Low Temperature Impact Toughness Research Articles

Effect of Pre-Weld Heat Treatment on the Microstructure and Properties of Coarse-Grained Heat-Affected Zone of a Wind Power Steel after Simulated Welding

The effect of pre-weld heat treatment on the microstructure and low-temperature impact toughness of the coarse-grained heat-affected zone (CGHAZ) after simulated welding was systematically investigated through the utilization of scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD). The Charpy impact test validated the presence of an optimal pre-weld heat treatment condition, resulting in the highest impact toughness observed in the CGHAZ. Three temperatures for pre-weld heat treatment (690, 720 and 750 °C) were used to obtain three different matrices (Steel 1, Steel 2, Steel 3) for simulated welding. The optimal pre-weld heat treatment is 720 °C for 15 min followed by water quench. Microstructure characterization showed that there is an evident microstructure comprising bainite (B) in Steel 1 and Steel 2 after pre-weld heat treatment, while the addition of martensite (M) with the pre-weld heat treatment temperature exceeds Ac1 by almost 60 °C (Steel 3). These differences in microstructures obtained from pre-weld heat treatment influence the refinement of high-temperature austenite during subsequent simulated welding reheating processes, resulting in distinct microstructural characteristics in the CGHAZ. After the optimal pre-weld heat treatment, Steel 2 subjected to single-pass welding thermal simulation demonstrates a refined microstructure characterized by a high density of high-angle grain boundaries (HAGBs) within the CGHAZ, particularly evident in block boundaries. These boundaries effectively prevent the propagation of brittle cracks, thereby enhancing the impact toughness.

Effect of Boron on Microstructures and Low-Temperature Impact Toughness of Medium-Carbon CrMo Alloy Steels with Different Quenching Temperatures

The effect of boron (B) on the microstructures and low-temperature impact toughness of medium-carbon CrMo steel quenched at 870~1050 °C and tempered at 600 °C was studied via Charpy impact testing and microstructure characterizations. The results showed that with an increasing B content from 0 to 50 ppm, the low-temperature impact toughness deteriorated significantly at quenching temperatures (Tq) lower than 950 °C but increased at a higher Tq of 1050 °C. Undissolved M2B particles remained and coarsened during the holding process due to the low Tq, decreasing the critical stress required for crack initiation and deteriorating the impact toughness accordingly. However, this detrimental effect of B could be mitigated by a higher Tq, and the favorable influences on the impact toughness improvement could be attributed to (1) the finer M2B particles formed during quenching effectively pinning the austenite grain boundaries (GBs), leading to a finer block size and a high density of high-angle grain boundaries, which reduced the critical stress for crack initiation; and (2) the fact that the coarsening of M23C6 on the GBs during tempering was slightly suppressed by the segregated B, eventually increasing the energy required for crack propagation. However, the degree of the favorable effect due to B was still lower than the negative effect of a high Tq.

Effect of carbon content on strength and toughness of deposited metal for 1000 MPa-grade high-strength welding wire

The microstructure and mechanical properties of the deposited metal of 1000 MPa-grade marine steel welding wire were observed and tested using characterization methods such as OM, SEM, TEM, XRD, and EBSD. The influence of carbon content (0.061–0.080 wt%) on the structure of the deposited metal was examined, with the strengthening and toughening mechanisms of the deposited metal elucidated. The results indicated that the deposited metal structure primarily consisted of lath bainite, martensite, and a small amount of retained austenite as the carbon content increased from 0.061 wt% to 0.080 wt%; the proportion of martensite increases, while bainite and retained austenite decrease. Additionally, the lath morphology changes from “interwoven” to “parallel.” The yield strength of the deposited metal increases from 915 MPa to 1007 MPa, while the impact energy at −50 ℃ decreases from 120 J to 59 J. The increase in martensite proportion contributed to the enhanced strength of the deposited metal. Solid solution and dislocation strengthening are the primary strengthening methods. The observation of the impact fracture section revealed that the bainite was segmented and refined. In contrast, the “intertwined” morphology of plate-striped bainite/martensite and high-angle grain boundaries hinders crack propagation, thereby improving the low-temperature impact toughness.

Research on the Low-Temperature Impact Toughness of a New 100-mm Ultra-Thick Offshore Steel Fabricated Using the Narrow-Gap Laser Wire Filling Welding Process.

Ultra-thick offshore steel, known for its high strength, high toughness, and corrosion resistance, is commonly used in marine platforms and ship components. However, when offshore steel is in service for an extended period under conditions of high pressure, extreme cold, and high-frequency impact loads, the weld joints are prone to fatigue failure or even fractures. Addressing these issues, this study designed a narrow-gap laser wire filling welding process and successfully welded a 100-mm new type of ultra-thick offshore steel. Using finite element simulation, EBSD testing, SEM analysis, and impact experiments, this study investigates the weld's microstructure, impact toughness, and fracture mechanisms. The research found that at -80 °C, the welded joint exhibited good impact toughness (>80 J), with the impact absorption energy on the surface of the weld being 217.7 J, similar to that of the base material (225.3 J), and the fracture mechanism was primarily a ductile fracture. The impact absorption energy in the core of the weld was 103.7 J, with the fracture mechanism mainly being a brittle fracture. The EBSD results indicated that due to the influence of the welding thermal cycle and the cooling effect of the narrow-gap process, the grains gradually coarsened from the surface of the welded plate to the core of the weld, which was the main reason for the decreased impact toughness at the joint core. This study demonstrates the feasibility of using narrow-gap laser wire filling welding for 100-mm new type ultra-thick offshore steel and provides a new approach for the joining of ultra-thick steel plates.

Microstructure evolution and enhanced cryogenic toughness of laser welded dissimilar SA645/AISI304L joints via intercritical tempering

In this study, we explored the impact of intercritical tempering on the microstructural evolution and mechanical properties of dissimilar welds formed through laser welding between 5%Ni steel (SA645) and austenitic stainless steel (AISI304L). Upon tempering at 500 °C, reversed austenite manifested within the weld metal. The volume proportion of reversed austenite in weld metal at room temperature increased first, exhibiting a maximum (∼10%) at ∼600 °C, and then decreased with increasing tempering temperature, which was associated with the diminished stability of austenite. Due to the change in tempering temperatures, two distinct types of reversed austenite were present, with one being located at the high-angle grain boundaries and the other within the martensite blocks. As the tempering temperature increased, the weld metal hardness decreased, reaching a minimum of ∼306 HV at 600 °C, and then increased. The dislocation density within the martensite matrix and the quantity of reversed austenite are the crucial factors to determine the hardness of weld metal. The cryogenic impact toughness of weld metal increased at first and then decreased as the tempering temperature increased. The maximum of ∼40 J for the cryogenic impact toughness was obtained when tempering at 600 °C. The low-temperature impact toughness of the weld metal was determined by the synergy of the TRIP effect of reversed austenite, the compatible deformation capability of dual-phase structure, and the crack propagation behavior.

The Influence of Centerline Segregation on Impact Toughness in Welding Heat-Affected Zone of X70 Pipeline Steel

The influence of centerline segregation on the low-temperature impact toughness of the heat-affected zone (HAZ) of welded joints was studied by welding experiments on X70 steel plates rolled from continuous casting slabs with segregation grades of class 2 and class 3. The experimental results show that the impact toughness at HAZ from class 2 slab steel plate is more stable and has excellent low-temperature toughness than that of class 3 slab steel plate. The impact toughness of the HAZ of the class 3 slab steel plate is low to 100 J at −40 °C and has a severe fluctuation range (~150 J), and the delamination phenomenon is also observed in the fracture cross-section. The reason for this phenomenon is due to the enrichment of C and Mn elements in the centerline segregation zone. The formation of abnormal microstructure (martensite/bainite) in the segregation zone leads to stress concentration, which easily weakens the low-temperature toughness of the joint.

Effect of Segregation Band on the Microstructure and Properties of a Wind Power Steel before and after Simulated Welding

This article uses scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD) to study the effect of C and Mn segregation on the microstructure and mechanical properties of high-strength steel with 20 mm thickness used for wind power before and after simulated welding. A Gleeble-3500 (GTC, Dynamic Systems Inc., Poestenkill, NY, USA) was used to study the microstructure evolution of the simulated coarse-grained heat-affected zone (CGHAZ) of experimental steel under different welding heat inputs (10, 14, 20, 30 and 50 kJ/cm) and its relationship with low-temperature impact toughness (−60 °C). The results indicate that alloy element segregation, especially Mn segregation, significantly affects the impact toughness scatter of the steel matrix, as it induces the formation of low-temperature martensite or hard phase, such as M/A (martensite/austenite) constituent. In addition, segregation also reduces the low-temperature impact toughness of the simulated welding samples and increases the fluctuation range. For high-strength steel with yield strength higher than 460 MPa used for wind power generation, there is an optimal welding heat input (~20 kJ/cm), which enables the simulated coarse-grained heat-affected zone (CGHAZ) to obtain the highest impact toughness due to the formation of lath bainite (LB) and the finest crystallographic block units. Excessive or insufficient heat input can induce the formation of coarse granular bainite (GB) or lath martensite (LM), leading to a larger size of crystallographic block units, reducing the hindering effect of brittle crack propagation and deteriorating low-temperature impact toughness.

Mechanism Analysis for the Enhancement of Low-Temperature Impact Toughness of Nodular Cast Iron by Heat Treatment.

The low-temperature impact toughness of nodular cast iron can be significantly enhanced by heat treatment, and thus meet the severe service requirements in the fields of high-speed rail and power generation, etc. In order to explore the enhancement mechanism, microstructure, hardness, composition and other characteristics of as-cast and heat-treated nodular cast iron is systematically tested and compared by optical microscopy, microhardness tester, EBSD, SEM, electron probe, and impact toughness testing machine in this study. The results show that heat treatment has little effect on the morphology and size of graphite in nodular cast iron, ignores the effect on the grain size, morphology, and distribution of ferritic matrix, and has little effect on the hardness and exchange of elements, while it is meaningful to find that heat treatment brings about significant decrease in high-angle grain boundaries (HAGB) between 59° and 60°, decreasing from 10% to 3%. Therefore, the significant enhancement of low-temperature impact toughness of nodular cast iron by heat treatment may result from the obvious decrease in HAGB between 59° and 60°, instead of other reasons. From this perspective, the study can provide novel ideas for optimizing the heat treatment process of nodular cast iron.

Effect of heat input on the mechanical properties of welded Q960E ultra-high strength steel butt joints

The influence of welding heat input on the tensile properties and low-temperature impact toughness of Q960E weld metals (WMs) and butt welded joints (BWJs) was investigated. Prediction models were established to correlate the heat input range (4–17.5 kJ/cm) with the yield strength, tensile strength, and elongation of both WMs and BWJs. Impact toughness was examined at different temperatures for the WMs, heat-affected zones (HAZs), and BM. Microstructure transformations were revealed in different regions of the BWJs, and their impact on the tensile properties and toughness. The results demonstrated that the yield strength, tensile strength, and elongation of the BWJs were 0.80–0.99, 0.9–1.0, and 0.44–0.82 of the BM, respectively; this was attributed to variations in the average effective grain size and Taylor factor in different regions of the BWJs, which led to distinct strength and plastic deformation capacities. The impact toughness of both the WMs and HAZs was notably diminished when compared to the BM. For the WMs, the Charpy impact absorption energy values were only 0.24–0.34, 0.50–0.59, and 0.40–0.50 of that of the BM for heat inputs of 4, 7.4, and 11.2 kJ/cm, respectively. For the same heat inputs, in the case of the HAZs, the Charpy impact absorption energy values were only 0.71–0.90, 0.63–0.70, and 0.63–0.80 of that of the BM, respectively. The presence of martensite–austenite (M–A) islands in the HAZ contributed to the decrease in the impact toughness. Increasing the heat input resulted in larger M–A components, thus further reducing the impact toughness of the HAZ. Conversely, a higher heat input for the WMs promoted the formation of acicular ferrite, thereby enhancing its impact toughness. However, owing to the abundant lath martensite present in the WM, its impact toughness was significantly inferior to that of the BM.

Comparative interplay of C and Mn on austenite stabilization and low temperature impact toughness of low C medium Mn steels

To expand the use of medium Mn steels for the production of automotive and structural components intended for cryogenic applications, the present study focuses on attaining an enhanced low temperature impact toughness in two inter-critically annealed medium Mn steels with varying C and Mn contents without sacrificing the strength. In both ‘low-C, high-Mn’ and ‘high-C, low-Mn’ steels, the key role in regulating the impact toughness is mainly attributed to the volume fraction of stable retained austenite. Mn is found to stabilize austenite more effectively than C while allowing the attainment of near-equilibrium composition. Except for the ‘low-C, high-Mn’ steel, inter-critically annealed at lower temperature, Mn redistribution occurs in other samples, leading to a reduction of stable austenite fraction. The filmy shaped Mn enriched retained austenite is more thermally and mechanically stable in ‘low-C, high-Mn’ steel sample on inter-critical annealing at lower temperature than the other samples. The presence of fine dispersion of the filmy austenite at various substructural boundaries of ‘low-C, high-Mn’ steel after low temperature inter-critical annealing results in significant structural (Bain width) refinement and thereby, remarkably improves its impact toughness.

Regulation Law of Tempering Cooling Rate on Toughness of Medium-Carbon Medium-Alloy Steel.

Temper embrittlement is a major challenge encountered during the heat treatment of high-performance steels for large forgings. This study investigates the microstructural evolution and mechanical properties of Cr-Ni-Mo-V thick-walled steel, designed for large forgings with a tensile strength of 1500 MPa, under different tempering cooling rates. Optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) were employed to analyze the microstructural features. The results demonstrate that the embrittlement occurring during air cooling after tempering is attributed to the concentration of impurities near Fe3C at the grain boundaries. The low-temperature impact toughness at -40 °C after water quenching reaches 29 J due to the accelerated cooling rate during tempering, which slows down the diffusion of impurity elements towards the grain boundaries, resulting in a reduced concentration and dislocation density and an increased stability of the grain boundaries, thereby enhancing toughness. The bainite content decreases and the interface between martensite and bainite undergoes changes after water quenching during tempering. These alterations influence the crack propagation direction within the two-phase microstructure, further modifying the toughness. These findings contribute to the understanding of temper embrittlement and provide valuable guidance for optimizing heat treatment processes to enhance the performance of high-performance steels in large forgings.

Refinement mechanism of large heat-input welding CGHAZ microstructure by N addition and its effect on toughness of a V-Ti-N microalloying weathering steel

The large heat input welding performance of uncoated weathering steel remains a great challenge since their coarse grain heat-affected zones (CGHAZs) are usually occupied by coarse and brittle microstructure consisting of granular bainitic ferrite (GBF) and martensite/austenite (M/A) constituents. In this article, a V-Ti-N microalloying weathering steel suitable for large heat input welding was achieved through the N addition, and the mechanism dominating the modified microstructure and the enhanced impact toughness of CGHAZ was deeply elucidated mainly by HRTEM. The results indicated that in the simulated CGHAZ with large heat input of 80 kJ/cm, the increase of N content from 0.0030 wt% to 0.0075 wt%, and 0.0115 wt% elevated the density of nano- and submicron-sized particles of (Ti, V)(C, N) containing VN, which in turn resulted in the refinement of prior austenite grains (PAGs) by the improved grain boundary pinning effect and the enhancement in heterogeneous nucleation of intragranular ferrite (IGF) by the decreased lattice disregistry (δ) between (Ti, V)(C, N) and IGF. The fraction of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) higher than 15° was thereby increased while the mean equivalent diameter (MED) of ferritic grains with HAGBs was decreased. Simultaneously, the N addition scattered the M/A constituents, promoted the internal substructure of dislocation-type M/A at the expense of twin-type M/A, and accordingly reduced the hardness difference between the M/A constituents and the matrix. All these factors retarded brittle crack initiation and propagation, and low-temperature impact toughness of simulated CGHAZ increased from 62.8 J to 141.2 J.

Crystallography of transformation products with different cooling rates in low-carbon alloy steel and its effect on low-temperature toughness uniformity of heavy plates

The crystallography of transformation products with different cooling rates has been investigated in a typical low-carbon alloy steel, and its effect on low-temperature impact toughness uniformity in the thickness direction of heavy steel plates has been elucidated. The results show that both the full lath martenite (LM) microstructure transformed at the cooling rate of 45 K s−1 and granular bainite (GB) + lath bainite (LB) + LM microstructure transformed at the cooling rates of 3∼6 K s−1 have higher density of high-angle grain boundaries (HAGBs). The density of HAGBs in the full GB microstructure transformed at the cooling rate of 0.5 K s−1 is the lowest, and that in the LB + LM microstructure transformed at the cooling rates of 15∼22 K s−1 is the middle. In the GB + LB + LM microstructure, the division of austenite by multi-stage transformation reduces the LB and LM transformation temperatures, contributing to a weakening of variant selection and a tendency to close-packed plane grouping. Therefore, the GB + LB + LM microstructure achieves the refinement degree similar to the full LM microstructure, but slightly higher than that of the LB + LM microstructure. For the 60 mm thick heavy steel plate with ultra-fast cooling, the cooling rate decreases from 41 K s−1 at the subsurface to 14 K s−1 at the 1/4-thickness and then to 4 K s−1 at the 1/2-thickness. The full LM microstructure, LB + LM microstructure, and GB + LB + LM microstructure are obtained in sequence, resulting in a more uniform density of HAGBs. Consequently, the relatively uniform impact toughness at 143 K in the thickness direction is achieved with a small difference of ∼30 J.

Crystallographic texture and the mechanical properties of API 5L X70 pipeline steel designated for an arctic environment

The aim of this research is to examine the effect of roughing and finishing reductions on crystallographic texture. Three rolling conditions were applied with the same total reduction but different combinations of the roughing and finishing reductions, identified by their finishing reductions as low (X7FL), medium (X7FM), and high (X7FH). Texture measurements showed significant inhomogeneity at the centreline region that had higher intensities of Epsilon, Alpha, and Gamma fibre textures compared to the quarter-thickness and surface layers. Drop Weight Tear Test (DWTT) indicated that X7FL and X7FM specimens, with a relatively higher fraction of texture components of {332}<113>, Gamma fibre, and Transformed Copper (TC), exhibited superior low-temperature impact toughness compared to the X7FH steel. The electrochemical hydrogen charging experiments showed that the internal hydrogen cracks appeared only in X7FL and X7FM steels due to the large presence of detrimental {001} grain orientation on surface layer, perpendicular to ND.

Obvious different formability and mechanical properties of warm caliber rolling Q345 steel with normalized and tempered states

The formability and strengthening-toughening behaviors of the warm rolled steels are highly dependent on the raw heat treatment state. Herein, the normalized and tempered Q345 steels were both subjected to caliber rolling at 500 °C with different rolling reduction ratios (18%, 44%, and 68%) to obtain elongation grains and carbide particles with different distribution and size. With the increase of caliber rolling reduction ratio, the strength and fracture elongation of the rolled normalized Q345 steels are gradually decreased, whereas the rolled tempered Q345 steels exhibit obvious improvement in these properties. The warm rolled normalized steel contains severe uneven grain distribution and coarse carbides, which leads to high stress concentration and low mechanical properties, as well as poor formability. On the other hand, warm rolled tempered steel contains nanoscale carbides and homogeneous elongation grains that help to release high stress and strain concentration, resulting in improved formability and mechanical properties to a certain extent. Moreover, the warm rolled tempered steel (Tempformed/TF) exhibited a significant improvement in the impact absorb energy, about 3.1 and 47 times higher than those of the quenched and tempered (QT) state at 20 °C and −80 °C, respectively. This remarkable improvement in low-temperature impact toughness can be attributed to the delamination behavior and subsequent induced super-high plastic deformation energy. These findings have important implications for the design and development of high-performance materials with superior strength-toughness combinations.

Insight into the Role of Mo Content on the Microstructure and Impact Toughness of X80 Thick-Walled Low-Temperature Pipeline Steel

In this manuscript, the effects of Mo content on the microstructure and impact toughness of X80 thick-walled low-temperature pipeline steel were studied. Two test steels with different Mo content (0.25% and 0.40%) were prepared by the thermo-mechanical control process. The impact properties were measured at −45 °C, and the microstructure evolution was observed via an optical microscope (OM), a scanning electron microscope (SEM), electron back-scattered diffraction (EBSD), and a transmission electron microscope (TEM). Each steel showed the formation of a mixed microstructure consisting of polygonal ferrite (PF), granular bainite (GB), and lath bainite (LB). Increasing Mo content resulted in the rise of LB at the expense of PF and GB. At the same time, the morphology of martensite/austenite (M/A) constituents changed from blocky to slender. The dislocation density in the ferrite matrix around the M/A constituents enhanced with an increase in Mo content. This also led to an increase in the microstrains around the M/A constituents. Also, the number fraction of the high angle grain boundary (HAGB) (MTA &gt; 15°) decreased with the addition of more Mo content. Furthermore, with an increase in Mo content from 0.25% to 0.40%, the low-temperature impact toughness decreased from 206 to 57 J. Both an increase in the slender M/A constituents and a decrease in the HAGB number fraction deteriorated the low-temperature impact toughness of the X80 thick-walled low-temperature pipeline steel.

Low-temperature impact toughness of laser–arc hybrid welded low-carbon bainitic steel

Low-temperature impact toughness of laser–arc hybrid welded low-carbon bainitic steel

Effects of Ti/N Ratio on Coarse-Grain Heat-Affected Zone Microstructure Evolution and Low-Temperature Impact Toughness of High Heat Input Welding Steel

Coarse-grain heat-affected zone (CGHAZ) properties of steel deteriorate when it is welded using high heat input, which always restricts the promotion and use of high heat input welding steel. TiN particles significantly inhibit the growth of austenite and improve the microstructure and properties of high heat input welding steel. Effects of different Ti/N ratios on the CGHAZ microstructure and properties of high heat input welding steel were studied using welding thermal simulations and in situ observations. Results showed that a higher Ti/N ratio led to the abnormal growth of austenite grains and promoted the nucleation and growth of lath ferrite, which made the microstructure of the CGHAZ heterogeneous. In contrast, austenite grains were more uniform at a lower Ti/N ratio. Thus, the microstructure was refined, the brittle structure was reduced, and the properties of the CGHAZ were improved. In addition, when Ti/N = 5.85, the impact absorption energy of the CGHAZ obviously fluctuated. However, when Ti/N = 2.82, the impact absorption energy of the CGHAZ was higher and more stable. These results provided a new idea for the development of high heat input welding steel based on TiN theory.

Effect of Heat Treatment on Microstructure and Properties of Quenched and Tempered Steel with High Vanadium Contents

After heat treatment, the microstructure of quenched and tempered (Q&T) steel with high vanadium (V) content was characterized using metallographic and scanning electron microscopes (SEM). The proto-austenite grains of the tested steel were compared at different quenching temperatures, and the precipitation behavior of precipitates and matrix microstructure characteristics at various tempering temperatures were studied. Tensile properties and low-temperature impact toughness of the tested steel with different heat treatments were measured using an electronic universal testing machine and an impact testing machine. The findings indicate that as the quenching temperature increases, the size of the proto-austenite grains gradually increases. When the quenching temperature surpasses 920 °C, the size of the proto-austenite grains obviously increases. With a constant tempering temperature, an increase in quenching temperature results in a decrease in the size of precipitates. With a constant quenching temperature, an increase in tempering temperature results in an increase in the size of precipitates. Therefore, the tested steel quenched at 920 °C for 1 hour and then tempered at 630 °C for 1.5 hours would obtain excellent comprehensive mechanical properties, with a tensile strength of 1233 MPa and a low-temperature impact value of 64 J.

The determining role of laminar heterostructures on the mechanical properties of low-density steels

The practical application of low-density steels is hindered because of poor low-temperature toughness. To overcome this limitation, this study is focused on a new low-density steel with exceptional low-temperature toughness performance using a combination of hot rolling, intercritical quenching, and tempering. We study here the different intercritical quenching temperatures on both the microstructure and mechanical properties by EBSD, SEM, Charpy impact experiments and tensile experiments. The results showed that the presence of laminar heterostructures significantly enhanced the steel's low-temperature impact toughness, which can lead to the Charpy impact work of 61 J for the experimental steels at −40 °C. By the observation of the fracture morphology and secondary cracking of the impact specimens, it is found that this improvement was primarily attributed to the formation of obvious anisotropic distribution of randomly oriented grain boundaries between ferrite-martensite laminar heterostructures and the ferrite, which had a strong {001} <110> texture. This led to the generation of more and longer secondary cracks, which increased the energy required for crack propagation (Ep). However, the proportion of ferrite had an optimal value, and as the intercritical quenching temperature increased, the ferrite phase gradually disappeared, resulting in an initial increase and then decrease in toughness. Once the quenching temperature was greater than the completely austenite forming temperature, the formation of a single martensite microstructure led to brittle fractures along the prior austenite grain boundaries, resulting in a significant decrease in toughness, especially at low temperatures. The underlying reasons for the observed behavior are interrogated and discussed in terms of process-structure-properties paradigm.