Study on the resistance spot welding technology of 22MnMoB hot stamping quenched steel

In this paper, the spot welding technology of a new kind of 22MnMoB hot stamping quenched steel sheet was systematically studied by power frequency spot welder. Through a series of technology and test experiments, we have obtained the optimal spot welding technological parameter condition. According to the results, the relations among spot welding technological parameter, welding nugget, mechanical property and fracture mode were discussed. The effects of all the welding parameters such as welding current, welding time and electrode force on the quality of joint can be boiled down to one thing--the diameter of welding nugget. The experimental results showed that welding nugget diameter determines the mechanical property of spot welding joint and the relation between welding nugget diameter and the mechanical property of joint presents a kind of linear mathematic representation. There are two typical fracture models of 22MnMoB hot stamping quenched steel sheet, i.e., interracial fracture and nugget pullout. Other than mild steel or normal high strength steel, in the shearing tensile test, hot stamping quenched steel has a great tendency to fail in interfacial mode due to the effects of high strength matrix structure, welding soft zone and the porosity level of fusion zone.

Rotating arc horizontal narrow gap welding of high strength quenched and tempered steel

Rotating arc borizontal narrow gap welding of quenched ＆ tempered （Q＆T） steel was innovatively performed for solving the bottleneck that the molten pool sagged due to the gravity. The shapely multilayer single pass horizontal joint could be obtairzed by using the rotating are welding process. The cold crack was not observed in the joint without controlling the heat input and selecting the consumables intentionally. Microstructure of the joint could be divided into three zones： base metal zone （BMZ） , heat-affected zone （ HAZ） and weld zone （WZ）. Because of the characteristic of the rotating arc horizontal welding process, the defects in the joints were slag inclztsion formed at the interlayer of lower side wall. Tbe tensile strength and hardness of HAZ and WZ were larger than those of BMZ. The impact toughness in WZ, HAZ and BM at 0 % is equal to 11.5, 212 and 236 J, respectively.

Calculation of the Hardness Space Distribution in the As Quenched Condition of a Medium Hardening Tool Steel

The calculation of the hardness profile is a powerful tool for the selection of the right steel for a given purpose. Computer programs INC-PHATRAN and INDUCTER-B were formerly developed by the authors for the calculation of hardness profiles after heat treatment processes of low alloy and carbon steels. The first one simulates quenching as well as through hardening operations, and the second one models electromagnetic induction heat treatments processes. These codes make use of the SAE Standard 3406 in order to obtain the hardness profile, with enhanced regression coefficients recently obtained by the authors. The present work broadens the field of application of this method, allowing to apply it for low hardenability tool steels such as the ASTM O1 Tool Steel. The method used for the calculation of the hardness profile is here summarized, and an example of application is described, which shows good correspondence between the calculated and measured values.

Effect of induction heating and tempering on properties and microstructures of quenched and tempered pipes

In this study,a C-Mn quenched steel tube was quickly tempered by induction heating,and the influence of the tempering temperature on its performance was studied and compared with that by traditional tempering. The results show that the yield strength of both is quite strong with regular changes in the tempering temperature,but that the tensile strength of the tube tempered by induction heating is higher than that tempered by traditional tempering by about 25 MPa,and the elongation after induction tempering is significantly higher than that after traditional tempering. The differences in the microstructures of tubes after induction and traditional tempering were compared by metallographic microscope,scanning electron microscopy,and transmission electron microscopy.Theoretical analysis was also performed. Compared with traditional tempering,a fine dispersion of precipitated carbides occurs after induction tempering,which is the main reason for the performance differences.

Cold Cracking of Flux Cored Arc Welded Armour Grade High Strength Steel Weldments

In this investigation, an attempt has been made to study the influence of welding consumables on the factors that influence cold cracking of armour grade quenched and tempered （Q＆.T） steel welds. Flux cored arc welding （FCAW） process were used making welds using austenitic stainless steel （ASS） and low hydrogen ferritic steel （LHF） consumables. The diffusible hydrogen levels in the weld metal of the ASS and LHF consumables were determined by mercury method. Residual stresses were evaluated using X-ray stress analyzer and implant test was carried out to study the cold cracking of the welds. Results indicate that ASS welds offer a greater resistance to cold cracking of armour grade Q＆T steel welds.

Effect of Direct Quenching and Partitioning Treatment on Mechanical Properties of a Hot Rolled Strip Steel

Three different online heat treatment processes were designed to study the effects on the mechanical properties of a 0.19C-1.6Si-1.6Mn（wt%） hot rolled strip steel.The microstructures were characterized by means of SEM,TEM,EPMA,and XRD.The mechanical properties were estimated by tensile tests.Results showed that a satisfying combination of strength and ductility could be obtained through the ferrite relaxation and direct quenching and partitioning process.Analysis was also focused on this process.The microstructure contained proeutectoid ferrite grains,martensite packets and blocky or interlath retained austenite,and also contained carbide-free bainite in the case of relatively high quench temperatures.The retained austenite fraction was increased through proeutectoid ferrite and partial bainite transformation,while the tensile strength was also consequently decreased.The most of retained austenite transformed to ferrite under deformation and the elongation was obviously improved.

Microstructural evolution and mechanical properties of a low-carbon quenching and partitioning steel after partial and full austenitization

In this work, low-carbon steel specimens were subjected to the quenching and partitioning process after being partially or fully austenitized to investigate their microstructural evolution and mechanical properties. According to the results of scanning electron microscopy and transmission electron microscopy observations, X-ray diffraction analysis, and tensile tests, upper bainite or tempered martensite appears successively in the microstructure with increasing austenitization temperature or increasing partitioning time. In the partially austenitized specimens, the retained austenite grains are carbon-enriched twice during the heat treatment, which can significantly stabilize the phases at room temperature. Furthermore, after partial austenitization, the specimen exhibits excellent elongation, with a maximum elongation of 37.1%. By contrast, after full austenitization, the specimens exhibit good ultimate tensile strength and high yield strength. In the case of a specimen with a yield strength of 969 MPa, the maximum value of the ultimate tensile strength reaches 1222 MPa. During the partitioning process, carbon partitioning and carbon homogenization within austenite affect interface migration. In addition, the volume fraction and grain size of retained austenite observed in the final microstructure will also be affected.

Experimental Investigation on the Performance of Armour Grade Q&T Steel Joints Fabricated by Flux Cored Arc Welding with Low Hydrogen Ferritic Consumables

Quenched and Tempered （Q＆T） steels are widely used in the construction of military vehicles due to its high strength to weight ratio and high hardness. These steels are prone to hydrogen induced cracking （HIC） and softening in the heat affected zone （HAZ） after welding. The use of austenitic stainless steel （ASS） consumables to weld the above steel was the only available remedy to avoid HIC because of higher solubility for hydrogen in austenitic phase. Recent studies revealed that low hydrogen ferritic （LHF） steel consumables can also be used to weld Q＆T steels, which can give very low hydrogen levels in the weld deposits and required resistance against cold cracking. Hence, in this investigation an attempt has been made to study the performance of armour grade Q＆T steel joints fabricated by flux cored arc welding with LHF steel consumables. Two different consumables namely （i） austenitic stainless steel and （ii） low hydrogen ferritic steel have been used to fabricate the joints by flux cored arc welding （FCAW） process. The joints fabricated by LHF consumable exhibited superior transverse tensile properties due to the presence of ferrite microstructure in weld metal. The joints fabricated by ASS consumable showed higher impact toughness due to the presence of austenitic phase in weld metal microstructure. The HAZ softening in coarse grain heat affected zone （CGHAZ） is less in the joints fabricated using LHF consumable due to the lower heat input involved during fabrication compared to the joints fabricated using ASS consumables.

Microstructural analysis of the softened zone in the welding joint of 100 kg class hot-rolled extra-high-strength steel

Using the Gleeble 3500 thermal-mechanical system to simulate thermal cycles with different peak temperatures, the hardness and microstructure in the heat-affected zones of two kinds of 100 kg class hot-rolled extra-high-strength steel were compared. When the peak temperature of the thermal cycle was 800℃ ,incomplete transformation occurred during quenching in both steels, and massive martensite and bainite grains were formed. The hardness was determined by the composition and distribution of the microstructure. The concentration of massive martensite was low, and hence the hardness was low,in steel #1. Conversely,the massive martensite content in steel #2 was high and uniformly distributed,resulting in a high hardness. These findings can provide a reference for improving the mechanical properties in the softened zone.

Competitive role of film-like austenite and transition carbides on hydrogen embrittlement resistance and impact toughness in bainite-containing quenched and partitioned steel

A microstructure composed of martensite matrix,lower bainite,and stable film-like austenite was designed by a quenching and isothermal bainitic holding process in a 0.30C–2.69Mn–1.71Si(wt.%)steel.The yield strength,tensile strength,and ductile-to-brittle transition temperature(DBTT)of the high-strength steel thus obtained were 1263 MPa,1521 MPa,and-33℃,respectively,and at-20℃,it showed superior low-temperature toughness,which reached 77.5 J/cm^(2).Meanwhile,it showed excellent hydrogen embrittlement(HE)resistance,and the total elongation loss is only 3.1%after 15 min of hydrogen charging.The excellent comprehensive performance is attributed to the fact that fine stable austenite with film-like morphology hindered the crack nucleation and propagation,and hindered hydrogen diffusion as a hydrogen trap.However,with a decrease in the isothermal temperature,transition carbide precipitation was accompanied by a further decrease in austenite grain size.For this condition,although transition carbides can act as effective hydrogen traps,excessive precipitation decreased the carbon content of retained austenite and increased the deformation heterogeneity between austenite and martensite matrix,leading to weakened austenite stability and HE resistance,a total elongation loss of approximately 39%(15 min hydrogen charging),a sharp decrease in impact toughness,and an increase in DBTT.The competitive role of film-like austenite and transition carbides on the comprehensive mechanical performance of steel is revealed,especially the suppression of crack nucleation and propagation that will provide a guide for the design of high strength steels with excellent impact toughness and HE resistance.

Dynamic Fracture Toughness of Armour Grade Quenched and Tempered Steel Joints Fabricated Using Low Hydrogen Ferritic Fillers

The armour grade quenched and tempered steel joints fabricated using low hydrogen ferritic steel （LHF） filler exhibited superior joint efficiency owing to preferential ferrite microstructure in the welds and also they offered required resistance to HIC. However, the combat vehicles used in military operations will be required to operate under a wide range of road conditions ranging from first class to cross country. Structural components in combat vehicles are subjected to dynamic loading with high strain rates during operation. Stress loadings within the vehicle hull of these vehicles are expected to fluctuate considerably and structural cracking especially in welds during the service life of these vehicles can lead to catastrophic failures. Under these conditions, fracture behaviour of high strain rate sensitive structural steels can be better understood by dynamic fracture toughness （K1d）. Hence, an attempt was made to study dynamic fracture toughness of the armour grade quenched and tempered steel and their welds fabricated using LHF consumables. The experimental results indicate that the K1d values of the joints fabricated by shielded metal arc welding （SMAW） are higher than those of the joints fabricated using flux cored arc welding （FCAW） process.

Solidification and heat transfer of molten steel slag particles during air quenching process

In order to effectively utilize the resources and energy of molten steel slag,the variation of precipitation phase and specific heat of air quenched steel slag(AQSS)particles during continuous cooling process was investigated by FactSage and thermogravimetry differential scanning calorimetry.The cooling and solidification process of molten AQSS particles was simulated by Fluent.The microstructure changes in AQSS particles in solidification process were analyzed using an ultrahigh temperature laser confocal microscope and a scanning electron microscope.The results indicated that in the cooling process of molten AQSS particles,the precipitation of Ca_(2)Fe_(2)O_(5) resulted in the largest change of specific heat.Under the condition of slow cooling,the cooling rate is more obviously affected by specific heat.When the initial air velocity was 300 m s^(-1),there was the highest temperature difference in AQSS particles during cooling process.What is more,the compactness of the boundary region of AQSS particles was obviously better than that of its central region.

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.

Influence of Initial Microstructure on Warm Deformation Processability and Microstructure of an Ultrahigh Carbon Steel

Various isothermal compression tests are carried out on an ultrahigh carbon steel （1.2% C in mass percent）, initially quenched or spheroidized, using a Gleeble-3500 system. The true stress is observed to decrease with increas ing temperature and decreasing strain rate. The true stress of the initially quenched steel is lower than that of the ini- tially spheroidized steel at high deformation temperature （700 ~C） and low deformation strain rate （0. 001 s-1 ）. The value of the deformation activation energy （Q） of the initially quenched steel （331.56 kJ/mol） is higher than that of the initially spheroidized steel （297.94 kJ/mol）. The initially quenched steel has lower efficiency of power dissipation and better processability than the initially spheroidized steel. The warm compression promotes the fragmentation and the spheroidization of lamellar cementites in the initially quenched steel. The fragmentation of lamellar cementites is the spheroidizing mechanism of the eementites in the initially quenched steel. Results of transmission electron microscope investigation showed that fine grains with high angle boundaries are obtained by deformation of the initially quenched steel.

Effect of Heat Treatment on Microstructure and Mechanical Properties of Quenching and Partitioning Steel

In order to investigate the effect of microstructural characterization on the mechanical properties and retained austenite stability, a different type of quenching and partitioning steel（I-Q＆P） through intercritical annealing before the quenching and partitioning process was designed, which can realize lamellar intercritical microstructure compared to the conventional quenching and partitioning（Q＆P） process. The morphology of ferrite and martensite/retained austenite is lamellar in the I-Q＆P steel while it is equiaxed after being heat-treated by conventional Q＆P process. The I-Q＆P steel is proved to have better formability and mechanical properties than conventional Q＆P steel, which is due to the highervolume fraction of retained austenite in the I-Q＆P steel and confirmed by electron backscattering diffraction patterns and X-ray diffraction. Furthermore, the stability of retained austenite in I-Q＆P steel is also higher than that in conventional Q＆P steel, which is investigated by tensile tests and differential scanning calorimetry.

Development of New Type Mn-Series Bainitic Steels in China

The alloying design idea,strengthening-toughening mechanism,microstructure,mechanical performances,development and application in China of new type Mn-series bainitic steels are introduced.Mn-series air-cooling bainitic steels including granular bainitic steels,FGBA /BG duplex steels,CFB/M duplex steels,medium carbon bainite/martensite steels,cast bainitic steels are presented.The invented idea mechanical performances,development and application of second generation of Mn-series bainitic steels,i.e.water quenching Mn-series bainitic steels invented by the authors newly are introduced.The water quenching Mn-series bainitic steels cover severe series steels containing ultra-low carbon,low-low carbon,medium-low carbon,and high-low carbon content etc,which can reduce the amount of alloying content,increase hardening capability and improve weldability.It should be pointed out that the application of both air cooling and water quenching Mn-series bainitic steels are complementary and mutually reinforcing,and the new type Mn-series bainitic steels can meet the performance requirements of most steels used in engineering structure.Some newest technologies of Mn-series bainitic steels in China are discussed in this paper.It is suggested that the significance of the development of the Mn-series bainitic steels can be summarized as:significantly reducing costs of both raw materials and production;good combination of strength and toughness;excellent weldability;simple procedure;large savings in energy resources and reduced environmental pollution.

Correlation of isothermal bainite transformation and austenite stability in quenching and partitioning steels

The possible decomposition of metastable austenite during the partitioning process in the high end quenching and partitioning （Q＆P） steels is somewhat neglected by most researchers. The effects of primary martensite and alloying elements including manganese, cobalt and aluminum on the isothermal decomposition of austenite during typical Q＆P process were studied by dilatometry. The transformation kinetics was studied systematically and resulting microstruc tures were discussed in details. The results suggested that the primary martensite decreased the incubation period of isothermal decomposition by accelerating the nucleation process owing to dislocations especially on phase and grain boundaries. This effect can be eliminated by a flash heating which recovered dislocations. Co addition significantly promoted the bainite transformation during partitioning while A1 and Mn suppressed the isothermal bainite transformation. The bainite transformation played an important role in carbon distribution during partitioning, and hence the amount and stability of austenite upon final quenching. The bainite transformation during partitioning is an important factor in optimizing the microstructure in Q＆P steels.

Tensile behavior and deformation mechanism of quenching and partitioning treated steels at different deforming temperatures

The effects of deforming temperatures on the tensile behaviors of quenching and partitioning treated steels were investigated. It was found that the ultimate tensile strength of the steel decreased with the increasing temperature from 25 to 100 ℃, reached the maximum value at 300 ℃, and then declined by a significant extent when the temperature further reached 400 ℃. The total elongations at 100, 200 and 300 ℃are at about the same level. The steel achieved optimal mechanical properties at 300 ℃due to the proper transformation behavior of retained austenite since the stability of retained austenite is largely dependent on the deforming temperature. When tested at 100 and 200 ℃, the retained aus tenite was reluctant to transform, while at the other temperatures, about 10 vol. % of retained aus- tenite transformed during the tensile tests. The relationship between the stability of retained austenite and the work hardening behavior of quenching and partitioning treated steels at different deforming temperatures was also studied and discussed in detail. In order to obtain excellent mechanical properties, the stability of retained austenite should be carefully controlled so that the effect of transforma tion-induced plasticity could take place continuously during plastic deformation.

Microstructure and Tensile Properties of a Nb–Mo Microalloyed 6.5Mn Alloy Processed by Intercritical Annealing and Quenching and Partitioning

The transformation behavior, microstructural evolution and mechanical properties were compared in a coldrolled Nb–Mo microalloyed 6.5Mn alloy after intercritical annealing（IA） and quenching and partitioning（Q ＆ P）,respectively. The thermodynamic calculation and theoretical analysis were used to determine the optimal heat treatment parameters. The Q ＆ P samples exhibited relatively higher strength with relatively low ductility, mainly due to the hard martensite matrix, which resulted in continuous yielding behavior upon loading, whereas the IA samples showed the significantly improved ductility, which benefited from the more sufficient transformation-induced plasticity（TRIP） effects and the softer ultrafine ferrite matrix. The dependence of yield point elongation（YPE） of IA samples on grain size demonstrated that the YPE value was in the reverse proportional relationship to the average grain size, which agreed well with theoretical analysis.