Structure heterogeneity and mechanical properties studied in thickness up to 100 mm of low-alloyed shipbuilding steel sheets with a yield strength not less than 420 MPa

This paper presents a study of changes in the structure and properties in thickness of rolled sheets up to 100 mm of low-alloyed shipbuilding steel with a yield point not less than 420 MPa. The fracture surface of samples after impact bending tests at low temperatures was investigated. It was found that the combination of the parameters of lath morphology bainite (fraction, areas average size and length) and the size of structural elements at given tolerance angles of 5 and 15° (indicating the presence or absence of a developed subgrain structure of deformation origin) determine the level of impact work at low temperatures testing.

Microstructure, Substructure and Mechanical Properties of 9CrNB Steel after Tempering

This paper deals with the analysis of microstructure and substructure of 9CrNB steel, after normalization at temperature of 1070 °C and tempering at 790 °C / 240 min. The tube was second time tempered at the following temperatures and holding times: 760 °C / 30 min (A1), 760 °C / 120 min (C1), 800 °C / 30 min (G1) and 800 °C / 120 min (I1). Microstructure after tempering consists of tempered martensite and bainite with lath morphology, while inhomogeneous redistribution of precipitates is visible. Substructure analysis of state A1 and I1 show, that a relatively large number of irregular, rod-shaped and oval carbide particles, often arranged in clusters, were precipitated at the primary original austenite grain boundaries. In case of state A1, the average size of these carbide particles is 300 nm and in case of state I1 the average size is 350 nm. A relatively large number of rod-shaped and oval shaped particles were found at the interface of the tempered martensite and bainite mainly in the form of clusters and also inside the tempered bainite with higher particle distribution. In the case of the state A1, they reached an average size of 150 nm. In some regions of substructure of the state I1, the fine carbide particles with an average size of 200 nm and coarse carbide particles with an average size of 400 nm were presented within the areas of tempered bainite. Particles were identified by EDX analysis and by selection electron diffraction. The mechanical properties after tempering were evaluated and compared with properties of P91 and P92 steel.

Microstructure and Mechanical Properties of 9CrNB Steel after Heat Treatment

This paper deals with the analysis of microstructure in the conditions after austenitising at high temperatures (1150 °C, 1180 °C a 1210 °C/45 min) for 9CrNB steel, and with the analysis of microstructure in the conditions after subsequent tempering at temperatures 740 °C, 770 °C, 800 °C/1 hr. After austenitising the microstructure was formed of lath martensite with secondary carbides. The average size of initial austenitic grains ranged from 144 μm (1150 °C) to 163 μm (1210 °C). Increasing the austenitising temperature led to insignificant improvement in strength properties, while the plasticity of the steel decreased. Microstructure in tempered conditions was formed from tempered martensite. This martensite had lath morphology, with inhomogeneus distribution of precipitates. Higher austenitising temperature and increased tempering temperature led to spheroidal and growth of secondary particles. The tempering effect on strength and plastic properties of steel are also described.

On Preparation of In Situ Composite Steel with Ultra-High Strength and its Thermal Stability

A new type of in-situ composite nano-multilayer plate with ultra-high strength (b 2112 MPa), Q235 steel plate with nano-layered structure of lath martensite produced by severe cold-rolling, was developed. After cold-rolling, subsequent annealing has great effect on the deformed lath morphology and grain refinement. Microstructure recrystallizing course have taken place after long time annealing at 350 °C. The recrystallization activation energy is 151 kJmol-1. Microstructure characteristics along rolling direction arrangement was decreased after annealing at 400 °C. In addition to the ultrafine ferrite grains, nano-carbides precipitated uniformly in the specimen annealed at 500 °C. Annealing at and above 600 °C resulted in coarse ferrite grains with spheroidized coarse carbides, causing grain growth. The average crystal size is about 4.7 m after annealing for 60 min at 600 °C.

Microstructural evolution of modified 9Cr-1Mo steel due to annealing and high-temperature fatigue

The microstructural evolution of Modified 9Cr-lMo steel has been investigated for a variety of conditions, including normalizing, tempering and subsequent annealing. The alleged advantages of the modified alloy for high temperature service, when compared to the standard 9Cr-lMo composition, have been examined for high temperature fatigue conditions; the only difference between the two alloys is the presence of micro additions of V and Nb in the modified alloy.Air cooling (normalizing) of Modified 9Cr-lMo from 1045°C results in the precipitation of fine (FeCr)3C particles within the martensite laths. Additional carbide precipitation and changes in the dislocation structure occur during the tempering of martensite at 700°C and 760°C after normalizing. The precipitation of M23C6 carbides occurs preferentially at lath interfaces and dislocations. The formation of Cr2C was detected during the first hour of tempering over the range 650-760°C, but was replaced by VC within one hour at 760°C. During prolonged annealing at 550°C-650°C, following tempering, the lath morphology remains relatively stable in the absence of simultaneously applied stresses; partitioning of the laths into subgrains and some carbide coarsening are beginning after 400 hr. annealing at 650°C (Fig. 1), but the lath morphology persists. The martensite lath stability is attributed primarily to the VC precipitates distributed along the lath interfaces (Fig. 2) and promotes improved performance of the Modified 9Cr-lMo alloy over the the standard alloy under static tensile and creep conditions.