Characterization of the evolution of recrystallization by fluctuation and fractal analyses of the magnetic hysteresis loop in a cold rolled non-oriented electric steel

Silicon steels with non-oriented grains are widely used in the fabrication of electrical motor nucleus where a low magnetic loss is an important point. The performance of these motors is affected by the level of recrystallization of these steels which can come from the steel plant in a semi-processed condition. In this condition, they have a partially deformed structure and are submitted to an adequate annealing heat treatment, after reaching the end shape, to get an appropriate magnetic property. In this study, samples of an electric steel, cold rolled 50% in thickness, were withdrawn during the industrial heat treatment at temperatures of 575, 580, 600, 620 and 730 °C with the objective of evaluating the evolution of recrystallization with temperature. Magnetic properties were measured at room temperature in a vibrating sample magnetometer and the changes in magnetic hysteresis loop with temperature have been identified by using two pattern classification techniques, principal-component analysis and Karhunen-Loève (KL) expansion, associated with statistical fluctuations and fractal analyses. The fluctuation and fractal analyses were used as preprocessing tools of the series which are built from each hysteresis loop, properly renormalized, whose values correspond to the amplitudes of the loop at given equally spaced values of the renormalized field interpolated between the experimental data. The samples have been classified in four sets corresponding to different temperatures, and to samples without annealing heat treatment and recrystallized ones. The classification of the different microstructures have been obtained by applying the two pattern classification techniques to the vectors obtained from the preprocessing, and in particular a 100% success rate has been reached by using KL expansion.     

Embrittlement of a superduplex stainless steel in the range of 550–700 °C

Duplex stainless steels are very attractive materials, combining high mechanical properties with improved corrosion resistance. However, these steels present technical limitations because they can experience embrittlement as a consequence of thermal cycles. Moreover, the higher level of alloying elements (Cr and Mo) in the new duplex generation, called superduplex, accelerates the precipitation kinetics of harmful intermetallic phases, which are the responsible of embrittlement. This fact raises the question of the influence of these cycles on the actual performance of duplex components and structures and on its possible failure during service.

In order to investigate this question, heat treatments in the range of 550–700 °C, with different exposure times and cooling rates, have been made on a superduplex stainless steel type EN 1.4507. The evolution of mechanical properties has been followed by means of hardness measurements, impact and fracture toughness tests, whereas microstructural changes have been identified by using optical and transmission electron microscopy analysis. A correlation between the degree of embrittlement and the different types of precipitates has been established.     

Effect of Mg and Sr-modification on the mechanical properties of 319-type aluminum cast alloys subjected to artificial aging

Aluminum-based 319-type cast alloys are commonly used in the automotive industry to manufacture cylinder heads and engine blocks. These applications require good mechanical properties and in order to achieve them through precipitation hardening, artificial aging treatments are applied to the products. The standard artificial aging treatment for alloy 319, as defined by the T6 heat treatment temper, consists in solution heat-treating the product for 8 h at 495 °C, water quenching at 60 °C, and then artificially aging at 155 °C for 2–5 h.

The present paper reports on aging heat treatments that were performed on four Al–Si–Cu–Mg 319-type alloys: 319 base alloy, Sr-modified 319 alloy, 319 alloy containing 0.4 wt% Mg, and the Sr-modified 319 + 0.4 wt% Mg alloy. This investigation was carried out in order to examine the effect of Sr-modification and additions of Mg on the microhardness, tensile strength and impact properties of 319-type alloys over a range of aging temperatures and times (150–240 °C, for periods of 2–8 h).

The results show that the best combination of properties is found in the Sr-modified alloy containing 0.4 wt% Mg (i.e. alloy 319 + Mg + Sr). Also, the optimum artificial aging temperature changes when Mg is present in the alloy.     

An Economical Heat Treatment Cycle for IMI Ti-230

IMI Ti-230 (Ti + 2.5% Cu) is a commercial age-hardening alloy which is capable of withstanding considerable cold deformation in the solution heat treated condition. Age-hardening is associated with the coherent Ti₂Cu precipitates. IMI recommends a duplex aging treatment comprising of nucleation (first aging) at 400°C for 24 hours followed by growth (second aging) for 8 hours at 475°C.

This investigation was aimed at designing a shorter and economical heat treatment cycle for achieving properties comparable to those obtained by the standard 32 hours duplex aging treatment as recommended by IMI. In addition, the effects of cold deformation at various stages in the aging cycle on the precipitation reaction were assessed by tensile property measurements.     

On Optimizing Batch Annealing for the Production of IF Steels

Batch annealing characterized by a long heating cycle and low heating temperature is widely used to produce sheet steels including interstitial-free (IF) steels. Optimizing batch-annealing conditions would be helpful in cutting lead time and saving energy while keeping the necessary deep drawability; however, little research has been done on it. The effects of batch-annealing parameters on the deep drawability of a Ti-stabilized IF steel and Nb+Ti-stabilized IF steel have been investigated. In general, a slight improvement in r_mvalue was found with the increase of annealing temperature. Because of the higher recrystallization temperatures of Nb+Ti-IF steel, it showed higher sensitivity to annealing temperature than Ti-IF steel. In addition, greater dependency of mechanical properties and deep drawability on soaking time was observed when annealing temperature was lower. Moreover, a heat transfer analysis showed that the annealing time could be reduced by 40% if the temperature decreases from 730°C to 650°C. The industrial production has verified that the analytical results are in good agreement with the recorded annealing temperature curve. Meanwhile, it has been shown that Ti-IF steels annealed at 650°C have very close deep drawability and other tensile properties compared with these annealed at 730°C.     

Microstructural stability of the directionally solidified γ′-base superalloy IC6

The microstructural stability of an Ni---Al---Mo directionally solidified γ′-base superalloy, IC6, was investigated and the effect on microstructure and tensile properties at room temperature and 760°C of high temperature aging at 900–1100°C for up to 200 h was studied. The experimental results show that the tensile strength decreases with aging time and that aging at 1000°C has the strongest effect on tensile properties. This is attributed to the precipitation of large amounts of the δ-NiMo phase. Microstructural changes during high temperature creep tests were also examined and are different from those observed during aging at the same temperature but without any applied stress.     

Indigenous development and airworthiness certification of 15–5 PH precipitation hardenable stainless steel for aircraft applications

In this paper, we discuss the optimization of chemical composition, processing (forging and rolling) and heat treatment parameters to obtain the best combination of mechanical properties in case of a Fe–15Cr–5Ni–4Cu precipitation hardenable stainless steel. The ε-copper precipitates that form during aging are spherical in shape and coherent with the matrix and principally provide strengthening in this alloy. The orientation relationship is found to be Kurdjumov–Sachs (K–S), which is common in fcc–bcc systems. Results obtained from metallurgical evaluation (mechanical property and metallography) on 15–5 PH alloy during type certification on 3 different melts were used for the optimization, attempted in this study. The mechanical properties following strain deformation has been carried out using optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). In the aged conditions, the 15–5 PH alloy exhibited brittle failure with extensive cleavage and/or quasicleavage fracture. This paper reports all results and also factually shows that indigenously developed and produced 15–5 PH stainless steel matches in its properties with the equivalent aeronautical grade precipitation hardening stainless steels globally produced by internationally renowned manufactures.     

Effect of microstructure on the fatigue of hot-rolled and cold-drawn NiTi shape memory alloys

We present results from a systematic study linking material microstructure to monotonic and fatigue properties of NiTi shape memory alloys. We consider Ni-rich materials that are either (1) hot rolled or (2) hot rolled and cold drawn. In addition to the two material processing routes, heat treatments are used to systematically alter material microstructure giving rise to a broad range of thermal, monotonic and cyclic properties. The strength and hardness of the austenite and martensite phases initially increase with mild heat treatment (300 °C), and subsequently decrease with increased aging temperature above 300 °C. This trend is consistent with transmission electron microscopy observed precipitation hardening in the hot-rolled material and precipitation hardening plus recovery and recrystallization in the cold-drawn materials. The low-cycle pseudoelastic fatigue properties of the NiTi materials generally improve with increasing material strength, although comparison across the two product forms demonstrates that higher measured flow strength does not assure superior resistance to pseudoelastic cyclic degradation. Fatigue crack growth rates in the hot-rolled material are relatively independent of heat treatment and demonstrate similar fatigue crack growth rates to other NiTi product forms; however, the cold-drawn material demonstrates fatigue threshold values some 5 times smaller than the hot-rolled material. The difference in the fatigue performance of hot-rolled and cold-drawn NiTi bars is attributed to significant residual stresses in the cold-drawn material, which amplify fatigue susceptibility despite superior measured monotonic properties.     

Strain aging and carbide precipitation in aluminium-killed steels

Abstract

The effect of 0·003–0·050 wt-%C additions on strain aging and carbide precipitation in batch-annealed aluminium–killed steels has been investigated. The low–carbon steels exhibited unusual strain–aging characteristics: steels containing 0·020–0·040 wt–%C strain aged at 160 but not at 50°C, whereas those containing 0.007–0·020 wt–%C aged at both temperatures. This behaviour is due to the presence of fine carbide particles, which form as a result of the supersaturation of carbon in the matrix during cooling and which raise the carbon solubility by the Thomson-Freundlich effect. Expressions are derived which give the carbon supersaturation as a function of temperature during cooling, and of the precipitation temperature of each class of carbide. Carbide precipitation alternates between the nucleation of new particles and the growth of existing ones, depending upon the particle spacing and the cooling rate. Strain aging in the 0·05 wt–%C steel is negligible up to 225°C, but then rises to a peak at 325°C, which has been attributed to effects of partition in the dislocation matrix.

MST/141     

Precipitation and hot deformation behavior of austenitic heat-resistant steels: A review

The austenitic heat resistant-steels have been considered as important candidate materials for advanced supercritical boilers, nuclear reactors, super heaters and chemical reactors, due to their favorable combination of high strength, corrosion resistance, perfect mechanical properties, workability and low cost.Since the precipitation behavior of the steels during long-term service at elevated temperature would lead to the deterioration of mechanical properties, it is essential to clarify the evolution of secondary phases in the microstructure of the steels. Here, a summary of recent progress in the precipitation behavior and the coarsening mechanism of various precipitates during aging in austenitic steels is made. Various secondary phases are formed under service conditions, like MX carbonitrides, M_(23)C_6 carbides, Z phase, sigma phase and Laves phase. It is found that the coarsening rate of M_(23)C_6 carbides is much higher than that of MX carbonitrides. In order to understand the thermal deformation mechanism, a constitutive equation can be established, and thus obtained processing maps are beneficial to optimizing thermal processing parameters, leading to improved thermal processing properties of steels.     

Quality control of heat treatment of oil and gas pipes using magnetic structuroscopy

The problem of the development of in-line nondestructive testing of mechanical properties of heat-strengthened oil and gas steel pipes remains topical. In this work, the effect of heat treatment conditions represented by the quenching temperature (in the range of 765–980°C) and cooling rate from the quenching temperature (in the range of 500–720°C, after quenching in water) on the structure, hardness, and magnetic properties (coercive force, residual induction, maximum magnetic permittivity, and saturation magnetization) is studied for the widely used pipe steels of 22KhG2A, 30KhMA, and 32G2 brands. The magnetic parameters are measured both in a closed magnetic circuit and using the attached magnetic devices on model and full-scale samples. A possible application of the coercimetric method as an alternative for the durametric one is shown for continuous underheating testing for quenching and the tempering quality of the pipes made from the steels under study during fabrication.     

Static strain ageing behaviour of dual phase steels

In this work an investigation was conducted into the cold deformation ageing susceptibility of a carbon steel and a microalloyed steel, both with dual phase micro-structure. Ageing experiments after different prestrains were carried out at temperatures ranging from 25 to 250 °C. It was found that yield strength (YS) and tensile strength (UTS) of the steels with different dual phase micro-structures exhibit maximum values at ageing temperature of 100 °C after different prestrains. It is assumed that the first rise is based on the formation of solute atom atmospheres around dislocations and the further strengthening in the second step is caused by the low-temperature carbide precipitation in ferrite. When the ageing temperature increased to 150, 200 or 250 °C, YS decreased due to tempering effect in martensite. It was also found that the ageing of the microalloyed steel occurred more slowly than that of the carbon steel. The slow occurrence of ageing was clearly observed at temperatures of 100, 150, 200 and 250 °C and was attributed to the chemical composition of the steels.     

马氏体时效不锈钢热处理工艺优化

利用热力学计算软件Thermo-Calc,研究了马氏体时效不锈钢Fe-13Cr-7Ni-4Mo-4Co-2W在不同温度下的基体组织和析出相的变化.通过TEM、SADP法分析研究了马氏体时效不锈钢在固溶处理与时效处理过程中显微组织与析出行为.热力学计算与实验研究结果一致表明,马氏体时效不锈钢高温析出Laves-Fe2Mo相,固溶温度超过1050℃,Laves-Fe2Mo相全部溶解;时效析出R相,其含量在8%左右.根据计算结果优化了相应的热处理工艺,力学性能研究结果表明,用所确定的时效工艺进行热处理后,马氏体时效不锈钢的强韧性最好.     

Effect of cryogenic treatment on tensile behavior of case carburized steel-815M17

The crown wheel and pinion represent the most highly stressed parts of a heavy vehicle; these are typically made of 815M17 steel. The reasons for the frequent failure of these components are due to tooth bending impact, wear and fatigue. The modern processes employed to produce these as high, durable components include cryogenic treatment as well as conventional heat treatment. It helps to convert retained austenite into martensite as well as promote carbide precipitation. This paper deals with the influence of cryogenic treatment on the tensile behavior of case carburized steel 815M17. The impetus for studying the tensile properties of gear steels is to ensure that steels used in gears have sufficient tensile strength to prevent failure when gears are subjected to tensile or fatigue loads, and to provide basic design information on the strength of 815M17 steel. A comparative study on the effects of deep cryogenic treatment (DCT), shallow cryogenic treatment (SCT) and conventional heat treatment (CHT) was made by means of tension testing. This test was conducted as per ASTM standard designation E 8M. The present results confirm that the tensile behavior is marginally reduced after cryogenic treatment (i.e. both shallow and deep cryogenic treatment) for 815M17 when compared with conventional heat treatment. Scanning electron microscopic (SEM) analysis of the fracture surface indicates the presence of dimples and flat fracture regions are more common in SCT specimens than for CHT and DCT-processed material.     

Influence of silver on structure and properties of low-carbon steel

Abstract

Silver has only slight solubility in low–carbon steel at elevated temperatures (~ 0·08 wt-% at the solidus) and precipitates on cooling as fine particles (< 10 nm) in the interphase mode. When present in as-cast steel at an estimated volume fraction of ~ 0·0006, silver can cause precipitation strengthening of up to 50 MN m^?2. In as–rolled and in normalized steels, silver (0·02 wt-%) improves both strength and toughness mainly through grain refinement, with only a small influence from precipitation hardening. An important observation is the improvement to the heat affected zone toughness of high heat input welds which can result from small silver additions (0·02 wt-%) to C–Mn–AI–V steels. However, because of its relatively high cost, it remains to be demonstrated whether silver will impart sufficient benefit to steel properties to be commercially acceptable as an alloying addition.

MST/44     

Failures Induced by Abnormal Banding in Steels

In industrial applications failures of mechanical parts made of carbon and alloyed steels may develop either during heat treatment steps or final finishing operations. Such failures have high impact costs for manufacturers, since heat treated steel products, in general, are high value products which increase in value with each step in the production process until the final life-cycle manufacturing steps are completed. This work highlights the selection of steels to avoid premature ruptures developing during either the heat treatment steps or finishing operations with emphasis on the role of banding in the failure process. Failure does not have to involve fracture but may simply imply a decrease in performance of surface treated components as consequence of surface properties, even in the presence of correct heat treatment parameters. The root causes for banding in steels, are described in literature, and banding has major effects on final product properties (and causal relationships). Therefore, the causes of banding are studied and classified. Conclusions suggest that microstructural defects such as (micro)segregation bands and other defects such as slag and oxides inclusions are developed in the early fabricating cycle steps and can cause premature failure of either semi-finished or finished products.     

Reverted austenite in PH 13-8 Mo maraging steels

The mechanical properties of maraging steels are strongly influenced by the presence of reverted austenite. In this study, the morphology and chemical composition of reverted austenite in a corrosion resistant maraging steel was characterized using transmission electron microscopy (TEM) and atom probe tomography (APT). Two types of austenite, i.e. granular and elongated, are present after aging at 575 °C, whereby the content of the latter increases during aging. The investigations revealed that the austenite phase is enriched in Ni, which prevents the transformation to martensite during cooling. Inside and next to the austenitc areas, Mo and Cr-rich carbides, which form during the aging treatment, were found. Various aging treatments were performed to obtain the activation energy for the formation of reverted austenite. Additionally, the experimental data are compared with thermodynamic and kinetic simulations. Based on these results and the chemical composition changes of the phases, a model for the formation of reverted austenite is presented. It is concluded that precipitation of B2-ordered NiAl and formation of reverted austenite take place simultaneously during aging and that dissolution of precipitates is not essential for the initial formation of reverted austenite.     

Comparative study of precipitation effects during aging in superaustenitic and superferritic stainless steels

Abstract

The concept of designing a steel that would match high corrosion resistance coupled with improved mechanical properties when exposed at elevated temperatures has always been a challenge for a metallurgist. The present paper relates the results of microstructural analysis of two commercial superaustenitic stainless steel grades and a novel experimental grade of superferritic stainless steel. A study of the microstructural stability and attendant mechanical property evolution has been carried out, employing a variety of aging experiments. Following prolonged aging up to 1000 h in the temperature range between 650 and 950°C, microstructure evolution was determined, while the mechanical properties were preliminary assessed via hardness testing. The present study helped clarify the effect of high temperature exposure on the precipitation sequences of both superaustenitic and novel superferritic stainless steels. The heat treatments performed indicate a number of precipitation sequences of intermetallic phases taking place, such as σ phase formation, clearly specifying a time–temperature regime where care must be taken during the fabrication and use of these materials.     

低温液体泵主轴材料的性能试验研究

通过对试验用钢与对比用钢的性能试验,探讨了在规定的热处理状态下马氏体沉淀硬化不锈钢OCr17Ni4Cu4Nb的室温和低温力学性能;指出对0Cr17Ni4Cu4Nb进行固溶和双时效处理是获得最佳低温性能的有效途径;得出了低温液体泵主轴选材应优先选用0Cr17Ni4Cu4Nb钢的结论