Secondary Hardening Steel Research Articles

Effects of Co on Mechanical Properties and Precipitates in a Novel Secondary-Hardening Steel with Duplex Strengthening of M2C and β-NiAl.

Synergistic strengthening of nano-scaled M2C and β-NiAl has become a new route to develop ultra-high secondary-hardening steel. At present, the effect of Co on the synergistic precipitation behavior of duplex phases of M2C and β-NiAl has been rarely reported. This paper revealed the effects of Co on the mechanical properties and duplex precipitates of M2C and β-NiAl in a novel 2.5 GPa ultra-high strength secondary-hardening steel. The tensile tests indicated that a 10% Co-alloy steel achieved a much stronger secondary-hardening effects compared to a Co-free steel during aging process, especially in the early-aging state. Needle-shaped M2C and spherical β-NiAl particles were observed in both Co-alloy and Co-free steels. However, the number density, and volume fraction of M2C were significantly enhanced in the 10% Co-alloy steel. The Mo contents in M2C carbide and α-Fe after aging treatment were both analyzed through experimental determination and thermodynamic calculation, and the results indicated that Co decreased the solubility of Mo in α-Fe, thus promoting the precipitation of Mo-rich carbides.

Revealing the evolution behavior of multiple carbides precipitation and mechanical response in M54 secondary hardening steel

As a new type of secondary hardening ultra-high strength steel, the evolution behavior of multiple carbides and its effect on the strength–toughness of M54 steel have been studied by subjecting to tempering duration (2– 10 h) at 520 °C. Within tempering of 2– 4 h, there is incompletely dissolved cementite with intra-granular distribution; however, it does not deteriorate the impact toughness. Without the observation of cementite, it shows the largest precipitation density of M2C carbides after 8 h-tempering, with the maximum density of 650 μm−2 and minimum inter-particle distance of 12.3 nm. Prolonged tempering of 10 h does not lead to coarsening due to the low diffusivity of constituent W element and coherent relationship of M2C/matrix interface. However, there is obvious segregation of MC carbides along the long-axis of M2C carbides in growth. The favorable precipitation of M2C carbides mostly contributes to the highest yield strength of 1747 MPa and ultimate tensile strength of 2037 MPa in case of 8 h-tempering. The stable ultrafined martensite blocks (∼0.30 μm) and high ratio of high-angle grain boundaries (∼50%) guarantee the desirable V-notched impact absorbed energy of ∼30 J for the 2– 8 h-tempering. Unfortunately, due to the insufficient ability of arresting propagating cracks, the possible preferential MC carbides-distributed sub-grain boundaries leads to an obvious degradation of impact toughness (∼18 J) in the 10 h-tempered samples.

Effect of Post-heat Treatment on Microstructure and Mechanical Properties of Laser Cladded High Co-Ni Secondary Hardening Steel Coating

The effect of post-heat treatment on the microstructure and microhardness of laser cladded high Co-Ni secondary hardening steel coating was investigated. The microstructures were analyzed using a SEM equipped with an EDS, and the microhardness was measured with a Vickers indenter. Decomposition of the retained austenite in the coating occurred during the post-heat treatment. As the temperature increased from 200 °C to 600 °C, the quantity of the retained austenite at the boundaries decreased significantly, while that of the needle-shaped M3C cementite and M2C carbides increased. The M2C carbides evidently coarsened when the temperature was higher than 500 °C. The microhardness of high Co-Ni steel coating increased as the temperature of post-heat treatment increased from 200 °C to 400 °C because the fine-scale M2C carbides were coherent with the matrix and increased distinctly in this temperature range. It decreased sharply when the temperature further increased from 500 °C to 600 °C due to both the incoherency of the coarsened M2C carbides and the recovery of dislocations in the carbon-supersaturated matrix.

Simultaneous improvement of strength and toughness via two-stage aging for a secondary hardening steel

A two-stage aging, involving initial low-temperature treatment followed by high-temperature treatment, was employed to induce a high number density M2C and inhibit the occurrence of chemical ordering and formation of large ellipsoidal particles at grain boundaries. The ultimate tensile strength, yield strength and Charpy impact energy increase by 233 MPa, 191 MPa and 3.3 J, respectively.

Design of a 2.7 GPa ultra-high-strength high Co–Ni secondary hardening steel by two-step nano-size precipitation tailoring

High Co–Ni secondary hardening steels are valuable in the field of aviation materials. With the increasing demand of lightweight, further enhancing its strength has received widespread attention. In this study, instead of using only M2C carbides as the reinforcements in the traditional high Co–Ni secondary hardening steels, nano-size NiAl phases were introduced to provide additional strength. To obtain the collaborative precipitation of these two reinforcements, systematically thermodynamic calculation, including the driving force, coarsening rate, etc., was made to quantitatively tailor the composition and the two-step aging parameters. The calculation results showed that adding 1 % Al into the standard composition of M54 is a plan with high potential and a second aging process at 450 °C should be added to promote NiAl precipitation. Based on the experimental validation, the target microstructure formed as the design. With the dual-phase strengthening of NiAl particles and M2C carbides, the yield and tensile strength are greater than 2.3 and 2.7 GPa, respectively, which increases the strength of M54 steel by >30 %, retaining sufficient elongation and relatively low cost. This new designed alloy increased the strength–ductility balance of high Co–Ni secondary hardening steels and aviation maraging steels from ∼12.5 to 13.5 GPa%.

Dual nanoprecipitation and nanoscale chemical heterogeneity in a secondary hardening steel for ultrahigh strength and large uniform elongation

Nanoprecipitates and nanoscale retained austenite (RA) with suitable stability play crucial roles in determining the yield strength (YS) and ductility of ultrahigh strength steels (UHSSs). However, owing to the kinetics incompatibility between nanoprecipitation and austenite reversion, it is highly challenging to simultaneously introduce high-density nanoprecipitates and optimized RA in UHSSs. In this work, through the combination of austenite reversion treatment (ART) and subsequent flash austenitizing (FA), nanoscale chemical heterogeneity was successfully introduced into a low-cost UHSS prior to the aging process. This chemical heterogeneity involved the enrichment of Mn and Ni in the austenite phase. The resulting UHSS exhibited dual-nanoprecipitation of Ni(Al,Mn) and (Mo,Cr)2C and nanoscale austenite stabilized via Mn and Ni enrichment. The hard martensitic matrix strengthened by high-density dual-nanoprecipitates constrains the plastic deformation of soft RA with a relatively low fraction of ∼15 %, and the presence of relatively stable nanoscale RA with adequate Mn and Ni enrichment leads to a marginal loss in YS but keeps a persistent transformation-induced plasticity (TRIP) effect. As a result, the newly-developed UHSS exhibits an ultrahigh YS of ∼ 1.7 GPa, an ultimate tensile strength (UTS) of ∼1.8 GPa, a large uniform elongation (UE) of ∼8.5 %, and a total elongation (TE) of ∼13 %. The strategy of presetting chemical heterogeneity to introduce proper metastable phases before aging can be extended to other UHSSs and precipitation-hardened alloys.

Aging response of a Fe–C–Co–Ni secondary hardening steel: The significance of matrix ordering

The evolution of precipitates and dislocations during aging dictates the mechanical properties of secondary hardening steels. Such microstructural response of a Fe–C–Co–Ni secondary hardening steel has been investigated via high-resolution transmission electron microscopy in this study. The primary source of strengthening is linked to coherent Molybdenum carbides, which form in a sequence of cementite, cluster and M2C. It is shown that the ordering of the martensitic matrix occurs rapidly upon aging and results in the formation of dispersed B2–FeCo precipitates. The co-location of ordered domains with the cluster and M2C carbides indicates that the ordering greatly affects the nucleation and growth of M2C and thus leads to a more homogeneous spatial distribution of M2C. This yields excellent mechanical properties including an ultimate tensile strength of 2150 MPa, yield strength of 1829 MPa, and total elongation of 12 %. It should be noted that the ordering not only contributes to the mechanical properties by promoting the dispersion of M2C, it also causes strengthening directly as demonstrated by an additive model of yield strength. However, the model fails to predict the yield strength of the as-quenched sample since it contains extra mobile dislocations which considerably modify the mechanical property.

IMPROVING THE WEAR RESISTANCE OF ECONOMICALLY ALLOYED STEELS

Problem statement. The influence of wear on the formation of a “white band” in metastable austenitic, martensitic-austenitic and secondary hardening steels of the Cr−Mn−Ti system, additionally alloyed with Mo, B, V, is studied. The influence of structure and phase composition on the wear resistance of economically alloyed metastable and secondary hardening steels is shown. Results. Surfacing of the studied materials was conducted in copper molds with different rates of forced cooling. Metastable austenitic, martensitic-austenitic and secondary hardening steels of the Cr−Mn−Ti system additionally alloyed with Mo, B, V are studied. Additional alloying of these steels with titanium in an amount of 2...5 % contributed to the prevention of spalling along the fusion zone. Near the fusion line there is a base metal zone with a width of 7...15 µm. After testing at the volume temperature of the working part of the specimen ТV = 553…573 K in the contact volumes for deposited metal of the 30Cr2W8V type, broadening of the grain boundaries, shear lines, finer grains compared to the underlying layers are revealed. Outside the zone of plastic deformation, the size of the grains corresponds to their sizes before the start of testing, the grain boundaries are relatively thin. The number and location of carbides observed at X430, X80O magnifications are also similar to the structural characteristics for deposited metal of the 30Cr2W8V. At close values of the contact pressure in the friction pair, the time of formation of a crack of critical length increases with an increase in the effective surface energy γе (including the energy of plastic deformation). Thus, the crack resistance indices (CR, j-integral, δС) and, consequently, the wear resistance of maraging steels are higher than those of metastable and tool steels. Conclusions. The conducted studies confirm the possibility of the formation of a "white band" both in alloys with a high concentration of elements − austenitizers (Mn, C, Ni), and when alloyed with carbide-forming elements with a relatively low affinity for carbon (V, Mo). The crack resistance indices (CR, j-integral, δС) and, consequently, the wear resistance of maraging steels is higher than those of metastable and tool steels.

Oxidation behavior and corrosion properties of 2-GPa grade Co-reduced ultrahigh-strength stainless steel

This study investigated the oxidation behavior and corrosion resistance of 2-GPa grade ultrahigh-strength stainless steel. Results revealed that the addition of V to Co-reduced FerriumS53 (7Co1V) accelerated the precipitation of MC carbide during aging, while inhibiting the precipitation of Cr-rich M23C6 carbide. A significant amount of Cr was predicted in the microstructure of the alloys, and this microstructure increased the partitioning of Cr to the alloy surface and provided enhanced resistance against corrosion pitting. These findings suggest that the addition of V into Co-reduced stainless secondary hardening steels can enhance their oxidation and corrosion resistance without compromising their mechanical properties.

Influence of structure and phase composition of economically alloyed steels on wear resistance

Problem. The paper considers the influence of wear on the formation of a "white band" in metastable austenitic, martensitic-austenitic and secondary hardening steels of the Cr-Mn-Ti system additionally alloyed with Mo, B, V., the influence of the structure and phase composition on the wear resistance of sparing. Goal. Based on the above, the purpose of this work was to study the influence of structure and phase composition on the wear resistance of economically alloyed metastable and secondary hardening steels. Methodology. Surfacing of the studied materials was carried out in copper molds with different rates of forced cooling. Metastable austenitic, martensitic-austenitic and secondary hardening steels of the Cr-Mn-Ti system additionally alloyed with Mo, B, V were studied. Additional alloying of these steels with titanium in an amount of 2...5% contributed to the prevention of spalling along the fusion zone. Near the fusion line there is a base metal zone with a width of 7...15 µm. Results. After testing at the volume temperature of the working part of the specimen ТV=553…573 K in the contact volumes of the deposited metal of the 30Kh2V8F type, broadening of the grain boundaries, shear lines, finer grains compared to the underlying layers were revealed. Outside the zone of plastic deformation, the size of the grains corresponds to their sizes before the start of testing, the grain boundaries are relatively thin. The number and location of carbides observed at X430, X80O magnifications are also similar to the structural characteristics of the deposited metal of the 30Kh2V8F type. Originality. At close values of the contact pressure in the friction pair, the time of formation of a crack of critical length increases with an increase in the effective surface energy γе (including the energy of plastic deformation). Thus, the crack resistance indices (CR, j-integral, δС) and, consequently, the wear resistance of maraging steels are higher than those of metastable and tool steels. Practical value. The crack resistance indices (CR, j-integral, δС) and, consequently, the wear resistance of maraging steels are higher than those of metastable and tool steels. Conclusions. The conducted studies confirm the possibility of the formation of a "white band" both in alloys with a high concentration of elements - austenitizers (Mn, C, Ni), and when alloyed with carbide-forming elements with a relatively low affinity for carbon (V, Mo).

STUDY OF THE EFFECT OF STRUCTURE AND PHASE COMPOSITION ON THE WEAR RESISTANCE OF SPARINGLY ALLOYED METASTABLE AND SECONDARY HARDENING STEELS

Introduction. The paper considers the influence of wear on the formation of a “white band” in metastable austenitic, martensitic-austenitic and secondary hardening steels of the Cr−Mn−Ti system additionally alloyed with Mo, B, V. The influence of the structure and phase composition on the wear resistance of sparing. Results. Surfacing of the studied materials was carried out in copper molds with different rates of forced cooling. Metastable austenitic, martensitic-austenitic and secondary hardening steels of the Cr−Mn−Ti system additionally alloyed with Mo, B, V were studied. Additional alloying of these steels with titanium in an amount of 2...5 % contributed to the prevention of spalling along the fusion zone. Near the fusion line there is a base metal zone with a width of 7...15 µm. After testing at the volume temperature of the working part of the specimen ТV = 553…573 K in the contact volumes of the deposited metal of the 30Kh2V8F type, broadening of the grain boundaries, shear lines, finer grains compared to the underlying layers were revealed. Outside the zone of plastic deformation, the size of the grains corresponds to their sizes before the start of testing, the grain boundaries are relatively thin. The number and location of carbides observed at X430, X80O magnifications are also similar to the structural characteristics of the deposited metal of the 30Kh2V8F type. At close values of the contact pressure in the friction pair, the time of formation of a crack of critical length increases with an increase in the effective surface energy γе (including the energy of plastic deformation). Thus, the crack resistance indices (CR, j-integral, δС) and, consequently, the wear resistance of maraging steels are higher than those of metastable and tool steels. Conclusions. The conducted studies confirm the possibility of the formation of a “white band” both in alloys with a high concentration of elements − austenitizers (Mn, C, Ni), and when alloyed with carbide-forming elements with a relatively low affinity for carbon (V, Mo). The crack resistance indices (CR, j-integral, δС) and, consequently, the wear resistance of maraging steels is higher than those of metastable and tool steels.

Effect of Cerium on Inclusion Modification in a Secondary-Hardening Steel.

Owing to the continuous increasing of steel strength, mechanical properties including toughness and fatigue performance are becoming increasingly sensitive to inclusions in ultra-high strength steel. Rare-earth treatment is considered as an effective method to reduce the harmful effects of inclusions, but is rarely applied in secondary-hardening steel. In the present study, different amounts of cerium were added in a secondary-hardening steel to investigate the modification effect of Ce on non-metallic inclusions in steel. The characteristics of inclusions were observed experimentally using SEM-EDS and the modification mechanism was analyzed based on thermodynamic calculations. The results indicated that the main inclusions in Ce-free steel are Mg-Al-O + MgS. Thermodynamic calculation indicated that MgAl2O4 is firstly formed in liquid steel and then successively transformed into MgO and MgS during cooling process. When the Ce content is 0.0030%, the typical inclusions in steel were individual Ce2O2S and MgO + Ce2O2S complex inclusions. When the Ce content was increased to 0.0071%, the typical inclusions in steel were individual Ce2O2S- and Mg-containing inclusions. Ce treatment modifies the angular magnesium aluminum spinel inclusions into spherical and ellipsoidal Ce-containing inclusions, thus reducing the harmful effect of inclusion on steel properties.

STUDY OF WEAR OF THE BUILDING-UP ZONE OF MARTENSITE-AUSTENITIC AND SECONDARY HARDENING STEELS OF THE Cr-Mn-Ti SYSTEM

The work deals with wear of the building-up zone of martensite-austenitic and secondary hardening steels of the Cr-Mn-Ti system (partially alloyed with Mo, B, and V). Additional alloying of the studied steels with titanium in the amount of 2…5% supported avoiding cleavages along the building-up zone. It was determined that there is a 7…15 µm wide parent metal zone next to the weld line. The influence of temperature on the weld toughness of the building-up zone was assessed according to the criterion of Malkin and Tetelman. The microhardness of the surface layers of built-up layers and the range of its dispersion are close to the results of laboratory tests, where ТСт ~ 823 K. This approves the formation of structure of metal of contact volumes with the collective effect of heating, plastic deformation and diffusion. It was experimentally approved that the change of microhardness in the sub-surface layers at the wear stage is justified simultaneously with the processes of mutual diffusion of friction pair materials, selective oxidation and thermodiffusion redistribution of the hardening phase under the influence of temperatures and deformations. The diffusion coefficient in the deformed sub-surface layer, which is up to some dozen micrometers thick, is one of the values providing the kinetics of growing of nuclear cracks and development of destruction cracks. The received results provide an opportunity to use martensite-austenitic and secondary hardening steels of the Cr-Mn-Ti system with built-up surface for the hot forming tools.

Зміцнення інструменту для холодної роликової прокатки корозійностійких труб

The work is devoted to a topical technical problem - to increase the wear resistance of pipe tools for cold roller rolling of corrosion-resistant pipes, due to its significant cost and difficult working conditions. It is proposed to improve the technology of heat treatment of rolled tools for the production of corrosion-resistant pipes (mandrels, rollers, support strips) from steels 4Х5МФ1С and 60С2ХФА. Instead of the usual technology - tempering with tempering, tempering with tempering and subsequent ionic nitriding in plasma of two-stage vacuum-arc discharge in a modified installation "Bulat-6" in the laboratory of plasma technology. The microstructure of special samples of these steels on a scanning electron microscope (SEM) at different magnifications was studied. Their microhardness was determined using a microhardness tester PMT-3. A comparison of the microhardness of the samples after traditional treatment and the proposed. The wear resistance of the actual tool (mandrels made of 60С2ХФA steel) for cold roller rolling of particularly thin-walled corrosion-resistant pipes was studied directly during industrial tests in the factory. Studies of the microstructure on the REM of the nitrided layer after ion-plasma nitriding revealed a finely dispersed very strong ε-phase. Due to its presence in the surface layer, the surface hardness of nitrided parts increases due to the formation of stable in the process of heating carbides, nitrides, carbonitrides. Steel acquires high hardness on the surface, high wear resistance, high endurance, corrosion resistance. Tests of mandrels in the factory, which were additionally subjected to ionic nitriding, showed an increase in their stability by 25-30%. Also reduced the cost of processing the manufacture of pipes and improved the quality of the inner surface of the pipes (absence of films, cuts and other defects of corrosion-resistant pipes). Replacement of steel 60С2ХФA with secondary hardening steel 4Х5MФ1С for the manufacture of rollers and support bars in addition to improving the quality of the inner surface of the pipes increased the stability of the tool by 2.7-3 times. Keywords: rollers, support laths, mandrels, cold rolling, heat treatment, ionic nitriding.

Influence of cooling rate during cryogenic treatment on the hierarchical microstructure and mechanical properties of M54 secondary hardening steel

Cryogenic treatment (CT) is an essential heat treatment process in improving the mechanical properties of secondary hardening ultra-high strength steel (SH-UHSS). The present study investigated the influence of cooling rate during CT on the hierarchical microstructure and strength-toughness of M54 SH-UHSS. Increasing cooling rate can synchronously refine martensite matrix and M2C-type (M = Mo, Cr, W, V) carbides, while there are the most refined martensite blocks and highest-density precipitation of the carbides at cooling rate of 3 °C·min-1. Further increased cooling rate weakens the effect of CT on the refinement and precipitation of martensite matrix. The refinement is related to the high-level segregation of carbon atoms and favorable equilibrium concentration of vacancies during CT and pinning effect of carbides on mobile dislocation during tempering. Besides, uniform carbon clusters and high-nucleation rate by the refined martensite matrix and high-density of dislocations mostly contribute to the above beneficial precipitation of carbides. Precipitation and martensitic matrix strengthening mostly contribute to the ultra-high strength (yield strength of 1730 MPa, ultimate tensile strength of 2018 MPa) at the cooling rate of 3 °C·min-1; meanwhile, the refined blocks as "effective" controlling unit is identified to be the major toughening mechanism contributing to a desirable impact toughness (V-notched Charpy impact energy of 30 J). This study would be instructive for processing the engineering components with large cross-section.

Comprehensive analysis of nitriding behavior and synergistic strengthening mechanism on quenched M50 steel

M50 secondary hardening steel is widely used as high-end bearing steel for engine shafts in the aerospace industry. In this study, plasma nitriding at different temperatures varying from 410 to 570 °C was performed on quenched M50 steel to investigate the response of the microstructure and mechanical properties to nitriding temperature and advance the scientific understanding of the mechanism involved. The nitrided samples were fully characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron microprobe analysis, Vickers hardness tester, and Pin-on-disc tribometer. The results revealed that the nitrided layer's thickness, microstructure, hardness, and wear resistance depended on the treatment temperature. When the nitriding treatment performs at secondary-hardening temperature, the samples were endowed with excellent wear resistance and gentle hardness profile, i.e., hardness profile with a gradually reducing hardness profile towards the core, attributed to the synergistic strengthening. Moreover, the thermodynamic calculation was involved to clarify the formation mechanism of coarse intergranular precipitation, which can be connected to form a wave shape line at higher temperatures.

Effect of structure and phase content on wear resistance of the economical alloyed metastable and secondary hardening steels of Cr-Mn-Ti system

Despite a large number of studies in the field of assessing the causes of the formation of hot and cold cracks during surfacing of wear-resistant alloys, today the issues of working out the use of economically alloyed wear-resistant materials and the technique of their surfacing remain relevant. Goal: The purpose of this work is to study the effect of the structure and phase composition on the wear resistance of economically alloyed metastable and secondary hardening steels of the Cr-Mn-Ti system, as well as with additional alloying with Mo, B, V. Mechanized surfacing was carried out with flux cored wires AN-22 and AN-20 with the supply of a de-energized additive to the head of the weld pool, which reduces the content of sulfur and phosphorus, the specific consumption of electricity and increases the assimilation of alloying elements and the relative mass of the flux. Cladding by manual arc welding was carried out with coated electrodes with the addition of a depleted CaF2-coated flux-cored wire filler. When surfacing with a de-energized additive, the ratio of the filler to the main electrode, the relative mass was determined by β = m1 / m2 (m1, m2 are the mass of the filler and the electrode rod, respectively). Submerged arc surfacing was carried out in the following modes: IN = 300 ... 350 A, UD = 26 ... 30 V, q = 6 ... 10 kJ / cm, with manual surfacing - IN = 180 ... 220 A, UD = 25 ... Results: The studies carried out confirm the possibility of the formation of a “white band” both in alloys with a high concentration of austenitizing elements (Mn, C, Ni) and when alloying carbide-forming elements with a relatively low affinity for carbon (V, Mo). The indicators of resistance to cracking (КС, j-integral, δС), and, consequently, resistance to wear of secondary hardening steels are higher than those of metastable and tool steels.

Direct achievement of ultra-high strength and good ductility for high Co-Ni secondary hardening steel by combining spark plasma sintering and deformation

A combination of spark plasma sintering and deformation was developed to fabricate ultra-high strength steel with good ductility in this investigation. Microstructure and mechanical properties of the produced M54 steel were subsequently investigated. Martensite matrix produced by distinctive mechanism of spark plasma sintering led to ultra-high strength of the specimen S and D. Also, the specimen D exhibited good ductility due to gradually closing of micro-pores during SPS compression. Thus, based on the combined procedures, the produced high Co-Ni steel directly achieved ultra-high strength and good ductility without complicated traditional heat treatment process.

Effect of structure and phase composition on wear resistance of economical alloyed metastable and secondary hardening steels of Cr-Mn-Ti system

Problem. Despite a large number of studies in the field of assessing the causes of hot and cold cracks in the surfacing of wear-resistant surfacing alloys, today the issues of developing relatively inexpensive alloyed wear-resistant surfacing materials and the technology of their application remain relevant. Purpose. The purpose of this work is to study the effect of structure and phase composition on the wear resistance of economically alloyed metastable austenitic and secondary-hardening steels of the Cr-Mn-Ti system, additionally alloyed with Mo, B, V. Methodology. Metastable austenitic, martensitic-austenitic and secondary-hardening steels of the Cr-Mn-Ti system additionally alloyed with Mo, V were investigated. Mechanized surfacing was carried out with alloyed flux-cored wires under fluxes AN-22 and AN-20 with the supply of a de-energized filler flux-cored wire to the head of the welding pool. The use of surfacing with a de-energized flux-cored wire made it possible to reduce the proportion of the base metal in the deposited metal, to reduce the specific consumption of electricity and flux, and to increase the assimilation of alloying elements in the deposited metal. Manual arc surfacing was performed with a CaF2-coated flux-cored wire electrode and a de-energized flux-cored wire in a copper die. Automatic submerged arc surfacing was carried out in the following modes: IN = 300 ... 350 A, UD = 26 ... 30 V, q = 6 ... 10 kJ / cm, with manual surfacing: IN =180 ... 220 A, UD = 25 ... 28 V. Results. The studies carried out confirm the possibility of the formation of a "white band" both in alloys with a high concentration of elements - austenizers (Mn, C, Ni), and when alloying with carbide-forming elements with a relatively low affinity for carbon. (V, Mo). The indicators of resistance to cracking (КС, j-integral, δС), and, consequently, resistance to wear, of secondary hardening martensitic steels are higher than of metastable and tool steels.

Influence of thermomechanical fatigue loading conditions on the nanostructure of secondary hardening steels

Abstract Dual hardening steels reach their well-balanced mechanical properties in terms of strength and toughness through the combination of secondary hardening carbide and intermetallic particle precipitation. This characteristic profile makes them well suited for hot-work applications. In this study, out-of-phase thermomechanical fatigue tests, recreating operating conditions present during hot-work applications, were performed on a dual hardening steel and a 5% Cr martensitic hot-work tool steel. Via high resolution analysis utilizing atom probe tomography and transmission electron microscopy, the behaviour of the different precipitate populations under combined thermal and mechanical loading conditions were compared. Coarsening of the different precipitates and partial dissolution of the intermetallic compounds was observed. It could be shown that with rising maximum fatigue test temperature, the dual hardening steel reaches an increased lifetime caused by its higher tempering resistance.