Microstructure and Property Relationships in Resistance Spot Weld between 7114 Interstitial Free Steel and 304 Austenitic Stainless Steel

Due to the differences in physical, chemical and mechanical properties of the base metals, the resistance spot welding of dissimilar materials is generally more challenging than that of similar materials. The influence of the primary welding parameters affecting the heat input such as peak current on the morphology, microhardness, and tensile shear load bearing capacity of dissimilar welds between 304 grades austenitic and 7114 grade interstitial free steel has been investigated in this study. The optimum welding parameters producing maximum joint strength were established at a peak current of 9 kA, where the electrode force is kept 6×10^-5 Pa and weld time is kept constant 17 cycles, respectively. The primary cause of weakening the weldment is identified as the excessive grain growing region of heat affected zone （HAZ） in case of 7114 grade interstitial free steel.

The combined influence of grain size distribution and dislocation density on hardness of interstitial free steel

Understanding the relationship between microstructure features and mechanical properties is of great significance for the improvement and specific adjustment of steel properties.The relationship between mean grain size and yield strength is established by the well-known Hall-Petch equation.But due to the complexity of the grain configuration within materials,considering only the mean value is unlikely to give a complete representation of the mechanical behavior.The classical Taylor equation is often used to account for the effect of dislocation density,but not thoroughly tested in combination with grain size influence.In the present study,systematic heat treatment routes and cold rolling followed by annealing are designed for interstitial free(IF)steel to achieve ferritic microstructures that not only vary in mean grain size,but also in grain size distribution and in dislocation density,a combination that is rarely studied in the literature.Optical microscopy is applied to determine the grain size distribution.The dislocation density is determined through XRD measurements.The hardness is analyzed on its relation with the mean grain size,as well as with the grain size distribution and the dislocation density.With the help of the variable selection tool LASSO,it is shown that dislocation density,mean grain size and kurtosis of grain size distribution are the three features which most strongly affect hardness of IF steel.

Corrosion fatigue behavior of friction stir processed interstitial free steel

In this study, interstitial free （IF） steel plates were subjected to double-sided friction stir processing （FSP）. The fine-grained structure with an average grain size of about 12 μm was obtained in tbe processed zone （PZ）with a thickness of about 2.5mm. The yield strength （325 MPa） and ultimate tensile strength （451 MPa） of FSP IF steel were significantly higher than those of base material （BM） （192 and 314 MPa）, while the elongation （67.5%） almost remained unchanged compared with the BM （66.2%）. The average microhardness value of the PZ was about 130 HV, 1.3 times bigher than that of the BM. In addition, the FSP IF steel showed a more positive corrosion potential and lower corrosion current density than the BM, exhibiting lower corrosion tendency and corrosion rates in a 3.5 wt% NaCl solution. Furtbernlore, FSP IF steel exhibited higher fatigue life than the BM both in air and NaCl solution. Corrosion fatigue fracture surfaces of FSP IF steel mainly exhihited a typical transgranular fracture with fatigue striations, while the BM predominantly presented an intergranular fracture. Enhanced corrosion fatigue performance was mainly attributed to the increased resistance of nucleation and growth of fatigue cracks. The corrosion fatigue mechanism was primarily controlled by anodic dissolution under the combined effect of cyclic stress and corrosive solution.

Effects of Refractories on the Total Oxygen Content and Inclusions in the IF Steel

The effects of refractories of quartz-corundum, consisting of 45mass% quartz and 55mass% corundum, fused corundum and fused magnesia on the total oxygen content (TOC) and the composition of inclusions in interstitial free (IF) steel were stud-ied. 3 mass % microsilica, Was added into the three refractories as binder. The refractories were shaped as lining of the graphite crucible by castable method., IF steel was placed in the crucibles, then they were heated at 1600℃ for 90min and 45min respectively. Oxygen contents of the steel samples were obtained by oxygen determinator . Inclusions were analyzed by SEM and EDS. The results show that the TOC of steel increased with increase of Index of Oxygen Potential (IOP) of the refractories. The steel sample coped with MgO refractories having the lowest IOP has the lowest TOC.

Effect of surface nanocrystallization on the electrochemical hydrogen evolution behavior of IF steel

Cathodic polarization curve and electrochemical impedance spectroscopy in 30% NaOH solution were utilized to investigate the hydrogen evolution （HE） behavior of interstitial free （IF） steel surface nanocrystallized （SNC） via ultrasonic particulate peening （USPP）. The surface morphology and grain size of the steel were analyzed by scanning electronic microscope （SEM） and X-ray diffraction （XRD）. It was found that the IF steel treated by SNC and SNC ＋ 1% roiling got reductions of 200 mV and 300 mV in HE over-potentials ,respectively. Their real surface areas are enlarged by about 20 times and the hydrogen evolution reaction activation flee energies are about 50% of the original IF steels＇ s activation free energy.

Precipitation Behavior of FeTiP in Ti-added Interstitial Free High Strength Steels

The precipitation behavior of FeTiP in interstitial free high strength（IFHS）steels has been studied by using a transmission electron microscope（TEM）.The results show that the TiC particles,appearing at earlier stage,are more stable than the FeTiP ones during recrystallization annealing at the two given temperatures（810℃ and 840℃）.Therefore,the FeTiP particles can only be observed in the steels with sufficient amount of Ti.There is a critical forming time for the FeTiP,which is between 90-120 sat 810 ℃ and 60-90 sat 840 ℃.The precipitation of FeTiP involves two steps,i.e.the formation of FeTi precursors and the diffusion of P.The former step determines the reliance of Ti content for the precipitation of FeTiP,whereas the latter step leads to the difference in the critical annealing time.

Extraction of Wind Stress from HF Radar Sea-Echo Doppler Spectra

This paper describes our preliminary results by using electron backscatter diffraction(EBSD) attachment in a scanning electron microscope. These studies revealed that the basal plane of 2H martensite originates from one {220} P plane of the parent phase, four variants A,B,C,D of a self accommodating plate group originate from various {220} P planes of the parent phase. Their orientation relationships can be determined by EBSD technique. The structural information provided by EBSD technique together with the composition information provided by energy dispersive X ray spectrometer leads to much convincible identification of micro area phases. By this method three phases were identified in as cast Al Cu Fe alloys with composition near that of icosahedral quasicrystal(IQC), namely IQC, λ Al 13 Fe 4 and β phases. Two types of grains were found in an interstitial free steel recrystallized at 750℃.Among them the flat grains possess γ fibre microtexture benifitial to the cold workability.

Resistance Spot Weldability of Dissimilar Materials: BH180-AISI304L Steels and BH180-IF7123 Steels

In this study, resistance spot weldability of 180 grade bake hardening steel （BH180）, 7123 grade interstitial free steel （IF7123） and 304 grade austenitic stainless steel （AISI304L） with each other was investigated. In the joining process, electrode pressure and weld current were kept constant and six different weld time were chosen. Microstructure, microhardness, tensile-shear properties and fracture types of resistance spot welded joints were examined. In order to characterize the metallurgical structure of the welded joint, the microstructural profile was developed, and the relationship between mechanical properties and microstructure was determined. The change of weld time, nugget diameter, the HAZ （heat affected zone） width and the electrode immersion depth were also investigated. Welded joints were examined by SEM （scanning electron microscopy） images of fracture surface. As a result of the experiment, it was determined that with increasing weld time, tensile shear load bearing capacity （TLBC） increased with weld time up to 25 cycle and two types of tearing occurred. It was also determined that while the failure occurred from IF side at the BHIS0＋IF7123 joint, it occurred from the BH180 side at the BHIS0＋AISI304L joint.