Softening Zone Research Articles

Self‐Piercing Riveting of Hot Stamped Steel and Aluminum Alloy Sheets Base on Local Softening Zone

Self‐piercing riveting (SPR) has been used to joint steel to aluminum alloy sheets in multimaterial car body. It is a challenge to joint high strength steel to aluminum alloy by SPR process, especially for materials with high strength and large thickness. Herein, the SPR process for 1500 MPa hot stamped steel and aluminum alloy sheets is studied by experiment and simulation method. The laser heat treatment process is applied to the hot stamped steel sheet before joining. The relationship between heat treatment process and hardness distribution is studied and the softening zone with hardness less than 300 HV is obtained. Furthermore, the local softening zone is joined to an aluminum alloy sheet by SPR process. In the process, the rivets with high hardness are used to pierce the steel sheet. The failure mode and maximum tensile shear force of the joint are studied. At the same time, the joints without laser heat treatment are compared. The results show that the interlock and the maximum shear force for laser heat treatment increase by 91% and 26%, respectively.

Mathematical Model for the Electric and Temperature Fields in the Heat-Affected Zone during Electrocontact Welding

The mechanism of formation of softened zones during electrocontact welding is considered. A calculation method is proposed, and the influence of the main electrocontact welding parameters on the surface and subsurface softened zone sizes and the decrease in the hardness of the metal in these zones is investigated.

Stress and load-bearing structure analysis of the surrounding rock in a soft broken roadway

After roadway excavation, the surrounding rock is the main bearing structure for the original stress and forms a supporting ring with the supporting structure. The automatic load-bearing ring is the main interactional structure between the surrounding rock and the supporting system. The rock mass strength, stress, and deformation characteristics were considered to establish the theoretical calculation model for the softened broken surrounding rocks. In the model, the strength, stress, and deformations were analyzed for the elastic-plastic softening and broken zones. The analytical solution was obtained for the stress field, displacement field, and plastic damage range of the surrounding rocks. Considering the postpeak strength softening, shearing dilatation effects, original rock stress, and supporting resistance, the roadway bearing structure was divided into a shallow bearing layer and a deep bearing layer. Moreover, the crucial bearing layer was vital to the bearing capacity and stability of the layer. The influence of the shallow bearing layer, deep bearing layer, and crucial bearing layer on controlling the deformation of the surrounding rock was also analyzed. Based on the evolution characteristics of the shallow and deep buried surrounding rock, the weaker the residual strength of the surrounding rock was, the thicker the shallow and deep buried surrounding rock; the thicker the shallow and deep buried layers were, the greater the surface displacement of the surrounding rock; the crucial bearing layer was farther away from the roadway, and the roadway surface displacement was great; the greater the supporting resistance was, the thinner the shallow and deep bearing layers; and the crucial bearing layer was closer to the roadway, and the roadway deformation control was effective. Furthermore, based on the specific calculation example, the influence of the residual stress and supporting measures on the evolution characteristics of shallow and deep layers and roadway deformation was analyzed in detail.

Study on the microstructure and mechanical performance for integrated resistance element welded aluminum alloy/press hardened steel joints

Resistance element welding (REW) is a recently developed hybrid joining process for aluminum (Al)/steel dissimilar materials. In this study, the microstructure and mechanical performance of the Al/press hardened steel (PHS) joints generated using an integrated REW process were investigated. The microstructure of fusion zone (FZ) between rivet and PHS was lath martensite, which was transformed from the fast cooling of austenite. The Al could be divided into four zones according to the microstructure and microhardness distribution: re-solidified zone (RZ), softening zone (SZ), transition zone (TZ) and base metal (BM). The dissolution and coarsening of the precipitates are responsible for the hardness reduction of Al sheet. The variation of mechanical performance was explained in light of the increasing FZ size and the softening Al sheet as heat input rising. Moreover, four block shear models were introduced to predict the peak load of REW joints using the average hardness of SZ in Al sheet, among which the models from Architectural Institute of Japan (AIJ) provided a relatively good prediction. Considering both the average hardness of SZ and sheet thickness, an analytical model was established to predict the variation of critical nugget sizes and explain the failure mode transition.

Investigation on laser welding of a novel hot-stamped steel with 2000 MPa

This present work is the first investigation on the laser welding of the novel hot-stamped steel with 2000MPa (HSS2000). Softening was present in mixed-grain HAZ and tempered zone, due to martensite transformation, precipitated carbons and partial recovery of martensite lath. Tensile strength of welded joint was ∼1500 MPa, 80% of base metal, due to the presence of soften zone. However, the softening cannot be eliminated with decreasing laser heat input but it did not influence the tensile strength of welded joints. Thus, the laser welding of HSS2000 is acceptable in the heat input range of 67∼86 J/mm because the tensile strength can reach 1500 MPa, which is superior to other ultra-high strength steels at present. HSS2000 has a potential application in the automotive manufactures in the future.

Borehole diameter shrinkage rule considering rheological properties and its effect on gas extraction.

To study the shrinkage rule of borehole diameter and its effect on gas extraction, a visco-elastoplastic model for boreholes considering strain softening and the dilatancy characteristic was established to obtain the expressions of the coal stress, variation in diameter, and pressure relief range. The stress distribution and pressure relief effect of the boreholes in soft and hard coal seams were comparatively analyzed. The shrinkage rule of the borehole diameter was studied. The reasons for the rapid reduction in the extraction concentration of the borehole in soft coal seams were described. A technology of improving the gas extraction effect in soft coal seams was developed. The research results showed that the radius of the plastic softening zone is 0.405 m for a borehole in a soft coal seam and 0.224 m for that in a hard coal seam. This indicates that the borehole in a soft coal seam has a better pressure relief effect. The boreholes in both hard and soft coal seams will incur a shrinkage phenomenon; however, the soft coal seam has low strength and a weak ability to resist damage, and thus the surrounding coal will have a more intense creep deformation, leading to an instability failure during a short period of time and thus, a blocking of the extraction channel, thereby causing a rapid reduction in the gas extraction concentration. The borehole in a hard coal seam also shows a shrinkage phenomenon, but remains in a stable state without a blockage; thus, high-concentration gas can be extracted from this borehole for a long period of time. The geo-stress and coal strength are the two main factors controlling the amplitude of borehole shrinkage. From an increase in stress, the borehole in a hard coal seam shows a more intense creep deformation in a deep mine, which may lead to blockage. The key to improving the gas extraction effect in soft coal seams is to maintain a smooth extraction channel. The full screen pipe is installed through the drill pipe to retain an extraction channel, leading to an average gas extraction increase from 0.043 m3/min to 0.12 m3/min, an increase of 2.77 times. These research results are consistent with actual production, and can provide theoretical guidance for determining the gas extraction parameters.

Microstructure and mechanical properties of fiber laser welded QP980/press-hardened 22MnB5 steel joint

Fiber laser welding of QP980 steel and press-hardened 22MnB5 steel in butt configuration was performed in this study. The effect of heat input on microstructure evolution and mechanical properties of dissimilar joint was investigated. Laser power employed was varied from 1.6 to 2.4 kW while welding speed was kept constant at 2 m/min. The microstructure of fusion zone (FZ) was changed with the increasing laser power. Martensite and few bainite were observed when laser power ranged from 1.8 to 2.2 kW. Out of this range, the microstructure in FZ consisted of mixed structure of martensite and ferrite. Transformation from liquid phase to ferrite was promoted by Al-Si coating on the surface of press-hardened 22MnB5 steel. Heat affected zones (HAZs) of dissimilar joint were composed of martensite and bainite. Softened zone (SZ) appeared at the HAZ of 22MnB5 steel due to tempering behavior. The hardness of SZ and QP980 steel base metal declined with microstructure evolution. The fracture occurred at QP980 base metal when dissimilar joints were produced at laser power of 1.8 and 2.0 kW. When laser power reached 2.2 kW, the tensile samples failed at SZ owing to the increased softening degree. With further increasing laser power, the weakest region was FZ since ferrite declined the whole joint strength. A better Erichsen value was obtained at 2.0 kW (4.04 mm), which was balanced by more crash energy absorption and better deformation resistance.

Modified trilinear stress-strain diagram of concrete designed for calculation of beams with fiberglass rebar

An improved technique for determining the strength of reinforced beams with polymer composite reinforcement based on a nonlinear deformation model using a three-line diagram of concrete deformation under uniaxial compression with an elastic, elastic-plastic zone and a concrete softening zone is considered and proposed. The features of the strength analysis of beams with fiberglass reinforcement are revealed. The calculation method presented in this article allows us to minimize the use of empirical coefficients to determine the height of the compressed zone of concrete and at the same time does not lead to labor-intensive calculated dependencies. The method is based on the use of the Euler – Bernoulli beam theory and the use of physical stress-strain relations for concrete. This calculation method allows you to set accurately the height of the compressed zone of concrete, and accordingly the strength of the beam elements in the normal section. Good results of comparison with experimental data were obtained.

Effect of Laser Beam Offset and Filler Wire on Laser Lap-Fillet Welding of Al–Si-Coated 22MnB5 Steel

Laser lap-fillet welding was used to weld Al–Si-coated 22MnB5 steel. The effect of laser beam offset and filler wire on microstructure and mechanical properties of the joint was investigated. Weld depression was observed with the low laser beam offset. Increasing the laser beam offset or adding filler wire was beneficial to good weld appearance. The fusion zone (FZ) consisted of martensite (in Al-deficient regions) and $$ \delta $$ -ferrite (in Al-rich regions). The amount of $$ \delta $$ -ferrite with filler wire was less than that without filler wire when the other welding parameters were the same. Three different zones existed in the heat-affected zone (HAZ) with microstructures of martensite (inner HAZ), mixed martensite and ferrite (middle HAZ) as well as tempered martensite (outer HAZ). The outer HAZ with tempered martensite exhibited the minimum hardness (about 250HV0.1). After the tensile test, the specimens with low laser beam offset were fractured at the FZ of the upper sheet, while the specimens with high laser beam offset (more than − 1 mm with filler wire and 1 mm without filler wire) were fractured at the HAZ (the soften zone, outer HAZ). The maximum tensile force was 9581 N with the laser beam offset of 2 mm and with addition of filler wire.

Stability analysis of cut slope with shear band propagation along a weak layer

Retrogressive spreading failure occur frequently in sensitive clay deposits due to triggering such as gradual natural erosion and degradation or artificial cut. The large scale of failure observed in retrogressive landslides have been attributed to quasi-horizontal shear band propagation along a weak layer. This study explores the critical conditions for such failure and develops the criteria for catastrophic propagation of quasi-horizontal shear band in infinite slopes accounting for unidirectional uphill shear band propagation and removal of downslope support. The critical conditions and criteria, with respect to pre-existing softened zone length and gravity shear stress ratio, for catastrophic shear band propagation in cut slopes are influenced by the front angle of cut slope, the softened zone length and the post-peak degradation relationship. The proposed criteria are justified by a large deformation finite element analysis. A framework for stability analysis of cut slopes in sensitive clays is established based on the criteria for catastrophic shear band propagation. The 1994 Sanite-Monique landslide in Quebec Canada is re-visited using the proposed framework, with the calculated factor of safety and failure mechanism compared with an upper bound limit analysis.

Study on the Interaction between Low-Viscosity High-Permeability Pregrouting Sealing Material and Coal and Its Application

In order to ensure the intactness of pressure-measuring boreholes and the accuracy of gas pressure determination, pregrouting treatment with polymer materials is frequently applied to bedding drilling in coal mines. However, the existing polyurethane materials are of high viscosity, low permeability, and poor safety, bringing great difficulties to their field promotion and application. In view of this problem, after optimization and experiments, polylactide polyol/polyether polyol 4110/isocyanate was determined as the target system. Bio-based benzoxazine (Boz-F), red phosphorus, and melamine with a mass ratio of 2 : 1 : 2 were used as the flame retardant, which then underwent mechanical modification by hollow glass bubbles. Finally, the pregrouting material with low viscosity and high permeability was compounded, and its interaction with coal was experimentally studied. The results show that compared with traditional polyurethane, the new material increases the effective consolidation distance in the coal seam by 40% on average. Its permeation radius is also larger than the calculated radius of the plastic softening zone of a borehole. In addition, the strengths of coal-new material consolidated products with different ratios fully surpass those of coal-polyurethane material consolidated products. The enhancement of compressive strength and bending strength is up to 153% and 161%, respectively. The field application indicates that after pregrouting treatment of boreholes in the coal seam with the new material, the borehole formation rate reaches 100%. Therefore, the new material is safe and practical for gas pressure measurement through bedding drilling on site.

Effect of the t8/5 Cooling Time on the Properties of S960MC Steel in the HAZ of Welded Joints Evaluated by Thermal Physical Simulation

The heat input into the material during welding significantly affects the properties of high-strength steels in the near-weld zone. A zone of hardness decrease forms, which is called the soft zone. The width of the soft zone also depends on the cooling time t8/5. An investigation of the influence of welding parameters on the resulting properties of welded joints can be performed by thermal physical simulation. In this study, the effect of the cooling rate on the mechanical properties of the heat-affected zone of the steel S960MC with a thickness of 3 mm was investigated. Thermal physical simulation was performed on a Gleeble 3500. Three levels of cooling time were used, which were determined from the reference temperature cycle obtained by metal active gas welding (MAG). A tensile test, hardness measurement, impact test with fracture surface evaluation, and microstructural evaluation were performed to investigate the modified specimen thickness. The shortest time t8/5 = 7 s did not provide tensile and yield strength at the minimum required value. The absorbed energy after recalculation to the standard sample size of 10 × 10 mm was above the 27 J limit at −40 °C. The hardness profile also depended on the cooling rate and always had a softening zone.

Numerical Simulation of Surface Softening Behavior for Laser Heat Treated Cu-Bearing Medium Carbon Steel

The critical temperature condition for softening behavior of AISI P21 steel during laser-assisted heat treatment is suggested by three-dimensional transient simulation with a finite different method. Temperature history of the cross-sectional region during laser-assisted heat treatment at 1273 K was simulated. The critical temperature condition for formation of the softening zone was assumed to range from 900 to 1008 K, based on this peak temperature history. Formation of the softening zone was simulated based on the assumed critical temperature condition. Morphology and area of simulated softening zone was compared with the experimentally obtained results, and these were well matched. In this regard, critical temperature condition for formation of the softening zone during laser-assisted heat treatment was identified to range from 900 to 1008 K.

Thermomechanical and Metallurgical Study of Laser-Welded AISI 1018 Steel

Non-conventional welding processes such as laser welding are being widely used by several industries. The great advantages they provide such as high welding speeds, lower thermal affectation and reduced discontinuities in the fusion zone allow its application in different materials. On the other hand, mild-/low-carbon steels such as AISI 1018 are being used by the automotive industry to manufacture different components. However, the properties of these materials are influenced by heat inputs. The softening of the heat-affected zones is the most important problem. Therefore, the aim of this study was to weld 12 coupons by laser beam of AISI 1018 steel of 8 and 16 mm thickness to analyze the mechanical and microstructural properties. The formation of a softening zone in heat-affected zone was determined. The softening zone caused cracking in this area during tensile tests. The cause of cracking in this zone was determined by phase quantification. Tensile and microhardness tests were performed to determine the mechanical properties of the welding joints. Microhardness increased in fusion zone and decreased in heat-affected zone. Tensile tests showed ductile fracture in heat-affected zone of the welding joints. The experimental results were verified with simulations showing a reasonable agreement.

Microstructural evolution and mechanical properties of laser-welded joints of medium manganese steel

The cold-rolled 5% medium Mn steel was butt-welded using a fiber laser. The microstructure, distribution of microhardness, and tensile properties of the base metal (BM) and welded joint were investigated. The results showed that the fusion zone of the welded joint had the highest microhardness due to the formation of 100% martensite. A finely mixed microstructure of martensite, ferrite, and austenite was formed in the heat-affected zone, and there was no softened zone in this area. The tensile test results indicated that the ultimate tensile strength and yield strength were higher for the joint than for BM. The joint efficiency was approximately 100%. All samples of the welded joint failed at the location of BM during tensile deformation. The fracture surfaces of the BM and welded joint were mainly ductile fractures. The BM and welded joint exhibited strain rate independence of the tensile strength and yield strength at strain rates of 0.01–1 s−1, while the yield strength of the BM and welded joint increased rapidly when the strain rate reached 5 s−1 due to changes in the dislocation movement mechanisms. The uniform elongation of the BM and welded joint decreased with increasing strain rate.

Ductile damage modelling of heterogeneous materials using a two-scale computational approach

The paper presents a new two-scale methodology for modelling of damage in ductile heterogeneous materials employing homogenization approach over the two microstructural models. The first microstructural model comprises elastoplastic behaviour without softening, while only microstructural softening is homogenized in the second model. The scale transition of the state variables is based on the first-order computational homogenization scheme. The microstructural damage response is governed by the nonlocal variable embedded into the gradient elastoplastic formulation using the implicit localization approach. The averaging computation over the microstructural softening zones is driven by the damage variable and the local equivalent strain. Therein discretization is performed by using the mixed finite element formulation including the nonlocal equivalent plastic strain interpolation, while the standard displacement based finite elements, employing the simple constitutive relation with damage variable, are used at the macrolevel. The capability of modelling material damage responses at both the micro and macroscale is demonstrated by the benchmark examples.

Temperature Field Distribution of a Dissected Blast Furnace

The temperature field distribution of the upper part of a blast furnace (BF) is the result of the combination of charge distribution and gas flow. It is of great significance for both researchers and operators to study the thermal state and the phenomenon of the BF. The graphite box method is used in this study to obtain the temperature field distribution in the upper part of a 125 m3 BF. Graphite boxes with a variety of different melting point metals were loaded into the BF with the charge. And the temperature field distribution was obtained after the boxes were taken out with position and temperature information during the dissection process. The results illustrate that the graphite boxes are unevenly distributed in the BF, which was related to the distribution of BF materials. Furthermore, the temperature field distribution is asymmetric, and the isotherms present an irregular “W” shape, which is caused mainly by the simultaneous development of the edge airflow and the central airflow. Moreover, the shape of the softening and melting zone observed in the dissection process has a good correspondence with the temperature field. We concluded that the deflection of the temperature field and the softening and melting zone is related to the strong gas flow in the direction of No. 3 tuyere, the existence of accretion in the hearth, and the filling of refractory materials in tuyeres before blowing out.

Necessity of multidimensional evaluation of the high-speed ball-end milled surface of hardened AISI D2 steel from a wear resistance perspective

This paper focuses on revealing the anisotropy of surface integrity and wear resistance of high-speed ball-end milled surfaces of hardened cold work die steel AISI D2. Dry milling tests were performed on a five-axis high-speed machining center by using a two-edged coated solid carbide ball-end milling cutter. Then, wear resistance of a hard milled surface was evaluated through block-on-ring wear tests under unlubricated condition. Results showed that ball-end milled surfaces present anisotropic characteristics in terms of surface topography, subsurface microhardness, and yield strength. Moreover, the wear resistance for ball-end milled surfaces shows dependence on sliding direction because of the anisotropy of surface integrity. Benefiting from surface work hardening, wear loss for the hard milled surface was at least 62% less than that for polished ones in the initial wear stage (10 min). However, the subsurface softening zone induced by a high-speed hard milling process has poor wear resistance due to the decrease of microhardness and yield strength. This study indicates the necessity of multidimensional evaluation for high-speed ball-end milled surfaces and the possibility of improving wear behavior by selecting appropriate sliding direction.

Effect of Coal-Dust Fuel and Compensating Measures on Blast Furnace Charge Softening Zone Gas Permeability

The effect of coal-dust fuel and the main compensating measures on the change in gas pressure drop in a blast furnace charge softening zone (Δpsz ) is estimated by calculation. The change in gas permeability in the charge softening zone is evaluated using a mathematical model based on contemporary concepts of the softening zone structure. Calculations show that an increase in coal-dust fuel consumption from 100 to 160 kg/ton of pig iron is accompanied by an increase in Δpsz . In order to reduce the Δpsz with injection of this fuel, compensating measures are used in the mathematical model. An increase in iron content in the charge from 56 to 60% provides a reduction in the Δpsz by 1.1–1.5%. An increase in coke strength from 40 to 50% makes it possible to reduce Δpsz by 20–25%. Use of iron-ore materials with a lower softening temperature range makes it possible to reduce Δpsz by 25%. In addition, formation of developed axial gas flow makes it possible to reduce Δpsz by a factor of 2.6.

Study on connection and properties of green assembled building steel structures

Abstract In order to meet the research and industrialization application needs of green building integration technology and key technology of green building envelope structure in affordable housing projects, the effects of laser power and welding speed on the forming quality, microstructure and hardness of welded joints of heterogeneous steels with unequal thickness in steel structures were studied. The results showed that, for the front weld width, the influence of laser power on the back weld width is more obvious, and the influence of welding speed on the back weld width is greater than that of the front weld width. With the increase of laser power, coarse lath martensite and ferrite are formed in laser welded joints; with the increase of welding speed, heat input decreases, heat dissipation speed increases, grain growth time shortens, and grain refinement occurs. With the increase of laser power and welding speed, the tensile strength of laser welded joint has little change compared with base metal, but the elongation of welded joint is lower than that of base metal. With the increase of laser power, the fracture position of welded joint will shift from base metal to heat-affected softening zone. With the increase of welding speed, the fracture position of welded joint will shift from heat-affected softening zone to base metal. The suitable laser power for HC550/DP780 heterogeneous steel plate is 1.2 KW and the welding speed is 2400 mm/min. At this time, the welded joint has good welding forming quality, the hardness of weld zone is high and there is no softening point in heat affected zone.