Effect Of Transformation Plasticity Research Articles

Transformation Plasticity in Carbonitrided PM-Steels: Quantification of Plasticity Effects in Dependence of the Part Density*

Abstract To optimize heat treatment processes of case hardened components, heat treatment simulations are used to predict surface layer conditions. Only a precise knowledge and modelling of the transformation processes allows a trustworthy prediction of the hardness and residual stresses in the surface zone. The transformation plasticity mechanism plays an essential role in the heat treatment process and its correct simulation has a significant influence on the resulting calculated residual stress profiles and component distortion. Without considering transformation plasticity, simulative residual stresses are significantly overestimated [1]. In this work, powder metallurgical components are pressed and sintered and subsequently carbonitrided for a dilatometric investigation to characterize the correlation between transformation plasticity effect and the density. The results show a dependence of the austenite-martensite volume change that led to a significant difference of 0.5 Vol-%. A model describing the martensite volume change with respect to density is proposed. This also affects the description of the transformation plasticity constants (K) between K = 5 – 6 × 10–5 MPa–1 in dependence of density. With currently available data, the effect of chemical composition and density cannot be separated and quantified and further studies are therefore necessary to allow such a refinement.

Effect of Transformation Plasticity on Gear Distortion and Residual Stresses in Carburizing Quenching Simulation

This paper addresses the effect of gear steel on distortion and residual stresses due to phase transformation in carburizing and quenching. In particular, the martensitic and bainitic phase transformation expansion and transformation plasticity properties of two automotive gearbox steels (20CrMnTiH and 20MnCr5) and their physical parameters are measured by experiments of transformation plasticity properties. Numerical simulations of the actual carburizing and quenching process of the gearbox spline helical gears were carried out in combination with the thermal and mechanical properties with temperature variations calculated by the material design software JMAT-Pro. In particular, the phase transformation properties of the two materials and their influence on the distortion and residual stresses after carburizing and quenching were verified by experiments of transformation plasticity and numerical simulations. A reliable basis is provided for predicting the distortion mechanism of gear steels in carburizing and quenching.

Optimizing Mandrel Dimensions for a Fixture Hardening Process of High-Strength Steel Aerospace Parts by Finite Element Simulation

A fixture hardening process for aerospace gear box components of alloy S156 (16NiCrMo16-5, 1.6722) is investigated by thermomechanical metallurgical 2D axisymmetric Finite Element simulations. Material parameters are experimentally determined. Contact and transformation plasticity effects among others are implemented in Abaqus® with user subroutines. The study aims to optimize the mandrel diameter of a fixture hardening tool for processing of planet gears. Plasticity, transformation plasticity, arising contact forces, and pressure depending on different mandrel sizes are investigated in depth. Distortion is evaluated and an optimal setting is derived from the calculations. Results show that cylindricity, defined here as the difference between maximum and minimum radius of the part, and maximum contact pressure, both can be reduced by increasing the mandrel radius. Physical effects and distortion evolve strongly nonlinear. Analysis methods highlighting cylindricity depending on the different mandrel diameters are developed to illustrate this nonlinear behavior and to enhance knowledge about the process.

Calculation of the transformation plasticity strain in the shape memory cylinder

A connected thermomechanical boundary problem for an infinite shape memory alloy cylinder loaded by an axial force and subjected to heating or cooling from the surface is solved. The evolution of stress-strain state is calculated for a process of the transformation plasticity. The mechanical properties of a material point are given by a micromechanical model accounting for the deformations due to elasticity, thermal expansion and phase transformation. The obtained problem was solved numerically using the iterative procedure with a variable iterative parameter. The influence of the surface temperature rate and radius of cylinder on the transformation plasticity is investigated. It is shown, that the elongation due to the transformation plasticity effect decreases with the increasing temperature rate. The elongation also decreases with the increasing cylinder diameter. This phenomenon can be explained by an inhomogeneity of the temperature and stress fields causing different conditions for the phase transformation in different points of the body. Stress in the local region can overtop more than twice the mean value.

Concentration-Dependent Transformation Plasticity Effect During Hydrogenation of Technically Pure Titanium Irradiated with an Electron Beam

Using the methods of X-ray diffraction analysis, the concentration-dependent transformation plasticity effect +TPE) is investigated during hydrogenation of the technically pure titanium (VT1-0) irradiated on one side with a low-energy high-current electron beam in a SOLO facility using three 50 μs pulses at the pulse repetition frequency 0.3 s–1 and the beam energy density W = 18 J/cm2. Its irradiation is performed in an argon atmosphere at the residual pressure 0.02 Pa. It is found out that the concentration-dependent TPE in the course of hydrogenation is due to the low stability state of the α-Ti crystal lattice formed in the specimen surface layer upon its e-beam irradiation. Relaxation of the elastic plane macrostresses occurs as a result of the development of concentration inhomogeneity of hydrogen or formation of α″-Ti and ω-Ti martensitic phases.

Effect of Pre-strain and High Stresses on the Bainitic Transformation of Manganese-boron Steel 22MnB5

During the last decade, the use of press-hardened components in the automotive industry has grown considerably. The so-called tailored tempering, also known as partial press hardening, employs locally heated tools seeking to obtain bainitic transformations. This leads to (seamless) zones within the formed parts with higher ductility. Due to the intrinsic nature of this process, phase transformations happen under the influence of high loads and in pre-deformed austenite. The austenite pre-strain state and applied stresses affect the kinetics of the bainitic transformation. Moreover, stresses have an additional relevant effect in this process, the so-called transformation plasticity. Linear transformation plasticity models have been successfully used to predict the behavior in the presence of low stresses. Nonetheless, because of the process’s severe conditions, these tend to fail. A strong nonlinearity of the transformation plasticity strain is observed for applied stresses above the austenite yield strength. Using thermomechanical tests on sheet specimens of a manganese-boron steel (22MnB5), widely utilized in the industry, the effect on the bainitic transformation of various degrees of deformation in the range of 0 to 18 pct, applied stresses in the range of 0 to 250 MPa and the transformation plasticity effect are investigated in this work.

Mechanical and tribological properties of “substrate–material” multifunctional composite with shape memory effect

The properties of steel–TiNi, TiNiCu, NiAl alloy multifunctional composite with shape memory effect are studied. The system is obtained under high-energy exposure (argon arc and laser surfacing, plasma and high-rate gas flame sputtering) with the formation of a structure with fine-grained to nanoscale dispersity. The experimental studies reveal the efficiency of the elaborated technique of the synthesis of composites to increase the wear resistance, fatigue strength, and endurance at frictionally cyclic low-cycle loading of material. The increase in fatigue and wear characteristics are explained by the processes caused by the combined cyclic loading and reverse friction. As is shown, the friction and mechanical fatigue in a surface-modified layer of the material undergoing the shape memory effect in the friction domain causes an increase in temperature that favors the martensite–austenite transformation, whereas the pressure arising in friction induces the transformation plasticity effect owing to the formation of stress martensite.

INVESTIGATION OF HARDENING AND TEMPERING DEFORMATIONS OF CARBURIZED LOW ALLOY STEEL

The article presents the investigation of plastic properties of carburized steel specimens during low tempering. Carburized articles often deform during quenching. The reasons of curvature are internal stresses produced by heterogeneity of structure, volume mismatch, different M S temperature of carburized and not carburized parts, uneven transformations, effect of transformation plasticity and other factors. The effect of asymmetrical carburization on steel transformation plasticity was examined evaluating the influence of structural compressive and tensile stress, which occurred because of volume mismatch. There was determined that internal stresses of carburized specimen exceed 300 MPa. The results of the experiment could be used for the development of heat treatment technologies of the carburized production – shafts, gears, cams, etc. DOI: http://dx.doi.org/10.5755/j01.mech.19.2.4155

Distortion of rings due to inhomogeneous temperature distribution

AbstractIn this paper, experimental results regarding the effect of asymmetric temperature distributions on shape changes of different ring types due to heat treatment will be presented. The main part of the distortion can be attributed to a distortion mechanism which is caused by the interaction of an inhomogeneous temperature distribution and the corresponding local phase transformation: In particular plastic strains by transformation plasticity dominate the change in roundness and the change of the angle of the raceways. To evaluate the portion on roundness deviation which can be attributed to sole thermal stress and/or transformation plasticity effects, experiments with rings produced from austenitic steel (material SAE 304) were examined. In addition, conical rings were heat treated, since the ring geometry has a large influence on the temperature fields during heat treatment. The experimental results were compared with the theoretical description of distortion potential by strain analysis, to identify the necessary inhomogeneous distribution of different carriers of the distortion potential which lead to the observed change in roundness deviation.

Mechanics and Characteristics of Transformation Plasticity and Metallo-thermo-mechanical Process Simulation

A phenomenological mechanism of transformation plasticity, or TP, is discussed, in the first part of the paper, to identify the characteristics of mechanical behavior of material. This is based on an assumption that the TP strain it is also a kind of plastic strain, a unified plastic flow theory is derived by introducing the effect of progressing new phase into the yield function of stress, temperature and plasticity related parameters. Thus obtained strain rate reveals to include the transformation plastic part in addition to mechanical and thermal plastic components. Applying the(2) formulated constitutive law with data of TP coefficient K identified, simulation of several cases of coupled metallo-thermo-mechanical processes by use of developed simulation code COSMAP. Special attention is paid on the effect of transformation plasticity to demonstrate that the effect makes a drastic influence for proper numerical simulation results.

The Effect of Transformation Plasticity on Prediction of Tooth Distortion of Heat-Treated Helical Gear

The displacement profile in a shank and dedendum-addendum circle of helical gear made of through-hardened S45C carbon steel and carburized-hardened SCr420H low alloy steel by gas carburizing was measured and predicted using a finite element method of DEFORM-HT. Both gears were quenched into same oil quenchant. Heat Transfer Coefficient (HTC) was taken by measuring thermal history inside of firstly silver probe and secondly stainless steel SUS304 gear blank. Uniform HTC of silver probe was calculated by lumped heat capacity method, whereas zone-based HTC of SUS304 gear blank was calculated by iterative modification method. Transformation plasticity was included and excluded to predict the tendency of distortion. Simulation results were analyzed and compared to the experimental results to validate the influence of transformation plasticity on the prediction of distortion during the martensitic and bainitic/pearlitic transformation. However, more accurate distortion profiles require detailed HTC by taking thermal history on the surface of the tooth.

A simple identification of transformation plastic behaviour and some data for heat treating materials

Transformation plasticity is known to play an important role in the course of heat treatment processes, and so affects the results of heat treatment simulations. For carrying out proper simulations, material data for this phenomenon (practically, the transformation plasticity coefficient) are required. In this paper, the authors introduce a new method by use of the four-point bending system of a beam to identify the transformation plasticity coefficient, taking advantage of its easiness and high accuracy compared with other conventional methods like tension test, etc. When the temperature reaches the transformation starting point, a tremendously large deflection takes place due to the effect of transformation plasticity, which gives fundamental data to identify the value of the coefficient. Some data measured for several heat treating steels under different applied stress are presented, which are compared with data taken by other methods in references.

Thermodynamic analysis of shape memory phenomena — I. Effect of transformation plasticity on elastic strain energy

The effect of plastic strains accompanying the thermoelastic transformation on shape memory phenomena is evaluated. It is shown that transformation plasticity can be formally included in Eshelby's macroscopic elastic analysis by defining a net transformation strain equal to the difference of the crystallographic transformation strain and the associated plastic strain. A two variant analysis for a thermoelastic martensite transformation is developed which enables calculation of the variant structure along the path of minimum elastic energy. It is shown that plastic strain accompanying the formation of a martensitic plate reduces the elastic energy stored during the forward transformation. The equilibrium variant structure is shown to be determined by both the applied stress and the fraction of martensite. If the plastic strains accompanying the forward and reverse transformations do not cancel, residual elastic stress fields remain after completion of the reverse transformation. The residual elastic strain fields influence subsequent transformation behaviour and provide the driving force for two-way shape memory behaviour.

Experimental determination of the stability of retained austenite in low alloy TRIP steels

The stability of retained austenite is the most important parameter controlling the transformation plasticity effects in multiphase low alloy TRIP steels. In this work the thermodynamic stability of the retained austenite has been determined experimentally by measuring the Mσs temperature as a function of bainite isothermal transformation (BIT) temperature and time in two low alloy TRIP steels. A single‐specimen temperature‐variable tension test technique (SS‐TV‐TT) has been employed, which allowed to link the appearance of yield points in the stress‐strain curve with the mechanically‐induced martensitic transformation of the retained austenite. The results indicated that the MσS temperature varies with BIT temperature and time. Higher austenite stability is associated with a BIT temperature of 400°C rather than 375°C. In addition, the chemical stabilization of the retained austenite associated with carbon enrichment from the growing bainite is lowered at short BIT times. This stability drop is due to carbide precipitation and comes earlier in the Nb‐containing steel. At longer BIT times the retained austenite dispersion becomes finer and its stability rises due to size stabilization. The experimental results are in good agreement with model predictions within the range of anticipated carbon enrichment of the retained austenite and measured austenite particle size.

浸炭焼入れ後の変形に及ぼす変態塑性の影響

Coupled metallo-thermo-mechanical analysis by use of heat treatment simulation code “HEARTS” is carried out for a ring-shaped model of Cr-Mo steel under carburized quenching: Diffusion of carbon carrier gas is calculated followed by oil-quenching from 850°C to estimate the distortion as well as structure distribution. To compensate the discrepancy between experimental data and calculated distortion, a special attention is paid on the effect of transformation plasticity. Several trials of simulation depending on the intensity of the effect shows that the transformation plastic strain plays an important role on the mode of distortion during carburized quenching process while a little effect is seen for normal quenching.

Retained austenite and mechanical properties in bainite transformed low alloy steels

Uniform ductility and formability of low alloy steels can be improved by the transformation plasticity effect of metastable retained austenite. In this work, intercritical annealing followed by bainite transformation resulted in the retention of austenite with sufficient stability for transformation plasticity interactions. The effect of retained austenite on mechanical properties was studied in two low‐alloy steels. Bainite transformation was carried out in the range of 400 to 500°C. The strength properties (yield strength and ultimate tensile strength) were more sensitive to bainite isothermal transformation temperature than holding time. Maximum strength properties were obtained for the lower transformation temperatures. On the other hand, high uniform and total elongation values were obtained at lower transformation temperatures but were sensitive to bainite isothermal transformation time. Variations in uniform elongation with holding time were linked to variations in retained austenite stability. Maximum values of uniform elongation occurred at the same holding times as the maximum amount of retained austenite. The same was true for total elongation and ultimate tensile strength. The above results indicate a strong correlation between retained austenite stability and uniform ductility and suggest that further optimisation regarding chemical composition and processing with respect to austenite stabilisation may lead to a new class of triple‐phase high‐strength high‐formability low‐alloy steels.

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 −5 mm 2 /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

A study of residual stresses in the surface hardening of a blade mould by high frequency induction heating

High frequency induction heating is a method of surface treatment which restricts the hardening area using the skin effect. Since the penetration depth of the magnetic field in the workpiece is dependent on the frequency, the required size of hardening area can be obtained by selecting an appropriate frequency. High frequency induction heating is able to harden a large area at once, in which the shapes of the coil and workpeice and the distance between them are important factors for the hardening area shape and the distribution of residual stresses. In this study, the transient heat flow and thermal stress were analysed for the high frequency induction surface hardening of a blade mould by using the modified two-dimensional finite element method. Besides the volume change in the phase transformation, the effect of transformation plasticity was also considered as an additional strain in the numerical analysis of the high frequency induction hardening process. The hardening area was fairly uniform in the mould surface except around the corner where the distance between the coil and workpiece was slightly larger than on the other parts. The thermal stress was induced mainly by the temperature gradient and martensitic phase transformation, while the latter was found to have a greater influence on the residual stress than the former. Simulation results revealed that compressive residual stresses occur in the hardening area, while the maximum tensile residual stress occurs near the boundary of the hardened zone.

Transformation Plasticity Effects on Residual Stresses in a Butt-Welded Pipe

Technical Briefs Transformation Plasticity Effects on Residual Stresses in a Butt-Welded Pipe B. L. Josefson, B. L. Josefson Division of Solid Mechanics, Chalmers University of Technology, Go¨teborg, Sweden Search for other works by this author on: This Site PubMed Google Scholar C. T. Karlsson C. T. Karlsson Division of Solid Mechanics, Chalmers University of Technology, Go¨teborg, Sweden Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information B. L. Josefson Division of Solid Mechanics, Chalmers University of Technology, Go¨teborg, Sweden C. T. Karlsson Division of Solid Mechanics, Chalmers University of Technology, Go¨teborg, Sweden J. Pressure Vessel Technol. Aug 1992, 114(3): 376-378 (3 pages) https://doi.org/10.1115/1.2929055 Published Online: August 1, 1992 Article history Received: October 9, 1991 Online: June 17, 2008

A study on residual stresses in surface hardening by high frequency induction heating

The transient heat flow and thermal stress during surface hardening by high frequency induction heating were analyzed using the finite element method. Using the modified two-dimensional finite model, the thermal and residual stresses during high frequency induction hardening were successively calculated. The thermal stresses were induced mainly by the temperature gradient and martensitic phase transformation; the phase transformation was found to have a greater influence on the residual stress than the temperature gradient. The simulation results revealed that there is a compressive residual stress within the hardening area and there is a maximum tensile residual stress near the exterior of the hardened zone. The effect of transformation plasticity was considered by using an additional strain in the numerical analysis of the high frequency induction hardening. The difference between the residual stresses considering and not considering the transformation plasticity was large, so it cannot be considered a negligible factor. In the non-hardening region, however, the stresses were distributed similarly.