Properties of cold-rolled high-strength steel sheets

For high-strength steel sheets, a new concept has become necessary,viz, the relation between strength and formability. When the relation between tensile strength and elongation is appraised for ranking in terms of the strengthening mechanism, it is found that the substitutional solid-solution hardening type is excellent and the precipitation hardening type is inferior. In batch annealing, the phosphorus-added aluminum-killed steel sheets are representative of the former type having excellent formability as indicated by a highr values despite their tensile strength of 450 N/mm². The titanium-added aluminum-killed steel sheets, which are representative of the latter type, have a tensile strength of 600 N/mm² and a relatively highr value. Continuous annealing of the highstrength steel sheets of the same chemical composition causes higher yield stresses and combinations of yield stress and elongation. Alternatively with continuous annealing the same level of strength can be achieved with smaller additions of alloying elements than with batch annealing. An additional advantage of continuous annealing is the uniformity of properties along the length of the coil. The rapid cooling possible after continuous annealing allows production of high strength steel sheets having excellent mechanical properties that are unobtainable in the batched annealed steels. For example, steel sheets of 0.4 pet Si and 1.4 pet Mn after continuous annealing, haven values and Erichsen values as higher than conventional low-carbon rimmed or capped steel sheets even while they have a tensile strengths of 550 N/mm².     

Properties of cold-rolled high-strength steel sheets

For high-strength steel sheets, a new concept has become necessary,viz, the relation between strength and formability. When the relation between tensile strength and elongation is appraised for ranking in terms of the strengthening mechanism, it is found that the substitutional solid-solution hardening type is excellent and the precipitation hardening type is inferior. In batch annealing, the phosphorus-added aluminum-killed steel sheets are representative of the former type having excellent formability as indicated by a highr values despite their tensile strength of 450 N/mm². The titanium-added aluminum-killed steel sheets, which are representative of the latter type, have a tensile strength of 600 N/mm² and a relatively highr value. Continuous annealing of the highstrength steel sheets of the same chemical composition causes higher yield stresses and combinations of yield stress and elongation. Alternatively with continuous annealing the same level of strength can be achieved with smaller additions of alloying elements than with batch annealing. An additional advantage of continuous annealing is the uniformity of properties along the length of the coil. The rapid cooling possible after continuous annealing allows production of high strength steel sheets having excellent mechanical properties that are unobtainable in the batched annealed steels. For example, steel sheets of 0.4 pet Si and 1.4 pet Mn after continuous annealing, haven values and Erichsen values as higher than conventional low-carbon rimmed or capped steel sheets even while they have a tensile strengths of 550 N/mm².      

Effect of heat input and preheating on hydrogen assisted weld joint cracking of a 0.13% C, 1.5% Mn, 0.032% Nb high strength steel of 50 mm plate thickness in the IRC test

The effects of preheating and heat input on hydrogen assisted weld joint cracking are investigated at a restraint intensity of 32 kN mm^?2 of a 0.13% C, 1.5% Mn, 0.032% Nb high strength steel of 50 mm thickness in the IRC test, using a high hydrogen experimental electrode of 530 N mm^?2 yield strength. For a heat input ranging from 0.6 to 1.05 kJ mm^?1 a critical preheating temperature of 140°C for almost complete crack prevention, for a range from 1.5 to 2.05 kJ mm^?1, 120°C were found respectively. Nominal stresses at the ends of the 70-80 mm long welds at the start of extensive cracking increase with heat input and preheat, the crack propagating from the HAZ into the weld metal quickly. Under conditions without or close to cracking, however, final stresses after 18 h are reduced with heat inputs. Consequently, crack critical combinations of preheating and heat input are linked to stresses decreasing with heat input but increasing with preheat. From the established IRC-test diagram required combinations of local preheat and heat input for either avoiding hydrogen cracking or overstressing of the weld metal can be determined. Currently used cracking prediction procedures do not consider the effect of heat input and preheat on stress sufficiently and, therefore, may provide unsafe conclusions.     

Consequences of transformation plasticity on the development of residual stresses and distortions during martensitic hardening of SAE 4140 steel cylinders

Residual stresses and distortions developing during martensitic hardening of steel can be quantitatively determined by finite element calculations, if the underlying processes are adequately modelled and the materials and process data necessary are known. In this context also transformation plasticity effects have to be taken into account. Model calculations for SAE 4140 steel cylinders demonstrate the influence of these effects on the developing residual stresses. Using a special device which allows martensitic transformation under constant external loads, for SAE 4140 the transformation plasticity constant K = 4.2 · 10^?5mm²/N is determined. With this constant and assuming realistic heat transfer conditions, the development of residual stresses and distortions of SAE 4140 cylinders with a diameter of 30 mm and a length of 90 mm is modelled. The calculated results are in good agreement with experimental findings.     

Fatigue crack propagation in carburized high alloy bearing steels

Fatigue cracks were propagated through carburized cases in M-50NiL (0.1 C,4 Mo, 4 Cr, 1.3 V, 3.5 Ni) and CBS-1000M (0.1 C, 4.5 Mo, 1 Cr, 0.5 V, 3 Ni) steels at constant stress intensity ranges, ΔK, and at a constant cyclic peak load. Residual compressive stresses of the order of 140 MPa (20 Ksi) were developed in the M-50NiL cases, and in tests carried out at constant ΔK values it was observed that the fatigue crack propagation rates,da/dN, slowed significantly. In some tests, at constant peak loads, cracks were stopped in regions with high compressive stresses. The residual stresses in the cases in CBS-1000M steel were predominantly tensile, probably because of the presence of high retained austenite contents, andda/dN was accelerated in these cases. The effects of residual stress on the fatigue crack propagation rates are interpreted in terms of a pinched clothespin model in which the residual stresses introduce an internal stress intensity, K_i where K_i, = σ_id _i ^1/2 (σ_i = internal stress, d_i = characteristic distance associated with the internal stress distribution). The effective stress intensity becomes K_e = K_a + K_i where K_a is the applied stress intensity. Values of K_i were calculated as a function of distance from the surface using experimental measurements of σ_i and a value of d_i = 11 mm (0.43 inch). The resultant values of K_e were taken to be equivalent to effective ΔK values, andda/dN was determined at each point from experimental measurements of fatigue crack propagation obtained separately for the case and core materials. A reasonably good fit was obtained with data for crack growth at a constant ΔK and at a constant cyclic peak load. The carburized case depths were approximately 4 mm, and the possible effects associated with the propagation of short cracks were considered. The major effects were observed at crack lengths of about 2 mm, but the contributions of short crack phenomena were considered to be small in these experiments, since the two steels were at high strength levels, and short cracks would be expected to be of the order of 10 μm. Also, the two other steels behaved differently and in a way which followed the residual stress patterns. Both M-50NiL and CBS-1000M have a high fracture toughness, with K_lc = 50 MPa · m^1/2 (45 Ksi · in^1/2), and the carburized cases exhibit excellent resistance to rolling contact fatigue. Thus, M-50NiL, carburized, may be useful for bearings where high tensile hoop stresses are developed, since fatigue cracks are slowed in the case by the residual compressive stresses, and fracture is resisted by the relatively tough core.     

THE FATIGUE PERFORMANCE OF SOME CONNECTING RODS MADE BY POWDER FORGING

Abstract

The fatigue properties of connecting rods made by the powder-forging process have been studied. Plain iron powder was used in their production and the carbon content adjusted to ～0·45% by addition of graphite. Heat-treatment raised the strength to ～700–850 N/mm². Fatigue performance under alternating tensile and compressive stress was investigated using a ‘push/pull’-type machine, standard drop-forged rods being tested for comparison. In addition, tests were made on parallel sided test pieces of forged iron-carbon powder and En 15 wrought bar stock.

The endurance limits of the powder-forged rods were superior to those of the drop-forged rods although the results for the latter showed considerable scatter. However, the fatigue performance of polished test pieces showed wrought steel to be slightly the better. Reasons for the differences are discussed. A low-alloy steel of higher fatigue performance is under development specifically for connecting-rod application.     

Economically alloyed high-strength steel for use in mine equipment

The development of low-temperature, high-strength weldable steels for very heavy-duty mine vehicles and pit props is considered. Steels of strength classes S70/60 (σ_y ≥ 590 N/mm²) and S80/70 (σ_y ≥ 690 N/mm²) are based on alloying with Cr, Mn, and Si, without added Ni and Mo; wear-resistant steels of strength class S100 (σ_y ≥ 950 N/mm²) are based on alloying with Cr, Mn, and B, with a small content of Ni and Mo. The bainitic–martensitic or martensitic structure with small lath size obtained after quenching and tempering ensures strength, plasticity, and low-temperature strength of the steels. Microalloying reduces the austenite grain size in recrystallizing rolling (microalloying with V) and in heating of the steel before quenching (microalloying with V and Nb) and also ensures precipitation hardening on account of the precipitation of vanadium- and niobium-carbonitride nanoparticles. The new steels may be used to replace imports of high-strength weldable steels of the same strength classes.     

Titanium-titanium boride (Ti-TiB) functionally graded materials through reaction sintering: Synthesis,microstructure, and properties

The study demonstrates an effective method to synthesize titanium-titanium boride (Ti-TiB) functionally graded material (FGM) tiles by exploiting the simultaneous TiB whisker formation in situ and the densification occurring during the reaction sintering process. The macrostructure of the graded material was designed to have a beta-titanium (β-Ti) layer on one side with the composite layers of Ti-TiB mixture having increasing volume fraction of the TiB through the thickness. The approach used an optimized tri-modal powder mixture consisting of α-Ti powder, a master alloy of the β-stabilizing-element powders (Fe-Mo), and TiB₂. The structure and properties of both of these FGMs were systematically characterized by X-ray diffraction, electron microscopy, and microhardness measurements. Interestingly, it has been found that two different kinds of TiB whisker morphologies were observed in the FGMs. The Ti-rich layers were found to have large and pristine TiB whiskers uniformly distributed in the Ti matrix. On the other hand, the TiB-rich layer was found to have a network of interconnected and relatively smaller TiB whiskers appearing as clusters. The layers of intermediate TiB volume fractions were found to consist of both the morphologies of TiB. The effectiveness of the X-ray direct comparison method for the determination of volume fractions of phases in the FGM layers was also demonstrated. The Vickers microhardness level was found to increase dramatically from 420 kgf/mm² in the β-Ti layer to 1600 kgf/mm² in the TiB-rich layer. The elastic residual stresses retained in the graded layers after fabrication were determined based on an elastic multilayer model. The nature of microstructure, the hardness variation, and the distribution of residual stresses in these novel FGMs are discussed.     

Metallography of fatigue crack initiation in an overaged high-strength aluminum alloy

Crack initiation was observed by optical microscopy using Nomarski interference contrast during fatigue cycling of an overaged 2024 aluminum alloy. The number of cracks more than five microns long at any given fraction of the fatigue life, and the distribution of cracks among various possible initiation sites, both depend on the applied stress amplitudeσa). The crack density at failure falls from approximately 300/mm² when σ^a is 90 pct of the yield strength, to less than I/mm² when σ^a is less than 60 pct of the yield strength. Cracks may begin in the matrix, in grain boundaries, or at constituent particles. At all stress amplitudes, however, the most common initiation sites areβ (Al₇Cu₂Fe) constituent particles. At low stress amplitudes in particular, fatigue cracks develop from the interface between closely-spaced fragments of β particles broken during prior processing (cluster sites). The stress-raising effect of voids which often occur at cluster sites may be responsible for their effectiveness in initiating fatigue cracks. Formerly Graduate Students in the Department of Metallurgical Engineering at Wayne State University During 1982–83 he is on leave as Associate Director of the Metallurgy Program, Division of Materials Research, National Science Foundation, Washington, DC 20550.     

A COMPARISON OF THE MECHANICAL PROPERTIES OF SOME POWDER-FORGED AND WROUGHT STEELS

Abstract

The tensile, impact, and fatigue properties of a range of powder-forged steels have been examined. A relationship has been found between the content of non-metallic inclusions and the fatigue performance. The properties of powder-forged steels at 900 N/mm² strength were compared with those of En16 wrought steel at the same level. The properties of wrought steel are demonstrably extremely variable, depending on the degree of hot work imparted during processing and on the relationship of the test-piece axis to the principal direction of working. The properties of powder-forged steel lie between the highest and lowest that can be expected in wrought steel; comparisons between the two types of material can be made only after careful consideration of their specific characteristics. Powder-forged steels were shown to be capable of developing useful properties over wide ranges of composition.     

Mechanical properties of multilayer steel-copper composites

Conclusions The tensile strength _t and fatigue limit _–1 of powder metallurgical and cast MLCs grow with decreasing mean layer thickness h starting from 5000 Å. In cast Type 45 steel/copper composites high tensile strength _t (222 kgf/mm²) combines with high impact strengtha _n (8 kgf-m/cm²) and hardness (42–45 HRC). With powder metallurgical and cast MLCs, at any given life NG the cycle stress necessary for fatigue fracture grows with decreasing mean layer thickness (within the h = 5000-500 Å range investigated).Translated from Poroshkovaya Metallurgiya, No. 11(203), pp. 33–37, November, 1979.     

Influence of vanadium and nitrogen on the structure formation and properties of K60 (X70) rolled steel coils

The production of microalloyed steel coils (thickness 8 mm) of strength category K60 (X70) with requirements on the low-temperature strength is studied in industrial trials. Microalloying of the steel with 0.10% V and 0.020% N and controlled rolling in the casting and rolling system, with a final deformation temperature of 800°C, results in the production of metal with high strength (σ_B ≥ 600 N/mm², σ_y ≥ 520 N/mm²) and plasticity (δ₅ = 28%) with satisfactory strength at –40°C according to impact-flexure and drop tests.     

Metallurgical aspects in cold rolled high strength steel sheets

Cold rolled high strength steel sheets with yield strength from 300 to 500 N/mm² have been developed by using conventional equipment for producing commercial cold rolled steel sheet, that is, cold rolling, box annealing, and temper rolling. Effective alloying elements for strengthening are carbon, silicon, manganese, phosphorus, niobium, etc. The sheets up to 400 N/mm² yield strength grade are easily produced by selecting appropriate chemical compositions. The sheets with higher yield strength grade than 450 N/mm² are obtained by introducing the new idea that the steel with more than 2 pct manganese is annealed between A₁ and A₃ transformation temperatures, and moderately temper rolled. Increase of tensile strength does not affect deep drawability while it deteriorates stretch-forming and stretch-flanging properties. As for electric resistance spot welding, shear tension strength increases in proportion to tensile strength, but cross tension strength hardly increases or tends to decrease. These sheets have been applied to door beams and bumper reinforcements.     

Directional solidification of aluminum-nickel eutectic alloys using electroslag remelting

Attempts were made to produce directionally solidified, specifically grain aligned Al-6 wt pct Ni eutectic alloy using a laboratory scale ESR unit. For this purpose sand cast alloy electrodes were electroslag remelted under different mold conditions. The grain structure of the ingots obtained from these meltings showed that insulated silica molds gave the best vertical alignment of grains along the length of the ingot. The NiAl₃ fibers within the grains tended to fan out and there was only a preferred alignment of fibers along the growth direction under the conditions of our experiments. The ESR parameters most suitable for vertical alignment of eutectic grains have been identified. In some electroslag remelting trials ingots were grown on a seed ingot. This resulted in a fewer vertical grains compared to the case when no seed ingot was used. The sand cast specimen of the eutectic exhibited a maximum tensile strength of around 88.2 MN/m² (9.0 kg/mm²) whereas conventional ESR using water cooled mold gave strength value of 98.0 MN/m² (10 kg/mm²). The directionally solidified ESR material showed longitudinal tensile strength as high as 213.7 MN/m² (21.8 kg/mm²) which could be further increased to 220.6 MN/m² (22.5 kg/mm²) by using the seed ingot. The average growth rate was varied between 5 to 25 mm/min during electroslag remelting in this study. The flow stresses, tangent modulus and ultimate tensile strength of directionally solidified eutectic increased with increasing growth rates. Formerly Research Fellow, Indian Institute of Science, Bangalore 560012 is now     

Heat treatment of tool steel with mixed martensite-bainite transformation of austenite

Heat treatment with mixed martensite-bainite transformation of austenite is optimized in terms of strength and fatigue-crack resistance. For the manufacture of components in impact mechanisms, the best combination of strength, hardness, and crack resistance is obtained for steel in which 40% martensite is formed on intermediate cooling, while isothermal heating converts the residual supercooled austenite to lower bainite. This increases the strength by 200–250 N/mm² in relation to quenching + tempering + isothermal quenching (with comparable crack resistance) and increases the resistance to fatigue-crack propagation by a factor of 1.3–2 (with analogous strength).     

Wrought aluminum-nickel alloys for high strength-high conductivity applications

Aluminum-Nickel alloys ranging from 0.06 pct to 6.1 pct (by wt) Ni have been developed for high strength-high conductivity applications. These alloys were produced by solidification in a permanent mold followed by homogenization, hot extrusion or hot rolling and cold drawing to wire form. This seguence of fabrication a) led to the production of fine fibrous dispersoids of NiAl₃ as part of the Al-NiAl₃ eutectic during the initial casting operation, b) permitted the retention of fine fibrous dispersiods of NiAl₃ produced during casting without any significant coarsening during processing and c) led to uniform dispersion and general alignment of these fibrous dispersoids along a given direction in the product without any measurable fiber-matrix separation, extensive fiber-fragmentation or crack production in the matrix. These alloys can be processed to wire form as easily as aluminum and when processed by the above sequence, possess very attractive combination of high strength-high electrical conductivity. Tensile strengths range from 173 N/mm² (at 0.6 pct Ni) to 241 N/mm² (at 6.1 pct Ni) in combination with corresponding conductivity values between 62 pct IACS and 55.5 pct IACS. The wires also possess attractive yield strength; for instance, the 0.2 pct off-set strength of Al-6.1 pet Ni wire is 213 N/mm². Using simple composite rules, the estimated strength and the conductivity of NiAl₃ fibers were found to be 1380 N/mm² and 18 pct IACS respectively, in these wires.     

Wrought aluminum-nickel alloys for high strength-high conductivity applications

Aluminum-Nickel alloys ranging from 0.06 pct to 6.1 pct (by wt) Ni have been developed for high strength-high conductivity applications. These alloys were produced by solidification in a permanent mold followed by homogenization, hot extrusion or hot rolling and cold drawing to wire form. This sequence of fabrication a) led to the production of fine fibrous dispersoids of NiAl₃ as part of the Al-NiAl₃ eutectic during the initial casting operation, b) permitted the retention of fine fibrous dispersiods of NiAl₃ produced during casting without any significant coarsening during processing and c) led to uniform dispersion and general alignment of these fibrous dispersoids along a given direction in the product without any measurable fiber-matrix separation, extensive fiber-fragmentation or crack production in the matrix. These alloys can be processed to wire form as easily as aluminum and when processed by the above sequence, possess very attractive combination of high strength-high electrical conductivity. Tensile strengths range from 173 N/mm² (at 0.6 pct Ni) to 241 N/mm² (at 6.1 pct Ni) in combination with corresponding conductivity values between 62 pct IACS and 55.5 pct IACS. The wires also possess attractive yield strength; for instance, the 0.2 pct off-set strength of Al-6.1 pct Ni wire is 213 N/mm². Using simple composite rules, the estimated strength and the conductivity of NiAl₃ fibers were found to be 1380 N/mm² and 18 pct IACS respectively, in these wires.     

Formation of a submicrocrystalline structure in austenitic 08Kh18N10T steel during equal-channel angular pressing followed by heating

The structure and mechanical properties of austenitic 08KhN10T steel subjected to equal-channel angular pressing (ECAP) at room temperature (? = 3.2) and subsequent heating are studied. In the course of ECAP, the steel undergoes a martensitic transformation; the martensite content reaches 45%. Upon heating, martensite (ferrite) transforms into austenite. The partly submicrocrystalline oriented structure of the 08Kh18N10T steel in the austenitic (55%)-martensitic (45%) state (formed upon ECAP) provides its high strain hardening (σ_0.2 = 1315 N/mm²), as compared to the initial state (σ_0.2 = 250 N/mm²), and high plasticity δ = 11%. After heating to 550°C, the steel has a predominantly submicrocrystalline austenitic (80%)-ferritic (20%) structure, σ_0.2 = 1090 N/mm², and δ = 11%.     

Evaluation of a threshold-based model of the elevated-temperature fatigue of impact-damaged γ-TiAl

Step-loading fatigue tests have been conducted on two γ-TiAl alloys with differing microstructures following quasi-static indentations intended to simulate assembly-related impact damage to low-pressure turbine blades. Fatigue tests were conducted at 600 °C using computer-controlled servohydraulic loading at a frequency of 20 Hz. Reasonably good agreement was achieved between the fatigue data and calculated fatigue strength based on the fatigue threshold and measured impact severity. In certain cases, the fatigue threshold model fails to completely describe the data. These discrepancies may be related to residual stresses, variations in crack-shape morphology, and small-crack effects. Residual stresses could not be directly measured, given the small size of the damage zones. However, a comparison of fatigue threshold approximations based on a through-thickness crack geometry and a corner-crack geometry suggests that these two models may represent the upper and lower bounds of the actual fatigue behavior. In addition, the behavior of small cracks was examined by modeling the stress-lifetime response of lightly damaged specimens of the duplex alloy. This effort indicates the need for small-crack fatigue threshold values when designing fatigue-critical γ-TiAl components.     

Effects of Processing Residual Stresses on Fatigue Crack Growth Behavior of Structural Materials: Experimental Approaches and Microstructural Mechanisms

Fatigue crack growth mechanisms of long cracks through fields with low and high residual stresses were investigated for a common structural aluminum alloy, 6061-T61. Bulk processing residual stresses were introduced in the material by quenching during heat treatment. Compact tension (CT) specimens were fatigue crack growth (FCG) tested at varying stress ratios to capture the closure and K _max effects. The changes in fatigue crack growth mechanisms at the microstructural scale are correlated to closure, stress ratio, and plasticity, which are all dependent on residual stress. A dual-parameter ΔK–K _max approach, which includes corrections for crack closure and residual stresses, is used uniquely to connect fatigue crack growth mechanisms at the microstructural scale with changes in crack growth rates at various stress ratios for low- and high-residual-stress conditions. The methods and tools proposed in this study can be used to optimize existing materials and processes as well as to develop new materials and processes for FCG limited structural applications.