AISI Type Research Articles

A Local Damage Approach to Predict Crack Initiation in Type AISI 316L(N) Stainless Steel

A local damage approach based on plastic strain equivalent to uniform strain and grain diameter of the material is proposed for prediction of crack initiation. Plane strain, plane stress, and 3D FEM simulations are carried out for compact tension (CT) geometry with blunt notch of different a/W ratios under mode-I loading. Elastic-plastic fracture parameters have been estimated based on certain assumptions on blunting at notch tip and micromechanisms of events leading to onset of crack. The various crack initiation parameters evaluated based on proposed local damage approach and initial assumptions have been verified by conducting experiments on CT specimens and subsequent scanning electron microscopy study on fracture surface. The laboratory scale experimental results of AISI 316L(N) stainless steel material are in good agreement with FEM-predicted fracture parameters for notch type of stress raisers. The local damage approach and FEM procedure established in the present study would be easily extendable to the analysis of stress raisers in components for the prediction of crack initiation under elastic-plastic condition.

Research of Joining Graphite by Use of Active Solder

The work deals with joining of graphite with stainless steel type AISI 321 by use of active solder type Sn3Ti. Structure of active solder type Sn3Ti is composed of a tin matrix in which the Ti6Sn5 phase is uniformly precipitated and the Ti3Sn phase is non-uniformly distributed. Wettability of Sn3Ti solder was determined in 10-4 Pa vacuum, at temperatures 860, 900 and 950 °C. Wetting angle of Sn3Ti solder on graphite is reduced with increasing soldering temperature and also with prolonged dwell time at soldering temperature. Temperature exerts greater effect on reduction of wetting angle. The lowest wetting angle on graphite 23° was observed at temperature 950 °C/20 min. The lowest wetting angle, observed on steel type AISI 321, attained value 18° at temperature 860 °C/20 min. Based on the results of wettability tests, the joints of graphite with stainless steel type AISI 321 were fabricated. The reactions after soldering resulted in formation of a continuous reaction layer with 5 μm average thickness on graphite/Sn3Ti solder boundary.

Fe Analysis of Residual Stresses Induced by Spot Welding of Stainless Steel Type Aisi 316

Fe Analysis of Residual Stresses Induced by Spot Welding of Stainless Steel Type Aisi 316

A model for predicting crack initiation in forged M3:2 tool steel under high cycle fatigue

AISI type M3 class 2 tool steel (or in German designation DIN: HS6-5-3 tool steel) is most commonly used in tooling industry, and also in some engine parts. Those components are usually subjected to cyclic stresses and mostly fail by fatigue. Fatigue crack initiation in this material occupies large fraction of total lifetime and strongly depends on microstructural features of primary and eutectic carbides, such as shape, shape ratio, volume fraction, the distribution of carbides as well as load ratio. To model fatigue initiation mechanisms of forged M3:2 tool steel, McDowell’s model was modified and developed for different length-scales. For fatigue crack formation and short crack growth, a hierarchical approach was used and the life time of these stages were estimated based on the local cyclic plasticity. Through this relation the effect of microstructural features on both fatigue crack formation and short crack growth in the material were identified. The results of the proposed model have explicitly reflected the influence of microstructural features on both fatigue crack formation and propagation in forged M3:2 tool steel. Moreover, the model can be used for improving the fatigue resistance of a tool steel component.

Study of creep cavitation in stainless steel weldment

A study of creep cavities near reheat cracking in AISI Type 316H austenitic stainless steel headers, removed from prolonged high temperature operation in nuclear power plants, is reported. It is shown how application of scanning electron microscopy (SEM), cryogenic fractography and small angle neutron scattering (SANS) can be applied, in a complementary way, to observe and quantify creep cavitation. Creep cavities in the vicinity of the crack are found to be mainly surrounding inter-granular carbides. Trends in the size and area fraction of creep cavities relative to the crack path are quantified using optimised metallography. The SANS technique is found to be a very suitable method of quantifying creep cavitation within the size range up to 600 nm averaged over a larger gauge volume. It is shown that the cavity size distribution peaks in the region 100–300 nm, and this correlates closely with the quantitative SEM observations.

Video based Process Observations of the Pulse Electrochemical Machining Process at High Current Densities and Small Gaps

Pulse Electrochemical Machining (PECM), a nontraditional process, using pulse-lengths in the low millisecond range as well as feed overlaid mechanical vibration, allows more precise tolerances and geometric precision through narrowing the working gap compared to conventional sinking ECM. With small working gaps in ranges down to 10μm, the anodic shape evolution during machining is getting difficult to monitor. Therefore understanding the shaping phenomena during the PECM process is key factor in achieving precision during the manufacturing of dies and molds, as well as precision parts in e.g. automotive or aircraft industry. In this contribution an experimental approach towards visual in-process observations of the PECM shaping process during the use of mechanical vibrations up to 50Hz and high pulsed current densities will be presented. Recording the process with a precisely clocked high speed camera system allowing precise μs shutter times, visual observations are conducted and being used as input for detailed downstream data analysis. The experimental study incorporates one of the most widely used flushing conditions in PECM as well as an outlook into the comparison between recorded in-process data and a static FEM simulation based on the monitored shape are given. In all experiments stainless steel of type AISI 304 (X5CrNi18–10) is used as anode and cathode material and for all PECM experiments a commercially available PEMCenter8000 with sodium nitrate as electrolyte was used. The concept presented will help to better link experiment and modelling of the PECM process, by simultaneously providing process relevant electrochemical data as well as the directly corresponding geometric shaping information during experiments.

Production of Dissimilar Metals Materials by the Method of Solid-State Joining

Mechanisms taking place while hot rolling in vacuum of dissimilar materials joined in solid phase are described. It is shown that at joining of materials in solid phase redistribution of atoms from one material into another occurs on the interface of joining. On the base of calculation and experimental results it is concluded that the ultimate strength of the interface of joint is always higher than the ultimate strength of less durable material. Pair zirconium-stainless steel SS (Type AISI 321) and carbon steel (Type C22E) those are used in nuclear power are investigated.

Local buckling strength of stainless steel beam webs subjected to a stress gradient

This paper investigates the local buckling strength of stainless steel beam webs, subjected to a stress gradient. Due to relatively low values of the proportional limit, stainless steels resist loads at stresses beyond the proportional limit. The design of such steels is therefore influenced by its inelastic buckling strength. To determine this strength, plasticity reduction factors are used in local buckling calculations to account for inelastic post-buckling behavior. Steels chosen for this study are AISI Type 304, 410 and Type 3CR12 stainless steels. Dimensions of the sections were selected to promote local buckling, and inhibit other forms of buckling, such as distortional and lateral-torsional buckling. The depth of the web was varied in order to study channels of different slenderness ratio. It was concluded in this investigation that an acceptable prediction of the local buckling strength of stainless steel beam webs may be obtained through the use of the effective width approach, using no plasticity reduction factor.

Effect of Cold Rolling on Sensitization of 304 Stainless Steel Weldments

The sensitization of stainless steel type AISI304 (304SS) is evaluated as a function of pre-welding cold work and welding heat input. 304SS is cold rolled to various percentages of thickness reduction of up to 50% at ambient temperature before being TIG welded. The susceptibility of 304SS to sensitization (weld decay) is evaluated using ASTM A262, practice A and electrochemical potentiokinetic reactivation tests. The results indicate that the influence of pre-welding cold work on the degree of sensitization (DOS) is distinct and appreciable within the range of 10% to 35% CR. In addition, within this range the optimum level of cold work after which the DOS starts to decrease is found to be inversely related to the amount of welding heat input. At higher levels of cold work of ≥ 35% CR, it is found that neither the cold work nor the heat input significantly influences the amount of DOS. Microstructural investigation showed that the predominant intergranular corrosion (IGC) occurs at austenite grain boundaries and less likely at ferrite / austenite interfaces of the weld metal. Meanwhile, transgranular attack at austenite bulk matrix along the defects is rarely observed.

Mechanical and anticorrosion properties of nanosilica-filled epoxy-resin composite coatings

Homogeneous, 50-μm-thick, epoxy coatings and composite epoxy coatings containing 2wt% of 130-nm silica particles were successfully synthetized on austenitic stainless steel of the type AISI 316L. The surface morphology and mechanical properties of these coatings were compared and characterized using a profilometer, defining the average surface roughness and the Vickers hardness, respectively. The effects of incorporating the silica particles on the surface characteristics and the corrosion resistance of the epoxy-coated steel were additionally investigated with contact-angle measurements as well as by potentiodynamic polarization and electrochemical impedance spectroscopy in a 3.5wt% NaCl solution. The silica particles were found to significantly improve the microstructure of the coating matrix, which was reflected in an increased hardness, increased surface roughness and induced hydrophobicity. Finally, the silica/epoxy coating was proven to serve as a successful barrier in a chloride-ion-rich environment with an enhanced anticorrosive performance, which was confirmed by the reduced corrosion rate and the increased coating resistance due to zigzagging of the diffusion path available to the ionic species.

A parametric study on creep‐fatigue endurance of welded joints

AbstractThis work is devoted to parametric study on creep‐fatigue endurance of the steel AISI type 316N(L) weldments defined as type 3 according to R5 Vol. 2/3 procedure at 550°C. The study is implemented using a novel direct method known as the Linear Matching Method (LMM) and based upon the creep‐fatigue evaluation procedure considering time fraction rule for creep‐damage assessment. Seven geometrical configurations of the weldment, which are characterised by individual values of a geometrical parameter ρ, are proposed. Parameter ρ, which represents different grades of TIG dressing, is a ratio between the radius of the fillet of the remelted metal on a weld toe and the thickness of welded plates. For each configuration, the total number of cycles to failure N* in creep‐fatigue conditions is assessed numerically for different loading cases including normalised bending moment $\tilde{M}$ and dwell period Δt. The obtained set of N* is extrapolated by the analytic function dependent on $\tilde{M}$, Δt and ρ. Proposed function for N* shows good agreement with numerical results obtained by the LMM. It is used for the identification of Fatigue Strength Reduction Factors (FSRFs) effected by creep and dependent on Δt and ρ. (© 2013 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantification of metal release from stainless steel electrodes during conventional and pulsed ohmic heating

Abstract Electrochemical reactions at the electrode-solution interface of an ohmic heater can be avoided or significantly limited by choosing appropriate processing conditions in relation to the food properties. In the present work the effect of the electrical parameters (electric field strength and frequency of the applied current signal) and product factors (halides concentration, electrical conductivity and pH) on metal release from stainless steel (type AISI 316 L) electrodes of a batch ohmic heater was investigated. In each experiment, the concentrations of the main constituents of stainless steel (iron, chromium and nickel) released in the heating medium were detected by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Atomic Absorption Spectrophotometry (AAS). Results showed that the rate of metal release from the electrodes to the heating medium depends on frequency and applied field strength. However, the use of ohmic heating at a higher frequency than conventional (50 Hz) can significantly (p ≤ 0.05) reduce the flux of metal ions from stainless steel electrodes. Moreover, it was also demonstrated that electrochemical phenomena occurring at the electrode-solution interface strongly depend on the composition, pH and electrical conductivity of the heating medium. Industrial relevance The magnitude of electrode material released into the heating medium during ohmic processing depends on many factors, whose effects should be known in order to define optimal treatment conditions, electrode material and food properties able to avoid or minimize the undesired phenomenon of contamination of the food product, electrode-fouling and electrode corrosion. This paper contributes to clarifying the effects of electric field strength applied as well as electrical conductivity, pH, and presence of halides in the heating medium on electrode corrosion or release of electrode materials during high frequency (25 kHz, bipolar square wave) pulsed power OH in comparison with conventional (50 Hz sine wave) OH. Interestingly, the use of sufficiently large frequencies may avoid or reduce the extent of electrochemical reactions at the electrode interface, minimizing corrosion and leakage of metals to the heating medium, even when electrode material of low cost and electrochemically active like stainless steel is used.

Effect of Equal Channel Angular Pressing on the Microstructure and Mechanical Properties of AISI Type 304 Austenitic Stainless Steel

The present paper describes the effect of severe plastic deformation on the microstructure and mechanical properties of AISI type 304 stainless steel by equal channel angular pressing (ECAP) at room temperature. The strain-induced martensitic transformation occurred in the specimens, and martensite phase increased with increasing strain up to 42% for three passes of ECAP. X ray diffraction was used to identify the strain-induced martensite phase and its volume fraction. The martensite phase, mechanical twins and micro hardness have increased with increasing passes of ECAP. Microstructures of specimens show that with increasing strain, subgrains less than microns are susceptible to be created by fragmentation of twins.

Wear and Sliding-Friction Characteristics of Stainless Steel SS316LN Without a Coating and With a CrN Coating at Elevated Temperatures and in a Vacuum

Stainless steel of type AISI 316LN - one of the structural materials of fast neutron reactors - must have a long service life under conditions that subject it to different types of wear (galling, adhesion, fretting, and abrasion). Cobalt-based hard facings are generally avoided due to induced radioactivity. Nickel-based hard facings are strongly preferred instead. One alternative to both types of coatings is a hard-alloy coating of CrN. This article examines wear and friction characteristics during the sliding of uncoated steel SS316LN and the same steel with a CrN coating. In addition, a specially designed pin-on-disk tribometer is used to perform tests in a vacuum at temperatures of up to 1000 degrees C in order to study the effect of oxygen on the wear of these materials. The morphology of the wear surface and the structure of the subsurface were studied by scanning electron microscopy. The formation of an adhesion layer and the self-welding of mating parts are seen to take place in the microstructure at temperatures above 500 degrees C. It is also found that steel SS316LN undergoes shear strain during sliding wear. The friction coefficient depends on the oxygen content, load, and temperature, while the wear rate depends on the strain-hardening of the surface of the material being tested.

Effect of Nitrogen Content on the Tensile and Stress Corrosion Cracking Behavior of AISI Type 316LN Stainless Steels

This paper deals with the effect of nitrogen on the tensile and stress corrosion cracking (SCC) behavior of type 316LN stainless steel. Yield stress (YS) and ultimate tensile stress (UTS) increased while the ductility [% total elongation (% TE)] decreased with increasing nitrogen content. Evaluation by conventional assessment parameters, such as ratios of UTS, % TE and SCC susceptibility index, derived by SCC testing using the slow strain rate testing (SSRT) technique indicated an improvement in SCC resistance on increasing the nitrogen content. However, crack growth rates, calculated from ratios of fracture stress from the SSRT tests in liquid paraffin and boiling 45 % magnesium chloride in SSRT tests, and the constant load tests at loads corresponding to 20 % YS in boiling 45 % magnesium chloride conclusively established that the SCC resistance of type 316LN stainless steel decreased with increasing nitrogen content.

Corrosion Resistance of 9 - 15% Cr ODS Steels and its Comparison with Austenitic Stainless Steel

An oxide dispersion strengthened steels are one of the most promising high temperatures, and high performance advanced structural material being developed for future fast reactors and high-temperature Generation IV reactors. In the present work, the corrosion resistance and its correlation with the passive film compositions of 11% Cr F/M and 9-15% Cr (with Zr or Hf) ODS steels is examined and compared with AISI type 304L stainless steel in boiling 60 - 62% (~13 M) HNO3. The corrosion rate measured in 62% HNO3 for 240 h of 11% Cr F/M, 9% Cr and 15% Cr (Zr) ODS steels show high corrosion rate. On the other hand, low corrosion rate was observed in 304L stainless steel (0. 21 to 23 mm y-1). However, severe intergranular corrosion attack was revealed in type 304L SS after 240 h exposure, but none in ODS steels. Such an intergranular corrosion attack seen in type 304L stainless steel is undesirable. On the contrary, low corrosion rate (0.04 0.15 mm y-1) of 15% Cr (Hf) ODS steel in 3 M, 6 M and 9 M HNO3, comparable to that of type 304L stainless steel was observed. The improved corrosion resistance of 15% Cr (Hf) ODS steel was attributed to enrich (20 at. %) and protective Al2O3 layer formation in addition to Cr2O3 in the passive film.

Development of Stainless Steels in Nuclear Industry: With Emphasis on Sodium Cooled Fast Spectrum Reactors History, Technology and Foresight

Stainless steels (SS) have earned a unique position as a widely accepted class of alloys with track record of steadily improving the performance and increasing applications from 1913 till date. This distinction is attributed to R&D, innovations and applications leading to harnessing the rare combination of properties of stainless steels, since its discovery hundred years back. Though the initial discovery of stainless steel is basically serendipity, and based on previous work, its indispensable position today, in many a wide range of applications is due to intense R&D efforts in understanding its physical, chemical, thermal and thermo-mechanical response for various chemistries and microstructures. A deliberate attempt to extend its application spectrum through various routes of manufacturing in the last century is another crucial aspect of the success story. The first part of the presentation would briefly review this exciting journey of unravelling the mysteries of stainless steels.The second part of the presentation would highlight the evolution of stainless steels in the nuclear industry, especially for the sodium cooled fast reactors. Early 70s have seen the application of stainless steels in first generation water based nuclear power plants and AISI types 304 and 316 SS was recommended for structural and core applications in fast spectrum reactors (FSR). Failure of some of the components even in the manufacturing stage and quest for improving mechanical properties and sensitisation and intergranular corrosion resistances resulted in the development of 304L, 304LN, 316L, 316LN SS during 1980-90 for further applications in FSRs. Towards core applications in intense radiation environments, three generations of stainless steels namely 20% cold worked 316 SS, D9, and D9I have been developed to yield high burnup and to triple the lifetime of the core components of the fast reactors. Towards closing the fuel cycle, again 304L SS was the workhorse material which was upgraded with newer varieties like nitric acid grade alloys for improved corrosion resistance and longer life. Manufacturing of special grades of SS and the developments in fabrication technologies was necessary in order to enhance the performance of components and to avoid failures. Welding, inspection, quality assurance and structural integrity of various components of SS for FSRs and fuel cycle facilities resulted in developments in areas like modelling, devices, methodologies and analysis. An opportunity existed for the development and application of innovative non-destructive testing techniques for robust examination of critical components.Nuclear industry is embarking a state of the art fourth generation reactors. The consequent newer generation of SS are evolving with improved properties to match the expectations of performance in increased temperature, pressure, chemical and other physical constraints. It is of paramount importance to consider the extension of the lifetime of the current reactors from 40-60 to 60-80 years for economic considerations, and in this regard innovations are necessary in the development of newer varieties of stainless steels with respect to modelling and life prediction, manufacturing, fabrication, testing and evaluation. Thus the management approach to knit a network of industry-research-academia is a key approach for way forward. A development of roadmap for robust science based technology development with foresight is a desired management strategy.

Microstructure and Mechanical Properties of 16Cr-2Ni Stainless Steel Fusion and Solid State Welds-Influence of Post Weld Treatments

Many critical applications in chemical equipment, aircraft and ordinance demand a material of construction with high strength and good corrosion resistance. Frequently the strength requirement exceeds that obtainable with austenitic or ferritic stainless steel and it is necessary to use one of the martensitic stainless steels. Since martensitic stainless steels are structural materials, weldability has been an important consideration in their development. AISI 431 is one of the most potentially attractive steels in this class used extensively for parts requiring a combination of high tensile strength, good toughness and corrosion resistance. Although this material has been used for many years, little information is available on the welding behavior of these steels. Further, data on electron beam (EB) welding and solid state welding process like friction welding are scarce. The lack of knowledge constitutes a potential drawback to the more widespread use of these steels. Hence, a study has been taken up to develop an understanding on the electron beam welding and friction welding aspects of martensitic stainless steel type AISI 431. Various kinds of post weld heat treatments (PWHT) were investigated to determine their influence on microstructure and mechanical properties. Weld center in EB welding resulted a cast structure consists of dendritic structure with ferrite network in a matrix of un-tempered martensite. In friction welding, the weld center exhibited thermo-mechanical effected structure consists of fine intragranular acicular martensite in equiaxed prior austenite grains. In both the welding processes, post weld tempering treatment resulted in coarsening of the martensite which increases with increase in tempering temperature. In the as-weld condition, welds exhibited high strength and hardness and poor impact toughness. Increase in impact toughness and decrease in strength and hardness is observed with an increase in tempering temperature. The hardness of EB welds increased with increase in the austenitizing temperature up to 1100 °C and a marginal decrease thereafter was observed. Double austenitization after double tempering resulted in optical mechanical properties i.e., strength, hardness and toughness.

Application of linear matching method to creep–fatigue failure analysis of cruciform weldment manufactured of austenitic steel AISI type 316N(L)

This paper presents a creep–fatigue life assessment of a cruciform weldment made of the steel AISI type 316N(L) and subjected to reversed bending and cyclic dwells at 550°C using the linear matching method (LMM) and considering different weld zones. The design limits are estimated by the shakedown analysis using the LMM and elastic–perfectly plastic material model. The creep–fatigue analysis is implemented using the material models: Ramberg–Osgood model for plastic strains under saturated cyclic conditions and power law model in ‘time hardening’ form for creep strains during primary creep stage. The number of cycles to failure N* under creep–fatigue interaction is defined by relation for cycles to fatigue failure N* dependent on total strain range Δϵtot for the fatigue damage ωf, long term strength relation for the time to creep rupture t* dependent on average stress during dwell Δt for the creep damage ωcr and non-linear creep–fatigue interaction diagram for the total damage. Numerically estimated N* for different Δt and Δϵtot shows good quantitative agreement with experiments. A parametric study of different dwell times Δt is used to formulate the functions for N* and residual life L* dependent on Δt and normalised bending moment .

Failure analysis: Chloride stress corrosion cracking of AISI 316 stainless steel downhole pressure memory gauge cover

The downhole pressure memory gauge cover which is the focal point of this failure case study was hung in the production tubing of a sour gas well at a depth of 3200m during a multiple rate productivity index test. Approximately 140h after start up, the tool cover failed. The memory gauge cover experienced repeated premature failures. One failed sample was investigated by chemical and microstructural analytical techniques to find out the failure cause and provide preventive measures. The gauge cover alloy was type AISI 316 stainless steel. During investigation many branched cracks were observed on the external surface of the gauge cover grown from the pits perpendicular to the cylinder axis. The results indicate that pressure memory gauge cover failed due to chloride stress corrosion cracking (CLSCC). Also according to slow strain rate test, it was evident that the best alternative for the downhole pressure memory gauge cover alloy is Nitronic 50 and the second option is 17–4 PH stainless steel.