Chromium Equivalents Research Articles

Investigation on the Correlation between Inclusions and High Temperature Urea Corrosion Behavior in Ferritic Stainless Steel

The influence of inclusion size and number density on high-temperature urea corrosion (HTUC) behavior of ferritic stainless steels was investigated in a simulated working environment of selective catalytic reduction (SCR) system in commercial vehicles. There is a positive correlation between the control level of inclusions and the resistance of HTUC. By slightly increasing the content of Nb in ferritic stainless steels, the inclusions, especially TiN, were significantly refined, and thus displayed an improvement in HTUC resistance. The interface between inclusions and the matrix becomes a fast channel for chromium precipitation during high-temperature nitriding induced by the decomposition of urea. Chromium nitrides will precipitate around the inclusions and wrap the inclusions, which will decrease the chromium equivalent of the matrix and reduce the resistance of ferritic stainless steels to HTUC. In addition, the high-temperature oxidation accompanied with thermal fatigue also makes the inclusions more likely to become the crack nucleation source, which can accelerate the material thinning and reduce its service life.

Solidification Behavior of Duplex Stainless Steel (UNS S32205) Welds as a Function of WRC–1992 Diagram Equivalents and Cooling Rate

Duplex stainless steels solidify as 100 % Ferrite, however solidification of Duplex Stainless steel welds is a function of Chromium equivalent and Nickel equivalent alloying elements, filler metal and base metal composition and associated weld thermal cycle. Resultant solidification mode behavior and hence final weld metal microstructure is always a matter of interest among researcher because mechanical properties and corrosion properties are depending on weld metal ferrite content. The present research is to investigate the solidification mode behavior of duplex stainless steel welds with varying Chromium equivalent and Nickel equivalent ratios and associated weld cooling rate. In order to meet this objective, weld coupon were prepared using experimental Low Nickel, Medium Nickel and High Nickel electrodes using SMAW process, step wise increased in Nickel content from commercially available standard 09 weight percentage Nickel in E2209 design. WRC-1992 Diagram has been used to calculate Chromium Equivalent and Nickel equivalent and predict Ferrite Number using compositions of Base metal and Nickel enriched experimental electrodes. Microstructure characterization of all weld coupons is carried out using ASTM A 923 Method. Calculated values were correlated with the different microstructures developed. According to the findings of the studies, weld produced with higher Nickel content electrode, when cooled with maximum rate approximately 100.8 °C/ Sec , considered under study, exhibited FA-mode of solidification with as low as 1.71 Cr eq /Ni eq ratio and there was no noteworthy presence of intermetallic phases and precipitations in as-weld microstructures at approximate retained ferrite level of 15 Ferrite Number.

Effect of Austenite Stability on Pack Aluminizing of Austenitic Stainless Steels

Aluminide coatings were applied to the surfaces of several austenitic stainless steels—UNS S30300, S30400, S30900, S31000, and S31600 (Type 303, 304, 309, 310, and 316)—by the halide activated pack cementation process. The coating compositions, microstructures, and hardness were determined for the different steels coated at 850°C for 25 h. The stability of the austenite phase for each type of steel was calculated by determining the ratio of the nickel to the chromium equivalents based on their nominal compositions. The thickness of the inner diffusion zone in the coating was shown to be inversely related to the austenite stability of the steels. Microhardness measurements were obtained across the coating thickness and into the substrate. The hardness values followed the same trends as the aluminum composition profile into the substrate.

Influence of Intrinsic Solidification Behaviour on Quality of Stainless Steel Cast and Rolled Products

Recent advances in continuous casting have facilitated remarkable improvement in the quality of cast steel. The focus of the present paper is directed towards understanding of the influence of chemistry and intrinsic solidification behaviour on the different quality issues of stainless steel. An attempt has been made, wherever possible in quantitative terms , to correlate the chemistry and the process parameters with the quality factors. The relevant theoretical background has been touched upon. Understanding has been developed on the role of the ratio of nickel equivalent and chromium equivalent. This factor can be used to represent the chemistry of any stainless steel grade. Under similar casting conditions, specific problem of mould sticking , strand bulging , or depression formation has been explained invoking this chemistry factor. Each quality issue owes its genesis to interaction between the high-temperature strength and ductility of the solidifying shell during casting. The important roles of micro-segregation , the effective shell thickness during the early stage of casting , and the temperature range of transformation for δ-ferrite to austenite have been highlighted. An approach of grade-specific casting practice has been suggested to formulate the specific casting parameters. The developed understanding has been utilised to address specific quality problems in continuously cast slabs and rolled products of different stainless steel grades. This integrated understanding can be useful for ensuring the quality of stainless steel of any chemistry.

Evaluation of the WRC 1992 diagram using computational thermodynamics

Ferrite content of a stainless steel weld metal is a vital parameter for ensuring that the microstructure and thus also corrosion and mechanical properties are adequate. A rapid way of estimating ferrite content and thus microstructure is by using weld metal composition and a Welding Research Council (WRC)-1992 diagram. The ferrite-forming alloying elements are estimated to a chromium equivalent, the austenitic to a nickel equivalent and the ferrite content is estimated by help of isoferrite lines. This diagram was derived by empirical work on a large number of commercial and laboratory stainless weld metals covering a wide alloy span. The present paper uses computational thermodynamics, i.e. Thermo-Calc, to evaluate the influence of temperature and key stainless alloying elements. The alloying range for duplex stainless weld metal was studied and the WRC 1992 diagram was compared with results from calculations. The chromium and nickel equivalents were evaluated and coefficients for Mo, Nb, C, N and Cu assessed. The results showed that while the coefficients for Mo and N proved accurate, the values for Nb, C and Cu would require an adjustment. Alternative diagrams with adjusted chromium and nickel equivalents and iso-ferrite lines in both ferrite numbers and volume percent were suggested.

Phase composition of nitrogen-containing low-alloy stainless steels of the system Fe–Cr–Ni–N–Si

The limiting solubility of nitrogen at atmospheric pressure in low-carbon stainless steels containing not more than 0.03% carbon, 13–25% chromium, 5–13% nickel, 0–5% silicon is calculated. Using Scheffler diagrams and the calculated limiting nitrogen content within steels of the system Fe–Cr–Ni–N–Si limits are determined for phase areas in relation to values of nickel and chromium equivalents. Temperatures are calculated for the start of martensitic transformation during cooling and plastic deformation of steels containing a different amount of chromium, nickel, nitrogen, and silicon.

Prediction of Ferrite Number in Stainless Steel Welds using Bayesian Neural Network Model

Bayesian neural network analysis has been used to develop an accurate model for predicting the ferrite content in stainless steel welds. The analysis reveals the influence of compositional variations on ferrite content for the stainless steel weld metals, and examines the significance of individual elements on ferrite content in stainless steel welds based on the optimised neural network model. This neural network model for ferrite prediction in stainless steel welds has been developed using the database used for generating the WRC-1992 diagram and our laboratory data. The optimised committee model predicts the ferrite number (FN) in stainless steel welds with better accuracy than the constitution diagrams and the other FN prediction methods. Using this generalised Bayesian Neural Network (BNN) model, the influence of variations of the individual elements on the FN in austenitic and duplex stainless steel welds has been determined. It is found that the change in FN is a non-linear function of the variation in the concentration of the elements, with elements such as chromium, nickel, nitrogen, molybdenum, silicon, titanium and vanadium are found to influence the FN more significantly than the rest of the elements in stainless steel welds. Manganese is found to have less influence on the FN. While titanium influences the FN more significantly than niobium, the WRC-1992 diagram considers only the niobium for calculating chromium equivalent. The role of silicon and titanium in influencing the FN in stainless steel welds has been brought out clearly, while these elements are not given due considerations in the WRC-1992 diagram.

The effect of the alloying elements of a CoNiCrAlY system on the corrosion resistance of alloys for protective coatings of gas turbine blades

Results from testing for resistance to sulfide-oxide corrosion of model alloys used to make possible CoNiCrAlY system-based protective coatings are presented. The effect exerted by Ni, Cr, Al, and Y on the corrosion resistance of these coatings at different temperatures is determined, and equations for calculating the chromium equivalents of these coatings are obtained.

Correlation dependences between the mechanical characteristics of corrosion-resistant steels

A stability factor of austenite A γ is proposed for corrosion-resistant steels. By using this factor, the equilibrium phase-structural state of steels can be quantitatively evaluated according to their chemical composition. The factor A γ is determined by the values of the chromium and nickel equivalents. This factor, together with the generalized Scheffler diagram, enables one to deduce the correlation equations for the mechanical characteristics of steels σu, σ0.2, δ, and ψ.

Nickel saving in a 0Kh17N12M2-type (AISI 316) steel due to nitrogen alloying

The possibilities of the maximum decrease in the nickel content in an AISI 316 (0Kh17N12M2-type) steel via partial substitution of nitrogen for nickel and the corresponding correction of the contents of other alloying elements are analyzed. The corrected chemical composition of an AISI 316L-based steel is ≤0.02% C, 0.1–0.3% N, 17.5–18.5% Cr, 2.2–2.7% Mo, 3–4% Mn, and 0.1–0.5% Si. Calculation of the chromium equivalents of martensite formation and ferrite formation using the Potak-Sagalevich structure-phase diagram shows that, after quenching, these steels should have a purely austenitic structure without martensite or ferrite.

Martensite microstructure of 9–12%Cr steels weld metals

Several new grades of steel have been developed in recent years, which have been based on the high-strength 9–12%Cr steels. These steels were developed to meet the proposed demand by power generation companies to increase efficiency to operating at higher temperatures by and thus burning less fossil fuel, principally coal, and thereby reducing costs and also meeting the increasingly stringent environmental requirements. The 9–12Cr steels used for high temperature components in power plants are generally required to possess good mechanical properties, corrosion resistance, creep strength and fabricability. Although such steels normally have a fully martensite microstructure, they are also susceptible to the formation of delta ferrite, mainly during the welding process. Delta ferrite has several detrimental effects on such properties as creep, ductility and toughness. Thus, it is important to avoid its formation. In this study the formation of delta ferrite in the weld metal of high-strength 9–12%Cr steels has been analysed for several samples with variations of key alloying elements. The results indicate that the most effective way to avoid delta ferrite in the weld metal is to reduce the chromium equivalent value to as low a value as possible. A fully martensite microstructure is obtained when both the Schneider chromium equivalent is lower than 13.5 and the difference between the chromium and nickel equivalent is lower than 8.

Hot cracking in austenitic stainless steel weld metals

Solidification cracking in austenitic stainless steel welds has been considered based on the solidification modes and the nickel and chromium equivalents in welding processes where the cooling rates are high. Taking a Creq/Nieq (Welding Research Council) ratio of 1.6 as a criterion for the final weld metal to avoid hot cracking, a methodology is proposed for the selection of filler or buffer material equivalents.

The chromium equivalents of ferrite stabilizers in commercial stainless steels

The chromium equivalents of ferrite stabilizers in commercial stainless steels

The chromium equivalents of selected elements in austenitic stainless steels

Using the well-established thermodynamic description of the Fe-Cr-Ni system, we have computed the austenite-ferrite equilibrium in Fe-Cr-Ni-X quaternary systems, where X is a ferrite-stabilizing element. The results are considered reliable, especially for small additions of X. From the computed equilibria, the Cr equivalents of a number of ferrite stabilizers, Al, Mo, Nb, Si, Ti, V, and W, have been derived. They show satisfactory agreement with the known experimental range. The distinguishing feature of the thermodynamic calculations is that the Cr equivalents are established as a function of annealing temperature as well as Ni content, in contrast to the fixed values reported in the empirical diagrams obtained under variable experimental conditions.

Ferritic-austenitic solidification mode in austenitic stainless steel welds

The macro-and microstructures of about fifty different stainless welds of the AISI/ AWS 300 series are analyzed. The results indicate that under conditions corresponding to a typical shielded metal arc (SMA) welding the welds with a ratio in the range 1.48≾Cr eq /Ni eq ≾1.95, where Ni eq and Cr eq are the nickel and chromium equivalents on the Schaeffler diagram, solidify in accordance with a duplex mode with the delta ferrite as the primary (leading) phase. The austenite forms between ferrite dendrites through a three-phase reaction between liquid, ferrite and austenite, and subsequently grows into the ferrite by either an equiaxial or an acicular mechanism, resulting in a drastic decrease in the volume fraction of the delta ferrite. The micro-structure at room temperature is characterized by a general irregularity and the varied morphology of the ferrite. The compositional differences observed at room temperature are a consequence both of the solidification and the solid state transformation.

Studies of Fabrication-Related Sensitization in Type 304 Stainless Steel

The Jominy end-quench hardenability specimen has been shown from earlier studies to serve as a useful screening tool for detecting sensitization in Type 304 stainless steel. The present study has investigated this technique for assessing sensitization developed in the various thermal cycles encountered in the fabrication and service of this type of stainless. The technique shows that the sensitization encountered is associated with heating and cooling from 800–950°C rather than higher temperatures; such fabrication or service should be above 1000°C to forestall sensitization. Differences in sensitization between test specimens and larger sections are attributed to the development of stresses that accentuate precipitation of M23C6 during cooling. The proper level of nitrogen can retard and reduce sensitization; these effects are related to chromium and carbon equivalents.

Austenitic solidification mode in austenitic stainless steel welds

The micro- and macrostructures of about 50 different stainless welds of the AISI/AWS 300 series are analyzed. The results indicate that in welding condition corresponding to a typical SMA welding those and only those welds in which the ratio Creq/Nieq≲1.48, where Nieq and Creq are the nickel and chromium equivalents on the Schaeffler diagram, solidify with the austenite as the primary or leading phase and the delta ferrite, if any, formed from the rest melt between growing cells or cellular dendrites of the austenite. At room temperature these welds are characterized by a regular general microstructure, soft forms of the ferrite and relatively large compositional differences mainly caused by solidification.

Single-phase ferritic solidification mode in austenitic-ferritic stainless steel welds

The macro- and microstructures of about 50 different stainless welds of the AISI/AWS 300 series are analyzed. The results indicate that under conditions corresponding to a typical SMA welding, those and only those welds in which the ratio Creq/Nieq≳1.95, where Nieq and Creq are the nickel and chromium equivalents on the Schaeffler diagram, solidify as single-phase ferrite: Austenite subsequently forms from ferrite in the solid state by a Widmanstatten mechanism. The microstructure at room temperature (RT) is characterized by straight phase boundaries and a distinct orientation relationship between the austenite and ferrite. The compositional differences observed at RT are mainly a consequence of the solid state transformation, not of solidification.

Deformation of stainless steels with austenite differing in stability

1. The stability of austenite in stainless steels, determining the rate of the γ → α transformation at different deformation temperatures, can be determined fairly accurately by calculating the chromium equivalent of martensite formation CreqvM from the structural diagram [5]. 2. The ultimate strength of austenitic steels increases with decreasing deformation temperatures (even if the γ → α transformation does not occur in the process of deformation), with increasing carbon concentrations (and, obviously, nitrogen), and with increasing rates of the γ → α transformation, determined by the value of CreqvM and the deformation temperature. 3. To obtain consistent mechanical properties of deformed stainless steels that undergo the γ → α transformation in the process of deformation, it is necessary to use steels having values of CreqvM within narrow limits or select the degree of deformation for different heats in relation to the value of CreqvM.

Even elongation of stainless steels

1. Even elongation, determining the ability of metals to change their shape, depends mainly on the rate of reduction of the relative hardening with increasing strain. For martensitic steels, including high-carbon steels, which are not inferior to austenitic steels in terms of notch toughness (an andap) and ductility (ψ), the rate at which the relative elongation decreases in the process of deformation is far higher than for steels of the austenitic and transition classes, and therefore even elongation is substantially smaller. 2. For martensitic and austenitic steels not undergoing phase transformations in the process of deformation an increase or decrease of deformation temperature leads respectively to a decrease or increase of relative hardening, and thus even elongation. 3. With formation of 20–30% martensite in unstable austenitic steels in the process of deformation the rate at which the relative hardening decreases during deformation is lower than for stable austenitic steels, and the even elongation reaches a peak. For each steel undergoing the γ→α transformation there is an optimal temperature (for a given strain rate) at which even elongation is highest. 4. The stability of austenite in stainless steels can be determined for practical purposes from the chromium equivalent of martensite formation (CreqvM) [6]. 5. Practically complete elimination of carbon and nitrogen sharply reduces the even elongation of martensitic steels but has no effect on the even elongation of austenitic steels. The even elongation of martensitic steels increases considerably after tempering at 200°.