Yiyin Shan

Continuous cooling transformation of undeformed and deformed low carbon pipeline steels

The continuous cooling transformation (CCT) behaviors of three low carbon pipeline steels containing the different carbon and alloy additions such as Mn, Nb, V, Ti and/or Mo were investigated in the undeformed and deformed conditions, respectively. The corresponding static (without hot deformation) and dynamic (with hot deformation) CCT diagrams were constructed, which almost involved the formation curves of bainitic ferrite, acicular ferrite, polygonal ferrite, and pearlite. It was found that with the exception of V, the aforementioned alloy additions played a significant role in suppressing the formation of polygonal ferrite and promoting the formation of acicular ferrite. Furthermore, hot deformation could also strongly promote the formation of acicular ferrite, that is, the temperature zone of acicular ferrite transformation was enlarged from 400-600 degreesC in the static CCT diagrams to 450-700 degreesC in the dynamic CCT diagrams. The corresponding cooling rate range of acicular ferrite transformation was significantly increased, and the island constituents in acicular ferrite became finer due to hot deformation

Comparison on strength and toughness behaviors of microalloyed pipeline steels with acicular ferrite and ultrafine ferrite

Abstract A laboratory smelted microalloyed pipeline steel was conducted by two different thermomechanical control process (TMCP) on a pilot rolling mill to produce two currently interesting microstructures, i.e., acicular ferrite and ultrafine ferrite. Strength and toughness behaviors of these two microstructures were investigated. Compared with commercial pipeline steels, the experimental steel with acicular ferrite and/or ultrafine ferrite possessed the satisfied strength and toughness behaviors, although this steel contained only about 0.025% carbon. Furthermore, acicular ferrite was better candidate microstructure for pipeline steels than ultrafine ferrite.

Effect of TiN inclusions on the impact toughness of low-carbon microalloyed steels

Microalloying with various elements, including titanium, coupled with thermomechanically controlled processing, has become a major technology for the manufacture of high-quality steel plate. In this research, the influence of TiN inclusions on the impact toughness of low-carbon plate steels microalloyed with titanium, vanadium, and boron was investigated. The three experimental steels had Ti/N ratios of 2.44, 3.5, and 4.2, and all three had a granular bainite microstructure. However, Charpy V-notch testing showed that steel A had very high toughness at both room temperature and −20 °C, whereas steels B and C showed very low toughness at −20 °C and moderate toughness at room temperature. Scanning electron microscope fractography revealed that coarse TiN inclusions had acted as cleavage fracture initiation sites in steels B and C. The effect of Ti and N levels on TiN formation and growth is analyzed using alloy thermodynamics. It is shown that not only is the Ti/N ratio important, but also the product of total Ti and N plays a most important role in TiN formation and growth. It is concluded that the product of the total Ti and N contents should not be greater than the solubility product of TiN at the solidus temperature to prevent the precipitation of TiN particles before solidification. Furthermore, the ratio of Ti to N should also be maintained lower than the stoichiometric ratio of 3.42 to ensure a low coarsening rate for the TiN inclusions during soaking before rolling.

Microstructural stability of 9–12%Cr ferrite/martensite heat-resistant steels

The microstructural evolutions of advanced 9–12%Cr ferrite/martensite heat-resistant steels used for power generation plants are reviewed in this article. Despite of the small differences in chemical compositions, the steels share the same microstructure of the as-tempered martensite. It is the thermal stability of the initial microstructure that matters the creep behavior of these heat-resistant steels. The microstructural evolutions involved in 9–12%Cr ferrite heat-resistant steels are elaborated, including (1) martensitic lath widening, (2) disappearance of prior austenite grain boundary, (3) emergence of subgrains, (4) coarsening of precipitates, and (5) formation of new precipitates, such as Laves-phase and Z-phase. The former three microstructural evolutions could be retarded by properly disposing the latter two. Namely improving the stability of precipitates and optimizing their size distribution can effectively exert the beneficial influence of precipitates on microstructures. In this sense, the microstructural stability of the tempered martensite is in fact the stability of precipitates during the creep. Many attempts have been carried out to improve the microstructural stability of 9–12%Cr steels and several promising heat-resistant steels have been developed.

Microstructure evolution of a 10Cr heat-resistant steel during high temperature creep

The microstructure evolution of a 10Cr ferritic/martensitic heat-resistant steel during creep at 600 degrees C was investigated in this work. Creep tests demonstrated that the 10Cr steel had higher creep strength than conventional ASME-P92 steel at 600 degrees C. The microstructure after creep was studied by transmission electron microscopy, scanning electron microscopy and electron probe microanalysis. It was revealed that the martensitic laths were coarsened with time and eventually developed into subgrains after 8354 h. Laves phase was observed to grow and cluster along the prior austenite grain boundaries during creep and caused the fluctuation of solution and precipitation strengthening effects, which was responsible for the two slope changes on the creep rupture strength vs rupture time curve. It was also revealed that the microstructure evolution could be accelerated by stress, which resulted in the lower hardness in the deformed part of the creep specimen, compared with the aging part.

Effect of Microstructure on Hydrogen Induced Cracking Behavior of a High Deformability Pipeline Steel

The hydrogen induced cracking (HIC) behavior of a high deformability pipeline steel was investigated with three different dual-phase microstructures, ferrite and bainite (F+B), ferrite and martensite/austenite islands (F+M/A) and ferrite and martensite (F+M), respectively. The HIC test was conducted in hydrogen sulfide (H2S)-saturated solution. The results showed that the steels with F+B and F+M/A dual-phase microstructures had both higher deformability and better HIC resistance, whereas the harder martensite phase in F+M microstructure was responsible for the worst HIC resistance. The band-like hard phase in dual-phase microstructure was believed to lead to increasing susceptibility to HIC.

Constitutive Modeling, Microstructure Evolution, and Processing Map for a Nitride-Strengthened Heat-Resistant Steel

Abstract A constitutive equation was established to describe the deformation behavior of a nitride-strengthened (NS) steel through isothermal compression simulation test. All the parameters in the constitutive equation including the constant and the activation energy were precisely calculated for the NS steel. The result also showed that from the stress-strain curves, there existed two different linear relationships between critical stress and critical strain in the NS steel due to the augmentation of auxiliary softening effect of the dynamic strain-induced transformation. In the calculation of processing maps, with the change of Zener-Hollomon value, three domains of different levels of workability were found, namely excellent workability region with equiaxed-grain microstructure, good workability region with “stripe” microstructure, and the poor workability region with martensitic-ferritic blend microstructure. With the increase of strain, the poor workability region first expanded, then shrank to barely existing, but appeared again at the strain of 0.6.

HIC and SSC Behavior of High-Strength Pipeline Steels

In this study, hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSC) behaviors of high-strength pipeline steels in four different strength grades (X70, X80, X90 and X100) with the microstructure of acicular ferrite were estimated. The results showed that both of X70 and X80 steels exhibited better HIC resistance, and their susceptibility to HIC increased with the strength grade. HIC parameters, including cracking length ratio, cracking thickness ratio (CTR) and cracking sensitivity ratio, were all increased, and among these, the CTR increased most, with the increase in the strength grade. HIC was found to initiate and grow along the hard boundaries such as large size martensite/austenite (M/A) islands and bainitic ferrite. In addition, the density of hydrogen-induced blister on the steel surface was increased with the decrease in pH value for the same-grade pipeline steels. SSC susceptibilities of X80, X90 and X90-C were revealed to subsequently decrease, which was related to the large size M/A islands.

9-12Cr heat-resistant steels

With China becoming a major force in steel research and development, this book highlights the work of a group from the Chinese Academy of Sciences, led by the first four authors. This group has the ideal knowledge base for writing this updated book on heat-resistant steels. The fifth author, Sha, based in the UK, has been collaborating with the Chinese group since 2009 and is the lead or sole author of four research books, all published in English. The last book, Steels: from materials science to structural engineering, was published by Springer in 2013. Within two months of its publication, researchers at the University of Science and Technology Liaoning had requested translation of the book into Chinese. Springer obliged, and the Chinese version was published by the Metallurgical Industry Press, Beijing, in August 2014. Sha has organized and completed the writing of the book, though the main research was done in China

Creep of Heat-Resistant Steels

The first part of this chapter addresses the overestimation of the time–temperature parameter method on the allowable creep strength of 9–12 %Cr heat-resistant steels. Creep data of 9 %Cr heat-resistant steels are divided into several ranges according to the creep controlling mechanism. The physically based continuum creep damage mechanics could provide a unified framework for predicting the creep life of steels working at elevated temperature. It is used to analyse the creep rupture properties of a heat-resistant steel used in the supercritical power generation, based on the current experimental database. In the microstructure after creep, the martensitic laths grow in size with time and eventually develop into a subgrain structure. Laves phase grows and collects along the prior austenite grain boundaries during creep and causes the fluctuation of solution and precipitation strengthening effects. The deformed part of the creep specimen has lower hardness than the aged part because stress can accelerate the microstructure evolution. In the final part of the chapter, the creep rupture mechanism of heat-resistant steel under different stress levels is discussed. Under conditions of high stress, the creep rupture mechanism is similar to the ductile fracture at room temperature. This changes to brittle fracture under low stress levels.