High Nitrogen Martensitic Stainless Steel Research Articles

Elucidating the dual effect of vanadium microalloying on hot deformation behavior of high nitrogen martensitic stainless steel

Precipitates always have a complex effect on the hot work hardening and recrystallization softening of materials. In this work, the dual effect of V-microalloying on hot deformation behavior and microstructure evolution of high nitrogen martensitic stainless steel (HNMSS) was investigated by hot compression experiments, modeling study and microstructure characterization. The results showed that V-microalloying enhanced the flow stresses and activation energy of 0.2V steel due to the fine grain strengthening effect and the blocking effect of dynamic precipitation on dislocation movement and grain boundary migration. On the other hand, the recrystallization model and deformed microstructure indicated the more intense dynamic recrystallization (DRX) softening effect in 0.2V steel, inducing the faster decrease of flow stress after the peak stress. V-microalloying promoted the dynamic precipitation of HNMSS during hot deformation, which reduced the size and increased the number of precipitates in 0.2V steel. The fine precipitates hindered dislocation movement, promoted dislocation accumulation and entanglement, formed a particle deformation zone (PDZ) with high dislocation density and large orientation gradient, and then induced the occurrence of particle-stimulated nucleation (PSN). In addition, the dislocation cells evolved from dislocation entanglement were also favorable nucleation sites for DRX.

Microstructure and Mechanical Properties of Heat-Treated High-Nitrogen Martensitic Stainless Steel

Microstructure and Mechanical Properties of Heat-Treated High-Nitrogen Martensitic Stainless Steel

Study on the mechanism of excellent strength and toughness combination of a high nitrogen martensitic stainless steel treated by Q-C-P-T process

In this study, a high nitrogen martensitic stainless bearing steel with ultra-high strength and excellent toughness combination treated by quenching-cryogenic cooling-partitioning-tempering (Q-C-P-T) process was obtained. The yield strength, ultimate tensile strength, uniform elongation and the impact energy of the sample after partitioning at 200 °C for 1 h followed by tempering at 400 °C for 1 h are up to 2034 MPa, 2217 MPa, 5.83% and 58.73 J respectively. The microstructure characterization shows that the interstitial atoms of C and N in martensite diffuse into adjacent austenite during the partitioning process, resulting in the high stability of austenite. Strain-induced martensitic transformation was suppressed in maximum, effectively avoiding the formation of hard martensite enriched in C/N and dimensional instability caused by volume expansion of martensitic transformation, which is crucial for bearing steels. And nano-scale precipitates in the martensite matrix effectively compensate for the strength loss of the martensite caused by lower solid solution of carbon and nitrogen, thus obtaining excellent comprehensive mechanical properties. This work paves a new way for the development of advanced bearing steels.

Nitrogen Solubility and Gas Nitriding Kinetics in Fe–Cr–Mo–C Alloy Melts under Pressurized Atmosphere

The precise control of nitrogen content in high nitrogen martensitic stainless steel is the guarantee of its excellent performance, and is a crucial issue for its manufacturing process using pressurized gas nitriding. In this paper, the effects of nitrogen pressure, temperature and alloy composition on nitrogen solubility and pressurized gas nitriding kinetics of Fe–Cr–Mo–C alloy melts were investigated. In order to correct the deviation of nitrogen solubility from Sieverts’ law under high nitrogen pressure, the first- and second-order interaction parameters of nitrogen on itself were obtained; accordingly, the nitrogen solubility model under pressure was established and well verified. The nitrogen solubility decreased with increasing temperature and C content, and increased with increasing Cr and Mo contents. The nitrogen mass transfer in liquid phase was the nitriding rate-determining step under different nitrogen pressures. The apparent mass transfer coefficient of nitrogen was approximately 0.0218 to 0.0230 cm·s−1, and showed a weak dependence with pressure. The apparent mass transfer coefficient of nitrogen increased significantly with the increase of temperature and electromagnetic stirring, and was rarely affected by the content of Cr, Mo and C. Considering the influence of pressure change on nitriding rate in pressurization stage, a novel kinetic model for gas nitriding under high pressure was established and exhibited effectiveness in practice.

Redistribution of C and N Atoms in High Nitrogen Martensitic Stainless Steel During Cryogenic Treatment

Redistribution of C and N Atoms in High Nitrogen Martensitic Stainless Steel During Cryogenic Treatment

Transition in wear behavior and mechanical properties of novel high nitrogen martensitic steel in cryogenic temperature regimes

In space engines with liquid cryogenic LH2/ LO2 propellants, bearings are used in cryopumps subjected to harsh working conditions. High nitrogen martensitic stainless (HNMS) steel is a newly developed steel modifying currently used AISI 440C steel by partial substitution of C with N and is a potential replacement for AISI 440C as an antifriction bearing material In the present study, HNMS samples are austenitized at 1075 °C followed by cooling to −80 °C (CT) and then double tempering at 500 °C followed by cryogenic treatment at −196 °C for 8 h and 1 h soft tempering at 100 °C (CTC). Hardness increment by 7.9% and 7.6%, wear rate decrement by 11.11% and 7.69% and shift in DBTT from − 60 °C to − 90 °C was observed for CTC samples compared to CT samples at −196 °C and RT respectively. The improved properties for CTC samples might be due to precipitation and uniform distribution of tertiary nano precipitates.

Hot Deformation Behavior and Processing Maps of 0.3C–15Cr–1Mo–0.5N High Nitrogen Martensitic Stainless Steel

Hot deformation behavior of 0.3C–15Cr–1Mo–0.5N high nitrogen martensitic stainless steel (HNMSS) was investigated in the temperature range of 1173–1473 K and at strain rates of 0.001–10 s−1 using a Gleeble 3500 thermal–mechanical simulator. The true stress–strain curves of the studied HNMSS were measured and corrected to eliminate the effect of friction on the flow stress. The relationship between the flow stress and Zener–Hollomon parameter for the studied HNMSS was analyzed in the Arrhenius hyperbolic sine constitutive model by the law of $$Z = 3.76 \times 10^{15} \sinh \left( {0.004979\sigma_{\text{p}} } \right)^{7.5022}$$. The processing maps at different strains of the studied HNMSS were plotted, and its flow instability regions in hot working were also confirmed in combination with the microstructure examination. Moreover, the optimal hot deformation parameters of the studied HNMSS could be suggested at T = 1303–1423 K and $$\dot{\varepsilon }$$ = 5–10 s−1 or T = 1273–1473 K and $$\dot{\varepsilon }$$ = 0.005–0.04 s−1.

Evolution of ultrafine precipitates and its influence on wear mechanism in cryoprocessed high nitrogen martensitic steel

ABSTRACTDevelopment of the space engines with liquid propellants necessitates the use of bearings of high quality. During recent years, a new family of high strength corrosion resistant steels, with partial replacement of carbon by nitrogen, known as high nitrogen steels were developed having higher impact strength and fracture toughness coupled with high hardness for antifriction bearing of cryopumps in rocket. In this research work, wear characteristics of cryogenically treated high nitrogen martensitic stainless steels were investigated by Pin-on-disc wear testing machine. After conventional hardening treatment, specimens were subjected to cryogenic treatment at −196°C with cryosoaking period varying from 8 to 32 h in the interval of 8 h followed by soft tempering. These specimens were analysed for hardness, wear resistance, subsurface and worn surface features by SEM. It is established that there is a profound influence of nucleation and growth of precipitates on the wear behaviour and the underlying wear mechanisms.

Designing for high corrosion-resistant high nitrogen martensitic stainless steel based on DFT calculation and pressurized metallurgy method

In this work, a new high corrosion resistant martensitic stainless steel (MSS) containing high nitrogen content (N > 0.33 wt.%) was designed based on density functional theory (DFT) calculation and pressurized metallurgy. The results indicate that Mo-N alloying significantly improve the corrosion resistance of MSS. This observation has been explained from point of view of crystal structural stability, wherein the reduced formation energy, bonding energy, density of state (DOS) and lattice distortion. It results in the increasing N content in solid solution and the decreasing fractions of Cr depletion induced by the precipitation of Cr2N.

Effect of partial replacement of carbon by nitrogen on intergranular corrosion behavior of high nitrogen martensitic stainless steels

The microstructure evolution and intergranular corrosion (IGC) behavior of high nitrogen martensitic stainless steels (MSSs) by partial replacing C by N were investigated by using microscopy, X-ray diffraction, nitric acid tests and double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests. The results show that the partial replacement of C by N first reduces and then increases the size and content of precipitates in high nitrogen MSSs, and converts the dominant precipitates from M23C6 to M2N, furthermore first improves and then deteriorates the IGC resistance. The high nitrogen MSS containing medium C and N contents provides good combination of mechanical properties and IGC resistance.

Optimization of Cryogenic Process for Enhancing the Wear Resistance of Novel HNMS Steel for Cryogenic Bearings of Launch Vehicle

Bearings are highly engineered components that have to tolerate vibratory stresses, bending moments, and high rotational speeds in cryogenic applications. AISI 440C is most widely used for antifriction bearings due to its good strength and high hardness but is prone to carbide banding and failure during service. High-nitrogen martensitic stainless steel (HNMS) was recently developed by partially substituting carbon with nitrogen. This article explores the possible improvement in wear properties by cryotreating HNMS steel. The hardening temperature was varied from 1050 to 1100 °C, cryosoaking period from 0 to 12 h at −196 °C, and tempering temperature from 100 to 300 °C for 1 h. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis revealed the presence of carbides and carbonitrides. The phase fraction and size of the precipitates was found to be influenced by the cryosoaking period. The carbide density variation with the cryosoaking period followed a sigmoidal curve. Eight hours of cryogenic treatment at −196 °C was observed to be the optimal cryosoaking period for HNMS steel. Maximum hardness (671 HV) and minimum wear rate (0.00714 mm3/m) and adhesive wear were observed for the optimum heat treatment condition. Compared to conventional hardening-tempering (HT) treatment, the optimized hardening-cryogenic-tempering (HCT) treatment cycle resulted in a 38% increase in hardness and a 31% decrease in the wear rate. The improved properties can be attributed to the presence of fine and uniform distribution of precipitates in the matrix and transformation of retained austenite to martensite.

Studies on Wear Behaviour and DBTT in Sub-zero Regimes of Cryo-Treated High Nitrogen Martensitic Stainless Steel (HNMS)

The development of space engines with liquid propellants require use of highly engineered bearings. High nitrogen martensitic stainless steel (HNMS) is the newly developed steel and is the potential candidate for anti-friction bearings in cryogenic engines in launch vehicles. The present paper aimed to assess the effect of cryogenic treatment on HNMS steel at sub-zero temperature regimes. HNMS steel specimens were treated using optimised heat treatment cycle comprising of hardening at 1075 °C, cryogenic treatment at − 196 °C for 8 h followed by soft tempering at 100 °C for 1 h. The specimens were tested for hardness, wear loss (test load 20–80 N), surface roughness, and impact toughness from room temperature (RT) to the cryogenic temperatures. Worn surfaces, subsurface features, and fractured surfaces were analysed using FE-SEM. It was established that the HNMS steel showed reduced wear rate by 28% and 60% for 20 N and 80 N wear test load, respectively, at − 196 °C as compared to the RT. Severe delaminated wear mechanism was observed at RT as compared to mild adhesive wear at − 196 °C. Impact testing revealed that the DBTT for HNMS steel lied between − 30 and − 60 °C .

Hot deformation behavior of high nitrogen martensitic stainless steels

The effect of deformation temperature and strain rate on hot deformation behavior of high nitrogen martensitic stainless steels was investigated by hot compression tests at temperatures of 900 °C–1150 °C and the strain rates of 0.1–10 s−1. Combining with microstructural evolution, the relationship of flow stress with strain rate and deformation temperature was analyzed. The results indicated that the true stress-true strain curves of the experimental material displayed dynamic recrystallization character under the present deformation conditions. The peak stress and flow stress decreased with increasing deformation temperature and increased with increasing strain rate. The thermal deformation activation energy of the experimental steel was 710 038 J · mol−1, and the constitutive equation of the plastic deformation of high nitrogen martensitic stainless steel was obtained by introducing the Zener-Hollomon parameter.

Influence of nitrogen on corrosion behaviour of high nitrogen martensitic stainless steels manufactured by pressurized metallurgy

Effect of nitrogen on microstructure and corrosion behaviour of high nitrogen martensitic stainless steels manufactured by pressurized metallurgy was investigated by microscopy, electrochemical and spectroscopy analyses. Results indicated that increasing nitrogen content significantly enhanced the corrosion properties of martensitic stainless steels, while excess nitrogen deteriorated the corrosion resistance. The impacts of increased nitrogen content could be summarized as three aspects: the change of precipitation content and conversion of main precipitates from M23C6 to M2N; the enhanced protection performance of passive film by enrichment of Cr, especially Cr2O3 and CrN; the improved repassivation ability by increased nitrogen content in solid solution.

ω-Fe particle size and distribution in high-nitrogen martensitic steels

Systematical transmission electron microscopy (TEM) studies of the martensitic substructure in quenched Fe–C binary alloys have revealed that the initially formed martensite has twinning structure, and the twin is body-centered cubic {112} -type twin regardless of the carbon concentrations. A metastable hexagonal ω-Fe(C) phase with an ultrafine particle-like morphology is distributed at the twin boundary region. In order to explore the common existence of the ω-Fe in nitrogen steels, the martensitic substructures in high-nitrogen martensitic stainless steels at various conditions (as-quenched, subzero-treated and tempered) have been investigated in detail by means of TEM. The ω-Fe with an ultrafine particle size of 1–3 nm has been observed in all the samples. TEM tilting experiment and electron diffraction analysis have revealed that each martensitic lath or plate is composed of {112} -type twin structure with the ω particles at the twin boundary region. The martensite morphology and the relationship between the twin and the ω phase particles have been discussed crystal geometrically. The ω particle size variation with subzero and tempering treatment has also been discussed based on a proposed dilation and split mechanism. The existence of large amount of twins up to 550 °C simply suggests that the nitrogen atoms have much stronger effect than carbon atoms on the ω phase stability. The present investigation will provide a very clear image about the martensitic substructure in high-nitrogen martensitic stainless steels.

Hot Deformation Behavior and Microstructural Evolution of High Nitrogen Martensitic Stainless Steel 30Cr15Mo1N

Hot deformation behavior of high nitrogen martensitic stainless steel 30Cr15Mo1N is investigated by isothermal compression tests at the temperature range of 900–1250 °C and the strain rate range of 0.01–10 s−1. The results indicate that the activation energy of this alloy (503.5 kJ mol−1) is higher than that of conventional martensitic stainless steels. The developed Arrhenius‐type constitutive equation considering strain compensation can predict flow stress of 30Cr15Mo1N with good consistency. The critical conditions for initiation of dynamic recrystallization (DRX) are determined based on the strain hardening rate versus flow stress curves. The DRX is inhibited and grain size decrease with increasing strain rate from 0.01 to 1 s−1. However, the fraction of DRX increased and grain size coarsened when strain rate is further increased to 10 s−1. Cr‐rich M23C6 and M2N precipitated at grain boundaries at lower temperatures, which can effectively pin grain boundaries and, thereby retard or even inhibit the process of DRX. In addition, dislocations tangled and piled up around precipitates, thereby hindering dislocation movement. The influence of precipitates on DRX can be eliminated due to their dissolution at higher temperatures.

Microstructure, Mechanical and Corrosion Properties of Friction Stir Welding High Nitrogen Martensitic Stainless Steel 30Cr15Mo1N

High nitrogen martensitic stainless steel 30Cr15Mo1N plates were successfully welded by friction stir welding (FSW) at a tool rotation speed of 300 rpm with a welding speed of 100 mm/min, using W-Re tool. The sound joint with no significant nitrogen loss was successfully produced. Microstructure, mechanical and corrosion properties of an FSW joint were investigated. The results suggest that the grain size of the stir zone (SZ) is larger than the base metal (BM) and is much larger the case in SZ-top. Some carbides and nitrides rich in chromium were found in BM while not observed in SZ. The martensitic phase in SZ could transform to austenite phase during the FSW process and the higher peak temperature, the greater degree of transformation. The hardness of SZ is significantly lower than that of the BM. An abrupt change of hardness defined as hard zone (HZ) was found in the thermo-mechanically affected zone (TMAZ) on the advancing side (AS), and the HZ is attributed to a combination result of temperature, deformation, and material flow behavior. The corrosion resistance of SZ is superior to that of BM, which can be attributed to less precipitation and lower angle boundaries (LABs). The corrosion resistance of SZ-bottom is slight higher than that of SZ-top because of the finer grained structure.

Effect of Deformation Ratio on Microstructures and Properties of High Nitrogen Martensitic Stainless Steel

To reveal the effect of deformation ratio on high nitrogen martensitic stainless steel, the room temperature mechanical properties, microstructures and precipitates morphology were studied by using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) as well as mechanical tests. The results indicate that the tensile strength, yield strength and impact energy were raised 241MPa, 180MPa and 10.0J respectively, at a higher deformation ratio. While, the average original austenite grain size was refined from 14.93μm to 9.57μm when the deformation ratio was increased from 0.65 to 0.83 as well as the martensite lath width thinned from 400nm to 250nm. Furthermore, at various deformation ratios, the precipitates morphology was different in quantity, grain size and dispersion. There are more eutectic precipitates and secondary precipitates dispersed along the grain boundary as continuous chains with lower deformation ratio. On the contrary, the precipitates in specimen with higher deformation ratio were uniform and fine without chain or network. It can be inferred that the improvements on mechanical properties, microstructures and precipitates morphology are mainly due to the deformation ratio.

Microstructure Evolution and Precipitation Behavior of a High Nitrogen 15Cr Martensitic Stainless Steel during Tempering

The microstructure evolution and precipitation behavior of a low carbon high nitrogen martensitic stainless steel containing 15%Cr-5%Ni-1%Mo-0.16%N (in mass percentage) after tempering at temperatures ranged from 300 °C~700 °C were investigated by hardness test, X-ray diffraction (XRD) and Transmission electron microscopy (TEM). The steel exhibits second hardening behavior after tempering at temperatures below 500 °C. Tempering at temperatures above 500 °C promotes the formation of reversed austenite and the precipitation of Cr2N. The coherent orientation relationship of Cr2N with martensite was revealed as , . Retained austenite originated from reversed austenite at room temperature results in the softening of the steel, and the retransformation of reversed austenite to untempered martensite after tempering at temperatures above 600 °C restores the hardness of the steel. σ phase was also found in specimens tempered at temperatures ranged from 500 °C to 700 °C.

Study of Hot Formability of High Nitrogen Martensitic Stainless Steel

Alloying high-chromium steels with Nitrogen leads to increase in strength, fatigue life and corrosion resistance, but reduce ductility and could induce cracks formation during forging. In order to address these problems, the hot working response of a high Nitrogen martensitic stainless steel (Fe-16.2%Cr-1.1%Mo-0.33%N-0.34%C) has been investigated by means of hot torsion tests up to rupture, in the temperature and strain rate ranges of 900-1200°C and 0.005-5 s-1 respectively. The peak stresses of the flow curves were related to strain rate (e&) and temperature (T) by the well known sinh equation. The ductility and the safe working conditions were presented in terms of processing map. The microstructure of the steel in the quenched state after deformation was analyzed by means of optical microscopy; the differences in term of morphology and distribution of the various constituents were discussed.