Showing papers on "Tempering published in 1995"

Heat treated martensitic steels: microstructural systems for advanced manufacture

Abstract: This paper describes the microstructure, deformation, and fracture of low-temperature-tempered (LTT) martensitic steels. The microstructural reasons for the ability of these steels to achieve ultrahigh strengths and the factors controlling ductility and toughness are described for low-carbon, medium-carbon, and high-carbon LTT martensitic steels. The key strengthening mechanism of LTT martensitic steels is the strain hardening provided by the transition carbide/dislocation substructure of the martensite crystals. In low- and medium-carbon steels, LTT microstructures fail by ductile fracture mechanisms, and ductility decreases as strain hardening rates increase with increasing carbon content. In high-carbon LTT steels, quench embrittlement associated with phosphorus segragation and cementite formation at austenite grain boundaries limits toughness and fatigue resistance. Approaches which permit the application of the high strengths of high-carbon LTT steels and minimize the effects of quench embrittlement are discussed.

The influence of heat treatment on the high-stress abrasion resistance and fracture toughness of alloy white cast irons

Abstract: The influence of a range of austenitizing and subcritical (tempering) heat treatments on the high-stress abrasion resistance and fracture toughness of four commercially significant grades of alloy white cast iron was investigated. Complementing an earlier study[1] on the influence of a more limited range of heat treatments on the gouging abrasion performance of the same alloys, the results showed that the effect of austenitizing temperature on high-stress abrasion pin test weight loss differed for each alloy. With increasing austenitizing temperature, these results ranged from a substantial improvement in wear performance and retention of hardness through to vir-tually no change in wear performance and substantial falls in hardness. Fracture toughness, however, increased markedly in all alloys with increasing austenitizing temperature. Tempering treatments in the range 400 °C to 600 °C, following hardening at the austenitizing temperature used commonly in industrial practice for each alloy, produced significant changes in both hard-ness and wear performance, but negligible changes in fracture toughness. Most importantly, the data showed that selection of the correct temperature for subcritical heat treatment to reduce the retained austenite content for applications involving repeated impact loading is critical if abrasion resistance is not to suffer.

Microstructures and Age Hardening Characteristics of Direct Quenched Cu Bearing HSLA Steel

Abstract: Microstructures and age hardening characteristics of 0.04C-3.5Ni-0.6Cr-1.8Cu-0.4Mo-0.03Nb Cu bearing HSLA steel manufactured by reheat quenching (RQ) and direct quenching (DQ) processes, were investigated. Especially, the effect of DQ treatment on quenched microstructures and niobium carbide and e-Cu precipitation were examined. A finer quenched microstructure was obtained in DQ processed steel compared with RQ processed steel, due to the heavily deformed austenite introduced by DQ. In DQ processed steel, lath and block sizes of martensite decreased and internal dislocation density increased with the increase of austenite deformation in the non-recystallization region. Hence, the strength of quenched DQ steel increased with the amount of deformation in non-recrystallization region. e-Cu precipitation characteristics in the aging stage changed very sensitively to the quenched microstructures. The size and growth rate of e-Cu precipitate decreased with the amount of deformation in the non-recrystallization region, because heavily dislocated structure supplied more preferred nucleation sites of e-Cu precipitates and retarded the precipitate growth. Also, DQ process affected the niobium carbide precipitation in this steel. In RQ steel, most of niobium precipitated as a coarse carbide in the reheating stage for austenitization. Whereas, in DQ steel, most of niobium precipitated very finely in the aging stage and thus highly contributed to the increase of the strength and tempering resistance. Therefore, it was councluded that DQ processed steel had higher strength and tempering resistance than RQ processed steel due to the finer e-Cu and niobium carbide precipitates.

Effect of alloying elements and dendrite arm spacing on the microstructure and hardness of an Al-Si-Cu-Mg-Fe-Mn (380) aluminium die-casting alloy

Abstract: The microstructure of aluminium alloys based on 380 die-casting alloy was studied in detail, as a function of the alloying elements iron, magnesium, copper and manganese, and the solidification rate Three methods of solidification were employed to simulate cooling rates obtained from investment, permanent, and die-casting processes, corresponding to ∼ 04, ∼ 12 and ∼ 260 °C s−1, respectively, with emphasis on the highest cooling rate Hardness measurements were carried out on samples obtained from the latter, in the as-cast and T5 tempered conditions (4 h at 25, 155, 180, 200 and 220 °C) The results have been discussed and the correlation between the hardness and microstructure as a function of alloying elements is presented The effect of solution heat treatment on the variations in the microstructure and hardness has also been discussed

Heat treatment of welded 13%Cr-4%Ni martensitic stainless steels for sour service

Abstract: For many years, the petroleum industry has employed martensitic stainless steels for wellhead and valve applications, and increasing use has been made of 13%Cr-4%Ni alloys. This material type was originally developed as a cast alloy (e.g., ASTM A487/A487M-89a Grade CA6NM). The combination of a low-carbon content and the addition of 3.5 to 4.5% nickel produces a fine, lath martensite structure which, after a tempering heat treatment, can exhibit superior mechanical properties. Thus, CA6NM and its forged variant ASTM A182/A182M-91 F6NM find application for production fluids containing CO{sub 2} and H{sub 2}S environments, particularly when hardening occurs, as is the case with fusion welds. Sensitivity to sulfide SCC increases at high material hardness levels, and the NACE MR0175 standard limits 13%Cr-4%Ni alloys to HRC 23 maximum for sour service. Attainment of such a hardness level requires careful consideration of tempering procedure. In this paper, the roles of welding procedure, material composition and postweld heat treatment are examined in relation to producing the minimum hardness levels in the weld zone.

Production of high strength seamless steel pipe excellent in sulfide stress cracking resistance

Abstract: PURPOSE: To provide a method for producing a seamless steel pipe having high sulfide stress cracking resistance, high strength and high toughness using a direct hardening method. CONSTITUTION: This is a method for producing a high strength seamless steel pipe excellent in sulfide stress cracking resistance in which, at the time of producing a seamless steel pipe by subjecting a billet having a prescribed chemical compsn. and satisfying Ti(%)-(48/14) N(%)-(14/91)Zr(%)}>0 to hot piercing and rolling, in succession to piercing, finish rolling of >=40% cross sectional compressibility is executed at 800 to 1050 deg.C finishing temp., after that, reheating is executed at a temp. T( deg.C) in the temp. range of 850 to 1100 deg.C for time t(h) to regulate the value of (T+273) (21+logt) to 23500 to 26000, immediately, direct hardening is executed, and next, tempering is executed at the Ac1 point or below. In the case the piercing is executed by a cross piercing machine or the contents of P and S in impurities are reduced, the effects are high. When reheating hardening is executed after the direct hardening, the effects are moreover high.

Rolling bearing for high-speed rotation at high temperatures

Abstract: One of component parts of an inner race, an outer race and rolling elements of a rolling bearing is composed of a bearing material that is prepared by working a high-speed steel, carburizing or carbonitriding the wrought steel in a temperature range from 800° C. to less than 1200° C. and subsequently hardening and tempering the carburized or carbonitrided steel, the steel composing of 0.2 wt %≦C≦2.3 wt %, Si≦1.0 wt %, Mn≦1.0 wt %, 2.0 wt %≦Cr≦7.0 wt %, 1.5 wt %≦W≦22.0 wt %, Mo≦10.0 wt %, 0.5≦V≦6.0 wt %, Co≦18.0 wt %, the balance Fe and incidental impurities. By carburizing or carbonitriding the steel at low temperatures less than 1,200° C., not only high core toughness is insured but also the dissolved C or N in the surface is compensated to increase the hot strength of the steel and the compressive stress that remains in the surface after heat treatment, so that the rolling bearing that rotate at high speed and high temperatures of more 400° C. extends the service life.

Plasma nitriding of microalloyed steel

Abstract: 3icroalloyed or high strength low alloy (HSLA) steels are carbon-manganese steels containing small amounts of Nb, V or Ti. The excellent mechanical properties of these alloys, particularly high yield strength, usually obviate the need for expensive quench and tempering operations. Furthermore, the presence of a significant amount of nitride-forming elements in some microalloyed steels has generated interest in the applicability of these alloys as a new generation of nitriding steels. In this paper, a study of the plasma nitriding behaviour of a commercially available microalloyed steel MAXIMATM is reported. A comparison is made with a traditional quenched and tempered nitriding steel (En19), plasma nitrided under similar conditions. Optical and scanning electron microscopy in conjunction with microhardness measurements and X-ray diffraction were utilized to characterize the nitrided surfaces. The observed differences in the thickness and structure of the compound layer and the diffusion zone are discussed in terms of chemical composition and microstructure of these steels.

Effect of modified heat treatment on mechanical properties of 300M steel

Abstract: The effects of modified heat treatment (MHT) on the mechanical properties of 300M steel have been studied to assess MHT steel for possible ultrahigh strength applications. The microstructure of MHT steel has variable amounts of martensite, carbide free upper bainite, and retained austenite. This is produced by partial isothermal transformation at 593, 623, or 673 Kfor the required times, followed by oil quenching and subsequent tempering at 473 K after 1173 K austenitisation. The optimum combination of plane strain fracture toughness with other relevant mechanical properties was obtained when 50 vol.-% bainite was associated with tempered martensite and retained austenite for MHT at 593 and 623 K. The MHT steel transformed at 593 K showed improved plane strain fracture toughness over conventional quenched and tempered steel at a similar tensile strength level, with little change of percentage elongation and Charpy 2 mm V-notch impact energy. Compared with conventional quenched and tempered steel, ...

Aluminum-silicon alloy sheet for use in mechanical, aircraft and spacecraft construction

Abstract: The invention relates to an aluminum alloy sheet heat treated by natural aging, quenching and possibly tempering so as to obtain a yield strength greater than 320 MPa, for use in mechanical, naval, aircraft, or spacecraft construction, with a composition (by weight) of: Si: 6.5 to 11% Mg: 0.5 to 1.0% Cu: <0.8% Fe: <0.3%

Process for producing a coil spring

Abstract: A process for producing a coil spring comprises steps of: forming a wire comprising: C in an amount of 0.55 to 0.75% by weight; Si in an amount of 1.00 to 2.50% by weight; at least two primary metals selected from the primary metal group consisting of: Mn in an amount of 0.30 to 1.5% by weight; Ni in an amount of 1.00 to 4.00% by weight; Cr in an amount of 0.50 to 2.50% by weight; Mo in an amount of 0.10 to 1.00% by weight; at least one secondary metal selected from the secondary metal group consisting of: V in an amount of 0.05 to 0.60% by weight; Nb in an amount of 0.05 to 0.60% by weight; and the balance of substantially Fe; oil quenching and tempering the wire treated by cold wire drawing; hot tempering the wire, thereby preparing the annealed wire whose tensile strength σb falls in the range of from 1370 to 1670 N/mm²; cold coiling; hardening and tempering; grinding; gas nitriding; high strength two-stage shot peening; and low temperature annealing the annealed wire. Accordingly the present process can product the coil spring whose endurance limit τm is 687 ± 560MPa and which has high strength and high fatigue resistance without breakage.

Kinetics of phosphorus segregation in 2.7Cr-0.7Mo-0.3V steels with different phosphorus contents

Abstract: The phosphorus grain boundary segregation kinetics during tempering at 680 °C and aging at 500 °C of 2.7Cr-0.7Mo-0.3V steels with phosphorus mass contents of 0.004, 0.014, and 0.027 % was investigated. To determine the grain boundary concentrations of phosphorus the Auger electron spectroscopy was used. Chemical compositions of carbide particles were determined by means of EDX/STEM. Xu Tingdong's and McLean's models of non-equilibrium and equilibrium segregations, respectively, were used to analyze experimental data. It was shown that a phosphorus grain boundary enrichment during tempering was mainly caused by non-equilibrium segregation. During aging the mechanism of the equilibrium grain boundary segregation was prevalent. Slow phosphorus segregation kinetics was observed in the experimental steels during aging.

Process for producing high-and low-pressure integral-type turbine rotor

Abstract: A rotor forging composed of Cr--Mo--V type alloy based on iron is normalizing-treated at a temperature of from 1000 to 1150° C., the temperature is maintained at 650°-750° C. on the way of cooling the temperature from the normalizing-treating temperature to pearlite transform the microstructure of the rotor forging, the portions of the rotor forging corresponding to a high pressure or middle pressure portion are quenched at 940°-1020° C. and the portion corresponding to the low pressure portion is quenched at 850°-940° C. after the heat treatment is carried out at 920°-950° C. once or more times, and the rotor forging is subjected to tempering at 550°-700° C. once or more times. A high creep strength at the high and middle pressure portions can be obtained and, at the same time, the toughness at the low pressure portion is drastically enhanced.

Production of seamless steel tube with high strength and high corrosion resistance

Abstract: PROBLEM TO BE SOLVED: To produce a seamless steel tube having high strength and excellent in toughness and corrosion resistance by successively subjecting a steel billet of specific composition to piercing, to rolling at respectively specified reduction of area and finishing temp., to reheating at specific temp. for specific time, to hardening, and then to tempering. SOLUTION: The steel billet has a composition which consists of, by mass, 0.15-0.50% C, 0.1-1.5% Si, 0. This steel billet is pierced, rolled at >=40% reduction of area at 800-1050 deg.C finish rolling temp., reheated at temp. T between 850 and 1100 deg.C for time (t), and, after the value of fn2 represented by equation II is regulated to 23500-26000, subjected, without delay, to direct hardening and then to tempering at a temp. not higher than the Ac1 point. By this method, the development cost for an oil well can be reduced.

High temperature decomposition of austempered microstructures in spheroidal graphite cast iron

Abstract: Spheroidal graphite (SG) cast iron is often plasma nitrided for corrosion resistance, and plasma nitriding has been proposed as a surface engineering treatment to improve wear resistance. However, the microstructure of austempered SG iron comprises constituents that may be unstable at nitriding temperatures. Therefore, the thermal stability of austempered SG cast iron has been studied at high temperature. Differential scanning calorimetry shows that microstructures obtained by austempering at low (300°C) and intermediate (380°C) temperatures, and which contained retained austenite, underwent a large exothermic transition during heating to typical nitriding temperatures. The transition began at approximately 470°C and peaked at 510–520°C, and was due to the decomposition of retained austenite to ferrite and cementite. A microstructure obtained by austempering at a higher temperature (440°C), and which consisted entirely offirst and second stage bainite, was stable up to nitriding temperatures. Afte...

Method for heating glass sheets to be tempered or heat-strengthened

Abstract: The invention relates to a method for heating glass sheets to be tempered or heat-strengthened, in which method glass sheets are heated in a preheating furnace (1) by applying a hot-air blast and convection heating produced thereby to the opposite sides of a glass sheet and the preheated glass sheet is transferred from the preheating furnace (1) into a radiation heating furnace (2) for heating the glass sheet to a tempering temperature. During the convection heating of a glass sheet, the rotating speed of a hot-air fan (5) is increased while adjusting the heating effect of heating resistances (6) so as to maintain the temperature of blasted air substantially constant. Thus, the diminishing temperature difference between glass and air is compensated for by controlling the coefficient of heat transfer.

Single cycle, combination layers with plasma assistance

Abstract: Alloy steel nitriding is currently employed to improve corrosion and wear resistance, to increase fatigue strength and to increase superficial hardness. The corrosion and wear properties of nitrided parts can be further improved by adding processing steps to the nitriding program. An oxidation step is added to produce a black Fe 3 O 4 layer to improve salt-spray corrosion resistance. Corrosion resistance is improved without surface roughening and this process does not require additional machining or polishing steps before completion. A CVD step is added, to deposit TiN hard-coatings on hardened and tempered alloy steels, for increased wear resistance. Nitriding provides enhanced support for this overlay coating. Deposition of these layers can be completed during a single cycle within the nitriding retort, by simply extending the process program. One pulsed plasma nitriding technology (ELTROPULS) offers a controlled plasma power duty-cycle that permits these combination layers to be added at low temperatures, that precludes geometrical distortion and dimensional change and does not require rehardening and tempering to restore core properties. Precursor reacting only within the plasma over the substrate surface permit mechanical masking and make for coating uniformity, independent of line-of-sight considerations.

Effects of welding parameters on weld zone toughness and hardness in 690 MPa steel

Abstract: Specifications for the welding of high-strength steels are generally intended to control hydrogen cracking and provide adequate weld zone toughness for resistance to fatigue cracking and shock loading. The specifications should also allow welding to be undertaken safely and profitably. The work described here was designed to identify the optimum match of welding parameters, notably preheat, interpass temperature and heat input, for the welding of a 690 MPa (100 ksi) microalloy quench and tempered steel. The paper covers investigations into two aspects of weldability: toughness and hardness. The first part involved shielded metal arc (SMA) butt joint welding of carefully designed plates at a range of preheat and interpass temperatures and heat input values, to identify welding procedures that give maximum HAZ and weld metal toughness. The second is a laboratory study of bead-on-plate submerged arc welds to clearly identify the relationship between hardness and welding parameters. The test procedure for the first investigation involved SMA welding at preheat and interpass (P and I) temperatures from -20°C to 220°C (-4° to 428°F) using heat input values of (approximately) 1.3, 2.9 and 4 kJ/mm (33, 74 and 102 KJ/in.) Charpy V-notch energy and fracture appearance transition curves were then generated with the Charpy notches being carefully located to give the lowest toughness values. Results showed that the minimum preheat and interpass temperature for control of weld metal hydrogen cracking was 60°C (140°F) and that low values of toughness in both weld and heat-affected zone consistently occurred in the weld root region. Low preheat techniques and one-sided welding should therefore be avoided for critical applications. For weld metal, greatest toughness occurred at high preheat and interpass temperatures (160°C; 320°F) combined with low welding heat input (1.2 kJ/mm; 30 KJ/in). For high heat input welding, preheat and interpass temperature had little influence on toughness over the range of temperatures examined. For heat-affected zone regions, levels of toughness obtained when using the high preheat/low heat input and low preheat/high heat input techniques were similar. It was found, however, that careful control of preheat and interpass temperature was essential, for each heat input value, because toughness can drop off rapidly on either side of the optimum interpass temperature. For some applications, optimum preheat and interpass temperatures were found to lie between 60° and 90°C (140° and 194°F). In the second study, a series of bead-on-plate submerged arc welds was deposited using a wide range of preheat and interpass temperatures, voltages, currents and travel speeds. In each case, the welding parameters were chosen so that the welding power input (V x I) could be held constant while varying heat input, or vice versa. For each weld, hardness readings were taken in both the weld deposit and HAZ at approximately 1 mm (0.04 in.) from the weld interface. It was found that: 1) HAZ hardness readings (390-430 HV 30 for heat input values up to 2 kJ/mm) were consistently higher than weld metal hardness readings. 2) Heat input has a marginal effect on HAZ hardness up to about 2 kJ/mm (51 kJ/in.), however, above 2 kJ/mm the hardness drops off at a rate of approximately HV 30 for each increase of 1 kJ/mm (25 kJ/in.). 3) Weld metal hardness dropped continuously over the range of 0.5 to 4.5 kJ/mm (13 to 114 kJ/in.). 4) Both HAZ and weld metal hardness drops approximately 1 HV 30 for every 4°C (7.2°F) increase in preheat and interpass temperature. The results of this work have been used to develop an optimized weld design for butt joint welding of 35-50 mm (1.4-2 in.) plate. This uses high heat input for all but the weld capping passes, where high interpass temperatures and low heat input techniques are employed.

Production of high strength and low yield ratio seamless steel pipe

Abstract: PROBLEM TO BE SOLVED: To provide a method for producing a high toughness seamless steel pipe having high strength and low yield ratio, furthermore excellent in SSC (sulfide stress cracking) resistance or moreover having a fine-grained structure by regulating the steel components and rolling heat treating conditions. SOLUTION: A slab contg., by weight, 0.02 to 0.2% C, 0.01 to 0.5 Si, 0.15 to 2.5% Mn, ≤0.02% P, ≤0.01% S, 0.005 to 0.1% Al, ≤0.01% N, 0.005 to 0.1% Ti and 0.005 to 0.1% Nb and furthermore contg. one or ≥two kinds among Cr, Mo, Ni, V, B, rare earth metals, Ca, Co and Cu according to necessity is formed into a hollow pipe stock by hot piercing continuous rolling, which is next subjected to finish rolling to produce a finished steel pipe. This steel pipe subjected to quenching treatment of executing rapid cooling from the Ar 3 point or above, is thereafter heated to between the Ac 1 point and the Ac 3 point, is rapidly cooled and is then subjected to tempering treatment of executing heating to the Ac 1 point or below and executing cooling. COPYRIGHT: (C)1997,JPO

Deep hardening boron steel article having improved fracture toughness and wear characteristics

Abstract: A deep hardening boron steel has a composition comprising, by weight, about 0.23 % to 0.37 % carbon, about 0.40 % to 1.20 % manganese, about 0.50 % to 2.00 % silicon, about 0.25 % to 2.00 % chromium, about 0.20 % to 0.80 % molybdenum, from 0.05 % to 0.25 % vanadium, from 0.03 % to 0.15 % titanium, from 0.015 % to 0.050 % aluminum, from 0.0008 % to 0.009 % boron, and 0.005 % to 0.013 % nitrogen. Also, the composition preferably contains less than about 0.025 % each of phosphorus and sulfur. After quenching and tempering, articles made from this material are substantially free of aluminum nitrides, have a fine martensitic grain structure, have a distribution of nanometer size background nitride, carbonitride, and carbide precipitates, and a combination of high hardness and fracture toughness. The deep hardening steel article embodying the present invention is particularly useful for ground engaging tools that are subject to breakage and wear, often at high temperature.

Production of martensitic heat resistant steel excellent in high temperature creep strength

Abstract: PURPOSE: To produce a martensitic heat resistant steel having high creep rupture strength by preventing the remarkable formation of retained austenite in a heat resistant steel largely contg. W and Co and having a martensitic single phase structure. CONSTITUTION: This heat resistant steel is produced in such a manner that a steel contg., by mass, 0.01 to 0.30% C, 0.01 to 0.80% Si, 0.20 to 1.50% Mn, 8.00 to <13.00% Cr, 0.01 to 3.0% Mo, 0.10 to 5.00% W, 0.05 to 6.00% Co, 0.002 to 0.800% V, 0.002 to 0.500% Nb and 0.002 to 0.150% N and moreover contg. at least one kind among 0.10 to 2.00% Ni, 0.10 to 2.00% Cu and 0.0005 to 0.01% B is used as a stock, which undergoes a producing process of executing tempering after normalizing.n an austenitic phase of holding at 600 to 650°C for ≥10min as primary treatment and holding at 750 to 800°C for ≥10min as secondary treatment. In this steel components, the content of P is regulated to≤0.030%, S to ≤0.010% and 0 to ≤.020%. COPYRIGHT: (C)1996,JPO

Microstructure and mechanical behaviour of nitrogen alloyed martensitic stainless steel

Abstract: Solid state gaseous nitrogenising was used to produce a wider range of nitrogen alloyed materials in a 10%Cr martensitic alloy. It was possible to achieve maximum nitrogen levels of 0·45 wt-%. A high solution treatment temperature was required to obtain a single phase martensitic structure. The effect of nitrogen content and heat treatment on microstructure in a variety of conditions was studied using transmission electron microscopy and X-ray techniques. CrN, VN, and Cr2N phases occurred during tempering ofmartensitic alloys in the range 400–750°C. High strength material is produced by introducing nitrogen; strength and hardness increased linearly with increasing nitrogen content and superior properties were observed on tempering. The presence of precipitation caused pronounced secondary hardening, and the increase and subsequent sharp decrease in strength with increasing temperature were assumed to be the effect of coherent precipitation and loss of coherency, respectively. The nitrogen alloyed ...

Production of high strength and high toughness steel tube excellent in workability

Abstract: PURPOSE: To produce a high strength and high toughness steel tube excellent in workability by subjecting a steel having a specified compsn. constituted of C, Si, Mn, P, S and Fe to tube making, thereafter executing specified hardening and tempering, subjecting it to cold drawing and thereafter executing annealing at a specified temp. CONSTITUTION: A steel having a compsn. contg. 0.05 to 0.15% C, <=0.50% Si, 0.50 to 2.00% Mn, <=0.020% P and <=0.020% S, contg., if necessary, one or more kinds among 0.05 to 0.50% Mo, 0.02 to 0.10% V, 0.05 to 0.50% Ni, 0.05 to 1.00% Cr, 0.05 to 0.50% Cu, 0.02 to 0.10% Ti, 0.02 to 0.10% Nb and 0.0005 to 0.005% B, and the balance Fe with inevitable impurities is subjected to tube making. This steel tube is hardened at 850 to 1000 deg.C and is tempered at 450 deg.C to less than the Ac1 transformation point to obtain fine and uniform austenitic grains. After that, the steel subjected to cold drawing of prescribed dimension. Next, the steel tube is subjected to softening at 450 deg.C to less than the Ac1 transformation point, preferably at about 500 to 650 deg.C.

Microstructural evolution and mechanical properties of AC8A/Al2O3 short-fiber composites after thermal exposure at 150 °C to 350 °C

Abstract: The microstructural evolution and mechanical properties of an AC8A/12 vol Pct A12O3 (sf) composite fabricated by squeeze casting were characterized. Thermal treatments included the normal T6 temper and thermal exposure at 150 °C, 250 °C, 300 °C, and 350 °C for 400 hours. The predominant strengthening phase in the matrix appeared to be β′ (Mg2Si) needles. Bulk pure Si particles and dendrites were commonly seen. Large particles, termed asB-type phase, might include hexagonal Al3(Ni, Cu, Fe, Si, Mg)2 and orthorhombic Al3(Ni, Cu, Fe, Si, Mg) phases. Both the Si andB dispersoids were not obviously affected by artificial aging at 150 °C to 350 °C. In certain cases, large cubic β (Mg2Si) particles, hexagonalQ′ orQ (Al4Cu2Mg8Si7) precipitates, and numerous small Al particles inside Si dispersoids were also seen. No interfacial reaction product was observed along the fiber/ matrix interface even after long exposure at 350 °C. Amorphous SiO2 gels, which were used as a binder during fabrication, were occasionally observed. The tensile and fatigue behavior of the AC8A alloys and composites after the preceding thermal exposures were evaluated over the temperature range of 25 °C to 350 °C. The composites showed similar strength as the matrix alloy at room temperature but exhibited higher strength at temperatures above 250 °C, with the sacrifice of the lower ductility. The strength levels of both the alloys and composites were significantly reduced after long thermal exposure, especially for temperatures higher than 250 °C. The loss of strength after long-term exposure at elevated temperatures may be attributed to age-softening of the matrix.

Improvement of Creep Rupture Strength of 9Cr-1Mo-V-Nb-N Steel by Thermo-Mechanical Control Process

Abstract: A 9Cr-1Mo-V-Nb-N steel was subjected to the thermo-mechanical control process (TMCP), more specifically the direct quenching and tempering process, and the influences of heating temperature (Ts) and finish-rolling temperature (Tf) on the mechanical properties, including the creep rupture strength (CRS), were examined. The results were analyzed by observation of the substructure and precipitates, then the reasons for the improvement in CRS were discussed.Raising Ts and lowering Tf improve CRS for separate reasons. Raising Ts augments the coherency strain around VN through lattice expansion owing to the increased solution of Nb to VN. This increase in coherency strain is the reason for the improvement of CRS. On the other hand, lowering Tf disperses VN more finely by serving dislocations as nucleation sites for VN, resulting in improved CRS through decreased interprecipitate distance.When TMCP is applied, thin disc-like (V, Nb)N which nucleates on dislocations becomes a dominant type of VN precipitation, instead of NbN/VN complex precipitate which prevails when the steel is normalized and tempered. Furthermore, successive rolling from sufficiently high temperature is important to avoid coarse precipitation of VN and to exert the precipitation hardening during tempering to its full extent.

An atom probe study of carbon distribution in martensite in Cr1Mo steel

Abstract: 2[1/4]Cr1Mo steel is used widely for superheater tubing in power plants, and as a filler material for joining [1/2]Cr[1/2]Mo[1/4]V steam piping. Components in power plants can be massive and therefore differences in cooling rates can result in a mixed microstructure of allotriomorphic ferrite, bainite and martensite. The creep strength of the steel is critically dependent on the carbide distribution within the microstructure. The position and nature of carbides within the microstructure is itself a critical function of the movement of carbon through the microstructure during the early stages of tempering. In this paper, atom probe field ion microscopy has been used to examine carbon segregation to lath boundaries in martensite in 2[1/4]Cr1Mo steel. Significant carbon enrichment was observed at the lath boundaries. This enrichment is consistent with the observation of retained austenite films at the lath boundaries in the transmission electron microscope, and with carbon levels previously found in retained austenite in low alloy ferrous martensites.

Steel for carburized gear

Abstract: A steel for darburized gear having softening resistance, consisting essentially of, in weight percentages, 0.18 to 0.25% C, 0.45 to 1.00% Si, 0.40 to 0.70% Mn, 0.30 to 0.70% Ni, 1.00 to 1.50% Cr, 0.30 to 0.70% Mo, up to 0.50% Cu, 0.015 to 0.030% A1, 0.03 to 0.30% V, 0.010 to 0.030% Nb, up to 0.0015% O, 0.0100 to 0.0200% N and the balance consisting of Fe and inevitable impurity elements, wherein quenching at 820° C. or higher after carburization does not cause any ferrite to be formed in a hardened structure of the core part of the carburized steel, and wherein, while tempering is generally performed at 160° to 180° C. after the quenching, reheating at any of temperatures inclusive of the tempering temperature and up to 300° C. does not cause the hardness of a carburized case of the carburized steel to decrease by HV 50 or more from the one after the carburization, quenching and tempering.