Multiple heat isothermal stress rupture correlations for type 316L(N) stainless steel and their use Part 1 – Development of reference correlations

Abstract

A low carbon, nitrogen alloyed version of stainless steel type 316 (SS 316L(N)) is the current choice as structural material for fast breeder reactors, and there is a need to derive reliable stress rupture correlations for life extrapolation. The present paper demonstrates the success of the heat correlation proposed by Ray, Sasikala and Rodriguez in deriving multiple heat isothermal correlations for stress rupture data for a large number of heats of SS 316L(N) from the European Commission's Working Group. This method of using multiple heat data as the knowledge base allows reliable correlations and extrapolations also for individual heats, even when the available data are inadequate for meaningful single heat stress rupture correlations. It is also shown, concatenating data for SS 316L(N) heats with those for two groups of SS 316 grade material (from the National Research Institute of Metals), that as far as heat dependence of stress rupture life is concerned, it is possible to consider SS 316L(N) heats as belonging to the SS 316 group. The isothermal 'reference' correlations thus derived are robust because of the large number of heats and the volume of data in the concatenated database for SS 316 and SS 316L(N) grades.     

The wear and corrosion resistance of shot peened–nitrided 316L austenitic stainless steel

316L austenitic stainless steel was gas nitrided at 570 °C with pre-shot peening. Shot peening and nitriding are surface treatments that enhance the mechanical properties of surface layers by inducing compressive residual stresses and formation of hard phases, respectively. The structural phases, micro-hardness, wear behavior and corrosion resistance of specimens were investigated by X-ray diffraction, Vickers micro-hardness, wear testing, scanning electron microscopy and cyclic polarization tests. The effects of shot peening on the nitride layer formation and corrosion resistance of specimens were studied. The results showed that shot peening enhanced the nitride layer formation. The shot peened–nitrided specimens had higher wear resistance and hardness than other specimens. On the other hand, although nitriding deteriorated the corrosion resistance of the specimens, cyclic polarization tests showed that shot peening before the nitriding treatment could alleviate this adverse effect.     

Explosive consolidation of 316L stainless steel powder – Effect of phase composition

Two stainless steel (SS) AISI 316L powders have been processed by explosive consolidation using a cylindrical configuration. Powders with d₅₀ of 9 and 5 μm and a phasic structure consisting of fcc and bcc are used. After shock processing (3.5 up to 4.9 mm/μs) hardness was evaluated. Powders with the lowest particle size and processed with the highest detonation velocities (4.9 and 4.1 mm/μs) gave rise to a bulk material where in the centre occurred a phase transformation of bcc to fcc phase. Nevertheless, the hardness values were dissimilar along the cross section depending on the macrodefects (centre hole and cracks) produced by detonation. After a pre-heating treatment (900 °C), this powder was full austenitic (fcc) and when submitted to explosive consolidation, it led a monolithic solid without cracks, with a density of 99% TMD (theoretical maximum density) and a hardness of 3.1 GPa. This value is lower than others measured, particularly when a centre hole is not present, revealing hardening by plastic deformation. Concerning powder with higher particle size (d₅₀ = 9 μm), the presence of mainly austenite induces after shock processing function of detonation parameters and localisation hardness values from 3.9 up to 5.0 GPa. The homogeneity of hardness reflex of absence of defects and low stress are almost achieved only for low particle size powders, using the lowest detonation velocities (3.4 GPa).     

Revisiting anisotropy in the tensile and fracture behavior of cold-rolled316L stainless steel with heterogeneous nano-lamellar structures

We produced a 316 L stainless steel with heterogeneous nanometer-thick lamellar structures by severe cold-rolling at room temperature,and conducted micro-scale tensile tests in different orientations to evaluate both the inplane(parallel to the nano-lamellae)and out-of-plane(normal and 45inclined to the nano-lamellae)mechanical anisotropy.The parallel orientation demonstrates the greatest tensile strength while the inclined orientation exhibits the least strength.The tensile tests in normal and inclined directions also indicate significant transient elastic-plastic response due to the strain path change.Fractographic examination demonstrates that the specimen fails in the normal direction by premature micro-void nucleation and growth,which restricts its tensile strength;however,we identified zig-zag cracking associated with lamellar shear cracking in the inclined direction.     

Failure In metal honeycombs manufactured by selective laser melting of 304 L stainless steel under compression

ABSTRACT

Cellular structures, specifically honeycombs, are commonly used as core materials in sandwich structures. This is especially true in aerospace applications where high bending and out-of-plane compressive stiffness coupled with low component weight is required. Additive manufacturing techniques are well suited for the manufacture of such cellular structures in a cost-effective manner. The current work focuses on honeycombs using selective laser melting of 304?L stainless steel. The mechanical behaviour of honeycombs was evaluated using out-of-plane compression tests. A numerical model was built to describe failure of the additively manufactured honeycombs. Compression tests were performed, on cylindrical samples to build the nonlinear material model. The material behaviour was found to be dependent on the build direction. Results of experiments and simulation show that failure occurs through a plastic buckling mechanism.     

Constitutive equations that describe creep stress relaxation for 316H stainless steel at 550°C

Abstract

The prediction of the stress relaxation behaviour of welding induced residual stresses in thick section 316H austenitic stainless steel welded component provides an input for quantifying reheat crack initiation observed in the heat affected zone. The cracks occur after service at a temperature range from 490 to 520°C. The present work reviews some of the widely applied stress relaxation models. The relative strengths and weaknesses of these existing models are discussed. An improved constitutive equation derived from a forward uniaxial creep deformation law is proposed. The relative importance of the parameters selected in the new constitutive model, when compared with experimental data, is discussed. The importance of a better understanding of the role of the internal stress and its measurement is highlighted.     

Martensitic transformation in SUS 316LN austenitic stainless steel at RT

The results of high-accuracy magnetic measurements on SUS 316LN austenitic stainless steel compressively deformed at room temperature (RT) are reported here. Even after the mild deformation of ∼25% true strain (ε _t ), the ferromagnetic phase (α′-martensite) could be clearly observed which increased sharply on further deformation. The amount of α′ was very small (0.18 vol% at ε _t ≈ 60%) when compared to the reported data for other grades of austenitic steels such as 304, 304L, 316, and 316L. The strain-induced α′-martensite is further studied by magnetic hysteresis loops. The coercivity (H _C) and remanence (M _r) were analyzed by subtracting the paramagnetic contribution of the bulk austenite structure. While H _C was found to decrease with α′, M _r remained the same (∼67 emu/g) when normalized to the volume fraction of α′. The decreasing H _C with increasing α′ and/or ε _t is presumed to be due to the domain wall pinning at the grain boundaries when the cluster size exceeds the domain wall width.     

Description of stress–strain curves for stainless steel alloys

There is a wide variety of stainless steel alloys, but all are characterized by a rounded stress–strain response with no sharply defined yield point. This behaviour can be represented analytically by different material models, the most popular of which are based on the Ramberg–Osgood formulations or extensions thereof. The degree of roundedness, the level of strain hardening, the strain at ultimate stress and the ductility at fracture of the material all vary between grades, and need to be suitably captured for an accurate representation of the material to be achieved. The aim of the present study is to provide values and predictive expressions for the key parameters in existing stainless steel material models based on the analysis of a comprehensive experimental database. The database comprises experimental stress–strain curves collected from the literature, supplemented by some tensile tests on austenitic, ferritic and duplex stainless steel coupons conducted herein. It covers a range of stainless steel alloys, annealed and cold-worked material, and data from the rolling and transverse directions. In total, more than 600 measured stress–strain curves have been collected from 15 international research groups. Each curve from the database has been analysed in order to obtain the key material parameters through a curve fitting process based on least squares adjustment techniques. These parameter values have been compared to those calculated from existing predictive models, the accuracy of which could therefore be evaluated. Revised expressions providing more accurate parameter predictions have been proposed where necessary. Finally, a second set of results, containing material parameters reported directly by others, with information of more than 400 specimens, has also been collected from the literature. Although these experimental results were not accessible as measured raw data, they enabled further confirmation of the suitability of the proposed equations.     

Enamel coating for protection of the 316 stainless steel against tribo-corrosion in molten zinc alloy at 460℃

Hot-dip galvanizing provides excellent corrosion and wear resistance for steels.However,the equipment itself,such as the steel roller,immerged in corrosive molten zinc suffers serious material loss during steel's production.Its protection has become the main technique problem in modern galvanizing line.In this study,an enamel coating was designed and prepared.Its tribo-corrosion in molten zinc alloy(Zn-0.2 wt%Al)at 460℃was investigated in comparison with the traditional WC-12 Co composite coating and the 316 stainless steel.Results indicate that the steel suffers serious material damage.Various corrosion products of Fe₂ Al₅ Znx form at the worn surface and the wear scar has reached 200μm deep after merely 5 h tribo-corrosion.Though the two coatings provide an improved tribo-corrosion resistance,for the WC-12 Co coating,its chemical reaction with the molten zinc increases brittleness and promotes cracking.The synergistic wear and corrosion cause its degradation.The enamel coating performs better during tribo-corrosion.It is chemically stable in molten zinc thus able to provide high corrosion resistance.In addition,the amorphous[Si04]network and the self-lubricating CaF₂ crystallite help it to build up an intact amorphous glaze layer readily at surface on sliding,leading to a reduced wear loss.During the whole tribo-corrosion process,the enamel coating is completely free of cracking,and the Zn penetration is inhibited.     

Corrosion and stress corrosion cracking behavior of 316L austenitic stainless steel in high H2S–CO2–Cl− environment

In oil and gas production environments, H₂S and Cl^? can coordinate to cause pitting or stress corrosion cracking (SCC) of stainless steels. There has been limited work conducted on corrosion and SCC of autenitic stainless steels in high H₂S–CO₂–Cl^? environments. In this paper, by four-point bending test method and scanning electron microscopy analysis, SCC of 316L steel was investigated under high H₂S–CO₂ pressures with 150,000 ppm Cl^? at 60 °C. The effect of high H₂S–CO₂ pressure was discussed. The results indicated that the higher H₂S–CO₂ pressure can accelerate anodic dissolution process, deteriorate passive films, and aggravate SCC sensitivity. Using cyclic potentiodynamic polarization measurements, the corrosion behavior of 316L steel was studied in high H₂S–CO₂–Cl^? environments. The effect of pH on pitting corrosion was discussed. Lower pH can promote both cathodic and anodic actions on 316L steel and facilitate passive film breakdown.     

Oxide/metal interface on 20Cr–25Ni–Nb stainless steel

Abstract

20Cr–25Ni–Nb stabilised stainless steel is used to contain the fuel in the advanced gas cooled reactor. During operation, this steel must withstand temperatures from 600 to 1073 K in CO₂ gas at 40 atm pressure. It is important that the oxide which forms on this steel is thoroughly characterised and the adherence of the oxide to the metal is understood. A technique of sputter ion plating has been used to remove the oxide from the metal without destroying either metal or oxide. This involves plating the oxide with nickel or molybdenum at a temperature of 600 K, while sputtering the surface with argon ions. On cooling, stresses set up between the oxide and the metal cause the oxide plus sputtered layer to peel off allowing both the metal and oxide sides of the interface to be examined. Results are presented from studies of the metal/oxide interface using scanning Auger microscopy. Analysis of grain centres and grain boundaries indicates that silicon and chromium play an important role in oxide/metal adhesion and, together with conventional analysis of the bulk oxide, assist in determining the oxidation mechanism.

MST/862     

Effect of dilution on GTAW Colmonoy 6 (AWS NiCr–C) hardface deposit made on 316LN stainless steel

Abstract

Nickel based Colmonoy 6 (conforming to AWS NiCr–C) hardfacing alloy finds application in hardfacing of various components made of austenitic stainless steel (SS) used in fast reactors. Owing to considerable difference in melting points of the SS and Colmonoy 6 alloys, significant dilution from substrate occurs during hardfacing using gas tungsten arc welding process. Dilution has a significant effect on microstructure, hardness and wear resistance of the deposit. To overcome the adverse effects of dilution on the hardness and, hence, the wear resistance of the deposit, often, the minimum thickness specified for the deposit on hardfaced components is high, which in turn increases the susceptibility of the deposit to cracking during deposition. In the present investigation, microstructure of different layers of multilayer Colmonoy 6 deposits on 316LN SS is characterised by optical and scanning electron microscopy, and the correlation between hardness and microstructure of the individual layers with dilution from the base metal has been established. The dilution from the base material is the highest in the first layer, and it progressively decreases in the subsequent layers. With progressive decrease in dilution, the precipitate fraction increases from about 16 to 20% from the first to the fifth deposit layers. This is accompanied by hardness increase from about 480 to 800 HV. The precipitates in the deposit consist of both borides and carbides, with the boride content varying more with dilution than the carbide content. The boride fraction increased from 5 to 8% with a decrease in dilution; however, layer to layer variation in carbide fraction was only marginal at about 11–12%. High dilution from the base material suppresses the formation of borides in the deposit and is responsible for low hardness of the deposit diluted with the austenitic SS compared to those of the undiluted deposit.     

Corrosion behaviour of single (Ti) and duplex (Ti–TiO2) coating on 304L stainless steel in nitric acid medium

Sputter deposited single titanium (Ti) layer, and duplex Ti–TiO₂ coating on austenitic type 304L stainless steel (SS) was prepared, and the corrosion performance was evaluated in nitric acid medium using surface morphological and electrochemical techniques. Morphological analysis using atomic force microscope of the duplex Ti–TiO₂ coated surface showed minimization of structural heterogeneities as compared to single Ti layer coating. The electrochemical corrosion results revealed that, titanium coated 304L SS showed moderate to marginal improvement in corrosion resistance in 1 M, and 8 M nitric acid, respectively. Duplex Ti–TiO₂ coated 304L SS specimens showed improved corrosion resistance as compared to Ti coating from dilute (1 M) to concentrated medium (8 M). The percentage of protection efficiency for base material increases significantly for duplex Ti–TiO₂ coating as compared to single Ti layer coating. The oxidizing ability of nitric acid on both the coatings as well as factors responsible for improvement in protection efficiency are discussed and highlighted in this paper.     

Effects of aging at 700°C on ductile fracture of AISI 316 stainless steel

Abstract

The micromechanisms of ductile fracture have been studied in a commercial AISI 316 austenitic stainless steel. Tensile, Charpy impact, and ductile fracture toughness testing have been performed on unaged material and samples aged at 700°C for times up to 4380 h. Examination of the specimens after testing has shown that the microstructural changes occurring at grain boundaries are responsible for the observed losses of ductility and crack growth resistance. The relative magnitude of the observed changes in mechanical properties has been explained using a simple model to describe the ductile fracture process.

MST/1001     

Strength—toughness relationship for austenitic stainless steel welds at 4 K

Cryogenic mechanical property data compiled at the National Bureau of Standards, USA, have been used to analyse the relationship between yield strength and fracture toughness for austenitic stainless steel welds at 4 K. This study demonstrates that there is an inverse linear correlation between yield strength and fracture toughness for the stainless steel welds at 4 K, and that the welds have significantly lower toughness than base materials of comparable strength.     

Evolution of texture and development of ∑3 n grain clusters in 316 austenitic stainless steel during thermal mechanical processing

A type 316 austenitic stainless steel sheet with various texture components along its thickness was slightly cold-rolled and then sequentially annealed at 1000 °C for 80 and 150 min. The evolution of texture and the development of grain clusters in which the grains are interfaced by ∑3 ⁿ (n = 0, 1, 2, 3…) boundaries (∑3 ⁿ GC) were studied by electron backscatter diffraction (EBSD). The results show that the texture difference along the thickness was minimized, and large (greater than 400 μ) ∑3 ⁿ GCs developed after cold rolling and annealing. In situ EBSD examination revealed that each ∑3 ⁿ GC was developed by the oriented-growth (OG) of brass ({110} $ \langle 112\rangle $ ), copper ({112} $ \langle 111\rangle $ ) and C1 ({114} $ \langle 041\rangle $ ) orientations (including their geometric variants). These orientations are intrinsically related to ∑3 ⁿ misorientations. The abutting ∑3 ⁿ GCs with grains of nearly the same or ∑3 ⁿ -related orientations usually merged into a larger ∑3 ⁿ GC or resulted in an effective disruption of the connectivity of the random high angle grain boundary network.     

Indigenous development and airworthiness certification of 15–5 PH precipitation hardenable stainless steel for aircraft applications

In this paper, we discuss the optimization of chemical composition, processing (forging and rolling) and heat treatment parameters to obtain the best combination of mechanical properties in case of a Fe–15Cr–5Ni–4Cu precipitation hardenable stainless steel. The ε-copper precipitates that form during aging are spherical in shape and coherent with the matrix and principally provide strengthening in this alloy. The orientation relationship is found to be Kurdjumov–Sachs (K–S), which is common in fcc–bcc systems. Results obtained from metallurgical evaluation (mechanical property and metallography) on 15–5 PH alloy during type certification on 3 different melts were used for the optimization, attempted in this study. The mechanical properties following strain deformation has been carried out using optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). In the aged conditions, the 15–5 PH alloy exhibited brittle failure with extensive cleavage and/or quasicleavage fracture. This paper reports all results and also factually shows that indigenously developed and produced 15–5 PH stainless steel matches in its properties with the equivalent aeronautical grade precipitation hardening stainless steels globally produced by internationally renowned manufactures.     

Data-driven prediction of the probability of creep–fatigue crack initiation in 316H stainless steel

Stainless steel components in advanced gas-cooled reactors (AGRs) are susceptible to creep–fatigue cracking at high temperatures. Quantifying the probability of creep–fatigue crack initiation requires probabilistic numerical simulations; these are complex and computationally intensive. Here, we present a data-driven approach to develop fast probabilistic surrogate models of creep–fatigue crack initiation in 316H stainless steel. We perform a set of Monte Carlo simulations based on the R5V2/3 high temperature assessment procedure and determine the sensitivity of the probability of crack initiation to loads and operating conditions. The data are used to train different supervised machine learning models considering Bayesian hyperparameter optimization. We discuss the relative performance of such models and show that a gradient tree boosting algorithm results in surrogate models with the highest accuracy.