The Effect of Nickel Contents on the Microstructure Evolution and Toughness of 800 MPa Grade Low Carbon Bainite Deposited Metal

In this work, three deposited metals with different nickel (Ni) contents were produced by active gas metal arc welding (GMAW) in order to explore the influence of Ni on the microstructure evolution and toughness of 800 MPa grade low carbon bainite deposited metal. The results showed that microstructure of the deposited metals mainly consisted of lath bainite, lath martensite, coalesced bainite (CB), and retained austenite (RA), and that the toughness was closely related to two factors: CB and RA. RA in deposited metal could improve the toughness, while the CB would deteriorate the toughness of deposited metal. As the Ni content increased, a large amount of CB was generated in the deposited metals. The RA content increased from 1.5% to 5.7% with the content of Ni increasing from 5.5% to 6.5%. However, the RA content did not increase when the Ni content increased from 6.5% to 7.5%. Additionally, the smallest control unit of toughness in 800 MPa grade low carbon bainite deposited metals is the Bain Packet (BP) from the perspective of crystallography characteristics. This work provided a reference for the chemical composition design of 800 MPa grade steel welding consumables and showed that the toughness of the deposited metal could be improved effectively by increasing the RA content while suppressing the formation of CB.

Microstructure, Mechanical Properties, and Corrosion Behavior of Ultra-Low Carbon Bainite Steel with Different Niobium Content

Four types of ultra-low carbon bainite (ULCB) steels were obtained using unified production methods to investigate solely the effect of niobium content on the performance of ULCB steels. Tensile testing, low-temperature impact toughness testing, corrosion weight-loss method, polarization curves, electrochemical impedance spectroscopy (EIS), and the corresponding organizational observations were realized. The results indicate that the microstructure of the four steels comprise granular bainite and quite a few martensite/austenite (M/A) elements. The niobium content affects bainite morphology and the size, quantity, and distribution of M/A elements. The elongation, yield strength, and tensile strength of the four types of ULCB steels are above 20%, 500 MPa, and 650 MPa, respectively. The impact toughness of the four types of ULCB steels at −40 °C is lower than 10 J. Steel with Nb content of 0.0692% has better comprehensive property, and maximum charge transfer resistance in 3.5 wt.% NaCl solution at the initial corrosion stage. The corrosion products on the surface of steel with higher niobium content are much smoother and denser than those steel with lower niobium content after 240 h of corrosion. The degree of corrosion decreases gradually with the increase of niobium content at the later stage of corrosion.

Effects of Final Cooling Temperature on Microstructure Transformation and Properties of Q550 Low Carbon Bainite Steel

The mechanical properties of low carbon bainite steel are closely related to the microstructure and proportion after phase transformation. The microstructure of the deformed austenite of low carbon bainite steel after isothermal transformation and continuous cooling transformation was studied by thermal simulation test. The metallographic structure was observed by optical microscopy (OM) and scanning electron microscopy (SEM). The metallographic and microhardness were used to judge the microstructure type, and the CCT (continuous cooling transformation) curve and TTT (time-temperature-transformation) curve of the test steel were drawn. It was found that at 700-430 °C isothermal, undergo a variety of medium-temperature microstructure transformations appeared for the test steels, such as ferrite, pearlite, granular bainite and lath bainite. The cooling rate and final cooling temperature have great influence on the type and performance of the final microstructure. The final cooling temperature was controlled at about 515°C. The mixed microstructures of granular bainite (GB) and fine martensite-austenite (M-A) island, a small amount of acicular ferrite and lath bainite were obtained. The yield and tensile strengths of this type of microstructure reached 639 MPa and 750 MPa respectively, the shrinkage rate reached 17%, and the better low-temperature impact performance was realized.

Study of the mechanism of structural state alteration in the strain ageing process and its influence on properties of bainitis type pipe steel

A pipeline is refered to metal structures of increased danger. To provide a high operational safety of pipe lines it is common to follow requirements to steels by level of strength, ductility, impact strength, cold resistance, fracture strength, weldability, corrosion resistance and other properties, forming of which is influenced by a complex of different factors (both technological and operational).At present structural low carbon low alloyed steels with increased level of properties, completely meeting all complex of presented requirements are used for production of sheet rolled products for pipes. However, during longtime operation of a pipeline, properties degradation demonstration is possible, that can result in decreasing of safety and originating of a structure destroying probability. A passing of strain ageing can be one of the reasons, responsible for state alteration of longtime operation pipes metal, resulted in increasing of yield strength to ultimate strength (σ0,2/σв) relation, decreasing of impact strength and cold resistance.Results of study of strain ageing influence on complex of properties and structural state of sheet rolled product of X80 strength class presented, the product being widely used for pipes production. It was shown, that in the process of the strain ageing, apart from decreasing of complex of standard properties, an increase of relaxation effect takes place. It speaks for an increased level of local strains, arising at loading and deformation as a result of fixing of dislocations by interstitial atoms. Concentration of the atoms is increasing in the process of preliminary deformation, which is confirmed by Snuk peak arising during measuring of internal friction.Study of structural state by different methods enabled to formulate peculiarities of strain deformation mechanism, typical for a steel with a structure of low carbon bainite type (bainite ferrite) with austenite insertions. Alteration of properties during strain ageing of bainite steel is linked with simultaneous arising of two processes in structure at different scale levels, having contrary influence on resistance to deformation. Increase of yield strength as a result of strengthening takes place because of fixing of dislocations by interstitial atoms. At the same time the weakening is caused by disappearing of M/A phase islands, which is strengthening the metal at the initials state, as a result of TRIP-effect during preliminary deformation and martensite disintegration during the following heating.

Microstructure and Properties of HAZ in Low-Carbon Bainite E550 Steel during Double-Pass Welding Thermal Cycle

Investigations on the microstructure and properties of the Coarse-Grained Heat-Affected Zone (CGHAZ) and intercritical reheated Coarse-Grained Heat-Affected Zone (ICCGHAZ) of a low-carbon bainite E550 steel were carried out using thermal simulation technology in this paper.Double-pass welding thermal cycle were performed on Gleeble-3800 thermal simulator, tempering heat treatment of the critical coarse crystal zone carried out in a box resistance furnace, low impact energies at -40 °C and Vickers hardness determined, and the microstructure were observed. The experimental results show that the microstructure of CGHAZ (Tp1 is 1320 °C) was dominated by coarse granular bainite and Lath bainite Ferrite, the impact toughness of CGHAZ was poor. The toughness of the CGHAZ was improved after second welding heat cycle except intercritical two-phase heating. When the peak temperature of the second thermal cycle(Tp2) was 650 °C, martensite-austenite (M-A) constituent of original CGHAZ wasdecomposed and refined, impact toughness and hardness were all higher than that of CGHAZ; When Tp2 is 750 °C, there was a ” necklace” distribution of massive M-A constituent in this ICCGHAZ, the impact energy at -40 °C prominently decreased and Hardness went up; When Tp2 was in the temperature range of 850 °C ~1100 °C, the microstructure was mainly finer granular bainite, the toughness of CGHAZ could be effectively improved; When Tp2 was over 1100 °C, M-A constituents become coarse, the toughness declined slightly . The changing of hardness was the opposite of toughness but the hardness fluctuation was comparatively small. After tempering at different temperature (520 °C~640 °C) , the grain boundary "necklace" structure of ICCGHAZ was still obvious, some of the M-A constituent were decomposed, the hardness decreased, the lowest hardness was obtained in 610 °C.