Research on microstructure and properties of boron/Q235 steel laser welded dissimilar joints under synchronous thermal field

The mechanical mismatch effect frequently occurs in the dissimilar materials welded joints, thus leading to plastic gradient at the interface between the weld and heat-affected zone(HAZ). In this work, the boron steel and Q235 steel were selected for laser tailor welding,which obtained boron/Q235 steel tailor-welded blanks(TWBs). The method of welding with synchronous thermal field(WSTF) was utilized to eliminate the mismatch effects in TWBs. The WSTF was employed to adjust cooling rates of welded joints, thereby intervening in the solidification behaviors and phase transition of the molten pool. Boron/Q235 steel was welded by laser under conventional and WSTF(300-600 ℃) conditions, respectively. The results show that the microstructure of weld and HAZ(boron) was adequately transitioned to ferrites and pearlites instead of abundant martensite by WSTF. Meanwhile, the discrepancy of microhardness and yield strength between various regions of welded joints was greatly reduced, and the overall plasticity of welded joints was enhanced by WSTF. It is indicated that WSTF can effectively contribute to reducing plastic gradient and achieving mechanical congruity in welded joints by restraining the generation of hardbrittle phase, which could significantly improve the formability of TWBs in subsequent hot stamping.

Formability of aluminum-silicon coated boron steel in hot stamping process

Flow behavior of the Al-Si coated boron steel was investigated with Gleeble-3500,in comparison with the uncoated one.Effect of deformation conditions on the coating integrity was characterized by optical microscopy.Facture surfaces of the coated steels were inspected under SEM.Experimental results indicate that the ultimate tensile strength and ductility of the Al-Si coated boron steel are lower than those of the uncoated steel under test conditions.Extensive cracks occur in the coating after tensile tests;the width and density of cracks are sensitive to the deformation temperatures and strain rates.The bare substrate exposed between the separate coating segments is oxidized.Appearance of the oxide degrades the Al-Si coating adhesion.Remarkable difference between formability of the coating layer and the substrate is confirmed.The formability of the Al-Si coating could be optimized by controlling the phase transformation of the ductile Fe-rich intermetallic compounds within it during the austenization.

MICROSTRUCTURE AND PROPERTIES OF HIGH BORON BEARING STEEL

High boron bearing steel, in which boron homogeneously distributed, wassuccessfully produced in the vacuum induction furnace. The microstructural observations of cast andhot rolled steels showed that the addition of titanium can eliminate the quantity of ferrous boridesprecipitated at the grain boundaries and break the net microstructure, as a result, its hotworkability is improved. The titanium boride TiB_2 homogeneously distributes in the matrix ofalpha-Fe. The parameters of hot rolling process, including preheated temperature, initial rollingtemperature, finished rolling temperature and the total deformation, have been optimized.

Effect of Boron Content on the Microstructure and Magnetic Properties of Non-oriented Electrical Steels

The effects of boron content in the range of 0-0.0082 wt%, on the inclusion type, microstructurc, texture and magnetic properties of non-oriented electrical steels have been studied. After final annealing, the addition of excess boron（w（B0〉0.004 1 wt%） led to the formation of Fe2B particles. As boron content increased, grain size increased and reached a maximum in steel with 0.004 1 wt% boron. Furthermore, steel containing 0.004 1 wt% boron had the strongest { 100} fiber texture, Goss texture and the weakest { 111 } fiber texture among the five tested steels. Flux density firstly rapidly increased and then suddenly decreased with increasing boron content and reached a maximum in steel with 0.004 1 wt% boron. Conversely, core loss first sharply decreased and then abruptly increased with the increase of boron content and reached a minimum in steel containing 0.004 1 wt% boron. Steel containing 0.004 1 wt% boron obtained the best magnetic properties, predominantly through the development of optimum grain size and favorable texture.

Microstructure and mechanical properties of a new type of austempered boron alloyed high silicon cast steel

In the present paper, a new type of austempered boron alloyed high silicon cast steel has been developed, and its microstructures and mechanical properties at different temperatures were investigated. The experimental results indicate that the boron alloyed high silicon cast steel comprises a dendritic matrix and interdendritic eutectic borides in as-cast condition. The dendritic matrix is made up of pearlite, ferrite, and the interdendritic eutectic boride is with a chemical formula of M2B （M represents Fe, Cr, Mn or Mo） which is much like that of carbide in high chromium white cast iron. Pure ausferrite structure that consists of bainitic ferrite and retained austenite can be obtained in the matrix by austempering treatment to the cast steel. No carbides precipitate in the ausferrite structure and the morphology of borides remains almost unchanged after austempering treatments. Secondary boride particles precipitate during the course of austenitizing. The hardness and tensile strength of the austempered cast steel decrease with the increase of the austempering temperature, from 250℃ to 400 ℃. The impact toughness is 4-11 J.cm^-2 at room temperature and the impact fracture fractogragh indicates that the fracture is caused by the brittle fracture of the borides.

IMPROVEMENT OF TYPE IV CRACKING RESISTANCE OF 9Cr HEAT RESISTING STEEL WELDMENT BY BORON ADDITION

Creep lives of high Cr ferritic heat resisting steel weldments decrease due to Type Ⅳ fracture, which occurs as a result of formation and growth of creep voids and cracks on grain boundaries in fine-grained heat affected zone (HAZ). Because boron is considered to suppress the coarsening of grain boundary precipitates and growth of creep voids, we have investigated the effect of boron addition on the creep properties of 9Cr steel weldments. Four kinds of 9Cr3WSCoVNb steels with boron content varying from 4.7×10-5 to 1.8×10-4 and with nitrogen as low as 2.0×10-5 were prepared. The steel plates were welded by gas tungsten arc welding and crept at 923K. It was found that the microstructures of HAZ were quite different from those of conventional high Cr steels such as P91 and P92, namely the fine-grained HAZ did not exist in the present steel weldments. Boron addition also has the effect to suppress coarsening of grain boundary carbides in HAZ during creep. As a result of these phenomena, the welded joints of present steels showed no Type Ⅳ fractures and much better creep lives than those of conventional steels.

Effect of Boron on Delayed Fracture Resistance of Medium-Carbon High Strength Spring Steel

The delayed fracture behavior of medium carbon high strength spring steel containing different amounts of boron （0. 000 5%, 0. 001 6 %） was studied using sustained load delayed fracture test. The results show that delayed fracture resistance of boron containing steels is higher than that of conventional steel 60Si2MnA at the same strength level and it increases with the increase of boron eontent from 0. 000 5% to 0. 001 6%. The delayed fracture mode is mainly intergranular in the boron containing steels tempered at 350℃, which indicates that the addition of boron does not change the fracture character. However, the increase of boron content enlarges the size of the crack initia tion area. Further study of phase analysis indicates that most boron is in solid solution, and only a very small quantity of boron is in the M3 （C, B） phase.

Influence of Boron Additions on Mechanical Properties of Carbon Steel

This work aims at the development of carbon steel AISI 1536 through the microalloying addition of boron. Three grades of this steel with different content of boron up to 0.0055% were melted in 100 kg induction furnace. The pro- duced steels were hardened at 960°C for 30 min., followed by tempering at different temperatures and durations. All hardened steels have martensite phase as illustrated with microstructures and X-ray diffraction. Hardness of all tem- pered steel samples was measured to calculate the activation energies of carbon migration through martensite phase. The results indicated that the activation energies of carbon migration through martensite phase decreases with the in- crease of boron content due to its positive effect on the crystallinity of martensite phase. Also, the results showed that the addition of boron up to 0.0023% can improve the steel properties at the lowest temperature and tempered time.

Redistribution of Boron during Ferrite Reaction in Nb-B Microalloyed Steel

By contrast of microstructure and boron distribution in the same area of samples, the behavior of boronduring ferrite reaction in Nb-B microalloyed steel was studied. Boron atoms have segregated to original austeniteboundaries betfore ferrite reaction starts during isothermal treatment. The concentration of boron in small Pieces offerrite foming along austenite boundaries may be remarkably higher than that in austenite matrix, but it will reducegradually when the ferrite continuously grows. There is no obvious boron segregation on interface of γ/α and α/α.These facts indicate that ferrite growth is not controlled by diffusion of boron in austenite.

Spheroidization process in large-deformed high-carbon steel

In this study,the spheroidization process of large-deformed steel under various conditions was researched.Steel with a high carbon content （1.0％ C） was first treated thermomechanically using multipass rolling.Then it underwent spheroidization treatments at different heating temperatures,using various heating times and cooling rates.Spheroidization processes with a lower heating temperature,shorter heating time,or faster cooling rate than those of the traditional process all showed good results,indicating that the spheroidization process was promoted significantly by the large deformation process.Grain refinement and fragmentation of cementite,along with the large deformation process,promoted this spheroidization process.

Effect of RE-Modifier on Microstructure and Mechanical Property of High-Carbon Medium-Manganese Steel

The effect of RE-modifier on the microstructure and mechanical properties of high carbon-medium manganese steel has been investigated in present work.The results showed that the RE-modifier can refine the crystalline grain of high-carbon medium-manganese steel.The shape and distribution of carbides are improved and the columnar grains and phosphide in grain boundary are eliminated.Consequently,the impact toughness of the steel is increased by more than one time,compared with no addition of RE-modifier.

Effects of carbon content on high-temperature mechanical and thermal fatigue properties of high-boron austenitic steels

High-temperature mechanical properties of high-boron austenitic steels （HBASs） were studied at 850 ℃ using a dynamic thermal-mechanical simulation testing machine. In addition, the thermal fatigue properties of the alloys were investigated using the self-restraint Uddeholm thermal fatigue test, during which the alloy specimens were cycled between room temperature and 800℃. Stereomicroscopy and scanning electron microscopy were used to study the surface cracks and cross-sectional microstructure of the alloy specimens after the thermal fatigue tests. The effects of carbon content on the mechanical properties at room temperature and high-temperature as well as thermal fatigue properties of the HBASs were also studied. The experimental results show that increasing carbon content induces changes in the microstructure and mechanical properties of the HBASs. The boride phase within the HBAS matrix exhibits a round and smooth morphology, and they are distributed in a discrete manner. The hardness of the alloys increases from 239 （0.19wt.% C） to 302 （0.29wt.% C） and 312 HV （0.37wt.% C）; the tensile yield strength at 850 ℃ increases from 165.1 to 190.3 and 197.1 MPa; and the compressive yield strength increases from 166.1 to 167.9 and 184.4 MPa. The results of the thermal fatigue tests （performed for 300 cycles from room temperature to 800 ℃） indicate that the degree of thermal fatigue of the HBAS with 0.29wt.% C （rating of 2-3） is superior to those of the alloys with 0.19wt.% （rating of 4-5） and 0.37wt.% （rating of 3-4） carbon. The main cause of this difference is the ready precipitation of M23（C,B）6- type borocarbides in the alloys with high carbon content during thermal fatigue testing. The precipitation and aggregation of borocarbide particles at the grain boundaries result in the deterioration of the thermal fatigue properties of the alloys.

Flow characteristics of aluminum coated boron steel in hot press forming

The flow characteristics of aluminum coated boron steel in hot press forming were investigated.Furthermore,the effects of aluminum coated layer on press forming were analyzed during deep drawing.The results show that aluminum coated boron steel exhibits a high sensitivity on temperature and strain rate.Aluminum coating layer appears in surface flaking in a temperature range of 700-800 ℃,but smooth surface is formed above 900 ℃.

Effect laws and mechanisms of different temperatures on isothermal tensile fracture morphologies of high-strength boron steel

The fracture behaviour and morphologies of high-strength boron steel were investigated at different temperatures at a constant strain rate of 0.1 s-1 based on isothermal tensile tests. Fracture mechanisms were also analyzed based on the relationship between microstructure transformation and continuous cooling transformation(CCT) curves. It is found that 1) fractures of the investigated steel at high temperatures are dimple fractures; 2) the deformation of high-strength boron steel at high temperatures accelerates diffusion transformations; thus, to obtain full martensite, a higher cooling rate is needed; and 3) the investigated steel has the best plasticity when the deformation temperature is 750 °C.

THE EFFECT OF BORON TREATMENT ON THE HYDROGEN ATTACK BEHAVIOR 0F 1.25Cr-0.5Mo STEELS

1.25Cr-0.5Mo steels with different Ti-B or Al-B treatment were exposed in 18 MPa hydrogen atmosphere at 480℃ fo f 100, 500 and 1000 h respectively. Tensile and bend properties of steels before and after hydrogen changing were tested. The results show that the different mechanical properties of the steels with different B content have different responses to high temperature high pressure hydrogen charging processes.After hydrogen changing, more loss of ductility can be found in the steel not treated with B than those in the steels treated with B. Metallographic analyses indicated that decarburization occurred in all the steels. Furthermore, many methane bubbles were found by SEM analysis in the surface decarburization layer in the steel without B,while only a few bubbles, the number and size of which decreased with increasing B content, were found in steels with B treatment. In addition, hydrogen determination results manifested that after being exposed in hydrogen atmosphere the steel containing no B had higher hydrogen content than that of steels containing B. All this suggests that B treatment improved the HA resistance of 1.25Cgr-0.5Mo steel, and the more B the steel contained the more perceptible the effect.

Titanium effect on the microstructure and properties of laminated high boron steel plates

High-boron steel is an important material used for thermal neutron shielding. The appropriate amount of added boron must be de- termined because excessive boron may deteriorate the steel＇s workability. A uniform microstructure can be formed by adding titanium to boron steel. In this study, casting and hot rolling were used to fabricate laminated high-boron steel plates whose cores contained 2.25wt% boron and 0wt%-7.9wt% titanium. The effects of titanium content and hot-rolling and heat-treatment processes on the microstructure and properties of the laminated plates were studied. The results indicated that the optimum titanium content was 5.7wt% when the boron content was 2.25wt%, and that the best overall properties were obtained after heat treatment at 1100℃ for 4 h. The tensile strength, yield strength, and elongation at the specified temperature and holding time were as high as 526.88 MPa, 219.36 MPa, and 29%, respectively.

Effect of boron on microstructure and toughness of simulated CGHAZ of HSLA steel

Effect of boron on microstructure and toughness of simulated CGHAZ of 790 MPa grade HSLA steel was studied.In the CGHAZ,boron improved the toughness of granular bainite(Bg),but deteriorated that of martensite.The major reason of boron improving toughness of Bg was to reduce the.quantity of M-A constituents.The reasons of martensitic brittleness in microstructure were discussed in detail.The experimental results proved that the order state of dislocations was an important factor of martensitic brittleness caused by boron, and the higher the order degree of dislocations was, the more brittle the martensite was.

Formation of FeMo_2B_2 phase in boron containing 9Cr-1.5Mo ferritic steels

The segregation and diffusion of boron during heat treatments were studied. The influence of boron contents, aging time and applied stress on FeMo2B2 formation was also studied. Finally, the effects of boron contents and FeMo2B2 formation on the high temperature strength were studied. Boron atoms were segregated to prior austenite grain boundary during normalizing treatment. And these boron atoms were slowly diffused into the grain interior during tempering and aging at 700 ℃. The FeMo2B2 phase was only formed after 1,000 h aging at 700 ℃ in alloy containing 196 ppm boron. The formation of FeMo2B2 phase is accelerated by the applied stress. It was expected that the formation of FeMo2B2 is closely related to the redistribution of boron atoms. The tensile strengths at 700 ℃ are increased with the increase of boron contents. However, the formation of FeMo2B2 phase results in lower tensile strength.

Boron metallurgy of advanced ferritic power plant steels and welded joints

This paper describes the alloy design philosophy for the improvement of long-term creep strength of tempered martensitic 9Cr steel,including welded joints.The creep life t_r is inversely proportional to the minimum creep rateε_(min) times the increase in creep rate by strain dlnε/dεin the acceleration region as t_r = 1.5/[(ε_(min)) (dlnε/dε)].The parametersε_(min) and dlnε/dεare closely correlated with the time to minimum creep rate t_m and the strain to minimum creep rateε_m,which characterize the creep deformation behavior in the transient region.The boundary and sub-boundary hardening is shown to be the most important strengthening mechanism in creep of 9Cr steel and is enhanced by fine dispersions of precipitates along boundaries.The addition of boron reduces the coarsening rate of M_(23)C_6 carbides along boundaries near prior austenite grain boundaries during creep.The enhancement of boundary and sub-boundary hardening increases the t_m and decreases theε_(min),which improves the creep life.The boundary and sub-boundary hardening is significantly reduced in fine-grained region of heataffected -zone(HAZ) of conventional steel P92 welded joints,promoting TypeⅣfracture.In NIMS 9Cr boron steel welded joints,the distribution of carbonitrides along boundaries are substantially the same between the HAZ and base metal,suppressing the TypeⅣfracture.

Effect of boron on inclusions morphology in tire cord steel

B_2O^3 was added into tire cord steel during refining in carbon tube furnace.The influence of boron which was added in tire cord steel on the deformability of oxide inclusions was studied by metallographic and scanning electron microscope(SEM) observation.The melting points of boron-bearing compound oxide inclusions were calculated by the software of Factsage.The results showed that the most part of inclusions were boron-bearing compound oxide and their deformation properties were obviously improved by adding B_2O_3 in steel.As the boron content was increased from 0.0046%to 0.039%,the proportion of long strip type inclusions changed a little and the number of inclusions decreased.The low-melting point areas of MnO-SiO_2-Al_2O_3 and CaO-SiO_2-Al_2O_3 ternary system were increased due to adding B_2O_3 in steel.Moreover,the areas increased with the increasing of B_2O_3 content in conclusions.Evident effect on low-melting point inclusion occurred when B_2O_3 content went up to 5% in CaO-SiO_2-Al_2O_3 inclusions system and to 10%in MnO-SiO_2-Al_2O_3 inclusions system.