidence of ma-

Nano-grain evolution in austenitic stainless steel during multi-directional forging

Abstract: Nano-grain evolution in an austenitic stainless steel (SUS 316) during multidirectional forging (MDF) was investigated at temperatures of 77 K and 300 K. The flow stress during MDF and the room-temperature hardness increased significantly with increasing cumulative strain. The initial grains were subdivided by mechanical twinning and martensitic transformation. The formation of packets , which are composed of lamellar-structured mechanical twins with a spacing of 10–300 nm, enhanced grain fragmentation. The packet size ranged from 40 nm to 100 nm depending on the MDF temperature and the cumulative strain. Tensile tests at ambient temperatures revealed a maximum proof strength of 2.1 GPa. While the proof strength increased with cumulative strain, the plastic strain at fracture was approximately 10% independent of the cumulative strain over ∑Δ ɛ = 2.4.