As-quenched Structure Research Articles

Sharp crack and blunt notch toughness behavior of quenched and tempered AISI 4340 steel

Precracked and blunt notch Charpy tests of AISI 4340 steel were carried out by instrumented Charpy tests for a variety of heat treatments. Scanning electron microscopy, transmission electron microscopy and optical microscopy were carried out to characterize microstructure and fracture surfaces. Results show that in precracked conditions high temperature austenitization at 1200 °C produces a twofold increase in the dynamic fracture toughness in the as-quenched condition compared with conventional austenitization at 870 °C, whereas in blunt notch testing conventional austenitization produces a better dynamic fracture toughness. High temperature austenitization, together with a large austenitic grain size, also results in substantial amounts of autotempered ϵ carbide in the as-quenched structure. These structural features are employed to seek an explanation for the anomalous toughness behavior in sharp crack and blunt notch testing of quenched and tempered AISI 4340 steel.

Laser surface heat treatment of AISI 4340 steel: A microstructural study

The effect of laser surface heat treatment on the microstructure of AISI 4340 steel was investigated and compared with conventional furnace heat treatments. The microstructural variables, i.e. prior austenite grain size, substructure of martensite, amount of retained austenite and its distribution and carbide precipitations, were carefully studied using transmission electron microscopy. The two significant observations were the presence of fine cementite particles of 200 Å size and large amounts of thin films of retained austenite in the as-quenched structures. The microstructure exhibited was fully martensitic, the substructure of which contained extensive twins. The average prior austenite grain size was measured to be 35 μm. A comparison with conventional furnace heat treatment (870°C treatment) showed that there was an enhancement in the amount of retained austenite and precipitation of fine cementite in the laser-heat-treated specimens. However, the prior austenite grain size and martensitic substructure remained essentially the same as those of conventional heat treatment.

The asquenched microstructure and tempering behavior of rapidly solidified tungsten steels

Transmission electron microscopy, and microhardness testing were used to examine the as-quenched structure and mechanical properties of a series of rapidly solidified (RS) iron-tungsten-carbon-alloys ranging from 6 to 23 pct tungsten with a constant W:C atomic ratio of 2:1, and Tl high speed tool steel. The RS iron-tungsten-carbon alloys were found to exhibit a significant change in microstructure and hardness as the tungsten and carbon content was increased. The change in morphology was from lath martensite in the lower tungsten alloys to a solidification structure of δ-ferrite cells surrounded by austenite and M6C carbide, in the higher tungsten alloys. A model is proposed to explain the morphologicl change. In addition, the tempering behaviors of RS Fe-6.2 wt pct W-0.21 wt pct C, Fe-23 wt pct W-0.75 wt pct C and Tl high speed tool steel were examined and compared to those observed for the conventional solution-treated and quenched alloys. A discussion is also included on the microstructural dependence upon cooling rate of RS high speed tool steels.

Structure and mechanical properties of Fe-Cr-Mo-C alloys with and without boron

A study of the structure and mechanical properties of Fe-Cr-Mo-C martensitic steels with and without boron addition has been carried out. Nonconventional heat treatments have subsequently been designed to improve the mechanical properties of these steels. Boron has been known to be a very potent element in increasing the hardenability of steel, but its effect on structure and mechanical properties of quenched and tempered martensitic steels has not been clear. The present results show that the as-quenched structures of both steels consist mainly of dislocated martensite. In the boron-free steel, there are more lath boundary retained austenite films. The boron-treated steel shows higher strengths at all tempering temperatures but with lower Charpy V-notch impact energies. Both steels show tempered martensite embrittlement when tempered at 350 °C for 1 h. The properties above 500 °C tempering are significantly different in the two steels. While the boron-free steel shows a continuous increase in toughness when tempered above 500 °C, the boron-treated steel suffers a second drop in toughness at 600 °C tempering. Transmission electron microscopy studies show that in the 600 °C tempered boron-treated steel large, more or less continuous cementite films are present at the lath boundaries, which are probably responsible for the embrittlement. The differences in mechanical properties at tempering temperatures above 500 °C are rationalized in terms of the effect of boron-vacancy interactions on the recovery and recrystallization behavior of these steels. Although boron seems to impair room temperature impact toughness at low strength levels, it does not affect this property at high strength levels. By simple nonconventinal heat treatments of the present alloys, martensitic steels may be produced with quite good strength-toughness properties which are much superior to those of existing commercial ultra-high strength steels. It is also shown that very good combinations of strength and toughness can be obtained with as-quenched martensitic steels.

Internal friction, tensile tests and hardness variations during the tempering of an as-quenched Cu-10wt.%Al alloy

A study of the mechanical properties of an as-quenched Cu-10wt.%Al alloy was undertaken between room temperature and 350°C. The results of internal friction and tensile tests and the hardness variations during heating and at room temperature after tempering were compared. The differences were explained mainly on the basis of the vacancy mobility in a complex martensitic structure. The vacancy behaviour was thought to be the reason for the observed differences between the mechanical tests. In particular, the as-quenched structure should be transformed to transitional phases by ordering effects. The room temperature internal friction measurements appeared to be closely related to the vacancy mobility but structural studies have yet to be completed.

非晶質合金の磁気的性質に及ぼす凝固条件の影響

The effects of processing conditions on the physical properties of Fe40Ni40B20 amorphous alloys produced by a single roll-casting technique were investigated. In producing the amorphous alloys, the temperature of the melt was changed while the rotating speed of the roll and the melt ejection pressure were held constant. The rotating speed was also varied at fixed melt temperatures and ejection pressures. The crystallization and Curie temperatures, saturation magnetization, and hardness of the resultant as-quenched amorphous alloys were not changed when either the temperature of the melt or the rotating speed of a roll was varied. The remanence Br and the ratio Br⁄B2 were also unchanged with the above processing conditions. However, a large monotonic increase in coercivity Hc was observed with increases in melt temperature, and it also decreased with roll speed. Isothermal annealing treatments of the above samples at 250°C revealed that: the higher the melt temperature, or the lower the rotating speed, the smaller is the amount of the change of Hc; i.e., these conditions produced a more stable as-quenched structure. These results are discussed in terms of the cooling rate in the casting and the state of disordered structure quenched into the amorphous alloy.

Structure and initial precipitation in a rapidly solidified nickel superalloy

A nickel base superalloy (Nimonic 80A) has been rapidly solidified at cooling rates of between 105 to 106 K.S-1 by pendant drop melt extraction and by chill block melt spinning in an evacuated chamber backfilled with helium or argon. The internal structure is described in terms of process variables pertaining to rotating chill block quenching techniques. Both transmission electron microscopy and atom-probe field-ion microscopy have been employed to give structural and constitutional data on quenched and aged specimens. The as-quenched structure is homogeneous apart from fluctuations in titanium concentration which upon aging undergoes a spinodal phase decomposition to form disordered Ni3(Ti,Al,Cr) precipitates in the matrix, which after prolonged aging produces ordered γ (Ni3(Ti,Al)). inin6 particles form readily on grain boundaries and also appear in conjunction with ordered γ, via a discontinuous reaction, after short aging times.

Splat-quenching of high speed tool steels

The solidification structure of rapidly quenched commercial high speed tool steels has been studied by means of two-piston splat-quenching and argon-arc plasma-spraying. X-ray studies of the as-quenched structure reveals an increase in austenite retention and almost complete suppression of carbide formation. After tempering, splat-quenched and plasma-sprayed specimens show a hardening peak at higher temperatures and higher hardness than the same steels heat-treated conventionally. X-ray diffraction work reveals that on tempering, austenite transforms to a bcc phase, with the austenite disappearing by 700°C, and no detectable carbide precipitation.

構造用高張力鋼中の水素拡散に及ぼす焼入れ焼もどし組織の影響

The effects of quenched and tempered microstructures on the diffusion coefficient and solubility of hydrogen in structural low-alloyed high strength steels have been investigated at room temperature by means of the electrochemical permeation technique. The results obtained are as follows:(1) The as-quenched martensitic structure gives a low diffusion coefficient, and the diffusion coefficient first increases with tempering temperature up to 200-300°C. Then the diffusion coefficient decreases until it reaches a minimum at about 450-550°C, after which it increases again at temperatures of 600°C and above. The mixed structure of bainite, martensite and ferrite obtained by furnace cooling of HT80 and HY130 steels above Acl gives a very low diffusion coefficient.(2) The solubility of hydrogen changes inversely with the diffusion coefficient.(3) The variation in diffusion coefficient or solubility can be interpreted in terms of the hydrogen trapping process at the lattice imperfections introduced by martensitic and/or bainitic transformation, such as dislocations, faults, subgrain boundaries and lattice vacancies, etc., or at the interface between the ferrite and carbide precipitates, depending upon the tempering temperature.

Precipitation Behavior in Fe–Ni–Mn Martensitic Alloys

The object of this study is to investigate precipitation behavior during aging of age-hardenable Fe–Ni–Mn martensitic alloys by means of specific heat measurement, thermal differential analysis, electron microscopic observation, and X-ray and electron diffractions. The as-quenched structure before aging is lath martensite. In the initial stage of aging, very fine zones with high solute concentration are homogeneously formed due to the two-phase decomposition. Subsequently the zone acts as a nucleus of the fct NiMn phase. As the zone changes to the fct NiMn particle the hardness of the alloys increases up to its maximum value. The main cause of the age-hardening is considered to be the strain hardening due to the coherency strain between zone or NiMn particle and matrix. In the over-aging and softening processes, NiMn particles grow into disc-like platelets on the (100) planes of the matrix, and the crystallographic orientation relationship with respect to the matrix is determined to be (011)α′4°(111)NiMn, [\bar1\bar11]α′8°[\bar101]NiMn. In the final equilibrium state, NiMn particles disappear and the structure is microduplex lamellae consisting of austenite and ferrite (or martensite) phases. The Kurdjumov-Sachs relationship holds between the austenite and ferrite.

Electron-Metallographic Identification of Retained Austenite in Steels

Recently it has been discovered by electron microscopy that even in medium carbon steels (Mf≳200°C) small quantities of interlath retained austenite (γ) is present in as-quenched structures. It is speculated that this small quantity of retained γ can have a profound influence on the mechanical properties of steel. This austenite is severely deformed and in most cases the amount is so small that detection by x-ray analysis becomes impossible. However, sophisticated electron microscopic techniques involving careful selected area diffraction, bright field and dark field imaging can unequivocally establish the presence of such small quantities of austentite. Extremely useful morphological and microstructural information on the austenite can also be obtained from the electron metallography.

Design of Fe/4Cr/0.4C martensitic steels eliminating quench cracking

Inter-granular cracking in the as-quenched structures of vacuum melted Fe/4Cr/0.4C steel has been investigated in terms of the heat treatment conditions. It has been shown that amount of carbon in solution and martensite packet size are the two most important factors influencing cracking tendency. Multiple heat-treatments have been designed in order to take advantage of higher austenitizing temperature and fine austenite grain size. Elimination of quench cracking and good mechanical properties were obtained after these treatments.

Aging behavior of Fe-30 Ni-1.0 Nb-0.1 C alloy

A study has been made of the precipitation reaction in Fe-30 wt% Ni-1.0 wt% Nb-0.1 wt% C alloy. The as-quenched structure of this alloy consists of austenite and martensite. On aging at 700° and 800°C the main precipitation reaction results in the formation of NbC. The precipitation of NbC occurs on dislocations, in austenite as well as in martensite, and on the martensitic interfaces and results in a considerable increase in the hardness of the alloy. Beside NbC small amounts of M 6C were also detected and in the specimens overaged at 800°C NbC, M 6C and Fe 2Nb precipitates were identified.

Quench-Ageing Behaviour of 40Co–38Ni–17Cr–5Ti Alloy

The quench-ageing behaviour of a new Co–Ni–Cr-base high-temperature alloy containing Ti has been studied. The as-quenched structure of the alloy was fcc, and on ageing between 700 and 900° C (973 and 1173 K) the main strengthening phase was observed to be an ordered fcc γ′ phase. On overageing, this phase transforms to a stable hexagonal Ni3Ti η phase. The grain boundaries provide nucleation sites for the cellular precipitation of η phase. TiC and M23C6 carbides were also observed. However, during overageing TiC transforms to M23C6 carbides.

Aging behavior of Fe-30 Ni alloys containing niobium

A study has been made of the precipitation reactions in Fe-30 wt pct alloys containing up to 5 wt pct Nb. The as-quenched structures of these alloys consist, of austenite, martensite in twinned as well as in massive form, and Ni3Nb and Fe2Nb precipitates. On aging at 700° and 800°C the main precipitation reaction results in the formation of hexagonal Laves phase Fe2Nb, but Ni3Nb in both bct and orthorhombic structures also precipitates. The precipitation of Fe2Nb is a heterogeneous process and results in a considerable increase in the hardness of the alloy.

微量のCを含むFe-5%V合金の時効におけるV炭化物の析出

Direct observations were carried out to study the morphology of vanadium carbide V4C3 and the crystallographic orientation relationship between the carbide and matrix in an iron-5% vanadium alloy, containing a small amount of carbon, which was aged at 600°C for various times after quenching from 1200°C into 10% iced brine. The results are as follows: (1) The as-quenched structure consists of simple ferrite and contains a few irregularly distributed dislocations only. Dislocation loops, helical dislocations and precipitates are not present. (2) Large numbers of square or rectangular-shaped thin platelets appear, accompanied by a slight increase in hardness on aging at 600°C. (3) These very thin platelets are identified as vanadium carbide V4C3 containing a small amount of nitrogen by chemical analysis and X-ray diffraction of extracted residues. (4) A definite crystallographic orientation relationship between vanadium carbide and ferrite matrix was determined by analysing a number of electron diffraction patterns as shown in the following: (100)_V4C3 \varparallel(100)_α-Fe, [010]_V4C3 \varparallel[011]_α-Fe, [001]_V4C3 \varparallel[0 \bar11]_α-Fe.