Stainless Grade Research Articles

Study on the deterioration of mechanical properties of stainless steel S35657 exposed to overall stage of fire

Fires often have destructive effects on steel structures, putting the safety of building structures at serious risk. As a new austenite stainless steel, grade S35657, its in-fire and post-fire mechanical behavior has not been reported, which limits its application. In this paper, a total 68 stainless steel S35657 coupons were designed and tested, with 34 coupons used for in-fire tests and 34 for post-fire tests. The effects of heating temperatures, loading rates, and heating soaking times on the mechanical properties were investigated. The failure modes, as well as the stress-strain curves and mechanical properties, were acquired. Electron microscopy scanning analysis were also conducted on the coupons to study the fracture mechanisms at elevated temperatures. The test results showed a rapid decline in in-fire mechanical properties when the temperature exceeded 600 °C. The yield strength of the coupon at 1000 °C is only 9.1 % of that at room temperature. As the tensile rate increased, the strength of the coupon increased gradually. For post-fire coupons, there is no significant change in the mechanical properties of coupons at temperatures ranging from 20 °C to 900 °C. However, after reaching a temperature of 1000 °C, the coupon experienced a reduction in yield strength (up to 27.8 %) and an expansion in elongation (up to18.0 %), compared to the coupon at ambient temperature. This related to the change in grain size of the coupons after exposure to high temperatures. The heating soaking time has an insignificant effect on the post-fire mechanical properties of stainless steel S35657. Existing constitutive models in the literature cannot be well applied to stainless steel S35657, new constitutive models to predict the in-fire and post-fire mechanical properties have been established. Moreover, reduction factors for the key mechanical properties were compared with existing values, and new unified equations for stainless steel S35657 were proposed. Finally, a reliability analysis was performed to evaluate the proposed design rules, and it was deemed that the proposed design rules were reliable. The findings of this study will provide a reference for engineers in evaluating the in-fire and post-fire residual strength of stainless steel S35657, and offer guidance for fire resistance design and high-temperature repair reinforcement.

Investigation of Corrosion Caused by Iron Contamination on the Surface of Stainless Steel Plates

Corrosion caused by iron contamination, also known as rouging, is a possible type of corrosion of stainless steels. This type of corrosion is often confused with the corrosion of stainless steel itself. Rusty discoloration on the surface of a material considered corrosion resistant is a problem in the construction and pharmaceutical industries. Repairing rouging afterwards is usually costly, but can be prevented by good manufacturing practices and control. In our research, we compared four commonly used stainless material grades with different surface treatments. We investigated the effect of scratches left by carbon steel on the surface of the samples and the time course of the process.

Comparative Study of the Drilling Operation in Duplex Stainless Steels with Drills of Two and Three Cutting Edges

The three cutting edge drills are not the most common drills, nevertheless, are characterized by superior stability, excellent precision and finishing, due to the intersection of three cutting edges at one point. In general, stainless steels are considered as difficult to machine materials due to their tendency to work harden, their toughness and relatively low conductivity, leading to poor surface finish, poor chip breaking and built-up-edge formation. In the case of duplex stainless grades, the high strength, and the very good corrosion resistance, only compared with austenitic steels, make these materials as an alternative to the austenitic stainless steels, with superior mechanical properties. In this study, the performance of the drills with two and three cutting edges were evaluated in the drilling of duplex stainless steels, when low-pressure external cooling or high-pressure internal cooling were applied. Whether used drills of two or three cutting edges, the most important factor to increase the number of holes made, is the use of high-pressure internal cooling, in detriment of external low-pressure external cooling. The drills of three cutting edges have better results in roughness and dimensional tolerance of the holes. However, these drills proved to be more fragile and more sensitive to the cutting parameters. The use of three cutting edges drills is recommended for situations where hole quality is more important, while two cutting edges drills is recommended for situations where productivity is the main objective.

Heat-affected zone ferrite content control of a duplex stainless steel grade to enhance weldability

Duplex stainless steels (DSS) have a microstructure consisting of 50% ferrite α and 50% austenite γ. This two-phase microstructure provides the duplex stainless steels with attractive mechanical and corrosion properties. However, the microstructure is very dependent on the heat treatment and chemical composition. During the welding process, the temperature reached in the heat-affected zone (HAZ) retransforms austenite into ferrite. Due to the high cooling rates, only a part of the ferrite is retransformed into austenite. This tends to modify the 50/50 phase equilibrium. The cooling rate is linked not only to heat input but also to the thickness of the welded plates. Most of the standards limit the ferrite content in the range from 30 to 70%. However, the 70% limit for ferrite content in HAZ may be difficult to obtain especially for heavy plate thicknesses. To limit the maximum ferrite content in HAZ, a duplex stainless grade with high nitrogen and nickel within the 2205 composition range, called Arctic 2205, was developed. This paper will provide details on ferrite content measurement by metallographic method and welding results obtained on industrial welds showing very low and stable ferrite content in HAZ. Mechanical properties obtained on these welds at low temperature will also be reported, pushing the limits of use of duplex from − 50 °C (− 58 °F) down to − 100 °C (− 148 °F).

Dimensional Stability of Low Temperature Surface Hardened Stainless Steel Components*

Abstract Stainless steels are commonly used for high precision components, which often are exposed to corrosive media. However, their inferior tribological behaviour restrict the use of these materials in many technical applications. Thermochemical surface hardening is one way to overcome these weaknesses. Solution nitriding in the austenitic range above 1000 °C is mainly used for hardening martensitic and ferritic stainless grades. In austenitic and duplex stainless grades, however, the hardening effect is limited. Additionally, the high process temperatures combined with a necessary rapid cooling may lead to non-desired dimensional changes. Low temperature surface hardening processing below 500 °C here offers interesting alternatives for increasing the wear properties, while maintaining the corrosion resistance. This paper demonstrates the influence of high and low process temperatures of thermochemical surface hardening treatments on the tight dimensional tolerances of a rotationally symmetrical precision component made from cold worked AISI 304. Based on these results, current and new industrial applications, which benefit from low temperature surface hardening, will be discussed.

Failure Analysis of the High Nitrogen Austenitic Stainless-Steel Boiler Tank Weldments

The paper investigates the reason for the cracking and leaking of stainless-steel (SS) tanks used to store processed hot water in rice boiling and husking plants. A detailed analysis of tank specimen samples using radiography, optical metallography, scanning electron microscopy, EDAX reveals that stress corrosion cracking (SCC) is the prime reason for the failure. The presence of favorable microstructure, environment and stress promoted the nucleation and propagation of SCC in the tank weldment. The evolution of the microstructure in the selected stainless grade was studied and discussed using Scheffler–Delong diagram. The formation of martensite in low nickel, high nitrogen austenitic stainless steel was observed to significantly influence the mechanical properties and promote SCC. The decreased pitting corrosion resistance of the material compared to SS 316 alloy was an important factor in the initiation of pits which acted as nucleation sites for SCC.

Sensor Development for Corrosion Monitoring of Stainless Steels in H2SO4 Solutions.

Equipment made of different stainless steels is often used in the hydrometallurgical processing industry. In this study, an electrical resistance sensor was developed for monitoring corrosion in acidic solutions at high temperature. Two types of stainless steel were used as the electrode materials, namely grade 316L stainless steel (EN 1.4404) and grade 2507 duplex stainless steel (EN 1.4410). The materials and sensors were exposed to a 10% H2SO4 solution containing 5000 mg/L of NaCl at various temperatures. Results from the sensors were verified using electrochemical techniques and postexposure examination. Results showed that the microstructure played an important role in the interpretation of corrosion rates, highlighting the importance of using an appropriate stainless steel for the production of sensors. Electrochemical tests and postexposure examination both showed that the grade 2507 had a significantly lower corrosion rate compared to the grade 316L. Under industrial‑process conditions, the results for the grade 2507 sensor were promising with respect to sensor durability and performance, despite the extremely harsh operating environment.

Hydrogen outgassing and permeation in stainless steel and its reduction for UHV applications

Stainless steel, grade 304L and 316L are widely used structural steel in most of the Ultra High Vacuum (UHV) systems in present-day research. Large size vacuum chambers and beam lines are used in much scientific applications, few examples are CERN accelerators, ITER, LIGO, Neutrino observatory, FAIR, Space research, etc. The selection and operation of the large size vacuum pumping system and the material to be used for UHV application is one of the crucial tasks for the vacuum system engineers as it relates the capital and operational cost linked to it. Any failure in operation has a direct impact on the project execution timeline and the huge refurbishment cost. Hydrogen in structural steel and its reduction for ultra-high vacuum systems is one of the major challenges for the present day. Therefore, prior understanding is required for knowing hydrogen content and its permeation and outgassing behaviour in vacuum system establishment and operation. Based on the available literature, a theoretical model for estimating permeation and outgassing rate is discussed. Relating the permeation and outgassing, the theoretical model contains first and second-order Fick's law and their solutions. Permeation and outgassing rate of 3 mm thick stainless steel is calculated for different thermal bake-out conditions and are discussed. A brief discussion (after going through various references) is mentioned for lowering the outgassing rate for hydrogen in steel.

Main Defects Observed in Titanium GRADE II and 316L Stainless Steel Elements Obtained by Selective Laser Melting

Selective Laser Melting SLM is one of the most popular three dimensional printing methods, which can be used for manufactured real elements (with high geometrical complexity) in many application, such as medicine, automotive or aerospace industries. The SLM final parts are characterized by high mechanical properties and satisfactory physicochemical properties. However, the properties of parts depend of process parameters optimization. In this paper, effects of processing parameters, such as laser power P, scanning speed SP, layer thickness t or point distance PD on defect formation and relative densities of manufactured elements are explored. For the purpose the stainless steel 316L and pure titanium Grade II are used. The process optimization were carried out according to the formula of energy density, which is delivered to the powder material. The stainless steel samples were divided into 12 groups, depends of the energy density. The titanium parts were printed at the same value of energy, and the process parameters are changed. The microscope observation and relative density measurements were carried out. Based on the obtained results, it can be confuted that the SLM parameters have a significant effect on the samples properties and the mechanism formed defect in both material are similar.

HARDNESS OF NITRIDED LAYERS TREATED BY PLASMA NITRIDING

Stainless steels, particularly the austenitic stainless grades are widely used in many industries due to good corrosion resistance, but very poor mechanical properties as surface hardness and wear resistance limit its possible use. Plasma nitriding is one of the few ways to increase the surface hardness of these steels, even though this will affect its corrosion resistance. This paper focuses on the description of the mechanical properties of nitrided layers in the two most widespread austenitic stainless steels AISI 304 and AISI 316L. The microstructure and properties of nitrided layers were evaluated by metallography and microhardness measurement. Surface properties of nitrided steels were characterized by Martens hardness. The results show that plasma nitriding created very hard nitrided layers with thickness about 40 μm and microhardness about 1300 HV0.05. Surface hardness measurements have shown that the maximum values for both steels are about 8.5 GPa, but have different behaviour under higher loads, when the AISI 316L nitrided layer began to crack on the surface and sink.

Low-cycle fatigue of stainless steel plates under large plastic strain demands

The low-cycle characteristics of structural subassemblies under large cyclic strains play an important role in contemporary seismic design. Obtaining these characteristics can lead to an understanding of a structure's degradation and nonlinear response behaviour, and can serve as a basis for developing efficient numerical models to predict seismic collapse mechanisms. The austenitic stainless tubular grade of steel has shown promise in terms of strain hardening character, structural overstrength and ductility, but existing test data is limited or constrained to small plastic strains, which is hardly useful in earthquake engineering applications. This article presents an experimental study designed to characterize the hysteresis of stainless steel plates under large inelastic cyclic strains and to assess their potential use in buckling-restrained brace components for seismic applications. Axial coupons machined from austenitic Grade 304L stainless steel and from regular carbon steel Grade 350WT were tested under uniaxial tensile loading, as well as constant and variable strain amplitude cyclic loadings. Results of the uniaxial tests confirmed higher ductility and strain hardening capacity for the stainless steel plates compared to those of carbon steels. These results were subsequently used to validate a novel technique based on image analysis to derive the true stress-strain characteristics. The stainless steel Grade 304L plates showed higher cyclic hardening but shorter low-cycle fatigue life compared to the carbon steel. Parameters for representing the low-cycle fatigue behavior of these materials, useful for developing a cyclic plasticity hardening numerical model, were derived from the test data.

Results of Thermal Tests of Recirculation Cooling Installations

The article presents the results from thermal tests of some recirculation installations for cooling air in nuclear power plant premises, including the volume under the containment. The cooling effect in such installations is produced by pumping water through its heat-transfer tubes. Air from the cooled room is blown by a fan through a bundle of transversely finned tubes and is removed to the same room after having been cooled. The finning of tubes used in the tested installations was made of Grade 08Kh18N10T and Grade 08Kh18N10 stainless steels or Grade AD1 aluminum. Steel fins were attached to the tube over their entire length by means of high-frequency welding. Aluminum fins were extruded on a lathe from the external tube sheath into which a steel tube had preliminarily been placed. Although the fin extrusion operation was accompanied by pressing the sheath inner part to the steel tube, tight contact between them over the entire surface was not fully achieved. In view of this, the air gap’s thermal resistance coefficient was introduced in calculating the heat transfer between the heat-transferring media. The air gap average thickness was determined from the test results taking into account the gap variation with temperature due to different linear expansion coefficients of steel and aluminum. These tests were mainly aimed at checking if the obtained thermal characteristics were consistent with the values calculated according to the standard recommendations with introduction, if necessary, of modifications to those recommendations.

Substantial improvement in sub-zero impact toughness in 12Cr stainless grade welding joints by applying double PWHT

Abstract A novel means to improve the sub-zero impact toughness in the welding joints of 12Cr stainless steels was developed. The results indicated that the optimal conditions for performing the double post-welding heat treatment (PWHT) procedures were as follows: heating at 1000 °C for 15 min, followed by air cooling and then heating at 780 °C for 15 min, and finally air cooling. The welding joints subjected to double PWHT exhibited excellent mechanical and enhanced corrosion-resistance properties. Furthermore, the transition of the substructure in the martensitic grains was identified as the decisive factor for the improvement in the sub-zero toughness.

Thermal tests of large recirculation cooling installations for nuclear power plants

The article presents the results from thermal tests of some recirculation installations for cooling air in nuclear power plant premises, including the volume under the containment. The cooling effect in such installations is produced by pumping water through their heat-transfer tubes. Air from the cooled room is blown by a fan through a bundle of transversely finned tubes and is removed to the same room after having been cooled. The finning of tubes used in the tested installations was made of Grade 08Kh18N10T and Grade 08Kh18N10 stainless steels or Grade AD1 aluminum. Steel fins were attached to the tube over their entire length by means of high-frequency welding. Aluminum fins were extruded on a lathe from the external tube sheath into which a steel tube had preliminarily been placed. Although the fin extrusion operation was accompanied by pressing the sheath inner part to the steel tube, tight contact between them over the entire surface was not fully achieved. In view of this, the air gap’s thermal resistance coefficient was introduced in calculating the heat transfer between the heat-transferring media. The air gap average thickness was determined from the test results taking into account the gap variation with temperature due to different linear expansion coefficients of steel and aluminum. These tests, which are part of the acceptance tests of the considered installations, were carried out at the NPO TsKTI test facility and were mainly aimed at checking if the obtained thermal characteristics were consistent with the values calculated according to the standard recommendations with introduction, if necessary, of modifications to those recommendations.

12.15: Structural performance of stainless lap bolted joints

ABSTRACTThe stainless steel employed in the structural elements present high corrosion resistance, durability, fire resistance and high aesthetic value compared to those constructed using carbon steel. In contrast, the stainless steel structures design is made following the requirements based on the carbon steel analogies. In fact, the structural performance of stainless steel is complex and dissimilar where some stainless grades presenting nonlinear tension versus strain curves without a defined yield plateau and large strain hardening. These properties influence on structural performance of the lap bolted joints. The Eurocode 3 pt. 1.4, one of the most adopted design codes, recommends two structural design criteria for the stainless steel elements under tension: ultimate resistance of the net cross‐section at holes or plastic resistance of the gross cross‐section. The present work aimed to investigate the structural response of lap bolted joints made of three stainless steel grades: austenitic, ferritic and duplex. Nonlinear numerical analyses has been carried out using ABAQUS 6.14 program where the response have shown a good agreement in terms of the maximum loads and failure modes when compared to the performed experiments. The results indicated a conservative level of safety from Eurocode 3 pt. 1.4 code provisions in comparison to the experimental results.

Thermophysical properties of Alloy 709

Alloy 709 (UNS S31025) has shown enhanced performance, particularly in creep strength, over current generation high temperature structural materials, specifically Type 316 stainless steel and Grade 91. Accurate knowledge of the thermal properties of the material is important to design efficient systems and to allow calculation of thermally induced stresses and stress gradients. Thermal diffusivity, thermal conductivity, specific heat capacity, and coefficients of thermal expansion have been determined for Alloy 709 as a function of temperature in the range of 20–850°C. The results do not vary significantly for the three heats of Alloy 709 tested, and compare well with those reported for other high Cr, high Ni austenitic stainless steels. Deviations from monotonic behavior have been observed for the specific heat capacity and to a lesser degree for the thermal conductivity, while the thermal expansion and thermal diffusivity increase nearly linearly with increasing temperature.

Improvement of mechanical properties on metastable stainless steels by reversion heat treatments

AISI 301LN is a metastable austenitic stainless steel that offers an excellent combination of high strength and ductility. This stainless grade is currently used in applications where severe forming operations are required, such as automotive bodies. When these metastable steels are plastically deformed at room temperature, for example by cold rolling, austenite transforms to martensite and, as a result, yield strength increases but ductility is reduced. Grain refinement is the only method that allows improving strength and ductility simultaneously. Several researchers have demonstrated that fine grain AISI 301LN can be obtained by heat treatment after cold rolling. This heat treatment is called reversion because it provokes the reversion of strain induced martensite to austenite. In the present work, sheets of AISI 301LN previously subjected to 20% of cold rolling reduction were treated and a refined grain austenitic microstructure was obtained. Mechanical properties, including fatigue limit, were determined and compared with those corresponding to the steel both before and after the cold rolling.

Researches about Mechanical Characteristics of some Steels Resistance Spot Welded

This work studies the influence of resistance spot welding (RSW) process upon the mechanic characteristics of three steel grades: S235JR, galvanized S235JR and austenitic stainless grade 304L. The variable parameter was the welding current. Constant parameters were the pressure (6 bars) and the welding time (5 cycles). Micro hardness and tensile-shear resistance tests were done. Welded joint hardness profile reveal a relative softer nuclei surrounded by the hardest zone and a progressive decrease throughout the heat affected zone (HAZ) to the unaffected material. The shear resistance depends upon the welding intensity as well as metal grade.

Effect of pH on in vitro biocompatibility of orthodontic miniscrew implants

BackgroundAlthough the clinical use of miniscrews has been investigated on a large scale, little is known about their biocompatibility. Since low pH can affect corrosion resistance, the aim of this study was to evaluate the cytotoxic effect of orthodontic miniscrews in different pH conditions.MethodsFour orthodontic miniscrews of stainless steel and grade IV and grade V titanium were immersed in a pH 7 and pH 4 saline solution for 1, 7, 14, 21, 28, and 84 days. Human osteogenic sarcoma cells (U2OS), permanent human keratinocytes (HaCat), and primary human gingival fibroblasts (HGF) were exposed to eluates, and the mitochondrial dehydrogenase activity was measured after 24 h to assess the cytoxicity. The results were analyzed using the Mann-Whitney U test (P < 0.05).ResultsWhen exposed to pH 7-conditioned eluates, the cell lines showed an even greater viability than untreated cells. On the contrary, the results revealed a statistically significant decrease in U2OS, HaCat, and HGF viability after exposure to eluates obtained at pH 4. Among the cell lines tested, HGF showed the most significant decrease of mitochondrial activity. Interestingly, grade V titanium miniscrews caused highest toxic effects when immersed at pH 4.ConclusionsThe results suggested that at pH 7, all the miniscrews are biocompatible while the eluates obtained at pH 4 showed significant cytotoxicity response. Moreover, different cell lines can produce different responses to miniscrew eluates.

Experimental investigation of microbiologically influenced corrosion of selected steels in sugarcane juice environment

In the current study, ferritic stainless grades AISI 439 and AISI 444 were investigated as possible construction materials for machinery and equipment in the cane-sugar industry. Their performance in corrosive cane-sugar juice environment was compared with the presently used low carbon steel AISI 1010 and austenitic stainless steel AISI 304. The Tafel plot electrochemical technique was used to evaluate general corrosion performance. Microbiologically influenced corrosion (MIC) behaviour in sugarcane juice environment was studied. Four microbial colonies were isolated from the biofilms on the metal coupon surfaces on the basis of their different morphology. These were characterized as Brevibacillus parabrevis, Bacillus azotoformans, Paenibacillus lautus and Micrococcus sp. The results of SEM micrographs showed that AISI 439 and AISI 304 grades had suffered maximum localized corrosion. MIC investigations revealed that AISI 444 steel had the best corrosion resistance among the tested materials. However from the Tafel plots it was evident that AISI 1010 had the least corrosion resistance and AISI 439 the best corrosion resistance.