Microstructural control of maraging steel C300

Abstract

The microstructure evolution and precipitation kinetics of maraging steel C300 have been studied in the aging temperature range from 400 to 600°C. The relation between mechanical properties and precipitation hardening response is explained, and modelling is used to optimise the properties. Ultrafine needle shaped Ni₃Ti phase is the main strengthening precipitate in maraging C300, and it shows very high resistance to coarsening. A spherically shaped Fe₂Mo phase is formed at higher temperatures and in the overaged condition. Inter- and intralath reverted austenite nucleates at higher temperature (～600°C). Rolling and aging treatment can produce the highest hardness by a combination of work hardening and precipitation strengthening. Microstructural evolution simulation using Monte Carlo modelling has been applied to this alloy, and the modelling has been validated by the experimental results.     

Fatigue-crack propagation in high yield-strength steels

Information on fatigue-crack growth in high yield-strength steels is necessary for toe prediction of service lives of structures subjected to fotigue loading. Therefore, as part of a long-range program to establish the mechanical behavior of high yield-strength steels, the fatigue-crack propagation in four high-strength steels-HY-80, HY-130, 10Ni-Cr-Mo-Co, and 12Ni-5Cr-3Mo-was investigated.

A technique was, developed to accurately measure incremental fatigue-crack growth by fofiowing the extension of the crack along uniformly spaced marks on the surface of a wedge-opening-loading (WOL) specimen that was cycled under constant maximum and minimum loads. The primary factor affecting crack growth rates in fatigue was found to be the applied stress-intensity range, ΔK_I. A conservative crack propagation rate for high yield-strength steels fatigue-tested at room temperature in an air environment was determined to be: .

The present results were compared with existing fatigue-crack-growth data. With one exception, all crack-growth data (for a total of 19 high yield-strength steels) could be represented by die above equation. Thus, it appears that fatigue-crack growth rates in high yield-strength steels tested in a room-temperature air environment can be represented by a single equation that is sufficiently accurate for engineering purposes.     

Fatigue-crack propagation in vinyl urethane polymers

The fatigue properties of a series of vinyl urethane polymers have been examined by determining the crack propagation rates as a function of the applied stress intensity factor. Measurements were also made of the craze lengths at the tips of the fatigue cracks, and the craze stress and crack opening displacement calculated. A model for fatigue-crack propagation based on cumulative damage within the crazed material has been proposed which is consistent with the observed results.     

Fatigue-crack propagation in a high-strength aluminum alloy

Fatigue-crack propagation experiments were carried out within the framework of linear-elastic fracture mechanics to examine the effect of water and its constituents on the rate of crack growth in a high-strength aluminum alloy over a range of test temperatures from 295 to 380° K. Dehumidified high-purity (99.9995 percent purity) argon was used as an inert reference environment. The results showed that water accelerated the rate of fatigue-crack propagation by about a factor of 10 in this temperature range, whereas dry oxygen and dry hydrogen had a negligible effect. They confirm the findings of Hartman, and Bradshaw and Wheeler that the cause for the large increase in the rate of crack growth is the formation of hydrogen gas at high pressure in the region ahead of the crack tip, driven in by the reaction of water with the-freshly created aluminum crack surfaces, as suggested by Broom and Nicholson. The results showed further that fatigue-crack propagation in water, as well as in the dry environments, is controlled by thermally activated processes, with apparent activation energies that depend strongly on the crack-tip stress-intensity parameter, K. The rate controlling process appears to be that associated with the creation of new crack surfaces in the range of crack growth rates 10^–6 to 10^–5 inch per cycle. The strong dependence of the apparent activation energy on K suggests that a careful study of the kinetics of fatigue-crack growth and of the crack growth laws is in order. Such a study should incorporate both the mechanical and chemical variables involved.

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Zusammenfassung Untersuchungen über die Bruchausbreitung beim Ermüdungsbruch wurden im Rahmen der linearelastischen Bruchtheorien ausgeführt, um den Einfluss von Wasser and seinen Komponenten auf die Bruchausbreitung pro Periode in einer hochfesten Aluminiumlegierung über einen Temperaturbereich von 295° bis 380°K zu prüfen. Getrocknetes, hochreines Argon (Reinheitsgrad: 99.9995%) wurde als träges Referenzgas benutzt. Die Resultate zeigten, dass Wasser die Ausbreitungsgeschwindigkeit des Fatiguebruchs innerhalb dieses Temperaturbereichs etwa um einen Faktor 10 heraufsetzte,während trockener Sauerstoff undtrockener Wasserstoff einen vernachlissigbaren Einfluss hatten. Dieses bestätigt die Ergebnisse von Hartman, and von Bradshaw and Wheeler, dass die Ursache (wie von Broom and Nicholson vermutet wird für das starke Anwachsen der Bruchgeschwindigkeit) die Bildungvon Wasserstoffgas bei hohem Druck in dem Gebiet vor der Rissspitze ist. Die Ergebnisse zeigten weiter, dass die Fatiguebruchausbreitung in Wasser ebenso wie in trockener Umgebung durch thermisch aktivierte Prozesse kontrolliert wird, mit einer auftretenden Aktivierungsenergie, die streng von dem Spansungsfaktor K and der Rissspitze abhängt. In einer Bereich von 2.5 × 10^–6 nis 2.5 × 10^–5 em pro Periode scheint der Steuervorgang für die Bruchausbreitung von der Art zu sein, dass er mit der Bildung von neuen Bruchflächen verbunden ist. Die strenge Abhängigkeit der auftretenden Aktivierungsenergie von K legt eine sorgfältige Untersuchung der Kinetik der Fatiguebruchausbreitung and der Bruchausbreitungsgesetze nahe. Solch eine Untersuchung sollte sowohl die mechanischen als auch die chemischen Parameter berücksichtigen.

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Résumé Des essais de propagation de fissures de fatigue ont été effectués pour étudier l'influence de l'eau et de ses composants sur le taux de croissance des fissures dans un alliage d'aluminium à haute résistance, entre 295 et 380°K. L'atmosphère de référence est obtenue au moyen d'argon sec (99.9995 %). L'eau accélère le taux de propagation par un factueur de 10 dans cette gamme de températures, l'influence de l'oxygène sec et de l'hydrogène sec étant négligeables. Nos essais confirment les résultats de Hartman, et Bradshaw & Wheeler; ils indiquent que la formation d'hydrogène gazeux à haute pression dans la région précédant immédiatement l'extrémité de la fissure cause la forte argumentation du taux de fissuration, comme suggéré par Broom & Nicholson. La propagation des fissures de fatigue dans l'eau et aussi en atmosphère sèche est contrölée par des processus activés thermiquement avec des énergies apparentes d'activation dépendent fortement de K. Entre 2,5 10^–6 et 2,5 10^–5 cm par période, le processus de contrôle parait être lie à la formation de nouvelles surfaces de fissures.Une étude approfondie de la einétique et des lois de croissance des fissures de fatigue semble indiquée par la forte influence de K sur l'énergie apparente d'activation. Les variables méchaniques et chimiques devraient être incorporées à cette étude.

     

Improvement in mechanical properties of C300 maraging steel by application of VAR process

The prime objective of this research was to improve the mechanical properties of C300 maraging steels originally cast in the vaccum induction melting (VIM) and AIM furnaces by application of vaccum arc remelting (VAR) process. For this purpose two sets of C300 maraging steel with different amounts of Ti were cast in two different types of furnaces. Three bars with 1, 1.5 and 2 wt% of Ti were produced in the argon induction melting (AIM) furnace and two bars with 0.65 and 1 wt% Ti in the VIM furnace. Then all of the bars were subjected to VAR process in order to study the effects of this process on the reduction of N and O gases and inclusions, hence improvement in the mechanical properties. The results show that the total reduction of N and O gases in the bars cast in the AIM furnace was up to 40%, the amounts of inclusions irrespective of their kind reduced up to 30% while ductility and impact energy increased up to 40%. However, these parameters in the bars cast in the VIM furnace changed as follows: total gas reduction decreased by 12%, ductility and impact energy increased by 30% and 47%, respectively. So this research provides a very informative data base for those who are interested in studying the effect of VAR process on the mechanical properties of this kind of cast steel.     

Fatigue-crack propagation characteristics of aluminum alloys in thick sections

Fatigue-crack propagation data have been obtained for a variety of aluminum alloys, tempers, and products in relatively thick sections of interest for large aircraft shapes. For the higher stress-intensity ranges, the alloys rate in about the same order with regard to resistance to fatigue crack propagation as with regard to plane-strain fractare toughness. However, for low stress intensity ranges (i.e. short cracks or low load ranges) the rate of fatigue-crack propagation was lowest for two alloys, 2020-T651 and 2014-T652. which have low plane-strain fracture toughness. The relative order for different specimen orientations was generally consistent with that based upon plane-strain fracture toughness. High humidity results in higher rates of fatigue-crack propagation, with the effect indicated to be largest for those alloys which are susceptible to stress corrosion cracking.     

Review: Fatigue-crack propagation in metallic and polymeric materials

Fatigue-crack propagation in metals and polymers is examined from the point of view of both the materials scientist and the engineer. Progress over the last few years is outlined, and possible future developments suggested.     

Fatigue-crack propagation behavior in monolithic and composite ceramics and intermetallics

We study microstructural mechanisms of fatigue crack growth in advanced monolithic and composite ceramics and intermetallics. Much attention is devoted to the contribution of cycling loading to the hindrance of mechanisms that lead to a considerable increase in toughness (crack-tip shielding) of these materials. For example, in intermetallics with a ductile phase, such as -TiNb-reinforced -TiAl or Nb-reinforced Nb₃Al, a significant increase in toughness caused by the presence of uncracked ductile phase inside a crack is retarded under cyclic loading because ductile particles immediately fail by fatigue. Similarly, in monolithic ceramics, e.g., in alumina (aluminum oxide) or silicon nitride, the significant plasticization appearing under monotonic loading is greatly diminished under cyclic loading due to gradual wear at the grain-matrix interface. In fact, the nature of fatigue in such low-plasticity materials differs essentially from the well-known mechanisms of fatigue in metals and is governed, first of all, by a decrease in shielding, which depends on the loading cycle and time. The susceptibility of intermetallics and ceramics to fatigue degradation under cyclic loading affects seriously the possibility of structural use of these materials in practice. In particular, in this case, it is difficult to apply strength calculation methods that take into account the presence of defects and to implement life-prediction procedures.Center for Advanced Materials, Lawrence Berkeley Laboratory, University of California, USA. Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 3, pp. 7–35, May–June, 1994.     

The preferred fatigue crack propagation mode in a M250 maraging steel loaded in shear

Macroscopic torsional fatigue cracks are shown to propagate in shear, in plain tubular specimens, in the M250 maraging steel, for stress ranges from 90% down to 40% of the yield stress. This cannot be explained in terms of microcrack coalescence for the smallest stress range, for which microcracks are scarce. The kinetics and mechanisms of mode II fatigue crack growth are thus investigated, using precracked CTS or tubular specimens. For a high Δ K _II , slowly decelerating mode II propagation takes place for a distance that increases with Δ K _II before branching occurs. Friction stresses along the crack flanks shield the applied load and explain this deceleration. An inverse analytical procedure is used to derive the effective stress intensity factor, allowance being made for friction effects, from displacement profiles measured from microgrids using a scanning electron microscope. The measured crack growth rates correlate much better with the effective stress intensity factor than with the nominal Δ K _II value. The crack paths observed in torsion are discussed in terms of maximum crack velocity.     

Study of the austenite quantification by X-ray diffraction in the 18Ni-Co-Mo-Ti maraging 300 steel

In this paper, quantifications of the austenitic phase in a maraging 300 steel heat treated at different temperatures and periods of time were carried out using the direct comparison method by X-ray diffraction. The influence of taking into account the chemical compositions of austenite and martensite phases in the results by the direct comparison method was evaluated. In order to analyze the instability of austenite under plastic deformation, the quantifications were carried out with and without previous grinding of the samples. The behavior of the austenite volume fraction against aging time at 560°C, 600°C and 650°C were determined. The variation of the martensite lattice parameter with aging time was also analyzed. The results show an increase of the austenite content with aging time at 560°C and 600°C. At 650°C, however, the austenite content present at room temperature decreases and the martensite parameter increases with the aging time above 1 h.     

Residual strength of maraging steel

Surface crack tension (SCT) specimens of 18 Ni 1800 MPa grade maraging steel (parent metal and welded) were used to evaluate the residual strength in 0.015 and 0.03m wide panels. For 0.03 m wide panels which satisfy infinite plate condition up to 0.006 m surface crack length, the residual strength variation between minimum and maximum crack sizes lie within 5% of the ultimate tensile strength (UTS) of weldments. Also for larger crack sizes both 0.015 and 0.03 m wide panels yields comparable values for residual strength. The results have significant bearing on flaw detection by non destructive testing (NDT) method which could be used for quality and reliability check on the fabricated component. Stress intensity values for these data makes an interesting observation on failure mode.     

Hydrogen embrittlement of maraging steel

Specimens of 18 Ni 1800 MPa (M250) grade maraging steel were charged with different quantities of hydrogen by an electrochemical method. The tensile properties and fracture characteristics have been correlated with the quantity of hydrogen picked up by the material. A drastic decrease in ultimate tensile strength from 1768 M Pa to 750 M Pa, elongation from 6% to less than 2%, and reduction in area from 55% to less than 5%, were observed as the hydrogen content of the steel increased from less than 2 p.p.m. to 7 p.p.m. However, hydrogen does not affect the hardness of the steel. The effect of baking at different temperatures on hydrogen embrittlement was also studied. A change in fractographic features from ductile dimples to mixed mode, intergranular separation and transgranular cleavage was observed as the amount of absorbed hydrogen increased.     

微波消解ICP-AES法检测马氏体时效钢18Ni300粉体成分

18Ni系马氏体时效钢广泛应用于日常生活、生产及尖端科技等多个领域,18Ni300作为金属增材制造技术(3D打印)中选区激光熔化技术(selective laser melting,SLM)的专用马氏体时效钢,已获得广泛关注。18Ni300中不同元素所起的作用不同,因此对18Ni300的元素成分测定尤为关键。该文提供一种快速准确的微波消解ICP-AES法测定马氏体时效钢18Ni300粉体中Ni、Co、Mo、Ti、Al含量的新方法,克服现有国标检测的不足。采用盐酸和硝酸进行试样消解,避免氢氟酸对仪器的腐蚀与污染,同时匹配酸度和基体含量,减少测试过程中的基体效应,采用内标法对待测元素进行测定,加标回收率在90%~110%,效果良好。     

Notch strength of maraging steel

A series of tensile tests was carried out on steel cylinders, on which had been machined circumferential notches of known root radii.

The properties of the notch ductility of Maraging steel were compared to similar properties of XC 10, a mild steel possessing a great ductility.

Two types of relationships were obtained: Firstly, it can be shown that the algebraic relationship between the elastic stress concentration factor and the notch strength ratio is also dependant on the root radius of the notch, and secondly, a connection was established between an extension parameter of the plastic zone around the notch and the ultimate tensile strength.

These expressions can be used to predict the ultimate tensile strength of a specimen of specific geometry.     

Sustained load failures in maraging steel weldments

To evaluate maraging steels for possible rocket motor case applications, sustained load tests in air were performed using fatigue pre-cracked partial-thickness crack tensile specimens. Welded sheets were of primary interest since they represented the most critical area.     

无钴马氏体时效钢的研究现状

综述了无钴马氏体时效钢的发展和研究现状,阐述了无钴马氏体时效钢的时效组织结构及强化机制,分析了合金元素在马氏体时效钢中的作用,并提出了无钴马氏体时效钢未来的发展趋势.     

Crack growth characteristics of maraging steel

Compact tension (CT) and surface crack tension (SCT) specimens of 18 Ni 1800 MPa grade maraging steel (parent metal and welded) were studied for the cyclic crack growth rate applying in sinusoidal (SL) and block loading (BL) conditions. The BL cycle was designed to simulate the stress intensity levels experienced during proof pressure and static test of a fabricated rocket motor. The threshold stress intensity for stage II crack growth for weldment is found to be about 0.6 that for the parent metal. For a given stress intensity, the crack growth rate for weldment and parent metal are in the same range under SL or BL. Through-thickness cracks (laboratory condition) in CT specimens and part-through thickness cracks (service condition) in SCT specimens show good correlation for cyclic crack growth rate. The results are of great significance for considering the reuse of the maraging steel rocket motor case.     

Environmental stress intensity for maraging steel

Stress intensity, K, values for 18 Ni 1800 MPa maraging steel stressed to 90% of its K _IC level and exposed to water, hydraulic oil (servo 317) and water inhibited with 0.5, 1.0, 1.5 and 2% dichromate (pH about 7.5) for various durations were determined using the modified wedge opening loading (WOL) technique. Experiments revealed that there is no appreciable deterioration in K in oil and water inhibited with 1.5% and 2% dichromate even after 25 days exposure. The threshold stress intensity is reached after 15 days in 0.5% dichromate inhibited water at 40 MPa m^1/2. Stress corrosion crack growth rate in water is about 40 times the rate in hydraulic oil. Crack path characteristics of the WOL-tested maraging steel specimens in different environments are briefly mentioned.