Effect of decreased hot-rolling reduction treatment on fracture toughness of low-alloy structural steels

Commercial low-alloy structural steels, 0.45 pct C (AISI 1045 grade), 0.40 pct C-Cr-Mo (AISI 4140 grade), and 0.40 pct C-Ni-Cr-Mo (AISI 4340 grade), have been studied to determine the effect of the decreased hot-rolling reduction treatment (DHRRT) from 98 to 80 pct on fracture toughness of quenched and highly tempered low-alloy structural steels. The significant conclusions are as follows: (1) the sulfide inclusions were modified through the DHRRT from a stringer (mean aspect ratio: 16.5 to 17.6) to an ellipse (mean aspect ratio: 3.8 to 4.5), independent of the steels studied; (2) the DHRRT significantly improvedJ _Ic in the long-transverse and shorttransverse orientations, independent of the steels studied; and (3) the shelf energy in the Charpy V-notch impact test is also greatly improved by the DHRRT, independent of testing orientation and steels studied; however, (4) the ductile-to-brittle transition temperature was only slightly affected by the DHRRT. The beneficial effect on theJ _Ic is briefly discussed in terms of a crack extension model involving the formation of voids at the inclusion sites and their growth and eventual linking up through the rupture of the intervening ligaments by local shear.     

Improved lower temperature fracture toughness of ultrahigh strength 4340 steel through modified heat treatment

A modified heat treatment has been suggested whereby lower temperature plane-strain fracture toughness (K _IC) of 4340 ultrahigh strength steel is dramatically improved in developed strength and Charpy impact energy levels. The modified heat-treated 4340 steel (MHT-4340 steel) consists of a mixed structure of martensite and about 25 vol pct lower bainite which appears in acicular form and partitions prior austenite grains. This is produced through isothermal transformation at 593 K for a short time followed by an oil quench (after austenitizing at 1133 K and subsequent interrupted quenching in a lead bath at 823 K). The mechanical properties obtained at room temperature (293 K) and 193 K have been compared with those achieved using various heat treatments. Significant conclusions are as follows: the MHT-4340 steel compared to the 1133 K directly oil-quenched 4340 steel increased theK _IC values by 15 to 20 MPa • m^1/2 at increased strength and Charpy impact energy levels regardless of the test temperature examined. At 193 K,K _IC values of the MHT-4340 steel were not less than those of the 1473 K directly oil-quenched 4340 steel, in whichK _IC values are significantly enhanced at markedly increased strength, ductility, and Charpy impact energy levels. The MHT-4340 steels compared to austempered 4340 steels at 593 K, which have excellent Charpy impact properties, showed superiorK _IC values at significant increased strength levels irrespective of test temperatures. The lower temperature improvement inK _IC can be attributed to not only the crack-arrest effect by acicular lower bainite but also to the stress-relief effect by the lower bainite just ahead of the current crack.     

Fracture behavior of stainless steel-toughened NiAl composite plate

Analysis of the tensile and fracture behavior of a composite system consisting of boron carbide particulate-reinforced NiAl with continuous 304 stainless steel toughening regions was performed. The composite was fabricated by extrusion, with the toughening regions extending along the length of the plate in the extrusion direction. Mechanical properties were determined as a function of orientation. Tensile testing revealed that the composite modulus varied only slightly as a function of testing direction, the strength was approximately 25 pct greater in the longitudinal relative to the transverse orientation, and the transverse failure strain was only 0.3 pct compared to values in excess of 10 pct for longitudinal testing. Notched Charpy impact testing indicated that the energy absorption values varied significantly as a function of specimen location and crack growth direction, ranging from 2 to 40 Joules. In addition,K _IC values measured on subsize compact tension samples were found to range from 17 to 27 MPa ⋅ m^1/2. It was also established that theK _max values determined from the maximum load measured during compact tension testing were similar to theK _Q values calculated from instrumented notched Charpy impact testing. Finally, the fatigue crack growth characteristics of the composite were determined as a function of orientation.     

Effect of volume fraction and shape of sulfide inclusions on through-thickness ductility and impact energy of high-strength 4340 plate steels

The effect of volume fraction and shape of sulfide inclusions on the tensile ductility and impact energy of high-strength AISI 4340 plate steels in the transverse and through-thickness testing directions was investigated for four tensile-strength levels of 930, 1210, 1410, and 1960 MPa (135, 175, 205, and 285 ksi). The volume fraction of sulfide inclusions was changed by varying the sulfur content from 0.002 to 0.022 pct. The shape of the sulfide inclusions was changed from lenticular MnS to spheroidal RE₂O₂S (RE_xS_y) by rare-earth treatment. Axisymmetric tensile ductility and charpy impact energy [22 °C (72 °F)] decreased much more rapidly in the through-thickness testing direction than in the transverse testing direction when the volume fraction of sulfide inclusions was increased. Changing the shape of the sulfide inclusions from lenticular MnS to spheroidal RE₂O₂S (RE_xS_y) by rare-earth treatment increased axisymmetric tensile ductility and impact energy in the through-thickness testing direction for tensile-strength levels at or below 1410 MPa (205 ksi), but not at 1960 MPa (285 ksi). Impact energy was also improved in the transverse testing direction but only for tensile strength levels at or below 1210 MPa (175 ksi). Changes in the volume fraction or shape of the sulfide inclusions had little or no effect on transition temperature. Plane-strain tensile ductility was much less affected than either axisymmetric tensile ductility or impact energy by changes in inclusion morphology or in testing direction because of the formation of macroscopic shear bands inclined at about 45 deg to the tensile axis. As a result impact energy correlated better with axisymmetric tensile ductility than with plane-strain tensile ductility. The results are discussed in terms of various models involving the formation of voids at inclusion sites and their growth and eventual coalescence by localized shear during plastic straining.     

Fracture toughness of calcium-modified ultrahigh-strength 4340 steel

Commercial and low-sulfur 4340 steels have been studied to determine the effect of calcium treatment on modifying the morphology of nonmetallic inclusions and plane-strain fracture toughness (K _IC ) of the ultrahigh-strength, low-alloy steels at commercial heat level. The significant conclusions are as follows: (1) for the low-sulfur 4340 steel, the addition of calcium in the molten steel gave rise to the formation of finely distributed, spherical, calcium-sulfide (CaS) inclusions with a mean diameter of 1.3 μm; (2) in comparing the calcium-modified 4340 steel with commercial 4340 steel, the calcium-modified steel not only had an improvedK _IC by about 25 MPa•m^1/2 in the longitudinal (L) orientation and by about 30 MPa • m^1/2 in the transverse (T) orientation, but also had increased fracture ductility and Charpy impact energy at similar strength levels; and (3) for the commercial 4340 steel, the calcium treatment was not very effective in modifying the morphology of the inclusions on improving the mechanical properties of the steel. The beneficial effect of calcium modification coupled with low sulfur content on theK _Ic is briefly discussed in terms of a crack extension model involving the formation of voids at the inclusion sites and their growth and eventual linking-up through the rupture of the intervening ligaments by localized shear.     

The effects of heat treatment on fracture toughness and fatigue crack growth Rates in 440C and BG42 steels

The fatigue crack growth rates,da/dN, and the fracture toughness, K_Ic have been measured in two high-carbon martensitic stainless steels, 440C and BG42. Variations in the retained austenite contents were achieved by using combinations of austenitizing temperatures, refrigeration cycles, and tempering temperatures. In nonrefrigerated 440C tempered at 150 °C, about 10 vol pct retained austenite was transformed to martensite at the fracture surfaces duringK _Ic testing, and this strain-induced transformation contributed significantly to the fracture toughness. The strain-induced transformation was progressively less as the tempering temperature was raised to 450 °C, and at the secondary hardening peak, 500 °C, strain-induced transformation was not observed. In nonrefrigerated 440C austenitized at 1065 °C,K _Ic had a peak value of 30 MPa m^1/2 on tempering at 150 °C and a minimum of 18 MPa m^1/2 on tempering at 500 °C. Refrigerated 440C retained about 5 pct austenite, and did not exhibit strain-induced transformation at the fracture surfaces for any tempering temperature. TheK _Ic values for corresponding tempering temperatures up to the secondary peak in refrigerated steels were consistently lower than in nonrefrigerated steels. All of the BG42 specimens were refrigerated and double or quadruple tempered in the secondary hardening region; theK _Ic values were 16 to 18 MPa m^1/2 at the secondary peak. Tempered martensite embrittlement (TME) was observed in both refrigerated and nonrefrigerated 440C, and it was shown that austenite transformation does not play a role in the TME mechanism in this steel. Fatigue crack propagation rates in 440C in the power law regime were the same for refrigerated and nonrefrigerated steels and were relatively insensitive to tempering temperatures up to 500 °C. Above the secondary peak, however, the fatigue crack growth rates exhibited consistently lower values, and this was a consequence of the tempering of the martensite and the lower hardness. Nonrefrigerated steels showed slightly higher threshold values, ΔK_th, and this was ascribed to the development of compressive residual stresses and increased surface roughening in steels which exhibit a strain-induced martensitic transformation.     

Effect of microstructure on plane-strain fracture toughness of aisi 4340 steel

Commercially available AISI 4340 steel has been studied to determine the effect of transformation structures on plane-strain fracture toughness (K _IC). Martensitic and bainitic steels with wide variation in the prior austenitic grain size, and steels having two different mixed structures of martensite and bainite were investigated. Microstructures were examined by optical and transmission electron microscopy. Fracture morphologies were characterized by scanning electron microscopy. The significant conclusions are as follows: in a martensitic or lower bainitic steel in which well-defined packets were observed, the packet diameter is the primary microstructural factor controllingK _IC. The steel's property is improved with increased packet diameter. If the steel has an upper bainitic structure, the packet is composed of well-defined blocks, and the block size controls theK _IC property. When the steel has a mixed structure of martensite and bainite, the shape and distribution of the second phase bainite have a significant effect on theK _IC property. A lower bainite, which appears in acicular form and partitions prior austenite grains of the parent martensite, dramatically improves theK _IC in association with tempered martensite. If an upper bainite appearing as masses that fill prior austenite grains of the parent martensite is associated with tempered martensite, it significantly lowers the K_IC.     

Effect of Retained Austenite on the Fracture Toughness of Quenching and Partitioning (Q&P)-Treated Sheet Steels

Fracture toughness K _IC was measured by double edge-notched tension (DENT) specimens with fatigue precracks on quenching and partitioning (Q&P)-treated high-strength (ultimate tensile strength [UTS] superior to 1200 MPa) sheet steels consisting of 4 to 10 vol pct of retained austenite. Crack extension force, G _IC, evaluated from the measured K _IC, is used to analyze the role of retained austenite in different fracture behavior. Meanwhile, G _IC is deduced by a constructed model based on energy absorption by martensite transformation (MT) behavior of retained austenite in Q&P-treated steels. The tendency of the change of two results is in good agreement. The Q&P-treated steel, quenched at 573 K (300 °C), then partitioned at 573 K (300 °C), holding for 60 seconds, has a fracture toughness of 74.1 MPa·m^1/2, which is 32 pct higher than quenching and tempering steel (55.9 MPa·m^1/2), and 16 pct higher than quenching and austempering (QAT) steel (63.8 MPa·m^1/2). MT is found to occur preferentially at the tips of extension cracks on less stable retained austenite, which further improves the toughness of Q&P steels; on the contrary, the MT that occurs at more stable retained austenite has a detrimental effect on toughness.     

Effect of shape of sulfide inclusions on anisotropy of inclusion spacings and on directionality of ductility in hot-rolled C-Mn steels

The effects of sulfur content (0.004 or 0.013 pct) and sulfide morphology (stringered or globular) on anisotropy of tensile ductility and Charpy shelf energy were investigated in a series of 0.1 and 0.2 pct carbon, 1.0 pct manganese steels. The effect of sulfide inclusions on fracture strain or Charpy shelf energy correlated with the projected area of inclusions per unit volume,A _v , on a plane perpendicular to the tensile direction and the mean free distance between inclusions, λ, in a direction parallel to the tensile direction regardless of the amount or the shape of the inclusions or the test direction: longitudinal, transverse, or through-thickness. The magnitude ofA _v is directly proportional to the volume fraction of inclusions and inversely proportional to the inclusion dimension parallel to the tensile direction. The mean free distance, λ, is inversely proportional toA _v . Approximate relations were obtained for the nearest-neighbor distances between inclusions on the longitudinal, transverse, and through-thickness planes. These distances were incorporated into a model for ductile fracture based on an adaptation of a previously proposed criterion for the linkage of voids nucleated at second-phase particles. The agreement between the observed and predicted fracture strains for longitudinal, transverse, and through-thickness specimens of the steels studied is encouraging. Formerly with United States Steel Corporation, Research Laboratory, Monroeville, PA 15146     

Effect of sulfide inclusion morphology and pearlite banding on anisotropy of mechanical properties in normalized C-Mn steels

The influence of sulfur content, sulfide shape, and pearlite banding on the anisotropy of mechanical properties was evaluated in a series of 0.2 pct carbon, 1.0 pct manganese steels containing either 0.004 or about 0.013 pct sulfur with and without rare-earth additions. Both globular and stringered sulfide inclusions had a detrimental effect on reduction of area and Charpy shelf energy; this effect was particularly evident in the deterioration of through-thickness properties and was much more severe for stringered inclusions than for globular inclusions. The effect of the sulfide inclusions in the different steels on the through-thickness reduction of area and Charpy shelf energy correlated with differences in their mean free distances or nearest-neighbor distances, both of which depended upon the inclusion characteristics of volume fraction, size, and aspect ratio. The projected length of sulfide inclusions per unit area on a plane perpendicular to the transverse direction seemed to reflect the overall effect of the inclusion characteristics on through-thickness reduction of area and Charpy shelf energy and appeared to be a useful parameter for assessment of steel’s susceptibility to fracture. Pearlite banding had no obvious effect on reduction of area or Charpy shelf energy in any of the steels studied. The improvement observed for the steel with stringered sulfide inclusions as a result of the removal of pearlite banding was a consequence of the inclusions coarsening during the short-time high-temperature treatment used to remove banding.     

The stress intensities for slow crack growth in steels containing hydrogen

A test technique has been developed to determine the stress intensity for slow crack growth in hydrogen precharged steels. Measurements on several grades of maraging steel and a 300M steel show that hydrogen contents on the order of 2 ppm reduce the stress intensity for slow crack growth by 50 pct or more of theK _Ic values. At equivalent hydrogen contents the 300M steel was more severely embrittled than the mar aging steels. Comparison of the present results with aqueousK _Iscc data indicates that the amount of hydrogen “picked up by the steels in stress corrosion increases with increasing yield strength. Formerly with International Nickel Co.     

Crack Arrest Toughness of Two High Strength Steels (AISI 4140 and AISI 4340)

The crack initiation toughness (K _c ) and crack arrest toughness (K _a ) of AISI 4140 and AISI 4340 steel were measured over a range of yield strengths from 965 to 1240 MPa, and a range of test temperatures from -53 to +74°C. Emphasis was placed onK _a testing since these values are thought to represent the minimum toughness of the steel as a function of loading rate. At the same yield strengths and test temperatures,K _a for the AISI 4340 was about twice as high as it was for the AISI 4140. In addition, theK _a values showed a more pronounced transition temperature than theK _c values, when the data were plotted as a function of test temperature. The transition appeared to be associated with a change in fracture mechanism from cleavage to dimpled rupture as the test temperature was increased. The occurrence of a “pop-in” behavior at supertransition temperatures has not been found in lower strength steels, and its evaluation in these high strength steels was possible only because they are not especially tough at their supertransition temperatures. There is an upper toughness limit at which pop-in will not occur, and this was found for the AISI 4340 steel when it was tempered to its lowest yield strength (965 MPa). All the crack arrest data were identified as plane strain values, while only about one-half of the initiation values could be classified this way.     

Heat treatment for improvement in lower temperature mechanical properties of 0.40 pct C-Cr-Mo ultrahigh strength steel

In the previous paper, it was reported that isothermal heat treatment of a commercial Japanese 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel (AISI 4340 type) at 593 K for a short time followed by water quenching, in which a mixed structure of 25 vol pct lower bainite and 75 vol pct martensite is produced, results in the improvement of low temperature mechanical properties (287 to 123 K). The purpose of this paper is to study whether above new heat treatment will still be effective in commercial practice for improving low temperature mechanical properties of the ultrahigh strength steel when applied to a commercial Japanese 0.40 pct C-Cr-Mo ultrahigh strength steel which is economical because it lacks the expensive nickel component (AISI 4140 type). At and above 203 K this new heat treatment, as compared with the conventional 1133 K direct water quenching treatment, significantly improved the strength, tensile ductility, and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel. At and above 203 K the new heat treatment also produced superior fracture ductility and notch toughness results at similar strength levels as compared to those obtained by usingγ α′ repetitive heat treatment for the same steel. However, the new heat treatment remarkably decreased fracture ductility and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel below 203 K, and thus no significant improvement in the mechanical properties was noticeable as compared with the properties produced by the conventional 1133 K direct water quenching treatment and theγ α′ repetitive heat treatment. This contrasts with the fact that the new heat treatment, as compared with the conventional 1133 K direct water quenching treatment and theγ α′ repetitive heat treatment, dramatically improved the notch toughness of the 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel, providing a better combination of strength and ductility throughout the 287 to 123 K temperature range. The difference in the observed mechanical properties between the above two ultrahigh strength steels is discussed on the basis of the effect of nickel content, fracture profile, and so forth.     

Effect of Sulfides and Sulfide Morphology on Anisotropy of Tensile Ductility and Toughness of Hot-Rolled C-Mn Steels

The effects of sulfur content (0.004 or about 0.013 pct) and sulfide morphology (stringered or globular) on anisotropy of tensile ductility and Charpy V-notch (CVN) shelf energy were investigated in a series of 0.1 and 0.2 pct carbon, 1.0 pct manganese steels. The effect of sulfide inclusions on fracture strain or CVN shelf energy correlated with a single parameter,P, regardless of inclusion shape, stringered or globular, or test direction, longitudinal, transverse, or through-thickness. The parameterP was defined as the total projected length of inclusions per unit area on a plane parallel to the fracture plane. The fracture strain and CVN shelf energy decreased with an increase inP. The magnitude ofP was directly proportional to the volume fraction of inclusions and inversely proportional to the inclusion dimension perpendicular to the fracture plane. The lower tensile ductility and CVN shelf energy in the 0.2 as compared with the 0.1 pct carbon steels was a consequence of the greater pearlite content in the former steels. This greater pearlite content had no apparent effect on the work-hardening rate,H, but decreased the strain-rate sensitivity of the flow stress,M, and the strain-rate sensitivity of the work-hardening rate,B. The decrease inM andB with increasing pearlite content is in accord with a decrease in tensile ductility according to recent models of neck development in tension tests. It appears that the decrease in tensile ductility with increasing pearlite content is a result of enhanced localized shearing, which promotes the coalescence of voids nucleated at second phases.     

Fabrication of ultrahigh nitrogen austenitic steels by nitrogen gas absorption into solid solution

For the purpose of fabricating ultrahigh nitrogen austenitic steels (>1 mass pct N), the phenomenon of nitrogen absorption into solid solution was thermodynamically analyzed and applied to Fe-Cr-Mn system ternary alloy. During the annealing of the steel in a nitrogen gas atmosphere of 0.1 MPa at 1473 K (nitrogen absorption treatment), the nitrogen content of the steel was increased with the absorption of nitrogen gas from the material surface and then saturated when the system reached a state of equilibrium. Effect of the steel composition on an equilibrium nitrogen content was formulated taking account of interactions among Cr, Mn, and N atoms, and the condition for fabrication of ultrahigh nitrogen austenitic steels was clarified. The nitrogen addition to ultrahigh content markedly increased proof stress and tensile stress of the austenitic steels without losing moderate ductility. For example, Fe-24Cr-10Mn-1.43N (mass pct) alloy has 830 MPa in 0.2 pct proof stress, 2.2 GPa in true tensile stress, and 75 pct in total elongation. As a result of tensile tests for various nitrogen-bearing austenitic steels, it was found that the proof stress is increased in proportion to (atomic fraction of nitrogen)^2/3.     

The effects of titanium additions on AF1410 ultra-high-strength steel

The mechanical properties and microstructure of two heats of AF1410 steel were compared. The first heat, heat 811, contained a titanium addition of 0.02 wt pct, while the second heat, heat 91, contained no titanium, manganese, or other strong sulfide formers. The sulfur in heat 811 was gettered as titanium carbosulfide, while in heat 91 the sulfides were chromium sulfide. The toughness of heat 811 was found to be much enhanced compared to heat 91, with Charpy impact energies of 176 J and 79 J and K_IC fracture toughness values of 235 MPa.m^1/2 and 170 MPa.m^1/2, respectively. This significant difference in fracture toughness is attributed to the fact that titanium carbosulfide particles are more resistant to void nucleation than the chromium sulfide particles, which appear to nucleate voids at the onset of plastic strain. In addition to altering the sulfide particle type, the titanium addition also results in the presence of undissolved MC carbides in the titanium-modified steel in addition to the M₂C carbides found in heat 91. These carbides act as grain growth inhibitors, resulting in a finer prior austenite grain size and martensite packet size in heat 811.     

Studies on the development of high-strength dual-phase steel sheets with high rm values

Experimental studies of cold-rolled sheets from a series of silicon-phosphorus (0.07 pct P, variable Si) steels showed that, depending on the silicon content, a dual-phase sheet can be produced with 70 ksi (483 MPa) yield strength, 100 ksi (690 MPa) tensile strength, 15 pct elongation (in one inch), and anr _mvalue of about 1.8. The relatively low elongation value is believed to be due to quench aging. Further development to increase hardenability and reduce the required cooling rates is expected to improve the ductility without reducingr _mvalues.     

Structure/property/continuum synthesis of ductile fracture in inconel alloy 718

Two microscopic ductile fracture processes have been established in a fracture tough superalloy, Inconel 718, aged to five strength levels. At yield strengths less than 800 MPa, the mechanism is a slow tearing process within large pockets of inhomogeneous carbides and nitrides, giving rise to plane strain fracture toughness (K _IC)values greater than 120 MPa-m^1/2. At yield strengths greater than 900 MPa, the mechanism involves fracture initiation at carbides and nitrides followed by off crack plane void sheet growth nucleated at the Laves (σ) phases. Here, the fracture toughness drops to about 80 MPa-m^1/2. A Mode I normal strain growth model for low yield strength conditions and a shear strain void sheet model for high yield strength ones are shown to model K_IC data obtained from a J-integral evaluation of compact tension results.     

Influence of sulfide inclusions and pearlite content on the mechanical properties of hot-rolled carbon steels

Sulfur content and sulfide shape are known to have a marked influence on the tensile ductility and notch toughness of plate steels. To investigate the initiation and growth of fractures at inclusions during plastic straining, a detailed study was conducted with a series of 0.1 and 0.2 pct carbon, 1.0 pct manganese steels containing either 0.004 or 0.013 pct sulfur with and without rare-earth additions. This paper describes the results of this study and evaluates the influence of sulfur content and sulfide shape on the anisotropy in tensile ductility and notch toughness in the steels and assesses the influence of other factors, such as pearlite content, affecting the ductility and toughness. Both globular and stringered sulfide inclusions had a detrimental effect on reduction of area, shelf energy, and transition temperature, which was particularly evident in deterioration of through-thickness properties and which was much more severe for stringered inclusions than for globular inclusions. Increased pearlite content was more detrimental to reduction of area and transition temperature when stringered inclusions were also present, whereas its effect on shelf energy appeared to be about the same regardless of the presence of inclusions or their morphology.     

Microstructure and mechanical properties of a 2000 MPa grade co-free maraging steel

The precipitation kinetics in the aging temperature range of 713 to 813 K in a 2000 MPa grade Co-free maraging steel (Fe-18.9 pct Ni-4.1 pct Mo-1.9 pct Ti, mass pct) has been studied. Study on microstructure and mechanical properties showed that a great deal of Ni₃Ti and a type of unknown spheroidal precipitates both with average diameter of 2 to 3 nm are formed in the early aging stage at 713 K, which results in a high strength and a relatively low fracture toughness. Ni₃Ti precipitates grow into needle or rod shape and become the main precipitation as the aging time is prolonged. Strength increases and fracture toughness (K _IC ) decreases with growth of the precipitates. The ultra-high strength of the maraging steel subjected to long-time aging at 713 K is attributed to the high resistance to coarsening of the precipitates. The strengthening in the underaged condition at 713 K is a combination of dislocations cutting through precipitates and the Orowan mechanisms. Aged at 813 K, the size of Ni₃Ti precipitates is seriously nonuniform at the early stage and a small amount of interlath reverted austenite is formed. Thereafter, Ni₃Ti precipitates coarsen sharply accompanied with the embrittlement. Intralath reverted austenite appears subsequently. In the later stage of aging, the coarsened Ni₃Ti precipitates dissolve into the striplike intralath reverted austenite that is disorderly embedded in the matrix. All of these result in a low strength and low fracture toughness under overaging condition. Analysis shows that the formation of reverted austenite contains the diffusion and Kudjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) shear mechanisms.