Chromium Martensitic Steel Research Articles

Cryogenic Machining Performance of M303 at High Cutting Speeds

The material M303 is commonly used in the fabrication of machinery, automotive components, locomotive axle housings, and injection moulds. It is a stainless martensitic chromium steel known for its high strength, wear resistance, and corrosion resistance. The primary purpose of this study is to investigate the machinability of M303 under cryogenic conditions, specifically focusing on high cutting speeds. By exploring the effects of cryogenic machining on M303, the study aims to provide insights into the performance and characteristics of this material under extreme cutting conditions. This study investigates the influence of cutting parameters on the machinability of M303 in a cryogenic environment using liquid nitrogen (LN2) and coated carbide cutting tools in a high-speed turning process. The study focuses on high cutting speeds and examines essential machinability factors, including cutting forces, surface finish, and tool life. The experimental design utilises a Taguchi L4 orthogonal array to systematically study feed rates (0.1-0.2 mm/rev), depth of cut (0.2-0.6 mm), and high cutting speeds (260-340 m/min). Notably, at a low cutting speed of 260 m/min, coupled with low feed rates and depth of cut, the study reveals the longest tool life of 48.57 mintues was achieved. This condition is characterized by a good surface finish and low cutting forces with Ra of 0.9 µm and cutting force of 100 N respectively. The predominant wear occurs on the flank face, primarily due to fracturing and chipping, especially under high combinations of cutting parameters. Conversely, gradual wear is observed under low combinations of cutting parameters, resulting in an extended tool life. In conclusion, the application of LN2 proves effective under conditions of low cutting parameters. The study suggests that the risk of fracturing the cutting tool increases at higher feed rates and depths of cut, especially when combined with elevated cutting speeds. This research provides valuable insights into optimizing the machining of M303 for enhanced efficiency and tool longevity.

Tribocorrosive Aspects of Tungsten Carbide, Silicon Nitride, and Martensitic Steel under Fretting-like Conditions

Water-based lubrication faces the common challenge of component lifetime extension which is impaired by tribocorrosion due to material surface depassivation. However, such mechanisms in a pH-neutral and low-halide electrolyte require additional understanding. A ball-on-flat configuration study of hard-phase materials in a low amplitude–high frequency sliding contact against martensitic chromium steel with contact pressures around 200 MPa is presented. Under lubrication by purified water, tungsten carbide-based metal matrix composite (MMC) with NiCr binder and silicon nitride-based ceramic (SiAlON) against DIN/EN 1.4108 steel yielded coefficients of friction above unity. Wear scar enlargement led to fretting-like conditions with adhesion becoming the fundamental wear mechanism. A tribocorrosion-induced depletion of tungsten carbide and nickel was determined for MMC. SiAlON materials suffered extreme wear under the formation of abrasive SiO2, while heat-treated DIN/EN 1.4125 steel showed lower friction and wear, but also showed signs of hydrogen embrittlement. Results from accompanying single-material corrosion experiments could not satisfactorily explain the phenomena. Including galvanic interaction and the influence of contact geometry, a new tribocorrosion model for fretting conditions is proposed. It describes an expanding anodic belt located at the inner-most crevice position of an otherwise cathodically polarized material. Low conductivity of the electrolyte is seen as a key player in this process, while the galvanic situation between two materials in contact was shown to invert when water was substituted by a wet organic phase.

Microstructure and mechanical properties of 9Cr18Mo martensitic stainless steel fabricated by strengthening-toughening treatment

Microstructure and mechanical properties of strengthening-toughening treated (1020–1060 °C argon quenching + −73 °C isothermal quenching + 140–240 °C tempering) 9Cr18Mo martensitic stainless steel (9Cr18Mo steel for short) were investigated by using Thermo-Calc software, microstructure characterization and mechanical tests. It was found that the size range of carbides in the 9Cr18Mo steel was wide and approximately 0.2–15.0 μm. Most of micron-carbides (>1 μm) and submicron-carbides (<1 μm) were located at the grain boundary and in the grain interior of original austenite, respectively; and the 9Cr18Mo steel with higher volume fraction of submicron-carbides showed a better combination of strength and impact energy. It was detected that the carbides in the 9Cr18Mo steel mainly consisted of M23C6 and M7C3 with primary element of Cr, Fe, and secondary element of Mo. There were core-shell (M7C3-M23C6) structured carbides with two morphologies of coarse irregular (>10 μm) and micron near-spherical, as well as individual micron-M7C3, micron- and submicron-M23C6 with near-spherical morphology. Quasi-equiaxed grains with body-centered cubic (bcc) structure formed via recovery and recrystallization, {112}<111> bcc twins accompanied with hexagonal ω-Fe phase at the twin boundaries, retained lath- and blocky-austenite were observed in the tempered martensitic matrix of 9Cr18Mo steel. Moreover, the twins and retained austenite were prone to decomposition at higher tempering temperature. It was indicated that the yield strength of 9Cr18Mo steel was mainly correlated with subgrain block size of tempered martensite and dispersion parameter of carbides via the analysis of statistical data, and the subgrain block contributed more to the yield strength of 9Cr18Mo steel. The M7C3-M23C6 core-shell structured carbides and {112}<111> bcc twins associated with hexagonal ω-Fe phase found in this study were also helpful to understand the correlation between microstructure and mechanical properties of other high carbon chromium martensitic steels.

Study on Intelligent Classification of Aging Heat-Resistant Materials.

High chromium martensitic heat-resistant steel is considered as a candidate material for pressure components of the next generation of incinerators of the subcritical level or above in China due to its excellent high-temperature and corrosion resistance, but in the long-term service, aging will significantly affect the service safety of materials. So, accurate identification of its aging state is important to enhance the safety of a power plant. In this paper, an automatic aging grading model of high chromium martensite heat-resistant steel based on the depth residual network is proposed according to different scales of metallographic data. A multiscale data set is constructed by image reduction to verify the accuracy of the model in identifying microstructure images of high chromium martensitic heat-resistant steel with different scales. The experimental results show that the model using multiscale data sets performs well, and then, through feature pyramid network model training, the accuracy rate is further improved, and a relatively good prediction accuracy model is obtained. The validity of the deep learning method for the classification of damage and aging of P91 steel with different scales is verified.

Surface Roughness in High Speed Turning of Alloy Steel M303 Using Carbide Tools in Dry Cutting Condition

M303 is a corrosion resistant martensitic chromium steel offering excellent toughness, corrosion, and wear resistance, characterized by improved machinability and polish ability. It is widely utilized in industries such as in mould and die making, machinery and automotive equipment, bearing housing, and tooling. Recently, this material has also been used in locomotive bearing housing. This paper presents surface roughness achieved in the turning process of M303 in dry cutting condition using coated and uncoated carbide tools. The turning parameters included a high cutting speed regime (260-340 m/min) and feed rate at 0.1-0.2 mm/rev, suitable for the finishing process. The experiment was conducted according to the Taguchi method (L18). Average surface roughness (R&lt;sub&gt;a&lt;/sub&gt;) was in the range of 0.395-1.356 μm, in which a mirror finish was achieved for certain cutting conditions that could eliminate the grinding process. Results of surface roughness were analysed using Analysis of Variance (ANOVA) for linear models and revealed that feed rate is the main significant factor contributing to surface roughness, followed by type of cutting tool, and cutting speed. These findings show that good and dition, therefore it is recommended to eliminate the acceptable R&lt;sub&gt;a&lt;/sub&gt; values for M303 turning are obtained in dry con use of flooding condition as normally practiced in the industry.

Tailoring the alloy composition for wire arc additive manufacturing utilizing metal-cored wires in the cold metal transfer process

In the present study, the application and tailoring of the alloy composition of chromium martensitic hot-work steels using metal cored wires (MCW) for wire arc additive manufacturing (WAAM) in a modified short-circuit metal transfer process is demonstrated. The nickel content was varied and the alloys were fabricated as tubular-cored wires with various powder fillings. By recording the material transfer at high speed during processing, evidence was gathered indicating the suitability of the fabricated cored wires for WAAM. Optimized process parameters were identified by taking a Design of Experiment (DoE) approach and additive manufacturing (AM) structures were fabricated from the chromium martensitic hot-work tool steel alloys. The microstructure and mechanical properties of the parts were subsequently characterized. The phase fraction of the polygonally shaped delta ferrite could be reduced and microstructural refinement could be achieved by adding nickel to the investigated hot-work tool steel. In addition to molybdenum-enriched precipitates that covered the grain boundaries, randomly scattered non-metallic inclusions and oxides were observed. Modifying the microstructure by adding nickel also affects the mechanical properties of the product: an increase in hardness, impact toughness and yield strength as the nickel content increased in the AM structures fabricated by WAAM was observed.

Comparison of coated and uncoated HSS drill bit on surface roughness, material removal rate and dimensional accuracy of SS410 stainless steel

Stainless Steel 410 (SS410) is 12% chromium martensitic steel which can be heat treated to obtain wide range of mechanical properties. Due to its high corrosion resistance and excellent combination of toughness and strength, it is used for furnace parts, steam turbine buckets, blades, pump, nozzle, automotive parts, aerospace application etc. Considering the challenges in machining of this material, drilling experiments were conducted on SS410 to analyse the effect of various parameters. Coatings always have a positive influence on the tool. It helps to reduce friction in the cut, reducing heat build-up in the drill bit, as well as improving the flow of material out of the drilling point. It also delivers protection against corrosion, rust and water damage. The AlTiN coating provides exceptional oxidation resistance and extreme hardness. This coating works well in very demanding cutting tool applications, especially when tools are being pushed to the max. This paper reports on the investigation of performance of Al-Ti-N coated and uncoated HSS drill bits when drilling SS410. Machining characteristics studied in this investigation include Material Removal Rate (MRR), Surface Roughness (Ra), Ovality. Drilling tests were carried out as per Taguchi’s L16 array using process parameters such as Speed (30, 40, 50, 60 m/min), Feed (F) (0.05, 0.075, 0.1, 0.125 mm/rev) and point angle (P) (100, 110, 120, 130 degree). Comparison of the performance of coated and uncoated drill bits showed that coated drill bits have higher performance than uncoated. By using coated drills surface Roughness and Ovality values of drilling are reduced to an extend of 48.5% and 2%. Material removal rate value is also improved to an extend of 23%.

Material Characterization and Modeling for Finite Element Simulation of Press Hardening with AISI\xa0420C

The process of press hardening is gaining importance in view of the increasing demand for weight reduction combined with higher crash safety in cars. An alternative to the established manganese-boron steel 22MnB5 is hot-formed martensitic chromium steels such as AISI 420C. Strengths of 1850 MPa and elongations of 12% are possible, exceeding those of 22MnB5. In industrial manufacturing, FE-simulation is commonly used in order to design car body parts cost-efficiently. Therefore, the characterization and the modeling of AISI 420C regarding flow stress, phase transformations as well as failure behavior are presented in this paper. Temperature-depended flow curves are determined, showing the low flow stress and hardening behavior at temperatures around 1000 °C. Cooling experiments are carried out, and a continuous cooling diagram is generated. Observed phases are martensite and retained austenite for industrial relevant cooling rates above 10 K/s. In addition, tests to investigate temperature-dependent forming limit curves are performed. As expected, the highest forming limit is reached at 1050 °C and decreases with falling temperature. Finally, a simulation model of a press-hardening process chain is set up based on the material behavior characterized earlier and compared to experimental values. The forming force, phase transformation and forming limit could be calculated with good agreement to the experiment.

Effects of Helium Production and Displacement Damage on Microstructural Evolution and Mechanical Properties in Helium-Implanted Austenitic Stainless Steel and Ferritic/Martensitic Steel

The effects of helium concentration and displacement damage on microstructural evolution at low dpa and low helium concentration were mainly investigated in specimens of austenitic stainless steel 316FR or SUS304 and a high chromium martensitic steel (HCM12A). The 316FR and HCM12A specimens were implanted uniformly with helium at 823 K up to 30 appm-He or 50 appm-He by 50 MeV cyclotron accelerator using energy degraders. After the helium implantation, the microstructures were examined by a transmission electron microscopy and positron annihilation lifetime measurements. Irradiation hardening behaviors were analyzed using SUS304 and HCM12A steels at 823 K implanted with He ion up to 100 appm with different He/dpa ratios in the HIT ion irradiation experiments and the hardening behaviors were examined by nano indentation method. In the irradiation and annealing specimens, these mechanical properties and microstructures were examined to understand the effects of helium production, displacement damage and annealing on microstructural development, and kinetic Monte Carlo (kMC) simulations were also performed to understand the microstructural development, and the results were compared with the results of TEM observation and positron annihilation lifetime measurements. Important some differences in the microstructural developments such as cavity formation and growth between austenitic stainless steel and martensitic steel were observed in low dpa and low helium concentration conditions.

Effect of vibration treatment during welding on technological strength of chromium martensitic steel P91

The paper shows the possibility of using the technology of vibration treatment in the process of welding of high-temperature chromium steel of martensitic class P91 applied to the product to increase the technological strength of welded joints and the effect of this technology on mechanical properties before and after thermal treatment. Application of vibration action on welding bath and weld seam in process of manufacturing of welded structure may allow to reduce inhomogeneity of weld metal structure in liquid phase, as well as increase its microchemical inhomogeneity within crystallites during crystallization, reduce grain size due to more efficient heat removal and increase of number of crystallization centers. On the example of manufacturing the turbine support housing of the gas turbine drive, it is shown that it is necessary to replace part of the heat treatment operations with other more energy - efficient and less labor-intensive operations. Results of studies of influence of vibration treatment of welded joint on tendency of welded joint metal to form cold cracks, strength, hardness and impact toughness are given. Based on the obtained results, diagrams of average values of parameters are constructed and conclusions are drawn on the possibility of using the technology of vibration treatment of the turbine casing in the process of welding in order to reduce the number of heat treatment operations during its manufacture.

Creep behaviour of a high chromium martensitic steel under stress varying creep loading conditions: Primary creep regeneration (PCR)

Primary creep regeneration (PCR) is an important phenomenon observed for some alloys loaded at high-temperature and under stress varying loading conditions. For example, for a specimen deforming in the secondary creep regime, a stress reversal might result in an enhanced creep rate upon reloading due to full/partial re-activation of the primary creep stage (i.e. primary creep regeneration). The extent of PCR activation is expected to be dependent on different loading parameters (e.g. magnitude and duration of reverse loading) and the characteristics of the loaded alloy. In this study, a comprehensive testing program was designed and conducted to investigate the PCR behaviour of an advanced 10%Cr martensitic steel at 600 °C. The experimental observations characterized the influence of different loading parameters on the extent of PCR activation. It was found that the inelastic strain accumulation during reverse loading periods is the most important parameter governing the extent of PCR activation after stress transients. Furthermore, the gathered experimental data was analysed to understand the effect of different loading parameters on anelastic recovery behaviour of the steel. It was shown that a larger amount of strain recovers after unloading for tests with higher forward stress magnitudes and/or larger amounts of pre-accumulated inelastic strain.

Effect of Boron in the Coarsening Rate of Chromium-Rich Carbides in 9%–12% Chromium Martensitic Creep-Resistant Steel: Experiment and Modeling at 650 °C

In this study, three martensitic creep-resistant steels with 100, 90, and 70 ppm of boron were investigated. The experimental data obtained from isothermal aging and creep test at 650 °C were compared with the results of simulation conducted using TC-PRISMA software. Tungsten was found to be the rate-controlling element in the coarsening of (Cr, Fe, W)23C6 carbides; however, this result differed in terms of boron-containing steel. Several studies indicate that the low solubility of boron in ferrite promotes boron enrichment in (Cr, Fe, W)23C6 carbide, thereby reducing its coarsening rate. However, this mechanism is not universally agreed upon. In the present study, a comparison between experimental and theoretical results revealed that in boron-containing steels, the coarsening of (Cr, Fe, W)23C6 carbide is controlled probably by boron volume diffusion or by trans-interface diffusion.

Detection of microstructural changes of 9% Cr‐martensitic steels during heat treatment and aging by polarization tests

Abstract9–12% chromium martensitic steels are widely used as pipe material in steam power plants. Their good creep behavior is based on a finely adjusted microstructure, which is strengthened by precipitates. During long‐term use at higher temperatures degredation mechanisms occur, which affect the initial microstructure. Subgrain and precipitate coarsening as well as phase formation like Laves‐phase ((Fe, Cr)2(Mo, W)) proceed and lead to a reduced creep resistance. For lifetime assessment a nearly nondestructive electrochemical test method would be beneficial to identify microstructural changes during periodical inspections. The paper is focussed on the usability of an adapted electrochemical potentiodynamic reactivation (EPR)‐method and deals with the adjustment of test conditions like electrolyte composition and polarization parameters. The results obtained under optimized test conditions show a clear indication of precipitate formation by changing the characteristics of the EPR‐curves for the well established alloy T/P91 (X10CrMoVNb9‐1) as well as for the boron containing advanced alloy MARBN (X10CrWCoVNb9‐3‐3).

Effect of Laser Beam Welding on the Cyclic Material Behavior of the Press-hardened Martensitic Chromium Steel X46Cr13

For the application of high-strength materials in welded joints, a point of principle is how the strength of the sheet metal is affected by cyclic loading and by welding. For the investigation of the cyclic material behavior of the press-hardened martensitic chromium steel X46Cr13, strain-controlled fatigue tests were performed and evaluated. The aim of compensating the limitations in the weldability of this press-hardened material is achieved by a reduced heat input of the laser beam welding and a defined heat treatment. The effect of laser beam welding on the fatigue properties is shown by the cyclic behavior of butt joints. The cyclic material behavior is the basis of strain-based fatigue assessment approaches. Both cyclic stress-strain curves and strain-life curves are used for the fatigue life estimation. No clear difference between the press-hardened base material and butt joints has been found in the cyclic stress-strain curves. Transient effects are found by comparison of hysteresis loops of the initial loading, at the cyclically stabilized state and at crack initiation. Cyclic hardening is concluded from the initial loading and the cyclically stabilized state. By comparison of strain-life curves, a difference in the number of cycles to crack initiation between the base material and butt joints is found. Cycles to crack initiation of butt joints tested under strain control result in over 50 % of the base material’s fatigue strength at 1·106 cycles to failure.

Effects of helium production, displacement damage on mechanical properties and surface acoustic wave in austenitic stainless steels and martensitic steel

The effects of helium production, displacement damage on mechanical properties and microstructures were investigated in some specimens such as austenitic stainless steels of SUS304 and SUS316 and a high chromium martensitic steel of HCM12A steels irradiated in a high fluence irradiation facility (HIT) of the University of Tokyo. The simultaneous dual ion (nickel plus helium ions) irradiations were adopted to simulate nuclear irradiation environments such as fast reactor and fusion reactor at HIT to 1 or 10 dpa and to 0, 1, 10, and 100 appm-He at 500, 550, and 600 °C. Thin foils for a transmission electron microscopy (TEM) were prepared with a focused ion beam (FIB) micro-sampling system. After the irradiation, the microstructures were observed by a transmission electron microscope and the irradiation hardening was measured by nano-indentation technique. In this study, the efficient of non-destructive measurement method of surface acoustic wave (SAW) excited by laser beam on the detection of radiation hardening behavior was also examined. Irradiation hardening was observed and the increment was tended to increase with dpa and He/dpa, which was especially effective in the condition of 100 appmHe/dpa even at 550 °C. Non-linear effect on amplitude of the excited SAW was also observed on the ion irradiated materials, and the propagation velocity of SAW was tended to be increased with irradiation dose. It was found that the behaviors of SAWs results were correlated with the changes of mechanical properties.

SEM Study of the Influence of Microstructure on Low Cycle Fatigue Crack Growth in Martensitic Steel I

Distribution of fatigue cracks in chromium martensitic steel after low cycle fatigue (LCF) tests at room temperature has been studied using SEM, and the experimental evidences of localized plastic flow (LPF) are presented. The influence of the location of LPF and the microstructure elements on the trajectory and growth of microcracks is also considered. The dimensions of plastic zones ahead of macrocrack tip as well as at its edges were measured in the process of crack propagation inside of the sample. The processes occurring in plastic zone, particularly ahead of macrocrack tip, were analyzed. Distribution, orientation and the reasons of slip bands’ formation as well as the microstructure elements at which they were nucleated have been studied. The impact of the slip bands’ orientation on the process of macrocrack growth was also analyzed. In addition the interactions of a crack with the boundaries of former austenite grains, martensitic packets, martensitic laths, slip bands and precipitates have been discussed.

Experimental studies on Drilling of 410 Stainless Steel

Stainless Steel 410 (SS410) is 12% chromium martensitic steel which can be heat treated to obtain wide range of mechanical properties. Due to its strong corrosion resistance and excellent combination of toughness and strength, it is used for furnace parts, micrometer parts, steam turbine buckets, blades, pump parts, petrochemical equipments, etc.However, during machining of SS410, difficulties are there due to its low thermal conductivity, high ductility and work hardening which can be minimized by using suitable machining parameters and cutting fluids. Therefore, now a day’s research works have been focused towards machinability of SS410 through different machining processes. Among different machining processes, many works have not been started on drilling of SS410. Drilled holes are used to receive screws, bolts, shafts, steam pipes, fitting of furniture and equipment. By considering the above applications and challenges in machining of this material, drilling experiments were conducted on SS410 to analyze the effect of drilling parameters and machining environment on surface roughness and machining time. Experiments were conducted in a jig boring machine with HSS twist drill of ∅10 mm on 5 mm thickness SS410 plate.Drilling tests were carried out as per Taguchi’s L9 array using three cutting speeds (Vc: 11, 16 and 21 m/min) and three feeds (f: 0.02, 0.05 and 0.08 mm/rev) using three different cutting fluids (castor oil, kerosene and coconut oil). Surface roughness (Ra) values were decreased from 7.326 to 2.423 µm with increase in 'Vc' and decrease in 'f' and machining time was increased from 7.4 to 31.5 seconds with increase in 'Vc' and 'f'. Optimum process parameters were identified and verified experimentally to improve the machinability of the SS410. Coconut oil medium gave good machinability results at higher 'Vc' and 'f'.

SEM Study of Fatigue Crack Propagation in Chromium Martensitic Steel after LCF

Crack propagation after low-cycle fatigue (LCF) deformation has been studied in the chromium martensitic structural steel. Although the study of a fundamental mechanism of fatigue crack growth has received much attention over the last decade, it still remains a sufficiently complex problem and needs full understanding. Moreover, the recent studies show that the cracks propagate discontinuously even on the millisecond timescale, and their growth rate significantly depends on a microstructure of the material. In the present work the boundaries of the former austenitic grains were revealed on the polished surfaces of the thermally treated samples, which subsequently were undergone low-cycle fatigue tests. The experimental studies show that fatigue macrocracks mainly grow along the boundaries of the former austenitic grains, and changetheir propagation direction when crossing the grain boundary, however, remain within 45 ̊ interval with regard the cycling axis. In particular cases, when the boundaries of a martensite packets and those of the former austenite grains lay along the length of a packet, the macrocrack is better formed and with regular borders. After a macrocrack reaches a definite length ~30-50μ, a microcrack is nucleated ahead of the macrocrack tip, and is oriented along the substructure element of the steel. Further deformation tests provide an increase in the length of the main crack via aggregation of microcracks initiated ahead of it during the LCF. In the cases when the macrocrack is deviated, slip bands are formed in martensitic structures along the boundaries of martensite packets (laths). A correlation is revealed between the microcrack components and the substructure elements of the steel as well. The same results were obtained by fractography of the tested and fractured samples. However, in the latter case correlation was established between the cleavage facets and the dimensions of packets.

Manufacturing of Axisymmetric Components out of Superalloys and Hard-to-Deform Steels by Roll Forming

Heat-resistant alloys are the basic material of gas turbine engine (GTE) design. Fine-grained structure in these alloys can be formed by isothermal forging and then different axisymmetric GTE components as wheels, shafts, rings can be superplastic roll formed. Examples of the superplastic and isothermal deformation use for manufacturing components out of superalloys and steels for critical applications are given. The possibility of roll forming parts as rings with a diameter up to 800 mm and as flange - cone with a diameter up to 600 mm out of superalloys (Inconel 718, EK79, EP741NP), accordingly, on SRZHD-800 and modified PNC-600 mills were showed. The macrostructure investigations of the components after the roll forming showed that the homogeneous structure was formed. The microstructure at the flange portion was fine-grained and at membrane zone was coarse-grained. Cone part was roll formed at isothermal condition from pre-stamped chromium martensitic steel sheet. Manufacturing technology of roll forming was tested by computer and physical simulation. Service properties of components were obtained by subsequent heat treatment. The effectiveness of the technology associated with increased service properties of components and decreases the labor content by automated equipment.

The Role of Microstructure in Creep Strength of 9-12%Cr Steels

Tempered martensite lath structure (TMLS) plays a vital role in creep resistance of high chromium martensitic steels. Under creep conditions the TMLS could be stabilized by three agents: (i) a dispersion of boundary M23C6 carbides and Laves phase; (ii) a dispersion of M(C,N) carbonitrides, which are homogeneously distributed within ferritic matrix; (iii) substitutional alloying element within ferritic matrix. The boundary particles exert a large Zener drag force which effectively hinders migration of low-and high-angle boundaries. A dispersion of M(C,N) carbonitrides both within ferritic matrix and lath boundaries provides the pinning of mobile dislocations. This process is responsible for reliving long-range elastic stress field originated from lath boundaries. In addition, M(C,N) carbonitrides provide high threshold stress. Substitutional elements as W and Mo effectively slowing down diffusion in ferritic matrix retard climb of lattice dislocation that also prevents the aforementioned knitting reaction. The suppression of knitting reaction between lattice dislocation and low-angle boundaries prevents their transformation to subboundaries by concurrent operation of all three agent types. Depletion of W and Mo from solid solution leads to the occurrence of static recovery and precipitation of Laves phase at boundaries under long-term aging. This process is responsible for creep strength breakdown. The strain-induced formation of Z-phase at the expense of V-rich M(C,N) carbonitrides highly facilitates this process. However, slow strain-induced coarsening of M23C6 carbides and M(C,N) carbonitrides provides the suppression of the knitting reaction between mobile lattice dislocations and intrinsic dislocations of lath boundaries and replacement of TMLS by subgrain structure. Ostwald ripening of boundary M23C6 carbides and Laves phase leads to rapid creep rate increase with strain in tertiary creep and premature rupture owing to the formation of subgrain structure replaced TMLS and further subgrain growth.