Showing papers on "Austenite published in 1985"

Structure and Corrosion Resistance of Plasma Nitrided Stainless Steel

Abstract: AISI316 stainless steel has been plasma nitrided at 570°C over a range of processing conditions, and the resultant corrosion properties have been investigated by a potentiodynamic polarization technique. X-ray diffraction and transmission electron microscopy studies have been used to map the process parameters under which a duplex surface compound layer of γ' phase and austenite is formed. This surface compound layer has better corrosion resistance than a plasma nitrided stainless steel surface, where the normal hardened layer consists of austenite' and chromium nitride precipitates. It has been found that the improvement in corrosion resistance is related to the presence of the γ' nitride. Furthermore; low temperature plasma nitriding at 400° C produces a nitrided layer which has a corrosion resistance equivalent to that of the original material.

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

Abstract: The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.

Mechanical properties of 0. 40 Pct C-Ni-Cr-Mo high strength steel having a mixed structure of martensite and bainite

Abstract: A study has been systematically made of the effect of bainite on the mechanical properties of a commercial Japanese 0.40 pct C-Ni-Cr-Mo high strength steel (AISI 4340 type) having a mixed structure of martensite and bainite. Isothermal transformation of lower bainite at 593 K, which appeared in acicular form and partitioned prior austenite grains, in association with tempered marprovided provided a better combination of strength and fracture ductility, improving true notch tensile strength (TNTS) and fracture appearance transition temperature (FATT) in Charpy impact tests. This occurred regardless of the volume fraction of lower bainite present and/or the tempering conditions employed to create a difference in strength between the two phases. Upper bainite which was isothermally transformed at 673 K appeared as masses that filled prior austenite grains and had a very detrimental effect on the strength and fracture ductility of the steel. Significant damage occurred to TNTS and FATT, irrespective of the volume fraction of upper bainite present and/or the tempering conditions employed when the upper bainite was associated with tempered martensite. However, when the above two types of bainite appeared in the same size, shape, and distribution within tempered martensite approximately equalized to the strength of the bainite, a similar trend or a marked similarity was observed between the tensile properties of the mixed structures and the volume fraction of bainite. From the above results, it is assumed that the mechanical properties of high strength steels having a mixed structure of martensite and bainite are affected more strongly by the size, shape, and distribution of bainite within martensite than by the difference in strength between martensite and bainite or by the type of mixed bainite present. The remarkable effects of the size, shape, and distribution of bainite within martensite on the mechanical properties of the steel are briefly discussed in terms of the modified law of mixtures, metallographic examinations, and the analyses of stress-strain diagrams.

Prediction of austenitic weld metal microstructure and properties

Abstract: Diagrams, such as the Schaeffler and DeLong diagrams, have been used to assist in the proper selection and use of austenitic filler materials and to predict weld metal microstructures and properties. These diagrams have been very successful in predicting the amount of delta ferrite in stainless steel weld metal. This paper is concerned with the predictability of austenitic weld metal microstructure and properties over a larger compositional range. In this paper, new expressions are introduced to predict the martensite start room temperature composition or the martensite start temperature. Some of these high manganese ferrous alloys are the basis for the new ''no chromium'' stainless steel. Various available diagrams, which allow for the prediction of weld metal microstructure, are given.

The Effect of Nitrogen on the Sensitization of AISI 304 Stainless Steel

Abstract: Thermodynamic calculations have been used to construct time temperature sensitization (TTS) diagrams for AISI 304 stainless steels (SS). The quantitative Stawstrom and Hillert model, which is based on chromium diffusion control of sensitization, was used to calculate the TTS diagrams. Electrochemical potentiokinetic reactivation (EPR) tests were performed on these steels, with various nitrogen additions, to obtain the experimental TTS curves. The calculated and experimental TTS diagrams have been compared to better understand nitrogen addition effects on the sensitization kinetics of AISI 304 SS. These results indicate that nitrogen additions below 0.16 wt% retard the sensitization kinetics; one possible mechanism by which this is achieved is an increase in chromium concentration adjacent to the grain boundaries, which decreases the chromium concentration gradient between the austenite matrix and the grain boundaries and hence retards carbide growth.

The development of some dual-phase steel structures from different starting microstructures

Abstract: The development of dual-phase structures with different morphologies has been studied in detail by intercritical annealing specimens of a steel containing 0.11 pct C and 1.6 pct Mn with different microstructures before annealing. The kinetics of formation of the two-phase structure at the annealing temperature and the redistribution of substitutional solute elements were measured in specimens quenched from the intercritical annealing temperature. The structure before annealing was either banded ferrite-pearlite, homogenized ferrite-pearlite, lath martensite, spheroidal cementite dispersed in ferrite, or austenite. No measurable partitioning of silicon or molybdenum, present in the steel in small concentrations, was found. However, close to equilibrium partitioning of manganese occurred on annealing specimens with either a ferrite-pearlite or a lath martensite structure, but during the separation of ferrite from austenite in step-quenched specimens there was no partitioning. Surprisingly, measurements of manganese concentrations using an electron beam of 1 nm diameter at intervals of 5 nm or less revealed the presence of narrow spikes in the concentration profile at many ferriteaustenite interfaces in specimens with a ferrite-pearlite or martensite starting structure as well as in those step-quenched from austenite. In some instances, a minimum in the concentration profile was found in ferrite, adjacent to a maximum at an interface. Thus, adsorption of manganese at ferriteaustenite interfaces produces concentrations in excess of the concentrations indicated by the equilibrium diagram. The probable diffusion processes controlling the kinetics of transformation in the different microstructures are identified.

Kinetics of austenite-ferrite and austenite-pearlite transformations in a 1025 carbon steel

Abstract: Isothermal and continuous-cooling transformation kinetics have been measured dilatometrically for the γ → α+ γ′ and γ′→ P reactions in a 1025 steel. The isothermal transformation of austenite for each reaction was found to fit the Avrami equation after the fraction transformed was normalized to unity at the completion of the reaction and a transformation-start time was determined. The transformation kinetics under isothermal conditions therefore were characterized in terms of then andb parameters from the Avrami equation together with the transformation-start times. The parametern was found to be independent of temperature over the range studied (645 to 760 ‡C) and to have values of 0.99 and 1.33 for the ferrite and pearlite reactions, respectively. This indicates that the nucleation condition is essentially constant and site saturation occurs early in the transformation process. The continuous-cooling experiments were conducted at cooling rates of 2 to 150 ‡C per second to determine the transformation-start times for the ferrite and pearlite reactions and the completion time for transformation to pearlite under CCT conditions. Both reactions were found to obey the Additivity Principle for continuous cooling provided that the incubation (pre-transformation) period was not included in the transformation time. The isothermal transformation data and CCT transformation-start times have been incorporated in a mathematical model to predict continuous-cooling transformation kinetics that agree closely with measurements made at three cooling rates.

The dependence of some tensile and fatigue properties of a dual-phase steel on its microstructure

Abstract: Five different dual-phase microstructures have been produced in a steel containing 0.11 C, 1.60 Mn, 0.73 Si. By careful design of the heat treatment schedules, it was arranged that all the specimens had the same fine austenite grain size before intercritical annealing and that they all contained close to 30 vol pct martensite and a negligible fraction of retained austenite after annealing. The microstructures were characterized by measurement of the mean ferrite path and the density and distribution of the extrinsic transformation accommodation dislocations in the substructure of the ferrite. Specimens representative of each of the microstructures were tested in tension at room temperature. The strength, work-hardening, and fracture of three of them were examined in detail and correlated with the microstructural parameters. The microstructural features which most influence the tensile properties are identified. Two specimens, representative of the finest and coarsest microstructures, were the subject of a detailed exploration of the initiation of persistent slip and microcracks and of crack propagation in bending fatigue. The fatigue endurance limit was also measured. It is shown that the initiation of persistent slip is primarily influenced by the heterogeneity in the density of the dislocations in the ferrite. The microstructural features of greatest importance in controlling the formation and propagation of microcracks are the local dislocation density and the mean ferrite path.

A theoretical model for the flow behavior of commercial dual-phase steels containing metastable retained austenite: Part I. derivation of flow curve equations

Abstract: A semi-mechanistic model for predicting the flow behavior of a typical commercial dual-phase steel containing 20 vol pct of ‘as quenched’ martensite and varying amounts of retained austenite has been developed in this paper. Assuming that up to 20 vol pct of austenite with different degrees of mechanical stability can be retained as a result of certain thermomechanical treatments in a steel of appropriate low carbon low alloy chemistry, expressions for composite flow stress and strain have been derived. The model takes into account the work hardening of the individual microconstituents(viz., ferrite-@#@ α, retained austenite- γr, and martensite -α′) and the extra hardening of ferrite caused by accommodation dislocations surrounding the ‘as quenched’ as well as the strain-induced(γr→ α′) martensite. Load transfer between the phases has been accounted for using an intermediate law of mixtures which also considers the relative hardness of the soft and the hard phases. From the derived expressions, the flow behavior of dual phase steels can be predicted if the properties of the individual microconstituents are known. Versatility of the model for application to other commercial steels containing a metastable phase is discussed.

Effect of Carbon on Hot Ductility of As-cast Low Alloy Steels

Abstract: The effect of C on hot ductility of low alloy steels has been studied in view of surface cracking of continuously cast (CC) slabs. As the ductility was not affected by C content in hot tensile test of the reheated specimens, the well-known C dependency of surface cracking susceptibility in CC slabs can be ascribed to the microstructural change during the solidification process. Austenite grain size of as-cast materials was found to depend largely on C content, i.e., the maximum grain size in 0.10-0.15% C region. This can be explained by the higher austenite formation temperature in these C region. Austenite grain growth rapidly occurred after the complete transformation or solidification into γ phase, as the strong pinning effect of the second phase such as δ-ferrite or liquid phase on γ grain boundary migration was relieved. Carbon dependency of γ grain size became more marked with increasing cooling rate up to that of ordinary continuous casting.Such coarsening of γ grains enhanced intergranular fracture, resulting in ductility loss inversely proportional to the γ grain size. Uneven surface solidification in the mold due to the peritectic reaction will produce much coarse γ structure because of the local delay of cooling. Surface cracking susceptibility will also be largely accelerated by this mechanism. Carbon range where surface cracking susceptibility was the largest varied with the chemical compositions. This shift can be explained in terms of the effect of alloying elements such as Mn on the peritectic composition.

The stability of precipitated austenite and the toughness of 9Ni steel

Abstract: A correlation was confirmed between the good low temperature Charpy toughness of 9Ni steel and the stability of its precipitated austenite against the martensitic transformation. Changes in the microstructure during isothermal tempering were studied in detail. The austenite/martensite interface is originally quite coherent over ∼100 A distances. With further tempering, however, the dislocation structure at the austenite/martensite interface changes, and this change may be related to the increased instability of the austenite particles. The reduction in austenite carbon concentration does not seem large enough to account for the large reduction in austenite stability with tempering time. The strains inherent to the transformation of austenite particles create dislocation structures in the tempered martensite. The large deterioration of the Charpy toughness of overtempered material is attributed, in part, to these dislocation structures.

Phase constitution and lattice parameter relationships in rapidly solidified (Fe0.65Mn0.35)0.83 Al0.17-xC and Fe3Al-xC pseudo-binary alloys

Abstract: A quasi-equilibrium temperaturevs carbon-concentration phase diagram of rapidly solidified pseudo-binary (Fe0.65Mn0.35)0.83Al0.17-xC alloys was determined after heat treatment in the 823 to 1323 K range. Lattice parameter relationships of rapidly solidified (Fe0.65Mn0.35)0.83Al0.17-xC and Fe3Al-xC alloys in the ferrite, austenite, and perovskite carbide phases were established as a function of the carbon concentration. This study shows that when a high concentration of carbon is present in the alloys a perovskiteL′l2 carbide is formed directly from the rapid solidification process. It is established also in this study that the carbon atom contribution to the lattice parameter increase in the fcc-based cubic crystal is greater in the disorderedγ-phase than in the ordered (L′l2 structure)κ-phase.

Anisotropy of tensile behaviour in a duplex stainless steel sheet

Abstract: Unusually large anisotropy of strength has been observed in commercially produced sheets of a duplex stainless steel, Fe–1·5Mn–22Cr–5·5Ni–3Mo–0·15N. The austenite and ferrite phases in this materia...

Correlation of microstructure and fracture toughness in two 4340 steels

Abstract: A correlation is made of plane strain fracture toughness and microstructure in two steels corresponding to AISI 4340 composition. The steels were deoxidized with aluminum and titanium-aluminum additions, respectively. In the case of the aluminum killed steel, austenitizing at temperatures above 950 °C led to large austenite grain sizes, whereas in the titanium steel grain sizes were maintained below about 70 μm even after austenitizing at temperatures approaching 1200 °C. This allowed a comparison of variations in plane strain fracture toughness with austenitizing temperature between microstructures that underwent large increases in grain size and those that did not. The results are interpreted using a simple fracture model which indicated that particle spacing is of primary importance in controlling toughness. The overall observed phenomenology, however, is not explainable using simple models that essentially require that either critical stresses or critical strains be achieved over distances scaling with microstructure. This finding suggests that more detailed crack tip models than presently exist are required if the full effects of heat treatment are to be understood and explained.

Diffusion-controlled growth of ferrite plates in plain-carbon steels

Abstract: The implementation of the theory of diffusion-controlled growth of ferrite plates in plain-carbon steels is critically assessed. It is found that the use of empirically extrapolated diffusion coefficients, phase boundaries, and thermodynamic functions leads to errors in calculations of growth rate. The errors become most important for low transformation temperatures, leading to exaggerated growth rates. Ways of avoiding these difficulties are suggested, and a new analysis of experimental data indicates that the lengthening of Widmanstatten ferrite plates in Fe–C alloys occurs at a rate which is influenced by the diffusion of carbon in the austenite ahead of the interface, assuming that the plates adopt a tip radius consistent with the maximum growth velocity. However, there is a systematic discrepancy between theory and experiment: plate-growth theory seems to underestimate the lengthening rate by some 5 μm s−1. This may have something to do with the lath shape of Widmanstatten ferrite, but an ana...

Structure of centrifugally cast austenitic stainless steels: Part 1 HK 40 as cast and after creep between 750 and 1000°C

Abstract: Centrifugally cast 0·4C–25Cr–20Ni (wt-%) steel (HK 40) has been subjected to detailed microscopic examination both in the as-cast condition and after long-term creep. The as–cast alloy contained a eutectic of M7C3 and austenite, which was subjected to energy-dispersive X-ray analysis. Observations by transmission electron microscopy revealed high dislocation densities near cell boundaries. After creep in the range 750–1000°C, the eutectic carbides coarsened and transformed from M7C3 to M23C6; moreover, the M23C6 carbide precipitated predominantly on dislocations as a finer dispersion within the grains. During coarsening, the M23C6 particles generated further dislocations in the austenite. The higher creep strength of the cast HK 40 compared to wrought alloys is discussed in terms of the detailed microstructural observations, in particular the M23C6 dispersion.MST/135

Effects of sulphur on hot ductility of low-carbon steel austenite

Abstract: The hot ductility of low-carbon austenite has been studied in order to reveal the hot-cracking mechanism related to sulphur segregation and sulphide precipitation during solution treatment and deformation. Ductility was greatly reduced during high-strain-rate tensile deformation in the temperature range 1073-1373 K. Fracture occurred via a typical intergranular mode, characterized by both shallow dimples caused by microvoid coalescence and rather smooth facets caused by intergranular decohesion. The former morphology is a result of the dense precipitation of sulphides, identified as Fe-rich (Fe, Mn) S, on the austenite grain boundaries. Finely dispersed sulphide particles within the grains and the existence of precipitate-free zones along grain boundaries enhanced strain localization in the vicinity of the boundaries during deformation. The intergranular decohesion arises from the decrease in grain-boundary strength produced by sulphur segregation. Hot ductility is improved on decreasing the solut...

Microstructure-mechanical property relationships of dual-phase steel wire

Abstract: The high strain hardening rate and formability of dual-phase steels makes them promising choices for drawing into high strength wire. As the fundamental part of an alloy design project, dual-phase steels with several different martensite volume fractions, particle shapes, particle sizes, compositions, and crystallographic relations with the ferrite matrix were studied. They were wire drawn with true strains of up to 6.1. The initial microstructure, void formation tendency, drawability, and mechanical properties of the various steels were compared and correlated. The Fe-2Si-0.1C alloy was found to be the most promising with a suggested reduction in the carbon level to 0.06 to 0.08 pct. The double heat treatment which consists of quenching from austenite to martensite followed by intercritical annealing and quenching produced the best microstructure for drawing into wire. The annealing temperature should be adjusted to yield 25 to 30 vol pct martensite in the final microstructure. Stress relief after drawing provided a substantial increase in ductility without significant loss in strength.

Effect of accelerated cooling after controlled rolling on the hydrogen induced cracking resistance of line pipe steel.

Abstract: The effect o f accelerated cooling after controlled rolling on the hydrogen induced cracking (HIC) resistance of line pipe steels produced from continuously cast slabs was examined, and the relation between HIC resistance and the microstructure in the segregated zone at the mid-thickness of plate was clarified.The optimization of accelerated cooling conditions reduced the volume fraction of high carbon martensite or upper bainite formed in the segregated zone, and this resulted in significant improvement of HIC resistance.The above result can be explained by the model that the high cooling rate as well as the optimized start and stop temperatures in accelerated cooling suppress the rejection of carbon from the non-segregated area to segregated zone during the austenite to ferrite transformation causing the more uniform profile of carbon distribution in the through-thickness direction of plate.

The effect of heat treatment, mechanical deformation, and alloying element additions on the rate of bainite formation in austempered ductile irons

Abstract: Several Alloys of Ductile Cast Iron containing various amounts of manganese, molybdenum, and nickel were austempered in the temperature range 316° to 427 °C. The rate and morphology of ferrite platelet formation (bainite reaction) were studied by optical metallography, x-ray diffraction, and hardness measurements. Austenitizing temperature, austempering temperature, and deformation by rolling were used as variables to control the kinetics of ferrite formation, stage I of the austempering reaction.

Austenitization during intercritical annealing of an Fe-C-Si-Mn dual-phase steel

Abstract: Partial austenitization during the intercritical annealing of an Fe-2.2 pct Si-1.8 pct Mn-0.04 pct C steel has been investigated on four kinds of starting microstructures. It has been found that austenite formation during the annealing can be interpreted in terms of a carbon diffusion-limited growth process. The preferential growth of austenite along the ferrite grain boundaries was explained by the rapid carbon supply from the dissolving carbide particles to the growing fronts of austenite particles along the newly formed austenite grain boundaries on the prior ferrite grain boundaries. The preferential austenitization along the grain boundaries proceeded rapidly, but the austenite growth became slowed down after the ferrite grain boundaries were site-saturated with austenite particles. When the ferrite grain boundaries were site-saturated with austenite particles in a coarse-grained structure, the austenite particles grew by the mode of Widmanstatten side plate rather than by the normal growth mode of planar interface displacement.

The Effect of Straining on the Transport of Hydrogen in Iron, Nickel, and Stainless Steel

Abstract: Hydrogen permeation transients for low-carbon iron were measured at room temperature; pure nickel and 17Cr-12Ni austenitic stainless steel (SS) specimens were electrochemically charged with hydrogen. The results for unstrained specimens were compared with those obtained under conditions of continuous stretching. A slight increase in the permeation rate for iron was observed in the range of elastic deformation, with no change in effective diffusivity. Plastic deformation caused a substantial reduction of both the diffusivity and permeability of hydrogen. The magnitude of these effects depended on the amount of strain but was independent of the strain rate; this suggested they were caused by an enhanced trapping of hydrogen. Only a slight influence of plastic deformation on the effective diffusivity and permeability of hydrogen was observed in nickel and austenitic SS.

Influence of deformation on the transformation of austenitic stainless steels

Abstract: Two commercial austenitic stainless steels of type 304 and 321 were deformed by reduction in thickness at room temperature or at liquid nitrogen temperature and subsequently annealed in the temperature range 473 to 1073 K. Microstructural changes were examined by electron microscopy. The deformation product in both the steels are different. An attempt has been made to correlate the mechanical properties with the microstructural changes.

In situ TEM study of martensitic NiTi amorphization by Ni ion implantation

Abstract: The amorphization of an equiatomic martensitic NiTi alloys is followed at room temperature using the Philips EM 400 transmission electron microscope (TEM) on line with the CSNM ion implantor of Orsay (France). During a 390 keV Ni + ion irradiation, it is observed that the initial martensitic structure does not transform to austenite (B2 structure) before the crystalline-amorphous transition. From a micrographic series of the same part of a thin foil, an estimate of the amorphous volume, as a function of the dose, is made. It turns out that the results are consistent with a cascade overlap mechanism. A comparison is made with the amorphization obtained by 1.5 MeV electron irradiation in a high voltage electron microscope (HVEM).

A theoretical model for the flow behavior of commercial dual-phase steels containing metastable retained austenite: Part II. calculation of flow curves

Abstract: The role of metastable retained austenite(γ R), its volume fraction, and mechanical stability on the flow characteristics of a dual phase steel containing 20 vol pct of ‘as quenched’ martensite in a ferrite matrix has been examined in this paper employing the flow curve expressions derived in Part I of this paper. It has been found that for a given volume fraction ofγ R, its mechanical stability plays a crucial role in enhancing the ductility. Whereas highly stableγ R does not contribute either to strength or ductility of the steel, highly unstableγ R which causes an increase in the strength is detrimental to ductility. Aγ R which is moderately stable and undergoesγ R → α′ transformation over a larger strain range is beneficial to enhanced ductility. Increasing amounts of moderately stableγ R significantly increase both the strength and the ductility of dual-phase steels through a sustained work-hardening due toγ R → α′ transformation. Load transfer which is determined by a parameterq has a significant contribution to work-hardening. A value of ∣|q∣|= 4500 MPa has been found to partition realistically the stress and strain in these steels.

Method of making aluminized strengthened steel

Abstract: A steel article which is made of carbon or low alloy steel, with a prescribed chemical composition, is heat treated so that its yield strength is increased to a minimum of 60,000 psi and is subsequently dipped or spray covered with aluminum and then heated to interdiffuse aluminum metal and the iron in the steel matrix, thereby forming a new intermetallic layer which is an integral part of the steel article and which is essentially an aluminum-iron alloy, all in a manner which does not lower the yield strength of the steel below 60,000 psi. In the preferred form of the invention, a steel article is austenitized above its critical temperature for about 15 minutes or more and is subsequently rapidly cooled to below 600° F. to transform the austenite structure to a much stronger martensitic structure and is then reheated to below 1341° F. to temper the martensite. Aluminum is applied to the outer surface of the cleaned article by dipping in molten aluminum or otherwise, and the article is again reheated to above 1000° F. but below 1341° F. whereupon a substantial quantity of aluminum interdiffuses with the iron in the steel matrix, thereby forming a new intermetallic alloy layer. The interdiffusion process is carried out without reducing the yield strength level below 60,000 psi. This method provides an inexpensive, high-strength steel tool with enhanced corrosion resistance.

Modified heat treatment for lower temperature improvement of the mechanical properties of two ultrahigh strength low alloy steels

Abstract: In the previous papers, a new heat treatment for improving the lower temperature mechanical propertise of the ultrahigh strength low alloy steels was suggested by the authors which produces a mixed structure of 25 vol pct lower bainite and 75 vol pct martensite through isothermal transformation at 593 K for a short time followed by water quenching (after austenitization at 1133 K). In this paper, two commercial Japanese ultrahigh strength steels, 0.40 pct C-Ni-Cr-Mo (AISI 4340 type) and 0.40 pct C-Cr-Mo (AISI 4140 type), have been studied to determine the effect of the modified heat treatment, coupled above new heat treatment withγ ⇆ α′ repctitive heat treatment, on the mechanical properties from ambient temperature (287 K) to 123 K. The results obtained for various test temperatures have been compared with those for the new heat treatment reported previously and the conventional 1133 K direct water quenching treatment. The incorporation of intermediate four cyclicγ ⇆ α′ repctitive heat treatment steps (after the initial austenitization at 1133 K and oil quenching) into the new heat treatment reported previously, as compared with the conventional 1133 K direct water quenching treatment, significantly improved 0.2 pct proof stress as well as notch toughness of the 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel at similar fracture ductility levels from 287 to 123 K. Also, this heat treatment, as compared with the conventional 1133 K direct water quenching treatment, significantly improved both 0.2 pct proof stress and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel with increased fracture ductility at 203 K and above. The microstructure consists of mixed areas of ultrafine grained martensite, within which is the refined blocky, highly dislocated structure, and the second phase lower bainite (about 15 vol pct), which appears in acicular form and partitions prior austenite grains. This newly developed heat treatment makes it possible to modify the new heat treatment reported previously so as to raise 0.2 pct proof stress to a higher level and keep notch toughness at the same level. The improvement in the mechanical properties is discussed in terms of metallographic observations and the modified law of mixtures and so forth.