High Carbon Austenite Research Articles

Simulation of Intercritical Austenization of a C-Mn Cold Rolled Dual Phase Steel

Formation of austenite strongly influences the microstructures and mechanical properties of dual phase steels. In present work, austenization process during intercritical annealing was studied in a Fe-C-Mn steel using Gleeble-1500 thermal simulator and quantitative microscopy. The experimental results show that austenite formation is separated into three different stages: (i) growth of high carbon austenite into pearlite rapidly until pearlite dissolution is completed; (ii) slower growth of austenite into ferrite; (iii) very slow equilibration between ferrite and austenite. The thermodynamic and kinetic analyses show that growth of austenite into ferrite is controlled by carbon diffusion in austenite in the primary stage and manganese diffusion in ferrite in the subsequent stage because diffusion coefficient of Mn in ferrite is several orders of magnitude smaller than that of C in austenite. The slow final equilibration between ferrite and austenite is obtained by manganese diffusion through the austenite. Based on the analysis, one dimensional diffusion model of intercritical austenization was developed and solved using finite volume method on the assumption that solute flux was local balance at interface, and the kinetics calculated was compared with experimental results. Simulated results indicate that growth of austenite reaches paraequilibrium in about one second, but remains thousands of seconds to reach final equilibrium. Simulated concentration profiles show that manganese atoms transferred from ferrite congregate in austenite near phase interface, which is consistent with the experimental phenomenon.

Effect of ausferrite volume fraction and morphology on tensile properties of partially austenitised and austempered ductile irons with dual matrix structures

In the present study, an unalloyed ductile iron containing 3·50%C, 2·63%Si, 0·318%Mn and 0·047%Mg was intercritically austenitised (partially austenitised) in the two phase region (α + γ) at temperatures of 795 and 815°C for 20 min, and then was quenched into a salt bath held at an austempering temperature of 365°C for various times to obtain various ausferrite volume fractions. Fine and coarse dual matrix structures were obtained from the two different starting conditions. Some specimens were also conventionally austempered from 900°C for comparison. Results showed that a structure having proeutectoid ferrite plus ausferrite (bainitic ferrite + high carbon retained or stabilised austenite) was developed. The phase previously described as new ferrite (also called epitaxial ferrite) was observed to form following heat treatment only in specimens with coarse austenite dispersion after austempering from the (α + γ) temperature range. It was observed that the parent austenite dispersion present at the intercritical austenitising temperature has an effect on the volume fraction of high carbon austenite following austempering. Finer austenite dispersions produced more stabilised high carbon austenite than coarse ones for a given austempering time. The volume fractions of proeutectoid ferrite, new ferrite, ausferrite can be controlled to influence the strength and ductility. Specimens austempered from the (α + γ) temperature range exhibited much greater ductility than conventionally austempered samples. Within each of the series austempered from the (α + γ) temperature range, the tensile strength increased and ductility decreased with increasing ausferrite volume fraction and decreasing proeutectoid ferrite volume fraction. However, specimens having a fine ausferritic structure without new ferrite exhibited lower ductility and higher strength than specimens with a coarse ausferritic structure under the same austempering conditions. The specimen with ∼60% AFVF (coarse structure) and ∼65% AFVF (fine structure) exhibited the best combination of high strength and ductility compared to pearlitic grades, but their ductility was slightly lower than ferritic grades. These materials satisfy the requirements for the strength of quenched and tempered grades and their ductility superior to that of this grade.

The Strain-Hardening Behavior of Partially Austenitized and the Austempered Ductile Irons with Dual Matrix Structures

In the current study, an unalloyed ductile iron containing 3.50 C wt.%, 2.63 Si wt.%, 0.318 Mn wt.%, and 0.047 Mg wt.% was intercritically austenitized (partially austenitized) in two-phase regions (α + γ) at different temperatures for 20 min and then was quenched into salt bath held at austempering temperature of 365 °C for various times to obtain different ausferrite plus proeutectoid ferrite volume fractions. Fine and coarse dual matrix structures (DMS) were obtained from two different starting conditions. Some specimens were also conventionally austempered from 900 °C for comparison. The results showed that a structure having proeutectoid ferrite plus ausferrite (bainitic ferrite + high-carbon austenite (retained or stabilized austenite)) has been developed. Both of the specimens with ∼75% ausferrite volume fraction (coarse structure) and the specimen with ∼82% ausferrite volume fraction (fine structure) exhibited the best combination of high strength and ductility compared to the pearlitic grades, but their ductility is slightly lower than the ferritic grades. These materials also satisfy the requirements for the strength of the quenched and tempered grades and their ductility is superior to this grade. The correlation between the strain-hardening rates of the various austempered ductile iron (ADI) with DMS and conventionally heat-treated ADI microstructures as a function of strain was conducted by inspection of the respective tensile curves. For this purpose, the Crussard-Jaoul (C-J) analysis was employed. The test results also indicate that strain-hardening behavior of ADI with dual matrix is influenced by the variations in the volume fractions of the phases, and their morphologies, the degree of ausferrite connectivity and the interaction intensities between the carbon atoms and the dislocations in the matrix. The ADI with DMS generally exhibited low strain-hardening rates compared to the conventionally ADI.

Transformation characteristics, microstructure and mechanical properties of austempered ductile iron welds

The isothermal transformation of austempered ductile iron (ADI) weld at 370°C for a time period ranging from 0·5 to 1440 min involves three stages. The welding process has the effect of accelerating the subsequent transformation during austempering to bainitic ferrite and high carbon austenite (stage 1) and delaying the transformation to carbide (Fe3C) and ferrite (stage 3). Within the austempering time range 15–240 min, the ADI welds consisting of bainitic ferrite and retained austenite had superior properties (namely tensile strengths between 1030 and 1060 MPa and elongations between 8·0 and 8·5%). As the austempering temperature increased from 310 to 400°C, the tensile strength of ADI weld decreased from 1141 to 1040 MPa and its elongation increased from 4·0 to 9·1%. Under tensile loading a large number of slip bands appeared in the retained austenite and the austenite/ferrite interfaces behaved as obstacles limiting the movement of dislocations. The superior properties of ADI weld are mainly associated with the presence of large amount of retained austenite (30–32%), grain boundary strengthening effect and solid solution strengthening effect.

Tensile properties of partially austenitised and austempered ductile irons with dual matrix structures

In the present study, an unalloyed ductile iron containing Fe–3·50C–2·63Si–0·318Mn–0.047Mg (wt-%) were intercritically austenitised (partially austenitised) in two phase region α+γ at various temperatures of 795, 805, 815 and 830°C for 20 min and then quenched into salt bath held at austempering temperature of 365°C for various times to obtain different ausferrite volume fractions (AFVFs). Results showed that dual matrix structure containing proeutectoid ferrite, new ferrite (also called epitaxial ferrite) and ausferrite (bainitic ferrite+high carbon austenite, which is retained or stabilised austenite) has been developed. Within each of the austempered series in α+γ temperature range, new ferrite volume fraction increased with increasing intercritical austenitising temperature (ICAT). Although, transforming percentage of new ferrite from parent austenite present at ICAT increased with decreasing ICAT. Some specimens were also conventionally austempered from 900°C for comparison. The new ferrite was absent in these samples. The volume fraction of proeutectoid ferrite, new ferrite and ausferrite can be controlled to determine the strength and ductility. Austempered specimens in α+γ temperature range exhibited much greater ductility than conventionally austempered ones. The tensile strength increased while ductility decreased with increasing AFVF. On the other hand, the ductility increased with increasing proeutectoid ferrite and new ferrite volume fractions at the expense of strength. The specimen with ∼47·2%AFVF exhibited the best combination of high strength and ductility. The strength and ductility of this material is much higher than that of ferritic grades. Its strength is at the same level as while ductility almost more than four times higher than that of pearlitic grades. Meanwhile, the specimen with ∼ 75%AFVF exhibited the best combination of high strength and ductility compared with those of pearlitic grades. The strength of this material is much higher and its ductility is almost more than two times higher than that of pearlitic grades yet slightly lower than that of ferritic grades. This material also meets the requirements for the strength of quenched and tempered grades and its ductility is higher than that of this grade.

Neutron diffraction analysis of martensite ageing in high-carbon FeCMnSi steel

Abstract A high-carbon metastable austenite phase was quenched in liquid nitrogen to obtain fresh athermal martensite. Different ageing treatments were performed in order to study the transformation of the martensite phase and the formation of carbides. Neutron diffraction experiments give detailed information on the transformations during the annealing treatment itself. The tetragonal athermal martensite transformed immediately, i.e. during the heating to the ageing temperature, to cubic martensite. Ageing at 400°C resulted in the decomposition of the austenite in ferrite and carbides and the formation of bainite. Those carbides could be determined as -carbides transforming in -carbides after extended annealing times. As was expected, ageing at 170°C resulted in the formation of a small amount of carbides while the austenite phase remained stable.

Ausforming of austempered ductile iron alloyed with nickel

The unique properties of austempered ductile iron (ADI) are related to the ausferrite matrix (stable high carbon austenite with fine acicular ferrite) formed during the austempering treatment of ductile iron castings. Ausforming of ADI adds a mechanical processing component to the conventional ADI heat treatment, thus increasing the rate of ausferrite production and therefore leading to a finer and more homogeneous ausferrite product. The present study aims at exploring the potential benefits of ausforming ADI alloyed with Ni, using a rolling operation, with reduction in height up to 25%. The ausforming was introduced into the austempering schedule just after the quench but before any substantial transformation of austenite. Alloying with up to 3·0% nickel as well as varying austempering times were investigated. The kinetics of ausferrite formation was studied using both metallographic and XRD techniques. Ausforming caused a dramatic increase in tensile strength for all investigated alloys and the ductility of Ni-alloyed irons increased, while the ductility of the unalloyed ADI decreased. Ausforming also leads to an increase in both hardness and wear resistance properties.

Effect of sintering atmosphere and carbon content on the densification and microstructure of laser-sintered M2 high-speed steel powder

In the present work, the role of graphite addition and sintering atmosphere on the laser sintering of M2 high-speed steel (HSS) powder was studied. The test specimens were produced using a continuous wave CO 2 laser beam at power of 200 W and at varying scan rate ranges from 50 to 175 mm s −1 under nitrogen and argon atmospheres. It was found that laser scan rate and sintering atmosphere strongly influence the densification of M2 HSS powder. Opposite to the conventional sintering that the sinterability of HSS is improved by graphite addition, the result of this work demonstrated that graphite addition decreases the sintered density of M2 HSS at relatively low laser scan rates, i.e. <100 mm s −1, whilst at higher scan rates the effect is not very conspicuous. Furthermore, sintering under argon atmosphere yielded better densification compared to nitrogen atmosphere, particularly at higher scan rates. The microstructure of laser-sintered parts consisted of large and elongated pores and a heterogeneous metal matrix. The matrix structure includes martensite, high carbon austenite and inter-cellular or inter-dendrite carbide network. Addition of graphite to the M2 HSS powder increases the heterogeneity of the microstructure, as it was noticed from the variation in the micro-hardness throughout the sintered specimens.

Effects of low-temperature coating process on mechanical behaviors of ADI

Austempered ductile iron (ADI) is an austemper-treated ductile iron with acicular ferrite and high-carbon austenite as the matrix of microstructure. In general, the austempering is isothermally treated about in the temperature range from Ms to 450°C, thus the traditional case hardening of high temperature cannot be available to treat ADI. In recent years, physical vapor deposition (PVD) technique using lower processing temperature has been widely adopted to coat various films, such as diamond-like carbon (DLC), CrN, TiN and so on, on the engineering material for surface modification. In particular, DLC film possesses excellent mechanical properties such as high hardness and low friction coefficient. Electroless nickel (EN), another lower temperature coating process, has also a wide field of application such as the industrial components and machine parts. Thus, the purpose of this study is to investigate the effect of EN and PVD–DLC surface coatings on mechanical behaviors of ADI, especially the tensile and fatigue properties. Analyses of the resulting films were also performed for correlating the mechanical properties attained to the coating characteristics.

Erosion and Corrosion Behaviors of ADI Deposited TiN/TiAlN Coatings by Cathodic Arc Evaporation

Austempered ductile iron (ADI) is a heat-treated ductile iron with acicular ferrite and high-carbon austenite as the matrix of the microstructure. The purpose of this research is to investigate the influence of different hard coatings (PVD-TiN and PVD-TiAlN) on the erosion and corrosion properties of ADI. Also, the coating structure and property were analyzed by using XRD, Rockwell C tester and SEM. The results showed that TiN and TiAlN films identified by XRD could be well deposited on the ADI substrate by the PVD method of cathodic arc evaporation (CAE). Adhesion between coating-layer and ADI substrate was better than that between coating-layer and DI substrate. It was found that the initial source of coated layer peeled for the both substrates occurred at the graphitic sites. The depositing rate of TiAlN was rapider than that of TiN, but it also resulted in the thicker coating thickness and rougher surface for the specimen coated with TiAlN film. After slurry erosion test, the result revealed that erosion resistance of coated specimens was better than that of uncoated specimen. In 3.5 mass% NaCl aqueous for polarization curve test, corrosion current of TiAlN film was smaller than that of TiN film. In 10 vol% HCl solution for immersion test, both TiN and TiAlN coatings could raise the corrosion resistance for ADI material.

Study of the austempering transformation kinetics in compacted graphite cast irons

In this work, the phases involved in the transformation austenite→ferrite+high-carbon austenite in cast irons were assessed by means of Mossbauer spectroscopy and hardness measurements for samples austempered at five different temperatures between 573 and 673 K with two Mn contents. The C content in the high-carbon austenite was found to have a dependence of t0.40±0.05 on the austempering time t, which evidences that diffusion of carbon has taken place. The kinetic parameter k determined using the Johnson-Mehl-Avrami equation has a maximum of 3.9×10−3 (s−1) at ≈623 K and corroborates that Mn slows the transformation rate.

Microstructural evolution in austempered ductile iron during non-isothermal annealing

This work investigates the thermal stability of ausferritic microstructures such as those found in NiCuMo austempered ductile iron (ADI). Typical ADI microstructures consisting of acicular ferrite and high carbon austenite were produced by heat treating NiCuMo ductile iron for 2 h at 315 °C and 370 °C. It was found that low temperature austempering led to the precipitation of a hexagonal epsilon-carbide phase. Hence, differential thermal analysis was used to closely follow the active phase transformations found in the experimental ADI during non-isothermal annealing. From these tests, it was apparent that the ausferrite stability is strongly influenced by the austempering conditions and heating rates. In particular, the experimental outcome shows that the ADI microstructures decompose at temperatures of 428°C and 459°C for irons austempered at 315°C and 370°C, respectively. Further evidence for the decomposition reactions involving acicular ferrite and high carbon austenite was provided by TEM observations. The TEM results indicate that ausferrite decomposes into ferrite-cementite during non-isothermal annealing via the precipitation of transition carbides. In the iron austempered at high temperature, orthorhombic η-Fe2C was identified as the transition carbide, whereas tricilinic silicon-carbide precipitates became dominant in ADI treated at 315°C.

Experimental Study of the Thermal Stability of Austempered Ductile Irons

A nonisothermal annealing was applied to austempered Ni-Cu-Mo alloyed and unalloyed ductile irons to determine the thermal stability of the ausferritic structure. Differential thermal analysis (DTA) results were used to build the corresponding stability diagrams. The transformation starting temperature of the high carbon austenite was found to be strongly dependent on the austempering temperature, the heating rate, and the chemical composition of the iron. The Ni-Cu-Mo alloying elements and high austempering temperature increased the stability. The transformation of the austenite to ferrite and cementite is achieved via the precipitation of transition carbides identified as silico-carbides of triclinic structure.

Austempering transformation kinetics of compacted graphite cast irons obtained by Mössbauer spectroscopy

Mossbauer spectroscopy has been applied systematically to study the processes occurring during the Stage I of the austempering transformation of compacted graphite cast irons at temperatures from 573 to 673 K for two Mn concentrations. The kinetics of transformation (γ→αFe+γhc) was followed determining the dependence of the high-carbon austenite percentage on austempering time for different austempering temperatures and Mn contents. The evolution of the C concentration and the total amount of C incorporated into high-carbon austenite were also monitored. The results are compared with those of other morphologies and discussed in the frame of Johnson-Mehl's and diffusion models.

Thermal dependence of austempering transformation kinetics of compacted graphite cast iron

The evolution of the relative fraction of high-carbon austenite with austempering time and temperature was analyzed in a compacted graphite (CG) cast iron (average composition, in wt pct: 3.40C, 2.8Si, 0.8Mn, 0.04Cu, 0.01P, and 0.02S) at five different austempering temperatures between 573 and 673 K. Samples were characterized by Mossbauer spectroscopy, hardness measurements, and optical microscopy. During the first stage of transformation, the kinetics parameters were determined using the Johnson-Mehl’s equation, and their dependence with temperature in the range from 573 to 673 K indicates that the transformation is governed by nucleation and growth processes. The balance between growth-rate kinetics and nucleation kinetics causes the kinetics parameter (k) to have a maximum at ≈623 K of 3.9×10−3(s−1). The evolution of the C content in the high-carbon austenite was found to be controlled by the volume diffusion of carbon atoms from the ferrite/austenite interface into austenite, with a dependence of t 0.40±0.05 on the austempering time (t).

Effect of Microstructural Features on the Austempering Heat Treatment Processing Window

Microstructural changes during the austempering heat treatment of alloyed ductile iron are described. The effect of heat treatment cycle and casting structure on mechanical properties of Austempered Ductile Iron, ADI, of composition 3.52%C, 2.64%Si, 0.25%Mo, 0.25%Cu, 0.67%Mn are studied. Comparison of EDX analysis with mechanical properties show that the optimised properties for an ADI are achieved when an ausferritic structure of high carbon austenite and ferritic plates with minimum martensite and carbides is formed. Processing Window is the period of austempering time over which the structure is fully ausferritic The results show that for a high Mn DI, single austempering process does not provide a fully ausferntic structure and usually further austempering process, double or stepped austempering process, is required. The experimental results demonstrate that such an ADI still might satisfy the ASTM standard with careful control over austempering parameters. This study confirms that some casting features like low nodules count and preferential segregation of alloying elements narrow the processing window This study also shows that the microstructural defects like microporosity and poor modularity have a detrimental effect on properties of ADI.

Formation of bainite in ductile iron

Abstract The subject of the present investigation was the standard Ni–Mn–Cu–Mo alloyed ductile iron. The kinetics of bainite transformation under isothermal conditions was measured by conventional dilatometry. The results of these experiments show that there are two C-shaped curves, which correspond to the formation of upper bainite (500–390°C) and lower bainite (390–250°C), on the TTT diagram. Optical microscopy shows that the morphology of the bainite changes from feathery-like in the upper temperature range to plate-like in the lower one. These facts are, probably, a result of different crystallographic shears during formation of upper and lower bainites. X-ray measurements were done on specimens cooled from isothermal temperatures just after ceasing of the bainite reaction. Comparison of dilatometrical data with X-ray results shows that bainite transformation ceases when the carbon content in low carbon austenite reaches a certain value. Probably, this concentration corresponds to the T 0 curve and the composition of the high carbon austenite is closer to the Ae 3 curve. The other fact is that an increase of the maximal volume fraction of bainitic α-phase promotes an increase in the volume fraction of high carbon austenite.

Study of high cycle fatigue of PVD surface-modified austempered ductile iron

Austempered ductile iron (ADI) is made from ductile iron by an austempering treatment, and its main microstructure is ausferrite that is composed of acicular ferrite and high carbon austenite. The purpose of this experiment is to investigate the influence of different coating layers and the size of casting (mass effect) on the high-cycle fatigue properties of ADI. Specimens in two casting sizes of the same chemical composition were subjected to a high-toughness austempering treatment, then coated with TiN or TiCN hard films by a physical vapor deposition (PVD) process. The results showed that the fatigue limit of the small casting size ADI is 292 MPa for ADI coated with TiN and 306 MPa for ADI coated with TiCN, which are 16% and 22%, respectively, higher than that of the ADI without coating (251 MPa). For the large casting size ADI, the fatigue limits are 200, 214 and 217 MPa for ADI without coating, ADI coated with TiN and ADI coated with TiCN, respectively. ADI coated with TiN and with TiCN are 7% and 9% better than the uncoated. Thus, it is concluded that TiN and TiCN coatings by PVD can improve the high-cycle fatigue strength of ADI. This is due to the high surface hardness and possibly the ADI surface compressive residual stress as well. For the small casting size ADI, TiCN-coated specimens have a bit higher fatigue strengths and this might be attributed to the higher hardness of TiCN than TiN films. As to the effect of mass, it is found that the small casting size has better fatigue properties and benefits more from the coating films. This could have stemmed from the higher nodule count and its associated benefits in thinner castings.

Effect of silicon content on transformation kinetics of austempered ductile iron

The present study investigated the effect of austenitising temperature (850, 900, and 950°C) and austempering time (0–7 h) on the volume of retained austenite of a 0.3 wt-%Mn ductile iron containing two different levels of silicon, namely 2.02 wt-% and 3.31 wt-%, and austempered at 360°C. The volume fraction of retained austenite and austenite carbon content results were then correlated with microstructural changes and impact toughness results. It is shown that the austenite stability is of great significance with respect to impact toughness, and that ferrite, when present in acicular form, can increase the mechanical stability of the austenite. It is observed that decreasing the austenitising temperature increases the driving force for the stage I transformation reaction in which mother austenite transforms to high carbon austenite plus acicular ferrite. However, the austenitising temperature has only a small effect on the kinetics of the stage II reaction in which high carbon austenite transforms to bainiticferrite plus carbides. In the low silicon iron, austenitising at 950°C results in a continuous network of intercellular and low carbon austenite which reduces impact properties. Intercellular austenite is attributed to the segregation of manganese and high austenitising temperature which decrease the carbon diffusion rate and delay ferrite nucleation and growth. Decreasing the austenitising temperature to 850°C increases the rate of transformation which results in a more uniform microstructure, stable high carbon austenite, and higher impact toughness. Silicon has the effect of modifying the Fe-C phase diagram such that a higher solution treatment temperature is needed to fully austenitise the iron. Furthermore, a three phase region of austenite-ferrite-graphite is introduced in the Fe-C-Si phase diagram. Consequently, austenitising at low solution treatment temperatures produces structures containing proeutectoidferrite. Increasing the austenitising temperature to 950°C leads to a more uniform acicular microstructure of stable high carbon retained austenite andferrite and results in optimum impact properties. Following short austempering times in irons containing 2.02% and 3.31% silicon, the carbon content of the retained austenite is low and on subsequent cooling to room temperature it transforms to martensite, resulting in low impact values. Optimum properties are obtained at intermediate austempering periods when both the amount of retained austenite and the austenite carbon content are maximum. Extending the austempering time causes the high carbon austenite to decompose to ferrite plus carbides, the stage II reaction, leading to a reduction in impact toughness.

Effect of silicon content on transformation kinetics of austempered ductile iron

The present study investigated the effect of austenitising temperature (850, 900, and 950°C) and austempering time (0–7 h) on the volume of retained austenite of a 0.3 wt-%Mn ductile iron containing two different levels of silicon, namely 2.02 wt-% and 3.31 wt-%, and austempered at 360°C. The volume fraction of retained austenite and austenite carbon content results were then correlated with microstructural changes and impact toughness results. It is shown that the austenite stability is of great significance with respect to impact toughness, and that ferrite, when present in acicular form, can increase the mechanical stability of the austenite. It is observed that decreasing the austenitising temperature increases the driving force for the stage I transformation reaction in which mother austenite transforms to high carbon austenite plus acicular ferrite. However, the austenitising temperature has only a small effect on the kinetics of the stage II reaction in which high carbon austenite transforms to bainiticferrite plus carbides. In the low silicon iron, austenitising at 950°C results in a continuous network of intercellular and low carbon austenite which reduces impact properties. Intercellular austenite is attributed to the segregation of manganese and high austenitising temperature which decrease the carbon diffusion rate and delay ferrite nucleation and growth. Decreasing the austenitising temperature to 850°C increases the rate of transformation which results in a more uniform microstructure, stable high carbon austenite, and higher impact toughness. Silicon has the effect of modifying the Fe-C phase diagram such that a higher solution treatment temperature is needed to fully austenitise the iron. Furthermore, a three phase region of austenite-ferrite-graphite is introduced in the Fe-C-Si phase diagram. Consequently, austenitising at low solution treatment temperatures produces structures containing proeutectoidferrite. Increasing the austenitising temperature to 950°C leads to a more uniform acicular microstructure of stable high carbon retained austenite andferrite and results in optimum impact properties. Following short austempering times in irons containing 2.02% and 3.31% silicon, the carbon content of the retained austenite is low and on subsequent cooling to room temperature it transforms to martensite, resulting in low impact values. Optimum properties are obtained at intermediate austempering periods when both the amount of retained austenite and the austenite carbon content are maximum. Extending the austempering time causes the high carbon austenite to decompose to ferrite plus carbides, the stage II reaction, leading to a reduction in impact toughness.