Bainitic Type Research Articles

Mechanical properties and microstructure of ULCB steels affected by thermomechanical rolling, quenching and tempering

A new family of high-strength low- and medium-alloyed bainitic steels with ultra-low carbon content and copper or micro-alloy additions has been developed. An increased amount of deformation below the austenite recrystalization temperature during finish rolling of ULCB–Mn and ULCB–Ni steels has contributed to the increase in the tensile properties of YS>690MPa and to the impact toughness being greater than 50J at 190K. This was attributed to the formation of dislocated lath martensite and acicular bainitic type ferrite from a homogenous austenite upon cooling in the air or water quenching. For steels having more carbon taken into solution, the tempering parameters after quenching are very important in the value of the fracture toughness stress intensity factor KC and the impact strength resistance to fracture KV. The local microsegregations of Ni, Mn and Cu at and above the AS temperature were superimposed on carbon segregation into the new austenite formed in the inter-critical range (α+γ). This newly formed austenite contributes to the retention of retained austenite islands and untransformed inter-laths when the steel is quenched or cooled down after rolling or ageing to room temperature. A marked improvement in toughness is associated with the formation of the new austenite, which gives the TRIP steel effect during deformation. For steel aged at temperatures of greater than 665°C, a second strengthening peak has been observed that is associated strongly with hardenability and transformation strengthening as well as particle strengthening caused by ε-Cu precipitation. Comparison of the mechanical properties and toughness of US and Polish grades of bainitic steels that have been produced recently has been made. Future applications of these steels might be in shipbuilding, in the constructions of offshore platforms, pressure vessels, and other high performance structural members, having as they do values of YS>650MPa and Charpy V energy 200J at 213K.

Effect of AustemperingTime on Mechanical Properties of a Low Manganese Austempered Ductile Iron

An investigation was carried out to examine the influence of austempering time on the resultant microstructure and the room-temperature mechanical properties of an unalloyed and low manganese ductile cast iron with initially ferritic as-cast structure. The effect of austempering time on the plane strain fracture toughness of this material was also studied. Compact tension and round cylindrical tensile specimens were prepared from unalloyed ductile cast iron with low manganese content and with a ferritic as-cast (solidified) structure. These specimens were then austempered in the upper (371 °C) and lower (260 °C) bainitic temperature ranges for different time periods, ranging from 30 min. to 4 h. Microstructural features such as type of bainite and the volume fraction of ferrite and austenite and its carbon content were evaluated by X-ray diffraction to examine the influence of microstructure on the mechanical properties and fracture toughness of this material.

The effects of microstructure, strength level, and crack propagation mode on stress corrosion cracking behavior of 4135 steel

The stress corrosion cracking (SCC) susceptibility of 4135 steel in a simulated sea water solution has been analyzed in an attempt to understand the effect that microstructural changes associated with the corresponding changes in strength level have on both intergranular (IG) and transgranular (TG) crack propagation modes. After a selection of heat treatments, the following different microstructural variables were studied: the effect of grain size on IG fracture processes; the influence of the grade of tempering on the SCC resistance and crack propagation mode; and the effect of type and content of bainite and the effect of ferrite in mixed microstructures. A global analysis shows that the typical SCC resistance-strength level inverse relationship can only be applied when the microstructure re-mains invariable. An important microstructural control of SCC behavior was found for TG processes at moderate and low strength levels. The data analysis showed the following: a beneficial effect of increasing the grain size when crack propagates at grain boundaries without precipitates; the existence of a critical tempering temperature so that a sudden IG-TG change happens without any apparent relation to microstructural changes; the beneficial effect of bainite presence as a substitute for mar-tensite and high SCC resistance of structures containing over 50 pct ferrite, associated with their low strength levels.

Effects of composition and structure on the state, fatigue, and damping properties of high-strength cast iron

Measurements have been made on the ultimate strength, yield stress, endurance limit with and without stress concentrators, and oscillation decrement for cast iron containing spheroidal graphite and alloyed (in mass %) with 0.30–1.08 Mn, 0–0.81 Cu, 0.45–0.62 Ni, and 0–0.32 Mo and having structures of ferrite, pearlite, bainite, and bainite-ferrite types (with 25–30% ferrite) and also martensite structure. Concentrations of those elements have been determined that provide the best combination of static and dynamic characteristics. The effects of the alloying elements on the properties of the cast iron have been related to the structure of the metal substrate. In particular, in pearlitic cast iron alloyed with 0.7–0.8% Cu, 0.4–0.5% Ni, 0.3% Mn, the addition of Mo is ineffective, while in cast iron having the bainite structure, that element raises the yield stress and the fatigue strength even in the absence of Cu, while the sensitivity to stress concentration is reduced.

Controlled hardening of large objects in water-air cooling installations

Hardening large massive shafts with complex configuration for welded rotors of turbines in nuclear and thermal power stations is a critical operation in the technological cycle of their heat treatment that creates the required complex of service properties. The devised regime of quenching has to ensure that the required strength, ductile properties and resistance to brittle failure over the entire section of the semiproduct are obtained together with a safe level of the arising quenching stresses. When in the process of hardening of rotor blanks traditional methods of cooling are applied with the use of water and oil tanks, considerable temperature gradient results, and the phase transformations along the object proceed nonuniformly. This leads to a high level of quenching stresses and to crack nucleation, a cause of the expensive products for important purposes having to be scrapped. A promising trend is the use of water--air mixtures ensuring the possibility of controlled cooling of massive objects [1]. The object of the present work is to devise a technology of controlled quenching of large objects on the example of shafts of welded rotors of high-speed turbines with a view to the design features of the cooling installation put into operation at the Kramatorsk plant "Energomashspetsstal ~' [2]. The installation is intended for the controlled quenching of rotor blanks weighing up to 20 tons, up to 1600 mm diameter and up to 4200 mm long, the coolants being water spray, water--air mixture, and fan-blown air. In regard to the technological possibilities the installation is unique among Soviet equipment for the heat treatment of large parts. A diagram of the shaft of the welded rotor is shown in Fig. 1. To choose the conditions of cooling the shafts, we investigated the effect of the rate of cooling from the quenching temperature on the phase transformations of supercooled austenite, on the mechanical properties and the resistance to brittle failure of steel 25Kh2NMFA that is used for making elements of welded rotors, and we also calculated the temperature fields in the process of hardening the shafts. The results of the investigations of the kinetics of the decomposition of supercooled austenite on a dilatometer "Formastor" show that in a wide range of cooling rates (8000-250"C/h) the decomposition of austenite is accompanied by the formation of structures of bainitic type, and only at cooling rates below 250°C/h do we find products of decomposition of austenite in the region of pearlitic transformation. We investigated the effect of the cooling rate on the mechanical properties on a laboratory installation designed at the Central Research Institute of Engineering which made it possible to heat and cool blanks 18 x 18 x 180 mm in size according to a specified program. The range of reproduced heating rates of blanks in the installation is 101000PC/h, and of cooling rates 10--3600"C/h. A detailed description of the design of the installation is presented in [3]. Heat treatment of blanks on the modeling installation included heating to 880°C, holding for 5 h, cooling at the specified rate, and tempering at 640°C for 25 h. Cooling from the quenching temperature was carried out at the rates 900, 700, 500, 300, 200, and 100*C/h. The chosen rates included the possible cooling rates of the central zones of shafts in hardening under industrial conditions. From the heat-treated blanks we made specimens for mechanical tests and for determining the critical brittle point (Tso).

Lower bainite with midrib in hypereutectoid steels

The isothermal transformations in five hypereutectoid steels (0.85 to 1.80 wt pct C) have been studied in the temperature range between 623 and 333 K. Two types of lower bainite and a thin plate isothermal martensite were observed. One of the lower bainites was the conventional lower bainite (CLB) formed at the high temperature range of 623 to 473 K, and the other was the newly named “lower bainite with midrib” (LBm) formed at the lower temperature range of 473 to 423 K. The thin plate isothermal martensite (TIM) was also observed below 373 K. This paper brought LBm into focus. Arrhenius plots (transformation ratevs l/T) for each steel revealed an abrupt change in kinetics at the temperature range between 483 and 443 K. This change was considered to correspond to the transition from CLB to LBm. The following two-stage process for the LBm formation is proposed: at the first stage a TIM is formed, which constitutes a midrib of LBm, and secondly the bainitic decomposition of austenite at TIM/austenite interfaces takes place. That is, an LBm is a composite of isothermal martensite and lower bainite.

Transmission electron microscopy characterizations of preheated and non-preheated shielded metal arc weldments of HY-80 steel

The effects of a preheating treatment on the microstructural changes that occur in the heat-affected zone (HAZ) during multipass shielded metal arc welding of HY-80 steel have been studied. Two weldments (pre-heated and precooled), employing identical welding conditions, were studied in order to compare the microstructural transformation effects in the HAZ. The microstructure in the HAZ of both welds has been characterized by optical, scanning and transmission electron microscopy in conjunction with microhardness traverse across the HAZ of the last weld pass. A wide variety of microconstituents are present in the base metal, HAZ and the weld metal. These microconstituents include tempered bainite-martensite duplex structures, tempered martensite, tempered bainite, three distinct types of bainite distinguished by the morphology of cementite precipitation within the ferrite laths, auto-tempered martensite, highly dislocated newly transformed fine-lath martensite, small amounts of retained austenite at the lath interfaces and a trace of transformation twinned martensite. The volume fraction of the transformation twinned martensite (the microstructure most susceptible to hydrogen-assisted cracking) was very small in both welds compared with other microconstituents present in the HAZ. However, the presence of twinned martensite in the preheated weldment was smaller than in the non-preheated weldment; this may reduce the susceptibility of the preheated material to hydrogen-assisted cracking. Microhardness traverses revealed very different hardness gradients in the two weldments. The microstructural data are used to explain these hardness gradient differences.

Mechanical properties of 0.40 pct C-Ni-Cr-Mo high strength steel having a mixed structure of martensite and bainite

A study has been systematically made of the effect of bainite on the mechanical properties of a commercial Japanese 0.40 pct C-Ni-Cr-Mo high strength steel (AISI 4340 type) having a mixed structure of martensite and bainite. Isothermal transformation of lower bainite at 593 K, which appeared in acicular form and partitioned prior austenite grains, in association with tempered marprovided provided a better combination of strength and fracture ductility, improving true notch tensile strength (TNTS) and fracture appearance transition temperature (FATT) in Charpy impact tests. This occurred regardless of the volume fraction of lower bainite present and/or the tempering conditions employed to create a difference in strength between the two phases. Upper bainite which was isothermally transformed at 673 K appeared as masses that filled prior austenite grains and had a very detrimental effect on the strength and fracture ductility of the steel. Significant damage occurred to TNTS and FATT, irrespective of the volume fraction of upper bainite present and/or the tempering conditions employed when the upper bainite was associated with tempered martensite. However, when the above two types of bainite appeared in the same size, shape, and distribution within tempered martensite approximately equalized to the strength of the bainite, a similar trend or a marked similarity was observed between the tensile properties of the mixed structures and the volume fraction of bainite. From the above results, it is assumed that the mechanical properties of high strength steels having a mixed structure of martensite and bainite are affected more strongly by the size, shape, and distribution of bainite within martensite than by the difference in strength between martensite and bainite or by the type of mixed bainite present. The remarkable effects of the size, shape, and distribution of bainite within martensite on the mechanical properties of the steel are briefly discussed in terms of the modified law of mixtures, metallographic examinations, and the analyses of stress-strain diagrams.

TEM characterization of HY-80 steel microstructures after small plastic strains

A systematic TEM characterization of HY-80 steel microstructures resulting from various thermomechanical treatments has been completed. The as-received material—50 mm thick plate—was water quenched and tempered. These quenched and tempered materials were given 0.03, 0.063 and 0.075 plastic strain by tensile loading in the long transverse direction. TEM thin foil examination revealed various microconstituents in the as-quenched as well as in the quenched and tempered materials. The microconstituents were typically highly dislocated lath martensite (lath width ⋍ 0.2 μm) with carbide in it, autotempered martensite, typical tempered bainite/martensite duplex structure, and three distinct types of bainite distinguished by the morphology of cementite precipitation within the ferrite laths. It was also found that the shape and size of the precipitated carbide/cementite particles were different in the various microconstituents. Small plastic strains produced tangling of dislocations around the precipitated cementite and tempered carbide/cementite particles within the ferrite laths and/or at the interface boundaries. The microconstituents in the as-received as well as in the prestrained materials are characterized.

Strengthening mechanisms in bainitic steels

An investigation has been made of the microstructures and mechanical properties of a series of 1%Cr-0.5 %Mo-0.002%B base steels following isothermal transformation to structures of the granular, upper and lower bainite types. The results indicate that in granular bainites the strength is raised above the base level of the matrix by the “islands” or “granules” which characterise the microstructure whereas in classical (i.e. ferrite-carbide) bainites the dislocation density of the ferrite matrix and the shape and spacing of the carbide precipitates exert important strengthening effects.

Particle size—amplitude relations for the ultrasonic atomizer: Lobdell, D.D. The journal of the acoustical society of America, vol 43, no 2 (February 1968) pp 229–231

The effects of a preheating treatment on the microstructural changes that occur in the heat-affected zone (HAZ) during multipass shielded metal arc welding of HY-80 steel have been studied. Two weldments (pre-heated and precooled), employing identical welding conditions, were studied in order to compare the microstructural transformation effects in the HAZ. The microstructure in the HAZ of both welds has been characterized by optical, scanning and transmission electron microscopy in conjunction with microhardness traverse across the HAZ of the last weld pass. A wide variety of microconstituents are present in the base metal, HAZ and the weld metal. These microconstituents include tempered bainite-martensite duplex structures, tempered martensite, tempered bainite, three distinct types of bainite distinguished by the morphology of cementite precipitation within the ferrite laths, auto-tempered martensite, highly dislocated newly transformed fine-lath martensite, small amounts of retained austenite at the lath interfaces and a trace of transformation twinned martensite. The volume fraction of the transformation twinned martensite (the microstructure most susceptible to hydrogen-assisted cracking) was very small in both welds compared with other microconstituents present in the HAZ. However, the presence of twinned martensite in the preheated weldment was smaller than in the non-preheated weldment; this may reduce the susceptibility of the preheated material to hydrogen-assisted cracking. Microhardness traverses revealed very different hardness gradients in the two weldments. The microstructural data are used to explain these hardness gradient differences.

The mechanism and transformation kinetics of the β → α phase change in the uranium/1 at % platinum alloy

Electrical resistivity investigations of the β → α transformation in a uranium/1 at % platinum alloy suggests three mechanisms of transformation in the temperature range 0° C to 660° C. One at higher temperatures is consistent with a thermally activated diffusion controlled reaction, an intermediate mechanism is probably of a bainitic type, whilst the low temperature reaction is characterised by a shear mechanism. All mechanisms show time-dependent characteristics. Dilatometric measurements are unable to separate the two high temperature mechanisms.

Structure, phase composition, and mechanical properties of 12Kh2MFSR steel

1. 12Kh2MFSR steel is a steel of the bainitic type. In this steel the equilibrium carbide phases are the vanadium carbide VC and the chromium carbide M7C3. 2. A change in the normalization temperature from 980 to 1080°C has little influence on the hardness, the tensile strength, and the impact strength. An increase in the normalization temperature to 1130°C decreases the ductility of the steel and increases the scattering of the values of impact strength. 3. Tempering at temperatures up to 650°C inclusively is insufficient for the formation of equilibrium carbide phases even when the tempering time is 25 h. Tempering at 700–780°C ensures the formation of equilibrium carbide phases; the strength is determined by the size and shape of the ferrite grains and also by the type of distribution of the carbides.