In this article, we will discuss ferritic stainless steel for services temperatures up to 1050 ◦C utilization intermetallic.
Extensive thermodynamic simulations have been used to design a new Nb-Ti ferritic stainless steels have excellent creep resistance at 1050°C, which is characterized by the leaves phase volume fraction was stabilized with Nb, Si, and Mo alloying elements.
With improvement the phase dissolution temperature, which also corresponds to the onset of rapid grain growth, Steel maintains its mechanical properties better at higher service temperatures. Lewis Stage Precipitates can also improve creep resistance by strengthening precipitation and grain boundaries Pinning depends on the dominant creep mechanism.
High temperature drop test is designed Compared to other common ferritic stainless steels, the results for the steel are significantly better. Current high-temperature applications Keywords: simulation, phase diagram, ferroalloys, intermetallic compounds, phase transformation.
Stainless steel is used in many different automotive parts due to its excellent corrosion and corrosion resistance. Heat resistance, good appearance, and mechanical properties. Use of stainless steel in cars Production began with decorative ornaments but later expanded to more functional parts.
Such as exhaust systems, where stainless steel makes up more than half of the stainless steel For automotive applications, increasing the use of stainless steel in exhaust systems The result is the tightening of exhaust regulations and the manufacturer’s warranty, as well as the desire Reducing fuel consumption and vehicle weight is the closest to the exhaust system Engines are exposed to the highest operating temperatures and are therefore the most demanding On the properties of high chromium ferritic stainless steel at the hot end of the exhaust system It has been introduced instead of AISI 409 or 304 steel which is more commonly used.
However, these steels lack Properties that need to be used at temperatures above 850°C, i.e., closest to the engine, require materials.
It has excellent high-temperature resistance, as well as thermal fatigue resistance, oxidation resistance and the corrosion of such a component is, for example, the exhaust manifold, which is directly connected to it Motor and sensitive to temperatures up to 1050 ◦C. AISI austenitic stainless steel 304 has excellent high-temperature resistance, but poor resistance to cyclic oxidation, while ferrite AISI 409 stainless steels have poor high-temperature strength.
To replace these, Nb-Ti is stabilized 18% ferritic chrome stainless steel, AISI 441, was introduced and is currently used because compared to AISI 409, it has better high-temperature properties. AISI 441 can be used at high temperatures but still falls short of fully satisfying the needs of contemporary exhaust systems.
up to 950 C. In this study, the authors presented a new ferritic stainless steel with a chromium content of 21%, which is relatively Designed to meet these growing needs, high Nb content can be used with the help of precipitation even at 1050°C.
Potential deposits are stable at temperatures above 700°C Laves and Chi are commonly encountered in high-temperature applications. If The precipitates are small enough that these phases can increase ambient temperature and high temperature The strength of steel increases with precipitation strengthening.
In addition, such deposits may increase Creep resistance is typically found in martensitic pipeline steels. However, in ferritic stainless-steel Precipitation of intermetallic phases to improve creep resistance has not been widely used.
In this study, the authors designed a new reinforced bistable ferritic stainless steel Creep resistance compared to high chromium ferritic stainless steels used at high temperatures Application. The increase in creep resistance is achieved by using phase precipitation and by controlling the number of phases, nucleation sites, and temperature of the solution line.
The phase content in the microstructure affects the high-temperature mechanical properties of the material Steel. The fact that the phase is on the grain boundary can improve the creep resistance or within the grains, depending on the creep mechanism and phasor and grain size.
It has been observed that phase nucleation in iron-based alloys occurs first at grain boundaries. then dislocations and finally -the Fe matrix. Phasor and solution line temperature It limits the use temperature of the steel but can be controlled by carefully balancing the alloy.
elements in the phase, such as Nb, assuming no interference in the Nb core settle down Furthermore, the nucleation sites of the phases can be controlled by applying precise annealing parameters such as peak temperature and holding time. Raise the 2 mm 150 150 size bars for a simple temperature drop test Image. This is a simple test to evaluate high-temperature creep resistance.
Place horizontally in the stove and place both ends 20 mm above the floor of the stove. The distance between the brackets is 140 mm. Following the placement of the sample in the furnace the deflection of the rebar is measured in the temperature range of 800 to 1050 degrees Celsius for 20 or 100 hours.
Crawl Deformation is caused by the weight of the sample itself and is largely dependent on the size of the rod. therefore, the dimensions, furnaces, and other conditions remained the same during the testing of all samples. The active creep mechanism in the drop test can be estimated using the deformation map, where Different creep mechanisms are in the normalized stress-temperature space.
The creep mechanisms considered are power law or displacement creep and diffusion creep the mechanisms are called Coble creep and Nabarro-Herring creep. It can be estimated using classical beam theory. Then the specimen is modeled as simply supported Light.
The problem is statically defined, so the distribution of bending moments can be It is determined directly according to the balance equation. At the beginning of the test, it is accumulated the inelastic strain can be neglected and the axial normal stress distribution can be assumed to be linear.
Sample thickness of the simulated interface can be calculated as modified by the pristine model as 0.14 to 0.16 J m2 in the temperature range 400 C to 1000 C. Nucleation is assumed to occur at grain boundaries with a grain size of 50 m (5.7 ASTM). It increases the temperature of phase solutes and thus the temperature of grain growth affects the service temperature of the steel initially, and eventually.
TC-Prisma and Thermo-Calc to simulate solution line temperature, equilibrium volume Fraction, and kinetics of η phase precipitation in fixed matrix composition (0.02C-0.02N-21Cr-0.32Mn-0.2Ti) weight percent with Nb content between 0.2 and 1.0 weight percent, Si between 0.2 and 0.8 weight percent, and Mo Between 0.03% by weight and 2.0% by weight.
The simulated interface can be calculated by modifying the pristine model to 0.14 to 0.16 J/m2 in the temperature range of 400°C to 1000°C. Nucleation is assumed to occur at grain boundaries with a grain size of 50 µm (5.7 ASTM).
For simulation, according to the simulation results shown in Figure 2, the alloy with Nb the solvus temperature increases the η phase and thus increases the grain growth temperature as shown by Nebiran et al., it affects the service temperature of the steel initially, eventually. Like the effect obtained with Nb, it can also be obtained by alloying with Si, however, has decreased significantly.
According to the thermodynamic simulation, increasing Si content from 0.4 to 0.8% by weight, it increases the temperature of the η phase solution by about 30°C, while an increase of 0.4% by weight in Nb content leads to a 0.35 and 1 wt% Nb as shown in Figure 2. Ferritic stainless steels usually have a significantly larger volume Mo is more abundant than Nb and Si, for example, AISI type 444 contains 2% by weight of Mo.
thermodynamics The simulation results show that Mo alloy has no significant effect on the solution line of η phase the temperature is shown in Figure 3. Therefore, it was decided to remove Mo from the alloy. However, no Increase in the volume fraction of η phase, if the amount of η phase increases, it can be considered a Mu alloy.
It is not enough to increase the creep resistance in the microstructure. Due to the optimization of Ti in steel, the deposition of carbonitrides containing Ti at Very high temperatures during casting must be considered. Ti value should be high enough the microstructure is depleted of free C and N, so that carbonitrides containing Nb are not formed, and Nb is separated for eta phase precipitation.
The ideal amount of titanium alloy is in small amounts Precipitation in the η phase, the level of carbon and nitrogen is kept as low as possible, and the amount of Ti The carbonitride is high enough to drain all the free interstitials in the matrix. However, too many alloys The Ti content causes surface defects and is usually kept at 0.2 wt.
Annealing Microstructures Stainless Steel
According to the thermodynamic simulation results and the above criteria, the test steel the chemical composition of 0.02C-0.02N-0.8Nb-0.8Si-0.2Ti (weight percent) prepared in the laboratory 65 kg bullion The ingot is heated to 1100°C for 75 minutes, which is typical for ferrite.
Stainless steel, then hot rolled 7 passes to finish at a starting temperature of 1160°C 6mm thickness then cold rolling was done with a 30% reduction and 4.2mm thickness.
Then the cold rolled strip is pre-annealed and the final cold rolling is done with a 48% reduction. The thickness after final annealing is 2 mm to achieve the best structure of the final product.
Pre-annealing simulation with parameters using Gleeble 3800 thermomechanical simulator the heating and cooling rates correspond to typical factory-scale annealing, as shown in Figure 4. The temperature range of 950°C to 1200°C was selected for simulation before annealing. isothermal the TC-Prisma calculated precipitation map in Figure 5 shows that 1% of the eta phase is predicted.
Precipitation in about 10 seconds at a temperature between 700 and 800 degrees Celsius. settle down Significant reduction in speed at higher temperatures due to reduced propulsion power. and high surface energies of the η phase.
A comparison of TC-Prisma predictions with Gleeble the simulated heating curve is shown in Figure 4, many η phases can Precipitation is precipitated during pre-annealing and some nucleation may also occur during cooling. There are two points to consider when choosing the peak temperature before annealing.
First, the microstructure must be fully crystallized after annealing, and second, any prey of the precipitated η is thus dissolved and allowed to precipitate after recrystallization to achieve good creep. Resist. The microstructure of steel is ferritic from ambient temperature to solution line temperature, so there are no phase Changes other than precipitation occurring at any temperature.
Thermodynamic and Gleeble simulations show that the recrystallization temp, in fact, also depends on the dissolution temperature of the eta phase. This dependency may be relevant Pinning effect of η phase particles in grain boundaries or Nb solid solution and its reducing effect Regarding the grain boundary mobility, however, the present study does not cover the confirmation of this.
The pre-annealing temperature should be set high enough to allow complete and complete recrystallization or almost complete dissolution of the eta phase.
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