Corrosion behavior of high-strength spring steel for high-speed railway

The corrosion resistance and evolution of corrosion products in medium-carbon high-strength spring steels were investigated in a neutral salt spray(5 wt% Na Cl solution). A formation model of γ-Fe OOH and a transformation model describing the conversion of γ-Fe OOH to α-Fe OOH were constructed. The results indicated that, at the initial corrosion stage, the corrosion resistance was gradually improved with the addition of Cr; however, with the addition of alloying element V, the corrosion resistance decreased. These results were attributed mainly to the initial corrosion stage being closely related to the matrix microstructure parameters such as grain-boundary character and dislocation density. After the rust layer was formed at a later corrosion stage, the corrosion resistance was reinforced with the addition of Cr and V because Cr strongly influenced the composition, structure, and morphology of the corrosion products. The results presented herein show that Cr was conducive to the transformation of γ-Fe OOH into α-Fe OOH. Moreover, V and Cr exhibited obvious synergy and were enriched in the inner layer of the corrosion products.

DEVELOPMENT OF EFFECTIVE TECHNOLOGICAL PROCESSES OF HIGH-STRENGTH STEEL WELDING

Composition and service properties of high - strength low-alloyed steels with 590-980 MPa yield strength,which find an application in Russia, Belorus,Ukraine and other countries of the former USSR in manufacture of welded structures of a powerful mining and transport machinery, are given. Electrodes and wires for main processes of arc welding of these steels have been devel- oped on the basis of a rational use of different systems of alloying (08KhN2GM,08KhNG2M, and also economical systems of type 10G25, ect. ). Main approaches to the technological provess- es of manufacture of structures of high - strength steels are formulated.They are mainly directed to the weakening of de offect of the factors which contribute to a delayed fracture of joints (diffusive hydrogen,unfavourable rates of cooling,level of residual stresses). When there are no stress concentrators (and at a low level of residual stresses) the welded joints of these steels have a good resistance to fatigue and brittle fractures.As a rule, they are prevented with the help of the known approaches.It is shown that in addition to them and due to a proper selection of conditions of welding the life of welded joints of the high - strength steels can be 1. 2 - 1. 4 times in- creased.

Drawing-upsetting-extrusion-clinching of high-strength steel and aluminum alloy

To realize good point connection between high-strength steel and aluminum alloy,a process of drawing-upsetting-extrusion-clinching was proposed.First,the sheets were drawn;then,the bottom of the protrusion part was extruded,forming the necessary initial interlock;and finally,the protrusion part was reverse press clinched,forming a certain height of the clinched head.Taking DP980 high-strength steel and A15083 aluminum alloy as the connection objects and using the method of numerical simulation combined with experimental test,the mold was made,and the experiment was carried out on the basis of numerical simulation.The experimental results proved the feasibility of the process and the effectiveness of the numerical model.The simulation and experimental results show that the necessary interlock in the drawing-upsetting-extrusion stage is the premise of effective connection.The relative protrusion height should be around 55.6%after reverse-press-clinching.The best comprehensive mechanical properties measured by strength test were shear resistance of 2644 N,fatigue life of 24,535 times and peel resistance of 1522 N.Through failure analysis,the relationship between the interlock Tu and the neck thickness Tn of the joint with the best comprehensive mechanical properties was established as Tu=0.35Tn.The design method of die and process parameters when the sheet thickness changes was researched by numerical simulation.The results show that the clearance between the punch and the die and the bottom thickness are directly proportional to the sheet thickness,and the drawing depth is directly proportional to the punch radius.

Strengthening Mechanisms for Ti-and Nb-Ti-micro-alloyed High-strength Steels

The strengthening mechanisms of hot-rolled steels micro alloyed with Ti （ST-TQS00） and Nh Ti （NT TQ500） were investigated by examining the microstructures of steels using optical microscope （OM）, scanning elec tron microscope （SEM） and transmission electron microscope （TEM）. The results revealed ahnost no differences in the solute solution strengthening and fine grained strengthcning of the two steels, whereas the contributions of pre cipitation strengthening and dislocation strengthening were different for ST-TQ500 and NT-TQ500. The measured precipitation strengthening effect of ST-TQ500 was 88 MPa higher than that of NT-TQ500： this difference was pri marily attributed to the stronger precipitation effect of thc Ti-containing nanoscale particles. The dislocation strengthening effect of ST TQ500 was approximately 80 MPa lower than that of NT-TQ500. This is tbought to be related to differences in deformation behavior during the finishing rolling stage; the inhibition of dynamic recrystallization from Nb in NT-TQ500 （Nb-Ti） may lead to higher density of dislocations in the microstructure.

Microstructural Characteristics and Mechanical Properties of Low-Alloy, Medium-Carbon Steels After Multiple Tempering

The microstructure and mechanical properties of NiCrMoV-and NiCrSi-alloyed medium-carbon steels were investigated after multiple tempering. After austenitising, the steels were hardened by oil quenching and subsequently double or triple tempered at temperatures from 250 to 500 °C. The samples were characterised using scanning electron microscopy and X-ray diffraction, while the mechanical properties were evaluated by Vickers hardness testing, V-notched Charpy impact testing and tensile testing. The results showed that the retained austenite was stable up to 400 °C and the applied multiple tempering below this temperature did not lead to a complete decomposition of retained austenite in both steels. It was also found that the microstructure, hardness and impact toughness varied mainly as a function of tempering temperature,regardless of the number of tempering stages. Moreover, the impact toughness of NiCrMoV steel was rather similar after single/triple tempering at different temperatures, while NiCrSi steel exhibited tempered martensite embrittlement after single/double tempering at 400 °C. The observed difference was mainly attributed to the effect of precipitation behaviour due to the effect of alloying additions in the studied steels.

Mechanical and fatigue properties of self-piercing riveted joints in high-strength steel and aluminium alloy

Static tensile and fatigue tests were performed on shear and tensile self-piercing riveted aluminium-steel structures to evaluate their mechanical and fatigue properties. The influences of the thickness and the strength of the high strength steel on mechanical and fatigue performances were investigated based on the tensile and F-N curves of the joints. The results show that mechanical and fatigue properties of the shear self-piercing riveted joints are much better than those of the tensile self-piercing riveted joints. Mechanical and fatigue performances of the two joints were significantly influenced by the thickness and strength of the steel sheet, and were markedly improved when the thickness of steel sheet increased. The steel strength showed significantly different effects on shear and tensile riveted structures, i. e. , when the steel strength increased, the strength of the shear structure greatly increased while the tensile structure just had a slight increase in the strength. Fatigue failure generally occurred in the sheet materials and the fatigue crack location changed with increasing the sheet thickness and the sheet strength.

Effect of adhesive on laser-arc hybrid welding of aluminum alloy to high-strength steel joint

Aluminum alloy 6061 and high-strength steel Q460 were joined by laser-arc-adhesive hybrid welding technology successfully.A Cu alloy interlayer was added between Al alloy and high-strength steel.The effect of the adhesive on laser-arc-adhesive hybrid welding of Al alloy to high-strength steel was discussed.The optical microscope,scanning electron microscope and electron probe micro-analysis were applied to observe the microstructural evolution and phase transition at Al-Fe interface of laser-arc-adhesive hybrid welded joints.The results showed the maximum tensile shear strength of the joint without adhesive was 256 MPa.After adding adhesive,the maximum tensile shear strength reached 282 MPa.The combination of the mechanical bonding and the metallurgical effect could improve the mechanical performances.The shape of the joint in Al alloy changed into a canine-like morphology.There was no porosity in welds because the molten pool of Al alloy and the special keyhole phenomenon of laser welding provided a channel for the decomposed gas to escape from fusion zone.

Interface reaction of high-strength low-alloy steel with Al-43.4Zn-1.6Si(wt.%)metallic coating

The microstructure,elemental distribution,phase composition,and thickness of intermetallic layers between high-strength low-alloy steel(H420)/mild carbon steel(DC51)and Al–43.4Zn–1.6Si(wt.%)(galvalume,GL)alloy were comparatively investigated.The experimental results reveal that the interfacial reaction layer was composed of Fe2Al5,Fe4Al13,and Al8Fe2Si intermetallic compounds.Moreover,the growth curves of the Fe2Al5 and Fe4Al13 intermetallic layers fit the parabolic law well,and the total thickness of the intermetallic layers of H420+GL was almost the same as that of DC51+GL.However,the thickness of the Fe2Al5 layer in H420+GL was thinner than that in DC51+GL.In addition,first-principle calculations were performed to explore the effect of Mn on the growth of the Fe2Al5 intermetallic phase,and the results indicate that Mn substitution in Fe2Al5 removes electronic charge from the Al atoms,thus decreasing the thickness of the Fe2Al5 interface layer.