As-quenched Martensite Research Articles

Mechanical properties and microstructural features of AISI 4340 high-strength alloy steel under quenched and tempered conditions

Abstract In this work, the mechanical properties and microstructures of AISI 4340 high strength alloy steel under different tempering conditions are investigated. The specimens are quenched and tempered to a martensite structure and loaded to fracture at a constant strain-rate of 3.3×10−4 s−1 by means of a dynamic material testing machine (MTS 810). The mechanical properties and strain-hardening exponent are considered as function of the tempering conditions. The morphological features of the as-quenched martensite and their evolution during tempering are described. Fractographs of the specimens are also made in order to analyse their fracture and embrittlement mechanisms. The results indicate that the mechanical properties and microstructural features are affected significantly by tempering temperature and holding time. The strength and hardness of tempered martensite drop as the tempering temperature and holding time are increased. However, the ductility increases with increasing tempering temperature and holding time, except when tempered martensite embrittlement occurs. Microstructural observations reveal that the carbide precipitates have a plate-like structure at low temperatures, but are spheroid-like at high temperatures. Under the tested conditions, the fracture appearances show that the material failed in a ductile manner except for the case of tempering at 300°C, where tempering martensite embrittlement occurs due to the existence of retained interlath austenite.

Eigenschaften von bei niedrigen Temperaturen angelassenen SAE-43xx-Stählen

Uniaxial tensile tests were performed on 4330, 4340 and 4350 steels in the as-quenched condition and after quenching and tempering at 150, 175 and 200°C for times of 10 minutes, 1 hour and 10 hours. 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. As-quenched 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 caused as, during tempering, transition carbides increasingly replace dislocations with and without carbon atom segregation in the as-quenched martensite.

The instability of plastic deformation in as-quenched martensite

The paper reports on an investigation of the temperature dependence of the rate sensitivity parameter in as-quenched martensite of Fe-24.1wt.%Ni-0.40wt.%C alloy. Most of the ascertained values of the rate sensitivity parameter were negative. Apparently, the rates of dislocation motion surpassed the rates at which the interaction forces attained their maxima. Simultaneously, the temperature ranges of the plastic deformation instabilities were determined. At low temperatures, the activation parameters of plastic deformation U 0 = 0.31 eV and v * = 10 b 3 were determined. The probable mechanism of plastic deformation in this temperature range was proposed.

Internal friction of Fe-N martensite at low temperature

The internal friction behaviour of a binary Fe-N martensite is obtained by using a torsion pendulum. Results show that during heating the as-quenched martensite from 82 to 350 K three major internal friction peaks appear. Activation energies of the corresponding processes are determined to be 0.37, 0.31 and 1.26 eV, respectively. A preliminary interpretation is proposed. Dislocation relaxation processes and interactions with interstitial atoms seem to be the major reason for the internal friction behaviour.

Fundamental Study on the Application of the As-Quenched Low Carbon Martensitic Steels to Structural Materials.

The mechanical properties of low-carbon martensitic steels were studied with the aim of their application to structural materials. The seven kinds of steels containing different carbon contents up to 0.3 mass percent were examined. The carbon content dependences of hardness, tensile properties and impact toughness in as-quenched martensite are shown. The low-carbon martensite is plastically deformable, i. e., more than 60% cold rolling is easily achieved in steels containing less than 0.2 mass percent carbon and the strength is explained reasonably from the viewpoint of dislocation density. The 0.1 mass percent carbon as-quenched martensite is revealed to show comparable impact value to those of quenched and tempered steels at strength level of more than 1 GPa. Several points relevant to practical use, including, residual stress and strain rate effect, are discussed.

焼入れままマルテンサイト組織を有する鋼の靱性に及ぼす化学成分の影響

焼入れままマルテンサイト組織を有する鋼の靱性に及ぼす化学成分の影響

Fatigue crack growth characteristics of laser-hardened 4130 steel

Laser surface hardening of steels is one of many successful applications in laser material processing. The effect of the microstructure on the da/dN of various steels has been reported by several investigators who concluded that tempered martensite has a higher resistance to da/dN than does as-quenched (AQ) martensite. In addition to the microstructure, the residual stress also has a significant influence on da/dN. Importantly, the distribution of residual stresses is not uniform through the depth of the hardened zone and may change as crack propagating takes place in the test. Owing to the complicated nature of residual stresses, it is difficult to quantitatively analyze such an influence on the da/dN of laser-hardened steels. The present study was to investigate the characteristics of da/dN in laser-hardened AISI 4130 steels. Residual stress measurements was performed on distinct laser-treated specimens in the evaluation process.

Effects of coherent interfaces in the freshly formed ironnickelcarbon martensites

Evidence for a coherent bond at the interfaces between retained austenite and as-quenched martensite in FeNiC alloys and the relation of coherent interfaces to the abnormally high tetragonality of martensite are presented. The iron based alloys with 20 Ni-0.73 C, 28 Ni-0.2 C, 20 Ni-1.2 C, 25 Ni-0.7 C, 30 Ni-0.37 C, 33.5 Ni-0.01 C (in wt%) were studied by means of X-ray and neutron diffraction, Mössbauer spectroscopy, internal friction, electrical resistivity and magnetic susceptibility. It is shown that the break of coherency occurs during heating of the freshly formed twinned martensite in the temperature range of 100–200 K and it is accompanied by a relaxation of stresses in the retained austenite and a decrease of tetragonality. A new internal friction peak centered at 145 K was observed and attributed to movement of coherent interfaces and to the subsequent break of coherency. The relation between abnormally high tetragonality and coherency at the interface was confirmed in experiments with external deformation of virgin martensite at temperatures around 100 K. It is shown that the plate morphology of martensite is a necessary condition for a coherent bond at the interface. A possible role of atomic ordering of austenite in abnormally high tetragonality is discussed. Mössbauer measurements gave evidence for nickel-rich regions in initial austenite. During quenching the regions with the highest nickel content were assumed to remain austenitic inside the martensite plates. The conclusion about the coherency at the interface between the freshly formed martensite and the ordered regions in the retained austenite and at the interface along the martensite plates as a reason for high tetragonality of the FeNiC martensite with plate morphology is made.

The redistribution of carbon interstitials during the ageing of martensite through the clustering and coarsening of carbon multiplets

The Mossbauer spectra of 9.5 at.% C as-quenched and aged martensite are analyzed in terms of seven Fe environments according to the the carbon multiplet model; in particular, one distinguishes the Fe atoms first nearest neighbours of an interstitial along or perpendicularly to the c axis. Recent13C NMR results are consistent with the model which leads to a B2 monoclinic iron carbide with parallelepiped cell of 5.73, 6.74 and 8.60 A.

The clustering and coarsening of carbon multiplets during the aging of martensite from mössbauer spectroscopy: The preprecipitation stage of epsilon carbide

Mossbauer spectra of as-quenched and aged martensite (9.5 at. pct C) are analyzed in terms of seven Fe environments, the abundances of which evolve with aging time. Besides the atoms of the matrix (A), Fe atoms which are first nearest neighbors of an isolated interstitial can only be distinguished by their quadrupole shift depending upon whether they lie in a direction perpendicular (Ba) or parallel to thec axis (Bc); classC atoms have two carbon neighbors whileD orE belong to an ordered phase; classF atoms have three carbon neighbors. The carbon multiplet model, based on interstitials residing exclusively in octahedral sites, an elastic stability of theC-C 1/2«111” doublet and a diffusion path for carbon in (001)α' can explain all the features observed: a first step attributed to the clustering of isolated multiplets, and a second step to the coarsening into extended multiplets which leads to the emergence of a new ordered iron carbide (B2 monoclinic with parallelepiped cell of 5.73, 6.74, and 8.60 A) which transformsin situ into e-carbide (~Fe9C4) (η carbide as long as it is coherent with the matrix). The positions of carbon atoms in Fe9C4 are specified and a synopsis with related phenomena is drawn.

Recrystallization and formation of austenite in deformed lath martensitic structure of low carbon steels

The effect of prior deformation on the processes of tempering and austenitizing of lath martensite was studied by using low carbon steels. The recrystallization of as-quenched lath martensite was not observed on tempering while the deformed lath martensite easily recrystallized. The behavior of austenite formation in deformed specimens was different from that in as-quenched specimens because of the recrystallization of deformed lath martensite. The austenitizing behavior (and thus the austenite grain size) in deformed specimens was controlled by the competition of austenite formation with the recrystallization of lath martensite. In the case of as-quenched (non-deformed) lath martensite, the austenite particles were formed preferentially at prior austenite grain boundaries and then formed within the austenite grains mainly along the packet, block, and lath boundaries. On the other hand, in the case of lightly deformed (30 to 50 pct) lath martensite, the recrystallization of the matrix rapidly progressed prior to the formation of austenite, and the austenite particles were formed mainly at the boundaries of fairly fine recrystallized ferrite grains. When the lath martensite was heavily deformed (75 to 84 pct), the austenite formation proceeded almost simultaneously with the recrystallization of lath martensite. In such a situation, very fine austenite grain structure was obtained most effectively.

13%Niマルエージング鋼におけるオーステナイト中の金属間化合物の析出と焼入マルテンサイトの靱性

13%Niマルエージング鋼におけるオーステナイト中の金属間化合物の析出と焼入マルテンサイトの靱性

The effect of structure on the deformation of as-quenched and tempered martensite in an Fe-0.2 pct C alloy

As-quenched and tempered martensite in an Fe-0.2 pct C alloy were subjected to tensile testing and structural characterization by light and transmission electron microscopy. The light temper, 400°C-l min, did not change packet morphology, but did reduce dislocation density, coarsen lath size and cause the precipitation of carbides of a variety of sizes. The yield strength of the as-quenched martensite was strongly dependent upon packet size according to a Hall-Petch relationship, but tempering significantly diminished the packet size dependency, a result attributed to packet boundary carbide precipitation and the attendant elimination of carbon segregation present in the as-quenched martensite because of autotempering. Examination of thin foils from strained tensile specimens showed that a well-defined cell structure developed in the as-quenched martensite, but that the random distribution of jogged dislocations and carbide particles produced by tempering persisted on deformation of the tempered specimens.

Volume expansion, pressure effects, and the strength differential in as-quenched iron-carbon martensite

To examine the possibility that the strength-differential (SD) effect in martensitic carbon steels is related to an unusually large volume change associated with yielding and flow, permanent density changes accompanying deformation have been measured in as-quenched AISI 4310, 4320, and 4330 steels. Though the observed rapid initial density decrease can be attributed to transformation of retained austenite, the rate of density change with strain continues to be unusually high at larger strains. The hydrostatic-pressure dependence of yield and flow stresses was found to be very high, especially for compression tests, though the dependence decreased with strain in both tension and compression. The presence of retained austenite, which is not intrinsically related to the SD effect itself, complicates the interpretation of the pressure effects. The volume expansion hypothesis is not supported by the present data.

200, 020, and 002 X-ray peaks in tempered Fe-18 Ni-C martensites

Separate 200, 020, and 002 X-ray peaks were recorded for 0.0, 0.4, and 0.8 wt pct carbon (18 pct Ni) martensites after tempering between 25 and 500°C. The carbon bearing martensites studied here have been tempered initially enough to eliminate the “high tetragonality” 002 peak usually recorded for as-quenched martensite and the present results apply to tempered martensite only. The peak maximum is taken to determine the lattice parameter and the peak shape is recorded. At all carbon levels and after all tempering treatments, the “crd parameter is larger than or equal to the “a” or “b”. The relative enlargement is very small (0.08 pct) for the lowest carbon level and for any carbon level after severe tempering (500°C for 15 min). For the two higher carbon alloys tempered at temperatures below 400°C (for 15 min) the “c” parameter is significantly larger than the “a” and “b” and for the 0.4 wt pct C alloy the “b” is significantly smaller than the“a” whereas in the 0.8 pct C alloy the “b” is slightly larger than the “a”. Within experimental error the mean volume of the unit cell does not change during the tempering studied here and is nearly unaffected by the initial carbon content. This indicates that little (at most 0.1 wt pct) carbon is dissolved in tempered martensite. In the low carbon alloy the peaks are symmetric and sharpen symmetrically during tempering. In the higher carbon alloys the peaks are nearly symmetric and sharp after severe tempering. After less severe tempering the 002 peak is asymmetrically broadened toward lower9 values (higher lattice parameters) whereas the 200 and 020 peaks are asymmetrically broadened toward higher 0 values corresponding to lower lattice parameters. This collection of results is tentatively interpreted as being due to strains in martensite due to transformation induced substructure and precipitated carbides.

Stress relaxation during the tempering of hardened steel

Stress relaxation during tempering was examined in hardened AISI 52100 steel and a series of plain-carbon steels by means of wedged split-ring specimens. Enhanced relaxation arising from the formation and growth of ϵ-carbide and cementite was found to be independent of the elastically applied stress at least within the range investigated. The percent stress relaxation increased with increasing carbon content of the as-quenched martensite, while the kinetics of relaxation,were essentially those associated with tempering. In addition to stress relaxation, a phenomenon of creepback was oblerved when a partially recovered sample was re-tempered in the unwedged condition. A mechanism for the enhanced relaxation during tempering is proposed. It is based on the generation of a plastic zone set up when a local volume change occurs, e.g., that accompanying the precipitation of a carbide particle in the martensitic matrix during tempering. This mechanism is not only consistent with the stress-relaxation results, but is also capable of explaining the creepback effect.

The effect of heat treatment on the tensile strength and hardness of a quenched ternary zirconium-base alloy (Zr-0.5 wt.% Nb-1 wt.% Cr)

The response of a ternary zirconium-base alloy, Zr-0.5 wt.% Nb-1 wt.% Cr, to hardening by thermal treatment has been investigated. A marked increase in the strength of the alloy was observed when it transformed to a martensitic structure after rapid cooling from the high temperature phase field, α + β or β. The structure of the as-quenched martensite was found to be hexagonal close-packed and it was supersaturated with respect to the solute elements (Cr and Nb). Tempering of this structure in the temperature range 400°–700°C lowered the hardness, an appreciable decrease being observed on tempering above 500°C. Factors responsible for the high strength of the as-quenched martensite are suggested.

Field-ion microscope investigations of fine structures in as-quenched and tempered ferrous martensite

The fine structures in as-quenched and tempered martensite in an Fe-0.8% C-0.4% Mn steel was investigated with hydrogen-ion microscopy. Field-ion image characteristics and contrast features of these materials were examined. In the images of as-quenched martensite both twinned and untwinned regions were observed and crystallographic as well as morphological data on the transformation twins were obtained. Variation in microstructural features occurring during tempering were studied and a comparison was made between microstructure and hardness. The contribution of various hardening mechanisms was discussed for the peak-hardness in the first stage of tempering.

Field-ion microscopy of ferrous martensite

Abstract Fine structures of as-quenched martensite in an Fe-0.8% C-0.4% Mn steel were investigated using field-ion microscopy techniques. Both untwinned and twinned martensite regions were detected. Differences in image contrast and regularity of net plane edges were found in the hydrogen-ion images of pure iron and untwinned martensite. It is proposed that in the twinned martensite images the twins appear as bright bands while dark bands represent the intersection of twin boundaries with the specimen surface. The twins were found to have a {112} orientation and a width of 25–100 A.

Tempering of martensite: A field ion microscope study

As-quenched martensite in an Fe-0.88% C-0.45% Mn steel was tempered in the field ion microscope. During the first stage of the tempering process, precipitation of two types of ε-carbide is observed—in an ordered fashion along twin planes, and as dispersed random particles in untwinned regions of high dislocation density. Further tempering causes cementite precipitation to occur most rapidly in those regions which contain random ε-carbide dispersions. Continued tempering causes the cementite thus nucleated to grow, and causes the transformation of low-carbon martensite to cementite and ferrite to proceed to completion. This is seen by the steady decrease and ultimate absence of the twin boundaries in the image.