Ac1 Temperature Research Articles

Effects of Heating and Cooling Rates on Phase Transformations in 10 Wt Pct Ni Steel and Their Application to Gas Tungsten Arc Welding

10 wt pct Ni steel is a high-strength steel that possesses good ballistic resistance from the deformation induced transformation of austenite to martensite, known as the transformation-induced-plasticity effect. The effects of rapid heating and cooling rates associated with welding thermal cycles on the phase transformations and microstructures, specifically in the heat-affected zone, were determined using dilatometry, microhardness, and microstructural characterization. Heating rate experiments demonstrate that the Ac3 temperature is dependent on heating rate, varying from 1094 K (821 °C) at a heating rate of 1 °C/s to 1324 K (1051 °C) at a heating rate of 1830 °C/s. A continuous cooling transformation diagram produced for 10 wt pct Ni steel reveals that martensite will form over a wide range of cooling rates, which reflects a very high hardenability of this alloy. These results were applied to a single pass, autogenous, gas tungsten arc weld. The diffusion of nickel from regions of austenite to martensite during the welding thermal cycle manifests itself in a muddled, rod-like lath martensitic microstructure. The results of these studies show that the nickel enrichment of the austenite in 10 wt pct Ni steel plays a critical role in phase transformations during welding.

Study of 13Cr-4Ni-(Mo) (F6NM) Steel Grade Heat Treatment for Maximum Hardness Control in Industrial Heats

The standard NACE MR0175 (ISO 15156) requires a maximum hardness value of 23 HRC for 13Cr-4Ni-(Mo) steel grade for sour service, requiring a double tempering heat treatment at temperature in the range 648–691 °C for the first tempering and 593–621 °C for the second tempering. Difficulties in limiting alloy hardness after the tempering of forged mechanical components (F6NM) are often faced. Variables affecting the thermal behavior of 13Cr-4Ni-(Mo) during single and double tempering treatments have been studied by means of transmission electron microscopy (TEM) observations, X-ray diffraction measurements, dilatometry, and thermo-mechanical simulations. It has been found that relatively low Ac1 temperatures in this alloy induce the formation of austenite phase above 600 °C during tempering, and that the formed, reverted austenite tends to be unstable upon cooling, thus contributing to the increase of final hardness via transformation to virgin martensite. Therefore, it is necessary to increase the Ac1 temperature as much as possible to allow the tempering of martensite at the temperature range required by NACE without the detrimental formation of virgin martensite upon final cooling. Attempts to do so have been carried out by reducing both carbon (<0.02% C) and nitrogen (<100 ppm) levels. Results obtained herein show final hardness below NACE limits without an unacceptable loss of mechanical strength.

Applying the Electron Concentration Calculation Method to Assess the Influence of Hydrogen on the Position of the Variable Solubility Lines in Ti-Cr System

A possibility of using the electron concentration calculation method to evaluate Acm and Ac3 temperatures in the Ti-Cr system alloys with different hydrogen content (0.15-0.93 wt %) is considered. It is found that the proposed method gives results similar to the experimental data only in the case of low hydrogen concentrations since it does not take into account the degree of hydrogen ion screening by electrons.

Extraordinary strength and ductility obtained in transformation-induced plasticity steel by slightly modifying its chemical composition

Extraordinary mechanical properties were obtained in a low-carbon transformation-induced plasticity (TRIP) steel by modifying its chemical composition and employing thermo-mechanical processing. It was found that Cu addition to the conventional CMnSiAl TRIP steel could improve the thermal stability of high temperature austenite by decreasing the Ac3 temperature and retarding the bainitic transformation, which led to the formation of retained austenite with higher volume fraction but lower carbon content. The combined effects of high strain hardening due to the TRIP effect, uniform strain distribution, higher volume fraction of thermal martensite, and nano-scale precipitation of ε-Cu particles in the Cu-TRIP steel led to a high strength of 1GPa and total elongation of approximately 50%. Moreover, a detailed scanning electron microscopy analysis showed that the copper had a noticeable effect on the morphology of the martensite phase created within the microstructure.

Understanding of Processing, Microstructure and Property Correlations during Spheroidizing Heat Treatment of 100Cr6 Steel

In the present study an investigation has been carried out for spheroidization of 100Cr6 bearing steel used in forging industry. Three different spheroidization processes were considered. The first one was the annealing of normalized steel under Ac1 temperature for a long time. The second one was the annealing of normalized steel above Ar1 temperature after heating between Ac1 and Acm for one hour. The third one was the annealing of hardened steel under Ac1 temperature for a long time. For evaluation of cold workability with different spheroidization annealing periods, the yield strength and percentage of reduction in area in uniaxial tension were recorded. The present results indicate that low alloyed carbon bearing steels can be easily processed to achieve unique microstructures and properties.

Martensite transformations in the surface layer at grinding of parts of hardened steels

The mechanism of direct and inverse martensitic transformation in the surface layer of a ground part of hardened steel under the influence of the grinding temperature was investigated. It is shown that the reverse martensitic transformation is carried out during grinding according to the martensite-perlite-austenite scheme. The possibility of high-speed tempering of martensite to perlite is shown, under the action of a contact grinding temperature, which, with a further increase in temperature, turns into austenite, forming an extremely harmful defect in the working surface –quenching burn. In the present work, it is shown that the quenching burn in the form of austenite is happened by the M–P–A diffusion mechanism. This made it possible to obtain graphical and analytical dependencies, using which it is possible to calculate the Ac1 temperature points of the formation of austenite for virtually any steel grade depending on the content of carbon and alloying elements. The latter circumstance makes it possible to create such grinding conditions (the type of abrasive, the characteristics of the wheel, the use of coolant, treatment regime), at which the austenite formation temperature is not attained and the quenching does not happen. Thus, as a result of the studies, it is possible to determine safe grinding temperatures for steels of different chemical compositions and treatment regimens that do not cause an increase in the grinding temperature above a certain level.

Effect of vanadium on dynamic continuous cooling transformation behavior of medium-carbon forging steels

Dynamic continuous cooling transformation (CCT) behavior of medium-carbon forging steels microalloyed with different V contents up to 0.29% was investigated by means of dilatometric measurement, microstructural observation and hardness measurement. The results showed that the dynamic CCT diagrams were similar and the main difference was that the fields of the transformation products were shifted to the right side of the diagrams with the increase of V content, and this effect was more noticeable with an addition of 0.29% V. The Ac1 and Ac3 temperatures showed increasing trends with increasing V content, while the critical cooling rates decreased with increasing V content. The increase of V content resulted in significant increase of hardness and this tendency was enhanced with increasing cooling rate until the formation of acicular ferrite (AF). A promising approach of remarkably improving the toughness of ferritic-pearlitic medium-carbon forging steels with suitable V addition and the introduction of AF without notable penalty on its strength level was suggested.

Atomic-scale investigation of carbon atom migration in surface induced white layers in high-carbon medium chromium (AISI 52100) bearing steel

The microstructure and chemical composition of white layers (WLs) formed during hard turning of AISI 52100 steel were studied using atom probe tomography (APT) and transmission electron microscopy (TEM). APT analyses revealed a major difference in the re-distribution of the carbon (C) atoms between WLs formed above and below the Ac1 temperature, i.e. T-WL and M-WL, respectively. In T-WL, the C-atoms segregate to grain boundaries (GBs) forming interconnected or isolated C-rich clusters, ∼5 nm, with a concentration of 9.8 ± 0.3 at.%C. Apart from the GB segregation, in M-WLs, large C-rich regions were found with 24.8 ± 0.4 at.%C. Owing to the chemical composition (stoichiometry) and element partitioning of such regions, they were assigned as θ-carbides (cementite). The APT results reveal that the original θ-carbides remain un-dissolved in the M-WLs, but might be plastically deformed due to the excessive strain that exists in hard machining process. The obtained results are in good agreement with the temperatures that are reached during formation of M-WLs. The isolated nano-sized C-clusters were assigned as off-stoichiometric carbides whereas the interconnected C-rich clusters were attributed to Cottrell atmospheres, evident by the linear shape of the C-enrichment as observed in the APT reconstructions. The C-contents in the nano-sized martensitic and ferritic grains were estimated to 0.50 ± 0.06 at.%C and ∼0.46 ± 0.02 at.%C, respectively. The C-content in the ferritic grains, beyond the C-solubility limit in ferrite (<0.1 at.%) is governed by the high dislocation density inside the grains, supported by the favorable binding energy between dislocations and C-atoms compared to C-atoms and Fe in carbides. No other evidence of redistribution of the substitutional alloying elements was observed. TEM analyses showed that T-WLs comprises of an equiaxed and nano-sized grains with well-defined cell boundaries, whereas the structure in the M-WLs comprised of elongated sub-grains formed via re-orientation of the original martensite followed by breakage/partitioning into elongated sub-grains.

Precipitation of Secondary Phases in a Supermartensitic 13Cr6Ni2.5MoTi Steel during Heat Treatment

The balance between strength and toughness of supermartensitic steels can be controlled by heat treatment parameters. Quality heat treatment usually consists of quenching and single or double tempering above the Ac1 temperature. Such a treatment results in stabilization of reverted austenite in the tempered martensite. Tempering can also be accompanied by intensive precipitation processes, especially in high alloyed grades. A detailed TEM characterisation of precipitation processes in a 13Cr6Ni2.5MoTi supermartensitic steel proved that single tempering at temperatures of 600°C and 690°C was accompanied by precipitation of three minor phases: MX (TiX), M23C6 and Laves phase (Fe2Mo type). Precipitation processes were more intensive at 690°C. Volume fractions of MX and M23C6 phases were low. Laves phase precipitation was intensive and particles of this minor phase grew fast. However, thermodynamic calculations using the Thermocalc software suggest that Laves phase is not an equilibrium phase in the steel under consideration.

Phase Transformation Behaviour in P91 During Post Weld Heat Treatment: A Gleeble Study

Grade P91 is a creep strength enhanced ferritic steel used widely at high temperature applications in thermal power plants. P91 weldments are subjected to post weld heat treatment (PWHT) at a typical temperature of 760 °C for stress relieving purpose and to obtain optimal mechanical properties. In general PWHT temperature is about 30–50 °C lower than the lower critical temperature (Ac1). Depending on the chemical composition (particularly Ni + Mn), heating rate and prior austenite grain size, Ac1 temperature can vary from 780 to 860 °C. In this investigation an attempt was made to understand the effect of surpassing the typical PWHT temperature of 760° and approaching Ac1 of the material. The investigation was based on physical simulation conducted using 3500 Gleeble thermo mechanical simulator. The study was performed on cylindrical specimens which were extracted from weld metal and parent metal. Specimens were subjected to normalizing treatment at 1050 °C for achieving austenitisation and to determine the Ac1 temperature and then to different PWHT temperatures in the range of 790–850 °C at suitable intervals. American Society for Testing and Materials (ASTM) A1033-10 based procedure was used to find Ac1 temperature for the parent metal and weld metal at slow heating rate condition to compare with practical heat treating condition. Dilatometry plots, microstructure and hardness tests performed on the specimen were used to analyse the phase transformation. Results indicated that alpha ferrite phase and fresh untempered martensite could be formed in P91 steel when PWHT temperature was about 12 °C less than the Ac1 temperature. It was also seen that the heating rate had strong influence on the Ac1 temperature. At the heating rate of 28°/hr, Ac1 was about 792 °C for weld metal, while Ac1 was about 812 °C for the heating rate of 220 °C/hr.

Study of martensitic-ferritic dual phase steels produced by hot stamping

The effects of heat treatment and initial microstructure on tensile properties of 22MnB5 and 30MnB5 high-strength hot stamping steels with martensite-ferrite matrix were investigated. Hot stamping steels possessed limited elongations of about 5% in a tensile strength ranging from 1300 to 1500 MPa when quenched at temperatures above A3 temperatures. The total elongations were tried to improve by partial austenization between Ac1 and Ac3 temperature and quenching. Ac1 and Ac3 temperatures were calculated via ThermoCalc. Microstructural characterization was made by using Light Microscope and Scanning Electron Microscope. Microstructure is composed of ferrite+martensite. It was seen that annealing temperature affects the volume fraction of phases. It was concluded that initial microstructure is an important parameter for the final microstructure. This method can be used for automobile parts which require higher TE with sufficient yield and tensile strength. Also this process may be a way of using Zn coated steel sheets in hot stamping process.

Effect of HF Welding Process Parameters and Post Heat Treatment in the Development of Micro Alloyed HSLA Steel Tubes for Torsional Applications

The aim of the present study is to investigate the effect of High frequency (HF) Electric resistance welding process parameters and post heat treatment on the torsional fatigue life of micro alloyed HSLA steel tubes. Micro alloyed grades exhibit higher strength and formability owing to the presence of fine recrystallized ferritic grains due to thermo mechanical treatment and presence of alloying elements like Vanadium, Niobium and Titanium. Welded tubular components made of micro alloyed HSLA steel grades are highly emerging and manufacturing them remains quite challenging. Weld bond width, HAZ width and bond angle are the significant factors that directly influences the Weld quality and strength. The effect of key welding parameters like Heat input, welding temperature, squeeze roll pressure, Vee-angle, Vee-length and Impeder diameter on the above mentioned significant factors was analysed. Narrow bond, Minimum HAZ width with pronounced Hour glass pattern and optimum bond angle resulted in superior bond strength and formability of HSLA tubes. Microstructural characterization of the samples was carried out using Light optical microscopy and Scanning electron microscopy. Residual stress was determined using X-ray diffract meter and tube slitting method. Higher tensile residual stress of magnitude 200 MPa was observed in the weld region. Since such high magnitude of tensile residual stress is detrimental to torsional fatigue life, stress relieving of the tubes was carried out at different subcritical temperatures 650°C and 700°C with soaking time of 45 minutes. Without significant drop in the tensile properties, compressive residual stress of magnitude 129 MPa was observed at a particular stress relieving cycle. As an effect of stress relieving heat treatment below Ac1 temperature, there is a significant improvement in the Torsional fatigue life of the HSLA steel. Thus, High Frequency welded micro alloyed HSLA steel tubes with enhanced torsional fatigue performance were successfully developed.

Effect of tempering time on the mechanical properties of P91 flux cored wire weld metal

ASME Grade 91 is one of the main materials for heavy-wall pipes in thermal power plants. Matching flux cored wires for welding P91 have already been available for several years and become more and more popular as flux cored arc welding (FCAW) offers several technical and economic advantages. To achieve high toughness of the weld metal at ambient temperature, temperature and/or time of post-weld heat treatment (PWHT) can be increased. As the temperature has to be kept below the transformation temperature Ac1, the range of suitable temperature is very small. In this contribution, the effect of increased tempering time on the mechanical properties at ambient temperature and elevated temperatures is investigated and the influence on long-term properties is discussed. Longer PWHT increases toughness of all-weld metal. MatCalc simulations and scanning electron microscopy (SEM) investigations indicate a coarsening effect of precipitates without detrimental effect on long-term creep properties.

Effects of α+γ Intercritical and γ Single-Phase Annealing on Texture Evolution in Cold-Rolled Dual-Phase Steel Sheets

Dual-phase (DP) steel sheets composed of both soft ferritic and hard martensitic phases are typical advanced high strength steel sheets applicable to a variety of automobile parts. The crystallite texture of the steel sheet is one of the important factors that influence press formability. However, the texture of the martensite itself in DP steels has not been discussed since the texture was generally measured by the X-ray diffraction method, which does not distinguish the texture of martensite from that of ferrite. The objective of this study is to investigate the effects of intercritical and γ single-phase annealing on the texture evolution in DP steels by a newly-developed analysis method using Electron Back-Scatter Diffraction (EBSD) to obtain the texture of each phase separately. The chemical composition of the steel used was 0.1%C-1.2%Si-2.3%Mn-0.1%Ti (mass%). The 1st-annealing was carried out at 948K, which is below the Ac1 temperature, in order to finish recrystallization after hot and cold rolling so as to focus on the transformation texture evolution itself. The steels were subsequently annealed both at 1123K in the intercritical region and at 1223K in the γ single-phase region to obtain DP microstructures with approximately 40% volume fraction of martensite. The overall texture including martensite in the case of intercritical annealing was similar to the initial texture before annealing, while the texture became randomized in the case of γ single-phase annealing. Moreover, our unique EBSD analysis method clearly showed that the textures of the martensite themselves were close to those of ferrite under the two annealing conditions.

Development of Armor High Strength Steel (HSS) Martensitic Plates for Troops Carriers

The present article is dealing with the conditioning of martensitic steel plates for armoring troops carriers. The steel alloy was processed into ingots. The ingots were hot flat rolled to 80 mm, followed by rolling to 6 mm. Specimens for detection of the critical transformation temperatures were taken. The Ac1, Ac3, Ms and Mf temperatures were detected as 716, 835, 356 and 218 °C respectively. The 80mm microstructure contains plate martensite colonies, while the 6mm plates reveal complete plate martensitic structure. Both cases contain embedded carbides. Heat treatment cycles were proposed for enhancing the ballistic resistance. A treatment was consisting of austentizing at 910 °C for 20 min., followed by water quenching. This process revealed fully fine lath martensitic packets. Tempering was done on the quenched as well as on rolled plates. Tempering at 350°C, causes dissociation of some martensite, and encourages migration of the entrapped carbon atoms forming carbide aggregates embedded between martensite plates packets. Tempering at 250°C for 20 min., relieves quenching induced stresses and reveals a lath martensitic structure with a small fraction of rosette like carbides. The process enhances elongation from 3% to 11.35%. The microstructure of as-rolled tempered plates at 300 oC for 20-30 minutes contains a thin ferrite layer on the boundaries surrounding the martensite packets, and few fractions of cotton wool shape carbides between the martensite packets. The treatment cycle improves the mechanical properties. Both treatments, at 250 and 300 oC, show successful ballistic resistance plates against shooting by 3 bullets.

Mechanical characterization of the welding of two experimental HSLA steels by microhardness and nanoindentation tests

The microhardness and nanohardness of the welding zone of two experimental HSLA steels were determined. The first steel has a microstructure of martensite and bainite, and the second one has a microstructure of quasipolygonal ferrite and acicular ferrite. In the bainitic - martensitic steel, softening of the heat affected zone was observed. This softening can be attributed to: the formation of polygonal ferrite in the recrystallization subzone, the formation of quasi-polygonal ferrite and the tempering of martensite in the intercritical subzone, and the tempering of martensite in the subcritical subzone. Besides the softening, with nanoindentation technique, hardening was observed at the position where the peak temperature reached the critical temperature Ac1, which can be attributed to a phenomenon of secondary hardening by precipitation of carbides of alloying elements. In the ferritic steel, a softening phenomenon did not appear since there was no martensite in its initial microstructure. Finally, it was noted that both polygonal ferrite and the bainite have similar behavior and nanohardness, this coincidence can be attributed to the effect of grain boundary.

Study of Phase Transformations in Steel X38CrMoV5-1 Using Dilatometry and Differential Thermal Analysis

The phase transformations of the X38CrMoV5-1 (AISI H11-1) steel are studied in the present work. Parts are usually manufactured by die-casting followed by a heat treatment, and the resulting microstructure as well as the mechanical properties are highly sensitive to the heat treatment conditions. The heat treatment consists on a quenching and tempering and it is not able to remove the casting dendritic microstructure or porosity but anyway, it is carried out to achieve an acceptable ductility, good hardness, and/or tensile strength in order to avoid catastrophic failures in service. These manufactured parts are used as safety braking parts. The authors use differential thermal analysis (DTA) and dilatometry techniques to obtain the phase transformation temperatures AC1 and AC3 and, therefore achieve an optimized heat treatment for this steel application. The microstructure obtained with this optimized heat treatment is totally martensitic providing the desired mechanical properties for this application.

Ferrite Decarburization of High Silicon Spring Steel in Three Temperature Ranges

Abstract Surface decarburization of high silicon spring steel in ambient air was studied. The experimental results confirmed the decarburized mechanism under AC1 temperature, in the temperature range of AC1-AC3 and AC3-G. Under AC1 temperature, pearlite spheroidization and surface decarburization are carried out simultaneously and pearlite spheroidization is reinforced. Considering the oxidation loss depth, the “true ferrite decarburized depth” at 850 °C (AC3-G) is still smaller than that at 760°C (AC1-AC3). That is because an “incubation period” must pass away before ferrite decarburization occurs in the temperature range of AC3-G, and the ferrite decarburized rate is limited to being equal to the partial decarburized rate.

Influence of tungsten on transformation characteristics in P92 ferritic–martensitic steel

Present study demonstrates by experiments and computation that a fully martensitic microstructure does not form in 9 Cr steels containing 1.9 wt% W (P92). The presence of a magnetic transition prior to the martensitic transformation during fast cooling supports the co-existence of ferrite and martensite. The formation of diffusional ferrite at high cooling rates is precluded by a systematic variation of three parameters namely cooling rate, austenitising temperature and hold times. However, complete martensite formation has not been experimentally feasible in P92 steel at all for all cooling rates (1–90 K min−1 in DSC and water quenching) employed from austenitisation temperatures (950–1150 °C), unlike in P91 steel where complete martensite forms during cooling at the rate of 30 K min−1 or higher. The experimental results have been substantiated by Thermo Calc simulation for varying W contents in P91 and P92 steels. Thermo Calc simulation revealed that P91 steel remains in the single phase γ domain during austenitisation at 950–1150 °C (1223–1423 K). It is observed that γ phase field gets shifted towards the right with addition of W in P91. P92 steel remains in the three phase domain of α + γ +MX region at 950–1150 °C (1223–1423 K), which explains the presence of ferrite above Ac3 temperatures.

Effects of Simulated Microstructure on the Creep Rupture of the Modified 9Cr-1Mo Steel

Microstructures of the heat-affected zone (HAZ) of a Gr. 91 steel weld were simulated to evaluate their effects on the creep life of the weld at elevated temperatures. The Ac1 and Ac3 temperatures of the Gr. 91 steel were determined by a dilatometer to be at 867 and 907 °C, respectively. An infrared heating system was employed to heat the samples to 860 (STOT), 900 (ICHAZ) and 940 °C (FGHAZ) for 1 min, followed by cooling to room temperature. The simulated specimens were then subjected to conventional post-weld heat treatment (PWHT) at 750 °C/2 h. After the PWHT, the tempered ICHAZ specimen had a shortest creep life among the specimens tested at 650 °C/60 MPa. Moreover, the simulated specimen heated to 860 °C (STOT) was more likely to fracture at 615 °C/80 MPa than others.