ON THE TENSILE MECHANICAL PROPERTY OF Si-Mn TRIP STEELS AT HIGH STRAIN RATE

Tensile mechanical properties of 1.6Si-1.58Mn-0.195C TRIP (transformation-induced plasticity) steels under high strain rate and effects of DP (dual-phase) treatments were studied and compared to the quasi-static tensile behavior. The results show that the increasing of strain rate leads to increasing in their strengths and decreasing in the uniform elongation remarkably. Because the stable retained austenite in TRIP steel can transform to martensite during tensile testing and the material exhibits excellent characteristic of transformation induced plasticity, the plastic deformation behavior is evidently improved and the combination of strength and elongation is superior to that of dual-phase steel, although its strength is smaller than that of DP steel. However, DP treated steel shown lower elongation under dynamic tension in spite of higher strength. A model was proposed to explain the excellent elongation rate of TRIP steel compared with DP steel on the basis of SEM analysis and the strength of the components in microstructure.

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.

Strain rate and cold rolling dependence of tensile strength and ductility in high nitrogen nickel-free austenitic stainless steel

The tensile strength and ductility of a high nitrogen nickel-free austenitic stainless steel with solution and cold rolling treatment were investigated by performing tensile tests at different strain rates and at room temperature. The tensile tests demonstrated that this steel exhibits a significant strain rate and cold rolling dependence of the tensile strength and ductility.With the increase of the strain rate from 10^-4s^-1to 1 s^-1, the yield strength and ultimate tensile strength increase and the uniform elongation and total elongation decrease. The analysis of the double logarithmic stress–strain curves showed that this steel exhibits a two-stage strain hardening behavior, which can be well examined and analyzed by using the Ludwigson equation. The strain hardening exponents at low and high strain regions（n2and n1） and the transition strain（εL） decrease with increasing strain rate and the increase of cold rolling RA. Based on the analysis results of the stress–strain curves, the transmission electron microscopy characterization of the microstructure and the scanning electron microscopy observation of the deformation surfaces, the significant strain rate and cold rolling dependence of the strength and ductility of this steel were discussed and connected with the variation in the work hardening and dislocation activity with strain rate and cold rolling.

Effect of Hydrogen Charging on the Tensile and Constant Load Properties of an Austenitic Stainless Steel Weldment

The effect of cathodic hydrogen charging on the tensile and constant load properties was deter- mined for an austenitic stainless steel weldment comprising a 304L steel in the solution treated condition as a base metal and a 308L filler steel as a weld metal. Part of the 304L solution treated steel was separately given additional sensitization treatment to simulate the microstructure that would develop in the heat affected zone. Tests were performed at room temperature on notched round bar specimens. Hydrogen charging resulted in a pronounced embrittlement of the tested materials. This was manifested mainly as a considerable loss in the ductility of tensile specimens and a decrease in the time to failure and threshold stress of constant load specimens. The 308L weld metal exhibited the highest, and the 304L solution treated steel the lowest, resistance to hydrogen embrittlement. Hydrogen embrittlement was associated with the formation of strain induced martensite as well as a transition from brittle to ductile fracture morphology onwards the centre of the specimens.

Effects of Nb and V on Microstructural Evolution,Precipitation Behavior and Tensile Properties in Hot-rolled Mo-bearing Steel

Microstructural evolution,precipitation behavior,and tensile properties of four experimental Mo-bearing steels were studied to elucidate the effects of Nb and V on microstructural properties.The results indicated that the microstructure of hot-rolled steels consisted of polygonal ferrite and degenerate pearlite,and the morphology remained same after holding at 600 ℃for 1h.The smallest grain size was obtained in Nb-V-Mo containing steel,followed by Nb-Mo and V-Mo steels.Precipitation was less in the hot-rolled Mo-bearing steel.The maximum volume fraction of precipitates was obtained in Nb-V-Mo bearing steel,and the Nb-V-Mo complex carbonitrides were highly stable at 600℃.In the Mo-bearing steel,the yield ratio was lowest at room temperature but highest at 600℃,which was attributed to the precipitation of Mo carbides and drag effect of Mo solute on dislocation movement.

Biaxial tensile behavior and yield loci of dual-phase steel sheets

The biaxial tensile tests were carried out to investigate the deformation behavior and yield loci of dual-phase （DP） steel sheets under biaxial tensile conditions. The true stress-true strain curves of DP steel sheets for different loading ratios were obtained, and the experimental yield loci were determined based on the equivalent plastic work principle. The experimental yield loci were compared to the theoretical yield loci based on Hi1148, Hi1190, Hi1193, Hosford, Barlat89, Yld2000-2d and Gotoh yield criteria. It is found that Yld2000-2d yield criterion can describe the general trends of experimental yield loci of DP steel sheets with comparably higher accuracy. Hosford criterion has the maximum error for DP590, whereas Hi1148 and Barlat89 have the maximum errors for DP780 and DP980.

Tensile properties and fracture behavior of friction stir welded joints of Fe-32Mn-7Cr-1Mo-0.3N steel at cryogenic temperature

This study investigates the cryogenic tensile properties and fracture behavior of fiction stir welded and post-weld heat-treated joints of 32Mn-7Cr-1Mo-0.3N steel. Cryogenic brittle fracture, which occurred in the as-welded joint, is related to the residual particles that contain tungsten in the joint band structure. Post-weld water toughening resulted in the cryogenic intergranular brittleness of the joint, which is related to the non-equilibrium segregation of solute atoms during the post-weld water toughening. Annealing at 55OC for 30rain can effectively inhibit the cryogenic intergranular brittleness of the post- weld water-toughened joint. The yield strength, ultimate tensile strength, and uniform elongation of the annealed joint are approximately 95%, 87%, and 94% of the corresponding data of the base metal.

Effect of Heat Input on Fume Generation and Joint Properties of Gas Metal Arc Welded Austenitic Stainless Steel

The effect of heat input on fume and their compositions during gas metal arc welding （GMAW） of AISI 316 stainless steel plates are investigated. Fume generation rate （FGR） and fume percentage were determined by ANSI/AWS F1.2 methods. Particle characterization was performed with SEM-XEDS and XRF analysis to reveal the particle morphology and chemical composition of the fume particles. The SEM analysis reveals the morphology of particles having three distinct shapes namely spherical, irregular, and agglomerated. Spherical particles were the most abundant type of individual particle. All the fume particle size falls in the range of less than 100 nm. Mechanical properties （strength, hardness and toughness） and microstructural analysis of the weld deposits were evaluated. It is found that heat input of 1.15 kJ/mm is beneficial to weld stainless steel by GMAW process due to lower level of welding fume emissions and superior mechanical properties of the joints.

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.

Effects of Processing Variables on Microstructure and Yield Ratio of High Strength Constructional Steels

The process of“controlled rolling＋relaxation＋ultra fast cooling（UFC）”for high strength constructional steel with low yield ratio was presented.Microstructure and corresponding relationship with low yield ratio were investigated.The results showed that the constructional steels with multiphase microstructure of bainitic ferrite,martensite-austenite（M-A）and lath bainite were obtained through the creative process.The grain size decreased with the decrease in finish rolling temperature,which enhanced the strength by the grain refinement strengthening.The proper relaxation treatment promoted the bainitic ferrite lath width and the formation of blocky M-A constituent.In addition,both the tensile and yield strength increased with the decrease in finish rolling temperature and UFC final temperature,but the yield strength increased more significantly than tensile strength,which caused the increase in yield ratio.By using the process of“controlled rolling＋relaxation＋ultra fast cooling”,the excellent comprehensive mechanical properties of 780 MPa grade constructional steels of 12-40 mm in thickness were achieved.

Experimental Investigation on Friction Welding of UNS S32205 Duplex Stainless Steel

UNS S 32205 duplex stainless steel specimens were joined by continuous drive friction welding process. The experiments were conducted as per the Taguchi（L16 orthogonal array） method. The friction welding process parameters such as heating pressure, heating time, upsetting pressure, upsetting time, and speed of rotation were fixed with low,medium, and high levels of range based on the machine capacity, and the required knowledge was acquired from the preliminary experiments. The joint characterization studies included micro structural examination and evaluation of mechanical properties of the joints. Microhardness variation, impact toughness, and tensile strength of the joints were evaluated. Neither a crack nor an incomplete bonding zone was observed. The tensile strength of the joints was higher than the strength of the base material, and the friction and upsetting pressures were found to influence the joint strength. The tensile strength of all the welds was observed to be increasing with an increase in the rotational speed. The toughness of the friction welds was evaluated at room temperature and also at subzero（cryo） temperature conditions. The toughness for friction welds was found to be superior to the fusion welds of duplex stainless steel at room temperature and cryo conditions. Weldments exhibited better corrosion resistance than the parent material.

Prediction and Experimental Validation of Forming Limit Curve of a Quenched and Partitioned Steel

Forming limit curve（FLC）is an effective tool to evaluate the formability of sheet metals.An accurate FLC prediction for a sheet metal is beneficial to its engineering application.A quenched and partitioned steel,known as QP980,is one of the 3rd generation advanced high strength steels and is composed of martensite,ferrite and a considerable amount of retained austenite（RA）.Martensite transformation from RA induced by deformation,namely,transformation induced plasticity（TRIP）,promotes the capability of work hardening and consequently formability.Nakazima tests were carried out to obtain the experimental forming limit strains with the aid of digital image correlation techniques.Scanning electron microscopy（SEM）was employed to examine the fracture morphologies of Nakazima specimens of the QP980 steel.The observed dimple pattern indicated that tensile stress was the predominant factor which led to failure of QP980 specimens.Therefore,maximum tensile stress criterion（MTSC）was adopted as the forming limit criterion.To predict the FLC of QP980 steel,Von-Mises yield criterion and power hardening law were adopted according to the tested mechanical properties of QP980 steel.Results were compared with those derived from other three representative instability theories,e.g.Hill criterion,Storen-Rice vertex theory and Bressan-Williams model,which shows that the MTSC based FLC is in better agreement with the experimental results.