Showing papers on "Tempering published in 1999"

Microstructural Evolution in a 17-4 PH Stainless Steel after Aging at 400°C

Abstract: The microstructure of 17-4 PH stainless steel at various stages of heat treatment, i.e., after solution heat treatment, tempering at 580 °C, and long-term aging at 400 °C, have been studied by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). The solution-treated specimen consists largely of martensite with a small fraction of δ-ferrite. No precipitates are present in the martensite phase, while spherical fcc-Cu particles are present in the δ-ferrite. After tempering for 4 hours at 580 °C, coherent Cu particles precipitate in the martensite phase. At this stage, the Cr concentration in the martensite phase is still uniform. After 5000 hours aging at 400 °C, the martensite spinodaly decomposes into Fe-rich α and Cr-enriched α′. In addition, fine particles of the G-phase (structure type D8a, space group Fm\(\bar 3\)m) enriched in Si, Ni, and Mn have been found in intimate contact with the Cu precipitates. Following spinodal decomposition of the martensite phase, G-phase precipitation occurs after long-term aging.

The effects of shear and temperature history on the crystallization of chocolate

Abstract: Experiments have been carried out on the tempering of chocolate using a temperature-controlled shearing rig with a concentric cylinder geometry. This design maximizes uniformity of shear rate during tempering in contrast to most tempering devices where shear is often concentrated in a small part of the chocolate mass. Samples were subsequently cooled in a differential scanning calorimeter (DSC) to monitor how the sample crystallized, and then reheated to gain further information on the melting points of the polymorphs formed. The results can be interpreted using established theories on the crystallization mechanism. It was found that at least two polymorphic forms could be generated. The higher melting form predominated provided the shear rate was high enough, the temper time long enough, the rewarm temperature low enough, and the DSC scan rate slow enough. For parameters affecting the nucleation stage (temper time and shear rate), it was found that the transition was a sharp one, reflecting the notion that seed crystals need to grow past a threshold point in order to be stably formed. Raising the rewarm temperature had the effect of destroying seed nuclei, but this was a more gradual process. The bimodal nature observed of the melting points compared to the more spread-out behavior of crystallization temperatures reflects the kinetic constraints found in crystallization which are not found on melting.

Light microscopy of carbon steels

Abstract: Containing over 1,200 representative micrographs and the information and explanatory text that makes them really useful, including composition, condition, etchant, magnification, and more than 100 graphs and tables, this 'how to' book not only gives everyday working examples, but also discusses the relationship between the constitution, metallurgy, and microstructure of various carbon steel products. Contents: Nomenclature of Phases and Constituents; Phase Transformations; Low-Carbon Irons and Steels; Annealing and Normalizing; Spheroidization and Graphitization; Austenitization; Transformation of Austenite; Tempering of Martensite; Welding; Surface Oxidation, Decarburation and Oxidation Scaling; Glossary of Terms; EtchingMethods; ConversionTables; Index.

Boron distribution in 9–12% chromium steels

Abstract: Martensitic 9–12% chromium steels are used in steam turbine power plants for large components such as turbines and pipes. The key property of these materials is long-term creep resistance at elevated temperatures (up to 600–650°C). Extensive alloy development has taken place worldwide over the last 20 years to improve the high-temperature properties of the 9–12% chromium steels. An alloying strategy that has proven to be very effective for increased creep strength is the addition of small amounts of boron. The mechanism for improved high temperature stability in this case is not yet fully understood. In this work a number of recently developed 9–12% chromium steels have been investigated with the atom probe technique to clarify the distribution of boron in the microstructure. It was found that only very small amounts of boron remain dissolved in the matrix after completed heat treatment (solution treatment and tempering). Instead almost all of the boron ends up in M23C6 carbides, possibly decreasing the coarsening rate of these precipitates.

Intergranular Corrosion in a Martensitic Stainless Steel Detected by Electrochemical Tests

Abstract: Quenched and tempered martensitic stainless steel UNS S41000 was tested electrochemically for susceptibility to sensitization in specimens quenched from 975°C (1,248 K) and tempered for 2 h at different temperatures between 300°C (573 K) and 700°C (973 K). Besides an oxalic acid etch test, the following tests were performed using a potentiostat and 1 N sulfuric acid (H2SO4) solution: potentiokinetic polarization, potentiostatic etch, and electrochemical potentiokinetic reactivation in the double-loop version (DL-EPR). Tested surfaces were observed metallographically. The maximum susceptibility to intergranular corrosion was observed in the condition tempered at 550°C (823 K), and a sensitized structure was detected. For lower tempering temperatures, steel was less sensitized, or not at all, and for higher tempering temperatures, steel was less sensitized. All tests except for the oxalic acid etch were able to evaluate quantitatively different degrees of sensitization as a function of tempering te...

Load-Bearing Capacity of Tempered Structural Glass

Abstract: This paper presents a method for the failure analysis of structural glass components of buildings. Structural glass is generally prestressed by tempering. In the method, residual stresses are first computed by simulation of this tempering process of soda-lime-silica glass plates using the finite-element method. The model includes both stress and structural relaxations. The edge effects of tempering are modeled. Then, the failure strength of annealed glass is obtained by a statistical analysis of tests performed on small specimens. Loading rate effects are also taken into account. Computational results are associated with the statistical approach for the failure prediction of large annealed and tempered glass plates. This prediction method is validated by experimental results from four-point bending tests up to failure.

Oil well steel excellent in toughness and sulfide stress corrosion cracking resistance, and its manufacture

Abstract: PROBLEM TO BE SOLVED: To provide a high strength oil well steel suitable for thick-walled products, excellent in toughness and SSC resistance, and having YS of 110 ksi class or above, and inexpensively manufacturing method therefor. SOLUTION: This steel is a low alloy steel which has a composition containing 0.15-0.3% C, 0.2-1.5% Cr, 0.1-1% Mo, 0.05-0.3% V, and 0.003-0.1% Nb and in which the total amount of precipitating carbides is 1.5-4% and an MC carbide comprises 5-45% of the total amount of the carbides, and further, the proportion of an M23C6 carbide is =1150 deg.C and to hot working at >=1000 deg.C finishing temperature, applying direct quenching from >=900 deg.C and tempering at a temperature between 550 deg.C and the Ac1 transformation point, and subjecting the steel to reheating up to 850-1000 deg.C and quenching and then to tempering at a temperature between 650 deg.C and the Ac1 transformation point at least once.

Microstructural evolution during tempering of a multiphase steel containing retained austenite

Abstract: Double cycle heat treatment has been imparted to a low carbon low alloy steel with a view to maximize volume fraction of retained austenite. e-Carbide has been found to occur in the austenite phase. Its unusual presence has been explained as a result of enrichment by interstitials during isothermal holding. The steel has been subjected to low temperature ageing treatment to assess the stability of retained austenite against thermal severity. Retained austenite phase remained stable up to a temperature of 100°C. Decomposition of the austenitic phase having predominantly chunky morphology commenced at and above 200°C. Tempering at a temperature of 300°C led to the rearrangement of dislocations into parallel arrays. The mismatch between parent austenite and product ferrite phase resulted in the formation of interfacial dislocations. Upon tempering at 400°C, e-carbide gave way to the formation of e′(η)-carbide. The observation is extraordinary in view of low carbon content of the steel. Tempering at 500°C eventually led to the formation of stable cementite. Thus a sequence of events leading to the precipitation of cementite (θ) from parent austenite phase can be traced as: γ R →α+e→α+e′ η →α+θ

Modelling of phase transformation kinetics in steels and coupling with heat treatment residual stress predictions

Abstract: One keypoint for the prediction of heat treatment residual stresses is the modelling of phase transformation kinetics and of their consequences on the thermomechanical behaviour of the material. In that paper, we focus on recent extensions of our model for calculating phase transformation kinetics in steels mainly for taking into account the chemical composition variation of the steel and the tempering/selftempering kinetics. This model is coupled to the calculation of temperature. stress and strain evolutions in a massive specimen submitted to chemical composition gradients (here carbon gradients). The consequences of the stress-phase transformation interactions on the microstructure and stress evolutions during cooling are analysed in the specific case of gaz quenching of a case hardened specimen. It is shown that the internal stresses can have a large effect on the bainitic transformation kinetics and on the stress evolutions during cooling but the effect on the residual stress state remains relatively small.

Effect of single and double austenitization treatments on the microstructure and mechanical properties of 16Cr-2Ni steel

Abstract: Double austenitization (DA) treatment is found to yield the best combination of strength and toughness in both low-temperature as well as high-temperature tempered conditions as compared to single austenitization (SA) treatments. Obtaining the advantages of double austenitization (DA) to permit dissolution of alloy carbides without significant grain coarsening was attempted in AISI 431 type martensitic stainless steel. Structure-property correlation after low-temperature tempering (200 °C) as well as high-temperature double tempering (650+600 °C) was carried out for three austenitization treatments through SA at 1000 °C, SA at 1070 °C, and DA at 1070+1000 °C. While the increase in strength after DA treatment and low-temperature tempering at 200 °C is due to the increased amount of carbon in solution as a result of dissolution of alloy carbides during first austenitization, the increased toughness is attributable to the increased quantity of retained austenite. After double tempering (650+600 °C), strength and toughness are mainly found to depend on the precipitation and distribution of carbides in the microstructure and the grain size effect.

Effect of Trace Elements on Creep Properties of 0.06C-2.25Cr-1.6W-0.1Mo-0.25V-0.05Nb Steel

Abstract: The effect of trace elements such as Mn and B on creep properties of 0.06C-2.25Cr-1.6W-0.1Mo-0.25V-0.05Nb has been investigated from the standpoints of a long-term microstructural stability. The chemical analysis of extracted residues and TEM observation show that the M23C6 carbide and/or M7C3 carbides, precipitated by tempering are replaced by M6C with a concentration of W; i.e., the amount of W in solution reduces during creep. On the other hand, MX type carbides such as VC and NbC are very stable during long-term aging and contribute to the creep strength by obstructing the dislocation annihilation. One of the most significant results is that a reduction in Mn-content lowers the minimum creep rate, resulting in an increase in the creep rupture time. The nucleation and/or growth of M6C are retarded with reducing Mn-content, thereby the increase in dissolved W seems to enhance the resistance to creep deformation. Another significant result is that an increase in B-content has delayed the transition from the primary creep to a tertiary creep stage. The principal roles of B are stabilizing M23C6 or M23(C, B)6 on former austenite grain boundaries and retarding the dynamic recrystallization during creep. In addition, in the specimens with higher amount of B, the bainite lath interface is covered by MX and some filmy precipitates with high density, thereby the softening resistance is enhanced.

Effect of Si on mechanical properties of low alloy steels

Abstract: The effect of Si content on mechanical properties in 0·6C–(1·0–2·5)Si–2Ni–0·2V (wt-%) steels was investigated using tensile tests, Charpy impact tests, and microstructural examination with transmission electron microscopy. The results showed that the tempering temperatures both for the maximum yield strength and for the softening of low alloy steel shifted to higher temperatures owing to the retardation of the conversion of ɛ carbide to cementite within martensite laths caused by Si addition. Additionally, it was found that increasing Si content shifted the tempered martensite embrittlement temperatures upwards, owing to the retardation of the formation and growth of cementite boundaries caused by the added Si.

An analytical electron microscopy study of paraequilibrium cementite precipitation in ultra-high-strength steel

Abstract: To support quantitative design of ultra-high-strength (UHS) secondary-hardening steels, the precipitation of cementite prior to the precipitation of the M2C phase is investigated using a model alloy. The microstructure of cementite is investigated by transmission electron microscopy (TEM) techniques. Consistent with earlier studies on tempering of Fe-C martensite, lattice imaging of cementite suggests microsyntactic intergrowth of M5C2 (Hagg carbide). The concentration of substitutional alloying elements in cementite are quantified by high-resolution analytical electron microscopy (AEM) using extraction replica specimens. Quantification of the substitutional elements in cementite confirms its paraequilibrium (PE) state with ferrite at the very early stage of tempering. The implications of these results are discussed in terms of the thermodynamic driving force for nucleation of the primary-strengthening, coherent M2C carbide phase. The ferrite-cementite PE condition reduces the carbon concentration in the ferrite matrix with a significant reduction of M2C driving force. The kinetics of dissolution of PE cementite and its transition to other intermediate states will also influence the kinetics of secondary hardening behavior in UHS steels.

Precipitates in tempered stainless martensitic steels alloyed with nitrogen, carbon or both

Abstract: The precipitation in martensitic steels alloyed with (wt. %) ∼15Cr, ∼IMo, 0.62N or 0.6 %C or 0.29C+0.35N is studied after tempering in the range of 100-650°C by means of transmission electron microscopy (TEM), Mossbauer spectroscopy (MS), internal friction (IF) and dilatometry. It is shown that the substitution of carbon by nitrogen shifts the precipitation towards higher temperatures and expands the formation of stable interstitial phases over a broad temperature range. TEM studies reveal the precipitation of orthorhombic ξ-nitride and hexagonal e-nitride in the nitrogen martensite while cementite and e-nitride are formed in the nitrogen+carbon martensite. A higher mobility of dislocations in the nitrogen martensite was observed by means of IF. The fraction of retained austenite (RA) in as-quenched martensite is increased in the order: C→N→C+N, and the stability of RA against decomposition during tempering is changed in the same order. According to MS data, the distribution of chromium atoms is characterised by a tendency to clustering in the carbon martensite and to short range ordering in the nitrogen and nitrogen+carbon martensites. The different precipitation behaviour of martensites during tempering is explained by the nitrogen effect on iateratomic bonding and short range atomic order in solid solutions.

High strength steel for oil well excellent in toughness and sulfide stress corrosion cracking resistance and its production

Abstract: PROBLEM TO BE SOLVED: To produce high strength steel for an oil well of >=140 ksi yield stress excellent in toughness sand sulfide stress corrosion cracking resistance and to provide a method for inexpensively producing it. SOLUTION: This is steel for an oil well composed of low alloy steel contg., by weight, 0.2 to 0.35% C, 0.2 to 0.7% Cr, 0.1 to 0.5% Mo and 0.1 to 0.3% V, in which the total content of precipitated carbides is 2 to 5 wt.%, the ratio of MC type carbides therein is 8 to 40 wt.%, and old austenite grain size is No. >=11 by the grain size number prescribed in ASTM, and the steel is producible by subjecting steel having the above chemical compsn. to quenching from the temp. equal to or above the A3 transformation point for at least two times and thereafter tempering it at 650 deg.C to the temp. equal to or below the Ac1 transformation point, or executing quenching from the temp. equal to or above the A3 transformation point and tempering under the above conditions for at least two times.

Rapidly solidified nonequilibrium microstructure and phase transformation of laser-synthesized iron-based alloy coating

Abstract: The rapidly solidified microstructural and compositional features, the precipitation and transformation of carbides during tempering, and the impact wear resistance of an iron-based alloy coating prepared by laser cladding are investigated. The clad coating alloy, a powder mixture of Fe, Cr, W, Ni, and C with a weight ratio of 10:5:1.1.1, is processed using a continuous wave CO, laser. Microstructural studies demonstrate that the coating possesses the hypoeutectic microstructure comprising the primary dendritic gamma-austenite and interdendritic eutectic consisting of gamma-austenite and M7C3 carbides. gamma-Austenite is a non-equilibrium phase with an extended solid solution of alloying elements. During high temperature tempering at 963 K for 1 h, the precipitation of M23C6, MC and M2C carbides in austenite and in situ carbide transformation of M7C3 to M23C6 and M7C3 to M6C respectively are observed. In addition, the microstructure of the laser-clad coating reveals an evident secondary hardening and a superior impact wear resistance.

Marforming and tempering of binary Ni–Ti alloys including precipitation effects

Abstract: The influence of plastic deformation in the martensitic state followed by tempering was studied. The amounts of true plastic deformation up to ϕ=1,1 were obtained by cold rolling. To achieve complete transformation into the martensitic state, Ni rich alloys with low transformation temperatures were cooled in a solution of liquid nitrogen and alcohol below Mf. The effects of workhardening on strength and transformation behaviour were investigated by differential scanning calorimetry, tensile tests and micro-hardness measurements, as well as by light and electron microscopy. Increasing deformation leads to higher reverse transformation temperatures (As, Af). Finally the alloys lose their martensitic retransformability (dead martensite), i.e. As and Af reach the temperature range in which diffusion effects become considerable. By tempering at 200

Manufacture of high strength steel product for oil well use, excellent in sulfide cracking resistance

Abstract: PROBLEM TO BE SOLVED: To manufacture the steel product with high energy efficiency by subjecting a steel stock which has a specific composition containing C, Si, Mn, P, S, Mo, Al, Ti, Nb, N, and B and in which yield strength and Mo content satisfy a specific relation to hot working, hardening treatment, and tempering under respectively specified conditions. SOLUTION: The steel stock contains, by weight, 0.10-0.25% C, =-18.8 (where YS is yield strength shown by ksi). The steel stock is heated to >=1,150 deg.C and hot worked. Finish working is completed at a temperature not lower than (Ar3 point +50 deg.C). Quenching is done without delay to perform hardening, and a metallic structure in which as-hardened hardness in the position at the longest distance from the cooling surface becomes >=95% of surface hardness is formed, followed by tempering at 660 to 720 deg.C.

High-strength valve spring and it's manufacturing method

Abstract: The high-strength valve spring uses, as the material, a steel containing 0.5-0.8% C, 1.2-2.5 wt % Si, 0.4-0.8 wt % Mn, 0.7-1.0 wt % Cr, balance Fe and inevitable impurities, where, in the inevitable impurities, Al is no more than 0.005 wt % and Ti is no more than 0.005 wt %, and the largest non-metallic inclusion is 15 μm. In the oil tempering treatment, the heating temperature at hardening is between 950-1100° C., and nitriding treatment is performed after coiling. It is preferable to nitride at a temperature no lower than 480° C. Since the material is a high-silicon steel, the tempering temperature can be set at a higher temperature, and the nitriding temperature can be so high. In another way, after coiling, the spring is subjected to shot peening at least twice with shot particles of hardness 720 Hv or higher to produce a compressive residual stress of 85 kgf/mm2 at around surface. These measures render high surface hardness, and produce high-strength valve springs having good fatigue strength, sag resistance and delayed fracture resistance.

Method of producing martensite-or bainite-rich steel using steckel mill and controlled cooling

Abstract: A steel rolling mill including a Steckel mill is provided with an in-line upstream quench station located downstream of the caster and upstream of the reheat furnace, a shear located downstream of the Steckel mill, and a temperature reduction station downstream of the shear. The upstream quench station has spray nozzles that quench a surface layer of the steel to transform same from an austentitic to a non-austentitic microstructure. The shear provides a precise transverse vertical face on the leading end of the steel. The temperature reduction station applies cooling fluid to the rolled steel so as to obtain a preferred microstructure that may be either bainite or martensite. If bainite, the temperature reduction station includes laminar-flow cooling apparatus; if martensite, the station also includes an initial rapid quench, in which latter case the station is followed by a tempering furnace.