Showing papers on "Tempering published in 2022"

Tensile strength improvement of martensitic ODS steels with Zr and Hf additions

Abstract: Aimed at evaluating the effectiveness of Zr and Hf additions on oxide particle refinement and tensile strength improvement of transformable oxide dispersion strengthened (ODS) steels, 9Cr–Zr and 9Cr–Hf ODS martensitic steels were fabricated by mechanical milling, spark plasma sintering and tempering heat treatment at 800 °C. The as-sintered 9Cr–Zr steel consisted of martensite and certain amount of transformed ferrite with M23C6 (M = Fe, Cr) precipites within ferrite interior. With respect to the as-sintered 9Cr–Hf steel, the microstructure consisted of martensite and small amount of residual ferrite. The residual ferrite is characteristics of higher number density of oxide nanoparticles than those in martensite. The added Zr or Hf cannot transform all Y2O3 into Y–Zr–O or Y-Hf-O complex oxides. Except for Y2O3, ZrO2 and Y4Zr3O12 were identified in 9Cr–Zr steel, and YxHfyOz and Y2H2O7 were found in 9Cr–Hf steel. The orientation relationship and interface structure between Y–Zr–O/Y-Hf-O and the matrix were studied in detail, as well as the size distributions of oxide nanoparticles in both as-sintered and tempered conditions. Tempering is mandatory to restore ductility of martensitic steels. The tensile strength of tempered 9Cr–Hf steel is superior to most reported results, while the tempered 9Cr–Zr steel exhibits a good combination of tensile strength and ductility. The ferrite phase in both tempered 9Cr–Zr and 9Cr–Hf steels has negligible effects on ductility degradation. The inferior strength of high-angle boundaries to tempered martensite itself is suggested to be responsible for limited ductility of steels. Compared to Al and Ti constituents, Zr and Hf can be considered as potential microalloying elements to produce ODS steels with favorable mechanical performance.

Investigation of the microstructure and mechanical properties of the laser welded joint of P22 and P91 steel

Abstract: The present manuscript investigates the microstructural aspects and mechanical properties of laser beam welded ASTM A335 Grade P91 and P22 steel joint for power plant applications. Detailed microstructure characterization of the weld metal (WM) and heat-affected zone (HAZ) were carried out in as-welded and post-weld heat treatment (PWHT) conditions. A variation in microstructure was observed along the welded joint. This resulted in inhomogeneity in mechanical properties. The PWHT resulted in the formation of the stabilized microstructure in weldments and reduced heterogeneity in mechanical properties along the weldments. Tensile strength of weld joint in as-welded (AW) and PWHT condition was found slightly higher (617 MPa and 628 MPa, respectively) than the tensile strength of P22 steel (610 MPa). The fracture location for both AW and PWHT was observed in the P22 base region, which indicates that welded joint is stronger than the base material. Maximum tensile strength of the WZ was found 864 MPa in the as-welded condition. The hardness of WM, P91 CGHAZ and P22 CGHAZ were found 376 HV, 420 HV and 302 HV, respectively. After the PWHT, the hardness of the WM, P91 CGHAZ, P91 ICHAZ, P22 CGHAZ and P22 ICHAZ were measured 237 HV, 264 HV, 212 HV, 208 HV and 190 HV, respectively. The reduction in hardness is attributed to the tempering reaction, which results in the formation of the tempered martensite in WM and HAZ of P91 and tempered bainite in P22 HAZ. The impact toughness of the weld metal in as-welded condition was measured 85 J, which was lower than P22 and P91 steel. PWHT resulted in a drastic increase in impact toughness of WM and it was 145 J. The impact toughness of the P22 and P91 HAZ was measured 146 J and 92 J, respectively, for AW and 168 J and 140 J, respectively, after PWHT. The optimum microstructure and mechanical properties of the dissimilar welded joint were obtained after the PWHT.

Tensile strength improvement of martensitic ODS steels with Zr and Hf additions

Abstract: Aimed at evaluating the effectiveness of Zr and Hf additions on oxide particle refinement and tensile strength improvement of transformable oxide dispersion strengthened (ODS) steels, 9Cr–Zr and 9Cr–Hf ODS martensitic steels were fabricated by mechanical milling, spark plasma sintering and tempering heat treatment at 800 °C. The as-sintered 9Cr–Zr steel consisted of martensite and certain amount of transformed ferrite with M23C6 (M = Fe, Cr) precipites within ferrite interior. With respect to the as-sintered 9Cr–Hf steel, the microstructure consisted of martensite and small amount of residual ferrite. The residual ferrite is characteristics of higher number density of oxide nanoparticles than those in martensite. The added Zr or Hf cannot transform all Y2O3 into Y–Zr–O or Y-Hf-O complex oxides. Except for Y2O3, ZrO2 and Y4Zr3O12 were identified in 9Cr–Zr steel, and YxHfyOz and Y2H2O7 were found in 9Cr–Hf steel. The orientation relationship and interface structure between Y–Zr–O/Y-Hf-O and the matrix were studied in detail, as well as the size distributions of oxide nanoparticles in both as-sintered and tempered conditions. Tempering is mandatory to restore ductility of martensitic steels. The tensile strength of tempered 9Cr–Hf steel is superior to most reported results, while the tempered 9Cr–Zr steel exhibits a good combination of tensile strength and ductility. The ferrite phase in both tempered 9Cr–Zr and 9Cr–Hf steels has negligible effects on ductility degradation. The inferior strength of high-angle boundaries to tempered martensite itself is suggested to be responsible for limited ductility of steels. Compared to Al and Ti constituents, Zr and Hf can be considered as potential microalloying elements to produce ODS steels with favorable mechanical performance.

Investigation of the microstructure and mechanical properties of the laser welded joint of P22 and P91 steel

Abstract: The present manuscript investigates the microstructural aspects and mechanical properties of laser beam welded ASTM A335 Grade P91 and P22 steel joint for power plant applications. Detailed microstructure characterization of the weld metal (WM) and heat-affected zone (HAZ) were carried out in as-welded and post-weld heat treatment (PWHT) conditions. A variation in microstructure was observed along the welded joint. This resulted in inhomogeneity in mechanical properties. The PWHT resulted in the formation of the stabilized microstructure in weldments and reduced heterogeneity in mechanical properties along the weldments. Tensile strength of weld joint in as-welded (AW) and PWHT condition was found slightly higher (617 MPa and 628 MPa, respectively) than the tensile strength of P22 steel (610 MPa). The fracture location for both AW and PWHT was observed in the P22 base region, which indicates that welded joint is stronger than the base material. Maximum tensile strength of the WZ was found 864 MPa in the as-welded condition. The hardness of WM, P91 CGHAZ and P22 CGHAZ were found 376 HV, 420 HV and 302 HV, respectively. After the PWHT, the hardness of the WM, P91 CGHAZ, P91 ICHAZ, P22 CGHAZ and P22 ICHAZ were measured 237 HV, 264 HV, 212 HV, 208 HV and 190 HV, respectively. The reduction in hardness is attributed to the tempering reaction, which results in the formation of the tempered martensite in WM and HAZ of P91 and tempered bainite in P22 HAZ. The impact toughness of the weld metal in as-welded condition was measured 85 J, which was lower than P22 and P91 steel. PWHT resulted in a drastic increase in impact toughness of WM and it was 145 J. The impact toughness of the P22 and P91 HAZ was measured 146 J and 92 J, respectively, for AW and 168 J and 140 J, respectively, after PWHT. The optimum microstructure and mechanical properties of the dissimilar welded joint were obtained after the PWHT.

Influence of heat treatment parameters on effectiveness of deep cryogenic treatment on properties of high-speed steels

Abstract: This study investigates the influence of heat treatment parameters on effectiveness of deep cryogenic treatment (DCT) and on properties of three high-speed steels (HSS) (AISI M2, AISI M3:2 and AISI M35) with different chemical composition. Within this research the same preselected soaking temperature, time, cooling/warming rate and placement of DCT, being found in the literature as the most effective, were applied to the selected HSS grades. The fatigue resistance, hardness, impact and fracture toughness, compressive strength and strain-hardening exponent were selected as the evaluators of DCT effectiveness. The results indicate that the influence and effectiveness of DCT is strongly subjected by the type (wrought or PM steel), chemical composition of the particular steel grade and heat treatment parameters, resulting in either degradation or improvement of the steel properties. Furthermore, the study elucidates the effect of DCT on different HSS grades based on the selected heat treatment temperatures (austenitization and tempering). The research confirms that DCT generally improves the hardness, fatigue properties, fracture and impact toughness, compressive strength and strain-hardening exponent for HSS grades. However, it may also have negative impact on HSS properties. For all HSS investigated, the level of DCT effect depended also on the heat treatment conditions, i.e. austenitization and tempering temperature. The lower austenitization and higher tempering temperature in general resulted in improvement of steel properties, as compared to higher austenitization and lower tempering temperature with predominantly negative effect.

Influence of heat treatment parameters on effectiveness of deep cryogenic treatment on properties of high-speed steels

Abstract: This study investigates the influence of heat treatment parameters on effectiveness of deep cryogenic treatment (DCT) and on properties of three high-speed steels (HSS) (AISI M2, AISI M3:2 and AISI M35) with different chemical composition. Within this research the same preselected soaking temperature, time, cooling/warming rate and placement of DCT, being found in the literature as the most effective, were applied to the selected HSS grades. The fatigue resistance, hardness, impact and fracture toughness, compressive strength and strain-hardening exponent were selected as the evaluators of DCT effectiveness. The results indicate that the influence and effectiveness of DCT is strongly subjected by the type (wrought or PM steel), chemical composition of the particular steel grade and heat treatment parameters, resulting in either degradation or improvement of the steel properties. Furthermore, the study elucidates the effect of DCT on different HSS grades based on the selected heat treatment temperatures (austenitization and tempering). The research confirms that DCT generally improves the hardness, fatigue properties, fracture and impact toughness, compressive strength and strain-hardening exponent for HSS grades. However, it may also have negative impact on HSS properties. For all HSS investigated, the level of DCT effect depended also on the heat treatment conditions, i.e. austenitization and tempering temperature. The lower austenitization and higher tempering temperature in general resulted in improvement of steel properties, as compared to higher austenitization and lower tempering temperature with predominantly negative effect.

Effects of hot air assisted radio frequency drying on heating uniformity, drying characteristics and quality of paddy

Abstract: In order to solve the big challenge of cracking during paddy drying, this research revealed the application of hot air assisted radio frequency (HA-RF) drying technology on paddy grains based on its volumetric heating advantage. Drying characteristics and paddy quality were investigated at various combinations of air temperatures (50 °C, 55 °C and 60 °C), air velocities (0.5 m/s, 1.5 m/s and 2.5 m/s) and electrode gaps (100 mm, 110 mm and 120 mm). A comparison between the tempering-intermittent hot air (HA) drying was investigated to demonstrate the applicability of HA-RF drying on paddy grains. Increase in temperature and air velocity, while decrease in electrode gap resulted in increased average heating rate. The RF assistance significantly enhanced the drying rate of paddy grains and reduced drying duration and energy consumption by up to 62.5% and 42.9%, respectively, as compared to HA drying. A higher color and taste value, while a lower crack ratio (CR) without damage in starch granules of microstructure which caused formations of internal fissures were observed in HA-RF dried samples than HA dried ones. The findings of this study demonstrated that HA-RF drying is a good alternative of HA for industrial-scale drying of paddy grains. • Hot air assisted radio frequency (HA-RF) has potential alternative for rice drying. • HA-RF decreased 62.5% of drying time and 42.9% of energy consumption than hot air. • HA-RF reduced crack ratio as well as preserved color and taste value than hot air. • RF heating was combined with hot-air drying for enhancing efficiency. • No damage was observed in starch granules microstructure of HA-RF dried samples.

Effect of tempering temperature on microstructure and mechanical properties of nanostructured bainitic steel

Abstract: The previous research on tempered nanostructured bainite has been mainly focused on the carbide precipitation at high temperatures with few details on nanostructured bainite tempered at lower temperatures. In this research, the microstructure evolution and the corresponding mechanical properties of the nanostructured bainite tempered at a temperature ranged from 180 °C to 400 °C were studied via X-ray diffraction, transmission electron microscope and high-resolution thermal dilatometer. Results show that retained austenite occurred bainitic transformation when the tempering temperature ranged from 210 °C to 320 °C. The amount of newly formed bainite increased at first and then decreased slightly with increasing tempering temperature. When the tempering temperature reached 380 °C and higher, the cementite precipitated from retained austenite and reduced the stability of over-stable retained austenite. As a result, it can be concluded that stress-induced martensitic transformation would occur easily in retained austenite during deformation process, leading to the optimum increase of retained austenite contribution to toughness. coefficient of carbon increased sharply, the dispersed cementite particles precipitated between the nanostructured bainitic plates, inhibiting coarsening of bainitic plates when the tempering temperature reached 380 °C. Finally, nanostructured bainitic plates only coarsened slightly. Therefore this high-carbon high-silicon nanostructured bainitic plate possess a high dimensional stability.

Effect of tempering temperature on microstructure and mechanical properties of nanostructured bainitic steel

Abstract: The previous research on tempered nanostructured bainite has been mainly focused on the carbide precipitation at high temperatures with few details on nanostructured bainite tempered at lower temperatures. In this research, the microstructure evolution and the corresponding mechanical properties of the nanostructured bainite tempered at a temperature ranged from 180 °C to 400 °C were studied via X-ray diffraction, transmission electron microscope and high-resolution thermal dilatometer. Results show that retained austenite occurred bainitic transformation when the tempering temperature ranged from 210 °C to 320 °C. The amount of newly formed bainite increased at first and then decreased slightly with increasing tempering temperature. When the tempering temperature reached 380 °C and higher, the cementite precipitated from retained austenite and reduced the stability of over-stable retained austenite. As a result, it can be concluded that stress-induced martensitic transformation would occur easily in retained austenite during deformation process, leading to the optimum increase of retained austenite contribution to toughness.

Effect of post-weld tempering pulse on microstructure and mechanical properties of resistance spot welding of Q&P1180 steel

Abstract: Resistance spot welds (RSWs) of quench and partitioning (Q&P) steels exhibit poor mechanical properties due to the brittleness of the nugget microstructure. To solve this problem, the nugget microstructure in RSWs of Q&P1180 steel was modified by applying a post-weld tempering pulse with different protocols. Effects of the post-weld tempering pulse on microstructural evolution and mechanical properties of RSWs were investigated. The results indicate that a proper post-weld tempering pulse (cooling time of 1000 ms and post welding current of 4 kA) achieved a 70% improvement in the cross-tension peak load and a 6% improvement in the tensile-shear peak load. The nugget, consisting of tempered martensite and retained austenite with low hardness and high toughness, hindered the crack propagation towards the nugget during the cross-tension test and improved cross-tension property. In the tensile-shear testing, a new failure mechanism was discovered. Despite the decrease in nugget hardness, the improvement of toughness helped enlarge the effective area subjected to the shear load during the tensile-shear test, resulting in the improvement of the tensile-shear property.

Effect of post-weld tempering pulse on microstructure and mechanical properties of resistance spot welding of Q&P1180 steel

Abstract: Resistance spot welds (RSWs) of quench and partitioning (Q&P) steels exhibit poor mechanical properties due to the brittleness of the nugget microstructure. To solve this problem, the nugget microstructure in RSWs of Q&P1180 steel was modified by applying a post-weld tempering pulse with different protocols. Effects of the post-weld tempering pulse on microstructural evolution and mechanical properties of RSWs were investigated. The results indicate that a proper post-weld tempering pulse (cooling time of 1000 ms and post welding current of 4 kA) achieved a 70% improvement in the cross-tension peak load and a 6% improvement in the tensile-shear peak load. The nugget, consisting of tempered martensite and retained austenite with low hardness and high toughness, hindered the crack propagation towards the nugget during the cross-tension test and improved cross-tension property. In the tensile-shear testing, a new failure mechanism was discovered. Despite the decrease in nugget hardness, the improvement of toughness helped enlarge the effective area subjected to the shear load during the tensile-shear test, resulting in the improvement of the tensile-shear property.

Microstructural variations and mechanical properties of deep cryogenic treated AISI M35 high-speed steel tempered at various temperatures

Abstract: Deep cryogenic treatment applied in high-speed steel yielded promising results, and it was found that tempering played a vital role. This paper systematically studied the effect of tempering temperatures on the microstructural composition (residual austenite, martensite, and carbides) and mechanical properties (Vickers hardness and impact toughness) of AISI M35 high-speed steel subjected to deep cryogenic treatment. An increase in the tempering temperatures facilitated the transformation of residual austenite and the formation of martensite blocks corresponding to a decrease in the number of dislocations. Moreover, the carbide precipitation (secondary carbides and nanoscale carbides) began at a tempering temperature of 350 °C, increased at 450 °C, and reached its maximum at 550 °C. The fracture mechanism on the micro-level could be interpreted as follows: cracks occurred in the carbides with larger sizes (primary carbides and large secondary carbides) and at the carbide/matrix interface, and small secondary carbides decohered at the interface, forming microvoids and facilitating plastic deformation. In addition, the specimen tempered at 150 °C exhibited the highest hardness of 880.4 HV 1 due to the highest number of dislocations. The impact toughness of the sample tempered at 550 °C was the best, namely 2.50 MJ m −2 , due to an increase in the number of martensite block boundaries and the more homogeneous carbide precipitation.

Heat treatment induced microstructural evolution, oxidation behavior and tribological properties of Fe-12Cr-9Ni-2Al steel (CX steel) prepared using selective laser melting

Abstract: A novel type of martensitic precipitation hardening stainless steel, Fe-12Cr-9Ni-2Al steel (CX steel), was successfully fabricated via selective laser melting (SLM) technology. The influence of the heat treatment regime on the surface microstructure, high-temperature oxidation resistance and tribological properties of the SLM Fe-12Cr-9Ni-2Al steel was studied. A series of vacuum heat treatment methods including austenitizing treatment and tempering treatment were determined by means of a differential scanning calorimetry (DSC) method. Reverted austenite (γ-Fe phase) would be induced by tempering treatment to generate along the martensite (α′-Fe phase) grain boundaries. During a 400 °C high-temperature oxidation experiment, a loose oxide layer consisted of Al2O3, Cr2O3 and Fe3O4 was formed on its surface. A high-temperature oxidation process model was put forward to illustrate this phenomenon. After austenitizing-tempering treatment, massive nano-NiAl precipitates enhanced the hardness of the SLM CX steel from 356 ± 14.3 HV0.2 in the as-built state to 527 ± 5.2 HV0.2 in the austenitizing-tempering treated state. Correspondingly, the coefficient of friction (COF) and wear rate of the SLM Fe-12Cr-9Ni-2Al steel samples decreased from 0.58 and 16.1 ± 3.1 × 10−5 mm3/(N·m) in the as-built state to 0.49 and 8.1 ± 3.3 × 10−5 mm3/(N·m) in the austenitizing-tempering treated state, respectively.

Strengthening-toughening mechanism of cost-saving marine steel plate with 1000 MPa yield strength

Abstract: One kind of cost-saving marine steel plate with 1000 MPa yield strength was successfully processed based on the alloying concept of nano-particles precipitation in a lath martensitic matrix. The conventional quenching and tempering heat treatment was adopted to optimize the mechanical properties. The experimental steel with lower alloy content exhibited excellent performance, i.e., the yield strength above 1030 MPa at room temperature, and the impact energy above 69 J at −40 °C. The microstructural evolution, precipitation behavior and dislocation density were systematically studied by optical microscopy, field emission scanning electron microscopy, X-ray diffraction and transmission electron microscopy. It was shown that the high strength resulted from grain refinement, dislocation strengthening, solid solution strengthening and precipitation strengthening. The role of every strengthening mechanism was quantitively evaluated so that the yield strength could be accurately estimated. Base on this study, it could be concluded that the precipitation strengthening had a significant contribution to yield strength. In addition, the excellent impact toughness of the steel was attributed to its unique microstructure characteristics of fine film-like stable austenite and delamination structure.

Residual stress evolution enhanced martensite phase transition and texture development in cryogenic-tempered WC-Co ultra-coarse grained cemented carbide

Abstract: The aim of this study is to clarify the effect of residual stress on the microstructure, martensite phase transition of Co binder phase, texture development of WC hard phase and mechanical properties enhancement in cryogenic-tempered WC-6wt.%Co ultra-coarse grained cemented carbides. Sin2(ψ)-2θ method was used to analyze the residual stress, and EBSD was used to reveal the phase transition of Co phase and texture of WC phase. The results showed that the density, grain size and carbon balance of cemented carbide revealed little difference after cryogenic-tempering treatment. The mean compressive residual stress of WC phase increased by 60.84% after cryogenic treatment, while tempering treatment resulted in a significant relaxation of the residual stress by 101.97% and even transformed the compressive residual stress to tensile stress. EBSD analysis showed that the mean ratio of hcp-to-fcc Co increased from 1.75% to 4.22% and 11.61%. It is difficult to distinguish the effect of residual stress increase and the temperature decrease on the martensite transition during the cooling process. However, the occurrence of martensite phase transition during tempering is not temperature-dependent but highly related to the residual stress evolution. After cryogenic-tempering treatment, {0001}<11 2‾ 0> preferred orientation of WC phase was concentrated, which was related to the basal plane slip and grain deflection of WC grains during the evolution of residual stress. Due to the martensite phase transition strengthening of Co and the preferred orientation of WC, the mean Vickers hardness increased from 1068.4 HV30 to 1149.4 HV30. The mean transverse rupture strength firstly decreased from 2747.5 MPa to 2515.3 MPa, and then increased to 2705.0 MPa, which was related to the increase and relief of residual stress during the post treatment. Furthermore, mean fracture toughness showed a slight increase from 22.5 MPa m1/2 to 23.4 MPa m1/2. The simultaneous increase of fracture toughness with hardness can be attributed to the residual stress evolution and martensite phase transition.

Effect of various post-extrusion tempering on performance of AA2024 tubes fabricated by shear assisted processing and extrusion

Abstract: • Extrusion speed of ShAPE for AA2024-T371 billets reaches 7.4 m/min. • The yield strength of the AA2024-T8510 ShAPE-extruded tube is 32% higher than typical ASM and minimum ASTM values. • The elongation at break of ShAPE-extruded AA2024-T8510 tubes is doubled than typical ASM and minimum ASTM values. Aluminum alloy 2024 tubes were extruded using the shear assisted processing and extrusion (ShAPE) method. Extrusions were produced at 7.4 m/min at 482 °C using the ShAPE technique which is more than doubles the previous highest extrusion speed for AA2024 using conventional extrusion methods. Standard T3510 and T8510 post extrusion heat treatments were applied to improve the mechanical performance of ShAPE extruded AA2024 tube. The ultimate tensile and yield strength of the AA2024-T8510 ShAPE-extruded tube are 522.0 ± 3.3 MPa and 510.7 ± 3.3 MPa, which are respectively 18% and 32% higher than typical ASM International and minimum American Society for Testing and Materials (ASTM) values. The elongation at break of ShAPE-extruded AA2024-T8510 tubes is two times higher than the typical ASM International and minimum ASTM values. This improved ductility is attributed to the refinement of grain size and secondary phases as well as the uniform dispersion of sub-micron strengthening precipitates formed during the ShAPE processing.

Strengthening-toughening mechanism of cost-saving marine steel plate with 1000 MPa yield strength

Abstract: One kind of cost-saving marine steel plate with 1000 MPa yield strength was successfully processed based on the alloying concept of nano-particles precipitation in a lath martensitic matrix. The conventional quenching and tempering heat treatment was adopted to optimize the mechanical properties. The experimental steel with lower alloy content exhibited excellent performance, i.e., the yield strength above 1030 MPa at room temperature, and the impact energy above 69 J at −40 °C. The microstructural evolution, precipitation behavior and dislocation density were systematically studied by optical microscopy, field emission scanning electron microscopy, X-ray diffraction and transmission electron microscopy. It was shown that the high strength resulted from grain refinement, dislocation strengthening, solid solution strengthening and precipitation strengthening. The role of every strengthening mechanism was quantitively evaluated so that the yield strength could be accurately estimated. Base on this study, it could be concluded that the precipitation strengthening had a significant contribution to yield strength. In addition, the excellent impact toughness of the steel was attributed to its unique microstructure characteristics of fine film-like stable austenite and delamination structure.

Selected Properties of RAMOR 500 Steel Welded Joints by Hybrid PTA-MAG

Abstract: The paper describes the selected properties of RAMOR 500 anti-ballistic (martensitic structure in the initial state) high yield strength steel (1450 MPa) welded joints produced by the hybrid PTA-MAG (Plasma Transferred Arc - Metal Active Gas) method. The welded elements were metal sheets 6.7 mm thick in a rectangular shape with dimensions of 200 mm x 350 mm. The tested butt weld joints have been made with process parameters selected according to criterion of lowest level of material weakening in the heat-affected zone (HAZ). The results of metallographic research of welds heat affected zone and base material, hardness distribution and XRD patterns of specific areas have been presented. Depth-sensing indentation (DSI) is used in this work to determine the distribution of mechanical properties affected by annealing/tempering by thermal cycle of hybrid PTA-MAG welding process. The investigation results show that the use of hybrid PTA-MAG heat source for welding of martensitic structure steel makes it possible to use high strength steel filler material (yield strength 890 MPa) without of cold cracks high risk. The hybrid plasma based welding method has a potential to become a beneficial alternative to other welding processes for ballistic protection steel due to its high efficiency, reduced amount of weld metal content or limited requirements for a preparation of edges of welded joints. The weakest area of welded joints is part of HAZ located close to the base material which has been secondarily tempered by heat of welding thermal cycle. The heat input about 0,57 kJ/mm required to achieve full penetration butt welded joints with no defects and with wide enough capillary channel to cover the welding gap during welding process. Hardness decrease in that area is about 25% in relation of base material. The width of the softened zone was approx. 4.5 mm.

Effect of Carbide Precipitation Behavior at High Temperatures on Microstructure and Mechanical Properties of M50 Steel

Abstract: By means of microstructure observation, phase analysis and mechanical-property tests, the effect of carbide precipitation behavior at high temperatures on microstructure and mechanical properties of M50 steel was studied. Results show that during the quenching process, the 5-min heat holding at 790 °C promotes the precipitation of carbides along grain boundaries. During the process, the carbon and alloying elements such as Cr, Mo and V are depleted near grain boundaries, where the solution strengthening effect and corrosion resistance are reduced, so after the corrosion treatment, the grain boundary characteristics are more obvious. After isothermal quenching, compared to the steel without short-term heat holding, the retained austenite in the steel with the short-term heat holding at 790 °C is more stable and the content is more. However, after tempering, the precipitation of grain boundary carbides reduces the stability of retained austenite, and the content of retained austenite in the steel with the short-term heat holding is lower than that without the holding. The short-term heat holding at the high temperature results in the formation of a carbide shell in grain boundaries, and when the steel is subjected to impact, tensile, and rotational bending loads, cracks are easy to initiate at grain boundaries, which significantly reduces the impact toughness, plasticity and rotational bending fatigue properties of the steel, and the hardness and the strength of the steel are also little reduced.