Tempered Lath Martensite Research Articles

Effect of quenching-super intercritical quenching-tempering process on the evolution of microstructure and the yield ratio of Ni–Cr–Mo steel

In the present study, X-ray diffraction, electron probe microanalysis, and transmission electron microscopy, among other advanced characterization methods, were used to analyze the microstructure and mechanical properties of the 9Ni steel treated at different heat treatment conditions. The goal was to reveal the dual-optimal strengthening mechanism of “low yield ratio and ultra-cryogenic toughness” of the investigated steel. The experimental results indicated that the 9Ni steel, designed with a ''Ni–Cr–Mo'' multi-elemental composition, balanced its hardenability and corrosion resistance. After the Q-Q' (710 °C) -T process, the diffusion of Cr and Mo atoms in the steel accelerated, while the Fe displacement diffusion formed a more stable FeNi face-centered cubic structure with a lattice energy of −76.98 eV. At this stage, the microstructure of the investigated steel primarily consisted of tempered sorbite (with an average effective grain size of 13.1 μm), tempered martensite lath clusters, reversed austenite, and precipitated Cr23C6 and Mo2C particles. This composition resulted in a low yield ratio. The concentration of Ni near the lath clusters promoted the formation of the reversed austenite with a volume fraction of 10.2%, ensuring the cryogenic toughness of the investigated steel.

Creep strength breakdown and the change in back stress strengthening in 10 % Cr martensitic steels during creep at 923 K

The creep strength breakdown in 10 % Cr martensitic steels appeared under creep conditions at a temperature of 923 K and the applied stresses below 120 MPa as a declination from linear regression obtained using short-term creep data. To reveal the origin of creep strength breakdown, the back stress strengthening conception was used. In the initial state, both 10 % Cr martensitic steels had a tempered martensite lath structure with a high dislocation density in the lath interiors and in the lath boundaries, which was stabilized by M23C6 carbides, NbX carbonitrides, and M6C carbides. The shear internal back stresses of 57.5–68.5 MPa and 35–37 MPa were obtained from the creep rate – stress relationship for the regions of high applied stress (above 120 MPa) and low applied stress (below 120 MPa), respectively. The transition from the high stress region to the low stress one in both steels was accompanied by a significant decrease in the dislocation density by an order of magnitude as well as a decrease in particle number density and an increase in the inter-particle spacing in the M23C6 carbide and Laves phase particles located at the low-angle boundaries of the martensitic laths/subgrains. The root mean square superposition of the dislocation and particle contributions to the shear internal back stresses gave the best coincidence with the values obtained from the creep data. The stability of the M23C6 carbide and Laves phase particles located at the low-angle boundaries was concluded to be the key factor suppressing the creep strength breakdown in the 10 % Cr martensitic steels.

Type IV cracking of P92 steel welded joint used in China power plants

ABSTRACT P92 (9Cr-1.8W–0.5Mo-NbV) martensite steel is widely used in high-temperature pipes of ultra-supercritical boilers in China. During high-temperature operation, microstructural evolution of the heat affected zone (HAZ) across welded joint leads to type IV cracking failure. In this article, microstructural evolution and type IV cracking of P92 steel welded joint in an actual 605℃ supercritical boiler were analysed. The results show the cracking appeared in the fine-grained heat affected zone (FGHAZ). Laves phase and M23C6 phase clustered around the cavity. Their precipitation and coarsening promote the nucleation and growth of cavities. The Z phase was also observed in the heated affected zone. Compared with other zones, FGHAZ has lower dislocation density and more degeneration of tempered martensite lath, the coarsening of Laves phase and M23C6 phase is more severer in FGHAZ. These factors led to the deterioration of the creep properties and type IV cracking.

Microstructural evolution and mechanical properties of an additively manufactured high-strength steel

In this study, the microstructural evolution and mechanical properties of a new type of steel fabricated by selective laser melting (SLM) with subsequent tempering treatment were discussed. The as-built sample consisted of cellular structures, lath martensite accompanied by carbides and little residual austenite. After tempering at 560℃ for 2 h, the matrix transformed from the supersaturated martensite to the tempered lath martensite, followed by residual austenite complete dissolution and more carbides precipitation. The as-built sample showed a high strength (yield strength:1335 ± 10 MPa, ultimate tensile strength:1555 ± 2 MPa) and a considerable ductility (elongation to failture:7.7 ± 1.4 %). The outstanding strength was attributed to fine grains and high number density of dislocations. A superior mechanical properties, including the yield strength of 1762 ± 25 MPa, ultimate tensile strength of 1850 ± 39 MPa and elongation of 5.6 ± 0.8 % have been achieved in the SLMed steel after tempering. The as-built sample exhibits a higher uniform elongation (∼7.0 %) compared with tempered 560℃ sample(∼2.6 %), which may be attributed to its excellent work hardening behavior.

Microstructural Evolution of a Re-Containing 10% Cr-3Co-3W Steel during Creep at Elevated Temperature

Ten percent Cr steels are considered to be prospective materials for the production of pipes, tubes, and blades in coal-fired power plants, which are able to operate within ultra-supercritical steam parameters. The microstructural evolution of a Re-containing 10% Cr-3Co-3W steel with low N and high B content during creep was investigated at different strains at 923 K and under an applied stress of 120 MPa using TEM and EBSD analyses. The studied steel had been previously normalized at 1323 K and tempered at 1043 K for 3 h. In the initial state, the tempered martensite lath structure with high dislocation density was stabilized by M23C6 carbides, NbX carbonitrides, and M6C carbides. At the end of the primary creep stage, the main microstructural change was found to be the precipitation of the fine Laves phase particles along the boundaries of the prior austenite grains, packets, blocks, and martensitic laths. The remarkable microstructural degradation processes, such as the significant growth of martensitic laths, the reduction in dislocation density within the lath interiors, and the growth of the grain boundary Laves phase particles, occurred during the steady-state and tertiary creep stages. Moreover, during the steady-state creep stage, the precipitation of the V-rich phase was revealed. Softening was in accordance with the dramatic reduction in hardness during the transition from the primary creep stage to the steady-state creep stage. The reasons for the softening were considered to be due to the change in the strengthening mechanisms and the interactions of the grain boundary M23C6 carbides and Laves phase with the low-angle boundaries of the martensitic laths and free dislocations.

Processability and Microstructural Evolution of W360 Hot Work Tool Steel by Directed Energy Deposition

Laser directed energy deposition (L-DED) was used to produce samples of the newly patented W360 hot work tool steel by Böhler. The process parameters were optimized to obtain nearly fully dense samples through the production and analysis of single deposited tracks and single layers. Subsequently, bulk samples underwent a hardening heat treatment, consisting of austenitizing, air quenching, and tempering. The samples were analysed in the as-built condition (AB), after quenching (Q) and following tempering cycles (HT) to observe the microstructural evolution. The microstructure was investigated using optical and scanning electron microscopes, energy dispersive X-ray analysis, and X-ray diffraction analysis. Furthermore, the microstructural evolution was analysed with differential scanning calorimetry, while the mechanical response was evaluated through microhardness test. It was found that the AB samples exhibited a dendritic-cellular microstructure with tempered martensite laths. The thermal history of the AB samples was completely modified by the austenitizing treatment followed by quenching, resulting in a fully martensitic Q sample that did not display the typical dendritic-cellular microstructure of the L-DED process. The completion of the heat treatment with tempering cycles revealed the presence of Mo-rich carbides dispersed in a martensitic matrix. The HT samples exhibited a mean microhardness of 634 HV, remaining constant along the entire building direction from the substrate to the last deposited layer, indicating a homogeneous microstructure. This high value, similar to other hot work tool steels such as H13, makes W360 a very promising candidate for tool build and repair purposes.Graphical

Low-temperature bonding between F82H and Cr assisted by metastable Cr interlayers applied by pulsed laser deposition

The joining of F82H and pure Cr metal has been demonstrated at temperatures as low as 798 K, assisted by the prior application of a thin metastable Cr interlayer to both bonding surfaces using pulsed laser deposition. Bonded F82H/Cr interfaces were characterised by SEM-EDS and TEM analysis, with respect to diffusion behaviour, chemical composition and microstructure. At higher bonding temperatures, an interface layer formed on the Cr side of the joint, which was rich in M23C6, though its structure and thickness was different depending on whether Cr interlayers were applied. As the bonding temperature was decreased stepwise, the interface layer thickness reduced to a minimum of ∼60 nm and appeared to adopt a film-like structure. Although the chemical composition of the thinner interface layers could not be determined, they were most likely M23C6 in a film-like form, which subsequently formed particles, and then columns, as the bonding temperature and interface layer thickness increased. Hardness around the interface increased as compared with the bulk F82H and Cr, though the peak hardness value decreased as the interface thickness decreased. The hardness of bulk F82H and Cr was similar before and after bonding at all temperatures, suggesting little impact on mechanical properties. Diffusion of Cr into F82H and Fe into Cr was not detected at any of the conditions tested, and the original tempered lath martensite microstructure of F82H appeared to have been retained.

Structural Evolution of 10% Cr–3% Co Steel Microalloyed with Re and Cu during Creep Au 923 К

Structural evolution of the tempered lath martensite of the 10% Cr--3% Co steel microalloyed with rhenium and copper with a low nitrogen content and a high boron content during creep at 923 K was investigated for the purpose of establishment of the rison of decrease in creep resistance of this steel under the low applied stress. The tempered martensite lath structure of 10%Cr-3%Co steel with an average lath size of 370 nm and a high dislocation number density of 2 ×1014 m–2 was observed after normalizing at 1323 K with the following tempering at 1043 K for 3 h. The structure was stabilized by M23C6 carbides, M6C carbides, and NbX carbonitrides. During long-term creep, the lath structure strongly experienced an evolution: the width of the martensitic laths increased significantly, dislocation density decreased, the Laves phase and Cu-enriched particles remarkably coarsen. Such structural evolution correlates with an appearance of creep strength breakdown on curves “Applied stress vs. Time to failure” and “Minimum creep rate vs. Applied stress”. Significant coarsening of the Laves phase particles and Cuenriched particles via formation of the large particles with sizes more than 250 nm along high-angle boundaries and full dissolution of the fine particles with sizes less than 50 nm along low-angle boundaries of martensite laths is considered to be the main cause of degradation of the creep resistance of the steel studied.

A phenomenological understanding of the novel design of hierarchical structure for 1 GPa ultrahigh strength and high toughness combination low alloy steel

In the present study we propose a novel design of hierarchical structure with the aim to obtain high strength of 1 GPa and low temperature toughness in a heavy low alloy steel plate by intercritical quenching and tempering (IQ&T). The hierarchical microstructure consisted of bimodal prior austenite grains with complex microstructure obtained via intercritical quenching from 810 °C and tempering at 500 °C. The large size grains with average diameter of ∼22 μm consisted of tempered martensite and fine intercritical ferrite. While the surrounding fine grains with average diameter of ∼12 μm consisted of tempered lower bainite and tempered lath martensite. Ultra-high yield strength of 998 MPa and tensile strength of 1059 MPa were obtained for the novel hierarchical microstructure steel, which was similar to the conventional quenched and tempered (Q&T) steel. Whereas, the low temperature impact energies at both −40 and −80 °C were ∼100% higher for the hierarchical microstructure steel than the conventional Q&T steel. The significant improvement of low temperature toughness is attributed to the synergistical effect of hierarchical microstructure. The fine intercritical ferrite in large grains increased the plastic deformation ability during impact and was beneficial for formation of dimples, and the surrounding fine grains offered high density of large mis-orientation grain boundaries to deflect crack propagation, leading to high impact energy at low temperature.

Research on thermal creep behavior of Chinese low-activation ferritic/martensitic steel

A series of thermal creep experiments are conducted on Chinese low-activation ferritic/martensitic steel (CLF-1 steel) at a temperature of 550, 575, and 600 °C with various normal stress (from 150 to 300 MPa), respectively. The results are analyzed by creep curves, microstructure, and creep properties. Finished samples display adequate fracture surface in short creep rupture tests (within 3000 h). The comparative creep curve shows no difference related to the thickness of CLF-1 steel plate (10 and 30 mm). Microstructure images before and after creep reflect a tempered lath martensite structure and carbides decorating both in prior austenitic grain boundaries and lath boundaries. Comparison is discussed between 30 mm plate and 10 mm plate. All current results reflect that all CLF-1 steel samples shows good creep behavior and proves that a series excellent progress of CLF-1 steel R&D work has been successfully achieved. Besides, collected data will contribute a lot to the design and R&D work of the Chinese Helium Cooled Ceramic Breeding Test Blanket Module (CNHCCB-TBM).

Effect of steam on the creep behavior of T92 steel at 650°C

One self-designed testing system was utilized to investigate the creep behavior of the T92 steel in steam environment. The creep tests were conducted at 650 °C with a certain stress range from 100 MPa to 160 MPa in the steam environment and air respectively. The specimens tested in steam environment shows typical multi-layered oxide scale consists of an outer scale of magnetite and an inner scale of Cr-rich spinel. Compared to the specimens in air, the steam specimens possessed larger minimum creep rate and reduced plasticity. The stress dependence of minimum creep rates exhibited a power law with similar exponents of 12.0 and 12.6 in steam environment and air respectively, indicating similar operating mechanism of creep. However, the activation energy in steam environment was smaller than that in air. Microstructural observation demonstrated that the degradation of tempered martensite lath structure and coarsening of the precipitations were mainly responsible for creep damage both in steam environment and air. The results concentrating on the possible interaction between the oxidation scale growth process and creep were discussed on the basis of oxidation process, a potential effect of hydrogen on creep and potential hydrogen embrittlement.

Strength-Toughness of a Low-Alloy 0.25C Steel Treated by Q&P Processing.

Quenching and partitioning (Q&P) treatments were applied to 0.25C steel to produce the microstructures that exhibit an improved balance of mechanical properties. The simultaneous bainitic transformation and carbon enrichment of retained austenite (RA) during the partitioning stage at 350 °C result in the coexistence of RA islands with irregular shapes embedded in bainitic ferrite and film-like RA in the martensitic matrix. The decomposition of coarse RA islands and the tempering of primary martensite during partitioning is accompanied by a decrease in the dislocation density and the precipitation/growth of η-carbide in the lath interiors of primary martensite. The best combinations of a yield strength above 1200 MPa and an impact toughness of about 100 J were obtained in the steel samples quenched to 210-230 °C and subjected to partitioning at 350 °C for 100-600 s. A detailed analysis of the microstructures and the mechanical properties of the steel subjected to Q&P, water quenching, and isothermal treatment revealed that the ideal strength-toughness combinations could be attributed to the mixture of the tempered lath martensite with finely dispersed and stabilized RA and the particles of η-carbide located in the lath interiors.

Effect of tempering process on the cryogenic impact toughness of 13Cr4NiMo martensitic stainless steel

The effects of tempering process on the cryogenic impact toughness of 13Cr4NiMo martensitic stainless steel (13-4MSS) were investigated by experiments and crystal plasticity finite element modeling (CPFEM). The tempering structure mainly includes tempered martensite lath, reversed austenite and M23C6 carbide. The tempered martensite lath is refined by the tiny new martensite transformed by reversed austenite, and the refinement degree increases with the tempering temperature. The nucleation of reversed austenite depends on the new martensite and carbide generated during tempering, and more reversed austenite is obtained by double tempering. The cryogenic impact toughness of the material changes nonlinearly with the increase of tempering temperature. It indicates that fine martensitic lath and inverse austenite are beneficial to improve the cryogenic impact toughness of the material, while carbide particles play the opposite role. The results of CPFEM show that reversed austenite coordinates the impact force of the surrounding martensite through large plastic deformation, and eases the stress concentration. Reversed austenite even occurs DIMT behavior under large plastic deformation to further reduce the stress concentration and prevent the initiation and propagation of cracks. As a hard particle, carbide causes the stress concentration, leading to cracks initiation and propagation along the martensite boundary.

Microstructure development and mechanical properties of a C-Mn-Si-Al-Cr cold rolled steel subjected to quenching and partitioning treatment

The microstructural evolution and mechanical behavior of Fe-0.2C-2.7Mn-1.2Si-0.8Al-0.3Cr (wt.%) quenching and partitioning steel were studied. Partitioning was performed at 400, 440, and 480 °C for various durations from 30 to 300 s to obtain different fractions of retained austenite. Scanning electron microscopy, dilatometry, and X-ray diffraction analysis as well as uniaxial tensile testing were employed to characterize the microstructural evolutions and mechanical properties. The resulting microstructures were primarily composed of tempered lath martensite, bainitic ferrite, retained austenite, and blocky fresh martensite. The lower partitioning temperature of 400 °C was appropriate for efficient carbon partitioning and stabilization of retained austenite at room temperature. However, the retained austenite fraction decreased with increasing partitioning temperature, due to accelerated bainite formation and carbide precipitation at higher partitioning temperatures. The tensile results revealed that all specimens had tensile strength values of more than 1128 MPa and yield strength values of over 830 MPa. Additionally, prolonging the isothermal holding time had a negative effect on the yield strength due to bainite formation, while it enhanced the elongation. Fractography analysis showed that specimens treated at the partitioning temperature of 400 °C were fractured in a relatively ductile mode, but the fracture nature altered to an intergranular mode after partitioning at higher temperatures.

Influence of microstructure on the micro-region fracture toughness of the 30Cr2Ni4MoV turbine rotor welded joint

The fracture toughness of each micro-region in 30Cr2Ni4MoV steam turbine rotor welded joint was studied, including the central zone of the SAW bead (SAW-C), the overlapping zone of the SAW bead (SAW-Z), and the central zone of the TIG weld metal (TIG), coarse-grained heating affected zone (CGHAZ), fine-grained HAZ (FGHAZ) and base metal (BM). The microstructure, fracture morphologies and cracking paths were observed by OM and SEM microscope, and the influence of microstructure on fracture toughness was analyzed. The research results show that the fracture toughness of the weld metal is lower than that of BM and HAZ, and the fracture toughness of the weld metal is non-uniform. TIG and SAW-C are composed of columnar tempered bainite, intergranular tempered lath martensite and carbides, and the fracture toughness is the worst. SAW-Z is zigzag columnar tempered bainite and equiaxed granular tempered bainite between weld passes, which has good fracture toughness. CGHAZ and FGHAZ are equiaxed tempered martensite with the best fracture toughness. By observing the fracture morphology and cracking path, SAW-C and TIG are dominated by brittle cleavage fracture mode, and SAW-Z, FGHAZ, CGHAZ and BM are in the ductile fracture mode. The straight columnar grains of SAW-C and TIG are prone to crack propagation, but the tempered bainite acts as a bridge. SAW-Z has a zigzag tempered bainite grains, and the crack propagation path is tortuous. For FGHAZ, CGHAZ and BM with equiaxed grains, the fracture toughness is related to the grain size and the dimple size around the crack tip. The crack propagation process is that the dimples are first formed in a certain range around the crack tip, but only the dimples in the lower strength micro-regions grow and merge preferentially, and rapidly form new cracks.

Carbon segregation and cementite precipitation at grain boundaries in quenched and tempered lath martensite

Tempering is widely applied to make carbon atoms beneficially rearrange in high strength steel microstructures after quenching; though the nano-scale interaction of carbon atoms with crystallographic defects is hard to experimentally observe. To improve, we investigate the redistribution of carbon atoms along martensite grain boundaries in a quenched and tempered low carbon steel. We observe the tempering-induced microstructural evolution by in-situ heating in a transmission electron microscope (TEM) and by compositional analysis through atom probe tomography (APT). Probe volumes for APT originate from a single martensite packet but in different tempering conditions, which is achieved via a sequential lift-out with in-between tempering treatments. The complementary use of TEM and APT provides crystallographic as well as chemical information on carbon segregation and subsequent carbide precipitation at martensite grain boundaries. The results show that the amount of carbon segregation to martensite grain boundaries is influenced by the boundary type, e.g. low-angle lath or high-angle block boundaries. Also, the growth behavior of cementite precipitates from grain boundary nucleation sites into neighboring martensite grains differs at low- and high-angle grain boundaries. This is due to the crystallographic constraints arising from the semi-coherent orientation relationship between cementite and adjacent martensite. We also show that slower quenching stabilizes thin retained austenite films between martensite grains because of enhanced carbon segregation during cooling. Finally, we demonstrate the effect of carbon redistribution along martensite grain boundaries on the mechanical properties. Here, we compare micro-scale Vickers hardness results from boundary-containing probe volumes to nanoindentation results from pure bulk martensite (boundary-free) probe volumes.

Effect of prior martensite formation on the bainite transformation kinetics in high-strength 3% Mn multiphase steel

The work presents results on the effect of prior martensite formation on bainite transformation kinetics in a 3% medium-Mn multiphase steel. The material was subjected to two isothermal holding temperatures: 400 °C (without martensite) and 350 °C (with prior martensite). According to obtained dilatometric results, the formation of prior martensite leads to the acceleration of bainite transformation kinetics. The bainite formation starts and finishes much faster, when the prior martensite was present before the isothermal holding. The microstructural investigation of the steel after heat treatment was carried out using light and scanning electron microscopy. The microstructures were composed of fine bainitic laths with retained austenite and small amount of martensitic-austenitic islands at 400 °C. At 350 °C the presence of large tempered martensite laths was detected. The bainite is composed of a mixture of fine and coarse laths. The increase of the bainitic lath thickness is attributed to the coalescence process occurring at the lower holding temperature. The differences in the steel hardness after the two heat treatments were relatively small (~ 13 HV10).

Ambient, elevated temperature tensile properties and origin of strengthening in friction stir welded 6 mm thick reduced activation ferritic-martensitic steel plates in as-welded and post-weld normalised conditions

Reduced Activation Ferritic-Martensitic (RAFM) steels are currently being considered for test blanket modules of International Thermonuclear Experimental Reactor. This study is aimed at understanding the microstructure and mechanical properties of Indian RAFM steel during friction stir welding (FSW) of 6 mm thick plates. The full penetration bead-on-plate welds were fabricated employing PcBN tool at a rotational speed of 200 rpm. The FSW joint consisted of stir zone, thermomechanically affected zone and BM. Microstructure, hardness and tensile properties were evaluated in as-received (base metal, BM), as-welded and post weld normalised + tempered states. The as-received microstructure was composed of Cr-rich M23C6 on prior austenite grain and tempered lath martensite boundaries with intra-lath V-and Ta -rich monocarbides. Substantial changes occurred during welding leading to destruction and dissolution of M23C6 and precipitation of Fe3C in SZ. The microstructure was restored to the BM level by giving a post weld normalising and tempering treatment. Comparative study on tensile properties has been carried out at room and elevated temperatures (up to 550 °C) in all the three states and the observed variations have been explained on the basis of initial microstructure and evolving substructures. The as-welded samples showed higher strength and low percentage elongation values. The ductility was minimum in all the states at an intermediate temperature and ascribed to the dynamic strain ageing. All the states revealed falling strength with increasing temperature up to 550 °C. Failure occurred in AW and PWNT states at the interface between TMAZ and BM. Irrespective of material state and tensile test temperature, the transgranular ductile fracture prevailed. The contribution of the different strengthening mechanisms to the yield strength of the SZ region has been estimated theoretically and it matches with the experimental results. The influence of low angle grain boundaries (LAGB) and high angle grain boundaries (HAGB) on the tensile behaviour of the SZ was well explained using the Electron Back Scattered Diffraction (EBSD) maps of these regions.

Heat treatment effects on the weld joint of CLF-1 fabricated by laser welding

In this paper, laser weld technology was applied to produce the welded joints of CLF-1 steel, which is the primary material for manufacturing test blanket module (TBM). The microstructures and mechanical properties of the joints were studied at as-welded state, post-weld heat treatment (PWHT) of 710 °C/2 h/air cooling (PWDT), and PWHT of 980 °C/1 h/air cooling (AC) followed by tempering at 710 °C/2 h/AC (PWNT). The results showed that abundant lath martensite phase with width 800 nm and small amount of δ-Fe phase were present in the weld at as-welded state. Its impact toughness was about 40 J. The microstructures of the weld at PWDT state included a large amount of tempered lath martensite with width 650 nm, few δ-Fe, M23C6 carbide (distributed along the boundary of martensite) and MX carbide (precipitated inside the martensite). Its impact toughness was about 237 J (equivalent to the base metal). The microstructures of the weld at PWNT state included a large amount of tempered lath martensite with width 350 nm, M23C6 carbide and MX carbide. Even though the δ-Fe phase disappeared, the impact toughness (about 156 J) was lower than that of the weld at PWDT state due to the aggregated growth of M23C6 and MX carbides.

Effect of heat treatment on microstructures and properties of electron beam welding CLF-1 steel

Vacuum electron beam welding was adopted for the reduced activation ferritic martensitic steel (CLF-1 steel) with a thickness of 32 mm, and a well-formed butt joint without metallurgical defects was obtained under the optimum technological parameters. After welding, the specimens were subjected to high temperature tempering (710 °C/120 min/AC (air cooling), abbreviation HTT) and incomplete annealing (840 °C/120 min/AC, abbreviation IA) heat treatment, and the microstructure and mechanical properties of the as-welded state and two different heat treatment states were compared. The results showed that the as-welded weld metal (WM) was composed of coarse lath martensite, and the heat affected zone (HAZ) was composed of a large amount of massive martensite and dispersed fine MX and M23C6. Compared with the welded state, the martensite structure of HTT state WM and HAZ were refined, nano-scale M23C6 was precipitated along the boundary of tempered martensite in WM and nano-scale MX was precipitated in tempered martensite lath structure, both of which played a role of dispersion strengthening. Compared with the HTT state, the martensite structure of the WM and HAZ in the IA state became coarser, and both M23C6 and MX tend to grow together. The fracture position of the tensile specimens of as-welded state, HTT state and IA state welded joints were all located on the base metal (BM), and the tensile strength decreased successively, while the yield strength and elongation increased successively. The average impact absorbed energy of the BM, as-welded state, HTT state and IA state welds were 236 J, 42 J, 241 J and 51 J, respectively. The impact performance of HTT state welds surpassed the BM.