Yield strength enhancement of martensitic steel through titanium addition

Yield strength enhancement for martensitic steel fabricated by vacuum induction melting is investigated. It is found that the addition of Ti can improve the yield strength property of the martensitic steel, which can be attributed to increase in precipitation hardening from formation of TiC precipitates in the martensitic matrix. Moreover, the yield strength can be further enhanced by tempering and reheat quenching process, which can be ascribed to the formation of a superfine sized (~8 μm) grains and large amount of freshly nano-sized (1–10 nm) precipitates in the final martensitic structure for martensitic steel containing Ti. The experimental and theoretical results on the contribution of TiC precipitates to hardening of the martensitic steel are in excellent agreement, showing that the precipitation hardening of 188 MPa caused by TiC precipitates is the main reason why the yield strength for martensitic steel is enhanced via titanium addition.     

Effect of direct quenching on the microstructure and mechanical properties of the lean-chemistry HSLA-100 steel plates

The influence of direct quenching on structure-property behavior of lean chemistry HSLA-100 steels was studied. Two laboratory heats, one containing Cu and Nb (C:0.052, Mn:0.99, Cu:1.08, Nb:0.043, Cr:0.57, Ni:1.76, Mo:0.55 pct) and the other containing Cu, Nb and B (C:0.04, Mn:1.02, Cu:1.06, Nb:0.036, Cr:0.87, Ni:1.32, Mo:0.41, B:0.002 percent) were hot-rolled into 25 and 12.5 mm thick plates by varying finish-rolling temperatures. The plates were heat-treated by conventional reheat quenching and tempering (RQT), as well as by direct quenching and tempering (DQT) techniques. In general, direct-quench and tempered plates of Nb-Cu heat exhibited good strength (yield strength ∼ 900 MPa) and low-temperature impact toughness (average: 74 J at −85 °C); the Charpy V-notch impact energies were marginally lower than conventional HSLA-100 steel. In Nb-Cu-B heat, impact toughness at low-temperature was inferior owing to boron segregation at grain boundaries. Transmission electron microscopy (TEM) and scanning auger microprobe (SAM) analysis confirmed existence of borocarbides at grain boundaries in this steel. In general, for both the steels, the mechanical properties of the direct-quench and tempered plates were found to be superior to reheat quench and tempered plates. A detailed transmission electron microscopy study revealed presence of fine Cu and Nb (C, N) precipitates in these steels. It was also observed that smaller martensite inter-lath spacing, finer grains and precipitates in direct-quench and tempered plates compared to the reheat quench and tempered plates resulted in their superior strength and good impact toughness.     

Effects of Ti and a twice-quenching treatment on the microstructure and ductile brittle transition temperature of 9CrWVTiN steels

The effects of Ti and a twice-quenching treatment on the microstructure and ductile brittle transition temperature (DBTT) of 9CrWVTiN steels have been studied. The results show that Ti addition reduces austenite grain size and martensitic lath, and moderate Ti (< 0.018%) content reduces the precipitates size. Microstructure, especially the coarse M₂₃C₆ precipitates is remarkably refined by twice quenching and by consuming C through preferential precipitation of MX precipitates in the furnace cooling period, thus contributing to the decrease of DBTT compared with quenching–tempering process. However, DBTT increases with increasing Ti content, and the increased DBTT reaches to as high as 36 °C compared with steel without Ti regardless of the refinement of microstructure. Through precipitate analyses, we find that, Ti strongly interferes with the precipitation of V(C,N). Ti(C,N) hardly exists in matrix alone. Instead, it acts as the core of quadrate (Ti,V)(C,N) particle. Big stress concentrations at the corners of coarse quadrate (Ti,V)(C,N) precipitates make them as crack initiators during impact tests, thus deteriorating the toughness. Also, formation of complex (Ti,V)(C,N) particles reduces the amounts of V, N, and C available for adequate fine V(C,N) particle precipitation. These two factors are the main reasons for high DBTT generated by Ti addition.     

A comparative study of precipitation effects in Ti only and Ti–V Ultra Low Carbon (ULC) strip steels

Two ULC steel grades were investigated, one based on combined vanadium and titanium additions and the other based on titanium only additions. It has been established that TiC formation during interphase precipitation retards grain growth of the {111} texture grains during continuous annealing and hence positively affects the r value of the Ti only steel. The formation of newly formed TiC precipitates on dislocations during continuous annealing has been found to result in an increase of the yield strength in both steel grades, as the annealing temperature is increased. It is also confirmed that VC particles formed during the coiling process dissolve during the continuous annealing cycles. Suitable continuous annealing cycles can be adopted to produce high formable steels with a bake hardening potential using the beneficial effects of combined Ti–V additions.     

Precipitation of MC phase and precipitation strengthening in hot rolled Nb–Mo and Nb–Ti steels

Hot rolled Nb–Mo steel of yield strength 600 MPa and Nb–Ti steel of yield strength 525 MPa with polygonal and acicular ferrite microstructure have been developed. Using physicochemical phase analysis, XRD, TEM and EDS, the distribution, morphology, composition, crystal structure and particle size of precipitates were observed and identified in these steels. The results revealed that the steels containing both Nb and Mo exhibited fine and uniformly distributed MC-type carbides, while the carbides were coarse and sparsely distributed in the steels containing Nb and Ti. The physicochemical phase analysis showed MC-type carbides contain both Nb and Mo, and the ratio of Mo/Nb was 0.41. Meanwhile, the mass% of the fine particles (<10 nm in size) of Nb–Mo steel was 58.4%, and higher than that of Nb–Ti steel with 30.0%. Therefore, the results of strengthening mechanisms analysis showed the higher strength of Nb–Mo steel than that of Nb–Ti steel is attributed to its relatively more prominent precipitation strengthening effect. The yield strength increments from precipitation hardening of Nb–Mo steel attained 182.7 MPa and higher than that of Nb–Ti steel.     

Experimental investigation on heat treatment of a high‐speed steel for hot rolling roll mill

The authors describe the researching results to optimise the hardening and tempering of the high carbon high‐speed steel for rolls containing 2.38%C, 5.07%V, 6.34%Mo, 5.09%Cr, 1.20%Ni, 1.17%Nb, 0.09%Ti and 0.05%RE by means of light optical microscope (LOM), scanning electron microscope (SEM), backscattered electron image (BSE), X‐ray diffraction (XRD), and hardness, tensile strength, impact toughness and wear testers. The results show that the microstructure of above casting high‐speed steel is given by a tempered martensitic matrix surrounded by eutectic carbides. Casting high‐speed steel has higher hardness quenching at 1280 K–1340 K, and it has higher hardness, tensile strength, impact toughness, and abrasive wear resistance tempering at 793 K–833 K. The comprehensive properties of casting high‐speed steel is the best while air‐cooling quenching about 1340 K and tempering about 813 K.     

Nano-sized precipitates in 11 % Cr ferritic-martensitic steel after thermomechanical treatment

9 %–12 % Cr ferritic/martensitic steels with a good long-term creep strength at temperatures up to 650 °C and higher are being developed in order to increase steam temperature of coal-fired power plants.Thermomechanical treatment can effectively enhance the mechanical properties of high-Cr ferritic/martensitic steels mainly due to plenty of nano-sized precipitates produced by thermomechanical treatment. Nano-sized precipitates in an 11 % Cr ferritic/martensitic steel produced by a thermomechanical treatment, including warm rolling at 650 °C plus tempering at 650 °C for 1 h, were investigated by transmission electron microscopy. The average size of precipitates in the steel after the thermomechanical treatment was determined to be about 30 nm in diameter, which is only one-third of the average size of precipitates in the steel with the normalized and tempered condition. A large number of Cr-rich precipitates having an average diameter of about 25 nm in the steel produced by the thermomechanical treatment were identified as Cr-rich M₂C carbide with a hexagonal crystal structure, rather than M₂₃C₆ or MX phase. The plenty of nano-sized Cr-rich M₂C carbides were dominant phase in the steel after the thermomechanical treatment. The reason why prior precipitate phase formed in the steel during the thermomechanical treatment was Cr-rich M₂C carbide is also discussed.     

Evolution of microstructure and mechanical properties during quenching and tempering of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel

Effects of quenching and tempering treatments on the development of microstructure and mechanical properties of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel were investigated. Samples were austenitized at 1123–1323 K for 2400 s and oil quenched (OQ) to produce mixed microstructures. Tempering was carried out at 473–773 K for 2–3 h. Phase transformation temperatures were measured using dilatometer. The microstructures were characterized using optical and scanning electron microscope. SEM–EDS analysis was carried out to determine the type and size of non-metallic inclusions. Volume percent of retained austenite was measured by X-ray diffraction technique. Hardness, tensile properties, and impact energies were also determined for all heat treated conditions. Fractography of impact specimens were done using stereomicroscope and SEM. The results showed that newly developed steel exhibited peak hardness, yield strength, and tensile strength of about 600 HV, 1760 MPa, and 1900 MPa, respectively, when OQ from 1203 K and tempered in between 473 and 573 K, combined with adequate ductility and impact toughness. Decrease in hardness and strength was observed with increasing tempering temperature whereas the impact energy was stable up to 623 K, however, impact energy was found to decrease above 632 K due to temper martensite embrittlement.     

回火方式对调质高强度钢组织和性能的影响

为改善高强度钢的塑性和韧性,对同一种低合金高强度钢进行两种不同回火方式的调质处理,淬火+缓慢加热回火的传统调质与淬火+感应加热回火的新调质工艺,分析该工艺对钢的组织与性能的影响.利用扫描电镜和透射电镜观察组织及析出物的变化,采用X射线衍射仪分析了钢中残余奥氏体体积分数.结果表明:两种工艺下,钢的组织均为板条宽300～500 nm左右的马氏体组织,感应加热回火调质工艺处理后,板条组织明显,析出物大多约为20 nm,比传统调质处理后的细小;两种不同热处理工艺均能提高钢的屈服强度.感应加热至500℃回火后试验钢具有16%以上的延伸率,-40℃冲击功达到32 J,优于传统调质工艺处理钢板的综合性能.感应加热回火能获得更多小尺寸析出物和更多的残余奥氏体,有利于改善钢的塑性和韧性.     

Effect of tempering temperatures on microstructures and properties of 0.28C–0.22Ti wear-resistant steel

The microstructures and properties of a 0.28C–0.22Ti low-alloy wear-resistant steel at different temperatures from 200 to 600°C was experimentally studied. It is shown that the wear resistance of the steel is not monotone changing with its hardness and strength. With the increase of the tempering temperature, the tensile strength and the hardness of the steels were gradually declined; however, the wear resistance was first decreased and then increased. The TiC particles can be divided into two classes: the small TiC particles (about 0.3–0.4?µm in diameter) and the coarse TiC particles (1–5?µm in diameter). The small TiC particles can improve the yield strength of the steels, and the coarse TiC particles can improve the wear resistance of the tested steels.     

Study on dislocation density,microstructure, and mechanical properties of P92 steel for different heat treatment conditions

The impact of various heat treatment procedures on microstructure, dislocation density, hardness, tensile characteristics, and impact toughness of P92 steel was examined in the current experiment. The martensitic microstructure and average microhardness of 463 HV 0.2±8 HV 0.2 of the normalized steel were prevalent. A tempering procedure was carried out at 760 °C for a range of 2 hours to 6 hours. Additionally, an X-ray diffraction examination was carried out, and the results were used to determine the dislocation density. The normalized sample was characterized by a high dislocation density. The dislocation density was decreased by tempering of normalized samples. With an increase in tempering time, the effect of the treatment coarsened the grains, precipitates, and decreased the area fraction of precipitates. After tempering, MX, M₂₃C₆, and M₇C₃ types precipitates were found to have precipitated, according to energy dispersive spectroscopy and x-ray diffraction research. The ideal tempering period was determined to be 4 hours at a tempering temperature of 760 °C based on the microstructure and mechanical characteristics. Steel that was tempered at 760 °C for 4 hours had a yield strength of 472 MPa, an ultimate tensile strength of 668.02 MPa, and an elongation of 26.05 %, respectively.     

Copper precipitation and its impact on mechanical properties in a low carbon microalloyed steel processed by a three-step heat treatment

The structure–mechanical property relationship, with particular focus on effect of tempering process on the microstructural evolution and mechanical properties was investigated in a low carbon Cu-bearing steel that was processed in three-steps, namely, intercritical annealing, intercritical tempering, and tempering heat treatment. The objective of adopting three steps was to elucidate the nature and evolution of microstructural constituents that contributed to high strength–ductility combination in the studied steel. The three-step processing led to a microstructure primarily comprising of ferrite, retained austenite, and small amount of bainite/martensite. The mechanical properties obtained were: yield strength > 720 MPa, tensile strength > 920 MPa, uniform elongation > 20%, total elongation > 30%, and low yield ratio of 0.78. The tempering step led to a significant increase in both yield and tensile strength and decrease in yield ratio, without reducing ductility, a behavior attributed to the precipitation of copper in retained austenite and ferrite. The precipitation of copper enhanced the stability of retained austenite and work hardening rate, leading to a high volume fraction of retained austenite (∼29%), with consequent increase in elongation and significant increase in yield and tensile strength during tempering.     

Study of tensile deformation behaviour of M250 grade maraging steel using acoustic emission

Tensile testing of solution annealed and thermally aged (755 K for various durations in the range of 0.25–100 h) specimens of M250 grade maraging steel has been carried out along with acoustic emission (AE) monitoring. Results have shown that strength increases and ductility decreases upon ageing up to 10 h and this has been attributed primarily to the precipitation of Ni₃Ti. Continued increase in strength up to 40 h of ageing has been attributed primarily to the precipitation of Fe₂Mo in addition to Ni₃Ti. Increase in ductility for 10–40 h of ageing has been attributed to dissolution of needle like Ni₃Ti precipitates and formation of fine spherical Fe₂Mo. Ageing beyond 40 h decreases strength and increases ductility due to the reversion of martensite to austenite and coarsening of the precipitates. The AE generated during tensile deformation depends on the ageing time. Increased occurrence of shearing of the precipitates by dislocations and increased brittleness of the matrix up to 10 h ageing increases the AE. The decrease in the AE beyond 10 h of ageing is due to the occurrence of deformation by Orowan looping, dissolution of Ni₃Ti precipitates and austenite reversion. The scanning electron microscopy (SEM) of the fracture surfaces has shown ductile fracture characterized by dimples and changes in the size and shape of the dimples with ageing time.     

Phase transition and mechanical properties of constraint-aged Ni–Mn–Ga–Ti magnetic shape memory alloy

Ni–Mn–Ga Heusler-type ferromagnetic shape memory alloys are attractive materials for micro-actuator, but the relatively poor ductility and low strength of Ni–Mn–Ga alloys have triggered a great deal of interest. In this study, we attempt to introduce some ductile second phase in the alloy by partially substituting Ti for Ga and constraint aging treatment. The results show that the martensitic transformation temperature first decreases and then increases slightly with the increasing of constraint-aging temperature, which can be attributed to the decrease of Ni content in the matrix and strengthening effect of the second particles. It is found that the amount of the Ni-rich precipitates by constraint-aged samples is more and the size of the second phase particle is smaller than that of the free-aged samples. The compressive stress and ductility can be significantly improved by the constraint-aging treatment, and the maximum compressive stress for constraint-aging alloy is about 1400 MPa, which is the highest value up to date compared with the 400 MPa in solution-treated Ni–Mn–Ga–Ti alloy and about 900 MPa in Ni–Mn–Ga–Ti alloy free-aged at 1073 K for 3 h. Scanning electron microscopy observations of fracture surfaces confirm that the Ni-rich second phase play a key role in improving the compression stress and ductility of Ni–Mn–Ga–Ti alloy.     

Precipitation behaviors of carbides and Cu during continuous heating for tempering in Cu-bearing medium C martensitic steel

The precipitation behaviors of carbides and Cu during continuous heating for tempering were investigated in Cu-bearing medium C martensitic steel by means of dilatometry, electrical resistivity, and transmission electron microscopy. The addition of 1.5 wt% Cu suppressed carbide precipitation during quenching from 900 °C, resulting in a large amount of solute C atoms in virgin martensite. The addition of Cu increased both the finish temperature of ε-carbide precipitation and the amount of ε-carbide precipitates during continuous heating. The precipitation of cementite was retarded and the amount of cementite precipitates increased by the addition of Cu. Retarded cementite precipitation in the Cu-bearing steel was attributed to sluggish Cu partitioning from cementite particles to the martensite matrix, the hindrance to the migration of cementite interfaces by Cu particles, and the slowed diffusions of C and Fe atoms. Cu precipitation was accelerated by cementite precipitation because cementite interfaces and the high Cu concentration near cementite particles provided nucleation sites for Cu precipitation. The hardness of the tempered Cu-bearing steel was higher than that of the tempered Cu-free steel at the temperatures of over 300 °C due to both Cu precipitation hardening and retarded cementite precipitation.     

Copper bearing microalloyed ultrahigh strength steel on a pilot scale: Microstructure and properties

In the present study, copper bearing low carbon microalloyed ultrahigh strength steel has been produced on a pilot scale. Transformation of the aforesaid steel during continuous cooling has been evaluated. The steel sample has been thermomechanically processed followed by either air cooling or water quenching. Variation in microstructure and mechanical properties at different finish rolling temperatures has been studied. A mixture of granular bainite, bainitic ferrite and precipitation of nano-sized (Ti, Nb)C particles is the characteristic microstructural feature of air cooled steel. On the other hand, predominantly lath martensitic structure along with the similar type of microalloying precipitates of air cooled steels and Cu precipitates are obtained in case of water quenched steel. The best combination of strength (1364-1403 MPa) and ductility (11-14%) has been achieved for the selected range of finish rolling temperature of water quenched steel.     

New observations of the twinning effect and austenite stability in intercritical quenched and tempered steel with high strength

To optimize the formability and strength of hot-rolled Fe-10Mn-0.4C-2Al-0.6 V medium Mn steel, intercritical quenching and tempering processes were carried out. The strength of the steel was enhanced, and the Lüdders platform was eliminated. The higher strength of the steel was attributed to the occurrence of a complex twinning effect, martensitic transformation and V-carbide precipitation during tensile deformation. In particular, the twin martensite structure retained after the quenching-tempering process served as another previous twin to accelerate the generation of nanomechanical twins in recrystallized austenite grain. The occurrence of transformation-induced plasticity (TRIP) of austenite with poor stability in non-recrystallized regions stimulated the TRIP and twinning-induced plasticity (TWIP) effects in austenite with high stability in recrystallized regions. Therefore, two pathways to improve the formability and optimize the mechanical properties of medium Mn steel by adjusting the quenching and tempering processes were proposed in this paper: (1) Manufacturing more martensite twin structures and (2) regulating the balance of austenite stability in both recrystallized and non-recrystallized regions.

     

Electropulsing strengthened 2GPa boron steel with good ductility

For the purpose of further enhancing the mechanical properties of a boron steel used in the automotive industry, the electropulsing is applied to strengthen the steel. The results show that the steel exhibits unexpected high strength and ductility. The engineering ultimate-tensile-strength and fracture strain are about 2022 MPa and 0.246, respectively. The microstructure examinations indicate that the martensitic was refined greatly during the electropulsing strengthening process. The fine martensitic with high-density dislocation makes the steel with good mechanical properties. The amazing results can be attributed to the thermal and athermal effects of the electropulsing. Due to the effect of rapid heating during electropulsing, a large overheating could be obtained, which could result in the high nucleation rate of austenite. Moreover, the pulse current itself can increase the austenite nucleation rate by decreasing the thermodynamic barrier. Finally, the fine lath-martensitic with high-density dislocation formed during the subsequent quenching process.     

Ferrite recrystallisation and characteristics of non-recrystallised ferrite grains in Ti added low carbon steels

The progress of ferrite recrystallisation in low carbon steel was slower than in ultralow carbon steel. The hardness of the non-recrystallised ferrite grains gradually decreased with increasing annealing time in ultralow carbon steel, but gradually increased with increasing annealing time in low carbon steel. The amount of Ti containing precipitates increased slightly during annealing in ultralow carbon steel, but increased remarkably with increasing annealing time in low carbon steel. These results suggest that the softening of non-recrystallised ferrite grains during annealing in ultralow carbon steel may reflect the progress of recovery and the Ostwald ripening of Ti containing precipitates formed during annealing. In contrast, the hardening of non-recrystallised ferrite grains in low carbon steel may be due to the precipitation hardening of TiC formed during annealing.     

两次淬火对HSLA钢组织和冲击韧性的影响

研究了两次淬火+回火和传统的一次淬火+回火热处理对HSAL钢的显微组织和力学性能的影响。结果表明,在不显著降低强度的条件下,两次淬火使实验钢的冲击功明显提高,还改善了低温韧性和稳定性。两次淬火回火热处理可细化钢的组织,使原始奥氏体晶粒的尺寸和有效晶粒尺寸减小、大角度界面的密度和解离裂纹的扩展偏折频率提高。组织的细化和大角度晶界的增多抑制了裂纹的扩展,使韧性大幅度提高。