Effect of Heat Treatment on Microstructure and Mechanical Properties of 2.25Cr–1Mo Steel

The effect of heat treatment on microstructure and mechanical properties of a ferritic heat‐resistant steel, 2.25Cr–1Mo, was investigated. The characteristics of carbides, i.e., size, morphology, distribution, and composition, in different stages of heat treatment were examined by using transmission electron microscopy (TEM) and energy dispersive X‐ray (EDX). As the heat treatment proceeded from normalizing and tempering (NT, the delivery state), simulated post‐welding heat treatment (PWHT), to step cooling (SC), the carbides evolved from M₃C, Mo₂C to M₇C₃, M₂₃C₆. The uniformly dispersed carbides, e.g., Mo₂C, M₇C₃, and M₂₃C₆, are responsible for the excellent mechanical properties and creep resistance of the steel. The result demonstrates that the coarsening of carbides deteriorates impact toughness and the morphology, crystal structure, and chemical composition of carbides are also important influencing factors.     

Effect of Tempering Temperature on Microstructure and Mechanical Properties of Steel Containing Ni of 9 %

  Mechanical properties of quenching, intercritical quenching and tempering (QLT) treated steel containing Ni of 9% were evaluated from specimens subject to various tempering temperatures. The detailed microstructures of steel containing Ni of 9% at different tempering temperatures were observed by optical microscope (OM) and transmission electron microscope (TEM). The volume fraction of austenite was estimated by XRD. The results show that high strength and cryogenic toughness of steel containing Ni of 9% are obtained when the tempering temperature are between 540 and 580 ℃. The microstructure keeps the dual phase lamellar structure after the intercritical quenching and there is cementite created in the Ni rich constituents when tempering temperature is 540 ℃. When tempering temperatures are between 560 and 580 ℃, the reversed austenites (γ′) grow up and the dual phase lamellar structure is not clear. The γ′ becomes instable at 600 ℃. When tempered at temperature ranging from 500 to 520 ℃, the increase of dislocation density in the lamellar matrix makes both tensile strength and yield strength decrease. When tempered at 540 ℃ and higher temperature, the yield strength decreases continuously because the C and alloying elements in the matrix are absorbed by the cementite and the γ′, so the yield ratio is decreased by the γ′. There are two toughness mechanisms at different tempering temperatures. One is that the precipitation of cementite absorbs the carbon in the steel which plays a major role in improving cryogenic toughness at lower temperature. Another is that the γ′ and the purified matrix become major role at higher tempering temperature. When the tempering temperature is 600 ℃, the stability of γ′ is decreased quickly, even the transformation takes place at room temperature, which results in a sharp decrease of Charpy V impact energy at 77 K. The tempering temperature range is enlarged by the special distribution of cementite and the lamellar structure.     

回火温度对Q960级高强结构钢组织及力学性能的影响

 以屈服强度960MPa级高强调质结构钢板开发为目标,研究了在相同轧制及淬火条件下,回火温度对试验钢显微结构及力学性能的影响。结果表明：随回火温度的升高,试验钢强度下降,韧塑性总体上呈现升高趋势,其中在300~450℃范围内出现一个韧塑性能的恶化区。当回火温度为600℃时,试验钢呈回火索氏体组织,屈服强度为1030MPa,抗拉强度为1080MPa,伸长率为15.9%,-40℃冲击功达144J,各项指标均满足国标GB/T 16270—2009要求。并对试验钢的拉伸力学性能进行了探讨。     

Effect of Tempering Temperature on Microstructure and Properties of E690 Offshore Plate Steel

 A new kind of Mn-Mo-Nb-Cu-B bainite steel which satisfied mechanical demands of E690 offshore plate steel was designed. The effect of two processes——thermomechanical control process (TMCP)+tempering (T) and thermomechanical control process (TMCP)+reheating and quenching (RQ)+tempering (T)——on microstructure and mechanical properties were studied by means of scan electron microscope (SEM), transmission electron microscope (TEM) and electron back scatter diffraction (EBSD). The results showed that optimal mechanical properties were available when tempering at 550 ℃ for both processes. The microstructure of the TMCP+T treated sample tempering in the range of 450 to 550 ℃ for 1 h did not change dramatically yet the lath in the TMCP+RQ+T treated sample merged together and transformed into polygonal ferrite. At the same time, the sub-structure of grain bainite transformed from lath to cell-shape to refine grains with tempering temperature mounting. Lots of sub-grain boundaries were located within bainite and adjacent bainite grain boundaries were high angle.     

回火温度对热轧双相钢组织和力学性能影响研究

研究了100~300℃回火对0.054C-1.18Si-1.16Mn-0.49Cr成分热轧双相钢DP600的显微组织和力学性能的影响。结果表明:回火温度主要影响热轧双相钢中铁素体位错密度和马氏体微观结构;随着回火温度的增加,热轧双相钢中铁素体可动位错密度降低,马氏体部分发生分解,析出碳化物;回火温度对抗拉强度影响不大,对屈服强度和屈强比的影响显著,175℃以上回火,热轧双相钢屈服强度显著提高,并出现屈服平台,150℃以下回火热轧双相钢屈服强度增加不明显,不出现屈服现象。     

Microstructure and Mechanical Properties of a Low Carbon Bainitic Steel

A low carbon bainitic steel with microstructure of granular bainite (GB), acicular ferrite (AF), and bainitic ferrite (BF) is obtained under different deformation and cooling rate. The effect of deformation and cooling rate on microstructural characteristics such as the type of the matrix, the size, and area fraction of the martensite–austenite (M–A) constituents is investigated. In addition, the nanohardness of these three kinds of matrix as well as that of the M–A constituents in them is characterized. Further, the effect of matrix and M–A constituents on strength–toughness balance is studied. Results indicate that deformation expands the transformation region. The size as well as the area fraction of the M–A constituent decreases with the increasing of the cooling rate. After deformation, the area fraction of the M–A constituents increases. Nanohardness of GB, AF, and BF increases orderly, but that of the M–A constituents in them decreases accordingly. The nanohardness of the M–A constituent is significantly affected by its carbon concentration. AF is the optimum microstructure having superior strength–toughness balance.