High-temperature Thermomechanical Treatment Research Articles

Effect of second phase on the fatigue crack growth in AlSl 4340 steel

Increasing applications of AISI 4340 steel necessitate more precise characterization of its mechanical properties so that designers can better ensure failsafe behaviour in service. The fatigue fracture process of a component includes crack initiation and crack propagation. C r a c k initiation behaviour can be evaluated by studying low cycle fatigue and high cycle fatigue. The crack propagation life depends on fatigue crack growth rate and fracture toughness. Many researchers have conducted fracture toughness measurement [1-3], dynamic fracture propagation [4-6] and bending and torsion fatigue tests [7] in 4340 steel. It has been shown that the fracture toughness can be improved by using higher austenitizing temperature (1100-1200 °C) or through high temperature thermomechanical treatment [3]. However, little work has been done on the fatigue crack growth rate of 4340 steel [8]. The second phase plays an important role in determining the mechanical properties. The second phase hard carbide particles in a tool steel will develop stresses at the boundary of the matrix and the inclusion during external cycling loading [9] and reduce the low cycle fatigue properties. Zaczyk et al. [10] indicate that the absorbed energy during stable crack growth depends upon the amount the morphology of second phase particles in ductile steels. The resistance to shear instability and fracture toughness also relies on second phase particle dispersions [11]. Tomita and Okabayashi have proposed two heat treatments that can produce a dutile second phase in the martensitic matrix of 4340 steel [12-15]. The first process is isothermal heat treatment (IHT), which produces a lower bainite second phase, and the second is interrupt quench treatment (IQT), which produces a highly tempered martensite second phase. The existence of the ductile second phase can significantly improve the mechanical behaviour such as tensile properties, impact energy [12-14] and plane strain fracture toughness [15]. However, Tomita and Okabayashi did not address the fatigue properties. In the study reported here we investigated the effect of types and volume fraction of the second phase on the fatigue crack growth rate in AISI 4340 steel. The results are compared to those of conventional quenched and tempered 4340 steel. A commercial AISI 4340 steel was used in this study. The chemical composition was (wt%: C 0.39, Si 0.24, Mn 0.61, P 0.02, S 0.01, Ni 1.46, Cr 0.67, Mo 0.17). The martensitic transformation temperature Ms determined by a dilatometer was 330 °C. Based on the Ms temperature, three different heat treatments were carried out to produce ductile second phase in the matrix: isothermal heat treatment, interrupt quench treatment and direct quench treatment (DQT). Table I lists these heat treatment processes and specimen identification. The microstructures of specimens etched with a 3% nital solution were observed by optical microscopy. Fatigue crack growth tests in accordance with the ASTM E647 standard [16] were performed in a computerized MTS servo-controlled testing system. Compact tension specimens of thickness 5.4 mm and width 43.2 mm were used. A sinusoidal load with frequency 40 Hz and load ratio R = 0.1 was applied throughout the test. A crack opening displacement gauge was attached close to the notch of the specimen, and the growi~ag crack length was measured by the compliance method. Fatigue cycles and cyclic stress intensity factor at every moment of stressing could be recorded automatically. Two duplicate fatigue tests were performed for each specimen. The broken surfaces were studied by scanning electron microscopy (SEM) to understand

Martensitic Transformations and Shape Memory Effect in Thermomechanically Treated Ti-Ni Alloys

The possibilities to regulate the martensitic transformation temperature range, mechanical and special properties of Ti-Ni-based shape memory alloys by high-temperature and low-temperature thermomechanical treatments were studied.

Low-alloy powdered steels obtained by metal oxide technology

We have studied and described the mechanism and kinetics of thermodiffusional alloying of iron powder by Cr, Ni, Mo oxides and carbon in halogen-containing media. Oxides of Cr, Ni, and Mo are easily reduced to the metal in the presence of metallic iron, ammonium chloride, and combination media (H2+C, H2+CO). We have studied the chemical compositions and physicotechnological properties of the powders produced. The homogeneity of the alloy formed meets the GOST 4543-71 standard (“alloyed structural steel”). We have studied the processes of compaction of powdered low-alloy steels by means of extrusion. A combination of extrusion and high-temperature thermomechanical treatment (HTMT) followed by low temperature treatment makes it possible to produce alloyed powdered steels with better characteristics: σ=2085 (P40Kh), 2420 MPa (P40KhN2MA); impact strength KC was respectively 465 and 655 kJ/m2. High strength and ductility are achieved by HTMT due to formation of a fragmentary substructure, i.e., the martensite inherits the dislocation structure of the deformed austenite. Our results were used to develop a technique for production of low-alloy powdered steels by extrusion combined with conventional thermal and high-temperature thermomechanical treatment.

Surface-alloyed powder components with high mechanical properties

Metallization of powder steels is optimized by mathematical planning and processing the results of an experiment. The possibility of combining hot compaction (stamping, extrusion) of articles with high-temperature thermomechanical treatment followed by low-temperature tempering is demonstrated. Surface-alloyed powder steels and production of parts from them are developed.

High temperature thermo-mechanical treatment of 12−0−2+C type high-speed steel

This investigation of the influence of high temperature thermo-mechanical treatment (HTTMT) involved 20 ∼ 60% deformation in the temperature range of 1000 ∼ 1150°C or conventional heat treatment (CHT), and examination of the phase transformations, the structure and the properties of W-Mo-V high-speed steel. The HTTMT, comprising austenitizing at 1190°C, drop forging at 1000 ∼ 1150°C and tempering at 540°C, may be employed for making particular tools from 12−0−2+C high-speed steel. The method can be recommended for the manufacture of parts of gear-cutting hobs. It is possible also to use HTTMT for parting-off tools and other high-speed steel tools of simple geometrical shapes. The method, with deformation by hot rolling, is suitable for the manufacture of twist drills, which are usually hot rolled. After HTTMT, as in the case of CHT, the structure of 12−0−2+C steel consists of fine lath martensite twinned in some regions, precipitations of cementite, retained austenite and undissolved M 4C 3 and M 6C carbides. In the martensite of the steel after the HTTMT, apart from the cementite there are fine M 4C 3 carbides, increasing the hardness with respect to CHT. The HTTMT of this steel at 1150°C caused refinement of the austenite grain by dynamic or metadynamic recrystallization of austenite, finally resulting in refinement of the martensite laths. After HTTMT carried out at 1000°C, the austenite grains were elongated and their size was similar to that after CHT. The martensite laths had an irregular bent shape caused by dynamic recovery and polygonization of the austenite. Tempering of the steel both after HTTMT and after CHT caused autonomous precipitation of fine M 4C 3 carbides during holding at the tempering temperature with a consequent secondary hardening effect, which was also influenced by martensitic transformation of retained austenite during cooling from the tempering temperature. After CHT a secondary hardness of 66.5 HRC was produced on tempering at 570°C. HTTMT caused the temperature of the maximum secondary hardness of 63 ∼ 65 HRC to shift to 540°C. Other properties after HTTMT were similar to those produced by CHT.

Effect of high-temperature thermomechanical treatment on hydrogen embrittlement of a high-strength aluminum alloy

Effect of high-temperature thermomechanical treatment on hydrogen embrittlement of a high-strength aluminum alloy

Low- Temperature improvement of mechanical properties of aisi 4340 steel through high-temperature thermomechanical treatment

The low-temperature mechanical properties of AISI 4340 ultrahigh-strength steel can be dramatically improved by high-temperature thermomechanical treatment (HTMT). A comparison was made with the mechanical properties developed by the conventional heat-treatment (CHT). When the steel was hot forged by 50 pct reduction at 1177 K followed by direct oil quenching and subsequent tempering at 423 K, the slow-bending fracture energy of fatigue-precracked steels was significantly improved, with notably increased strength, owing to a moderate increase in ductility over the temperature range of 123 to 293 K, and the ductile-to-brittle transition temperature (DBTT) in the Charpy impact test was remarkably lowered. The marked development was achieved in the slow-bending fracture energy, with moderate improvement in strength, owing to significantly increased ductility over the temperature range of 123 to 293 K, when the steel was deformed by 50 pct reduction at 1473 K followed by direct oil quenching and subsequent tempering at 453 K. The treatment also improved the shelf energy and DBTT in the Charpy impact test. The improved mechanical properties are attributed to the subcell structure introduced in austenite by processing HTMT and inherited by martensite. The present work shows that an HTMT steel is attractive for low-temperature ultrahigh-strength applications when suitable combinations of deformation temperatures with tempering conditions are applied to the steels.

Effect of microstructure on mechanical properties in 0.5 carbon-steel processed by high temperature thermomechanical treatment

Effect of microstructure on mechanical properties in 0.5 carbon-steel processed by high temperature thermomechanical treatment

Effect of microstructure on mechanical properties in 0.5 carbon-steel processed by high temperature thermomechanical treatment

The effect of microstructure on the slow bending stress and fracture energy in 0.5C-steel processed by high temperature thermomechanical treatment (processed by forging) (HTMT) was studied to understand which microstructural factors contribute to the strength and toughness of a HTMT steel. Significant improvement was achieved in the slow bending fracture energy, with moderate increase in the slow bending stress when the steel was deformed by 50% at 1473K followed by direct water quenching and subsequent tempering at 453 K. When the steel was deformed by 50% reduction at 1173 K followed by direct water quenching and subsequent tempering at 423 K, the slow bending stress significantly increased though the increase in the fracture energy was not as great as that of the 1473 K forged steel. However, an abrupt reduction occurred in the fracture energy above suitable tempering temperatures, so above these temperatures, there was little difference between the properties of the HTMT and conventional heat-treated steels. Microstructural factors contributing to the mechanical properties are discussed in terms of thin-foil transmission electron microscopy, non-isothermal dilatometry, and X-ray measurements.

Change in the properties of high-damping copper-aluminum-zinc ?-alloys after HTMT

The effect of fractional high-temperature thermomechanical treatment, i.e., HTMT (overall degree of deformation 20–70%, duration of the post-deformation period 0–10 sec), on the structure and physicomechanical properties of β-alloys of the copper-aluminum-zinc system is studied. The promising nature is demonstrated of using HTMT with the aim of improving mechanical properties with retention of high damping capacity for β-alloys based on copper-aluminum with reverse martensitic transformation. Dependences of physicomechanical properties on treatment schedule are plotted and explained.

Friction welding of steels subjected to high temperature thermomechanical treatment

Friction welding of steels subjected to high temperature thermomechanical treatment

Effect of cleanness of steel on the susceptibility to delayed failure after high-temperature thermomechanical treatment

Effect of cleanness of steel on the susceptibility to delayed failure after high-temperature thermomechanical treatment

Structural changes during hot deformation of austenite in alloy steels

In austenite of alloy steels, intensive strain hardening takes place during hot deformation to strains of 0.07–0.1. These processes lead to the formation of a hot-worked substructure which is characterized by a high density of dislocations, distributed uniformly or forming a cell-like substructure. With an increase in the degree of deformation (up to ε max , corresponding to the setting up of a dynamic equilibrium between the processes of strengthening and softening), the processes of dynamic polygonization develop, although net strengthening still prevails. Oriented polygonal subboundaries are formed, and with an increase in the strain, the volume fraction of polygonized regions increases, as does that of the equiaxed subgrains. The subgrain boundaries become more perfect, and the density of free dislocations decreases. During steady-state flow, a stable equiaxed polygonized substructure is formed, whose parameters do not change on further increases in strain. The structural changes occurring during hot deformation by compression or by rolling under comparable conditions are similar. The structures formed during rolling at ϵ ̇ g3′ > 1 s −1 correspond to the unsteady stage of hot deformation. The relatio established between the flow curve and the structural changes taking place during the hot deformation of austenite in alloy steels makes it possible to choose optimum conditions for the high temperature thermomechanical treatment of industrial alloys which soften by dynamic polygonization.

Improving the finishing practice for rolled products subjected to high-temperature thermomechanical treatment

Improving the finishing practice for rolled products subjected to high-temperature thermomechanical treatment

Etude enthalpimetrique des differentes etapes du revenu dans les aciers silico-manganeux

Compared to dilatometry an analysis of enthalpies is ideally suited for investigating the tempering of 61SC7 steel. In conjunction with crystallography, classification on an enthalpy scale and a stability scale of the quenched or partially tempered states may be formed: (α″ + η) is always metastable with respect to (α′' + χ) which has the lowest enthalpy level. Magnetic properties and ordering lead to positive excess C p values for (α″ + η) and (α″ + χ) with regard to (α + θ). The enthalpy differences represent at the same time the chemical enthalpy of carbide transformation and the accompany lattice relaxation. The transition α′ → α″ + η accounts for 90% of the total tempering enthalpy. Through short periods of tempering at high temperatures the sequence of carbide precipitation is the same as in anisothermal tempering and the relaxation is more advanced for the same carbide structural states. A high-temperature thermo-mechanical treatment introduces additional energy because of strain hardening of austenite, accelerates the matrix relaxation on tempering and stabilises the η phase.

Study of factors governing steel toughness after high-temperature thermomechanical treatment

1. Formation during HTTT of recrystallized austenite grains may lead to a sharp reduction in fracture toughness KIc with unchanged values of strength and ductility. A reduction in fracture toughness may be overcome by steel purification. 2. Temperature-time schedules for deformation with HTTT should provide austenite grain refinement, a specific austenite dislocation structure, and a reduction in the level of internal residual microstresses. However, toughness properties of steel after HTTT depend to a considerable degree on the concentration and distribution of harmful impurities forming segregations along austenite grain boundaries. Grain-boundary embrittlement as a result of enrichment in harmful impurities may reduce the structural strength of steel after HTTT.

Effect of high temperature thermomechanical treatment on the mechanical properties of vanadium – modified AISI 4330 steel

The relationships between the recovery and recrystallization of hot-deformed austenite grains and the precipitation of carbide on tempering in a 0.3C-Ni-Cr-Mo-V steel have been studied to examine closely the favourable effects of high-temperature thermomechanical treatment (HTMT) on the mechanical properties in steel. The results show that the strength and toughness increase with increasing degree of deformation by HTMT, particularly in which the ductile-brittle transition temperature as a measure of toughness decreases steeply with increasing degree of deformation. From the analysis of line broadening on (110)M and hardness change with holding time after hot deformation, the dynamic recovery and recrystallization during HTMT are believed not to occur. Owing to this, the random distribution of fine carbide precipitates in martensite structure is hardly affected. Consequently, the improvement of mechanical properties by HTMT is due to the distribution of fine carbide precipitates within martensite lath and the refinement of the martensite structure.

Effect of high temperature thermomechanical treatment on the mechanical properties of vanadium ? modified AISI 4330 steel

Effect of high temperature thermomechanical treatment on the mechanical properties of vanadium ? modified AISI 4330 steel

Hot plasticity of structural steel in relation to deformation conditions

The existence of general rules governing changes in the hot plasticity, the similar values of resistance to deformation at corresponding temperatures and deformation rates of steel St3 austenitized at 1150°C and not austenitized, and the similarity in the formation of structure at tdef>850°C leads to the assumption that not deformation of similar steels is possible without preliminary high-temperature heating (1150–1200°C) during treatment by such methods as high-temperature thermomechanical treatment or controlled rolling.

High-temperature thermomechanical surface treatment of long cold-rolling mill rolls

High-temperature thermomechanical surface treatment of long cold-rolling mill rolls