Showing papers on "Maraging steel published in 1974"

An investigation of the plastic fracture of AISI 4340 and 18 Nickel-200 grade maraging steels

Abstract: The mechanisms of plastic fracture (dimpled rupture) in high-purity and commercial 18 Ni, 200 grade maraging steels and quenched and tempered AISI 4340 steels have been studied. Plastic fracture takes place in the maraging alloys through void initiation by fracture of titanium carbo-nitride inclusions and the growth of these voids until impingement results in coalescence and final fracture. The fracture of AISI 4340 steel at a yield strength of 200 ksi (1378 MN/mm2) occurs by nucleation and subsequent growth of voids formed by fracture of the interface between manganese sulfide inclusions and the matrix. The growth of these inclusion-nucleated voids is interrupted long before coalescence by impingement, by the formation of void sheets which connect neighboring sulfide-nucleated voids. These sheets are composed of small voids nucleated by the cementite precipitates in the quenched and tempered structures. The sizes of non-metallic inclusions are an important aspect of the fracture resistance of these alloys since the investigation demonstrates that void nuclea-tion occurs more readily at the larger inclusions and that void growth also proceeds more rapidly from the larger inclusions. Using both notched and smooth round tensile specimens, it was demonstrated that the level of tensile stress triaxiality does not effect the void nu-cleation process in these alloys but that increased levels of triaxial tension do result in greatly increased rates of void growth and a concomitant reduction in the resistance to plastic fracture.

Increased fracture toughness in a 300 grade maraging steel as a result of thermal cycling

Abstract: Systematic changes in the fracture toughness of a 300 grade commercial maraging steel were obtained using a non-standard heat-treating process. Microstructures consisting of variable amounts of retained austenite in an aged martensitic matrix were produced. A mathematical model is presented relating these toughness values to the properties of the individual constituents. Increases in fracture toughness resulting from the non-standard heat-treatment were attributed to the nature of the distribution of the tough phase (retained austenite) in a brittle matrix of precipitation hardened martensite. In some cases, a strain-induced transformation to martensite was observed which greatly added to the toughness. Some improvements in fatigue crack propagation characteristics also resulted from this heat treatment.

A study of grain boundary segregants in thermally embrittled maraging steel

Abstract: Auger electron spectroscopy was employed to directly determine the cause of thermal embrittlement of 250 series maraging steel. A drop in CVN energy with introduction of an embrittling intermediate anneal was directly correlated with a build-up of both Ti and C on the prior austenite grain boundaries. By following grain boundary concentration of Ti as a function of time at a given temperature, the diffusion coefficient for Ti in the maraging steel at 1600°F was determined. Finally, the further drastic drop in CVN energy on aging an embrittled specimen was shown to be due to a segregation of B to the already embrittled prior austenite grain boundaries, causing a further loss of cohesion.

Improved fatigue resistance of 18Ni (350) maraging steel through thermomechanical treatments

Abstract: The influence of thermomechanical treatments, ausforming and marforming, on the fatigue resistance of 18 Ni (350) maraging steel has been examined. Although the low cycle fatigue resistance of this material is essentially unaffected by these treatments, an increase of 30 pct in the low-stress, high-cycle fatigue resistance can be achieved. This increase can be explained by considering the influence of processing on the resulting precipitate and dislocation substructures. Differences in texture, residual stress level and inclusion morphology have no effect on the improved fatigue resistance.

Dynamic Fracture Toughness Measurements of High-Strength Steels Using Precracked Charpy Specimens

Abstract: The dynamic fracture toughness, K I d , was measured in a number of ferrous alloys using precracked Charpy specimens and an instrumented impact machine. The alloys investigated included quenched and tempered steels (H-11, D6AC, and 4340), 18Ni maraging steels (grades 200, 250, and 300) and a high-temperature, stainless maraging steel (Pyromet X-15). Standard Charpy specimens were precracked in fatigue and tested at either 72°F (22°C) or -65°F (-54°C). Values of K I d were determined as a function of yield strength and microstructure, and correlations were established between K I d and both the energy to initiate fracture, W m /A, and the total energy of fracture, W/A. The instrumented, precracked Charpy test is shown to be a convenient method of determining relative fracture toughness; under proper conditions this test procedure can be used to determine the dynamic plane-strain fracture toughness.

Properties of maraging steel 300 produced by powder metallurgy

Abstract: Powders of 300-grade maraging steel have been produced by two different atomizing processes and consolidated by hot isostatic pressing, extrusion, hot rolling, or a combination of these methods The tensile properties of the PM alloys were equivalent or superior to those of the conventionally cast and forged alloys, but no improvement in fatigue and fracture-toughness properties was gained Fine grain sizes (1–5 μm) were obtained by hot rolling either conventionally processed or PM bars of grade-300 maraging steel at 760, 820, and 870°C, followed by air-cooling In all cases, the tensile properties and fracture toughness were improved by grain-size refinement

Phase composition, structure, and properties of maraging steel Kh14K9N6M5

Abstract: 1. Three stages of aging are observed in cast maraging steel Kh14K9N6M5 quenched to martensite and retained austenite. In the first state (up to 550°) martensite decomposes into two solid solutions — ferromagnetic, rich in iron, and nonferromagnetic, rich in chromium, with highly dispersed R phase in the latter. In the second state (550°, 3–50 h) the two solid solutions and R phase that can be detected in powder patterns are retained. In the third stage only the intermetallic R phase is precipitated (at temperatures 600 and 550° for 100 h). 2. The highest strength corresponds to decomposition of the solid solution with formation of α phase rich in chromium and with highly dispersed R phase in it.

The effect of microstructure and strength on the fracture toughness of an 18 ni, 300 grade maraging steel

Abstract: Fractography and metallographic sectioning were used to investigate the influence of microstructure and strength on the fracture toughness (KIc) and fracture mechanism of an 18 Ni, 300 grade maraging steel. Increased yield strength from 1442 to 2070 MN/m squared through precipitation hardening results in a KIc loss from 143 to 55 MN/m superscript 3/2. Ti (C,N) Ti2S, and TiC inclusions in sizes from 1 to 8, 1 to 15, and 0.1 to 2 microns respectively serve as sites for void nucleation and lead to fracture by the dimpled rupture process in all strength levels considered. TiC nucleated dimples occupy more than half the fracture in all conditions. Void nucleation rate and resultant number of dimples per unit area of fracture increase with increasing yield strength. Average dimple size decreases with increasing strength and/or overaging which follows from the decreasing amount of stable void growth measured by sectioning tensile specimens. Void growth is assisted by crack branching along a path of TiC inclusions. Coalescence occurs in the highest strength materials by a combination of TiC void nucleation and premature separation at strengthening precipitates.

The fracture toughness and microstructure of the heat affected zone of a welded maraging steel

Abstract: The fracture toughness and microstructure of the heat-affected zone of welded 300 grade maraging steel have been studied. A wide range of welding processes has been covered using a thermal simulation technique. This approach has been justified by correlating real and simulated heat-affected zone properties and structures. Fracture toughness has been found to be a function of peak temperature, but independent of heating and cooling rates. Increases in fracture toughness were found and related to peak temperature, specific values being 75 MNm−3/2; (67·8 ksi√in) at 650°C and 66–;77·5 MNm−3/2 (59·6–70·1 ksi√in) over 12000–1400°C, as compared with the parent material value of 60 MNm−3/2 (54·2 ksi√in). These increases inf racture toughness were accompanied by decreases in tensile strength in all cases. The metallographic, fractographic and fracture toughness results have been correlated to explain the observed changes in properties. It is shown that reverted austenite gives increased toughness at 650°C ...

Fracture toughness study of a grade 300 maraging steel weld joint

Abstract: The plane strain fracture toughness (K sub Ic) of a gas tungsten-arc (GTA) welded joint prepared from 10 mm ().39 in.) thick, grade 300, maraging steel was determined. All critical K sub I values less than about 270 kp mm to the -3/2 (76 ksi in. to the 1/2) satisfied ASTM validity criteria for K sub Ic; these included determinations for all microstructures except one that was characterized by a very high stable austenite content. The effect of microstructural and property variations characteristic of the HAZ (Heat Affected Zone) on the determination of K sub Ic was studied using "weld simulation" specimens thermally treated in a Smit weld simulator. Results were correlated with those obtained from actual welds on the basis of equivalent microstructures, mechanical properties, and thermal treatments. From this study, a tendency towards embrittlement due to short time exposure at 750 C was detected. It was also noted that K sub Ic appeared to be independent of grain size.

Delayed Failure Crack and Fracture Toughness of 18 Ni Maraging Steel with Aging Structure at Low Temperature

Abstract: prior T grain size and the delayed failure crack was observed in front of the fatigue precrack. This study has been made in order to investigate these phenomena in more details and to discuss the mechanism by which KIC is increased. It is made clear that an increment in KIC is completely associated with slow crack growth of delayed failure which precedes the unstable fracture. However, its increment is not an increment in true KIC as a material constant but an increment in apparent KIC This may be due to the blunting at crack front caused by the microbranching of the delayed failure crack, the degree of which also depends upon the prior ƒÁ grain size. Tensile ductility also markedly decreases under the same condition in which an apparent KIC increases. This is attributed to the occurence of the delayed failure crack at the specimen surface adjacent to or in the necking region. Thus, the 18Ni margaing steel with low temperature aged structure shows the complicated behavior on toughness and ductility because of the high sensitivity to delayed failure. (Received Oct. 18, 1973)

Aging structure of maraging steel

Abstract: Transmission electron microscopy and electron diffraction studies were carried out in order to clarify the crystallographic properties of strengthening precipitates and aged structure in an 18Ni maraging steel.By using various zonal diffraction patterns, the co-existence of the precipitates Ni 3 Mo, Ni 3 Ti and the dispersed reverted austenite was identified, and their orientation relation-ships with respect to the martensite matrix determined.It was shown that the close-packed plane and direction of the precipitates and the reverted austenite are parallel to those in the matrix. The Widmanst?ttentype morphology in the transmission electron micrographs of the aged structure is in fact a three-dimensional skeleton structure in which the length direction of the rod-like precipitates is parallel to one of the four directions of the matrix. The true existence of the observed extended and piled-up dislocations needs further experimental verifications.The process of precipitation along dislocation lines is discussed and a possible mechanism of the cobalt-molybdenum interaction is proposed from the structural point of veiw.

Structural changes during aging of maraging steel 03Kh11N10M2T

Abstract: Electron microscopic and Mossbauer spectroscopic studies showed that aging of steel 03Kh11N10M2T produces complex structural changes due to redistribution in martensite of titanium, molybdenum, nickel, and chromium atoms, and precipitation of Ni3Ti even with brief holding (5 sec at 525°). Also, a partial α→γ transformation occurs in previously deformed samples during aging at 525°. There are good correlations between the structural changes and the changes observed in the properties of steel 03Kh11N10M2T during aging.

Aging structure of maraging steel

Abstract: Transmission electron microscopy and electron diffraction studies were carried out in order to clarify the crystallographic properties of strengthening precipitates and aged structure in an 18Ni maraging steel. By using various zonal diffraction patterns, the co-existence of the precipitates Ni3Mo, Ni3Ti and the dispersed reverted austenite was identified, and their orientation relation-ships with respect to the martensite matrix determined. It was shown that the close-packed plane and direction of the precipitates and the reverted austenite are parallel to those in the matrix. The Widmanstattentype morphology in the transmission electron micrographs of the aged structure is in fact a three-dimensional skeleton structure in which the length direction of the rod-like precipitates is parallel to one of the four 111 directions of the matrix. The true existence of the observed extended and piled-up dislocations needs further experimental verifications. The process of precipitation along dislocation lines is discussed and a possible mechanism of the cobalt-molybdenum interaction is proposed from the structural point of veiw.

Use of maraging steels for parts of metal working machines

Abstract: 1. During prolonged aging of maraging steel 03Kh11N10M2T the dimensions change considerably. 2. The wear resistance of steel 03Kh11N10M2T is low after aging, and can be increased by nitriding. 3. Two hardening heat treatments are recommended for critical machine parts: a) nitriding combined with aging at 540° for parts such as guides; b) preliminary tempering at 580°, followed by grinding and nitriding at 540°, for parts such as hypoid indexing gears. 4. Aging combined with nitriding at 540° and holding for 36 h reduces the linear dimensions by 1.2 μ/mm, and preliminary tempering at 580° for 3–5 h makes it possible to reduce this value to 0.4 μ/mm.

Mechanical pro perties of high-strength steels

Abstract: Under optimal conditions, steel 30N9K4 has the same toughness as steel 18Kh2N4VA and a higher strength (20kg/mm2 higher). The combination of mechanical properties is the same for steel 30N9K4 as for maraging steel N18 K8M3T, although it contains only half as much nickel and cobalt.

Stress corrosion cracking susceptibility of 18 Ni maraging steel

Abstract: The stress corrosion cracking (SCC) resistance of 18Ni maraging steel (grades 200, 250, 300, and 350) was determined in 3.5 percent salt (NaCl) solution, synthetic sea water, high humidity, and outside MSFC atmosphere. All grades of the maraging steel were found to be susceptible to SCC in varying degrees according to their strengths, with the lowest strength steel (grade 200) being the least susceptible and the highest strength steel (grade 350), the most susceptible to SCC. The SCC resistance of 250 grade maraging steel was also evaluated in salt and salt-chromate solutions using fracture mechanics techniques. The threshold value, K sub SCC, was found to be approximately 44 MN/sq m square root m, (40 ksi square root in.) or 40 percent of the K sub Q value.

Radiation resistance of maraging steel

Abstract: 1. The radiation resistance of maraging steels is superior to that of pearlitic steels. After irradiation with an integral neutron flux of 8·1020N/cm2 at 70–100° the ductile-brittle transition temperature of the pearlitic steel rises from −80 to +100°, while that of the maraging steel remains below −80°. The changes in the strength, ductility, and toughness are smaller than in pearlitic steels. 2. The radiation resistance of maraging steel increases with the original strength after heat treatment and with the concentration of alloying elements causing an increase in strength during aging.

Phases precipitated in grain boundaries during slow cooling of maraging steels

Abstract: Maraging steels EP678, EP679, and EP699 cooled slowly from 1250° show precipitates of various phases in the grain boundaries, mainly titanium carbonitrides, the size and quantity of which increase with cooling. With cooling to 950–850° there are also particles that consist of titanium with oxygen, nitrogen, and carbon. These particles in the grain boundaries can cause embrittlement of the steels.