Showing papers on "Tempering published in 1997"

Influence of the amount and morphology of retained austenite on the mechanical properties of an austempered ductile iron

Abstract: High Si contents in nodular cast irons lead to a significant volume fraction of retained austenite in the material after the austempering treatment. In the present work, the influence of the amount and morphology of this phase on the mechanical properties (proof stress, ultimate tensile strength (UTS), elongation, and toughness) has been analyzed for different austempering conditions. After 300 °C isothermal treatments at intermediate times, the austenite is plastically stable at room temperature and contributes, together with the bainitic ferrite, to the proof stress and the toughness of the material. For austenite volume fractions higher than 25 pct, the proof stress is controlled by this phase and the toughness depends mainly on the stability of γ. In these conditions (370 °C and 410 °C treatments), the present material exhibits a transformation-induced plasticity (TRIP) effect, which leads to an improvement in ductility. It is shown that the strain level necessary to initiate the martensitic transformation induced by deformation depends on the carbon content of the austenite. The martensite formed under TRIP conditions can be of two different types: “autotempered” plate martensite, which forms at room temperature from an austenite with a quasi-coherent epsilon carbide precipitation, and lath martensite nucleated at twin boundaries and twin intersections.

Modelling precipitation sequences in power plant steels Part 1 – Kinetic theory

Abstract: The ability of steels to resist creep deformation depends on the presence in the microstructure of carbides and intermetallic compounds which precipitate during tempering or during elevated temperature service. The precipitation occurs in a sequence which leads towards thermodynamic equilibrium. The present paper deals with an extension of the Johnson-Mehl-Avrami theory for overall transformation kinetics. The modification permits the treatment of more than one precipitation reaction occurring simultaneously, afeature which isfound to be essential for representing the reactions observed experimentally in a wide range of secondary hardening steels.

Deformation, fracture, and mechanical properties of low-temperature-tempered martensite in SAE 43xx steels

Abstract: Uniaxial tensile tests were performed on 4330, 4340, and 4350 steels in the as-quenched (AQ) condition and after quenching and tempering at 150 °C, 175 °C, and 200 °C for times of 10 minutes, 1 hour, and 10 hours, respectively. Strength parameters decreased and ductility parameters increased continuously with increasing tempering. Mechanical properties are presented as a function of tempering conditions and steel carbon content, and hardness and ultimate strength changes are given as a function of Hollomon—Jaffe tempering parameters. All tempered specimens, except for some lightly tempered 4350 specimens, deformed plastically through necking instability and failed by ductile fracture. The stresses required for the ductile fracture, estimated from an analysis of the interfacial stresses at particles in the neck at fracture, showed no systematic variation with carbon content or tempering conditions despite significant variations in deformation and strain hardening. The AQ specimens of the 4340 and 4350 steels, and some of the lightly tempered 4350 steels, failed by brittle mechanisms. The deformation and fracture of the low-temperature-tempered 43xx steels are discussed in terms of the changes in fine structure, namely, the formation of transition carbides and a rearranged dislocation substructure that evolve from an AQ martensitic substructure consisting of dislocations with and without carbon atom segregation.

Influence of microstructure on fracture toughness of austempered ductile iron

Abstract: An investigation was carried out to examine the influence of microstructure on the plane strain fracture toughness of austempered ductile iron. Austempered ductile iron (ADI) alloyed with nickel, copper, and molybdenum was austenitized and subsequently austempered over a range of temperatures to produce different microstructures. The microstructures were characterized through optical microscopy and X-ray diffraction. Plane strain fracture toughness of all these materials was determined and was correlated with the microstructure. The results of the present investigation indicate that the lower bainitic microstructure results in higher fracture toughness than upper bainitic microstructure. Both volume fraction of retained austenite and its carbon content influence the fracture toughness. The retained austenite content of 25 vol pct was found to provide the optimum fracture toughness. It was further concluded that the carbon content of the retained austenite should be as high as possible to improve fracture toughness.

Calculation and experimental determination of dimensions of hardening and tempering zones in quenched U7A steel irradiated with a pulsed electron beam

Abstract: Based on metallographic data and microhardness depth distributions measured in preliminarily quenched and tempered U7A steel 1 (0.7% C) after irradiation with an electron beam of duration 10 −5 s, electron energy 170–180 keV and energy density 40–150 J/cm 2 , the dimensions of the hardened and tempered zones have been determined. The experimental results are compared with those obtained by solving numerically the heat equation taking into account the processes of melting, evaporation, and crystallization. Good agreement between experimental and theoretical data is observed.

Aging and Tempering Behavior of Iron-Nickel-Carbon and Iron-Carbon Martensite

Abstract: The aging at room temperature (RT) and the tempering behavior in the temperature range 293 to 973 K of ternary iron-nickel-carbon martensite (containing 14.4 at. pct Ni and 2.35 at. pct C) was investigated principally by using X-ray diffractometry to analyze changes in the crystalline structure and differential scanning calorimetry to determine heats of transformation and activation energies. These techniques also were used in the parallel study performed in this work of the tempering behavior of FeC martensite (containing about 4.4 at. pct C) in the temperature range 298 to 773 K. Analysis of the structural changes revealed that in both FeNiC and FeC the following processes occurred: (1) formation of carbon enrichments and development of a periodic arrangement of planar carbon-rich regions up to 423 K; (2) precipitation of e/η transition carbide and transformation of a part of the austenite into ferrite under simultaneous enrichment with carbon of the remaining austenite (between 423 and 523 K); (3) decomposition of the retained austenite into ferrite and cementite between 523 and 723 K (only partly for FeNiC); (4) precipitation of cementite between 523 and 723 K; and (5) for FeNiC, reformation of austenite from ferrite and cementite above 773 K. A short comparative discussion concerning the first stage of martensite decomposition for FeC, FeNiC, FeN, and FeNiN martensites is given.

Ultra high strength, secondary hardening steels with superior toughness and weldability

Abstract: High strength steel is produced by a first rolling of a steel composition, reheated above 1100 °C, above the austenite recrystallization, a second rolling below the austenite recrystallization temperature, water cooling from above AR to less than 400 °C and followed by tempering below the AC transformation point. The steel comprises predominantly martensite/bainite phase containing precipitates of the carbides, nitrides, or carbonitrides of vanadium, niobium, and molybdenum, wherein the concentrations of vanadium +niobium ⊃ 0.1 wt.%, the carbon content ranges from about 0.03 to 0.12 wt.%, and chromium is present in amounts ranging from 0.3-1.0 wt.%.

Method and apparatus for carburizing, quenching and tempering

Abstract: Method and apparatus for carburizing, quenching and tempering workpieces, by which the step of carburizing can be incorporated into a machining and manufacturing process to be performed by an on-line machining and manufacturing system as one stage thereof, and by which high-quality treatment or processing can be achieved without occurrences of variation in quality among heat-treated products. In this apparatus, a preheating chamber (2), six carburizing chambers (5a to 5f), a cooling chamber (6), a reheating chamber (7), a quenching chamber (8) are placed around an intermediate chamber (4) having a transport device (12). Further, a high-frequency induction heating technique is used for heating each of major ones of the treatment or processing chambers. Moreover, a plasma carburizing method is performed in the carburizing chambers. Furthermore, workpieces, which are successively carried into this apparatus through a machining and manufacturing line, are assigned and distributed to the chambers, respectively, and are then treated or processed therein. Thus, the process of carburizing, quenching and tempering can be performed without stagnation.

Effect of the Ti/N ratio on the hardenability and mechanical properties of a quenched-and-tempered C-Mn-B steel

Abstract: Ten experimental 0.18 pct C-1.2 pct Mn- 0.002 pct B steels with various Ti/N ratios were evaluated in this study. The hardenability of these steels was first determined using Jominy tests. Slab sections were then rolled to produce 12.5-mm-thick plates, and subsequently quenched and tempered for mechanical property evaluation. The volume fraction of coarse (greater than 1 µm) TiN particles was measured in all steels using quantitative metallographic techniques. Scanning transmission electron microscopy was used to investigate fine precipitates, and scanning electron microscopy was used to examine the fracture surface of Charpy specimens. The results show that a complete boron (B) hardenability effect is obtained with Ti/N ratios ≥2.9, a value slightly below the stoichiometric Ti/N ratio of 3.4. Any excess Ti, above that which combines with N, provides an additional increase in hardenability on quenching (effect of Ti in solution) and an increase in strength on tempering (Ti (C,N) precipitation). Steels with a higher (Ti)(N) product develop a higher volume fraction of coarse TiN particles during solidification. These coarse TiN particles result in reduced toughness levels of the heat-treated plates evaluated in the present study.

High surface pressure resistant steel parts and methods of producing same

Abstract: High surface pressure resistant steel parts and their producing methods are disclosed. These steel parts are useful as gears, cams, bearings and similar high-strength compact steel articles which are required to have wear resistance and strength to withstand fatigue in rolling or rolling-slipping applications. In a steel part formed according to the invention, a fine nitride and/or carbonitride having at least an average grain size of 0.3 μm or less is dispersed in the contact surface structure; a multi phase structure composed of martensite, which is divided into extremely fine pieces, forming a disordered shape, by the nitride and/or carbonitride, is formed; and a carbide having a grain size of 3 μm or less is dispersed to increase the hardness of the surface. Such a steel part is produced by carrying out carbonitriding or carburization/carbonitriding so as to precipitate extremely fine AIN, using nitrogen permeating from the surface and by carrying out quenching or quenching/tempering, starting from a temperature region where the parent phase is austenite

The effect of strontium on the Mg2Si precipitation process in 6201 aluminum alloy

Abstract: A transmission electron microscopy (TEM) study of a 6201 aluminum alloy to which controlled strontium additions were made has revealed important differences compared to the same alloy free of strontium. In the as-cast state, strontium favors the formation of α-AlFeSi (Al8Fe2Si) rather than β-AlFeSi (Al5FeSi) phase, resulting in a greater quantity of excess silicon present in the strontium-treated alloy. During heat treatment, the excess silicon allows a greater density of finer β″-Mg2Si precipitates to form, leading to increased tensile strength values and increased electrical resistivity. Strontium also retards the growth of the precipitates formed during heat treatment and inhibits formation of the equilibrium β-Mg2Si phase. As a result, the strontium-treated alloy exhibits a resistance to overaging.

Microstructure and character of laser remelting of plasma sprayed coating (Ni-Cr-B-Si) on Al-Si alloy

Abstract: In this paper, a plasma sprayed coating (Ni-Cr-B-Si) on an Al-Si alloy surface was remelted by a 5 kW CO 2 laser. Structure and chemical composition of different parts of the laser melted zone were investigated. Hardness distribution in the laser melted zone was measured. Experimental results showed that chemical composition of the sample is not a well-distributed gradient because there is compositional segregation in the laser melted zone. In the surface of the laser-melted zone, a concentration of aluminium exists; and there is a concentration of Fe, Cr and Ni in the sub-surface of this zone. Corresponding to chemical compositional segregation, a aluminium-rich phase Al 3 Ni emerges in the surface and the nickel-rich phases AlNi and AlNi 3 are found in the sub-surface. There are some amorphous regions with super-hardness in the sub-surface of the laser-melted zone. Owing to the tempering effect of laser scanning heat transfer, many nano-crystallites of AlNi 3 separate out from the amorphous structure. The hardness distribution in the laser-melted zone corresponds with the structure and solidification conditions. The highest hardness is in the amorphous and nano-crystallite regions. The higher hardness is in the Al + Al 3 Ni eutectic structure region that is in the rapidly solidified surface and is in the fine needle-like Al 3 Ni 2 structure region that is adjacent to the amorphous region. The thick needle-like Al 3 Ni 2 structure has lower hardness.

Process and plant for hardening glass plates

Abstract: A system for tempering glass plates has preheating zone (1) in which several glass plates (7) stacked vertically in compartmented car (6) are heated together to a temperature below the tempering temperature of for example 650° C. (for example 300° C.). Preheated glass plates (7) are moved individually from preheating zone (1) into heating zone (2). In heating zone (2) glass plates (7) are heated to the tempering temperature, their being inclined to the vertical at an acute angle and being held by air cushions between two heating plates. On the lower edge of the two heating plates of heating zone (2) there is transport device which supports glass plates (7) simultaneously to the bottom. Glass plates (7) heated to the tempering temperature are moved into cooling zone (3) which has cooling plates aligned parallel to the heating plates and between which glass plate (7) is pushed for quenching. The cooling plates can also be placed on the two sides of glass plate (7) to be tempered for purposes of quenching. The quenched glass plates which still have a temperature of for example 300° C., are transported into after-cooling zone (4) in which they are slowly cooled to room temperature after being deposited in compartmented cars (6). Because glass plates (7) are placed in heating zone (2) and cooling zone (3) obliquely, not horizontally, there are no problems with support and transport of glass plates (7). Joint preheating of glass plates (7) and joint cooling of glass plates (7) to room temperature after quenching thereof save considerable energy since the heating and cooling power in preheating zone (1) and aftercooling zone (4) can be low.

Microstructure and mechanical properties of a 10% chromium steel with improved creep resistance at 600°C

Abstract: A powder metallurgical 10%Cr steel containing molybdenum, tungsten, vanadium, niobium, and boron was studied. Results from tensile, impact, and creep rupture tests are reported. The alloy studied had a considerably higher creep resistance than the standard steel X 21 CrMoV12 1. After tempering, the microstructure consisted of tempered martensite with M23 C6 and M2N precipitates at grain and lath boundaries and inhomogeneously distributed disc shaped fine MX precipitates a few nanometers thick and 5–20 nm in diameter. Boron was found inside the M23 C6 precipitates after tempering and after creep testing at 600°C. Laves phase (Fe,Cr)2(Mo,W) precipitated at grain and lath boundaries during creep. Atom probe analyses of the matrix in the tempered and creep tested material indicated the presence of very small precipitates rich in chromium, vanadium, iron, and nitrogen. A latent creep resistance of the material, giving rise to a dynamic nucleation of small precipitates at dislocations, is suggested.

Production of high strength and high corrosion resistance seamless steel pipe

Abstract: PROBLEM TO BE SOLVED: To obtain a high strength high corrosion resistance seamless steel tube, which is excellent in SSC resistance, has a high yield point and is usable for an oil well and its related equipment by subjecting a billet having a specified element composition to piercing/stretch rolling under a specified condition and then direct quenching/tempering after auxiliary heating treatment. SOLUTION: A billet has a composition consisting of, by weight, 0.2-0.35% C, 0.05-0.5% Si, 1-5% Mn, 0.2-1% Mo, 0.005-0.5% Ti, 0.0001-0.1% B, 0.005-0.1% Nb, 0.005-0.5% sol. Al, others of Cr, V, W, Zr, Ca and the balance Fe with inevitable impurities, further P, S, Ni, O in the impurities of specified quantity or lower. When piercing/stretch rolling the billet in hot, after the billet, in a final finish rolling, is rolled at a thickness reduction ratio of >=40% in a range of 850-1100 deg.C, an auxiliary treatment to heat/hold in the temp. range for 10 sec-30 min is conducted. Further direct quenching is conducted and then tempering.

The Susceptibility to the Hydrogen Embrittlement of Low Alloy Cr and CrMo Steels

Abstract: Low alloy Cr and CrMo steels have been studied to determine their susceptibility to hydrogen embrittlement. The steels were quenched at a constant cooling rate of 30 Ks -1 and tempered at 673 and 973 K. Hydrogen charging of steels was carried out in 0.5 M H 2 SO 4 solution without and with addition of propargylic alcohol as corrosion inhibitor. In spite of the current density of 2-3 mA cm -2 and the applied load of 40% yield strength was found that the steels tempered at 673 K were highly sensitive to hydrogen embrittlement. The steels tempered at 973 K showed significantly higher resistance to hydrogen embrittlement. In spite of higher strength, the CrMo steel showed higher resistance to hydrogen embrittlement than the Cr steel. The fracture of steels was initiated at the second phase particles followed by transgranular fracture from iron carbide or iron-manganese sulphide inclusions. Since the inclusion distributions are similar in both steels, the enhanced resistance of CrMo steel to hydrogen embrittlement is due to finer prior austenite grain and presence of MO 2 C in the ferrite matrix. With the addition of propargylic alcohol in 0.5 M H 2 SO 4 solution the resistance of steels to hydrogen embrittlement was increased and fracture surface showed mostly dimple fracture.