[en] Highlights: • First report on hot cracking behavior of carbide-free bainitic (CFB) weld metals • CFB weld metals show comparable hot cracking resistance to most austenitic steels. • Strategies for optimization of CFB weld metal compositions discussed In this work, hot cracking behavior of a carbide-free bainitic weld metal was investigated using Varestraint tests and Gleeble hot ductility tests. The results show that the carbide-free bainitic weld metal is as resistant to hot cracking as many of the standard austenitic stainless steel weld metals. The effects of composition, solidification mode, and impurity content on hot cracking susceptibility of carbide-free bainitic steels are discussed. Some guidelines for optimizing their compositions for superior hot cracking resistance are also presented.

[en] The kinetics of the bainitic transformation in a polycrystalline Fe-Cr-Mo-C alloy designed for applications in energy generation systems has been studied, with particular attention to the influence of mild tensile stresses on transformation behaviour. The steel was found to exhibit the incomplete reaction phenomenon, in which transformation to bainite stops well before the residual austenite acquires its paraequilibrium carbon concentration. It was found that even in the absence of an applied stress, the growth of bainitic ferrite caused anisotropic changes in specimen dimensions, consistent with the existence of crystallographic texture in its austenitic condition and, significantly, with the nature of the invariant-plane strain shape change that accompanies the growth of bainitic ferrite. Thus, transformation induced plasticity could be detected in fine grained polycrystalline samples, even in the absence of applied stress. The application of an external stress was found to alter radically the transformation behaviour, with clear evidence that the stress tends to favour the development of certain crystallographic variants of bainite, even though the stress may be well below the single phase yield strength. It is concluded that the transformation is influenced significantly by stresses as low as 45 MN m^-2, even though the effect may not be obvious in metallographic studies. The results are analysed and discussed in terms of the mechanism of the bainite transformation. (author)

[en] The effect of microstructure on the fracture toughness of ferrite-martensite -bainite steels was investigated with Fe-0.11C-1.64Mn-0.78Si composition. One inch compact tension specimens (1T-CTSs) were machined from hot rolled plates. The microstructure of ferrite-martensite-bainite was introduced to the specimens by the heat treatment of intercritical annealing at 800deg C and isothermal holding at 350deg C. Holding at 350deg C increased volume fraction of bainite, while decreased that of martensite, and refined martensite particles. Single specimen unloading compliance method was used in fracture test to obtain J-resistance (J-R) curve and to determine the fracture toughness(J_IC). Introduction of bainite to the ferrite-martensite steel improved the fracture toughness due to the deformation of bainite which relaxed the stress concentration on the interface of ferrite and martensite. Observation of fracto-graphs through the scanning electron microscope(SEM) identified the fracture mechanism of ferrite-martensite-bainite steels as dimple nucleation and crack growth by decohesion of ferrite matrix and second phase particles and by microvoid coales cence. (Author)

[en] The displacive growth of the partially carbon supersaturated plates of ferrite during the transformation of austenite have been examined, using a theory for nonequilibrium transformation, for Fe-0.3 C wt.% alloy. The calculations indicate that at bainite-start temperature of the plates of ferrite grow with full supersaturation of carbon even though carbon may partition during the nucleation. Both processes tend to become completely diffused less at the martensite-start temperature. The time temperature transformation (TTT) diagram has also been predicted and compared with that obtained from the other established models for the displacive transformation of austenite. (author)

[en] Strength of carbide-free bainitic steels primarily depends on the thickness of bainitic ferrite and its volume fraction in the microstructure. The morphology and volume fraction of phases formed in bainitic transformation are governed by the steel’s composition and transformation temperature. Higher carbon concentration in bainitic steels is expected to refine the bainitic microstructure even when the austempering temperature is kept to be the around 300 °C due to the enhanced strength of austenite. However, there is a concomitant loss in the volume fraction of bainite formed. Therefore, predicting the optimal composition for getting the highest strength and hardness in nanostructured carbide free bainitic steels is not possible without systematic experimental studies. We have made steels with different carbon concentrations and have demonstrated that after austempering at 300 °C, maximum hardness as well as tensile strength are attained in a wide range of carbon concentration ranging from 0.5–0.9 wt% C but drops significantly outside this range. The experimental results have been compared with the empirical strength in terms of lath thickness and volume fraction formulated from data extracted from several past studies with different carbon concentrations. (paper)

[en] Bainitic steels with intensive lath boundaries demonstrate an excellent combination of strength and toughness. The bainite lath has an inhomogeneous distribution of dislocations across its lath thickness. The present work investigates quantitatively the effect of bainite lath boundaries with inhomogeneous dislocation distribution on the evolution of dislocation density in bainitic steels by both modeling and experimental works. For the first time, the Kocks-Mecking (K-M) model is extended by including two new terms to probe the boundary induced dislocation multiplication and annihilation mechanisms in bainitic steels. The initial dislocation density and the lath thickness of bainite are obtained by synchrotron X-ray diffraction analysis and microscopy observation, respectively. Based on the modeling and experimental works, it is found that the enrichment of dislocations at the lath boundary exhibits significant influence on multiplication as well as annihilation of dislocations during tensile deformation. The dislocation density at the bainite lath boundary is estimated by the present model as twice as the value in the bath interior, which is consistent with experimental observations.

[en] To optimize 20Kh13 steel heat treatment regimes it is proposedto apply stepped quenching instead of usual quenching and tempering. It is shown that cooling by a jet of compressed air from 1070 to 400 deg C, subsequent decelerated cooling down to 200 deg C and cooling in the air during 4 hours lead to the production of bainitic structure with (Cr, Fe)₂₃C₆ type carbided distributed in it. Stepped quenching allows one to obtain the required mechanical properties, reduce the treatment time and power consumption. 2 refs

[en] The variant selection phenomenon during the austenite to bainite phase transformation in hot-rolled TRIP-aided steels was quantitatively characterized at the level of individual austenite grains. The reconstruction of the electron backscatter diffraction maps provided evidence that bainite grows by packets of laths sharing a common {1 1 1}_γ plane in the austenite. The affect of hot deformation is to reduce the number of packets that form. It is suggested that slip activity is important in understanding this effect.

[en] A new method to determine the orientation relationship between martensite and bainite with the parent austenite is developed based on electron backscatter diffraction analysis. This method can determine the orientation relationship accurately without the presence of retained austenite, and is applicable to lath martensite and bainite in low-alloyed carbon steels. The angles between close-packed directions are about 3^o for lath martensite regardless of the carbon content, while the angles between close-packed planes become smaller with increasing carbon content.

[en] Spheroidisation is well studied process in steels with pearlitic structures. Bainite as initial microstructure for spheroidisation treatment is not so frequent. The article describes rapid carbide spheroidisation of bainitic structure of 51CrV4 spring steel. The experimental steel was treated in quenching dilatometer capable of rapid temperature changes. Isothermal austenitization was studied at different temperatures with respect to its kinetics and microstructural features. Special attention was paid to the morphology of growing austenite grains and carbide morphology during their dissolution in austenite. Rapid partial austenitization was repeatedly applied to achieve accelerated carbide spheroidisation. This phenomenon is known for pearlitic structures. The article shows that bainite can be spheroidised in similar way as pearlite. Accelerated carbide spheroidisation can be applied as replacement of conventional time-consuming spheroidisation annealing. (paper)