Heat Resistant Austenitic Stainless Steel Research Articles

Understanding the high-temperature oxidation resistance of heat-resistant austenitic stainless steel with gradient nanostructure

The underlying high-temperature oxidation mechanisms of the heat-resistant austenitic stainless steel with gradient nanostructured surface layer is revealed through systematic analysis of the microstructure and composition. Nanoscale oxide grains and high-density grain boundaries promoted the formation of pronounced spinel oxides, which suppressed elemental diffusion and CrO3 volatilization. High level of residual stress in the GNS layer facilitated the formation of high-density precipitates at the oxide/matrix interface and grain boundaries, which hindered the growth of oxide scale and reactive elements diffusion. The enhanced high-temperature oxidation resistance resulted from the synergistic combination of spinel oxides, precipitates, and high-density grain boundaries.

Solidification Characteristics and Eutectic Precipitates in 15Cr–22Ni–1Nb Austenitic Heat-Resistant Stainless Steel

Solidification Characteristics and Eutectic Precipitates in 15Cr–22Ni–1Nb Austenitic Heat-Resistant Stainless Steel

Temperature-frequency wear mechanism maps for a heat-resistant austenitic stainless steel

A systematic experimental investigation is used to establish the influence of temperature and cycling frequency on the friction and wear behavior of the high-Cr, high-Ni heat-resistant austenitic stainless steel 310S (25%Cr–20%Ni) under gross slip fretting wear. The contact configuration is cylinder-on-flat using a like-on-like tribopair. Temperatures range from 20 °C to 700 °C and cycling frequencies range from 1 Hz to 50 Hz. A workflow to generate the wear maps with minimal number of tests, namely hierarchical probing and sampling strategy, is used to clearly establish the severe-to-mild wear transition associated with glaze layer formation. The critical temperatures related to this transition along with the number of cycles needed to establish effective glaze layer protection are obtained. A recent model reported in the literature that determines the number of cycles needed to establish effective glaze layer protection at different temperatures and frequencies gives good prediction if the temperature rise at the contact surface due to Coulomb heating is accounted for.

Effect of Al<sub>2</sub>O<sub>3</sub> on Viscosity and Refining Ability of High Basicity Slag for Heat-resistant Austenitic Stainless Steel

To optimize Al2O3 content in high basicity CaO-SiO2-Al2O3-8%MgO-8%CaF2 slag (%CaO/%SiO2=6) used for the refining of heat-resistant austenitic stainless steel, the effect of Al2O3 on the viscosity and refining ability of the slag was systematically investigated, and the relevant mechanism was clarified based on the analyses of solid phase precipitation behaviour and ionic unit structure transformation. The results demonstrated that the slag viscosity dramatically decreased and marginally varied in the Al2O3 ranges of 10%–20% and 20%–30%, respectively. At 1833–1873 K, both the slag viscosity and the activation energy reached the lowest values at 25% Al2O3. Al2O3 addition not only significantly inhibited the precipitation of CaO phase but also enhanced the polymerization degrees of aluminate and silicate networks. Furtherly, the inhibiting of Al2O3 on the solid phase precipitation dominated the viscosity reduction at 10%–25% Al2O3, while the enhancing of Al2O3 on the polymerization degree of networks led to the viscosity increase with further increasing Al2O3 to 30%. Finally, we optimized the reasonable high basicity slag containing 25% Al2O3 for Sanicro 25 steel due to the lowest viscosity and optimum refining ability of this slag.

Precipitate Evolution in 22Cr25NiWCuCo(Nb) Austenitic Heat-Resistant Stainless Steel during Heat Treatment at 1200 °C.

In this study, 22Cr25NiWCuCo(Nb) heat-resistant steel specimens with high Cr and Ni contents were adopted to investigate the effect of Nb content on thermal and precipitation behavior. Differential scanning calorimetry profiles revealed that the melting point of the 22Cr25NiWCuCo(Nb) steel specimens decreased slightly with the Nb content. After heat treatment at 1200 °C for 2 h, the precipitates dissolved in a Nb-free steel matrix. In addition, the Z phase (CrNb(C, N)) and MX (Nb(C, N), (Cr, Fe)(C, N), and NbC) could be observed in the Nb-containing steel specimens. The amount and volume fraction of the precipitates increased with the Nb content, and the precipitates were distributed heterogeneously along the grain boundary and inside the grain. Even when the heat treatment duration was extended to 6 h, the austenitic grain size and precipitates became coarser; the volume fraction of the precipitates also increased at 1200 °C. The Z phase, rather than the MX phase, became the dominant precipitates at this temperature.

Microstructural Investigation of the Magnetic Behavior of a Modified HP-NbTi Heat-Resistant Cast Austenitic Stainless Steel

Microstructural Investigation of the Magnetic Behavior of a Modified HP-NbTi Heat-Resistant Cast Austenitic Stainless Steel

Evolution of the microstructure and mechanical properties of HR3C austenitic stainless steel after ageing for up to 30,000 h at 650–750 °C

After long-term ageing of heat resistant austenitic stainless HR3C steel at 650, 700, and 750 °C for up to 30,000 h, microstructural investigations, identification of precipitates, and testing of mechanical properties were conducted. In the as-received condition, the steel had austenitic microstructures with annealed twins and numerous, large primary NbX and also Z-phase precipitates. Long-term ageing of the steel mainly resulted in numerous precipitation processes: at the grain boundaries, M23C6 carbides and σ phases occur, whereas inside the grains, Z-phase precipitates, M23C6 carbides, and σ phases are observed. The amount and size of σ phases in the structure of the HR3C steel depend on the time and temperature of ageing. The precipitation processes and coarsening decreased the plastic properties and impact energy and resulted in alloy overageing, and the yield strength and tensile strength after 30,000 h ageing were similar to the properties of the as-received steel. In the steel, σ phase precipitates had only a small effect on the toughness, and the dominant impact was shown by M23C6 carbides.

Investigations on drilling performance of high resistant austenitic stainless steel

The aim of the investigations presented in this research work is to optimize the cutting geometry for drilling of Heat Resistant Austenitic Stainless Steel (HRASS) based on the tool life and strain hardening generated on the work material during cutting process. In particular, an attempt has been made to understand the effect of cutting edge preparation on both the cutting forces and the strain hardening of the work material. Besides, chips in flutes are also analyzed by tomography technique to better understand their evacuation in-situ in correlation with the cutting forces. A Quick-Stop Device (QSD) in drilling is also employed to analyze the influence of the cutting edge radius, the coating material, and the cutting fluid nature on the strain hardened layer using micro hardness filiations.

Investigations on Strain Hardening During Cutting of Heat-Resistant Austenitic Stainless Steel

Abstract This study presents a novel analysis of the machined subsurface layer formation dealing with strain hardening phenomenon which results from complex mechanisms due to cutting edge multiple passes in drilling. On the one hand, the hardened layer during drilling is characterized in relation with the local cutting geometry and thanks to a quick-stop device (QSD) to suddenly interrupt the operation. Micro hardness is used to determine the hardened thickness of the machined subsurface layers along the local cutting edge geometry. On the other hand, orthogonal cutting performed with a complex self-designed planing experiment is used to investigate in details the hardening accumulation aspects. Then, dedicated methodologies are proposed to quantify the strain hardening as well as the incremental plastic strain generated by consecutive tool passes. In addition to the subsurface hardness evolution, the work material strain is observed during the steady-state cutting process thanks to the high-speed camera. The digital image correlation technique is exploited to analyze not only the plastic strain remaining on the workpiece after the cut but also the effect of the incremental plastic strain generated by the consecutive planing passes as the cutting edges in drilling do. One of the outcomes is that the hardened layer thickness can reach from two to three times the cut thickness in drilling or in planing. As a consequence, this work demonstrates that the cutting process affects itself by hardening. Thus, the studied austenitic stainless steel in such a way that this last is never cut in its initial state.

The Effect of Service on Microstructure and Mechanical Properties of HR3C Heat-Resistant Austenitic Stainless Steel.

The physical metallurgical tests were performed on the test samples made of HR3C steel, taken from a section of a pipeline in the as-received condition and after approximately 26,000 h of service at 550 °C. In the as-received condition, the test material had austenitic microstructure with numerous large primary Z-phase precipitates inside the grains. The service of the test steel mainly contributed to the precipitation processes inside the grains and at the grain boundaries. After service, the following precipitates were identified in the microstructure of the test steel: Z-phase (NbCrN) and M23C6 carbides. The Z-phase precipitates were observed inside the grains, whereas M23C6 carbides - at the boundaries where they formed the so-called continuous grid. The service of the test steel contributed to the growth of the strength properties, determined both at room and elevated temperature (550, 600 °C), compared to the as-received condition. Moreover, the creep properties of HR3C steel after service were higher than those of the material in the as-received condition. The increase in the strength properties and creep resistance was connected with the growth of strengthening of the test steel by the precipitation of Z-phase and M23C6 carbides.

Application of Cold Metal Transfer Welding for High Pressure Die Casting Mold Restoration

This paper presents results of the research focused on the possibility of the restoration of the shape parts of molds made of X15CrNiSi20-12 (EN 100 95) heat-resistant austenitic chromium-nickel stainless steel working in high-pressure die casting of aluminum alloys by clad welding. There were tested two welding wires—E Ni 6625 and E 18 8 Mn B 2 2—deposited on X15CrNiSi20-12 (EN 100 95) tool steel using cold metal transfer (CMT) welding in a protective atmosphere of Ar. The resistance of welds was tested against dissolution in molten aluminum alloy ENAC-AlSi9 and the testing procedure was designed. The resistance of welds against dissolution were assessed by exposition of welded clads in an aluminum melt for 120 and 300 min. The EDX semi-quantitative microanalyses of element distribution were performed at the welding–melt interface, and build-ups were also observed on the surface of welded clads.

Mechanical analysis of local cutting forces and transient state when drilling of heat-resistant austenitic stainless steel

In the present research work, mechanical aspect of transient state in drilling operation is investigated by analyzing together the cutting forces and the chip formation. Particularly, a sudden peak occurring on cutting forces during transient state is observed and deeply studied. To do so, new experimental methodologies to analyze local cutting forces when drilling heat-resistant austenitic stainless steel are introduced. The chisel edge and the cutting edge of each studied drill are numerically discretized as series of elementary cutting edges. Therefore, local cutting force evolution is correlated with the local cutting geometry variation along the cutting edge. On one hand, results have shown that for high ductile materials, local cutting geometry affects deeply not only the chip shape but also the chip flow direction. This strongly disrupts the monotony of the cutting forces evolution in transient state and lead to a sudden peak occurrence on cutting forces. On the other hand, linear local cutting forces could be determined by numerical decomposition of the global cutting forces measured during the drill tip engagement.

Comparative Study on the Oxidation Behavior of Austenitic and Ferritic Heat-Resistant Stainless Steels at High Temperatures

A comparative study of the high-temperature oxidation behavior and mechanism of 0Cr25Ni20 austenitic heat-resistant stainless steel (AHSS) and 0Cr18AlSi ferritic heat-resistant stainless steel (FHSS) at 800°C, 900°C, and 1000°C in air up to 140 h was performed using isothermal oxidation tests. The oxidation kinetics of 0Cr25Ni20 AHSS and 0Cr18AlSi FHSS followed the parabolic law. The oxide films on 0Cr25Ni20 AHSS were composed of continuous and dense Cr2O3, MnCr2O4, and a small amount of NiMn2O4, whereas silicon exhibited internal oxidation and deteriorated the adhesion between the oxide film and substrate. Nickel-free 0Cr18AlSi FHSS exhibited good oxidation resistance at 800°C and 900°C due to dense, continuous, and well-adhered multicomponent oxide films containing Al2O3, Cr2O3, MnCr2O4, and a small amount of MnFe2O4. The oxidation resistance of 0Cr18AlSi FHSS declined at 1000°C, mainly due to the formation of nonprotective Fe2O3 and severe internal oxidation of aluminum.

Influence of solidification induced composition gradients on carbide precipitation in FeNiCr heat resistant steels

Secondary precipitation of Cr-rich carbides in heat resistant austenitic stainless steels has been investigated both experimentally and using finite element simulations. The microstructural evolutions in two commercial grades were characterized using electron microscopy. A special emphasis was given on the peculiar spatial distribution of M23C6 secondary carbides exhibiting precipitate free zones surrounding primary carbides, and high density precipitate zones extending with longer aging times. Solidification induced chemical composition gradients were clearly exhibited in the as-cast alloys with significant chromium depletion in the vicinity of primary M7C3 carbides. It is then proposed that these gradients play a major role in the secondary precipitation mechanism. Classical nucleation and growth theories have been adapted to account for (i) the flux of solutes with large difference in diffusion coefficients, (ii) the initial composition gradients in the matrix and, (iii) the chemical driving force for nucleation and growth of M23C6 carbides. Within this framework, the whole kinetics has been reproduced. It clearly shows that the spatial distribution of secondary carbides that play a key role in the creep resistance of these alloys is the result of a complex interaction between initial composition gradients in as-cast alloys and solute flux resulting from phase transformation during aging.

Precipitate Evolution in a Modified 25Cr-20Ni Austenitic Heat Resistant Stainless Steel During CreepRupture Test at 750 ℃

Precipitate Evolution in a Modified 25Cr-20Ni Austenitic Heat Resistant Stainless Steel During CreepRupture Test at 750 ℃

Fracture and Damage Behavior in an Advanced Heat Resistant Austenitic Stainless Steel During LCF, TMF and CF

Future advanced ultra-supercritical power plant will be run at higher temperature and pressure. New materials will be used to meet the requirements. However, the structure integrity of these materials needs to be evaluated. Sanicro 25 is a newly developed advanced austenitic heat resistant stainless steel with the aim to be used in future 700 °C or 650 °C power plants to replace part of Ni based alloys. This paper provides an overview on the fracture and damage behavior in this material during LCF, TMF and CF. The cyclic hardening and fatigue life during LCF, TMF and CF will be discussed. The influence of prolonged service degradation has been analyzed by the use of pre-aged material for TMF and CF loading conditions.

Mechanical study in drilling of heat resistant austenitic stainless steel

Machining of difficult-to-cut-materials like heat resistant stainless steels leads to the rapid tool wear and tool failure. The analysis of drilling process of these steels is even more difficult as it is not easy to investigate the cutting process at the drill tip. Therefore for this critical operation, modelling of cutting forces is very important. The study of the mechanical loading is based on the cutting tool geometry variation along the cutting edge. This study aims to investigate the drilling of heat resistant stainless steels with two geometries of twist drills, by analyzing specific cutting energy and forces at a global and a local scale.

Dissolution of Grain Boundary Carbides by the Effect of Solution Annealing Heat Treatment and Aging Treatment on Heat-Resistant Cast Steel HK30

Decreasing the weight of heavy-duty vehicles is an ongoing concern. However, the need to deal with high temperatures in components such as manifolds imposes, by itself, some restrictions regarding material selection, being further limited when other required properties (e.g., functional, manufacturing or cost requirements) are taken into account. Cast austenitic stainless steels may represent a good choice in this context but the existence of concentrated chromium carbides can generate undesirable results. A good combination of heat treatments can be applied to cast heat-resistant austenitic stainless steels, in an effort to achieve the dispersion of fine carbides, consequently improving their microstructure, mechanical properties and creep resistance. In this work, an austenitic stainless steel usually used in high temperature applications was characterized and subjected to solution annealing and aging heat treatments. The material analyzed was the austenitic cast stainless steel HK30 and the goals of the work were to evaluate the effects of solution annealing heat treatments on the dissolution of grain boundary chromium carbides and the effects of aging treatments on creep resistance. The results show that the elimination of grain boundary chromium carbides is possible by applying a solution annealing heat treatment. Additionally, the precipitation of fine dispersed carbides is obtained after the aging treatment with an increase of hardness and, consequently, an expected improvement of creep resistance. Thus, the novelty presented by this work consists of selecting the best heat treatment combination in order to promote dispersion of carbides, thus avoiding further crack nucleation phenomena when parts are cyclically subjected to load and unload; this work also found the most adequate mechanical properties and achieved corrosion resistance regarding the application in heavy-duty vehicle components subjected to mechanical and thermal fatigue. By discovering methods of improving the properties of cast materials, large savings can be made both in terms of production costs as well as in the overall weight of the components.

Studies on Hot Cracking Susceptibility and Establishment of Welding Procedure of Austenitic Stainless Steel Grade UNS S31035 for Power Plant Application

Austenitic stainless steel Sandvik Sanicro 25 (UNS S31035) was developed in European Advanced ultra super critical boiler program called Thermie-AD700 project. Compared to all other commercial heat resistant austenitic stainless steels, Sanicro 25 has the highest creep strength. Hence, this material has been identified as one of the candidate materials in reheater and super heater lines of advanced ultra super critical boiler for up to 700 °C temperature usage. In this austenitic stainless steel, along with Fe, Ni and Cr, solid solution hardening element, like W, Co & N and precipitation hardening elements like Nb and Cu are added to enhance the creep and high temperature oxidation resistance. Addition of more alloying elements and full austenitic microstructure may lead to the possibility of solidification cracking susceptibility during welding. Hence, investigation on the weldability of this material becomes inevitable to establish the welding procedure. In this paper, solidification cracking behaviour of UNS S31035 has been studied with varestraint test. From the test results, it is concluded that solidification cracking susceptibility is higher in this steel and similar composition filler cannot be used for welding. Based on this conclusion, Ni based super alloy filler ER NiCrCoMo-1 is used for establishing welding procedure in gas tungsten arc welding process. Test results are presented and discussed in this paper.

Evolution of intergranular corrosion resistance for HR3C heat-resistant austenitic stainless steel at elevated temperature

ABSTRACTAn attempt has been made to investigate the evolution of intergranular corrosion (IGC) resistance of HR3C heat-resistant austenitic stainless steel. The double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests were conducted to evaluate the IGC resistance of HR3C steel. The results show that the side peaks can be observed in active–passive region of DL-EPR curves due to inhomogeneous distribution of the chromium dissolved in matrix. The variation of the degree of sensitisation (DOS) during sensitising at 800°C can be divided into three regions (sensitisation dominant region, desensitisation dominant region and equilibrium region). When HR3C steel is sensitised at 800°C for over 24 h, the equilibrium state in DOS can be reached. Simultaneously, the IGC morphologies after DL-EPR tests can correlate well with the DOS. On the basis of the equilibrium state in DOS, proper heat treatments and corrosion protection measures can be taken to avoid IGC of HR3C steel.