Hot deformation behavior of microstructural constituents in a duplex stainless steel during high-temperature straining

The hot deformation characteristics of 1.4462 duplex stainless steel （DSS） were analyzed by considering strain partitioning between austenite and ferrite constituents. The individual behavior of ferrite and austenite in microstructure was studied in an iso-stress condition. Hot compression tests were performed at temperatures of 800-1100~C and strain rates of 0.001-1 s-1. The flow stress was modeled by a hyperbolic sine constitutive equation, the corresponding constants and apparent activation energies were determined for the studied alloys. The constitutive equation and law of mixture were used to measure the contribution factor of each phase at any given strain. It is found that the contribution factor of ferrite exponentially declines as the Zener-HoUomon parameter （Z） increases. On the contrary, the austenite contribution polynomially increases with the increase of Z. At low Z values below 2.6. x 1015 （lnZ---35.5）, a negative contribution factor is determined for austenite that is attributed to dynamic recrystallization. At high Z values, the contribution factor of austenite is about two orders of magnitude greater than that of ferrite, and therefore, austenite can accommodate more strain. Microstructural characterization via electron back-scattered diffraction （EBSD） confirms the mechanical results and shows that austenite recrystallization is possible only at high temperature and low strain rate.

Uniaxial Time-Dependent Ratcheting of SS304 Stainless Steel at High Temperatures

The uniaxial time-dependent strain cyclic behaviors and ratcheting of SS304 stainless steel were studied at high temperatures （350 ℃ and 700 ℃）. The effects of straining and stressing rates, holding time at the peak and/or valley of each cycle in addition to ambient temperature on the cyclic softening/hardening behavior and ratcheting of the material were discussed. It can be seen from experimental results that the material presents remarkable time dependence at 700 ℃, and the ratcheting strain depends greatly on the stressing rate, holding time and ambient temperature. Some significant conclusions are obtained, which are useful to build a constitutive model describiog the time-dependent cyclic deformation of the material.

Strain rate and cold rolling dependence of tensile strength and ductility in high nitrogen nickel-free austenitic stainless steel

The tensile strength and ductility of a high nitrogen nickel-free austenitic stainless steel with solution and cold rolling treatment were investigated by performing tensile tests at different strain rates and at room temperature. The tensile tests demonstrated that this steel exhibits a significant strain rate and cold rolling dependence of the tensile strength and ductility.With the increase of the strain rate from 10^-4s^-1to 1 s^-1, the yield strength and ultimate tensile strength increase and the uniform elongation and total elongation decrease. The analysis of the double logarithmic stress–strain curves showed that this steel exhibits a two-stage strain hardening behavior, which can be well examined and analyzed by using the Ludwigson equation. The strain hardening exponents at low and high strain regions（n2and n1） and the transition strain（εL） decrease with increasing strain rate and the increase of cold rolling RA. Based on the analysis results of the stress–strain curves, the transmission electron microscopy characterization of the microstructure and the scanning electron microscopy observation of the deformation surfaces, the significant strain rate and cold rolling dependence of the strength and ductility of this steel were discussed and connected with the variation in the work hardening and dislocation activity with strain rate and cold rolling.

High temperature oxidation resistance of plasma sprayed NiCrAl+(ZrO_2+Y_2O_3) gradated coating on stainless steel surface

The plasma sprayed gradated coating with the bottom layer of NiCrAl and the top layer of(ZrO2+Y2O3) was prepared by the plasma spraying technique. The phase structure and morphology of the gradated coating were analyzed by means of X-ray diffractometry(XRD) and scanning electron microscopy(SEM). The high temperature oxidation resistance of the plasma sprayed samples at 800 ℃ was investigated. The oxidation kinetics curve was obtained. The results show that the thickness of gradated coating is about 410 μm. The plasma sprayed gradated coating on stainless steel surface can improve the high temperature oxidation resistance of stainless steel. The oxidation rate of stainless steel is less than that of plasma sprayed gradated coating. The oxidation film of plasma sprayed sample is very dense and is not easily exfoliated. The dense oxidation film prevents the stainless steel from more oxidation.

High-temperature oxidation behavior of heat resistant stainless steel 1.4828

The kinetic curves of the high-temperature oxidation of austenitic heat resistant stainless steel 1. 4828 at 1 050 ℃ were measured using a weighing method. It is shown that the oxidation curves at 1 050 ℃ followed the parabolic line law, and after 250 h of oxidation, the mass gain was about 80 g/m2. The surface morphology and structure of the oxide layers were studied by scanning electron microscopy and X-ray diffraction. A complicated oxide layer obtained at 1 050 ℃ was mainly composed, from inner to outer, of （FeSi） 3 04, Cr2 03, Fe2 03, and spinel oxides FeCr204 and NiMn204.

Effects of tungsten on the oxidation resistance and high-temperature strength of 18CrNbTi ferritic stainless steel

18CrNbTi ferritic stainless steel is a low-cost material mainly used for the fabrication of manifolds, which usually work at temperatures below 950℃. With the development of engine technology, exhaust manifolds tend to work above 1 000 ℃ and this may be even higher in the future. For developing a new kind of steel to satisfy these requirements,the effects of tungsten （W）addition on the high-temperature strength and oxidation resistance of 18CrNbTi ferritic stainless steel are discussed in this study. The test results show that W enhances high-temperature strength at 1 000 ℃ and significantly improves oxidation resistance. However, W addition tends to degrade oxide layer adhesion,causing spalling during alternate hot and cold conditions.

Fabrication of high nitrogen austenitic stainless steels with excellent mechanical and pitting corrosion properties

A series of high nitrogen austenitic stainless steels were successfully developed with a pressurized electroslag remelting furnace. Nitride additives and deoxidizer were packed into the stainless steel pipes, and then the stainless steel pipes were welded on the surface of an electrode with low nitrogen content to prepare a compound electrode. Using Si3N4 as a nitrogen alloying source, the silicon contents in the ingots were prone to be out of the specification range, the electric current fluctuated greatly and the surface qualities of the ingots were poor. The surface qualities of the ingots were improved with FeCrN as a nitrogen alloying source. The sound and compact macrostructure ingot with the maximum nitrogen content of 1.21wt% can be obtained. The 18Cr18Mn2Mo0.9N high nitrogen austenitic stainless steel exhibits high strength and good ductility at room temperature. The steel shows typical ductile-brittle transition behavior and excellent pitting corrosion resistance properties.

MAGNETIC PROPERTY CHANGE IN AN AUSTENITIC STAINLESS STEEL SUBJECTED TO DAMAGE AT ELEVATED TEMPERATURE-MICROSTRUCTURE RESPONSIBLE FOR MAGNETIC PROPERTY

It has been found that magnetic property changes in austenitic stainless steel subjected to creep at high temperature. The change of magnetic property is mainly due to decrease the chromium concentration in the vicinity of grain boundary and transform into martensite in the area. However this result is for short-term creep. It is necessary to evaluate the long-term creep in order to develop non-destructive technique for plants. Moreover it is important to evaluate the fatigue. The change of magnetic property for those damages at high temperatures is investigated. The transformation into martensite is observed for both the long-term creep and fatigue. The magnetic regions are observed in sever deformed area and near crack. Then the formation of magnetic phases is related to the damage. The damage at high temperature can be universally evaluated.

Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels

Pitting corrosion and crevice corrosion behaviors of high nitrogen austenitic stainless steels （HNSS） were investigated by electrochemical and immersion testing methods in chloride solution, respectively. The chemical constitution and composition in the depth of passive films formed on HNSS were analyzed by X-ray photoelectron spectrum （XPS）. HNSS has excellent pitting and crevice corrosion resistance compared to 316L stainless steel. With increasing the nitrogen content in steels, pitting potentials and critical pitting temperature （CPT） increase, and the maximum, average pit depths and average weight loss decrease. The CPT of HNSS is correlated with the alloying element content through the measure of alloying for resistance to corrosion （MARC）. The MARC can be expressed as an equation of CPT=2.55MARC-29. XPS results show that HNSS exhibiting excellent corrosion resistance is attributed to the enrichment of nitrogen on the surface of passive films, which forms ammonium ions increasing the local pH value and facilitating repassivation, and the synergistic effects of molybdenum and nitrogen.

High Temperature Corrosion of Water Wall Materials T23 and T24 in Simulated Furnace Atmospheres

Candidate materials for water wall of supercritical and ultra-supercritical utility boilers,T23 and T24,were chosen as the experimental samples and exposed to oxidizing atmosphere,reducing atmosphere and oxidizing/reducing alternating atmosphere separately.The corrosion temperature was 450-550?C.The effects of oxygen con-tent and temperature on the corrosion in reducing atmosphere and alternating atmosphere were investigated.The scanning electron microscope(SEM) and energy dispersive spectrometer(EDS) were used to examine the corroded samples.The results show that the corrosion kinetics of T23 and T24 can be described by the double logarithmic equation and parabolic equation respectively.To describe the corrosion of materials accurately it is not sufficient to analyze the macro-mass gain and the macro-thickness of the corroded layer only,but the EDS should be applied to examine the migration depth of corrosive elements O and S.It is revealed that the corrosion becomes more severe when H2S is present in the corrosive gas.S is more active than O,and Cr can reduce the migration of oxygen but not S.The combination corrosion of S and O and pure [S] has a stronger corrodibility than pure H2S.T24 suffers the most severe corrosion at oxygen content of 0.8%.Corrosion is aggravated when the corrosion temperature is above 450 ℃ in the alternating atmosphere.T23 has better corrosion resistance than T24 and W contributes a lot to the corrosion resistance of T23.

High performance nano-structured stainless steel sheet

High-strength steels have been attracting more and more attention of people,Unfortunately.deterioration of ductility limited their applications.To solve this problem,a nano-structured stainless steel sheet is developed to combine high strength and high ductility.Processing of the surface mechanical attrition treatment(SMAT) was introduced to obtain a nano-grain layer on the double surface of the stainless steel sheet.The microstructure of the nanostructured steel sheet is characterized by an alternate distribution of coarse grained layer and nanocrystalline layer.Then the dual surface nano-crystallized stainless steel sheets were co-warm rolled at 500℃.The experimental results reveal that the mechanical properties of the nanostructured steel exhibit high yield strength in the range of 700 -950 MPa and tensi le strength higher than 930 MPa.Moreover,elongation to fracture reaches to 15%-48%, together with a uniform elongation stabilized to 13%-45%.

Investigation and Analysis of High Temperature Corrosion and Degradation of Marine Boiler Combustion Swirler

The present paper investigated and analyzed swirler material consisting of mild steel which was subjected to service for the period of one year in a 30 MW marine boiler. Due to the presence of high temperatures in the furnace coupled with the corrosive marine environment swirler material showed accelerated degradation and material wastage. An investigation into the feasibility of manufacturing the existing swirler with an alternate material or coating the swirler material with a thermal barrier coating was undertaken. Based on their properties and performance, SS 304 and SS 316 were proposed as the replacement materials for the swirler. The other alternative of coating the existing swirlers with a form thermal barrier coating to observe for any improvement in their performance at elevated temperatures was also tested. Stellite, which is a Ni-Co based coating, was carried out on the MS samples and the same were exposed to same temperatures mentioned above. The performance of the available options was evaluated with respect to the grain structure of the material, the hardness value of the materials and deterioration at elevated temperatures. Investigation showed the proposed materials/ coatings like SS 304, SS 316 and Stellite coating revealed that SS 316 is the material best suited for high temperature application.

Key role of niobium stabilization in ultra-pure ferritic stainless steels for construction and home-appliance applications

The beneficial effects of niobium addition on properties such as high-temperature strength, toughness, and formability of ferritic stainless steels have been addressed. Based on the Thermo-Calc analysis, precipitation of niobium carbonitride and solubility of niobium have been predicted and characterized via scanning electron microscope （SEM） and transmission electron microscope （TEM） observations. It is shown that addition of niobium has a beneficial effect on improving the high-temperature strength, toughness, formability, and corrosion resistance of ferritic stainless steel. Soluted niobium is very effective in improving the high-temperature strength, which is beneficial to reducing the sticking propensity during hot rolling. Although niobium increases the recrystallization temperature, niobium-added ferritic stainless steels show a high mean r value, or a high plastic strain ratio, as long as the annealing temperature is high enough. Furthermore, because niobium helps to inhibit the formation of chromium carbides, ferritic stainless steel can keep an effective chromium content in the matrix, leading to improved corrosion resistance. Applications of these ferritic stainless steels for construction and home appliances have also been presented.

Development of heat-resistant ferritic stainless steels for exhaust lines at Nisshin Steel

Exhaust emission regulations of the automotive are enforced in each country to prevent air pollution and global warming,and the restriction standard tends to become severer.Various techniques such as the combustion improvement of gasoline,upgrades of the catalyst,and the thermal capacity decreases in the exhaust lines are adopted to suit the regulations,and these lead to an increase of the maximum temperature of the exhaust gas. Recently,ferritic stainless steels are mainly used to parts of exhaust lines,as their thermal expansion coefficient is small,and the cyclic oxidation resistance and the thermal fatigue property are better than austenitic stainless steels. This paper presents newly developed heat-resistant stainless steels from Nisshin Steel for exhaust lines usage,and describes the currents of the steel development that could be envisaged in the future.With regard to improving the high-temperature strength of ferritic stainless steels,the addition of Nb,Mo and Cu is effective in solution hardening and precipitation hardening at 700℃,while the addition of Nb,Mo and W is effective in mainly solution hardening at 900℃.The addition of Cr,Si and Mn suppress the breakaway oxidation in air at 950℃up to 200 h of ferritic stainless steels containing 14%Cr.Especially,the addition of 0.8%or higher Mn would effectively improve the adherence of oxide scale.It is confirmed that ferritic stainless steels,NSSHR-1(14Cr-lMn-0.9Si-Nb) and NSSHR-2(10Cr-0.9Si-Nb-Ti ),is having a superior heat resistance,formability and cost performance compared to conventional Type441 and Type439 respectively.

Synergic Effect of Shot Blasting and Deposit RE on Corrosion Behavior of TP304H steel in SO_2-O_2 at 923K

The corrosion behavior of TP304H steel with combined treatment of shot blasting and electrophoresis deposited RE coating in l%SO2+14%O2+85%Ar mixture gaseous has been studied at 923K for 150 hours. The results show that both RE coating and shot blasting can improve the corrosion resistance of TP304H steel, and combined treatment has the best effect. Combined treatment reduces 50% mass gain, enhances the continuous and compactness of scale and changes the scale phase forms from FeaOs/Fe^S/Ni^/S to Cr2O3/FeCr2S4/Ni3_xS2. The synergistic effects of combined treatment on corrosion resistance and the effect of shot blasting on corrosion kinetics are discussed. Shot blasting increases the outward diffusion and surface concentration of Cr, Ce coating promotes the selective oxidation of Cr.

Microstructure and properties of composite of stainless steel and partially stabilized zirconia

To fabricate the metal-ceramics multi-layer hollow functionally gradient materials(FGMs) that might meet the requirement of repeated service and long working time of high temperature burners, such as spacecraft engine, the microstructure and properties of composite of stainless steel and partially stabilized zirconia were investigated. Samples of different proportions of stainless steel to partially yttria-stabilized zirconia were fabricated by powder extrusion and sintering method. Shrinkage, relative density, microstructure, micro-Vickers hardness, compression strength, bending strength, fractography morphology and electrical resistivity of sintered samples with different proportions of stainless steel were measured. The results show that threshold of metallic matrix composite(MMC) is approximately equal to 60%(volume fraction) stainless steel. The samples with 0 to 50%(volume fraction) stainless steel indicate ceramic brittleness and non-cutability, and the samples with 70% to 100%(volume fraction) stainless steel indicate metallic plasticity and cutability.

Effect of grain boundary characteristic distribution on brittle cracking of ferritic stainless steel

In order to better understand the relation between grain boundary characteristic distribution （GBCD） and the brittle cracking of ferritic stainless steel, the GBCD, impact test and bend test were investigated using scanning electron microscopy （SEM） and the electron backscatter diffraction （EBSD） technique. The results show that a crack occurs preferentially at high angle boundaries, and that low angle and low-∑ coincidence site lattice（CSL） boundaries can offer resistance to the propagation of cracks. It is suggested that an optimum GBCD, i.e. a high frequency of low angle or low-∑ CSL boundaries and discontinuous high angle boundaries network can offer the potential for decreasing the ductile-to-brittle transition temoerature （DBTT） of ferritic stainless steels.

Effect of Preheating Temperature on Surface Cracking of High Nitrogen CrMn Austenitic Stainless Steel

18Mn18Cr0.5N steel specimens were preheated at the temperatures from 1100 to 1250°C for 5 min,and then cooled to 950°C and compressed.The cracking behaviors were investigated using optical microscopy and scanning electron microscopy.The results showed that the hot workability of 18Mn18Cr0.5N steel gradually decreased with increasing preheating temperature between 1100 and 1200°C,and quickly deteriorated up to 1250°C.Above 1200°C,delta ferrite particles appeared in 18Mn18Cr0.5N steel,promoted cavity coalescence on grain boundary,and accelerated surface crack formation during the hot working process.

Effect of Si on high temperature oxidation of 30Cr13 stainless steel

The effect of Si on the high temperature oxidation behavior of the 30Cr13 martensitic stainless steels was investigated. The underlying mechanism was further discussed by oxidation kinetics curves, scanning electron microscopy （SEM） and X-ray diffraction （XRD） analysis. It was ob- served that the addition of Si can significantly reduce the mass gain per unit area and the oxidation rate of the experimental steels. With the increase of Si content, the film surface composed of the wheat-like oxides turned into the small granular oxides after oxidation for 120 h. The SiO_2 film formed on the metal substrate inhibited the outward diffusion of the metal cation and the inward diffusion of the oxygen anion; thus, the high temperature oxidation resistance was enhanced.